N-Heterocyclic Carbene Complexes of Copper, Nickel, and Cobalt

Review Cite This: Chem. Rev. XXXX, XXX, XXX−XXX

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N‑Heterocyclic Carbene Complexes of Copper, Nickel, and Cobalt Andreas A. Danopoulos,*,†,‡ Thomas Simler,*,‡,§ and Pierre Braunstein*,‡ †

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Laboratory of Inorganic Chemistry, National and Kapodistrian University of Athens, Panepistimiopolis Zografou, Athens GR 15771, Greece ‡ Université de Strasbourg, CNRS, Institut de Chimie UMR 7177, Laboratoire de Chimie de Coordination, Strasbourg 67081 Cedex, France ABSTRACT: The emergence of N-heterocyclic carbenes as ligands across the Periodic Table had an impact on various aspects of the coordination, organometallic, and catalytic chemistry of the 3d metals, including Cu, Ni, and Co, both from the fundamental viewpoint but also in applications, including catalysis, photophysics, bioorganometallic chemistry, materials, etc. In this review, the emergence, development, and state of the art in these three areas are described in detail.

CONTENTS 1. General Introduction 2. NHC Copper Complexes 2.1. Introduction 2.2. Mononuclear CuI Complexes 2.2.1. Monodentate NHC Ligands 2.2.2. Complexes with Bidentate bis(NHC) Ligands 2.2.3. Complexes with Bidentate Heteroatom Functionalized NHC Ligands (Lig) 2.2.4. Complexes with Tridentate Heteroatom Functionalized NHC Ligands 2.2.5. Complexes with Multidentate Heteroatom-Functionalized NHC Ligands (Lig) 2.3. Binuclear and Polynuclear CuI Complexes 2.3.1. Homometallic Binuclear and Polynuclear CuI Complexes 2.3.2. Heterometallic Multinuclear Complexes 3. NHC Nickel Complexes 3.1. Introduction to Ni 3.2. Mononuclear Complexes 3.2.1. Mononuclear Ni0 Complexes 3.2.2. Mononuclear NiI Complexes 3.2.3. Mononuclear NiII 3.3. Binuclear and Polynuclear Complexes 3.3.1. One-Atom Halide Bridges and Related Complexes 3.3.2. One-Atom Group 16 Bridges and Related Complexes 3.3.3. One-Atom Group 15 Bridges and Related Complexes 3.3.4. One-Atom Group 14 Bridges and Related Complexes 3.3.5. Two- and More-Atom Bridges and Related Complexes 4. NHC Cobalt Complexes © XXXX American Chemical Society

4.1. Introduction to Co 4.2. Mononuclear Co0 Complexes 4.2.1. Monodentate Carbene Ligands 4.3. Mononuclear Co−I Complexes Monodentate Carbene Ligands: Type [Co(NHC)2L2]−(Cation+) 4.4. Mononuclear CoII Complexes 4.4.1. Monodentate Carbene Ligands 4.4.2. Bidentate Bis-Carbene Ligands 4.4.3. Tridentate and Multidentate Tris- and Polycarbene Ligands 4.4.4. Functionalized NHCs 4.5. Mononuclear CoI Complexes 4.5.1. Monodentate Carbene Ligands 4.5.2. Functionalized NHCs 4.6. Mononuclear CoIII Complexes 4.6.1. Monodentate Carbene Ligands 4.6.2. Tris-Carbene Ligands 4.6.3. Functionalized NHCs 4.7. Mononuclear CoIV and CoV Complexes 4.7.1. Monodentate Carbene Ligands 4.7.2. Functionalized Carbene Ligands 4.8. Polynuclear Homometallic 4.8.1. Binuclear Complexes 4.8.2. Tetranuclear Complexes 4.9. Polynuclear Heterometallic 5. General Conclusion Associated Content Special Issue Paper Author Information Corresponding Authors ORCID Present Address

B C C D D BE BE BF BH BI BI BY CA CA CB CB CL CV EC EC EE EF EF EG EI

EI EJ EJ EN EN EN EN FF FF FH FN FN GF GK GK GK GL GR GR GT GT GT HB HC HC HE HE HE HE HE HE

Received: August 10, 2018

A

DOI: 10.1021/acs.chemrev.8b00505 Chem. Rev. XXXX, XXX, XXX−XXX

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Review

in various review articles, including on coinage metal NHC complexes,7 coinage metal hydrides,8 copper NHC complexes in catalysis,9 binuclear copper complexes in catalysis,10 CuNHC complexes as NHC transfer reagents,11 Fe NHC complexes and catalytic applications,6,12,13 NiI complexes,14 C−C and C-heteroatom cross-coupling with Ni-NHC species,15,16 NHC ligand design,17 pincer complexes with NHC donors,18 multidentate NHC complexes with 3d metals,19 oxygen, sulfur or phosphorus atom functionalized NHC complexes,20 cAAC complexes including those of 3d metals,21,22 and mesoionic NHC complexes including those of 3d metals.23 The review is partitioned according to the metal (Cu, Ni, Co) and further into sections on mononuclear and polynuclear complexes with ascending nuclearity and ligand denticity of the NHC containing ligands (monodentate, bidentate, polydentate exclusively with NHC donors, bidentate, polydentate heteroatom functionalized, etc.). For the mononuclear complexes featuring monodentate NHC ligands, a more detailed classification is introduced according to the metal oxidation state (in particular for Ni and Co complexes), their homoleptic or heteroleptic nature, and the column of the Periodic Table to which the donor atom(s) of the non-NHC ligand(s) belong. Within heteroleptic complexes, all coligands (not the NHCs) are grouped by adhering to the LX ligand description as defined for neutral ligands (despite the use of metal oxidation states vide supra), while the notion of the coordination number for classification purposes is relaxed, even though coordination geometries are discussed for the majority of the complexes throughout the text. Separate sections are dedicated to mononuclear complexes featuring ligands with more than one NHC donor and to complexes with ligands featuring NHC donors functionalized with heteroatom donors. Bi- and poly nuclear complexes form a separate section classified according to the number of atoms involved in the bridge and ordered according to the group of the Periodic Table to which the latter belong, starting from Group 17. Although this “hybrid” ligand-based grouping approach constitutes one of the few possible empirical classifications of the relevant coordination chemistry, it is advantageous in highlighting comparable patterns of structure and reactivity as a function of the type and number of ligands forming the coordination sphere. Needless to mention that the outline detailed above frequently cannot be rigidly adhered to in view of interconversion of complexes belonging to different classes (e.g., mononuclear vs binuclear, ligand associations-dissociations, redox reactivity, etc.); in this case, to avoid repetition, the relevant sections and schemes are cross-referenced within the document. The generic term NHC is used for all types of Nheterocyclic carbenes, irrespective of the nature of the heterocycle where the divalent C donor is integrated. Heteroatom (B, Si, etc.) NHC analogues are treated as coligands and therefore appear only in combination with NHCs and not in the absence of the latter. In the description of NHCs, widely used acronyms are maintained and used throughout without assignment of additional numbering (e.g., IPr, SIPr, IMes, Me2cAAC, etc.) (see also list of abbreviations). Superscripts Cu, Ni, and Co in the compound numbering are referring to entries found in Cu, Ni, and Co sections, respectively. Sequential numbering is used throughout with arabic numerals combined (or not) with letters from the latin alphabet and occasionaly accompanied by the superscripts s and u. The latter refer to the presence of imidazol(in)-2-

HE HE HE HE HG

1. GENERAL INTRODUCTION Although some of the first attempts to synthesize complexes from stable or transient N-heterocyclic carbenes (NHC) involved 3d base metals,1−4 the first years of the systematic study of the coordination and catalytic chemistry of NHC complexes focused on heavier transition metals and in particular platinum group metals. Plausible reasons that may be responsible for this include the ease of study of diamagnetic, usually air stable species comprising strong, inert metal-CNHC bonds, the efficiency of the popular Ag transmetalation reactions5 with the heavier metals, and the generally strong interest at that time in specific catalytic applications based on heavier transition metals, like C−C and C-heteroatom crosscoupling (Pd), alkene metathesis (Ru), transfer hydrogenation (Rh, Ir), and various carbonylations (Rh, Ir). The often-used comparison of the σ-donor properties of NHC ligands and trialkylphosphines without resorting to systematic computational work in order to elucidate the nature of the metal-NHC bonding also oversimplified the perception of this bond with metals across the Periodic Table. However, soon it became clear that NHCs were not simply mimicking phosphines but constituting a class of versatile ligands with unique donor and steric characteristics showing a potential for unprecedented chemical reactivity but also supporting complexes with unique physical and medicinal properties. In addition, the development of NHC donors based on a range of heterocyclic rings (mainly 5- and 6-membered), the modulation of electrondonor characteristics by mesoionic NHCs, the emergence of cyclic alkyl-amino carbenes (cAACs), and the numerous possibilities for accessing new ligand designs incorporating more NHC donors or other “classic” functionalities, fermented the interest and curiosity in the chemistry of NHCs with 3d metals. Today, there is a substantial amount of information on complexes that comprise at least one NHC donor to 3d metals (in particular Cu, Ni, Co, and Fe), spanning aspects of structural, stoichiometric, and catalytic reactivity, bioinorganic chemistry, materials chemistry, and photophysical and magnetic properties. The research area is vibrant and branching out to concepts and directions of fundamental or applied interest that were unforeseen before, including the stabilization of low coordination numbers, unusual oxidation states, reactive or transient species coordinated on metals, small molecule coordination and activation, synthesis of fine and commodity chemicals, photocatalysis, photophysics to mention a few. This coincides with the strong desire to expand the use of base metals for sustainable chemistry. In this contribution, we provide a detailed account of the NHC complexes with Cu, Ni, and Co, stretching back to the description of the first complexes of this type. The relevant coverage of Fe complexes has recently appeared and is not pursued further here.6 The review deals exclusively with welldefined complexes; species formed in situ in catalytic systems by mixing (pro)ligands with metal precursors are not systematically considered. Diverse aspects and properties of NHC complexes in part dealing with 3d metals have appeared B


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Scheme 1. General Synthetic Methods to Access Cu-NHC Complexes

number 3, and 15% coordination number 4 (cf. 65% of the bonding interactions) originating from the Coulomb attraction between the positively charged Cu atom and the σ-electron pair of the C(NHC). The covalent part of the bonding shows little πbackbonding, which is comparable to π-backbonding in classical Fischer carbene complexes bearing two π donors on the carbene carbon.54,55 The comparative study of the bonding in [CuCl(NHC)] [NHC = simplified “normal” imidazolylidene (bound from C2) and “abnormal” imidazolylidene (bound from C4)] showed also that the total strength of the Cu-C(NHC) bond depends on the orbital interactions and electrostatic attraction. The calculated bond dissociation energy trend, aNHC > NHC (C2-coordinated), was accounted for by the higher energy level of the σ-lone-pair orbital in the abnormal NHCs, as well as the increased electrostatic attraction in this case.56 2.2.1.1.2. Type [CuF(NHC)]. Copper(I) fluoride complexes are rare, owing to the lability of the Cu-F bond, in part originating from the mismatch between the hard fluoride and the softer copper center. In the realm of CuI−NHC complexes, mononuclear, linear two-coordinate compounds [CuF(NHC)] were prepared from well-defined or in situ generated [Cu(OtBu)(NHC)] by the reaction with “NEt3·3HF” in anhydrous benzene57,58 or by PhC(O)F in toluene;59 the reaction of [Cu(μ-OtBu)(ICy)]2 with “Et3N·3HF” also provided mononuclear [CuF(SICy)].60 Alternatively, [Cu(OTf)(IPr)] can be converted to [CuF(IPr)] by the reaction of CsF or KF in dioxane at 45 °C for 10 min (Scheme 10).61 Adjustment of the stoichiometry and carrying out the fluorination in THF resulted in the formation of the mononuclear copper(I) bifluoride complexes 22aCu and 22bCu, which could be obtained directly from the corresponding [CuCl(NHC)] by the action of AgHF2 in THF (Scheme 11).62

Scheme 12. Dinuclear Cu(I) Complexes with a Bridging μ2Fluoride Ligand

fluoride in the dinuclear species is highly labile, undergoing facile hydrolysis, and can also be displaced by polar solvents. Complex 23bCu reacts easily with silanes by displacement of F by H (see section 2.3.1.1.3) and shows astonishing subtle differences in reactions toward the substrates in the Cu(NHC)-catalyzed fluorination of triflates, the hydroalkylation of alkynes, coupling reactions, etc.63 2.2.1.1.3. Type [CuX(NHC)] (X = Group 16 donor OH, OR, Carboxylates, HS, SR, SAr): Alkoxides and Phenoxides. The first mononuclear two-coordinate Cu alkoxide [Cu(OtBu)(IPr)] (27auCu) was prepared in good yields by salt metathesis of [CuCl(IPr)] with one equivalent of NaOtBu (route A, Scheme 13).64 Direct reaction of two equivalents of NaOtBu with IPr·HCl and CuCl led to the isolation of the homoleptic [Cu(IPr)2]+ (described below in section 2.2.1.2). The complex [Cu(OtBu)(IPr)] represents a milestone in the chemistry of Cu-NHC complexes related to homogeneous catalysis because of its versatility and the wide range of transformations in which the OtBu group can be involved under mild stoichiometric or catalytic conditions. Since the discovery of 27auCu, other analogues including 27asCu, 27bu/sCu, and 27gu/sCu species have been accessed by essentially the same methodology.60,65,66 Surprisingly, 27gsCu adopts a dinuclear structure with bridging tBuO and has limited stability, decomposing after a few hours at room temperature.60 Moreover, additional indirect transformations have been shown to lead to mononuclear twocoordinate [Cu(alkoxides)(NHC)] and [Cu(phenoxides)(NHC)]. Although they are of limited preparative interest, they map out important reactivity patterns of the twocoordinate CuX(NHC) system: alkanolysis of [CuMe{(S)-

Scheme 11. Synthesis of Mononuclear Copper(I) Bifluoride Complexes

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Scheme 13. Synthesis of Various [CuX(NHC)] Complexes (X = Anionic O Donor)

Scheme 14. Synthesis of [CuX(NHC)] Complexes with Diamido- or Diamino-Expanded Ring NHCs (X = Anionic O Donor)

27auCu, opened new ways for the derivatization of twocoordinate Cu(NHC) centers with Cu-H, Cu-B, Cu-C, and Cu-Si reactive functionalities and subsequent applications in catalysis. The reactivity of 27auCu is summarized in Scheme 15. Seminal work described the synthesis of the boryl complex 30aCu from 27auCu and [B(pin)]2; 30aCu catalyzed the reduction of CO2 to CO with concomitant transfer of one oxygen atom of CO2 to boron (see section 2.2.1.1.8, Reaction Involving Reduction of CO2 to CO, for discussion).73 Reaction of 27auCu with SiH(OEt)3 gave the dinuclear complex 31auCu with symmetrical hydride bridges and limited stability in solution at ambient temperature (Cu-H at δ = 2.67 ppm in C6D6); this, in combination with the high solubility of 31auCu in nonpolar solvents, hampered its isolation in pure form and in good yields.64 Arylation or alkynylation of Cu in 27auCu by the corresponding boronic esters BR(pin) (R = aryl, alkynyl) gave selectively the copper alkynyls and aryls 32auCu, 25huCu, and 25nuCu, respectively,70,74,75 without competing formation of diorganocuprates. The insertion of silylene 33Cu into the Cu-OtBu bond of 27auCu gave the addition product 34aCu; a family of related complexes was studied in the context of the effect of heteroatom-substituted anionic silyl group nucleophilicity on the reduction of CO2 to CO in well-defined complexes. 76 Reaction of 27a uCu with tetrafluoro- or pentafluoro-benzene gave after C−H activation the fluoroaryl complexes of type 35Cu and 36Cu.77 Reaction of 27auCu with the simple silyl-boranes (pin)B-ER3 (ER3 = SiMe2Ph, SiPh3), or the stannyl-borane [(C2H4)(iPrN)2]B−SnMe3 (37Cu), gave the complexes [Cu(ER3)(IPr)] (38aCu, 38bCu) and 39aCu, respectively (see also Scheme 37).70,78 The silylation reactions using the silyl- and stannyl-boranes most likely involve σ-bond metathesis and selectively produce silyl- or stannyl-complexes but only traces of boryl species. This has been attributed to the preferred quaternization of R3Si-B(pin) rather than pentacoordination at R3Si-B(pin) in the metathesis transition state.79 The complexes 38aCu and 38bCu are convenient sources of nucleophilic silyl groups, for example in conjugate additions to unsaturated carbonyl compounds80 and other catalytic applications. The complex 27auCu served as starting material for the synthesis of trifluoromethyl complexes; initial efforts using one equivalent of SiMe3CF3 as source of CF3 led to the formation of 40Cu, which despite featuring the mononuclear two-coordinate Cu-CF3 moiety, also showed remote functionalization of the IPr ligand by SiMe3 (see also section 2.2.1.1.5).

IPr}] (25au/sCu and 25buCu) with phenol or ethanol leads to phenoxides 27cu/sCu, 27duCu, and ethoxides 27eu/sCu and 27fuCu, respectively, with release of CH4 (route D); in all cases, the mononuclear nature of the products was established crystallographically.67,68 Preparative methodologies for complexes 25Cu are discussed later (section 2.2.1.1.5 and Scheme 28). The mononuclear hydroxide [Cu(OH)(IPr)] (26Cu) is available in good yields from the reaction of [CuCl(IPr)] with CsOH in THF. It can participate in a range of heteroatom-H activation reactions under release of H2O (see below); with MeOH and tBuOH, 26Cu gives the corresponding alkoxides almost quantitatively (route B).69 Following an alternative preparative strategy, the [Cu(OtBu)(NHC)] complexes were obtained by the reaction of the free NHCs with [Cu(OtBu)]4 (route C). The method has a good scope with respect to the nature of the NHC for the synthesis of tert-butoxides.70 Recently, route D has been applied to the synthesis of [Cu(OtBu)(NHC)] (29aCu and 29bCu), where NHCs are the diamido- or diamino-expanded ring NHCs (RE-NHCs). Although 29bCu has the same stability as the imidazol-2ylidene analogues, 29aCu is thermally and hydrolytically less stable, undergoing decomposition by ring-opening reaction; 29aCu and 29bCu can be accessed indirectly via the mesityl complexes 28aCu and 28bCu, which in turn can be easily prepared by the reaction of the isolated free NHCs with [Cu(Mes)]5 (Scheme 14).71 Exchange of the -OtBu with -OSiPh3 by the reaction of 29aCu with Si(OH)Ph3 gave the siloxide species 29cCu.72 Reactivity of [Cu(OR)(NHC)] and [Cu(OH)(NHC)]. The study of the reactivity of complexes 27aCu−27iCu, in particular I


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Scheme 15. Diverse Reactivity of the Complex [Cu(OtBu)(NHC)] (27auCu)

Mechanistic details on this process have not been established.81 Alkenes substituted with electron-withdrawing groups insert into the Cu-O bonds of Cu(IPr) alkoxides or phenoxides with anti-Markovnikow regioselectivity; these reactions are best described as nucleophilic additions of the coordinated alkoxides to the alkene.82−84 The aforementioned transformations have been extended to the catalytic intramolecular hydroalkoxylation of alkynes (Scheme 16).84

Scheme 17. Diverse Reactivity of the Hydroxide Complex [Cu(OH)(IPr)] (26Cu)

Scheme 16. Intramolecular Hydroalkoxylation of Alkynes

The complex [Cu(OH)(IPr)] (26Cu) exhibits diverse X−H (X = C in all hybridization forms with C−H acidic protons in the range of pKa = 27−30) and E−H (E = N, P, O, S) activation chemistry by virtue of the basicity and the steric exposure of the coordinated hydroxyl group. The involvement of a protonolysis rather than a free radical mechanism during the X−H activations was verified experimentally in a few cases by the course of the reactions being unaffected in the presence of TEMPO. Selected Si-N and Si-C bonds were also cleaved. Typical examples are shown in Scheme 17.69,85 The cAAC complex 11eCu was used for the synthesis of the copper hydroxide 46Cu by salt metathetical reaction with KOH in the presence of tBuOH; in a way analogous to 26Cu, complex 46Cu underwent a clean reaction with phenylacetylene

to give the copper acetylide 47Cu. Complexes with anionic heteroatom donors such as phenoxides, thiophenoxides and anilides were easily prepared from 11eCu by in situ formation J


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Scheme 18. Reactions of the Copper Hydroxide cAAC Complex 11eCu

postulated in the reaction of [Cu{B(pin)}(IPr)] with trifluoromethylketones. In this case, in the β-B(pin)substituted alkoxide formed by the insertion of the trifluoromethylketone into the Cu-B(pin), a β-F-B(pin) elimination provides the route to the reactive enolate formation, which nucleophilically adds to give a product of net C−F activation of the trifluoromethyl group of the ketone (Scheme 19).87 Cu(NHC)-carboxylates. There is a wide range of [CuI(carboxylate)(NHC)] complexes that have been obtained as products of the insertion of CO2 into (NHC)Cu-X bonds (X = H, C) that are described in (section 2.2.1.1.8). Some [CuI(carboxylate)(NHC)] complexes are available by the reaction of the NHC with copper carboxylate salts; the acetates have found use as precursors to [CuMe(IPr)] by reaction with AlMe2(OEt) or AlMe3, generally providing cleaner substitution than the halides.27,68 Thioalkoxides and thiophenoxides. CuI thiolates are usually polymeric or well-defined polynuclear complexes. However, the use of bulky monodentate NHCs with strong Cu-CNHC bonding and reduced neutral NHC ligand dissociation opened the way for a wide range of two-coordinate Cu(NHC) alkyl-, aryl-, and silyl-thiolates and initiated the study of their reactivity, especially in relation to the role of low coordination number copper sites in metalloenzymes. In addition, they served as simple models for, or constituent blocks of, higher nuclearity copper−sulfur clusters, which feature interesting photophysical properties, like quantum confinement88 and photoluminescence.89 In the first mononuclear, two-coordinate thiolate complexes, the ligand combinations employed were IMes and I(S)Pr with PhS- and BnS-thiolates (Bn = benzyl) (e.g., 51bCu, 51eCu, 51fCu, and 51gCu) (Scheme 20). The complexes were prepared either from [CuCl(NHC)] or [CuMe(NHC)] by alkali metal salt metathetical or protonolysis reactions, respectively.90 Extension to combinations of the less bulky Me2IiPr with the sterically encumbered (terphenyl)S-thiolate, (terphenyl = 2,6-bis(2,4,6triisopropylphenyl)phenyl-) 51aCu,91 of the (S)IPr with tBuS-92 51jCu and SSiMe3-thiolate 51hCu,93 of the IPr* with SSiMe3-thiolate93 51lCu, and of the IMes with triptycenyl-S51eCu and Si(iPr)3S- thiolate,94 51iCu was achieved following salt metathesis methodologies with alkali metal (Li, Na, K) or

of [Cu(OtBu)(cAAC)] through reaction with NaOtBu; protonolysis of the latter with a range of substituted phenols, anilines, and thiophenols provided a clean route to the respective phenoxides, anilides, and thiophenoxide (Scheme 18).46 All these complexes show good thermal stability and have been studied in the context of their photoluminescence properties. Cu(NHC)-enolates. There is a limited number of welldefined [Cu(enolate)(NHC)] complexes that have been postulated or isolated. They occur as intermediates in the (Cu-NHC)-catalyzed reaction of α,β-unsaturated carbonyl compounds (ketones, aldehydes, and esters) with the silyl borane B(pin)-SiMe2Ph which gives after hydrolysis β-silyl carbonyl/carboxyl species (Scheme 19).80 Stoichiometric Scheme 19. Formation of Cu-NHC Enolates

reactions of a range of α,β-unsaturated carbonyl substrates with [Cu(SiMe2Ph)(IPr)] proceeded to the formation of the O-bound enolate, except with α,β-unsaturated esters, where the C-bound regioisomer was obtained. A range of complexes were identified in solution by NMR techniques and in the solid state crystallographically.86 An O-bound enolate has been K


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analogue has been used as a method to construct bi- or trinuclear Cu complexes with μ2-S or μ3-S bridging atoms, which are insightful as models of the heterobimetallic Cu-Mo active site of carbon monoxide dehydrogenase (CODH), catalyzing the reduction of CO2 to CO. They are also useful as model/constituent blocks of the tetranuclear copper site in nitrous-oxide reductase (N2OR), which catalyzes the reduction of nitrous oxide to dinitrogen and water. Thus, reaction of 51hCu with [Cu(IPr)(NCMe)]+ leads to the dinuclear 53Cu by substitution of the labile MeCN; further reaction with 21dCu gave after SiFMe3 elimination the trinuclear 54Cu (Scheme 21). All three transformations proceed cleanly and in high yields. The Cu centers in 53Cu and 54Cu have nearly linear coordination geometries (169−171°) and bridging pyramidalized S (angle sums at S ca. 325 and 340°, respectively); complex 54Cu features a C3 symmetric molecular structure.93,95 The dinuclear complex [Cu(μ2-S)(IPr*)]2 (55Cu) could also be obtained by intermolecular elimination of tBuOH from the SH of 51mCu and [Cu(OtBu)(IPr*)]. Similarly, the trinuclear [CuI(μ3-S)(IPr)]3 became available via a synthetic sequence involving [Cu{S(SiMe3)}(IPr)], [Cu(IPr)(NCMe)]+, and [CuF(IPr)] precursors, coordinating sequentially at a single S atom (Scheme 21).95 The complex 51dCu with the ligand η1κSC(O)CH3) (Scheme 20) has been prepared as a synthon to analogues of the postulated intermediate Mo(μ-OSCO)Cu in the catalytic cycle of carbon monoxide dehydrogenase (CODH).96 Mononuclear 51gCu and 51jCu catalyze the rapid, reversible transnitrosation of S-nitrosothiols RSNO, which constitute biological sources of NO, without catalyzing the loss of NO gas. Although the complexes were unreactive toward NO, they reacted with NO+ to give dinuclear thiolato-bridged species 56Cu after nitrosothiols formation and release (Scheme 22).92 The complexes 56Cu were prepared also by a method analogous to 53Cu. 2.2.1.1.4. Type [CuX(NHC)] (X = Group 15 Donor Amido, Anilido, Phosphido): Amido and Anilido Complexes. The reaction of N−H acidic compounds like aniline and pyrrole

Scheme 20. Synthesis of Cu-NHC-Thioalkoxides and -Thiophenoxides

Tl thiolates. A unique complex featuring the hydrothiolato-SH and IPr* 51mCu was obtained from [CuCl(IPr*)] and KHS in THF/methanol (Scheme 20).93 Finally, the only Cu thiophenolato complex supported by cAAC 52Cu was obtained by the in situ reaction of the corresponding [Cu(OtBu)(cAAC)] with thiophenol.46 The complexes 51gCu and 51fCu, their SIPr analogues, and 51bCu and 51cCu have been studied as active catalysts in the anti-Markovnikov hydrothiolation of alkenes bearing electronwithdrawing groups; the complexes with bulky IPr NHC reacted slower than the IMes analogues 51bCu and 51cCu.90 The controlled intermolecular elimination of SiFMe3 from 51hCu or 51lCu and the fluoride complex 21dCu or its IPr*

Scheme 21. Cu NHC Complexes with μ2-S or μ3-S Bridging Atoms

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Scheme 22. NHC Complexes with Cu-S Bonds

Scheme 24. Reactivity of the Anilido NHC Complex 57Cu

have been used to prepare the two-coordinate air sensitive Cu anilides 57aCu−57cCu (Schemes 23 and 24) and the pyrrolide 58Cu by alkanolysis of the analogous methyl and ethyl complexes (Scheme 23).67,68 Spectroscopic and computational studies evidence that there is no favored conformation of the phenylamido in 57aCu−57cCu even at −80 °C, which has been attributed to the absence of any Cu-N π-bonding (N to Cu(4p)). Surprisingly though, crystallographic studies reveal planar geometry around the anilido N atoms, which is commonly found in early transition metals featuring strong π-bonding. Computational studies suggest that this planarity may be due to electronic effects associated with the phenyl amido substituent rather than the Cu center. Conversely, the anilido ligand in 57aCu−57cCu is nucleophilic and reacts with Et-Br, leading to alkylation of the anilines and concomitant formation of [CuBr(NHC)] (Scheme 24). The reactivity toward the nucleophile follows the order 57cCu > 57bCu > 57aCu and was ascribed to steric factors.

Attempts to coordinate one tBuNC to 57aCu led to the detection in solution of 61aCu (by NMR), which could not be isolated (Scheme 24). However, reaction of 57aCu with EtBr in the presence of excess of tBuNC did not influence the reaction rate; the weak binding of tBuNC does not trigger any electronic change affecting the reaction rate. The complex 57aCu was studied as a candidate for the C−H activation of simple molecules like C6H6; however, no reaction was observed up to 130 °C. The results imply that when associated with the CuI(IPr) fragment, the (IPr)Cu-NHPh bond is thermodynamically favored over (IPr)Cu-Ph, which is a typical behavior of the early rather than the late transition metals. Computational studies support an unusually strong (for late transition metal) Cu-N(anilido) bond that may be due to a combination of an sp2-hybridized N(anilido) moiety with the larger 4s(Cu) orbital, leading to better metal-anilido σ-overlap. It is also likely that the low coordination number due to the bulky NHC ligands may also strengthen Cu-anilido bonding. Stoichiometric reactions of 57aCu with excess aryliodides [Ph-I, 3,5-Xyl-I (Xyl = 3,5-dimethylphenyl)] led to NHPh2 or NHPh(Xyl) and [CuI(IPr)] by a mechanism which likely involves oxidative addition of Caryl-I and formation of CuIII intermediates, as supported by computational methods.97 The activation of highly acidic N−H bonds by 26Cu or 11eCu/NaOtBu as methods of forming [Cu(amido)(NHC)] complexes (NHC = IPr and cAAC, respectively) has been mentioned previously (Schemes 17, 18, and 23).46,69 Addi-

Scheme 23. NHC Copper Complexes with Cu-N Bonds

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tional two-coordinate Cu-NHC complexes with anionic N donors have been obtained by salt metathesis reactions. Thus, reaction of LiNPtBu3 with [CuCl(IPr)] led to complex 59Cu (Scheme 23), which serves as a good source of “CuH(IPr)” by reacting with BH(pin).98 In situ reactions of CuCl with a range of imidazol-2-ylidenes and imidazolin-2-ylidenes, generated from the corresponding azolium chlorides and 2-fold excess of LiN(SiMe3)2, the latter serving also as amido source, gave the two-coordinate [Cu{N(SiMe3)2}(NHC)] (62aCu−62fCu) (Scheme 25).99 The N(silylamide) in all the complexes

Scheme 26. Two-Coordinate NHC Complexes with Cu-P Bonds

Scheme 25. Synthesis of the Two-Coordinate [Cu{N(SiMe3)2}(NHC)]

62aCu−62fCu is planar, and the torsion angles between the amido plane and the plane of the NHC range between ca. 35 and 88°. A detailed study of the thermal stability of 62aCu− 62fCu showed that the most stable species feature imidazolin-2ylidenes. Phosphido Complexes. A limited number of two-coordinate NHC complexes with anionic phosphido groups has been reported. The reaction of the acetate complex 24Cu with P(SiMe3)3 resulted in a clean formation of [Cu{P(SiMe3)2}(IPr)] (63Cu), which is more stable thermally (up to 85 °C) and toward solvent loss than the homoleptic hexameric [Cu6{P(SiMe3)2}6] (Scheme 26).100 It features an upfield 31 P NMR signal at δ = −268 ppm, characteristic of terminal −P(SiMe3)2 groups. The P atom at the coordinated phosphido adopts trigonal pyramidal geometry, indicative of no involvement of the P lone pair in Cu-P bonding. The reaction of 26Cu with PHPh2 was reported to give 45aCu, after deprotonation of the P-H by the coordinated hydroxide base.69 It was formulated as mononuclear; however, the 31P NMR signal at δ = −27.8 ppm may point to bridging phosphido groups, as found in [Cu(μ-PPh2)(NHC)]3 (NHC = IiPr, ItBu) (45bCu and 45cCu) with similar 31P NMR chemical shifts (−30.5 ppm).101 The two-coordinate complex 64Cu was obtained by salt metathesis reaction from [CuCl(IPr)] with the P-metalated phosphine-borane.102 The P−B bond in 64Cu is persistent in the presence of reagents used for the deprotection of phosphine-borane adducts (NEt 3 , DABCO) but on thermolysis converts to a mixture of products, which includes the phosphalkene 65Cu by an unspecified mechanism. A cAAC ligand was found to stabilize complexes of Cu with the phosphaethynolate anion (PCO)−. The anion, although synthetically accessible as an alkali metal salt (e.g.,

Na+(PCO)−), has displayed limited coordination chemistry with transition metals due to its strong reducing power.103 However, reaction of Na(PCO) with [CuCl(cAAC)] (11dCu) gave the complex 66 C u , featuring a η 2 -(κC,κP)phosphaethynolate ligand. The 31P NMR signal of the coordinated PCO at δ = −387 ppm is slightly downfield shifted compared to Na(OCP) (δ = −392 ppm), giving evidence of minor electronic reorganization of the ligand on coordination. Interestingly, 66Cu reacts further with cAAC, giving the salt 67Cu containing the cationic homoleptic species [Cu(cAAC)2]+ and (PCO)− as counteranion; the latter does not show any close contact with cationic or solvent sites in the lattice. This feature may persist in solution as evidenced by the upfield shift of the anion signal (δ = −400 ppm) in the 31P NMR spectrum (Scheme 26).104 2.2.1.1.5. Type [CuX(NHC)] (X = Group 14: C, Si, Sn Donor Ligands): [CuX(NHC)] (X = Alkyl, Aryl, Alkenyl). The number of complexes featuring anionic C donor ligands in combination with NHCs at two-coordinate Cu centers has increased dramatically in the recent years, in particular due to their rich reactivity related to catalysis. Despite the coordination of one neutral NHC and one anionic C donor group at the same metal center, in most reactions studied the NHC adopts a spectator role. This may be related to the BDEs of the two different Cu-C type bonds but also to the higher migration aptitude of the anionic C donor compared to the NHC; supportive evidence is provided by the fact that the catalysisrelated reactivity is based on elementary insertion reactions. In N


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Scheme 27. Synthesis of Alkyl and Allyl NHC Copper Complexes

Scheme 28. Synthesis of Aryl NHC Copper Complexes

and gave a mixture of alkylated and arylated Cu(IPr) products (ca. 8:1 ratio), separable by fractional crystallization.106 The complexes 25euCu and 25cuCu have also been prepared by using [CuCl(ICy)] or [CuCl(IPr)] and MeLi or EtLi as alkylating agents, respectively.107,108 Finally, [Cu(η1-allyl)(IPr)] (25fuCu) was prepared by transmetalation from Si(allyl)(OMe)3 to the Cu in [CuF(IPr)] following SiF(OMe)3 elimination, in a method of wider scope.57 Heating of the methyl complexes 25au/sCu and 25bu/sCu at temperatures between 100 and 130 °C in C6D6 produced methane, ethane, and ethylene, in addition to a black precipitate or pink plating and other unidentified Cu(NHC) complexes but not free NHCs or imidazolium salts. When the thermolysis was performed in the presence of radical traps (TEMPO and cyclohexadiene), no change was observed concerning the course of the reaction, thus eliminating the possibility of homolytic decomposition paths.68 Interestingly, oxidation of the complexes 25au/sCu or 25cu/sCu with AgOTf to

the following discussion, the C-donor anionic complexes are classified according to the hybridization type of the C donor atom (in the order sp3, sp2, and sp) and the method used for the Cu-C bond formation; (per)fluorocarbyls are grouped separately. [CuX(NHC)] (X = anionic C donor with sp3-hybridized atom). The first alkyl complex [CuMe(IPr)] (25auCu) was prepared in good yields by the reaction of [Cu(OAc)(IPr)] with AlMe2(OEt).27 The methodology was extended to SIPr and (S)IMes analogues using the same Cu starting material and AlMe3 or AlEt3 alkylating agents.67,68,90,105 In work that was aimed to elucidate the mechanism of tandem catalytic C− H activation and carboxylation of substituted aromatics with [Al(iBu)3(TMP)]Li/CO2 (TMP = tetramethylpiperidine) (see below section 2.2.1.1.8, Reactions Involving Hydrocuparation of Alkynes), [Cu(OtBu)(IPr)] was reacted stoichiometrically with Li[Al(iBu)3(anis)] (anis = 2-anisyl) O


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the putative [CuII(alkyls)(NHC)]+ resulted in rapid reductive elimination of ethane or butane, respectively, and formation of [Cu(OTf)(NHC)]. It has been suggested that the alkane elimination from the CuII species follows a bimolecular pathway and not Cu-C bond homolysis. In contrast to the thermolysis of 25ausCu and 25busCu, the decomposition of CuII alkyls does not produce olefins.105 It has been contended that although the mechanisms of decomposition of the CuI and CuII species are different, none of them include free radical intermediates. DFT calculations estimate the CuI-Me and the CuII-Me bond dissociation energy at ca. ∼80 and 38 kcal/mol, respectively, in line with the general tendency of weaker M−C bonds with open shell systems. Complex 25duCu is light sensitive and of limited stability at room temperature, decomposing to ethane (Scheme 27).108 There is one example of sp3-bound 1-hexyl to the Cu(RE-6DiPP) moiety (RE-6-DiPP = extended ring, NHC 1,3-bis(2,6diisopropylphenyl)-3,4,5,6-tetrahydropyrimidin-2-ylidene), obtained by reaction of excess 1-hexene with the binuclear [Cu(μ-H)(6-DiPP)]2 at room temperature. The complex was characterized by NMR spectroscopy and underwent further insertion chemistry with CO2.43 It is interesting to note that despite the insertion of alkynes into a Cu-H bond being welldocumented and representing a key elementary step in many catalytic processes, the 1,2-insertion of unactivated alkenes into copper hydrides is rare; most (NHC)Cu-Calkyls are accessed by salt metathesis or transmetalation methods. [CuX(NHC)] (X = anionic C donor with sp2-hybridized atom). Two-coordinate complexes with a Cu-C(sp2) σ-bond of aromatic or vinylic origin (excluding fluoro-aromatic and fluoro-vinylic species) are numerous and have been obtained by the methods given in Scheme 28, initially to study the nature and stability of Cu-C(sp2) bonding and furthermore to establish the reactivity of well-defined model organometallics in catalytic transformations. The first [Cu(aryl)(NHC)] was prepared by salt metathesis from [CuCl(NHC)] and MgPh2 (Method C in Scheme 28). Although the complexes 25hu/sCu were stable and characterized by NMR spectroscopy, they were not obtainable analytically pure by this method.68 Heating 25hu/sCu in C6D6 to 120−130 °C resulted in decomposition, but unambiguous identification of the organic or organometallic decomposition products failed. DFT calculations aimed at gaining insight into the stability and reactivity of 25hsCu revealed that the BDEs of Cu-Ph are higher than that of Cu-Me (by ca. 15 kcal/mol). Despite this, exchange of the Me substituent in 25au/sCu by Ph, for example via a possible C−H activation of C6H6 (homolytically or by a σ-bond metathesis mechanism) was not feasible, presumably due to unfavorable activation barriers.68 The use of aryl-boronates or aryl-silanes as transmetallating agents aimed at milder and more selective arylation methods, with good functional group tolerance, that can be applied in the presence of electrophiles and catalytic amounts of Cu complexes. Thus, the clean and nearly quantitative conversion of [Cu(OtBu)(NHC)] or [CuF(NHC)] to a range of [CuAr(NHC)] complexes was achieved by the reaction with aryl-boronates or aryl-silanes, respectively (Methods A and B, Scheme 28). High yields of [Cu(4-methoxyphenyl)(IPr)] (25nuCu) were obtained by the reaction of [Cu(OtBu)(IPr)] with 4methoxyphenylboronic acid 2,2-dimethyl-1,3-propanediol ester. Interestingly, no reaction was observed when [CuCl(IPr)] was used as a source of Cu, implying that the driving force was the formation of the B−OtBu bonds.74 The broad

scope of these transmetalations has been exploited by integrating them in a range of catalytic applications, including the Suzuki-type (sp3-sp2) coupling by nucleophilic substitutions of allyl chlorides with arylboronic esters75 and the carboxylation of the latter with CO2.74 The sterically exposed two-coordinate complex 25iuCu was accessible under mild conditions following the same methodology.70 Similarly, a range of [Cu(aryl)(IPr)] (25juCu−25nuCu) was obtained in high yields and purity by the “transmetalation” reaction between the organosilanes Si(aryl)(OEt)3 and [CuF(IPr)].57 The less electrophilic SiMe2(aryl)(OEt) also is effective in the transformation.57 As previously mentioned, there is only one case of transmetalation from Li[Al(iBu)3{2-(OMe)phenyl}] to [Cu(OtBu)(IPr)], leading to a mixture of two organocopper complexes arising from the transfer of iBu and aryl groups.106 Finally, diverse free NHCs have been used to deaggregate polynuclear aryl-copper species to well-defined two-coordinate mononuclear complexes (Scheme 28). Thus, the reaction of [Cu(Mes)]5 with SIMes or Me2IiPr led to the isolation of [Cu(Mes)(SIMes)] (68aCu) or [Cu(Mes)(Me2IiPr)] (68bCu), respectively, in excellent yields,109,110 and the preformed aggregate [(ArNN)2Cu8Br6] (ArNN = 2,6-(RN = CH)2-4tBuC6H2, R = 2,6-iPr2C6H3) reacted with Me2IiPr to give [Cu(Me2IiPr)(ArNN)] (69Cu).111 The complexes [Cu(Mes)(RE-NHC)], where RE-NHC are the diamido- or diaminoexpanded ring NHCs, have been prepared by analogous deaggregation methodology (Scheme 14).71 The number of well-defined mononuclear [Cu(heteroaromatic)(NHC)] complexes featuring Cu-C(sp2) bonds is limited and their involvement in catalytic transformations emerges as more intricate than it was initially thought. The benzoxazolyl complex 70Cu was obtained by the C−H activation reaction of 27auCu with benzoxazole (pKa = 24.8) (Scheme 29). Complex 70Cu inserted CO2 to afford the Scheme 29. Synthesis of [Cu(heteroaryl)(NHC)] Complexes

2-benzoxazole carboxylate 71Cu; both transformations proceeded with excellent yields and constituted model steps for the catalytic direct carboxylation of C−H bonds of benzoxazole heterocycles.112 Mononuclear triazolide complexes with Cu-Csp2 bonding were initially targeted in the search for well-defined model “click intermediates” for the Cu azide−alkyne cycloaddition (CuAAC). Thus, the phenylacetylide 32asCu (obtained from the reaction of LiCCPh with [Cu(OAc)(SIPr)]), underwent a clean cycloaddition with the azide RN3 at room temperature over 24 h to give the triazolide 72aCu the structure of which reveals a mononuclear species (Scheme 30).113 Despite the plausible implication that monomeric Cu complexes may be suitable model intermediates for the click reaction, additional experimental evidence was accumulated, pointing toward dinuclear Cu sites as most likely catalytically relevant species. The copper acetylide 32asCu with BnN3 in the presence of P


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Scheme 30. Copper NHC Complexes Relevant to CuAAC

[Cu(NCMe)4](PF6) serving as a Cu+ source with labile ligands, amenable to the formation of μ-(σ,π-acetylide) dinuclear complexes, resulted in markedly accelerated reaction leading to the complex 72bCu with the coordinated triazolide, which is analogous to 72aCu. This, in combination with the observed scrambling of isotopically labeled Cu reactant complexes, was interpreted as evidence of involvement of binuclear sites in the catalytic transformation.114 Defining evidence in the form of isolated binuclear model complexes was obtained by employing cAAC ligands as stabilizing spectators to the catalytically active Cu centers. The acetylide complex 32bCu, obtained by salt metathesis reaction from [Cu(OAc)(cAAC)], was cleanly converted to the isolable binuclear complex 73Cu, the solid state structure of which revealed the presence of a bridging σ-,π-acetylide supporting the dinuclear core (Scheme 30). Despite the complex being thermally stable, its structure in solution was nonrigid since the 1H NMR spectrum showed the presence of only one type of cAAC ligand, even at −80 °C. The complex 73Cu reacted further with BnN3 to the binuclear 74Cu. Unexpectedly, in the structure of 74Cu, the triazolyl cyclo-

addition product is bridging two distinct Cu(cAAC) termini, one bound through a Cu-Csp2 and the other through a dative Cu-N bond to the aromatic triazolyl ring. The catalytic competence of the 73Cu and 74Cu over the mononuclear acetylide 32bCu was demonstrated by the favorable conversion kinetics to products when used in the presence of suitable substrates. Importantly, the weakly coordinating OTf− was crucial for the successful isolation of 74Cu, always maintaining coordinatively unsaturated Cu centers. Complex 74Cu served as a catalyst resting state that can be converted to 32bCu by the action of phenylacetylene via the alkanolysis of the Cu-C(sp2triazolyl) bond (Scheme 30).115 NHC complexes with σ-alkenyl bonds are mainly accessible via the insertion of alkynes into the “H−Cu(NHC)” moiety (hydrocupration). They serve as nucleophiles in many Cucatalyzed transformations. Initial studies in stoichiometric reactions provided evidence for the feasibility of the insertion of alkynes into [CuH(IPr)], which was considered as a mononuclear active species formed in solutions of the binuclear [Cu(μ-H)(IPr)]2. The hydrocupration of alkynes is a key elementary step of known catalytic hydrofunctionalizaQ


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tion reactions, including the semireduction,116 the hydrocarboxylation,58 the hydrobromination,117 and the hydroalkylation118 of alkynes. Recently however, there is accumulating evidence that bridging alkenyls and binuclear complexes may also be catalytically relevant (see section 2.3.1.1.3 below). The initial report described syn-hydrocupration of 3-hexyne by the [Cu(μ-H)(IPr)]2, leading to the E-alkenyl complex 75Cu in excellent yields (Scheme 31); 75Cu was obtained either by

In addition to their involvement in CuAAC catalytic cycles as outlined previously, [Cu(alkynyl)(NHC)] complexes have also been invoked in hydrophosphination of alkynes by PHPh2101 and the alkylation of terminal alkynes with nonactivated alkyl triflates;120 they have been identified as catalyst deactivating species in the semireduction of alkynes to Z-alkenes by silanes in iPrOH.116 [CuX(NHC)] (X = anionic organof luoride C donor). A few Cu(NHC) fragments served as platforms to stabilize unique mononuclear CF3 complexes, offering a versatile entry to and providing reactivity models for the Cu catalytic chemistry that leads to fluoroalkyl-substituted derivatives with medicinal applications. The initial attempts to access [Cu(CF3)(IPr)] by the reaction of 27auCu with SiMe3(CF3) gave instead the mononuclear complex [Cu(CF3)(NHC)] (40Cu) (Scheme 33), in which the IPr was also remotely functionalized by the

Scheme 31. Cu NHC Complexes with a σ-Alkenyl Bond

Scheme 33. Synthesis of Fluoroalkyl NHC Copper Complexes

trapping [Cu(μ-H)(IPr)]2 after generating it from [Cu(OtBu)(IPr)] and SiH(OEt)3 or by using the isolated dinuclear hydride.64 Analogous reactivity was observed by trapping the unstable [Cu(μ-H)(RE-NHC)]2 with an internal alkyne to give 76Cu (Scheme 31).71 [CuX(NHC)] (X = anionic sp-hybridized C donor). Some methods leading to mononuclear [Cu(alkynyl)(NHC)] complexes with Cu-C(sp) bonds have already been mentioned in conjunction with the reactivity of [CuX(NHC)] species (X = Me, (i);67 OtBu, (ii);70 OH, (iii);69 PPh2, (iv)101) and the synthesis or generation under catalytic conditions of “click intermediates” (v)119 and (vi).113 They are summarized in Scheme 32. A few [Cu(alkynyl)(cAAC)] species have been prepared by alkanolysis of reactive Cu-C or Cu-OH bonds with terminal alkynes46,115 or salt metathesis reactions (Scheme 18 and Scheme 30).115 Scheme 32. Syntheses of [Cu(alkynyl)(NHC)] Complexes

electrophilic SiMe3 group of the reagent (see also Scheme 15). Use of saturated NHC instead of IPr led to clean substitution of the tBuO by CF3 and the isolation of the mononuclear 77a− cCu.81,121 The complex 77bCu has been shown to be involved in a redistribution equilibrium in THF (KeqRT ca. 1.2) leading to the formation of the ion pair 78bCu with homoleptic [Cu(SIMes)2]+ cation and [Cu(CF3)2]− anion.122 The R


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Scheme 34. Synthesis of Fluoroaryl NHC Copper Complexes

a product of the nucleophilic attack of the coordinated fluoroaryl to the electrophilic N reagent (Scheme 34). The intermediacy of the fully characterized σ-fluorovinyl species 84Cu in the course of the stoichiometric monodefluoroborylation of C2F4 and other gem-difluoro-alkenes (e.g., Ar(H)CCF2) has been studied (Scheme 35). These species

energetics of the redistribution have not been detailed, and the transformation has not been described with other analogues of 78bCu. By a similar methodology, the [Cu(CHF2)(NHC)] complexes 79u/sCu have been obtained (Scheme 33).123 Interestingly, in this case the CHF2, analogues of 77aCu and 77bCu, were not formed in appreciable quantities, and equilibria involving ion-pairs were not observed. This may be related to the different steric requirements of the two fluoroalkyls. An alternative approach based on the availability of the “ligand-free” CuC2F5 (obtained by cupration of C2HF5 with [K(DMF)][Cu(OtBu)2]) was employed for the synthesis of [Cu(C2F5)(IPr*)] (80Cu). The reaction with the isolated IPr* provided the product (Scheme 33).124 A comparison of the oxidation potential of the organometallic complexes 77cCu and 25asCu bearing trifluoromethyl- and methyl-bound Cu centers, respectively, revealed that substitution of methyl for trifluoromethyl raised the potential by approximately +0.6 V versus ferrocene/ferrocenium (Fc/Fc+) couple, as expected due to the high electron-withdrawing properties of the trifluoromethyl ligand.121 The involvement of 77aCu has been implicated in the trifluoromethylation of aryl iodides by SiMe3CF3 in DMF in the presence of the alkoxide precursor. Stoichiometric trifluoromethylations of aryl iodides or bromides (at elevated temperatures) using 77aCu−77cCu and 78bCu concluded that the active species are heteroleptic complexes rather than ion pairs.122 Stoichiometric difluoromethylations of aryl electrophiles (i.e., diaryliodonium salts, aryl iodides, and aryl bromides) with 79u/sCu have been described; furthermore catalytic cross-coupling of aryl iodides with (difluoromethyl)trimethylsilane to difluoromethylated arene products using [CuCl{(S)IPr}] as catalyst precursor should proceed via the formation of the reactive 79u/sCu (Scheme 33).123 Tetra- and penta-fluoroaryls have been introduced to Cu(NHC) moieties by C−H activation using the t-butoxide, hydroxide, and mesityl complexes as precursors and C6F4H2 or C6F5H as substrates to give 35Cu, 36Cu, and 81Cu, respectively (Schemes 14, 15, 17, and 34).29,72,77,85 35Cu and 36Cu served as intermediates in the amidation of fluoroarenes with BOCNCl(Na) as source of electrophilic nitrogen in stoichiometric and catalytic reactions, the latter in the presence of NaOtBu as chain transfer agent.77 The complex 82Cu was characterized as

Scheme 35. Model Reaction for the Catalytic DefluoroBorylation of Fluoroalkenes

and reactions were used as models for the mechanistic understanding of the corresponding successful catalytic systems. Complex 84Cu was obtained by the borylative cleavage of one F−C vinylic bond via a fluoroalkene 1,2-addition-β elimination mechanism, starting from the weakly nucleophilic Cu-B(pin) complex 30Cu, and proceeded by the postulated 83Cu addition adduct of boryl-cupration. Two distinct addition regioselectivities applicable to C2F4 and gem-difluoro-alkene substrates, respectively, promote elimination of either B(pin)F or [CuF(IPr)] to give products of vic- or gem-defluoroborylation 85aCu and 85bCu, respectively. The initially formed S


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Scheme 36. Complexes Featuring Cu-Si Bonds Obtained by Salt Metathesis or Transmetallation

fluorovinylboranes of type 85aCu and 85bCu were converted to borate salts through the action of excess of KHF2 and the latter isolated.125 This elegant method opens ways to access rare fluorovinyl-substituted derivatives (Scheme 35). An extension of this methodology to the defluorosilylation of C2F4 in order to provide access to fluorinated vinylsilanes made use of silaborane Me2PhSi-B(pin) to form fully characterized β-silyl-ethyl copper species 86Cu, which underwent selective β-fluorine elimination promoted by the Lewis acidic BF(pin), generated in situ in the course of the reaction (Scheme 35).126 [CuX(NHC)] (X = anionic heteroatom f unctionalized C donor). [Cu(alkyl)(NHC)] complexes where the alkyl group is functionalized in the α- or β-positions by heteroatom groups have been obtained as products of 1,2- or 1,1-insertions of unsaturated molecules in reactive Cu-Si, Cu-B, and Cu-H bonds. Some functionalized alkyl species have been fully characterized, while others have transient existence in catalytic reactions. There are examples that appear in the discussion of the reactivity of specific Cu-X bonds (see Schemes 48, 62, 63, and 64). The complex [CuCl(IPr)] was reacted with the lithiated 2,6-(dimesitylphenyl)diazomethane to give a rare αdiazoalkyl Cu-IPr complex.127 [CuX(NHC)] (X = anionic silyls or stannyls). The number of complexes featuring anionic silyl and stannyl donor ligands in combination with NHCs at two-coordinate CuI centers has increased dramatically in recent years. Instrumental to this was the development of transmetalation methods to Cu(NHC)

moieties using silyl boranes. The initial attempts to prepare [Cu(SiPh3)(IPr)] consisted of reaction of [CuCl(IPr)] with KSiPh3 in THF; KSiPh3 was generated in situ from (SiPh3)2 and 2 equiv of K.128 The salt metathesis methodology introduced the mononuclear [Cu(SiPh3)(IPr)] and gave evidence for the nucleophilicity of the coordinated SiPh3 through reactivity studies with CO2 (see section 2.2.1.1.8, Reaction Involving Reduction of CO2 to CO); the salt metathesis methodology was used again much later for the synthesis of a series of Cu-NHC complexes with bulkier silyl groups -Si(SiMe3)2R (R = SiMe3, Et, 88aCu−88cCu) (Scheme 36) and a range of NHC ligands differing in bulk.129 Following the transmetalation methodology, the [Cu(SiPh3)(IPr)] obtained featured a mononuclear two-coordinate Cu center, but it was shown that decreasing the steric demands of the NHC, as when going from 88aCu to 88dCu, resulted in the formation of binuclear species with bridging silyl groups and cuprophilic interactions. All complexes described showed good thermal stability under inert atmosphere; in fact, 88aCu−88cCu were volatile under vacuum without decomposition and studied as volatile organometallic precursors for the deposition of Cu (Scheme 36).129 As alluded to above, the significant development leading to [Cu(SiMe2Ph)(IPr)] complexes was the introduction of silyl boranes B(pin)SiMe2Ph in combination with [Cu(OtBu)(IPr)], initially for the generation of the complexes in situ under catalytic conditions to carry out conjugate additions of silyl nucleophiles to unsaturated carbonyl compounds;80,130 T


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later the catalytically active complexes were prepared and isolated.79 The transmetalation of silyl boranes B(pin)SiMe2Ph with [Cu(OtBu)(IPr)] proceeds via a σ-bond metathesis mechanism and is selective for the silylation (rather than borylation) of Cu due to the favorable transition states leading (pin)BOtBu rather than the (pin)BOSiR3 and is driven thermodynamically by the strength of the B−O bond of the (pin)B−OtBu side product (see also above section 2.2.1.1.3). The structures of the isolated dinuclear organometallic species (for the less bulky NHCs) revealed close intermetallic contacts and cuprophilic interactions.70 The transmetalation methodology developed to an important route for the use of silyl nucleophiles in organic transformations through silacuprations of unsaturated organic molecules. The broad scope was demonstrated by employing silyl boranes (pin)B-SiPh3 and (pin)B-SiMe2Ph with Cu precursors bearing NHC ligands of diverse steric demands. Triphenylstannyl complexes have been prepared from HSnPh3 and [Cu(OtBu)(IPr)] or less conveniently [Cu(μH)(IPr)]2 by elimination of tBuOH and H2, respectively;78 trimethylstannyl complexes have been prepared by the reaction of the trimethylstannylated amino borane 37Cu. In the limited number of trimethylstannyl analogues that have been fully characterized, the degree of aggregation is dependent on the sterics of the NHC: dinuclear and trinuclear species were obtained with small NHC ligands (Scheme 37).70

Scheme 38. Complexes with Heteroatom Substituted-Silyl Ligands

upfield trend of which follows the order X = OtBu, OH, and H, owing to the shielding of Si by the electron-donating groups. Furthermore, the rates of the reaction of 34aCu−34dCu with CO2 giving CO and 90aCu−90dCu were also measured and correlate with the nucleophilicity and the upfield 29Si NMR shifts. The results are in support of a deoxygenation mechanism that includes, as a first step, nucleophilic attack of the silyl group to CO2, which was postulated based on DFT calculations.76,131 2.2.1.1.6. Type [CuX(NHC)] (X = Group 13 Donor B, Ga): [CuX(NHC)] (X = Anionic Boryl). The first fully characterized two-coordinate Cu-boryl complex [Cu{B(pin)}(NHC)] (30aCu) was prepared in good yields by the reaction of [Cu(OtBu)(IPr)] with [B(pin)]2 in a σ-bond metathetical process (Scheme 39). The linear at Cu molecule features a nucleophilic boryl group, which exhibits interesting reactivity for the reduction of CO2 to CO and simultaneous transfer of one oxygen atom to boron.73 The analogues of 30aCu (i.e., 30bCu and 30cCu) were prepared by identical methodology, starting from suitable precursors.66 The borylation reaction of [Cu(OtBu)(IPr)] by B(pin)(dambBn) and B(pin)(dambMe) (damb = diaminobenzene), which are nonsymmetric [B(pin)]2 analogues, led to the complexes 30dCu and 30eCu with amidosubstituted B(dambBn) fragments on the Cu center.132 Alternatively, salt metathesis of [CuCl(IMes)] with the isolated anionic boryl lithium species Li[B(IuDiPP)] and Li[B(IsDiPP)], which are isoelectronic to neutral (S)IPr, led to 30fu/sCu albeit in low yields (Scheme 39).133 The nucleophilic nature of coordinated boryl in the Cu complexes 30aCu−30cCu has been used advantageously in the borylcupration of unsaturated organic molecules with applications in catalysis (see section 2.2.1.1.8, Reactions Involving Borylcupration). [CuX(NHC)] (X = anionic gallyl donor). Salt metathetical reactions have been employed for the synthesis of the thermally stable [Cu(gallyl)(NHC)] complexes 91aCu−91cCu analogous to 30fu/sCu but comprising the heavier Ga congener and featuring rare Cu-Ga direct bonds (Scheme 40).134 The anionic Ga ligand is a strong σ-donor with a sp-hybridized Ga lone pair and a weak π-acceptor ability to an empty Ga porbital orthogonal to the heterocycle plane. Interestingly, probing the reactions of 91bCu toward PhCCH reveals facile protonolysis of the Cu-Ga bond rather than gallyl cupration; reaction with CNtBu leads to the formation of the adduct 92Cu with three-coordinate Cu center and with CO2 leads to decomposition of the Ga heterocycle and formation of 93Cu;

Scheme 37. Complexes Featuring Cu-Sn Bonds

Insertion of the free silylene 33Cu into the Cu(IPr)-X bond was developed as a unique method to access complexes with silyl ligands bearing heteroatom substituents at silicon 34aCu− 34dCu (Scheme 38). By changing the nature of the migrating group X (X = OtBu, OH, H, and OC6F5), the nucleophilicity at the silyl in 34aCu−34dCu was modulated. Initially, it was correlated with the observed 29Si NMR chemical shifts, the U


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Scheme 39. Synthesis of Two-Coordinate Cu-Boryl Complexes

Scheme 40. Synthesis and Reactivity of Cu-Gallyl Complexes

offer. Thus, the reaction of 27auCu with SiH(OEt)3 gave the complex 31auCu of limited stability in solution and the solid state at ambient temperature but isolable as yellow solid (Scheme 41). Its structural characterization at lower temperatures by X-ray diffraction revealed a binuclear arrangement in the solid state, with inferred but not experimentally located and refined bridging hydrides. Supporting spectroscopic evidence for the presence of hydrides was gained by IR spectroscopy [ν(Cu-H) absorption at 881 cm−1, shifted to 638 cm−1 for ν(Cu-D)], by 1H NMR spectroscopy [Cu-H at δ 2.67 ppm (s)], and by reactivity studies, mainly the hydrocupration of alkynes.64 Analogous stability was found for 31asCu generated from CuF by reaction with a Si−H moiety (Cu-H at δ 1.93 ppm).118 Increasing the size of the NHC ligand using IPent and IHept resulted in the complexes 31bCu and 31cCu, respectively, the former being the most stable in the series 31aCu−31cCu, although quantitative stability data as well as the decomposition mechanism are not established.30 Computa-

the latter transformation is mechanistically vague (Scheme 40).135 2.2.1.1.7. Type [CuX(NHC)] (X = Hydride and Related Binuclear Analogues). The history of CuH(L) moiety, L = 2e donor ligand, as an elusive mononuclear entity or building block of higher aggregates and the associated reactivity is long and still under development. A recent review of the coinage metal hydrides provides detailed description of the advancements with diverse ligands L, focusing on synthetic, structural, bonding and catalytic aspects; it also describes the impact of the NHC ligands in this area of chemistry.8 Herein selected milestones and very recent progress in the field of Cu-NHC hydrides is given. Following the discovery of the first well-defined molecular copper hydride [CuH(PPh3)]6136 and its catalytic applications as a hydrogenation catalyst,137 attempts were made to synthesize molecular NHC CuI hydride complexes, in view of the steric and electronic benefits that the NHC ligands may V


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whereby both hydrides are at virtually equal distances from each Cu center, but their distances to the two different Cu centers are unequal. The ν(Cu-H) vibration in 94Cu was observed at 981 cm−1 [cf. ν(Cu-H) 881 cm−1 for 31auCu]. The 13 C signal of the CcAAC nucleus appears at δ = 259.9 ppm, but the hydride signals in the 1H NMR spectrum were masked by other resonances due to the ligand backbones. It has been contended that the stability and degree of aggregation of 94Cu is due to the presence of the bulky menthyl substituent in the Menthyl cAAC ligand; the less bulky CycAAC does not lead to stable binuclear species. Addition of ligands L (L = PCy3, PMe3, tBuNC) to 94Cu led to the migration of the hydride to the CcAAC, resulting in overall insertion of the cAAC into the Cu-H bond (95Cu). This transformation has been attributed to the localization of higher electron density at the Cu due to the stronger σ-donor properties of the cAAC ligand, rendering the dissociation of H− easier (Scheme 42).44 The use of bulky, encapsulating IPr** in the context of stabilizing mononuclear Cu-H complexes led to the formation of species 96aCu and 96bCu. Despite a binuclear structure being adopted in the solid state (i.e., 96bCu), in solution 1H- and 13C NMR spectra are clearly in agreement with an equilibrium mixture of mononuclear and binuclear complexes (Scheme 43).139 The 13C{1H} spectrum of 96Cu showed two carbene

Scheme 41. Synthesis and Equilibria of NHC Copper Hydrides

tional studies estimated that the dimerization of simplified “CuH(NHC)” models (formally obtained by replacing the DiPP in the IPr ligands by Ph) was almost thermoneutral and slightly disfavored entropically (by ca. 5 kJ/mol). Consequently, based on this evidence and on kinetic studies of the catalytic hydrosilylation of ketones by the CuI(NHC)/ trialkoxysilanes system, the presence of mononuclear species in solution at room temperature seemed most likely.138 In contrast, DOSY NMR experiments of 31aCu−31cCu in solution support the presence of dimers as the predominant species (Scheme 41).30 Overall, these data point to the existence of a facile equilibrium between binuclear and mononuclear complexes, the former being dominant; the stability of the binuclear species determines the overall stability in solution. Optimum steric protection within the group of 31aCu−31cCu hydrides is provided in 31bCu; the marginally bulkier IHept rendered dimer 31cCu formation energetically costlier and therefore the overall stability of the system lower (Scheme 41). More stable binuclear CuI hydrides were obtained by replacing (S)IPr with RRcAAC as supporting NHC ligands. Treatment of [Cu(OtBu)(RRcAAC)] (RR = menthyl) with Li[HBEt3] at −60 °C led to the isolation of the binuclear [Cu(μ-H)(RRcAAC)]2 (94Cu), which is stable at room temperature as a solid or in solution.22,44 The complex was characterized in the solid state by a X-ray diffraction study, which revealed its binuclear structure with short intermetallic distance (ca. 2.31 Å) and Cu-C(RRcAAC) (ca. 1.86 Å) bond lengths. Location and refinement of the bridging hydrides showed an unexpected nonsymmetric bridging arrangement,

Scheme 43. CuI Hydrides with Bulky NHC Ligands

signals at δ 185.8 and 192.8 ppm. In the 13C proton-coupled spectrum, the signal at 192.8 ppm is a triplet (J = 3.6 Hz), assignable to the dinuclear 96bCu; the doublet at 185.8 ppm (J = 40 Hz) was assigned to the mononuclear 96aCu. Correlation NMR spectroscopy supported the previous assignments. Crystallization of 96bCu followed by dissolution resulted in

Scheme 42. CuI Hydrides with cAAC-Type Ligands

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Scheme 44. CuI hydrides with Ring Expanded NHCs Ligands

the re-establishment of the equilibrium mixture (Scheme 43). The metrical data of 96bCu (CNHC−Cu, ca. 1.89; Cu-Cu, 2.32 Å) are comparable to those of the cAAC-stabilized dimer 94Cu. Six- and seven-membered ring-expanded NHCs (RE-6- and RE-7-) have also been employed for the stabilization of complexes with Cu-H functionalities and low aggregation. Toward this goal, precursors with the sterically similar and electronically different 29aCu and 29bCu, with electrondeficient RE-6-Mes(DAC) and electron-rich RE-6-Mes, respectively, reacted with SiHEt3 (Scheme 44). The products obtained with the electron-deficient RE-6-Mes(DAC) implied the transient formation of a reactive CuH(NHC) species which was too unstable to be isolated; however, the hydride was trapped by nucleophilic sites with the NHC undergoing an insertion into the reactive Cu-H bond: as nucleophiles could serve the coordinated base OtBu in 29aCu leading to the dinuclear 97Cu or externally added P(o-tolyl)3 leading to 98Cu. The mechanism of formation of this species has not been further explored (Scheme 44).71 The products obtained with the electron-rich RE-6-Mes were also rationalized by slow insertion of the RE-6-Mes into the Cu-H bond of the transient 99Cu, possibly within the binuclear bridging hydride. The increased stability of the intermediate hydride 99Cu (compared to [CuH(RE-6-Mes(DAC)]) and in situ insertion reactions allowed trapping of [CuH(RE-6-Mes)]: insertion of Cu(RE-6-Mes) into the Cu-H bond gave 100Cu and consecutive insertions of 1-phenylpropyne and CO2 gave the salt 102Cu via the hydrocupration intermediate 101Cu (Scheme 45).71 Further insight into the stability and reactivity of the transient 99Cu was gained by using LiAlH4 as more reactive hydride source that could be employed at lower temperatures. This approach led to the generation of 99Cu at −95 °C and its characterization by low temperature NMR spectroscopy (CuH at δ = 0.96 ppm), including DOSY experiments, the latter providing evidence for the binuclear structure (Scheme 45). 99Cu that persists up to −65 °C, converting at higher temperatures to 100Cu.140 The use of Li[BHEt3] as hydride source gave after reaction with 29bCu at −95 °C to the mononuclear borohydride complex 103Cu which was stable below −35 °C for prolonged periods but decomposed at higher temperatures with deposition of Cu0. The structure of 103Cu revealed a rare κ1HBEt3 complex featuring two-coordinate Cu, C-Cu-H and CuH-B angles of 162.4(13)° and 110.2(18)°, respectively, and

