Mesoionic and Related Less Heteroatom ... - ACS Publications

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

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Mesoionic and Related Less Heteroatom-Stabilized N‑Heterocyclic Carbene Complexes: Synthesis, Catalysis, and Other Applications Á ngela Vivancos,†,‡ Candela Segarra,†,§ and Martin Albrecht*,† †

Department of Chemistry and Biochemistry, University of Bern, Freiestrasse 3, CH-3012 Bern, Switzerland Departamento de Química Inorgánica, Universidad de Murcia, Apartado 4021, 30071 Murcia, Spain § Instituto de Tecnología Química, Universitat Politècnica de València-Consejo Superior de Investigaciones Científicas, Avenida de los Naranjos s/n, 46022 Valencia, Spain Downloaded via UNIV OF SOUTH DAKOTA on July 17, 2018 at 13:44:27 (UTC). See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles.

‡

ABSTRACT: Mesoionic carbenes are a subclass of the family of N-heterocyclic carbenes that generally feature less heteroatom stabilization of the carbenic carbon and hence impart specific donor properties and reactivity schemes when coordinated to a transition metal. Therefore, mesoionic carbenes and their complexes have attracted considerable attention both from a fundamental point of view as well as for application in catalysis and beyond. As a follow-up of an earlier Chemical Reviews overview from 2009, the organometallic chemistry of N-heterocyclic carbenes with reduced heteroatom stabilization is compiled for the 2008− 2017 period, including specifically the chemistry of complexes containing 1,2,3triazolylidenes, 4-imidazolylidenes, and related 5-membered N-heterocyclic carbenes with reduced heteratom stabilization such as (is)oxazolylidenes, pyrrazolylidenes, and thiazolylidenes, as well as pyridylidenes as 6-membered N-heterocyclic carbenes with reduced heteroatom stabilization. For each ligand subclass, metalation strategies, electronic and steric properties, and applications, in particular, in metal-mediated catalysis, are compiled. Mesoionic carbenes demonstrate particularly high activity in (water) oxidation, hydrogen transfer reactions, and cyclization reactions. Unique features of these ligands are identified such as their dipolar structure, their specific donor properties, as well as stability aspects of the ligand and the complexes, which provides opportunities for further research.

CONTENTS 1. Introduction 2. Complexes with 1,2,3-Triazolylidene Ligands 2.1. General Aspects 2.2. Synthesis of Triazolylidene Metal Complexes 2.2.1. Transmetalation 2.2.2. Free Carbene Route 2.2.3. Direct Triazolium Metalation 2.2.4. Ligand Postmodification 2.2.5. Other Methods 2.3. Properties and Reactivity 2.3.1. Donor Properties 2.3.2. Structural Features 2.3.3. Triazolylidene Chelation 2.3.4. Stability and Reactivity 2.4. Catalytic Applications 2.4.1. General Overview 2.4.2. Triazolylidene Palladium Catalysis 2.4.3. Triazolylidene Iridium Catalysis 2.4.4. Triazolylidene Ruthenium and Osmium Catalysis 2.4.5. Triazolylidene Rhodium Catalysis 2.4.6. Triazolylidene Gold and Silver Catalysis 2.4.7. Triazolylidene Copper Catalysis 2.4.8. Catalysis with Triazolylidene Complexes of Fe, Co, Ni, and Mo © XXXX American Chemical Society

2.5. Other Applications 2.5.1. Photophysical Applications 2.5.2. Biological and Bioinspired Applications 3. Complexes with Mesoionic Imidazolylidene Ligands 3.1. General Aspects 3.2. Synthesis of Mesoionic Imidazolylidene Metal Complexes 3.2.1. Transmetalation 3.2.2. Free Carbene Route 3.2.3. Isomerization from Free 2-Imidazolylidene Ligands 3.2.4. Direct Imidazolium Metalation 3.2.5. Oxidative Addition 3.2.6. Mesoionic Carbene Formation in the Metal Coordination Sphere 3.2.7. Other Methods 3.3. Properties and Reactivity 3.3.1. Electronic and Steric Properties 3.3.2. Reactivity 3.4. Catalytic Applications 3.4.1. Imidazolylidene Palladium Catalysis

B C C C D G K N O O O R T U W W X AB AG AJ AK AM

AP AP AR AR AR AS AS AS AT AV BC BC BC BD BD BF BF BF

Special Issue: Carbene Chemistry Received: March 6, 2018

AO A

DOI: 10.1021/acs.chemrev.8b00148 Chem. Rev. XXXX, XXX, XXX−XXX

Chemical Reviews 3.4.2. 3.4.3. 3.4.4. 3.4.5.

Imidazolylidene Iridium Catalysis Imidazolylidene Rhodium Catalysis Imidazolylidene Ruthenium Catalysis Imidazolylidene Catalysis with Miscellaneous Metals 3.5. Other Applications 4. Complexes with Other 5-Membered N-Heterocyclic Carbene Ligands 4.1. Pyrazolylidene Complexes 4.1.1. Metalation Procedures 4.1.2. Properties and Reactivity 4.1.3. Catalytic Applications 4.2. Oxazolylidene and Thiazolylidene Complexes 4.3. Isoxazolylidene Complexes 4.4. Tetrazolylidene Complexes 5. Complexes with Pyridylidenes and Related Ligands 5.1. General Aspects 5.2. Metalation Procedures 5.2.1. C−H Bond Activation 5.2.2. Oxidative Addition 5.2.3. Miscellaneous Methods 5.3. Donor Properties and Reactivity 5.3.1. Donor Properties 5.3.2. Stability toward Acids and Bases 5.3.3. Stoichiometric Bond Activation 5.4. Catalytic Applications 5.5. Other Applications 6. Conclusions Author Information Corresponding Author ORCID Notes Biographies Acknowledgments References

Review

BH BI BJ BK BL BL BL BL BN BO

Figure 1. Generic representation of Arduengo-type 2-imidazolylidene complexes (A) and variations with less heteroatom stabilization of the carbenic site, including 5-membered (B−E) and 6-membered heterocycles (F−H) as well as remote heterocycles (I). Variations comprising a sulfur or oxygen atom instead of a substituted nitrogen (such as (is)oxazole- and thiazole-derived systems) constitute an extension of these carbene ligands.

BO BP BP BR BR BR BR BW BX BY BY BZ CA CB CC CC CD CD CD CD CD CE CE

stabilization by heteroatoms (C−I). The less direct influence of the heteroatoms in these carbenes generally induces a higher pKa of the proton at the precarbenic site,7,8 with obvious consequences for carbene formation and the stability of the free carbene. When coordinated to a metal center, this lower heteroatom stabilization induces a higher basicity of the ligand and hence stronger donor properties. Due to the large variability of N-heterocyclic scaffolds with less heteroatom stabilization, these donor properties and accordingly the reactivity of the coordinated metal center can be varied over a large range. These factors and the new synthetic opportunities that arise from the presence of a carbon as opposed to nitrogen nuclei adjacent to the carbenic site are arguably the key drivers that have promoted and continue to promote research into these less heteroatom-stabilized carbene complexes. Different nomenclatures have been used to describe carbenes with less heteroatom stabilization. Imidazolylidenes bound via C4/5 rather than C2 were initially termed abnormal carbenes,9 as they were not binding through their normal position. Later, this definition has been expanded to any Nheterocyclic carbene that cannot be represented by a neutral uncharged covalent structure, thus also including, for example, 3-pyridylidenes and 4-pyrazolylidenes.10 As a common feature, these “abnormal” carbene complexes are represented by a dipolar 5- or 6-membered heterocycle in which the positive and negative charge are at least partially delocalized and which cannot be represented by a single resonance form. These properties are congruent with the IUPAC definition of mesoionic compounds,11 and therefore, these carbenes should be termed mesoionic carbenes (MICs). Historical factors as well as the parallels to normal carbenes have prevented a radical change of nomenclature.12,13 For example, mesoionic and “normal” 1,2,3-triazolylidene complexes do not differ in properties or reactivity.14 Another type of division of NHCs is focusing on the presence or absence of heteroatoms adjacent to the carbene center. Thus, N-heterocyclic carbenes without any directly carbene-linked heteratom are termed remote carbenes, such as 4-pyrazolylidenes or 4-pyridylidenes. Obviously, this division of (none) vs remote carbenes is orthogonal to that of (none) vs mesoionic/abnormal carbenes, and there are remote normal as well as remote abnormal NHCs and also nonremote normal and abnormal/mesoionic carbenes (Figure 2).

1. INTRODUCTION One of the major transformations in organometallic chemistry and homogeneous catalysis of the last decades has emerged from the use of stable carbenes1 and, in particular, Nheterocyclic carbenes (NHCs)2 as ligands for transition metals.3−6 Their facile availability, synthetic flexibility, and beneficial impact on (catalytic) reactivity of the metal center have been key parameters that have continued to stimulate research in carbene metal complexes and has disclosed highly diverse fields of applications. Initially, synthetic variations have predominantly focused on modifications of so-called Arduengo-type carbenes,2 i.e., NHCs based on the 2-imidazolylidene scaffold (A, Figure 1), either by modifying the substituents at the two nitrogen atoms (wingtip modification) or by modulating the backbone, for example, by partial saturation (imidazolines) or conjugation (benzimidazolederived carbenes). Despite precedents in the literature from the 1980s and 1990s, metal carbene chemistry focusing on modulation of the actual heterocycle has become popular only at the beginning of this century through investigation of different bonding patterns of Arduengo carbenes (4- and 5-imidazolylidenes as opposed to 2-imidazolylidenes, B in Figure 1) and different heterocyclic scaffolds that provide carbene sites with less extensive B


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Figure 2. Selection of normal and abnormal/mesoionic and remote/ nonremote carbenes.

Figure 3. Number of publications of triazolylidene metal complexes per metal (2008−2017).

This review covers all NHC complexes that contain remote NHCs as well as nonremote carbenes that are stabilized by only one heteroatom adjacent to the carbenic site, except for (amino)(alkyl)carbenes and (amino)(aryl)carbenes, which have been reviewed recently and are included elsewhere in this themed issue.15,16 The different NHC ligands with one or zero heteroatom directly bound to the carbenic site is compiled under the term “less heteroatom-stabilized NHC ligands”. This focus also excludes phosphorus−heterocyclic carbenes (PHCs)17 and all carbocyclic carbenes.18 As a followup of an earlier review,19 particular emphasis is directed here toward the literature that has appeared in the 2008−2017 time span, and earlier work is only considered when required to place novel results into context. For the origins of various developments in the field of less heteroatom-stabilized NHC complexes, the reader is referred to the earlier review.19 The material in this review has been arranged by ligand class and is sorted by the number of publications that appeared over the last 10 years. Accordingly, triazolylidene complexes are surveyed first followed by complexes of imidazolylidenes, miscellaneous 5-membered NHC scaffolds such as pyrazolylidenes and oxazolylidenes and concludes with pyridylidenetype complexes. Each of these sections describes first the different synthetic strategies available to synthesize the corresponding NHC complexes, second the reactivity and specific properties imparted by the NHC ligand, and then catalytic and other applications of the complexes. Some aspects have been reviewed recently with a more specific focus, including reviews on triazolylidene complexes20−24 and abnormal/mesoionic palladium complexes.25

materials chemistry to use earth-abundant base metals as active centers has only started to involve triazolylidene chemistry, with notable achievements in catalysis for copper, nickel, and cobalt, (see sections 2.4.7 and 2.4.8) and photochemical applications of triazolylidene iron complexes (see section 2.5).26 2.2. Synthesis of Triazolylidene Metal Complexes

Generally, the ligand precursor of triazolylidene complexes is the triazolium salt. This approach is particularly attractive as the triazolium ligand precursor is readily available through a [2 + 3] cycloaddition of an alkyne and an azide (“click reaction”) to produce the corresponding 1,2,3-triazole, which is then selectively alkylated at the N3 position as the most nucleophilic site. The click reaction is highly functional group tolerant, providing access to a variety of substitution patterns on the triazole scaffold. Moreover, this click reaction is highly regioselective and affords exclusively the 1,4-disubstituted triazole when catalyzed by copper27−29 or the 1,5regioisomer when the reaction is catalyzed by a ruthenium complex30 or base.31 One limitation of this method pertains to the substitution of the N3 position, which requires an electrophile such as an alkyl halide or a triflate but does not allow for facile introduction of an aryl substituent. The synthesis of 1,3-diaryl triazoles is accomplished from triazenes via the formation of a 1,3-diaza-2-azoniaallene followed by 1,3dipolar cycloaddition of an alkyne or an alkyne equivalent.32 This adaption of the click reaction is particularly useful for the production of triazolium salts with identical aryl substituents at N1 and N3. The accessibility of triazolylidene precursors further benefits from the many attractive applications disclosed for triazolium salts, for example, in the area of ionic liquids, anion recognition and supramolecular chemistry, and organocatalysis.22,23 Triazolylidene metal complex formation from the triazolium salt is accomplished by different methods, and the best procedure is often dependent also on the nature of the metal center (Table 1). The most frequently applied methods so far have been the (i) transmetalation via in situ formation of a triazolylidene silver intermediate, (ii) deprotonation to form the free triazolylidene followed by metal coordination, and (iii) direct metalation involving concerted metalation/deprotonation with either an external base or a basic ligand bound to the metal precursor salt (such as in [Pd(OAc)2] or [Rh(OMe)(OMe)]2, COD = 1,5-cyclooctadiene). Other procedures include carbene generation from an ammonia adduct,33 formal tautomerization of a N-bound triazole within the metal coordination sphere,34 and alkylation of triazolyl ligands in

2. COMPLEXES WITH 1,2,3-TRIAZOLYLIDENE LIGANDS 2.1. General Aspects

The transition metal chemistry of triazolylidenes has experienced a great boost during recent years. The most popular metals in triazolylidene complexes have been iridium, palladium, rhodium, ruthenium, and gold with more than 20 publications each (Figure 3) due to a large extent to the high stability of the metal carbene bond in these complexes, as well as the intrinsic catalytic activity of the metal center. Copper has also received considerable attention (14 publications), while only about a handful of publications have appeared thus far for triazolylidene complexes of iron, nickel, silver, osmium, and platinum. Bonding of triazolylidene to rhenium, molybdenum, cobalt, and manganese has been just introduced (≤2 publications). Hence, the current trend in catalysis and C


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Table 1. Methods Available for the Synthesis of Triazolylidene Complexesa

a

methods are used very frequently (solid square), sometimes (darker outlined square), occasionally (less dark outlined square), or rarely (open square); transmetalation refers to metalation via a triazolylidene Ag intermediate. bCu TM = transmetalation from triazolylidene Cu complex; NH3 loss = abstraction of NH3 from the corresponding triazolylidene adduct.

