High-Oxidation-State 3d Metal (TiâCu ... - American Chemical Society

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High-Oxidation-State 3d Metal (Ti−Cu) Complexes with N‑Heterocyclic Carbene Ligation Jun Cheng, Lijun Wang, Peng Wang, and Liang Deng* State Key Laboratory of Organometallic Chemistry, Center for Excellence in Molecular Synthesis, Shanghai Institute of Organic Chemistry, University of Chinese Academy of Sciences, Chinese Academy of Sciences, 345 Lingling Road, Shanghai 200032, People’s Republic of China

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ABSTRACT: High-oxidation-state 3d metal species have found a wide range of applications in modern synthetic chemistry and materials science. They are also implicated as key reactive species in biological reactions. These applications have thus prompted explorations of their formation, structure, and properties. While the traditional wisdom regarding these species was gained mainly from complexes supported by nitrogen- and oxygen-donor ligands, recent studies with N-heterocyclic carbenes (NHCs), which are widely used for the preparation of low-oxidation-state transition metal complexes in organometallic chemistry, have led to the preparation of a large variety of isolable high-oxidation-state 3d metal complexes with NHC ligation. Since the first report in this area in the 1990s, isolable complexes of this type have been reported for titanium(IV), vanadium(IV,V), chromium(IV,V), manganese(IV,V), iron(III,IV,V), cobalt(III,IV,V), nickel(IV), and copper(II). With the aim of providing an overview of this intriguing field, this Review summarizes our current understanding of the synthetic methods, structure and spectroscopic features, reactivity, and catalytic applications of high-oxidation-state 3d metal NHC complexes of titanium to copper. In addition to this progress, factors affecting the stability and reactivity of high-oxidation-state 3d metal NHC species are also presented, as well as perspectives on future efforts.

CONTENTS

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1. Introduction 2. Challenges in the Stabilization of High-Oxidation-State 3d Metal Species 3. High-Oxidation-State 3d Metal Complexes with NHC Ligation 3.1. Nature of the Metal−Carbon(NHC) Bond in High-Oxidation-State 3d Metal Species 3.2. Complexes of Titanium 3.3. Complexes of Vanadium 3.4. Complexes of Chromium 3.5. Complexes of Manganese 3.6. Complexes of Iron 3.7. Complexes of Cobalt 3.8. Complexes of Nickel 3.9. Complexes of Copper 4. Summary and Perspective Associated Content Supporting Information Author Information Corresponding Author ORCID Notes Biographies Acknowledgments Abbreviations © XXXX American Chemical Society

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1. INTRODUCTION The 3d metals (Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn) are abundant in the earth’s crust. Some of these metals are essential for biochemical functions and occur in metalloenzymes, which, to a certain degree, also reflects their biocompatibility. With these features, 3d transition metals are thought to be ideal for the development of economical and sustainable materials.1 Hence, research interest in 3d metal complexes is at an all-time high.2−4 Transition metals can usually exhibit multiple stable oxidation states in their compounds. The multiple accessible oxidation states, in combination with their versatile coordination geometries and spin states, as well as versatile ligand environments, jointly engender the rich physical and chemical properties of 3d metal complexes. Ever since the birth of modern coordination chemistry, interest in 3d metal species in different oxidation states has been extremely high. High-oxidation-state 3d metal species play important roles in the areas of synthetic chemistry, biology, and materials science.

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Special Issue: Carbene Chemistry Received: February 9, 2018

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DOI: 10.1021/acs.chemrev.8b00096 Chem. Rev. XXXX, XXX, XXX−XXX

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review in 200741 and Herrmann and Kühn’s review in 2008.42 At that time, high-oxidation-state transition metal complexes were mainly known for the early and middle 4d and 5d metals. In 2011, Cazin43 and Sigman44 independently reviewed oxidation reactions catalyzed by NHC complexes. However, both reviews focused on organic transformations, instead of high-oxidation-state transition metal NHC complexes. Readers who are interested in these topics are encouraged to read these reviews for related information. The following section of this Review will start with a brief introduction of the challenges inherent to the study of highoxidation-state 3d metal species. The features of NHCs that might be beneficial for the stabilization of high-oxidation-state 3d metal−NHC species, and the current understanding of the metal−carbon(NHC) bond in high-oxidation-state 3d metal− NHC species, will then be discussed. Followed by these general views are the advances in the chemistry of highoxidation-state 3d metal−NHC species. As the main body of this Review, this section is categorized by the type of 3d metal involved, and introduces their synthesis, structure, spectroscopic features, reactivity, as well as catalytic application. As scandium and zinc usually exist in their compounds in their highest oxidation states (i.e., scandium(III) and zinc(II)), and thus only rarely undergo redox reactions, studies of their NHC complexes are not covered herein.45,46 We should also state that because each metal differs in its redox properties, the high oxidation states covered in this Review include formal oxidation states equal to (for Ti and V) or higher than (for the other 3d elements) their common oxidation states. For the late 3d metals, NHC complexes of iron(III), cobalt(III), and copper(II) are included as these organometallic compounds have been proven to be very reactive. For readers’ convenience, the charts and schemes illustrate the formal oxidation states of the metals as superscript roman numerals. In addition, representative characterization data of these complexes are compiled in Table S1. In the final section, a perspective on possible future directions and important unanswered questions in this research field is presented.

In organic synthesis, high-oxidation-state 3d metal oxides, for example, V2O5, CrO3, MnO2, and KMnO4, are commonly used oxidants.5,6 There are also numerous 3d-metal-catalyzed organic transformations employing dioxygen, hydrogen peroxide, nitrene precursors, and electrophilic halogen reagents as oxidants, wherein high-oxidation-state 3d metal species are the putative oxidizing intermediates.5,6 In the search for efficient and cheap water oxidation catalysts, 3d-metal-based materials and complexes are viewed as potential candidates,7−9 and many of them are recognized to have high-oxidation-state metal oxo species as the key intermediates. For example, cobalt(IV) terminal oxo species and mononuclear iron(V) oxo species were thought to be the key intermediates in Nocera’s cobalt-catalyzed water oxidation systems10,11 and Costas’ (amine-pyridine)iron-catalyzed reactions,12 respectively. As many metal-catalyzed oxidation reactions suffer from low catalytic efficiency and poor selectivity, and 3d-metal-based water oxidation catalysts frequently suffer from high overpotential and low stability, there is great demand for deeper knowledge of the formation, structure, and reactivity of highoxidation-state 3d metal species.13,14 In biology, a wide range of enzymatic processes are believed to have high-oxidation-state 3d metal species as intermediates.15−22 Examples of these conversions include the oxidation of methane with O2 to form methanol catalyzed by iron- and copper-methane monooxygenases,23−25 the versatile C−H bond activation reactions mediated by cytochromes P450,26−28 the decomposition of H2O2 to H2O and O2 by iron-catalases,29 the dismutation of superoxide (O2−) into O2 or H2 O2 by manganese-superoxide dismutase,30,31 the reduction of N2O to N2 by copper-containing nitrous oxide reductase,32,33 and the oxidation of H2O to produce O2 in the oxygen-evolving complex during the light-mediated reactions of photosynthesis.34−36 As the proposed high-oxidation-state 3d metal intermediates could be highly reactive, the validation of their identity and roles in enzymatic reactions is among the most challenging tasks of modern bioinorganic chemistry. In materials science, high-oxidation-state transition metal oxides represent a class of important solid-state materials.37−40 In addition to their use as catalysts for oxidation reactions, the highly polarized Mn+−O2− covalent interaction of highoxidation-state 3d metal oxides could induce intriguing properties such as charge ordering, spin transition, and superconductivity.38 A well-known example is TiO2, which can undergo photoinduced charge-separation, making it useful for photocatalysis. With their relevance to the important oxidation reactions in synthetic chemistry and biology, as well as applications in materials science, high-oxidation-state 3d metal species have been a major focus of study in modern chemistry. Among the many efforts in the area, recent exploration of high-oxidationstate 3d metal species with N-heterocyclic carbene (NHC) as ligands has proved particularly fruitful, as evidenced by the diversity of such complexes, which currently encompass all of the 3d metals in their higher oxidation states (e.g., Ti(IV), V(V), Cr(V), Mn(V), Fe(V), Co(V), Ni(IV), and Cu(II)) bearing different ancillary ligands (e.g., halides, oxo, imido, and nitride). This Review aims to present an overview of the status quo of the chemistry of high-oxidation-state 3d metal species with NHC ligation. It should be mentioned that, while there are many studies on NHC complex-catalyzed oxidation reactions, reviews of high-oxidation-state transition metal complexes with NHC ligation are rare and restricted to Stahl’s

2. CHALLENGES IN THE STABILIZATION OF HIGH-OXIDATION-STATE 3d METAL SPECIES Being mismatched with their importance, our knowledge of the chemistry of high-oxidation-state 3d metal species, particularly the late transition metals, is quite limited. This situation is related to the difficulty in accessing isolable complexes of the type and the complexity of 3d metal chemistry. While the early 3d metals can readily form complexes in which the oxidation-state of the metal center is equal to the corresponding group number, it is recognized that highoxidation-state 3d metal species are, in general, less stable than their 4d and 5d congeners.47−50 The situation can be discerned from the known highest oxidation states of late 3d metals, which are lower than the corresponding group numbers and also lower than those of their heavier congeners.50 For example, the heavier group 8 metals Ru(VIII) and Os(VIII) can form binary oxides of the form MO4, whereas FeO4 is unknown and Fe(VI), as in [FeO4]2−, is the highest oxidation state observed for Fe.47,50 For the group 9 metals, while Ir(IX) and Rh(VI) species are known,51,52 Co(V), as in [Co(1norbornyl)4][BF4],53,54 represents the highest oxidation state of this element. This situation parallels the fact that the 3d metals have higher third and fourth ionization enthalpies than the corresponding 4d and 5d metals.47,50 On the other hand, B


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the metal 3s/3p orbitals and ligand atomic orbitals, which could prevent the metal and ligand coming close enough.55 The balance of the two factors results in an inefficient 3d metal−ligand interaction. The situation could be even more severe in high-oxidation-state metal species, as their 3d orbitals have more contracted radial distributions.47 Kaupp even denoted 3d metal−ligand interactions as “stretched” metal− ligand bonds.56 A stretched bond means a small energy gap between bonding and antibonding interactions, which renders the excited states of high-oxidation-state 3d metal species easy to access, adding complexity to 3d metal chemistry.56 Related to their ease of accessing excited states, high-oxidation-state 3d metal complexes might show intense ligand-to-metal charge transfer bands in the absorption spectra.47 In addition to charge-transfer-induced excited states, 3d metal complexes could also have 3d-to-3d transitions as the crystal field splitting energies of 3d metal ions are generally lower than those of the corresponding 4d and 5d metal ions. One consequence of a high density-of-state is the capability of high-oxidation-state 3d metal species to involve excited states in reactions.57−68 As an excited state might have different charge distribution and/or spin density distribution on metal and ligand fragments as compared to the ground state, the excited-state species could have distinct reactivity.69 The effect of spin-states of metal oxo

the Pauli repulsion between the 3s/3p orbitals of 3d metals and the atomic orbitals of the ligand could be among the causes.50,55,56 It is recognized that the limited spatial extension of metal 3d orbitals relative to those of 4d and 5d orbitals necessitates short metal−ligand distances to achieve effective bonding, which however might cause Pauli repulsion between the orbitals of the metal and its ligands (Figure 1). In their

Figure 1. Schematic illustration of the Pauli repulsions between ligand orbitals and the outermost core (n − 1)s,p shell in a transition metal complex, leading to a stretched bond and poor overlap between (n − 1)d orbital and ligand orbital even at equilibrium distance. Reproduced with permission from ref 56. Copyright 2007 John Wiley & Sons, Inc.

theoretical study on the metal−ligand bond in MnO4−, Buijse and Baerends pointed out that the comparable size of 3s, 3p, and 3d shells of the metal could cause Pauli repulsion between

Chart 1. Representative N-Based Chelating Ligands Employed in the Preparation of High-Oxidation-State 3d Metal Complexes

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Chart 2. Representative NHC Ligands Used in the Study of High-Oxidation-State 3d Metal Complexes and Their Designations

the halides, F−, Cl−, and Br−, oxygen-based ligands such as oxo (O2−), carboxylates (RCOO−), and alkoxides (RO−), and the nitrogen-based anions nitride (N3−), imido (RN2−), and amido (R2N−).70 From a covalent bond classification,71 these are LXtype ligands that provide both σ- and π-donation to a metal ion, and consequently alleviate the electron deficiency of highoxidation-state transition metal center. In addition to LX-type ligands, charge-neutral L-type ligands are also applied as ancillary ligands in the synthesis of high-oxidation-state late 3d metal complexes. Nitrogen-based ligands, for example, amines, pyridines, imines, and oxazolines, constitute the majority of this type of ligands.

species on their reactivity in C−H bond activation has been the subject of vigorous discussion in the bioinorganic chemistry community.57,61−68 As the stabilization of high-oxidation-state metal species necessitates suitable ligands, and, in return, ligands affect the physical and chemical properties of the metal species, examining the effect of ligands on the stability and properties of high-oxidation-state 3d metal complexes is one of the major goals of the field. For the stabilization of high-oxidation-state transition metal species, the knowledge accumulated thus far has shown that anionic ligands with strongly electronegative coordinating atoms are effective.50 This type of ligand includes D


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Bi- and multidentate ligands, as compared to monodentate ligands, have enforced bonding with a metal center and are commonly applied to support high-oxidation-state 3d metal species.14,72 Chart 1 lists some of the representative examples of such ligands. N-Based chelating ligands, for example, cyclams (1,4,8,11-tetraazacyclotetradecane),73−75 TPA (tris(2-pyridylmethyl)amine),76 TMG3 tren (1,1,1-tris{2-[N2(1,1,3,3-tetramethylguanidino)]ethyl}amine),77 terpyridine,76 2,6-diiminopyridine,78,79 H3buea (tris[(N′-tert-butylureayl)-Nethylene]amine),80 tris(pyrazolyl) borates,81 corroles, 82 N4Py,83 Bn-tpen,84 porphyrin,73,85−87 phthalocyanine,85−87 corrolazine,88 TAML (tetraamido macrocyclic ligand),18,89 nacnac (β-diketiminate),90−92 and dipyrrin,93−96 belong to this category. As compared to nitrogen ligands, phosphines and arsines are more susceptible to oxidation. However, when coordinated with transition metals, chelating phosphine and arsine ligands, particularly those having bulky substituents, seem to be less prone to oxidation. Nyholm and Levason et al. showed that bisphosphine and bisarsine ligands can be used for the preparation of high-oxidation-state 3d metal halides.48,97 Peters and co-workers have demonstrated the applicability of multidentate phosphine ligands in stabilizing terminal imido and nitrido complexes of Fe(III), Co(III), and even Fe(IV).98−100 In addition to these ligands, carbon-based ligands, which are widely used for the preparation of organometallic compounds, are rarely used for the synthesis of high-oxidation-state 3d metal complexes. From this point of view, the large body of high-oxidation-state 3d metal complexes with NHC ligation reported in the previous two decades is extraordinary.

transition metal is large, either the M−C(NHC) interaction could be ionic in nature or a redox reaction could occur between the NHC and the high-oxidation-state metal species. The dissociation of NHCs from certain titanium(IV)−NHC complexes,119 which draws parallels with f-block NHC complexes due to “hard−soft” bonding,45 and the reduction of FeCl2 by Et2-cAAC129 and CuCl2 by IPr,130 reflect the situation. When the energy levels are comparable, effective NHC−metal bonding can then take place, and the stabilization of high-oxidation-state 3d metal complexes by NHC ligands is achieved. Fortunately, the availability of versatile NHC ligands with variable σ-donating abilities, for example, imidazolin-2ylidene, imidazolin-4-ylidene, imidazolidin-2-ylidene, and cyclic(alkyl)(amino)carbene, provides a great opportunity for the desired match.108,125,131,132 In this case, the strong σdonating nature of the NHC, to a certain degree, decreases the electron-deficiency of the high-oxidation-state metal center, and consequently reduces the possibility of decomposition via reductive elimination or redox reactions with external reducing reagents. In addition, the steric bulk of NHCs might exert steric shielding on the metal centers and/or the reactive ancillary ligands, for example, oxo, nitrido, and imido, hindering the attack of external reagents. Thus, with these unique steric and electronic features, it can be anticipated that NHCs would make useful ligands for the stabilization of highoxidation-state 3d metal species. Indeed, recent studies have shown that NHCs are well suited for this job. Since Herrmann’s report on the preparation of MCl4(IMe2H2)2 (M = Ti, Zr, Hf, Nb, Ta; IMe2H2 denotes 1,3-dimethylimidazol-2-ylidene, Charts 2 and 3) and ReO3Me-

3. HIGH-OXIDATION-STATE 3d METAL COMPLEXES WITH NHC LIGATION This section summarizes the status quo of the chemistry of high-oxidation-state 3d metal−NHC complexes. The first part summarizes the current understanding of the metal−carbon(NHC) bond of high-oxidation-state 3d metal−NHC complexes. The chemistry of the complexes of titanium to copper will then be introduced. The discussion for each metal is categorized in the following order: homoleptic complexes, complexes with halides or carboxylates as ancillary ligands, and complexes featuring metal−ligand multiple bonds, for example, imido, nitrido, and oxo complexes. As we shall see, the known high-oxidation-state 3d metal−NHC complexes display very rich structural diversity. As a reflection of this diversity, Chart 2 lists representative ligands used in the study of high-oxidationstate 3d metal species.

Chart 3. High-Oxidation-State 3d Metal−NHC Complexes Subjected to Bonding Analysis by Frenking and Jacobsen142−144

3.1. Nature of the Metal−Carbon(NHC) Bond in High-Oxidation-State 3d Metal Species

(IMe2H2)2 in 1994,133 a number of high-oxidation-state early to middle transition metal complexes with NHC ligation have been reported.104−122 Following the achievements in early to middle transition metals, the number of reported highoxidation-state late transition metal complexes with NHC ligation has also experienced rapid growth,42,104−122 particularly with late 3d metal complexes.105 As milestones of the field, Fehlhammer reported homoleptic hexa(NHC)iron(III) and hexa(NHC)cobalt(III) complexes;134,135 the groups of Meyer and Smith showed that tris(NHC) ligands could support terminal imido and nitrido species of iron and cobalt, in which the highest formal oxidation state of the iron center reaches +V; 136−138 Deng and co-workers found that monodentate NHCs could be used for the stabilization of formal cobalt(IV) and cobalt(V) imido complexes;139 Meyer

A new development in the chemistry of high-oxidation-state 3d metal complexes is the emergence of complexes featuring NHC ligands. Since the first report of NHC complexes in the 1960s,101−103 NHCs, including the cyclic alkylaminocarbenes (cAACs), have proved to be useful ligands in the preparation of low- and middle-oxidation-state transition metal complexes.104−126 As carbon-based ligands, NHCs are strongly σdonating in nature, and their HOMOs are more diffuse and relatively higher in energy than those of nitrogen- and oxygenbased ligands.127,128 This strong σ-donating nature could mean effective orbital overlap between the HOMO of the NHC and the 3d orbitals of metal ions. Ideally, when the energy gap between the HOMO of an NHC and the 3d orbitals of a E


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While the ligand-to-metal π-interaction energy in TiCl4(IH2) can be related to the donation from the HOMO−1 orbital of IH2 to the 3d orbital of Ti, the occurrence of the metal-to-ligand π-interaction energy is unusual when noting the formal d0 count of the metal center. An explanation for this could be the charge flux involving the ligand (Cl) trans to the NHC and the metal center. This argument is nicely demonstrated by the Cl(3p)−Nb(4d)−NHC(π*) interactions in Ciancaleoni’s calculation of the 4d complexes [NbVCl6−x(NHC)x]x−1 (Chart 5).147 Whether or not such a

et al. achieved the preparation of iron(IV) oxo species with a cyclic tetra(NHC) ligand;140 and Fout et al. demonstrated the accessibility of nickel(IV) complexes [(DippCCC-2)NiX3] with NHC-based pincer ligands.141 The accessibility of a large variety of high-oxidation-state 3d metal−NHC complexes raised questions regarding the nature of their metal−carbon(NHC) bonds, which remains poorly understood. Very few bond dissociation energy data are available for metal−carbon(NHC) bonds of high-oxidationstate 3d metal complexes. To our knowledge, the calculated bond dissociation energies of 111 and 140 kJ/mol for the M− C(NHC) bonds in the d0 complex TiCl4(IH2) and the abnormal NHC complex TiCl4(a-IH2), respectively, as reported by Frenking and co-workers, are the only examples (Chart 3).143 The higher bond dissociation energy of the Ti− C(a-IH2) bond over that of the Ti−C(IH2) bond is consistent with the stronger σ-donating nature of abnormal NHCs over normal NHC ligands. These bond energies are lower than those of the analogous zirconium and hafnium complexes by ca. 30 kJ/mol, and much lower than those of the d6 complex Cr(IH2)(CO)5 (218 kJ/mol), the d8 complex Fe(IH2)(CO)4 (252 kJ/mol), and the d10 complex Cu(IH2)Cl (284 kJ/ mol).143 No experimental data have been reported for the M− C(NHC) bond energy of high-oxidation-state 3d metal complexes. The few theoretical studies on the orbital interactions of metal−carbon(NHC) bonds of high-oxidation-state metal complexes are also restricted to early transition metal complexes with d0 electronic configuration. Jacobsen and coworkers performed an energy decomposition study on the 3d0 species [TiCl5(IH2)]−, [Cp2TiMe(IH2)]+, and VCl3O(IH2) (Chart 3).128,144 The bond snapping energy (BEsnap) of the metal−carbon(NHC) bonds, which was defined as the energy required for the dissociation of a M−L bond giving two fragments that possess both the local equilibrium geometry and an electronic structure suitable for bond formation,145,146 were calculated to be ca. 110, 240, and 180 kJ/mol, respectively. Further energy decomposition analysis revealed corresponding orbital interaction energies ΔEorb of ca. −220, −240, and −310 kJ/mol, corresponding σ-contributions to the orbital interaction of 89%, 82%, and 89%, and π-contributions to the orbital interaction of 11%, 18%, and 11%. These data clearly indicate the dominant role of σ-interactions in the metal− NHC bonds (Chart 4). Accordingly, Frenking’s analysis of TiCl4(IH2) revealed σ- and π-interaction energies of the Ti−C bond of −214 and −32 kJ/mol, respectively, and that the weak π-interaction can be further divided into ligand-to-metal and metal-to-ligand π-interactions (−16 and −16 kJ/mol, respectively).143

Chart 5. Cl(3p)−Nb(4d)−NHC(π*) and Cl(3p)− NHC(π*) Interactions in High-Oxidation-State Metal− NHC Species

situation is present in the aforementioned high-oxidation-state 3d metal−NHC complexes is yet to be examined. Relevant to this, Abernethy and Cowley showed the presence of novel Cl− C(carbene) interactions in the five-coordinate 3d0 complexes [VCl3(O)(NHC)]148 and [TiCl2(NMe2)2(NHC)].149 Such an interaction is characterized by the short Cl−C(NHC) distances, which are within the sum of the van der Waals radii, and can be viewed as the interaction of Cl(3p) lone pair with the empty NHC(π*) (Chart 5). This interaction is relevant to the transition state of the reductive elimination of imidazolium salts from transition metal NHC complexes.150 In addition to the orbital considerations, Frenking et al. pointed out that electrostatic interaction could play an important role in the metal−carbon(NHC) interaction in NHC complexes.143 Their study of the 3d0 species TiCl4(IH2) revealed that the interaction energy (ΔEint) between a preorganized TiCl4 motif and the NHC ligand IH2 was 232 kJ/mol at the BP86/TZ2P level.143 Energy decomposition analysis, which can split ΔEint into the electrostatic energy ΔEelstat, the orbital interaction energy ΔEorb, and the Pauli repulsion energy, indicated values of −468, −246, and 482 kJ/ mol, respectively. The large absolute value of ΔEelstat, which accounts for two-thirds of the attractive interactions (ΔEelstat + ΔEorb), suggests that the electrostatic energy ΔEelstat plays a dominant role in the attractive interactions. As for the abnormal NHC complex TiCl4(a-IH2), the interaction energy between the preorganized TiCl4 motif and a-IH2 is calculated to be 266 kJ/mol, with the contributions from the electrostatic energy, the orbital interaction energy, and the Pauli repulsion energy being −503, −268, and 505 kJ/mol, respectively. The decomposition analysis again revealed the decisive role of electrostatic attraction in determining the metal−C(carbene) bond strength in the abnormal NHC complex.143 Notably, this phenomenon is also observed in other early to late transition metal−NHC complexes that have high d-electron counts. The observation has thus led to the claim that electrostatic interaction constitutes the main energetic component of metal−carbon(NHC) bonds.143

Chart 4. Orbital Interactions of NHC Ligand with a 3d Metal Ion

3.2. Complexes of Titanium

The oxidation state +IV is the highest and the most common observed for titanium in its compounds. A large number of titanium(IV) complexes exist with NHC ligation. Herrmann and Kühn’s review42 on high-oxidation-state metal−NHC F


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Chart 6. Examples of Ti(IV) NHC Complexes

complexes in 2008 covered the early studies on titanium− NHC complexes. In 2015, Zhang and Zi presented a more specific and detailed review of group 4 metal−NHC complexes.119 Since 2015, only a few studies on titanium complexes with NHC ligation have been reported.151,152 To avoid unnecessary overlap with the existing reviews, only brief summaries of the structural diversity, the properties of the Ti(IV)−C(carbene) bonds, and the catalytic applications of titanium(IV)−NHC complexes are presented herein. A large number of titanium(IV) NHC complexes featuring either monodentate or chelating NHC ligands have been reported. The latter type predominates in quantity. Chelating NHCs, particularly those having anionic donors, can bind tightly with titanium(IV) centers, which can alleviate the lability of Ti(IV)−C(carbene) bonds. Chart 6 lists some examples of the reported titanium(IV)−NHC complexes.153−165 The known titanium(IV) NHC complexes display very rich structural diversity (Figure 2). Both inorganic ligands (e.g., halides, alkoxides, aryloxides, amides, carboxylates, azides, and imides) and organometallic hydrocarbyl ligands155,156,159,161,164 (e.g., benzyl and methyl162,165) are compatible with NHCs in the coordination sphere of titanium(IV). Chelating NHC ligands found in the known titanium(IV) complexes include bidentate alkoxy-NHC, aryloxy-NHC, imido-NHC, tridentate NHC-amide-NHC, NHC-aryl-NHC, aryloxy-NHC-aryloxy, as well as cyclopentadienyl-NHC ligands.153−157,161,163,165 The wide use of oxygen-based anion-functionalized NHCs is likely due to the well-known oxophilicity of early transition metals. As titanium(IV) is the most common and stable oxidation state of the element, titanium(IV) precursors can be used directly in the preparation of titanium(IV) NHC complexes. As shown in Scheme 1,153,160,164 traditional synthetic methods