Scheme 45. Insertion of the Electron-Rich RE-6-Mes into the Cu-H Bond of the Transient 99Cu

Cu-H distances of 1.56(3) Å. Computational study of the bonding and electronic structure of 103Cu indicate a virtually ionic interaction between [Cu(RE-6-Mes)]+ and [HBEt3]−. Reaction of 29bCu with SiHMe2Ph as source of hydride in the presence of the Lewis acidic B(C6F5)3 gave 104Cu with a hydride bridging the Cu and B atoms and a short Cu-Cipso contact to the C6F5 ring (Scheme 46). Evidence that 103Cu and 104Cu cannot be regarded as Lewis acid adducts of “CuH(RE-6-Mes)” was provided by their attempted reactions Scheme 46. Complexes With Bridging Hydride Between Cu and B Atoms

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heptanoate 108Cu, respectively, and 1,1-insertion of benzylisocyanide to formidoyl-species 109Cu (Scheme 48).43 2.2.1.1.8. Homogeneous Catalysis and Stoichiometric Reactivity with [CuX(NHC)] (X = C, Si, B, H donors): General Considerations. Catalytic applications with [CuX(NHC)] complexes have increased and diversified dramatically in the past decade. The aim of this section is to correlate coordination chemistry and organometallic reactivity of welldefined Cu complexes with elementary steps in the catalytic cycles. The list of the catalytic transformations cited and the diversity of organic substrates/products targeted is not comprehensive. There is overlap and/or complementarity between the catalytic properties of NHC and phosphine complexes of Cu; however, the latter are not covered here. Specialized reviews which focus on aspects of the activity/ selectivity and scope of CuI catalytic transformations with respect to the targeted organic products are cited in the General Introduction. Few key Cu-NHC organometallic functionalities are frequently involved in the catalytic transformations under discussion. There are close analogies among the sequence of elementary steps in many Cu-NHC-based catalytic cycles (Scheme 49): they comprise σ-bond metathesis reactions (step I) leading to the Cu-NHC-stabilized reactive organometallics [e.g., Cu(NHC)-Nu (Cu-H, Cu-C, Cu-B, and Cu-Si)] that undergo insertion of unsaturated organic molecules (alkenes, alkynes, allenes, carbonyl compounds, CO2, etc.) (step II) to give intermediates which are further functionalized with exogenous electrophiles, a step that may also release the products from the copper center and simultaneously regenerate the precatalyst (step III); alternatively, precatalyst regeneration is taking place after an additional ligand exchange at Cu (step IV), which may involve reaction with alkali metal alkoxides, fluorides, alcohols, etc. The insertion reactions in step II (hydro-cupration, carbo-cupration, boryl-cupration, silyl-cupration, etc.) mostly reside on the nucleophilicity of the moieties coordinated to the d10 CuI. However, the Cu(NHC)-stabilized nucleophiles show better functional group compatibility and selectivity compared to the harder anionic nucleophiles with harder electropositive metals and to cuprates. Reactions Involving Cu(NHC)-H Intermediates: Reduction of Carbonyl Compounds. [CuX(NHC)] complexes in the presence of variable amounts of alkali metal t-butoxides were studied as precatalysts in the hydrosilylation of carbonyl compounds (ketones, aldehydes, esters).28,119,142 The involvement of “CuH(NHC)” obtained by the action of silane on the in situ formed [Cu(OtBu)(NHC)] species was implicated. More recently, the design of complexes 2Cu and 110Cu with bulky and chiral NHCs led to successful robust and enantioselective hydrosilylation catalysts for a wide range of carbonyl compounds, including diaryl- and dialkyl-ketones and prochiral ketones, respectively (Scheme 50).38,143 Highly efficient hydroboration of carbonyl compounds with BH(pin) catalyzed by [Cu(OtBu)(IPr)] has also been described. Despite its high activity (catalyst loadings as low as 0.1 mol % and in some cases turnover frequencies up to 6000 h−1), the system has a broad scope and functional group tolerance (except to alkynes, where hydroboration predominates); electron-releasing ketones and aldehydes react slower.144 The postulated mechanism has similarities to the hydrosilylation of ketones, with the intermediacy of [CuH-

with the alkyne PhCCMe, in which case no simple hydrocupration products could be observed.140 It is interesting to contrast the reactivity of 29bCu toward Li[BHEt3] and SiHMe2Ph/B(C6F5)3 to yield the Cu-H-B complexes 103Cu and 104Cu, with the reactivity of [Cu(OtBu)(MenthcAAC)] and [CuCl(Et2cAAC)]; the latter on reaction with Li[BHEt3] and NaBH4 yield the bridging hydride 94Cu and the κ2-borohydride 105Cu, respectively (Scheme 47).141 In Scheme 47. Formation of Cu-H-B Complexes With cAAC Ligands

these cases, the relevant thermodynamic parameters to be considered in the postulated mononuclear intermediate precursors are the energies of [H-BEt3]− and [H-BH3]− bonds and the energies of dimerization of a species [CuH(NHC)] succeeding H−B cleavage and leading to 94Cu and 99Cu. The computed values for the energies of [H-BEt3]− are rather small, leading to facile H−B cleavage and formation of 94Cu thanks to favorable dimerization energetics; 103Cu is observed due to the unfavorable dimerization energies. [HBH3]− and [H−B(C6F5)3]− are strongly bound fragments and permit the isolation of 104Cu and 105Cu. Analogous studies with the bulkier RE-6-DiPP led to the isolation of the binuclear hydride [Cu(μ-H)(RE-6-DiPP)]2 (106aCu) (δ Cu-H = 0.77 ppm, ν(Cu-H) at 909 cm−1), which is stable at room temperature in solution (C6D6) or in the solid state (Scheme 48). The [Cu(μ-H)(RE-7-DiPP)]2 (δ Cu-H = Scheme 48. Reactivity of Copper Hydrides

0.47 ppm, ν(Cu-H) at 912 cm−1) 106bCu was also stable at room temperature. The existence of the binuclear hydrides 106aCu and 106bCu in solution was further demonstrated by solution IR spectroscopy, which showed unchanged ν(Cu-H) absorptions at wavenumber values that are quite different from the experimentally observed for terminal Cu-H absorptions (at ca. 1860 cm−1). The hydride 106aCu undergoes consecutive 1,2-insertion of 1-hexene and CO2 to 1-hexyl 107Cu and Y


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Scheme 49. Elementary Steps in Cu-NHC-Based Catalytic Cycles

Scheme 50. Hydrosilylation of Carbonyl Compounds

Scheme 52. Hydrosilylation of CO2

with the hydrocarboxylation of alkynes (see also sections 2.2.1.1.8, Reactions Involving Hydrocupration of Alkynes and Reactions Involving Insertion of CO2 into Cu-C Bonds) point to a slightly exergonic transformation, with the rate-limiting step being the σ-bond metathesis between the Cu-formate complex and SiH(OEt)3 to generate a catalytically active Cu-H species (Scheme 52).146 The hydroboration of CO2 by BH(pin) also occurred using as catalyst [Cu(OtBu)(IPr)] leading to borate formate complexes, which were hydrolyzed to formic acid in high yields (Scheme 53).147 The proposed mechanism involves the

(IPr)] and two σ-bond metathesis transition states, as shown in Scheme 51. Scheme 51. Hydroboration of Carbonyl Compounds

Scheme 53. Postulated Mechanism for the Hydroboration of CO2

Hydrosilylation and Hydroboration of CO2. The hydrosilylation of CO2 by SiH(OEt)3 is very efficiently catalyzed under mild conditions by [Cu(OtBu)(IPr)] (TOF = 1250 h−1, TON = 7590).145 The proposed mechanism involves insertion of CO2 into the Cu-H bond giving the κ1-formate complex 111Cu followed by σ-bond metathesis between this and SiH(OEt)3 to release the product and regenerate Cu-H; in stoichiometric reactivity studies 111Cu was isolated, fully characterized and proved to be catalytically competent (Scheme 52). DFT calculations of the reaction in conjunction

insertion of CO2 into the (IPr)Cu-H bond, with conversion to borate formate and regeneration of the Cu-H moiety by the action of BH(pin). Stoichiometric experiments evidenced the formation of Cu-H and its reaction with CO2 led to [Cu(κ1O2CH)(IPr)]. However, isolated [Cu(κ1-O2CH)(IPr)] failed to produce [CuH(IPr)] by reaction with BH(pin), although it is a competent catalyst for the hydroboration of CO2 under Z


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confirmed as being catalytically competent. Recent work on the hydroalkylation of alkynes, initially believed to involve mononuclear catalysts, has shown that the transformation in fact proceeds via binuclear intermediates. The stoichiometric syn-hydrocupration of [CuH(IPr)] with 3-hexyne to (E)[Cu(3-hexenyl)(IPr)] was studied experimentally and served as a model in this elementary step of the catalytic cycles.64 Synhydrocupration involving other CuH(NHC) species and alkyne partners have been carried out, and the products were characterized spectroscopically in situ or isolated. The insertion of PhCCtBu into the Cu-H bond of [CuH(Cl2IPr)] resulted in the alkyne carbon bearing the phenyl group ending α to the metal.149 A limited number of DFT calculations of this step describe it as concerted with regioselectivity (of relevance to the insertion of nonsymmetrical or terminal alkynes) being controlled by steric and electronic parameters at the Cu.146,150,151 A summary of the catalytic transformations based on the hydrocupration of alkynes is given in Scheme 55. Semireduction of alkynes. The semireduction of terminal or internal alkynes to Z-alkenes has been accomplished by catalytic systems based on the Cu(IPr) or Cu(Cl2IPr) moieties using the very reactive PMHS (polymethylhydrosiloxane) as hydride source, in the presence of iBuOH, tBuOH, or NaOtBu, respectively (Scheme 56).116,149 The reaction is postulated to proceed via Cu(IPr)-H intermediates and syn-hydrocupration of alkynes. Interestingly, the catalytic system development was based on careful optimization by promoting the productive and suppressing deactivation reactions. Although the stoichiometric syn-hydrocupration had been previously described, the competing deactivation reactions (A−C) due to the high acidity of the terminal alkynes and alcohols had to be suppressed. This was achieved by using high activity hydride (to suppress A and B) and bulky IPr and alcohols (to suppress C). Alternatively, terminal alkynes were semireduced using a PMHS, Cu(Cl2IPr) and NaOtBu system. Conversely, with internal alkynes as substrates, the more acidic iBuOH increased the rate of release of the σ-vinyl adducts from the organocopper intermediate and the formation of the semireduction products (Scheme 56).116 Overreduction of the alkene to alkane is avoided by the lack of insertion reactivity of alkenes under the reaction conditions. Stoichiometric reactions resulted in the spectroscopic observation of the hydride

similar conditions. On the basis of these data, the postulated mechanism is shown in Scheme 53. The reduction of alkyl triflates and iodides by a [Cu(OtBu)(IPr)]-catalyzed protocol with reductant TMDSO and stoichiometric quantity of CsF was also postulated to involve [CuH(IPr)] and nucleophilic substitution of the triflate or iodide groups by hydride; radical pathways were minor and relevant only in the reduction of iodides (Scheme 54).148 Scheme 54. Reduction of Alkyl Triflates

Reaction Involving Hydrocupration of Alkynes: Overview. The involvement of alkynes as addenda to hydrocupration intermediates stabilized by NHC ligands has led recently to the development of versatile catalytic transformations. Although the details of the generation of the Cu(NHC)-H species under catalytic conditions vary and are of crucial importance for the overall activity and selectivity, in the majority of the cases, Cu-H reactive species are formed by the reaction of CuI alkoxides (occasionally of phenoxides or fluorides) with hydridic sources. In a few cases, stoichiometric reactions under controlled conditions have successfully been used to pin down or isolate intermediate complexes that were Scheme 55. Hydrocupration of Alkynes

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were also reactive under these conditions but regioselectivity can suffer depending on the nature of the alkyl substituents.58 A plausible mechanism was compiled based on the isolation and characterization of the σ-vinyl- and κ1-carboxylatocomplexes (B and C, respectively) in stoichiometric reactions (for NHC = Cl2IPr); both B and C were catalytically competent (Scheme 58).

Scheme 56. Semireduction of Terminal or Internal Alkynes

Scheme 58. Hydrocarboxylation of Diarylalkynes

An intriguing issue of selectivity in the alkyne hydrocarboxylation reaction arises because the reactive components present in the catalytic system under different conditions can lead to chemically diverse products. In particular, hydrosilylation of CO2 is a very efficient process that is shut down in the presence of alkyne. DFT calculations were carried out to probe the origin of selectivity.146,151 It was found that for the model systems studied employing a small NHC ligand, the CO2 hydrosilylation with SiH(OEt)3 is exergonic (by ca. 7 kcal/mol) as well as the hydrocarboxylation of diphenylacetylene (by ca. 27 kcal/mol). The calculated rate-determining step for the hydrosilylation of CO2 is the σ-bond metathesis between Si(OEt)3-H and the Cu formate complex (ca. 20.0 kcal/mol) and for hydrocarboxylation, the σ-bond metathesis between the Si(OEt)3-H and the vinyl-carboxylate analogous to C in Scheme 58, with a computed free energy barrier of ca. 18.0 kcal/mol. On the basis of these data, both reactions are thermodynamically and kinetically feasible under the reaction conditions. The fact that the former does not occur during hydrocarboxylation was attributed to kinetic factors related to the reaction medium and the biphasic nature of the hydrosilylation. Hydrobromination of alkynes. The hydrobromination of terminal alkynes occurs with anti-Markovnikov E-selectivity. The hydride source was SiH2Ph2 and the electrophile 1,2dibromo-1,1,2,2-tetrachloroethane. The phenoxide KO(2tBuC6H4) acted as turnover reagent. The terminal alkyne scope includes alkyl and aryl substituents, also with functional groups.117 The proposed mechanism for the reaction is shown in Scheme 59. The steps I−IV are transmetalation of Cu-phenoxide by SiH2Ph2, alkyne insertion, electrophilic substitution, and ligand exchange, respectively. Of importance is the success of the substituted phenoxide as turnover reagent. Its use was dictated by the need to suppress direct metalation of the terminal alkyne with the commonly used stronger base KOtBu; the metalated alkyne could in turn consume the electrophile; alkyne metalation does not proceed with the aryloxide base. However, the transmetalation of Cu-OAr required the use of SiH2Ph2 since the transmetalations with PMHS or SiH(OEt)3 were much slower. Interestingly, under these conditions the

[CuH(Cl2IPr)] and its conversion to the σ-vinyl complex by reaction with PhCCtBu; the former could be isolated and proved to be catalytically competent.149 Modifications of the catalytic system allowed the use of H2 as reducing agent, without added protic sources. For this purpose, the hydroxy-functionalized imidazolium salt was employed as proligand to the alkoxide-functionalized Cu complex 112Cu (Scheme 57). Heterolytic H2 splitting on 112Cu Scheme 57. Formation of an Alkoxide-Functionalized Cu Complex

leading to Cu-H and an alcohol-functionalized NHC was postulated, the latter facilitating intramolecular proto-decupration of the vinyl-copper intermediate leading to the desired alkene (Scheme 57). All postulated catalytic species were formed in situ by the reaction of [Cu(Mes)]5 with a range of imidazolium salts.152 In a simpler system, the H2 splitting used for the semihydrogenation of internal alkynes was catalyzed by a combination of [Cu(OtBu)(SIMes)] and catalytic amounts of NaOtBu.153 Despite the plausibility, no intermediates were established in support of the proposed mechanisms. Hydrocarboxylation of alkynes: the hydrocarboxylation of diarylalkynes has been carried out by using [CuF(IMes)] catalyst and SiH(OEt)3 as hydride donor. Alkyl aryl-alkynes that were unreactive with this system could be converted by switching to [CuF(Cl2IPr)] as catalyst precursor; in all cases, the E-alkene was obtained with high selectivity. The regioselectivity of insertion resulted in the aryl group positioned α-to the Cu center. Nonsymmetrical dialkyl-alkynes AB


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competent intermediates for the hydroalkylation.63 Thus, the formation of the binuclear A is facilitated by the limited concentration of [CuF(SIPr)] due to slow phase transfer of F− from CsF to solution; fast reaction of [CuF(SIPr)] with the solution abundant [Cu(OTf)(SIPr)] leads to A. Complexes A, B, and C were fully characterized, and their interconversions were established experimentally while maintaining the integrity of the binuclear arrangement. The conversion of C to the product by the reaction with R1OTf was also demonstrated experimentally, even though the exact mechanistic nature of the transformation is not known. Interestingly, complex B does not reduce triflates as does “Cu[(S)IPr]-H” and complex A does not convert triflates to fluorides (Scheme 61).

Scheme 59. Hydrobromination of Terminal Alkynes

Scheme 61. Hydroalkylation of Terminal Alkynes hydrosilylation of alkynes is not competing with the bromination. DFT studies gave mechanistic insights into this transformation: the σ-bond metathetical transmetalation (step I) is exergonic, the syn-addition of the Cu-H bond is concerted, and the anti-Markovnikow regioselectivity arises from the charge distribution at the Cu-H and CC. The brominating agent uses the bromonium end to attack the Cu-C bond in the vinylic intermediate.150 Hydroboration of Alkynes. The hydroboration of terminal alkynes with BH(dan) to E/Z vinyl boranes has been described using [CuCl(NHC)] as precatalysts (NHC = ICy, IMes, SIPr) or in situ generated phosphine catalysts. The stereochemistry of the product boranes is opposite with the two types of ancillary ligands: [CuCl(SIPr)] gives the E-isomers and the complex with the chelating DPEphos the Z-isomers (Scheme 60). Although no attempts were made to pin down any Scheme 60. Hydroboration of Terminal Alkynes

Hydroboration of substituted allenes by BH(pin) leading to isomeric allyl boronates also involves hydrocupration steps and has been studied in detail for a range of NHC ligands.66 Reactions Involving Borylcupration: Overview-Stoichiometric Reactions. The synthesis of the [Cu(boryl)(NHC)] (30aCu− 30cCu) complexes from [Cu(OtBu)(NHC)] (see sections 2.2.1.1.3 and 2.2.1.1.6) and the development of their reactivity has opened new directions in the copper organometallic catalysis and the utility of the boryl nucleophiles in borylative organic transformations. Organoboronic acids and their derivatives have a great potential for carbon−carbon and −heteroatom bond formation reactions, amplified by their ease of handling; borates can stabilize masked forms of enolates. Aspects of boryl cupration and related transformations, mainly with NHC and phosphine complexes of copper, have been reviewed.155 Boracarboxylations involving reactions of the nucleophilic copper boryls with CO2 have also been covered.156 The diverse chemistry of diboron compounds including [B(pin)]2 in synthetic and catalytic applications has been reviewed.157 The stoichiometric insertions into the CuB(pin)(NHC) bond that led to well-defined complexes are summarized in Scheme 62. They laid the foundations of catalytic hydroborations and carboborations of alkenes, alkynes, and allenes that are discussed below. The insertion of alkenes into the Cu-B bond of 30aCu to βboroalkyls has been studied in some detail.158 The reaction

catalytic intermediates, it is proposed that insertion of the alkynes into copper hydrides leads to vinyl copper species leading to the boranes by B(dan)-Η σ-bond metathesis. Stereodifferentiation may involve interaction of the aromatic rings of BH(dan) with the ancillary SIPr or DPEphos (Scheme 60).154 Hydroalkylation of Alkynes. The hydroalkylation of terminal alkynes with excess of the electrophilic primary alkyl triflates, excess of (SiHMe2)2O as hydride source in the presence of CsF as turnover reagent catalyzed by [Cu(OTf)(SIPr)], and leading to E-alkyl-alkenes by anti-Markovnikov addition was described.118 The reaction conditions are similar to those employed for the conversion of alkyl triflates to alkylfluorides and the reduction of alkyl triflates to alkanes,61,148 except that alkyne is present. A careful experimental mechanistic study, including stoichiometric reactions and competition experiments, aimed at understanding the selectivity toward hydroalkylation and revealed that binuclear complexes are AC


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process involving β-hydride elimination followed by reinsertion of the resulting olefin into a Cu-H bond. Scrambling experiments with substituted styrenes supported the involvement of a displaceable olefin on the Cu center at some point of the rearrangement. Complex 114aCu with α-heteroatom boryl substituents was fully characterized (Scheme 63). DFT calculations of the alkene insertions into simplified Cuboryl model complexes have been carried out.159 The insertion process is described as an intramolecular nucleophilic attack of the boryl to the coordinated alkene in the intermediate preceding the reaction. Furthermore, the orbital interaction between the Cu-B σ- and the alkene π*-orbitals points to the importance of the boryl nucleophilicity and the alkene electrophilicity in the insertion reaction, the latter being augmented by electron-withdrawing substituents on the alkene that facilitate 1,2-insertion. The calculations show that the βborylalkyl is formed under kinetic control in an irreversible 2,1insertion and the α-borylalkyl is the thermodynamic product. The relative stability of the α-borylalkyl-Cu may be attributed to hyperconjugation between the Cu-C σ bond and the “empty” pz orbital on boron. The insertion of mesitylaldehyde into the Cu-B bond of 30aCu was also studied in stoichiometric reactions.65 This led to the α-boryloxy alkyl complex 115aCu in high yields (Scheme 64). Attempts to establish whether 115aCu constituted the primary insertion product or resulted from a rearrangement of the alkoxide 115a’Cu were not conclusive: attempted synthesis of 115a’Cu by alternative routes led only to the isolation of 115aCu. This has been considered as indirect evidence of a “bora-Brook” rearrangement in the (α-boryl-benzyloxy)copper complex, resulting in the formation of a (boryloxy)benzylcopper complex with the oxophilic B atom bound to the neighboring oxygen rather than the benzylic C.160 The integration of this stoichiometric transformation to a catalytic diborylation of mesitylaldehyde was initially attempted by reacting 115aCu with [B(pin)]2, which gave low conversion the diborylated aldehyde over 20 h and regeneration of 30aCu. However, in the presence of excess of both [B(pin)]2 and mesitylaldehyde, 30aCu acted as a competent catalyst for the 1,2-diborylation. Further refinement ensued by employing the less bulky precursor of 27guCu, which could effectively catalyze the diborylation of a range of aldehydes under mild conditions (Scheme 64). The mechanism and the energies of intermediates and transition states of the stoichiometric and catalytic transformations shown in Scheme 64 were studied in

Scheme 62. Stoichiometric Borylcupration Reactions

with styrene resulted in 2,1-syn-insertion with the phenyl group α- to the Cu to give 113Cu; substituted styrenes reacted similarly, while cis-stilbene gave a mixture of syn- and antiinsertion products (Scheme 63). Despite the presence of βScheme 63. Insertion of Alkenes into the Cu-B Bond

hydrogens in the products of type 113aCu, they are thermally stable at room temperature indefinitely, thus constituting rare examples of fully characterized Cu-alkyls with β-hydrogens and β-heteroatoms. Qualitative insight into the role of electronic effects during the insertion reaction was gained by competition experiments with p-substituted styrenes, revealing that electron-donating p-substituents slow down the reaction in support of incipient carbanion character on the Cu-bound carbon atom during the insertion; the alkene substrates behave as electrophiles. Heating of 113aCu to 70 °C in C6D6 solution resulted in a slow rearrangement to 114aCu presumably via a Scheme 64. Insertion of Mesitylaldehyde into the Cu-B Bond

AD


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detail by DFT methods. The results pointed to an initial aldehyde insertion into Cu-B that leads to Cu-O-C-B(pin) as a kinetically favored species in view of the high nucleophilicity of the Cu-B(pin) bound to the electron-rich Cu. In the presence of excess [B(pin)]2 (in a catalytic setup), a σ-bond metathesis involving the Cu-O-C-B(pin) complex and [B(pin)]2 ensues, leading to the diborylated aldehyde and regeneration of the catalyst. In the absence of excess [B(pin)]2 (i.e., in a stoichiometric setup), the kinetic product rearranges to the thermodynamic product with Cu-C-O-B(pin) arrangement. The boryl migration to the metal-bonded oxygen proceeds via a SE2-like transition state.161 The stoichiometric insertion of alkynes into the [Cu(IPr)B(pin)] bond was studied in conjunction with the boracarboxylation of alkynes with CO2. Thus, insertion of PhCCPh into Cu-B by syn-addition proceeded in an almost quantitative yield to give the β-boryl alkenyl-copper complex 116Cu which was isolated and fully characterized (Scheme 62). Exposure of 116Cu to the electrophilic CO2 led to 117Cu, which featured a cyclic “β-bora-lactone” structure and the NHC− copper moiety bonded to an O atom of the boronic ester.162 These complexes constitute models for the catalytic boracarboxylation, but catalytically active complexes were only obtained with the SIMes ligand (see below). The stoichiometric insertion of substituted allenes led stereoselectively to the Z-σ-allyl complexes 118aCu−118cCu, which on methanolysis gave a mixture of isomeric allyl-boronic esters.66 Catalytic Reactions: Hydroboration of Alkenes or Alkynes. Catalytic systems for the hydroboration of unsaturated organic molecules using [B(pin)]2 and alcohols (as source of H+) were developed, based on the previously discussed borylcupration model reactions. The mechanism of Scheme 65 was supported by the stoichiometric reactions of Scheme 62. Substituted styrenes, in particular E-isomers, were hydroborated stereoselectively in high yields (Scheme 65, eq 1). The use of catalysts prepared in situ from chiral imidazolinium salts led to enantioselective hydroboration of the β-substituted styrenes.163 Broad scope, site selective hydroboration of terminal alkynes, to the internal or α-vinylboronates has been accomplished by a similar catalytic system (Scheme 65, eq 2). The α-selectivity is maximized with alkynes bearing electron-withdrawing substituents and NHC bearing aryl groups. In contrast, SIAd supporting ligands afford high βselectivity.164 This has been rationalized by the energetically favored orientation of the terminal alkyne prior to the syn addition step, which is dictated by electronic and steric factors involving the alkyne and the NHC substituents.164 The use of (pin)B−B(dan) instead of (pin)B−B(pin), resulting in -B(dan) over -B(pin) addition, also favors α-selectivity, irrespective of the alkyne substituent, in view of the reduced electrophilicity of the B(dan) group (dan = 1,8-diaminonaphthalene).165 The hydroboration of substituted allenes can lead to the isomers shown in Scheme 65 in variable yields and selectivity, depending on the nature of the NHC ligand and the reaction conditions, which influence the site of protonation of σ-allylCu intermediate.66 Diboration of Alkenes. The diboration of styrenes using as precatalyst [Cu(NHC)(NCMe)]BF4 (NHC = (S)IPr, IMes) and bis(catecholato)diboron [B(cat)]2 as the boron source was also described, providing high conversion of diborated product in refluxing THF when the ratio of [B(cat)]2 to styrene is

Scheme 65. Catalytic Hydroboration of Unsaturated Organic Molecules

1:1.5. Use of excess of styrene at room temperature led to monoboration. A mechanistic scheme involving the insertion of the alkene into the Cu-B(cat) bond was postulated, giving rise to β-borylalkyl-Cu intermediates which lead to the diboration products by σ-bond metathesis/transmetalation with excess [B(cat)]2.166 The use in these reactions of [B(cat)]2 as borylating agent instead of [B(pin)]2 is also crucial: the former being more electrophilic favors transmetalation reactivity by σ-bond metathesis with the βborylalkyl-Cu which is involved in the diboration.167 Hydrocarboxylation of Acetylenes with CO2 via Carboboration. The carboboration of unsaturated organic molecules catalyzed by Cu-NHC boryl complexes with [B(pin)]2 and carbon electrophiles follows a mechanistic path analogous to the hydroborations discussed above, except that a C electrophile reacts with the primary hydrocupration intermediate. The hydrocarboxylation of acetylenes carried out in the presence of [CuCl{(S)IMes}] and LitOBu in THF at 80 °C follows this mechanistic scheme. The postulated catalytic cycle based on the isolation of the key intermediates A−C (as IPr complexes that were interconvertible via the catalysis relevant reagents) is shown in Scheme 66.162 Rare catalytic carboboration of alkenes and allenes using benzyl chloride and aryliodide as a source of electrophilic carbons, respectively, and [CuCl(SIMes)] as a catalyst leading to alkyl- and vinyl-boronic esters have been described (Scheme 67).168−170 Bimetallic Cu/Pd catalytic systems were developed for the intermolecular carboboration of alkenes with PhBr electrophiles. In these cases, the in situ generation of the nucleophilic β-borylalkyl copper intermediates from [CuCl(NHC)] complexes (e.g., [CuCl(IPr)], [CuCl(SIMes)]), base (NaOMe or AE


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Scheme 66. Carboboration of Unsaturated Organic Molecules

Scheme 68. Intermolecular Carboboration of Alkenes by a Bimetallic Cu/Pd System

Scheme 69. Dehydrogenative Borylation of Styrenes

Scheme 67. Carboboration of Alkenes

NaOtBu), [B(pin)]2, and alkene (e.g., substituted styrene, etc.) in the presence of Ph-Br and catalytic [Pd(OAc)2]/phosphine systems, resulted in the cross-coupling of the β-borylalkyl with Ph-Br. The intermediacy of (NHC)Cu-β-borylalkyl originating in the “Cu-cycle” and the requirement for Pd/phosphine for the activation of the Ph-Br (“Pd-cycle”), that are both essential for the transformation, were consolidated by resorting to the study of stoichiometric reactions. Optimization, scope, and selectivity of the catalytic system, which comprises two cooperating catalytic cycles working in tandem have been described (Scheme 68).171−173 Dehydrogenative Borylation of Styrenes. Dehydrogenative borylation of styrenes has also been developed for the synthesis of vinyl boronic esters. In this case, ketones (benzophenone or dialkylketones) are used as in situ oxidants of the “Cu(NHC)-H” species that results at higher temperatures by β-hydride elimination from the β-borylalkyl copper intermediates and liberation of the vinyl boronic esters (Scheme 69).174 Reaction Involving Insertion of CO2 into Cu-C Bonds. Cu(NHC)-C bonds formed by transmetalation from aryl- or alkenyl-boronic esters undergo insertion of CO2 leading to carboxylates.74 A catalytic cycle was proposed based on stoichiometric reactions and isolation and characterization of the intermediates A and B (for R = p-anisyl) (Scheme 70).

A related selective carboxylation of alkyl-boranes was accomplished only by using LiOMe as base. On the basis of isolation, characterization, and the proof of catalytic competence, the species A and B were proposed as intermediates in a modified catalytic cycle; CO2 insertion takes place in the organocopper complex obtained after the adduct formed between the methoxy group and the boron atom of the alkylborane (Scheme 71).175 DFT calculation studies provide computational evidence of the steps postulated in Scheme 71 and the feasibility of alkoxide-bridged intermediates preceding the transmetalation step. Insertion of CO2 was considered as nucleophilic attack by the coordinated aryl in a rate-determining step; the studies also predicted the reduced reactivity of alkyl-boranes in the transmetalation step.176 The facilitation of the CO2 insertion in Cu-B(OR)2 versus Cu-Ph was also rationalized by the DFT AF


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Scheme 70. Carboxylation of Alkyl Boronates Catalyzed by Cu(NHC) Complexes

carboxylated product was isolated. On the basis of these observations, the mechanism of Scheme 72 was postulated. DFT calculations of the postulated mechanistic steps uncovered a lower-energy pathway involving rearrangement of the heteroaryl complex A to the isomeric nucleophilic metallacarbene A′, which can react with CO2 in a reversible manner, as known for many free NHC ligands, to the carboxylated product B.177 In the intermediate A′, the CNHC

Scheme 71. Carboxylation of Alkyl-Boranes Catalyzed by Cu(NHC) Complexes

Scheme 72. Catalytic C2-Heteroaryl Carboxylation with CO2

work: an analogous interaction of CO2 with [Cu(NHC)B(OR)2] prior to insertion was not found in [Cu(NHC)-Ar] due to the superior σ-donor character and nucleophilicity of the B(OR)2 compared to σ-aryl. Cu-C2 bonds with heteroaryls, formed by selective direct metalation with [Cu(OtBu)(NHC)], underwent catalytic C2heteroaryl carboxylation by reaction with CO2 in the presence of KOtBu.112 A follow-up reaction of the potassium carboxylates with alkyl halides led to esters. Weakly acidic heteroaryls gave reduced or no yields. Stoichiometric reactions were employed to establish the reaction mechanism: the metalation of benzoxazole with one equivalent [Cu(OtBu)(IPr)] gave quantitatively the complex of type A, which was fully characterized; this step is considered as protonolysis of the C−H bond by the basic coordinated tBuO. Further reaction of A with CO2 at room temperature gave B in high yields, but interestingly, the latter under vacuum was decarboxylated to A over 12 h, implying that this step is reversible. In the presence of KOtBu, the salt of the AG


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reaction is the formation of (pin)B-OB(pin) bond (ca. 190 kcal/mol), compensating for the energetic cost of OCO cleavage (ca.127 kcal/mol) rendering the whole cycle exergonic. The crucial controlling factor of the reaction is the nucleophilicity of the Cu-B(pin), which determines the regiochemistry of the reaction (i.e., attack on the electrophilic C atom of CO2 rather than attack of the OCO to the oxophilic B center). The rate-determining step is the migration of the boryl group from C to O. An interesting comparison was made with the insertion of CO2 into the Cu(NHC)-Me bond, which proceeds readily to form the acetate complex from which elimination of CO through a methyl migration from C to O is energetically difficult, underlining the critical role of the empty p-orbital on boron in the boryl moiety for facilitating the migration. The strong trans influence of the NHC ligands also weakens the Cu-B(pin) (or Cu-Me) bonds (Scheme 74).181

may be stabilized by electron donation from the electron-rich 3d10 Cu(IPr) moiety (Scheme 72). Substrates bearing acidic C−H bonds were carboxylated by CO2 using as catalyst [Cu(OH)(IPr)] (26Cu) in the presence of CuOH. Direct cupration of the acidic C−H sites by 26Cu, followed by insertion of CO2 into the Cu-C bond, were postulated as crucial steps in the reaction mechanism.178 The transformation showed a broad scope for C−H acidic substrates (pKa ca. 23−27) (e.g., fluoroaromatics and heteroaromatics). A heterobimetallic catalytic system comprising the lithium aluminate Li[Al(iBu3)(TMP)] and [CuCl(NHC)] (NHC = IPr, SIPr, ICy) complexes was developed for the direct carboxylation of C−H bonds of aromatics with CO2 in the presence of KOtBu additives.106 The aryl-aluminum species, obtained by direct C−H activation, underwent in situ cupration with concomitant carboxylation by nucleophilic attack of CO2. The presence of an o-directing group in the aromatic substrate is required for the initial regioselective alumination. Best performance was recorded with IPr complexes in the presence of KOtBu, resulting in good-toexcellent yields (Scheme 73). Heteroaromatic compounds

Scheme 74. Reduction of CO2 to CO

Scheme 73. Direct Carboxylation of Aromatic C−H Bonds

were carboxylated similarly. Stoichiometric reaction of [Cu(OtBu)(IPr)] with [Al(iBu3)(aryl)] (aryl = 2-anisyl) gave competitive arylation and alkylation of Cu, both organometallics being very reactive toward CO2 (see also Scheme 26). The factors responsible for the selectivity were not established. Other carboxylation reactions of diverse organic substrates involving organocopper reagents and CO2 have been reviewed.9,179,180 Reaction Involving Reduction of CO2 to CO. Facile and rapid reduction of CO2 to CO was achieved by the reaction of [Cu{B(pin)}(IPr)] (30aCu) with CO2 at room temperature in the presence of [B(pin)]2. The only Cu-containing product that was obtained and fully characterized was the complex 118dCu featuring a κ−ΟB(pin) ligand; therefore, [B(pin)]2 also adopts the role of the sacrificial oxygen atom acceptor from the CO 2 . Treatment of 118d Cu with [B(pin)] 2 regenerated 30aCu and (pin)B−O−B(pin). The reduction reaction can be run catalytically with 0.1 mol % [Cu(OtBu)(IPr)] (27auCu) and 100-fold excess [B(pin)]2 under an atmosphere of CO2 at room temperature.73 At higher temperatures (100 °C) and 0.1 mol % [Cu(OtBu)(IPr)] loading, turnover numbers up to 1000/Cu were achieved. Decreasing the size of the ligand (i.e., switching to ICy) produced thermally less stable catalysts that could only operate at lower temperatures. The mechanism of the transformation was studied computationally by DFT means.181 The main findings of this study were that the reaction started by insertion of CO2 into the Cu-B bond leading to Cu-OC(O)-boryl species with Cu-O and C−B bonds. In a subsequent step, boryl migration from the carboxylate C to O took place and finally a σ-bond metathesis between [B(pin)]2 and [Cu{OB(pin)}(NHC)] regenerated the catalyst. The driving force for the

Initial studies also showed that stoichiometric reduction of CO2 to CO was feasible, albeit under more forcing conditions by the reaction with the silyl complex [Cu(SiPh3)(IPr)] (38bCu); the Cu-containing product isolated after the reduction was the siloxide 118fCu. A plausible intermediate 118eCu was independently synthesized and shown to convert to 118fCu by intramolecular extrusion of CO (Scheme 75).128 A further advancement in this direction was described by the use of (pin)B-SiMe2Ph as transmetallating agent and source of nucleophilic silyl groups for the preparation of a series of silyl complexes 38aCu−38fCu (see also Scheme 36). Furthermore, the stoichiometric reaction of [Cu(SiMe2Ph)(IPr)] (38aCu) with CO2 led to the formation of the new complex 119Cu, which slowly converted to 120Cu and CO; the transformation was conveniently monitored by NMR spectroscopy. The catalytic running of this reaction using an excess of (pin)BAH


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Scheme 75. Stoichiometric Reduction of CO2 to CO with the Silyl Complex 38bCu

Scheme 76. Carboxylation of Silyl Complexes

Sn).131 The preferred mechanism is concerted with the relative nucleophilicity and strength of the Cu-E and E-Ph bonds controlling the nature of the products. The reactivity of the Cu-E bond toward CO2 decreases as E becomes heavier, while the reactivity of the E-Ph bond toward CO2 increases as E becomes heavier. Thus, in a transition state, as in Scheme 77, the reactivity of the Cu-E bond toward the electrophilic CO2 is highest for E = Si and leads to the formation of the strong Ph3Si-CO2Cu interaction which drives the reaction thermodynamically. The driving force for the elimination of CO is the formation of strong E−O bonds (see also above for the availability of accessible empty orbitals for the migration which

SiMe2Ph and CO2 in the presence of a catalytic amount of [Cu(OtBu)(IPr)] showed slow catalytic conversion of CO2 to CO (Scheme 76). However, additional complexity became apparent due to the decarboxylation of 119Cu by the excess (pin)B-SiMe2Ph, leading to the adduct 121Cu.79 Finally, the reaction of [Cu(SnPh3 )(IPr)] with CO2 produces SnPh2 and the benzoate complex [Cu{κ-OC(O)Ph}(IPr)] as the sole Cu-containing species in high yield in a transformation that involves Sn-Ph bond cleavage.78 DFT calculations were carried out to rationalize the nature of products obtained and the reactivity trends in the interaction of CO2 with the complexes [Cu(EPh3)(NHC)] (E = Si, Ge, AI


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As mentioned previously, CO2 hydrosilylation or hydroboration by SiH(OEt)3 or BH(pin) are high yielding reactions catalyzed by [Cu(OtBu)(IPr)] (Schemes 52 and 53); in contrast, CO2 reactions with [B(pin)]2 and the same catalyst lead selectively to deoxygenation and formation of CO catalytically. A class of well-defined bimetallic complexes of the type Cu(IPr)-[M] were synthesized ([M] = Fp, FeCp(CO)2; Wp, WCp(CO)3; Mp, MoCp(CO)3) that, on reaction with BH(pin)/CO2 and depending on the nature of [M], yield mixtures of CO and HC(O)OB(pin) in ratios ranging from 0.7:1 to >20:1, respectively; the highest ratio was obtained for [Cu(IPr)-MoCp(CO)3].182 The control of reduction selectivity by the nature of bimetallic pairing was rationalized by cooperative tandem catalytic cycles: a Cu-cycle leads to hydroboration of CO2 and formation of (pin)B-formate and a “heterometal” cycle leads to the decarbonylation of the latter to CO and [(pin)B]2O. The ratio of the boryl formate to CO depends on the relative concentration of the mononuclear Cu and heterometal species, which in turn relates to the [M] intrinsic lability that is responsible for releasing the [M] complex for decarbonylation. 2.2.1.1.9. Type [Cu(NHC)L]+(A−) (L ≠ NHC). A limited number of [Cu(NHC)L]+ complexes with simple neutral σdonors has been described. The rare aquo complexes [Cu(IPr*)(H2O)][SbF6] (122Cu) was prepared from [CuBr(IPr*)] and AgSbF 6 in wet dichloromethane.183 The complexes [Cu(NHC)L](A) (123aCu, 123bCu, L = phosphine donor) have been synthesized either by the deprotonation reaction of a phosphonium salt with 26Cu or by the chloride abstraction from [CuCl(IPr)] by KPF6 in the presence of PPh3.184,185 A mononuclear dicoordinate Cu-NHC complex with a zwitterionic bulky tetrel Zintl cluster functionalized phosphine led to the charge neutral zwitterionic Cu complex 123c with NHC and P donors (Scheme 78).186 There is growing interest in isolating and studying complexes that model intermediates in late transition metal catalyzed reactions involving as reactive species coordinated

Scheme 77. Reactions of [Cu(EPh3)(IPr)] with CO2

is the case for SiPh3 and B(pin)). Conversely, the reactivity of E-Ph is highest for E = Sn and leads to the formation of the benzoate and release of SnPh2. There are some additional insightful comparisons of the CO2 reduction with [Cu{B(pin)}(IPr)]/[B(pin)]2, [Cu(SiPh3)(IPr)], and [Cu(OtBu)(IPr)]/[B(pin)(SiMe2Ph)] systems. Only the [Cu{B(pin)}(IPr)]/[B(pin)]2 and [Cu(OtBu)(IPr)]/[B(pin)(SiMe2Ph)] have been developed to catalytic reactions; in all three cases, the initial step is the nucleophilic attack of the boryl or silyl group to the electrophilic C atom of CO2. The driving force of this step is the formation of strong C−B and C−Si bonds. However, only in the [Cu(OtBu)(IPr)]/[B(pin)(SiMe2Ph)] case was the initial product of the attack silylcarboxylate fully characterized and its conversion to the siloxide with concomitant elimination of CO observed. The formation of boryl- and silyl-carboxylate complexes was attributed to the high (kinetic) nucleophilicity of the boryl and silyl moieties, respectively. The extrusion of CO is the ratedetermining step; the calculated barrier is the lowest for the B(pin) system (ca. 22 kcal/mol), followed by SiPh3 and SiMe2Ph (24.6 and 31.3 kcal/mol, respectively), in agreement with the fact that only in the latter case the intermediate preceding the CO release step was observable. The driving force for this step was attributed to the formation of B−O and Si−O bonds.

Scheme 78. Complexes of the Type [Cu(monodentate-NHC)L]+ (L ≠ NHC)

AJ


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carbenes with or without α-heteroatom donors. The electronic structure of the reactive carbene and its interaction with the electron-rich metal constitute a challenging extension to the diverse range of carbene-metal bonding across the Periodic Table; the topic has recently been reviewed.187 One case has been described where Cu-NHC moieties have been used to stabilize mononuclear reactive carbenes in Cu-catalyzed transformations. The reaction of the complex [Cu(NTf2)(IPr**)] with dimesityldiazomethane gave as sole product [Cu(IPr**)(CMes2)](NTf2) (124Cu) as greenish yellow, diamagnetic, thermally unstable, and water-sensitive solid after elimination of N2. Although clean and complete conversions to the products were not achieved, satisfactory characterization of the species was accomplished.188 Complex 124Cu constitutes a rare example of a complex with the two types of carbene ligands: the heteroatom-stabilized IPr** acting as spectator and the reactive dimesityl carbene. The ligands showed distinct NMR spectra. The 13C NMR signal of the dimesityl carbene carbon is very deshielded (δ = 332 ppm, cf. 13C NMR signal of CNHC of IPr** at δ 177.8 ppm). This, in conjunction with a comparative analysis of 13C NMR data of 124Cu and those of Ag and Au analogues that were also described (chemical shifts values and 1JAg−C coupling constants), led to the conclusion that a higher carbenoid character is found in the (Mes)2C carbene with minimal πinteraction with the metal; therefore, it can be considered as a coordinated carbenoid. The structural data of 124Cu are discussed below (Scheme 79).

Scheme 80. Cyclization of a Sterically Demanding NAlkynyl Formamidine

Scheme 81. Formation of the Mesoionic 125bCu in the Presence of Base

Scheme 79. Complexes With Two Types of Carbene Ligands

extended to include a range of imidazol(in)ylidene ligands obtained by their reaction (frequently generated in situ from imidazolium salts and NaOtBu) with [Cu(NCMe)4](A) precursors (126aCu−126eCu Scheme 82). The complexes Scheme 82. Homoleptic CuI NHC Complexes

The coordinated carbene stabilized on a Cu(IPr*) moiety was obtained by a Cu-promoted cyclization of a sterically demanding N-alkynyl formamidine and subsequent transformation into an alkylidene ligand coordinated on Cu. Out of the possible resonance forms of the isolated complex (A− C), the carbene form B seems to be in agreement with experimental structural information: the presence of C−Cu single bond and CO double bond distances support the description of a coordinated carbenoid complex.189 The complexes have been postulated as intermediates in the catalytic amination of alkynes (Scheme 80). In the presence of base, the mesoionic carbene product 125bCu was isolated (Scheme 81). 2.2.1.2. Homoleptic Complexes [Cu(NHC)2](A). The first Cu-NHC complex, the homoleptic [Cu(IMes)2]OTf, was prepared by the direct reaction of two equivalents of free IMes with [Cu(OTf)].2 The family of similar complexes was later

were evaluated in comparative catalytic studies for the hydrosilylation of ketones, where both the nature of the NHC and the counterion showed a significant influence on the catalytic performance.119,142 The homoleptic [Cu(SIMes)2][Cu(CF3)2] (78bCu) is proved to be in equilibrium with [Cu(CF3)(SIMes)] (see also Scheme 33).122 One homoleptic complex (127Cu) with the rigid pyrimidine-functionalized NHC ligand was prepared by electrochemical methods involving imidazolium salts or the corresponding silver−NHC complexes as carbene sources and AK


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cAAC and KC8 in toluene. The complexes 130aCu and 130bCu are stable up to ca. 130 °C in the solid state under inert atmosphere but slowly decompose in solution, even in toluene or hexane to unidentified species. Both complexes were characterized by diffraction methods which corroborated for their charge neutral nature and atom connectivity.193 The electronic structure of 130aCu and 130bCu was studied by computational methods, EPR spectroscopy, magnetic measurements, and CV. Initial DFT calculations suggest that the unpaired electron is localized over the p(π) orbitals of the NC-Cu-C-N coordination vector, with the highest probability on the coordinated cAAC carbon atoms. The analysis of the room temperature EPR spectra gave data in agreement with the computed structure: isotropic g = 1.997−1.999, close to the free electron value that confirms minimal contributions from Cu to the SOMO. This picture supports a three-center carbene/copper/carbene bonding and predominantly cAAC radical character. The small 63/65Cu hyperfine coupling and N superhyperfine constants in the EPR spectra are in line with this description. Finally, the magnetic moment of 130aCu and 130bCu is 1.74 μB at 155 K, close to the spin-only value of 1.73 μB for one unpaired electron. The cyclic voltammogram of 130bCu in DMF exhibits a quasi-reversible reduction wave at ca. E1/2 = −1.35 V versus Cp*2Fe+/Cp*2Fe, attributable to a reduction (Scheme 83). More detailed theoretical analyses aiming at understanding the bonding in 129bCu and 130bCu or simplified models thereof (R = H) have also appeared. The calculated Cu-cAAC bond distances show that the Cu-C separations in the cationic complexes analogous to 129bCu are significantly longer than in the neutral species analogous to 130aCu and 130bCu. However, the cations have much higher bond dissociation energies than the neutral molecules.194,195 In 129bCu, the interaction energy is due to Cu+ in the 1S electronic ground state and (cAAC)2, while in 130bCu it involves the Cu atoms in the excited 1P state resulting in strong Cu p(π) → (cAAC)2 π backdonation, which is absent in 129bCu. The calculations also suggest that in 130bCu the cAAC are stronger π acceptors than σ donors.194 NBO analysis of the bonding in 130aCu describes the Cu cation as bound to two “non-innocent” cAAC ligands that are each reduced by 0.5 electron. The unpaired spin delocalizes over a π network spanning the two ligands.195 The topical area of the homoleptic low oxidation state 3d cAAC complexes has been reviewed.21,22 “Pseudohomoleptic” complexes (i.e., comprising two disparate NHC ligands in a mononuclear two-coordinate Cu center) (131Cu and 132aCu−132cCu) have been synthesized by the reaction of various imidazol(in)ium tetrafluoroborate salts with [Cu(OH)(NHC)] or chloride abstraction from [CuCl(NHC)] and substitution by free NHC ligands or substitution in the presence of imidazolium salt and exogeneous base (NaOH) in MeCN under microwave irradiation (Scheme 84).184,185,196 They have been tested as precatalysts in azide− alkyne cycloaddition reactions to 1,2,3-triazoles. Investigations of the catalytic mechanism suggest one NHC is abstracted from the precatalysts to afford mono-NHC complexes that are involved in the catalysis. A family of heteroleptic bis-carbene complexes bearing one anionic and zwitterionic NHC and a range of imidazol-2ylidenes (133Cu) were obtained by a one-pot, three step procedure from the pyrimidinium betaines, a copper source (typically CuBr·SMe2), and the imidazolium salts. Two equivalents of base were added to generate the free anionic

electrolytes and Cu plate as sacrificial anode. The methodology, despite its specialized nature, has a broad scope in terms of both the functionalized ligands and other 3d metals (Scheme 82).190 Homoleptic complexes have been described with other types of NHCs: they include the substituted 1,2,3-triazol-5-ylidenes, which were obtained by the reaction between two equivalents of the corresponding triazolium tetrafluoroborates and [Cu(NCMe)4](BF4) in the presence of base. Interestingly, use of halide-containing starting materials led to heteroleptic NHC halide complexes.191 The homoleptic 1,2,3-triazol-5-ylidenes were shown to be very efficient catalysts for the [3 + 2] cycloaddition reaction between azides and alkynes, in some cases with loadings as low as 0.005 mol %; the halide-free homoleptic species outperformed the halide-containing precatalysts. The homoleptic NHC complex with an extended ring NHC ligand, [Cu(1,3-dimesityl-3,4,5,6-tetrahydropyrimidin-2ylidene)](CuBrCl) was obtained as a product from the reaction of CuCl with 1,3-dimesityl-3,4,5,6-tetrahydropyrimidin-2-ylidene (RE-6-Mes) generated in situ from the azolium bromide and NaOtBu as base. iPr-Substituted azolium salts led to heteroleptic halide-substituted species.192 The same homoleptic cation in combination with a bis(carboxylate)cuprate was obtained as disproportionation product from the reaction of the in situ generated [Cu(H)(RE-6-Mes)] with sequential reaction with PhCCMe and CO2.71 The use of the strongly σ-donating and π-accepting R2cAAC (R = Me, Et) to form homoleptic NHC complexes led to the isolation and characterization of the cationic [(Et2cAAC)2Cu]I (129Cu) in good yields by the reaction of the free Et2cAAC with CuI. The white diamagnetic complex was characterized analytically and by NMR spectroscopic methods. Most interestingly, reduction of 129Cu with Na in THF gave the red, charge neutral [Cu(Et2cAAC)2] (130bCu) in low yields (Scheme 83). An analogous complex, the red [Cu(Me2cAAC)2] (130aCu) could be obtained in low yields in one step by the reduction of a mixture of CuCl2 in the presence of the 2 equiv. Scheme 83. Reduction of CuI cAAC Complexes

AL


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Scheme 84. “Pseudo-Homoleptic” Complexes

complexes in sections 2.2.1.1 and 2.2.1.2 are also included in Tables 6, 7, 8, 9, 10, and 11. Perusal of the data shows that the nature of the NHC is the determining factor for the chemical shift region where δCNHC is found; the nature of the X ligand has a minor effect. It is also noteworthy that in some complexes, in particular bromides, iodides, alkyls, and hydrides, the resonances due to the CNHC were not observed or reported. In certain cases, this is due to thermal instability or lability. The most deshielded CNHC are found in the cAAC and RE-7 complexes and the most shielded in the BAC, 1,2,3-triazolylidene, and unexpectedly some aNHC complexes; there is a significant deshielding effect on moving from imidazol-2-ylidene to imidazolin-2-ylidene complexes. The effect of solvent on the chemical shift is probably of secondary importance. However, few deviations from these generalizations may be due to solvent associations or higher aggregations of the complexes in solution. 2.2.1.4. [Cu(NHC)L2](A), L = Group 15 and 14 Donors. Mononuclear complexes of this type, where L2 is a N donor bidentate chelate, were initially targeted for photophysical applications. They were easily prepared by the reaction of [CuCl(IPr)] or [Cu(OH)(IPr)] with the chelating ligand in the presence of AgOTf or KPF6 or HBF4, respectively.245,246 Distorted trigonal planar coordination geometries and nearly coplanar arrangement of the ligand and NHC heterocycles are common structural features. It has been suggested that the conformational stability of the complexes originates from the interaction of the H atom situated α to the N atom of the heterocyclic ring of the bidentate ligand with the aromatic electron density of the NHC wingtips. The absorption spectra of the complexes show transitions at ca. 240−260 and 280−320 nm, which were ascribed to π−π* and dπ−π* metal-to-ligand charge transfer (MLCT) transitions, respectively. Interestingly, in the solid state, the dipyridyl amine complexes exhibit broad emission around 436−488 nm corresponding to blue emission. Specific structural features of the complexes have been associated with higher φem, which for 134fuCu and 134esCu can reach ca. 86% (τem = 78 μs) (Scheme 86). The large π−π* and singlet− triplet energy gaps in the imidazol(in)-2-ylidene coligands imply that the emission energy in the complexes 134aCu− 134hCu is controlled by variations in the L2 chelate coligand; furthermore, imidazolin-2-ylidenes increase this gap. Blue-emitting electrochemical cell devices were built based on selected complexes from Scheme 86 and their derivatives, showing moderate electrochemical stabilities, which under low currents provide stable blue emissions.247 The Me2cAAC complexes 135aCu and 135bCu prepared by analogous methods from [CuCl(Me2cAAC)] and bidentate bipy or phen adopt also distorted trigonal planar geometries and show emission spectra with peaks bathochromically shifted by 50 nm into the NIR compared to that of 134aCu (Scheme 87). This has been attributed to the σ-donating properties of the cAAC compared to IPr, which destabilize the Cu 3d orbitals.242 The NHC ligand 136Cu with the anionic remote siliconate functionality was used to prepare the overall neutral 3coordinate Cu bipy complex 137Cu. Complex 136Cu was obtained by reaction of the bulky IPr with Martin’s spirosilane, concomitant rearrangement to the abnormal aIPr coordination mode, and isolation of the zwitterionic siliconate-substituted imidazolium. Deprotonation of the latter with nBuLi in the presence of [Cu(NCMe)4](BF4) and bipy led to 137Cu (Scheme 88).248

NHC and substitute the Br by N(SiMe3)2 in the ensuing Cu complex. In the final step, the coordinated silylamide deprotonates the imidazolium salt and introduces the second NHC in the coordination sphere (Scheme 85).197 The zwitterionic complexes are efficient and robust precatalysts for the intramolecular cyclopropanation reaction of diazo esters. Scheme 85. A “Pseudo-Homoleptic” Complex with Zwitterionic NHC

2.2.1.3. Selected Metrical Data of [CuX(NHC)] and [Cu(NHC)2](A). The majority of the complexes described in this section feature 2-coordinate linear or quasi-linear geometries. They are tabulated with reference to their Cu-CNHC and Cu-X bond distances and the coordination angle at Cu (Table 3). In specific cases, (i) the torsion angle between the NHC plane and planar coligands trans to them, (ii) the pitch, and (iii) the yaw of the coordinated NHC (as given in the scheme below, Table 3). The aim of the compilation is to extract trends and highlight/rationalize if possible unusual structural features. Comprehensive coverage of all known complexes is not intended. In the Tables 4, 5, 6, 7, 8, 9, 10, and 11 are given the chemical shifts of selected two-coordinate Cu-NHC complexes with a diverse range of NHCs; the data were extracted from the literature discussed in sections 2.2.1.1 and 2.2.1.2 for halide complexes; [CuBr(SIMes)], [CuCl(RE-6-Mes)],244 and [CuI(IPr)]97 were recently reported. NMR data for other AM


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Table 3. Summary of Metrical Data of [CuX(NHC)] (X = Halide)

AN


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Table 3. continued

AO


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Table 3. continued

AP


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Table 3. continued

AQ


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Table 3. continued

AR


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Table 3. continued

AS


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Table 3. continued

AT


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Table 3. continued

AU


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Table 3. continued

AV


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Table 3. continued

AW


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Table 3. continued

AX


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Table 3. continued

AY


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Table 4. δCNHC in the 13C-NMR Spectra of Selected Complexes [CuX(NHC)] (NHC = Imidazol(in)ylidene, X = F, Cl, I) ICy F Cl Br I

SICy b

ItBu

IAd

IMes

b

197.4 197.8b 174.2d 176.6e 177.9d

197.4 197.8c

SIMes

b

172.9c

171.2c

173.7c 175.6e

172.1c 174.4c

179.1 179.4c

202.8c

180.0c

157.2d

IPra

Cl2

SIPr

b

180.7 180.2d

203.7 204.3c

181.4c 183.3c

203.1c 204.5c

IPr

b

IPr*

179.9b 179.7b

180.9b 180.7c 181.2c

The azide tagged and heteroatom functionalized imidazol(in)-2-ylidene complexes give signals in ca. δ 178 ppm and ca. δ 201 ppm, respectively, in agreement with the parent complexes; in complex 3Cu, the CNHC appears at ca. δ 212 ppm. bIn CD2Cl2. cIn CDCl3. dAcetone-d6. eIn C6D6.

a

Table 5. δ CNHC in the 13C-NMR Spectra of the Complexes [CuX(NHC)] (NHC = triazol; X = Cl, Br, I) RR

Cl

Et2

cAACa

RE-6-Mes

c

250.3 253.0b Menth 251.0c Ad 253.3b Ad 254.1b

RE-6-DiPP

RE-7-DiPP

c

c

200.8

210.3

201.1c

210.5

cAAC, RE-NHCs, BAC, aNHC, 1,2,3-

RR

BAC 148.4

e

aNHC

1,2,3-triazol

c

161.3

155.9−166.4c

167.1c

165.0−167.1b

Ad

Br I

198.8c

a The heteroatom functionalized cAAC Cu-Cl complexes with κC(cAAC) give δCNHC in the region 242−249 ppm (in C6D6 or CDCl3). bIn CD2Cl2. cIn CDCl3. dAcetone-d6. eCD3CN.

reduced pressure for several hours, the complex [Cu(IPr*)(H2O)]SbF6 was obtained. Conversely, bubbling CO through a CH2Cl2 solution of [Cu(IPr*)(H2O)]SbF6 led to 139bCu. 2.2.1.5. [Cu(NHC)(LX)]. Complexes of this general type have been obtained as catalytically relevant intermediates in sequential stoichiometric C−H activation/carboxylation reactions of heterocycles catalyzed by [Cu(OtBu)(NHC)] or targeted as tunable organocopper species for photophysical applications. The reaction of [Cu(OtBu)(IPr)] with benzoxazole gave 70Cu which on carboxylation with CO2 led to the complex 71Cu, which features a chelate κN,κO-2-benzoxazolyl-carboxylate and distorted trigonal planar geometry at Cu (Scheme 29).112 Amidation of [Cu(IMes)(2,3,5,6-C6F4H)] with the electrophilic amidating agent Na(BOC)N(Cl) led to the isolation of copper fluoroarylcarbamate species 82Cu (Scheme 34), which features a 3-coordinate Cu center of highly distorted T-shape geometry due to the 4-membered chelate ring strain and the dative nature of the Cu-OtBu interaction.77 As a development to the interesting photophysical properties of the cationic 3-coordinate complexes of type 134aCu−134hCu attempts were focused on the design of neutral Cu-NHC 3coordinate complexes obtainable by replacing one L donor as part of the bidentate chelate by X. A family of [Cu(pyridylazolide)(IPr)] complexes 140aCu−140eCu were prepared by the reaction of [CuCl(IPr)] with the anionic pyridyl-azolate obtained by deprotonation of the pyridylazole with NaH (Scheme 90).250 Invariably the Cu center in complexes 140aCu−140eCu adopts distorted trigonal planar coordination geometries. A distinctive feature in the solid-state structures of 140b−140eCu as opposed to 140aCu is the magnitude of the interplanar angle between the coordination plane (which coincides with the plane of the heterocyclic LX) and the imidazole plane of IPr: in the former group, it is approaching 70−80°, while in 140aCu, it is only 9°. The reason behind this conformational preference has not been established but may be related to intramolecular interactions (see also complexes 134aCu−134hCu) or crystal packing. Cu-Npy distances are longer than the corresponding anionic Cu-Nazolate, which could be due to the σ-donor ability

Complexes of the type [Cu(IPr)(P−P)](A), where P−P is one of the chelating diphosphines dppe or dppf, have been prepared from [CuCl(IPr)] and P−P in the presence of KPF6 in acetone.185 The distorted trigonal planar geometry found in the solid state is persistent in solution. The coordination and NHC planes are almost mutually perpendicular. The complexes show a reduced catalytic activity in the Cu-catalyzed alkyne azide cycloaddition reaction. A range of 3-coordinate κN-κE Cu-IPr complexes with the mixed donor ligands N-E, where N and E are pyridine and S donors of N-(2-pyridinyl)amino-diorganyl-phosphine sulfides, respectively, have been prepared by the reaction of [CuCl(IPr)] with the free N-E ligand in the presence of AgSbF6 as chloride abstractor (Scheme 89). In the structure of the complexes, the six-membered chelate rings are puckered, and the NHC plane is nearly perpendicular to the coordination plane.249 Stable 3-coordinate [Cu(SIPr)(CO)2]SbF6 (139aCu) and [Cu(IPr*)(CO)2]SbF6 (139bCu) were synthesized by the reaction of [CuCl(SIPr)] and [CuBr(IPr*)] with AgSbF6 under a CO atmosphere (1 atm) in noncoordinating CH2Cl2, respectively. The ν(CO) stretching frequencies in the IR spectra of the complexes, which are a measure of metalto-ligand dπ−π*(CO) backbonding, are found in the region of “nonclassical” metal carbonyls at 2149 and 2168 and 2151 and 2171 cm−1, respectively; these values are higher than the stretching frequency of the free CO (2143 cm−1), implying that the CuI−CO interaction is dominated by electrostatic and OC → Cu σ-donor components. Complexes 139aCu and 139bCu add to the limited family of [CuX(CO)n] complexes (X = N(OTf)2, n = 2; X = 1-benzyl-CB11F11, n = 4). The structure of 139aCu features a trigonal planar coordination geometry with inter-CO angle at 119.8°.183 In the crystal structure, a weak interaction Cu···F-SbF5 between the Cu center and the anion was also detected. The structure of 139bCu exhibits a more distorted trigonal geometry and unequal Cu-CO distances, attributable to interactions of CO with the bulky IPr*. Complex 139bCu is less stable than 139aCu: upon exposure to traces of moist air, it converts to [Cu(IPr*)(CO)(H2O)]SbF6; by placing the latter under AZ


252.5b

SR

Et2

Ad

Table 7. δCNHC in the 13C-NMR Spectra of Selected Complexes [CuX(NHC)] (X = SR, SH)

(Me)

253.5b

253.1c

253.2 ; 254.4b Et2 251.5b; Ad

cAAC

b Menth

RR

Et2

Review

IMes

IPr

SIPr

IPr*

(Ph) 182.8c (SiiPr3) 180.7c

(SiMe3) 182.8c (Bn) 181.7a 182.8f

(Ph) 203.3a

(SiMe3) 181.7c

a

(H) 210.1b

213.1

RE-7-DiPP

SH b

c

(H) 200.6b 175.9a

RE-6-DiPP

203.5 184.2

f

IPr*

NHPh N(SiMe3)2 pyrrolyl NTf2 PPh2 P(SiMe3)2

(Arylvinyl) 180.2b

183.6 205.2 205.0

206.4

184.4 182.3b 182.8 205.0 182.7; −183.8c (Me) 182.6b

IPr 182.2

Ad

SIPr

c

c

cAAC

254.6c

204.7

201.6c 163.3c 178.7a 183.5f (δP − 27.8) 186.6c (δP − 268.0) 146.5a (δP 8.8) 178.6b (δP 66.9)

In CDCl3. bIn CD2Cl2. cIn C6D6. dAcetone-d6. eCD3CN. fTHF-d8.

IMes

SiPh3 SiMe2Ph SnMe3 a

IPr

SIPr

RR

cAAC

c

Me Et Ph o-tolyl o-anisyl Allyl C2Ph CHF2 CPh2


202.5 OtBu OH OPh OEt O-enolate OOCR H2O

177.5

b

181.2

ItBu

c

180.3

182.8c 183.7c

186.5 186.3c 186.2c 185.3a 185.7c 185.3c 184.2c 182.8b 177.8 (δCHPh2 332.0) 184.7c 185.7c

207.7c

256.0b

(Ad)

183.4c b

c

d

6 e

In CDCl3. In CD2Cl2. In C6D6. Acetone-d . CD3CN. fTHF-d8.

of the azolate ligand. In solution, there is a fast dynamic process involving rotation around the Cu-CIPr bond, which was demonstrated by variable temperature 1H NMR spectroscopy in CDCl3 and other (including coordinating) solvents.250 All complexes display broad, featureless yellow-orange luminescence with emission maxima between 555 and 632 nm. The emissions are assigned to a mixed triplet MLCT to ligandcentered transition associated with the LX. Interestingly, DFT calculations showed that the introduction of the electronwithdrawing CF3 group in complex 140dCu led to equal stabilization of both the HOMO and LUMO compared to 140cCu, while introduction of a third nitrogen in 140eCu resulted in additional stabilization along these lines. The substituent manipulation of the energy of the HOMO and LUMOs points to a way of fine electronic tuning in view of potential applications in organic electronics devices. An alternative strategy for modulating the photophysical properties of 3-coordinate Cu complexes by using the NHC

a

(Me) 180.9c

181.1

182.4

6 e

Table 9. δCNHC in the 13C-NMR Spectra of Selected [CuX(NHC)] (X = Group 14 Donor) and of [Cu(IPr*)(CPh2)]+

(Me) 203.5a

IPr Cl2 c c

SIPr IPr

181.7c d

IMes

PPh3 PtBu3

c c b

IMes

cAAC

(Ph) 250.1b

Table 8. δCNHC in the 13C-NMR Spectra of Selected [CuX(NHC)] and [Cu(NHC)L]X (L = Group 15 Donors)

a

SICy

Et2

In CDCl3. In CD2Cl2. In C6D6. Acetone-d . CD3CN. fTHF-d8.