Scheme 1. Transmetalation with Various Metal Precursors To Form Representative Triazolylidene Metal Complexesa

a

Note that the triazolylidene silver complex may be the ion pair shown or the neutral complex [Ag(trz)X].

palladium, platinum, or ruthenium complexes.35,36 All of these metalation methods parallel the synthetic approaches available for the formation of classic Arduengo-type NHC complexes. We are not aware of any reports on triazolylidene complex formation via oxidative addition,37 decarboxylation,38 or carbene formation via cycloaddition within the metal

coordination sphere.39 Unpublished work in our laboratories has demonstrated, however, that oxidative addition and decarboxylation are feasible to form triazolylidene metal complexes. 2.2.1. Transmetalation. Among the various methods used for the formation of triazolylidene complexes, transmetalation D


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Figure 4. Structural diversity of triazolylidene silver complexes.

Scheme 2

fonate). Typically, the formed silver complexes are not isolated, and their formation is inferred by the disappearance of the low-field triazolium resonance in the 1H NMR spectrum of the reaction mixture. Similar to Arduengo NHC silver complexes,40 triazolylidene silver complexes are structurally diverse and evidence has been obtained for both cationic and neutral complexes of type [Ag(trz)X] and [Ag(trz)2][AgX2]

via a triazolylidene silver complex is by far the most widely employed route (Scheme 1). The silver carbene complex is synthesized by reaction of the triazolium salt with silver(I) oxide. Addition of a chloride source such as Me4NCl is beneficial for stabilizing the product when the anion in the triazolium salt is a weakly or noncoordinating anion such as OTf−, BF4−, or PF6− (OTf− = CF3SO3−, trifluoromethylsulE


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mixtures of trans and cis isomers 12 and 13, respectively, with the trans isomer as the major species.46,48,50,52,54,57,122 Since the dissymmetric triazolylidene in these complexes is oriented essentially orthogonally with respect to the palladium coordination plane, these complexes exist as mixtures of syn and anti conformers.59 When the same palladium precursor [PdCl2(NCR)2] is added in the presence of NaI or another halide additive, the dimeric species 14 with bridging halide ligands is obtained.58 When this transpalladation is complemented with a further ligand such as pyridine or imidazole, the dimeric species is cleaved and monomeric complex 10 with two different heterocyclic ligands is formed (L = pyridine, imidazole).62,131 The effect of an added halide to promote formation of the dimetallic complex has not been observed when using a transmetalation procedure to prepare the palladium complex of the chiral triazolium salt 15 (Scheme 3). In the presence of a KX, monometallic trans-[Pd(trz)2X2]

(trz = triazolylidene), respectively, though with no distinct consequence for the subsequent transmetalation step.41 Presumably, the two species are in a dynamic equilibrium in solution, as reported for Arduengo NHC silver complexes.42,43 Despite their widespread application for transmetalation, triazolylidene silver complexes have not been investigated extensively and only a relatively small number of complexes have been characterized in detail (Figure 4). The two first isolated and well-characterized silver complexes were reported in 2011. These complexes are stabilized by bulky triazolylidene ligands. For example, complex 1 features triazolylidene bonding sites substituted with pyrrole functional groups that coordinate to silver as well, hence resulting in a tetrameric structure containing eight silver centers.44 The silver complex 2 also contains a ditriazolylidene ligand yet lacks chelating groups and therefore forms a dinuclear structure.45 Related diand tris-triazolylidene ligands like the bi(naphthylamine)based ditriazolylidene silver complex 346 and complex 4 with a rigid triphenylbenzene core also form cationic silver complexes of type [Ag(trz)2]+, with the latter ligand system forming a 2:3 ligand:metal assembly that shapes up into a macromolecular cylinder.47 In recent years, also simpler triazolylidene silver complexes have been reported. Kü hn and co-workers synthesized and characterized the neutral [Ag(trz)Cl] complex 5 with a 1,2,4-substituted (normal) triazolylidene.48 Complex 6 has been obtained in our laboratories and crystallizes as a neutral complex with a mixture of cyanide and iodide anions due to MeCN activation during the reaction of the triazolium salt with Ag2O.41 The monocationic bis(triazolylidene) silver complex 7, which contains a sulfoxide wingtip group, is a cationic bis(triazolylidene) species according to NMR and HRMS analysis, yet X-ray diffraction reveals a neutral monocarbene silver complex in the solid state.49 These results further support the presence of an equilibrium between neutral and ionic species. The silver carbene either reacts after isolation or more often just generated in situ with a variety of metal precursors and has been successfully applied for the synthesis of triazolylidene complexes with Pd(II),48,50−64 Ir(I),65−67 Ir(III),35,65−90 Ru(II),36,44,48,66,74,79−81,83,89,91−110 Rh(I),48,58,66,111−114 Rh(III),70,83,90 Au(I),47,49,62,76,112,115−122 Cu(I),123−127 Pt(II),128,129 Os(II),74,79,96,103 Mo(I),48 and Co(III).130 Many of the complexes also form when Ag2O and the appropriate metal precursor are added simultaneously to the triazolium salt rather than sequentially via formation of the triazolylidene silver intermediate. Palladium. The importance of the choice of the transmetalation conditions (metal precursor and use of additives) is particularly relevant in the synthesis of triazolylidene palladium complexes, for which the conditions are crucial in determining the selectivity of product formation (Scheme 2). Transpalladation of the carbene silver complex 8 with one equivalent of [Pd(allyl)Cl]2 affords the monocarbene complex 9.60 Related species with “normal” triazolylidenes are obtained using the same metal precursor.48 Similar monocarbene species 10 are formed when the transpalladation is performed with PdCl2 in the presence of another coordinating ligand such as pyridine,51,55,56,131 imidazole,131 or excess of MeCN.57,62 Addition of 1.2 equiv of [Pd(OAc)2] to the silver intermediate produces the cyclometalated complex 11 when the triazolylidene contains a N-phenyl substituent.59 In contrast, the metal precursor [PdCl2(NCR)2] favors the formation of the bis(triazolylidene) complexes [PdCl2(trz)2], generally as

Scheme 3

complexes 16 are obtained in enantiomerically pure form.50 Likewise, palladium complex 17 with a trans-chelating dicarbene ligand (Figure 5) is obtained via transmetalation with PdCl2 at room temperature.52

Figure 5. Palladium complex 17 with a trans-chelating bis(triazolylidene) ligand.

Other Platinum Group Metals. The transmetalation procedure has also been extensively used for the synthesis of Ir, Ru, and Rh complexes. The use of [M(COD)Cl]2 (M = Rh, Ir) allows for the synthesis of Rh(I) and Ir(I) species, while the use of [Cp*MCl2]2 (M = Rh, Ir) gives Rh(III) and Ir(III) complexes (Cp* = C5Me5−, pentamethylcyclopentadienyl). Only a few triazolylidene iridium(III) complexes feature monodentate triazolylidene ligands (18, Figure 6),76,82,89,90

Figure 6. Monodentate and chelating triazolylidene iridium(III) complexes. F


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tetrachloroplatinate salt 25 reacts in the presence of Ag2O, presumably through the transient formation of the silver complex followed by immediate in situ transmetalation with the platinum anion at 140 °C to yield the bis(carbene) pincer complex 26. Molybdenum. The transmetalation procedure has allowed for the synthesis of one of the two unique examples of triazolylidene molybdenum complexes. Silver complex 27 contains a 1,2,4-substituted 1,2,3-triazolylidene, i.e., a formally “normal” triazolylidene (Scheme 7). Transmetalation with

while the majority of known triazolylidene iridium(III) complexes contain a chelating carbene ligand. Chelate formation is, on one hand, promoted by the incorporation of appropriate substituents on the triazole heterocycle such as a pyridyl group (19) and, on the other hand, a direct consequence of the high activity of the triazolylidene iridium(III) unit toward bond activation.73 In the +1 oxidation state, triazolylidene iridium complexes are relatively rare, though this oxidation state is prevailing for triazolylidene rhodium complexes. Similar transmetalation proceeds smoothly with [RuCl2(cym)]2 complexes and other ruthenium arene precursors (cym = p-cymene).66 The mild reaction conditions for the formation of the triazolylidene silver intermediate and the subsequent transmetalation is demonstrated by the synthesis of the Rh−Pt bimetallic species 21 (Scheme 4).113

Scheme 7

Scheme 4 [CpMo(CO)3Cl] gives the Mo(II) complex 28 in good yields and under mild reaction conditions (45 °C, 16 h). The same method has also been used for the synthesis of “normal” triazolylidene complexes of rhodium(I), palladium(II), and ruthenium(II).48 Transmetalation through Triazolylidene Cu Intermediates. The transmetalation procedure is not limited to silver intermediates. Metalation has also been accomplished via a triazolylidene copper complex by Bertrand and co-workers. Deprotonation of the triazolylidene copper complex 29 produces a presumably polymeric species which is cleaved to a distinct bimetallic complex upon transmetalation with a suitable metal precursor (Scheme 8).132 This protocol provides access to ortho 4,5-dimetalated carbanionic 1,2,3-triazolylidene complexes of Pd, Ir, and Rh. Cazin and co-workers expanded this synthetic procedure to other carbenes including cyclic (alkyl)(amino)carbenes (cAACs), normal and mesoionic imidazolylidenes, and triazolylidenes. A series of PdII, RhI, IrI, and AuI complexes is obtained in good yields from the corresponding CuI carbene precursor.133 Similarly, transmetalation of the bis(copper) complex 33 containing an imidazolylidene and a triazolylidene ligand site with Rh or Pd affords complexes 34 and 35 with a chelating ditopic dicarbene ligand (Scheme 9).134 This transmetalation method through a copper intermediate provides an alternative to the more common silver route and may be particularly attractive for triazolylidene ligands with oxidation-sensitive functional groups, since copper(I) is a much weaker oxidizing agent than silver(I). 2.2.2. Free Carbene Route. In 2010, Bertrand and coworkers synthesized 37a as the first free 1,2,3-triazolylidene by deprotonation of the corresponding triazolium salt 36a with KN(SiMe3)2 (KHMDS; Scheme 10).135 The free carbene is stable at room temperature and has been analyzed by X-ray

The reaction starts from a triazolium salt 20, which contains an organoplatinum substituent at C4, which tolerates reaction with Ag2O followed by transrhodation with [Rh(COD)Cl]2. Two different pathways have been reported for the transplatination reaction. One involves the use of the precursor [Pt(COD)Cl2] by a standard procedure consisting of treatment of the triazolium salt 22 with Ag2O followed by transmetalation of the intermediate triazolylidene silver complex with [Pt(COD)Cl2] (Scheme 5).128 Treatment with a β-diketone in the presence of base forms the cyclometalated complex 23. Scheme 5

Alternatively, a platinate counterion of the triazolium salt 24 has been formed through an anion metathesis of the triazolium salt and K2PtCl4 (Scheme 6).129 The formed ditriazolium Scheme 6

G


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Scheme 8

deprotonation with KOtBu and reacts readily with [Rh(COD)Cl]2 and [Pd(Br)2(iPr-bimy)]2 to give the Rh(I) complex 42 and the bis(carbene) Pd(II) complex 43 containing a normal and a mesoionic NHC ligand, respectively (bimy = 1,3-diisopropylbenzimidazol-2-ylidene; Figure 8).136 The free bis(triazolylidene) 40, obtained from deprotonation with KHMDS, affords the rhodium(I) complex 44 upon coordination to [Rh(COD)Cl]2. The same product is obtained from the reaction of the bis(triazolium) salt with [Rh(COD)(OEt)]2 in the presence of ethanolic EtONa, thus providing two complementary methods for preparing bis(triazolylidene) Rh(I) complexes (see section 2.2.3).137 The pyridyl-bridged bis(triazolylidene) ligand 41 reacts with FeCl2 to form the homoleptic bis(CNC-pincer) iron(II) complex 45; however, it is isolated in very low yield (2%).138 The first isolation of a free triazolylidene has paved the way for the free carbene route, and this method is widely used for synthesis of triazolylidene complexes. It has been applied successfully for preparing triazolylidene complexes with Pd,34,136,139,140 Ir,32,70 Ru,32,141,142 Rh,46,70,134,136,139,140,143−145 Au,139,140,146−148 Cu,132,149−152 Ni,152,153 Fe,143,154−156 and Mn.153 Generally, a strong base such as KHMDS, LiNiPr2 (LDA), or KOtBu and the appropriate metal precursor are added simultaneously to the triazolium salt. While this protocol does not allow for the free carbene to be identified and its formation is generally just assumed, the in situ procedure avoids the accumulation of the free carbene and hence suppresses secondary processes such as alkyl group transfer (see Scheme 10). Dealkylation of the N3 site is particularly favored with benzyl and methyl substituents at this position. When the triazolium salt 36 is treated with KHMDS at low temperature in the presence of the metal precursors [Pd(allyl)Cl]2, [Rh(COD)Cl]2, or AuCl(SMe2), the corresponding Pd(II), Rh(I) and Au(I) complexes 46−48 are obtained in excellent yields (>90%, Scheme 11).139 Likewise, a series of iridium(I) and ruthenium(II) complexes 49 and 50 is accessible by KHMDS-mediated deprotonation to the stable free carbene 37, followed by metal coordination.32 Di- and trinuclear triazolylidene species have also been obtained through the same methodology.139,145 The free carbene route has been used for the metalation of heteroditopic imidazolium−triazolium salts to form, depending on the conditions, either chelating or heterobimetallic

Scheme 9

Scheme 10

diffraction, though at 50 °C it rearranges through a formal methyl group transfer to give the 1,4,5-trisubstituted triazole 38.32,135 This carbene quenching process is suppressed when the N3-bound methyl group is replaced by an aryl substituent. Other free triazolylidenes have been isolated and fully characterized by X-ray diffraction analysis, including the ferrocenyl-substituted triazolylidene 39 and the bis(triazolylidene) compounds 40 and 41 (Figure 7).136−138 The ferrocenyl-substituted triazolylidene 39 is obtained via

Figure 7. Isolated and crystallographically characterized free triazolylidenes 39−41. H


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Figure 8. Complexes 42−45 obtained from isolated free carbenes.