Figure 2. Molecular structure of a titanium(IV) complex reported by Hahn and co-workers. Hydrogen atoms are omitted for clarity. The figure was generated on the basis of a cif file reported in ref 164.

are frequently applied for the preparation of titanium(IV)− NHC complexes, that is: (i) direct coordination of free NHC ligands to the titanium(IV) center, or ligand-replacement reactions between free NHC ligands and titanium(IV) precursors, (ii) amine or alcohol elimination reactions of imidazolium salts with titanium amides or alkoxides, and (iii) salt elimination reactions of the alkaline metal salts of alkoxyfunctionalized NHC ligands with titanium(IV) halides. The last type of reaction can also be viewed as a transmetalation process. A side range of titanium(IV) NHC complexes have been characterized by single-crystal X-ray diffraction studies. The G


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olefin polymerization (Scheme 3).154,165,166,169 For instance, the bis(aryloxy)−NHC complex 14 (with MAO as cocatalyst) can catalyze the polymerization of ethylene to give polyethylene with no methyl or long-chain branches. Under optimized conditions ([Al]/[Ti] ratio of 1:1000, 50 °C, 50 psig of ethylene pressure), the activity of the catalytic system can reach ca. 90 kg PE mol−1Ti h−1 atm−1ethylene, but the number-average molecular weight of the resulting polymer is less than 3230 g mol−1.169 Grubbs and co-workers154,166 found that high molecular weight polyethylene and highly syndiotactic polystyrene could be obtained using 6 and MAO as the catalytic system. In the case of styrene polymerization, the catalytic system only showed moderate activity (1.3 kg PS mol−1Ti h−1), and the polymer had a low number-average molecular weight (10.2 kg mol−1) and a broad molecular weight distribution of 2.4. Le Roux and co-workers applied their bis(phenolate)− NHC−titanium(IV) complexes (ROCO-2)TiX1X2 (X1, X2 = OBn, OAc, N3, OSi(OBut)3, OPri; R = H, But) to the polymerization of cyclohexene oxide with CO2.151,156 With [Ph3PNPPh3]Cl or [Ph3PNPPh3]N3 as the activator and a cyclohexene oxide:Ti ratio of 2500:1, the catalytic reactions at 60 °C under a CO2 pressure of 0.5 bar in 24 h produced selectively atactic poly(cyclohexylene carbonate) in 15−33% yields (relative to cyclohexene oxide), which correspond to TONs of 432−930 and TOFs of 20−39 molCHO molTi−1 h−1. The mechanisms of these catalytic reactions are not clear. As an explanation for the fine performance of these (ROCO2)Ti(IV) catalysts, the authors proposed that the strongly σdonating nature of the NHC ligands may reduce the Lewis acidity of the titanium(IV) center and hence facilitate coordination and dissociation of the epoxide/cocatalyst trans to the NHC. Titanium(IV)−NHC complexes have also been used for the ring-opening polymerization (ROP) of lactides.153,161,168 As a representative and early exploration, Arnold and co-workers showed that the alkoxy−NHC−titanium(IV) complex 5 displays fast polymerization catalysis at room temperature, and is more active than [TiCl(OPri)3] (Scheme 3).153 With a catalyst/monomer ratio of 1:100, the reaction at room temperature gave polylactide in 85% yield after 2 min. The polymer has an averaged molecular weight of 2300, and a narrow molecular weight distribution of 1.2. The presence of imidazolium groups as the terminal groups of the polymer chains, as determined by MALDI mass spectrometric analysis, suggests that the alkoxy−NHC−titanium(IV) complex might function as a bifunctional catalyst system. In this case, the polymerization was thought to start from the nucleophilic attack of the labilized NHC fragment on the titaniumcoordinated monomer. Thus, the hemilabile nature of the titanium(IV)−NHC interaction could be an important factor contributing to the good performance of the NHC−titanium(IV) catalyst. In addition, titanium(IV) halide complexes featuring tridentate bis(aryloxy)-NHC ligands also proved to be effective catalysts for the ring-opening polymerization of lactides.161,168 However, end-group analysis indicated the presence of an isopropoxide group at the ester end, suggesting an initiation mechanism different from that of Arnold’s system.153

Scheme 1. Representative Synthetic Methods Used for Ti(IV) NHC Complexes

titanium centers of these complexes usually have coordination numbers of five or six. The Ti(IV)−C(carbene) bond distances in the titanium(IV)−NHC complex span the range 2.16−2.35 Å,151−153,157,159,161−169 and are in general longer than Ti(IV)−C(hydrocarbyl) bonds.162,165 For example, the Ti(IV)−C(carbene) and Ti(IV)−C(methyl) bonds in [Cp2Ti(IPr2H2)(Me)][BPh4] have corresponding distances of 2.29 and 2.18 Å,162 and the Ti(IV)−C(carbene) and Ti(IV)− C(benzyl) bonds in [(t‑BuOCO-4)Ti(CH2Ph)2] are 2.19 and 2.13 Å in length,165 respectively. While in many reactions of titanium(IV)−NHC complexes the Ti(IV)−C(carbene) bonds remain intact, a few reactions demonstrate reactivity of the Ti(IV)−C(carbene) bonds. Fischer found that the interaction of [(IPr)TiCl4] with ZnMe2 at room temperature leads to the transfer of IPr from titanium to zinc to yield [(IPr)ZnCl2(THF)].167 Bellemin-Laponnaz and Dagorne showed that the treatment of [Ti(CH2Ph)4] with imidazolinium chloride gave the unexpected rearranged dimer product that might come from the migration of the benzyl group from the titanium center to the carbene carbon (Scheme 2).152 In addition, the moisture-sensitive nature of titanium(IV)−NHC complexes may result from hydrolysis of the Ti(IV)− C(carbene) bonds to give imidazolium salts. Catalytic applications of titanium(IV) NHC complexes have been found in polymer synthesis. Kawaguchi, Zhang, and Grubbs have independently showed that aryloxy-functionalized NHC−titanium(IV) complexes can serve as precatalysts for Scheme 2. Reactions of Ti(IV)−C(NHC) Bonds

3.3. Complexes of Vanadium

Vanadium(V) is the highest oxidation state observed for complexes of this metal. Organometallic vanadium(V) H


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Scheme 3. Catalytic Applications of Ti(IV) NHC Complexes

complexes are not rare, but there are only a few reports of vanadium(V) complexes with NHC ligation in the literature. Even more rare are vanadium(IV) complexes bearing NHC ligands. Studies on both types of vanadium−NHC complexes are included in this section. In 2003 Abernethy and co-workers reported the first example of a vanadium(V) NHC complex [(IMes)V(O)Cl3] (16), prepared by the direct interaction of VOCl3 with IMes (Scheme 4).148 The high isolated yield (76%) indicates the Scheme 4. Synthesis of Vanadium(V) Complexes [(NHC)V(O)Cl3] (16−19)

Figure 3. Molecular structure of 16. Hydrogen atoms are omitted for clarity. The figure was generated on the basis of a cif file reported in ref 148.

sterically demanding nature of IMes, are thought to be contributing factors to the high stability of 16. Applying a similar synthetic method, Zhang and Wu prepared the analogous vanadium(V)−NHC complexes [(NHC)V(O)Cl3] (NHC = 1,3-di(2,6-dimethylphenyl)imidazol-2-ylidene, 17; 1,3-di(2,6-diethylphenyl)imidazol-2ylidene, 18; and 1,3-di(2,6-diisopropylphenyl)imidazol-2-ylidene (IPr), 19) and used them as catalysts for ethylene/ propylene copolymerization.171 These vanadium(V)−NHC complexes were found to be stable in air. They show broad UV−vis absorption bands at 420−475 nm and 51V NMR signals at ca. −80 ppm (referenced to VOCl3). With Et3Al2Cl3 as cocatalyst, 16−19 proved effective precatalysts for the copolymerization of ethylene with propylene in hexane at −45 °C. The four complexes exhibit different activity, and require different Al/V molar ratios to achieve the highest activity. The highest catalytic activity of ca. 38 kg copolymer (molV−1 h−1) was observed with 16 as catalyst and an Al/V molar ratio of 125. The resulting polymer had molecular weight parameters of Mw = 732 000, Mn = 295 000, Mw/Mn = 2.5, and C3incorporation of 35.5%. The different catalytic behaviors of these vanadium(V) NHC complexes are attributed to the different electronic and steric effects of the NHC ligands. Importantly, the structure of the NHC ligands also affected the propylene incorporation in the resulting copolymers. 13C NMR

inability of IMes to reduce the vanadium(V) precursor. In contrast to a number of other trichloro-oxo-vanadium adducts that are readily hydrolyzed in air,170 for example, [(Hpycan)V(O)Cl3] (Hpycan = N-(2-nitrophenyl)picolinamide), 16 shows exceptional air stability in both the solid state and dichloromethane solution. It is also noted that 16 can undergo a reversible one-electron-reduction at a potential of 0.5 V, being lower than the standard potential of V(V)/V(IV). X-ray diffraction analysis reveals a distorted tetragonal pyramidal geometry for 16, in which the oxo ligand sits at the apical position and the V(V)−C(carbene) bond length is 2.137(2) Å (Figure 3). The complex features an unusual interaction between the carbene carbon and two chlorides cis to the NHC ligand, as the Cl−C(carbene) distances (2.849(2) and 2.887(2) Å) are within the sum of the van der Waals radii for carbon and chlorine (3.45 Å). DFT calculations suggest that the bonding overlap of the lone-pair electrons of the chloride ligand and the vacant p-orbital of the carbene carbon atom might cause the short contact (Chart 5). Such a bonding interaction, in combination with the electron-donating and I


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spectroscopic analysis of the copolymers indicated the random distribution of the two types of monomer units in the polymer chains. Vanadium(V)−NHC complexes bearing imido and alkylidene ligands are also known. In 2008, Nomura and co-workers reported the preparation of the (alkyl)(imido)vanadium(V) alkylidene complexes [(IPr)V(NAd)(CHSiMe3)(CH2SiMe3)] (20) and [(IPr)V(NXyl)(CHSiMe3)(CH2SiMe3)] (21) from the reactions of the corresponding (imido)vanadium(V) trialkyl complexes with NHC ligands (Scheme 5).172 No

Scheme 6. Polymerization of Norbonene Catalyzed by Vanadium(V)−NHC Complex 20

Under the same conditions, 21 exhibited a turnover number of 390 in 1 h. The activities of the two vanadium(V) NHC catalysts were found to be lower than that of [V(CHSiMe3)(NAr)(NCBut2)(PMe3)].173 Recently, Nomura, Tam, and co-workers reported (imido)vanadium(V) complexes featuring unique anionic NHCs bearing a weakly coordinating borate moiety (WCA−NHCs) (Scheme 7).174 The salt elimination reaction of lithium salt

Scheme 5. Synthesis of (Imido)vanadium(V)−Alkyl, Alkylidene Complexes 20 and 21

Scheme 7. Synthesis of Vanadium(V) Complexes 22−24 with WCA−NHC Ligation

reaction occurred when [V(NAd)(CH2SiMe3)3] was treated with the more bulky NHC ligand IBut. The carbene carbon nuclei show 13C NMR signals at 162 ppm, alkylidene hydrogen 1 H NMR signals at 13.4 ppm, and 51V NMR resonances at ca. 350 ppm (referenced to VOCl3). An X-ray diffraction study showed the metal center in 20 to have a distorted tetrahedral geometry with V(V)−C(carbene), V(V)−C(alkyl), V(V)− C(alkylidene), and V(V)−N distances of 2.172(2), 2.069(3), 1.829(3), and 1.637(2) Å, respectively (Figure 4). The V(V)−

[Li(WCA−NHC)(toluene)] with the (imido)vanadium(V) precursors [V(NR)Cl3] gave the four-coordinate (imido)vanadium(V) complexes [V(NR)Cl2(WCA−NHC)] (R = 1adamantyl, 22; Ph, 23; 2,6-Me2C6H3, 24). These complexes have characteristic 51V NMR signals at 45.4, 185.6, and 302.7 ppm, respectively. The difference in the chemical shifts of the 51 V NMR signals is significant; however, no explanation was provided in the report. One could guess that the different electronic nature of the substituents of the imido moieties might contribute to the difference, but this requires further theoretical study to confirm. Presumably because of the significant peak-broadening caused by the 51V nuclei, no 13C NMR signals were reported for the carbene carbons. The crystal structures indicated that the vanadium(V) centers of these complexes are all in a distorted tetrahedral geometry. Their V(V)−C(carbene) distances (2.039(3)− 2.049(2) Å) are apparently shorter than those of the vanadium(V)−NHC complexes 16 (2.137(2) Å) and 20 (2.172(2) Å),148,172 and are comparable to the V(V)−C(alkyl) bonds in 20.147 These structural data signify that the electronic nature of the WCA− NHC ligand is somewhat different from those of IMes and IPr. Continuing their interests in vanadium-catalyzed olefin polymerization, Nomura et al. examined the catalytic performance of 22−24 with MAO or AlBui3 as cocatalyst in ethylene polymerization, which proved their effectiveness. A notable feature of these vanadium(V)−WCA−NHC catalysts is that they are able to function with AlBui3 as cocatalyst, which contrasts to the ineffectiveness of other (imido) vanadium(V) dichloride complexes (e.g., [V(NR)Cl2(O-2,6-Me2C6H3)]) under similar conditions. Among the vanadium(V)−WCA− NHC complexes, the xylimido complex 24 shows the highest catalytic activity. The activity of the catalyst 24 under optimized conditions (25 °C, 8 atm of ethylene, AlBui3 as


C(NHC) distance is comparable to the corresponding distance of congener [(IMes)V(O)Cl3].148 The V(V)−C(alkyl), V(V)−C(alkylidene), and V(V)−N(imido) distances are close to the corresponding distances of the tetrahedral vanadium(IV) complex [V(CHSiMe 3 )(NAr)(NCBu t 2 )(PMe3)].173 Notably, the vanadium(V) NHC complexes 20 and 21 proved to be effective catalysts for the ring-opening metathesis polymerization of norbornene (Scheme 6). The report briefly noted that 20 showed a turnover number of 740 in 30 min when the reaction was conducted in toluene at 80 °C. The resulting polymer has Mw = 3.74 × 105, Mw/Mn = 2.0. J


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the cocatalyst, Al/V ratio of 50 in toluene) reached 11 000 kg molV−1 h−1, and the resulting polyethylene had values of Mn = 18 000 and Mw/Mn = 1.76. This impressive performance was ascribed to the capability of the anionic WCA−NHC ligand in stabilizing cationic alkyl species, which are thought to be the genuine catalytically active species in the polymerization reaction. It has been proposed that vanadium(II) or vanadium(III) species might be formed in these catalytic reactions; however, this proposal requires further study to be validated. Another interesting vanadium(V) complex bearing anionic donors is the bis(aryloxy)−NHC complex [(t‑BuOCO-2)V(O)Cl] (25) reported by Bellemin-Laponnaz and Dagorne.175 The synthesis of 25 involves an alcohol elimination resulting from the reaction of [(PriO)3V(O)] with an imidazolinium salt (Scheme 8). Single-crystal X-ray diffraction studies revealed a

Scheme 9. Synthetic Route to Vanadium(IV) Complexes 26−28 Bearing a Bis(NHC)−Pyridine Ligand

Scheme 8. Synthetic Route to a Vanadium(V) Complex Bearing a Bis(aryloxy)−NHC Pincer Ligand (25)

3.4. Complexes of Chromium

Among the reported examples of chromium complexes with NHC ligation, the most common oxidation states of the chromium centers are I, II, and III.178 High-oxidation-state chromium NHC complexes are rare. In 2011, Zhu and Zhang reported the chromium(V)−NHC complex [(IPr2Me2)2Cr(N)Ph2] (29), which was isolated in 39% yield from the reaction of a chromium(II) complex [(IPr2Me2)2CrPh2] with adamantyl azide (Scheme 10).179 The

distorted square-pyramidal geometry for the five-coordinate complex with the oxo moiety sitting in the apical position. The shorter V(V)−C(carbene) bond (2.095(3) Å) in 25, as compared to that of [(IMes)V(O)Cl3] (2.137(2) Å),148 was attributed to the geometrical constraints caused by the tridentate chelate. Analogous to [(IMes)V(O)Cl3], the distance of the carbene carbon atom to the neighboring V(V)O oxygen atom (2.77(1) Å) is shorter than the sum of van der Waals radii of the two atoms, which led the authors to propose a bonding overlap between the lone-pair electrons of the oxo ligand and the vacant p-orbital of the carbene carbon atom (Chart 5). Vanadium(IV)−NHC complexes represent another type of high-oxidation-state vanadium−NHC complexes. So far, only one report on such complexes has been made. In his effort to access high-oxidation-state transition metal complexes with bis(NHC)−pyridine ligands,176 Danopoulos found that the vanadium(II) and vanadium(III) complexes [(DippCNC)VCl3] and [( D i p p CNC)VCl 2 (THF)] ( D i p p CNC = 2,6-bis(arylimidazol-2-ylidene)pyridine, aryl = 2,6-diisopropylphenyl) can be oxidized by 4-methylmorpholine N-oxide (NMO) to form V(IV)−oxo complex [(DippCNC)V(O)Cl2] (26). Further chloride abstraction from 26 with AgBF4 generated the cationic vanadium(IV) complex [(DippCNC)V(O)(MeCN)2][BF4]2 (27) (Scheme 9). Both 26 and 27 show an octahedral geometry with the oxo ligand located trans to the pyridine site of the mer-CNC pincer. The V(IV)O bonds in the two complexes are ca. 1.60 Å in length, and the IR absorption band at 974 cm−1 was assigned to the V(IV)O stretching mode of 26. The stretch is at the lower end of the range observed in reported vanadium(IV)−oxo complexes,177 and the strong σdonating character of NHC ligands was thought to be the cause. The study also included the preparation of the (imido)vanadium(IV) complex [(DippCNC)V(NC6H4-Mep)Cl2] (28) from the reaction of [(DippCNC)VCl2(THF)] with p-tolylazide (Scheme 9).

Scheme 10. Synthetic Route to the Chromium(V) Nitride Complex 29

mechanism for the formation of 29 is not clear. Nevertheless, the use of an organic azide as a nitride source appears to be a novel route to form transition metal nitride species, whose preparation usually entails amine, N3−, and N2 as the nitrogen source.180−182 Complex 29 is paramagnetic. The metal center of 29 exhibits a square-pyramidal geometry in its solid-state structure (Figure 5). The nitride ligand occupies the apical position with a Cr(V)−N distance of 1.546(2) Å, which is close to those of other structurally characterized chromium nitride species.183−185 The Cr(V)−N bond stretching mode of complex 29 was found at 1049 cm−1 in the IR spectrum. The average Cr(V)−C(carbene) bond distance in 29 is 2.110(2) Å, being slightly shorter than those in the chromium(II) complex [(IPr2Me2)2CrPh2] (2.168(5) Å on average). Jenkins and co-workers reported a series of chromium(IV) oxo and imido complexes bearing novel cyclic borate-bridged tetra(NHC) ligands (Scheme 11).186 The synthesis of these chromium imido and oxo complexes utilizes group-transfer reactions between a chromium(II) complex [(BMe2,EtTCH)Cr] and oxo/imido precursors. The chromium(IV) oxo complex [(BMe2,EtTCH)Cr(O)] (30) prepared from the reaction of [(BMe2,EtTCH)Cr] with Me3NO or O2 shows a squarepyramidal geometry with the oxygen atom sitting at the apical position. The chromium(IV) oxo complex is paramagnetic and K


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Figure 6. Molecular structure of 31. Hydrogen atoms and solvent molecules are omitted for clarity. The figure was generated on the basis of a cif file reported in ref 186. Figure 5. Molecular structure of 29. Hydrogen atoms are omitted for clarity. The figure was generated on the basis of a cif file provided in ref 179.

Notably, the Cr(IV)−N(imido) lengths of 1.703(4) and 1.686(2) Å observed in 31 and 32, respectively, are at the long end of the length range of Cr−N(imido) bonds of known Cr(IV) imido complexes. In addition, the Cr−N(imido)− C(Mes/Ad) angles were found to be bent (166.1(4)° and 149.8(2)°, respectively), in sharp contrast to the linear Cr− N(imido)−C alignments of known chromium(IV) imido complexes supported by other ancillary ligands. An interesting difference in reactivity between the chromium(IV) imido and oxo complexes is their group-transfer reactions with 1-decene and triphenylphosphine. While the chromium(IV) oxo complex 30 was found to be inert toward both substrates, the imido complex 31 can react with 1-decene and triphenylphosphine to give group transfer products 1(mesityl)-2-octylaziridine and N-mesityl-1,1,1-triphenylphosphanimine in 86% and 65% yield, respectively (Scheme 11). The authors proposed that the different group-transfer reactivity is related to the different π-donating nature of imido versus oxo ligands in these chromium(IV) complexes.186 In addition to the terminal imido complexes, the metallotetrazene complexes [(BMe2,EtTCH)Cr(RN4R)] (R = p-tolyl, 33; n-octyl, 34) and a nitride-bridged dinuclear chromium complex [(BMe2,EtTCH)Cr]2(N) (35) were also obtained in the reactions of [(BMe2,EtTCH)Cr] with p-tolyl azide, n-octyl azide, and trimethylsilyl azide, respectively (Scheme 11). Structural analysis suggested that the tetrazene ligands in 33 and 34 were dianionic in nature. Hence, both complexes are chromium(IV) complexes. The nitride-bridged dinuclear chromium complex [(BMe2,EtTCH)Cr]2(N) (35) has a linear Cr−N(nitride)−Cr arrangement. SQUID magnetometry studies suggested weak ferromagnetic coupling between the two inequivalent chromium centers. The aziridination reaction of the chromium(IV) imido complexes 31 prompted exploration of alkene aziridination reactions using tetra(NHC)chromium complexes as catalysts. While tests with the dianionic borate-bridged tetra(NHC) chromium complexes proved unsuccessful, the recent report of Jenkins et al. showed that the chromium(III) complex [(Me,EtTCPh)Cr(Cl)2][PF6], which bears a charge-neutral tetra(NHC) ligand, can serve as an effective catalyst for the aziridination of a series of electron-rich alkenes with aryl azides (Scheme 12).189 The study represents the first example of aziridination catalysis with a chromium complex as catalyst, and a chromium(V) imido species [(Me,EtTCPh)Cr(NAr)-

Scheme 11. Synthetic Route to Chromium Oxo and Imido Complexes with Tetra(NHC) Ligation

features a short Cr−O distance of 1.564(3) Å and short Cr− C(carbene) distances of 2.026(5)−2.094(5) Å. Associated with the short Cr−O distance, the Cr−O stretching vibration appears at 1031 cm−1. The chromium(IV) oxo complex proved inert toward 1-decene and triphenylphosphine, which is consistent with Che’s porphyrin Cr(IV) oxo complex,187 but differs from Theopold’s tris(pyrazolyl)borate Cr(IV) oxo complex, which can abstract hydrogen atoms from weak C− H bonds.188 The reactions of [(BMe2,EtTCH)Cr] with the organic azides MesN3 and AdN3 yielded paramagnetic chromium(IV) imido complexes [(BMe2,EtTCH)Cr(NR)] (R = Mes, 31; Ad, 32). Structural analysis revealed that both Cr(IV) centers of 31 and 32 adopt a distorted square-pyramidal geometry (Figure 6). L


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metal center, these Mn(IV)−C(carbene) bonds are shorter than those of the manganese(I) and manganese(II)−NHC complexes,176,191,192 and are comparable to those of known manganese(IV) alkyl complexes.193 An unusual property of 36 is its low reduction potential to the manganese(III)−NHC species, with a half-wave potential of −0.77 V (referenced to the [Cp2Fe]0/1+ couple). This potential is significantly lower than those of the tris(pyrazolyl)borate complexes (0.99 and 0.97 V, referenced to the [Cp2Fe]0/1+ couple for [Mn(HB(pz)3)2Mn]2+/1+ and [Mn(HB(3,5-Me2pz)3)2Mn]2+/1+, respectively).194,195 The low potential reflects the capability of the tris(NHC)borate ligand to stabilize high-oxidation-state metal species due to its strong electron-donating nature. Terminal nitride complexes consitute an important type of high-oxidation-state manganese−NHC complex. Meyer and co-workers reported a series of manganese(III,IV,V) complexes bearing amine-linked tris(NHC) ligand XylTIMEN (Scheme 14).196 The preparation routes for these manganese nitrido

Scheme 12. Catalytic Aziridination of Protic-Functionalized Alkenes with Organic Azides

(Cl)]2+ (Chart 7) was proposed as the intermediate in the key step of alkene aziridation.