ItBu

ICy

Table 6. δCNHC in the 13C-NMR Spectra of Selected Complexes [CuX(NHC)] (NHC = imidazol(in)ylidene, X = OR, OH, O(O)CR, O-enolate, and [Cu(NHC)(H2O)]+)

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Table 10. δCNHC in the ICy

13

C-NMR Spectra of [Cu(NHC)2]+(A−) Salts ItBu

IAd

IMes

SIMes

174.2d

ICy

IPr

SIPr

R2

cAAC

RE-6-Mes(DAC)

1,2,3 triazolylidene

171.7 179.3a 171.7d

ItBu IAd IMes

169.8a 178.8d 201.4a

SIMes

176.2 179.3a 179.0 202.2a 199.8a

SIPr R2 cAAC

Me2 Et2

b

248.6 249.3a,b 199.3/208.6a

RE-6/7-Mes RE-6-Mes(DAC) 1,2,3 triazolylidene a

RE-6/7-Mes

212.7a 162.0−166.0a c

d

6 e

f

8

In CDCl3. In CD2Cl2. In C6D6. Acetone-d . CD3CN. THF-d .

Table 11. δCNHC in the 13C-NMR Spectra of the Complexes [CuX(NHC)] (X = Group 13 Donor) B(pin) 1,3-DiPP Boryl Bdbab Bdmab Ga(NR)

IMes

SIMes

185.3c

185.2c

Scheme 86. Diverse Co-Ligands Influencing Emission Energy in the Complexes 134aCu−134hCu

IPr 183.2c 186.9c 187.1f 181.2c

a


ligands as tunable chromophores through modifications of the associated singlet−triplet gap was tested. In the [Cu(NHC)(LX)] complexes, the symmetric di-(2-pyridyl)dimethylborate LX chelate possesses high energy triplet and acts as an ancillary ligand (Scheme 91).251 The complexes 141aCu−141cCu reveal trigonal planar “Yshaped” coordination geometries with puckered six-membered chelate rings. The conformation in 141aCu places the NHC and pyridine rings opposite and perpendicular to the DiPP planes, while in 141bCu and 141cCu, they bisect the coordination plane. The emission spectra of the complexes in the solid differ, resulting in different colors: 141aCu is light blue, 141bCu yellow, and 141cCu orange. The bathochromic shift is attributed to the nature of the NHC ligand: the expanded π-system in 141bCu and the N substitution in 141cCu result in lowering the LUMO, which is associated with the emission; emission efficiencies of 0.16−0.80 in the solid state are also tunable by the nature of the NHC. Metalation of the 2-(2,3,4,5-tetrafluorophenyl)pyridine with [Cu(OH)(IPr)] in THF at lower temperatures in the presence of molecular sieves gave the 3-coordinate complex 142Cu, featuring a metalated fluoroaromatic ring attached to coordinated pyridine onto a trigonal planar Cu center. Reactions in different solvents or in the absence of sieves led to decomposition, presumably involving hydrolysis by the liberated H2O during the reaction (Scheme 92).233 142Cu is a weak emitter in solution but intensely luminescent in the solid state, exhibiting orange-red phosphorescence at room temperature, most likely from a triplet state. The remote functionalization of NHCs was extended to integrating an anionic acetylacetonate backbone moiety onto the framework of the IMes. Reaction of the mesoionic zwitterion 143Cu with [CuCl(IPr)] in the presence of AgBF4

as chloride abstractor led to the complex 144Cu with a trigonal planar Cu center. Furthermore, deprotonation of 143Cu with KN(SiMe3)2, followed by reaction with [CuCl(IPr)] gave the “pseudohomoleptic” neutral complex 145Cu with linear coordination geometry. Interestingly, 144Cu in the presence of KN(SiMe3)2 rearranges to 145Cu (Scheme 93).243 BB


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Scheme 87. Cu(I) cAAC Complexes Emitting in the NIR

Scheme 90. Complexes of the Type [Cu(pyridylazolide)(IPr)]

Scheme 88. Complex Containing a NHC Ligand with an Anionic Remote Siliconate Functionality

Scheme 91. Complexes with the Symmetric Di-(2pyridyl)dimethylborate Chelate

Scheme 89. 3-Coordinate Copper Complexes with Mixed Donor Ligands

Scheme 92. Complex with a Metalated Fluoroaromatic Ring Attached to Coordinated Pyridine

Comparative photophysical studies involving [CuCl(Et2cAAC)], [CuCl(AdcAAC)], the pyrazolyl-borate 3-coordinate 146Cu, and the related 4-coordinate 147Cu were performed. The synthesis of 146Cu and 147Cu was successful, following straightforward routes with KTp as reagent, although care should be taken to avoid comproportionation of [CuCl(Et2cAAC)] to [Cu(Et2cAAC)2](CuCl2) which can take place in polar solvents and during the salt metathesis reactions; slow recrystallization gave pure products. Although the atom connectivity of 146Cu and 147Cu in the solid state was established crystallographically (Scheme 94), the solution structures are nonrigid as demonstrated by VT 1H NMR spectroscopy. It appears that dynamic processes based on the hemilability of the coordinated pyrazole heterocycles combined with CcAAC-Cu rotation are responsible for the symmetrization of the NMR spectra, which exhibit three peaks associated with protons on the pyrazoles.252 Both 146Cu and 147Cu and their two-coordinate precursors are phosphorescent (orange, yellow, respectively), but previous reports that the emission from [CuCl(cAAC)]-type complexes was

exclusively from prompt fluorescence, with nanosecond lifetimes, could not be confirmed. Complex 148Cu, prepared by the reaction of [CuClEt2 ( cAAC)] with NaBH4 or BH3·NH3, constitutes a unique example of borohydride on a monoligated Cu fragment; other Cu(BH4) complexes with low coordination number are also rare. 148Cu features slightly nonsymmetrical κ2-BH4 coordination with Cu-CcAAC distances in the usual range (ca. 1.88 Å). The accessibility of 148Cu from BH3·NH3 was rationalized by a salt elimination reaction involving [BH2(NH3)2][BH4], a postulated unstable intermediate in the dehydrogenation of BH3·NH3, and [CuCl(Et2cAAC)]. The complex [CuCl( Et2 cAAC)] is an efficient catalyst precursor for the BC


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structures of the complexes show trigonal planar geometry at Cu with Cu-CNHC bonds (1.94−1.98 Å) significantly longer than in the two-coordinate [Cu(κ1-OAc)(NHC)] (ca. 1.85 Å) described previously; an analogous trend is seen for the Cu-O bond distances. Interestingly, in the structures of 149aCu, 149buCu, and 149cCu, the plane of one of the acetate groups is almost perpendicular to the coordination plane, which positions one distal oxygen atom of the acetate at distances of ca. 2.25−2.30 Å, corroborating interaction with the metal. 149aCu has been used as a precatalyst for the 1,2- and 1,4reduction of carbonyl compounds under hydrosilylation conditions. 2.2.1.7. [CuX(NHC)2]. A limited number of complexes of this type have been described, including imidazol-2-ylidenes,218 functionalized imidazol-2-ylidenes,255,256 1,2,3-triazol-5-ylidene,191 and 1,2,4-triazol-5-ylidene with remote substituted NHCs related to Nitron257 ligands (Scheme 95). All feature

Scheme 93. Complexes with Remotely Functionalized NHCs Including Fused Anionic Acetylacetonate Backbone

Scheme 95. Complexes of Type [CuX(monodentateNHC)2]

Scheme 94. Cu(I) cAAC Complexes With Borate Ligand

dehydrogenation of BH3·NH3 in acetone/water mixtures, releasing 2.6 mol of H2 per mole BH3·NH3 with estimated TOF of ca. 3100 mol H2 mol cat−1 h−1. Substitution of [CuCl(Et2cAAC)] by 148Cu increased TOF to ca. 8400, while showing good catalyst recyclability. The activity of 148Cu in the dehydrogenation reaction matches the one of noble metal catalysts.141 2.2.1.6. [CuX2(NHC)]. This class is dominated by the complexes of type [CuII(OAc)2(NHC)]. In contrast to the instability of [CuII(halide)2(NHC)], [CuII(OAc)2(NHC)] species have been prepared by the reaction of [CuII(OAc)2] with free NHC (NHC = IPr, 149aCu; (S)IMes, 149bu/sCu; RE6-Mes, 149cCu; RE-7-Mes, 149dCu) in toluene.253,254 The

halide as X ligand, except 150eCu.129 The geometries at Cu are T-shaped,255 or distorted trigonal planar,129,218,256 with CuCNHC bond distances slightly longer than in 2-coordinate Cu(NHC)-halides, presumably due to increased steric congestion. 2.2.1.8. [Cu(NHC)(L2X)]. The 4-coordinate distorted tetrahedral Cu-Et2cAAC tris-pyrazolylborate complex 147Cu was previously mentioned in conjunction with studies on phosphorescent Cu-cAAC complexes (Scheme 94).252 AddiBD


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by transmetalation from the Ag complexes with CuBr.218 The metrical data in 151aCu−151dCu are unremarkable; however, it appears that the mononuclear nature is promoted by the rigidity of the ligand and the ratio of the added reactants; the allyl wingtip does not interact with Cu. Thus, reaction of CuBr with the two equivalents of the pyridine-NHC silver complex gave the binuclear species 152Cu with short Cu-Cu separation (ca. 2.525 Å) (Scheme 98).

tion of 1 equiv of phen to [CuCl(SIMes)] solutions in tBuOH/H2O solvent mixtures increased dramatically the catalytic activity of the system in the CuAAC reaction with benzyl azide and phenylacetylene. The 4-coordinate distorted tetrahedral complex 149eCu was isolated and fully characterized; interestingly, it was denoted as a “weak complex” since the Cu-CNHC, Cu-Cl, and Cu-Nphen bond distances are elongated compared to [CuCl(SIMes)] and [Cu(phen)]+ or [Cu(phen)2]+, respectively, which render a dissociation of phen a facile process. In the mechanism proposed to account for the acceleration effect of phen, a binuclear catalytic species was postulated, where a Cu-SIMes center with σ-phenylacetylide and azide coordination is further activated by η2alkyne coordination of a [Cu(SIMes)(phen)]+ moiety.258 Finally, the formally 4-coordinate [CuCp(NHC)] complexes 149fCu were isolated by the reaction of the NHCs generated in situ from imidazolium salts and nBuLi, followed by LiCp addition in one pot (Scheme 96). There is a

Scheme 98. T-Shaped Mononuclear Complexes with a Pyridine-Functionalized NHC Ligand

Scheme 96. Structures of the Phen and Cp Complexes

noticeable Cu-CCp disparity of the coordinated cyclopentadienyl ligands in the complexes, although the complexes have been assigned a η5-hapticity. The angle at Cu subtended by the Cp centroid and the CNHC ranges between 165 and 177°.259 2.2.2. Complexes with Bidentate bis(NHC) Ligands. The complex 150dCu with the trans-spanning bis-NHC ligand was prepared by deprotonation of the bisimidazolium salt with Cs2CO3 in the presence of [Cu(NCMe)4]+ as CuI source (Scheme 97). Its unusual conformation resulting in the

Lutidine-functionalized NHC ligands led to binuclear species 153aCu−153eCu or 154Cu, with cuprophilic interactions, and/or the polymer 155Cu, depending on the type of wingtip substituent and the reaction and crystallization conditions; 154Cu contains unusual NHC donors bridging the two adjacent Cu centers (Scheme 99).24,261,262 Complexes 154Cu and 155Cu are based on the same mononuclear building unit with the lone pairs of the mixed donor ligands in syn- and anti-conformation, respectively. The chloride analogues of type

Scheme 97. Synthesis of the Complex 150dCu with TransSpanning Bis-NHC

Scheme 99. Complexes with Lutidine-Functionalized NHC Ligands

proximity of one aromatic C−H to the Cu+ center raised the question of the additional stabilizing interactions being responsible for the observed experimental structure. However, computational studies trace the origin of the conformation to a combination of the chelating ligand structure, the metal preferred coordination geometry, and the Pauli repulsion between the d10 metal and the aromatic ring.260 2.2.3. Complexes with Bidentate Heteroatom Functionalized NHC Ligands (Lig). 2.2.3.1. [CuX(Lig)] (Lig = Neutral Bidentate Heteroatom Functionalized NHC Ligand). The 3-coordinate T-shaped mononuclear 151aCu with the pyridine-functionalized NHC ligand was prepared by the reaction of the corresponding imidazolium salt with Cu2O in CH2Cl2, in the first demonstration of the use of Cu2O as a convenient Cu precursor for the synthesis of Cu-NHC complexes;24 the analogues 151bCu−151dCu were obtained BE


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153bCu−153eCu and 155Cu with substituted pyridine rings have been obtained by the reaction of Cu precursors with the corresponding imidazolium salts in aqueous ammonia. Detailed photophysical studies show that in complexes of the binuclear type 153bCu−153eCu, cuprophilic interactions responsible for strong spin−orbit coupling, result in an increase of the radiative rate constant from the T1 state; this can lead to emissions comparable to those from thermally activated delayed fluorescence (TADF), which originate from thermally induced reverse intersystem-crossing (RISC) T1 → S1 and emission from the singlet excited state S1 → S0. An analogous class of complexes 156Cu−158Cu has been obtained with oxazoline-functionalized NHC ligands for a range of R and Ar substituents. The complexes were prepared from the imidazolium salts by generation of the oxazoline NHC in situ through reaction with KOtBu followed by complexation with CuBr·SMe2. They are mononuclear in solution (156Cu) but form binuclear or polymeric aggregates in the solid state (157Cu and 158Cu) (Scheme 100).263

Scheme 101. Pyridine-Functionalized NHC Complexes of CuII

Scheme 100. Complexes with Oxazoline-Functionalized NHC Ligands

2.2.3.2. [CuX2(Lig)], [Cu(Lig)L2]+, [CuX(Lig)2]+, and Related (Lig = Neutral Bidentate Heteroatom Functionalized NHC Ligand). A rare family of pyridine-functionalized NHC complexes of CuII in a range of coordination numbers and geometries has been studied.262 They were prepared by transmetalation from the silver complexes to CuBr2 (Scheme 101). The nature of the isolated complexes depended on the mole ratio of NHC to CuBr2, the substituents of the pyridine functionality, and the presence (during crystallization) of additional donors. The products obtained with ligand-to-metal ratio of 1:1 can be dinuclear (for unsubstituted pyridine) or mononuclear (for Y = Me or OMe), adopting distorted trigonal bipyramidal or tetrahedral geometries, respectively. The complexes with ligand-to-metal ratio of 2:1 are trigonal bipyramidal cationic species with trans-disposition of the NHCs. The Cu-CNHC bond distances are rather long (1.95− 2.00 Å). The remarkable stabilization of the CuII centers under NHC ligation has been attributed to the pyridine functionality in a rigid structure, which hampers NHC dissociation. Hard− soft interactions between CuII and CNHC have been invoked to explain the relative scarcity of CuII-NHC complexes, in particular with monodentate NHCs. The 4-coordinate complexes 164aCu, 164bCu, and 165Cu were targeted as candidates to study photophysical properties of Cu-NHC complexes in these coordination numbers. They were prepared by the oxidative transmetalation of the NHC ligands from the Ag complexes to Cu0 in the presence of the chelating phosphine DPEphos (Scheme 102). The cationic complexes comprise pseudotetrahedral Cu centers with elongated Cu-CNHC distances (ca. 1.96−1.97 Å). The emission spectra at ambient temperature of 164aCu feature peaks at 520 nm (green emission) and of 164bCu (with fused aromatic

rings) at 570 nm (yellow) with high efficiency in the solid state. Replacement of the Ph in the benzyl wingtip in 165Cu by 2-naphthyl- or anthracenyl- resulted in deterioration of the emission quantum yields due to π−π stacking.264,265 2.2.3.3. [Cu(Lig)2] (Lig = Anionic Bidentate Heteroatom Functionalized NHC Ligand). The rare CuII 4-coordinate NHC complex featuring an anionic N-substituent was obtained by transmetalation from the easily accessible Ag complexes with CuCl2·2H2O. Interestingly, the N-anionic and the Oanionic forms of the ligand were observed in the Ag complex 166Cu and the Cu complex 167Cu, respectively (Scheme 103).266 The complexes 168aCu and 168bCu featuring alkoxyfunctionalized bidentate NHC ligands were obtained by reaction of the corresponding preformed dimeric lithium carbene alkoxide reagent with CuCl2 in a 1:1 and 2:1 ligand-toCu ratio, respectively (Scheme 103). Both complexes were paramagnetic, exhibiting broad featureless ESR spectra, and did not provide X-ray quality crystals; they were characterized only analytically and by mass spectroscopy. The complexes served as precatalysts in the conjugate addition reaction of ZnEt2 to cyclohexenone in toluene.267 2.2.4. Complexes with Tridentate Heteroatom Functionalized NHC Ligands. 2.2.4.1. [Cu(Lig)]+, [Cu(Lig)]X, and Related (Lig = Neutral Tridentate Heteroatom Functionalized NHC Ligand). The cyclic NHC pincer ligand with a central lutidine donor and a dodecamethylene spacer was developed as a means to increase stability and control the structures of Platinum Group Metal complexes. The study of this ligand system with coinage metals led to the synthesis of complexes including the Cu species 169Cu and 170Cu, which were used as well-defined transmetalation reagents. Although deprotonation of the imidazolium proligand with KOtBu in the BF


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Scheme 102. Synthesis of 4-Coordinate CuI Complexes

Scheme 103. O-Anionic Functionalized NHC Ligands that Stabilize Mononuclear CuII Centers

presence of excess CuBr gave the tetranuclear 169Cu with an interesting structure comprising bridging- and terminalcoordinated NHCs and cuprophilic interactions, exchange of the bromides in 169Cu with the noncoordinating [BArF4]− led to the mononuclear complex 170Cu with a 3-coordinate Tshaped Cu center and equal Cu-CNHC bond distances (Scheme 104).268 The NHC-based pincer ligand with a central lutidine donor and alkyl wingtips gives a family of Cu complexes, the nature of which depends on the type of the R wingtip. Mononuclear 3coordinate complexes 171Cu with distorted T-shaped geometry were obtained when R = Et and tBu;269,270 the Cu-Npy interaction is weak, especially for R = Et, and the pyridine heterocycle plane is perpendicular to the plane defined by the two imidazole rings. It has been suggested that the stabilization of the mononuclear structures involves intermolecular π−π stacking of the aromatic NHC and pyridine rings to suppress

Scheme 104. Use of a Cyclic NHC Pincer Ligand

BG


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cuprophilic interactions. Conversely, the binuclear 172aCu and 172bCu were obtained when R = Me, iPr, in which case cuprophilic interactions predominate the π−π stacking; the two complexes 172aCu and 172bCu differ in the magnitude of the angle between the NApy-CuA-CuB and NBpy-CuB-CuA planes with values 180° and −130°, giving rise to antiperiplanar and anticlinal conformations, respectively (Scheme 105).

Scheme 106. Pincer-Type Framework Bearing Two Mesoionic NHC Donors and Acting as Bridging Ligand

Scheme 105. Pincer-Type Complexes with Cuprophilic Interactions

Scheme 107. Formation of the CuII Complex 175Cu

The complexes 176Cu with an asymmetric tridentate ligand contain a chiral bicyclic NHC framework which shows an unusual diastereoselectivity on complexation with CuI halides: when X is Cl, the SCu isomer is formed selectively, whereas when X = I, the RCu isomer is preferred.274 The Cu centers adopt pseudotetrahedral coordination geometries with generally longer than usual Cu-CNHC bond distances, possibly due to the presence of large chelate rings (Scheme 108). Scheme 108. Complex with a Chiral Bicyclic NHC Framework The NHC-containing pincer ligands with a central pyridine donor under the same reaction conditions give complexes 173aCu−173cCu, where the ligand is semibridging two proximal Cu centers which also feature cuprophilic interactions. An analogue of 173aCu with nBu imidazol wingtips has been prepared from the imidazolium salt and Cu powder in MeCN in air. It exhibits a similar semibridging ligand arrangement. A multitude of other CuI complexes have been prepared by this method.271 In contrast, the same pincer framework but bearing the two mesoionic NHC donors 1,2,3-triazole-5-ylidenes has been used to synthesize silver complexes by the reaction with Ag2O, which can transmetallate the ligand to CuCl, leading to a binuclear complex 174aCu featuring noninteracting twocoordinate Cu(NHC)Cu moieties attached to the pyridine linker. CV measurements showed that the two CuI centers can be oxidized in a stepwise manner. Complex 174aCu catalyzed the hydroboration of trans-β-methyl-styrene with [B(pin)]2 for the β-selective formation of 1-methyl-2-phenylethylboronate (Scheme 106).272 The paramagnetic CuII NHC complex 175Cu with the symmetric tridentate mixed donor ligand has been prepared directly by the reaction of CuSiF6 with the imidazolium chloride in methanol in air. The distorted octahedral complex features a rather short CuII-CNHC which could be due to the chelating ligand or to π-bonding (Scheme 107).273

2.2.4.2. [Cu(Lig)], [CuX(Lig)] (Lig = Anionic Tridentate Heteroatom-Functionalized NHC Ligand). The mononuclear CuI and Cu II complexes 178 Cu and 179 Cu with the monoanionic rigid carbazolide pincer ligand bearing σ-/πdonating amido bridgehead and mesoionic 1,2,3-triazol-5ylidene wingtips were prepared by salt metathesis of the K+ salt of the fully deprotonated bis(1,2,3-triazol-5-ylidene)carbazolide 177Cu with CuI or CuCl2, respectively. The ligand precursors were obtained by 1,3-dipolar cycloaddition between the 1,3-diaza-2-azoniaallene salt and a 1,8-diethynylcarbazole and deprotonated using excess of KN(SiMe3)2 to the potassium complex 177Cu.275 Attempts to obtain a CuII-H complex by the reaction of 179Cu with LiHBEt3 led to 178Cu instead (Scheme 109). 2.2.5. Complexes with Multidentate HeteroatomFunctionalized NHC Ligands (Lig). The mononuclear complex 180Cu with the functionalized tripodal triscarbene BH


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Scheme 109. Complexes with the Monoanionic Carbazolide Pincer Ligand

Scheme 111. Complex with a Rigid Pentadentate Ligand

2.3. Binuclear and Polynuclear CuI Complexes

In a number of bi- and poly-nuclear Cu(NHC) complexes, short Cu−Cu bonding distances have been ascribed to (i) net attractive cuprophilic interactions between 3d10 metal centers, (ii) three-center-two-electron bonding, and (iii) geometric constraints imposed by bridging ligand(s). In a multitude of cases, the Cu centers are placed further apart by the ligand design, excluding any bonding interactions. Simple monatomic bridges can provide complexes with net attractive cuprophilic interactions and short Cu−Cu internuclear distances. A recent review described catalytic properties attributed to binuclear Cu N-heterocyclic carbene complexes.10 Below, binuclear and polynuclear complexes are described, initially starting with single atom bridging ligands that are not integrated to a multidentate ligand framework; donor atoms of the bridging ligands are from groups 17 to 14 and hydrides. This category is followed by complexes bearing a combination of single atom bridging ligands being part of and attached to a larger chelating ligand structure; mixed donor ligands with one atom bridging and one or more chelating functions are included here. Finally, complexes with chelating or functionalized ligands, where Cu centers are placed further apart by the ligand design, are described. Some multinuclear complexes have been already mentioned above in conjunction with the synthesis and reactivity of mononuclear species, and in this section are cross referenced. 2.3.1. Homometallic Binuclear and Polynuclear CuI Complexes. 2.3.1.1. With Single Atom Bridges, Isolated or Being Part of Chelating Ligand. 2.3.1.1.1. Halide Bridging Ligands. Simple binuclear mono-μ-fluoride complexes have been obtained by the reaction of mononuclear fluorides [CuF(NHC)] with one mol equiv [Cu(OTf)(NHC)] or with 0.5 mol equiv Ph3COF (Scheme 12). The complex [Cu(μI)(IAd)]2 features a symmetric Cu2I2 core with rather long Cu···Cu separation (2.95 Å) and weak intermetallic interaction. The “Cu(ICy)Br” and “Cu(ICy)I” are polynuclear and in the solid state crystallize as hexamers and trimers, featuring one and two bridging ICy, respectively.29 Other binuclear complexes with virtually symmetrical [Cu2(μ-X)2] bridges are shown in Scheme 112. They were prepared by transmetalation from the corresponding silver salts to CuI, CuBr, and CuBr2 precursors or reaction of the in situ generated NHC from the imidazolium salt and NaN(SiMe3)2 with CuI (for 183Cu).218,223,262,278 The complex 184bCu features a NHC with remote substitution related to the mesoionic analytical reagent Nitron.257 The intermetallic interactions in all these cases are very weak. The binuclear and trinuclear species 185aCu and 185bCu with the RE-6-Mes(DAC) N-heterocyclic carbene were obtained by the reaction of the isolated ligand with 1 and 1.5 equiv. CuCl, respectively; in situ generated RE-6Mes(DAC) from the corresponding azolium and NaN(SiMe3)2 led to mixtures (Scheme 113). In the structures of the complexes, there are no Cu···Cu interactions. In solution,

ligand was prepared by the reaction of the free tris(NHC) ligand with CuI sources (Scheme 110). The geometry at Cu is Scheme 110. Complexes with Functionalized Tripodal Tricarbene Ligand

trigonal planar with insignificant interaction of the metal with the bridgehead N. Oxidation of 180Cu (R = Bn) led to the isolable CuII species 181Cu which is also obtainable directly from the free ligand and [Cu(OTf)2]. The paramagnetism of 181Cu was established by EPR spectroscopy and SQUID measurements. The electronic structure of the trigonal planar CuII studied by DFT features the unpaired electron in 3dx2−y2 orbital and Jahn−Teller distortion.276 The complex 182Cu with the rigid pentadentate ligand containing four equatorial pyridines and one axial NHC enforces an octahedral geometry at CuII. Its structure reveals short Cu-CNHC bonds and unequal Cu-Npy distances due to Jahn−Teller distortions. The S = 1/2 state is confirmed by EPR spectroscopy (Scheme 111).277 BI


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Scheme 112. Complexes with Symmetrical [Cu2(μ-X)2] Bridges

Scheme 113. Bi- and Tri-Nuclear Complexes Obtained with the Ligand RE-6-Mes(DAC)

Scheme 114. NHC Complexes with Hydroxide (μ-OH) or Alkoxide (μ-OR) Bridges

respectively (Scheme 114). The binuclear 186aCu features a bent structure (Cu-O-Cu = 127.8°) and short Cu-CNHC bonds (1.87 Å). It has been used as a catalyst in the Cu-catalyzed AAC of benzyl azide with substituted phenylacetylenes in the presence of 4,7-dichloro-1,10-phenanthroline; it showed enhanced catalytic efficiency compared to [CuCl(IPr)] or [Cu(OtBu)(IPr)], a fact that was attributed to the absence of coordinating halides or other strongly bound ligands.279

there is evidence that exchange of RE-6-Mes(DAC) takes place to form complexes of higher nuclearity. The complex [Cu{RE6-Mes(DAC)}2]PF6 was obtained by reaction of [Cu(NCMe)4]PF6 with RE-6-Mes(DAC) as described above.212 2.3.1.1.2. Hydroxy-, Alkoxy-, and Thioalkoxy-Bridging Ligands. One atom hydroxide- (μ-OH) or alkoxide- (μ-OR) bridged polynuclear Cu-NHC complexes were obtained from hydrolysis of the in situ generated [Cu(IPr)(NH3)](BF4), or by using chelating mixed donor alkoxide NHC ligands, BJ


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The binuclear cationic complex 186bCu with bridging trimethylsiloxide was prepared by the reaction of [Cu(OSiMe3)(IPr)] with Ph3C+BF4−. It is moisture sensitive and has been used as a starting material for the preparation of other cationic binuclear species with single atom bridges (Scheme 114).280 The complex [Cu(OtBu)(SICy)] (27gsCu) (prepared as outlined in Scheme 13) has been shown crystallographically to adopt a symmetric binuclear structure with bridging alkoxides.81 The complex 187Cu was obtained by transmetalation reaction from the Ag complex; the Cu center shows an uncommon square planar geometry.281 The ligand combines the hard donor alkoxide with soft NHCs (Scheme 114). The complex 188Cu was prepared from the imidazoliumsubstituted pyrazole by reaction with Ag2O followed by Cu0 oxidative transmetalation in MeCN in air; exclusion of air and moisture significantly retarded the reaction (Scheme 115). It

corresponding ones with PR3 and exhibit good solubility in organic solvents. Scheme 116. Thiolato-Bridged Tetranuclear Complexes

2.3.1.1.3. Hydride, Alkenyl, Alkyl, NHC, and Other Group 14 Donor Bridging Ligands. Attempts to isolate mononuclear complexes of type [CuH(NHC)] led to binuclear species instead with two hydride bridges in the solid state and of variable stabilities. The most stable complexes were obtained for NHC = MenthcAAC. In solution, the dinuclear species act as sources of reactive [CuH(NHC)] that has been implicated in a number of catalytic transformations; for NHC = IPr**, an equilibrium between the binuclear and homonuclear complex has been detected by 13C NMR spectroscopy. These complexes have been described previously (Schemes 41, 42, 43, 45, 47, and 48) and summarized in Scheme 117). Selected diagnostic NMR characterization data are given in Tables 12 and 13. The binuclear cationic complexes 191aCu and 191bCu with a single bridging hydride have been obtained by the reaction of [Cu(μ-OSiMe3)(IPr)]2 and [Cu(μ-F)(IPr)]2 with BH(pin) and (SiHMe2)2O, respectively (Scheme 118).63,284 In the dimer, the C-Cu-Cu angles (ca. 156 to 163°) are consistent with the presence of a triangular [Cu2H] core; short Cu−Cu distances (ca. 2.53 Å), and Cu-H-Cu angles (ca. 122°) support intermetallic interaction. DFT calculations on a simplified model of 191aCu and 191bCu show that the HOMO is Cu−Cu antibonding in character and best describe the [Cu2H]+ bonding as a three-center, six-electron system with Cu-H bond orders of ca. 0.53. Despite 191aCu being cationic, the reactivity of the Cu-H-Cu unit is hydridic, reacting with electrophilic MeO-H and CO2 to give the μ-OMe 192Cu and μ-formate 193Cu, respectively, while maintaining the binuclear structure; insertion of PhCCH also occurs easily to give (trans-phenylvinyl)-bridged dicopper(I) complex 194Cu with slightly elongated Cu−Cu interaction (2.63 Å) and trigonal Cu2(μ-Cvinyl) arrangement involving an open three-center interaction (like 191aCu).284 The selectivity implications of the formation of 191aCu and 191bCu in the catalytic systems for the hydroalkylation of alkynes with alkyltriflates have been described previously.63 Reaction of 191bCu with p-BrPh(CH2)2-allene gave rise to a binuclear species which cocrystallized in two different configurations 195aCu and 195bCu. The minor component (195aCu) contains symmetrically arranged Cu(IPr) fragments bonded to a bridging η3-π-allyl fragment from opposite faces. In the major component (195bCu), one Cu atom is σ-bonded to the η1-allyl fragment while the other is η2,π-bonded to the terminal double bond of the η1-allyl. This nonsymmetrical configuration implies higher reactivity toward electrophiles compared to the structurally different μ-alkenyl-Cu2 complexes and may account for selectivity in catalysis.285

Scheme 115. Complexes with Pyrazolyl-Functionalized NHC Ligands Featuring Isolated OH Bridges

was suggested that the hydroxide group originated from moisture. The complex comprises two CuII centers and is paramagnetic. In its structure, the two 4-coordinate Cu centers are bridged by a pyrazolate and a hydroxide, forming a fivemembered metallocyclic ring. The Cu···Cu separation at 3.350 Å is relatively long, excluding any significant metal−metal interaction. The complex 189Cu was obtained by in situ transmetalation from the Ag complex to Cu powder; a pyrazine substituted (Y = N) imidazolium salt can also be used in the place of the pyridine-substituted analogue with the same structural outcome. The core of 189Cu is composed of two hydroxide-bridged symmetrically related [Cu 2(μ3−OH)(ligand)] units, in which the two metals are held together by the hexadentate ligand and hydroxide. The complexes 188Cu and 189Cu show good activity as catalysts for the N-arylation of imidazoles and aromatic amines with arylboronic acids under mild conditions in methanol and low catalyst loadings.282 Some one-atom thioalkoxide-, thiophenoxide-, and phosphido-bridged polynuclear Cu-NHC complexes have been described previously in conjunction with the synthesis and reactivity of mononuclear thioalkoxides (Schemes 20, 21, and 26). The complex [Cu(μ2-SPh)(Me2cAAC)]2 is easily obtainable from the reaction of [CuCl(Me2cAACl] with NaSPh.242 The reaction of [Cu(StBu)]n with NHC substituted with iPr wingtips gave complexes 190Cu with cyclic tetrameric structures in the solid state (Scheme 116).283 The clusters 190Cu are more stable under ambient light than the BK


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Scheme 117. Characterized Binuclear Hydrides

Table 12. δHCu‑H in the 1H-NMR Spectra of the Complexes “CuH(NHC)” in Solution RE-6-DiPP

RE-7-DiPP

0.77

0.47

Menth

cAAC

not observed

[(Cu(IPr))2(μH)] 4.13 (THF)

Scheme 118. Reactivity of the Cu-H-Cu Unit in 191a,bCu

IPr** 4.26/2.14 (benzene)

Table 13. δCNHC in the 13C-NMR Spectra of the Complexes “CuH(NHC)” RE-6-DiPP a,b

213.5

RE-7-DiPP 223.9

a,b

Menth

cAAC

259.9a,b

IPr** 185.8 192.8a,c

a In C6D6. bDinuclear. cEquilibrium mixture of mononuclear and dinuclear species, with the signal at δ 192.8 ppm assigned to the binuclear; for [CuH(S)IPr]2, no 13C NMR data were reported, presumably due to thermal instability.

There are a few complexes with bridging NHC ligands. Some have already been described above: 154Cu 262 (Scheme 99), 169Cu 268 (Scheme 104), and [CuBr(ICy)] and [CuI(ICy)];29 the latter two, together with the thiazol-2-ylidene complex 196Cu,286 represent examples of bridging nonfunctionalized NHC ligands (Scheme 119). Complex 197Cu features nonrigid diphosphine functionalized NHC pincer and 198Cu nonrigid bis-lutidine functionalized (benz)imidazol(in)2-ylidenes with unique trinuclear D3 symmetric triangular arrangement and three bridging NHC ligands (Scheme 119).287−289 The complexes 198Cu are also accessible by transmetalation from the analogous triangular Ag3 species to CuCl. In all reported examples, the Cu···Cu bond distances span a narrow range of 2.36−2.51 Å (Cu−Cu distance of 2.556 Å in Cu metal); the CNHC has a tetrahedral environment, and the NHC plane is at a right angle to the Cu···Cu vector. Cu centers are among the most suitable transition metals to

support bridging NHC structures due to their small size (covalent radius of Cu 1.32 Å). The bridging NHC bonding BL


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Scheme 120. Reactivity of Bridging μ-Boryl Functionality

Scheme 119. Complexes with Bridging NHC Ligands

presence of CsOH; from a range of pyrazoles used, bridging structures were observed in the solid state for those shown in Scheme 121 which lack substituents in the proximity of the has been described by either formation of a σ-bond with one Cu atom and π-interaction with the second or, alternatively, a three-center-two-electron bond. It is interesting to relate this bonding mode to that of bridging phosphines.290 DFT calculations on systems related to 196Cu show that the bridging NHC is a better π-acceptor and can be stabilized by strongly π-basic metal environments. The bridging NHC in a dinuclear environment is rather a carbonyl than a phosphine mimic. It has been suggested that elementary reactions such as transmetalations may involve bridging NHC ligands as intermediates. Copper NHC complexes with bridging silyl and stannyl ligands have been described above in conjunction with the synthesis and structures of [CuE(NHC)] (E = group 14 donor (Schemes 36 and 37). 2.3.1.1.4. Group 13 Bridging Ligands. The bent core [Cu2B(cat)]+ in 199Cu, with bridging μ-boryl functionality, was prepared by the reaction shown in Scheme 120. It is stable as a solid at −30 °C, but solutions in CD2Cl2 at room temperature slowly deposit metallic copper. The complex was characterized spectroscopically (even though the bridging boryl does not give rise to a 11B-NMR resonance) and crystallographically unveiling an acute Cu-B-Cu angle (ca. 72.1°) and a very short intermetallic Cu−Cu distance (2.41 Å); this distance is shorter than the Cu−Cu separation of ca. 2.541(2) Å found in [Cu(μH)(SIPr)]2]OTf. Complex 199Cu does not react with CO2 but readily with MeOH, forming [Cu(μ-H)(SIPr)]2]BF4 and (cat)B-OMe after alcoholysis and hydride transfer. It also reacts with phenylacetylene to yield the bridging vinyl dicopper cation in 200Cu. DFT calculations point to a three-center bond between the two Cu atoms and the B of the boryl group.280 2.3.1.2. With Two and Three Atom Bridges, Isolated, or Being Part of a Chelating Ligand. The use of 1,2-pyrazole moieties bridging two Cu(IPr) units has been studied in conjunction with accelerating the catalytic hydrosilylation of ketones. The complexes 201aCu−201dCu were easily prepared by the reaction of the free pyrazoles with [CuCl(IPr)] in the

Scheme 121. Complexes with Bridging 1,2-Pyrazolyl Moieties

metals or bear distant substituents (201aCu−201cCu). In solution, DOSY NMR measurements give evidence of the presence of entities with molecular weight close to the mononuclear species. It has been established that certain types of pyrazoles exert an accelerating effect in the hydrosilylation catalysis, but evidence for the involvement of intact binuclear species was not found.227 Similarly, 1,2,4triazolide bridged complex 201dCu (and analogues with other types of NHCs) were prepared by the same synthetic methodology. Complex 201dCu adopts a distorted-trigonal geometry around each copper. The Cu···Cu separation is 3.77 Å; the flat bridging ring structure is perpendicular to the NHCCu-Cu-NHC plane. In the catalytic hydrosilylation of carbonyl compounds, the bridging triazolate could act as an internal base, eliminating the need for external bases, which activate the silane and the precatalyst. 201dCu proved as an efficient catalyst for the reaction with SiH(Me)(OEt)2 at low catalyst loading (0.05 mol %) and with PMHS at higher loadings (0.25 mol %).291 BM


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Reaction of 2 equiv [Cu(Mes)(SIMes)] with oxalic acid in THF gave the binuclear complex 202Cu (Scheme 122). It

Scheme 125. Bridging Anionic 1,2,3-Triazole-4,5-Diylidene Ligand

Scheme 122. Binuclear Complex with Bridging Oxalate

complex and transmetalation with CuI (Scheme 126). The ligand adopted a bridging coordination mode with virtually adopts a Ci-symmetrical arrangement with two SIMes ligands coordinated to distorted trigonal-planar CuI centers that are linked by the dianionic oxalato bridge in a μ-1,2,3,4 coordination mode. Thermal decomposition of the complex occurs in the range 210−350 °C and cleanly converts the complex to Cu0 with release of CO2.109 Reaction of [CuCl(NHC)] (NHC = IMes, IXyl) with Na(NCS) gave the centrosymmetric binuclear complexes 203Cu with 1,3 bridging thiocyanate ligands; with the bulkier IPr, mononuclear complexes were obtained (Scheme 123).234

Scheme 126. Complexes with Bridging PyridazineAnnelated bis(N-Heterocyclic Carbene) Ligand

Scheme 123. Binuclear Complex with Bridging Thiocyanates

linear Cu centers and Cu···Cu separation of ca. 2.50 Å, indicative of d10-d10 interactions. Interestingly, the planes of the two pyridazine ligands coordinated to the (Cu2)2+ core are perpendicular. Even though the pyridazine NHC has a rigid backbone, the distance of the NHC donor sites has been compared to 2,2-bipyridine (ca. 2.97 Å) (Scheme 126).293,294 The binuclear Cu complexes 206Cu were obtained in two steps by the reaction of the bis-triazolium proligand with Ag2O followed by transmetalation to CuI. The Cu centers have linear coordination geometry and the separation between them is ca. 2.8 Å. All complexes display excellent activity as precatalysts in the azide−alkyne cycloaddition reaction, almost twice as high as that of analogous mononuclear complexes for the same amount of copper (Scheme 127).295

Binuclear complexes with 1,3-dimetalated bridging triazolide and [(cAAC)Cu-σ-(μ−η2 alkynyl-Cu)] complexes have been isolated as intermediates in the CuAAC of azides and alkynes (Schemes 30 and 124).115 Scheme 124. Binuclear Complexes with Bridging Triazolide

Scheme 127. Binuclear Bis(Bis-triazolylidene) Cu Complexes

The polymeric Cu complex 204Cu with the two-atom bridging anionic 1,2,3-triazole-4,5-diylidene has been prepared in two steps, involving complexation of CuX (X = Cl, I) with the mesoionic 1,2,3-triazol-ylidene, followed by deprotonation with KN(SiMe3)2. The complex was characterized by mass spectroscopy and further reactivity by transmetalation from Cu, yielding a range of mononuclear dimetalated complexes with Platinum Group Metals (Scheme 125).292 2.3.1.3. Complexes with Chelating and/or Functionalized NHC Ligands Placing Cu Centers Further Apart due to the Ligand Design. 2.3.1.3.1. Complexes with Mono- or Bis-NHC Donors Linked Directly or via Linkers Attached to the Ν α-to the CNHC: −CH2−, −CH2X−, or −BMe2 Linkers. The binuclear Cu complexes 205aCu and 205bCu with the pyridazine annelated bis(NHC) ligand were obtained by direct cupration with Cu2O (R = Bn) or in two steps through the silver

The dinuclear complexes with the bis-tBu-NHC ligand and (−CH2−) linker 207aCu with a ligand-to-Cu ratio 1:2 were prepared by the deprotonation of the bis-imidazolium salts in the presence of CuBr in the suitable stoichiometry. The complexes have a polymeric nature as shown in Scheme 128. They react with N-bases, including imidazoles to break the BN


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Scheme 128. Di- and Polynuclear Complexes Based on CH2- and BMe2-Linked Dicarbenes

Scheme 129. Cu/O2 Mediated Degradative Rearrangement of Bis-NHC Ligands

polymeric structure giving adducts of the type 207bCu (Scheme 128).296 The dinuclear complex 208bCu was obtained by transmetalation from the corresponding Ag complexes 208aCu. Its structure comprises a “shape-of-O” arrangement with a 12membered dimetallamacrocycle. The geometries at Cu depart from linearity (ca. 169.5°), which has been attributed to cation-π interactions within the complex between each Cu+ center and the CNHC of one of the adjacent imidazol-2-ylidenes not coordinated to it. For the first time, the emission properties of a Cu-NHC complex were observed with 208bCu. A powdered sample of 208b Cu showed a bright green

phosphorescence (374 and 482 nm at rt and 500 nm at 77 K). The occurrence of the dual emission at room temperature has been attributed to changes of the interaction between two copper atoms. Compared with emission properties of Cuphosphine complexes, the nature of the electronic transitions in Cu-NHC species is different, sometimes with molecular orbitals of the NHC ligands contributing to the absorption and emission.297 A range of dinuclear complexes with the bis-NHC ligands featuring the “isoelectronic” linkers (−CH 2 −) and (−BMe2−−), 208cCu and 208dCu, respectively, and ligand-toCu ratio 1:1 were prepared by the deprotonation of the bisBO


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imidazolium salts in the presence of CuI sources (Scheme 128). The linear coordination geometries at Cu are slightly distorted. In the conformation observed in the solid state, the planes of the NHCs of the same ligand are perpendicular to each other with a boat-like arrangement of the metallamacrocycle. The Cu−Cu distances fall in the range of 2.87−3.42 Å, which excludes strong intermetallic interactions. In solution, NMR spectra show nonrigidity.241 Similar preparative methods were employed for 208eCu.256 C−N cleavage and simultaneous C−C formation leading to the CuII complexes 209fCu and 209gCu with tetrahedral Cu centers was observed in the attempted reaction of imidazolium salts with Cu2O in air (Scheme 129). It has been proposed that the air sensitive complexes 209bCu, 209fCu, and 209gCu are involved in this unusual reaction sequence that comprises C− H bond activation, C−N bond cleavage, and C−C bond formation. Insight into the mechanism was obtained by carrying out the cupration under N2, when 208bCu, 208fCu, and 208gCu were formed, followed by oxidation with O2 which was accompanied by formation of formaldehyde. Analogues of 208fCu with −CHPh− bridges afforded in the same reaction sequence 209dCu and benzaldehyde. However, the presence of methylene linkers is essential: ethylene-bridged imidazoliums did not react similarly. The reaction of 208dCu, 208eCu, and 208fCu with lithium chloride and O2 led to 209dCu, 209eCu, and 209fCu. The Cu−Cu separation in 209fCu of 2.90 Å may suggest a weak metallophilic interaction. This complex is luminescent: when excited at 580 nm in aqueous solution, an emission band in the visible at 630 nm was observed.298 The hydroxymethyl-functionalized bis-NHC 210Cu was obtained by transmetalation from the analogous Ag complex (Scheme 130). It adopts an anti-configuration of the OH at the

arrangement was adopted in the solid state for R = Me, Et, nBu, and p-MeOPh (211aCu, 211bCu, 211cCu, 211dCu); in this case, the two Cu centers are in proximity (2.69−2.90 Å) and interact through cuprophilicity. This affects the luminescent properties of the dinuclear complex. All complexes were prepared by silver transmetalation to CuI. The complexes 211aCu, 211bCu, and 211cCu are strongly emissive, 211dCu is a weaker emitter due to the presence of the anisyl group. Complexes 211aCu, 211bCu, and 211cCu in methanol show intense orange luminescence on UV excitation (two bands at ca. 420 and 590 nm). The powdered samples exhibit bluegreen photoluminescence. DFT calculations show that the Cu−Cu interactions are important in shaping the nature of the frontier orbitals that are responsible for the observed transitions.300 The complexes 212aCu and 212bCu were prepared by alcoholysis of the corresponding Cl2IPr(OtBu) with bifunctional thiols of variable linear linker length (Scheme 132). The low coordination number of the Cu centers allowed the crystallization of conformers featuring a sterically protected “closed” Cu2S2 solid state structure for 212aCu with two CuI three-coordinate centers; “open” isolated remote two-coordinate CuI centers are found, for example, in the structure of the binuclear 212bCu. The arrangements in 212aCu and 212bCu have been tested as artificial model complexes with low coordinate Cu centers of the reduced state of CuA electron transfer sites; the latter are found in redox Cu-containing enzymes. The CuA resting state is charge-delocalized featuring a [Cu1.5Cu1.5]+ core with rigid Cu2S2 diamond core and participates in electron transfer with the [CuICuI] sites. Complexes 212aCu and 212bCu can be oxidized by [FeCp2]+ to transient species, which show UV−vis and EPR spectral characteristics that resemble more closely those of the open and close forms of CuA site than the bridged Cu2S2 complexes with four-coordinate Cu centers, although at the expense of stability.231 The functionalized 4,5-diazafluorenide with the 2(diphenylphosphino)ethyl tail has been used for the synthesis of binuclear Cu complexes (Scheme 133). The reaction of the potassium salt of the hybrid ligand with [CuCl(NHC)] (NHC = IPr, IMes) gave the complex 213aCu, which exists in solution in equilibrium as a head-to-tail dimer and a monomer with dangling phosphine arm and as a dimer in the solid state. This was evidenced by variable-temperature 31P NMR spectroscopy. Complex 213aCu features tetrahedral Cu centers; the planes of the two diazafluorenide moieties are parallel to each other with a distance of ca. 4.54 Å. Reactions of 213aCu and 213bCu with Platinum Group Metal precursors (e.g., [RhCl(PPh3)3], [Rh(μ-Cl)(COD)]2, and [AuCl(SMe2)]) result in the transfer of the difunctional ligand with elimination of [CuCl(IPr)] (Scheme 133).301 Other Hydrocarbyl Linkers. A potentially binucleating bis(NHC) ligand was designed to situate proximally two CuI centers in a constrained ligand backbone with a bridging pphenylene linker. The imidazolpyridine-based NHC afforded a ligand related to para-terphenyl diphosphines previously described. In the dinuclear CuI2 complex 214Cu prepared by deprotonation of the imidazolium salt with KOtBu in the presence of CuCl, the two Cu linear centers are occupying opposite faces of the arene, excluding any intermetallic interaction (Scheme 134).302 A bis(NHC) proligand with a rigid m-phenylene linker has been synthesized as a potential precursor to pincer bis-NHC

Scheme 130. Hydroxymethyl-Functionalized bis-NHC Complex 210Cu

bridge; the Cu centers are virtually linear, and the dimetallamacrocycle adopts a chairlike conformation. Spectroscopic data of 210Cu show that the bridge functionalization does not affect the core electronic properties. The hydroxymethyl-functionalized bridge design and its lack of interference with the Cu2 electronics may point to its use as an anchoring tether for catalytic applications by immobilized derivatives.299 (−CH2−)n, n > 1, or Chains with Heteroatom Tail Donors. Elongation of the hydrocarbyl linker to propylene gave access to two different types of binuclear isomeric arrangements, depending on the nature of the wingtip of the imidazolylidenes (Scheme 131). The “shape-of-O” arrangement (also observed with the shorter −CH2− linker) was obtained when R = Mes (211eCu); in this case interaction of the Cu centers being part of the 16-membered dimetalla-macrocycle is not observed in the solid state (Cu···Cu = 6.164 Å). The “shape-of-8” BP


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Scheme 131. Isomeric Binuclear Arrangements with (CH2)3-Linked Dicarbenes

Scheme 132. Influence of the Length of the Linker in Bifunctional Thiols on Binuclear NHC Complex Conformation

adopted conformation and polymerization precluded any cupration of the aromatic ring (Scheme 135).303

Scheme 133. Complexes with 2-(Diphenylphosphino)ethylFunctionalized 4,5-Diazafluorenide

Scheme 135. Phenylene-Linked Bis-NHC Ligand-Based Binuclear Complexes

Scheme 134. Binuclear Complex Based on the Rigid pPhenylene Dicarbene Ligand Coumarin-substituted bis-imidazolium salts with an anthracene linker were metalated with Cu2O to give organometallic dicopper rectangles 216aCu with terminal coumarin wingtips. Upon irradiation (λ = 365 nm) of the latter, the dicopper complexes underwent photochemical photodimerization through a [2 + 2] cycloaddition reaction of two adjacent coumarin moieties, leading to the macrocyclic tetra(NHC) complex 216bCu (Scheme 136). The [2 + 2] cycloaddition of the coumarin was stereoselective to syn-head−head isomers. Irradiation at λ = 254 nm led to photocleavage of cyclobutanes and reformation of 216aCu with coumarin pendants. The

complexes. Attempted cupration using [CuN(SiMe3)2] led to a Cl-bridged polymer, the repeat unit of which comprises a binuclear Cu complex 215Cu with linear geometry at Cu; the BQ


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Scheme 136. Photochemical Rearrangement of CoumarinSubstituted Ligands

Scheme 138. Synthesis of the Bis-Triazolylidene Complex 218Cu

Scheme 139. Binuclear Complex Supported by Remote-Site Linked Bis NHCs

reported complexes may serve as chemical sensors in biological applications (Scheme 136).304 The binuclear complex 217Cu was prepared from the corresponding imidazolium salt by an electrochemical method: the imidazolium as the hexafluorophosphate salt in acetonitrile was used as the electrolyte with Cu plates as both sacrificial anode and cathode (Scheme 137). At the anode, the copper was oxidized to CuI, and at the cathode, the imidazolium was reduced to free NHC and combined with the Cu cation. 217Cu features linear geometry at Cu and “folded back” conformation of the macrocyclic dicarbene (Scheme 137).305 The 1,2,4-triazol-5-ylidene NHCs attached to 1,3-phenylene spacer have been prepared via the reaction of the corresponding triazoles with two equivalents adamantyl bromide, followed by generation of the stable free 1,2,4triazol-5-ylidenes by deprotonation with KOtBu or NaH. The free bis-NHC ligand reacted with CuI to give the binuclear complex 218Cu.306 On the basis of the separation between the Cu centers (2.66 Å), only weak interaction between them is expected (Scheme 138). 2.3.1.3.2. Complexes with NHC Donors Linked via Substituents at Remote Sites. The bis-NHC ligands linked via remote silyl substituents have been prepared by the interaction of free NHCs with SiR2Cl2. Deprotonation of the resulting bisimidazolium salts and reaction with CuCl sources gave binuclear copper complexes 219Cu (Scheme 139).206,307 A limited number of binuclear complexes with NHC ligands functionalized with classical donors at the remote positions to

the NHC has been reported. With Cu, 220cCu was prepared by the addition of one equiv CuCl to 4-PPh2-substituted 220bCu under kinetic control (Scheme 140). The former was also Scheme 140. Complexes with Donors at the Remote Positions to the NHC

Scheme 137. Electrochemical Synthesis

BR


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Scheme 141. Binuclear Complexes with Phenanthroline Functionalized NHC Ligands

available in a “one-pot” by the reaction of the C2-substituted imidazolium precursor 220aCu in the presence of CuCl; this transformation involves fast migration of the PPh2 group, from the C2 to C4 remote position, accompanying complexation. Finally, the homodinuclear CuI complex 220dCu was easily accessible from 220bCu and CuCl. Heterometallic complexes can be obtained by substituting CuCl in the last step with other metal precursors (e.g., [AuCl(SMe2)], [Pd(allyl)(μCl)]2, etc.) (Scheme 140).308 2.3.1.3.3. Linkers Attached to the Ν α-to the CNHC and Functionalized by Heteroatom Donors. Binuclear Cu complexes supported by 2,6-pyridine- and 2,6-lutidinedicarbenes have been described above in conjunction with their mononuclear chemistry (Scheme 105). Phenanthrolinefunctionalized imidazolium and triazolium salts with diverse wingtips were prepared as proligands to the semirigid, potentially three-coordinate functionalized NHCs. The synthesis of Cu complexes was accomplished by the reaction of the imidazolium (PF6−) salts with Cu powder at room temperature in MeCN. Dinuclear doubly bridged Cu complexes were obtained with the phen-NHC ligands being arranged head-to-head or head-to-tail, depending upon the

steric repulsion of the NHC wingtips (Scheme 141). The complexes 221aCu−221eCu with smaller NHC wingtip R comprise two CuI centers bridged by two ligands arranged in a head-to-tail manner, rendering each copper coordinated by one phenanthroline and one NHC of the second ligand. The bond distances of Cu-C bond are ca. 1.88 Å and the separation between the Cu ca. is 2.71 Å, implying weak metal−metal interaction. In 221fCu, with the bulkier NHC wingtip DiPP, the two ligands are arranged in head-to-head manner, which may be due to reduction of steric congestion. In the complexes 221aCu−221eCu, interconversion between head-to-head and head-to-tail arrangements is possible in solution as concluded by NMR spectroscopy. Use of imidazolium salts with halide counterions leads to complexes 222aCu with one bridging halide between the two Cu centers (Scheme 141).309 Two rare binuclear complexes were obtained when using 1,2,3-triazole wingtips in the imidazolium proligand, rendering the ligand potentially tetradentate and tridentate when substituting the imidazol-2-ylidene functional group by 1,2,4triazol-2-ylidene. In the former case, the complex 222bCu was obtained with bridging NHC ligands and, in the latter, the complex 223Cu featuring a (μ2-NCMe) bridging acetonitrile BS


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Scheme 142. Binuclear Complexes Obtained with Phenanthroline-Functionalized NHCs with 1,2,3-Triazole Wingtips

Scheme 143. Double-Helical Complex 224Cu

(Scheme 142). Some complexes from this family (e.g., 221Cu) are good catalysts for the CuAAC of benzyl azide and phenylacetylene.309 The flexible heterocyclic ligand with a 6,6′-bis-NHC-2,2′bipyridine framework has been used to access the binuclear 224Cu.310 Its solid state structure unveiled a double helical arrangement, with the two copper centers in a distorted tetrahedral environment comprising two NHC and two pyridine donors originating from two different ligands (Scheme 143).310 The synthesis of the proligands 2,9-di(3-R-1H-imidazolium1-yl)-1,10-phenanthroline salts opened the way to access a range of bi-, tri-, and tetra-nuclear Cu complexes generally obtainable by the reaction of the imidazolium salt with Cu powder in acetonitrile. The exact nature of the isolated complexes was dictated by the type of the wingtips of the imidazolium and the nature of its counterion. Thus, from the bis-imidazolium proligand bearing allyl wingtips, the complex 225Cu was isolated in low yields, consisting of a zigzag Cu4 chain with Cu···Cu separations between 2.64 and 2.90 Å. The coordination sphere of the external Cu atoms comprises one NHC and two acetonitrile ligands in a trigonal geometry and of two internal Cu centers by the phenanthroline and one NHC donor of the second ligand also in a trigonal arrangement (Scheme 144).311

From the bis-imidazolium and benzimidazolium proligands bearing benzyl wingtips, the complexes 226aCu and 226bCu were obtained, respectively. The structure of 226aCu comprises one linear tricationic chain with two terminal Cu centers ligated by NHCs in a slightly bent geometry with Cu (ca. 172− 175°), while the internal copper atom is tetracoordinated to two phenanthroline moieties; the Cu separations (ca. 2.7 Å) indicate weak metal−metal interactions. Similar features are found in the structure of 226bCu. From the mesityl-substituted bis-benzimidazolium proligand, the binuclear complex 227Cu was obtained. The cation in 227Cu consists of two bent CuI centers coordinated by two carbenic carbons (Cu-CNHC ca. 1.91 Å) and weakly interacting with the phenanthroline N donors (ca. 2.55 Å). Starting from the mesityl-substituted bis-imidazolium proligand, complex 228Cu was obtained, the structure of which comprises a triangular Cu3 core with two Cu centers in trigonal pyramidal geometry (one NHC and phenanthroline of the other ligand) and the third dicoordinated by two carbenic carbon atoms in a bent geometry; the separation between the copper centers is 2.58−2.72 Å (Scheme 144).311 Lutidine wingtips in the bis-imidazolium phenanthroline proligands gives rise to a potentially hexadentate ligand, which on reaction with Cu powder in MeCN afforded two types of products, the nature of which depends on the counteranion of the imidazolium salt. Complex 229aCu comprises one BT


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Scheme 144. Structural Diversity in Phenanthroline Functionalized Bis-NHC Complexes

with two types of Cu centers, one 3- and one 4-coordinate. The two Cu atoms in the Cu2 substructures are bridged by one iodide (Scheme 145).311 Studies with heteroatom-functionalized NHC ligands obtained by formal replacement of one or both wingtips of the NHC core by −PR2 or −CH2PR2 donors have been carried out. In particular, comparisons of these ligand environments with the bis-NHC ligands (205aCu, 205bCu, 206Cu, and

triangular Cu3 core; furthermore, it exhibits two nonsymmetrical μ2-bridging carbenic ligands with longer Cu-CNHC bond distances (ca. 1.96 and 2.45 Å) and Cu···Cu separations of 2.55−2.65 Å; all three Cu centers are in a distorted tetrahedral environment. In contrast, the presence of coordinating I− as counteranions in the original bisimidazolium salt results in the formation of 229bCu in which the ligand compartmentalized two Cu2 substructures, each BU


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Scheme 145. Complexes from Lutidine Wingtips in the Bis-Imidazolium Phenanthroline

208bCu−208eCu) are interesting. Thus, the N-phosphanylNHCs (featuring a −PR2 donor directly attached to the NHC ring, Scheme 146) with smaller wingtip substituents (Me or Mes), when reacted with [Cu(NCMe)4](PF6), gave the binuclear C2h symmetric complexes 230Cu with heteroleptic Cu centers; analogs substituted with the bulkier tBu group under the same conditions gave the mononuclear 231Cu. In both types of complexes, the Cu centers adopt virtually linear geometries; in 230Cu, the coordination and NHC planes coincide and the internuclear distances are shorter than in 208Cu (2.57−2.61 Å).241 A complex analogous to 230Cu with saturated NHC backbone (R = Mes) has been prepared by transmetalation from the corresponding dinuclear Ag salt and CuI; it shares the same structural features as 230Cu.312 The use of CuBr·SMe2 as a CuI source led to the complexes 232aCu and 232bCu for Me- and Mes-substituted NHCs, respectively. 232bCu features a centrosymmetric Cu4 cluster with a nearly square planar Cu4 frame (Cu···Cu = 2.77−2.78 Å), μ4-bridged on both sides by two Br ligands. With the N-phosphinomethylfunctionalized NHCs (with the more flexible −CH2PR2 donor attached to the NHC ring), the binuclear products 233aCu and 233bCu were obtained depending on the NHC substitution. The Cu centers in these complexes are distorted linear, and the internuclear distances fall in within 2.64−2.74 Å. Thus, enlarging the ring size of the dinuclear metallacycles resulted in an increase of the metal−metal separation. The nature of the products is possibly dictated by minimization of interligand repulsions. Symmetrization of the 1H NMR spectra at room temperature was ascribed to dynamic processes involving interconversion of dimetallacyclic conformers, for example as shown for 233Cu, etc. The magnitude of free energies of activation for these processes has been calculated by variabletemperature NMR spectroscopy experiments (Scheme 146).