Different triazolylidene Cu(I) complexes have been synthesized through the free carbene route, including neutral monocarbene complex 56,149,150 cationic dicarbene complex 57,149,151 and the trigonal dicarbene complex 58 (Scheme 13).151 The selectivity toward mono- vs bis(triazolylidene) complex formation is dictated by the metal precursor. While metalation with CuI generally affords the monocarbene complexes 56, metalation of triazolium 36l with Mes wingtip groups is an exception and yields the bis(carbene) species 57l. With smaller Ph wingtip groups, the halide from the triazolium salt coordinates to the copper center upon metalation with [Cu(MeCN)4]BF4 with formation of the three-coordinate complex 58. This method also provides access to pincer-type triazolylidene metal complexes when using the bis(1,2,3-triazolium)carbazole 59 as ligand precursor (Scheme 14). This ligand is obtained through cycloaddition of the corresponding 1,3diaryl-2-azoniaallene salt with 1,8-diethynylcarbazole. Remarkably, the addition of 3 mol equiv of KHMDS to 59 does not yield the free carbene but only induces carbazole deprotonation, leaving the triazolium units unaffected. Triazolium deprotonation is accomplished in the presence of 5 mol equiv of KHMDS and leads to the formation of the potassium adduct 60.152 The triazolylidene exists in the solid state as an intriguing dimer in which the pincer ligand is κ3-bound to one potassium ion, with one triazolylidene residue bridging two potassium ions in a μ2-coordination mode (Figure 9). This coordination mode in fact mimics the transition state of a transmetalation process. The potassium adduct 60 reacts, either after isolation or when formed in situ, with CuI or CuCl2 and affords T-shaped Cu(I) complex 62 and the paramagnetic Cu(II) 63, respectively. This method also enables the metalation with precious metals to produce related bis(triazolylidene) carbazolide pincer complexes of Au(I),147 Ru(II) (65),142 and Rh(I) (64).144 The latter forms a rare O2 adduct upon exposure to oxygen. Moreover, this procedure is also suitable for the preparation of complexes with other firstrow transition metals. Reaction of 60 with KHMDS in the presence of the nickel(II) precursor [Ni(dme)Cl2] at −78 °C (dme = dimethoxyethane) affords the nickel hydride complex 61. Complex 61 represents the first example of an isolated neutral nickel carbene hydride species. Likewise, nickel complexation is achieved via the free carbene route with the tris(triazolium) borate salt 66 (Scheme 15). Deprotonation with LDA and subsequent metalation with [Ni(NO)(PPh3)2Br] yields the tris(carbene)borate complex 67. This procedure is also applicable to other first-row transition metals. When using the manganese(I) precursor [Mn(CO)3(CNtBu)2Br]2 instead of the nickel salt, the tris(carbene)borate complex 68 is formed. This complex

Scheme 11

complexes. When the precursor salt 51 is reacted with 2.5 mol equiv of KHMDS, both heterocycles are deprotonated and addition of a rhodium or palladium precursor induces chelation to afford the monometallic mixed carbene complexes 34 and 35, respectively (Scheme 12). In contrast, addition of a Scheme 12

slightly substoichiometric amount of base induces the selective deprotonation of the imidazolium fragment and allows for selective metalation to yield the monometallic imidazolylidene complexes 52 and 53 with a pendant triazolium substituent. Addition of a second equivalent of KHMDS in the presence of an additional metal precursor does not induce chelation but coordination to the second metal precursor and yields the dimetallic complexes 54 and 55 with one metal bound to an Arduengo carbene and the other one to a triazolylidene.134 This approach is useful for the formation of heterobimetallic complexes and demonstrates a sufficiently large pKa difference of triazolium and imidazolium salts.157 I


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Scheme 13

Scheme 14

represents the first and to date only example of a triazolylidene complex of manganese.153 Also, for iron complexation to triazolylidenes, the free carbene route is the method of choice. All four publications that have appeared so far on triazolylidene iron complexes report on metalation that involves the generally in situ deprotonation of the triazolium salt with KOtBu or NaOtBu followed by coordination of the triazolylidene to the iron precursor. According to this general protocol, iron complexes containing bidentate bis(triazolylidene) ligands (69,154 70155) and tridentate bis(carbene) ligands with a pyridyl bridge (45138) have been prepared from the corresponding triazolium ligand precursor and [FeCl2(bpy)] (for 69) and FeX2 (for 45 and 70) (bpy = 2,2′-bipyridine; Figure 10). Similarly, monocarbene complexes, both supported (71) and unsupported by chelation (72),156 are readily obtained upon reaction

Figure 9. Bimetallic complex 60 with two μ2 -coordinating triazolylidene ligands as frozen models of an intermediate of transmetalation.

J


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Palladation has been investigated most thoroughly. Direct metalation with [Pd(acac)2] requires long reaction times (2 days) and high temperatures (>100 °C; Scheme 16).169 In

Scheme 15

Scheme 16

comparison, [Pd(OAc)2] is more reactive and metalation is easier; however, a mixture of mono- and dicarbene species is formed, revealing considerable less selectivity than transmetalation.58 The product selectivity of the direct palladation is strongly influenced by the applied reaction conditions. Palladation of the 1,4-diphenyl-substituted triazolium salt 36p is representative (Scheme 17).160 Sankararaman and co-

of the appropriate triazolium salt with [CpFe(CO)2I] in the presence of KOtBu. Despite the large variety of piano-stool Fe(NHC) complexes known,158 complexes 71 and 72 represent the first examples of this class of compounds bearing triazolylidene ligands. 2.2.3. Direct Triazolium Metalation. Direct C−H bond activation of triazolium salts with a suitable metal precursor has been particularly popular as a method for the synthesis of triazolylidene complexes of Pd, 5 0 , 5 6 , 5 8 , 6 9 , 7 5 , 1 5 9 − 1 6 9 Ir,35,87,168,170−172 Rh,137,167,168,172 and Ni.145,173−175 In addition, a few triazolylidene complexes have been prepared by direct C−H bond activation with Cu,111 Pt,170 Mo,176 and Re177 species. This synthetic procedure can be divided in three different domains: (i) activation mediated by a metal precursor that contains a sufficiently basic ligand; (ii) activation induced by addition of an external proton scavenger; (iii) chelating groups as a driving force for C−H activation (cyclometalation). Thermally induced metalation of triazolium salts with metal precursors that contain an internal base is a relatively mild method. Precursors suitable for such C−H activation have been demonstrated to be [Pd(OAc)2],58,69,159,160,162,165,167 [Pd(acac)2] (acac = acetylacetonato),169 [M(COD)(OMe)]2 (M = Rh, Ir),167,168 [Rh(COD)(OEt)]2,137 [Rh(COD)(OAc)]2,167 Ag2O (see section 2.2.1), and CuO.111

Scheme 17

workers have shown that palladation of the triazolium salt in the presence of a halide source such as KCl or KI results in the predominant formation of a dimeric anion-bridged complex

Figure 10. Triazolylidene iron complexes obtained via the free carbene route. K


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Scheme 18

Scheme 19

Scheme 20

Scheme 21

studies159,165 and seems to be a general feature of direct palladation of triazolium salts. While transmetalation provides access to the same diversity of products, direct metalation tends to be less selective and usually produces mixtures of complexes. Depending on the triazolylidene substituents, separation is straightforward due to the different solubility of mono- and (bis)carbene complexes. In a similar study,

with one triazolylidene ligand bound to each palladium center. With soft iodide, the dimer is halide bridged (14), while harder chloride anions promote cyclometalation of one of the phenyl substituents and formation of the acetate-bridged dimer 11. When the reaction is performed in a 2:1 triazolium/palladium stoichiometry, the monometallic bis(carbene) complex 74 is formed. This product diversity has been observed in related L


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Scheme 22

Mendoza-Espinosa and co-workers obtained bimetallic, cisand trans-dicarbene complexes, and pyridine-containing species from palladation with [Pd(OAc)2] at 30 °C.159 Additives have been used to influence the course of direct palladation. Cyclometalation of the phenyl substituent of the BODIPY-functionalized triazolium salt 75 with [Pd(OAc)2] is favored in the presence of carbonate as base and yields the dimeric complex 76, which is readily converted to a monometallic complex upon addition of a further ligand (BODIPY = boron dipyrromethene; Scheme 18).69 Similar chelate formation is accomplished in the reaction of [Pd(OAc)2] with a pyridyl-substituted triazolium salt,163 though obviously the energetically most demanding step in this latter process pertains to the triazolium C−H bond activation, while in the formation of complex 76, the activation of the phenyl C−H bond in 75 is assumed to be considerably harder. The direct metalation procedure is also suitable for the synthesis of PEPPSI-type palladium complexes 77 (PEPPSI = pyridine-enhanced precatalyst preparation, stabilization, and initiation; Scheme 19).50,56,75,161,162 Reaction of the triazolium salt 36, PdCl2, and K2CO3 as an external base in the presence of pyridinethe latter generally employed as the reaction solventaffords the triazolylidene pyridine palladium complex 77. This protocol has also been applied for the synthesis of triazolylidene Pt(II) complexes.170 These reaction conditions are relatively mild and have therefore been used successfully for the second metalation of the bis(triazolium) salt 78a to produce the heterodimetallic bis(triazolylidene) complex 80 (Scheme 20).170 The intermediate triazolylidene iridium complex 79 with a pendant triazolium unit is formed through a double C−H activation using [Cp*IrCl2]2, K2CO3, NaOAc, and KI. Direct metalation of triazolium salts with iridium and rhodium is common. For example, Cowie reported the synthesis of dinuclear Ir2, Rh2, and IrRh complexes by direct metalation of the meta-phenylene-linked bis(triazolium) salt 78b with [M(COD)(OMe)]2 (M = Rh, Ir; Scheme 21).168 The reaction is stepwise and affords the monometalated

complex 81 in good yields. Addition of a second equivalent of the same or a different metal precursor provides the homo- and heterobimetallic complexes 82. Similarly, this metal precursor reacts with the pendant triazolium substituent of the imidazolylidene palladium complex 83 and yields the Pd,Rh bimetallic complex 84 bearing a bridging imidazolylidene− triazolylidene ligand.167 The rhodium or iridium metal precursor for direct metalation, [M(COD)(OMe)]2, does not need to be prepared first, and the reaction outcome is identical when the internal base is generated in situ by reaction of the metal chloride [M(COD)Cl]2 (M = Ir, Rh) with NaH in the presence of MeOH.172 Distinct reactivity patterns have been observed in the metalation of polytopic ligands. For example, iridation of the tris-triazolium salt 85 with [Cp*IrCl2]2 in the presence of K2CO3 as base affords the dinuclear iridium complex 86 with one triazolium unit unaffected (Scheme 22).35 In contrast, complexation via a transmetalation procedure through the in situ formation of the tris(silver) carbene intermediate and subsequent transmetalation with [Cp*IrCl2]2 yields the trinuclear complex 87. These reactivity schemes underline the differences of the various metalation methods. Direct palladation of the tris(triazolium) salt 85 with PdCl2 in the presence of pyridine produces the tris(palladium) PEPPSI complex 88.164 Similar tris(triazolylidene) palladium complex formation takes place when starting from an analogous tris(triazolium) precursor with a 1,3,5-triphenylbenzene core unit.75 Direct iridation of the pyridyl-functionalized triazolium salt 89 selectively produces the iridium complex 90 as exclusive product, even though two triazolium C−H bonds are available for activation (Scheme 23).171 The selective formation of complex 90 suggests a chelate-directed bond activation process as typical for cyclometalation reactions. Hence, iridium coordination to the pyridyl site directs the iridium to the triazolium C5 position selectively. In agreement with a donor group-directed bond activation process, attempts to use a transmetalation procedure via a silver intermediate are less M


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Scheme 23

Scheme 25

selective and give a mixture of products instead since silver carbene formation is controlled by the acidity of the triazolium protons and considerably less by chelate stabilization. Heteroatom coordination is not always dictating the selectivity of metalation though. For example, iridation of the triazole 91 containing pyridine and pyridinium substituents is expected to give a complex similar to 90 if the pyridyl group is directing metalation. However, iridation with [Cp*IrCl2]2 in the presence of NaOAc affords the bimetallic complex 92 (Scheme 24).87 This complex features two nonidentical Scheme 24

Scheme 26

iridium centers: one is N,N coordinated and the other is C,C bound by two carbenic ligand sites, viz. a pyridylidene and a (normal) triazolylidene. Sarkar and co-workers demonstrated that [Re(CO)5Cl] induces direct metalation of the mono- and bis(triazolium) salts 93−95 in the presence of Et3N and affords the rhenium carbene complexes 96−98 containing a chelating triazolylidene ligand (Scheme 25).177 Base-assisted direct metalation has generally been using oxygen-containing bases, either incorporated in the metal precursor as, for example, in [Pd(OAc)2] or [Ir(COD)(OMe)]2 or as external base such as NaOAc or K2CO3. Other oxo-base-mediated metalations of triazolium salts include of course the formation of triazolylidene silver complexes by reaction of the triazolium salt with Ag2O. This method has been recently expanded to copper as the lighter congener of the coinage metals. Reaction of the triazolium salt 99 with Cu2O yields the copper(I) complex 100 (Scheme 26).111 While the reaction with Ag2O proceeds at room temperature, copper complexation requires elevated temperature and microwave irradiation. Other bases have rarely been employed so far for the direct metalation of triazolium salts. One notable exception is the reaction of triazolium salts with nickelocene, which provides a reliable and convenient access to triazolylidene nickel complexes.145,173−175 Reaction of the triazolium salts 36 with [Ni(Cp)2] yields mixtures of mono- and bis(triazolylidene) nickel complexes 101 and 102, respectively (Scheme 27).173−175 Reaction conditions (time, temperature) and additives (halide sources) strongly influence the product selectivity and can be adjusted to produce predominantly the mono- or the bis(triazolylidene) complex. Introduction of a

Scheme 27

suitable donor group as substituent on the triazolium precursor induces chelation, as illustrated with the pyridyl−triazolylidene nickel complexes 103a and 103b. This procedure has also been successfully applied for nickelation of a tris-triazolium salt containing a 1,3,5-triphenylbenzene backbone.145 2.2.4. Ligand Postmodification. This route entails bonding of the triazole to the metal center, followed by heterocycle modification to form the neutral triazolylidene within the metal coordination sphere. Two different approaches have emerged to this end. In one approach, the metal−carbon bond is installed first to produce a complex with an anionic triazolide ligand, which is then alkylated in a N


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Scheme 28

Scheme 29

as the reaction proceeds under very mild conditions, at low temperature, and without the need of additives. This procedure might thus be applicable for the complexation of a wide range of transition metals. The most severe limitation probably relates to the preparation of the starting material.