Scheme 14. Syntheses and Conversions of Manganese(IV− V) Nitride Complexes Supported by Tris(NHC) Ligand TIMEN

Chart 7. Proposed Intermediate for the Tetra(NHC)chromium-Catalyzed Aziridination Reaction

3.5. Complexes of Manganese

Manganese complexes with NHC ligation mostly feature manganese in its (I) and (II) oxidation states.107,111 In terms of the higher oxidation states, only a few manganese(IV) and manganese(V) complexes with tris(NHC) ligation have been reported. Smith and co-workers examined the oxidation reaction of the manganese(I) complex [PhB(MeIm)3Mn(CO)3] with AgOTf or O2 in the presence KOTf, thereby achieving the preparation of the first manganese(IV)−NHC complex, [(PhB(MeIm)3)2Mn][OTf]2 (36) (Scheme 13).190 The study Scheme 13. Synthesis of the Manganese(IV) Complex 36

complexes start from the manganese(II) azide complex [(XylTIMEN)Mn(N3)][PF6]. The exposure of the azide complex to ultraviolet radiation led to dinitrogen release and yielded the deep red manganese(IV) nitride complex [(XylTIMEN)Mn(N)][PF6] (37). The observed bands at 315, 385, and 488 nm in the absorption spectrum of 37 agree with the observed color. TD-DFT calculations led to the assignment of the band at 488 nm to a metal-to-ligand charge transfer, and the other two bands to ligand-centered transitions. The data obtained from SQUID magnetometry (μeff = 2.01 μB at 300 K) and EPR spectroscopy (g1 = 1.97, g2 = 1.98, g3 = 2.22, Figure 7) suggested a doublet ground state for the manganese(IV) nitride complex (S = 1/2), which was also supported by DFT calculations. IR spectroscopy showed bands

also revealed that the reaction of PhB(MeIm)3− with 0.5 equiv of MnBr2, followed by treatment with 2 equiv of AgOTf, could be an alternative synthetic route to 36. The complex showed a solution magnetic moment of 3.1 μB, which led to the assignment of an S = 3/2 state as its ground spin-state. The molecular structure established by single-crystal X-ray diffraction showed the complexation of two tris(NHC)borate ligands [PhB(MeIm)3]1− to the manganese(IV) center, rendering the complex a distorted octahedral geometry with Mn(IV)−C(carbene) bond lengths in the range 2.024(2)− 2.053(3) Å. Consistent with the higher-oxidation state of the M


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Figure 8. Structure of the cation [(XylTIMEN)Mn(N)]2+ in 39. Solvent molecules, counteranions, and hydrogen atoms are omitted for clarity. The figure was generated on the basis of a cif file reported in ref 196.

Figure 7. X-band EPR spectrum of 37 in a frozen 2-methyl THF solution at 11 K and corresponding simulation with g1 = 1.97, g2 = 1.98, g3 = 2.22, A1 = 77 MHz, A2 = 168 MHz, and A3 = 268 MHz. Adapted with permission from ref 196. Copyright 2012 American Chemical Society.

oxidation state going from Mn(III) to Mn(IV) and to Mn(V). It is also noted that the incremental increase in the maxima of the Kβ1,3 bands of the X-ray emission spectra of these manganese nitride complexes is in accord with the proposed half-integer spin increase through the series manganese(III), manganese(IV), and manganese(V) nitrides. More recently, the Meyer group found that the pentacoordinate nitride complexes (XylTIMEN)Mn(V) can coordinate additional ligands to form the six-coordinate nitride complexes (XylTIMEN)Mn(V).198 Variable-temperature UV−vis and 1H NMR spectroscopy studies revealed that [(XylTIMEN)Mn(N)][PF6]2 (39) in its MeCN solution can bind MeCN to give the six-coordinate complex [(XylTIMEN)Mn(N)(NCMe)][PF6]2 (40). The conversion between 39 and 40 is reversible, temperature-dependent, and is accompanied by a spin-state change from S = 1 (for 39) to S = 0 (for 40) (Scheme 14). Evaporation of a concentrated acetonitrile solution of 39 in the presence of NaBPh4 at room temperature resulted in the formation of [(XylTIMEN)Mn(N)(NCMe)][BPh4]2 (41). In addition, the reaction of the manganese(IV) nitride complex [(XylTIMEN)Mn(N)][BPh4] (38) with AgBPh4, followed by the addition of ButNC, [NBun4]CN, or NaNCS, resulted in the preparation of the six-coordinate (XylTIMEN)Mn(V) nitrides [(XylTIMEN)Mn(N)(CNBut)][BPh4]2 (42), [(XylTIMEN)Mn(N)(CN)][BPh4] (43), and [(XylTIMEN)Mn(N)(NCS)][BPh4] (44) in moderate to good yields. The analogous complex bearing a fluoride ligand, [(XylTIMEN)Mn(N)(F)][BPh4] (45), and the silver-bridged dimanganese(V) complex, [{(XylTIMEN)Mn(N)}2μ-{Ag(CN)2}][BPh4]3 (46), were also obtained from the corresponding reactions of 38 with AgF and AgCN. These complexes were characterized by various spectroscopic methods that suggest their diamagnetism (lowspin, S = 0). In these six-coordinate manganese(V) nitride complexes, the Mn(V)−N bond distances (1.520−1.529 Å) are shorter than that of 39 (1.546 Å). The average lengths of the Mn(V)−C(carbene) bonds (2.038(3)−2.064(2) Å) are comparable to that of 39 (2.054(2) Å). Electronic absorption spectra and theoretical calculations reveal that the d-orbital splitting of these tetragonal nitride complexes could be in the sequence d(xy)2{d(xz)d(yz)}0d(x2−y2)0d(z2)0 with a split between the d(xz) and d(yz) orbitals of less than 1000 cm−1. It is noteworthy that, while the manganese(V) complexes have a low d-electron count (d2), AOC DFT calculations on the hypothetical species [(XylTIMEN)Mn-

at 1033 and 1008 cm−1, which were attributed to the Mn(IV)14N and Mn(IV)15N stretches, respectively. The anion-exchange reaction of 37 with NaBPh4 led to the preparation of [(XylTIMEN)Mn(N)][BPh4] (38), the structure of which was confirmed by single-crystal X-ray diffraction study. The manganese(IV) center in 38 shows a trigonal pyramidal geometry with the metal located 0.42 Å above the trigonal plane of the three C(carbene) atoms. The short Mn(IV)−N distance (1.524(3) Å) indicates its multiple bond nature. It was noted that the lengths of the three Mn(IV)− C(carbene) bonds differ significantly (2.102(5), 1.932(6), and 1.990(5) Å), which was attributed to a Jahn−Teller-active degenerate electronic ground state of {d(x2−y2)d(xy)}3{d(xz),d(yz)}0d(z2)0. In accordance with the observation of a reversible oneelectron redox wave with a half-wave potential of −1.1 V (versus the [Cp2Fe]0/1+ couple) in the cyclic voltammogram of the manganese(IV) complex 37, the oxidation of 37 with NOBF4 led to the preparation of the manganese(V) nitride complex [(XylTIMEN)Mn(N)][PF6]2 (39). In contrast to the manganese(IV) complex 37, the metal center in the manganese(V) nitride complex 39 features an Mn−amine(anchor) interaction, as evidenced by the Mn(V)−N distance of 2.448(3) Å, which is much shorter than that in 37 (3.078(3) Å). Thus, the manganese center in 39 has a trigonal bipyramidal geometry (Figure 8). The Mn(V)−N(nitride) bond length of 1.546(3) Å in 39 is on the long end of the range of corresponding bond distances of well-characterized terminal manganese(V) nitrides,197 and is even longer than its congener in the manganese(IV) complex 37 (1.524(3) Å). The Mn(V)−C(carbene) bond lengths were observed to be 2.054(2) Å in length. Significantly, DFT calculations and SQUID measurements established a triplet ground state {d(x2−y2)d(xy)}2{d(xz),d(yz)}0d(z2)0 for 39, thus making it the first example of an open-shell manganese(V) nitride complex. The study has also applied Mn K-edge X-ray absorption spectroscopy (XAS) and Kβ X-ray emission spectroscopy (XES) to probe the physical oxidation states of the manganese nitride complexes. The pre-edge absorption appears near 6540 eV, and is attributable to 1s to 3d excitations observed in the complexes, while the observed increases in energy of the maxima correspond to increases in N


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applied direct field with an energy gap of 3.5 cm−1. Theoretical calculations point out that the origin of the single-molecule magnetism property in 47 is likely dominated by quantum tunneling, direct, and/or Raman mechanisms. Being distinct from 38, cyclic voltammetry revealed that 47 could not be oxidized to the corresponding manganese(V) nitride species. This difference suggests that the amine anchor in the TIMEN ligand plays an important role for the stabilization of Meyer’s manganese(V) nitride complexes.

(N)]2+, whose structure was derived from the structure of [(XylTIMEN)Mn(N)(F)][BPh4] (45), indicated the π-accepting nature of the tris(NHC) ligand. The capability of tris(NHC) ligands to support high oxidation-state manganese nitride complexes is not restricted to the TIMEN ligand; the tris(NHC)borate ligand is also amenable to the task. Smith and co-workers found that the reaction of the N-mesityl-substituted manganese(II) complex [PhB(MesIm)3MnCl] with NaN3 under UV irradiation could furnish the manganese(IV) nitride complex [PhB(MesIm)3Mn(N)] (47) (Scheme 15).199 Complex 47 has Mn(IV)−N

3.6. Complexes of Iron

Among the known NHC complexes of 3d transition metals, iron−NHC complexes are the most abundant.105,111,200−203 The formal oxidation states of the iron centers in iron−NHC complexes range from 0 to +V. While the metal centers in isolable organo-iron complexes usually have oxidation states lower than III,200,201,203 the high-oxidation-state iron−NHC complexes covered in this section encompass iron(III), iron(IV), and iron(V) complexes. The potential of NHC ligands to support high-oxidationstate iron species was first demonstrated by Fehlhammer in his pioneering studies on tris(NHC)borate complexes.134,135 Upon alkylation of tris(imidazolyl)borate, Fehlhammer and co-workers achieved the preparation of tris(imidazolium)borate salts that could be deprotonated by BunLi to generate tris(NHC)borate ligands in situ. The interaction of the in-situgenerated tris(NHC)borate ligand with anhydrous FeCl2 in THF, followed by treatment with NaBPh4 or NaBF4, resulted in the formation of red solutions, from which the red iron(III)−NHC complexes [Fe(HB(MeIm)3)2][X] (X = BPh4, 48; BF4, 49) and [Fe(HB(EtIm)3)2][BF4] (50) were isolated in low yields (Chart 8). These six-coordinate

Scheme 15. Synthesis of a Manganese(IV) Nitride Complex with Tris(NHC)borate Ligation

(1.523(2) Å) and Mn(IV)−C(carbene) distances (1.938(2)− 2.006(2) Å) (Figure 9) comparable to those in [(XylTIMEN)-

Chart 8. Fehlhammer’s Homoleptic Iron(III)−NHC Complexes


homoleptic iron(III)−NHC complexes have solution magnetic moments of ca. 2.1 μB, reflecting their ground spin-state of S = 1/2. The Fe(III)−C(carbene) distances (1.984(3) and 1.994(4) Å on average for those in 48 and 50, respectively) are at the long end of those observed in low-spin iron(III)− NHC complexes (vide infra). The isolation of these iron(III)− NHC complexes, rather than iron(II)−NHC complexes, was unexpected as no apparent strong oxidant exists in the reaction system. As the reduction wave of the iron(III) species [Fe(HB(RIm)3)2]1+ to [Fe(HB(RIm)3)2] in the voltammogram occurs with a half-wave potential (ca. −0.90 V versus SCE and −1.45 V versus that of the [Cp2Fe]0/1+ couple) significantly lower than those of the [Cp*2Fe]0/1+ and [(Tp)2Fe]0/1+ couples (−0.45 and −0.20 V versus that of the [FeCp2]0/1+ couple),204 the authors speculated that the observation of the iron(III)−NHC complexes in these reactions might be associated with the strongly electron-donating nature of the tris(NHC)borate anion, which renders the iron(II) species [Fe(HB(RIm)3)2] highly reducing and able to be readily oxidized by an (as-yet-unidentified) oxidant. In addition to the

Mn(N)][BPh4] (38). Jahn−Teller distortion is also apparent in this d3 tetrahedral complex. However, the higher rigidity of the tris(NHC) borate ligand than that of Meyer’s amineanchored XylTIMEN results in the Mn(IV)−C(carbene) bonds being more similar in length than those in 38, whereas the B− Mn−N vector was found to be bent (174.7°). Magnetic susceptibility measurements and EPR spectroscopy collaborated the low-spin nature of 47. Its EPR spectrum showed axial symmetry, and was simulated as an S = 1/2 system with g values of g1 = 2.35, g2 = 1.973, g3 = 1.965 and 55 Mn couplings of A1 = 300, A2 = 74, A3 = 202 MHz. These data, in addition to the X-ray photoelectron spectroscopy data, led to the assignment of the electronic structure of 47 as a low-spin d3 manganese(IV) species. Another important property of 47 is its dynamic magnetic property. Despite its low-spin nature, the complex shows slow relaxation of its magnetism under an O


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conversion to an iron(II) species, the cyclic voltammetric study also revealed the ability of the tris(NHC)borate ligand to stabilize iron(IV) species. The quasi-reversible redox waves, with half-wave potentials of ca. 1.2 V (versus SCE) in the voltammograms of 49 and 50, were assigned to the [Fe(HB(RIm)3)2]1+/2+ redox couple. The study noted the bulk electrolysis of the solutions of 49 and 50 at high potential resulted in the formation of green solutions of presumed iron(IV)−NHC complexes that seem to be stable in solution for hours. However, no further synthesis or characterization of the [Fe(HB(RIm)3)2]2+ species was reported. Another interesting homoleptic iron(III)−NHC complex is the bis(1,2,3-triazol-5-ylidene) complex [Fe(btz)3][PF6]3 (51, btz = 3,3′-dimethyl-1,1′-bis(p-tolyl)-4,4′-bis(1,2,3-triazol-5ylidene)), reported by Wärnmark and co-workers.205 The complex was prepared from the reaction of FeBr2 with 3 equiv of the in situ-generated btz ligand, followed by anion-exchange and recrystallization (Scheme 16). As in the preparation of

Figure 10. Steady-state absorption and emission spectra of 51 in CH3CN at room temperature. The green line shows the transient absorption spectrum at about 1 ps with ground-state bleach at 528 and 558 nm. Adapted with permission from ref 205. Copyright 2017 Nature Publishing Group.

Scheme 16. Synthetic Route to Wärnmark’s Iron(III)−NHC Complex 51

48−50, the species that oxidizes the ferrous center to a ferric center is unknown. Spectroscopic characterization suggested the low-spin iron(III) nature of 51. Specifically, it features an axially anisotropic EPR spectrum with g⊥ = 2.32 and g∥ = 1.95. The Mössbauer spectrum shows a quadrupole doublet with an isomer shift of δ = 0.03 mm/s and a quadrupole splitting of ΔEQ = 2.22 mm/s. The Fe(III)−C(carbene) distances (1.95− 1.98 Å) are also typical of iron(III)−NHC species. The iron(III)/iron(II) redox wave of 51 appears at a low potential, E1/2 = −0.58 V (versus [Cp2Fe]1+/0), significantly lower than that of the [Fe(bpy)3]3+/2+ couple (0.68 V), and higher than that of the [Fe(HB(RIm)3)2]1+/0 couple (−1.45 V versus [Cp2Fe]1+/0). The different charge and σ-donating/π-accepting properties of btz versus [HB(RIm)3]1− are likely the causes of the different redox property. The oxidation of the species [Fe(btz)3]3+ occurs at high anodic peak potentials of Epa = 1.16 V, and the process is irreversible, which also differs from the reversible [Fe(HB(RIm)3)2]2+/1+ couple. An important feature of [Fe(btz)3][PF6]3 is its photoluminescence at room temperature. The acetonitrile solution of the complex features characteristic absorption bands with maxima at 528 and 558 nm, assignable to the ligand-to-metal charge-transfer (LMCT) transitions from the iron(III) ground state (Figures 10 and 11). Excitation of these LMCT bands at room temperature gave an emission spectrum with the first peak around 600 nm, which was assigned to the decay of the lowest 2LMCT state to the ground state of the iron(III)−NHC complex. Significantly, the decay has a lifetime of 100 ps, which is the longest reported to date for an iron-based charge-transfer state. It is proposed that the excellent photoluminescent properties of the iron(III)−NHC complex could be attributed to the strong σdonating properties of the btz ligand, which stabilize the

Figure 11. Emission kinetics from time-resolved photoluminescence experiments at room temperature (■, τ ≈ 100 ps) and 100 K (blue △, τ ≈ 420 ps), overlapped with room-temperature transient absorption kinetic (red ●, τ ≈ 100 ps). The kinetics are plotted at the maximum of the emission signal: 575 nm for 100 K and about 600 nm for room temperature. The kinetics are normalized to their maxima. Adapted with permission from ref 205. Copyright 2017 Nature Publishing Group.

excited state to realize a long charge-transfer lifetime. This discovery points out the potential utility of NHC ligands for the design of new earth-abundant materials for use as light emitters and photosensitizers. Related to Wärnmark’s 1,2,3-triazole-based NHC−iron(III) complex, Chen and co-workers attempted the synthesis of a bis(1,2,4-triazole-5-ylidene)iron complex from the reaction of the bis(triazolium) salt with FeCl2 and base. However, this led to the isolation of iron(III) complexes bearing the tridentate NHC−amide−oxygen ligand [(MeCNO)2Fe][I] (52) and [(n‑BuCNO)2Fe][PF6] (53) (Scheme 17).206 The cleavage of the triazoyl rings might be caused by the hydrolysis of the in situ-generated NHC. Again, the reagent that oxidizes the ferrous ion to a ferric species remains unknown. Both 52 and 53 were characterized by X-ray diffraction study. Their Fe(III)−C(carbene) distances in the range 1.888(3)− 1.916(3) Å are much shorter than those in 51. Magnetic susceptibility measurements suggest their low-spin iron(III) nature. The voltammograms of 52 and 53 show two reversible one-electron redox processes with half-wave potentials at ca. −0.76 and +0.75 V (versus [FeCp2]1+/0). These redox waves P


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ligand will occur. Ligand exchange of IPr by THF was also noted when 55 was dissolved in THF.207 The redox properties of the ferric NHC complexes were also examined by cyclic voltammetry studies.207,209 The corresponding half-wave potentials (E1/2) of 55−58 (−0.48, −0.48, −0.36, and −0.35 V, versus [FeCp2]/[FeCp2]+) are lower than the corresponding [FeX4]1−/0 couples (−0.41 and −0.21 V for the [FeCl4]1−/0 and [FeBr4]1−/0 couples, respectively). The low potentials are likely associated with the strongly donating nature of NHC ligands, and indicate the utility of these complexes in atom-transfer radical polymerization. Indeed, Matyjaszewski and co-workers’ study showed that 55−58 exhibited high catalytic activity for the polymerization of methyl methacrylate (MMA) and styrene.209 Among these iron(III) complexes, the bromide derivatives 56 and 58 appeared to be more efficient activators. For instance, in the case of polymerization of MMA, with a [MMA]/[EBrPA]/ [58]/[AIBN] ratio of 200:1:0.01:0.2, the reaction in the presence of 50% (v/v) anisole at 60 °C over 24 h gave PMMA with 64% conversion with a molecular weight distribution (Mw/Mn) and a molecular weight (Mn) of 1.20 and 12 900, respectively. Also, ICAR ATRP of styrene with the same [monomer]/[EBrPA]/[58]/[AIBN] ratio in the presence of 50% (v/v) anisole at 60 °C over 72 h resulted in the formation of PS with 53% conversion, along with Mw/Mn and Mn values of 1.19 and 9500, respectively. The direct complexation of NHCs to ferric halides has been used for the preparation of polymer-supported iron catalysts. Cho and Lee prepared the polystyrene-supported iron(III)− NHC complex 62 from the reaction of FeCl3 with KOBut and polystyrene-supported imidazolium salts (Scheme 18).210 The resultant polymer-supported iron(III)−NHC species was characterized by various spectroscopic methods. Energy dispersive spectroscopy and inductively coupled plasma emission spectroscopy analysis indicated that the loading of the iron atom on the ligand was 1.6−16 mol % relative to the total imidazolium amount. X-ray powder diffraction indicated the absence of crystalline iron nanoparticles or iron oxides, and X-ray photoelectron spectra revealed the presence of iron(III) species and the reduced energy of the N 1s orbital. These results support the formation of an iron(III)−NHC complex. The study also demonstrated the ability of 62 to catalyze dehydration of fructose into hydroxymethylfurfural (HMF). With a PS−NHC−FeIII/fructose ratio of 0.02:1, reaction at 100 °C over 3 h yielded HMF in 73% yield, along with a

Scheme 17. Synthetic Route to the Triazole-Based NHC− Amide−Iron(III) Complexes 52 and 53

were ascribed to the Fe(II)/Fe(III) and Fe(III)/Fe(IV) couples, respectively. Attempts to prepare the corresponding iron(IV) species were unsuccessful. In addition to the aforementioned homoleptic iron(III)− NHC complexes 48−51, a number of heteroleptic iron(III)− NHC complexes are known. Ferric halide complexes of the form (NHC)FeCl3 constitute a large subclass of such species. The tetrahedral complexes [(NHC)FeCl3] (54−61) (Chart 9) Chart 9. Examples of Iron(III)−NHC Halide Complexes

reported by Tonzetich, Tatsumi, Matyjaszewski, and coworkers belong to this category.207−209 All of these complexes were prepared from the direct interaction of anhydrous FeCl3 with corresponding free NHCs. These complexes seem to have a high-spin ground state as reflected by solution magnetic moments of ca. 5.8 μB (by Evans’ method) and long Fe− C(carbene) bond distances (2.08−2.12 Å).207,208 The NHC ligands in these complexes seem to bond to the iron center loosely. NMR studies indicated that, when 55 is treated with an excess of IPr, rapid exchange of the free and bound IPr

Scheme 18. Prepration of the Polymer-Supported Iron(III)−NHC Complex 62 and Its Catalytic Application in Fructose Dehydration

Q


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recorded turnover frequency (TOF) of 12 h−1. This polymersupported iron(III)−NHC catalyst even showed good performance when reused in successive tests, resulting in ca. 70% yield and TOF values of 11−12 h−1 after 10 successive uses. In addition to the reactions with monodentate NHC ligands, the reaction of FeCl3 with bis(NHC)pyridine ligands can also afford iron(III)−NHC complexes. Gibson and co-workers showed that the interaction of the preformed bis(NHC)pyridine ligand 2,6-bis(N-2,6-diisopropylphenylimidazol-2ylidene)pyridine (DippCNC) with FeCl3 led to the sixcoordinate iron(III) complex [(DippCNC)FeCl3] (63, Scheme 19).211 Attempts to use this complex as a catalyst for ethylene polymerization (in combination with cocatalyst MAO) indicated its inability to promote the conversion.

([Th][PF6 ]) (Scheme 21). The structure of 65 was characterized by a single-crystal X-ray diffraction study. The Scheme 21. Synthetic Route to the Tetra(NHC)iron(III) Complex 65

iron(III) center adopts an octahedral geometry, with the four C(carbene) atoms and iron center mutually coplanar. The Fe(III)−C(carbene) bond distances are in the range 1.937(3)−1.957(2) Å. As a mimic of porphyrin−iron species, the iron(III) complex 65 appears to have exceptionally high activity in catalyzing olefin epoxidation using H2O2 as the oxygen source. Specifically, with H2O2 (aq 50%, 150 mol %) as oxidant and a catalyst 65 concentration of 0.03 mol %, the epoxidation of cis-cyclooctene at −30 °C in 60 min produced the product in 96% yield. Further reduction of the catalyst loading to 0.005 mol % still provided a yield of 22%, corresponding to a turnover number of 4300, a performance rarely achieved for oxidation catalysts. Intriguingly, when conducted at 25 °C for only 10 s, the reaction produced the epoxidation product in 51% yield, showing an exceedingly high turnover frequency value of 183 600 h−1. A substrate scope study indicated the effectiveness of the iron(III) catalyst for the epoxidation of a variety of cyclic and acyclic alkyl and aryl alkenes, for example, cyclooctene, cyclohexene, 1-hexene, 1octene, 1-decene, cis-2-octene, trans-2-octene, 1-phenyl-cyclohexene, styrene, and norbornene, among which the reactions with cyclic olefins showed high yields. The detailed mechanism of the reaction is as yet unclear. In 2015, Kühn and Meyer reported a new iron(III) complex bearing a macrocyclic bis(NHC)bis(pyridine) ligand, [(N2C2)Fe(MeCN)2][PF6]3 (66). The complex was also prepared from the oxidation of its iron(II) precursor by the thianthrene cation radical (Scheme 22).214 An electrochemical study of its

Scheme 19. Synthesis of the (DippCNC)Iron(III) Complex 63

Another interesting pincer-type iron(III) complex featuring NHC ligation is Byers’ carbenodiamidine iron complex.212 Upon the oxidation of the iron(II) precursor [(CDAPri)FeCl2] (CDAPri denotes N,N-bis(N-2,6diisopropylphenylethanimino)trihydropyrimid-2-ylidene) with the acetylferrocenium cation, the iron(III)−NHC complex [(CDAPri)FeCl2][BF4] (64) was obtained and fully characterized by a range of spectroscopic methods (Scheme 20). Scheme 20. Synthesis of the (CDAPri)Iron(III) Complex 64

Scheme 22. Synthetic Route to the (N2C2)Iron(III) Complex 66

Being much different from the aforementioned iron(III)− NHC complexes, 64 features a trihydropyrimid-2-ylidene donor. More significantly, it exhibits a spin-transition from an intermediate spin (S = 3/2) state to a high spin state (S = 5/2) when the temperature rises above 100 K. Along with the temperature change from 80 to 298 K, the Fe(III)− C(carbene) distance in 64 changes from 1.908(2) to 2.033(2) Å, the Mössbauer parameters change from δ = 0.18 mm/s, ΔEQ = 2.21 mm/s to δ = 0.21 mm/s, ΔEQ = 1.58 mm/ s, and also the magnetic susceptibility changes from 1.88 to 3.13 cm3 K mol−1. The unique properties of 64 are believed to originate from the unique strength of its CN2Cl2 ligand field. Macrocyclic ligands are commonly used for the stabilization of high-oxidation-state transition metal complexes. Accordingly, there are also reports of iron(III) complexes bearing macrocyclic NHC ligands. As an example, Kühn et al. reported the iron(III) complex [(Me,MeTC)Fe(MeCN)2][PF6]3 (65) with methylene-bridged tetradentate NHC ligand (Me,MeTC).213 The complex was synthesized in high yield from the reaction of its iron(II) precursor [(Me,MeTC)Fe(MeCN)2][PF6]2 with thianthrenyl hexafluorophosphate

iron(II) complex indicated a half-wave potential of 1.4 V (versus NHE) for the iron(II)/iron(III) redox couple, which is higher than that of the tetra(NHC)iron system 65 (0.79 V versus NHE). The large cathodic shift of the oxidation potential from the tetra(NHC)iron system to the (N2C2)iron system again suggests the capability of NHCs to stabilize highoxidation-state iron species. Complex 66 displays a deep blue color, and its absorption spectrum exhibits a broad asymmetric band with a maximum of 660 nm. TD-DFT studies suggest that this low-energy absorption band originates from a NHC(π)-to-Fe(III)(3d) LMCT transition. Accordingly, the aforementioned tetra(NHC)iron(III) complex 65 exhibits a R