Homometallic binuclear and trinuclear complexes with CuI centers are accessible by the use of the rigid mixed donor bisdiphosphanyl-NHC. The trinuclear 234aCu (D2h), obtained by the reaction of the free NHC ligand with [Cu(OTf)], comprised a symmetrical ligand arrangement leading to three homoleptic Cu centers with one all-NHC and two all-P donations; the binuclear 234bCu (C2h) obtained by the reaction of the imidazolium proligand with [CuN(SiMe3)2] comprised two heteroleptic C centers and dangling phosphines; an unusual bridging triflate mode was found in the structure of 234aCu. The Cu···Cu separation in the latter is short (ca. 2.58 Å) and shorter than the corresponding one in 234bCu (2.68 Å). The geometries at Cu are almost linear (171−172°), and the orientation of the lone pairs of the dangling P donors in 234bCu is toward the metal. The NHC rings and the coordinated Cu centers are coplanar; there are some dynamic processes taking part in solution that render the NMR spectrum at room temperature rather broad (Scheme 147).313 The higher nuclearity complex 235Cu was obtained by using simultaneously two copper sources, [Cu(OTf)] and CuI, in optimized stoichiometry. It features two parallel Cu3 arrays bridged on one side by the diphosphanyl NHC and on the other by iodide anions. Hexanuclear 235Cu displays moderate photoluminescence centered at λ = 710 nm (Scheme 148).314 Complexation of the bifunctional ligands, formally obtained by the replacement of one NHC functionality in m-phenylenelinked bis-NHC ligands by −CH2PR2, was carried out by the reaction of the phosphine imidazolium salt with [Cu(Mes)]5. This led to the centrosymmetric binuclear dicopper metallamacrocycle 236Cu, which displays a fluxional behavior, most likely associated with conformational changes in the dimetallamacrocyclic ring (Scheme 149).315 In the structure BV


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Scheme 146. Complexes with N-Phosphinomethyl-Functionalized NHCs and N-Phosphanyl NHCs

Scheme 147. Complexes Obtained with a Rigid Mixed Donor bis-Diphosphanyl-NHC

Scheme 148. Higher Nuclearity Complexes Based on the Rigid Bis-Phosphanyl Carbene

of 236Cu, there is a “head-to-tail” arrangement for the NHC and P donors. The three-coordinate Cu centers adopt a distorted planar T-shaped geometry, the third donor being a bromide. The large separation, in the range of 6.84−7.14 Å, between the two CuI centers is due to the large 1,3-phenylene spacer linking the NHC and phosphine donors. Interestingly, transmetalation of the same NHC-phosphine hybrid ligand BW


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pending them on linkers of trigonal symmetry (e.g., 1, 3, 5substituted aromatics) of variable rigidity. Finally, strictly linear topologies have been realized with rigid mixed donor phosphine-NHC ligands. The complex 239aCu with two capping tris(imidazolyl-2ylidene)borate ligands was initially prepared by the reaction of the free ligand, obtainable from the tris-imidazolium triflate precursor and nBuLi, with [CuBr(PPh3)3] or [Cu(NCMe)4](PF6). Complex 239bCu with benzyl (Bn) wingtips was available by transmetalation from the corresponding Ag complex to [CuBr(SMe2)] (Scheme 151). The structures of

Scheme 149. Bi- and Tetra-Nuclear Complexes Based on the Semi-Flexible Mixed Donor Phosphine NHC Ligand

Scheme 151. Complexes with Capping Tris(Imidazolyl-2ylidene)borate Ligands

from the Ag cubane complex let to the 237Cu in which two Cu centers are linked by two bromide bridges supported by a bridging NHC-phosphine hybrid ligand (cf. complex 232bCu Scheme 146). Transmetalation of both the NHC and P ends from Ag to Cu occurred without being able to observe the products corresponding to the individual steps (Scheme 149).315,316 2.3.1.3.4. With More than Two NHC Donors and Diverse Linkers. The flexible 24-atom macrocyclic ligand with four (imidazol-2-ylidene) functionalities and four trimethylenic linkers supports binuclear dicopper complexes. The cupration was carried out by the reaction of the tetra-imidazolium salts with Cu2O in DMSO in the presence of acetate anions. The complex 238Cu comprises two linear Cu centers coordinated to the diametrically opposite NHCs; the separation between the Cu centers is within the range of cuprophilic interactions (Scheme 150).317 Trinuclear Cu3 triangular complexes have been described in conjunction with the bridging NHC ligands (Scheme 114). Alternative Cu3 trinuclear topologies have been accessed by introducing the NHC donors in tripodal environments or by

the complexes show an overall twisted, chiral structure with the Cu atoms being dicoordinated and departing from linearity (CNHC-Cu-CNHC ca. 170°). The Cu−Cu separations of ca. 2.62−2.70 Å imply minor intermetallic interactions. The complexes have been studied as catalysts in the Ullmann and Sonogashira reactions.318 Tripodal 1,1,1-[tris(3-alkylimidazol-2-ylidene)methyl]ethane ligands can also support binuclear or trinuclear arrangements. Reaction of one equiv of the tert-butylsubsituted ligand with [Cu(NCMe)4](PF6) gave the complex 240Cu in which each Cu adopts a trigonal planar geometry with two normal and one abnormal NHC donors (Scheme 152). The mechanism of normal to abnormal rearrangement was not studied in detail.319

Scheme 150. Binuclear Complexes Supported by Macrocyclic NHC Ligands

BX


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Scheme 152. Dinuclear Complex with a Tripodal Ligand

Scheme 154. Formation of the Cu3 Oxo Complex 242bCu

Reaction of the methyl-substituted ligand, generated in situ from organometallic bases and the tris-imidazolium salt, with CuOTf gave the complex 241Cu, which could also be obtained by transmetalation from the corresponding Ag complex to CuBr. The structure of 241Cu is D3 symmetric and exhibits three Cu ions bridging two tripodal ligands. Each metal ion is coordinated to two of the NHCs in an approximately linear geometry. DFT calculations show that in addition to NHC to Cu σ donation, there is a degree (ca. 15%) of π backbonding from the metal to the π* orbital of the NHC (Scheme 153).320 Reaction of the tris-imidazolium salt 242aCu with [Cu(NCMe)4]X (X = BF4, PF6) led to the unexpected Cu3(μ3oxo) complexes 242bCu. The monocations have pseudo-C3 symmetry with an axis running through the μ3-O ligand and the center of the Cu3 plane and the centroid of the supporting arene ring (Scheme 154). The μ3-O atom lies 0.8 Å above the Cu3 plane. The origin of the oxo ligand was not established but assumed to be the base (KOtBu), the THF, or the deprotonation side products.321 A related but more rigid ligand design with the imidazolium rings directly attached to the aromatic base was used to prepare cylinder-type complexes with Cu by transmetalation to the corresponding silver complexes 243aCu (Scheme 155). The Ag complexes are also cylinder-type and formed via metalcontrolled self-assembly; the same molecular structure is maintained in the Cu species 243bCu which features linear Cu centers. The intramolecular separation between the phenyl ring centroids depends on the torsion angles between the planes of the phenyl ring and the NHC heterocycles rather than the M-CNHC bond distances and the CNHC-M-CNHC bond angles.322 In an attempt to increase the internal volume of the cavity while maintaining the same overall arrangement of the metal sites and their geometry, the central trisubstituted phenyl ring in the ligands of the previous scheme was replaced by the 1,3,5-triphenylphenyl backbone. The Ag complex 244aCu was prepared from the imidazolium salts and Ag2O in a process that involves a metal-controlled self-assembly of the nanometer-sized trinuclear cylinder. The three Ag centers are

Scheme 155. Cylinder-Type Trinuclear Complexes 243a,bCu

sandwiched between the extended planar polyaromatic ligands, which maintain an approximate C3 arrangement. Transmetalation with CuI gave the hexanuclear Cu complex 244bCu, which retained the structure of the Ag precursor. The separation of the two average ligand planes in 244bCu is ca. 4.24 Å. The Cu···Cu separation is ca. 1.4 nm (Scheme 156).323 2.3.2. Heterometallic Multinuclear Complexes. The heterodinuclear complexes 245aCu−245gCu with direct Cu-M bonds were prepared by salt elimination reactions from [CuCl(IPr)] and Na carbonylmetallates (Scheme 157).239,324 The different nucleophiles (Nu) used span a range of 7 × 107 relative nucleophilicity units, allowing for tuning of the Cu-M bond in the heterometallic species. The complex 245cCu was also prepared by dehydration from [Cu(OH)(IPr)] and [MoHCp(CO)3].69 The presence of bulky IPr is necessary to suppress ligand comproportionation reactions. The Cu-M distances were compared using formal shortness ratio (FSR) calculations for the two partners A and B given by FSRAB = DAB/(R1A + R1B), where R1A and R1B are Pauling’s atomic radii of A and B, respectively, and DAB is the A−B bond length. In the above cases, the FSR range is 1.004−1.048, which are values expected for single bonds. Inspection of the data points to the trend that the stronger the nucleophile, the shorter the Cu-M bond; exceptionally, 245gCu showed a short bond despite its weak nucleophilicity. All CO coligands in the complexes 245a−gCu were virtually linear, terminal, or “semibridging” leaning toward the Cu center. Computational studies showed that the Cu-M bonds are polarized with the Cu

Scheme 153. Trinuclear Complex with a Tripodal Ligand

BY


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Scheme 156. Cylinder-Type Trinuclear Complexes 244a,bCu

[Cu(OTf)(IPr)] (Scheme 158). Its structure displays a short Pd-Cu distance (2.55 Å) and a strong interaction between Cu

Scheme 157. Complexes with Direct Cu-M Bonds

Scheme 158. Heterobimetallic Pd-Cu Complexes

and one of the carbons of the aromatic ligand (Cu-C ca. 2.03 Å). The complex 246Cu constitutes an example of d10-d8 heterobimetallic species and may serve as a model for the common Sonogashira cross-coupling reaction involving Pd and Cu components. DFT calculations suggest that the metal− metal interaction is responsible for additional stabilization of 246Cu (ca. 9 kcal/mol), which could imply that in transmetalations during catalysis a bimetallic interaction energy may be responsible for stabilization of transition states.326 The heterobimetallic PtII/CuI complexes 247aCu and 247bCu based on the 9-(2-(diphenylphosphino)ethyl)-4,5-diazafluorenide were prepared by sequences involving substitution or salt metathesis reactions (Scheme 159). In 247aCu, there is a thermodynamic preference for coordination of the cation Cu(NHC)+ and the PtPh2 fragments to the N atoms of the diazafluorenide and the tethered phosphine, respectively. It appears that the d10 CuI has no strong preference for N- or Pcoordination, while the PtII prefers the P coordination and determines the overall regioselectivity. Similarly, the preferred binding site for PtPh2 in 247bCu is the N chelate of diazafluorenide; these preferences may be a demonstration of the soft nature of Pt and the chelate effect.327 The heterobimetallic complexes 248Cu and 249Cu were obtained by stoichiometry controlled partial transmetalation of the rigid tridentate ligand from Cu to Pd. Both complexes feature slightly distorted linear heterometallic chains (ca. 178.6°) almost coplanar with the six donor atoms. From the crystallographic characterization and the unit cell contents, 248Cu was formulated as CuI2Pd0 species. Complex 249Cu on the basis of magnetic measurements, DFT calculations, and computed nuclear shieldings, was formulated as containing a Pd0−CuI−Pd0 chain with an electron hole delocalized over the whole cation, including the metal chain. The coordination geometries at the metals depart from linearity, a fact

end maintaining a positively charged electrophilic character and the other metal a nucleophilic character. Out of the three highest filled MO’s, one possesses Cu-M σ-character and the other two Cu-M π*-character. Despite the differences in the nucleophilicities and reduction potentials of the various M fragments, the calculated Cu atomic charges were found invariant within the series. Complex 245aCu catalyzes the dehydrogenative borylation of unactivated arenes under photochemical conditions. The proposed mechanism postulates reaction of the bimetallic complex with the BH(pin) resulting in fission of the Cu-Fe bond to generate [CpFe(CO)2{B(pin)}] and [CuH(IPr)]; the former under photolytic conditions is the active borylating catalytic component of the arene. Recombination of [CuH(IPr)] and [CpFe(CO)2H] affords the original bimetallic complex and H2, completing the catalytic cycle.325 A heterometallic Pd-Cu complex 246Cu was obtained from the reaction of bis(1,10-benzo[h]quinolinato)-palladium with BZ


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Scheme 159. Heterobimetallic PtII/CuI Complexes Based on the 9-(2-(Diphenylphosphino)ethyl)-4,5-diazafluorenide

Scheme 160. Heterobimetallic Pd/Cu Complexes Based on a Rigid Bis-phosphanyl Carbene Ligand

Scheme 161. General Preparative Methods for the Homoleptic Ni(NHC)2 Complexes

rationalized by invoking heterometallic d10-d10 interactions

3. NHC NICKEL COMPLEXES 3.1. Introduction to Ni

The field of N-heterocyclic carbene (NHC) complexes of Ni has flourished after the introduction of isolable NHCs by Arduengo. Before this, work by Lappert329 and Sellmann330

(Scheme 160).328 CA


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(Keq = 1), although the conditions were not specified;332 poor and nonreproducible yields were attributed to this equilibrium.333 Furthermore, an inhibiting effect of 1,5-COD in certain catalytic reactions (e.g., C−H functionalization of fluoroaromatics) based on the in situ generation of “Ni(NHC)” catalysts from [Ni(1,5-COD)2] and NHC has been studied in detail and was ascribed to the formation of stable dormant [NiIIX(η3-cyclooctadienyl)(NHC)] (see below, section 3.2.3.1.2).334 The use of [Ni(acac)2] and NaH in the presence of IMes or IPr has been developed to a high yield synthesis of 1auNi and 1buNi. Carrying out the reaction of IPr· HCl with [Ni(acac)2] in the presence of stoichiometric NaH led to the five-coordinate complex [Ni(acac)2(IPr)], which was converted to 1buNi either by the reaction with additional NaH or by addition of IPr·HCl and NaH, in 40% and 90% yields, respectively. Although the competence of [Ni(acac)2(IPr)] for the synthesis of the 1buNi was thus demonstrated, the exact reaction mechanism was not established in view of the general lability of the NHC ligands in Ni(NHC)2-type complexes. For example, the IPr ligand in 1buNi was substituted at room temperature by the addition of one equiv of IMes giving 1auNi.335 A two-step procedure for the preparation of 1au/sNi and 1buNi recently described involves the reduction of [NiCl2(NHC)2] species, obtainable in high yields from [NiCl2(PPh3)2] and NHC (NHC = (S)IMes, IPr) with KC8 in THF.333 Reaction of excess of ItBu with Ni metal vapor gave the analogous complex 1cuNi in poor yields (Schemes 161 and 162);336 attempts to perform the synthesis of 1cuNi in solution

have given examples of Ni-NHC complexes prepared from electron-rich alkenes and functionalized saturated NHCs, respectively. There are important breakthroughs in the synthesis of low coordination number complexes and the homogeneous catalysis based on the use of bulky NHC ligands. Although Ni is commonly found in oxidation states Ni0 and NiII, there is recent interest in the “unusual” NiI complexes, which have been implicated as catalytic reactive intermediates. The NiI complexes are stabilized by a range of ligands including NHCs. A recent review of the NiI chemistry has appeared.14 The organization of the section follows the general guidelines outlined in the introduction: it is divided based on the nuclearity of the complexes, the metal oxidation state, and the nature of the coligands (following the LnXm ligand classification). Although most of the general synthetic methods described in the Cu section (except those using basic metal oxides as combined sources of metal and basic deprotonation equivalents) are applicable for the synthesis of Ni complexes, their frequency of occurrence and scope are variable and also dependent on the oxidation state of the targeted Ni complexes. Thus, the majority of Ni0 complexes are obtained by the interaction of the isolated or in situ generated NHCs with Ni0 sources, mainly [Ni(1,5-COD)2] or less commonly [Ni(CO)4], etc.; NiII complexes are obtainable from the isolated or in situ generated NHCs but also via the Ag transmetalation protocols, especially when heteroatom-functionalized NHC ligands are involved. C−X activation methodologies (X = H, halide, etc.) using Ni0 precursors and oxidative addition reactions, or NiII organometallics and alkanolysis protocols, have also been employed for the synthesis of NiII complexes. NiII precursor complexes with coordinated bases include [Ni(OAc)2] and [Ni(acac)2], while K2CO3/Cs2CO3 as external bases are gaining popularity. An important entry to NiI species involves disproportionation of Ni0 and NiII precursors, although single-electron reduction protocols of NiII have also been successful under careful steric control of the coordination sphere. All these methods and others of limited scope are detailed and detailed below.

Scheme 162. Attempts to Access [Ni(ItBu)2] from ItBu and [Ni(1,5-COD)2]

3.2. Mononuclear Complexes

3.2.1. Mononuclear Ni0 Complexes. 3.2.1.1. Complexes with Monodentate NHC Ligands. 3.2.1.1.1. Homoleptic [Ni0(NHC)2] and [Ni0(NHC)3]. The only well-defined strictly two-coordinate NHC Ni0 complexes are homoleptic; they have been obtained using bulkier NHCs. The original method for the synthesis of [Ni(IMes) 2] (1auNi) resided on the substitution of 1,5-COD in [Ni(1,5-COD)2] by free IMes; the diamagnetic 14-electron purple complex was obtained in good yields (Scheme 161). Diagnostic NMR spectroscopic data of 1auNi (CNHC: δ = 193.2 ppm, HC4/HC5: δ = 6.0 ppm corresponding to shielding relative to the IMes) have been attributed to a degree of π-back-bonding from the Ni to the NHC ring. The complex is linear at Ni with the imidazole rings twisted by 53° relative to each other.3 Attempts to prepare well-defined analogues of 1auNi with other bulky monodentate NHCs initially gave mixed and unexpected results. Thus, by the reaction of [Ni(1,5-COD)2] with two equiv of IPr, good yields of 1buNi were obtained; the complex was characterized spectroscopically and analytically.331 Later work pointed out that in the preparation of 1buNi from [Ni(1,5-COD)2] and IPr, an equilibrium is established involving all three components

led after long reaction times (ca. 2 weeks) to the binuclear products 2Ni and 3Ni, depending on the presence or not of adventitious silicone grease contaminants, respectively. Shorter reaction times (ca. 5 days) afforded low yields of the unstable three-coordinate [Ni(ItBu)(η2-1,3-COD)2] (4Ni), which converted to complex 5Ni by C−H activation of the tBu substituents. The mechanism postulated for the transformations involving ItBu is shown in Scheme 162, although the exact nature of the steps was not clarified.337 An alternative approach, which also led to an extension of the family of [Ni0(NHC)2] complexes, originated from studies CB


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of the reductive elimination reaction in simple cis-[NiMe2(NHC)2] species: with bulky NHC ligands, spontaneous reductive elimination of ethane at room temperature took place. In this way, the species 1au/sNi and 1bu/sNi were obtained as crystalline solids that were fully characterized. Interestingly, also 1csNi was obtained which is not available by any other of the previous methods. With smaller NHC ligands (e.g., Me2 IiPr), cis-[NiMe2(NHC)2] was isolable after reaction at room temperature. Although the method involves the handling of [NiMe2(TMEDA)], which has limited thermal stability, it provides a means of generating the [Ni(NHC)2] in the absence of competing coordinating alkene or other ligands (Scheme 161).338 The reaction of 6Ni (see below section 3.2.1.1.5, Scheme 183) with free IPr in THF led to 1buNi and with stoichiometric quantities of IMes to 1duNi.339 Interestingly, stirring 1buNi in d8-toluene led to the deuterated analogue of 6Ni, which provided further evidence for the lability of the second NHC in the [Ni(NHC)2] structures (Scheme 163).

Computational evidence supported a reduction with LiNiPr2 involving β-hydride elimination from a transient [NiCl(NiPr2)(Me2cAAC)2], followed by base-promoted reductive elimination of HCl. Computational methods also showed that the ground state of 8Ni is a closed-shell singlet; the calculated atomic charges corroborate a character intermediate between NiI and Ni0 and strong delocalization along the N-CNHC-NiCNHC-N bonds due to π-backdonation from the d(Ni) to the p(CNHC) orbitals (cf. the copper analogs [Cu(cAAC)2]n+ n = 1, 0, section 2.2.1.2, Scheme 83). In contrast, comparative calculations of the electronic structure of a 1buNi model revealed exact Ni0 character.341 The complexes 1buNi and 8Ni show high activity for the Kumada coupling reactions or the amination of chloroarenes. The reaction of [Ni(1,5-COD)2] with three equivalents of the small IMe gave the homoleptic three-coordinate [Ni(IMe)3] (9Ni); in contrast, the bulkier InPr, IiPr led to the binuclear complexes 10Ni with bridging 1,5-COD (Scheme 165). In the solid state, 9Ni has a distorted trigonal planar Ni center with the NHC planes out of the coordination plane in a propeller-like arrangement. Despite the longer Ni-CNHC distances in 9Ni compared to the two-coordinate 1Ni (1.86− 1.89 vs 1.83 Å), the IMe ligands are not labile (which is the case in 1aNi, 1bNi), supporting associative substitution mechanisms in the latter cases.342 DFT studies attribute the scarcity of [Ni0(NHC)n] (n > 2) to energetic terms associated with steric and electronic factors. In the latter category belongs the high energy required to deform the linear NHC-Ni0-NHC fragment, which is associated with the strong σ-donor and poor π-acceptor characteristics of the NHC ligands; this energy cost must be compensated by additional metal ligand interaction energy and medium polarity effects; conversely, it is predicted that smaller NHC ligands with better π-acceptor properties may favor complexes of type [Ni(NHC)n] (n = 3, 4).343 The range of reactions of 9Ni includes substitution of one IMe with CO or alkynes, the oxidative addition of the C−C bond of biphenylene, and the catalytic insertion of alkynes to biphenylene to phenanthrenes; they are discussed below (Schemes 217, 219, 220). 3.2.1.1.2. Heteroleptic [Ni(NHC)L]. The simple mononuclear η2-imine complexes 11aNi and 11bNi were synthesized in order to study the sequence of steps in the Ni-catalyzed [2 + 2 + 2] cyclization reactions of alkynes and imines to N-hydropyridines. They were prepared in quantitative yield by the reaction of [Ni(1,5-COD)2] with IPr and the imine in a 1:1:1 ratio (Scheme 166). The products obtained feature a nonsymmetrical η2-imine coordination and a distorted Tshape geometry. The Ni-Nimine distances (ca. 1.85 Å) are shorter than the Ni-Cimine (1.94 Å), and the CNHC-Ni-Nimine angle is ca. 173°; the Ni-CNHC bond distance is 1.84 Å. Although there is significant elongation of the coordinated imine bond (1.37 Å), the complex is described as Ni0 with a η2coordinated neutral imine (L).344

Scheme 163. Homoleptic [Ni(NHC)2] and [(η6-arene)Ni(NHC)]; Equilibria and Substitution Reactivity

The purple, closed-shell, diamagnetic [Ni(RE-6-Mes)2] (7bNi) was obtained by the reduction of the NiI complex 7aNi with KC8 in THF (see also below sec 3.2.2.1.1).340 The electronic structure of 7bNi comprises a frontier orbital region with five occupied metal-based orbitals with approximate 2:1:2 splitting: the near-degenerate HOMO set (with Ni dxz and dyz character) lying above a single σ-type orbital (with Ni dz2 character) and further below is found a degenerate set (with Ni dx2−y2 and dxy characters) (Scheme 164). Scheme 164. Homoleptic [Ni0(NHC)2] by Substitution/ Reduction of NiI Precursors

The diamagnetic [Ni(Me2cAAC)2] complex 8Ni was obtained by reduction of [NiCl2(cAAC)2] with LiNiPr2 or KC8 in THF.

Scheme 165. Homoleptic [Ni(NHC)3] Complexes from [Ni(1,5-COD)2] and the Small IMe

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Scheme 166. [Ni(NHC)(η2-Imine)] by Substitution at [Ni(1,5-COD)2]

and the lability of the alkene coligands have rendered [Ni(NHC)(alkene)2] indispensable in Ni catalytic applications. For catalytic purposes, most complexes have been accessed in situ by direct reactions of the alkenes with [Ni(1,5COD)2] in the presence of NHCs. The 1:2 NHC-to-alkene ratio in the product complexes is dictated by the sterics of the NHC and the selected reaction conditions: with less bulky NHCs, high alkene-to-NHC reactant ratios are needed to suppress the formation of [Ni(NHC)2(alkene)] species. The first reported complex 4Ni has limited lifetime and was obtained in low yields by the reaction of ItBu with [Ni(1,5COD)2] (Scheme 162). The Ni adopts a trigonal planar geometry with Ni-CNHC and Ni-Calkene distances in the ranges of 1.95 and 2.01−2.05 Å, respectively. Notably, the fact that the presence of coordinated η2-1,3-COD was established crystallographically implies that 5Ni is a secondary product succeeding the isomerization of 1,5-COD; however, the mechanistic details of the transformation have not been elucidated.337 The reaction of NHC with [Ni(1,5-COD)2] in the presence of two equivalents of dimethylfumarate349,350 and (CH2CH)SiMe3351 or excess of styrene,352 norbornene,353 and (CH2CH)SiMe3351 has been used for high yielding syntheses of the complexes 13aNi−13fNi (Scheme 168). The common features of the structures of complexes 13aNi− 13fNi are distorted trigonal planar geometry (defined by the CNHC and the CC centroids), with the coordination plane tilted from the NHC plane, Ni-CNHC bond distances of ca. 1.91−1.95 Å, elongated η2(CC) bonds compared to the free alkenes (supporting Ni π-basicity), and usually C2 molecular symmetry implying coordination of the same face of prochiral alkenes; in 13fNi, the noncentrosymmetric form was obtained in the solid state, while in solution both isomers existed in ca. 5:1 ratio, in addition to fluxional rotamers. An alternative method for accessing a range of complexes of type [Ni(NHC)(alkene)2] was developed by the reaction of NiCl2 with two equivalents of (C3H5)MgCl (allyl magnesium chloride) and in situ transformation of the intermediate formed after addition of the NHC; coupling of the allyl ligands in the plausible [Ni(η1-allyl)(η3-allyl)(NHC)] gave bis-η2-1,5hexadiene complexes 13g Ni−13j Ni ; 13g Ni served as a convenient starting material for diverse alkene complexes (e.g., 13bNi and 13kNi−13mNi) and performed as precatalyst better than a combination of Ni(1,5-COD)2/NHC (Scheme 169).334,354,355 An analogue of 13kNi with the bulkier IPr* (i.e., [Ni(IPr*)(η2-C2H4)2]) has been identified as a product of the imido transfer from a NiII imido complex to ethylene (Scheme 203 below).356

3.2.1.1.3. Heteroleptic [Ni(NHC)L2]. The reaction of [Ni(CO)4] with the sterically demanding ligands IAd or ItBu led to the three-coordinate substitution products [Ni(NHC)(CO)2] (NHC = IAd, ItBu 12aNi and 12bNi) after dissociation of two CO ligands from [Ni(CO)4]; 12aNi and 12bNi adopt distorted trigonal structures, with Ni-CNHC bond distances at ca. 1.95 Å. Interestingly, monitoring the reactivity of 12aNi and 12bNi under a CO atmosphere by IR spectroscopy (1 atm) did not establish any associative CO reactivity to [Ni(NHC)(CO)3] species, which are commonly found with smaller NHCs; instead, direct conversion to [Ni(CO)4] was observed, in a reaction that is the reverse of the preparation of 12aNi and 12bNi. These observations corroborate the thermodynamic instability of [Ni(NHC)(CO)3] (NHC = IAd, ItBu) (Scheme 167). The extraction of the thermodynamic parameters ΔH Scheme 167. Three-Coordinate [Ni(NHC)(CO)2] by Substitution at [Ni(CO)4]

and ΔS from the equilibrium shown in Scheme 167 allowed the calculation of Ni-NHC BDEs: 43 ± 3 kcal mol−1 and 39 ± 3 kcal mol−1 for IAd and ItBu, respectively. Accordingly, the IR spectra of 12aNi and 12bNi display the expected A1 and B2 vibration modes (A1 at 2007.2 and 2009.7 cm−1 for 12aNi and 12bNi, respectively).345,346 Three-coordinate 16-electron complexes of type [Ni(NHC)(alkene)2] have been prepared as precatalysts for various Nicatalyzed reactions, in particular the cycloaddition of unsaturated organic molecules, carbonyl-ene, hydroalkenylation, and amination reactions; this is still a topical subject and has been reviewed.347,348 The strong Ni-NHC binding in the [Ni(NHC)] fragment, the possibility of facile steric tuning,

Scheme 168. Three-Coordinate [Ni(NHC)(L)2], L = Alkene, by Substitution at [Ni(1,5-COD)2]

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equilibrium in solution. The equilibrium reaction mixture 13nNi/13oNi on heating in dioxane liberated 2,2′-bipy (Scheme 170). The reaction of 13cNi or 6Ni with 2-chloro-6-tBupyridine provided only the mononuclear complex, which was fully characterized, including structurally. In the mononuclear 13nNi, the chloride can be substituted by indolyl via a salt metathesis reaction giving quantitatively 13pNi. The catalytic competence of the latter was established by providing the indolyl-pyridine (ca. 60%) on heating in dioxane. On the basis of these experimental evidence and QM/MM computational studies, a Ni0/NiII catalytic cycle was postulated, involving facile oxidative addition on the 12-electron species Ni0(IPr) followed by transmetalation and rate-limiting reductive elimination of the aminated product; the binuclear complex 13oNi was catalytically inactive. The scope of the proposed mechanism in other Ni-catalyzed amination reactions was not explored. The reaction of 13cNi with isothiocyanates and isocyanates led to coupling of the cumulenes at the coordination sphere of the metal and formation of the species 13qNi and 13rNi (Scheme 171).

Scheme 169. Non-Redox Substitution Reactivity at [Ni(NHC)(bis-η2-1,5-hexadiene)]

In addition to the catalytic usage of [Ni(NHC)(alkene)2] complexes alluded to previously, stoichiometric reactions have been studied. The oxidative addition reaction of 13cNi or 6Ni with 2-chloro-pyridine and 2-chloro-6-tBu-pyridine was explored as part of a study of stoichiometric reactions related to the Ni-catalyzed amination of heteroaryl halides with indoles and carbazoles (Scheme 170).357 The reaction at room

Scheme 171. Reactivity of [Ni(NHC)(η2-styrene)2] with Isocyanate and Thiocyanate

Scheme 170. Oxidative Addition Reactivity of [Ni(NHC)(L)2] with 2-Chloro-pyridine

The molecular structure of 13qNi unveiled a new dianionic [SC(NPh)N(Ph)CS]2− ligand moiety obtained by the reductive coupling of two PhNCS molecules on the coordination sphere of Ni, through the formation of a C−N single bond; the ligand is bound to the Ni via an anionic S donor and an anionic η2-thiocarbonyl, and the Ni is in a planar coordination geometry. In contrast, reaction of 13cNi with one equiv PhNCO resulted in the formation of a γ-lactam nickelacycle 13rNi featuring a T-shaped Ni center after the reductive coupling of the isocyanate with one coordinated styrene.358 The catalytic applications of Ni0(NHC) moieties have been extended to include carbonyl and imine functionalities of organic molecules and their activation by η2-“side-on” coordination to the Ni. In this respect, mechanistic studies of the good scope intramolecular hydroacylation/cyclization of benzaldehydes, o-substituted by olefinic groups, led to the isolation of the well-defined complex 14aNi as resting catalytic species featuring one η2-alkene and one η2-aldehyde functionalities coordinated to Ni0ItBu. Complex 14aNi converts at room temperature over longer periods (ca. 30 d) to an isomeric mixture of the oxa-nickelacycles 14bNi; the latter on heating gives the cyclic ketone (Scheme 172).359 In 14aNi,

temperature with 2-chloro-pyridine gave a mixture of the mononuclear and binuclear species 13nNi and 13oNi, respectively. Although their presence in solution was verified by 1H NMR spectroscopy, isolation and crystallization of the binuclear 13oNi proved only possible when using 6Ni since cleaner reaction mixtures were obtained. The binuclear 13oNi features distorted square planar geometries, o-metalated pyridine ligands resulting in a (η2-N,C)-coordination with short Ni-Cpy bonds (ca. 1.82 Å) and IPr plane perpendicular to the coordination plane; long intermetallic distances exclude Ni−Ni interactions. Interestingly, dissolution of the isolated crystals of 13oNi in C6D6 gave again the mixture of binuclear and mononuclear species, implying the establishment of an CE


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Scheme 172. Complexes with [Ni(NHC)(η2-aldehyde)] Moiety

Scheme 173. [Ni(NHC)L2] from 2-Hydroxy-1,1′-biphenyl and Related Complexes

geometry at Ni is pyramidalized, and the σ-coordinated C− Nimine bond distance is much shorter than the π-coordinated C−Nimine (ca. 1.29 vs 1.37 Å). The weakly π-basic imine is easily displaced under catalytic conditions by the alkyne substrate. Intramolecular attack of the σ-imine to the π-alkyne was not implicated based on the products isolated (Scheme 174).362

both types of double bonds are elongated on coordination to the trigonal planar Ni (ca. 1.32 and 1.39 Å for CO and C C, respectively); the NHC plane is perpendicular to the coordination plane. In the centrosymmetric anti 14bNi, the NiII environment is planar; the long Ni−Ni separation supports the possibility of facile dimer dissociation to a reactive monomeric species on the way to the hydroacylated product. Mechanistic studies on the Ni-catalyzed formation of benzoxasiloles by the cyclization of silyl-substituted aldehydes led to the isolation of the well-defined unusual complex 15Ni as an off-cycle inactive species featuring two η2-aldehyde functionalities coordinated to Ni0IPr.360 In a study of the Ni-catalyzed hydrogenolysis of diarylethers to arenes and phenols, in the presence of base NaOtBu, the NiII complex 16aNi was found to be a competent catalyst for the reaction. Reaction of 16aNi with H2 gave after hydrogenolysis the complex 16bNi which could be independently synthesized by the reaction of [Ni(SIPr)(η6-benzene)] with the 2-hydroxy-1,1′-biphenyl. The Ni0 in 16bNi is coordinated in a bidentate fashion by a dative O−Ni bond and an η2-arene interaction. The presence of the phenolic hydrogen was spectroscopically evidenced (Scheme 173).361 Interestingly, the catalyst resting state [Ni(SIPr)(η6-C6H6)] reacted with excess diphenyl ether in hexamethyldisiloxane to give the complex 16cNi with η6-bound diphenyl ether (see also section 3.2.3.1.2, Scheme 204). Starting from the development and optimization of a catalytic system to effect the intermolecular hydroimination of alkynes with aromatic N−H ketimines as substrates, [Ni(1,5-COD) 2], IPr, and Cs2CO3 were reacted with diphenylacetylene and benzophenone-imine to afford Zenamine species after workup. On the basis of the observed product stereochemistry, it was postulated that the catalytic reaction proceeded by external nucleophilic attack of the ketimine to a π-alkyne-NiIPr complex. Catalyst optimization led to the synthesis of complex [Ni(IPr)(η1-HN = CPh2)(η2HN = CPh2)] (17Ni) as a potential catalyst precursor featuring a formally three-coordinate Ni center with one σ-imine and one π-imine ligands, in addition to the IPr spectator. The

Scheme 174. [Ni(NHC)(Imine)2] Complexes and Intermolecular Hydroimination of Alkynes

The three-coordinate [Ni(IPr)L2] (L2 = dppe, L = tBuNC) were prepared from 13gNi and dppe or tBuNC, respectively. Significantly, reactions with more basic trialkylphosphines resulted in the displacement of the IPr too.355 The threecoordinate [Ni(RE-6-Mes)(PPh3)2] (18aNi) was conveniently prepared by the addition in a 1:2 ratio of RE-6-Mes and PPh3 to a solution of [Ni(1,5-COD)2]. Complex 18aNi serves as a convenient precursor for a multitude of three-coordinate complexes by substitution of one PPh3 with another L donor (Scheme 175).363 It also can effect clean and facile C−F activation of C6F6. Reaction of 18aNi with smaller NHCs like Me2 IMe resulted in complete dissociation of the RE-6-Mes and Ph2P-Ph activation reactions.363 A family of nitrosyl complexes was synthesized aiming at stabilizing low coordination numbers by taking advantage of the steric properties of bulky NHCs (Scheme 176). The reaction of [Ni(NO)I(THF)2] with IPr provided 19aNi and, after halide exchange, its triflate analogue 19bNi, which are useful starting complexes for further derivatizations. Importantly, the planar environment of the Ni in 19bNi adopts an approximate T-shaped geometry with a linear NO (Ni-N-O ca. 163.8°), featuring a short N−O distance (1.95 Å). Thus, the ligand is best described as NO+ and the metal Ni0; the ν(NO) CF


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prepared by the reaction of [Ni(IMes)2], formed in situ from [Ni(1,5-COD)2] and IMes in THF, with alkene after changing the solvent from THF to toluene. Complex 20aNi is stable in solution at room temperature and displays NMR spectra in accord with C2 molecular symmetry originating from the interaction of the NHCs with the alkene CO2Me substituents. The symmetry is maintained in the solid state where the Ni adopts a trigonal planar coordination geometry with Ni-CNHC distances at ca. 1.95 Å and elongated olefinic C−C distances (ca. 1.45 Å), indicative of Ni-to-alkene πbackbonding. Studies of the speciation in the system Ni(IMes)2/dimethyl-fumarate, as a function of the ratio of the added reactants, showed that with equimolar amounts in benzene at room temperature, 20aNi was formed; with two equivalents of alkene, the initially formed species are dinuclear (with Ni/alkene ratio of 1:1), the most stable of which, 20bN, was structurally characterized. Finally, with three equivalents of alkene, the complex 13aNi was obtained (Scheme 177).350

Scheme 175. Reactivity of Heteroleptic [Ni(NHC)(PPh3)2]

Scheme 176. Transformations with [Ni(NO)X(NHC)]

Scheme 177. Substitutions at [Ni(IMes)2] with Activated Alkenes

The reaction of 9Ni or 10Ni with diphenyl acetylene (1:1 ratio per Ni) gave the three-coordinate complexes 21Ni with distorted trigonal planar geometry (assuming that the acetylene occupies one coordination site). 13C NMR spectroscopy gives evidence for departure from sp-hybridization of the η2-alkyne C atoms, indicative of π-backdonation from the Ni to the alkyne. This is also confirmed from the solid state structure which reveals alkyne bond elongation and deviation from linearity; the alkyne vector lies in the coordination plane (Scheme 178).342 The reaction of ethylene, 4-vinyl-pyridine, and mesityl-oxide with 10Ni (R1 = iPr) gave the complexes 22Ni, 23Ni, and 24Ni where the Ni coordination geometry is best described as trigonal planar (assuming that the η2-alkene is occupying one coordination site). NMR spectroscopy and crystallography give evidence for strong backdonation from the metal fragment to the coordinated η2-alkene moiety; the alkene vector lies within the Ni coordination plane.365,366 Finally, reaction of 10Ni with tBuCP gave the mononuclear complex 25 Ni with η2coordination of the phosphalkyne. The elongation of P−C bond and the positioning of the P−C bond in the coordination plane were attributed to the π-basicity of the Ni(IiPr)2 moiety.367

in the IR spectrum also corroborates a linear NO. Similar conclusions can be drawn for 19aNi based on spectroscopic analogies. Substitution of the iodide by SCPh3 using TlSCPh3 maintains the same structural features of the Ni(NHC)(NO) moiety.364 Reaction of 19bNi with NaCp led to the complex 19dNi with η5-Cp coordination and bent NO to accommodate for the electron-rich metal, formally NiII. Reduction of 19aNi with Na/Hg provided binuclear NiI complexes with bridging iodide and NO, while attempts to form the two-coordinate [Ni(IPr)(NO)]+ led to the rearranged trigonal planar species 19fNi, the cation of which comprises one “normally” and one “abnormally” coordinated NHCs; here too the coordinated NO is linear as evidenced by crystallography and spectroscopy. Thus, despite the strong donor properties and favorable sterics of the IPr, the mononuclear [Ni(IPr)(NO)]+ is too reactive to be isolated. 3.2.1.1.4. Heteroleptic [Ni(NHC)2L]. The complex [Ni(IMes)2(alkene)] (alkene = dimethylfumarate) 20aNi was CG


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η2-Complexes of the Ni(NHC)2 moiety with alkene analogues of the higher group 14 congeners (i.e., disilenes and distannylenes) have been described. The reaction of the NHC-stabilized silylene hydride 29dNi with [Ni(1,5-COD)2] gave the dihydrodisilene complex 30Ni (represented by the two limiting valence bond structures 30aNi and 30bNi in Scheme 180) after dehydrogenative dimerization and migration of the NHC from the Si to the Ni. The values of the chemical shift and the 1JSi−H in the 1H NMR spectrum of 30Ni gave evidence for the dihydrodisilene formation and coordination, and the 29 Si NMR spectrum shows a peak at ca. δSi = −115 ppm falling in the range of coordinated disilene to transition metals. The geometry around the Ni atom is planar; the Si−Ni distances (ca. 2.30 Å) are longer than those in silylene−Ni complexes (2.04−2.25 Å). From the structural and NMR data, the preferred formulation of 30Ni is the nickelacycle 30aNi rather than the coordinated η2-silylene 30bNi (Scheme 180).370 In an analogous manner, the NHC adduct of the stannylene SnTrip2 31Ni reacted with [Ni(1,5-COD)2] to give the complex 32Ni with coordinated distannene, which is stable in solution (Scheme 180). Multinuclear NMR spectroscopy, structural characterization, and DFT calculations indicate the presence of a π-interaction between the distannene moiety and the Ni0 center, in contrast to 30aNi, which was formulated as disilanickelacycle.371 Three-coordinate complexes of the Ni(NHC)2 moiety with group 15 donor atoms were obtained by the reaction of diphenyl-diazomethane with [Ni(NHC)2] or complex 10Ni, the latter acting as a source of [Ni(IiPr)2]. In these reactions, the steric properties of the Ni(NHC)2 moiety dictated the manner the diphenyldiazomethane coordinated to the Ni: the less encumbered IiPr gave the complex 33Ni with side-on η2-Ncoordination, while IMes and IPr led to 34Ni with κ1coordination (Scheme 181). In 33Ni, the Ni adopts trigonal planar coordination geometry, with long Ni−N and N−N distances (of ca. 1.86 and 1.27 Å, respectively, the latter compared to the uncomplexed diazoalkanes at ca. 1.12 Å). The metric trends were attributed to the substantial π-backbonding to the diazo group from the π-basic Ni, accompanied by a stabilization of the N−C bond. As a result, complex 33Ni is very stable and does not undergo under inert atmosphere N2 loss thermally or photochemically.365 Complexes 34Ni were obtained by the reaction of [Ni(NHC)2] (NHC = IMes, IPr) with diphenyldiazomethane. Their structures also feature planar, three-coordinate Ni centers, in which the N2CPh2 is coordinated in an end-on κ1-fashion. The Ni−N bond distances (ca. 1.73 Å) fall between Ni−N and NiN singleand double-bond distances (Scheme 181). Complexes 34Ni are stable thermally and photochemically but can undergo facile dissociation reverting to [Ni(NHC)2] starting material. In addition, they can stoichiometrically or catalytically transfer the CPh2 group to alkenes giving substituted cyclopropanes; [Ni(NHC)2] in the presence of Ph2CN2 also catalyze the reaction.333 The reaction of adamantyl azide with [Ni(IMes)2] gave also a stable adduct of the azide with η2-N-coordination; on heating, the azide underwent dissociation regenerating [Ni(IMes)2]; this contrasts the behavior of complexes with chelating NHC which, following the formation of adducts with azides, undergo N2 elimination affording nickel imido complexes (see Scheme 185).333 The reaction of [Ni(1,5-COD)2] with the NHC-stabilized acyclic silylene 35Ni, in the presence of one additional

Scheme 178. Substitution Reactivity Leading to [Ni(NHC)2(η2-L)]

The reaction of R2cAAC with [Ni(CO)4] in a 1:1 ratio gave the substitution product [Ni(R2cAAC)(CO)3] (26Ni), which features tetrahedral coordination geometry; the product 27Ni of decomposition of the ketene formed by the direct reaction of the free R2cAAC with CO was also obtained. On the basis of the Tolman Electronic Parameter (TEP) of 26Ni, it was deduced that R2cAAC is a strong σ-donor: the A1 vibration mode at ν = 2042−2046 cm−1 is similar to that of analogous complexes with common imidazol-2-ylidenes. In addition, the π-acceptor properties of the ligand Me2cAAC were evaluated from the phenylphosphinidene adduct 28Ni. It has been suggested that the 31P NMR shift of the phophinidine adducts of the NHCs can be correlated to the π-acidity of the NHC; in Me2 cAAC, δP = 67.2 ppm, a value associated with good πacceptors. Further reaction of 26bNi and 26cNi with R2cAAC (R= Cy, Me) gave the rare three-coordinate [Ni(R2cAAC)2CO] (29bNi and 29cNi). Interestingly, 29bNi and 29cNi could also be obtained from the reaction of R2cAAC with [Ni(ItBu)(CO)2] by a sequence involving substitution of the ItBu by R2cAAC. In 29bNi and 29cNi, the metal is in a distorted trigonal planar environment with short Ni-CNHC bonds (ca. 1.88 Å) (Scheme 179).368,369 CH


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Scheme 179. System [Ni(CO)4] with cAAC

(Scheme 182). In the 29Si NMR spectrum, the SiII signal appeared at ca. δ = 123 ppm, deshielded relative to 35Ni. The Ni in 35Ni has a trigonal planar environment with a short Ni− Si bond distance of ca. δ 2.08 ppm, suggesting a multiple bonding character in the Ni-Si interaction due to the πacceptor character of the Si ligand. DFT calculations show that the HOMO and LUMO in 36Ni have Si−N π and 3p Si characters, respectively, with a narrow HOMO−LUMO gap. The reactivity of 36Ni with small molecules stems from this electronic property: it cleaves molecular dihydrogen to complex 37Ni and catecholborane to 38Ni. The structure of 38Ni revealed the first example of terminal hydroborylene (formally BI) coordinated to the trigonal planar NiII and to two NHCs that migrated from Ni. The metrics of the Ni-B-Si moiety and DFT calculations imply the presence of an “agostic” Ni-B-Si interaction. The formal reduction of BIII in BH(cat) to BI and the concomitant cleavage of strong B−O bonds takes place via a sequence of σ-bond metathesis steps and is compensated by the formation of two strong Si−O bonds (Scheme 182).372 3.2.1.1.5. Heteroleptic [Ni(NHC)L3]. Four-coordinate tetrahedral species 39Ni of type [Ni(NHC)(CO)3] (NHC = (S)IMes, (S)IPr, IPent, IPr*, (S)IPr*(OMe), and ICy) were obtained by the reaction of the free NHCs with [Ni(CO)4]; IAd and ItBu gave complexes of stoichiometry [Ni(NHC)(CO) 2 ] (see above, section 3.2.1.1.3 and Scheme 167).346,373−376 In contrast to the reaction of IAd and ItBu, the displacement of CO from [Ni(CO)4] is not reversible. In all complexes, the Ni-CNHC falls in a narrow range (ca. 1.96− 1.98 Å) despite the electronic difference of saturated and unsaturated NHC. The A1 IR active vibration mode (Tolman Electronic Parameter, TEP) of 39Ni also fell within a narrow

Scheme 180. Formation of Disilene and Distannylene Complexes

equivalent of NHC, led to the rare three-coordinate, 16valence-electron complex [Ni(NHC)2 (silylene)] (36 Ni) CI


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Scheme 181. Complexes of Type [Ni(NHC)2(Diphenyl-diazomethane)]

similar to those in complexes with imidazole-based NHCs; slightly lower TEP value for the RRcAAC complexes than the (S)IPr was also observed (2046 vs 2052 cm−1).368,369 The reaction of [Ni(1,5-COD)2] with three equiv of 2,6xylyl-isocyanide in the presence of one equiv IPr gave the complex [Ni(IPr)(2,6-xylyl-isocyanide)3] (ν(CN) 1960 cm−1 cf. ν(CN) 2123 cm−1 for free CNXyl), with Ni−CNHC and Ni−Cisocyanide bond distances ca. 1.97 and 1.82 Å, respectively.377 The one pot reaction of [Ni(1,5-COD)2] with NHCs in the presence of H2 in aromatic solvents (toluene, benzene, etc.) has been developed to a single-step, multigram scale method for the synthesis of [Ni(NHC)(η6-arene)] complexes 6Ni for a good range of NHCs (IPr, SIPr, IPr*) and arenes (toluene, benzene). The use of a combination of imidazolium salts and KOtBu can also be used in place of the free NHC. In 6Ni, the η6-arene can be displaced by a variety of reagents (Scheme 163, Scheme 173, and Scheme 183) and used as a source of the Ni0(NHC) moiety in catalytic applications (see also above).339 The complex [Ni(IPr*)(η6-toluene)] was also prepared in lower yields by the heterogeneous reduction of [NiCl2(IPr*)(THF)] with Mg turnings in toluene.356 3.2.1.1.6. Heteroleptic [Ni(NHC)2L2]. A family of tetrahedral 18 valence electron complexes of type [Ni(NHC)2(CO)2] (41Ni) (NHC = IMe or IiPr, InPr) were obtained by the reaction of the three-coordinate homoleptic 9Ni (Scheme 165) or the 1,5-COD-bridged binuclear [Ni(NHC)2)2(1,5-COD)]2 10Ni (Scheme 165), respectively, with CO at atmospheric pressure. Comparison of the electronic properties of the complexes by means of IR spectroscopy shows little variation: the A1 and B2 IR active stretching modes vary within the range of 1927−1940 and 1847−1851 cm−1, respectively, with the lower values for each mode associated with the IiPr ligands, indicative of marginally better σ-donation compared to the

Scheme 182. Reaction of [Ni(1,5-COD)2] with the NHCStabilized Acyclic Silylene

range (2050−2052 cm−1), indicating almost equal donor strengths of the NHCs toward the Ni(CO)3 fragment. In contrast, a comparison with the TEP of [Ni(CO)3(PR3)], of the same molecular symmetry, demonstrates the better σdonor strengths of NHCs relative to even trialkylphosphines, the latter also displaying variation depending on the P substituents (2056−2069 cm−1). The diverse reactivity behavior of the NHCs toward [Ni(CO)4] giving three- and four-coordinate complexes in reversible and irreversible CO displacement reactions, respectively, has been attributed to BDE and steric factors, the latter quantifiable by the % buried volume (% Vbur). Complexes 39Ni by virtue of the electron-rich nature of the metal undergo facile polar oxidative additions (e.g., with allyl bromides), leading to η3-allyl complexes. Analogous four-coordinate tetrahedral complexes [Ni(RRcAAC)(CO)3] (RR = Me2, menthyl, cyclohexyl) (40Ni) were only prepared in moderate yields by the reaction with the free RRcAAC. The Ni−CNHC bond distances measured are CJ


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imido complexes of Ni. Thus, the reaction of 43bNi with [Ni(1,5-COD)2] gave the distorted square planar threecoordinate Ni0 bis-carbene η2-COD complex 46Ni. Reaction of 46Ni with ArN3 (Ar = DiPP) gave the η2-side-on bound Ni0 adduct 47Ni, which was stable under thermal and photolytic conditions and structurally comparable to the labile adducts [Ni(IMes)2(η2-AdN3)] obtained with monodentate IMes and AdN3 described above (Schemes 181 and 185). Reaction of

Scheme 183. Synthesis and Reactivity of [Ni(NHC)(η6arene)] Complexes

Scheme 185. [Ni(Chelating-Bis-NHC)(Aryl Azide)] and [Ni(Chelating-Bis-NHC)(Imido)] Complexes and Their Reactivity

InPr and IMe analogues. Furthermore, in comparison to isostoichiometric complexes [NiL2(CO)2] with other donors L = PMe3, PiPr3, PPh3, or L2 = diethylphosphinoethane, bipy, there is clear evidence of the superior σ-donor properties of the NHCs for this metal fragment.342 As mentioned in the previous section, bulkier aryl-substituted NHC ligands on reaction with [Ni(CO)4] provide complexes of the type [Ni(NHC)(CO)3]. However, the tetrahedral complexes [Ni(IMes)2(CO)2] and [Ni(ICy)2(CO)2] were prepared by following an alternative strategy, from the three-coordinate [Ni(NHC)(CO)2] (NHC = ItBu, IAd) with two equivalents of IMes or ICy. The A1 stretching mode of the ICy complex in the IR spectrum appears at 1948 cm−1. Calorimetric measurements based on the substitution of [Ni(NHC)(CO)2] (NHC = ItBu, IAd, by IMes and ICy) gave an estimation of the BDE of Ni−ICy of ca. 30.1 ± 3.0 kcal/mol and Ni−IMes of ca. 27.5 ± 3.0 kcal/mol, in accordance with IR spectroscopy.378 3.2.1.2. Chelating bis-NHC and tris-NHC Ligands (Lig). 3.2.1.2.1. Type [Ni(Lig)L], [Ni(Lig)L2]. The complex 45aNi with the bidentate bis-NHC ligand 43aNi (Ar = DiPP) was prepared by the reduction of the NiII complex 44aNi under 1 atm CO (Scheme 184). The NiII complex 44aNi was in turn available from [NiBr2(DME)] and the NHC 43aNi either generated in situ from the imidazolium salt and base or preformed and isolated.379 The analogous bidentate dicarbene 43bNi has been used to access Ni0 complexes that subsequently were used as precursors for the synthesis of low-coordination-number

46Ni with the bulkier terphenylazide Ar′N3 resulted in N2 elimination and formation of the three-coordinate nickel imido 48Ni, which features a planar Y-shaped formally NiII center, terminal bent imido ligand (ca. 127° at the Nimido) and short Ni-Nimido bond distance (1.73 Å); the geometrical constraints in 48Ni localize the “out-of-plane” nitrogen pz-electrons on the anilide ring. Reaction of 48Ni with FeCp2+ results in an imido ligand-centered one-electron oxidation to give 50Ni, presumably via dimerization of the unobserved mononuclear cationic quinone imine 49Ni.380 The bidentate bis-carbene complexes 51aNi and 51bNi prepared by the reaction of the corresponding in situ generated free bis-carbene ligands with [Ni(1,5-COD)2] differ in the hapticity of the COD and the coordination number at Ni; for R = DiPP, a η4-COD is observed in solution and the solid state, while for R = tBu, a η2-COD is preferred, presumably due to the increased steric congestion at the Ni and the small ligand bite angle (Scheme 186). Complex 51bNi is similar to 46Ni, which also features η2-COD. Fluxionality in solutions of 51aNi was attributed to boat-to-boat inversion of the sixmembered nickelacycle chelate. The solid-state structure of 51aNi shows a distorted tetrahedral geometry with a bite angle of the chelate at ca. 94°. The solid-state structure of 51bNi confirms the η2-coordination of 1,5-COD and a chair chelate conformation. In both 51aNi and 51bNi, the COD ligand is readily replaced by electron-deficient olefins: fumaronitrile

Scheme 184. Complexes with Chelating Bis-Carbene Ligands

CK


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intense absorptions in the 360−600 nm range, which were assigned to metal-to-ligand charge transfer transitions. Analysis of the coordination geometry of the metal using the geometric index τ4 as a tool to classify the four-coordinate geometries led to the description of the geometry in 55Ni as a distorted tetrahedral and that of 57Ni as a C2v sawhorse. The rigidity of the ligand systems and the steric proximity of the methyl wingtips implies that the geometrical rearrangement during the reduction is associated with high activation energy corresponding to the lengthening or breaking of Ni-CNHC bonds. This is reflected in the irreversibility and very negative reduction potential (−2.0 to −2.3 V vs Fc/Fc+) of the NiII complexes found by cyclic voltammetry.382 The rare tris-NHC Ni0 complex 58Ni was prepared by the interaction of the C3 symmetric tripodal tris-NHC ligand with [Ni(1,5-COD)2] (Scheme 188). The C3 symmetry was maintained after complexation, as established by the symmetry of the NMR spectra of 58Ni and by X-ray crystallography in the solid state. The coordination geometry at the Ni is trigonal (coordination angles at 118.7°) with Ni−CNHC bond distances of ca. 1.89 Å; a long Ni−N distance of 3.20 Å excludes nickel− amine interaction. An agostic interaction involving the C(CH3)3 wingtips and Ni may account for slight pyramidalization. Complex 58Ni reacts with one-electron oxidants (BnCl, CH2Cl2, etc.) to form the cationic NiI complex 59Ni which maintains trigonal pyramidal geometry except for the anchoring nitrogen atom now occupying the axial position. The average Ni−CNHC bond distances in 59Ni are longer (ca. 2.00 Å) than in 58Ni, which was tentatively attributed to πbackbonding in the latter complex.383 3.2.2. Mononuclear NiI Complexes. 3.2.2.1. Monodentate NHC Ligands. 3.2.2.1.1. Homoleptic [Ni(NHC)2]+ and [Ni(NHC)3]+. The synthesis of the NiI homoleptic, twocoordinate, formally 13 valence electron complex [Ni(RE-6-

Scheme 186. Substitution and Oxidative Addition Reactivity of [Ni(Chelating-Bis-NHC)(1,5-COD)2]

reacts with 51aNi to give 52aNi which was characterized spectroscopically, and PhCN reacted with 51bNi to give the η2side-on coordinated benzonitrile 52bNi. The coordinated nitrile in the latter undergoes fast C−C activation (65 °C, 6 h) to 53bNi thanks to the highly nucleophilic nature of the Ni0 center coordinated by two alkyl substituted NHCs (Scheme 186).381 3.2.1.2.2. Type [Ni(Lig)2]. The homoleptic square planar NiII complexes 54aNi, 54bNi and 56aNi, 56bNi served as starting materials for the synthesis of the homoleptic Ni0 species 55Ni and 57Ni via the reduction of the square planar (PF6)− salts 54bNi and 56bNi with KC8 (Scheme 187). All NiII and Ni0 complexes were diamagnetic; in addition, the latter showed Scheme 187. Synthesis of [Ni(Chelating-Bis-NHC)]n+ (n = 2, 0)

CL


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Scheme 188. Ni0 and NiI Complexes with a Tripodal Tris-NHC Ligand

Mes)2]Br (7aNi) was mentioned in conjunction with the description of the Ni0 species 7bNi (Scheme 164). It represents a rare two-coordinate NiI species and the only NiI homoleptic NHC complex. Despite its electron deficiency and low coordination number, it is relatively air-stable, presumably due to the metal center protection provided by the mesityl wingtips. The structure of 7aNi revealed a highly linear system with Ni−CNHC distances of ca. 1.94 Å. DFT calculations initially focusing on 7bNi gave an insight into the manifold of the five occupied Ni orbitals with a 2:1:2 splitting pattern and two groups of two approximately degenerate orbitals; removal of one electron maintains the degeneracy, which is important to explain the magnetic properties of 7aNi; however, DFT is limited on how to describe exactly the electronic structure of 7aNi. The orbital degeneracy results in unquenched first-order orbital angular momentum, which generates magnetic anisotropy and single-ion magnet (SIM) behavior for 7aNi. The magnetic moment in solution (Evans’ method) and the solid state (Gouy balance) gave μeff values of 2.2 μB and 2.7 μB, respectively; in addition, 7aNi is EPR silent, a fact also attributable to the unquenched orbital angular momentum leading to large g shifts. SQUID measurements gave χT = 1.12 cm3 K mol−1 at room temperature, exceeding the theoretical spin-only value (0.375 cm3 K mol−1 for a 3d9 center). The discrepancy can originate from the anisotropy of the NiI ion: the partially filled, degenerate highest lying dxz and dyz can interconvert by rotation of 90°, and the orbital angular momentum is not quenched by the ligand field ensuing in magnetic anisotropy. The magnetic relaxation in 7aNi was studied by ac magnetic susceptibility measurements.340 3.2.2.1.2. Heteroleptic [NiX(NHC)], [Ni(NHC)LX], [Ni(NHC)L2]+, and [NiX(NHC)2]. In lower coordination number geometries, the availability of suitable symmetry metal orbitals that can be involved in π-bonding opened the way to the study of two-coordinate heteroleptic Ni(NHC)(amido) complexes and their reactivity. The reaction of the binuclear NiI complex 60Ni (see also section 3.3.1) with NaN(SiMe3)2 gave the paramagnetic (μeff = 1.9 μB) mononuclear complex 61Ni; it features a linear (178.8°), two-coordinate geometry at Ni (Scheme 189). Similarly, the paramagnetic (μeff = 2.3 μB) 63Ni with the primary arylamido ligand was obtained, the structure of which deviates from linearity (ca. 163−167°). Chemical oxidation of complex 61Ni with (FeCp2)(BArF4) gave the diamagnetic NiII complex 62Ni. It features a T-shaped threecoordinate κN-iminosilane, arising from β-Me migration to the Ni from the targeted cation [Ni{N(SiMe3)2}(IPr)]+; 62Ni constitutes a rare example of 14-electron NiII three-coordinate alkyl cation. Oxidation of 63Ni with (FeCp2)(BArF4) gave the NiII complex 64Ni in which the amide adopts an η3heterobenzylic coordination involving N, the ipso-C and one of the o-C atoms; a labile THF molecule was also incorporated in the coordination sphere. The latter can be removed by

Scheme 189. [NiI(Amido)(NHC)] Complexes and their Reactivity

crystallization from CH2Cl2, leading to the binuclear species 65Ni. Upon dissolution of 65Ni in THF, it reverts to 64Ni.384 In an attempt to suppress η3-heterobenzylic formation and observe clean 1 electron redox events, bulkier terphenylsubstituted primary amido ligands were employed. Thus, the reaction of 60Ni with Li(Mes-terphenyl) and Li(DiPPterphenyl) gave the paramagnetic (1.88 μB) complexes 66Ni and 67Ni, respectively (Scheme 190). Although both adopt mononuclear dicoordinate structures, the geometry at Ni deviates substantially from linearity (smallest coordination angle of ca. 112−116°) due to interaction of the metal with the ipso-C of the terphenyl framework. Interactions of the ipso-C of terphenyls attached to other donors or directly to 3d metals have been described. The steric congestion at the Ni coordination sphere is also inferred by the elongated Ni− CNHC bond distances (ca. 1.94 Å, compared to 1.83 Å in the less congested 63Ni). Chemical oxidation of 66Ni with ferrocenium salts gave the unusual 14-electron T-shaped NiII benzyl aniline complex 68Ni featuring a 7-membered “nickelacycle” and Ni-κ-benzyl-κ-amine bonds; the reaction formally constitutes an intramolecular deprotonation of a o-Me of the Mes-terphenyl by the basic amide. The neutral IPr and NH2R ligands occupy mutually trans positions. The cyclic voltammogram of 67Ni shows a quasi-reversible oxidation (E1/2 = −0.63 V vs Fc/Fc+); the chemical oxidation with (FeCp2)(BArF4) gave the paramagnetic NiII d8 complex 70Ni. It has a triplet CM


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ingly, other classical donors, on reaction with 71Ni, lead to the formation of the three-coordinate T-shaped adducts without the intervention of reduction.387 A broader scope transformation under mild conditions employed the homoleptic “ato” complex 71bNi and the weakly acidic NEt3·HCl in the presence of a two electron donor L ligand, including IPr.388 Further aminolysis of the residual N(SiMe3)(DiPP) in 72Ni with a bulky phenol gave cleanly the two-coordinate NiI phenoxide 74Ni. Both 72Ni and 74Ni are one electron paramagnets (2.12 and 1.80 μB, respectively, the deviation from expected value ascribed to spin−orbit coupling). The structures of 72Ni and 74Ni show distorted linear geometries (coordination angle ca. 173.0°), a fact attributable to electronic factors and crystal packing; the Ni−CNHC bond distance was in the range of 1.86−1.88 Å (Scheme 191). The complexes 75Ni and 76Ni feature one bulky alkyl and one bulky terphenyl attached to the Ni(IPr) fragment, respectively (Scheme 192). They were prepared from 60Ni by salt metathesis reactions with ClMg−CH(SiMe3)2 and Li(Mes-terphenyl), respectively; smaller alkyls (e.g., Bn) led to homocoupling of the benzyl group and the complex [Ni(IPr)]2 (Scheme 266). Both 75Ni and 76Ni are paramagnetic in solution (1.8−1.9 μB) and adopt distorted linear geometries (angles at Ni ca. 174.8/175.8°) with Ni−CNHC and Ni−Calkyl bond distances of 1.91/1.92 and 1.97/1.95 Å, respectively. DFT calculations show that the SOMO is Ni-localized and therefore the metal oxidation state NiI. Association of 75Ni with tBuNC led to the paramagnetic three-coordinate adduct 77Ni. Reactions of 75Ni with alkyl halides were studied in conjunction with the Ni-catalyzed cross-coupling. Reaction of 75Ni with 1-bromo-1-phenylethane gave only the homocoupled 2,3-diphenylbutane and no cross-coupling, suggestive of a radical reaction; in addition, the Y-shaped (rather than the more common T-shaped) NiII complex 78Ni was the only Nicontaining product which could form by addition of the Br to 75Ni after homolysis of the C−Br bond of the alkyl bromide. Reaction of Bn-Br with 75Ni gave as Ni-containing products a mixture of the complex 78Ni and [Ni(μ-Br)(IPr)]2 (in ca. 1:4 ratio) and as organic products the homocoupled dibenzyl and 1,1,2,2-tetra(trimethylsilyl)ethane and the heterocoupled 1,1bis(trimethylsilyl)-2-phenylethane (in ca. 1:2:4 ratio). Finally, reaction of 75Ni with CHBr(SiMe3)2 gave 78Ni and [CH(SiMe3)2]2. The nature of the organic products as a function of the structure of the secondary alkyl bromide used and the absence of alkyl scrambling (e.g., when using CDBr(SiMe3)2 with 75Ni) points to the involvement of radical species rather than mononuclear oxidative addition (NiI to NiIII) or bimolecular oxidative addition (Scheme 192).389 Metrical data in the NiII complex 78Ni show Ni−CNHC and Ni−Calkyl bond distances shorter than in the NiI 75Ni. The two- and three-coordinate NiI complexes 80Ni and 81Ni were prepared by the reaction of one equiv of the corresponding sodium amidate with [NiCl2(IPr)(PPh3)] leading to the isolable NiII species 79aNi and 79bNi, which in a subsequent step were reduced with Na/Hg (Scheme 193). Alternatively, 80Ni and 81Ni were prepared from the binuclear complex 60Ni by the reaction of one equiv (per Ni) with the corresponding sodium amidate; the analogous 82Ni with R2 = iPr was only available by the latter method. All NiI complexes were mononuclear and one electron paramagnets. Their exact structures were dependent on the substituents of the anionic amidate. Thus, with the DiPP-substituted amidate, κ1-Ocoordination and η2-π interaction of the aromatic ring with the

Scheme 190. [NiI(Terphenyl-amido)(NHC)] Complexes and their Reactivity

ground-state (3.20 μB), which was also confirmed by DFT. It is plausible that the iPr in the DiPP-terphenyl prevents C−H activation as seen in 66Ni on steric grounds. Interestingly, geometry optimization of the singlet state by DFT shows similar Ni-N interactions and metrics except the presence of an η6-coordination of one of the 2,6-diisopropylphenyl groups of the terphenyl ligand to the NiII, which corresponds to an 18electron complex (Scheme 190).385 The two-coordinate complex [Ni{N(SiMe3)(DiPP)}(IPr)] (72Ni) with the bulky N(SiMe3)(DiPP) amido ligand was prepared by the reaction of the homoleptic NiII bis-amido complex 71aNi with IPr at elevated temperatures (Scheme 191).386 The Ni is reduced in a reaction that formally constitutes substitution of N(SiMe3)(DiPP) by IPr. InterestScheme 191. Synthesis of [Ni(NHC)(Amido)] Complex from a “Ni-ate” Precursor

CN


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Scheme 192. [NiI(Alkyl)(NHC)] and Related Complexes and their Reactivity

Scheme 193. [Ni(Amidate)(NHC)] System

supporting δ-bis(C−H) agostic (or bifurcated η3-H2C) interactions, resulting in an overall pseudo-T-shaped coordination geometry (in 81Ni). Finally, with the iPr-substituted

NiI center were observed (in 80Ni). With the tBu-substituted amidate, the structure of the isolated complex revealed κ1-N amidate coordination in addition to the presence of dualCO


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amidate, an unsupported nonchelating κ1-N amidate was obtained (in 82Ni). The absence of any agostic interaction in 82Ni was interpreted as arising from the release of steric congestion that may be responsible for bringing the C−H bonds in the proximity of the NiI center. The group of complexes 80Ni−82Ni provides evidence for the versatility of the amidate ligand on NiI centers (Scheme 193).390 Reaction of 81Ni with one equivalent of (2,6-xylyl)NC led to the T-shaped NiI adduct 83Ni after cleavage of the agostic interactions, while addition of a second equivalent led to clean disproportionation of the latter to the Ni0 complex 84Ni and the NiII complex 85Ni. Mechanistic evidence discounts the involvement of radicals and supports a bimolecular mechanism in which the amidate acts as hemilabile ligand (Scheme 193).377 Complexes of type [NiIX(NHC)L] have been obtained with a limited number of bulky NHCs. Addition of L (L = PPh3, P(OPh)3 or pyridine) to 60Ni resulted in the generation of the three-coordinate Y-shaped monomeric NiI complexes 86aNi− 86cNi (Scheme 194). In solution, PPh3 can easily dissociate

complexes. In situ NMR spectroscopic studies show that when isolated 86aNi−86cNi were dissolved in benzene, 60Ni is formed by dissociation of L; the latter reverts to 86aNi−86cNi by addition of excess L (3−5 equiv). Due to the unavailability of the IMes analog of 60Ni, complex 87Ni was prepared by the reaction of IMes with a mixture of [Ni(1,5-COD)2] and [NiCl2(PPh3)2]. Magnetometry, EPR, and computational methods show that 86aNi−86cNi and 87Ni are single-electron paramagnets with the SOMO being mainly of Ni character. Complexes 86aNi−86cNi were active in the Suzuki coupling and the amination of aryl bromides with the best catalyst being 86aNi (Scheme 194).391 In particular, the unusual steric and electronic properties of extended ring NHCs proved instrumental for the isolation of mononuclear, low coordination number NiI complexes. Thus, the distorted trigonal planar, paramagnetic 88aNi−88gNi with a NiI center were prepared from [Ni(1,5-COD)2] and various extended ring NHCs in the presence of [Ni(PPh3)2Br2] via a comproportionation reaction (Scheme 195). In solution, Scheme 195. Synthesis and Reactivity of [Ni(Halide)(NHC)(PPh3)] (NHC = Ring Expanded NHC)

Scheme 194. Transformations Related to [Ni(Halide)(NHC)L] (NHC = Imidazolylidene)

from 86aNi to form the monomeric two-coordinate moiety [NiCl(IPr)] in situ or the dinuclear 60Ni; 86aNi can thus be considered as a possible resting state of reactive [NiCl(IPr)] under catalytic conditions. The reaction of 60Ni with the diphosphines 1,2-bis(diphenylphosphino)ethane (dppe) or 1,3-bis(diphenylphosphino)propane (dppp) gave mixtures containing [Ni0(diphosphine)2] and [NiIICl2(IPr)2]; however, the reaction with 1,4-bis(diphenylphosphino)butane gave the binuclear NiI complex 86dNi, which converts only slowly to [Ni0(diphosphine)2] and [NiIICl2(IPr)2]. These results may be accounted for by the need for close proximity of the NiI centers to disproportionate via electron transfer to Ni0 and NiII

88aNi−88gNi did not show any evidence of a monomer− dimer equilibrium or dissociation of the NHC or PPh3; in this respect, they differ from the T-shaped [NiCl(IPr)2] (see below). The EPR spectra of 88aNi−88gNi display a rhombic g parameter and are broadened due to large superhyperfine coupling to the 31P and 79,81Br nuclei. The EPR spectral parameters as a function of the distortion in the trigonal planar CP


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Scheme 196. Transformations Involving the [NiI(Halide)(NHC)(PPh3)] (NHC = Ring Expanded NHC)