subsequent step to yield the neutral triazolylidene species. Formation of the M−C bond is promoted by cyclometalation. For example, metalation of the pincer ligand precursor 104 induces C−H bond activation and yields the triazolide complex 105, which is transformed into the triazolylidene upon alkylation at the N3 position (complex 106, Scheme 28a).178 A similar cyclometalation alkylation sequence affords the tridentate bipyridyl−triazolylidene ruthenium complex 109 starting from the triazolyl-functionalized bipyridine 107 (Scheme 28b).36 In an extension of this approach, the triazole can be formed directly in the rhenium coordination sphere (Scheme 29).34 Starting from the pyridyl−imine rhenium complex 110 with a pendant azide functionality, copper-catalyzed cycloaddition of an alkyne directly furnishes the triazolylidene rhenium complex 111. In this complex, the N3 position is not alkylated but metalated by copper to yield a dimeric species. Alkylation readily cleaves this dimer and yields the monometallic triazolylidene rhenium complex 112. 2.2.5. Other Methods. Normal 1,2,3-triazolylidene Ir(I), Rh(I), Cu(I), and Au(I) complexes 114−117 are accessible via ammonia abstraction from the 1,2,4-substituted triazolylidene ammonia adduct 113 (Scheme 30).33 This procedure is comparable to the decarboxylation of imidazolium carboxylates

2.3. Properties and Reactivity

2.3.1. Donor Properties. By far the most commonly used metric to evaluate the electronic properties of NHCs is the Tolman electronic parameter (TEP), based on CO stretch vibrations of carbonyl compounds.179 While originally designed for nickel complexes [NiL(CO)3], correlations have been established also for rhodium and iridium complexes of general formula cis-[MCl(L)(CO)2] (M = Rh, Ir).180−182 Early IR measurements on a series of triazolylidene iridium complexes suggest that 1,2,3-triazolylidenes are stronger donors (TEP 2045 ± 2 cm −1 ) than Arduengo-type imidazole-2-ylidenes (TEP 2050 ± 2 cm−1), though less strongly donating than other mesoionic carbenes such as 4imidazolylidenes (TEP 2033 ± 2 cm−1).21 Various more recent measurements on triazolylidene rhodium complexes confirm this notion. Complexes 118 and 119 feature an average CO vibration frequency νav = 2027 cm−1, which translates into a TEP of 2042 cm−1 (Figure 11).140 This average CO vibrational frequency is lower than in Arduengo-type carbenes (νav = 2039−2041 cm−1) or cAACs (νav = 2036 cm−1) yet higher than in imidazolylidenes and related mesoionic carbene ligands (νav = 2016−2025 cm−1), hence indicating that triazolylidenes are less strongly donating ligands than mesoionic imidazolylidenes but considerably stronger than Arduengo carbenes.32 Complex 32 (cf. Scheme 8) represents a special case as the bridging nature of the heterocycle implies that only one carbon is formally binding as a mesoionic carbene, while the other carbon is formally a carbanionic ligand. Therefore, the carbon ligands are expected

Scheme 30

O


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Figure 11.

donor strength of the ferrocenium-substituted triazolylidene that is similar to Arduengo-type carbenes. Remarkably, the TEP increase is the same for complexes 120 and 121, despite the fact that the ferrocenyl unit is bound either to the triazole N1 or C4 position. The slightly lower TEP of the triazolylidene with a C-bound ferrocene reflects the different connectivity and indicates a stronger influence of the C-bound substituent compared to the N-bound group. This rationale is in agreement with a partially separated π electronic system in the triazolylidene heterocycle, divided into a cationic triazonium unit and a vinyl anion as limiting resonance structure.183 Moreover, the identical shift difference of 120 and 121 upon ferrocene oxidation might suggest that the ferrocenyl/ferrocenium substituent has a predominantly inductive effect on the electron density of the triazolylidene ligand. Another rationale for these observations may be a stereoelectronic effect, since the bending of the ferrocenium unit is considerable and entails a steric modification that impacts the coordination of the cis-positioned CO ligand, resulting in an altered CO stretch vibration. Obviously, such a steric change of the wingtip group is not related to a change of the triazolylidene donor strength. A similar effect has been noted with sterically rigid cyclophane-substituted 2-imidazolylidenes.184 Strong donor properties of the triazolylidene ligand are also deduced from analysis of the CO stretch vibration in rhenium complexes 96−98, 122, and 123 (Figure 12).177 While the diimine ligands in complexes 122 and 123 result in an average CO vibration νav = 1948 ± 2 cm−1, substitution of the imine donors with one or two triazolylidene units successively lowers the stretch frequency by some 10 cm−1. Hence, the imine− triazolylidene ligands in complexes 96 and 97 lower the average CO vibration to 1937 ± 3 cm−1, while the di(triazolylidene) in complex 98 induces an even lower CO vibration (νav = 1928 cm−1). No efforts have been made so far to correlate the rhenium CO vibrations with those of the iridium or rhodium scale. The donor properties of tris(triazolylidene) borate ligands bound to either a Ni(NO) center or to a Mn(CO)3 unit have been measured by IR spectroscopic monitoring of the νNO and νCO band, respectively, and compared to those of similar tris(2-

to be even stronger donors in this complex. Indeed, evaluation of the CO stretching frequencies of rhodium complex 32 reveals a marked downfield shift compared to other monoanionic 1,2-dimetalated ligands, confirming that carbanionic 1,2,3-triazolylidenes are stronger donors than pyrazolate, triazolate, and imidazolate ligands.132 The influence of the various substituents on the triazolylidene ligand has been discussed controversially. A series of triazolylidene iridium complexes with different aryl groups at C4 and N1 displays essentially the same TEP irrespective of the aryl substitution pattern,32,60,135 hence suggesting a very moderate influence of the substituents on the ligand donor properties. Moreover, mesoionic 1,3,4-trisubstituted triazolylidene ligands as in complex 49 have the same TEP as the nonmesoionic 1,2,4-trisubstituted triazolylidene homologues (117, Figure 11),33,66 suggesting a negligible influence of the triazo fragment of the triazole heterocycle on the electronic nature of the carbene center. This model implies that a distinction between normal and mesoionic/abnormal 1,2,3-triazolylidene ligands is not meaningful. However, the ferrocenyl-substituted triazolylidenes in the iridium complexes 120 and 121 considerably change their average CO vibration energy upon ferrocene oxidation (Table 2).76,120 The TEP value increases by 7 cm−1, indicating a Table 2. IR Stretch Vibrations and TEP Values for Complexes 120 and 121a

complex

FeII νav (cm−1)

FeII TEP (cm−1)

FeIII νav (cm−1)

FeIII TEP (cm−1)

120 121

2020.1 2018.2

2047.0 2045.4

2028.7 2026.8

2054.3 2052.7

a

From ref 120.

Figure 12. P


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Figure 13. (a) Triazolylidene rhodium(I) complexes with characteristic 13C NMR spectroscopic modulation based on wingtip changes. (b) Triazolylidene ruthenium complexes with significant 13C NMR variation demonstrating the relevance of other factors than donor properties on the 13 C NMR chemical shift.

imidazolylidene) complexes.153 The triazolylidene system imparts stretch vibrations (e.g., νNO = 1709 cm−1) that are between those of the methyl-substituted 2-imidazolylidene analogue (νNO = 1697 cm−1) and that of a mesityl-substituted 2-imidazolylidene borate ligand system (νNO = 1724 cm−1). Compelling data analysis is, however, hampered due to the fact that the heterocycles feature different substituents. Complementary, the donor strengths of this ligand have also been evaluated by electronic structure calculation of nitrosyl nickel species.185 The results compare well with the experimental data of the complexes and validate the model, which may prove useful for predicting the donor strength of such ligands. NMR analysis has been used to support the evaluation of donor properties by IR spectroscopy.58,99,118,172,186 Analysis of the 13C NMR frequency of the metal-bound carbon of the triazolylidene rhodium complexes 47 shows a correlation with the nature of the wingtip group (Figure 13a) . With mildly donating alkyl wingtip groups, the carbene resonance doublet (1JRhC = 46.5 ± 3 Hz for all complexes) appears at highest field while the presence of one phenyl group induces a 2 ppm downfield shift. This effect is cumulative, and when a second phenyl group is incorporated, the resonance is shifted by another 2 ppm to lower field.58 Moreover, an increase of the coupling constant (1JRhC) is observed when less electrondonating aryl wingtip groups are present, whereas alkyl substituents result in smaller coupling constants. However, the carbenic carbon is generally not a suitable probe for assessing the carbene donor properties, as this resonance is highly susceptible to a variety of factors, in particular, the substitution pattern at C5. This complexity is demonstrated, for example, in the series of triazolylidene ruthenium complexes 124−126 containing a chelating pyridyl or picolyl substituent (Figure 13b).99 The carbenic C5 nucleus resonates at considerably different frequency in these three complexes (δC = 159.6, 169.5, and 174.0 for 124a, 125a, and 126a, respectively). When modifying the para substituent R of the phenyl unit to electron-donating OMe or electron-withdrawing CF3 groups, the resonance of the carbenic nucleus shifts by a mere 0.3 ppm up- or downfield, respectively. While the direction of the chemical shift changes is consistent with the electronic nature of the substituent, clearly the 13C NMR

frequency is governed much stronger by the triazole substituent pattern, suggesting absolute chemical shifts of the carbenic center to be poor probes of the carbene donor properties. To avoid any of these complications, Huynh developed a trans-[PdBr2(bimy)] synthon for binding to a variety of ligands.186 Their donor strength is then evaluated on the basis of their trans influence, which is monitored by the 13C NMR frequency of the carbenic resonance of the bimy reporter ligand. This probe has been useful for a set of imidazolylidene and phosphine ligands. When using triazolylidenes as in complex 127 (Figure 14), the IR spectroscopic conclusions are

Figure 14.

confirmed as the NMR shifts for alkylated and arylated triazolylidene palladium complexes are between those of Arduengo-type carbenes and mesoionic imidazole-4-ylidenes. Subtle changes in the wingtip composition induce small NMR shifts, which reflect the inductive effect of the substituents on the triazolylidene ligand.58 Electrochemical analyses offer a method for the direct quantification of the electronic impact of ligands on the metal center. Parametrization of this effect leads to the Lever electronic parameter (LEP), which is complementary to the spectroscopic analysis that is underlying TEP. Comparative electrochemical studies of triazolylidene and Arduengo-type imidazolylidene complexes indicate a marked difference in electron density at the metal center. For example, the triazolylidene iron(II) complex 71a is oxidized at a 70 mV lower potential than its analogue 128 featuring a sterically identical bis(mesityl)imidazolylidene (IMes) ligand (Figure 15).156 The oxidation potential for those two complexes translates187 into a LEP of +0.18 for the imidazolylidene ligand and +0.12 for the triazolylidene analogue. Likewise, the Q


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binding energies in the imidazolylidene complex 131 are 0.5 eV lower than those in the related triazolylidene complex 9a, indicating a higher electron density on the metal center imparted by the 1,2,3-triazolylidene than by the Arduengo-type imidazolylidene.60 Trends in the electronic structure of five-membered NHC ligands with a varying number of nitrogen atoms are also revealed by DFT.189 These calculations are in agreement with the conclusions from IR, NMR, and XPS analysis and indicate that the HOMO is the carbene lone pair, with an energy eigenvalue that correlates with the first proton affinity. DFT emphasizes the crucial role of the heteroatom number relative position to the carbenic carbon. These analyses also predict a certain π basicity, though π donation only plays a role when the carbene is bound to electron-poor, early transition metals. Moreover, these calculations predict a substantial negative charge at the carbenic carbon, which shows remarkable similarity to aromatic carbanions, a model that is in agreement with only partial conjugation of the π electrons in the triazolylidene heterocycle. All of these analyses therefore consistently indicate that 1,2,3-triazolylidenes are stronger donors than Arduengo-type imidazolylidenes and cAACs yet weaker donors than mesoionic imidazolylidenes (see section 3). These donor properties are not surprising when considering the impact of heteroatoms that are positioned adjacent to the carbene and when taking into account the fact that heteroatoms generally reduce the electron density in aromatic systems. The donor strength difference of triazolylidenes with respect to Arduengotype NHCs is quantified by various terms, such as the acidity (ΔpKa = ca. 2), the Tolman electronic parameter (ΔTEP = ca. 7 cm−1), the Lever electronic parameter (ΔLEP = ca. 0.1 V), the electron binding energy (ΔE = ca. 0.5 eV), and by different NMR spectroscopic methods such as 13C NMR shifts of transpositoned benzimidazolylidene. 2.3.2. Structural Features. All crystallographically analyzed triazolylidene complexes have been collated to evaluate their bond lengths. Average of M−C bond lengths of published complexes are presented in Table 3. These lengths are not markedly different from classic Arduengo-type NHC complexes. A few general aspects are worth noting: while unsurprisingly the M−Ctrz bond lengths in complexes of 4d and 5d transition metals are all within a very similar range, 2.02

Figure 15.

[Ru(bpy)3]2+ analogue 129 with one pyridyl donor substituted by a triazolylidene is oxidized at 140 mV lower potential than the corresponding complex 130 containing a 2-imidazolylidene ligand site (Figure 15).105,188 Evaluation of the observed oxidation potentials of complexes 129 and 130 (+1.47 and +1.33 V, respectively, vs normal hydrogen electrode) affords a LEP value for the triazolylidene ligand unit in 129 of −0.01 and +0.13 for the imidazolylidene residue in 130.187 This LEP difference is bigger than that in the iron couple, presumably because the impact of the carbene on the dicationic ruthenium complex is more pronounced than in the neutral iron system. The donor properties of the triazolylidene ligand in the Pd(II) complex 9a have been assessed by X-ray photoelectron spectroscopy (XPS; Figure 16). The palladium 3d electron

Figure 16.

Table 3. Overview of M−C Bond Lengths (Angstroms) in Triazolylidene Complexes M

average M−Ctrz

Mo Mn Co Re Os Pt Fe Ag Ni Cu Au Rh Ru Ir Pd

2.221 2.035 1.932 2.156 2.042 1.973 1.971 2.081 1.887 1.903 1.997 2.044 2.046 2.037 1.992

shortest M−Ctrz

2.135 2.021 1.946 1.928 2.064 1.865 1.876 1.962 1.969 1.979 1.974 1.901

longest M−Ctrz

no. of complexes

ref

2.174 2.068 2.003 2.010 2.105 1.960 1.983 2.060 2.089 2.120 2.124 2.062

1 1 1 4 3 3 9 6 11 13 43 20 38 57 64

48 153 130 34, 177 74, 103 128, 129, 170 138, 154, 155, 190 41, 44, 45, 47−49 152, 153, 173−175 111, 123, 124, 127, 149−152 33, 49, 76, 112, 116−120, 122, 133, 139, 146−148, 191−193 45, 48, 66, 70, 83, 132, 133, 136, 137, 143, 144, 167 32, 44, 79, 80, 83, 92−94, 97−102, 104−106, 108, 110, 194 33, 35, 65−68, 71, 73, 74, 79, 80, 82−85, 88, 89, 171 48, 50−53, 55, 56, 58−61, 69, 75, 118, 131−134, 136, 159−161, 164, 169, 178, 186 R


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Figure 17. Diversity of chelation motives of functionalized triazolylidene ligands.