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[(IPr2Me2)Fe(μ2-NDipp)2Fe(IPr2Me2)]2+. However, the reactions of [(IPr2 Me 2)Fe(μ2-NDipp) 2 Fe(IPr2 Me2)] with [Cp2Fe][BF4] or benzyl chloride led only to the isolation of the diferric halide complexes [F(IPr2Me2)Fe(μ2-NDipp)2Fe(IPr2Me2)F] (71), [F(IPr2Me2)Fe(μ2-NDipp)2Fe(IPr2Me2)(BF4)] (72), and [Cl(IPr2Me2)Fe(μ2-NDipp)2Fe(IPr2Me2)Cl] (73), as well as the mixed-valence complex [(IPr2Me2)Fe(μ2NDipp)2Fe(IPr2Me2)Cl] (74) (Scheme 23). The authors’ failure to access the cationic three-coordinate iron species could be due to their high Lewis acidity. The dinuclear complexes 71−74 are paramagnetic, and their structures were established by single-crystal X-ray diffraction studies. The Fe(III)−C(carbene) distances (2.08−2.17 Å) are comparable to those of the four-coordinate NHC−iron(III) trihalides 54− 61. Interestingly, all of the dinuclear iron−NHC complexes were found to be effective catalysts for the cross-coupling of alkyl fluorides with aryl Grignard reagents.219 While the detailed catalytic mechanism is unclear, a catalytic reaction using cyclopropylmethylene fluoride indicated the formation of a ring-opening product, which hints at the involvement of the cyclopropylmethylene radical in the reaction. Cyclopentadienyl ligands can support iron(III) and even iron(IV) species.220−223 Considering this, the accessibility of cyclopentadienyliron(II) complexes bearing NHC donors is not unusual, although examples of this type of complex are scarce.203 As rare examples, Royo showed that the oxidation of an iron(II) complex bearing a phenylethylene-linked cyclopentadienyl-NHC ligand [(Cp*-NHC)Fe(NCMe)(CO)][BF4] by AgBF4 or tert-butyl hydroperoxide (TBHP) under different conditions gives the iron(III) complexes [(Cp*NHC)FeCl][BF4] (75), [(Cp*-NHC)Fe(NCMe)2][BF4]2 (76), and [(Cp*-NHC)Fe(H2O)][BF4]2 (77) (Scheme 24).221 The chloride ligand in 75 was thought to come from

characteristic absorption band at 540 nm, signifying the stronger ligand field of the tetra(NHC) ligand versus the bis(NHC)bis(pyridine) ligand. The Fe(III)−C(carbene) distances in 66 are 1.923(4) and 1.929(4) Å, being slightly shorter than those in 65. Mössbauer (δ = 0.13 mm/s, ΔEQ = 2.48 mm/s) and SQUID studies unequivocally established the low-spin iron(III) nature of 66. In addition to the bis(NHC)bis(pyridine)iron complex 66, iron(III) species that have acyclic bis(NHC)bis(pyridine) ligand scaffolds, 67−69 (Chart 10), have been proposed to be Chart 10. Examples of Iron(III) Species Featuring Acyclic Pyridine−NHC Ligands

accessible as the cyclic voltammograms measured for their iron(II) precursors all show reversible one-electron-oxidation waves assignable to the iron(II)/iron(III) redox event.215,216 However, the isolation of these iron(III) species was not achieved. Related to the iron(III) species featuring pyridinefunctionalized NHC ligands, the tetra(pyridine)NHC complex [(PY4Im)Fe(MeCN)][PF6]3 (70 in Chart 10) was synthesized by the oxidation of [(PY4Im)Fe(MeCN)][PF6]2 with thianthrenyl hexafluorophosphate, and was thoroughly characterized.217 Its short Fe(III)−C(carbene) distance (1.889(5) Å) and small magnetic moment (2.5 μB) suggested its low-spin iron(III) nature. While the majority of the aforementioned iron(III)−NHC complexes have charge-neutral donors or halides as ancillary ligands, there are also several examples of complexes bearing bridging imido, cyclopentadienyl, and even aryl anions. Deng et al. synthesized a series of imido-bridged dinuclear iron(III) NHC complexes from the oxidation reactions of a dinuclear three-coordinate iron(II) imido complex [(IPr2Me2)Fe(μ2NDipp)2Fe(IPr2Me2)] (Scheme 23).218 The voltammogram of this iron(II) complex shows two quasi-reversible redox processes with the half-wave potentials of −0.74 and 0.26 V (versus SCE), which suggested the accessibility of the cationic species [(IPr 2 Me 2 )Fe(μ 2 -NDipp) 2 Fe(IPr 2 Me 2 )] 1+ and

Scheme 24. Synthesis of the Cyclopentadienyliron(III)− NHC Complexes 75−77

Scheme 23. Synthesis of the Imido-Bridged Dinuclear Iron(III)−NHC Complexes 71−74 halide abstraction of the iron(III) species from CH2Cl2, which implies the high Lewis acidity of the in situ-generated iron(III) intermediate. Structural analysis revealed that the dicationic complex 76 has a distorted three-legged piano stool geometry and a Fe(III)−C(carbene) distance of 1.987(4) Å. The Mössbauer data of 75 (δ = 0.44 mm/s, ΔEQ = 0.99 mm/s) and 77 (δ = 0.48 mm/s, ΔEQ = 0.87 mm/s) hint at their lowspin nature. The molecular structure of 77 was not established by XRD. Iron(III) hydrocarbyls are putative intermediates in ironcatalyzed cross-coupling reactions;224−226 however, the synthesis of related iron(III) compounds proved challenging. Deng and co-workers attempted to synthesize the di(hydrocarbyl)iron(III) complexes [(IPr2Me2)nFeR2]+ (n = 2, S


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R = Ph, CCBut; n = 1, R = CPhCPh2) from the reactions of the corresponding iron(II) complexes with the ferrocenium cation, which merely resulted in C−C bond-forming reductive elimination products R−R.227−229 In contrast, Meyer and coworkers demonstrated the readily accessibility of six-coordinate iron(III) aryl complex [(Tol‴TIMEN)Fe] (78) upon reduction of the iron(II)−NHC complex [( Tol TIMEN)Fe][BF 4 ] 2 (Scheme 25).230 The formation of 78 can be understood as

Scheme 26. Synthetic Route to the Iron(III) and Iron(IV) Imido Complexes with Tris(NHC)borate Ligation 80−82

Scheme 25. Synthetic Routes to the Cyclometalated Iron(III) Aryl Complex with NHC Ligation

iron(III) imide 80, and the intermediate-spin (S = 1) ground spin-state of the iron(IV) imido species 81 and 82. Despite the different oxidation state of the metal center, the Fe−N bond lengths in 80 and 82 are close (1.625(4) and 1.618(3) Å, respectively), and are comparable to those in four-coordinate iron(III) and iron(IV) imido complexes supported by phosphine and nitrogen ligands.236,247 The Fe−N−C(imido) units in both complexes are linear. However, the average Fe− C(carbene) distances (1.927(4) and 1.969(9) Å for 80 and 82, respectively) are even longer than that of the iron(IV) species. The cause of this difference is unclear. The tris(NHC)borate iron(IV) imido species 81 displays astonishing stability as it is air stable and could be heated at 100 °C for days without decomposition. The one-electron redox process of [PhB(MesIm)3Fe(NAd)]1+/0 has a low half-wave potential of −0.98 V (versus [Cp2Fe]1+/0), which indicates the weak oxidizing power of the iron(IV) species. Despite this, it was found that the tris(NHC)borate iron(IV) imido species could still react with 9,10-dihydroanthracene at 100 °C to form anthracene in 25% yield. This reactivity is in accordance with the estimated N−H BDFE (free energy) of [PhB(MesIm)3Fe−N(H)Ad]+ of 88(5) kcal/mol. It was noted that the driving force for hydrogen-atom abstraction by [PhB(MesIm)3Fe(NAd)]1+ is the high basicity of the reduced imido complex [PhB(MesIm)3Fe(NAd)]. This basicity is likely related to the strongly electrondonating nature of the tris(NHC)borate ligand. While the majority of the reported iron terminal imido complexes utilize chelating ligands to achieve their stabilization, Deng and co-workers recently showed that monodentate NHC ligands can support iron(IV) terminal imido complexes.244,245 The three-coordinate iron(IV) bisimido complexes [(NHC)Fe(NR)2] (83−88) shown in Scheme 27 were synthesized by the reaction of the corresponding iron(0)− alkene complexes with 2 equiv of organic azides at room temperature. Probably due to their low-coordinate nature, these iron(IV) imido complexes are air- and moisture-sensitive. However, under N2 atmosphere, they exhibit good stability in both solid and solution states. The bis(imido)iron(IV) NHC complexes are diamagnetic in nature, which indicates their lowspin ground state. The 13C NMR signals of the iron-bound carbene carbon atoms of 83 and 84 were observed at ca. 200 ppm, and those of the Me2-cAAC complexes 85−88 appeared

a series of sequential reduction-induced cyclometalation reactions. Interestingly, the N-substituents affected the reaction outcome, as the reduction of an iron(II) complex with N-3,5dimethylphenyl substituents [(XylTIMEN)Fe][PF6]2 only gave a double-cyclometalated iron(II) species [(Xyl″TIMEN)Fe] featuring an iron···C−H agostic interaction. The subsequent interaction of the iron(II) species with AgOTf led to the formation of the iron(III) complex [(Xyl″TIMEN)Fe][PF6] (79), in which the iron···C−H agostic interaction is retained. The molecular structures of 78 and 79 were established by single-crystal X-ray diffraction study. Their Mössbauer (78: δ = 0.03(1) mm/s, ΔEQ = 2.37(1) mm/s) and EPR (78, g∥ = 2.29, g⊥ = 1.94; 79, g1 = 2.42, g2 = 2.20, g3 = 1.94) data support their common low-spin iron(III) nature. Iron imido species have received continuous attention because of their rich electronic structures as well as their relevance to important chemical transformations, such as olefin aziridation, C−H bond amination, and dinitrogen reduction.231−235 Along with success in the stabilization of terminal imido complexes of iron(III) and iron(IV) with tris(phosphine), nacnac, dipyrromethene, amidinate, and other nitrogen-based chelating ligands,231−242 recent explorations have shown that NHC ligands can also stabilize iron imido complexes.243−246 The ability of NHCs to stabilize terminal iron imido species was first disclosed by Smith in 2008. In a reactivity study of the iron(I) species [PhB(MesIm)3Fe(cot)] (cot = cyclooctene), the authors found that the interaction of the iron(I) alkene complex with 1 equiv of adamantyl azide in THF provided the iron(III) imido complex [PhB( Mes Im) 3 Fe(NAd)] (80) (Scheme 26).243 Further treatment of the iron(III) imido complex with ferrocenium salts then led to the clean formation of iron(IV) imido complexes [PhB(MesIm)3Fe(NAd)][X] (X = OTf, 81; BPh4, 82). Magnetic susceptibility measurements indicated the low-spin (S = 1/2) ground spin-state for the T


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Scheme 27. Iron(IV) Bis(imido) Complexes with NHC Ligation

between 307 and 320 ppm. The distances of the Fe(IV)− C(carbene) bonds in these complexes fall within a small range, 1.867(2)−1.905(4) Å, which is consistent with the low-spin nature of the complexes. Accordingly, the distances of the Fe(IV)−N bonds (1.612(2)−1.636(2) Å) also fall at the short end of the range of the Fe−N bonds of reported iron imido complexes. The Fe−N−C(imido) alignments (161−173°) are all close to linear (Figure 12). Presumably related to the high

Figure 13. 80 K zero-field 57Fe Mössbauer spectra of 85 (top) and 88 (bottom). The data (dots) and best fit (solid line) are shown. Reproduced with permission from ref 245. Copyright 2015 American Chemical Society.

quadrupole splitting of 1.31 mm/s. The Mössbauer spectrum of 90 shows similar fitting parameters. These parameters, in addition to the large solution magnetic moment of 4.0 μB, clearly show the open-shell electronic structure of the fourcoordinate iron(IV) imido complexes. The bis(imido) iron(IV) complex 83 can transfer both nitrene moieties to CO and ButNC to give DippNCO and DippNCNBut, respectively, in high yields (Scheme 28). In the case of the reaction with CO, the resultant iron(0) species [(IMes)Fe(CO)4] was also


Scheme 28. Reactivity of the Bis(imido)iron(IV) NHC Complex 83

oxidation state of the metal center and also its low-coordinate nature, the isomer shifts of 83−88 (δ = −0.29 to −0.52 mm/s, Figure 13) are negatively shifted as compared to that of the low-spin iron(IV) imido complex [(pyrr2py)Fe = NAd] (pyrr2py = bis(pyrrolyl)pyridine, isomer shift δ = −0.09(1) mm/s).239 As a representative example of bis(imido)iron(IV) complexes, complex 83 was subjected to a reactivity study. Complex 83 reacted with N,N-diphenylcarbodiimide and pisopropylphenylisocyanate to form the [2π+2π]-addition products 89 and 90 (Scheme 26). Complexes 89 and 90 are new NHC-containing iron(IV) imido complexes, and were characterized by a variety of spectroscopic methods. The structure of 89 was unambiguously established by single-crystal X-ray diffraction study. The monoimido iron(IV) complex 89 has a Fe(IV)−N(imido) distance of 1.708(2) Å and a Fe(IV)− C(carbene) distance of 2.086(3) Å, which are longer than those of 83. The single quadrupole doublet in the Mössbauer spectrum of 89 has an isomer shift of 0.17 mm/s and a U


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94) (Scheme 30).246 Complex 94 shows a tetrahedral geometry with Fe(IV)−C(carbene), Fe(IV)−C(alkyl), and

isolated. In addition to the reactions with unsaturated substrates, 83 can also activate σ-bonds. By heating in toluene at 100 °C, it can undergo intramolecular alkane chain dehydrogenation to form the corresponding iron(II) complex [(IMes)Fe(NHDipp)(NHC6H3-2-Pri-6-CMeCH2)]. Its reaction with PhSiH3 can take place at room temperature, from which the iron(II) silylamide complex [(IMes)Fe(NHDipp)(NDippSiPhH2)] was obtained (Scheme 28). The rich reactivity shown above makes the bis(imido)iron(IV) NHC complex stand out from many of the other reported iron imido complexes. The reactivity is presumably associated with the unique features of the complex: low-coordination number, formal iron(IV) oxidation-state, and the coexistence of two imido moieties. DFT calculations indicated that 83 has a singlet ground state, which is located below the triplet (S = 1) and quintet states (S = 2) by ca. 3 and 11 kcal/mol, respectively. The NHC ligand mainly undergoes σ-interactions with the iron center, whereas π-interactions between the 2p orbitals of N(imido) and the 3d(yz, xz) orbitals of iron are observed. The authors suggested that the trigonal planar geometry makes the orbital overlap of the π-interactions less effective, which might be the cause of the small triplet−singlet energy gap. Mechanistic calculations on the Si−H and C−H bond activation reactions indicated that the former reaction might start from a [2σ+2π] addition step, with the latter involving a hydrogen atom abstraction process (HAA). Notably, the readily accessibility of the triplet state of 83 plays an important role in these conversions, and the reaction pathways on the triplet energy surfaces were found to be energetically favorable for both reactions. More intriguingly, the energetically favored pathways for both reactions were found to involve similar formal organoiron(IV) intermediates: hydridoiron(IV) imido 91 and silyliron(IV) imido 92, respectively (Scheme 29). Reductive elimination from these

Scheme 30. Synthetic Routes to the Bis(alkyl)iron(IV) Imido Complexes 93 and 94, and Their Reductive Elimination Reactions

Fe(IV)−N distances of 2.074(2), 2.015, and 1.672(2) Å, respectively. Magnetic susceptibility measurements and theoretical calculations revealed an intermediate spin ground state (S = 1) for the tetrahedral iron(IV) imido species. Unfortunately, complexes 93 and 94 further underwent reductive elimination or migratory insertion, rather than αelimination to give an alkylidene complex, providing alkyl(amido)iron(II) complexes [(IPr2Me2)Fe{N(Ad)R}(R)]. Interestingly, the production of these iron(II) amido complexes is in accordance with the products of the calculated mechanisms of the C−H and Si−H activation reactions by 83 (Scheme 29). The ability of NHCs to support iron imido species has been utilized for the development of iron-catalyzed olefin aziridation reactions. Jenkins and co-workers found that the tetra(NHC)iron(II) complex [(Me,EtTCPh)Fe(NCCH3)2][PF6]2 serves as an effective catalyst for the aziridation reactions of electronrich olefins (Scheme 31).250 For example, with a catalyst

Scheme 29. Calculated Pathways for the Dehydrogenative C−H Bond Activation Reaction of 83 and Its Reaction with PhSiH3

Scheme 31. Catalytic Aziridation of 1-Nonene Catalyzed by a Tetra(NHC)iron(II) Complex

monoimido iron(IV) intermediates accounts for the formation of the iron(II) products. Apparently, the low-coordinate nature of 83 plays a crucial role in the proposed formation of 91 and 92. There is great research interest in the development of ironbased olefin-metathesis catalysts, and iron alkylidene imido species with NHC ligation are among the targets in this direction.246,248,249 Pertinent to this goal, Wolczanski et al. examined the reactions of three-coordinate dialkyliron(II) NHC complexes [(IPr2Me2)FeR2] (R = neo-pentyl, 1norbonyl) with 1-adamantyl azide, which afforded the fourcoordinate dialkylmono(imido)iron(IV) NHC complexes [(IPr2Me2)FeR2(NAd)] (R = neo-pentyl, 93; 1-norbornyl,

loading of 0.1 mol %, the catalytic reaction of p-tolyl azide with a 29-fold excess of 1-decene at 90 °C for 18 h gave 2-octyl-(ptolyl)aziridine in 70% yield. The catalyst is also effective for the aziridation of di-, tri-, and even tetra-substituted olefins, for example, cis-4-cyclooctene, 1-methylcyclohexene, and 2,3dimethyl-2-butene, albeit with lower yields. Aiming to shed light on the reaction mechanism, the group of Jenkins has performed a detailed study of the reactions of V


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Scheme 32. Synthetic Route to the Tetra(NHC)iron(IV) Tetrazene Complex 95 and Its Conversions

the iron(II) catalyst [(Me,EtTCPh)Fe(NCCH3)2][PF6]2 with organic azides. It turned out that the reaction of the ferrous complex with 2 equiv of tolyl azide afforded iron(IV) tetrazene complex [(Me,EtTCPh)Fe((p-tolyl)N4(p-tolyl))][PF6]2 (95) (Scheme 32).251 Complex 95 is diamagnetic, and the 13C NMR signal of its carbene carbon nuclei was observed at 170 ppm. The Mössbauer data for the complex (δ = −0.01 mm/s, ΔEQ = 0.62 mm/s) are also in the common range of the data of low-spin iron(IV) complexes. Interestingly, an X-ray diffraction study revealed that the tetra(NHC) ligand in 95 has a saddle-type conformation. The N−N bond lengths of the tetrazene moiety (1.345(3), 1.292(3), and 1.343(3) Å) are indicative of the dianionic tetrazene ligands. The Fe(IV)− C(carbene) distances (1.952(3)−2.011(2) Å) in the iron(IV) complex are comparable with those of low-spin iron(III)− NHC complexes. Complex 95 can be viewed as the [2 + 3]addition product of the iron(IV) imido species [(Me,EtTCPh)Fe(N(p-tolyl))]2+ with p-tolyl azide. Further reactivity studies revealed that it can function as a nitrene source in its reaction with cyclooctene. This reaction, performed at 90 °C, gave the aziridation product 9-(p-tolyl)-9-azabicyclo[6.1.0]nonane in addition to 4,4′-dimethylazobenzene and the iron(II) species [(Me,EtTCPh)Fe(NCCH3)2][PF6]2 (Scheme 32). In the absence of the olefin, heating the solution of 95 in CD3CN led to the production of the azo compound 4,4′-dimethylazobenzene and a ferrous complex. In addition, 95 also proved to be an effective catalyst for olefin aziridation. These results led to the proposal that 95 would be a competent catalyst for the aforementioned aziridination reaction. In addition to these data, the study also revealed the accessibility of a tetra(NHC)iron(III) tetrazene complex [(Me,EtTCPh)Fe((p-tolyl)N4(ptolyl))][PF6] (96) upon the reduction of 95 by cobaltocene (Scheme 32). Characterization data suggested its low-spin iron(III) nature. Another important class of high-oxidation-state iron complexes with NHC ligation are the iron terminal nitrido complexes. Both iron(IV) and iron(V) nitrido complexes with NHC ligation are known.137,138,241,252−259 It should be reiterated that due to the high covalency of the metal−ligand multiple bond, the formal oxidation states of the metal center are stated here.

In 2008, Meyer and co-workers reported the synthesis of the iron(IV) nitrido complexes [(ArTIMEN)FeN][BPh4] (Ar = Mes, 97; Xyl, 98), which are among the first structurally wellcharacterized iron(IV) terminal nitrides (Scheme 33). The Scheme 33. Synthesis of the Iron Nitrides 97 and 98

synthetic route to these complexes involves photolysis of the corresponding iron(II) azide precursors, and is similar to that used for the manganese(IV) nitrido complex [(XylTIMEN)Mn(N)][BPh4] (38).137 Ignoring the difference in the Nsubstituents, 97 and 98 are isostructural. The iron centers adopt a trigonal-pyramidal geometry with short Fe(IV)−N distances of ca. 1.53 Å and an average Fe(IV)−C(carbene) distance of ca. 1.95 Å (Figure 14). These distances are comparable to the corresponding distances in their manganese analogue [(XylTIMEN)Mn(N)][BPh4] (38). The iron(IV)

Figure 14. Structure of the cation [(XylTIMEN)FeN]1+ in 98. Solvent molecules, counteranion, and hydrogen atoms are omitted for clarity. The figure was generated on the basis of a cif file reported in ref 137. W


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Scheme 34. Synthesis and Reactivity of Iron(IV,V) Nitride Complexes Supported by Tris(NHC)borate Ligands

(1.512(1) Å) (Figure 15). However, the average Fe(V)− C(carbene) bonds in 101 (1.949(2) Å in length) are longer

nitride complexes are purple in color, and the absorption spectra exhibit intense absorption bands centered at 520 nm. Spectroscopic data for 97 also revealed a Fe(IV)−N stretching band at 1008 cm−1, and a sharp quadrupole doublet with δ = −0.27 mm/s and ΔEQ = 6.04 mm/s in its Mössbauer spectrum. These spectroscopic data, in addition with DFT calculations, suggest a {d(xy)2d(x2−y2)2}{d(z2)0d(xz)0d(yz)0} electron configuration for the iron(IV) nitrido complexes. The report noted that the NHC-supported iron(IV) nitride complexes differ from Peters’ phosphine-supported iron(IV) nitride species [(PhBPriP3)Fe(N)] not only in color (purple versus tan) and geometry (trigonal pyramidal versus tetrahedral), but also in the reluctance of the former to undergo dimerization to give dinitrogen-bridged metal species. The difference presumably comes from the different electronic and steric properties of the supporting ligands (TIMEN versus [PhBPriP3]1−). Smith’s iron nitrido complexes supported by tris(NHC)borate ligands represent further elegant examples of the stabilization of high-oxidation-state transition metal species by NHC ligands.241,259,260 With N-mesityl- and N-But-substituted phenyltris(NHC)borate anions as ligands, two four-coordinate iron(IV) nitride complexes [PhB(RIm)3Fe(N)] (R = But, 99; Mes, 100) were synthesized by irradiation of the corresponding light-sensitive iron(II) azide precursors with UV light (Scheme 34). More intriguingly, the N-But-substituted phenyltris(NHC)borate can even support the formation of an iron(V) nitride complex, as indicated by the reversible oneelectron redox couple in its voltammogram (E1/2 = −0.53 V, versus [Cp2Fe]0/1+ couple). The oxidation of the iron(IV) nitrido complex 99 by [Cp2Fe][BArF4] at −78 °C in diethyl ether affords the dark purple iron(V) nitride complex [PhB(t‑BuIm)3Fe(N)][BArF4] (101) in high yield (Scheme 34).138 The iron nitride complexes supported by the tris(NHC)borate anions show pseudotetrahedral structures that differ from the trigonal pyramidal geometries of 97 and 98. The key interatomic distances and angles of the iron(IV) and iron(V) complexes 99 and 101, respectively, are found to be very close. The Fe(V)−N bond distance of the iron(V) complex 101 (1.506(2) Å) is slightly shorter than that of 99

Figure 15. Structure of the cation [PhB(t‑BuIm)3Fe(N)]1+ in 101. Solvent molecules, counteranion, and hydrogen atoms are omitted for clarity. The figure was generated on the basis of a cif file reported in ref 138.

than those in 99 (1.924(1) Å). The authors suggested that the opposite trend of bond-length change might be due to the enhanced iron−nitrido interactions in the iron(V) species, which however weaken the Fe(V)−C(carbene) interactions. Complex 101 is unstable at room temperature, and shows a half-life of ca. 4 h at 25 °C. Despite its instability, the iron(V) nitride complex was fully characterized by various spectroscopic methods. Its absorption spectrum shows characteristic bands at 452 and 563 nm that were assigned as LMCT transitions. The 57Fe Mössbauer (δ = −0.45 mm/s, ΔEQ = 4.78 mm/s at 80 K, Figure 16) and g-values of the X-band EPR spectra (g∥ = 2.30, g⊥ = 1.97, Figure 17) are indicative of the low-spin nature of the iron(V) nitride species (S = 1/2). Theoretical calculations gave further support to this assignment, and revealed the unpaired spin located essentially on the iron(V) center. As a preliminary exploration of the reactivity of the iron(V) nitride complex, it was found that treatment of 101 with water (15 equiv) and cobaltocene (3 equiv) at −78 °C led to the release of ammonia in 89% yield. Interestingly, X