90Ni, which could not be isolated free from TlBr (Scheme 196). Reaction between 88aNi and NaBArF4 led to the unusual cationic, paramagnetic (2.51 μB in ether solution), monobromide bridged dimer 91Ni, featuring a single distorted linear Br bridge. Magnetic measurements with 91Ni are consistent with the presence of two NiI centers, which by DFT calculations were predicted to couple weakly antiferromagnetically with a negative exchange coupling. EPR spectroscopy of 91Ni supported dissociation of the dimer in ether to two complexes, 88aNi, and a second species, the solvated [Ni(RE-6Mes)(PPh3)(etherate)]+. Treatment of 88aNi with an equimolar amount of [(Et3Si)2(μ-H)][B(C 6F5 )4] gave the mononuclear 92Ni which features an η2-bound toluene ligand that originated from the crystallization of an initially formed species from toluene. Exposure of a THF solution of 90Ni or 91Ni to 1 atm CO gave the green/yellow T-shaped 93Ni (ν(CO) 2032 cm−1). Reaction of NaBH4 with 88aNi in the presence of ethanol gave the rare binuclear borohydride complex 94Ni with two asymmetric borohydride bridges as deduced by the metrical data; each borohydride can be described as adopting either a μ2,η1:η1 or a μ2,η2:η1 coordination mode (Scheme 196). The intermetallic separation (ca. 2.43 Å) implies the presence of a Ni−Ni bond in agreement with the observed diamagnetism; however, the borohydrides appear to be fluxional. The first mononuclear NiI cyanotrihydroborate 95Ni was obtained by the reaction of 88aNi with NaBH3CN. Finally, reaction of 88aNi with NaOtBu in the presence of NHPh2 provided the trigonal planar NiI amido complex 96Ni with Ni−N bond length of 1.92 Å; the adopted coordination geometry and the pyramidalization at the N were attributed to steric factors. EPR spectra and

geometry were analyzed by theoretical methods. It was found that the SOMO in 88aNi−88gNi has a mixed 3dz2 and dx2−y2 character, the degree of admixture being dependent on the ligand nature and the geometrical parameters imposed, which are also reflected by the magnitude of the g-tensor components. The complex 88iNi featuring the electronwithdrawing DAC NHC was prepared similarly. Interestingly, reaction of one equiv of the bulky RE-6-Mes with [Ni(1,5COD)2] gave the C−H activated 6-membered N-heterocyclic carbene 6-Mes 88hNi; the η3-cyclooctenyl coordination, presumably arising from the insertion of the 1,5-COD in the Ni−H bond formed after C−H activation. Thermolysis of 88hNi at 70 °C afforded an N-alkyl indole derivative.392,393 Activation of O2 by the complexes 88aNi and 88eNi gave the binuclear NiII complexes 89aNi and 89bNi after fast C−H oxygenation of the o-methyl of the Mes wingtips. However, reaction of O2 with the less bulky analogs 88cNi and 88dNi gave the mononuclear NiII complexes [NiBr2(RE-6-o-tolyl)(PPh3)] and [NiBr2(RE-7-o-tolyl)(PPh3)], respectively; oxygenation of the complex 88iNi gave PPh3O and oxidized DAC-NHC.394 Complex 88eNi was found to be a precatalyst for the reductive dehalogenation of fluoroaryl-halides in the presence of NaOiPr as source of hydrogen. Aryl C−F bonds reacted with difficulty: no hydrodefluorination was observed for either p-BrC6H4F or p-ClC6H4F (Scheme 195).392,393 Attempts to prepare cationic, low coordination number complexes by halide abstraction from 88aNi led to a range of complexes, the exact nature of which was dependent on the type of halide abstractor employed and the reaction/ crystallization conditions. The use of TlPF6 led to the mononuclear, paramagnetic, distorted T-shaped THF adduct CQ


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Scheme 197. Synthesis and Reactivity of NiI(η5-Cp)(NHC)-Type Complexes

(DAC)}] (97iNi).398 Lastly, the complexes 97eNi, 97fNi with aryl-substituted Cp were obtained by the reduction of the binuclear-substituted cyclopentadienyl bromide with KC8 in the presence of NHCs.399 Importantly, a salt metathetical methodology with one equivalent of NaCp or LiInd gave the diamagnetic binuclear complexes 99aNi, 99bNi and 100aNi, 100bNi featuring a NiI2(μ-Cl) core bridged by one cyclopentadienyl and one indenyl group, respectively, and inseparable minor amounts of 97aNi, 97bNi and 98aNi, 98bNi. The 1H NMR spectra of 97aNi−97iNi and 98aNi, 98bNi are paramagnetically shifted (in the range from δ +20 to −50 ppm). Their structures in the solid state are not symmetrical: they comprise planar η5-Cp or -indenyl ligands with equal C− C distances within the five-membered rings, CNHC-Ni-Cp(centroid) or CNHC-Ni-indenyl(centroid) angles subtended at Ni in the range of 154−165° (with higher values observed with the bulkier RE-Mes NHCs and Cp*) and NHC yaw of ca. 7−

calculations were used to differentiate the electronic structures of T-shaped and Y-shaped geometries within the above family of complexes. The SOMO in T- and Y-shaped complexes has dx2−y2 and dxy character, respectively, and the T-shape is generally favored for d9 complexes, but a Y-shape results in increased charge transfer from the ligands to the metal.395 The salt metathetical reaction of the binuclear [Ni((S)IPr)(μ-Cl)]2 (60Ni) with two equivalents of sodium cyclopentadienyl or lithium indenyl gave the mononuclear paramagnetic, 17 valence electron complexes 97aNi, 97bNi and 98aNi, 98bNi (Scheme 197). An alternative method to access 97aNi, 97bNi and the related IMes and Cp* complexes 97cNi, 97dNi consisted of reduction of the NiII precursors [NiCl(η5Cp)(NHC)] or [NiCl(η5-Cp*)(NHC)] with KC8.396,397 Reduction of [NiBr(η5-Cp)(RE-Mes)] and [NiBr(η5-Cp){RE-6-Mes(DAC)}] with KC8 led to the complexes [Ni(η5Cp)(RE-Mes)] (97gNi, 97hNi) and [Ni(η5-Cp){RE-6-MesCR


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8°. The magnetic susceptibilities of solid samples of 97aNi, 97bNi and 98aNi, 98bNi at room temperature fall in the range of 1.9−2.1 μB, in agreement with one electron paramagnets. Furthermore, the EPR spectra for all 97aNi−97iNi and 98aNi, 98bNi display rhombic tensors g1 ≅ 2.36, g2 ≅ 2.30, g3 ≅ 2.05, implying that the SOMO has some metal character with dxz orbital contribution (Scheme 197). This was also confirmed by DFT calculations. A systematic study of the EPR spectra of complexes 97gNi, 97hNi, and 97iNi revealed the strong dependence of the spectral parameters on the nature of the NHC ligand. The sensitivity of the g-tensor components to the electronic properties of the ligands on NiI highlighted the πaccepting properties of Mes(DAC) and revealed the presence of an additional NiI site at lower temperatures which appeared as disorder in the crystal structure of 97iNi.398 The diamagnetic binuclear species 99aNi, 99bNi and 100aNi, 100bNi have approximate Cs symmetry with planar Cp or indenyl ligands bridging, through a (−CH−)3 segment of the five-membered ring; the Ni-CCp bond distances suggest that only the central C atom of the (−CH−)3 segment is bound to both centers of the binuclear Ni2 core while the two adjacent C atoms bind to only one Ni center. However, in solution the structure is fluxional even at lower temperatures as corroborated by the single peak observed for the Cp ring in both the 1H and 13C NMR spectra. The Ni−Ni bond distances are ca. 2.40−2.44 Å, consistent with the presence of a Ni−Ni single bond (Scheme 197). The presence of small inseparable amounts of 97aNi and 97bNi during the synthesis of the binuclear 99aNi and 99bNi by salt metathetical reactions pointed to the establishment of equilibria involving the diamagnetic binuclear 99aNi and 99bNi, the paramagnetic mononuclear 97aNi and 97bNi, and the “Ni(NHC)Cl” moiety (Scheme 198). For example, the 1H

bridging indenyl-containing 100aNi and 100bNi undergo dissociation at temperatures up to 70 °C or can be involved in crossover products, even though the mononuclear 98aNi can react with [Ni(μ-Cl)(NHC)]2 to form inert diamagnetic binuclear species, even isolable nonsymmetrical analogues after a suitable choice of the reactants.397 Complex 97aNi exhibits diverse stoichiometric redox reactivity leading to Ni0 or NiII complexes. The reaction of 97aNi with the chelating dienes divinyltetramethyldisiloxane (dvtms) and diallylether led to Ni0 NHC complexes 13lNi and 13mNi, respectively (Scheme 199), with concomitant dissociation of the Cp ligand and dimerization to various isomers of dicyclopentadiene (C10H10). The complex [Ni0(IPr)(CO)3] (39Ni) was obtained from the reaction of 97aNi with CO (see also above). The reaction with MeI led to a 1:1 mixture of NiII complex products 101Ni and 102aNi. A plausible mechanism for this reaction involves oxidative addition of MeI to 97aNi to generate a transient NiIII species, which could undergo comproportionation with additional 97aNi to generate the two NiII products; alternatively, a homolytic radical mechanism is possible. Surprisingly, 97aNi does not react with other strong σ-donors like PMe3 or pyridine. Chemical oxidation of 97aNi with (Fc)(PF6) in THF gives the cationic NiII THF adduct 103Ni and the corresponding THF-free cation 104Ni after exposure to vacuum. Complexes 97aNi−97fNi (Scheme 197) act as precatalysts in the Suzuki-Miyaura coupling reaction of phenyl chloride with phenylboronic acid. The binuclear species 99aNi, 99bNi with bridging Cp ligands (Scheme 197) are also catalytically active, but those with indenyl bridging groups 100aNi 100bNi are not, which may be related to their diminished lability and difficult generation of the active catalyst. Although the activity of the catalytic system is not high, it raises interesting mechanistic questions, in particular the possible involvement of NiI−NiIII catalytic cycle.397 The complex 97aNi thanks to its metalloradical character easily reacts phenyl disulfide to give [Ni(SPh)(η5-Cp)(IPr)]. With the persistent free radical TEMPO, it afforded the adduct 105Ni with side-on η2-coordinated TEMPO and concomitant η1-Cp rearrangement giving rise to a distorted square planar NiII center (Scheme 199). On the basis of the N−O, Ni−N, and Ni−O bond distances, the coordinated TEMPO is formally considered as an anionic ligand. However, the appearance of the cyclopentadienyl protons as a sharp singlet even at lower temperatures points to a facile haptotropic rearrangement. Complex 97aNi reacts with P4 to give selectively the binuclear complex 106Ni with a butterfly tetraphosphide bridge. The solid-state structure of 106Ni unveils an exo/exo arrangement for the two Ni(η5-Cp)(IPr) units with P−P bond distances (ca. 2.21−2.23 Å) similar to those found in free P4 (ca. 2.21 Å), implying negligible activation. The reaction 97aNi with S8 (1/8 equiv) gave a mixture of products (107Ni and 108Ni) which could be separated due to their different solubilities; the structure of 108Ni with a (S3)2− bridge is particularly uncommon. An analogous product mixture [but enriched in the (Se2)2− complex] was obtained by the reaction with Se∞ with 97aNi, while with metallic Te∞ only the ditelluride was obtained in a slow and low yielding reaction (Scheme 199).396 The reaction of 97aNi with PhNCS gave a mixture of products: the complex 13qNi described previously (Scheme 171) and the NiII species [Ni(η5-Cp)(η1-Cp)(IPr)] (142Ni)

Scheme 198. Exchange and Dimerization Equilibria in the [Ni(η5-Cp)(NHC)] System

NMR spectrum of a crystallized sample of 99aNi at −50 °C was consistent with the presence of a single diamagnetic species but at 70 °C with the presence of the paramagnetic 97aNi as the sole Cp-containing component. Indirect evidence for the existence of an equilibrium was obtained by a crossover experiment (Scheme 198) although the nonsymmetrical crossover product was not isolated. Furthermore, reaction of the mononuclear 97aNi with 60Ni led to the formation of the diamagnetic binuclear 99aNi. In contrast to bridging Cpcontaining 99aNi and 99bNi, there is no evidence that the CS


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Scheme 199. Diverse Reactivity of [Ni(η5-Cp)(NHC)] Complexes

Scheme 200. Reaction of [Ni(η5-Cp)(NHC)] with Na(PnCO) (Pn = P, As)

butterfly like cores [Ni(NHC)(CO)]2(μ2:η2:η2-(Pn)2); the P−P bond distance in 110aNi (2.08 Å) is close to the value associated with PP double bond (2.04 Å) (Scheme 200). The As−As bond distance in 111aNi is ca. 2.30 Å. NMR spectroscopic data support the presence of the solid-state structures in solution. Reaction of 97aNi with 0.5 equiv of [Na(dioxane) x ][PCO] gave the diamagnetic [{Ni(IPr)}2(μ2:η5,η5-Cp)(μ2:η2,η2-PCO)] (112aNi), the structure of which features a symmetrical binuclear species with NiI2

(Scheme 213). It is plausible that after association of the isothiocyanate reagent with 97aNi, the η5-Cp undergoes rearrangement to η1-Cp followed by homolysis of the Ni-CCp bond to provide a Ni0NHC species, on which the two isothiocyanates would undergo a reductive coupling as described previously for 13qNi.358 Reaction of 97aNi or 97cNi with the anions of the [Na(dioxane)x][PnCO] (Pn = P, As) salts (one equiv) gave the complexes 110aNi, 110aNi and 111aNi, 111aNi with CT


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core and bridging Cp− and PCO− ligands; it is related to 99aNi since the bridging chloride in the former was replaced by the pseudohalogen (PCO)−. The bridging P and C atoms are equidistant from the two NiI centers. The 31P NMR resonance of P atom in 112aNi is found at δ −293.1 ppm, in the same spectral region as the metallaphosphaketenes. The side-on coordination of a (PCO)− ion has been observed before in the Cu(cAAC) complex (Scheme 26). Although the binuclear complex 112aNi is stable enough to be fully characterized, the analogue 112bNi has transient existence and its structure was deduced from the similarity of its NMR data to those of 112aNi. However, the reaction of 97cNi with 0.5 equiv. [Na(dioxane)x][AsCO] gave the stable final product 113bNi, which is a NHC-phosphinidenyl bridged binuclear species with two nonsymmetrical NiI centers, featuring 16 and 18 valence electron counts (Scheme 200). The Ni−Ni intermetallic separation (2.40 Å) supports the presence of Ni−Ni bond. It was suggested that the transient 112bNi is intermediate to the formation of 113bNi, the latter being obtainable by decarbonylation of the bridging PnCO to a bridging phosphinidyne, followed by migration of the IMes from the NiI to the latter. In an analogous manner, the reaction of 97cNi with 0.5 equiv of [Na(dioxane)x][AsCO] gave the binuclear complex 114bNi. Attempts to prepare mixed μ2:η2,η2-PAs bridged analogues of 110Ni or 111Ni by reacting the binuclear 112Ni with the [AsCO] anion resulted only in 31P NMR spectroscopic characterization of the desired species; however, the undesired formation of As2 and P2 complexes rendered the separation and full characterization difficult (Scheme 200).400 The complexes [NiIX(NHC)2] (NHC = IMes, IPr, X = halide) have been prepared during attempts to study the oxidative addition of aryl halides to [Ni(NHC)2] complexes; the latter reaction is an important step in the homogeneous Nicatalyzed cross-coupling reactions. Unexpectedly, the reaction with activated or deactivated aryl chlorides, bromides, and iodides in THF gave the T-shaped, three-coordinate, air sensitive and paramagnetic mononuclear complexes 115aXNi and 115bXNi (Scheme 201). Although the metal in 115aXNi and 115bXNi is found in the oxidation state NiI, there is no evidence for further reaction of the NiI species with ArX. However, rearrangement of 115aXNi, 115bXNi to the binuclear NiI complex 60Ni and oxidation to trans-[NiCl2(IPr)2] has been observed in C6H6 and chlorinated solvents, respectively. In the molecular structures of the T-shaped mononuclear complexes 115aXNi, 115bXNi, the Ni−CNHC bond distances and the obtuse coordination angle are in the range of 1.91− 1.93 Å and ca. 165−168°, respectively. DFT calculation show that the SOMO has a dx2−y2 Ni character.401 More detailed studies on the accessibility of 115aXNi, 115bXNi showed that they can also be formed by the disproportionation of an equimolecular mixture of [Ni(NHC) 2 ] and [Ni(halide)2(NHC)2] (NHC = IPr, IMes, halide = Cl, Br, I), the substitution of PPh3 by IPr or IMes in the adducts trans[Ni(Ar)(X)(PPh3)2], or the photolysis of trans-[Ni(H)X(IMes) 2], which is accompanied by evolution of H2. Importantly, the reaction of [Ni(1,5-COD)2] with the smaller Me2 IMe and aryl bromides gave the square planar NiII species, implying that the preferential formation of 115aXNi, 115bXNi occurs to avoid sterically congested square planar NiII complexes (Scheme 201).402,403 The oxidative addition of aryl halides to [Ni(PEt3)4] complexes was studied earlier by Kochi, giving rise to nickel(I)-aryl-halide radical anions, which

Scheme 201. Synthesis of Complexes [Ni(Halide)(NHC)2]

can convert to [NiIIArXLn] or [NiIXLn] complexes stabilized by the phosphine. The stoichiometric reactivity of 115aXNi (X = Br) was studied in relation to elementary steps postulated in Kumada and Suzuki cross-coupling reactions (Scheme 202). Reaction of 115aXNi with one equiv MgBrMes in C6D6 led to a mixture of unidentifiable Ni species and mesitylene as the sole organic product (reaction B). However, arylation of 115aXNi with BPh(OH)2 and BPh(OH)2/KOtBu gave biphenyl in 30 and Scheme 202. Stoichiometric Reactions Related to CrossCoupling by [NiX(NHC)2]

CU


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Scheme 203. Synthesis and Reactivity of [NiII(Imido)(NHC)] Complex

structure comprises a linear (ca. 174.2°) Ni center with very short Ni−N bond distance (1.66 Å) and nearly linear imido group (171.6°). These metrical data are in agreement with the presence of π-bonding; the heterocyclic plane of the IPr* and the plane of the central C6H3 aromatic group of the Mesterphenyl form a dihedral angle of ca. 41°. SQUID magnetometry measurements show that 119Ni has a triplet ground state in the solid-state (2.77 μB) with large zero field splitting (D = 24 cm−1). DFT calculations demonstrated that one σ* antibonding (dz2) orbital was stabilized (becoming nonbonding) by a symmetry-allowed admixing with the 4s Ni orbital; the two degenerate SOMO π* molecular orbitals (3dxz, 3dyz) are responsible for the multiple bonding. Based on the d orbital occupation, the bond order is 2. Complex 119Ni reacts with excess CO, transferring the imido group to give the corresponding isocyanate with concomitant reduction of the Ni to [Ni(IPr*)(CO)3] (Scheme 203). The N-(Mes-terphenyl) can also insert into the C−H bond of ethene (1 atm), via the aza-nickelacyclic intermediate 121Ni, formed by [2 + 2] cycloaddition of the CC and NiN bonds that is observable by NMR spectroscopy. The N-vinyl aniline that was isolated as organic product indicates that the productive fragmentation of the aza-nickelacycle involves either 1,2-hydride shift or β-hydride elimination followed by N−H reductive elimination; homolytic H−CHCH2 abstraction pathways involving the “diradical” 119Ni leading to the Nvinyl aniline are not likely. Interestingly, the facile reactivity of 119 Ni with ethene should be contrasted to its slow aziridination by the three-coordinate [Ni(=NDiPP)(PP)] (PP = (tBu)2PCH2CH2P(tBu)2). It is plausible that steric constraints may prevent the attainment of T-shaped geometries in 121Ni that are necessary for the C−N reductive elimination leading to aziridination. Therefore, H transfer reactions are more favorable (Scheme 203).356 Mononuclear, three-coordinate NiII(NHC) complexes are rare. Complex [Ni(alkyl)Br(NHC)] (78Ni) was discussed above in conjunction with the reactivity of two-coordinate 75Ni (Scheme 192).389 A systematic study of the nickel-catalyzed selective hydrogenolysis of diaryl ethers to arenes and phenol in the system comprising [Ni(1,5-COD)2]/SIPr·HCl/NaOtBu/H2 was carried out in order to gain insight into the nature of the catalytic

100% yields, respectively (reactions D and E). The origin of mesitylene may be traced to mesityl radical formation catalyzed by [NiIBr(IMes)2], which then abstracts a H· radical from the solvent. It has been proposed, based on the evidence presented above, that the reaction of [NiIBr(IMes)2] with BPh(OH)2 gave the unstable [NiIPh(IMes)2], which is susceptible to the formation of Ph radicals that dimerize to biphenyl. The stoichiometric reactivity presented here can eliminate the involvement of two electron oxidative addition steps for the Kumada-type cross-coupling in the system Ni/ bulky NHC. The involvement of NiI/NiIII cycle (e.g., by the reaction of aryl halides with [NiI(Ar)(IMes)2]) or bimolecular mechanisms is plausible.403 Under catalytic conditions, 115bXNi (X = Cl), in the presence of excess IPr, is an active catalyst for the Kumada coupling of Ph-Br or p-OMe-C6H4Br with Ph-MgBr, resulting in high yields (89−93%) of heterocoupled product (1 mol % of 115bXNi, 5 mol % IPr, THF, room temperature, 18 h);401 in contrast, it is inactive in the Suzuki cross-coupling reactions. Interestingly, the mononuclear Ni complex 86aNi (see above Scheme 194) was active in Suzuki cross-coupling reactions.391 The only NiI complex stabilized by three NHC donors is 59Ni with a tripodal ligand arrangement. It was obtained by oxidation of the Ni0 precursor and has been described above (Scheme 188).383 3.2.3. Mononuclear NiII. 3.2.3.1. Monodentate NHC Ligands. 3.2.3.1.1. Heteroleptic [Ni(NHC)X2]. The concept of accessing stable but reactive late transition metal complexes with multiply bonded ligands under low coordination number environments, while sterically preventing aggregation, was showcased by the synthesis of the two-coordinate [NiII{=N(Mes-terphenyl)}(IPr*)]. The lower valence electron counts and the availability of nonbonding orbitals of suitable symmetry and energy for π-bonding justifies the rationale for this approach. The olive-green 14 valence electron Mes-terphenyl imido complex 119Ni was prepared in good yields by the reaction of Mes-terphenylazide with [Ni0(IPr*)(η6-toluene)] (118Ni) under elimination of N2 (Scheme 203). The latter reagent was obtained by the heterogeneous reduction of [NiIICl2(IPr*)(THF)] with Mg turnings. Complex 119Ni is thermally stable under inert atmosphere in the solid state or in solution in hydrocarbon solvents or THF. The molecular CV


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possibly via a multistep sequence, of the paramagnetic (two unpaired electrons) NiI dimer 122Ni, which was catalytically inactive (Scheme 204).361 3.2.3.1.2. [NiX2(NHC)L], [NiX2(NHC)L2], [NiX2(NHC)2]. Partially or fully fluorinated penta- and hepta-nickelacycles stabilized by bulky NHCs were isolated and characterized in attempts to study mechanistically catalytic derivatization reactions of tetrafluoroethylene or expand the reactivity of nickelaperfluorocyclopentane complexes, which are already established with P donor (e.g., phosphines and phosphites) or bipy ligands, respectively. The cross trimerization reaction of tetrafluoroethylene, ethylene, and aldehydes was developed as a versatile method for the synthesis of fluorinated ketones catalyzed by species formed from [Ni(1,5-COD)2] and monodentate phosphine or NHC ligands. The efforts to isolate catalytically relevant or competent organometallics focused on the employment of IPr as spectator ligand. Thus, the reaction of TFE and ethylene in the presence of an equimolar mixture of [Ni(1,5-COD)2] and IPr resulted in the isolation of a seven-membered nickelacycle, [Ni(CF2CF2CH2CH2CF2CF2)(IPr)] (123Ni) featuring distorted T-shape geometry and two “cis-”Ni fluoroalkyl bonds; it was formed by the oxidative coupling of two TFE and one ethylene molecules (Scheme 205). In the absence of aldehyde, the postulated catalytically important five-membered nickelacycle formed by the coupling of one TFE and one ethylene molecule could not be observed or isolated. Under forcing conditions, complex 123Ni proved to be unreactive and catalytically irrelevant (Scheme 205).404 The reaction of the four-coordinate nickelaperfluorocyclopentane complexes stabilized by the phosphite ligands P(OR)3 (R = iPr, o-tolyl) with ItBu and SIPr gave the four-coordinate mixed phosphite-ItBu or three-coordinate T-shaped SIPr nickelaperfluorocyclopentanes 124aNi and 124bNi, respectively. Complex 124bNi undergoes abstraction of the α-F to Ni by reaction with the electrophilic TMSOTf, which is followed by an unusual migration of the fluoroalkyl to the reactive carbon center giving the novel perfluorocyclobutyl complex 125Ni via Ni-Cfluoroalkyl bond cleavage; no dissociation or migration of the NHC was observed during these sequences. With the less acidic trifluoroacetic and mesitylacetic acids, functionalized nickelacycles 126Ni were obtained (Scheme 205).405 Heteroleptic NiII complexes with one NHC and another classical donor L (L = monodentate phosphines or substituted pyridines) have been prepared and studied for various catalytic applications. The initial synthetic method using selective substitution of one PPh3 in [NiX2(PPh3)2] (X = Cl, Br) worked satisfactorily with IPr, where reaction with one and two equivalents of IPr led sequentially to the complexes trans[NiX2(IPr)(PPh3)] (127aCl/BrNi) and trans-[NiX2(IPr)2] (128aCl/BrNi), respectively (Scheme 206).406 Better yields of 127aCl/BrNi as well as the complexes trans-[NiX2(ItBu)(PPh3)] (127bCl/BrNi) were obtained by using [NEt4][NiX3(PPh3)] (X = Cl and Br) as NiII source. Selective substitution of the PPh3 in 127aBrNi and 127bBrNi with the stronger σ-donating PCy3 led to the species trans-[NiX2(IPr)(PCy3)] (127cBrNi) and trans-[NiX2(ItBu)(PCy3)] (127 dBr Ni ), respectively. 407 Alternatively, 127cCl Ni , trans[NiX2(ItBu)(PCy3)] (127dClNi) and [NiX2(IPr*)(PCy3)] (127eCl N i ) were obtained from the reaction of [NiCl2(PCy3)2] with one equiv of in situ generated IPr, ItBu, or IPr*, respectively.408 Reaction of [NiCl2(PPh3)2] with in situ generated CycAAC gave only transiently trans-

species and the role of the base, which was not only limited to the generation of the free SIPr.361 Initial attempts by 1H NMR and UV−vis spectroscopy confirmed that at 100 °C in C6D6 (conditions similar to those employed during catalysis), the preformed complex [Ni(SIPr)2] converted to a (1:1) mixture of [Ni(SIPr)(η6C6D6)] and free SIPr. [Ni(SIPr)(η6-C6D6)] was also found in aliquots from the reaction of Ph2O with H2 catalyzed by the combination of [Ni(1,5-COD)2] and SIPr with excess NaOtBu, implying that it can be considered as catalyst resting state. Similar complexes were prepared by the reaction of [Ni(1,5-COD)2] with NHCs in the presence of H2 in aromatic solvents (Scheme 183). Furthermore, the substrate Ph2O can displace η6-arenes from [Ni(SIPr)(η6-C6H6)], producing complexes of type [Ni(SIPr)(η6-C6H5−OPh)] (16cNi, Scheme 173), which can be considered as the species preceding the activation of Ph2O by the Ni0. Despite characterizing 16cNi, a plausible product from the activation of C6H5−OPh by the oxidative addition of the C−O bond (i.e., [NiIIPh(OPh)(SIPr)]) was difficult to obtain. However, the three-coordinate T-shaped complex 16aNi of limited thermal stability, which can be considered as a product of the oxidative addition of benzofuran to [Ni0(IPr)], was independently synthesized by a combination of salt metathesis and C−Br activation reactions and was fully characterized (Scheme 204). It featured the SIPr Scheme 204. Reactivity of Complexes 16aNi and 16bNi as Models for the Hydrogenolysis of Diaryl Ethers to Arenes and Phenol

donor trans to the phenoxide rather than the σ-phenyl donor by virtue of the difference in corresponding trans-influences. Under 1 atm of H2 in C6D6, complex 16aNi underwent hydrogenolysis of the Ni−Cphenyl and Ni−Ophenyl bonds and was converted to [Ni(SIPr)(η6-C6D6)] and the stable 16bNi, which could also be independently synthesized from [Ni(SIPr)(η6-C6H6)] and 2-hydroxy-1,1-biphenyl; it constituted a rare phenol complex of Ni0 (see also Schemes 173 and 183). Although 16bNi was stable in C6D6 in the presence of NaOtBu, it readily dissociated in the sodium salt of 2-hydroxy-1,1biphenyl, regenerating [Ni(SIPr)(η6-C6H6)]; the lability under these conditions implied that the role of NaOtBu in the catalytic system may be to facilitate dissociation from the Ni0 of coordinated phenol as a phenoxide. Notably, the reaction of PhOH with [Ni(SIPr)(η6-C6H6)] led to the formation, CW


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Scheme 206. Synthesis and Reactivity of trans[Ni(Halide)2(NHC)(PPh3)]

Scheme 205. Stoichiometric and Catalytic Reactions of Fluorinated Nickelacycles Stabilized by the [Ni(NHC)] Moiety

boronic esters in the presence of KOtBu, where a good degree of activity and selectivity is observed with the complex 127dClNi. The complex trans-[NiCl2(IPr)(2,6-lutidine)] (130Ni) has been obtained during studies of the photoactivation of [Ni(μCl)Cl(IPr)] 2 (129 Ni ) toward dehalogenation (Scheme 207).410 Thus, the binuclear 129Ni, which is obtainable by Scheme 207. Synthetic Pathways to trans[Ni(IPr)(lutidine)Cl2]

[NiCl2(CycAAC)(PPh3)] (127fClNi), which was converted to the ion-pair (cAAC-H)+[NiCl3(PPh3)]− by adventitious acidic impurities (Scheme 206).409 Complexes 127a−eXNi were diamagnetic solids, soluble in organic solvents. The geometry at Ni is slightly distorted square planar (with increasing distortion when bulkier NHCs are present) and trans arrangement of the NHC and phosphine ligands. In the PCy3 complexes, the Ni−CNHC and Ni−P bond distances are longer. The complexes 127a−dXNi were tested as catalysts for the Kumada-type reaction of aryl Grignard reagents with various electrophiles. Complex 127bCl/BrNi showed the highest catalytic activity for the cross-coupling of aryl chlorides and fluorides with aryl Grignard reagents, which is higher than with the corresponding [NiCl2(NHC)2] or [NiCl2(PPh3)2]; 127 dBrNi showed the highest catalytic activity for the crosscoupling of aryl methyl ethers with aryl Grignard reagents.407 Furthermore, the complexes 127a−eXNi have been studied as catalysts for the Markovnikov addition of styrenes to benzyl

crystallization as square-planar diamagnetic (from dichloromethane/pentane) or tetrahedral paramagnetic (from toluene/ hexanes) species, reacted with 2,6-lutidine with cleavage of the binuclear structure and formation of the trans-[NiCl2(IPr)(2,6lutidine)] (130Ni). In addition, photolysis of 129Ni led to the binuclear 60Ni, which oxidatively added HCl from lutidinium hydrochloride to give again 130Ni (Scheme 207). A range of neutral complexes [NiX(η3-allyl)(NHC)] (131aNi − 131lNi, X = halide) have been described. They were prepared by the in situ reaction of two equivalents of NHC ligands with the binuclear [Ni(η3-allyl)(μ-X)]2 (X = Cl, Br), using 1,5-COD as solvent; the iodide complexes were accessed by halide exchange with NaI in THF. The starting material [Ni(η3-allyl)(μ-X)]2 was easily obtainable in situ from the reaction of [Ni(1,5-COD)2] with C3H5-X (X = Cl, Br).411−413 The complex 131jNi was isolated as the product from the reaction of [Ni(Me2cAAC)(CO)3] with allyl bromide; allyl chloride was unreactive with [Ni(Me2cAAC)(CO)3], but the chloride analogue 131iNi was obtained by the in situ CX


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Scheme 208. Synthesis of [Ni(Halide)(η3-allyl)(NHC)] and Related Cationic Derivatives

method from [Ni(η3-allyl)(μ-X)]2 and Me2cAAC, as outlined for the imidazol-2-ylidenes (Scheme 208).369 Furthermore, the cationic complexes [Ni(NHC)(η3-allyl)L](BArF4) (133Ni and 134Ni, NHC = IPr, Me2IMe, L = H2O, MeCN) were synthesized in two sequential steps, involving cleavage of the binuclear [Ni(η3-allyl)(μ-OAr)]2 (Ar = 2,6-iPr2C6H3) with two equivalents NHC to give 132Ni followed by the reaction with [H(Et2O)](BArF4).414 Finally, 131aNi was obtained by the reaction of 13gNi with allyl- or methallyl-chlorides.355 The complexes 131 Ni are chiral and adopt square planar coordination geometries at Ni; they crystallize as mixture of enantiomers. In the solid-state structures, the plane of the NHC forms angles with the coordination plane that are dependent on the size of the ligands at Ni (NHC and halides). Due to the electronic differences of the donors trans to the termini of the η3-allyl moiety, the two Ni-Callyl bond distances are unequal, with the one trans to the NHC being longer. The Ni−CNHC distances are shorter in the Me2cAAC complexes than those in the almost isosteric analogue with ItBu (ca. 1.88 vs1.96 Å), presumably due to the electronic differences (better π-acceptor cAAC) of the NHCs. The structures of 131Ni in solution are nonrigid. In 131dNi and 131hNi, three dynamic

processes are discerned by VT 1H NMR spectroscopy with increasing temperature: (i) rotation of the NHC around Ni−CNHC single bond; (ii) formal allyl rotation around the Niη3-allyl axis, which results in the equivalence of the allyl protons cis−trans to the NHC; and (iii) π−σ−π allyl rearrangements, which occur at higher temperatures and render the syn- and anti-allyl protons equivalent (Scheme 208). The reaction of the saturated NHC precursor 135Ni with [Ni(1,5-COD)2] led to the isolation of the NiII η3-cyclooctadienyl complex 136Ni (Scheme 209). It was proposed that the expected reactive Ni0(SIMes)(η4-COD) moiety was formed along with one equivalent of C6F5H, the C-H of which transfers to 1,5-COD, leading to NiII aryl cyclooctenyl species. Hydride migration within the bound cyclooctenyl ligand and chain walking ultimately formed π-allyl complex 136Ni. In support of the proposition, 136Ni was also formed from a mixture of C6F5H, IMes, and [Ni(1,5-COD)2]. The isolation and characterization of 136Ni initiated a computational and experimental study of its role as an off-cycle deactivated form of Ni(NHC) organometallic moiety, showing sluggish reactivity in transformations based on intermolecular C−H bond activation steps; reversion of 136Ni to catalytically CY


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efficiency of specific NHC ligand structures in blocking the axial metal coordination site(s) which are postulated to be involved in the activation of the O2 by coordination prior to attack of the allyl C−H bonds. This rationalization originates from a mechanism involving the adoption of a nonplanar geometry at Ni as a requirement for reaction with O2 (Scheme 210).415 Complexes with η3-benzylic coordination on NiII have been obtained in two steps: reaction of IPr and SIPr with [Ni(η3benzyl)Cl(PMe3)] gave the species 138Ni featuring a σ-benzyl; subsequent PMe3 abstraction by reaction with B(C6F5)3 was accompanied by η1-to η3-benzyl rearrangement to 139a,bNi (Scheme 211). Interestingly, the η3-benzyl complexes 139a,bNi

Scheme 209. Inhibiting Action of 1,5-COD in Catalytic Reactions with [Ni(1,5-COD)2] and SIMes

Scheme 211. Complexes with η3-Benzylic Coordination

active forms may require forcing conditions. In contrast, complexes 13g−jNi present a higher kinetic barrier for the formation of inactive η3-allyl complexes. Experimental proof of these trends was provided by the higher activity of 13iNi in the hydroarylation of alkynes, which proceeded under milder conditions than when using [Ni(1,5-COD)2] and IMes as catalyst precursors (Scheme 209).334 Robust Ni(η3-cyclooctenyl) complexes with metalated ItBu have been obtained by the reaction of ItBu with [Ni(1,5-COD)2] (Scheme 162). The reaction of complexes 131Ni with dioxygen led to the hydroxo-bridged binuclear species 137Ni (Scheme 210); a

showed good catalytic activity toward the vinyl-type norbornene polymerization in the absence of any electrophilic activator. The catalysts were thermally robust, and the activity increased at higher temperatures (Scheme 211).416 The complexes 131k,lNi with the bulky flexible IPr* and OMe IPr* have shown high activity in the Buchwald−Hartwig arylamination reactions in the presence of NaOtBu and catalyst loadings of ca. 1 mol % with diverse activated and deactivated aryl chlorides as substrates; OMeIPr*-based catalysts were more active than those containing IPr*. The same complexes also catalyzed efficiently the arylation of thiophenols.413 NHC complexes with azallylic and allylic moieties have been synthesized in order to gain insight into the mechanism of (i) the Ni catalyzed dehydrogenative [4 + 2] cycloaddition of 1,3dienes with nitriles; (ii) the intermolecular [2 + 2] cycloaddition of conjugated enynes with alkenes; and (iii) [2 + 2+2] cycloaddition of imines with alkynes to N-aryl-1,2-dihydropyridines (Scheme 212). The stoichiometric reaction of benzonitrile with 2,3-diphenyl-1,3-butadiene at room temperature in the presence of [Ni(1,5-COD)2] and IPr (falling under case (i)) resulted in the formation of the corresponding aza-nickelacycle 140Ni. From the two possible tautomers A and B, A was characterized crystallographically. The treatment of 140Ni with CO led to the quantitative formation of a mixture of the dihydropyridines 3,4,6-triphenyl-2,5-dihydropyridine and 3,4,6-triphenyl-1,2-dihydropyridine, in support of the catalytic competence of 140Ni. Although the optimized catalytic system employs PCy3 in the place of IPr, the better stability of Ni-NHC complexes favored the characterization of the intermediate (Scheme 212, A).417 The reaction of conjugated cyclic and acyclic enones with an equimolar amount of the conjugated ene-yne, [Ni(1,5-COD)2], and IPr (falling under case (ii)) gave the nickelacycles 141aNi and 141bNi, respectively (Scheme 212, B). The structure of 141aNi features a coordinated η3-butadienyl attached to the alkynyl carbon. A carbon−carbon bond is formed between the alkynyl carbon distal to the alkenyl group and the β-C of the cyclohexenone, while the α-C is bound to Ni giving rise to a Cbound enolate. Conversely, in the reaction with the acyclic

Scheme 210. Action of O2 on [Ni(Halide)(η3-allyl)(NHC)] and Derivatives

related reaction with the 1-phenyl-substituted allyl ligand was employed to identify cinnamaldehyde and phenyl vinyl ketone as the less volatile organic oxidized products formed in an analogous reaction. The proposed mechanism of the oxidation of the Ni-bound allyl involves reversible binding of O2 to the Ni followed by the formation of a NiIII peroxide intermediate, in which an oxygen radical abstracts homolytically an H atom from the allyl group; this leads to intramolecular hydroxylation and the formation of α-hydroxy allyl species, which is set up for a hydrogen atom transfer to yield the carbonyl product and a mononuclear hydroxy-nickel complex; the latter dimerizes via hydroxide bridging (Scheme 210).411 The study of structure− reactivity relationships established facile oxidation of IPr and IMes complexes, complete inertness of ItBu and IAd complexes but moderate reactivity (i.e., O2 stability for >30 min) with smaller NHCs like IpTol. Furthermore, an attempt to correlate the reactivity with the %Vbur of the NHC ligand failed. The discrepancy was rationalized by considering (i) the conformational freedom of Ni-NHC originating from unrestricted rotation around the Ni-CNHC bond and (ii) the CZ


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Scheme 213. Synthesis of [NiIIX(η5-Cp)(NHC)] Derivatives

Scheme 212. Allylic and Aza-Allylic Complexes Related to the Intermolecular Cyclization of Unsaturated Molecules with Nitriles and Imines to Pyridine Derivatives

found as counteranion; the overall geometry at the cation is C2 and at the Ni center approximately trigonal planar (assuming one coordination site is occupied by the Cp centroid); the NHC planes are inclined at dihedral angles of 82° to the coordination plane, and the Ni−CNHC bond length is ca. 1.88 Å. The facile η5- to η1-rearrangement and the good leaving group role that the Cp− adopts are probably related to the weakness of the Ni-CCp bonds due to reduced Ni-CCp bond order, in view of the occupation of antibonding orbitals in the parent nickelocene; the use of smaller NHC may favor kinetically full dissociation of the Cp−. In a related system aiming to access organonickel polymers by ring-opening polymerization of a strained nickelophane, the reaction of the latter with IMes gave 142cNi, where the η1-bound Cp remained tethered to the η5-bound donor.420 The group [NiX2L2(NHC)] is dominated by “halfsandwich” complexes of the type [NiX(η5-Cp)(NHC)] (143aNi−143gNi, NHC = imidazol-2-ylidenes, benzimidazol2-ylidenes), the equivalent cationic [Ni(η5-Cp)(NHC)L]+ (L = MeCN, acetone, IMe and IiPr 144aNi−144dNi), the related [NiX(η5-indenyl)(IMes)] (145aNi), the complexes with the extended ring NHCs 146aNi−146eNi,421 and the 1,2,3-triazolylidenes 147aNi−147dNi.422,423 In all the above classes, the complexes attain 18 valence electron counts (Scheme 214). The wide range of reported examples have been prepared by the reaction of [NiCp2] with imidazolium salts resulting in alkanolysis and elimination of one CpH;201,421−442 a Cp* analogue was obtained by the reaction of [Ni(η5-Cp*)(acac)] with imidazolium salts (Scheme 214).427 The cationic species 144aNi, 144bNi were obtained by abstraction of the halide in the presence of acetonitrile or acetone donors; the cationic 144cNi, 144dNi can be prepared from 144aNi by substitution of the coordinated MeCN with the less bulky IMe or IiPr. Larger NHCs did not react accordingly (Scheme 214). The complexes 143aNi−143gNi, 144aNi−144dNi, 145aNi, 146aNi−146eNi, and 147aNi−147dNi (see also below) have “piano stool” arrangements with the Ni−CNHC bond distances confined in the narrow range of ca. 1.85−1.89 Å. The hapticity of the Cp rings is five. In solution, structural fluxionality is associated with hindered rotation of the NHC around the Ni−

enone leading to 141bNi, the O-bound enolate was obtained. Reductive elimination gave the cyclobutene. It was postulated that the alkene originating from the enones and the conjugated enyne simultaneously coordinate to Ni0 and undergo oxidative cyclization to give the η3-butadienyl nickelacycle intermediate. The catalytic competence of 141Ni was demonstrated by its reaction with excess ethyl acrylate.418 Finally, reaction of the η2-coordinated N-aryl-imine with alkynes (falling under case (iii)) led to the aza-nickelacycle 142Ni featuring a T-shaped 14electron NiII center. It models the intermediate in the first step of the Ni-catalyzed [2 + 2 + 2] cycloaddition of alkynes and imine leading to N-aryl-1,2-dihydropyridines (Scheme 212, C); addition of one equivalent of alkyne at higher temperatures results in the formation of the substituted pyridines.344 The complexes of type [NiIIX2(NHC)L] (79a,bNi) have been described above in conjunction with the synthesis and reactivity of NiI(NHC) amidates (Scheme 193). One of the initial contributions in this area was the reaction of nickelocene with one equivalent of two different imidazol-2ylidenes giving the products 142aNi and 142bNi (Scheme 213).419 Complex 142aNi is diamagnetic with one η1- and one η5-bound Cp rings and one NHC moiety. Its structure features dihedral angles of the aryl substituents with the NHC plane of ca. 56°, bending at Ni of ca. 130° and Ni−CNHC bond distance of 1.885 Å. The species 142bNi comprises a cationic bis(NHC) derivative with one η5-Cp having acted as a leaving group and DA


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Review

Scheme 214. Synthesis of [NiII(η5-Cp)X(NHC)] Neutral and Cationic Derivatives

Ni−Ctriazolylidene bond, which enables facile carbene dissociation and coordination to a different nickel center.422 Complexes from the series 143Ni, 143aNi−143gNi, also catalyze the hydrosilylation of aldehydes and ketones with SiH2Ph2 at room temperature in the presence of a catalytic amount of sodium triethylborohydride. The Ni hydride species [NiH(η5Cp)(IMes)], independently synthesized by the reaction of [NiCl(η5-Cp)(IMes)] with KHBEt3 and structurally characterized, was found to be the catalytically active species.432 Finally, 143auNi is an efficient catalyst for the α-arylation of acyclic ketones with aryl halides (chlorides, bromides, and iodides) in the presence of a stoichiometric amount of base (NaOtBu). Although polar mechanisms and the plausible Cenolate or aryl complexes [Ni(η5-Cp)(κC-PhC(O)=CHMe)] and [Ni(η5-Cp)Ph], respectively, were studied as catalyticrelevant species for the arylation reaction, strong evidence was gathered in support of radical mechanisms.439 In the class of [NiX2(NHC)2] are found square planar NiII complexes: coligands X can be (i) halides, giving rise to trans[NiX2(NHC)2], usually obtained by reactions of NiII starting materials with NHC sources (isolated or generated in situ NHCs and Ag complexes); and (ii) at least one X is a σ-bound organometallic, hydride, silyl, or occasionally anionic heteroatom donor group, the other often halide, σ-organometallic or

CNHC bonds, which slows down for the bulkier NHCs and the Cp* analogues. In the complexes 144aNi and 144bNi, deprotonation of the acidic C−H bonds α- to the nitrile or the carbonyl functions leads to metalation at these positions, described as C−H activation (147eNi, 147fNi); acetone can also be doubly deprotonated giving rise to a bridging “oxyallyl” species 147gNi, while nitrile-functionalized wingtips can also be metalated α- to the functional group to 147hNi (Scheme 215).428,431 Selected complexes from the series 143aNi−143gNi and related species have been studied as catalysts for the BuchwaldHartwig amination of aryl halides, with best results obtained with the bulky OMeIPr* complexes.424,436 In addition, cationic complexes from the series 144aNi−144dNi were used as catalysts for the Suzuki coupling of bromoacetophenone with phenyl boronic acid in the absence of additives, where the species with the bulkier Cp* performed better, although there was no noticeable difference in activity and rate between the cationic and neutral complexes.429,433 1,2,3-Triazol-5-ylidene analogues of 143aNi were also tested for this type of catalytic transformation; although they showed high initial activities, they suffered from limited thermal stability under the catalytic conditions, a fact attributed to the lability of the DB


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reaction of [Ni(halide)(η5-indenyl)(NHC)] with the second imidazolium salt leads to the desired products. A straightforward optimization of the procedure for the one-pot synthesis of symmetrical [Ni(halide)2(NHC)2] was also described.444 The OAc− has been employed as a coordinated base for the straightforward synthesis of thiazol-2-ylidenes from thiazolium salts and [Ni(OAc)2].450 Complexes of the type [AgX(NHC)] have only occasionally been used as sources of NHC in a transmetalation reaction sequence, with no report for NHCs lacking heteroatom functionalities.445,446,448,449 In one case (128lNi vs 128nNi), the products obtained from the Ag transmetalation and the reaction of the free NHC generated in situ differ in the coordination geometries at Ni (cis and trans, respectively).446 There is only one report of isolation of trans[Ni(halide)2(NHC)2] from the oxidative addition of the C−H of imidazolium salts to [Ni(1,5-COD)2] probably via a multistep sequence;278 reaction of [Ni(1,5-COD)2] in the presence of Me2IMe and excess aryl bromides leading to trans[NiBr2(Me2IMe)2] (116Ni) has been previously mentioned (Scheme 201). As alluded above, the majority of the characterized [Ni(halide)2(NHC)2] complexes adopt a trans-geometry. The Ni−CNHC bond distances fall in the range of 1.90−1.94 Å, with the less bulky NHCs featuring the shorter distances; interestingly, in the cis-isomer 128nNi, the Ni−CNHC distances are shorter than in the trans-isomer (ca. 1.87−1.88 Å), indicative of the impact of the trans-influence on the bond distances. In all reported examples, the carbene ring planes are oriented almost perpendicular to the Ni coordination plane. In the complexes 128iNi and 128jNi, with the nonsymmetrically substituted NHCs bearing one acetylated D-glucopyranosyl substituents, a dynamic behavior is observed in solution attributed to the interconversion of anti- and syn-rotamers, by the rotation of the NHC ligands around the axes of the Ni−CNHC bonds. The CNHC chemical shift in the 13C NMR spectra of complexes 128Ni fall within the range of δ163−178 ppm, the most shielded observed for IiPr and most deshielded for IPr complexes. Specific complexes in the series 128aNi− 128r Ni have been tested as precatalysts in ethylene oligomerization (dimerization) reactions in the presence of MAO as an activator or in the addition polymerization of norbornene generally showing moderate or low activity.443,446,448,449 Low or no activity was also observed in Kumada-type cross-coupling reactions.406 The complex 10Ni acts as a source of the reactive 14 valence electron nucleophilic Ni0(NHC)2 moiety under thermal conditions. This property has been widely employed to study the nucleophilic oxidative addition of strong, unreactive Chalide bonds (C−F, C−Cl, C−Br) of aryl halides to zerovalent Ni. In particular, the reactivity of [Ni0(NHC)2] with arylfluorides, which proceeds smoothly under mild conditions, has been studied in detail both stoichiometrically and in relation to the derivatization or hydrodefluorination of aryl fluorides. A summary of the stoichiometric transformations leading to complexes trans-[NiF(ArF)(NHC)2] and their derivatives is given in Scheme 217.366,451−454 Complex 10Ni reacts readily with polyfluorinated aromatics (but not with fluorobenzene) giving well-defined products selectively. The reactions with hexafluorobenzene and decafluorobiphenyl (ratio of Ni to substrate 1:1) in THF at room temperature led to the products 148aNi and 150aNi in good isolated yields, both featuring square planar geometries, with the NHC planes perpendicular to the coordination plane,

Scheme 215. Metallation of Acetone and Nitriles by [NiII(η5-Cp)(NHC)] Derivatives in the Presence of KOtBu

anionic heteroatom donor group. In case (ii), the complexes [NiX2(NHC)2] are usually obtained as products of oxidative organometallic transformations involving Ni0(NHC)2 sources as starting materials, as detailed below. The associated stoichiometric reactivity is crucial for the understanding of certain catalytic reactions. Square planar [NiX2(NHC)2] complexes (X = halide) constitute an important entry point for further comparative studies. In most described examples, the NHC is imidazol-2ylidene406,426,443−447 or heteroatom-functionalized imidazol-2ylidene with dangling heteroatom donor278,446,448,449 and, in a limited number of cases, 1,2,3-triazol-5-ylidene and thiazol-2ylidene.422,450 The synthetic methods leading to the complexes are summarized in Scheme 216.406,443 The reactions of free IPr and IMes (isolated or generated in situ) with [NiCl2(PPh3)2] or [NiBr2(PPh3)2] led to the complexes trans-[Ni(halide)2(NHC)2] (128aNi and 128bNi, respectively), in isolated yields of up to 40% [methods (i)/ (ii)]). As mentioned previously (see also Scheme 206), with IPr the first and second substitution steps could be controlled by regulation of the stoichiometry of the addition and the reaction conditions; with the smaller IMes only, the trans[Ni(halide)2(NHC)2] could be obtained. Generation of other small NHCs in situ from the imidazolium salts and organometallic bases in the presence of Ni sources such as [Ni(NCMe)4]2+, [Ni(Hal)2(PPh3)2], or [Ni(Hal)2(DME)2] constitutes a method of broad scope for accessing the complexes [method (ii)]. A range of trans-[Ni(halide)2(NHC)(NHC′)] complexes comprising two different NHCs has been prepared based on the stepwise and sequential alkanolysis of indene from [Ni(η5-indenyl)2] by two disparate imidazolium salts, the indenyl acting as a coordinated base [method (iii)]. In the methodology aiming to access the nonsymmetrical species, isolation of the [Ni(halide)(η5indenyl)(NHC)] intermediate after the first alkanolysis is crucial to avoid the formation of a mixture of products; DC


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Scheme 216. Synthetic Approaches to [NiX2(NHC)2]

complex 148eNi appears at δ −13.7 ppm as a deceptively simple septet, attributed to coupling with the fluoride atoms of the trans situated C6F5.452 Complex 148eNi adopts a squareplanar, albeit more distorted structure, due to the reduced steric requirements of the hydride ligand. Further metalation of the C6F5 groups of 148Ni and 150Ni was carried out by addition of one-half more equivalent of 10Ni, leading to the binuclear species 149Ni and 150Ni (Scheme 217). In the binuclear species, replacement of the fluoride by chloride and derivatization of the Ni−F bonds by aryl chalcogenides (leading to 149cNi, 149dNi and 150cNi, 150dNi, respectively) was also made possible by the use of trimethylsilylated reagents.454 The complex 10Ni and the related mononuclear analogue [Ni(IiPr)2(η2-C2H4)] (22Ni) also activate the C−F bonds in fluorinated heteroaromatics (e.g., pentafluoropyridine), leading to trans-[NiF(C5NF4)(IiPr)2]; at lower temperature, the para-regioselectivity is maintained, while at room temperature, a mixture of o- and p-substituted complexes (2:1) is observed by 1H NMR spectroscopy.453 Attempts to gain insight into the mechanism of the oxidative addition of the Ni0(NHC)2 fragment across the CAr−F bond, in particular the detection of any intermediates preceding it,

Ni−CNHC bonds of ca. 1.93 Å and Ni−CArF of ca. 1.91 Å; 148aNi was crystallographically characterized only as aquosolvate featuring Ni-F···H-OH hydrogen bonding (Scheme 217). Solution fluxionality of 148aNi studied by 1H NMR spectroscopy was attributed to restricted rotation of the IiPr around the Ni−CNHC bond. When using decafluorobiphenyl as a substrate, the reaction took place at the C−F bond para to the C6F5 substituent; analogous regioselectivity was observed with octafluorotoluene and (pentafluorophenyl)-trimethylsilane, both leading to the p-metalated complexes 151Ni and 152Ni, respectively.366,451 The chemo- and regio-selectivity of the reaction was studied in more detail with a range of partially fluorinated isomeric fluoroarenes; in all cases, C−F (and not C−H) activation was only observed, leading to complexes 153Ni−157Ni presumably due to superior Ni−F bond strength (compared to Ni−H).453 A range of derivatives of 148aNi were obtained by reactions with trimethylsilylated reagents of nucleophilic alkali metals, leading to substitution of the fluoride at Ni and the products 148bNi−148eNi, after trimethylsilyl fluoride or LiF elimination. All derivatives except the hydride 148eNi feature good thermal stability; all maintain the square planar geometry at Ni. Interestingly, the Ni-H of DD


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Scheme 217. Stoichiometric Oxidative Addition Reactivity of the in Situ Generated Fragment [Ni(IiPr)2]

fluxionality; also, the symmetry could be rationalized by the existence of a Ni(IiPr)2(η2-fluoroaromatic) adduct, which persisted up to 273 and 293 K for experiments with hexafluorobenzene and octafluoronaphthalene, respectively; above these temperatures, the NiII oxidative addition products

were focused on observation of reactions of 22Ni with hexafluorobenzene and octafluoronaphthalene in toluene at 193 and 243 K, respectively. Under these conditions, the appearance of broadened 19F-NMR spectra of a postulated adduct formed prior to oxidative addition was attributed to DE


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were formed. At 213 K, the octafluoronaphthalene adduct 158Ni, originating from the η2-coordination of the octafluoronaphthalene, was crystallized and structurally characterized, corroborating the η2-bonding mode. A VT 19F-NMR study of this adduct gave insight into the nature of a dynamic process occurring at lower temperatures: it involved a suprafacial 1,3shift of the Ni(IiPr)2 moiety from 1,2-η2 to 3,4-η2-coordination with no migration to the second aromatic ring of the naphthalene. This constitutes one of the dynamic processes taking place; another may include rotation of the C2v-shaped Ni(IiPr)2 moiety around the Ni-polyene bond (Scheme 218).453

The stoichiometric oxidative addition and derivatization reactivity displayed in Schemes 201 and 202 are related to elementary steps in the cross-coupling of aryl halides. In fact, Suzuki and Kumada cross-coupling of octafluorotoluene or decafluorobiphenyl with phenyl boronic acid and 10Ni (ca. 2− 3 mol %) in the presence of triethylamine or K2CO3 has been described;451 [Ni(IPr)2] and in situ formed catalysts from [Ni(acac)2] and imidazolium salts catalyze the Kumada coupling of p-substituted activated fluoroarenes and aryl Grignard reagents.331 Complex 10Ni is also a very active catalyst for the Suzuki coupling of aryl bromides and chlorides with BPh(OH)2 (ca. 0.6 mol % in toluene); the choice of base is critical for good conversion, the best results being obtained with KOtBu. Catalyst deactivation products 161Ni and [(IiPr− Ar)+]2[NiBr4]2− were identified as described above.457 The Suzuki cross-coupling system under discussion based on the use of the less bulky [Ni(IiPr)2] operates with Ni0/NiII cycles and has to be contrasted with systems based on bulkier [NiX(IPr) 2 ], which are believed to operate via Ni I intermediates (Schemes 201 and 202). With the use of complex 10Ni as a source of Ni0, stoichiometric oxidative addition (C−C activation) of the strained 2,2’-C−C bond of the biphenylene has been described to give the biphenyl complex 162Ni, which was fully characterized (distorted square planar with Ni-CNHC = 1.89− 1.91 Å and Ni-Caryl = 1.93−1.94 Å) (Scheme 220). DFT

Scheme 218. Non Rigid Adduct of Octafluoronaphthalene with [Ni(IiPr)2]

The oxidative addition of aryl halides to the [Ni(1,5COD)2]/Me2IMe system leading to the trans-[NiBr2(Me2IMe)2] (116Ni) (Scheme 201) has been mentioned previously. Oxidative addition of the Ar−Y bonds (Y = Cl, Br; Ar = substituted aryl) to the Ni(IiPr)2 moieties originating from 10Ni have also been studied (Scheme 219): for Y = Cl, the reactions at room temperature gave selectively the adducts trans-[NiCl(Ar)(IiPr)2] (159Ni) for a range of unsubstituted or p-substituted with electron-withdrawing groups aryls; for Y = Br, reactions at −78 °C in dilute solutions were necessary to obtain the analogous 160Ni and avoid the competing formation of trans-[NiBr2(NHC)2] (161Ni), presumably via bimolecular pathways involving 160Ni and ArBr. Once 160Ni were formed, they showed good stability toward disproportionation, even at higher temperatures in aromatic solvents, provided excess of free aryl bromide is not present. Additional disproportionation reactions to [(IiPr−Ar)+]2[NiBr4]2− were observed from 160Ni and ArBr. Similarly, reactions of 159Ni with a series of aryl halides at higher temperatures led to the decomposition of the Ni(IiPr)2 moiety and formation of the imidazolium salts [(IiPr−Ar+)]2[NiBr2(halide)2]2− in high yields.455−457 The reaction of [Ni(PPh3)2] with imidazolium salts in the presence of simple alkenes proceeded via the oxidative addition of Cimid−H bond, leading to phosphine-stabilized carbene hydrides that insert alkene into the Ni−H bond and reductively eliminate 2-alkyl-imidazolium salts.458

Scheme 220. C−C Bond Activation of Biphenylene by the [Ni(IiPr)2] Moiety

calculations show that a plausible intermediate preceding the C−C cleavage is a complex in which Ni(IiPr)2 is η4coordinated to the four-membered ring of biphenylene. The reaction of biphenylene with diphenylacetylene is efficiently catalyzed by 10Ni to give 9,10-di(phenyl)phenanthrene.342 The oxidative addition of Ni(IiPr)2 fragment into the C− S(O) bonds of dimethyl- and methyl-phenyl-sulfoxides (featuring formally SIV centers) led to the formation of the trans-[Ni(R){S(O)Me}(IiPr)2] (R = Me, Ph, 163Ni) with S bound (i.e., Ni-S(O)Me) metal sulfinyl moiety (SO distance

Scheme 219. Oxidative Addition of Aryl Halides to [Ni(IiPr)2] Moiety

DF


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in 163Ni ca. 1.53 Å, longer than the SO bond in DMSO of ca. 1.49 Å; the S lone pairs are localized on the S atom). In contrast, the oxidative addition of diphenylsulfoxide led to complex 164Ni, also via C−S(O) cleavage but to a Ni-O-bound (i.e., Ni-OSPh) sulfinyl moiety (Scheme 221).459 The

interaction of the Ni(IiPr)2 moiety with benzothiophene-1,1dioxide and methyl phenyl sulfone (both featuring SVI) stops after η2-coordination at the Ni0 center (165Ni and 166Ni). Finally, the oxidative addition of the Ni(IiPr)2 moiety into the S−C bond of thioethers (including cyclic thioethers, for example, benzothiophene and dibenzothiophene), the S−H bond of tBuS-H, and the S−S bond of alkyl- and phenyldisulfides led to C−S, S−H, and S−S bond cleavage reactions and formation of aryl thiolates, thiolate hydrides, and bisthiolates (i.e., complexes 167Ni−170Ni, respectively) (Scheme 221).460 All NiII adducts feature square-planar, trans-arrangements at Ni, except the adducts of the cyclic thioethers benzothiophene and dibenzothiophene, where cis arrangement was favored. Aspects of chemoselectivity were rationalized based on thermodynamic (i.e., the relative strength of the bonds broken and formed) and kinetic [i.e., precoordination of the Ni(IiPr)2 moiety prior to oxidative addition] factors. For example, the preference for S−Caryl over S−Calkyl bond activation in 167Ni may be due to the formation of the stronger Ni−Caryl bonds compared to Ni−Calkyl bonds, while the preferential insertion of Ni(IiPr)2 into the S−Cvinyl over S− Caryl bond of benzo- and dibenzo-thiophenes (i.e., in 168Ni) was attributed to η2-precoordination before C−S activation (Scheme 221). All complexes isolated are stable and do not show any tendency for desulfurization.460 The reaction of silanes with 10Ni led to cis-[NiII(IiPr)2disilyl] or cis-[NiII(IiPr)2-silyl hydride] complexes, depending on the nature of the silane employed. Thus, reaction with excess SiH3Ph and SiH2Ph2 at room temperature gave quantitatively 171aNi and 171bNi, respectively, with liberation of dihydrogen; reaction with excess of SiH2R2 or SiH3Cy also led to disilyl complexes 171cNi−171eNi. Reactions with the bulkier SiHPh3 and SiHMePh2 led to the hydrido silyl complexes 172aNi and 172bNi. X-ray diffraction data for 172bNi (despite the known difficulty in accurately locating H atoms by this technique) imply elongated Si−H separation (1.979 Å) and short Ni−H and Ni−Si bond distances; experimental and theoretical charge density studies in 172bNi

Scheme 221. Reactions of [Ni(IiPr)2] Moiety with Organosulfur Compounds

Scheme 222. Stoichiometric Reactions of [Ni(IiPr)2] Moiety with Silanes

DG


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point to the formulation of this complex as a symmetric oxidative addition adduct, with established M−H and M−Si bonds but featuring some residual Si−H bonding interaction. In support of this, the HOMO of 172bNi displays simultaneous Ni−Si and Si−H bonding character, but spectroscopically, the hydrides are clearly observable in the 1H NMR spectrum with 2 JSiH = 11 Hz.461,462 In solution, as studied by 1H NMR spectroscopy, complex 172bNi undergoes a NHC exchange process with coalescence in the range from −20 to 0 °C that renders the two inequivalent NHCs (trans to H and silyl) equivalent; H/D exchange of the hydride ligand with C6D6 is also observable, accompanied by formation of C6D5H. Finally, irreversible cis−trans rearrangement to the thermodynamically more stable isomer is occurring at higher temperatures, which is stable up to 60 °C.462 Heating of 10Ni with two equivalents of SiH2Ph2 in toluene gave the binuclear diamagnetic NiI complex 174Ni, presumably via silyl hydride or disilyl intermediates. Interestingly, in the 1H NMR spectrum of 174Ni, one hydride resonance with Si satellites is observed with JSiH = 20.3 and 62.5 Hz for the trans and cis coupling, the latter lies at the lower limit of coupling constants observed for σsilane complexes; the magnitude of the JSiH serves as a measure of the degree of the Si−H oxidative addition to the metal. The Ni−Ni distance in 174Ni at ca. 2.51 Å is indicative of a weak Ni−Ni single bond. Finally, the reaction of 10Ni with SiH(OEt)3 led to the trans disilyl complex 173Ni. All these transformations are shown in Scheme 222.461,462 On the basis of the H/D exchange reactivity of 172bNi with C6D6 outlined above, the catalytic incorporation of deuterium in SiHEt3 using 10Ni as catalyst and C6D6 as source of deuterium atoms was studied. At 2.0 mol % catalyst loading at 70 °C, conversions up to 95% were observed after 11 h; however, SiHMePh2 under these conditions was unreactive, presumably due to competing cis-trans isomerizations. Mechanistically, the exchange is thought to proceed via a Ni0/II cycle with the H/D exchange step to involve a σ-bond metathesis in a σ-coordinated C6D6 complex on a cis[NiH(SiEt3)(IiPr)2] fragment. Finally, 10Ni has been used as a catalyst for the dehydrogenative coupling of SiH3Ph to polysilanes and the hydrogenation of (SiR12R2)2 to SiHR12R2 (R1, R2 = Me, Ph).461,462 The catalytic hydrodefluorination of perfluoroaromatics with 10Ni using SiHEt3 or SiHPh3 as fluoride acceptor and related stoichiometric reactions have also been described. Thus, the reaction of C6F6 with 5 equiv of SiHPh3 in the presence of 5 mol % of 10Ni resulted in monodefluorination to 1,2,4,5tetrafluorobenzene (48 h at 60 °C), followed by 1,4difluorobenzene (96 h at 80 °C); perfluorotoluene was defluorinated to 1-(CF3)-2,3,5,6-C6F4H using 5 equiv of SiHEt3 (4 d at 80 °C) (Scheme 222). Two groups of stoichiometric reactions were carried out in order to gain insight into the reaction mechanism: (i) the silyl hydrides 172aNi and 172bNi reacted with C6F6 at room temperature to form 148aNi and 148eNi with elimination of silane or fluorosilane, respectively; (ii) the complexes 148aNi and 151aNi reacted with SiH3Ph or SiH2Ph2 to give the hydrides 148eNi and 151bNi, respectively, which converted to the trans[NiF(IiPr) 2 (2,3,5,6-F 4 C 6 H)] and trans-[NiF(IiPr) 2 (3CF3C6F3)] and trans-[NiF(IiPr)2(2-CF3C6F3)].463 In the case of rearrangement of 151bNi, the intermediate 175Ni was detected spectroscopically (Scheme 223). This intermediate is subject to substitution of the defluorinated arylfluoride with perfluoroarene in a reaction that models the chain transfer

Scheme 223. Stoichiometric Reactivity Related to Hydrodefluorination of Fluoroaromatics

step. On the basis of these experimental findings and additional theoretical calculations, it is postulated that the mechanism of hydrodefluorination involves the C−F activation of the fluoroarene and subsequent reaction of the resulting nickel fluoride with the silane to give nickel fluoroaryl hydrides that rearrange to the defluorinated arene and Ni(IiPr) (fluoride route); this is preferred to the activation of silane to silyl hydride followed by reaction of the latter with perfluoroarene (silane route).463 The rare cis-[NiMe2(IiPr)2] has been targeted as a model for the intermediates involved in the synthesis of [Ni0(NHC)2] by reductive elimination of ethane from cis-[NiMe2(TMEDA)] and two equivalents of NHC (Scheme 161).338 3.2.3.2. Complexes with Chelating bis-NHC and tris-NHC Ligands. Monocationic NiII complexes 176aNi and 177aNi using the methylene- and ethylene-bridged bidentate bis NHC ligands (in Scheme 224, L1tBu and L2, respectively) were prepared by the reaction of [NiCl2(PMe3)2] with one equivalent of the free ligand. Anion exchange in 176aNi and 177aNi with TlBPh4 gave the (BPh4) salts 176bNi and 177bNi. The geometry around the NiII in all these complexes is distorted square-planar with the six- and seven-membered chelate rings formed, adopting a boat-like conformation. Reaction with two equivalents of L1tBu gave the homoleptic dicationic 178aNi. Interestingly, a homoleptic dicationic complex with the ethylene-bridged L 2 could not be obtained.464,465 Alternative approaches to 178aNi analogues were described, including the reaction of bis-imidazolium iodides with [Ni(OAc)2]; in this case, the acetate is acting as coordinated base, the electrochemical oxidation of Ni in the presence of bis-imidazolium salts, the reaction of the free NHCs L1R with [NiBr2(DME)], and the reaction of Ag NHC complexes with [NiBr2(DME)].190,466,467 Furthermore, homoleptic chelating dicationic bis-benzimidazole NHC analogues of 178aNi with the same structural motif have been described previously (prepared from [Ni(OAc)2] and (NnBu4)Br in the melt); their electrochemical and chemical reductions have also been mentioned (Scheme 187). All complexes of type 178aNi− 178dNi are square-planar with puckered chelate rings in transdouble boat conformation, resulting in acute twist dihedral angles between the planes of the imidazol-2-ylidenes of the same ligand (ca. 55−57°). The Ni−CNHC bond distances are ca. 1.89−1.90 Å (Scheme 224). The carbonate complex 179Ni (κO, κO-CO3) was obtained directly by the reaction of the imidazolium salt with NiCl2 and excess K2CO3 as external base.468 A chelating, chiral NHC complex 180Ni based on NHC donors anchored to the axially chiral BINAP skeleton via DH