± 7 Å, the Re−Ctrz and Mo−Ctrz bonds are markedly longer (2.15 ± 2 and 2.22 Å, respectively). These longer bonds may point to a smaller attraction between the triazolylidene ligand and the transition metals with lower d-electron count. The M− Ctrz bonds in first-row transition metal complexes are typically shorter, 1.91 ± 6 Å for triazolylidene complexes of Co, Ni, and Cu, though the bond is considerably longer in the corresponding iron complexes (1.97 ± 4 Å), and the only structure of a triazolylidene manganese complex is even longer. The 2.035 Å length is the longest M−Ctrz bond of a first-row transition metal, though obviously a generalization is not possible based on the single structure known to date. A high trans influence has been established in several complexes. This trans influence is most obvious when comparing trans-bis(triazolylidene) complexes with related monocarbene analogues or isomers with a cis arrangement. For

example, cis-[Pd(trz)2Cl2] reveals a Pd−Ctrz bond of 1.995(5) Å, while this bond is markedly longer in the trans analogue (2.049(3) Å).58 Likewise, the Au−Ctrz bond is slightly longer in cationic gold(I) complexes than in neutral [Au(trz)Cl] complexes (2.03 vs 1.99 Å).117,191 The longest Au−Ctrz bond so far has been observed in a bis(triazolylidene)carbazolide pincer ligand with a pronounced T-shaped geometry.147 No structural distinction is observed when comparing structural features of mesoionic with nonmesoionic triazolylidene complexes.48,191 The reported structures of nonmesoionic triazolylidene Rh(I), Pd(II), and Au(I) complexes all display M−Ctrz bond distances close to the average of their mesoionic triazolylidene analogues. Various studies furthermore demonstrate that there is no obvious correlation between the observed M−Ctrz bond lengths in the solid state and the electronic nature of the trz ligand.118 S


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2.3.3. Triazolylidene Chelation. The synthetic versatility of the copper-catalyzed click reaction has been widely exploited for the preparation of a variety of bi- and polydentate triazolylidene ligands. In particular, bis(triazolylidene) complexes have been extensively investigated during recent years, including complexes of Re, 177 Ir, 68,70,71,80 Ru, 46,80,95 Rh,46,70,137,195 Fe,154 Ag,46 and Pd.46,52,64,165 These species can be divided in C4-linked bis(triazolylidene) species M64,68,70,71,80,95,137,154,165,177 and N3-linked bis(triazolylidene) complexes of type N (Figure 17).46,52,95,195 The linker between the triazolylidene heterocycles can be tailored over a wide range, including systems with no linker at all (i.e., directly connected heterocycles),68,71,80,95,137,154,177 alkyl linkers ranging from mono- to tetramethylene units,68,95,195 ethercontaining linkers, 68,70 sterically rigid ortho-phenylene groups,64 biphenyl linker that produces a chiral environment after metalation,52,165 and even a chiral binaphthyl backbone.46 Intriguing opportunities emerge when the linker between the two triazolylidene units is functional, e.g., because it contains a coordination site. This concept leads to Ctrz,E,Ctrztridentate coordinating dicarbene pincer complexes, in which the pincer ligand is typically meridionally coordinating. This coordination mode is obtained in complexes with pincer ligands comprised of two triazolylidene fragments that are connected through a pyridine (O; M = Ru,36,91,108,109 Pt,129 and Fe138) or a carbazole (P; M = Au,147 Ni,152 Cu,152 and Rh144). A central imidazole unit gives a tris(NHC) Ru complex Q,92 with a facially coordinating C,C,C-tridentate ligand. A similar bonding mode is accessible with a tris(triazolylidene) borate ligand, as demonstrated with the nickel complex 67 and the manganese analogue (68; see Scheme 15).153,185 Triazolium salts with pyridyl substituents give Ctrz,Nbidentate chelating complexes after metalation, irrespective of whether the pyridyl substituent is positioned either at N1 (R; M = Ir,13,74,86,171 Ru,74,93,95,99,100 Os,74 Ni173) or at C4 (S; M = Ir,65,69,71,77,88,196−198 Re,177 Ru,95,99,100,107 Ni,173 Pd,163 and Co130). Interestingly, when both C4 and N1 contain a pyridyl substituent, only the N-bound pyridyl site forms a chelate while the other pyridyl unit is idle (coordination type R).86 This selective bonding is attributed to the higher basicity of the N-bound pyridyl ring due to the electron-donating effect of the triazole nitrogen into that pyridine ring. The bite angle of the triazole−pyridyl chelate is acute, about 77°, which positions the metal closer to the chelating pyridyl and too remote to the second pyridyl for coordination. Related Ir, Ru, and Os complexes with a bidentate chelating pyrimidine- rather than a pyridine-substituted triazolylidene ligand are also known.96 Other Ctrz,N-bidentate chelating triazolylidene ligands coordinated to ruthenium or iridium include N-donors derived from pyrrole (T),44 triazole (U),71 BOC-protected amines (V),143 N-(methylpyridylidene)-amide (W),94 and benzoxazole or thiazole rings (X).67 Likewise, chelated dicarbene Rh, Pd, and Ir complexes Y bearing a linked normal−abnormal/ mesoionic NHC ligand have been studied.134,172 Triazolium salts with carboxylate functionalities afford C,Obidentate chelating species Z in the absence of any other donor group.100 Similar weak chelation is observed with olefins,110 though other donor groups substitute these chelation modes. For example, ruthenation of a triazolium ligand precursor with a carboxylate and a pyridyl donor group produces exclusively the Ctrz,N-bidentate chelate, leaving the carboxylate site unaffected. Alkoxide bonding and formation of a Ctrz,Obidentate chelating triazolylidene complex is accomplished

from hydroxyl- and ether-functionalized triazolylium salts (AA; M = IrIII).67,81 Related complexes with a Ctrz,S-bidentate chelate have been established with ruthenium, iridium, and osmium (AB).79 Similarly, tridentate systems with just one triazolylidene ligand site and either two N-donors (AC, AD)34,36 or two P-donor arms (AE)178 have been developed. Triazolylidene complexes that contain an aryl wingtip group undergo cyclometalation depending on the metal precursor and the reaction conditions (see section 2.3.4). Cyclometalation yields Ctrz,CPh-bidentate chelating triazolylidene complexes of type AF (C-bound aryl, M = Ru,97,102,104 Ir,75,82,85,170) or AG if the aryl group is linked to N1 (M = Pd,58,59,69,83 Pt,128 Ru,83,100,199 Ir,70,73,83,88,90 Ni,174 Rh83,90). Ditriazolium salts with a phenyl backbone give bimetallic Ctrz,CPh-cyclometalled species AH.85 Likewise, bis(triazolium) ligand precursors with phenyl substituents afford the Ctrz,Ctrz,CPh-tridentate coordinating bis(triazolylidene) complex AI upon metalation with Rh or Ir.70 Related cyclometalation with iridium(III) takes place if the phenyl substituent is replaced by a pyridinium group, which affords either a heterodicarbene complex AJ featuring a triazolylidene and a pyridylidene donor or an isomer comprised of a pyridine−ylide donor site (AK, see section 2.3.4).73,78,84,88 In addition to the large variety of potentially chelating groups that are readily incorporated into the triazolylidene scaffold, the versatility of the “click” cycloaddition has allowed for the synthesis of triazolium precursors with functionalities such as carboxylates,93 sulfinyl groups,49 estrone moieties,192 carbohydrates,89 ferrocenyl substituents,76,112,120,136 and fluorescent markers.69 Such functionality may become very useful for selected applicatons such as redox-switchable properties, molecular recognition, reaction monitoring, and possibly also chiral induction. While ditopic ligands with proximal donor groups or a flexible linker tend to produce chelating triazolylidene complexes, more rigid linkers prevent chelation and lead to dimetallic products. For example, a p-phenylene-bridged bis(triazolium) ligand precursor affords dimetallic complexes AL after complexation with palladium,74,139,164 platinum,170 rhodium,139 and gold (Figure 18).119,139 This ligand also

Figure 18.

provides access to a heterobimetallic [Ir,Pd] complex, which undergoes cyclometalation selectively at the iridium site.170 The diiridium species undergoes spontaneous double cyclometalation and affords complex AH (cf. Figure 17).85 Interestingly, while most metals bind to the directly linked bis(triazolylidene) ligand with chelate formation (cf. M, n = 0 in Figure 17), copper coordination leads to a bimetallic Cu2 species AM (n = 0, Figure 17).124 With longer aliphatic linkers, the reaction conditions dictate whether the bis(triazolylidene) is chelating or bridging between two metal centers (AM, AN).68,70,78 Likewise, triazolium salts with amide or amine functionalization give dimetallic species when complexed with gold or silver due to efficient coordination of both the triazolylidene as well as the N-donor ligand (for an example, see complex 1; Figure 4). T


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Polytopic NHC ligands have attracted much attention.200 The straightforward synthesis of triazoles and triazolium salts also provides access to polytopic triazolylidene complexes such as AO (Figure 19), in which a 1,3,5-substituted phenyl

Scheme 31

and for extended reaction time without any detectable loss of the triazolylidene ligand (Figure 20).171 Likewise, the presence

Figure 19.

backbone is present.35,164 The triazolylidene units may be flexibly attached by a −OCH2− unit139 or more rigidly connected to the phenyl core, either linked with another phenylene unit or no linker at all.75,145 In the absence of a linker, iridation again leads to spontaneous arene C−H bond activation to produce a triply cyclometalated product. Trigold species AP have been prepared as well as related tetrametallic triazolylidene gold units embedded in a macrocyclic steroidal environment containing four estrone residues192 or interlinked through a central pentaerythritol core.201 2.3.4. Stability and Reactivity. This section highlights aspects related to the stability of the triazolylidene metal complexes. Three major reactivity trajectories are considered: (i) intrinsic stability of the complexes toward decomposition; (ii) programmed rearrangements of the triazolylidene ligand or its substituents; (iii) specific reactions within the metal coordination sphere. Intrinsic Stability of Triazolylidene Metal Complexes. When considering the very soft base character of formally neutral C-donor ligands, it is not surprising that the most stable complexes are formed with soft Lewis acid metals such as the platinum group metals. In contrast, harder first-row transition metals are expected to be more susceptible to degradation. Such behavior is indeed noted for triazolylidene metal complexes. For example, triazolylidene iron(II) complexes such as 71 are stable in the solid state but decompose in solution to a paramagnetic species within a few hours if not manipulated under inert atmosphere.156 The corresponding nickel(II) complexes are more stable under neutral conditions. However, in the presence of a weak acid such as phenyl boronic acid, the nickel complex 101 (Scheme 27) decomposes instantaneously and no presence of triazolium salt or triazole is detected.174 Triazolylidene ruthenium complexes show a higher stability, and decomposition requires the addition of a stronger acid. Bertrand, Grubbs, and co-workers used this stability regime to prepare a ruthenium olefin metathesis catalyst with latent catalytic activity that is initiated by protonolysis of the more reactive Ru−Ctrz bond in complex 132 to produce the active catalyst 133 (Scheme 31).141 Similarly, the acetate-protected carbohydrate residue of the triazolylidene complex 134 is deprotected with HCl and leads to Ru−Ctrz bond cleavage and complex decomposition.89 These reactivity patterns contrast the extraordinarily high robustness of the IrIII−Ctrz bond, especially when chelate stabilized. Iridium complexes such as 90 can be exposed to 10 M HCl, even at elevated temperatures

Figure 20.

of KOH as a base does not lead to any notable reaction in these Ctrz,N-chelate complexes. This robustness has been utilized to develop an acid-based protocol for the deprotection of acetylated pyranose as a functional group bound to the triazolylidene in iridium complex 136 (Scheme 32).89 Scheme 32

Deacylation in methanolic HCl leads to complete loss of the acyl groups and formation of the carbohydrate-functionalized triazolylidene iridium complex 137, demonstrating the high stability of the Ir−Ctrz bond. The stability of cobalt complex 135 toward acids is demonstrated by its inertness in the presence of 50 equiv of acetic acid (Figure 20). No decomposition is observed after 24 h by NMR spectroscopy. The robustness of the M−Ctrz bond allows the use of complex 135 as catalyst under acidic conditions.130 Programmed Rearrangement of the Triazolylidene Ligand or Its Substituents. Exposure of the triazolylidene copper(I) complex 29 to CsOH results in the rapid cleavage of the copper center from the triazolylidene scaffold and formation of the mesoionic oxide 138 (Scheme 33).126 This reaction is selective and is assumed to proceed through a Cu− OH intermediate. Triazolylidene gold complexes are susceptible to carbene transfer, demonstrated by the formation of complexes [Au(trz)2]+ from the neutral complex [Au(trz)Cl] (Scheme 34).117 When exposing a homoleptic bis(triazolylidene) gold complex 139 to the neutral monotriazolylidene gold chloride complex, carbene migration is induced and affords the heteroleptic bis(triazolylidene) gold complex 140. This carbene migration is slow in the absence of additives; however, U


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Scheme 33

Scheme 34

temperature do not compromise the stability of the Pd−Ctrz bond. When using the 1,4-diphenyl-substituted triazolylidene ligand, the N1-bound phenyl group is cyclometalated exclusively.58 This same selectivity has been observed with all metal complexes that undergo cyclometalation and has been attributed to the higher electron density in the N-bound phenyl group, which is beneficial for an electrophilic activation process. Cyclometalation of the C-bound phenyl group has been achieved, though typically this C−H bond activation process requires harsher conditions (complexes AF; Figure 17). Deuteration experiments carried out with Ir complex 143 reveal metalation of both aromatic rings taking place during the reaction with hydrogen, with isotope exchange also on the pendant phenyl ring to eventually yield 143-d3 (Scheme 37).

it is very fast in the presence of catalytic amounts of silver(I). Presumably, this carbene transfer entails a retro-transmetalation from gold to silver, followed by the formation of the bis(carbene) gold complex. A similar carbene migration is observed for the triazolylidene nickel species 101 (Scheme 35).174 Upon heating the Scheme 35

Scheme 37 monotriazolylidene nickel complex 101 in solution or in the solid state in vacuum, a disproportionation is induced, yielding the bis(triazolylidene) nickel complex 141 along with nickelocene, [Ni(Cp)2]. Triazolylidene complexes that are comprised of a phenyl substituent are prone to undergo cyclometalation (see section 2.3.3). This bond activation process has been observed with several metals and depends on whether the phenyl group is bound to N1 or C4 of the triazolylidene ligand. Cyclometalation is spontaneous in triazolylidene iridium(III) complexes and occurs with N-bound phenyl, benzyl, or pyridinium substituents.66,73,83,90 Similarly, cyclometalation with ruthenium(II) or nickel(II) is favored and occurs spontaneously.83,104,174 Cyclopalladation of complex 14 requires NaOAc as an additive and is a reversible process (Scheme 36).58 Of note, the presence of base and elevated

In the ground state, however, exclusive cyclometalation of the N-bound phenyl ring is observed.202 Of note, the presence of H2 is essential for the isotope exchange. Heating complex 143 just in the presence of D2O and iPrOH-d8 does not lead to any deuterium incorporation into the complex. The triazolylidene iridium scaffold obtained after metalation of the triazolium salt 144 is a particularly active platform for C−H bond activation of pyridinium and benzyl substituents (Scheme 38). The selectivity of this cyclometalation is dictated by the electron density of the two substituents. Functionalization of the benzyl group with electron-donating groups favors the cyclometalation of this residue over the pyridinium site. The formed metalacycle is not stable, and a spontaneous oxidative addition and reductive elimination process then produces the ylide 146. In contrast, electron-withdrawing substituents on the benzyl moiety direct metalation toward the pyridinium ring and favor the formation of the triazolylidene− pyridylidene complex 145.73

Scheme 36

V


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Scheme 38

Specific Reactions within the Metal Coordination Sphere. The reactivity properties of ancillary ligands in the iridium complex 19a containing a chelating pyridyl−triazolylidene ligand have been investigated in detail (Scheme 42). While the

The triazolylidene gold(I) complex 147 exhibits yet another type of programmed transformation due to an intrinsic instability of the complex toward visible light, which promotes a disproportionation reaction and the formation of the (bis)triazolylidene Au(III) complex 148 (Scheme 39).146 This complex is a rare example of a triazolylidene gold(III) complex.