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consequence making the H atom transfer to the iron nitride species favorable. Smith and co-workers performed a detailed reactivity study on a range of iron(IV) nitride complexes, which were found to be reactive toward triphenylphosphine,259,262 styrene,263 CO,264 and isonitriles264 to form group-transfer products (Schemes 34 and 35). The reactions of 99 and 100 with Scheme 35. Reactivity of the Iron(IV) Nitride Complex 100

Figure 16. 78 and 200 K zero-field 57Fe Mössbauer spectra of 101. The solid lines represent the best fit obtained with parameters. At 78 K, δ = −0.45(1) mm s−1, |ΔEQ| = 4.78(1) mm s−1, Γfwhm = 0.44(1)/ 0.90(1) mm s−1; at 200 K, δ = −0.49(1) mm s−1, |ΔEQ| = 4.73(1) mm s−1, Γfwhm = 0.28(1)/0.36(1) mm s−1. fwhm, full width at halfmaximum. Reproduced with permission from ref 138. Copyright 2011 American Association for the Advancement of Science.

phosphines gave the iron(II) complexes [PhB(RIm)3Fe(NPPh3)].262 A Hammett study on the reactions of 100 with para-substituted triarylphosphines indicated that electronwithdrawing substituents can increase the rate of reaction, while the rates of reaction of 100 with PPh3, P(OPh)3, and P(OMe)3 were found to decrease stepwise in that order. The relative rates are explained by the dual orbital interactions of the σ donation from the P lone pair to the nitride LUMO and the π donation from the N lone pair to the P−X σ* orbitals (Chart 11). Thus, the nitrido ligand in the iron(IV) nitrido Chart 11. Simplified Orbital Interactions of PhB(RIm)3FeN with Phosphine

Figure 17. X-Band EPR spectrum of 101 in frozen toluene measured at 15 K. Conditions: frequency 8.9564 GHz, power 1 mW, modulation 1 mT/100 kHz. The fit (dashed line) was obtained with S = 1/2 and g⊥ = 1.971, g∥ = 2.299. Reproduced with permission from ref 138. Copyright 2011 American Association for the Advancement of Science.

complex should feature both electrophilic and nucleophilic character. The reactions of 99 and 100 with styrene derivatives gave high-spin iron(II) aziridino complexes [PhB(RIm)3FeN(CH2CRAr)]. On the other hand, both iron(IV) imido complexes are unreactive toward aliphatic olefins. Because the alkyl-substituted aziridino iron(II) complex is accessible via another synthetic route, it is believed that the inertness of the iron nitrido species toward aliphatic olefins is due to kinetic factors. Related to the reactions with styrenes, complex 100 reacted with cyclohexa-1,3-diene to produce an iron(II) pyrrolide complex (Scheme 35). This transformation was thought to proceed via an azabicyclic intermediate formed from a [4+1] addition reaction of the nitride moiety and the diene.265 The nitrogen-transfer reactions of 99 and 100 with CO and isocyanides gave the iron(II) complexes [PhB(t‑BuIm)3Fe(NCX)(CO) 2 ] (X = O, NBu t ), [PhB( M e s Im) 3 Fe(CNBut)2(NCO)], and [PhB(MesIm)3Fe(CNBut)3][NCNBut] (Schemes 34 and 35). The different steric properties of the NHC ligands are thought to be the origin of the different reaction outcomes. While all of the aforementioned reactions can be rationalized as formal two-electron reduction reactions

the corresponding iron(IV) nitride complex 99 does not react with water under the same conditions. More recently, Smith, Kirk, and Hoffman presented a more detailed study of the electronic structure of 101 via EPR and ENDOR spectroscopy in combination with DFT/CASSCF calculations.261 The Q-band ESE-EPR spectrum of 101 exhibits signals of g∥ = 2.30 and g⊥ = 1.98, which are consistent with the X-band EPR data. Interestingly, ENDOR experiments indicated the highly rhombic nature of the 14/15Nnitride ligand’s hyperfine tensor, which suggests the nearly spherical electron density for the nitride ligand with donation from N(2pσ) to iron equal to that of N(2pπ) to iron. Analysis of the g-tensor revealed that the strong first-order Jahn−Teller distortion of the complex might induce markedly strong vibronic coupling, being even stronger than the spin−orbit coupling. The small g anisotropy is thought to be related to a large value of the formal primary Jahn−Teller distortion parameter. Accompanying this distortion is the significant ea(d(xy), d(x2−y2))−eb(d(xz), d(yz)) mixing that can mix the N(pz) orbital into the Jahn−Teller split ea orbital, as a Y


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those of 99 and 100. Surprisingly, 103 was found to react with 2 equiv of BAC to give a low-spin iron(II) cyanide complex [PhB(Pri2Im)3Fe(N2)(CN)(BAC)] and diaminoacetylene (Scheme 37), which represents an unsual example of a fourelectron carbon-atom-transfer reaction. The author proposed that the cyanide complex might be formed via the initial formation of an iron(II) ketiminate complex, which then extrudes alkyne to generate an isocyanide ligand.267 Iron oxo species are pivotal intermediates in certain synthetic and enzymatic iron-catalyzed oxidation reactions.268,269 However, structurally well-defined iron oxo complexes are scarce due to the difficulties associated with their synthesis. While a handful of reported isolable iron terminal oxo species utilize multidentate N-donor ligands,270−276 recent studies of Smith and Meyer have shown that multidentate NHC ligands are also amenable to this task.140,277 On the basis of their success in the preparation of iron nitrido complexes with tris(NHC)borate ligands, Smith and co-workers also attempted the synthesis of iron terminal oxo complexes with this type of ligand. Their thermolysis of TEMPO complex [PhB(MesIm)3Fe(TEMPO)] at ca. 70 °C indicated that stoichiometric amounts of 2,2′,6,6′-tetramethylpiperidine were formed, along with paramagnetic products. The results suggest the occurrence of homoleptic cleavage of the O−N bond in the thermolysis reaction to give the iron(III)−oxo species [PhB(MesIm)3Fe(O)] (104) and an aminyl radical (Scheme 38).277 Indeed, thermolysis of the

of the iron(IV) nitrides, there are also examples involving oneelectron reduction of the iron(IV) nitride species. The interaction of 100 with Gomberg’s radical dimer resulted in the formation of low-spin iron(III) imido complex [PhB(MesIm)3Fe(NCPh3)] (102).241 In addition, treatment of 100 with an excess of TEMPO-H (1-hydroxy-2,2,6,6-tetramethylpiperidine) gave [PhB(MesIm)3Fe(TEMPO)], ammonia, and TEMPO in high yields. The latter reaction was thought to involve hydrogen-atom abstraction from TEMPO-H by 100 to give the parent imido species PhB(MesIm)3Fe(NH) and TEMPO as a key step. A kinetic study on the reaction of 100 with TEMPO-H in the temperature range 288−318 K revealed activation enthalpy and entropy values of ΔH# = 11 kcal/mol and ΔS# = −37 eu. In contrast to the reaction with TEMPO-H to produce ammonia, no reaction was observed between the iron(IV) nitride species and 9,10-dihydroanthracene or xanthene. The nucleophilicity of the nitride ligand in the tris(NHC)iron(IV) nitride complex can also be discerned from its reaction with with V(Mes)3(THF), wherein a bimetallic complex [PhB(MesIm)3Fe−N = V(Mes)3] featuring a linear nitride bridge moiety was formed (Scheme 36).266 The data Scheme 36. Reaction of 100 with V(Mes)3(THF)

Scheme 38. Synthesis and Reactivity of the Iron(III) Oxo Intermediate 104

obtained from single-crystal X-ray diffraction, magnetic studies, and X-ray photoelectron spectroscopy (XPS) revealed the vanadium(V) and high-spin iron(II) nature of the metal center in the bimetallic complex, confirming the occurrence of a redox reaction. Intriguingly, the bimetallic complex complex displays strong magnetic anisotropy and slow magnetic relaxation behavior in the presence of an applied direct current field with a reversal barrier of ca. 10 K. In addition to the aforementioned group-transfer reactions, Smith and co-workers have very recently found that a tris(NHC)iron(IV) nitride complex [PhB(Pri2Im)3Fe(N)] (103) can react with a stabilized carbene ligand bis(diisopropylamino)cyclopropenylidene (BAC) to give an iron cyanide complex (Scheme 37).267 The new iron(IV) nitrido complex 103 was prepared by irradiating the mixture of PhB(Pri2Im)3FeCl with NaN3. This complex has its Fe−N distance, as well as Mössbauer and EPR data, consistent with

TEMPO complex in the presence of PMe2Ph and Ph2C C6H5CPh3 led to the production of the corresponding oxygentransfer product OPMe2Ph and the radical-trapping product [PhB(MesIm)3FeOCPh3], supporting the proposed formation of the intermediate 104. While the high reactivity of 104 prevents its isolation and spectroscopic characterization, theoretical calculations were employed to gain insight into its structural features. Calculations showed that the tris(NHC)borate iron(III) oxo species has a lowest energy spin-state of S = 5/2. In this state, the optimized structure of 104 contains a longer Fe(III)−O bond (1.717 Å) than other Fe(IV)−O bonds (1.639(5)−1.680(1) Å) in iron(IV) oxo com-

Scheme 37. Reaction of 103 with BAC

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plexes.271−275 The high-spin state imparts the three highestlying molecular orbitals substantial Fe(III)−O antibonding character and an Fe(III)−O bond order of 3/2. There is a spin density of 0.85 at the oxo ligand, which indicates the radical character of the oxyl ligand and is consistent with the observed radical rebound reaction with the triphenylmethyl radical (Scheme 34). The successful application of the macrocyclic tetra(NHC) ligand in iron-catalyzed oxidation reactions also prompted explorations of the reactive iron−oxygen intermediates.278,279 As a representative effort, Meyer et al. found that the tetra(NHC) ligand Me,EtTC can support iron(IV) terminal oxo species.140 Upon combination of the low-spin iron(II) complex [( Me,EtTC)Fe(MeCN) 2][OTf]2 with excess 2(ButSO2)C6H4IO at −40 °C, the authors synthesized two iron(IV) oxo complexes [(Me,EtTC)Fe(O)(L)][OTf]2 (L = MeCN, 105; EtCN, 106) (Scheme 39), on which they

Figure 18. Structure of the cation [(Me,EtTC)Fe(O)(NCEt)]2+ in 106. Counteranions and hydrogen atoms are omitted for clarity. The figure was generated on the basis of a cif file reported in ref 140.

Scheme 39. Preparation of the Tetra(NHC)iron(IV) Oxo Species [(Me,EtTC)Fe(O)(RCN)]2+ and Its HAA Reactivity

the weak C−H bonds in xanthene, 9,10-dihydroanthracene, 1,4-cyclohexadiene, and fluorene, to give μ-oxo diferric complex 107 and the corresponding organic products (Scheme 39).280 A detailed kinetic study indicated that the formation of 107 follows pseudo-first-order kinetics, and the reaction of 105 with deuterated dihydroanthracene exhibits a large kinetic isotope effect value of 32. More interestingly, the rate constants of these C−H bond activation reactions (2.2, 0.76, 0.48, and 6.4 × 10−3, respectively, at −40 °C in MeCN) are larger than those of the HAA reactions mediated by [(cyclam)Fe(O)(MeCN)]2+ (cyclam = 1,4,8,11-tetramethyl-1,4,8,11-tetraazacyclotetradecane) by about 2−3 orders of magnitude. The difference was attributed to the single-state reactivity of 105 in the HAA reaction, whereas a two-state reactivity exists in the reaction mediated by [(cyclam)Fe(O)(MeCN)]2+. Recently, Ye and co-workers performed a detailed study on the electronic structure of the iron(IV) oxo species [(Me,EtTC)Fe(O)(NCMe)]2+.281 Helium tagging infrared photodissociation spectroscopy revealed an Fe(IV)−O stretching vibration of 832 cm−1 for the iron(IV) oxo species. The combination of magnetic circular dichroism (MCD) spectroscopy and DFT calculations enabled the establishment of the electronic configuration of the species as σ(O-pz)2π(Opx,y)4NB(Fed(xy))2π*(Fe-d(xz,yz))2σ*(Fe-d(z2))0σ*(Fe-d(x2−y2))0 (NB denotes a nonbonding orbital) with a large triplet−quintet energy gap (28.3 kcal/mol estimated by CASSCF/NEVPT2 calculation). The higher-lying Fe-d(x2−y2) orbital is due to the destabilization by the strongly σ-donating tetra(NHC) ligand. Consequently, it is believed that the C−H activation reaction of the tetra(NHC)iron(IV) oxo species might operate exclusively on the triplet energy surface, in contrast to the two-state reactivity of the N-donor-supported iron(IV) oxo species. Another important study of the stabilization of reactive iron oxygen species by NHC ligands is Kühn’s investigation of the reactions of the methylene-bridged cyclic tetra(NHC)iron(II) complex [(Me,MeTC)Fe(MeCN)2][PF6]2 with O2 gas (Scheme 40).282 The study revealed that the reaction outcome is strongly affected by the reaction conditions, as the reaction in acetonitrile gave the oxygen-free tetra(NHC)iron(III) complex [(Me,MeTC)Fe(MeCN)2][PF6]3 (108), while that in acetone at −40 °C initially afforded an intermediate, presumably the iron(III) superoxide [(Me,MeTC)Fe(O2)][PF6]2 (109), which further converted into the diferric complex [((Me,MeTC)Fe)2(μ2-O)][PF6]4 (110) when warmed to room temperature

performed a full spectroscopic characterization.140 It was noted that the oxidant plays an important role in the preparation of iron(IV) oxo species, as the reaction with either PhIO or O2 as oxidant led to the formation of the oxo-bridged diiron(III) species [((Me,EtTC)Fe)2(μ2-O)][OTf]4 (107) (Scheme 39). The iron(IV) oxo species 105 and 106 display good stability at −40 °C, enabling further spectroscopic characterization. A magnetic susceptibility study revealed an intermediate spin (S = 1) ground state for the iron(IV) oxo species, and a 57Fe Mössbauer study on 105 supported this assignment. Notably, the Mössbauer data of 105 feature a large quadrupole splitting (δ = −0.13 mm/s, ΔEQ = 3.08 mm/s at 80 K) that is unusual for iron(IV) oxo species. The large quadrupole splitting was ascribed to the strong electron-donation of the equatorial tetra(NHC) ligand to the d(x2−y2) orbital of the iron center. A single-crystal X-ray diffraction study revealed a nearly octahedral coordination geometry of the iron(IV) center in 106 (Figure 18). The Fe(IV)−C(carbene) bond distances fall in the range of 1.979(5)−2.045(5) Å, and the Fe(IV)−O bond distance is 1.661(3) Å. The latter bond length is comparable to those of characterized high-spin iron(IV) oxo complexes (1.661−1.680 Å).274,275 A reactivity study further revealed that 105 could perform a hydrogen-atom abstraction (HAA) reaction at −40 °C with AA


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species. A detailed theoretical analysis even predicted that the oxidation ability of the five-coordinate species 111 is comparable to that of the P450 Compound I species (Por)Fe(O)(SH), while the six-coordinate species [(Me,MeTC)Fe(O)(MeCN)]2+ (112) was proposed to have a lower oxidation activity as the coordination of the sixth ligand increases the energy of the σ*(d(z2)) orbital involved in the HAA step.284

Scheme 40. Reactions of Kühn’s Tetra(NHC)iron(II) Complexes with O2

3.7. Complexes of Cobalt

Organocobalt complexes are dominated by cobalt(II), cobalt(I), cobalt(0), and cobalt(−I) species, a feature that also holds true for cobalt complexes with NHC ligation. In this section, the published studies of cobalt(III), cobalt(IV), and cobalt(V) NHC complexes are summarized. As will be seen, cobalt(III) NHC species are plentiful, whereas cobalt(IV) and cobalt(V) species with NHC ligation are exceedingly rare. Fehlhammer and co-workers reported the first cobalt(III)− NHC complexes in 1985.285 In this seminal study on homoleptic transition metal−NHC complexes, the synthesis of the cobalt(III)−NHC complexes 113 and 114, of the form [Co(NHC)6]Cl3, was achieved by the reactions of CoCl2 with 2-hydroxyalkylisocyanides and O2 (Scheme 41). In the

(Scheme 40). Alternatively, the interaction of the iron(III) species 108 with KO2 at low temperatures also provided 109. Complex 109 was found to be diamagnetic, which might be due to the antiferromagnetic coupling of a low-spin iron(III) center with a O2− ligand. While the molecular structure of 109 was not established by X-ray diffraction study, geometry optimization suggested the side-on coordination mode of the iron-bound O2 unit, with a O−O bond length of 1.369 Å. This bond length is close to the O−O distance of superoxide.283 57 Fe Mössbauer spectroscopy and spin-density distribution analysis by calculation may provide information regarding the electronic structure of the intermediate. A preliminary reactivity study indicated that the interaction of the superoxide species 102 with PPh3 or DMSO in acetonitrile did not effect the oxidation of the substrates, but instead led to the formation of iron(II) complexes [(Me,MeTC)Fe(L)2][PF6]2 (L = PPh3, DMSO) and O2 (Scheme 40). On the other hand, the oxobridged diiron complex 110 underwent an oxygen-transfer reaction with PPh3 to give OPPh3 and [(Me,MeTC)Fe(MeCN)2][PF6]2. Synthetic efforts toward (Me,MeTC)iron(IV) oxo species have not yet been reported. However, a theoretical study by de Visser and co-workers pointed out the similar structure features of [(Me,MeTC)Fe(O)]2+ (111) and [(Me,EtTC)Fe(O)]2+, their common triplet ground-spin-state (S = 1), and also the large triplet-quintet energy gap of 111 (Chart 12).284 The study even predicted that [(Me,MeTC)Fe(O)]2+ could perform epoxidation reactions with styrene, as well as the hydroxylation of propene to give allyl alcohol and an iron(II)

Scheme 41. Synthetic Route of Fehlhammer’s Homoleptic Cobalt(III)−NHC Complexes

syntheses, the NHC ligands are formed by nucleophilic addition to metal-coordinated isocyanides, and O 2 is responsible for the oxidation of the cobalt(II) center. Interestingly, anion-exchange reactions of 113 and 114 with [NH4][PF6] led to the production of the corresponding hexafluorophosphate salts [Co(NHC)6][PF6]3 (115 and 116), whereas reactions with NaBPh4 resulted in the formation of [Co(NHC−H)(NHC)5][BPh4]2 (117 and 118), which bear a deprotonated NHC-based anion [NHC−H]− (Scheme 41). The outcome suggests the considerable acidity of the N−H protons in these cobalt(III)−NHC complexes. The characterization data show that 113−118 are diamagnetic, and 13C NMR signals for the carbene carbon nuclei were observed in the range 200−220 ppm. The molar conductivities of the complexes support their identity as 3:1 (for 113−116) and 2:1 (for 117−118) electrolytes. Despite the poor quality of the crystal structure data, an X-ray diffraction study unambiguously confirmed that the central cobalt atom in 117 is surrounded by six five-membered NHC ligands with an average Co(III)− C(carbene) distance of 1.95 Å.

Chart 12. Structures of [(Me,MeTC)Fe(O)]2+ and [(Me,MeTC)Fe(O)(NCMe)]2+

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ligands 122−126 (Chart 14).291,292 The synthetic routes to these complexes are noteworthy. While 3d transition metals were thought to have weaker affinities to NHC ligands than the 4d and 5d transition metals, complexes 122−126 were synthesized via transmetalation reactions of CoCl2(PPh3)2 with the corresponding in situ-generated silver−NHC species, followed by aerobic oxidation. These cobalt(III)−NHC complexes were characterized by NMR spectroscopy and Xray crystallography. 1H NMR spectroscopy indicated the presence of two isomers for 122, whereas the 1H NMR spectrum of 123 exhibited only one set of signals for the functionalized NHC ligand, suggesting the rapid dissociation and association of the N-donors in solution despite their lowspin nature. These cobalt(III)−NHC complexes possess an octahedral coordination-geometry. Their Co(III)−C(carbene) distances span a wide range (1.85−1.95 Å), probably due to the different steric properties of the chelating ligands. Notably, the cobalt(III) complexes 122, 123, and 125 were found to be effective catalysts for the cross-coupling of aryl halides with aryl Grignard reagents, among which 122 shows higher activity. At room temperature and with a catalyst loading of 2 mol %, the reactions of aryl bromides/chlorides with (p-tolyl)MgBr or (otolyl)MgBr catalyzed by 122 gave the biaryl products in moderate to high yields (Scheme 43). In addition, this complex also catalyzed the coupling of an alkenyl bromide with an aryl Grignard reagent. The authors proposed a traditional oxidative addition/reductive elimination pathway involving a cobalt(I)/cobalt(III) cycle293 for the cobalt-catalyzed C−C bond formation reaction. Evidence supporting this proposed mechanism has not yet been provided. Another interesting catalytic application of N-donorfunctionalized NHC−cobalt(III) species is in hydrogenevolution reactions. Using a similar route developed by Chen, Yamauchi and Sakai et al. prepared a cobalt(III) complex bearing a macrocyclic methylene-bridged bis(NHC)bis(pyridine) ligand of the form [(N2C2)CoCl2][Cl] (127) (Scheme 44).294 This cobalt(III) complex served as an effective precatalyst for photochemical hydrogen-evolution. With EDTA as the redox donor, [Ru(bpy)3][NO3]2·3H2O as photosensitizer, and methylviologen as a redox acceptor, the exposure of an aqueous acetate buffer solution (0.03 M CH3COOH and 0.07 M CH3COONa, pH 5.0) of 127 (0.1 mM) to a 300 W Xe lamp produced 0.133 mL of H2, with an initial H2 evolution rate of 4.7 × 10−4 mL/min (Scheme 45). The catalytic activity was found to be comparable to those of carboxylate-bridged dirhodium(II) catalysts reported previously by the author.295 As electrochemical studies on 127 conducted in aqueous solutions with a pH value of 5.0 revealed the presence of the Co(III)/Co(II) process at a potential of 0.22 V (versus NHE) and the absence of Co(II)/Co(I) redox wave, and given that the cyclomethylviologen radical cation is less likely to reduce cobalt(II) species to cobalt(I), it was proposed that the hydrogen-evolution reaction catalyzed by 127 proceeds via a proton-coupled electron transfer (PCET) mechanism involving the conversion of a cobalt(II) species to a cobalt(III) hydride. Considering the large body of reported cobalt(III)−NHC complexes in the literature, this poses an interesting question related to their catalytic performance in the hydrogen-evolution reaction, which is worthy of exploring in the future. Likely prompted by the success of bis(aryloxy)−NHCs in stabilizing titanium(IV) and vanadium(V) complexes, in addition to the high research interest in redox-noninnocent

In a subsequent study, Fehlhammer and co-workers prepared the homoleptic cobalt(III)−NHC complexes [Co(HB(RIm)3)2][BF4] (R = Me, 119; Et, 120, Chart 13) by the Chart 13. Fehlhammer’s Cobalt(III) Complexes with Tris(NHC)borate Ligation

reactions of in situ-generated tris(NHC)borate ligands with CoCl2 followed by oxidation with air.135 The molecular structure of 120 was unambiguously established by a singlecrystal X-ray diffraction study, which revealed the approximate S6 geometry of the cation [Co(HB(EtIm)3)2]+, analogous to the structures of the iron(III) complexes 48−50. The Co(III)−C(carbene) distances in 120 (1.950(5) Å on average) are comparable to those of [Co(NHC−H)(NHC)5][BPh4]2 (117). Complexes 119 and 120 also exhibit a low-spin ground spin-state, and their 13C NMR spectroscopic carbene carbon signals appear at around 180 ppm. Studies of the redox properties of these homoleptic cobalt(III)−NHC complexes were not reported. A large number of cobalt(III) complexes bearing donorfunctionalized NHC ligands exist in the literature. Because simple cobalt(III) salts are not readily accessible, most of these reported cobalt(III)−NHC complexes are synthesized either by the oxidation of well-defined cobalt(II)−NHC complexes or by the one-pot reaction of cobalt(II) precursors with imidazolium salts and oxidants. In contrast, the direct reaction of cobalt(III) precursors with free carbenes, a route commonly used for the preparation of high-oxidation-state earlier 3d transition metal−NHC complexes,135,286−289 is rarely used for cobalt(III)−NHC complexes. Cobalt(III) tribromide is unstable under ambient conditions; however, with a chelating pincer-type NHC−pyridine−NHC ligand, the cobalt(III) tribromide complex [(DippCNC)CoBr3] (121) proved to be stable.290 Danopoulos’ synthesis of 121 involves the oxidation of its cobalt(II) precursor (DippCNC)CoBr2, which was obtained by aminolysis of the amide complex [Co[N(SiMe3)2]2] with a bis(imidazolium)pyridine salt, followed by addition of BrN(SiMe3)2 in THF (Scheme 42). Complex Scheme 42. Synthetic Route to the (DippCNC)Cobalt(III) Complex 121

121 is diamagnetic and its cobalt center adopts an octahedral geometry. The Co(III)−C(carbene) bond length (1.962(7) Å) is comparable to those of the six-coordinate cobalt(III)−NHC complexes 117 and 120. Chen reported a series of cobalt(III) complexes bearing pyridine-, pyrimidine-, or phenanthroline-functionalized NHC AC