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Scheme 224. NiII Complexes with Bidentate Chelating Bis-Carbene Ligands

Scheme 225. Trans-Spanning Chelating Bis-NHC Ligands Based on the BINAP Framework

flexible methylene linkers has been prepared: the imidazolium salts prepared by quaternization of the BINAP iodide precursor with methylimidazole were subjected to reaction with [Ni(acac)2] in NMP at 200 °C, giving rise to the transbis(NHC) Ni complex (Scheme 225).469 The synthesis of complex 44aNi from [NiBr2(DME)] and the free dicarbene 43aNi was previously described (Scheme 184).379 Increasing the length of the linker (i.e. using a 1,3-propylene linker, as in the bis-imidazolium salts 181aNi and 181cNi) has a dramatic effect on the observed coordination chemistry. The favored formation of the dicationic homoleptic complexes seen with shorter linkers is suppressed, and the 182aNi and 182cNi cis-dibromide complexes are formed by reacting the

imidazolium salts with [Ni(OAc)2] in molten N(nBu4)Br. Furthermore, abstraction of the bromides with NaPF6 and in the presence of 2,2′-bipy led to the cationic 183aNi, 183bNi and 183cNi, 183dNi, which constitute rare examples of [Ni(NHC)2L2]2+ species (see also above). The study of the redox behavior of 183aNi, 183bNi and 183cNi, 183dNi revealed that reversible redox couples are attained with 183cNi, 183dNi, which was attributed to structural ligand rigidity, which inhibits dramatic geometrical changes and reorganizations during the reduction, thus facilitating reversibility. The first reduction event (−1.54 and −1.59 V vs Fc+/Fc) was assigned to bipy ligand-centered reduction, while the second was metal-based. The assignments were confirmed by DFT calculations; in the DI


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first reduction, the bipy-based LUMO is singly occupied while the optimized geometry remains planar (Scheme 226).470

The square-planar diamagnetic NiII complex 186Ni was obtained by the reaction of the anionic bis-carbene borate lithium with Ni II precursors (e.g., [NiCl 2 (DME)], [NiCl2(PPh3)2], or [NiCl2(PMe3)2]). The centrosymmetric complex isolated and characterized is structurally relevant to the cationic 178Ni featuring square-planar Ni and a trans double-boat conformation of the chelating ligands. DFT calculations confirm that the square planar, S = 0 electronic structure is stabilized relative to octahedral (S = 1) by the strong Ni-CNHC bonding interactions (Scheme 228).471

Scheme 226. NiII Complexes with Bis-NHC Ligands with Long Flexible Linkers

Scheme 228. NiII Complex with Bis-NHC Ligands with (BH2)− Linker

The bulky tris(carbene)borate ligand was used to stabilize four-coordinate pseudotetrahedral nickel bromide and nitrosyl complex 187Ni and 188Ni, respectively. They were obtained by salt metathetical reaction with [NiBr2(PPh3)2] and [NiBr(NO)(PPh3)2], respectively (Scheme 229).472 In the paramagnetic 187Ni, the tris(carbene)borate ligand is coordinated to Ni in a tridentate fashion with Ni-CNHC ca. 1.99 Å; the bulky tBu substituents create a C3 symmetric pocket and hamper the coordination of a second scorpionate ligand to the metal center. Complex 187Ni is related to the isostructural hydrotris(3-tert-butylpyrazolyl)borate nickel bromide. High frequency EPR spectra of 187Ni were analyzed in detail and are characteristic for a triplet state powder pattern. The diamagnetic 188Ni, formally a [NiNO]10 complex, features a linear nitrosyl ligand which could imply a Ni0 center. However, based on metrical data (mainly the short Ni−NO bond of ca. 1.62 Å) and IR spectroscopic data [ν(NO) = 1703 cm−1], the preferred formulation of NO is (NO)3− and the oxidation state of the metal NiIV. The complexes 191Ni and 194Ni featuring the linear, flexible, open-chain tetradentate NHCs with all-methylene and methylene-propylene linkers have been obtained by transmetalation from the Ag complexes 190 Ni and 193Ni, respectively (Scheme 230). They both feature slightly distorted square-planar geometries at Ni and are diamagnetic. The versatility of the ligand design stems from the variable length of the organic linkers, which can adjust flexibility and stabilize desired coordination geometries.473 A limited number of Ni complexes with macrocyclic ligands bearing only NHC donors has been described. The 24membered macrocyclic tetra-imidazolium iodide 195Ni was deprotonated in the presence of NaOAc in DMSO to yield the mononuclear Ni tetracarbene cation combined with I− or PF6,− after anion exchange (Scheme 231). The structure of 196aNi reveals a square-planar Ni atom coordinated by four NHC donors with a conformation topologically similar to the cover of a tennis ball. Reduction of 196aNi with Na/Hg in DMF gave the neutral molecule 197Ni, which could be formally viewed as a Ni0 complex (Scheme 231). CV measurements of 196bNi recorded a reduction wave at a highly negative reduction potential (−2.4 V vs Ag/AgCl) corresponding to a

The reaction of L2 with [NiMe2(bipy)] gave the cis-dimethyl complex 184Ni featuring distorted square-planar geometry and cis methyls; in contrast, reaction with L1tBu led to intractable mixtures, even though at temperature above −50 °C signals attributable to [NiMe2(L1tBu)] could be observed, that were persistent in solution for 24 h. Reaction of trans[NiMe2(PMe3)2] with L1tBu gave the binuclear complex 185Ni which was characterized spectroscopically and crystallographically. When 185 Ni was warmed above 50 °C, decomposition was observed with concomitant exclusive formation of methane and surprisingly no ethane (Scheme 227).464,465 Scheme 227. σ-Alkyl NiII Complexes Supported by Flexible Chelating Bis-NHC Ligands

DJ


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Scheme 229. Ni Nitrosyl Complexes with Anionic Tripodal Tris-NHC Ligands

Scheme 230. Complexes with Tetradentate Tetra-NHC Ligands

single-electron transfer; no oxidation of 196bNi was observed in the CV. The highly negative potential observed implies that the neutral tetracarbene ligand inhibits the reduction of the NiII ion. Importantly, DFT calculations provide support that the addition of an electron to the 196bNi populates the LUMO of the complex which resides primarily on the macrocyclic ligand; thus, the monocationic species formed after a singleelectron reduction features a NiII center and singly reduced, radical anionic, redox noninnocent macrocyclic tetra-NHC. A two electron reduction results in the entering of the second electron in ligand-based orbitals too. Complex 197Ni carried out successfully difficult reductions, like the Birch reduction of aromatics, the reduction of toluenesulfonamides, and the reduction of carbonyl compounds.474

Broad scope transmetalation of a related tetracarbene macrocycle from the tetranuclear silver complex 199Ni, the latter obtained by the reaction with AgOTf or AgPF6 and the imidazolium salt 198Ni in DMSO in the presence of 5 equiv NEt3, led to the salt 198Ni with the cation featuring a diamagnetic, distorted square planar Ni center (Scheme 231).475 Finally, macrocyclic tetra-imidazolium diborate ligand precursors with two different ring sizes have been deprotonated with nBuLi, and subsequent addition of NiCl2 led to the neutral macrocyclic tetracarbene complex 203Ni only with an 18-atom macrocycle; the 16-atom variant 201Ni did not react. The incorporation of borates into the macrocyclic rim should improve the electron-donating ability of the carbenes to the Ni DK


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Scheme 231. Complexes with Macrocyclic Tetra-NHC Ligands

of the R1 substituent at the imidazol-2-ylidene. The Ag transmetalation methodology was applied to the synthesis of pyrimidine-functionalized complexes, while the use of [Ni(η3allyl)(1,5-COD)](BArF4) as Ni source gave the cationic η3allyl species (Scheme 232).477−479 The larger bite of picoline-NHC gave rise to (1:1) paramagnetic tetrahedral (206aNi−206cN) and (2:1) diamagnetic square planar (205Ni) complexes, while the pyridineNHC favored (1:1) paramagnetic tetrahedral (209Ni) and (2:1) paramagnetic octahedral (204aNi, 204bNi, and 208Ni) complexes, the latter as dinuclear species with halide bridges. The Ni−CNHC bond distances are shortest in the square-planar (ca. 1.91 Å) and longest in the octahedral geometries (ca. 2.01 Å), despite the disposition of the NHC donors trans to ligands with weak trans-influence (halides); in tetrahedral geometries, Ni−CNHC short-to-intermediate distances (slightly longer than ca. 1.91 Å) were observed. The allyl complex 207Ni is diamagnetic with puckered C−N chelate; it shows activity toward dimerization and hydrosilylation of styrenes. Cyclopentadienyl NiII complexes with benzothiazole- or pyridine-functionalized NHC ligands 210Ni and 211aNi, 211bNi, respectively, have been described; the former feature imidazol-2-ylidene and the latter, 1,2,3-triazol-5-ylidene NHC

and provided a neutral complex soluble in nonpolar organic solvents (Scheme 231).476 3.2.3.3. Heteroatom-Functionalized NHC Ligands. 3.2.3.3.1. Bidentate Ligands. Bidentate heteroatom-functionalized NHCs as ligands to mononuclear NiII centers have been described with L- and X-type donors. The former that are presented below first comprise pyridine, pyrimidine, amine, phosphine, and silylene donors. The discussion is followed by X-type donors, which include anionic group 16 (enolates, phenoxides), group 15 (mainly amido), and group 14 donors (mainly cyclometalated NHC wingtips). NHC ligands with heteroatom functional groups not coordinated in any reactive transformation to the metal are not considered in this section. Pyridine- and picoline-functionalized NHC (mainly imidazol-2-ylidene) ligands have been employed right from the beginning of the NHC ligand development. The first study of this family of complexes employed two methods for their synthesis (i.e., ligand transmetalation to NiII from well-defined Ag-NHC complexes or reaction of simple NiII sources with free NHCs isolated or generated in situ). The nature of the complexes obtained depended on the preferred preparative method, the ligand bite angle (i.e., pyridine or picoline functionalization), the NHC-to-Ni reactant ratio, and the type DL


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Scheme 232. NiII Complexes with Bidentate Pyridine and Picoline-Functionalized NHC Ligands

solid state. The preferred conformation was attributed to weak N−H−N hydrogen-bonding and π−π interactions involving the amino and Mes wingtips of the NHC ligand in 213Ni; in the absence of these stabilizing interactions, the anti conformer would be sterically preferred. Furthermore, catalytic hydrogenation of the imidazolium nitrile 214Ni in the presence of Ni sources led to the diamagnetic square-planar complex 215Ni featuring a bidentate functionalized NHC with primary NH2 functionality. The complex is axially chiral about the two chelating ligands, analogous to BINAP chirality; in solution, it exists as a racemic mixture. The use of enantiopure Δ-TRISPHAT as an NMR chiral shift reagent allowed the observation of diastereotopic ion pairs experimentally. Transmetalation of the ligand from 215Ni to [RuCl2(p-cymene)]2 gave Ru complexes with the functionalized NHC ligand (Scheme 234).481,482 The air stable, square-planar ethylene-phosphine functionalized NHC complexes in a ligand-to-Ni ratio of 2:1 were prepared by the reaction of NiCl2 in DMSO with NaOAc as base or, more generally, by the reaction of [Ni(OAc)2] with the corresponding imidazolium chloride and tetraphenylborate

donors. They were prepared by the reaction of the corresponding imidazolium precursors with NiCp2 (Scheme 233).423,480 Complexes of type 210Ni are efficient catalysts for the homocoupling of benzyl bromide in the presence of MeMgCl with good functional group tolerance and the catalytic oxidative homocoupling of Grignard reagents with 1,2dichloroethane as an oxidant. Complex 211bNi is a very active catalyst for the hydrosilylation of aldehydes (but not ketones) with high catalyst stability and turnover frequencies of up to ca. 20000 h−1. Two types of NiII complexes with amino-functionalized NHC ligands have been reported: first, the pseudotetrahedral paramagnetic and the square planar, diamagnetic 212Ni and 213Ni share the same ligand framework. Complex 212Ni featured chelating κC,κN ligand coordination and was obtained by the reaction of the LiBr adduct of the functionalized NHC ligand with [NiBr2(PPh3)2] in a 1:1 ratio. Complex 213Ni was obtained similarly by reacting the same reagents in 2:1 ratio. Detailed NMR studies on 213 Ni support a syn-transconformation in solution, which was also observed in the DM


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(dppp)2]2+ (depp = diethylphosphinopropane, dppp = diphenylphosphinopropane)] has been carried out. Substitution by NHC of one of the phosphine donors in a diphosphine ligand resulted in negative shifts in the reduction potentials (E°) (by 0.6 to 1.2 V) relative to the corresponding nickel bisdiphosphine complexes; this points to the possibility of redox tuning by interchanging between phosphine and NHC donors. Computational studies were used to rationalize the unreactive nature of 216eNi and 216fNi toward H2 in the presence of strong organic bases, contrasting the behavior of [Ni(depp)2]2+ and [Ni(dppp)2]2+, which were shown experimentally to form the cationic monohydride species. The inability of 216eNi and 216fNi to heterolytically cleave H2 was attributed to the large and unfavorable free energy for H2 addition as well as the thermodynamic instability of the plausible product of H2 heterolysis (i.e., [NiH(NHC-P)2]+), due to the weak Ni−H homolytic bond dissociation free energy; furthermore, the hydride donor aptitude in the [NiH(NHC-P)2]+ complex with the hybrid ligand bearing NHC and PPh2 donors was computed to improve by 32 kcal/mol relative to the estimated hydride donor aptitude for the analogous nickel complex of the chelating diphosphine ligand 1,3-bis(diphenylphosphino)propane. This combined picture renders the potential of 216eNi and 216fNi, as catalysts for H2 splitting and CO2 activation, rather low.484 Complex 217Ni featuring pseudo square-planar Ni (Scheme 235) was described as catalytically competent species in the direct Suzuki cross-coupling reactions of allylic alcohols with phenylboronic acid esters in the presence of K3PO4. It was contended that the P donors of the ligand help stabilize the Ni catalyst under the reaction conditions, while the NHC donor helps to create an electron-rich Ni center to enable oxidative addition of the weakly electrophilic C−O bond; furthermore, the bidentate nature of the ligand hinders deactivation of the reactive Ni0 intermediates.485 The thermally unstable (above −20 °C) diamagnetic complex 218Ni with the small bite angle, rigid N-phosphanylimidazol-2-ylidene was prepared by the reaction of [NiMe2(TMEDA)] with the free N-phosphanyl-imidazol-2ylidene and characterized at lower temperatures, inter alia crystallographically. It features a strained four-membered chelate ring, a ligand bite angle of ca. 70°, and Ni−CNHC and Ni−Me bond distances falling within the same narrow range (1.92−1.95 Å) (Scheme 235).486 NiII and Ni0 complexes with the N-heterocyclic silylene (NHSi)-functionalized imidazol-2-ylidene chelate ligands were accessed via NiII-mediated tautomerization of the N-heterocyclic hydrosilyl-NHC 219Ni, the latter obtained by the SiIIbased sp3 C−H activation of the 1,3,4,5-tetramethylimidazol-2ylidene by a zwitterionic NHSi stabilized by the nacnac framework (Scheme 236). The facile formation of the mixed NHSi−NHC chelate complex 220Ni succeeded by the hydrogen-atom migration from the Si atom to the exocyclic methylene group of the NHSi, presumably triggered by coordination of the NHC moiety of 219Ni to NiBr2.487 Complex 220Ni was characterized spectroscopically and crystallographically and features a square-planar Ni center, with a Ni−Si bond distance of ca. 2.16 Å, which is longer than the distances seen in NHSi-supported Ni0-(η6-arene) complexes (ca. 2.08 Å), presumably due to the stronger Ni−SiII πbackdonation in the Ni0 complexes. The reduction of 220Ni with KC8 in the presence of PMe3 depended on the reactant ratio, with one equivalent of KC8 giving the unexpected silyl-

Scheme 233. CpNiII Complexes with Bidentate HeteroatomFunctionalized NHC Ligands

Scheme 234. NiII Complexes with Bidentate AminoFunctionalized NHC Ligands

(Scheme 235). The complexes were characterized by multinuclear NMR spectroscopy, mass spectrometry, and X-ray diffraction studies. The observed structures displayed distorted square-planar geometry at the metal and mutually transdisposition of the P and NHC donors, respectively; in the series of benzylic NHC analogues 216aNi−216dNi, the 216bNi has the longest Ni−CNHC and the shortest Ni−P bond distances, which correlate with the lowest activity is Suzuki cross-coupling (see below). The complexes in the series 216aNi−216dNi have been studied systematically as catalysts for the Suzuki-catalyzed cross-coupling of aryl chlorides with phenyl boronic acids in the presence of K3PO4 or Cs2CO3 as bases, where 216aNi is the best performing analogue.483 A study of how the presence of the NHC donor as part of a ligand chelate affects the electrochemical and thermodynamic properties of the complex compared to analogs with all phosphine chelating donors [i.e., [Ni(depp)2]2+ and [NiDN


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Scheme 235. Ni Complexes with Bidentate Phosphine-Functionalized NHC Ligands

Scheme 236. NiII and Ni0 Complexes with the N-Heterocyclic Silylene (NHSi)-Functionalized NHC Ligands

NHC NiII complex 221Ni following a dehydrobromination reaction and with two equivalents of KC8 and PMe3 the hydrosilyl-NHC-Ni0 complex 222Ni stabilized by an agostic

(Si−H)-Ni0 bonding interaction, presumably preceded by intermediate reduction to the NiI analogue of 221Ni. The Si-H in the 1H NMR spectrum appeared at δ 7.8 ppm (JSi−H = 87.5 DO


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Scheme 237. Complexes with O−Donor Functionalized NHC Ligands

Hz); the value of the coupling constant is higher than values for Si-M-H complexes obtained with complete oxidative addition of Si−H bonds to transition metals; the large value of the minimum T1 (1319 ms) and IR data were in accord with a η2-(Si−H) agostic coordination. Use of the chelating dmpe (bis(dimethylphosphino)ethane) instead of PMe3 afforded the tetrahedral mixed NHSi−NHC chelate stabilized Ni0 complex 225Ni. Furthermore, the carbonyl complex 224Ni was obtained by the reduction of 220Ni with K(BHEt3) under a CO atmosphere. Complex 224Ni features a Ni0 center with Ni−Si distances longer than in 225Ni, possibly due to the stronger πacidity of CO compared to the P donors of dmpe, which results in decreased π-backdonation from the Ni to the silicon. The IR stretching vibrations for the CO ligands are seen at ν = 1952 and 1887 cm−1 (cf. ν = 1991, 1927 cm−1 and 2050, 1877 cm−1 in [Ni(dmpm)(CO)2]2488 and [Ni(IMes)2(CO)2], respectively [dmpm= bis(dimethylphosphino)methane)]. This suggests that the NHSi−NHC is a stronger σ-donor than two phosphine or IMes ligands. Complex 220Ni shows a high activity for the Kumada cross-coupling reactions of aryl halides (halide = Cl, Br, I) with p-tolyl- or tBu-MgCl.487

The O-enolate functionalized NHC nickel phenyl complexes 226aNi and 226bNi were prepared in one pot by the reaction of [Ni(1,5-COD)2] with PhCl, two equivalents of NaN(SiMe3)2, and the corresponding α-arylimidazolium-substituted acetophenone proligand, which bears enolizable methylene protons (Scheme 237). The complexes are square-planar with very short Ni−CNHC and Ni−Oenolate bond distances of 1.85 and 1.89 Å, respectively; the latter are slightly shorter than those observed in analogous diphenylphosphinoenolate complexes (ca. 1.92 Å). The aryl wingtip is almost perpendicular to the NHC and coordination planes. Toluene solutions of 226aNi and 226bNi are moderately active for the polymerization of ethylene in the absence of cocatalyst; their activity is comparable to that of SHOP-type catalysts but is lower than that of the neutral Ni salicylaldimine or cationic catalysts. A striking difference between the Ni diimine and 226aNi and 226bNi is the linearity of the polyethylene obtained with the latter, in particular at low pressures; this was attributed to the absence of “chain walking”.489 Attempts to induce the formation of chelate NHC-alkoxide complexes by using the β-hydroxyethyl-functionalized imidazolium or imidazol-2-yliDP


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Scheme 238. Ni Complex with Amidate-Functionalized NHC

KN(SiMe3)2 in the presence of [NiBr(Mes)(PPh3)2] led to the targeted complex 231Ni (Scheme 237). Interestingly, the aforementioned decomposition route is distinct from more common decomposition of NHC complexes of alkyl- or arylorganometallics involving migration of the organic group to the CNHC followed by elimination of 2-alkyl- or 2-aryl-imidazolium salts.493 A series of rigid phenyl(triethylphosphine)Ni complexes 232aNi−232bNi bearing the ligand imidazo[1,5-a]quinolin-9olate-1-ylidene were synthesized by reacting [NiCl(Ph)(PEt3)2] with the corresponding imidazolium proligand and 2 equiv KH. The stronger donor PEt3 in the Ni starting material was selected to suppress uncontrolled ligand dissociation during the synthesis leading to intractable mixtures of products. The complexes were characterized inter alia crystallographically and display a trans arrangement of the NHC and the PEt3 donors. Due to the strong trans effect of the NHC, the PEt3 is labile. The Ni−CNHC and Ni−Oaryloxide bond distances (ca. 1.90 Å) are slightly elongated compared to 226Ni, and the aryl wingtip is nearly perpendicular to the NHC and coordination planes. The rigid 232aNi−232bNi catalyze ethylene polymerization to linear polyethylene at 50−100 °C with good activity in the absence of cocatalysts; most known nickel-based catalysts are deactivated at this temperature. At lower temperatures (50 °C and even at 30 °C), ethylene polymerization activity was observed in the presence of [Ni(1,5-COD)2], a known phosphine scavenger. Co-oligomerization of ethylene with allyl acetate at 100 °C or allyl ether, affording co-oligomers, were also feasible; while other comonomers, such as methyl acrylate and acrylonitrile, suppressed the polymerization reaction.494 Finally, the N-acyliminoimidazolium ylide 233Ni reacted with AgOAc and Na2CO3 to afford the binuclear Ag complex 234Ni, which in turn transmetallates with [NiBr2(DME)] to give the mononuclear square-planar complex 235Ni featuring an κ-Ο-amidate functionalized NHC (Scheme 238). The anionic amide-functionalized diamagnetic complexes of type 236Ni were obtained by the reaction of the amidesubstituted imidazolium salts with NiCl2 in the presence of K2CO3 as a base (Scheme 239). Their solid-state structures revealed square-planar Ni centers and mutually cis-arrangement of the anionic N and the NHC donors. However, in the Bn-substituted analogue, the nature of the isomer at Ni is dependent on the solvent polarity, with the more polar cisisomer formed in highly polar DMSO and the trans-isomer found exclusively in CHCl3; the isomerization is reversible.495 The complexes were evaluated as catalysts in the base-free

dene (pro)ligands (to access complexes which would formally constitute analogues of 226aNi and 226bNi with saturated ethylene spacers at the functionalized wingtip) were not successful.448 Chelate NiII-complexes with NHC ligands functionalized with anionic aryloxides have been studied in detail with various linker lengths and rigidity (Scheme 237). Thus, the reaction of [NiBr2(PPh3)2] with the NHC ligand, generated in situ from two equivalents of NaN(SiMe3)2 and the imidazolium salt, gave complexes 227aNi and 227bNi, irrespective of the ligandto-Ni ratio.490 In the solid state, 227aNi and 227bNi adopt an arrangement with the NHC and aryloxide donors cis to each other, the Ni center in square-planar geometry and Ni−CNHC and Ni−O bond distances at ca. 1.85 and 1.89 Å, respectively. The complexes 228aNi−228dNi with aryloxide-functionalized NHC ligands were obtained from o-hydroxyaryl imidazolium proligands by two different methodologies consisting of either in situ generation of the NHC with nBuLi in the presence of NiII precursors or reaction of [Ni(OAc)2] in the presence of excess K2CO3 as base. The complexes always comply to a 2:1 ligand-to-metal stoichiometry, irrespective of the ratio of the added reagents, and adopt mutually cis-arrangement of NHC and aryloxide donors.491 The related 228eNi prepared by transmetalation from the corresponding Ag complex led to a bidentate coordination mode (κC,κO, with dangling pyridyl donor), while complexation via the reaction of the imidazolium salt with NiCp2 provided complexes, where the ligand adopts a tridentate coordination mode (κC,κO,κN), discussed below.492 With methylaluminoxane (MAO) as cocatalyst, 228aNi−228dNi showed moderate catalytic activities in the addition polymerization of norbornene, while the activity of 228eNi was low. Attempts to prepare analogs of the series of complexes 228aNi−228dNi bearing (an) additional σ-Ni−C bond as actor ligand(s) in catalytic applications, supported by aryloxidefunctionalized saturated NHCs were focused on the reaction of cis-[NiCl(Ph)(PPh3)2] with the imidazolinium salt 229Ni in the presence of two equivalents of the base KN(SiMe3)2 (Scheme 237). The isolated product 230Ni featured a new heterocyclic ring derived from N−C cleavage of the imidazoline ring and ring expansion of the heterocycle, accompanied by migration of the σ-phenyl from the Ni to the CNHC; in this way, a tridentate ligand on Ni with Fischer carbene, amide, and phenoxide moieties was obtained. In order to eliminate completely the migration aptitude of the coordinated σ-aryl group, the Ph was replaced by the bulkier Mes on the Ni starting material. Treatment of the imidazolinium salt with DQ


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imidazolium salt 238Ni and either in situ NHC generation with LiN(SiMe3)2 in the presence of [NiBr2(PPh3)2], followed by reduction with KC8 and spontaneous oxidative addition of the Br-aryl to the in situ generated Ni0 species, or in a one-pot reaction by deprotonation of 238Ni with LiN(SiMe3)2 in the presence of [Ni(1,5-COD)2] and PMe3 followed by spontaneous Br-aryl oxidative addition; the species obtained by the two variations were the square-planar PPh3 240aNi and PMe3 240bNi complexes, respectively (Scheme 240). In the first variation, the trans-[NiBr2(NHC)(PPh3)] (239Ni) could be isolated prior to Ni0 reduction and was fully characterized spectroscopically and crystallographically. Reduction of 239Ni to a transient Ni0 complex was followed by oxidative addition of Caryl−Br to the complex 240aNi featuring the metalated aromatic wingtip of the NHC and PPh3 coordination on Ni. The analogue 240bNi was obtained directly from the imidazolium salt after deprotonation, a Ni0 precursor and PMe3. The structures of 240Ni were established by X-ray diffraction, both adopting a distorted-square-planar/“sawhorse” configuration. The σ-aryl−Ni bond is reactive toward insertion of isocyanides, leading to the η2-imino-acyl complexes 241Ni, the exact nature of which depended on the R substituent of the isocyanides employed. The η2-2,6-xylyliminoacyl can be oxidatively removed from the Ni using a mild oxidant (e.g., CBrPh3) and recoordinated by reduction of the NiII complex with KC8.498 In attempts to understand the mechanism of hydroheteroarylation of vinylarenes catalyzed by a system consisting of [Ni(1,5-COD)2] and stable abnormal NHCs (aNHC), the two components were reacted stoichiometrically at room temperature giving rise to the cyclometalated complexes 243Ni and 244Ni, the nature of which depended on the substitution pattern of the aNHC (Scheme 241). Lack of o-iPr substituents on the DiPP wingtip led to its preferential cyclometalation, otherwise C−H activation took place at the phenyl ring adjacent to the CNHC; in both cases, the Ni coligand was an anionic η3-cyclooctenyl moiety. Both complexes were catalytically competent, possibly resting states, but 243Ni gave better conversions and yields.499 Insight into the initial changes

Scheme 239. NiII Complexes with Anionic Amido Functionalized NHC Ligands

Michael addition reactions of representative cyclic 5membered β-dicarbonyl and β-ketoester substrates.496 The amido-functionalized diamagnetic 237Ni was also obtained by the reaction of the amido-substituted imidazolium salts with [NiCl2(PPh3)2] in the presence of K2CO3 (Scheme 239). Here too a cis-arrangement of the anionic and NHC donors was confirmed crystallographically. The complexes were evaluated as catalysts for the Kumada cross-coupling of aryl chlorides with aryl Grignard reagents leading to nonsymmetrical biphenyls.497 The diamagnetic NiII complexes 240aNi, 240bNi featuring the bidentate anionic aryl functionalized NHC were prepared in one or two steps from the aryl-bromide functionalized

Scheme 240. NiII Complexes with Anionic Aryl-Functionalized NHC Ligands and the Insertion of Isocyanides

DR


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fluxionality of the molecule originating from reversible associative MeCN exchange. On the basis of integration of the 1H NMR spectra and mass spectrometric data, it was deduced that one MeCN is coordinated per Ni. Three plausible structural models were considered to account for the observed spectroscopic data (i.e., dimeric species with bridging acetonitriles, monomeric species with side-on MeCN coordination, and three-coordinate species with end-on MeCN coordination). The latter structure was preferred based on DFT calculations. Complexes 247Ni show modest activity in the coupling of heteroaryls with aryl halides which mechanistically may involve initial reduction of NiII by sacrificial heteroaryl substrate (Scheme 242).500 3.2.3.3.2. Complexes with Tridentate and Tetradentate Ligands. In this section, complexes with heteroatom(s) functionalized (X-type and/or L-type donors) NHC(s) ligands are included. The group is dominated by symmetrical “pincer” bis NHC ligands with neutral pyridine or anionic carbazole bridgehead donor atoms; nonsymmetrical pincer ligands constitute recent additions. The description starts with symmetrical ligands with one NHC donor, followed by symmetrical ligands with two NHC donors, nonsymmetrical ligands and finally the limited number of tetradentate and higher denticity ligands. The symmetrical bis-picoline functionalized NHC complex 248Ni was prepared by transmetalation of the ligand from the trinuclear Cu complex 200Cu using three equivalents of [NiCl2(PPh3)2] (Scheme 243).288 The complex was characterized spectroscopically (except 13C NMR due to poor solubility issues) and crystallographically, the latter revealing a square-planar Ni center with Ni−CNHC bonding distance of ca. 1.84 Å and Ni−Npyridine of ca. 1.94 Å (Scheme 243). The azolium salt 249Ni featuring picolyl wingtips attached to a chiral bicyclic (six- and seven-membered) framework, originating from the (1R)-camphor was used to construct potentially chiral functionalized NHC complexes; reaction of 249Ni with [Ni(1,5-COD)2] led via oxidative addition to the cationic Ni η3-cyclooctenyl, formally square-pyramidal (with one axial pyridine) species 250Ni after isomerization and insertion of 1,5-cyclooctadiene to a transient Ni II -H. Conversion of 250Ni to the square-planar Ni chloride 251Ni proceeded quantitatively by the reaction of the former with CHCl3 or CH2Cl2 (Scheme 243).501 The 1H NMR spectrum of 250Ni showed the presence of two isomers possibly originating from the syn- or anti-disposition of the η3cyclooctenyl ring relative to the axial pyridine; the CNHC signal in the 13C NMR spectrum falls in the region characteristic of the RE-NHCs and downfield to the region associated with imidazol-2-ylidenes. The azolium proligand 252Ni originating from cyclization with CH(OEt)3 of the N,N′-pyridine disubstituted 1,3-diamine could not be deprotonated to an isolable pyrimidin-2-ylidene pincer. However, reaction with KN(SiMe3)2 and CuI afforded the tetrahedral copper complex 253Ni with two dangling pyridine donors, from which the potentially tridentate ligand can be transmetalated to various metals, including to the Ni of [NiBr2(DME)], leading to the green, paramagnetic (μeff = 3.5 Evans’ method) distorted trigonal bipyramidal (axial pyridine donors) 254Ni (Scheme 243).502 The proligand 255Ni obtained by optimized reactions of 1,4,5,6-tetrahydropyrimidine and N-DiPP-benzimidoyl chloride, on treatment with [Ni(1,5-COD)2] produced the NiII species 256Ni, which based on 1H NMR spectroscopy and X-

Scheme 241. NiII Complexes with Anionic Aryl Functionalized Abnormal NHC Ligands

occurring when 243Ni was reacted at elevated temperatures with the heteroarene substrate was gained by NMR spectroscopy; the disappearance of the signal assigned to the cyclometalated C-atom and the liberation of cyclooctadiene was interpreted as evidence to the formation of a 12-electron Ni0(aNHC) reactive species which undergoes oxidative addition with the C−H bonds of the heteroaryl substrate. Furthermore, the mechanism of formation of 243Ni and 244Ni was postulated to involve C−H activation/orthometalation of the species [Ni(aNHC)(1,5-COD)], followed by insertion of the coordinated COD into the Ni−H bond. Interestingly, DFT calculations showed a difference in the electronic structures of 243Ni and 244Ni. In 243Ni, the HOMO is localized on Ni and the aNHC ligand, while in 244Ni, it is localized on Ni and the cyclooctenyl ligand; furthermore, the HOMO of 243Ni is at higher energy. These results show that in 243Ni the aNHC has more electron-donating substituents and the imidazole ring higher charge population which would make the Ni center more reactive in oxidative addition reactions.499 Interaction of the half-sandwich complexes 245Ni bearing dangling alkylnitrile side arms with KN(SiMe3)2 led after abstraction of one of the α-methylene protons to the series of half-sandwich nickelacycles 246Ni in low-to-moderate yields (Scheme 242). The complexes exhibit structures with slightly Scheme 242. NiII Complexes of Functionalized NHC Ligands with α-Metalated Nitrile Wingtip

puckered metallacycles and ligand bite angle dependent on the length of the functionalized wingtip. Treatment of 246Ni with stoichiometric HCl at 0 °C and KPF6 gave after elimination of an anionic Cp the nickelacycles 247Ni, which are sparingly soluble in organic solvents and could only be characterized spectroscopically. The 13C NMR signals of the metalated C atoms in 247Ni appear as broad singlets presumably due to the DS


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Scheme 243. NiII Complexes with Tridentate Heteroatom Functionalized NHC Ligands

Scheme 244. NH3 Activation with a Ni Complex with NHC Diimine Tridentate Ligand

stabilization from the π-electrons of the N flanking atoms, which are engaged in delocalization over the imines. However, deprotonation of 256Ni with KN(SiMe3)2 led to the Ni0 “ato” complex 257Ni, described by the limiting resonance forms 257aNi and 257bNi, which adopted a dimeric arrangement with a K2Cl2 core bridging the Ni(κN,κC,κN) pincer subunits. On the basis of the observed metrical data in 257Ni, in particular the short CNHC−N distance (1.39 Å, cf. 1.44 Å in 256Ni), a

ray crystallography was assigned a pincer structure with anionic pyramidalized C-sp3-central bridgehead donor and two κN imine wingtips (Scheme 244). Compared to known Ni− CHAr2 and Ni−CHR2 σ-bonds, the Ni−Cpyrimidinyl bond distance in 256Ni is significantly shorter (1.98 vs 1.85 Å, respectively); the preferential formation of 256Ni rather than an isomeric carbene hydride was ascribed to an energetically inaccessible NHC structure, due to the lack of C NHC DT


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Scheme 245. NiII Complexes with Tridentate Pyrazole Functionalized Tridentate NHC Ligands

Scheme 246. NiII Complexes Bearing Tridentate Ligands with a Central NHC and P or O Wingtip Donors

degree of C−N double bond character was associated with the molecule, which is better represented with the resonance structure 257bNi. Due to the anionic “ate” character, Ni in 257Ni is expected to show high basicity/nucleophilicity and this was demonstrated by the reaction with NH3 gas in toluene at room temperature, which yielded a mixture of two isomeric species. On the basis of 1H NMR spectroscopy, the two products were assigned the structures 258aNi and 258bNi. Thus, the coordinated NH3 in 258aNi was seen at δ = −0.11 ppm in d8-toluene (0.45 ppm in d8-THF), while in 258bNi, the NH2 was found at δ −2.97 ppm in d8-toluene (δ −3.63 ppm in d8-THF) and the N2CH at δ 5.13 ppm. Finally, exposure of the equilibrium mixture to ND3 reduced the intensity of NH3, NH2, and the N2CH signals due to deuterium scrambling. DFT calculation demonstrated a slight energetic preference for 258bNi. The current system represents a rare case where a Ni0 complex rapidly activated ammonia in a ligand-assisted process with the CNHC acting as proton acceptor, with simultaneous observation of the component featuring the coordinated ammonia and the product of the N−H cleavage being equilibrium partners.503 The square planar complex 261Ni featuring the symmetrical tridentate ligand with one central NHC and two pyrazole donor arms linked via methylene spacers was prepared by a reaction sequence starting from the imidazolium 259Ni and Ag2O giving the silver complex 260Ni with ligand to Ag stoichiometry of 2:1 and pendant pyrazole arms, followed by transmetalation to Ni by reaction with [NiCl2(PPh3)2] (Scheme 245). The broadness of the 1H NMR spectrum of 260Ni was ascribed to dynamic processes involving change of conformations of the pyrazol arms rather than paramagnetism (e.g., tetrahedral−square−planar interconversion); this was based on electronic spectroscopy and the absence of paramagnetism in solution (by NMR). In the solid state, a κN,κC,κN symmetric structure was unveiled with the Cl coligand terminally bound to the NiII center, in a trans position relative to the NHC.504 The symmetrical tridentate ligand with one central saturated NHC and two diisopropyl phosphine donor arms linked via ophenylene spacers was used as building block for stable NiII complexes. Thus, the reaction of the proligand imidazolinium salt 262Ni with [Ni(1,5-COD)2] gave the square-planar cationic NiII-H complex 263Ni after activation of the imidazolinium C−H bond (Scheme 246). The structure of 263Ni in the solid state displays a chiral conformation originating from twisting of the ligand framework above and below the plane defined by the metal and the PCP donors; however, in solution, dynamic processes lead to facile interconversion of the enantiomers as judged by 1H NMR spectroscopy (e.g., the protons of the backbone ethylene unit of the NHC appear as a sharp singlet), indicating equivalence on the NMR time scale. The Ni−CNHC (ca. 1.86 Å), Ni−P (ca.

2.10 Å), and Ni−H (ca. 1.34 Å) bond distances are unremarkable.505 Two symmetrical tridentate ligands with one central NHC and two anionic oxygen donors have been used for the synthesis of Ni complexes. The potassium bis-fluoroalkoxidefunctionalized NHC 266Ni was obtained by two successive deprotonations of the bis fluoroalcohol imidazolium salt 264Ni and the zwitterion 265Ni with NaOMe in methanol and KOtBu in THF, respectively (Scheme 246). The electron-withdrawing nature of the geminal CF3 groups renders the alcohol more acidic than the imidazolium functionality. Transfer of the dianionic ligand to NiII was accomplished by the reaction of 266Ni with [NiCl2(PPh3)2], in which case, the complexes 267Ni and 268Ni (2:1 ratio) were isolated, the latter after chromatographic workup. Both feature square-planar Ni center with puckered six-membered “fused” chelate rings. In 268Ni, the plane of the NHC functionality of the nonchelating ligand is perpendicular to the coordination plane and locked in the observed conformation by intramolecular H bonding involving the O atoms of the two ligands. The CNHC signals in the 13C NMR spectra appear are shielded (ca. δ 159 ppm) relative to DU


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Review

Scheme 247. NiII Pyridine Dicarbene and Related Complexes

reaction of the bis-imidazolium and [Ni(OAc)2] initially at room temperature followed by heating at 130 °C provided the dinuclear species 273aNi, 273bNi in which both bridging and chelating forms of the pyridine dicarbene are present; the different outcome as a function of the temperature was attributed to initial irreversible formation of the binuclear mono-NHC complex framework, on which two additional κC,κN,κC-pyridine dicarbene ligands coordinate. In the complexes 272bNi and 273bNi, which were structurally characterized, the metal adopted a square-planar coordination geometry; the structures seen in the solid state were maintained in solution as evidenced by the symmetry of the NMR spectra.508 In a variation of the above methodology, the bisbenzimidazolium bromide or iodide analogues of 271aNi were reacted with [Ni(OAc)2]·4H2O in a 1:1 ratio in DMSO at 160 °C for 5 min, affording after precipitation from the reaction medium at room temperature, the sparingly soluble, dark purple, paramagnetic, trigonal bipyramidal 274aNi (Scheme 247). Dissolution of the latter in methanol resulted in a color change to yellow, which was attributed to isomerization to square-planar, ionic species with one κC,κN,κC-pyridine dicarbene and one halide ligands coordinated to Ni center,

those of complexes with simple imidazol-2-ylidenes (δ at ca.166 ppm).506 Finally, the rigid bis-aryloxide NHC was complexed to Ni by the reaction of the imidazolium or benzimidazolium salts 269Ni with NiCl2 in the presence of K2CO3 in pyridine to give the square-planar complexes 270Ni with one pyridine coordinated trans to the NHC donor (NiNpyridine = 1.95 Å due to trans influence of the NHC). The NiCNHC bond distance (1.79 Å) is unremarkable, but the chelate defined by the donor atoms and Ni significantly deviates from planarity, presumably due to ligand imposed strain (Scheme 246).507 The impact of 2,6-pyridine dicarbene and related ligands in the Ni coordination chemistry has been considerable with applications in catalytic polymerizations, CO2 reduction, crosscoupling, etc. The reaction of the bis-imidazolium 271aNi with [Ni(OAc)2] as Ni source at higher temperatures (ca. 160 °C) in the presence of (NnBu4)Br gave the pincer complex 272aNi and, after anion exchange with silver triflate, 272bNi.508 The halide-free complex 272cNi featuring the pyridine dicarbene pincer Ni moiety and one end-on coordinated MeCN was prepared by the reaction of the corresponding bis-imidazolium triflate with [Ni(OTf)2] in MeCN and fully characterized (Scheme 247).509 Attempts to affect complexation by the DV


Chemical Reviews Scheme 248.

Ni

Review

NiII Pyridine Dicarbene and Related Complexes

Ag and the isolation of complex 278Ni. This behavior was attributed to comparable Ag− and Ni−CNHC bond energies with the direction of transmetalation being overall determined by additional parameters (e.g., solvation, sterics, and nonbonding interactions). Since the appearance of this report, more reverse transmetalations, in particular involving Ni and Ag, have appeared. Both ionic and coordinated bromides in 275eNi, 275fNi could be abstracted by treatment with two equivalents of AgBF4 in MeCN, leading to the square-planar acetonitrile complex 276Ni, with an acetonitrile ligand coordinated trans to the pyridine. Alternatively, reaction with excess of AgOTf in THF afforded the paramagnetic octahedral THF complex 277Ni with two mutually trans-disposed κ1O(SO2)CF3 ligands and one THF-coordinated trans to the central pyridine (Scheme 248).512 Attempts to study alkyl complexes of Ni stabilized by the pincer pyridine dicarbene ligand involved the interaction of the free pincer ligand with [NiMe2(TMEDA)], which surprisingly gave as the only isolable Ni species the methyl complex 279eNi which featured one NHC ring-opened under the reaction conditions (Scheme 249). Although the transformation was unprecedented, it was mechanistically rationalized by the formation of the transient five-coordinate 279aNi, followed by Ni-to-CNHC migration of the axial methyl, deprotonation of the methyl α to the metal by the external base TMEDA and imidazole ring opening. The metal-to-CNHC migration step of

with the second halide being counteranion. A few of the species 274bNi were fully characterized by exchanging the halide counteranion with larger anions [i.e., (PF6)−]; the pure dichloride 274cNi comprising a coordinated and ionic chloride was obtained by substituting the halides of 274aNi with Cl− originating from the DOWEX resin in methanol. On concentration, the ionic complex was converted to the fivecoordinate chloride analogue 274aNi with two coordinated chlorides on the five-coordinate Ni center (Scheme 247).510 A series of ionic complexes of the type 275aNi−275hNi (with pyridine or picoline central donor) were also prepared by transmetalation from the corresponding silver carbene complexes using [NiCl2(DME)] as source of Ni (Scheme 248).511 A methodology based on the reaction of the free pincer NHC with [NiBr2(DME)] in a 1:1 ratio was applied for an entry to the class of square-planar complexes 275Ni, with pyridine central donor, bulky DiPP wingtips, and ionic bromides.512 Selective exchange of the anionic bromide without substitution of the coordinated bromide was accomplished by the reaction with one equiv of TlOTf or KPF6 leading to the square-planar diamagnetic 275gNi, 275hNi. Interestingly, and in contrast to the silver to nickel transmetalations that were used occasionally for the synthesis of 275Ni (discussed above), attempts to substitute the remaining coordinated Br in 275gNi, 275hNi with OTf, by treatment with excess of AgOTf, led to rare reverse transmetalation from Ni to DW


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evolution. Investigations suggest that the CO2 reduction may occur via two consecutive one-electron reductions from NiII to Ni0 and dissociation of MeCN at the Ni0 oxidation state allowing coordination of CO2. Subsequent proton facilitated steps may lead to the formation of CO. A group of chelate NiII complexes was accessed using the rigid, potentially anionic tridentate bis-NHC carbazole/ide ligand skeleton. Thus, the bis-imidazolium carbazole 280Ni was converted directly to the corresponding distorted squareplanar, diamagnetic complexes 281aNi−281cNi by treatment with [Ni(OAc)2] in the presence of NEt3 as base (Scheme 250). In the solid state, the Ni−CNHC and Ni−Ncarbazole bond distances are ca. 1.94 and 1.83 Å, respectively, while the acetate ligand adopts a κ1-coordination. The complexes 281aNi− 281cNi were tested as one-component catalysts for the coupling of cyclohexene oxide and CO2 leading regioselectively to cis-cyclohexene carbonate (>90%), the best results were obtained with 281bNi featuring the better electron-donating NHC, which in turn enhanced the nucleophilicity of the coordinated acetate, which is instrumental for the epoxide ring opening. At higher catalyst loadings and longer reaction times, the catalyst 281bNi could afford narrowly dispersed poly(cyclohexene carbonate) copolymers.514 The rare neutral Ni−H complex 284Ni was accessed preferentially by a rational approach involving oxidative addition of one imidazolium C−H bond of the partially deprotonated 282Ni to [Ni(1,5-COD)2]; alternatively, the

Scheme 249. NHC Ring Opening of the Pyridine Dicarbene Ligand in a Ni-Me Intermediate

σ-alkyl organometallics has been observed on a limited number of chelate NHC-stabilized σ-alkyls (Scheme 249).512 Complexes from the series of pincer complexes 275aNi− 275hNi have been used as catalysts for the Suzuki coupling of aryl bromides and chlorides with aryl- and alkenyl-boronic acids in the presence of K3PO4, providing a range of biphenyls and stilbenes in high yields (at ca. 1 mol % catalyst loading at 100−120 °C in dioxane or tBuOH).511,513 Complex 272cNi is an efficient catalyst for the selective electrocatalytic CO2 reduction in the presence of H2O without competing H2

Scheme 250. Ni Complexes with the Anionic Carbazole Bis-Imidazolylidene

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same product was obtained by an “unusual oxidative addition” reaction of [Ni(1,5-COD)2] to the fully deprotonated 283Ni. The latter reaction may involve anionic zerovalent metalate stabilized by the pincer ligand although the species could not be isolated for Ni (Scheme 250). Reaction of 284Ni with CHCl3 afforded the square-planar diamagnetic chloride species 281dNi.515 The pincer complexes 286aNi−286dNi featuring protic NHC wingtips were prepared from the bis-imidazole ligands by carboxylate-assisted direct metalation of the imidazoles at higher temperatures in benzene. The favorable stabilization of the metal center within the chelate framework may contribute to the success of this reaction, which is uncommon with the lighter 3d Ni. The chloride 286aNi, obtained by adding NaCl in the reaction mixture of the metalation, was isolated and could be converted to the acetate, triflate, and cationic carbonyl complexes 286bNi−286dNi, by treatment with AgOAc, AgOTf, and CO, respectively (Scheme 250); complex 286dNi loses CO under vacuum. The coordination geometries at Ni for all the above complexes are close to ideal square-planar in part due to the absence of any steric constraints arising from the NHC wingtips. The substitution of the chlorides by acetato- and triflato-ligands was accompanied by the establishment of intramolecular H-bonding involving the O atoms of the oxyanions and the N−H of the protic NHC. This was confirmed by 1 H-, 15 N NMR, and IR spectroscopic techniques.516 The related pincer ligand with mesoionic 1,2,3-triazol-2ylidene NHC functionalities was complexed to Ni by the reaction of the bis-triazolium salt with [NiCl2(DME)] in the presence of KN(SiMe3)2. The unexpected and rare nickelhydride 287Ni (δNi‑H = −6.3 ppm) featuring a distorted squareplanar geometry at Ni was presumably formed via a triazolium C5−H activation. The NHC heterocycles deviate from the plane of the carbazole; the DiPP wingtips that are at right angles to the coordination plane certainly contribute to the stabilization of the Ni−H bond (Scheme 251).275 Finally, the complex 288Ni with the less rigid linear tridentate pincer bearing anionic anilido donor and imidazol2-ylidene wingtips was prepared by transmetalation from the relevant Ag complex using [NiCl2(DME)] as Ni source or,

preferentially, by the reaction of the bis-imidazolium salt with [NiCl2(PPh3)2] in the presence of excess of base (Scheme 251). The complex was shown to be an active catalyst for the Kumada and Suzuki cross-coupling reactions.497,517 The symmetrical 1,3-bis(benz)imidazolium phenylene proligand 289Ni has been used as precursor to pincer complexes featuring an anionic σ-aryl bridgehead donor and two (benz)imidazol-2-ylidene wingtips (Scheme 252). Complex 290aNi was obtained by one-pot sequential metalation/ transmetalation procedure using [Zr(NMe2)4] as selective metallating agent for the C2aryl-H and the two C2-H of the imidazolium salts; once metalation was completed, addition of [NiCl2(DME)] resulted in transmetalation of the pincer ligand from the Zr to the Ni leading to 290aNi. Exchange of Cl for MeCN was also possible by the reaction with AgPF6 in MeCN. Complexes 290aNi and 290bNi were evaluated for the electrocatalytic reduction of CO2 under a variety of conditions, providing fast rates and CO2 substrate selectivity (CO2 vs H+). Rates improved in the presence of water and catalysis occurred at the first reduction potential corresponding to the involvement of NiI oxidation state.518 The deprotonation of the symmetrical 1,3-bis-benzimidazolium-phenylene proligand with KCH2Ph provided the stable bis-carbene, which on reaction with NiCl2 led to the pincer complex 293aNi; the latter could also be obtained by the in situ deprotonation of the bis-benzimidazolium salt with two equivalents of LiN(SiMe3)2 in the presence of [NiCl2py4], which is soluble in organic solvents. Crystallographic characterization of 293aNi confirmed a square-planar NiII center (Ni−CNHC and Ni−Caryl at ca. 1.92 and 1.85 Å, respectively). The coordinated halide of 293aNi can be substituted by σalkyls (Me and CH2SiMe3) using the corresponding alkyl lithiums as transfer reagents; Grignard reagents or attempted complexation of the free NHC to preformed [NiR2(py)2] were inefficient. The diamagnetic alkyls were stable under inert atmosphere and characterized spectroscopically and crystallographically, maintaining the square-planar geometries (more distorted in the case of 293cNi due to steric reasons) and similar Ni−CNHC bond distances as in the parent chloride. A direct method to access the pincer-stabilized Ni-H was developed and consisted of the reaction of the free NHC with [Ni(1,5-COD)2] at −35 °C in toluene (Scheme 252). The diamagnetic, square-planar hydride (δNi‑H = −6.5 ppm) displays shorter Ni−CNHC and Ni−Caryl distances (1.89 and 1.87 Å, respectively) in support of space availability within the “pincer cavity”, provided there are no constraints imposed by the coordinated ligands; controlled protonolysis of the Ni-H bond by treatment of 292Ni with HCl in ether constituted an alternative higher yield method of preparing 293aNi.519 There is a limited number of mononuclear NiII complexes with linear tridentate-functionalized NHC ligands, bearing two disparate L-type donors or one L- and one X-donors, and topologies, where the NHC is situated in terminal of internal sites. Most of the designs have arisen from the aim to attain precise control of the metal coordination sphere and hemilability. The relevant systems with terminal and internal NHC donors are given in the Schemes 253 and 254, respectively. The salicylaldimine-functionalized imidazolium salt 294Ni was converted to the square-planar 295Ni featuring a “CNO donor” chelate by the sequential reaction with NaN(SiMe3)2 and [NiBr2(PPh3)2] or by alkanolysis with [NiCp2] or [Ni(Ind)2]. The complex shows good activity for the

Scheme 251. Complexes with the Anionic Carbazole Bistriazolylidene

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Scheme 252. Complexes with the Anionic Phenyl Bis-Imidazolylidene

Scheme 253. Complexes with the Non-Symmetrical Pincer Ligands Featuring Terminal NHC Donor

296Ni were converted to the square-planar diamagnetic 297Ni, or octahedral paramagnetic species 298Ni featuring “CNN

polymerization of styrene in the presence of NaBPh4 at 80 °C.520 The phenanthroline-functionalized imidazolium salts DZ


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Scheme 254. Complexes with Non-Symmetrical Pincer Ligands Featuring Internal NHC Donor

The complexes with nonsymmetrical linear tridentate ligands where the NHC occupies the internal position are shown in Scheme 254. The nature of the complexes obtained comprising the nonsymmetrical NHC functionalized with 1,2,3-triazol and pyridine/picoline/pyrimidine wingtips depended on the reaction stoichiometries and the size of the spacers between the NHC and the pyridine-type functionalities. All complexes were obtained following the transmetalation methodology and using [NiCl2(PPh3)2] as Ni source. With pyridine/pyrimidine donors as the second type of wingtip, mononuclear species with 1:1 310Ni or 2:1 ligand-toNi ratio 311Ni and square-planar or octahedral geometries, respectively, were obtained. With picoline as the second type of wingtip donor, binuclear species featuring one hydroxo bridge and square-planar Ni centers (312Ni) were isolated in low yields.525 The benzimidazo-2-ylidene complex 310bNi acts as catalyst for the Suzuki coupling of aryl bromides and phenylboronic acids at 110 °C. The aryloxy-NHC complexes 313Ni and 314Ni were prepared by the reaction of the imidazolium salts with

donor” chelates via a Ag transmetalation protocol using one or half mole equivalent of [Ni(PPh3)2Cl2], respectively. The square-planar 297Ni exhibits good activity in the Kumada cross-coupling reaction of aryl chloride at room temperature.521 “CNN donor” chelates were also constructed from NHC, pyridine, and substituted iminophosphine donors (299Ni and 301Ni) leading to the square-planar complexes 300Ni and 302Ni, respectively.522 Finally, the rigid “CCP donor” chelates 307Ni and 308Ni were accessed by a multistep sequence starting from the direct metalation of the imidazolphosphine phosphinite 304Ni with [NiBr2(iPrCN)2] (iPrCN = isobutyronitrile) and leading to the P′CP pincer complex 305Ni. Upon quaternization of the imidazol-phosphine at the imidazole-N and reaction of the resulting cationic phosphine complex 306Ni with the nucleophilic NaOEt, the CCP donor pincer complex 307Ni was obtained. Its cation 308Ni was formed after reaction of 307Ni with AgOTf in acetonitrile.523,524 The cationic species 308Ni catalyzed the hydroamination of nitriles to give unsymmetrical amidines (Scheme 253). EA


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Scheme 255. Complexes with the Tetradentate Functionalized NHC Ligands

nickelocene; they feature square-planar Ni centers and exhibit moderate to high activity toward the homopolymerization of norbornene and copolymerization of norbornene with 1octene with B(C6F5)3 as the cocatalyst.492 Finally, the chiral nonsymmetric imidazolium proligands with phosphine and pyridine donor wingtips were used for the synthesis of Ni-η3octenyl and Ni-Cl complexes 316Ni and 317Ni by methods similar to those described for the symmetrical pyridinefunctionalized tridentate ligand described previously (Schemes 253 and 254).526

The tetradentate ligands bis(N-(benz)imidazolylpyridine)methane, bis(N-imidazolylpyridine)-ethane, bis(N-imidazolylpyridine)-propane, α,α′-bis(N-imidazolylpyridine)-o-xylylene, α,α′-bis(N-imidazolylpicoline)-o-xylylene, and bis(Nimidazolylpicoline)methane were used to prepare the series of Ni complexes 318aNi−318kNi by reactions involving the corresponding imidazolium salts and [Ni(OAc)2] (in the presence or not of (NnBu4)Br, Raney-Ni, Ni powder, or following a Ag transmetalation methodology from isolated or in situ generated Ag complexes and [NiCl2(PPh3)2] or EB


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[NiCl2(DME)] as Ni source (Scheme 255).527−532 The unusual C−C oxidative addition of the highly strained heterocyclic dication 319Ni with [Ni(1,5-COD)2] in a reaction of limited preparative scope gave also the complexes 318c,lNi.533 The structures of complexes 318aNi−318kNi feature a distorted square-planar Ni center with increasing distortion to tetrahedral geometries when increasing the length of the linker G (Scheme 255). Due to the geometric requirements of the linkers, the coordinated NHC and pyridine rings are not coplanar with the central nickel, again depending on the length of the spacers; for example, the dihedral angles between the rings of the two types of donors and the coordination plane range from ca. 20° (NHC) to 85° (py) in the less rigid 318aNi. Of interest in conjunction to the catalytic electroreduction of CO2 versus H2O is the electrochemical behavior of the complexes and its association with the nature and the stability of reduced and oxidized species formed as a function of specific structural features. Cyclic voltammograms of 318cNi, 318dNi, 318eNi in MeCN solution show one reduction peak at −0.84 V (irreversible for 318cNi), −0.86 V (reversible for 318dNi), −0.80 (reversible for 318eNi), and a second reduction wave at −1.69, −1.54, and −1.46 V, all reversible for 318cNi, 318dNi, 318eNi, respectively (relative to SCE). All these reductions were assigned to metal-based events. In particular, the observed irreversible feature of 318cNi at −0.84 V may be a result of NiII to NiI reduction, with the rigid methylene bridge disfavoring square-planar NiI complexes; the additional flexibility of the ligand scaffold confers the ability on the reduced complex to distort in a tetrahedral fashion. Exposure of all three complexes to CO2 causes the appearance of a catalytic wave at the second reduction process. Stoichiometric reductions of the 318cNi, 318dNi, 318eNi with KC8 in THF resulted in the formation of diamagnetic products: 318cNi gave the binuclear 320Ni with a “‘butterfly’” dimeric structure and the two tetradentate ligands coordinating two formally NiI centers; reduction of 318dNi led to 321Ni with a Ni2C2 diamond-core and unusual carbene bridges (Schemes 256

adopt different conformations can rationalize the reversible electrochemical behavior described above. Finally, controlledpotential electrolysis at the peak potential of the second reduction process under an atmosphere of CO2 showed the formation of CO as the major reduction product and no H2 originating for H+ reduction.529 Complexes 318Ni in the presence of [Ir(ppy)3] (ppy = 2-phenylpyridine) and an electron donor reduce CO2 to CO in a visible-light photoredox system that proceeds with high selectivity toward CO2 and activity (turnover numbers and turnover frequencies reaching 98000 and 3.9 s−1, respectively).531 Complexes 318jNi and 318kNi are catalysts for the electrocatalytic H+ reduction in the presence of water with better acid tolerance shown for 318jNi, although its overpotential is high.532 The paramagnetic 322Ni with the rigid pentadentate ligand containing four equatorial pyridines and one axial NHC enforces an octahedral geometry at NiII (Scheme 258). Its structure reveals short Ni−CNHC bonds (1.919 Å).277 Scheme 258. Complex with a Pentadentate NHC Ligand

3.3. Binuclear and Polynuclear Complexes

3.3.1. One-Atom Halide Bridges and Related Complexes. There is a limited number of binuclear or polynuclear species with one-atom-halide bridges. The symmetrical NiII species 129Ni (Scheme 207) and NiI species 60Ni (Schemes 189, 190, 192, 193, 194, 201, 207, 261, 263, and 264) have been discussed in conjunction with the mononuclear complexes. Complex 129Ni could also be obtained preparatively by the reaction of 60Ni with Cl2 or N-chlorosuccinimide (Scheme 263). Furthermore, the complexes 19eNi (Scheme 176), 91Ni (Scheme 196), 99Ni, 100Ni (Scheme 197), 204Ni, 208Ni (Scheme 232), 338Ni (Scheme 262), 341Ni, 342Ni, and 343Ni (Scheme 263), 348Ni, 349Ni, and 350Ni (Scheme 264), and 357Ni (Scheme 267) have already been encountered or are presented further below. Complex 60Ni is a versatile precursor to the NiI(S)IPr moiety.534 Furthermore, the cAAC complex 323Ni has been obtained by the reaction of cAAC and [NiBr2(DME)] (Scheme 259).535 The synthesis and reactivity of binuclear complexes with one μ-chloride and one μ-aryl was attempted as a means to deepen the understanding of Kumada aryl cross-coupling, catalyzed by Ni in the presence of IPr ligands. The stoichiometric synthetic transformations are given in Scheme 259.536 The reaction (oxidative addition) of aryl chlorides (aryl = ptolyl, p-anisyl, 1:1 ratio) with a Ni0(IPr) species in situ generated from [Ni(1,5-COD)2] and IPr (also in 1:1 ratio) gave the coordinatively unsaturated dinuclear NiI complex 324Ni featuring one bridging μ-η2-aryl and one bridging chloride. Interestingly, the addition of 2 equiv of IPr to [Ni(1,5-COD)2] followed by p-chlorotoluene resulted in the formation of the mononuclear product [NiCl(IPr) 2 ] (115bXNi) described above (Scheme 201). Complex 324Ni could also be obtained by the reaction of 60Ni with one equiv of p-tolyl-Grignard reagent, while excess of p-tolyl chloride

Scheme 256. Reduction of a NiII Complex Featuring a Pyridine-Functionalized bis-NHC Ligand with a C1 Linker G (see Scheme 255)

and 257). The 1H NMR spectra of 321Ni in the temperature range from 40 to 75 °C demonstrate fluxionality; its ability to Scheme 257. Reduction of a NiII Complex Featuring a Pyridine-Functionalized bis-NHC Ligand with a C2 Linker G (see Scheme 255)

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Scheme 259. Halide-Bridged Binuclear Complexes

Scheme 260. Binuclear Complexes with Nonsymmetrical Pincer Ligand

reverts 324Ni to 60Ni. Furthermore, a salt metathetical reaction with the aryl Grignard reagent at the bridging chloride yielded the corresponding biaryl complex 325Ni [Ni(σ-η2-C6H4CH3)(IPr)]2 with two symmetrically bridging aryl groups. Both 324Ni and 325Ni are diamagnetic and structurally characterized crystallographically. The Ni−Ni distance of ca. 2.39 Å implies that a Ni−Ni single bond is present; an additional feature of the observed binuclear structures 324Ni and 325Ni is the bonding mode of the bridging aryl with σ-bonding with one Ni center and η2-π-bonding with the second. This pattern results in unequal bond distances within the aromatic ring. In cross-coupling reactions, 324Ni and 115bXNi (Scheme 210), both featuring NiI centers, exhibit different catalytic performance, with the binuclear species being more active even with the deactivated 4-chloroanisole and phenylmagnesium

chloride. Interestingly, stoichiometric competition experiments using 324Ni (R = Me) and p-chloro-anisole led to higher yields of cross-coupling products than using 324Ni and the corresponding Grignard reagent, leading to the conclusion that oxidative addition processes on 324Ni are more likely to lead to cross-coupling than transmetalation. The slow transmetalation may be due to disfavored interaction of the nucleophilic Grignard with electron-rich NiI centers; however, the product from the oxidative addition to 324Ni (possibly a NiII species) could not be identified.536 The precursor to the tritopic NHC ligand, imidazolium salt 326Ni, on reaction with [Ni(1,5-COD)2] led to the unusual NiI dimer 327Ni (Ni−Ni ca. 2.41 Å), featuring one bridging halide and one rare bridging η3-cyclooctenyl ligand, presumably formed after oxidative addition of the imidazolium to a NHC ED


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Scheme 261. Binuclear Complexes with S-Donor Bridges and their Reactions

electronic factors were considered responsible for its stability. Reaction of 331Ni with a range of isocyanides resulted in the nonsymmetric cleavage of the disulfide bridging structure and the formation of a 1:1 mixture of the mononuclear 333Ni and 334Ni, the former containing a Ni(η2-S2) core and a coordinated isocyanide; the latter is a square-planar NiCl2IPr isocyanide adduct. Alternatively, 333Ni could be obtained by the reaction of three-coordinate Y-shaped paramagnetic [NiCl(IPr)(CNR)] with S8, leading again to a 1:1 mixture of the complexes 333Ni and 334Ni; other bulky isocyanides reacted similarly. The IR spectra of 333Ni and 334Ni were characterized by ν(CNAd) at 2136 and 2210 cm−1 , respectively (cf. 2123 cm−1 for CNAd), and therefore constitute examples of nonclassical isocyanide complexes with σ-donation and minimal π-backbonding. Reaction of 331Ni with two equiv KC8 resulted in reductive cleavage of the coordinated S−S bond and the isolation of the binuclear 336Ni (Ni−Ni = 2.36 Å), which adopts a structure with planar Ni centers and the two IPr heterocyclic rings being perpendicular to each other, one coplanar with the planar, diamond-shaped Ni2S2 core and the other perpendicular to it. Reaction of 336Ni with H2 resulted in the splitting of the latter and the formation of the binuclear 335Ni with two hydrosulfide moieties bridging the Ni2 core. Complex 335Ni reverted to 336Ni by hydrogen atom abstraction with two equiv of 2,4,6-tBu3-phenoxy radical.442 Further mechanistic studies of the cleavage of H2 by 336Ni using NMR spectroscopy and computational methods revealed that the formation of 335Ni proceeds via a rate-limiting heterolytic addition of H2 across a Ni−S bond of intact dinuclear 336Ni, followed by cis/trans isomerization at Ni, and H migration from Ni to S. Heterolytic cleavage of the B−H bond of BH(pin) by 336Ni led to the unusual, diamagnetic [{Ni(IPr)}2(μ-SH)(μ-SB(pin)] (337Ni) with a short Ni−Ni distance (2.36 Å) and antiferromagnetically coupled NiI paramagnets (Scheme 261).538

stabilized Ni-H complex and subsequent insertion and comproportionation steps (Scheme 260). Interestingly, the related symmetrical pincer ligand precursor 328Ni on reaction with [Ni(1,5-COD)2] resisted oxidative addition leading instead to the pincer sp3 C−H imidazolyl complex 329Ni, with all metrical data in agreement with the anionic imidazolyl formulation. On the basis of theoretical calculations, this behavior was attributed to steric constraints suppressing the Ni-H bond formation and maintaining the planarity of the imine wingtips. Remarkably, ethylene inserted into the C(sp3)−H leading to 330Ni without any apparent intermediacy of Ni-H intermediates (Scheme 260).537 3.3.2. One-Atom Group 16 Bridges and Related Complexes. Some binuclear or polynuclear species with one-atom group-16 bridging donors as part of chelating multidentate ligand structures have been described above: 2Ni (Scheme 162), 14bNi (Scheme 172), 89aNi/89bNi (Scheme 195), 122Ni (Scheme 204), 137Ni (Scheme 210), 312Ni (Scheme 254), and 361Ni and 365Ni (Schemes 268 and 269). A series of binuclear complexes comprising the Ni(IPr) fragment and bridging sulfide, disulfide, and hydrosulfide donors were prepared in an attempt to elucidate the structures and reactivity of Ni species and are of interest and relevance in catalysis and bioinorganic chemistry (Scheme 261). The reaction of 60Ni with S8 gave the bridging disulfide complex 331Ni, which is thermally stable up to 90 °C. In its structure, the Ni centers adopt distorted square planar geometries, with the two square planes sharing an edge, the S22− moiety. This results in a butterfly structure (Scheme 261). The NHC heterocycles are rotated from the corresponding Ni square planes, while the distance between the two sulfur atoms is ca. 2.01 Å, in agreement with a S−S single bond. Comparison of the structure of 331Ni with the related complexes [{Ni(nacnac)}2(μ2-η2,η2-S2)] and [{Ni(PP)}2(μ2η2,η2-S2)]2+ (PP = 1,8-bis(diisopropylphosphino)naphthalene) showed less hindered arrangement in 331Ni, and therefore, EE