Scheme 42

Scheme 39

Another specific transformation of a triazolylidene substituent involves the decarboxylation of complex 149 (Scheme 40). Thermal and base-induced decarboxylation of complex chloride is robustly bound to the metal center in MeCN, changing the solvent to H2O results in rapid complex solvolysis and formation of the aqua complex 153a.197 Likewise ammonia and aniline adducts have been formed. Ligand competition experiments reveal that the stability of the ligand increases along the series H2O < MeCN < aniline < NH3 < Cl.203 The unusual T-shaped gold(I) complex 154 with a bis(triazolylidene)carbazolide pincer ligand exhibits intriguing reactivity patterns (Scheme 43).147 In CH2Cl2, C−Cl bond activation takes place and affords the formal Au(III) complex 155 as a square-planar complex. Treatment of complex 154 with either excess trifluomethanesulfonic acid or trifluoroacetic acid leads to a formal oxidative ligation producing Au(III) hydride complex 156. The hydridic character is supported by the high-field NMR signal. However, the stability of this hydride toward acids is remarkable, and its reaction with a hydride to form H2 suggests a pronounced protic character of this hydrogen. Oxidative ligation of a CH3+ electrophile produces the gold(III) methyl complex 157 or the gold(I) complex 158 with an alkylated carbazole unit as a consequence of different steric hindrance of the Mes and Dipp substituents.

Scheme 40

149 provides complex 150a with the C5 position unsubstituted.93 Attempts to trap the decarboxylation process with a metal center have not been successful so far. Finally, the cyclometalated triazolylidene iridium complex 151 features an Ir−CPh bond that is sufficiently reactive for insertion of an alkyne to yield complex 152 (Scheme 41).90 When using an internal alkyne for the insertion, the configuration at the metal center is maintained during the alkyne insertion. With methyl propiolate two regioisomers are formed, yet both are enantiomerically pure. Scheme 41

2.4. Catalytic Applications

2.4.1. General Overview. This subsection aims to provide a brief qualitative overview of the catalytic application according to the type of reactions catalyzed by triazolylidene metal complexes. The subsequent sections compile the catalytic applications sorted by type of metal, and more specific details are collected there. 1,2,3-Triazolylidene metal complexes are efficient catalyst precursors for a great variety of W


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Scheme 43

complexes of Cu,149 Ni,173 and Fe.156 Other hydrogen-related transformations include electrochemical proton reduction and catalytic hydrogen formation which are catalyzed by a triazolylidene cobalt complex.130 Catalytic oxidation of alcohols and amines is imparted by triazolylidene Ru, Os, Rh, and Ir complexes, in some processes without the need of any additive or terminal oxidant (i.e., direct dehydrogenation). Triazolylidene Ru and Ir complexes furthermore demonstrate high activity in water oxidation, a key challenge en route to artificial photosynthesis. Moreover C(sp3)−H bond activation and oxidative transformation is catalyzed by Ir complexes,71 and a proof of concept for noctane oxidation has been demonstrated with a triazolylidene Ni complex.175 Epoxidation of cyclooctene takes place with a “normal” triazolylidene molybdenum complex.48 The application of triazolylidene metal complexes in oxidation catalysis is particularly attractive,205 as these mesoionic carbenes constitute a rare class of ligands that combine a soft ligand character with high resistance toward oxidation and with strong donor properties for the stabilization of high-valent intermediates. A list of additional catalytic reactions that are mediated by triazolylidene metal complexes is compiled in Table 5. This list may serve as a point of reference for the subsequent sections, in which the catalytic activity of triazolylidene complexes are discussed according to the nature of the metal center. 2.4.2. Triazolylidene Palladium Catalysis. Suzuki− Miyaura Cross-Coupling. Many triazolylidene palladium complexes have been investigated in cross-coupling reactions, especially in Suzuki−Miyaura-type aryl−aryl cross coupling.50,51,55,60,61,63,75,131,160,162,169 Inspired by the high activity of Arduengo-type NHC palladium complexes with allyl and pyridine ancillary ligands,208−210 analogues with a triazolylidene rather than an Arduengo-type NHC ligand have been developed. Complexes 9 and PEPPSI-type complexes 77 and 159 indeed catalyze aryl cross-couplings (Scheme 44), though activities are orders of magnitude lower than the benchmark complexes with 2-imidazolylidene ligands. Allyl complex 9 (R = R′ = Dipp; R′′ = Ph; Dipp = 2,6-diisopropyl phenyl) catalyzes the coupling of various aryl chlorides with boronic acids in the presence of KOtBu at room temperature, reaching TON up to 1140 after 15 h.60 The catalytic species may also be formed in an in situ procedure involving the addition of the triazolium salt, [Pd2(dba)3] (dba = dibenzylidene acetone), and Cs2CO3 and the substrates without preformation of the triazolylidene complex 9 as catalyst precursor.166 This mixture

reactions including C−C bond formation, oxidations, and reductive transformations. Table 4 summarizes the different Table 4. Catalytic Reactions Catalyzed by Various Triazolylidene Metal Complexes major reaction type

triazolylidene metal complexes

cross coupling olefin metathesis cyclizations polymerization hydrogenation transfer hydrogenation hydrosilylation alcohol/amine oxidation oxidative coupling water oxidation

Pd, Ni Ru Au, Ag Ru, Mo Pd, Ru Ir, Ru, Rh, Os Ir, Rh, Cu, Ni, Fe Ir, Ru, Rh, Os Ru, Ir Ir, Ru

classes of reactions and the metals that have been used. When considering the major types of reactions, viz, redox-neutral reactions (cross-coupling, metathesis, substitution), reductions (hydrogenation, transfer hydrogenation, hydroelement additions), and oxidations (including oxidative couplings), a few general patterns emerge. The area of redox-neutral reactions unveils little surprises in terms of metals employed. Cross-coupling catalysis, predominantly Suzuki−Miyaura-type aryl−aryl coupling, has been studied with triazolylidene palladium complexes and more recently also with nickel analogues.174 Ruthenium complexes with triazolylidene ligands are active catalysts for olefin metathesis,32,141 and polymerization reactions are catalyzed by ruthenium (norbornene polymerization)44 and molybdenum complexes (dicyclopentadiene polymerization).176 Cyclization reactions are accomplished with triazolylidene silver and gold complexes, including the condensation of aldehydes and isonitrile esters to produce oxazolines. Hydrogenation of organic substrates, in particular, via transfer hydrogenation, is catalyzed by various triazolylidene complexes of Ir, Ru, Rh, Pd, and Os.204 Specific triazolylidene Ru complexes also catalyze the direct hydrogenation of esters,91 alkenes, and alkynes in the presence of H2.97,102 Hydrosilylation as a valuable alternative to (transfer) hydrogenation of ketones and aldehydes is catalyzed by precious metal triazolylidene complexes (Ir,171 Rh46), though recently also highly efficient procedures based on earth-abundant transition metals have been developed, including triazolylidene X


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PEPPSI complex 159 containing a triazolylidene ligand with mesityl substituents and 3-chloropyridene as a “throw-away” ligand shows moderate catalytic activity for aryl bromide conversion at 50 °C (TOF50 about 70 h−1).55 At higher temperatures, complex 159 decomposes. Variation of the wingtip groups of the triazolylidene ligand reveal that smaller substituents induce higher activity,55,56,60 a trend opposite to 2imidazolylidene-based catalysts where steric congestion improves catalytic activity.211 For example, complex 159 with R = Ph and R′ = Me accomplishes a TOF50 up to 270 h−1. These PEPPSI-type complexes are also effective in the arylation of aryl chlorides, reaching up to 800 TON when the reaction is performed in EtOH at room temperature and with KOtBu as base.131 On the basis of the successful conversion of aryl chlorides, a molecular catalyst is surmised. Mechanistic investigations indicate no poisoning effect of phosphine, yet complete inhibition of catalysis in the presence of mercury and transmission electron microscopy (TEM) and MS demonstrate the formation of nanoparticles.55,75,160 These results provide evidence for a critical role of a heterogeneous phase, presumably as a reservoir for the catalytically active species. A molecular species as active catalyst is further supported by a recent study with triazolylidene palladium complexes that comprise chiral ferrocenyl wingtip groups. These ligands induce very high enantioselectivities in the crosscoupling of naphthyl substrates.212 Complexes 12, 14, 160, and 161 represent variations of the PEPPSI system in which the chloropyridine “throw-away” ligand is substituted. Dimeric complex 14 (R = R′ = Dipp) displays slightly lower activity than the pyridyl analogue 159 in the room-temperature arylation of aryl chlorides,131 while the catalytic performance of the bis(triazolylidene) complex 12 is essentially identical to that of the PEPPSI system 159. This latter result suggests that one of the triazolylidene ligands in 12 is lost during catalyst activation similar to pyridine dissociation from complex 159. Triazolylidene dissociation is in agreement with a strong trans influence of the triazolylidene and furthermore points to a limited stability of the palladium− carbene bond in these complexes. Similar reactivity patterns

Table 5. Additional Catalytic Reactions Arranged by Metal metal

applications

Pd

hydroarylation C−N bond formation (N-aryl amination) oxidation of sp3 C−H bonds norbornene polymerization olefin metathesis hydroformylation hydrothiolation alkynes formylation amines with CO2 and Ph2SiH2 enanstioselective hydrosilylation ketones dimerization−hydrothiolation tandem hydrohydrazination alkenes and alkynes carbene transfer cyclizations allylic etherification hydroamination hydroalkoxylation of allene cycloisomerization−dimerization cascade hydroboration of alkenes click chemistry

Ir Ru Rh

Au

Cu

Ni Ag Mo Co

C−H carboxylation oxidation of n-octane oxazoline synthesis epoxidation polymerization (dicyclopentadiene) electrocatalytic H2 formation

ref 59 159 71 44 32, 141 136 143 195 46 144 146, 148, 201 116, 119, 192 115−120, 122, 139, 193 118 191 119 206 123 111, 124, 127, 150, 151, 207 125 95 41 48 176 130

provides a catalytically active species, though remarkably, a control experiment with [Pd2(dba)3] only reveals identical conversion rates. Therefore, carbene dissociation from 9 is plausible as a catalyst activation pathway. Nonmesoionic triazolylidene Pd(II) analogues are only mediocre catalysts for the coupling reaction due to the limited stability of the complex under the catalytic conditions.48 Scheme 44