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Chart 14. Examples of Cobalt(III) Complexes Bearing N-Donor-Functionalized NHC Ligands

ligands,151,156,161,165,169,175 Soper and co-workers performed a detailed study on cobalt complexes featuring bis(phenolate) NHC ligands.296 Upon the interaction of imidazolium salts with NaOMe (3 equiv) and CoCl2, square-planar cobalt(II) complexes [(t‑BuOCO)Co(solv)] (solv = MeCN or THF) featuring different NHC moieties were synthesized (Chart 15). The voltammograms of these cobalt complexes exhibited three sequential one-electron redox processes, assignable to the sequential one-electron redox processes of [(t‑BuOCO)Co(solv)]0/1+/2+ (Chart 15). The half-wave potentials of the corresponding redox waves of the three complexes are all different. Of these, the complex with a saturated NHC backbone [(t‑BuOCO-2)Co(THF)] has relatively lower halfwave potentials (−0.322, 0.299, and 0.773 V versus the [Cp2Fe]0/1+ couple). A later synthetic study from Soper et al. indicated that the chemical oxidation of the cobalt(II) complex [(t‑BuOCO2)Co(THF)] by AgOTf (1 equiv) and [N(p-C6H4Br)3][PF6] (2 equiv) led to the preparation of the monocationic complex [(t‑BuOCO-2)Co(THF)2][OTf] (128) and the dicationic complex [(t‑BuOCO-2)Co(THF)3][PF6]2 (129), respectively (Scheme 46).296 A further anion-exchange reaction of 128 with NaBPh4 gave [(t‑BuOCO-2)Co(THF)2][BPh4] (130). Attempts to prepare the corresponding [(t‑BuOCO-2)Co]3+ species were unsuccessful. The compounds 130 and 129 are formally cobalt(III) and cobalt(IV) complexes, respectively. Single-crystal X-ray diffraction studies revealed the octahedral and pseudosquare-pyramidal coordination geometries of their cobalt centers. Their Co−C(carbene) distances (1.840(2) and 1.849(3) Å, respectively) are slightly longer than that of the cobalt(II) precursor (1.789(3) Å), whereas the Co−O distance of the cobalt(II) species (1.812(2) Å) locates between those of 130 and 129 (1.802(2) and 1.821(2) Å on average, respectively). Comparison of the C−C and C−O bonds of the phenolate moieties indicated that those in the monocation

Scheme 43. Cross-Coupling Reactions Catalyzed by the Pyrimidine-Functionalized NHC−Cobalt(III) Complex 122 (top), and the Organic Halides Used in These Reactions (bottom)

Scheme 44. Preparation of the (N2C2)Cobalt(III) Complex 127

Scheme 45. Photochemical H2 Evolution Catalyzed by 127

Chart 15. Possible Redox Processes of Soper’s Bis(phenolate)NHC−Cobalt Complexes

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Scheme 46. Synthetic Route to Formal Cobalt(III) and Cobalt(IV) Complexes Supported by a Redox-Active [t‑BuOCO-2] Ligand

Scheme 47. Preparation of the Cobalt(III) Complex 131 Bearing a Thiolate-Functionalized 1,2,4-Triazole-5-ylidene Ligand

scaffold, the determination of the true electronic structure of the complex will require further study. Amido-functionalized NHC ligands have been successfully applied in the preparation of early to middle transition metal complexes. However, cobalt complexes of this type are scarce. The six-coordinate cobalt(III) complex bearing a tridentate NHC-amide-oxygen ligand reported by Chen, [(MeCNO)2Co][I] (132), was obtained from the reaction of a bis(triazolium) salt with CoCl2 (Scheme 48), analogous to

[(t‑BuOCO-2)Co(THF)2]+ are comparable to those in the neutral complex [(t‑BuOCO-2)Co(THF)], and that those in the dication [(t‑BuOCO-2)Co(THF)3]2+ are in line with the distances expected for phenoxyl groups. Both of the cationic complexes are paramagnetic (μeff = 2.9 and 2.5 μB at room temperature, respectively). With the aim of probing the electronic structures of these complexes, the authors further performed studies of their solid-state magnetic properties, as well as EPR measurements and calculations on the cationic complexes. The collective evidence suggested that the formal cobalt(III) species [(t‑BuOCO-2)Co(THF)2]+ might have two possible formulations, an intermediate-spin Co(III) bound to a closed-shell (t‑BuOCO-2)2− moiety or a low-spin Co(II) center ferromagnetically coupled with a radical anion ligand ((t‑BuOCO-2)•)−. No unambiguous assignment of the metal and ligand oxidation states could be achieved for the monocationic complex. On the other hand, characterization data in combination with a theoretical study suggested that the dicationic complex [(t‑BuOCO-2)Co(THF)3]2+ could be described as a low-spin cobalt(II) species bearing a chargeneutral [t‑BuOCO-2] ligand. The discovery demonstrates the complexity of the electronic structures of high-oxidation-state transition metal complexes, and also poses questions about other high-oxidation-state late 3d metal complexes, particularly those having conjugated donor−NHC scaffolds. Thiolate-functionalized NHC ligands are envisioned as strongly electron-donating ligands. However, the inherent difficulties in accessing thiolate-functionalized NHC ligands mean that metal complexes bearing thiolate-functionalized NHC ligands are rare.297−299 As some of the few available examples, Straub and co-workers reported novel nickel, palladium, platinum, cobalt, and molybdenum complexes bearing a thiolate-functionalized 1,2,4-triazolylidene ligand.297 The cobalt(III) complex [(SC)3Co] (131) was synthesized from the reaction of anhydrous CoCl2 with 4 equiv of the triazolium salt in the presence of K2CO3 (Scheme 47). Characterization data obtained by X-ray crystallography and NMR spectroscopy revealed the octahedral geometry of the cobalt center with three SC ligands in a meridional formation. The corresponding Co(III)−C(carbene) distances (1.918(3)− 1.969(3) Å) are typical of low-spin cobalt(III)−NHC complexes. The carbene carbon signals of 131 in its 13C NMR spectrum were found at 177 and 180 ppm, comparable to those of Fehlhammer’s homoleptic cobalt−NHC complexes.134,285 Considering the conjugated nature of the ligand

Scheme 48. Preparation of the Cobalt(III) Complex 132 Bearing an Amido-Functionalized 1,2,4-Triazole-5-ylidene Ligand

the synthesis of iron(III) complexes 52 and 53.206 Complex 132 was characterized by NMR spectroscopy and X-ray crystallography, which indicated its low-spin cobalt(III) nature. The voltammogram of the complex exhibits one reversible oneelectron redox wave with a half-wave potential of 0.92 V (vs [Cp2Fe]0/1+). The author noted that this redox process might be related to the cobalt(III)/cobalt(IV) couple. However, the attempts to prepare the desired cobalt(IV) species by oxidation of 132 with NOPF6 or Ce(NH4)2(NO3)6 were unsuccessful. As all of the aforementioned cobalt(III)−NHC complexes employ chelating NHC ligands, one might question the stability of cobalt(III) complexes bearing monodentate NHCs. To answer this question, oxidation reactions of cobalt(II) complexes featuring monodentate NHCs could be examined. Unfortunately, no such studies have been published thus far. Related to this, Albrecht’s report of porphyrin cobalt(III) complexes featuring 1,3-dimethyl-imidazol-2-ylidene, 133− 137 (Scheme 49), suggested the accessibility of this type of complex.300 Albrecht’s preparation of the cobalt(III)−NHC complexes starts from the direct coordination of the in situgenerated NHC ligand from the decomposition of an imidazolium carboxylate with the cobalt(III) complex mesotetraphenylporphyrin cobalt(III) chloride [(TPP)CoCl]. The resultant air-stable cobalt(III)−NHC complex [(TPP)CoCl(IMe2H2)] (133) was subsequently converted to the five- and six-coordinate cobalt(III)−NHC complexes 134−137 via anion abstraction and ligand coordination reactions (Scheme 49). These porphyrin cobalt(III)−NHC complexes are diamagnetic, and the structures of the alcohol-coordinated complexes were verified by X-ray diffraction studies. The AE


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distances in a narrow range of 1.958(4)−2.000(2) Å. The Co(III)−C(aryl) distances (1.880(2) and 1.871(4) Å, respectively) are within the range of reported cobalt(III)− C(aryl) bonds. The 13C NMR signal of the carbene carbon nuclei appeared at approximately 196 ppm. These data are comparable to those of Danopoulos’ complex [(DippCNC)CoBr3] (121).290 Using these cobalt(III) complexes as starting material, pincer-type cobalt(II) and cobalt(I) complexes ( Ar CCC-2)CoCl(py), ( Ar CCC-2)Co(N 2 )(PPh 3 ), and (DippCCC-2)Co(N2) were found to be accessible upon the reduction of 138 and 139 with the appropriate reducing reagents (Scheme 50). A more intriguing example of a (MesCCC-2)cobalt(III) complex is the hydride compound [(MesCCC-2)CoHCl(PMe3)] (140), which was prepared by the oxidative addition reaction of the cobalt(I) complex [(MesCCC-2)Co(N2)(PMe3)] with HCl·Et2O (Scheme 51).305 The hydride

Scheme 49. Conversions of Porphyrin Cobalt(III) Complexes with NHC Ligation

Scheme 51. Reactivity of the (ArCCC-2)Co(III) Complex 140 Co(III)−C(carbene) distances (ca. 1.93 Å) are typical of cobalt(III)−NHC complexes. Pincer-type ligands possess the important features of strong electron-donation and high rigidity, which are useful for the synthesis of reactive transition metal species and also new catalyst design.301−303 On the basis of these factors, there is high interest in cobalt complexes supported by NHC-based pincer ligands.290,304−307 As the representative study in this area, Fout found that the bis(NHC)-aryl ligand 2,6-bis(Narylbenzimidazol-2-ylidene)phenyl (ArCCC-2) was able to support cobalt(I, II, and III) species that are useful catalysts for the hydro-functionalization of olefins.304,305 As the preparation of the bis(NHC)-aryl lithium salts from the reactions of the bis(NHC)benzene with BunLi was unsuccessful, the author developed a one-pot metalation procedure, the reactions of the 1,3-benzene-bridged di(benzimidazolium) salts [H3(ArCCC-2)]Cl2 with [Co(N(SiMe3)2)2(py)2], LiN(SiMe3)2, and ClCPh3. Using this protocol, the green pincer complexes [(ArCCC-2)CoCl2(py)] (Ar = Dipp, 138; Mes, 139) were prepared in high yields (Scheme 50).304 The crystal structures revealed the perfect planarity of the (ArCCC-2)Copincer fragments in 138 and 139 with Co(III)−C(carbene)

complex was isolated as an orange solid. It shows a characteristic 1H NMR signal at −10 ppm and a Co−H IR stretch at 1828 cm−1. The molecular structure established by a single-crystal X-ray diffraction study indicated the mutually trans position of the Cl and C(aryl) units. The Co(III)−C distances are close to those of 138 and 139. As high-oxidationstate transition metal hydride species may be prone to decomposition via reductive elimination pathways, the

Scheme 50. Preparation of the (ArCCC-2)Co(III) Complexes 138 and 139 and Their Conversions to Cobalt(I,II) Complexes

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isolation of this stable cobalt(III) hydride species is noteworthy. The stability apparently benefits from the strong electron-donating nature and high rigidity of the bis(NHC)aryl pincer ligand. The study also revealed that the cobalt(III) hydride species could further react with HCl·Et2O to form the cobalt(III) dichloride complex [(MesCCC-2)CoCl2(PMe3)] (141), along with H2. However, attempts to prepare cobalt(III) dihydride species via the interaction of 140 with Cp2ZrHCl and the reaction of [(MesCCC-2)Co(N2)(PMe3)] with H2 were unsuccessful (Scheme 51), whereby only the corresponding cobalt(I) complexes [(MesCCC-2)Co(N2)(PMe3)] and [(MesCCC-2)Co(H2)(PMe3)] were formed. By taking advantage of the ability of the bis(NHC)−aryl pincer to support cobalt(I) dihydrogen and cobalt(III) hydride species, Fout’s group subsequently developed (MesCCC-2)Co catalysts for olefin hydrogenation. Combined experimental observations and NMR studies, including parahydrogen-induced polarization experiments, suggested the possible involvement of transient cobalt(III) dihydride species in the catalytic cycle.305 In addition to Fout’s study, Hollis and co-workers investigated complexes of the n-butyl-substituted pincer ligand 2,6-bis(3-n-butylimidazol-2-ylidene)phenyl (n‑BuCCC-1).306 By inducing transmetalation from the preformed (n‑BuCCC-1) zirconium complexes to cobalt with careful selection of the zirconium and cobalt precursors, the high-yield synthesis of two (n‑BuCCC-1)Co(III) complexes was achieved, [(n‑BuCCC1)2Co][Cl] (142) and [(n‑Bu CCC-1)Co(acac)I] (143) (Scheme 52). The monoanionic pincer ligand [n‑BuCCC-1]1−

Scheme 53. A Catalytic Application of 143 in the Hydroboration of Styrene

pronounced electron-donating properties. However, the incompatibility of the Lewis acidic silanes with nucleophilic NHC ligands renders the direct synthesis of this type of ligand difficult. By utilizing a one-electron-oxidation-induced coupling of benzyl−NHC and silyl−NHC moieties on a preformed cobalt(II) complex, Deng and co-workers achieved the synthesis of the cobalt(III) complex [(CSiC)CoH(OEt2)][BPh4] (144). (Scheme 54). The characteristic 29Si NMR Scheme 54. Preparation of the Pincer (CSiC)Co(III) Complex 144 and Its Conversion to a (CSiC)Co(I) Complex

Scheme 52. Preparation of the (n‑BuCCC-1)Cobalt(III) Complexes 142 and 143 via Transmetallation

resonance of 144 (40 ppm referenced to SiMe4) and the small Si−H constant (less than 28 Hz) support the identity of 144 as a (CSiC)Co(III) hydride complex, rather than a Co(I) silane complex. Moreover, the molecular structure of the analogue with an acetonitrile ligand [(CSiC)CoH(NCMe)][BPh4] (145), which was prepared through the ligand-exchange reaction of 144 with MeCN, confirmed the proposed structure. Induced by the strong trans-effect of the (CSiC) pincer ligand, the cobalt(III) complex displays a distorted square-pyramidal geometry with the silicon atom sitting at the apical position. The observed Co(III)−C(carbene) distances (ca. 1.92 Å) in 145 are consistent with the low-spin cobalt(III) nature. To gain insight into the electronic properties of the pincer ligand, a (CSiC)Co(I) dinitrogen complex [(CSiC)Co(N2)] was prepared from the reaction of 144 with LiBEt3H (Scheme 54). The lower N−N stretch (1937 cm−1) observed in its IR spectrum as compared to those of analogous cobalt(I) dinitrogen complexes supported by [CCC]1− and [CNC]1 pincer ligands confirms the pronounced electron-donating nature of the [CSiC] pincer ligand. Intriguingly, benefiting from the strong electron-donating nature of the [CSiC] pincer ligand, the cobalt(I) dinitrogen complex was found to undergo oxidative addition reactions with C−H, N−H, and O−H bonds, affording the corresponding (CSiC)Co(III) complexes

differs from the [ArCCC-2]1− ligands in Fout’s complexes both in the backbone of the NHC moieties and in the nature of the N-substituents. The sterically less demanding n-butyl group disfavored the preparation of a mono-CCC-NHC cobalt complex, as demonstrated by the formation of 142. Fortuitously, treatment of the isolated zirconium species [(n‑BuCCC-1)ZrCl2(NMe2)] with Co(acac)3 led to the successful preparation of [(n‑BuCCC-1)Co(acac)I] (143). As a demonstration of the synthetic utility of these (n‑BuCCC-1) Co complexes, 143 in combination with LiBHEt3 was used as the catalytic system for the hydroboration of styrene with HBpin, wherein the Markovnikov addition product was obtained in near quantitative conversion (Scheme 53). Very recently, Deng and co-workers presented another type of cobalt(III) complexes supported by NHC−silyl−NHC ligands.307 Silyl-functionalized NHCs are thought to have AG


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d(xy)2d(x2−y2)2d(z2)2d(xz)0d(yz)0 electronic configuration. The molecular structure of 151 established by XRD showed a pseudotetrahedral geometry for the cobalt center, which has Co(III)−C(carbene) bond distances of 1.938(3)−1.962(3) Å and a Co(III)−N(imido) distance of 1.675(2) Å. Interestingly, one of the carbene carbon atoms has a short C−N(imido) distance of 2.982 Å. The distance is smaller than the sum of the van der Waals radii (3.25 Å). DFT calculations revealed the presence of orbital interactions between the σ-orbital of the carbene carbon atom and the empty π*-orbital at the imido nitrogen atom. Related to this structural feature, these imido complexes can undergo decomposition in solution at room temperature, giving intramolecular nitrene-transfer products (Scheme 56). The occurrence of this nitrene-transfer reaction indicated the electrophilicity of the imido ligand. The detailed mechanism for the formation of the cobalt(II) species is unclear. The authors proposed that the disproportionation of the corresponding cobalt(I) intermediates may be taking place. Likely due to the steric hindrance caused by the bulky substituents of NHC moieties, treatment of these cobalt imido complexes with nucleophiles (such as styrene and tetramethylimidazol-2-ylidene) did not yield intermolecular nitrenetransfer products. Smith and co-workers also demonstrated the ability of the tris(NHC)borate ligand to support cobalt(III) terminal imido species.310 In constrast to Meyer’s synthetic method, a cobalt(III) imido complex supported by tris(NHC)borate ligand, [PhB(t‑BuIm)3Co(NBut)] (153), was prepared by hydrogen-atom abstraction from the high-spin cobalt(II) amido complex [PhB(t‑BuIm)3Co(NHBut)] by the 2,4,6tri(tert-butyl)phenoxy radical (Scheme 57). Complex 153

146−148 (Scheme 55), whose structures were unambiguously established by X-ray diffraction studies. Scheme 55. Formation of (CSiC)Co(III) Pincer Complexes via Oxidative Addition Reactions of [(CSiC)Co(N2)] with X−H Bonds (X = C, N, O)

Cobalt imido complexes are the other important class of higher-oxidation-state cobalt complexes. As studies of cobalt imido species have been much less extensive than those of their iron congeners, only a handful of higher-oxidation-state cobalt imido complexes with NHC ligation have been reported. After Peters’ report of the first structurally characterized cobalt terminal imido complex supported by a tris(phosphino)borate ligand in 2002,308 synthetic efforts were also made toward cobalt terminal imido complexes supported by tris(NHC) ligands. Meyer’s study utilizing TIMEN ligands showed that the reaction of the cobalt(I) complexes [(ArTIMEN)Co]Cl with aryl azides indeed provided the monomeric cobalt(III) imido complexes [(ArTIMEN)Co(NArR)][X] (149−152, X = Cl, BPh4) in near quantitative yields (Scheme 56).309 Spectroscopic characterization and theoretical studies indicated that these cobalt(III) imido complexes have a low-spin ground state (S = 0) with a

Scheme 57. HAA of a Cobalt(II) Amido Complex with Tris(NHC)borate Ligation Leading to the Formation of the Cobalt(III) Imido Complex 153

Scheme 56. Synthesis of (TIMEN)Cobalt(III) Imido Complexes and Their Decomposition Reactions

adopts a low-spin ground state. Its Co(III)−C(carbene) (1.949(4)−1.988(4) Å) and Co(III)−N (1.660(3) Å) distances are in accordance with reported Co(III) imido complexes with ArTIMEN and tris(phosphine)borate ligands.247,308,309,311−314 Theoretical calculations suggested a concerted proton-coupled electron-transfer mechanism for the hydrogen-atom abstraction reaction. Along with the successful preparation of high-oxidation-state bis(imido)iron NHC complexes, Deng and co-workers found that similar bis(imido)cobalt NHC complexes are also accessible. 139 Upon the treatment of three-coordinate cobalt(0) precursor [(IMes)Co(dvtms)] with 2 equiv of DippN3, the first cobalt(IV) imido complex [(IMes)Co(NDipp)2] (154) was prepared in high yield (Scheme 58). The cyclic voltammogram of 154 features a reversible oneelectron redox wave with a half-wave potential of −0.16 V (versus SCE). Hence, subsequent treatment of 154 with [Cp2Fe][BArF4] resulted in the formation of the first cobalt(V) imido complex [(IMes)Co(NDipp)2][BArF4] (155) in 88% AH


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cobalt(V) complex is diamagnetic and showed a characteristic C NMR signal for the carbene carbon nucleus at 176.8 ppm. The low-coordinate nature and the high formal oxidation states of the metal centers of 154 and 155 engender the high covalency of the metal−ligand interactions in these complexes. Accordingly, theoretical calculations indicated that the frontier molecular orbitals of the complexes have near-equal contributions from both the cobalt center and the imido ligand orbitals. Consequently, the unambiguous assignment of the spectroscopic oxidation states of their cobalt centers is difficult. Another interesting finding related to these bis(imido)cobalt NHC complexes is their distinct tendency to undergo C−H activation reactions. It was found that heating the cobalt(IV) imido complex 154 resulted in the formation of a C−H amination product, a cobalt(II) amido complex. In contrast, the cobalt(V) imido complex 155 is inert under the same conditions. This contrasting reactivity indicated that a high oxidation state of a late 3d metal imido complex does not necessarily mean high activity of toward C−H activation reactions. Later, Chen et al. performed detailed mechanistic calculations on the system.315 It turned out that the C−H amination reaction of 154 seems to involve sequential steps of hydrogen atom abstraction, radical rebound, and migratory insertion, with a pathway on a doublet (S = 1/2) energy surface being energetically favored (Scheme 59). The failure to

Scheme 58. Synthesis and Reactivity of Bis(imido)cobalt(IV) and Cobalt(V) Complexes with NHC Ligation

13

yield (Scheme 58). Noting the spontaneous decomposition of the cobalt(III) imido complex [(MesTIMEN)Co(NArOMe)][Cl] via reductive elimination, the accessibility of these unusual high-oxidation-state bis(imido)cobalt complexes presumably benefits from the use of the NHC ligand IMes, which is strongly electron-donating and sterically demanding, kinetically stabilizing the formal high-oxidation-state species. The lowcoordinate nature of the metal center is likely an additional factor in the stabilization of the complex. As the first examples of high-oxidation-state cobalt imido complexes, 154 and 155 were subjected to detailed spectroscopic study. X-ray crystallography established the trigonal planar coordination geometry of their cobalt centers (Figure 19). Interestingly, the Co(IV)−N(imido) distance in

Scheme 59. Calculated Pathway for the C−H Bond Amination Reaction of 154

observe a similar C−H activation reaction with 155 was attributed to the higher HAA barrier (33 kcal/mol versus 24 kcal/mol for 154) of the bis(imido)cobalt(V) species on its energetically favored triplet energy surface (S = 1). Efforts have also been made toward stabilizing cobalt nitride species with NHC ligands. Because of their intrinsically high activity, so far no isolable complex of this type has been reported. However, Meyer’s investigation with an amineanchored bis(NHC)−phenolate chelate ligand implies that such a species could be accessible after judicious ligand design. Meyer and co-workers found that photolysis of the Co(II) azide precursor at 10 K produced a new paramagnetic species featuring nearly axial EPR signal at g = 2.01. In the presence of a proton source, for example, 2,4,6-tri(tert-butyl)phenol, the complex quickly converted to the cobalt(II) species [(NHBIMPNMes,Ad,Me)Co][BPh4] bearing an imine ligand (Scheme 60).316 This observation, in combination with theoretical calculations, suggested that the intermediate was the cobalt(IV) nitrido complex [(BIMPNMes,Ad,Me )Co(N)] (156) (Scheme 60). The calculations predicted that the cobalt(IV) nitride species has a doublet ground state. The CoC2ON core displays a near-trigonal pyramidal geometry with Co(IV)− C(carbene) and C−N(nitride) distances of 1.91 and 1.59 Å, respectively. Importantly, the calculated EPR parameters of 156 are fully consistent with those observed experimentally

Figure 19. Structure of the cation [(IMes)Co(NDipp)2]1+ in 155. Counteranion and hydrogen atoms are omitted for clarity. The figure was generated on the basis of a cif file reported in ref 139.

154 (1.665(3) Å) is slightly longer than that in 155 (1.641(3) Å on average), whereas the Co(IV)−C(carbene) bond in the former is shorter (1.879(5) and 1.941(4) Å, respectively). The trans effect is thought to play a role in the observed pattern of bond lengths. The cobalt(IV) complex is paramagnetic. It shows a solution magnetic moment of 2.2(1) μB and an axial X-band EPR spectrum with gav = 1.98 (at 103 K). These data, in addition to DFT calculations, indicate an S = 1/2 ground spin-state for 154. Importantly, an atomic spin-density distribution analysis revealed that the spin resides not only on the cobalt center but also on the two imido moieties. The AI


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Scheme 60. Formation of the Transient Cobalt(IV) Nitride Species 156 by Photolysis of Its Cobalt(I) Azide Precursor, and the Subsequent Decomposition via Migratory Insertion

Scheme 61. Synthesis of a Cobalt(III) Peroxo Complex 157 Supported by Tris(NHC)−Amine Ligand and Its Reactions with Electrophiles

(Figure 20). The calculations also revealed a very low barrier for the migratory insertion step (2.2 kcal/mol), which accounts

diamagnetic. The lengths of its Co(III)−C(carbene) bonds (1.962(4) Å in average) are typical of low-spin cobalt(III)− NHC complexes (Figure 21). The side-on peroxo ligand has

Figure 20. X-band EPR spectrum of 156 in a frozen 2-methyl THF solution at 10 K (black trace) and its simulation (red trace). Experimental conditions: microwave frequency ν = 8.960 GHz, modulation width = 2.0 mT, microwave power = 2 mW, modulation frequency = 100 kHz, time constant = 0.1 s. Simulation parameters: effective spin S = 1/2, effective g-values g⊥ = 4.18, g∥ = 2.02, Wx = 11.6 mT, Wy = 43.5 mT, Wz = 41.1 mT. Adapted with permission from ref 316. Copyright 2014 American Chemical Society.