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to elucidate the mechanism of the mesitylimido transfer, 60Ni or 340Ni were treated stoichiometrically with alkylisocyanide (alkyl = Bn, tBu) to give the monomeric trigonal planar, paramagnetic adducts 344Ni (μeff ∼ 2.1 μB), which were also detected by paramagnetic 1H NMR spectroscopy in the catalytic imido transfers mentioned before. Furthermore, treating solutions of 344Ni with MesN3 gave the corresponding carbodiimides and 340Ni, which points to the intermediacy of the latter in the catalytic imido transfer; tosylN3 did not react accordingly. Finally, on reaction of 340Ni with excess CO, the coordinated MesN was transferred to CO giving MesNC O with concomitant regeneration of 60Ni, although the reaction did not occur catalytically. This was rationalized by the reactivity of 60 Ni and excess CO, which led to disproportionation to the binuclear paramagnetic in solution 129Ni (also seen in Scheme 207) and [Ni(IPr)(CO)3]. Complex 129Ni could be obtained preparatively by the reaction of 60Ni with Cl2 or N-chlorosuccinimide. Although the structure of 129Ni features two square-planar moieties in the solid state, the solution paramagnetism implies tetrahedral distortions or dimer dissociation on dissolution. Reaction of 340Ni with PMe3 led to the formation of the corresponding phosphinimine and 60Ni; with excess of PMe3, the complexes 347Ni and the binuclear species [Ni(IPr)]2 were obtained (Scheme 263 cf. Scheme 266).540 Other binuclear complexes with one-atom group-15 (N, P, As) bridging donors have been described in conjunction with the mononuclear complexes from which they originate: 19eNi (Scheme 176), 110Ni, 111Ni, 112Ni, and 113Ni (Scheme 200). 3.3.4. One-Atom Group 14 Bridges and Related Complexes. Binuclear NHC complexes with one-atom group 14 bridging atom have been described above (324Ni, 325Ni, Scheme 259, and 327Ni, Scheme 260). The bridging silyl species 174Ni has been described (Scheme 222). The dinuclear diamagnetic, cationic Ni complexes with bridging diphenyl-carbene and trimethylsilyl-carbene 348Ni and 349Ni were obtained by the reaction of the binuclear 60Ni with diphenyldiazomethane or trimethylsilyldiazomethane in the presence of NaBArF4 after elimination of N2, respectively (Scheme 264). In the solid state, 348Ni adopts an unsymmetrical μ,η3-bonding structure in which the bridging diphenylcarbene binds with one Ni center in a π-fashion involving the ipso- and one ortho-C of a phenyl ring in addition to the carbene carbon. In contrast, the (trimethylsilyl)carbene unit in 349Ni is bound symmetrically to both Ni centers; the Ni−Ccarbene bond distances are short (ca. 1.85 Å), and the Ni− Ni separation (2.43 Å) is in accord with Ni−Ni bonding. Both complexes in solution exhibit NMR spectra with symmetrical structures consistent with C2v symmetry on the NMR time scale. The Ccarbene appear highly deshielded (δ 274 and 294 ppm for 348Ni and 349Ni, respectively, more deshielded than the CNHC at δ 177 ppm). The complex 348Ni exhibits interesting carbene group transfer reactivity: it reacts with excess PhCHN2 to give a 1:1 mixture of Ph2CCPh2 and the azine Ph2C−NN−CPh2; with MesN3 to the ketimine Ph2CNMes (carbene-nitrene coupling) and 343Ni; with CO to the ketene OCCPh2 and the dinuclear NiI−NiI carbonyl complex 350Ni; and with tBuNC to the carbeneisocyanide coupling product Ph2CCNtBu and the isocyanide complexes 344Ni and 351Ni after dissociation of the binuclear species. Attempts to isolate neutral carbene analogues of 348Ni and 349Ni were unsuccessful due to

In a comparative study of the catalytic trifluoromethylselenylation of aryl iodides using in situ generated and well-defined Ni0 and NiI species, respectively, the diamagnetic NiI binuclear species were prepared by the reaction of [Ni(1,5-COD)2] with [NiI2(DME)] and SIPr to give the iodide-bridged binuclear complex 338Ni, analogous to 60Ni (Scheme 262). The reaction Scheme 262. Binuclear Complexes with Se-Donor Bridges and their Reactions

of 338Ni with (NMe4)SeCF3 gave the selenolato-bridged binuclear 339Ni. Complex 339Ni is a competent catalyst for the trifluoromethylselenylation of aryl-iodides by a transmetalation/oxidative addition sequence, which keeps the binuclear core intact. The system shows superior activity and selectivity for the specific catalytic cross-coupling transformation than an in situ formed Ni0 catalyst operating via a Ni0/NiII cycle, which is deactivated over the productive cross-coupling by reaction with the product ArSeCF3 produced at the early stages of the reaction (Scheme 262).539 3.3.3. One-Atom Group 15 Bridges and Related Complexes. Bridging NiII imido complexes have been obtained by the reaction of the binuclear NiI species 60Ni with two organic azides. The reaction with mesityl azide (MesN3) led to the bridging d8−d8 imido, diamagnetic 340Ni (Ni−Ni 2.57 Å) with two terminal chlorides per Ni(IPr) moiety and short Ni−N bond distances (1.76 Å), intermediate between the values expected for Ni−N double and single bonds (Scheme 263). Tosylazide reacted with 60Ni to give the paramagnetic 341Ni (μeff = 2.4 μB), which adopts a nonsymmetrical structure with one Ni center in tetrahedral and the other in square-pyramidal geometries; the tosylimido bridging binds as N,O-chelate to the square-planar Ni. Reduction of 340Ni with KC8 yields the neutral d8−d9 mixed-valence complex 342Ni (μeff = 2.0 μB, 1e paramagnet) and chloride abstraction to 343Ni; the last two complexes exhibit similar geometries and metrical data with slight lengthening of the Ni−Ni separation in 342Ni versus 343Ni (ca. 2.33 vs 2.29. Å). Reaction of alkyl isocyanides with 340Ni (alkyl is Bn or tBu) results in the transfer of the bridging imido to the isocyanide carbon with formation of the nonsymmetrical mesityl-alkyl carbodiimide and regeneration of 60Ni. The reaction can be run catalytically using equimolecular amounts of MesN3 and alkylisocyanides and catalytic 60Ni (10 mol %). In an attempt EF


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Scheme 263. Binuclear Bridging Imido Complexes and their Reactivity

decomposition of the diazoalkane initiated by 60Ni or transient formation of the carbene complexes (Scheme 264).541 3.3.5. Two- and More-Atom Bridges and Related Complexes. Binuclear complexes with two-atom C,N bridging imidazole ligands originating from functionalized NHC ligands have been described. The reaction of [Ni(1,5COD)2] with ItBu proceeded slowly giving the binuclear species 3Ni (Scheme 162) after tBu-N bond cleavage. Bridging coordination modes of chelating tridentate or tetradentate ligands incorporating NHC donors have been encountered in 273aNi (Scheme 247) and 312Ni (Scheme 257). Binuclear complexes with bridging ligands not including NHC donors have also been seen, e.g., 10Ni (Schemes 165, 178, 181, 217, 219, 220, 221, and 222) and (bridging quinoline, Scheme 183) 50Ni (bridging diquinone-diimine, Scheme 185), 65Ni (bridging imino-hexadienyl, Scheme 189), 86dNi (bridging diphosphine, Scheme 194), 106Ni, 107Ni, and 108Ni (bridging cyclotetraphosphine, bridging chalcogenides, Scheme 199), 94Ni (μ-H-B-H, Scheme 196), 99Ni and 100Ni (μ-C5H5, μ-indenyl, Schemes 197, 198), and 147gNi (Scheme 215). In an attempt to gain understanding of the Ni(NHC)catalyzed cycloaddition reactions of nitriles and diynes for the formation of pyridines, the stoichiometric reactivity of [Ni(1,5COD)2] with IPr and a range of not bulky nitriles was explored. Reactions in 1:1:1 molar ratio in hexane gave the

species 352Ni which, in the solid state, adopt binuclear structures with one nitrile bound to two Ni atoms via both η1and η2-binding bonding motifs (Scheme 265). The N−Cnitrile bond distance, of ca. 1.23 Å, is indicative of CN double bond in the η2-bound nitrile; the Ni−CNHC distances are short (ca. 1.86 Å). In benzene solution, the dinuclear structures are maintained (as evidenced by NMR spectroscopy and determination of the molar lass of solutes by the Singer method). The complexes 352Ni are catalytically competent for the cycloaddition reaction of nitriles with diynes (1:1 in toluene), leading to pyridines in good yields. However, stoichiometric reactions of 352Ni with diynes gave good yields of pyridines only if exogenous nitrile was added. Experiments with 352Ni and diynes using isotopically labeled exogeneous nitriles demonstrate the incorporation of the label in the pyridine product. Further evidence from kinetic data of the reaction with first-order dependence solely on dimer and the lack of dimer-crossover products suggest a cycloaddition mechanism involving the presence of the integral binuclear core with possible partial dimer opening as the ratedetermining step while both endogenous nitriles remain coordinated to the binuclear core. Immediate binding of an exogenous nitrile and subsequent reaction with diyne can lead to pyridine product. After the cycloaddition step, the binuclear nature of 352Ni was preserved (Scheme 265).542 EG


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Scheme 264. Binuclear Bridging Carbene Complexes and their Reactivity

Scheme 265. Binuclear Bridging Nitrile Complexes and their Reactivity

Scheme 266. Binuclear [Ni2(IPr)2] and its Reaction with CO2

The reaction of 60Ni with NaOtBu followed by [B(pin)]2 or by SiH(OEt)3 or with Li(BHEt3) gave the Ni0 binuclear coordinatively unsaturated species 353Ni (Scheme 266). In the solid-state structure of 353Ni, each Ni center is coordinated by one IPr and one η6-bound DiPP from the IPr coordinated to

the adjacent Ni atom. In toluene or benzene solution, the dimer slowly dissociates forming [Ni(IPr)(η6-arene)]. Reaction of 353Ni with CO2 gave the binuclear [{Ni(IPr)}2(μCO)(μ-η2, η2-CO2)] (354Ni), in which a CO2 ligand is bridging two Ni atoms. The coordinated CO2 is bent at ca. 133°, and the C−O distances are elongated to ca. 1.26 Å (cf. 1.16 Å in the free CO2). In the 13C NMR, the signals due to the coordinated CO2 and CO appear at δ 172 and 246 ppm, respectively. The partial deoxygenation of CO2 evident from the structure of 354Ni, which comprises also one coordinated CO, raised the question of the identity of the O atom acceptor. Since no NMR evidence of additional species in solution was found, it was proposed that a NMR silent paramagnetic NiL(CO3)n species may be accompanying the formation of 354Ni (Scheme 266).543 The binuclear diamagnetic Ni02 complexes 355Ni, 356Ni and the NiI2-containing 357Ni were constructed on a rigid dicarbene scaffold bringing the metals in close proximity with the potential for dinuclear reactivity (Scheme 267). Complexes 355Ni and 357Ni were obtained by reactions involving complexation of the bis NHC ligand generated in situ with [Ni(1,5-COD)2] (for Ni02) or in situ comproportionation in the presence of [Ni(1,5-COD)2] and [NiCl2(DME)] (for NiI2). A striking difference between the structures of 355Ni and 357Ni is the localization of the metals in the opposite or EH


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Scheme 267. Rigid p-Phenylene Bis-NHC Supported Binuclear Complexes and the Reactions with CO2

Scheme 268. Multinuclear Complexes with Pyrazole Functionalized NHC Ligands

congestion. On the basis of 1H NMR spectroscopy, nonplanarity is maintained in solution.544 Complexes 361Ni feature ligand-to-Ni stoichiometry of 1:2 and bridging hydroxide between the two Ni centers which adopt square planar geometries (Scheme 268). Treatment of the macrocyclic 362Ni with [NiCl2(PPh3)2] and Cs2CO3 led to the concave bowl-shaped dinuclear complex 363Ni; the appearance of the NMR spectra points to maintaining of the structure in solution and the presence of high inversion barriers (Scheme 268).545 Trinuclear complexes with two-atom pyrazolate bridges were obtained by using the bidentate pyrazole functionalized NHC originating from the proligand 364Ni. Depending on the R substituent of the NHC wingtip, trinuclear linear or triangular arrangements were obtained, the latter featuring μ3-hydroxo bridges. All Ni centers are found in square-planar environments with long separations precluding intermetallic interactions (Scheme 269).546

the same face of the aromatic spacer, respectively, yielding a closely interacting dinuclear core only in 357Ni (ca. 2.39 Å); in both structures, each metal center interacts in a η2-fashion with the central aromatic ring spacer. In benzene solution, 355Ni exists in equilibrium with 356Ni; pure 356Ni can be obtained in benzene in the presence of H2 which hydrogenates 1,5-COD; however, it has limited stability in solution or in the solid state, precluding structural characterization. In addition, the broadness and symmetry of the 1H NMR spectrum of 355Ni manifests nonrigidity in solution. Treatment of benzene solutions of 355Ni or 356Ni with CO2 led to a mixture of products, the major species being 358 Ni , which was characterized spectroscopically and crystallographically. Its structure features two closely interacting nickel centers located on the same face of the central arene (Ni−Ni ca. 2.25 Å) that are bridged by two μ-CO ligands; the Ni centers also interact with the vicinal diene of the central arene in a η2-fashion. As with 353Ni, reduction of CO2 to CO on the electron-rich Ni02 core took place, even though the identity of the oxygen atom acceptor was not pinned down (Scheme 267).302 A series of binuclear NiII2 complexes with two atom bridges as part of the pyrazole-functionalized bis-NHC ligands were prepared by the reaction of the proligands 359Ni with NaN(SiMe3)2 in the presence of [NiCl2(DME)] or via the synthesis of the Ag complexes followed by transmetalation to [NiCl2(DME)] or [NiCl2(PPh3)2] (Scheme 268). Pyridine- or picoline-substituted derivatives of 359Ni under silver transmetalation conditions led to Ni2(μ-OH) cores with the bridging pyrazolyl-functionalized NHC ligand. The structure of 360Ni reveals a ligand-to-Ni stoichiometry of 1:1, with two nickel atoms bound to two carbenes and two pyrazole nitrogens in a severely distorted square-planar geometry; the twist angle between the two Ni square planes is greater for the bulky DiPP-substituted structures as a way to alleviate steric

4. NHC COBALT COMPLEXES 4.1. Introduction to Co

A large literature covers the field of N-heterocyclic carbene (NHC) complexes of cobalt. Early work includes studies by Lappert where NHC-Co complexes were accessed from electron-rich alkenes.329,547−549 These reports have not been included in this review. Although the largest part is concentrated on the three “easily” accessible oxidation states CoI, CoII, and CoIII, recent reports describe the synthesis of highly reduced Co0 and Co−I complexes or of highly oxidized complexes (formally CoIV or CoV) stabilized by NHC donors. Noninnocent ligands sometimes play a role in stabilizing such complexes, rendering the “observed oxidation-state” of the metal center ambiguous. In the classification used here, the different complexes are sorted according to their “formal” oxidation state. The organization of the section follows the general guidelines outlined in the introduction: it is divided based on the nuclearity of the complexes, the metal oxidation state EI


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4.2.1. Monodentate Carbene Ligands. 4.2.1.1. Homoleptic: Type [Co(NHC)2]. The only examples of homoleptic NHC-Co0 complexes correspond to the bis-cAAC complexes 1aCo and 1bCo (Scheme 270).551,553 The synthetic accessibility to the formally Co0 complex 1aCo was indicated by the cyclic voltammogram of the CoI precursor complex, 2aCo, which exhibited a quasi-reversible one-electron reduction at E1/2 = −0.57 V in DMF versus (Cp*2Fe)/(Cp*2Fe)+. The synthesis of 1aCo was achieved by reduction of 2aCo with KC8 in THF in 98% yield or, alternatively, by reaction of 2aCo with 2.2 equiv LDA as reducing agent. The solid-state structure of 1aCo was established by X-ray crystallography, revealing a twocoordinate bent structure. The Co0 center is solely coordinated by two Me2cAAC ligands with a CcAAC-Co-CcAAC angle of ca. 170.1°, resulting in a bent “2-metallaallene” structure. Upon reduction, a slight shortening of the Co-CcAAC bond distance is observed [from 1.920(2) and 1.932(2) Å in 2aCo to 1.871(2) and 1.877(2) Å in 1aCo], as well as an elongation of the CcAAC−N bond; both changes were attributed to π-backdonation. The EPR spectrum of the paramagnetic 1aCo exhibits a broad unresolved resonance both in the solid state and in THF solution, arising from very rapid relaxation in the quasilinear structure. Theoretical calculations were carried out and support a doublet S = 1/2 ground state. The CcAAC-Co-CcAAC bending was shown to lead to more effective overlap of the ligand orbitals with the 3d orbitals on the Co atom. Complex 1bCo was obtained by halide abstraction in 2bCo using NaBArF4, followed by reduction with one equiv of Na/ Hg.553 The room-temperature solution magnetic moment of 1bCo (2.0 μB) is consistent with a low-spin S = 1/2 center. Analysis by EPR spectroscopy revealed a well-resolved signal at low temperature (77 K) with clear hyperfine coupling to the 59 Co nucleus, suggesting metal-centered spin character. However, the electron density is partially delocalized on the cAAC ligand, as evidenced by relatively long C−N bonds [1.355(5) Å]. DFT calculations on 1bCo confirmed only partial spin delocalization on the ligand and mainly metal-centered spin density (60%). 4.2.1.2. Heteroleptic. 4.2.1.2.1. Type [Co(NHC)L2]. The Co0 complex 3Co bearing a chelating divinyltetramethyldisiloxane (dvtms) olefin ligand was obtained in good yield (75%) by the one-pot reaction between IMes, CoCl2, dvtms, and KC8 (Scheme 271).554 Similarly, 4aCo−4fCo were synthesized in moderate-to-good yield by reaction of the free carbene with CoCl2 and Na/Hg or KC8 in the presence of vinyltrimethylsilane (vtms).555−557 In practice, an excess of vtms was used in order to maximize the isolated yields of the Co0 complexes. The solution magnetic moments of 3Co and 4aCo− 4fCo are in the range of 2.8−3.3 μB, consistent with low-spin Co0 centers. These values are substantially larger than the spinonly value for an S = 1/2 system, probably due to strong spin− orbit coupling and/or the presence of trace amounts of paramagnetic impurities in the samples. The 1H NMR spectra of these paramagnetic complexes reveal that the idealized C2 symmetry observed in the solid state is retained in solution. Interestingly, the preferred NHC-to-vtms 1:2 ratio in 4aCo− 4fCo seems to be mainly governed by the steric properties of the NHCs, as attempts to prepare [Co(CyIDep)2(vtms)] only resulted in the isolation of 4eCo (Scheme 271). The reactivity of 3Co and 4aCo−4fCo was investigated toward bulky organic azides and hydrosilanes (Scheme 272).554−557 The vtms complexes exhibited a higher reactivity toward organic azides than the chelated dvtms 3Co complex.

Scheme 269. Multinuclear Complexes with PyrazoleFunctionalized NHC Ligands

Scheme 270. Synthesis of a Co0 Bis-cAAC Complex

and the nature of the coligands (following the LnXm ligand classification). Selected structural, magnetic, and spectroscopic data for the different NHC Co complexes are compiled in Tables 14, 15, 16, 17, 18, 19, and 20. In addition, the characteristic IR absorption bands of selected CO, N2, and related complexes are presented in Table 21 and Table 22. 4.2. Mononuclear Co0 Complexes

The chemistry of reduced Co0 complexes with NHC ligands is recent with the first examples published in 2014. These highly reactive species have been accessed either by ligand exchange from a Co0 precursor,550 by reduction of the corresponding NHC-CoI complexes,551−553 or by reduction of in situ generated NHC-CoII complexes in the presence of olefin ligands.554,555 Selected structural and magnetic data of NHC Co0 complexes are compiled in Table 14. EJ


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Scheme 271. Synthesis of the Co0 NHC Complexes 3Co−5Co

Scheme 272. Reactivity of Co0 NHC Complexes with Hydrosilanes and Organic Azides

Accordingly, reaction of 3Co with DiPPN3 resulted in the CoIV complex 6Co, while the reaction with DmpN3 (Dmp = 2,6dimesitylphenyl) was unsuccessful. In contrast, reaction of the vtms analogue 4aCo with DmpN3 afforded the cyclometalated CoII amido complex 7Co.556 The outcome of the reaction is determined by the nature of the NHC wingtip substituents as treatment of 4bCo−4dCo with DmpN3 afforded the CoII imido complexes 8aCo−8bCo.556,557 The vtms Co0 complexes [Co0(NHC)(vtms)2] react with hydrosilanes, leading to the dinuclear cobalt silyl complexes 9Co and 10aCo or 10bCo, depending on the nature of the NHC ligand and on the stoichiometry of the added SiH2Ph2 reagent.555 4.2.1.2.2. Type [Co(NHC)L 3 ]. The paramagnetic Co 0 complexes 11aCo−11cCo bearing a bis(olefin)-amino ligand were obtained by substitution of the PPh3 donor in the

corresponding zerovalent phosphine complexes (Scheme 273).550 The solution effective magnetic moment of 11aCo− 11cCo (μeff = 1.8−1.9 μB) is consistent with a S = 1/2 spin ground-state, which is further supported by SQUID magnetic susceptibility measurements. The crystal structures of the three complexes reveal a distorted tetrahedral coordination environment for the Co0 center containing the NHC, the amine, and two olefin ligands. Strong π-backdonation from the metal to the olefins is indicated by a significant elongation of the coordinated CC bonds (average 1.43 Å, in comparison to 1.34 Å for an unperturbed CC bond). The complexes were further characterized by EPR spectroscopy, and the spectra recorded at room temperature were consistent with a low-spin d9 metal center in a distorted tetrahedral geometry. The EK


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Table 14. Selected Structural and Magnetic Data of NHC Co0 Complexes

EL


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Table 14. continued

a

Solution magnetic moment.

Scheme 273. Synthesis of Paramagnetic Co0 NHC Complexes with a Bis(olefin)-Amino Ligand and Application in Selective Phosphine Oxidation

ful, further suggesting that the NHC-to-vtms stoichiometry is mostly governed by the steric properties of the NHC ligand. The crystal structures of 5aCo, 5bCo reveal a trigonal planar coordination environment with marginally longer Colefin-Colefin separations in comparison with the mono(NHC) complexes 4aCo−4fCo, probably due to enhanced Co-to-olefin backdonation in the bis(NHC) complexes. The reactivity of 5aCo toward hydrosilanes was found to depend on the experimental conditions (Scheme 274).555

activity of the complexes was examined in the selective oxidation of secondary and allylic phosphines with N2O. 4.2.1.2.3. Type [Co(NHC)2L]. The bis(NHC) Co0 complexes Co 5a , 5bCo were synthesized in moderate yield (34−53%) applying the same procedure as for 4aCo−4fCo but using 2 instead of 1 equiv of the NHC ligand (Scheme 271).555 The synthesis only succeeded with the moderately bulky IMesCy and ICy ligands. In addition, attempts to synthesize the mono(NHC) complex [Co(IMesCy)(vtms)2] were unsuccessEM


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Scheme 274. Reactivity of the Bis-NHC Co0 Complex 5aCo Toward SiH2Ph2

ligated N2. The N−N stretching vibration of the coordinated dinitrogen ligand was detected at 1917 and 1921 cm−1 (in KBr and THF, respectively), indicating substantial backdonation from the electron-rich Co center to the N2 ligand. Analysis of 14Co by cyclic voltammetry revealed two quasi-reversible oneelectron processes at −2.10 and 0.34 V versus SCE, probably corresponding to formation of the anionic [Co(ICy)3(N2)]− and the cationic [Co(ICy)3(N2)]+ species, respectively. Accordingly, the chemical oxidation of 14Co with [FeCp2](BF4) afforded the trigonal planar CoI complex 16Co without any coordinated N2 ligand. In an alternative synthesis, 16Co was obtained by the chloride abstraction reaction from 15Co with NaBF4. 4.3. Mononuclear Co−I Complexes

Monodentate Carbene Ligands: Type [Co(NHC)2L2]−(Cation+). As indicated by cyclic voltammetry studies, complexes of the type [Co(ICy)n(N2)m]− may be accessed by the one-electron reduction of 14Co (Scheme 276).552 The bis(dinitrogen) Co−I complexes 17aCo−17cCo were obtained in moderate-to-good yield by the reaction of 14Co with the corresponding alkali metal. They decomposed quickly in THF solution, and their 1H NMR spectra are consistent with an idealized C2 symmetry in solution. Two bands, corresponding to the symmetric and asymmetric stretching vibrations of the two dinitrogen ligands, were detected at ca. 1800 and 1900 cm−1 in the IR spectra (Table 22), indicating a stronger N2 activation in the Co−I complexes than in the Co0 complex 14Co. The solid-state structures of 17aCo−17cCo revealed nearly isostructural complexes where the Co center has a distorted tetrahedral coordination geometry defined by two ICy ligands and two end-on N2 ligands. The Co−N bond distances, in the range 1.747(4)− 1.765(4) Å (Table 15), are shorter than in 14Co [1.798(5) Å], further implying a stronger activation of the N2 ligand in 17aCo−17cCo. The reaction of 17aCo−17cCo with triflic acid generated N2H4 in 24−30% yields (relative to Co). These yields are relatively high for such a reaction, which can be traced back to the strong N2 activation in these complexes. In contrast, only traces of hydrazine were formed by the reaction of 14Co with triflic acid under similar conditions. In order to get some insight into the mechanism of the N2 activation and reduction reaction, 17aCo was treated with 1 equiv of SiClR3 (R = Me, Et), resulting in the low-spin CoII diazene complexes 18aCo and 18bCo which were isolated in moderate yield (Scheme 276) and were characterized spectroscopically (including EPR) and structurally by X-ray diffraction. Combined experimental and theoretical results support the formulation of 18aCo and 18bCo as low-spin CoII complexes with a coordinated dianionic [η2-R3SiNNSiR3]2− ligand. The dinuclear complex 19Co was obtained by addition of 18crown-6 to 17aCo. Its crystal structure revealed a tetrahedral coordination environment for the Co−I center, the latter surrounded by two ICy and two end-on coordinated N2 ligands. One of the N2 ligands is bridging the Co center and the chelated K ion. The overall spectroscopic data of 19Co are very similar to those of 17aCo.

Reaction of 5aCo with SiH2Ph2 at low temperature led to the square-planar CoII silyl hydride complex 12Co, isolated in 21% yield. Analysis of its crystal structure revealed a cis arrangement for the two NHC ligands and short Co−H and Co−Si distances (ca. 1.47 and 2.25 Å, respectively). The steric bulk created by the presence of the two NHC ligands probably prevents the formation of dinuclear complexes, previously observed in the reaction of the mono(NHC) [Co0(NHC)(vtms)2] complexes with SiH2Ph2 (Scheme 272). The silyl hydride complex 12Co is not stable at room temperature and converts to 13Co, which features a silyl-functionalized NHC chelate and a cyclometalated [IMes′Cy]− ligand. A distorted square-planar coordination environment is observed for the CoII center in 13Co with the two NHC ligands trans to each other. The synthesis of 13Co also proceeds directly by reaction of 5aCo with SiH2Ph2 at higher temperature. 4.2.1.2.4. Type [Co(NHC)3L]. The Co0 dinitrogen complex 14Co was obtained by reduction of the CoI-Cl analogue 15Co with 1 equiv KC8 under N2 atmosphere and isolated in 87% yield (Scheme 275).552 The solution magnetic moment of 14Co [μeff = 2.6(1) μB] is higher than the spin-only value for an S = 1/2 system, possibly due to contribution from the orbital moment. Characterization by EPR spectroscopy supports a doublet ground state with small g-value and large 59Co nuclear hyperfine constants. The crystal structure of 14Co revealed a distorted tetrahedral coordination environment with an end-on Scheme 275. Synthesis and Reactivity of the Co0 Dinitrogen Complex 14Co

4.4. Mononuclear CoII Complexes

4.4.1. Monodentate Carbene Ligands. 4.4.1.1. Homoleptic: Type [Co(NHC)4]2+(A−)2. Only one example of a homoleptic CoII NHC complex, 20Co, has been described in the literature (Scheme 277).558 This complex has been EN


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Scheme 276. Synthesis of the Co−I Complexes 17a-cCo

Table 15. Selected Structural and Spectroscopic Data of NHC Co−I Complexes

a

Polymeric structure with two subunits in the asymmetric unit. bTwo crystallographically independent molecules in the asymmetric unit.

obtained in 52% yield by reaction of Me2IEt with [CoCl(PPh3)3] and NaBF4, followed by one-electron oxidation with [FeCp2](BF4). An alternative higher yielding procedure consists of the direct reaction of Me2IEt with CoCl2 in the presence of NaBF4. The solution magnetic moment of 20Co, μeff = 2.4 μB, is consistent with a low-spin four-coordinate CoII center. A square-planar coordination geometry for the Co center was established by X-ray crystallography, and the complex was further characterized by EPR spectroscopy. Complex 20Co can engage in one-electron redox reactions, as evidenced by its reaction with aryl Grignard reagents (ptolylmagnesium bromide), which led to the reduction of the metal center. As a result, the corresponding CoI complex 21aCo was formed along with biaryl coupling products. Conversely, treatment of 21aCo with organic halides gave back 20Co,

suggesting that [Co(Me2IEt)4]1+/2+ is a privileged platform for one-electron redox reactions. 4.4.1.2. Heteroleptic. 4.4.1.2.1. Type [Co(NHC)X2] and Related Complexes. Imido complexes. Reaction of the Co0 complexes 4bCo−4dCo with DmpN3 afforded the twocoordinate CoII imido complexes 8au/sCo and 8bCo in good yields (71−82%) (Scheme 278).556,557 These complexes are air-, moisture-, and heat-sensitive and slowly decompose in solution at room temperature. However, in the solid state, they can be stored under inert atmosphere at −30 °C for several months without any decomposition. The crystal structures of 8au/sCo and 8bCo reveal nearly linear CNHC−Co−N arrangements with short Co-Nimido distances (1.675(3)−1.691(6) Å, Table 16), consistent with the formulation of a Co−N multiple bond. In comparison, the Co−Namido separations reported for EO


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Scheme 277. A Rare Example of Homoleptic CoII NHC Complex

ethylene and CO resulted in imido group transfer reactions. In the case of ethylene, transfer of DmpN led to the formation of the imine DmpNCH−CH3 along with the putative Co0 complex 22Co, identified by 1H NMR spectroscopy. With the use of excess CO, the dinuclear Co0 carbonyl complex 23aCo and the isocyanate DmpNCO were isolated in very good yield and fully characterized. Reaction of 8auCo with SiH2Ph2 and SiH3Ph led to the lowspin CoII complexes 24Co and 25Co, respectively. Both complexes feature a β-agostic interaction between the Co center and the Si−H moiety. The CoII hydride complex 24Co and the cyclometalated 25Co result from a Si−H bond activation reaction, where a bond is created between the less electronegative Si atom and the nucleophilic N center. The absence of cyclometalation in 24Co may be traced back to steric reasons. Reaction of 8auCo with the terminal alkyne p-TolCCH resulted in a C−H activation reaction, leading to the amido alkynyl CoII complex 26Co in 65% yield. Analysis of the crystal structure of 26Co reveals a pseudo three-coordinate Co center surrounded by one NHC, one alkynyl, and one amido ligands. Accordingly, the Co-Namido bond distance [1.918(1) Å] in 26Co is substantially longer than the Co-Nimido separation in 8auCo [1.691(6) Å]. Amido complexes. The cobalt bis(trimethylsilyl)amido complex, [Co{N(SiMe3)2}2],559 has proved to be a valuable precursor for the synthesis of 3-coordinate CoII NHC complexes (Scheme 279).560,561 The highly air-sensitive complexes 27au/sCo and 27bCo were obtained in good yield by the aminolysis reaction of [Co{N(SiMe3)2}2] with 1 equiv of the corresponding imidazolium salt. Their solution magnetic moments, in the range of 4.8−5.0 μB (Table 16), indicate a high-spin CoII center. These values are larger than the spin-only moment for an S = 3/2 spin ground-state but in the typical range for related trigonal-planar high-spin CoII complexes. The outcome of the reaction was found to depend on the steric bulk of the NHC ligand. Reaction of the bulkier SIPr·HCl imidazolium salt with [Co{N(SiMe3)2}2] did not provide the expected Co NHC complex but instead the ion pair [SIPrH][Co{N(SiMe3)2}2Cl]

two-coordinate CoII amido complexes are much longer and fall in the range 1.84−1.91 Å. The Co−CNHC bond is slightly longer in 8asCo, probably due to the different steric properties of the saturated NHC ligand. The solution magnetic moment of 8au/sCo and 8bCo is in the range μeff = 4.6−5.1 μB, which is larger than the spin-only value for a high-spin CoII center (S = 3/2) probably due to spin−orbit coupling. DFT calculations were in agreement with a high-spin ground state and indicated a Mayer bond order of 1.55 for the Co−Nimido bond in 8auCo. The magnetic properties of the 8au/sCo and 8bCo were investigated, and a slow relaxation of magnetization with high effective relaxation barriers was observed under zero applied direct current (dc) field.557 For 8asCo, an effective relaxation barrier of 413 cm−1 was obtained, which is the largest value to date for transition-metal based single-molecule magnets (SMMs). Ab initio theoretical investigations indicated that the large magnetic anisotropy in 8asCo mainly originates from the short CoNimido bond. The reactivity of the two-coordinate CoII imido complex 8auCo was investigated toward CO, ethylene, hydrosilanes, and terminal alkynes (Scheme 278).556 Reaction of 8auCo with

Scheme 278. Reactivity of the Two-Coordinate CoII Imido Complex 8auCo

EP


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Table 16. Selected Structural and Magnetic Data for CoII Complexes

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EY


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Table 16. continued

a Magnetic moment in solution, unless otherwise stated. bTwo or more crystallographically independent molecules in the asymmetric unit. cIn the solid state.

Scheme 279. Synthesis of 3-Coordinate CoII NHC Complexes

moments of 29au/sCo−29gCo, in the range of 4.5−5.3 μB, suggest high-spin CoII centers (S = 3/2) with a substantial contribution from the orbital moment to the effective magnetic moment. The 1H NMR spectra of 29au/sCo−29dCo indicate that the idealized C2 symmetry observed in the solid state is retained in solution. Some structural distortions are observed in the complexes bearing nonsymmetrical ligands (29eCo and 29gCo), possibly due to weak secondary interactions. The thermal stability of 29auCo, 29buCo, and 29cCo was investigated (Scheme 280).562 Heating a solution of 29buCo in toluene resulted in the formation of 31Co where rearrangement of the NHC from the “normal” C-2 to the “abnormal” C-4 coordination mode occurred. Complex 31Co was isolated in moderate yield (30%) and corresponds to the first “abnormal” NHC-Co complex. The X-ray structure of 31Co established a trigonal-planar environment with a slightly shorter Co−CNHC bond distance than in the starting 29buCo [2.059(2) vs 2.119(3) Å]. In addition, the steric bulk exerted by the IPr ligand in its “abnormal” coordination mode is significantly reduced. The less sterically hindered IMes analogue, 29auCo, does not undergo thermally induced “normal-to-abnormal”

was isolated along with small amounts of [SIPrH][CoCl3(SIPr)].560 In an alternative synthesis, 27bCo was obtained by reaction of the dinuclear [{CoCl2(IPr)}2] complex 28auCo with NaN(SiMe3)2.561 The bis(amido) complexes 29au/sCo−29gCo were synthesized in good-to-excellent yield by addition of the corresponding free carbene to [Co{N(SiMe3)2}2] (Scheme 279).562−565 Metalation of the in situ generated CycAAC free carbene with [Co{N(SiMe3)2}2] led to the corresponding complex 30Co, which was however isolated in lower yield (20%).563 The structures of all nine complexes were established by X-ray crystallography, and all feature a three-coordinate CoII center in a slightly distorted trigonal-planar coordination environment. A slight decrease in the Co-CNHC bond distance from ca. 2.15 to 2.06 Å (Table 16) was observed in the series SIPr, SIMes, IPr, IMes, and ItBu, consistent with a decrease in the steric demand of the NHC ligands. The Co−CNHC bond distance of 2.130(1) Å in the CycAAC complex 30Co is similar to the Co−CNHC separation in the NHC complexes 29au/sCo− 29gCo, revealing only a weak contribution of π-bonding interaction with the CycAAC ligand. The solution magnetic EZ


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Scheme 280. Thermal Behavior and Reactivity of the Bis(amido) Complexes 29Co

Scheme 281. Reactivity of Three-Coordinate Mono(amido) Complexes 27Co

rearrangement, further supporting a sterically induced reactivity. Heating the ItBu complex 29cCo at 80 °C led to the activation of a tert-butyl substituent and the formation of a 1-tert-butylimidazole CoII complex, indicating the only moderate thermal stability of 29cCo. The mono(silylamido) complexes 27asCo and 27bCo were used to access other NHC-Co complexes by aminolysis of the

remaining N(SiMe3)2 or by ligand exchange with bulky amides (Scheme 281).561,566 Reaction of 27asCo with N,N’-bis(cyclohexyl)acetamidine and 2,6-diisopropylaniline resulted in the aminolysis of the N(SiMe3)2 group and the formation of 32Co and 33Co, respectively.566 The cobalt center in 32Co lies in a distorted tetrahedral environment, coordinated by one NHC, one chloride, and one κ2-amidinato ligand. Only a very FA


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40Co (Scheme 280).564 The solid-state structure of 40Co revealed a centrosymmetric dimer with two bridging phosphinidene ligands. In the 31P NMR spectrum of 40Co, a singlet was observed at δ 449.1 ppm, supporting the formulation of the bridging phosphorus ligands as phosphinidenes. Surprisingly, 40Co is diamagnetic, which was also confirmed by variable-temperature SQUID magnetization measurements. Such diamagnetism may be the result of a strong antiferromagnetic exchange between the two metal centers through the μ-phosphinidene ligands. With the use of the C2-arylated imidazolium salt IPrPh·HI, an interesting strategy to access “abnormal” NHC-CoII complexes was recently described by the group of Ghadwal (Scheme 282).567 The reaction of IPrPh·HI with [Co{N-

weak interaction with the amidinato C atom is observed [CoCamidinato = 2.433(3) Å]. The solid-state structure of the dinuclear 33Co reveals a distorted tetrahedral coordination geometry around the two Co centers. Each of them is coordinated by bridging chlorides, one SIMes ligand and one terminal bent anilido ligand. No direct metal−metal interaction is observed, owing to the long Co···Co intermetallic distance (ca. 3.35 Å). The aminolysis of the coordinated amide was further exemplified by the reaction of 27bCo with 2,6-ditert-butyl-4-methylphenol (BHT-H).561 As a result, the threecoordinate trigonal-planar phenoxide complex 34Co was isolated in 71% yield. Reaction of 27b Co with 2,6diisopropylaniline [NH2(DiPP)] or with the deprotonated Li[NH(DiPP)] did not lead to the corresponding monosubstituted 35Co, but the bis(aryl-amido) complex 36Co was isolated instead. A disproportionation of 35Co into 36Co and [{CoCl2(IPr)}2] (28auCo) may account for this result. The crystal structure of 36Co reveals a pseudo trigonal-planar coordination environment with close Co-H contacts involving the CHMe2 atoms of two aryl-amido groups. Reaction of 27b Co with the bulkier lithium 2,6-dimesitylanilide (LiNHDmp) resulted in an amide exchange with formation of 37Co. Contrary to 35Co, 37Co is stable toward disproportionation and could also be prepared by reaction of [{CoCl2(IPr)}2] (28auCo) with 2 equiv of LiNHDmp. In the solid-state structure of 37Co, a pyramidalization of the cobalt center is observed due to an additional interaction with a flanking aryl group of the NDmp ligand (Co−Cipso separation of 2.577(3) Å). The solution magnetic moment of 37Co (μeff = 3.7(1) μB) is substantially lower than that in the threecoordinate amido complexes 29Co, 30Co, and 36Co (Table 16), likely resulting from the pyramidalization of the cobalt center. The N(SiMe3)2 aminolysis strategy was further exemplified using the bis(silylamido) 29Co to access the aryl-amido complexes 38Co and 39aCo−39bCo,563 and the phosphinidene complex 40Co (Scheme 280).564 Interestingly, a sequential substitution of the N(SiMe3)2 groups by NH(DiPP) anilido ligands is possible. Reaction of 29buCo with 1 equiv NH2(DiPP) led to the mixed silylamide/aryl-amide complex 38Co, while using an excess of NH2(DiPP) resulted in the formation of the symmetrical bis(anilido) complex 39auCo. In the case of the phosphine-functionalized PCNHCP complex 29dCo, only the symmetrical 39bCo was isolated, using either 1 equiv or excess NH2(DiPP). Similarly, only the synthesis of the bis(anilido) complex 39asCo proceeded cleanly. The various complexes were characterized by X-ray diffraction studies, and all feature a trigonal-planar coordination geometry with relatively long Co−CNHC separations. Substitution of the N(SiMe3)2 ligands by NH(DiPP) groups leads to a decrease in the Co−CNHC bond distances, probably due to steric reasons. In addition, similarly to what was observed in the mono(anilido) complex 37Co, close contacts between the Co center and CHMe2 protons of the NH(DiPP) groups were observed. Magnetic and EPR studies of the different complexes are consistent with high-spin CoII centers (S = 3/2) featuring a strong anisotropy and large values for the zero-field splitting parameter D. It is interesting to note that the transamination reactivity from N(SiMe3)2 to NH(DiPP) does not follow simple acid−base considerations but may be due to a combination of electronic and steric effects. The reaction of 29fCo (isolated or prepared in situ) with PH2Mes resulted in a similar reactivity, leading to the formation of the phosphinidene-bridged dinuclear complex

Scheme 282. Synthesis of the “Abnormal” NHC-CoII Complexes 41Co and 42Co

(SiMe3)2}2] led to the bis “abnormal” NHC complex 41Co isolated in good yield (79%); it will be described in the corresponding section (type [Co(NHC)2X2]). The bis(amido) “abnormal” NHC complex 42Co was obtained from an indirect route involving NHC transfer from an “abnormal” NHCborane adduct. The latter was synthesized in good yield by reaction of IPrPh·HI with NaBHEt3. The solid-state structure of 42Co reveals a distorted trigonal-planar coordination geometry, similar to that observed in the “normal” NHC analogues 29Co. The Co−CNHC bond distance of 2.075(2) Å is comparable to that in the “abnormal” IPr analogue 31Co. Alkyl complexes. The three-coordinate bis(alkyl) complex 43Co was obtained by alkylation of the dinuclear [{CoCl2(IPr)}2] (28auCo) with 4 equiv Mg(CH2SiMe3)Cl (Scheme 283).568 Scheme 283. Synthesis of the Bis(alkyl) Complex 43Co

The 1H NMR spectrum of 43Co reveals paramagnetically shifted resonances and is consistent with an average C2v symmetry in solution. The solution magnetic moment of 5.1 μB indicates a high-spin CoII center with significant orbital contribution, further supported by EPR measurements. In the solid-state structure of 43Co, a trigonal-planar coordination environment is observed for the CoII center, coordinated by FB


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Scheme 284. Synthesis of The Four-Coordinate CoII Complexes 44aCo−44dCo

Scheme 285. Reactivity of 45Co Toward Disulfur Compounds

two alkyl and one IPr ligands. No reaction was observed upon exposure of 43Co to CO over short times (15−30 min), suggesting a relative low reactivity of this high-spin CoII alkyl complex. The cyclic voltammetry of 43Co revealed several irreversible reduction events, suggesting that the in situ generated reduced complexes are prone to further chemical events. 4.4.1.2.2. Type [Co(NHC)LX2]. The four-coordinate CoII complexes 44aCo−44cCo bearing an IPr ligand were obtained in moderate yield (30−59%) by cleavage of the corresponding [{CoX2(IPr)}2] (X = Cl, Br, I) dimeric complexes (28aCo− 28cCo) in the presence of pyridine (Scheme 284).569 Dissociation of the dimer did not occur using other donor solvents such as THF, acetonitrile, and acetone. The complexes were characterized by a series of spectroscopic methods, including XPS and EPR spectroscopy. SQUID magnetization measurements indicated a high-spin S = 3/2 ground state for 44aCo−44cCo, consistent with a tetrahedral geometry. All complexes were characterized by X-ray diffraction studies, confirming the tetrahedral coordination environment around the CoII center. Only very slight variations in the Co−CNHC and Co−Npy bond distances were observed by changing the nature of the halide ligand (Table 16). The IMes analogue, 44dCo, was obtained in 26% isolated yield by the one-pot reaction of IMes with CoI2 in THF, followed by addition of pyridine (Scheme 284).570 Complex 44dCo showed similar spectroscopic and metrical data to 44aCo−44cCo. The NHC amidinato complex 32Co (Scheme 281) was formed by aminolysis of the Co−N(SiMe3)2 bond in 27asCo upon reaction with N,N′-bis(cyclohexyl)acetamidine.566 4.4.1.2.3. Type [Co(NHC)L2X2]. The mixed donor CoI complex 45Co was prepared by substitution of one phosphine ligand in [CoCp(PPh3)2] by an NHC donor, and its reactivity toward disulfur compounds was explored (Scheme 285).571 Reaction of 45Co with diphenyldisulfide (PhSSPh) led to the monothiolate CoII complex 46Co in high yield (87%). This paramagnetic complex (μeff = 1.73 μB in solution) features a distorted trigonal-planar coordination geometry in the solid state. Oxidation attempts of 46Co with (1S)-(+)-(10camphorsulfonyl)oxaziridine did not lead to the formation of the corresponding CoII cobaltosulfoxide complex, but instead the more oxidized CoII cobaltosulfone complex 47Co was isolated in 48% yield and crystallographically characterized. Reaction of 45Co with the mixed benzenesulfonothioic acid (PhSSO2Ph) resulted accordingly in a mixture of 46Co and 47Co.571

With the use of a linked cyclopentadienyl-carboranyl [η5:σMe2C(C5H5)(C2B10H10)] ligand, the NHC complex 48Co was obtained by substitution of the phosphine ligand in [Co{η5:σMe2C(C5H5)(C2B10H10)}(PPh3)] (Scheme 286).572 The CoII center is η5-bound to the Cp ring, σ-bound to the cage carbon atom, and further coordinated to the NHC ligand. Scheme 286. Synthesis of the Cp-Carboranyl CoII Complex 48Co

4.4.1.2.4. Type [Co(NHC)X3]−(Cation+). The mononuclear CoII complex 49Co was obtained as a side-product in attempted syntheses of [CoBr2(IPr)2] (Scheme 287).573 While the reaction of [CoBr2(THF)2] with one equiv of IPr led to the dinuclear 28bCo in moderate yield (53%), treatment with two or more equiv of IPr afforded 49Co in high yield. The crystal structure of 49Co reveals an ion pair complex with a tris(bromo)metalate [CoBr3(IPr)]− anion and a reprotonated [IPrH]+ imidazolium cation. The cobalt center in the [CoBr3(IPr)]− anion lies in a distorted tetrahedral coordinaFC


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Scheme 287. Complexes Obtained by Reaction of [CoBr2(THF)2] with IPr

Scheme 288. Synthesis and Reactivity of the Bis(NHC) Complexes 50aCo−50eCo

tion environment. The reason for the reprotonation of one IPr ligand is not clear and may be due to adventitious proton impurities. Small amounts of 49Co and of its chlorido and iodido derivatives were also isolated as decomposition products in the synthesis of [{CoX2(IPr)}2] (X = Cl, Br, I).569 Furthermore, formation of the SIPr analogue, [SIPrH][CoCl3(SIPr)], was detected in the reaction of SIPr·HCl with [Co{N(SiMe3)2}2].560 4.4.1.2.5. Type [Co(NHC)2X2]. Although the reaction of 2 equiv IPr with [CoBr2(THF)2] resulted in an unexpected sideproduct, the addition of 2 equiv IMes, ICy, or Me2IiPr to CoCl2

or [CoBr2(THF)2] in THF led to the formation of the desired bis(NHC) complexes 50aCo−50dCo (Scheme 288).568,573−575 All complexes feature a high-spin tetrahedral CoII center (S = 3/2) and very similar magnetic moments in the range of μeff = 3.9−4.2 μB (Table 16). The redox-activity of 50aCo was investigated by cyclic voltammetry, and a reversible oxidation process was observed in CH2Cl2 at +0.310 V versus Fc/Fc+.568 Attempts to oxidize 50aCo with Ag+ or NO+ did not result in the expected CoIII complex, but the dimeric chloro-bridged complex 28duCo was formed instead, although in low yield. In addition to the anodic event, several cathodic irreversible FD


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Scheme 289. Synthesis of the Bis(NHC)borate Complexes 55Co and 56Co

Scheme 290. Synthesis and Reactivity of the Tris(NHC)borate CoII Complexes 57aCo and 57bCo

2.5(2) μB] indicates a low-spin (S = 1/2) CoII complex. Analysis of the crystal structure of 53Co revealed a fourcoordinate square-planar coordination geometry with the two methyl groups trans to each other. Shorter Co−CNHC separations are observed in 53Co in comparison with 50aCo (ca. 1.92 vs 2.08 Å, respectively), originating from the change from high-spin to low-spin CoII. In the synthesis of 53Co, the mono(methyl) complex 54Co was identified as a minor byproduct by 1H NMR. The latter complex was synthesized in moderate yield by reaction of 50aCo with 1 equiv of MeMgCl. A preliminary X-ray structure determination of 54Co established a square-planar coordination geometry, but the quality of the data was hampered by severe disorder between the CH3 and Cl ligands. Monitoring the reaction of 53Co with excess CO by 1H NMR revealed the immediate formation of a new diamagnetic product, assigned to the Co0 dimer 23bCo, along with free IMes and acetone. A possible mechanism involves the insertion of CO into one of the Co-Me bonds, followed by reductive elimination and dimerization of the resulting Co0 carbonyl complex. This reactivity contrasts with the relative inertness of the high-spin 43Co toward CO, highlighting that the spin-state of CoII alkyl complexes highly affects their reactivity.

processes were observed at −2.19 V and −2.5 V, possibly corresponding to successive reductions of the metal center and further reaction. The electronic structure and bonding in 50aCo and 50bCo were further investigated by combined magnetic circular dichroism (MCD) and DFT calculations.575 In these CoII complexes, the nature of the NHC wingtip substituents induces slight changes in the geometry, resulting in relatively large effects in the ligand field. Although two NHCs are slightly better donors than bisphosphines, a weakening of Co-Cl bonds owing to a trans-like influence occurs, resulting in an overall reduced ligand field splitting. Substitution of the chlorides by thiolate donors was achieved by addition of 2 equiv. NaStBu to in situ generated 50cCo, yielding the high-spin tetrahedral CoII complex 51Co.574 In contrast, reaction of 50cCo, generated in situ, with 2 equiv of PhMgBr led to the formation of the low-spin [μeff = 2.4(2) μB] square-planar diaryl complex 52Co in good yield (65%).576 Alkylation of 50aCo by reaction with 2 equiv of MeMgCl in THF proceeded cleanly and afforded the dimethyl complex 53Co.568 However, the attempted synthesis of other alkyl/aryl analogues by reaction of 50aCo with EtMgCl and PhMgCl was not successful, and partial reduction of the metal center was observed. The solution magnetic moment of 53Co [μeff = FE


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Scheme 291. Tetra-NHC CoII Complex

borate CoII complexes 57aCo and 57bCo were obtained in good yield by deprotonation of the corresponding phenylborane dications and metalation using [CoCl2(THF)1.5] (Scheme 290).579 Interestingly, clean deprotonation was only possible using bulky lithium bases such as LDA, LiN(SiMe3)2, or LiTMP, while bases of other alkali or alkaline-earth metals led to complex mixtures. Analysis of the X-ray structure of 57aCo revealed a Co metal center surrounded by the triponal ligand and one additional chloride anion. The solution magnetic moment of 4.2(3) μB is consistent with the formulation of a high-spin d7 CoII complex (S = 3/2). Reaction of 57aCo and 57bCo with traces of acid resulted in the formation of the zwitterionic complexes 58aCo and 58bCo where one of the NHC donors has been reprotonated. Due to its high sensitivity to traces of acid, 57bCo could only be generated in situ, and isolation attempts led to the partially reprotonated 58bCo. In the solid-state structure of 58aCo and 58bCo, the cobalt center is in a tetrahedral coordination environment, coordinated by two NHC donors and two chloride anions. The solution effective magnetic moment of 58aCo [μeff = 3.9(3) μB] is consistent with a high-spin CoII center, and the 1H NMR spectrum reveals an apparent higher symmetry of the complex in solution, due to rapid proton shuttling between the three carbene ligands. Deprotonation of 58aCo with bulky lithium bases gave back 57aCo, proving the reversible protonation/ deprotonation of the scorpionato ligand. Reaction of 57aCo and 58aCo with, respectively, 1 and 2 equiv of MeLi or MeMgBr resulted in the alkylation of the metal center and the formation of 59Co. The Co−Me bond distance of 2.042(2) Å is comparable to the distance reported for four-coordinate CoII alkyl complexes. The solution magnetic moment of 4.1(3) μB confirms the presence of a high-spin CoII metal center. Reaction of 57aCo with LiNHtBu led to the paramagnetic high-spin CoII (S = 3/2) amido complex 60Co in high yield (77%) (Scheme 290).580 The crystal structure of 60Co contains three crystallographically independent molecules with Co−N bond distances in the range of 1.886(7)−1.88(2) Å and angles around the anionic N atom in the range of 152.5(2)− 172.4(9)°. No decomposition was observed upon heating to 100 °C for several days, indicating the high thermal stability of the amido complex 60Co. Treatment of 60Co with the stable 2,4,6-tri(tert-butyl)phenoxy radical resulted in the formation of the diamagnetic CoIII imido complex 61Co along with 2,4,6-tri(tert-butyl)phenol. The X-ray structure of 61Co establishes a short Co−N bond distance (1.660(3) Å) and a linear Co−N−C bond angle (179.7(3)°), consistent with the occurrence of an imido complex. The formation of 61Co involves both a protontransfer (PT) and an electron-transfer (ET) step, likely occurring through a concerted proton-coupled electron transfer (PCET) pathway. Attempts to generate intermediates arising solely from a PT or ET step were unsuccessful, pointing

The air-sensitive high-spin di-tert-butoxy complex 50eCo was obtained by the one-pot reaction of the IiPr imidazolium salt with CoCl2 and KOtBu (4 equiv) (Scheme 288).577 The solidstate structure of 50eCo was determined by X-ray diffraction studies and revealed an overall four-coordinate tetrahedral geometry around the CoII center. Additional secondary interactions between the O atoms of the OtBu groups and the CH(CH3)2 protons of the iPr wingtips were noted. Other examples of mononuclear CoII complexes of the type [Co(NHC) 2 X 2 ] include the previously described 12 Co (Scheme 274)555 and 18aCo and 18bCo (Scheme 276).552 Complex 41Co (Scheme 282) features a four-coordinate cobalt center in a distorted tetrahedral geometry.567 The Co−CNHC bond distances in 41Co [2.021(2) and 2.037(2) Å] are slightly shorter than, but comparable with, the corresponding separation in the “abnormal” NHC complex 31 Co (2.059(2) Å, Table 16), reflecting the reduced steric pressure induced by the NHC in its “abnormal” coordination mode. Temperature-dependent magnetic susceptibility measurements by SQUID magnetometry are consistent with a high-spin (S = 3/2) CoII center in a tetrahedral environment. 4.4.2. Bidentate Bis-Carbene Ligands. The fourcoordinate 55Co and 56Co complexes bearing bis(NHC)borate ligands were obtained by deprotonation of the imidazolium salt proligand with 2 equiv LDA followed by treatment with anhydrous CoCl2 (Scheme 289).578 Two structural isomers, the square-planar 55Co and the tetrahedral 56Co, crystallized simultaneously and were manually separated. In the crystal structure of 55Co and 56Co, the CoII center adopts a distorted square-planar and distorted tetrahedral geometry, respectively, the distortion being due to the constraints imposed by the chelate ring. The simultaneous crystallization of both isomers under the same conditions implies a very similar stability of 55Co and 56Co, further supported by the unexpected isolation of a cocrystal containing both isomers. Equilibrium between both isomers is observed in solution, leading to similar values for the solution magnetic moments of 55Co (3.47 μB) and 56Co (3.62 μB). These values lie between the spin-only value for a high-spin and low-spin CoII center (3.87 and 1.73 μB, respectively) and suggest that the high-spin tetrahedral complex 56Co is predominant in solution. The magnetic properties of the complexes were further examined by direct-current (dc) and alternating-current (ac) susceptibility measurements at different temperatures. The dc magnetic susceptibility data in the solid state indicate an effective magnetic moment μeff = 1.95 μB for 55Co, close to the spin-only value for an S = 1/2 ground state. The effective magnetic moment of 56Co, μeff = 4.16 μB at 300 K, is slightly higher than the theoretical value for a high-spin CoII (S = 3/2) center, probably due to spin−orbit coupling. 4.4.3. Tridentate and Multidentate Tris- and Polycarbene Ligands. The four-coordinate phenyltris(carbene)FF


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Scheme 292. Synthesis of 64Co by Transmetallation from Ag

Scheme 293. Reactivity of the Cyclometalated CoII Complex 65Co

voltammetry. A single reversible redox process was observed at −1.15 V versus Fc/Fc+, corresponding to the reduction to the CoI analogue, but no oxidative process was observed up to +2 V. One-electron chemical reduction was carried out by treatment of 63Co with sodium amalgam, and the in situ generated CoI complex was used in the reduction/cyclization of a series of functionalized aryl halides. In a rare example of transmetalation from silver to 3d transition metals, the CoII complex 64Co was obtained in good yield (68%) by reaction of the corresponding silver complex with CoCl2 (Scheme 292).475 In the solid-state structure of 64Co, the CoII center features square-pyramidal coordination geometry, ligated by the four NHC donors of the macrocyclic ligand and one triflate anion. A free coordination site is left in the apical position, trans to the coordinated triflate. The latter remains bound in CD3CN solution as evidenced by two

toward a concerted mechanism, which is further supported by DFT calculations. The gas-phase N−H BDE (enthalphy) of 60Co was estimated at 75 kcal/mol (to be compared with the O−H BDE of 2,4,6-tri(tert-butyl)phenol of 81.2 kcal/mol), consistent with a spontaneous reaction. Interestingly, TEMPO (TEMPO-H, O−H BDE = 69.7 kcal/mol) does not react with 60Co, possibly due to a high kinetic barrier. The complex 62Co bearing a tetra-NHC ligand was obtained in low yield by deprotonation of the imidazolium salt precursor followed by reaction with anhydrous CoCl2 and recrystallization from methanol (Scheme 291).581 The X-ray structure of 62Co features a tetrahedral coordination environment around the metal center, and the solid-state magnetic moment of 4.9 μB is consistent with the formulation of a high-spin CoII center. In an improved one-pot procedure, the yield of 62Co could be increased up to 60%. Anion exchange with silver tetrafluoroborate afforded 63Co which was characterized by cyclic FG


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Scheme 294. Reactivity of the Silyl-Functionalized NHC Complexes 70aCo and 70dCo

resulting low-spin CoII complexes 70aCo−70dCo were isolated in 46−66% yield and fully characterized. The solid-state structure of 70aCo−70cCo revealed a square-planar CoII complex bearing one anionic silyl-donor-functionalized NHC chelate and one cyclometalated [IMes′] ligand.576 The Co−Cbenzyl bonds in 70aCo−70cCo [2.048(5) to 2.070(2) Å, Table 16] are slightly longer than those in 65Co [2.025(2) Å], reflecting the stronger trans influence exerted by the silyl moieties. Formation of the silyl-donor-functionalized NHC complexes 70aCo−70dCo corresponds to the formal insertion of a silylene moiety into the Co-Cbenzyl bond, but the precise mechanism is not clear. 4.4.4.1.2. Silyl-Functionalized NHC Ligands. Monitoring the reaction of 65Co with SiH3Ph by 1H NMR spectroscopy revealed the formation of a paramagnetic intermediate, which was assigned to the CoII hydride complex 71Co (Scheme 294).576 This paramagnetic species can also be detected by exposure of 70aCo to H2 and results from the hydrogenolysis of the Co−Cbenzyl bond. However, an equilibrium mixture was observed and, due to the fast elimination of H2, 71Co could not be isolated. Reaction of 70aCo with BH3·THF yielded the borylated low-spin CoII complex 72Co. The crystal structure of 72Co established a distorted tetragonal-pyramidal coordination geometry with one anionic silyl-donor-functionalized NHC ligand and one newly formed anionic NHC-borate chelate. A η2-coordination mode was observed for the benzylborate anion with a Co−B separation of 2.169(4) Å. The reactivity of the silyl-donor-functionalized complexes 70aCo and 70dCo was also examined toward 2-pyridone (Scheme 294).583 Low yields of the paramagnetic CoII complex 73Co were obtained by reaction of 70aCo with 1 equiv of 2-pyridone. The solid-state structure of 73Co features a five-coordinate CoII complex bearing an anionic tridentate NHC-silyl-pyridine ligand. Reaction of 70aCo and 70dCo with 3 equiv 2-pyridone resulted in the formation of the diamagnetic, formally CoIII, complexes 74aCo and 74bCo which were isolated

distinct resonances in the 19F NMR spectrum of the complex, assignable to coordinated and free counteranions. 4.4.4. Functionalized NHCs. 4.4.4.1. Bidentate. 4.4.4.1.1. Alkyl Functionalized (Cyclometalated) NHC Ligands. The cyclometalated NHC CoII complex 65Co (Scheme 293) was obtained in good yield by reduction of [CoCl(IMes)2] (66uCo),582 or by the one-pot reaction between 2 equiv IMes, CoCl2, and sodium amalgam in the absence of olefin coligands (cf. section 4.2.1, Mononuclear Co 0 Complexes).576 The solution magnetic moment of 65Co [μeff = 2.6(2) μB] is consistent with the formulation of a low-spin CoII complex. A distorted square-planar coordination geometry around the metal center was established by X-ray diffraction studies, with the presence of two bidentate anionic [IMes′] ligands. The Co−Cbenzyl bonds are relatively long [2.025(2) Å], probably due to geometric constraints. The formation of 65Co is thought to occur through intramolecular C(sp3)-H bond activation reactions on the putative Co0 intermediate [Co(IMes)2]. The high reactivity of the cyclometalated 65Co was evidenced by its reaction with CO, isocyanide, diazo, and hydrosilyl compounds (Scheme 293).582 Treatment of 65Co with CO, isonitrile (XylNC), and diazo (XylOCOCHN2) reagents led to migratory insertion reactions, resulting in the CoI complex 67Co and the CoII complexes 68Co and 69Co, respectively. All complexes were characterized by spectroscopic methods, and their molecular structures were established by Xray crystallography. Although the mechanism for the formation of 67Co is not clear, a first migratory-insertion step of CO into one CoII−Cbenzyl bond may be invoked. The reaction of 65Co with hydrosilyl compounds led to the formation of new silyl-donor-functionalized NHC complexes (Scheme 293).576,583 While the reaction of 65Co with SiH3Ph or SiH3CH2Ph proceeded neatly at room temperature, heating to 70 °C was found necessary to reach full conversion when using the bulkier SiH2RPh (R = Me, Ph). No reaction occurred with SiH(OEt)3 and SiHPh3 under the same conditions. The FH


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Scheme 296. Synthesis of 76Co by Transmetallation from the Ag NHC Complex

in moderate yield (25−30%) and fully characterized. The solid-state structures of 74aCo and 74bCo revealed the formation of new silane-functionalized NHC complexes featuring a five-coordinate hypervalent silicon atom and short Co−Si bond distances [2.275(1) and 2.267(1) Å, respectively]. These Co−Si separations are shorter than the sum of the covalent radii of low-spin cobalt and silicon (2.37 Å), suggesting a covalent interaction between the two fragments. Theoretical calculations support this view of the bonding situation, in contrast to the metal-to-Si dative bond observed in metallasilatranes. Attempts to prepare the cationic analogues of 74aCo and 74bCo by chemical oxidation were unsuccessful, which was corroborated by cyclic voltammetry measurements showing only an irreversible oxidation wave at +0.37 V. 4.4.4.1.3. Nitrogen-Donor Functionalized Ligands. Formation of the CoII bis(imino)NHC complex 75Co was achieved by the one-pot reaction of the imidazolium salt precursor with CoCl2 and KN(SiMe3)2 (Scheme 295).584 High

Scheme 274; section Mononuclear CoIV and CoV Complexes, Scheme 336, respectively).