Y


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Complexes 77w,x, 163, and 164 catalyze the fluoride-free Hiyama coupling of aryl bromides and iodides with PhSi(OMe)3 in the presence of NaOH (Scheme 47).51,161 Complex 77w with a trans-positioned pyridine ligand exhibits consistently higher conversions than the trans-PPh3 analogue 163. This higher performance has been attributed to the stronger coordination of the triazolylidene ligand in complex 77w as indicated by the shorter Pd−Ctrz bond distance in 77w and the higher bond dissociation energy as predicted by DFT (80 kcal mol−1 for 77w vs 65 kcal mol−1 for 163).161 Furthermore, pyridine is more labile than PPh3 in these complexes. Remarkably, the best performance is achieved with complex 164, which is the cis homologue of 163, reaching up to 97% conversion at a 2 mol % catalyst loading (80 °C). PEPPSI-type complexes related to 77w comprised of a triazolylidene ligand with one or two adamantyl substituents are also precatalysts for the Hiyama coupling, with performance similar to 77w.51 The mono- and bimetallic PEPPSI-type complexes 165 and 16656 as well as the bis(triazolylidene) palladium complexes 167 catalyze the arylation of olefins (Mizoroki−Heck crosscoupling; Scheme 48).63,213 Arylation of methyl acrylate with p-iodoacetophenone is catalyzed by complexes 165 or 166 and reaches essentially full conversion within 2 h when the reaction is performed at 80 °C and with 2 mol % catalyst loading.56 Longer reaction times and higher temperatures (125 °C) are required for converting electron-deficient aryl bromides. Under these conditions, however, catalyst decomposition and formation of palladium black is observed. Moreover, elemental mercury effectively poisons the catalytic reaction, pointing to the presence of a heterogeneous phase that is relevant for catalytic turnover, much as revealed for Suzuki−Miyaura crosscoupling (see above). Complex 168 containing two chelating pyridyl-triazolylidene ligands catalyzes the Sonogashira reaction in water without the need of any additive (Scheme 49).163 Under these copper-free conditions, full conversion of a large variety of substrates including both electron-poor and electron-rich (hetero)aryl bromides is achieved with 1 mol % 168 at reflux temperature (1−4 h reaction time). In comparison, complex 167a with two monodentate bonding triazolylidene ligands (cf. Scheme 48) is considerably less active and only converts electron-poor aryl bromides, while aryl halide with electron-donating substituents gives moderate to low conversion.213 The mixed phosphine triazolylidene complex 169 catalyzes the carbonylative alkynylation of aryl iodides to afford fluorescent 1,3-diarylpropynones (Scheme 50).62 Under an atmosphere of CO (1 bar), phenyl, naphthyl, carbazolyl, and pyrenyl derivatives are obtained in good yields within 24 h at 80 °C using 5 mol % of catalyst loading. Under these conditions, formation of the diarylethyne products due to competitive Sonogashira coupling is suppressed, indicating that carbonylation of the aryl palladium intermediate after oxidative Ar−I addition is much faster than the transmetalation step en route to the reductive C−C coupling. Traces of products derived from the oxidative dimerization of the terminal alkynes are detectable. The mixed triazolylidene benzimidazolylidene complexes 127 are precatalysts for the direct arylation of pentafluorobenzene containing an activated C−H bond (Scheme 51).186 Direct C−H activation is relevant as it avoids the prefunctionalization of one coupling partner. Catalysis with complex 127 requires elevated temperatures (120 °C) and

are observed with diiodo analogues of complexes 12, 14, 159, and 160 containing phenyl-substituted triazolylidene ligands, which catalyze the room-temperature coupling of 4-bromoanisol with phenyl boronic acid.160 Complex 12 catalyzes multiple Suzuki−Miyaura coupling reactions when polybromoarenes are used as substrates. In the presence of PPh3 and NaOH, TONs up to 1260 are reached after 13 repetitive substrate addition cycles when using phenylboronic acid and 1,4-dibromobenzene as coupling partners.106 The related palladium complex 17 bearing a trans-chelating bis(triazolylidene) ligand (cf. Figure 5) shows considerably less activity in the arylation of 1,4-dibromobenzene (88 TON),52 presumably because chelation disfavors triazolylidene dissociation and hence hampers catalyst activation. The chiral triazolylidene palladium complex 161 catalyzes the coupling of several aryl bromides with arylboronic acids, although a temperature of 75 °C is needed.64 Sterically more demanding naphthyl substrates fail to undergo cross-coupling reactions because of a competing deboronation process. Triazolylidene palladium complexes 73 containing an acac ligand catalyze the Suzuki−Miyaura cross-coupling reaction, though these systems are less active than the PEPPSI-derived complexes. At room temperature and using a 0.5 mol % catalyst loading and K2CO3 as a base, they reach about 80% conversions (160 TON).169 Other C−C Cross-Coupling Reactions. Triazolylidene allyl Pd(II) complexes 9 produce efficient catalysts for the αarylation of propiophenone with aryl bromides (Scheme 45). Up to 90% conversion is reached with a variety of aryl bromides at a 3 mol % catalyst loading and using NaOtBu as a base.140 Scheme 45

Similarly, complexes 16, 162, and the trinuclear PEPPSI species 88 (cf. Scheme 22) provide active catalysts for the αarylation of cyclic amides in the presence of NaOtBu (Scheme 46). The trinuclear complex displays the best performance of this set, accomplishing 91% conversion for the arylation with 4bromotoluene (5 mol % Pd, 120 °C, 16 h).50,75 Scheme 46

Z


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Scheme 47

Scheme 48

Scheme 49

Scheme 50

leads to detectable amounts of Pd black. This course of action points to a heterogeneous mechanism, which may be related to the mechanistic conclusions drawn from Heck and Suzuki-type coupling reactions. C−N Cross Coupling, C−H Activation, and Hydrogenation Reactions. Complexes 14, 74, 77, and 170 are precatalysts for the amination of aryl bromides (Buchwald− Hartwig amination, Scheme 52).159 PEPPSI-type complex 77x shows the best performance, with almost quantitative conversion to N-phenylmorpoline at 3 mol % catalyst loading (50 °C, 6 h). No palladium black has been observed under these conditions. Despite the established success of palladium

complexes in catalyzing hydro-element additions to unsaturated substrates, this area of catalysis has barely been explored in triazolylidene palladium chemistry, with the exception of an early example of a hydroarylation of electron-deficient terminal alkynes catalyzed by the acetate-bridged palladium dimer 11 featuring a Ctrz,CPh-bidentate coordinating triazolylidene ligand (Scheme 53).59 Direct hydrogenation of alkenes and alkynes is mediated by palladium complex 171 containing a chiral [2.2]paracyclophane-functionalized triazolylidene ligand, Scheme 54. Complex 171 with a labile MeCN ligand shows good performance even though aldehydes and nitro groups are AA


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Scheme 51

Scheme 53

hydrogenated as well, whereas the PPh3 analogue 172 is catalytically silent.57 For example, complex 171 hydrogenates E-stilbene completely within 7 h when using 2 mol % precatalyst at room temperature. A color change of the reaction mixture from yellow to black is noted during the transformation only if 171 is used but not with 172. Similar observations have been noted in the olefin hydrogenation catalyzed by a mesoionic imidazolylidene palladium complex, which has been demonstrated to be associated with the formation of colloidal palladium as catalytically active species (cf. section 3.4).214 Summative Considerations. Triazolylidene palladium catalysis has been dominated so far by a remarkably small set of structural motifs and catalytic reactions. The general structural motif is defined by a [PdX2(trz)L] with L either a (substituted) pyridine, another carbene, or one of two bridging halides. These complexes have been applied for a variety of cross-coupling reactions, with the common feature that in most of these reactions, a catalyst heterogenization appears as the most likely mode of action. Apart from Fukuzawa’s chiral triazolylidene system that imparts very high asymmetric induction,212 no compelling evidence for a molecularly defined active species has been provided. Phosphine spectator ligands (L = PR3) and a more creative ligand design may enhance the stability of the active species (or its resting state). Likewise, palladium catalysis beyond C−C cross-coupling has been only little explored so far. 2.4.3. Triazolylidene Iridium Catalysis. Transfer Hydrogenation. Triazolylidene iridium(III) and iridium(I) complexes have been extensively used as precatalysts for the transfer hydrogenation of different substrates such as ketones, aldehydes, and nitro compounds. Generally, these reactions use iPrOH as the hydrogen donor and require the presence of

an external base such as KOH or KOtBu.35,67,68,74,75,80,81,85,163,172 Even though efforts in transfer hydrogenation are currently directed toward developing processes that eliminate the need for an external base, to the best of our knowledge there has not been a triazolylidenecontaining catalyst developed so far that operates under basefree conditions. Transfer hydrogenation catalysis with triazolylidene iridium(III) precursors shows a high dependence on the coordination mode of the triazolylidene ligand. A variety of bonding motifs has been evaluated including monodentate triazolylidene complexes (173, 174), C,N- and C,C-bidentate chelating triazolylidene systems (175−178), as well as polytopic scaffolds leading to bi- and trimetallic complexes (86, 87, 179−181), Scheme 55. Comparative analyses are limited by the fact that different conditions and model substrates are used in different laboratories. Nonetheless, some trends are apparent. The monodentate iridium(III) complex 174 is only moderately active in the transfer hydrogenation of benzophenone (TOF50 = 12 h−1 with 174a).81 The activity of 177 with a precoordinated ether group is not significantly different, in agreement with the activity of other monodentate carbene iridium complexes and suggests no substantial influence of the oxygen donor in this reaction. Complexes 176 with a chelating pyridyl group show somewhat enhanced activity under similar conditions.74 Introduction of functional groups to the C-bound phenyl group efficiently tailors the catalytic activity of the iridium center. While electron-donating substituents lower the turnover frequency, electron-withdrawing groups enhance the activity, with complex 176g reaching 93% conversion in 3 h at 0.5 mol % catalyst loading. The same electronic influence has been noticed in the catalytic reduction of olefins.96 Changing

Scheme 52

AB


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Scheme 54

Scheme 55

AC


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iridium(III) analogues. The methoxy-substituted triazolylidene iridium(I) complex 182 transfer hydrogenates a large variety of substrates including ketones, aldehydes, and imines with initial rates reaching up to 2300 h−1 using 0.5 mol % catalyst loading and NaOiPr as a base (5 mol %; Scheme 55).67 This complex is also a rare example of a catalyst that is active in the transfer hydrogenation of nonpolarized double bonds as in mono- and disubstituted olefins, including (E)- as well as cyclic (Z)olefins. Catalytic rates are lower than for CO or CN reduction, though still reaching appreciable TOFini up to 260 h−1. Even trisubstituted olefins are hydrogenated efficiently, though extended reaction times are required to reach high conversions. The related complex 183 with a benzoxazole substituent at the triazolylidene ligand rather than the methyl ether is about 3 times less active, which may be attributed to inhibiting imine chelation. More robust chelation as in the heterodicarbene iridium(I) complex 184 comprised of a imidazolylidene and triazolylidene ligand induces even lower catalytic activity with maximum TOFs in the double digits (90% conversion after 3 h at 1 mol % catalyst loading).172 Of note, the scope of the catalytically active species is similar to complex 182, and reduction of an imine and an alkene has been demonstrated. Hydrogenation, Hydrosilylation, and Related Reactions. The chelating triazolylidene complexes 19a and 143 catalyze the direct hydrogenation of quinolines (Scheme 57).202 At a

the chelating group from a pyridyl to a second triazolylidene unit as in complex 175 further enhances the catalytic activity (TOF50 = 60 h−1 for the bis(triazolylidene) complex without a linker, n = 0).68 Optimization of the linker to a tris(methylene) unit more than doubles the catalytic activity (TOF50 = 140 h−1). The analogous monometallic complex 173 (n = 0) with a pendant triazolium salt displays about the same activity, pointing to a similar active species. Presumably, the pendant triazolium unit is metalated under the basic catalytic conditions. The dinuclear complex 180, with a doubly cyclometalated phenylene core, displays considerably higher catalytic activity than the mononuclear analogue 178 and reaches 70% vs 24% conversion of acetophenone after 1 h at 0.5 mol % iridium concentration, albeit at rather high base loading (20 mol % KOH). The diverging activity of these two complexes points to an electronic activation of the iridium centers in the dimetallic complex as a potential mode of cooperativity.85 Such effects are much less pronounced in the tri-iridium complex 181, which is only slightly more active when compared to the corresponding monometallic species containing the same Ctrz,CPh-bidentate triazolylidene chelate (40% vs 24% conversion).75 This attenuated cooperativity may be rationalized by the weaker electronic communication of the iridium centers through a triphenyl linker in 181 as compared to the phenylene linker in 180. In line with such a model, the nonchelated di- and tri-iridium complexes 86 and 87 show moderate transfer hydrogenation irrespective of the halide at iridium,35 and the catalytic activity is not significantly different from that of monometallic analogues (see above). Likewise, the bimetallic complex 179 (n = 3) shows only modest activity in the transfer hydrogenation of benzophenone with TOFini of 20 h−1, which is almost an order of magnitude lower than the chelating and monodentate analogues 175 and 173, respectively. Hence, a polymetallic catalyst system is only prolific if the iridium centers are electronically interacting, as in phenylene-linked complex 180. However, the gain of activity is on a moderate level, and the activity of any of these triazolylidene iridium(III) complexes is more than 2 orders of magnitude lower than the most active iridium-based transfer hydrogenation catalysts known to date.215 It is worth noting that while complexes 86 and 87 do not show particularly high activity, these complexes exhibit diverging selectivity in the reduction of nitrobenzene via transfer hydrogenation (Scheme 56).35 With the dinuclear

Scheme 57

moderate pressure of 5 bar H2 and with 1 mol % catalyst loading, full hydrogenation of the heterocycle is achieved. Interestingly, while complex 19a is active in H2O and iPrOH, complex 143 with a Ctrz,CPh-bidentate chelate requires iPrOH as medium and is inactive in H2O. In related reduction catalysis, the triazolylidene iridium complex 185 catalyzes the hydrosilylation of acetophenone (Scheme 58) reaching a TOF50 of 2000 h−1.171 The catalytic activity is further enhanced when replacing the mesoionic triazolylidene by a stronger donating 4-imidazolylidene, which achieves conversion rates up to 6000 h−1 (see section 3.4.2). The presence of a labile solvento ligand is essential as the corresponding monocationic iridium chloride complex 90 is catalytically silent. Changing the nature of the silane from Et3SiH to Ph2SiH2 modulates the product selectivity of complex 185 and induces a dehydration process to form the symmetric ether.216 The same types of products are also formed when using the reduced benzylic alcohol as starting material; however, reactions are substantially slower and suggest that ketone reduction is not the first step in the ether formation. Aliphatic

Scheme 56

Ir(III) complex 86, azoxybenzene is formed predominantly whereas the trinuclear complex 87 favors the formation of azobenzene. Interestingly, the bidentate chelating ditriazolylidene complex 175 (n = 0) is selective for azoxybenzene, suggesting that the pendant triazolium site in the dimetallic complex 86 may not be innocent under the basic reaction conditions. The product selectivity with these iridium(III) complexes contrasts with those of related ruthenium complexes, which yield aniline and azoxybenzene in a 2:3 ratio.80 Low-valent triazolylidene iridium(I) complexes provide access to substantially more active catalysts than their AD


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Scheme 58

favors the formation of imine products. Spectroscopic and crystallographic analyses indicate cooperative amine substrate bonding through hydrogen bonding of the amine N−H to the noncoordinating pyridyl site. Carbohydrate-functionalized triazolylidene iridium (and ruthenium) complexes are also active catalysts for the homocoupling of benzylamine under base-free conditions.89 Turnover frequencies of 16 h−1 are obtained with complex 137 containing a deprotected carbohydrate unit, considerably higher than the 9 h−1 reached with 136 containing an acylprotected sugar moiety. The activity of the protected ruthenium analogues is considerably higher (see section 2.4.4); however, the influence of the triazolylidene ligand is more pronounced with iridium, as the [IrCp*Cl2]2 precursor is essentially inactive, and catalytic activity is only induced upon triazoylidene coordination. Alcohol oxidation under base-free conditions is catalyzed by the mono- and bimetallic complexes 92 and 187−190 (Scheme 60). The bimetallic complex 92 containing a bridging ligand featuring a N,N-bidentate and a C,C-bidentate coordination site is less active than the monometallic analogues containing either only the N,N- or only the C,C-chelate (complexes 187 and 188, respectively), yet selectivity toward benzaldehyde formation is enhanced.87 The monometallic complexes yield considerable amounts of the ether product, while the bimetallic system produces the dehydrogenation product with good selectivity and in high yields. Spectroscopic and electrochemical analysis indicate a substantial interaction between the two iridium centers, corroborating a more enhanced activity of phenylene-bridged diiridium complexes

ketones and alcohols are converted much slower, a reactivity difference that enables the formation of methyl ethers from ketones and MeOH, when the latter is employed as a cosolvent. Alcohol and Amine Oxidation. A variety of triazolylidene iridium complexes display interesting catalytic activity in the acceptorless and oxidant-free dehydrogenation of amines and alcohols. For example, complexes 176a and 186 containing either an inert phenyl group or a basic pyridyl substituent catalyze the oxidative coupling of amines with concomitant production of H2 gas (Scheme 59).86 Under standard Scheme 59

conditions, that is, 5 mol % iridium loading and 150 °C reaction temperature, a mixture of products is obtained that consists of benzylidene benzylimine, dibenzylamine, and traces of the tertiary amine. Complex 186 with a pendant pyridyl unit Scheme 60