Figure 21. Structure of the cation [(XylTIMEN)Co(O2)]1+ in 157. Hydrogen atoms, counteranion, and solvent molecules are omitted for clarity. The figure was generated on the basis of a cif file reported in ref 287.

an O−O bond length of 1.429(3) Å, which is comparable with other well-known cobalt(III) peroxo complexes.321−324 Accordingly, the O−O bond stretching frequency (890 cm−1) is in the range of known peroxo species (930−740 cm−1).325 Noting the rarity of transition metal−NHC complexes with O2 coordination, the author ascribed the success in the preparation of 157 to the strong electrondonating nature of the NHC-based ligand, which stabilizes the cobalt(III) species, and the sterically demanding xylene substituents, which block the bimolecular decomposition pathway of the peroxide species. DFT calculations on the cation of 157 further indicated that the origin of the diamagnetism of 157 is the low-spin nature of the cobalt(III) center. Importantly, the HOMO of the species is found to be predominantly composed of dioxygen σ* orbitals, which suggests the nucleophilicity of the peroxide ligand in 157. Related to this electronic feature, 157 was found to react with electron-deficient organic substrates such as benzoyl chloride and benzylidenemalonitrile to produce phenyl benzoate and benzyl aldehyde, respectively, along with the cobalt(II) complex [(XylTIMEN)Co(CH3CN)][BPh4]2 (Scheme 61).

for the difficulty in isolating the cobalt(IV) nitride species. It is interesting to noted that, prior to Meyer’s study, Smith had examined the photolysis of the cobalt(I) azide species [PhB( t‑BuIm) 3Co(N3 )] with the aim of preparing the corresponding cobalt(IV) nitride species. This reaction, however, appeared only to give an azide radical and a cobalt(I) species.317 This outcome also differs from the formation of an iron(IV) nitride observed in the photolysis of [PhB(t‑BuIm)3Fe(N3)].138 Cobalt oxygen species are implicated as the reactive intermediates in cobalt-catalyzed oxidation reactions in organic synthesis, as well as water oxidation reactions,10,11,318−320 thus stimulating considerable research interest in such complexes. So far, only very limited studies on cobalt oxygen species with NHCs as supporting ligands have been reported. Meyer et al. studied the reaction of the (XylTIMEN)cobalt(I) complex [(XylTIMEN)Co][Cl] with dioxygen and achieved the preparation of a pale pink (XylTIMEN)cobalt(III) complex featuring a side-on-bound peroxo ligand, [(XylTIMEN)Co(O2)][BPh4] (157) (Scheme 61).287 Complex 157 is AJ


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support d6 cobalt(III) species.304,305 Extension of their studies to nickel indicated the accessibility of d6 nickel(IV) complexes [(DippCCC-2)NiX3] (X = Cl, 161; Br, 162) via the interaction of the nickel(II) complexes [(DippCCC-2)NiX] with halogens or halogen surrogates PhICl2 and BTMABr3 (Scheme 63).141

On the other hand, 157 is inert toward 2-cyclohexene-1-one, 1,4-naphthoquinone, 2-methyl-1,4-naphthoquinone, styrene, cyclohexene, and triphenylphosphine. Thus, the tris(NHC)− amine ligand-supported cobalt(III) peroxide complex was classified as a class II nucleophilic peroxo species as defined by Valentine and co-workers.326 As part of a rare example of a study on cobalt oxygen species with NHC ligation, Radius et al. found that the reaction of a pentamethylcyclopentadienylcobalt(I) carbonyl complex [(C5Me5)Co(IPr2H2)(CO)] with O2 gas produced the cobalt(III) carbonato complex [(C5Me5)Co(IPr2H2)(κ2CO3)] (158) (Scheme 62).327 The formation of 158

Scheme 63. Synthesis and Reactivity of (DippCCC2)Nickel(IV) Halides

Scheme 62. Reaction of a NHC−Cobalt(I) Carbonyl Complex with O2

The resulting nickel(IV) complexes were found to be diamagnetic. X-ray diffraction studies established the octahedral coordination geometry of their nickel(IV) centers (Figure 22). The Ni(IV)−C(carbene) (1.936(3) and 1.979(2) Å for

represents the first example of O2 activation of CO on a 3d metal complex. The authors applied low-temperature timeresolved UV/vis spectroscopy and DFT calculations to elucidate a possible reaction mechanism. The UV/vis spectra recorded at −90 °C led to the detection of an intermediate species with a characteristic absorption band at 585 nm. DFT calculations suggested this intermediate to be a peroxoacyl intermediate [(C 5 Me 5 )Co(IPr 2 H 2 )(κ 2 -C,O−C(O)OO)] (160) formed from an intramolecular nucleophilic attack of the distal oxygen atom of the O2 ligand of the initial dioxygen intermediate (C5Me5)Co(IPr2H2)(CO)(O2)] (159) (Scheme 62). In contrast to the cobalt(III) side-on-bound peroxide complex 157, detailed calculations on its electronic structure indicated that the optimized structure of 159 has an open-shell singlet configuration with an end-on O2 ligand, with O−Co and O−O distances of 2.100 and 1.313 Å, respectively, and a O−O−Co angle of 110.46°. Thus, 159 is better described as a cobalt(II) terminal superoxide species. The observation of the nucleophilic attack at CO rather than at the NHC ligand may be related to the sterically demanding nature of the NHC.

Figure 22. Molecular structure of 161. Solvent molecules and hydrogen atoms are omitted for clarity. The figure was generated on the basis of a cif file reported in ref 141.

161 and 162, respectively), Ni(IV)−C(aryl), and Ni(IV)−X bonds were found to be longer than those in the nickel(II) precursors. The lengthening of the bonds was attributed to the higher coordination number of the nickel(IV) complexes as compared to the nickel(II) precursors. Notably, despite the stabilization by the bis(NHC) aryl pincer ligand, the two nickel(IV) complexes are still highly reactive. In THF, the trichloride complex 161 decomposes spontaneously at room temperature to give the nickel(II) complex (DippCCC-2)NiCl and the paramagnetic product [H3(DippCCC-2)][NiCl4]. The tribromide complex 162 was found to be stable in THF. However, its interactions with styrene, cyclohexene, 2mesitylmagnesium bromide, and lithium hexamethyldisilazide led to the bromo-transfer products 1,2-dibromo-1-phenylethane, 1,2-dibromocyclohexane, mesityl bromide, and BrN-

3.8. Complexes of Nickel

Nickel NHC complexes are mostly known for the oxidation states II, I, and 0. While high-oxidation-state nickel NHC species were thought to be involved in the electrochemical oxidation of nickel(II) NHC species328 as well as nickel− NHC-catalyzed reactions,329−331 the reported examples of isolable high-oxidation-state nickel complexes with NHC ligation are limited to Fout’s nickel(IV) halide complexes, which are supported by the NHC−aryl−NHC pincer ligand [DippCCC-2].141 Fout’s study on (ArCCC-2)cobalt complexes revealed the ability of the electron-rich bis(NHC)−aryl pincer ligand to AK


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(SiMe3)2, along with the nickel(II) complex (DippCCC-2)NiBr (Scheme 63). Determining whether these conversions proceed through a concerted reductive elimination of Br2 followed by halogenation of the substrates, or the direct interaction of the nickel(IV) bromide species with the substrates via a radicaltype mechanism, will require further study. Meyer et al. proposed the formation of a nickel(III) species bearing tetradentate bis(NHC)-bis(amide) ligands, of the form [(C2N2)Ni]+ (163 in Chart 16), in the electrochemical

reactions to form the dinuclear complex (Scheme 64). DFT calculations on the hypothetical species 164 indicated that one unpaired electron-spin on [(TMTBM)Ni(Ndippp)]+ resides mainly on the nitrogen and ortho- and para-carbon atoms of the NAr moiety. Thus, 164 is best viewed as a nickel(II) species featuring a quinoneimine radical. The instability of [(TMTBM)Ni(Ndippp)]+ (164) contrasts with the stable species [(dtbpe)Ni]N(dmp)]+ (dtbpe = 1,2-di(tert-butyl)phosphino)ethane; dmp = 2,6-dimesitylphenyl),334 and was thought to be related to the different steric properties of the bis(NHC) and bis(phosphine) ligands. In the latter case, the central aryl ring on the imido moiety is perpendicular to the coordination plane of the nickel center, which may be the cause of the attenuated spin-density distribution on the aryl ring. No synthetic studies on (NHC)nickel nitrides and oxides are known thus far. Considering the higher 3d electron count of nickel than those of the earlier 3d metals, the high activity of this type of complex could be assumed, because a high 3d electron count might lead to greater occupation of antibonding orbitals. Smith and Wang explored the electronic structure of hypothetical nickel(IV) nitrido complexes supported by tris(carbene)borate ligands.335 Theoretical calculations revealed that the nickel(IV) nitrido species [PhB(t‑BuIm)3Ni(N)] (165 in Chart 16) is similar in energy in both its triplet (S = 1) and its singlet (S = 0) states, and that both states are lower in energy than the S = 2 state. Interestingly, optimized structures in the triplet and singlet states possess bent B−Ni−N units (141° and 147°, respectively). While the Ni−N distances (1.676 and 1.610 Å in the triplet and singlet states, respectively) reflect the multiple nature of the corresponding bonds, they are significantly longer than those in the iron(IV) and iron(V) nitrido complexes (1.512(1) and 1.506(2) Å for [PhB(t‑BuIm)3Fe(N)] and [PhB(t‑BuIm)3Fe(N)]+, respectively). The long Ni−N bond is presumably related to the occupation of the Ni−N π*-orbitals. It should be mentioned that while attempts to access nickel(III) NHC complexes have thus far been unsuccessful, the accessibility of this type of complex can still be anticipated when noting the large amount of known nickel(III) phosphine complexes.334,336−338 Thus, the judicious design of the ligand set may be the key to addressing this challenge.

Chart 16. High-Oxidation-State Nickel Amido, Imido, and Nitrido Species with NHC Ligation

oxidation of its nickel(II) precursor.328 Cyclic voltammetry in aqueous solution revealed a half-wave potential of +0.89 V (versus NHE) for the quasi-reversible one-electron redox process of [(C2N2)Ni]0/1+. The authors noted that this potential is comparable with those of nickel complexes of Nsubstituted cyclam derivatives [(N4)Ni(OH2)2]2+/3+.332 EPR spectroscopy (g1 = 2.247, g2 = 2.245, g3 = 2.011, g⊥ > g∥ ≈ 2.0) supported the nickel(III) nature of the one-electron-oxidation product. Considering the tendency of Ni−N(amide) bonds to undergo hydrolysis, one might suspect that the redox event corresponds to that of the [(C2N2H2)Ni(OH2)2]2+/3+ couple. No reports of isolable nickel(III) imido species with NHC ligation are known. In one unsuccessful attempt, Hillhouse and co-workers found that the oxidation of three-coordinate nickel(II) imido complex [(TMTBM)Ni(Ndippp)] (TMTBM = 3,3′-methylenebis(1-tert-butyl-4,5-dimethylimidazoliylidene), dippp = 2,6-bis(2,6-diisopropylphenyl)phenyl), with [Cp 2 Fe][B(C 6 F 5 ) 4 ] gave a dimeric complex [{(TMTBM)NiNC6Ar2H2}2][B(C6F5)4]2 (Scheme 64).333 The formation of the dinuclear complex was thought to be due to the radical character of the imido moiety in the formal nickel(III) species [(TMTBM)Ni(Ndippp)]+ (164), which might undergo C−C bond coupling and dehydrogenation

3.9. Complexes of Copper

As an important subclass of coinage metal complexes, copper NHC species have received much attention, partly due to their applications in catalysis and photoluminescent materials.339−350 Lin’s review on coinage metal NHC complexes published in 2009348 covered the advances made in copper NHC chemistry up to that point. Since 2009, a large amount of new studies on copper NHC complexes have appeared.351−354 While many of these focus on copper(I) NHC complexes, studies involving copper(II) and copper(III) NHC species were also undertaken. With the aim of preparing copper(II) complexes with monodentate NHC ligands, Yun and Nechaev examined the reactions of simple copper(II) salts with free monodentate NHC ligands, revealing that copper(II) acetate is a suitable copper(II) precursor.130,355 Upon the direct interaction of copper(II) acetate with free NHC ligands (IMes, sIMes, IPr, sIPr, 6-sIMes, 7-sIMes) in toluene or diethyl ether, blue or green copper(II)−NHC complexes [(NHC)Cu(OAc)2] (NHC = IMes, 166; sIMes, 167; IPr, 168; sIPr, 169; 6-

Scheme 64. Hillhouse’s Attempt To Synthesize the Formal Nickel(III) Imido Species 164

AL


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reaction of trimethylene-linked 1,10-diethyl-3,30-trimethylene-dibenzimidazolium diiodide with copper(I) iodide and KOBut in a mixture of THF and acetonitrile in air (Scheme 67).357 The crystal structure of the complex indicated a

sIMes, 170; 7-sIMes, 171) were synthesized in moderate to high yields (Scheme 65). Whether similar complexes bearing Scheme 65. Synthetic Route to the (NHC)Copper(II) Acetate Complexes 166−171 and the Reaction of 161 with Water

Scheme 67. Synthetic Route to the Polymeric (NHC)Copper(II) Iodide Complex 173

sterically less-demanding NHCs are accessible or not is as yet unknown. Complexes 166−171 are paramagnetic in nature. The molecular structures of 166, 168, and 170 were established by X-ray structure analysis. The Cu(II)−C(carbene) distances in the three complexes (1.960(4), 1.942(4), and 1.981(3) Å, respectively) are comparable to each other, and the acetate ligands bind with the copper centers in a chelating fashion. Notably, these (NHC)copper(II) acetate complexes exhibited distinct stability toward moisture: the complex bearing the ligand IPr (168) is air stable and could be kept for months without any noticeable decomposition at room temperature,355 whereas that containing sIPr (169) decomposes quickly in wet chloroform to form the open-ringed formamide (Scheme 65).130 Similarly, Marder et al. found that treating copper(II) acetylacetonate with the free NHC ligand IMes also provided the stable copper(II) NHC complex [(IMes)Cu(acac)2] (172).356 In contrast to the successful preparation of (NHC)copper(II) acetate and acetylacetonate complexes, similar (NHC)copper(II) dihalide complexes were difficult to access under ambient conditions. Nechaev and co-workers found that neither the direct interaction of copper(II) dihalides with monodentate NHC ligands, nor the transmetalation reaction between (NHC)silver(I) halides and copper(II) dihalides, gave the desired (NHC)copper(II) dihalide complexes, instead providing copper(I) NHC complexes and haloamidinium salts (Scheme 66).130 The authors proposed that the formation of

distorted tetrahedral geometry for the copper center, to which is bound one NHC ligand, two bridging iodide anions, and one terminal iodide ligand. The Cu(II)−C(carbene) distances (1.923(2) and 1.989(2) Å) are typical of copper−NHC complexes. In this case, the bis(NHC) ligand binds as a monodentate ligand to each copper site. The study also noted that 173 exhibits double emission bands with maxima at 425 and 500 nm in its fluorescent emission spectrum. The complex shows well-defined 1H and 13C NMR signals, despite its expected paramagnetic nature. Considering the presumed instability of copper(II) dihalide complexes supported by monodentate NHC ligands, the NMR signals could instead correspond to the (NHC)copper(I) iodide species generated from the decomposition of 173 in solution. Despite the limited number of copper(II) complexes with monodentate NHC ligation, there are useful catalytic applications of these complexes. Yun and co-workers found that the copper(II) complex 168 can serve as precatalyst for 1,2- and 1,4-reduction of carbonyl compounds by hydrosilanes (Scheme 68).355 Marder and co-workers showed that the Scheme 68. Organic Transformations Using Copper(II) Salts and Monodentate NHCs as Precatalysts

Scheme 66. Unsuccessful Attempts To Synthesize (NHC)Copper(II) Dihalides

combination of CuCl2 with IPr or IMes can be used for the catalytic borylation of unactivated alkyl bromides and chlorides by bis(pinacolato)diboron (Scheme 68).356 It should be noted that, while copper(II) compounds were used as precatalysts, the genuine catalytic active species in these reactions were thought to be (NHC)copper(I) species formed from the reactions of (NHC)copper(II) species with hydrosilanes or bis(pinacolato)diboron. While copper(II) complexes with monodentate NHC ligands are mainly restricted examples bearing carboxylate ligands, those bearing donor-functionalized NHC ligands are

the haloamidinium salts and copper(I) complexes might be due to a bimolecular reductive elimination reaction of (NHC)copper(II) dihalide intermediate [(NHC)CuX2]2. The details of the mechanism of this reaction are unclear. As a rare example of a copper(II) halide complex bearing a monodentate NHC ligand, Liu and co-workers isolated the polymeric compound [(NHC)2Cu2I4]n (173) from the AM


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Chart 17. Examples of Copper(II) Complexes Bearing Neutral Chelating NHC Ligands

The long Co(II)−N distance (2.54 Å) indicates weak Co− amine interaction in the cation. Related to the distorted trigonal planar geometry, calculations indicated a nondegenerate ground-state electronic configuration of d(xz)2d(yz)2d(xy)2d(z2)2d(x2−y2)1 for [(MeTIMEN)Cu]2+. The coordination of the tris(NHC) ligand to the copper(II) ion appears to attenuate the oxidizing power of the copper(II) complex. As discerned from the half-wave potential of the copper(II)/ copper(I) redox couple, the one-electron oxidation of the copper(I) species [(BnTIMEN)Cu][PF6] occurs at −0.10 V (versus [Cp2Fe]0/1+). Liu’s bis(NHC)copper(II) complex 175 was prepared by the combination of anhydrous copper(I) iodide, KOBut, and the corresponding ether-linked diimidazolium salt, followed by addition of anhydrous mercury(II) iodide.359 An X-ray diffraction study indicated a slightly distorted trigonal bipyramidal geometry for the copper(II) center in 175. The molecule features a long Cu(II)−C(carbene) distance (2.52(3) Å) and a uniquely weak Cu···Hg interaction, as indicated by a Cu−Hg distance of 3.513(2) Å. The Cu(II)− C(carbene) distances are significantly longer than those of the aforementioned five-coordinate (NHC)copper(II) iodide complex [(NHC)2Cu2I4]n (173) (1.923(2) and 1.989(2) Å).357 Willans and co-workers found that transmetalation of pyridine-functionalized NHC silver(I) complexes with copper(II) complexes featuring pyridine−NHC ligation is a reliable synthetic route to pyridine-functionalized NHC copper(II) complexes.360 Using this method, four- and five-coordinate copper(II) complexes bearing 1-(3-R-1-pyridyl)-3-allylimidazol-2-ylidenes, [(CN)2Cu2Br4] (176), [(CNR)CuBr2] (R = Me, 177; OMe, 178), and [(CNR)2CuBr][PF6] (R = H, 183; Me, 184), were prepared in high yields (Scheme 69). Further interaction of the dibromide complexes with coordinating solvent, for example, pyridine and DMSO, led to the formation of five-coordinate complexes [(CNR)CuBr2L] (179−182). The structures of these complexes were unambiguously established by single-crystal X-ray diffraction studies. In general, the copper(II) centers in the four-coordinate complexes display a distorted square-planar geometry, and the copper centers of the five-coordinate complexes exhibit distorted trigonal bipyramidal configuration. The Cu(II)− C(carbene) distances fall in the range 1.934(5)−2.014(8) Å, the longer of which being observed in 183 and 184. The strong trans effect of NHCs was thought to be the cause.

plenty. In terms of copper(II) complexes with neutral, chelating NHC ligands, Meyer’s [(BnTIMEN)Cu][OTf]2 (174),358 Liu’s ether-linked bis(NHC) complex [(C2O)CuI2HgI2] (175),359 Willans’ pyridine−NHC complexes,360 for example, [(CN)CuBr2]2 (176), [(CNMe)CuBr2] (177), and [(CN)2CuBr][PF6] (183), Singer’s bis(pyridine)−NHC complexes [(CN2)Cu(MeOH)2(SiF6)] (185) and [(CN2)Cu(Cl)(SO(CD3)2)][X] (186),361 and Long’s tetra(pyridine)− NHC complex [(PY4Im)Cu(MeCN)][PF6]2 (187)217 are representative examples, and Chart 17 depicts their structures. Meyer’s [(BnTIMEN)Cu][OTf]2 (174) complex can be prepared from either the interaction of the in situ-generated functionalized NHC ligands with copper(II) triflate or oxidation of a preformed copper(I) NHC complex by silver triflate or dioxygen.358 The g-values (g1 = 2.005, g2 = 2.060, g3 = 2.275), the characteristic copper hyperfine (53Cu, I = 3/2, 100%) coupling constant of 397 MHz, and magnetic moment (1.86 μB at room temperature) all suggest its copper(II) nature (Figure 23). As the molecular structure of 174 was not determined by XRD, the authors employed DFT calculations to probe the structural features of the tris(NHC)copper(II) species. It was found the copper(II) center in the simplified cation [(MeTIMEN)Cu]2+ has a trigonal geometry with different Cu(II)−C(carbene) distances (1.965, 1.936, and 1.935 Å) and C−Cu−C angles (139.7°, 112.5°, and 107.4°).

Figure 23. X-band EPR spectrum of 174 in a frozen acetonitrile/ toluene solution at 8 K. Experimental conditions: microwave frequency ν = 9.4666 GHz; power = 0.63 mW; modulation amplitude = 10 G. Simulated parameters: g1 = 2.005 (W1 = 20.00 G); g2 = 2.060 (W2 = 55.00 G); g3 = 2.275 (W3 = 42.00 G); A3 = 132 × 10−4 cm−1 (397.00 MHz). Adapted with permission from ref 358. Copyright 2003 American Chemical Society. AN


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copper(II) nature, the 1H NMR spectrum of 185 showed broad signals. A single-crystal X-ray diffraction study indicated that the Cu(II)−C(carbene) distances (1.915(3) and 1.932(2) Å, respectively) fall at the short end of those of the known copper(II) NHC complexes. The shorter Cu(II)−C(carbene) distance might be due to geometric constraints exerted by the NCN pincer ligand. Accordingly, the Cu(II)−C(carbene) bond in Long’s copper(II) complex [(PY4Im)Cu(MeCN)][PF6]2 (187) (1.889(4) Å) is even shorter, although the copper center is six-coordinate.217 In addition to these structurally characterized complexes, Scholz and co-workers reported the preparation of [(CNC)CuBr2] (188, CNC denotes 1,6-bis(3-methyl-imidazol-2-ylidene)pyridine) and [(NCN)CuBr2] (189, NCN denotes 1,3-bis(pyridin-2ylmethyl)imidazole-2-ylidene) from the reactions of pyridinefunctionalized NHC ligands with copper(II) dibromide.362 Despite the absence of structural data, the accessibility of the aforementioned copper(II) complexes bearing pyridine−NHC ligands lends support to their successful synthesis. Another important class of copper(II)−NHC complexes are those featuring anionic alkoxy- and amido-functionalized NHC ligands. Chart 18 lists representatives of this class of complex. The synthesis of these copper(II) NHC complexes can be classified into two routes: (i) the transmetalation between alkaline metal salts of the functionalized NHC ligands and copper(II) salts, and (ii) one-pot reactions of imidazolium salts with Ag2O and copper in air. Arnold’s alkoxy-functionalized NHC complexes [(R3COR1 R2)CuCl(THF)] (190) and [(R3COR1 R2)2Cu] (191−193) were obtained from the reaction of imidazolium salts with copper(II) salts in the presence of lithium hexamethyldisilazide (Scheme 71).363 Complex 190 was

Scheme 69. Synthetic Routes to Copper(II) Complexes Bearing Pyridine−NHC Chelating Ligands

The aforementioned copper(II) NHC complexes were generally prepared by transmetalation, the coordination of NHC ligands with copper(II) salts, or the oxidation of copper(I) NHC complexes. Thus, Singer’s preparation of the blue bis(pyridine)−NHC−copper(II) complexes [(CN2)Cu(MeOH)2(SiF6)] (185) and [(CN2)Cu(Cl)(SO(CD3)2)]X (186) from the direct interaction of 1,3-bis(pyridin-2ylmethyl)-1H-imidazolium chloride with copper(II) hexafluorosilicate is unusual (Scheme 70).361 The methanol ligand in

Scheme 71. Arnold’s Preparation of Alkoxy-Functionalized NHC Copper(II) Complexes 190−193

Scheme 70. Synthesis of the Copper(II) Complexes 185 and 186 Bearing a Bis(pyridine)−NHC Ligand

185 suggested its stability toward moisture. Indeed, the complex is air- and moisture-stable. Consistent with its Chart 18. Examples of Anionic Alkoxy- and Amido-Tethered NHC Ligands Used in the Preparation of Copper(II) Complexes

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nature as discerned from their paramagnetically broadened 1H NMR spectra. The molecular structures indicated that the copper centers in these dinuclear complexes have distorted square planar geometries, and that the aryloxyl groups function as bridging ligands. The lengths of the Cu(II)−C(carbene) bonds (1.926(8)−1.964(9) Å) are marginally shorter than those of the Cu(II)−C(carbene) bonds (1.99(3) Å) in Arnold’s four-coordinate alkoxyl−NHC copper(II) complexes 194,363 which might be due to the stronger electron-donating nature of imidazolidin-2-ylidenes over imidazolin-2-ylidenes.108,125,131 Notably, using the copper(II)−NHC complexes as catalysts, the catalytic allylic alkylation of organic zinc reagents with allylic phosphates afforded the corresponding products with high enantioselectivity (up to 98% ee) and yields (up to 98%) (Scheme 73). Whether a copper(II) alkyl or copper(I) alkyl species is involved in the catalytic process is currently not known.

isolated as a green solid, while 191−193 were found to be purple. The structures of these copper(II)−NHC complexes have not been confirmed by XRD. Fortunately, the bis(NHC) complexes are air stable, allowing observation of the parent ion [[(R3COR1 R2)2Cu + H]+ in their electrospray mass spectra, which supports their proposed composition. The axial X-band EPR spectra of 191−193, which feature clear 53Cu hyperfine coupling, are also characteristic of copper(II) species. Supporting the ability of the alkoxy-functionalized NHC ligand to stabilizing copper(II) species, the study also noted the isolation of tetranuclear complex 194 (Chart 19) from the Chart 19. Arnold’s Tetranuclear Copper(II) Complex 194