Scheme 295. Synthesis of the Imine-Donor Functionalized Complex 75Co

4.4.4.2. Tridentate. 4.4.4.2.1. Symmetrical CNHCNCNHC Pincers. The air-sensitive CoII complex 78Co bearing a CNHCNCNHC pincer ligand was obtained by the aminolysis reaction of [Co{N(SiMe3)2}2] with the pincer imidazolium salt proligand (Scheme 297).586 Reaction of 78Co with thallium triflate afforded the air-stable 79Co featuring a distortedoctahedral geometry with the triflate anions coordinated trans to each other. Treatment of 78Co with NBr(SiMe3)2 resulted in the oxidation of the metal center and formation of the diamagnetic tris-bromo CoIII complex 80Co. Reduction of the metal center occurred upon treatment of 78Co with Na/Hg, leading to the diamagnetic CoI complex 81aCo, or by reaction of 78Co with different organolithium or organomagnesium reagents. The reduction of 78Co to 81aCo was also achieved using a stoichiometric amount of NaBHEt3.587 The high air-sensitivity of 81aCo is highlighted by the instantaneous formation of the diamagnetic [CoBr(η2O2)(CNHCNCNHC)] complex upon exposure to air. Direct reaction of 78Co with excess MeLi (4 equiv) led to the squareplanar Co-alkyl complex 82Co which can also be synthesized by alkylation of 81aCo with 1 equiv MeLi. Interestingly, the 1H NMR spectra of the reduced Co complexes 81aCo and 82Co display an unusual downfield shift for the para pyridine hydrogen (δ 9.55 and 10.65 ppm, respectively), which may be ascribed to redox noninnocence of the ligand. The chemistry of the reduced CoX-CNHCNCNHC (X = halide, methyl, hydride) complexes and their electronic structures will be further discussed in the section dealing with Mononuclear CoI Complexes. The cationic CoII alkyl complex 83Co was obtained in low yield by oxidation of 82Co with [FeCp2](BArF4) and exhibited a solution magnetic moment of 1.8(5) μB, consistent with an S = 1/2 ground state. The solid-state structure of 83Co confirmed a square-planar coordination environment around the Co center. Analysis of 83Co by EPR spectroscopy revealed a remarkable anisotropy of the g tensor and large hyperfine coupling constants, consistent with a cobalt-centered spin and a square-planar CoII complex.

yields (>84%) were obtained upon pretreatment of cobalt dichloride with KN(SiMe3)2 at low temperature, followed by addition of the imidazolium salt proligand. Transmetalation from the corresponding silver or copper complexes proved unsuccessful. Complex 75Co is paramagnetic with a solution magnetic moment of 4.24 μB, suggesting three unpaired electrons and a high-spin CoII center. In the IR spectrum of 75Co, two stretching bands were detected at 1685 and 1652 cm−1, indicating that only one imine group is coordinating to the metal center. In contrast, the IR spectrum of the imidazolium salt proligand displays one single absorption band at 1698 cm−1 for both CN groups. The solid-state structure of 75Co was established by X-ray diffraction and confirmed a bidentate coordination mode of the ligand. The metal center is in a distorted tetrahedral geometry, coordinated by one imine, one NHC, and two chloride ligands. As expected, the CN bond distance for the coordinated imine [1.273(4) Å] is slightly longer than for the dangling imine [1.253(4) Å], due to π-backdonation. Tridentate coordination of the ligand is presumably prevented, owing to combined steric and electronic considerations. The hexacoordinated complex 76Co was obtained in 56% yield by reaction of the corresponding naphthyridine-based triazolium salt proligand with Ag2O in acetonitrile, followed by transmetalation with [CoCl2(PPh3)2] (Scheme 296).585 This paramagnetic complex was characterized by EPR spectroscopy, revealing a low-spin CoII center. Structural analysis by X-ray diffraction studies established a distorted octahedral coordination environment with two chelating naphthyridine-functionalized NHC ligands and two additional acetonitrile ligands located trans to the NHC donors. Complexes 7Co, 13Co, and 77Co are mentioned elsewhere (section Mononuclear Co0 Complexes, Scheme 272 and FI


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Scheme 297. Reactivity of the Pentacoordinated CoII Complex 78Co

4.4.4.2.2. Symmetrical CNHCCCNHC Pincers. The CoII and CoIII complexes 84aCo, 84bCo and 85aCo, 85bCo, respectively, bearing an anionic CNHCCCNHC pincer ligand were obtained using a similar route as described above for the CNHCNCNHC complex 78Co (Scheme 298).588 The “aminolysis procedure” was selected due to failed attempts to cleanly generate the lithiated anionic pincer ligand by addition of nBuLi to the CNHCCCNHC free carbene. The one-pot reaction of the imidazolium salt precursor with [Co{N(SiMe3)2}2(py)2] and one equivalent of base, in the absence or presence of trityl chloride as oxidant, afforded in good yield (75−93%) the CoII and CoIII complexes 84aCo and 85aCo, 85bCo, respectively. Surprisingly, the mesityl-substituted CoII complex 84bCo could not be accessed using the same procedure. The synthesis of 84bCo was achieved in low isolated yield (28%) by reduction of the CoIII analogue 85bCo with half an equivalent of the magnesium anthracene complex [Mg(C14H10)·3THF]. The 1H NMR spectra of the diamagnetic CoIII 85aCo, 85bCo indicated C2-symmetric species in solution. Structural characterization by X-ray diffraction established an octahedral coordination environment for the CoIII center, the latter surrounded by one anionic tridentate CNHCCCNHC chelate, two chlorides trans to each other, and one additional pyridine ligand. The paramagnetic low-spin CoII complexes 84aCoand 84bCo were characterized by EPR spectroscopy and by X-ray diffraction studies. The cobalt center lies in a square-pyramidal coordination environment, with the CNHCCCNHC ligand and one pyridine in the basal plane and one chloride in the apical position. Cyclic voltammetry measurements revealed a reversible cathodic event at E1/2 = −1.46 and −1.56 V versus Fc/Fc+ for 84aCo and 84bCo, respectively, suggesting the formation of the corresponding reduced CoI complexes. Chemical reduction of 85aCo was achieved by treatment with

magnesium anthracene and afforded the square-planar diamagnetic CoI-N2 complex 86Co in good yield. In the presence of triphenylphosphine, reduction of 84aCo, 84bCo with magnesium anthracene or Na/Hg led to the squarepyramidal low-spin CoI complexes 87aCo, 87bCo. In the IR spectra of 86Co, 87aCo and 87bCo, the coordinated N2 ligand gave rise to a strong band at 2063, 2117, and 2112 cm−1, respectively, indicating very low activation of the dinitrogen ligand. Addition of one or two equivalents of trityl chloride (CClPh3) as oxidant gave back the corresponding CoII and CoIII complexes, showing that the interconversion between the three oxidation states can be easily achieved. 4.4.4.2.3. Symmetrical OCNHCO Pincers. The redox noninnocent OCNHCO pincer ligand featuring two di-tert-butylphenolate moieties and a central NHC core was used to stabilize a series of Co complexes in high oxidation states (Scheme 299).589 The Co pincer complexes 88u/sCo and 89Co were obtained by deprotonation of the imidazolium salt precurors with 3 equiv NaOMe and subsequent treatment with a stoichiometric amount of CoCl2. In all three complexes, the cobalt center has an approximate square-planar geometry, coordinated by the anionic OCNHCO chelate and an additional solvent molecule (MeCN or THF) bound trans to the NHC donor. The labile THF ligand is easily replaced by an acetonitrile donor upon dissolution in MeCN. The relatively short Co−CNHC bond distances (Table 16) can be traced back to geometric constraints imposed by the chelating ligand and the strong σ-donor ability of the NHC donor. Slight variations in the metrical data are observed for the unsaturated and saturated chelates in 88u/sCo and 89Co. A shorter Co−CNHC bond distance and an elongation of the Co−N bond to the MeCN ligand was observed in the saturated 88sCo, reflecting enhanced π-backdonation from the Co to the saturated FJ


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Scheme 298. Synthesis and Reactivity of the Pincer Complex 84aCo, 84bCo and 85aCo, 85bCo

Scheme 299. Cobalt Complexes with the Redox Non-Innocent OCNHCO Pincer Ligand

in the range of 1.82−1.90 μB. Such values are higher than the

carbene. For all three complexes, the bonding metrics within the phenolate groups are consistent with the occurrence of two fully reduced phenoxide donors, rendering the ligand dianionic and the Co center in a formal +II oxidation state. All three complexes are paramagnetic, with solution magnetic moments

spin-only value for an S = 1/2 center but consistent with a square-planar d7 CoII center exhibiting a substantial orbital contribution to the magnetic moment. Computational data FK


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Scheme 300. Synthesis and Reactivity of the CoII Complex 92Co Bearing a Tridentate Phosphino-Picoline-NHC (PNCNHC) Ligand

further support the formulation of 88u/sCo and 89Co as CoII centers bound to closed-shell [OCNHCO]2− dianions. To investigate whether oxidized deriatives of 88sCo can be accessed, cyclic voltammetry measurements were performed and revealed three quasi-reversible one-electron oxidations at potentials below 1.2 V versus Fc/Fc+, indicating formation of a formally CoV species. However, the true oxidation state, as observed by spectroscopic and physical methods, is much lower (vide infra). Interestingly, a strong variation (up to 400 mV) in the oxidation potentials was observed depending on the degree of unsaturation in the NHC backbone. The redoxpotential can thus be conveniently tuned for further reactivity investigations toward small molecules. The synthesis of the different oxidized analogues of 88sCo was attempted, and the monocationic complex 90Co was obtained by treatment of 88sCo with one equiv of AgOTf followed by anion metathesis with NaBPh4. Analysis of the crystal structure of 90Co revealed a Co center in a pseudo square-pyramidal geometry with an elongation of the Co−CNHC bond distance from 1.789(2) Å in 88sCo to 1.849(3) Å in 90Co. Analysis of the metrical data within the phenolate rings reveals quinoid-type bond alternations, pointing to some degree of ligand oxidation and the presence of phenoxyl radicals. The solution magnetic moment μeff = 2.88 μB is consistent with the spin-only value for an S = 1 center, suggesting either the formulation of 90Co as a closedshell [OCNHCO]2− dianionic ligand bound to an S = 1 CoIII center or as a low-spin CoII ion ferromagnetically coupled to a monoanionic [OCNHCO·]−. In the latter case, a single unpaired electron is delocalized over the ligand system. Computational and solid-state magnetism data are consistent with both formulations, rendering the ligand in 90Co truly noninnocent. Synthesis of the dicationic complex 91Co succeeded by oxidation of 88sCo with 2.1 equiv of [N(p-C6H4Br)3](PF6). The crystal structure of 91Co established a pseudo-octahedral coordination geometry around to Co center with three THF molecules completing the coordination sphere. The ligand metrical data within the phenoxide moieties clearly reveal a ligand-based oxidation and a quinoid-type pattern. Complex 91Co is therefore best described as a charge-neutral, closedshell, doubly oxidized ligand coordinated to a formal CoII

metal center. This view of the electronic structure of the complex is further supported by computational data. The solution magnetic moment μeff = 2.51 μB is substantially higher than that for a low-spin d7 S = 1/2 ion, suggesting strong spin− orbit coupling. Synthesis of the tricationic complex observed by cylic voltammetry revealed unsucessful, resulting only in intractable mixtures under various conditions. 4.4.4.2.4. Nonsymmetrical PNCNHC Pincers. Using the nonsymmetrical phosphino-picoline-NHC (PNCNHC) ligand, additional reactivity arises from the acidity of the α-CH2P protons (Scheme 300).590 Upon side arm deprotonation, the neutral ligand is transformed into the anionic dearomatized P*NaCNHC pincer ligand which features one vinylic phosphine (P*) and one anionic nitrogen (Na) donors. The corresponding P*NaCNHC-CoBr pincer complex, 92Co, was accessed either by transmetalation of the dearomatized potassium salt with [CoBr2(THF)2] or by double aminolysis of [Co{N(SiMe3)2}2] using the imidazolium bromide salt precursor. The CoII complex 92Co is paramagnetic in solution with an effective magnetic moment μeff = 2.1(2) μB, consistent with the formulation of a low spin d7 CoII center. Analysis of the Xray structure of 92Co revealed a CoII center in a nearly squareplanar coordination geometry, chelated by the anionic pincer ligand and further coordinated by a bromide donor. Dearomatization of the picoline moiety is evident by perusal of the bond distances within the heterocycle, with alternating double and single bonds. Monitoring the reaction between the imidazolium salt precursor and [Co{N(SiMe3)2}2] by EPR and NMR spectroscopy revealed the formation of an intermediate species, assigned to the 5-coordinate CoII complex [CoBr{N(SiMe3)2}(PNCNHC)].591 Reaction of 92Co with one additional equivalent of [CoBr 2(THF) 2] led to an unusual metalation at the nucleophilic α-CHP side arm and afforded the dinuclear CoII complex 93Co bearing a rearomatized PNCNHC ligand. The central CoII metal is in a distorted square-pyramidal environment, chelated by the neutral ligand PNCNHC ligand and further coordinated by one terminal and one bridging bromide. The lateral CoII center is in a tetrahedral environment, coordinated by one sp3-hybridized α-C benzylic donor, one bridging and one terminal bromide, and one THF ligand. The FL


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Scheme 301. Synthesis and Reactivity of 96Co Bearing a Hybrid Tripodal Ligand

Scheme 302. Synthesis of 99Co by Transmetallation from Ag

in different solvents suggested that the Cl− and N3− anions coordinate to the metal center in THF solution but are not coordinated in MeCN and chlorofom solutions. Variabletemperature SQUID magnetization measurements were carried out, and the data are consistent with high-spin CoII complexes (S = 3/2). The magnetic moments for 95Co and 96Co (4.3−4.4 μB) are significantly larger than the spin-only value for a triplet ground-state (3.87 μB), suggesting a large contribution from spin−orbit coupling. Photolytic cleavage of the azide was observed upon irradiation of the complex at λ = 310 nm, in the presence of 2,4,6-tris-tert-butylphenol as a sacrificial hydrogen donor.593 The CoII complex 97Co was isolated after metathesis with NaBPh4 and exhibits a modified ligand structure, resulting from an N-migratory insertion. The CoII center displays a fourcoordinate trigonal pyramidal geometry showing a Co-N separation with the anchoring N atom of 2.106(2) Å, in line with a bonding interaction. The transformation of the azido complex 96Co into 97Co is evident by comparison of the corresponding IR spectra, where the intense azide vibration bands have been replaced by bands at 3340 and 1608 cm−1, respectively, corresponding to the imine N−H and CN vibrations. Analysis of 97Co by EPR spectroscopy and SQUID magnetization measurements supported a high-spin CoII center with an S = 3/2 ground state. The isolation of 97Co upon irradiation of 96Co involves the formation of a transient nitrido complex, 98Co, followed by the insertion of the nitrido ligand into one of the two Co−NHC bonds, and a final H atom abstraction step. Low-temperature (10 K) photolysis experiments on 96Co were monitored by

value of the solution magnetic moment of 93Co [μeff = 4.8(2) μB] is lower than the value for two noninteracting high-spin CoII centers (5.48 μB), suggesting some degree of antiferromagnetic coupling between the two metals. Alkylation of the 92Co with LiCH2SiMe3 proceeded neatly and led to the square-planar low-spin CoII complex 94Co which was characterized by X-ray diffraction and EPR spectroscopy. The reduction chemistry of 92Co will be described in the section Mononuclear CoI Complexes, section 4.5.2.2.3. 4.4.4.3. Higher Denticity. 4.4.4.3.1. Tripodal Ligands: Phenolate/bis(NHC)amine. The CoII complex 95Co bearing a hybrid tripodal bis(NHC)-monophenolate ligand was obtained by reaction of the potassium salt of the ligand with CoCl2 and fully characterized (Scheme 301).592,593 The solidstate structure of 95Co reveals a distorted trigonal-pyramidal coordination geometry around the cobalt center, solely coordinated by the tripodal ligand. The Co−N bond separation of 2.141(2) Å suggests the presence of a coordination bond with the anchoring N atom, while no coordination with the chloride counteranion is observed. Salt metathesis with sodium azide yielded the CoII complex 96Co. Azide coordination was confirmed by IR spectroscopy with the presence of intense absorption bands centered at 2081, 2044, and 1999 cm−1. In the molecular structure of 96Co, the CoII center displays a trigonal-pyramidal coordination geometry, chelated in its base by the two NHC and the phenolate O donors, while the azide is located in the axial position. The Co−N separation of 2.659(2) Å involving the anchoring N atom in 96Co is too long to suggest coordination to the metal. Analysis of the 1H NMR spectra of 95Co and 96Co FM


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Scheme 303. Synthesis of the Homoleptic CoI NHC Complexes 100au/s−cCo

were obtained in good yield by salt metathesis reactions starting from the corresponding tricoordinate [CoCl{(S)IMes}2] (66u/sCo) complexes (Scheme 303).582,596 Synthesis of the IAd analogue, 100bCo, was achieved by reaction of 101Co with equimolar amounts of NaBArF4 and IAd free carbene.596 The 1H NMR spectra of the paramagnetic 100au/s−bCo indicated an idealized C2 symmetry for the [Co(NHC)2]+ cation in solution. Interestingly, the effective magnetic moments of 100au/sCo at room temperature (μeff = 5.40 and 5.10 μB, respectively) are much higher than that in the IAd complex 100bCo (μeff = 3.94 μB, Table 17), indicating a substantial contribution of unquenched orbital angular momentum in the former complexes. The solid-state structure of 100au/s−bCo was established by X-ray crystallography and revealed structural similarities in the three complexes, all featuring a linear CNHC-Co-CNHC arrangement. While no secondary intra- or intermolecular interactions were observed in the (S)IMes complexes, secondary interactions between the Co center and the adamantyl group were found in 100bCo. Another interesting difference between the three structures lies in the value of the dihedral angle between the two trans located NHC rings. With the bulky IAd ligand, a perpendicular arrangement of the NHC planes is observed, while, with the less sterically hindered IMes and SIMes ligands, smaller dihedral angles (39.6 and 35.0°, respectively) are obtained. Ab initio calculations showed that changes in the dihedral angle between the two NHC rings dramatically affect the spin−orbit coupling splitting, due to modifications in the Co-NHC πinteractions. An especially large magnetic anisotropy in the IMes complex 100auCo was observed by SQUID magnetization measurements. In addition, 100auCo exhibits a slow magnetic relaxation behavior under an applied dc field and was considered to be the first d8 single-ion magnet.596 In contrast, the magnetic

EPR spectroscopy and supported the formation of the highly reactive nitrido complex 98Co. The EPR spectrum is consistent with the formulation of a S = 1/2 low-spin d5 CoIV nitrido complex, which is further supported by DFT computational analysis. The nitrido complex 98Co readily reacts at temperatures higher than 50 K, preventing the isolation of the complex in the solid state. The mechanism of the transformation was investigated by DFT calculations. The release of N2 from 96Co leading to 98Co involves a high activation barrier of 46.8 kcal mol−1, consistent with a photochemically induced N2 elimination step. The following N-migratory insertion step into the Co−C bond, leading to a Co-imido intermediate, has a very low activation barrier of 2.2 kcal mol−1. Finally, the more stable carbene-imine-phenolate complex 97Co is obtained after abstraction of an H atom. 4.4.4.3.2. Pentadentate Ligands. The CoII complex 99Co bearing a pentadentate pyridine-substituted ligand was accessed by transmetalation from the corresponding silver complex with CoII triflate under mild conditions (Scheme 302).277 Attemps to generate the free NHC carbene by deprotonation of the imidazolium salt precursor did not proceed cleanly. In the solid-state structure of 99Co, the pentadentate ligand features a square-pyramidal coordination mode with the NHC donor, the latter located in the apical position. Analysis by EPR spectroscopy established a low-spin CoII center, showing the tendency of the ligand to stabilize low-spin electronic configurations. The CoII complexes 150Co, 151Co, 153Co and 155a−dCo are described in the following section Mononuclear Co I Complexes. 4.5. Mononuclear CoI Complexes

4.5.1. Monodentate Carbene Ligands. 4.5.1.1. Homoleptic. 4.5.1.1.1. Type [Co(NHC)2]+(A−). The homoleptic twocoordinate CoI complexes 100au/sCo bearing (S)IMes ligands FN


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Table 17. Selected Data for the NHC CoI Complexes

FO


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Table 17. continued

FP


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Table 17. continued

FQ


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Table 17. continued

FR


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Table 17. continued

FS


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Table 17. continued

FT


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Table 17. continued

FU


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Table 17. continued

FV


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Table 17. continued

a

Magnetic moment in solution, unless otherwise stated. bTwo or more independent molecules in the asymmetric unit. cIn the solid state. dSee text, redox-active ligand.

anisotropy in 100asCo and 100bCo is lower and these complexes do not show single-ion-magnet behavior under applied dc fields. Such a disparity in the magnetic properties may originate from slightly different Co-NHC interactions, depending on the saturated/unsaturated character of the NHC ligand, the dihedral angle between the two NHC planes and secondary metal−ligand intramolecular interactions. The high-spin (μeff = 3.2 μB) homoleptic two-coordinate CoI complex 100cCo bearing two Et2cAAC ligands was obtained upon halide abstraction from 2bCo using NaBArF4 (Scheme 303).553 Its crystal structure revealed a slightly bent CcAAC− Co−CcAAC bonding angle, in contrast to the linear CNHC−Co− CNHC arrangement observed in 100auCo [168.35(9)° vs

178.58°, respectively]. Cyclic voltammetry measurements on 100cCo revealed a reversible reduction process at E1/2 = −1.79 V versus Fc+/Fc, corresponding to the formation of the formal Co 0 complex 1b Co (see Scheme 270 in section on Mononuclear Co0 Complexes). 4.5.1.1.2. Type [Co(NHC)3]+(A−). The homoleptic threecoordinate 16Co was already described in the section Mononuclear Co0 Complexes (Scheme 275).552 The structure of the trigonal-planar CoI complex 102Co featuring an extremely bulky amido counteranion has been published, but no associated experimental data have been reported.597 4.5.1.1.3. Type [Co(NHC)4]+(A−). The homoleptic CoI complexes 21a−cCo were obtained in good yield by reaction FW


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bent arrangement [N−Co−C angle of 140.2(1)°], with an additional intramolecular interaction with the ipso carbon of one phenyl group from the amide ligand. The Co−CNHC separation in the two-coordinate 103Co [1.879(2) Å] is slightly shorter than in the arene-coordinated 104Co [1.979(3) Å]. Both complexes exhibit similar paramagnetically shifted resonances in their 1H NMR spectra, suggesting an easy interconversion between the two isomers in solution. Their solution magnetic moment at room temperature [μeff = 2.6(1) μB] is consistent with a high-spin S = 1 ground state. Very recently, the synthesis of the two-coordinate, formally CoI, complex 105Co was reported using different synthetic strategies (Scheme 306).565,598 Starting from the mono(amido) complex 27bCo, reduction with KC8 or NaBHEt3 afforded the paramagnetic 105Co in moderate yield after recrystallization (40−50%). In an alternative higher yielding procedure, 105Co was obtained by reaction of the bis(silylamido) complex 29buCo with PH2Mes* [Mes* = 2,4,6(tBu)3C6H2], along with an organic phosphaindane byproduct. No further reaction between 105Co and excess PH2Mes* was noted, in contrast to the aminolysis reactivity between PH2Mes and [Co{N(SiMe3)2}(Me2IMe)] in Scheme 280. Interestingly, the attempted reduction of the IMes-substituted [CoCl{N(SiMe3)2}(IMes)] (27auCo) with KC8 did not yield the expected [Co{N(SiMe3)2}(IMes)] complex, but the threecoordinate CoI complex [CoCl(IMes)2] (66uCo) was isolated instead. The solid-state structure of 105Co was established by X-ray diffraction studies and revealed a nearly linear coordination geometry at the metal center [N−Co−CNHC angle of 178.83(7)°]. The Co−C NHC bond distance in 105 Co (1.942(2) Å) is significantly shorter than in the bis(silylamido) precursor 29buCo [2.119(3) Å]. The magnetic properties of 105Co were investigated by SQUID magnetometry, which revealed high magnetic moments. In addition to the magnetometry data, combined spectroscopic (XPS, XANES) and computational (DFT) analyses indicated that the electronic structure of 105Co is best described by a high-spin CoII center coupled to one electron delocalized on the IPr ligand. Consequently, the IPr ligand is electronically noninnocent and exhibits a radical anionic character. Interestingly, the formal CoI complex 105Co could also be accessed by treatment of 29buCo with SiH(OEt)3, generating the silylamide Si(OEt)3[N(SiMe3)2] and dihydrogen as byproducts (Scheme 306).565 This reaction corresponds to a rare example of CoII to CoI reduction by hydrosilane reagents. A possible mechanism for the formation of 105Co involves the generation of the CoII hydride intermediate [CoH{N(SiMe3)2}(IPr)], followed by homolytic cleavage of the CoH bond. Reaction of 105Co with an additional equivalent of

of the corresponding free NHCs with [CoCl(PPh3)3], followed by anion exchange with NaBPh4 (Scheme 304).558 Analysis of the X-ray structure of 21a−cCo established a square-planar coordination geometry around the CoI center, which is surrounded by four monodentate NHC ligands. A slight elongation of the Co−CNHC separations (Table 17) is observed when increasing the steric bulk on the NHC alkyl wingtips [on average 1.913(4), 1.931(3) and 1.970(2) Å for the Me-, Et-, and iPr-substituted complexes, respectively]. Cyclic voltammetry studies on 21aCo revealed two quasireversible oxidation processes at −1.26 and 1.37 V, corresponding to the oxidation of the metal center in its divalent and trivalent states, respectively. In addition, an irreversible oxidation process was observed at ca. 0.90 V, which is associated with the oxidation of the borate anion. As already described in the section on Mononuclear CoII Complexes (section 4.4.1.1), the corresponding CoII complex 20Co could be obtained by oxidation of 21aCo with organic halides such as 3,5-dimethylphenyl iodide, benzyl bromide, and 2-methyl-1,2-dichloropropane (Scheme 277), indicating the easy interconversion between the + I and + II oxidation states in the [Co(Me2IEt)4]+/2+ core. 4.5.1.2. Heteroleptic. 4.5.1.2.1. Type [Co(NHC)X]. The first neutral two-coordinate CoI complex 103Co bearing a Me2IiPr donor and a highly bulky amide ligand was published in 2015 by the group of Jones and was obtained in moderate yield by substitution of the coordinated benzene in the corresponding [Co(amide)(η6-C6H6)] precursor (Scheme 305).597 This complex exhibits an almost linear coordination geometry around the CoI center, with an N-Co-CNHC angle of 175.2(1)°. With dependence on the crystallization conditions, another isomer (104Co) was isolated. Its crystal structure revealed a

Scheme 304. Synthesis of the Homoleptic NHC CoI Complexes 21a−cCo

FX


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Scheme 305. Synthesis of the Two-Coordinate CoI Complex 103Co

Scheme 306. Synthesis and Reactivity of the Two-Coordinate, Formally CoI, Complex 105Co

Scheme 307. Synthesis and Reactivity of the CoI Alkyl Complex 108Co

SiH(OEt)3 in benzene afforded the diamagnetic CoI hydride complex 106Co in high yield. In the 1H NMR spectrum of 106Co, the hydride resonance was observed at δ −21.6 ppm. The driving force for both steps may be related to the formation of the Si(OEt)3[N(SiMe3)2] byproduct featuring a strong Si−N bond. In further reactivity studies, the reaction of 105Co with NH2Mes was investigated and afforded the dinuclear 107Co after aminolysis of the Co-N(SiMe3)2 bond.598 The structure of the centrosymmetric complex was established by X-ray crystallography and revealed the presence of bridging (NHMes)− anilido ligands between the two CoI centers. The

Co−Co separation of 2.5765(4) Å is consistent with some degree of metal−metal interaction. 4.5.1.2.2. Type [Co(NHC)LX]. The pseudo three-coordinate CoI complex 101Co was prepared by reaction of IAd with [CoCl(PPh3)3] and isolated in good yield (70%) after recrystallization (Scheme 307).599 Its solid-state structure, determined by X-ray diffraction studies, revealed secondary interactions between the Co center and two H atoms from the adamantyl NHC wingtips. The corresponding CoI alkyl complex 108Co was obtained by alkylation of 101Co with LiCH2SiMe3 and fully characterized. The solution magnetic moment of this paramagnetic complex, μeff = 3.7(1) μB, is considerably larger than the spin-only value for a high-spin S = FY


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Scheme 308. Synthesis and Reactivity of the Coordinatively Unsaturated 16-Valence Electron Complex 111Co

Scheme 309. Synthesis and Reactivity of Piano-Stool NHC CoI Complexes

coordinated alkyne donor and a σ-alkyl group. The Calkyne− Calkyne bond distance for the bound diphenylacetylene is relatively long [1.271(3) Å, to be compared to ca. 1.20 Å in free PhCCPh], with a bent CPh−Calkyne−Calkyne arrangement (148°), resulting from strong metal-to-ligand back-donation. As observed in 101Co, short-contact interactions between the Co center and one or two H atoms of the adamantyl wingtip substitutents can be noticed in the crystal structures of 108Co− 110Co. 4.5.1.2.3. Type [Co(NHC)L2X]. The coordinatively unsaturated 16-valence-electron complex 111Co was obtained by addition of free IPr to [(CoCp*)2-μ-(η4:η4-toluene)] (Scheme

1 CoI center (μso = 2.83 μB). A trigonal-planar arrangement around the metal is observed in the molecular structure of 108Co with a large CNHC−Co−Calkyl angle of 137.32(7)°. The CoI alkyl complex 108Co was used to access other (pseudo) tricoordinate CoI complexes (Scheme 307).599 Reaction of 108Co with SiH2Ph2 led to the first threecoordinate CoI silyl complex, 109Co, in moderate yield. The crystal structure of 109Co and elemental analysis data confirmed the identity of the complex but further characterization was hampered by a fast decomposition in C6D6 solution. Treatment of 108Co with PhCCPh led to the CoI-alkyne-alkyl complex 110Co featuring both an η2FZ


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Scheme 310. Reaction of 114bCo with P4

308).600 Even in the presence of excess IPr, the mono-NHC complex 111Co was obtained, owing to the important steric demand of the IPr ligand. The solution magnetic moment μeff = 2.6(1) μB is consistent with a high-spin CoI center, as confirmed by computational analysis, but is slightly lower than the expected value for an S = 1 ground state. Complex 111Co proved to be thermally stable with no C−H activation observed upon heating to 100 °C for a week, in accordance with the large calculated singlet−triplet gap. The reactivity of this coordinatively unsaturated CoI complex was explored toward small molecules (H2, ethylene, CO, and PMe3). Reaction of 111Co with excess CO resulted exclusively in the carbonyl addition product 112fCo, while reaction with excess PMe3 or ethylene led to displacement of the NHC ligand and formation of [CoCp*(PMe3)2] and [CoCp*(en)2], respectively. Steric reasons may account for such a substitution of the IPr ligand. Complex 111Co reacts reversibly with H2 through an oxidative addition process, yielding the diamagnetic CoIII complex 113Co. The presence of hydride ligands was confirmed by 1H NMR spectroscopy with an upfield resonance integrating for two protons at δ −17.8 ppm. However, isolation attempts only gave back 111Co, due to an easy loss of H2. 4.5.1.2.4. Type [Co(NHC)L3X]. Piano-stool Cp complexes. The diamagnetic CoI complex 45Co (Scheme 309) has already been mentioned in the section on Mononuclear CoII Complexes (Scheme 285) and was prepared by substitution of one phosphine ligand in [CoCp(PPh3)2] by an IiPr donor.571 Various [Co(η5-C5R5)(NHC)(CO)] (R = H, Me) complexes (112aCo, 112c−eCo) were prepared by substitution of CO in [Co(η5-C5R5)(CO)2] with the corresponding free NHC (Scheme 309).571,601,602 Reactions using the Cp* (R = Me) cobalt precursor typically required more forcing conditions to reach completion.602 In the IR spectra, the CO stretching vibration was detected at lower frequencies in the Cp* complexes compared to the corresponding Cp species, reflecting the stronger electron donation ability of the former ligand (Table 21). The νCO stretching frequency in 112eCo was observed at 1921 cm−1 (pentane), at a similar value to that in the PMe3 analogue (1923 cm−1, pentane) but at considerably lower frequency than that in [CoCp(PPh3)(CO)] (1937 cm−1, pentane).601 The solid-state structure of 112cCo and 112eCo, established by X-ray diffraction, revealed two-legged pianostool structures with the cobalt center surrounded by one NHC, one CO, and one η5-C5R5 (R = H, Me) ligand. In solution, hindered rotation about the Co−CNHC bond in 112eCo was evidenced by 1H VT NMR experiments. Coalescence of the IiPr CH3 signals was observed at 3 °C, which indicates a rotational barrier of ca. 14 kcal/mol, in accordance with the value determined by DFT calculations.571

The air-sensitive ethylene complexes (114a−dCo) were obtained by reaction of the corresponding free carbene ligand with the [Co(η5-C5R5)(C2H4)2] (R = H, Me) bis(ethylene) complexes (Scheme 309). 602,603 As observed for the substitution of CO in [Co(η5-C5R5)(CO)2], the reaction was found to be more sluggish using the Cp* substituted complex. Interestingly, exposure of 114cCo to ethylene resulted in an equilibrium mixture with partial regeneration of [CoCp*(C2H4)2] and free IMes, possibly driven by a release of steric constraint.602 For the synthesis of 114aCo, a convenient approach consisted in the addition of IiPr to [CoCp(C2H4)2], the latter generated in situ owing to its low thermal stability.603 Hindered rotation of the ethylene ligands was observed in 114a−bCo, but no coalescence was detected in the VT 1H NMR experiments at temperatures up to 90 °C. The crystal structures of 114a−bCo revealed two-legged piano-stool arrangements with an increased CC separation for the coordinated ethylene in case of the Cp*-substituted complexes [1.427(3) vs 1.417(3) Å].603 Reaction of 114a−bCo with carbon monoxide led to the corresponding carbonyl complexes 112a−bCo,603 in higher yield (66−99%) than by direct substitution using the [CoCp(CO)2] precursor (30% in case of 112aCo).571 The carbonyl complexes 112a−bCo are very air-sensitive and readily react with oxygen both in solution and in the solid state. The Cp-substituted complex 112aCo decomposes upon reaction with air or O2, leading to intractable products (Scheme 309).603 In contrast, the Cp* complex 112bCo is oxidized by air into the CoIII carbonato complex 115Co, isolated in 93% yield. The carbonato ligand in 115Co gives rise to a resonance at δ 199.0 ppm in the 13C{1H} NMR spectrum and a CO stretching band centered at 1612 cm−1 in the IR spectrum. This oxidation reaction is too fast to be monitored by NMR spectroscopy, but insights into the mechanism were obtained by low-temperature time-resolved UV−vis spectroscopy. An intermediate species was detected with a characteristic absorption band at 585 nm and was assigned to a peroxo acyl complex based on DFT calculations. The calculated mechanism for the transformation of 112bCo in 115Co supports a strongly exothermic reaction with two kinetics barriers corresponding to the generation of the peroxo acyl intermediate and its rearrangement into the carbonato complex 115Co.603 The reaction of 112cCo with I2 did not lead to the expected oxidative addition product [CoI2Cp(IMes)], but instead the cationic and air-stable CoIII complex 116Co was isolated (Scheme 309).602 The nature of 116Co is based on elemental analysis data and its formation may be due to the steric pressure exerted by the IMes and iodido ligands. Decomposition of the complex was observed upon heating in tolueneGA


Chemical Reviews

Review

analysis of the X-ray structure of 120Co revealed a shorter ethylene CC bond distance than in the phosphine starting material [1.355(13) vs. 1.396(5) Å, respectively], suggesting competing backdonation to both the carbene and the ethylene ligands.605 Carbonyl and nitrosyl complexes. A series of 4-coordinate [Co(NO)(NHC)2(CO)] (121a−hCo) and [Co(NO)(NHC)(CO)2] (122a−jCo) complexes was prepared by reaction of [Co(NO)(CO)3] with the corresponding free NHCs (Scheme 312).606 With sterically modest NHC ligands, the metal-toNHC ratio is mainly governed by the stoichiometry of the two reagents. In constrast, with bulky NHCs, the mono-NHCsubstituted complexes are exclusively obtained, even in the presence of excess NHC ligand. In the bis(carbene) complexes 121a−hCo, two CO ligands have been displaced by two NHC donors. Further substitution of the remaining CO ligand was not successful, even using the less sterically demanding IMe ligand under forcing conditions. When using the bulkier ItBu ligand, only the mono-NHC-substituted [Co(NO)(ItBu)(CO)2] complex could be isolated. The 1H NMR spectrum of the product revealed the presence of two isomers, 122kCo and 123Co, corresponding to a “normal” (C-2) and “abnormal” (C-4) coordination mode of the ItBu ligand, respectively. Formation of the “abnormal” 123Co is driven by a release in the steric congestion around the metal center, but only a very low energy difference between the two isomers was established by DFT calculations. With the sterically demanding IPr and IMes ligands, the mono(NHC) complexes 122i−jCo were exclusively obtained, even in the presence of excess NHC ligand. Similarly, reaction of Me2cAAC with [Co(NO)(CO)3] only resulted in the formation of the monosubstituted 124Co, irrespective of the metal-to-ligand stoichiometry. The crystal structures of the bis(NHC) complexes 121a− hCo invariably featured a tetrahedral coordination geometry around the CoI center, the latter surrounded by two NHCs, one carbonyl, and one (linear) nitrosyl ligand. A tetrahedral coordination environment was also observed for the monosubstituted complexes 122i−jCo and 124Co, which feature one carbene, two carbonyl, and one nitrosyl ligands. A slight elongation of the Co−CNHC bond can be noticed when increasing the steric demand of the NHC ligands (Table 17). In constrast, no major deviation of the Co−CNHC bond distance is observed between the mono- and bis-NHC complexes. The Co−CcAAC bond in 124Co [1.9585(15) Å] is slightly shorter than the Co−CNHC separation in the corresponding mono-NHC complexes 122a−jCo (1.961− 2.011 Å), in line with the stronger σ-donor and π-acceptor capabilities of the cAAC ligand. All complexes are diamagnetic, and the CNHC resonance was detected at ca. 200 ppm in the corresponding 13C{1H} NMR spectra (the CcAAC resonance in 124Co was not observed). Surprisingly, analysis by IR spectroscopy did not reveal any direct correlation between the CO stretching frequency (Table 21) and the reported TEP value for the corresponding NHC ligands. The totally symmetric A1 vibration of the CO ligands in the mono(NHC) complexes 122i−jCo was found in the range of 2007−2011 cm−1, at considerably higher frequencies than in the bis(NHC) analogues 121a−hCo (in the range 1865−1878 cm−1). This red shift can be explained by a higher electron density at the metal in the latter complexes due to the high σ-donating and low πaccepting abitilies of the NHC ligands. In the Me2cAAC complex 124Co, the A1 CO stretching vibration was detected at 2004 cm−1, at a slightly lower wavenumber than in the

d8 solution and resulted in the formation of 2-iodo-1,3dimesitylimizadolium triiodide (IMesI·HI3). The reactivity of the CoI ethylene complex 114bCo was investigated toward P4 and allowed the isolation and characterization of relevant reaction intermediates (Scheme 310).604 The activation of P4 by 114bCo proceeded cleanly at room temperature, leading to the CoIII complex 117Co which was isolated in 78% yield. The formation of 117Co results from the decoordination of the ethylene ligand and the insertion of the metal into one of the P−P bonds. In contrast, the reaction of P4 with other cobalt half-sandwich complexes does not usually proceed cleanly. Characterization by multinuclear NMR spectroscopy revealed a complex signal in the 31P NMR spectrum, composed of three sets of resonances at δ −165.9, −296.9, and −347.6 ppm for the A2XY spin system. The X-ray structure of 117 Co indicates a tetrahedral coordination environment for the CoIII center and the formation of a reduced dianionic P42− ligand. It can be noticed that the P−P bond distances between the P atoms coordinated to Co [2.6398(11) Å] are significantly longer than all other P− P separations in the molecule [2.1799(15)−2.2220(12) Å]. Cleavage of another P−P bond occurred upon addition of a second equivalent of the CoI complex 114bCo. As a result, the dinuclear 118Co was isolated in 82% yield and features a μ,η2:2cyclo-P44− ligand bridging the two cobalt centers. In the 31P NMR spectrum of 118Co, two new sets of resonances for the AA′XX′ spin system were detected at δ −58.16 and −107.1 ppm. Quantitative transformation of 118 Co into 119 Co occurred upon heating in toluene. The formation of this new dinuclear complex results from the elimination of one IiPr ligand and the rearrangement of the cyclo-P4 ligand with dissociation of a P−P bond. In 119Co, a P44− chain is coordinated to one [CoCp*] fragment in an η4-fashion and further η2-coordinated to one [CoCp*(IiPr)] moiety. In the 31 P NMR spectrum, different sets of resonances were observed downfield at δ 142.2 and 252.3 ppm, allowing convenient monitoring of the reaction by 31P NMR. Further thermolysis of 119Co by heating a xylene solution of the complex at 135 °C for 7 days led to the quantitative formation of [(CoCp*)2P4]. In this complex, two isolated P22− phosphide ligands bridge the two [CoCp*] fragments.604 Surprisingly, the reaction of free Me2IiPr with a CoI ethylene complex bearing a chelating phosphine-functionalized Cp ligand did not result in displacement of the ethylene ligand (Scheme 311).605 In contrast, decoordination of the phosphine arm occurred, leading to the CoI ethylene complex 120Co which was isolated in 42% yield. In its 13C NMR spectrum, the carbene resonance was detected as a broad signal at δ 241.4 ppm. The dangling phosphine donor gave rise to a singlet at δ 29.7 ppm in the 31P{1H} NMR spectrum of 120Co (vs δ 92.6 ppm in the starting Cp-P chelated complex). Surprisingly, Scheme 311. Synthesis of 120Co Bearing a PhosphineFunctionalized Cp Ligand

GB


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Review

Scheme 312. Synthesis of Carbonyl Nitrosyl NHC CoI Complexes

mono(NHC) complexes 122a−jCo. All complexes are stable upon sublimation in vacuo (ca. 10−2 mbar) and were characterized by DTA/TG analysis. The bis(carbene) complexes (121a−hCo) showed an increased stability toward thermal decomposition compared to the mono(NHC) analogues (122a−jCo). The nonsymmetrically substituted CoI complexes 125a−gCo were obtained by selective substitution of one carbonyl by a phosphine ligand in the corresponding [Co(NO)(NHC)(CO)2] complexes 122Co (Scheme 313).607 In some cases, UV irradiation was necessary to achieve quantitative substitution of the ligand. Interestingly, the synthesis of 125dCo and 125gCo did not require photochemical conditions and occurred smoothly at room temperature. The different complexes are chiral at the metal center, but a racemic mixture was obtained. All complexes were characterized spectroscopically by multinuclear NMR and IR spectroscopy (see Table 21 for the CO and NO stretching vibrations). The crystal structures of 125cCo and 125gCo were established by X-ray crystallography and revealed a distorted tetrahedral coordination geometry for both complexes. The thermal properties of the different complexes were investigated by thermogravimetric studies (DTA/TG). The phosphine complexes 125a−gCo exhibited a higher thermal stability compared to the [Co(NO)(NHC)(CO)2] precursors (122Co). All complexes are volatile and stable upon sublimation. Vapor-phase deposition experiments were carried out with 125aCo as well as 121aCo, 122aCo, and 122hCo. The 5-coordinate 126a−cCo were obtained in good yield by displacement of the phosphine ligand in the corresponding [Co(R)(PPh3)(CO)3] (R = Me, Et, CF3) complexes (Scheme 314).608,609 In contrast, the direct reaction of SIPr with

Scheme 313. Formation of the Phosphine Complexes 125a− gCo

[Co(CF3)(CO)4] led to a mixture of products.609 The boryl functionalized 126dCo was prepared by substitution of one CO ligand in the corresponding tetra-carbonyl [Co(boryl)(CO)4] complex (Scheme 313).610 While the latter reagent is highly sensitive toward air and moisture, the NHC complex 126dCo can be exposed to air without any decomposition. The solidstate structures of 126auCo, 126csCo, and 126dCo were determined by X-ray crystallography and revealed a distorted trigonal bipyramidal geometry around the CoI center. The GC


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Scheme 314. Synthesis of 5-Coordinate CoI NHC Carbonyl Complexes

three CO ligands are located in the equatorial plane, while the NHC and alkyl/boryl groups occupy the axial positions. In the IR spectra, three bands were observed for the CO stretches (Table 21), with a significant blue shift in case of 126csCo, due to the low σ-donation ability of the CF3 substituent. Reaction of 126auCo and in situ generated 126buCo with CO led to a migratory insertion of the carbonyl in the metal−alkyl bond, resulting in the quantitative formation of the acyl complexes 127aCo and 127bCo, respectively (Scheme 314).608 Their solid-state structures are similar to that of the alkyl analogues, with the acyl and NHC donors located in the axial positions, trans to each other. The IR spectrum of 127aCo revealed similar CO stretching bands to those observed in the alkyl complex 126auCo but at slightly higher energy (Table 21). This blue shift is due to a lower electron density at the metal in case of the acyl complex. Both acyl complexes 127aCo and 127bCo react with H2, leading to the CoI hydride complex 128Co (Scheme 314).608 Due to slow decomposition in solution, 128Co could only be characterized by NMR and IR spectroscopic methods and by elemental analysis. In the solidstate and under inert atmosphere, no decomposition of 128Co was observed at room temperature, which contrasts with the low thermal stability of the corresponding phosphine complexes. Alkyl/hydride complexes. The diamagnetic hydride complex [CoH(η6-C6H6)(IPr)] (106Co) was already mentioned above in Scheme 306.565

As indicated in Scheme 337 (section Polynuclear Complexes, vide supra), the very air-sensitive CoI complex 129Co was obtained after addition of 4 equiv [Mg(benzyl)2(THF)2] to [{CoBr2(IPr)}2] (28bCo).566 This result contrasts with the synthesis of [Co(CH2SiMe3)2(IPr)] by alkylation of [{CoCl2(IPr)}2] (28auCo) with Me3SiCH2MgCl, as described by Tonzetich (Scheme 283).568 Although the mechanism of the formation of 129Co is not clear and involves reduction of the metal center and C−C coupling between the two benzyl ligands, generation of the [Co(benzyl)2(IPr)] intermediate may be invoked. In the crystal structure of 129Co, the CoI center is σ-coordinated by one benzyl group and further bound to a second benzyl moiety in an η6 fashion. A fluxional behavior was observed by variable-temperature 1H NMR studies and may be ascribed to conformational interconversion in the chelating benzylic ligand. 4.5.1.2.5. Type [Co(NHC)2L3]+(A−). The formation and properties of the carbonyl complex 130Co are mentioned in the section on Polynuclear Homometallic Complexes (Scheme 339).611

4.5.1.2.6. Type [Co(NHC)2X]. The three-coordinate CoI complexes 66u/sCo were obtained in very good yield by reaction of 2 equiv (S)IMes with [CoCl(PPh3)3] (Scheme 315).582,596 The complexes are paramagnetic in solution, and the effective magnetic moment of 66uCo [4.4(1) μB] is GD


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Scheme 315. Synthesis and Reactivity of the 3-Coordinate 66u/sCo

Scheme 316. Synthesis and Reduction of the

Me2

cAAC Complex 2a−bCo

which further reacts via σ-bond metathesis or oxidative addition/reductive elimination processes. As described in Scheme 338 (vide supra, section on Polynuclear Complexes), the three-coordinate Me2cAAC complex 2aCo was prepared by reduction of the dinuclear CoII complex 132bCo (Scheme 316).551,612 Cyclic voltammetry measurements indicated a quasi-reversible one-electron reduction process, corresponding to the formation of a reduced, formally Co0, complex. Accordingly, 1aCo was synthesized in excellent yield by reduction of 2aCo with KC8 (see Scheme 270, section Mononuclear Co0 Complexes).551 Complex 2bCo was obtained by reaction of free Et2cAAC ligand with CoCl2 and in situ reduction with one equiv Na/Hg.553 Its crystal structure featured a distorted trigonal-planar geometry, similar to that of 66u/sCo. The solution magnetic moment of 2bCo (μeff = 2.9 μB) is consistent with an S = 1 spin state. Halide abstraction with NaBArF4 and reduction with Na/Hg afforded the two-coordinate, formally Co0, complex 1bCo. 4.5.1.2.7. Type [Co(NHC)2L2X]. The synthesis and properties of the bis-NHC carbonyl/nitrosyl CoI complexes 121a−hCo were described in Scheme 312.606

consistent with the formulation of a high-spin S = 1 center. The crystal structures of both complexes were reported and revealed a distorted trigonal-planar arrangement around the CoI center, with a wider CNHC−Co−CNHC angle for the saturated 66sCo [152.5(1)° vs 129.6(2)° in 66uCo]. These complexes were further used to access the corresponding homoleptic two-coordinate complexes 100au/sCo after salt metathesis with NaBPh4, as previously described in Scheme 303.582,596 Treatment of 66uCo with MeLi, under N2 atmosphere resulted in an intramolecular C(sp3)−H bond activation and formation of the cyclometalated CoI dinitrogen complex 131Co (Scheme 315).582 A similar reactivity was observed using Me3SiCH2Li or (p-tolyl)MgBr. Complex 131Co is diamagnetic and features a CoI center in a square-planar environment, coordinated by one neutral IMes, one anionic bidentate [IMes′]−, and one terminal N2 donor. The latter ligand gives rise to a stretching band at 2006 cm−1 in the IR spectrum (Table 22), indicating only weak activation of the N2 ligand. The formation of 131Co is thought to be preceded by the generation of the CoI alkyl intermediate [Co(CH3)(IMes)2] GE


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Scheme 317. Reactivity of Reduced CNHCNCNHC-Co Pincer Complexes

Scheme 318. Synthesis and Reduction of the Dinitrogen Complex 137Co

4.5.1.2.8. Type [Co(NHC)3X]. The paramagnetic tris-carbene CoI complex [CoCl(ICy)3] (15Co) was obtained in 61% yield by reaction of CoCl2 with 3 equiv ICy, followed by in situ reduction with sodium amalgam. Its crystal structure revealed a tetrahedral coordination environment around the CoI center. Reduction of 15Co with 1 equiv KC8 under N2 atmosphere afforded the Co0 complex 14Co (see Scheme 275 in the section on Mononuclear Co0 Complexes).552 4.5.2. Functionalized NHCs. 4.5.2.1. Bidentate: Alkyl Functionalized (Cyclometalated). The cyclometalated CoI dinitrogen complex 131Co was previously described in Scheme 315.582 4.5.2.2. Tridentate. 4.5.2.2.1. Symmetrical CNHCNCNHC Pincers. Access to the formal CoI pincer complex 82Co was already described in Scheme 297 (section on Mononuclear CoII Complexes). In this section, the reactivity and electronic structure of the reduced CNHCNCNHC cobalt complexes are discussed in more detail. The high activity of 82Co in the catalytic hydrogenation of alkenes prompted the synthesis and study of a series of reduced Co-CNHCNCNHC pincer complexes (Scheme 317).587 The diamagnetic hydride complex 133Co was prepared by hydrogenation of 82Co, resulting in the liberation of CH4 as sole byproduct. Interestingly, in its 1H NMR spectrum, the resonance of the hydrogen at the 4position of the pyridine was detected downfield at δ 11.3 ppm,

similarly to what was observed in the corresponding methyl complex 82Co. However, 133Co is not stable in solution, and migration of the metal hydride to the 4-position of the pyridine ring was observed in benzene-d6 solution under N2 atmosphere over 3 h at 22 °C, leading to the Co-N2 complex 134Co. The presence of the N2 ligand was confirmed by IR spectroscopy, with a strong band detected at 2048 cm−1. Modification of the pyridine ring was evidenced by the presence of resonances in the range of δ 3.5−4.3 ppm in the 1H NMR spectrum. No crystal structure of 134Co could be obtained due to its low stability in solution, even at low temperatures (−35 °C). Nevertheless, treatment of freshly generated 134Co with CClPh3 led to the formation of 81bCo along with CHPh3, resulting from hydride abstraction and cooperative metal− ligand reactivity. The crystal structure of 81bCo revealed very similar overall molecular geometry and metrical parameters to those in the bromo derivative 81aCo. An analogous migration reaction was observed when 133Co was reacted with the bulky 1,1-diphenylethylene, leading to the CoI-N2 complex 135Co. The latter complex exhibits similar spectroscopic features to those in 134Co (N2 vibration at 2050 cm−1 in the IR spectrum and signals at δ 4.3−4.9 ppm in the 1 H NMR spectrum due to modification of the pyridine ring). Analysis of the X-ray structure of 135Co established an idealized square-planar environment for the CoI center and GF


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Review

Scheme 319. Reactivity of Dinitrogen CNHCCCNHC Pincer Complexes

Careful analysis of the metrical data in 81a−bCo and 82Co revealed distortions of the central pyridine ring, resulting from the reduction of the chelating ligand. The Co II X(CNHCNCNHC)(1‑) (X = alkyl, halide) family of compounds is best described as a chelate radical anion coordinated to a lowspin CoII center. This view was further supported by X-ray absorption spectroscopy (XAS) studies and by the anomalous downfield chemical shifts of the para-pyridine proton in the 1H NMR spectra of these complexes. Broken-symmetry DFT calculations confirmed the presence of an anionic radical chelate in 81aCo and 82Co, the radical being essentially localized in the pyridine ring, which may explain the observed hydride and alkyl migration reactivity. 4.5.2.2.2. Symmetrical CNHCCCNHC Pincers. Driven by the high catalytic activity of 87bCo in olefin hydrogenation, the reactivity of the CoI pincer complex 87bCo (cf. Scheme 298)588 and its PMe3 analogue 87cCo was investigated toward H2 and HCl (Scheme 319).613 Complex 87cCo was prepared in good yield by reduction of the corresponding CoIII dichloro complex 85bCo (cf. Scheme 298)588 with 2 equiv KC8 in the presence of PMe3. The crystal structure of 87cCo is akin to that of the PPh3 complex 87bCo, with a square-pyramidal coordination environment around the CoI center. Other spectroscopic data are also very similar to those observed in 87bCo. Exposure of 87b−cCo to H2 gas (4 atm) at room temperature resulted in the decoordination of the N2 ligand and formation of the dihydrogen complexes 139b−cCo. The coordination mode of the H2 ligand was examined by NMR spectroscopic methods (T1 relaxation studies) and is in favor of a “nonclassical” η2-coordination of the dihydrogen ligand. No conversion into the CoIII dihydride oxidative addition product was observed, even upon heating to 80 °C. Removal of the H 2 atmosphere and exposure of 139b−c Co to N 2 regenerated 87b−cCo, indicating a reversible coordination of the H2 ligand.

confirmed a modification of the pyridine ring with the formation of a sp3-hybridized carbon at the 4-position. The observation of hydride and alkyl migrations to the paraposition of the pyridine ring point to the occurrence of ligandcentered radicals in these reduced cobalt complexes, which was confirmed by a combination of structural, spectroscopic, and computational studies. The use of alkenes with a lower steric hindrance resulted in a different regioselectivity of the insertion reaction. Treatment of 133Co with 1-butene or isobutene led to the 1,2-insertion of the alkene into the cobalt-hydride bond and formation of 136a−bCo. However, both compounds were not stable in solution under N2 atmosphere and could only be characterized by 1H and 13C{1H} NMR. To investigate the possibility of a radical character and potential redox-(non)innocence in the CNHCNCNHC pincers, additional complexes were synthesized (Scheme 318) and the electronic structure of CNHCNCNHC-CoX (X = H, alkyl, halide) was thoroughly investigated.587 The diamagnetic cationic complex 137Co was obtained by reaction of 81aCo with NaBArF4 under N2 atmosphere. Dinitrogen coordination was evidenced by a strong band at 2141 cm−1 in its IR spectrum, and the X-ray diffraction structure confirmed a planar dinitrogen cobalt complex. The neutral complex 138Co was generated in situ by reduction of 137Co with [CoCp*2]. Reduction attempts using sodium amalgam or sodium/ naphthalene only resulted in mixtures of products. The coordinated N2 ligand was detected at 2047 cm−1 in the IR spectrum of the neutral 138Co, but no crystal structure could be obtained due to rapid decomposition in solution. Analysis by EPR spectroscopy established an S = 1/2 compound and revealed an isotropic signal with eight hyperfine lines arising from the coupling of the mainly ligand-centered radical to the I = 7/2 59Co nucleus. The EPR data support the electronic structure description of a low-spin CoI ion with a chelate radical anion, indicating a redox-active CNHCNCNHC ligand. GG


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Review

Scheme 320. Synthesis and Reactivity of the Reduced Pincer Complexes 86Co and 142Co

Scheme 321. Reduction of 92Co with SiH2Ph2 and KC8

Reaction of 87cCo with 1 equiv HCl·Et2O resulted in the formation of the CoIII oxidative addition product 140Co in good yield (Scheme 319).613 In the 1H NMR spectrum of 140Co, a large 2JHP coupling constant of 109 Hz was observed, suggesting that the hydride ligand is located trans to the phosphine group. The X-ray structure of 140Co confirmed the oxidative addition of HCl and revealed a distorted octahedral coordination geometry around the CoIII center. Treatment of 140Co with [ZrCp2(H)Cl] under N2 atmosphere resulted in the formation of [ZrCp2Cl2] and H2 and yielded back 87cCo. This reaction probably occurs via generation of a cis-CoIIIdihydride intermediate, followed by reductive elimination of H2. Addition of a second equiv of HCl resulted in the formation of the dichloro complex 141Co and the liberation of H2 gas. Alternatively, 141Co can be prepared by oxidation of 87cCo with 2 equiv CClPh3. Reduction of 85bCo with 2 equiv KC8 in the absence of phosphine afforded the new N2-free CoI complex 142Co (Scheme 320).614 The absence of N2 coordination was indicated by IR spectroscopy and further confirmed by analysis of the crystal structure of the complex. The cobalt center in

142Co lies in a four-coordinate square-planar coordination environment, with a pyridine ligand located trans to the anionic Caryl donor. The presence of mesityl NHC wingtip substituents play a major role in the isolation of 142Co. Using the more sterically demanding DiPP-substituted ligand, no pyridine coordination was observed under the same conditions, and the CoI-N2 complex 86Co was exclusively isolated.588 The pyridine ligand in 142Co can be easily displaced, for example by nitrile donors. Addition of 4-methoxybenzonitrile to 142Co yielded the CoI complex 143Co (Scheme 320)614 which was characterized spectroscopically. In its IR spectrum, an intense absorption band was detected at 2184 cm−1, indicating end-on coordination of the nitrile ligand. Upon further addition of PPh3, the more crystalline 144bCo complex was obtained and structurally characterized. The corresponding benzonitrile complex 144aCo was prepared by substitution of the N2 ligand in the CoI phosphine-dinitrogen complex 87bCo (Scheme 319). In both cases, the cobalt center lies in a distorted square-pyramidal environment, with the nitrile ligand located trans to the anionic aryl donor. In the IR spectra of GH


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Review

Scheme 322. Reactivity of 148a−bCo Bearing a Tripodal Ligand

observed in the 1H NMR spectra of both reduced complexes, ruling out redox noninnocence of the ligand in these complexes.587 Surprisingly, reaction of 92Co with excess (>4 equiv) SiH2Ph2 led to reduction of the cobalt center and formation of the diamagnetic 147Co in low yield (Scheme 321).591 The crystal structure of 147Co indicated rearomatization of the heterocycle and silylation of the pincer backbone at the α-CHP position. A possible mechanism involves, first, heterolytic cleavage of the Si−H bond through a metal−ligand cooperative process, generating a CoII-hydride intermediate. Homolytic cleavage of the CoII−H bond and concomitant reduction of the metal center leads to the formation of 147Co. Such a homolytic Co−H bond cleavage has also been postulated in the formation of the CoI amido complex [Co{N(SiMe3)2}(IPr)] (105Co) from the [CoH{N(SiMe3)2}(IPr)] intermediate (cf. Scheme 306).565 4.5.2.3. Higher Denticity: Tripodal Ligands: Tris(NHC)amine. The tris-carbene CoI complexes 148a−bCo were synthesized by reaction of the corresponding free carbene ligand with the CoI precursor [CoCl(PPh3)3] (Scheme 322).594 The 1H NMR spectra of the complexes are consistent with C3-symmetric species in solution. The tripodal ligand in 148a−bCo creates a well-protecting cavity which allows the coordination of an additional donor. Exposure of 148aCo to excess CO gas (1 atm) yielded the CoI-CO complex 149Co. The presence of the terminal CO ligand was confirmed by IR spectroscopy with the detection of a stretching band centered

144a−bCo, the coordinated nitrile gave rise to an intense band at 2191 and 2202 cm−1, respectively. 4.5.2.2.3. Nonsymmetrical PNCNHC Pincers. Reduction of Co 92 (cf. Scheme 300) bearing an anionic dearomatized P*NaCNHC ligand led to two different complexes depending on the reaction conditions (Scheme 321).590 Reaction with KC8 in N2-saturated solvents and evaporation of the volatiles under a stream of N2 led to the isolation of the mononuclear CoI complex 145Co. In its solid-state structure, the Co center is in a square-planar coordination environment, chelated by the anionic dearomatized pincer ligand, and further coordinated by a terminal N2 ligand. In the IR spectrum of 145Co, the N N stretching frequency was detected at 2057 cm−1, indicating only weak activation of the N2 ligand. When the reaction was carried out under N2-depleted conditions, the dinuclear 146Co was isolated instead and corresponds to the first “anionic dicarbene” (NHDC) cobalt complex. The solid-state structure of 146Co consists in two subunits featuring each a chelated CoI center in a distorted square-planar geometry. In the first subunit, the cobalt center is coordinated by a dearomatized P*NaCNHC pincer ligand, with similar coordination environment and metrical data to those in 145Co. In addition, C−H activation and metalation at the “abnormal” (C-4) position of a second NHC ligand occurred. Perusal of the bond distances within the picoline heterocycle revealed reprotonation of the α-CHP side arm and rearomatization of the PNCNHC ligand in the second and upper subunit. Interestingly, no anomalous chemical shift was GI


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Review

Scheme 323. Oxidative Addition Reactivity of Cp NHC CoI Complexes

at 1927 cm−1. The magnetic moments of 148aCo (3.65 μB) and 149Co (3.49 μB) at room temperature were determined by SQUID magnetization measurements and are consistent with high-spin CoI complexes (d8, S = 1) with a large orbital angular momentum contribution (spin-only value of 2.83 μB for a triplet ground state).594 Although stable in the solid state under inert atmosphere, oxidation of 148aCo into the high-spin CoII complex 150Co readily occurred in solution, especially in the presence of benzyl chloride or chlorinated solvents such as dichloromethane or chloroform. Treatment of 150Co with NaBPh4 in acetonitrile led to the dicationic high-spin CoII complex 151Co. The latter could also be obtained by reaction of 148aCo with NaBPh4, but, in this case, several days were required for the reaction to reach completion. The molecular structures of 150Co and 151Co display a distorted trigonalpyramidal geometry around the CoII center, with the chloride or the acetonitrile ligands located in the axial position, respectively. The different complexes were further characterized by SQUID magnetization measurements and UV−vis absorption spectroscopy. Clean formation of the diamagnetic CoIII-O2 addition product 152Co occurred upon exposure of 148aCo to O2 at room temperature. Side-on coordination of the peroxo ligand was indicated by an O−O stretching frequency at 890 cm−1 in the IR spectrum of 152Co and further confirmed by analysis of the crystal structure of the complex. The O−O distance of 1.429(3) Å lies in the typical range for peroxide complexes (1.4−1.5 Å). DFT calculations support the formulation of the complex as a low-spin CoIII metal center (d6, S = 0) with a dative peroxo ligand. The main contribution in the HOMO arises from the dioxygen π* orbitals, suggesting some nucleophilic character for the coordinated dioxygen ligand and a possible reactivity toward electron-deficient organic substrates. The reaction of 152Co with benzoyl chloride in acetonitrile led to an oxygen-transfer reaction with quantitative formation of phenyl benzoate and the CoII complex 151Co. The reaction of 152Co was also investigated with electrondeficient alkenes. However, only the reaction with benzylidenemalonitrile occurred cleanly, leading to 151Co and benzyl

aldehyde. The lack of electrophilicity of 152Co was confirmed by the absence of reactivity toward styrene, cyclohexene, or triphenylphosphine. Accordingly, 152Co can be classified as a Class II nucleophile.615 In the attempted synthesis of a CoIII imido complex, 148aCo was treated with trimethylsilyl azide (Scheme 322).595 However, no oxidation of the metal center into CoIII was observed, but formation of the one-electron oxidized azido species 153Co occurred instead. Reaction of 148a−bCo with aryl azides at low temperature (−35 °C) led to the desired monomeric CoIII imido complexes 154a−dCo in almost quantitative yields. The 1H NMR spectra of these diamagnetic CoIII complexes suggest a C3-symmetry in solution. Analysis of the crystal structure of 154cCo reveals a pseudotetrahedral coordination geometry around the Co metal center. The short Co−Nimido distance of 1.675(2) Å, in the expected range for CoIII imido complexes, reflects a strong Co−N multiple bond character. DFT calculations indicate that the LUMOs of the complex consist of the empty dxy and dyz orbitals of the metal which are destabilized by a strong π-bonding interaction with the imido π-lone pairs. The Co-N bonding in 154cCo is best described as a formal double bond. Although stable in the solid-state and in solution at −35 °C, 154a−dCo were found to react at room temperature via insertion of the imido group into one of the cobalt-carbene bonds, leading to the CoII imino complexes 155a−dCo. This reaction possibly involves a CoI imino intermediate that further disproportionates into 155a−dCo, metallic Co0, and other unidentified organic products. The solid-state structures of 155bCo and 155cCo were established by X-ray diffraction studies and confirmed coordination of two carbene units, the anchoring nitrogen atom, and the newly formed imine N donor. The observed imido insertion reactivity shows that the cobalt-imido complexes 154a−dCo are highly electrophilic. However, no intermolecular imido transfer was observed upon treatment of 154a−dCo with nucleophiles such as styrene or tetramethylimidazol-2-ylidene (Me2IMe), suggesting that the intramolecular process is faster. GJ


Chemical Reviews

Review

4.6. Mononuclear CoIII Complexes

complex features an intense purple color. Coordination of the NHC ligand in 157Co was established by NMR spectroscopy. A high-field shift of the signal for the Me wingtip substituents of the NHC donor (δ −0.63 ppm) was observed and is due to the porphyrin ring current. Even in the presence of excess imidazolium carboxylate, only the mono-NHC-substituted complex 157Co was isolated. Treatment of 157Co with AgBF4 led to halide abstraction and formation of 158Co. Addition of MeOH or EtOH led to the alcohol complexes 159a−bCo, respectively, which were characterized crystallographically. The CoIII center lies in an octahedral coordination environment with the porphyrin ligand occupying the equatorial positions, while the NHC and alcohol ligands are located in the axial positions. Distortion of the porphyrin macrocycle is observed, probably due to the short Co−CNHC bond (1.96 Å), preventing the coordination of a second NHC ligand. Addition of 1,2-dimethylimidazole or 2,4-dimethylimidazole to 157Co led to the CoIII NHC-imidazole complexes 160a−bCo, which were isolated after purification by preparative thin-layer chromatography and characterized spectroscopically. 4.6.2. Tris-Carbene Ligands. The CoIII tris(carbene)borate complexes 161a−bCo bearing tripodal NHC ligands were obtained by in situ deprotonation of the corresponding tris(imidazolium)borate salts, followed by addition of cobalt dichloride (Scheme 326).618 Air was passed through the reaction mixture in order to ensure oxidation of the metal to its trivalent state. In the IR spectrum of 161a−bCo, the B−H stretching vibrations were detected at 2400−2500 cm−1. The solid-state structure of 161bCo was unambiguously established by X-ray diffraction studies, revealing a CoIII metal center in an approximate S6 geometry, coordinated by six NHC donors. The CoIII imido complex 61Co was already mentioned in the section on Mononuclear CoII Complexes (Scheme 290).580

4.6.1. Monodentate Carbene Ligands. 4.6.1.1. Heteroleptic: Type [Co(NHC)L3X3]. The CoIII oxidative addition complexes 113Co,600 115Co,603 116Co,602 and 117Co (Scheme 323)604 were obtained starting from [Co(η5-C5R5)(NHC)(L)] (R = H, Me; L = no ligand, CO, C2H4), as described in the section on Mononuclear CoI Complexes. Among them, 115Co and 117Co were structurally characterized. Reaction of 116Co with MgMe2 led to the CoIII dimethyl complex 156aCo in low yield (

N-Heterocyclic Carbene Complexes of Copper, Nickel, and Cobalt

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