Scheme 73. Asymmetric Allylic Addition Catalyzed by the Aryloxy-Functionalized NHC Copper(II) Complexes 195 and 196

aerobic oxidation of the copper(I) species [Cu{OC(Ph)(CH2{1-C[NCHCHNBut]})2}]2. A single-crystal X-ray diffraction study indicated that the metal ions in 194 adopt a slightly distorted square planar geometry with an average Cu(II)−C(carbene) bond length of 1.99(3) Å. In addition to their synthesis and characterization, Arnold showed that these copper(II)−NHC complexes could catalyze the asymmetric conjugate addition of ZnEt2 to cyclohexenone. Hoveyda and co-workers developed highly efficient coppercatalyzed asymmetric allylic alkylation reactions using a combination of copper(II) salts and silver(I) complexes featuring chiral aryloxy-functionalized NHC ligands as catalysts.364,365 Their examination of the reactions of the silver(I) NHC complex with CuCl2·2H2O led to the preparation of the dark red dinuclear copper(II) complexes 195 and 196, whose structures were established by XRD (Scheme 72). The copper(II) complexes are paramagnetic in

Transmetalation has also been applied in the synthesis of a bis(alkoxy-NHC)copper(II) complex. Charette and co-workers reported the preparation of the bis(imidato-NHC)copper complex 197 from the reaction of a dimeric silver(I) benzoylimino−NHC complex with CuCl2·2H2O at room temperature (Scheme 74).366 Complex 197 was isolated as Scheme 74. Transmetallation Reaction of a Benzoylimino− NHC−Silver(I) Complex with Copper(II) Dichloride

Scheme 72. Synthetic Route to Hoveyda’s AryloxyFunctionalized NHC Copper(II) Complexes 195 and 196

an air-stable green solid, which was characterized by mass spectroscopy and a single-crystal X-ray diffraction study. The molecular structure in the solid state clearly showed a distorted square planar geometry for the copper(II) center, which is coordinated by two imidato-NHC ligands with Cu(II)− C(carbene) and Cu(II)−O distances of 1.945(4) and 1.950(3) Å, respectively. The establishment of the structure of 197 gives credence to Arnold’s bis(alkoxy-NHC)copper(II) complexes 191−193,363 as well as Boydston and Bielawski’s polymeric bis(aryloxy-NHC)copper(II) complex 198, prepared from the interaction of the bis(imidazolium) salt with copper(II) dichloride and base (Scheme 75).367 AP


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different structures of the resultant copper(II) NHC complexes may be associated with the different steric features of these multidentate ligands. Intriguingly, further studies by the group showed that the dicopper(II) hydroxides 199−201 could also be synthesized by electrochemical methods in acetonitrile, whereby a copper plate was used as the sacrificial anode, a platinum electrode acted as the cathode, and the imidazolium salts play the dual role of ligand source and electrolyte.369 Along with these interesting synthetic methods, these copper(II)−NHC complexes were found to be efficient catalysts for the N-arylation of imidazoles and anilines by phenylboronic acid.368 The pyrazolato-bridged copper(II) NHC complexes 199− 202 contain Cu(II)−N(pyrazolato) bonds that could conceivably render the complexes sensitive to water. However, these complexes are air- and moisture-stable, which may be due to the enforcement of their rigid di- and tetra-nuclear structures. In contrast to Chen’s copper(II)−NHC complexes, the mononuclear copper(II) complexes supported by bis(NHC)−carbazolido pincer ligands were found to be air- and moisture-sensitive. Bezuidenhout and Bertrand developed an anionic CNC-tridentate ligand featuring two 1,2,3-triazol-5ylidenes and a central carbazolido group, and synthesized the copper(II) complex [(triazCNC)CuCl] (203) from the reaction of the bis(1,2,3-triazolium)carbazole with KN(SiMe3)2 and CuCl2 (Scheme 77).370 Complex 203 represents a rare example of amido copper(II) NHC complexes. Its molecular structure exhibits a seesaw geometry for the four-coordinate copper center with a Cl−Cu−N angle of 123.53(6)° and Cu(II)−C(carbene) and Cu(II)−N distances of 1.983(3) and 1.956(3) Å, respectively (Figure 24). The nonplanar geometry might result from the combination of the trans effect of the carbazolido ligand and the steric repulsion between the chloride ligand and the two vicinal Dipp substituents. An attempt to prepare copper(II) hydride species by treating 203

Scheme 75. Preparation of the Polymeric Bis(aryloxyNHC)copper(II) Complex 198

While transmetalation reactions dominate the syntheses of alkoxyl−NHC copper(II) complexes, the preparation of pyrazolato-bridged copper(II) NHC complexes by Chen, employing imidazolium salts, silver oxide, and copper powder, is unusual and highly practical (Scheme 76).368 With this method, copper(II) hydroxide complexes bearing pyrazolatobridged bis(pyridyl/picolyl−NHC) ligands, 199−201, and a dicopper(II) complex bearing pyrazolato-bridged bis(thiophenyl−NHC) ligands, 202, were obtained in 47−73% isolated yields as red solids. The structures of these copper(II) NHC complexes were confirmed by single-crystal X-ray diffraction studies. Their Cu(II)−C(carbene) distances are in the range 1.911(5)−1.989(5) Å. The Cu(II)−N(pyrazolato) distances are between 1.95 and 1.98 Å, being slightly shorter than the Cu(II)−N(pyridine) bonds (2.02−2.12 Å). The

Scheme 76. Synthesis of Chen’s Copper(II) Complexes Bearing N-Donor-Functionalized NHC Ligands

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Scheme 77. Bezuidenhout and Bertrand’s Copper(II) Carbene Complex

the interatomic distances and angles around the copper centers in 203 and 204. The lengths of the Cu(II)−C(carbene) bonds are typical of copper(II)−NHC complexes and longer than those of copper(I)−NHC complexes (1.90−1.93 Å). The key bond distances and angles of the pincer fragments in 205 are similar to those of 204. Complex 204 was also characterized by EPR spectroscopy. Its axial EPR spectrum (g1 = g2 = 2.0357, g3 = 2.1285) showed hyperfine coupling to both the copper (63/65Cu: A1 = A2 = 5.3 G, A3 = 86.9 G) and the nitrogen nuclei (14N: A1 = A2 = 17.5 G, A3 = 30.9 G), implying the distribution of the unpaired spin on both copper and nitrogen atoms. Indeed, DFT calculations revealed that both the copper d(x2− y2) orbital and a carbazolide nitrogen orbital contribute to the SOMO. This electronic feature implies the high reactivity of copper(II) amido complexes. Consequently, one could speculate that the chelating effect of the pincer ligand and the strong electron-donating nature of NHCs must contribute to the stabilization of 203−205. Along with the successful preparation of copper(II)−NHC complexes, there has also been much interest in stabilizing copper(III) species with NHC ligands. Unfortunately, synthetic efforts have thus far been unsuccessful, and our current understanding of these complexes has been gained mainly from theoretical studies. (NHC)copper(III) intermediates have been proposed in the oxidation reactions of some (NHC)copper(I) compounds. For instance, Stack and coworkers examined the reactions of (NHC)copper(I) halides with different oxidants, and noticed that no observable reaction occurred when (IPr)CuCl was treated with [Ph2I][PF6] or [Cp2Fe][PF6]. However, reactions of (IPr)CuX (X = Cl, Br, I) with Selectfluor or Cu(OTf)2 led to the formation of 2haloimidazolium salts (Scheme 79).150 The authors performed DFT calculations to probe the mechanism of the reaction with

Figure 24. Molecular structure of 203. Solvent molecules and hydrogen atoms are omitted for clarity. The figure was generated on the basis of a cif file reported in ref 370.

with LiBHEt3 led to the isolation of the T-shaped copper(I) species [(triazCNC)Cu]. In analogy to complex 203, bis(1-methyl-imidazl-2-ylidene)carbazolido copper(II) complexes [(diazCNC)CuCl] (204) and [(diazCNC)2Cu2Cl][CuCl2] (205) were reported by Kunz et al. (Scheme 78).371 The red complex 204 was prepared in Scheme 78. Kunz’s Copper(II) Carbene Complexes

Scheme 79. Formation of 2-Haloimidazolium Salts in the Oxidation Reactions of (NHC)Copper(I) Complexes and a Calculated Pathway for Their Formation

42% isolated yield from the reaction of CuCl2 with the in situgenerated bis(1-methyl-imidazl-2-ylidene)carbazolido ligand, and 205 was obtained as green crystals from the reaction of CuCl with the bis(1-methyl-imidazol-2-ylidene)carbazolido ligand. The formation of 205 was unexpected. Aside from its crystal structure, no further characterization data were reported. Similar to 203, the structure of 204 also shows a seesaw geometry at Cu. Its Cu(II)−C(carbene) and Cu(II)−N distances are 1.986 and 1.904 Å, respectively. As the steric nature of the NHC moieties in 203 and 204 is different, one should refrained from drawing conclusions about the electronic nature of the two types of NHC ligands merely on the basis of AR


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of bis(imidazolium) salts in the reactions of (1-pyridinylimidazol-2-ylidene)silver(I) bromide with copper(II) dibromide (Scheme 81).374 Willans et al. found that the equimolar

Selectfluor, which suggested a reductive elimination pathway from (NHC)copper(III) species to form 2-haloimidazolium salts. As shown in Scheme 74, upon interaction with Selectfluor, the (NHC)copper(I) species (IPr)CuCl is oxidized into a (NHC)copper(III) intermediate [(IPr)CuClF]+ (206). The (NHC)copper(III) species then coordinates an acetonitrile molecule to form four-coordinate species [(IPr)CuClF(NCMe)]+ (207), which then undergoes C−Cl bond-forming reductive elimination to give the chloroimidazolium cation and copper(I) species. The overall reaction is exothermic with a free energy change of −54.2 kcal mol−1. The low activation barrier of the copper(III)/copper(I) reductive elimination step (3.5 kcal mol−1) suggests the feasibility of the calculated route. In contrast to this route, the reductive elimination from the (NHC)copper(II) species [(IPr)CuCl(NCMe)2]+ to give the 2-chloroimidazolium cation and copper(0) species is endothermic (ΔG = 44.1 kcal/mol). The occurrence of the reductive elimination reaction was attributed to the electrophilic nature of the copper(III) center, which renders the C(carbene) atom susceptible to nucleophilic attack, as suggested by the close C(carbene)−Cl separation (ca. 2.7 Å) and the orbital interactions between the lone pair on chloride and the pπ orbital of C(carbene) (Scheme 79). In parallel with Stack’s finding, a study by Willans, Ariafard, and Fairlamb on the reactions of a (NHC)copper(I) bromide complex with iodobenzene, affording a 1-phenyl-imidazolium salt, also suggests the involvement of a (NHC)copper(III) intermediate of the form [cis-(NHC)CuBrPhI] (208) (Scheme 80).372 As an explanation for the selective formation of 2-

Scheme 81. Reactions of Silver(I)−NHC Complexes with Copper(II) Dibromide and Possible Copper(III)−NHC Intermediates 210 and 211 Leading to the Formation of the Imidazolium Salts

Scheme 80. Formation of 2-Phenylimidazolium Salts in the Reactions of (NHC)Copper(I) Complexes with PhI

reaction of (1-pyridinyl-3-allylimidazol-2-ylidene)silver(I) bromide with copper(II) dibromide produced the copper(II)− NHC complex 176 in good yield, whereas changing the molar ratio into 1:2 suppressed the isolation of 176 and resulted in the formation of the 2-bromo-1-pyridinyl-3-allylimidazolium salt (Scheme 81). More intriguingly, the reaction of a 3mesityl-substituted NHC silver(I) complex with 2 equiv of copper(II) bromide led to the formation of a bis(imidazolium) salt (Scheme 76). The authors performed detailed theoretical studies to gain insights into the mechanism of this reaction.374 It was found that the 2-bromoimidazolium salt may originate from the C(carbene)−Br bond-forming reductive elimination reaction of a dinuclear copper(III)−copper(I) species [(CN)CuBr2(μBr)CuBr] (210 in Scheme 81), which can be viewed as the product of the redox disproportionation of a dicopper(II) species. As an experimental study indicated that the 2-bromo1-pyridyl-2-mesitylimidazolium salt could also react with the copper(I)−NHC complex (CN)CuBr to form the bis(imidazolium) salt (Scheme 82), it was suggested that the bis(imidazolium) salt may originate from the C(carbene)−

phenylimidazolium salts instead of 2-haloimidazolium salts in the experiments, computational studies revealed that C−I and C−Br reductive elimination via the copper(III) intermediate [trans-(NHC)CuBrPhI] (209) is energetically disfavored.372 The reluctance of the (NHC)copper(III) species 209 to undergo C−X reductive elimination forms a stark contrast to the facile reductive elimination from 207. This difference could be due to the different electronic nature of the ancillary ligand, as a study by Willans, Ariafard, and Fairlamb on the C(carbene)−Br reductive elimination from the copper(III) species [trans-(IMe2H2)CuBr2X]+ (210, X = benzyl, Me, vinyl, Ph, CF3, CN, I, F, Br, and Cl) indicated that the stronger σdonating nature of the X ligands results in higher activation energies of the reductive elimination step.373 In addition to the aforementioned studies, copper(III) species were also implicated as intermediates in the formation

Scheme 82. Reaction Accounting for the Formation of a Bisimidazolium Salt

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tion-state 3d metal hydrocarbyl NHC species have been proposed as the key intermediates in some of these reactions. Examples include the proposed (NHC)iron(III) aryl species in the iron-NHC-catalyzed cross-coupling of alkyl halides with aryl Grignard reagents,227 the iron(IV)−NHC intermediates in Nakamura’s iron−NHC-catalyzed cross-coupling of aryl chlorides with aryl Grignard reagents,225 the high-oxidationstate cobalt species in cobalt−NHC-catalyzed C−H bond functionalization reactions,247,309−311 and the (NHC)nickel(III) species in nickel−NHC-catalyzed C−C and C−N bondforming reactions.315−317,321 The validation of the proposed catalytic cycles and the further development of new 3d metal catalysts with higher catalytic efficiency will rely on further exploration of high-oxidation-state 3d metal chemistry. The aforementioned studies have demonstrated the great progress made in the chemistry of high-oxidation-state 3d metal NHC complexes with imido and nitrido ligands. However, the progress made in high-oxidation-state late 3d metal NHC complexes with alkylidene and oxo ligands is rather limited. High-oxidation-state late 3d metal alkylidene species could potentially form the basis of alternatives to Schrock and Grubbs’ olefin metathesis catalysts.375,376 However, synthetic attempts to realize these goals were unsuccessful.246,377,378 As for high-oxidation-state late 3d metal oxo complexes with NHC ligands, Meyer’s iron(IV) oxo complex supported by a tetra(NHC) ligand is still the only successful example.140 When considering the nature of the metal center, high-oxidation-state nickel and copper NHC complexes featuring alkylidene, imido, oxo, or nitrido ligands have also remained elusive. The causes of the apparent inability to access these high-oxidation-state 3d metal NHC species are difficult to determine. As a transition metal complex with more than five d electrons in a tetragonal geometry will have difficulties maintaining metal−ligand multiple bonds, a phenomenon referred to as the “oxo wall”,379 it has been reasoned that reducing the number of ancillary ligands might allow the oxo wall to be crossed. Betley et al. pointed out that the application of a tetrahedral ligand field to metal−oxo complexes could lower the energy level of the d(x2−y2) and d(xy) orbitals to become nonbonding, and may allow the accommodation of four electrons in a low-spin configuration before the metal’s π* and σ* orbitals are populated.380 Holland found that an iron(III) imido in a trigonal planar ligand field has more π-bonding than the π-bonding in an octahedral iron(IV) oxo species.90 The utilization of this protocol has led to the successful preparation of isolable cobalt(IV) and cobalt(V) imido species.139 Thus, its validation for the stabilization of analogous high-oxidation-state late 3d metal species with metal−ligand multiple bonds deserves further exploration. In addition to the open questions related to their synthetic chemistry, our limited knowledge of the chemical and physical properties of high-oxidation-state 3d metal NHC complexes also urges further exploration. As 3d metal species, these complexes embody the features of high density-of-state.57−60 The high oxidation state of the metal center may induce charge-transfer between metal and ligand.47 The combination of these two features could endow high-oxidation-state 3d metal NHC complexes rich magnetic and photochemical properties. While known explorations of this possibility are limited, Wärnmark’s study of the photoluminescence of homoleptic iron(III) NHC complexes205 and Smith’s exploration of the single-molecule-magnet properties of tri(NHC)-

C(carbene) bond-forming reductive elimination from the mononuclear copper(III) species [(CN)2CuBr2]+ (211). It was proposed that π−π interactions between the mesityl and pyridine substituents in 211 contribute to its transient stabilization. Hence, the lack of similar π−π interactions in the allyl-substituted NHC complexes would make the formation of the bis(NHC)copper(III) species difficult, which explains the absence of the bis(imidazolium) salt in the reaction of the allyl-substituted NHC silver(I) complex with copper(II) dibromide. The diverse C−C and C−X reductive elimination products obtained from these reactions demonstrate the potential synthetic utility of copper(III)− NHC species. On the other hand, it also reflects the challenges inherent in pursuing this type of high-oxidation-state 3d metal complex and urges further synthetic efforts.

4. SUMMARY AND PERSPECTIVE As is elaborated in the above sections, NHCs have proved to be useful ligands for the stabilization of high-oxidation-state 3d metal complexes. The ability of NHCs to stabilize highoxidation-state 3d metal complexes was demonstrated by the large number of such complexes reported in the literature, including examples based on titanium(IV), vanadium(IV,V), chromium(IV,V), manganese(IV,V), iron(III,IV,V), cobalt(III,IV,V), nickel(IV), and copper(II). These complexes exhibit large structural diversity as reflected by the existence of both homoleptic and heteroleptic complexes. For the latter type, the diversity is further enriched by the wide range of ancillary ligands, including halides, carboxylates, alkoxides, amides, imides, nitride, peroxide, oxide, and many more. This structural diversity also benefits from the convenient accessibility of versatile donor-functionalized NHC ligands. Listed in Chart 2 are the representatives employed in highoxidation-state 3d metal chemistry. The availability of these high-oxidation-state 3d metal NHC complexes then opens a new door to probing high-oxidation-state metal chemistry. Pertinent to this, the reported studies have shown that the magnetic, photoluminescence, and redox properties, as well as the group-transfer reactivity, of these oxo, imido, and nitrido complexes can be different from those of analogous complexes supported by oxygen, nitrogen, and/or phosphorus ligands. Again, the strongly σ-donating property of NHCs, and their sterically demanding nature, are likely among the key factors contributing to these unique properties. Along with these developments, it should be stated that the exploration of the chemistry of high-oxidation-state 3d metal NHC species is still in its infancy, and many open questions remain. From the view of synthetic chemistry, it should be noted that the highest oxidation states of the metal centers of known middle to late 3d transition metal NHC species are generally lower than the highest oxidation states known for the respective metal. For example, so far no isolable NHC complexes of chromium(VI), manganese(VI,VII), iron(VI), or copper(III) have been reported. Considering the reducing power of NHCs and their potential to undergo C(carbene)−X bond-forming reductive elimination in high-oxidation metal species, the preparation of very-high-oxidation-state 3d metal species such as those mentioned above will likely be challenging and may require more judicious ligand design. Equally interesting are the higher-oxidation-state 3d metal NHC complexes featuring hydrocarbyl ligands, particularly those of the late 3d metals. NHCs have been widely applied in 3d-metal-catalyzed cross-coupling reactions, and high-oxidaAT


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borate manganese(IV) nitrido species199 have highlighted the prospects of high-oxidation-state 3d metal NHC complexes as new, functional coordination compounds. From their rich electronic structures, one could assume the presence of similarly rich chemical properties. While the reported C−H activation and group-transfer reactions of manganese, iron, and cobalt imido and nitrido species have shown the tip of the iceberg,137−139,196−199,241,243−246,259,262−265,309,310,315,316 other attractive reactivity patterns of 3d metal−ligand multiple bonds beckon, for example, olefin metathesis and inert C−H bond insertion. The development of the untapped reactivity and vast potential catalytic applications of these complexes will continue to motivate research in this fascinating field.

Journals Award (2013), the ACS Organometallics Young Investigator Fellows program (2014), and the National Science Fund for Distinguished Young Scholars of China (2017). He is a member of the editorial boards of Chinese Chemical Letters, Chinese Journal of Chemistry, Chinese Journal of Organic Chemistry, Organometallics, and Inorganic Chemistry.

ACKNOWLEDGMENTS We thank the financial support from the National Key Research and Development Program (2016YFA0202900), the National Natural Science Foundation of China (nos. 21725104, 21690062, and 21432001), and the Strategic Priority Research Program of the Chinese Academy of Sciences (no. XDB20000000).

ASSOCIATED CONTENT S Supporting Information *

ABBREVIATIONS acac acetylacetonate Ac acetyl Ad adamantyl AIBN azodiisobutyronitrile AOC average-of-configuration ArF 3,5-di(trifluoromethyl)phenyl ATRP atom transfer radical polymerization av average BDFE bond dissociation free energy Bn benzyl bpy 2,2′-bipyridine BTMABr3 benzyltrimethylammonium tribromide btz 3,3′-dimethyl-1,1′-bis(p-tolyl)-4,4′-bis(1,2,3-triazol-5-ylidene) Bun n-butyl But tert-butyl ca. circa (latin) cAAC cyclic (alkyl)(amino)carbene cot cyclooctene Cp cyclopentadienyl Cp* C5Me5 [pentamethylcyclopentadienyl] cyclam 1,4,8,11-tetraazacyclotetradecane Da dalton DFT density functional theory Dipp 2,6-diisopropylphenyl Dipp CNC 2,6-bis(2,6-diisopropylphenylimidazol-2-ylidene)pyridine DMSO dimethyl sulfoxide dvtms divinyltetramethyldisiloxane EDTA ethylenediamine tetraacetic acid EBrPA ethyl 2-bromo-2-phenylacetate EPR electron paramagnetic resonance equiv equivalent Et ethyl Et2-cAAC 3 , 3 - d i e t h y l - 5 , 5 - d i m e t h y l - 1 - ( 2 ′ , 6 ′ diisopropylphenyl)pyrrolidin-2-ylidene Fc ferrocene G Gauss HAA hydrogen atom abstraction HBPin 4,4,5,5-tetramethyl-1,3,2-dioxaborolane HMF 5-hydroxymethyl-2-furfural HOMO highest occupied molecular orbital Hpycan N-(2-nitrophenyl)picolinamide H3buea tris[(N′-tert-butylureayl)-N-ethylene]amine ICAR initiators for continuous activator regeneration Im imidazole

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.chemrev.8b00096. Table of characterization data of selected high-oxidationstate 3d metal complexes with NHC ligation (DOCX)

AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected]. ORCID

Liang Deng: 0000-0002-0964-9426 Notes

The authors declare no competing financial interest. Biographies Jun Cheng was born in Anhui province, China, in 1990. He received his B.S. degree from Anhui Normal University in 2013. He subsequently joined Prof. Liang Deng’s laboratory at Shanghai Institute of Organic Chemistry (SIOC) to pursue his Ph.D. degree. His research focuses on the synthesis and reactivity of two- and threecoordinate 3d metal imido complexes with NHC ligation. Lijun Wang was born in Shanxi province, China, in 1993. She received her B.S. degree from Natong University in 2014. She then joined Prof. Liang Deng’s laboratory at SIOC to pursue her Ph.D. degree. Her research focuses on the synthesis and reactivity of open-shell organo− iron complexes with NHC ligation. Peng Wang was born in Hubei province, China, in 1993. He received his B.S. degree from Beijing University of Chemical Technology in 2016. He then joined Prof. Liang Deng’s laboratory at SIOC to pursue his Ph.D. Degree. His research focuses on iron imido species supported by arsenic and antimony ligands. Liang Deng was born in Hunan Province, China, in 1980. He received his B.S. degree from Peking University in 2002 under the supervision of Prof. Zhenfeng Xi, and his Ph.D. degree from the Chinese University of Hong Kong in 2006 under the supervision of Prof. Zuowei Xie. After working as a postdoctoral fellow in the groups of Prof. Zuowei Xie at the Chinese University of Hong Kong and Prof. R. H. Holm at Harvard University, he joined the faculty of SIOC in 2009, where he is currently a professor. His research interests are focused on the synthetic, structural, and reactivity aspects of 3d metal chemistry. He has been honored by the Hundred Talents Program of the Chinese Academy of Sciences (2010), the National Science Fund for Excellent Young Scholars of China (2012), the Thieme Chemistry AU


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IMes IPr IPr2Me2 IR LMCT LUMO MALDI MAO MCD Me Me2-cAAC

1,3-bis(2,4,6-trimethylphenyl)imidazol-2-ylidene 1,3-bis(2,6-diisopropylphenyl)imidazol-2-ylidene 1,3-diisopropyl-4,5-dimethylimidazol-2-ylidene infrared ligand-to-metal charge-transfer lowest unoccupied molecular orbital matrix-assisted laser desorption/ionization methylaluminoxane magnetic circular dichroism methyl 3,3,5,5-tetramethyl-1-(2′,6′-diisopropylphenyl)pyrrolidin-2-ylidene Mes mesityl MMA methyl methacrylate nacnac β-diketiminate NHC N-heterocyclic carbene NHE normal hydrogen electrode NMO 4-methylmorpholine N-oxide NMR nuclear magnetic resonance oorthopparaPCET proton-coupled electron transfer PE polyethylene Ph phenyl PMMA polyMMA Por porphyrin Pri isopropyl PS polystyrene p-tolyl 4-methylphenyl py pyridine pyrr2py bis(pyrrolyl)pyridine PY4Im 1,3-bis(bis(2-pyridyl)methyl)imidazol-2-ylidene pz pyrazolyl ROP ring-opening polymerization SCE saturated calomel electrode solv solvent SOMO singly occupied molecular orbital SQUID superconducting quantum interference device TAML tetraamido macrocyclic ligand TBHP tert-butyl hydroperoxide TD-DFT time-dependent density functional theory TEMPO (2,2,6,6-tetramethylpiperidin-1-yl)oxyl TEMPO-H 1-hydroxy-2,2,6,6-tetramethylpiperidine Tf trifluoromethylsulfonyl THF tetrahydrofuran TMG3tren 1,1,1-tris{2-[N2-(1,1,3,3-tetramethylguanidino)]ethyl}amine TOF turnover frequency TON turnover number TPA tris(2-pyridylmethyl)amine TPP 5,10,15,20-tetraphenylporphyrin UV ultraviolet vis visible ν frequency XAS X-ray absorption spectra XES X-ray emission spectra XRD X-ray diffraction Xyl 2,6-dimethylphenyl δ isomer shifts

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