AE


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Water Oxidation. Iridium complexes 19a,b, 90, 146, and 193−196 bearing triazolylidene ligands have demonstrated high activity as catalyst precursor for water oxidation using cerium ammonium nitrate, [Ce(NO3)6](NH4)2 (CAN), or NaIO4 as sacrificial oxidant (Scheme 62).84 This reaction requires ligands that are resistant toward oxidation and strongly donating in order to stabilize high-valent metal centers such as metal−oxo species. Mesoionic carbenes fulfill these criteria very well and have emerged as highly reliable ligands to iridium for catalytic water oxidation. The catalytic properties of triazolylidene iridium complexes strongly depend on the substitution pattern of the triazolylidene ligand and provide guidelines for further catalyst design. For example, steric bulk on the N1 position decreases the turnover frequency. Moreover, coordination of a C-bonding pyridylidene chelating group (193) imparts higher activity than a Nbound pyridyl chelate (90).88 Despite a contradictory report,217 a series of (sub)stoichiometric experiments, in situ DLS measurements, and kinetic analysis strongly support a molecularly defined catalytic system, at least in the time range of highest turnover frequency.88 Robust chelation is advantageous, though monodentate complex 194 achieves high catalytic activity in CAN-mediated water oxidation, and is also operational in a photoelectrochemical device using hematite as the photoelectrode.82 In contrast, the ylidic chelate 146 does not lead to high turnover numbers.84 Complex 193 shows excellent catalyst integrity and reaches turnover numbers of around 40 000, though turnover frequency is moderate (about 8 min−1).88 High robustness is also supported by the stability of the Ir−C bonds even under harsh acidic conditions (10 M HCl, 80 °C), indicating stability of this complex in the acidic environment generated by aqueous CAN.171 Comparison of monometallic catalyst precursors 193 with the homologous dimetallic complex 196 demonstrates diverging activity, which suggests a change in the mode of action of the two systems.78 Moreover, this comparative investigation also evidences that complex degradation is not relevant for generating the active species, as the dimetallic and monometallic species should both provide the same decomposition products.

in transfer hydrogenation (cf. 180, Scheme 55) than the related monometallic species. Complexes 136 and 137 with a carbohydrate-functionalized triazolylidene ligand (cf. Scheme 59) displays moderate conversion (TOF = 1 h−1) though reach full conversion at 5 mol % catalyst loading.89 The C,N-bidentate chelating triazolylidene Ir(III) complexes 189 and 190 show slightly higher initial turnover frequencies of 4 h−1, though high catalyst loading and extended reaction times are still needed to reach high conversions.67 Related triazolylidene Ir(III) complexes with a methyl ether wingtip group as well as analogous iridium(I) systems impart slower rates, and the lowvalent precursors also display limited stability under catalytic conditions.67 The C,S-bidentate chelating triazolylidene iridium(III) complex 191 has been tested as catalyst for the oxidation of benzyl alcohol in the presence of N-morpholine oxide (NMO) as sacrificial oxidant (Scheme 61).79 However, under these Scheme 61

conditions, only 15% conversion is reached after 3 h reaction, and both benzaldehyde and benzoic acid are formed in a 7:2 ratio. The related osmium(II) complexes show higher activity than the Ir complex.79 Interestingly, the N,S-bidentate chelating triazole iridium(III) complex 192 is a better catalyst than its triazolylidene analogue. Scheme 62

AF


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Scheme 63

Scheme 64

Other Oxidation Reactions. The bis(triazolylidene), triazolylidene−pyridyl, and triazol−triazolylidene chelated iridium complexes 19d, 175, and 197 are catalyst precursors for the oxidation of cyclooctane in the presence of NaIO4 as sacrificial oxidants (Scheme 63).71 Among these complexes, the pyridyl-substituted triazolylidene in complex 19d imparts the highest activity, yielding 55% of cyclooctanone and minor quantities of the cyclooctadione after 24 h at 0.5 mol % catalyst loading. When using 3-chloroperoxybenzoic acid (m-CPBA) as terminal oxidant, cyclooctanone is also the major product (40%), though substantial quantities of cyclooctanol (19%) are formed. 2.4.4. Triazolylidene Ruthenium and Osmium Catalysis. Ruthenium triazolylidene complexes have been widely

Replacement of the triazolylidene substitution in 19a from a methyl to an octyl group in 19b induces a remarkable increase of catalytic activity by more than 1 order of magnitude.77 After an initial period of a few minutes, during which both catalysts 19a and 19b display the same activity (TOF ca. 10 min−1), complex 19b enhances activity by nearly a factor of 10 and reaches turnover frequencies of almost 120 min−1. Exhaustive analysis of the reaction indicates degradation of the Cp* ligand at early stages218,219 and supports a robust chelation of the Ctrz,Npyr-bidentate ligand. Moreover, diffusion NMR spectroscopy suggests the formation of tri- or tetrameric aggregates, which may rationalize the high activity after a short induction period. However, no higher aggregates such as micelles have been detected. AG


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studied as catalysts for both (transfer) hydrogenation and oxidation reactions yet less so for olefin metathesis. Transfer Hydrogenation. Complex 198a containing a Cbound pyridyl as chelating substituent is an active catalyst for transfer hydrogenation of ketones, reaching TOF50 values up to 1400 h−1 in the presence of base (0.1 mol % catalyst loading; Scheme 64).100 Considerably lower activity was reached with related complexes with a monodentate triazolylidene, or analogues containing a Ctrz,CPh-cyclometalated ligand, or chelating ether, hydroxyl, carboxylate, or pyridylideneamide (PYA, 199) groups, or a N-bound pyridyl substituent.81,94,100 Variation of the latter ligand set by incorporation of an aryl group as the C-bound substituent on the triazolylidene ligand considerably improved catalytic activity.74 Variation of the electronic nature of the aryl substituent of complex 124 has revealed that electron-donating substituents enhance the catalytic activity, leading up to 10 000 TONs. Note that the electronic effect is opposite to that observed for related iridium(III) complexes. While these trends insinuate a correlation between electron density at ruthenium and the catalytic activity, no such trend is obvious when changing the nature of the chelating group (see above). For example, replacing the pyridine chelate by a less electronrich pyrimidine improves catalyst activity. Related osmium(II) complexes show the same electronic effect as complex 124. However, lower conversions are obtained (e.g., TON up to 1760 for the reduction of acetophenone with the Os(II) analogue of 124c vs 8900 with the ruthenium species under identical conditions).96 Further catalyst optimization has aimed at introducing functional groups to explore potentially synergistic ligand− metal interactions in an attempt to facilitate the hydrogen transfer. However, lower catalytic activity is shown with complexes 200 and 149 containing a pendant pyridyl or carboxylate functionality as potential hydrogen shuttle.93 The tridentate tris(carbene) ruthenium complex 201 containing a 2-imidazolylidene ligand with two adjacent triazolylidene units is a more robust catalyst for transfer hydrogenation than the related N,C,N-chelate species with two N-bound triazole groups instead of the triazolylidenes.92 While full conversion is reached within 2 h with complex 201 (0.05 mol % catalyst), the reaction does not exceed the 80% conversion with the imidazole-derived catalyst. This observation may be attributed to a stronger chelation and a higher electron density at the metal center in complex 201. The ruthenium(0) complex 202 containing a triazolylidene ligand is catalytically silent on its own, though it catalyzes the transfer hydrogenation of 4-fluoroacetophenone when activated with CAN as oxidizing agent.98 In the reduced state, the soft ruthenium(0) center may disfavor interactions with oxygen, which however is essential for transfer hydrogenation to proceed, both for iPrOH activation and for binding of the carbonyl substrate. Direct Hydrogenation and Reductive Coupling Reactions. Triazolylidene ruthenium complexes have been employed as precatalysts for the direct hydrogenation of esters,91 alkenes, and alkynes97,102 in the presence of molecular hydrogen. The hydride species 203, prepared by reaction of the dichloride analogue with Et3SiH, hydrogenates alkenes under moderate H2 pressure (4 bar; Scheme 65).102 This transformation is also catalyzed by mono- and bidentate triazolylidene ruthenium(II) complexes 204 and 205 containing phosphine ligands. Highest activity is achieved with complex 205 containing PCy3 ligands,

Scheme 65

which reaches quantitative conversion of hexene within 2 h at a 2 mol % catalyst loading.97 The C,N,C-tridentate pyridylbis(triazolylidene) ruthenium complex 206 actively hydrogenates esters; however, high pressure (50 bar) is required to achieve high conversion (Scheme 66).91 Scheme 66

The triazolylidene ruthenium complexes 150b, 198b, 207, and 208 with specifically modulated chelating groups catalyze the dehydrogenative coupling of nitrobenzene with benzyl alcohol (Scheme 67).95 With an excess of benzyl alcohol (10 equiv/mol of nitrobenzene) and in the presence of substoichiometric quantities of base (0.6 equiv/mol of nitrobenzene), nitrobenzene is converted into N-benzyl aniline selectively. Essentially quantitative conversions are achieved when the reaction is run with 3 mol % catalyst loading at 120 °C for 7 h. Alcohol and Amine Oxidation. The monodentate triazolylidene ruthenium(II) complex 209a is an efficient catalyst precursor for the base- and oxidant-free oxidation of alcohols and amines (Scheme 68).194 When 5 mol % of complex 209a is employed, full conversion of benzyl alcohol into benzaldehyde and of benzyl amine into imine is achieved in 16 and 20 h, respectively. A wide variety of primary and secondary alcohols can be oxidized to the corresponding aldehydes and ketones, respectively. The influence of the triazolylidene ligand framework has been investigated with complexes 149, 200 (cf. Scheme 64), 209b, 210, and 211 as well as with the tethered triazolylidene complex 212.93,101,194 These ligand variations indicate some useful structure−activity trends. Specifically, aryl wingtip groups induce lower activity than alkyl groups, and longer alkyl substituents lead to slightly better performance than shorter alkyl chains.106 Introduction of a chelating pyridyl substituent deactivates the metal center substantially.100 Amine homocoupling with these complexes proceeds selectively to the imine, while related iridium complexes produce a mixture of imine and amine (see section 2.4.3).89 AH


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Scheme 67

Scheme 68

Ruthenium-catalyzed alcohol oxidation has also been achieved with a sacrificial oxidant rather than dehydrogenation as described above. The C,S-bidentate chelating triazolylidene ruthenium and osmium analogues of the iridum complexes 191 and 192 (cf. Scheme 61) oxidize benzyl alcohols in the presence of NMO as terminal oxidant.79 The reaction shows mediocre selectivity and affords a 3:1 mixture benzaldehyde and benzoic acid, suggesting considerable overoxidation under these conditions. Such a side reaction is suppressed in the oxidant-free dehydrogenation. Similarly, the triazolylidene ruthenium complexes 124−126 constitute catalyst precursors for the oxidation of alcohols in the presence of tert-butyl hydroperoxide (tBuOOH) in water at room temperature

Complex 209a oxidizes a mixture of alcohol and amine in the presence of NaH as an external base to produce amides (Scheme 69). For example, the 209a-catalyzed reaction of Scheme 69

benzyl alcohol with benzylamine affords phenyl benzyl amide.194 In the absence of base, homocoupling is the predominant pathway and the corresponding imine is formed. Scheme 70

AI


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However, acidolysis of the triazolium ligand with HCl or trifluoroacetic acid (but not with HBF4) generates a highly active catalyst. Protonation of the triazolylidene instead of the 2-imidazolylidene ligand indicates stronger donor basicity and higher carbanionic character of the triazolylidene ligand. Complex 214 catalyzes the ring-opening metathesis polymerization of norbornene upon activation with trimethylsilyl−diazomethane (Scheme 73).44 This complex is a rare phosphine-free olefin metathesis catalyst that does not contain an alkylidene ligand.

(Scheme 70). Formation of ketones from secondary alcohols is readily achieved; however, benzyl alcohol oxidation generates a product mixture containing inter alia benzoic acid and tertbutyl-perbenzoate.99 Water Oxidation. Ruthenium complexes 198 and 213 catalyze the CAN- and NaIO4-mediated water oxidation (Scheme 71).107 The catalytic activity is increasing with Scheme 71

Scheme 73

2.4.5. Triazolylidene Rhodium Catalysis. Chelated rhodium species 34 (Scheme 74) have been tested as

smaller N-substituent in the triazolylidene ligand. Turnover frequencies correlate with the influence of the N-substituent on the RuII/RuIII oxidation potential. This oxidation is anything but the rate-limiting step, though the trend in substituents may be preserved also in stabilizing oxidation states higher than +3. The cymene complex 198 is considerably less active than the tetrakis(MeCN) complex 213, and it produces substantial amounts of CO2 as a result of oxidative ligand degradation. In contrast, complex 213 selectively generates O2 and achieves initial rates of about 1.1 min−1 and ca. 6000 TONs. This activity is more than 1 order of magnitude lower than that of iridium complexes, both in terms of both activity and catalyst lifetime (see section 2.4.3). Olefin Metathesis. The catalytic performance of triazolylidene ruthenium complexes 50 and 132 was evaluated against several olefin metathesis reactions.32,141 The performance of complex 50 in the ring-closing metathesis (Scheme 72a) and ring-opening metathesis polymerization (ROMP) (Scheme 72b) is strongly influenced by the substituent at N3 with respect to their initiation and resistance to deactivation and in some cases may rival the catalytic activity of the wellestablished Grubbs’ GII catalyst. Kinetic analysis suggests steric hindrance to be a key factor for the lower activity. Complex 132 features an unfavorable di(carbene) ligand array, which renders the ruthenium center catalytically inactive.

Scheme 74

precatalyst in the transfer hydrogenation of different ketones, though in all cases better results have thus far been achieved with the corresponding iridium analogues (see section 2.4.3).172 Rhodium(I) complexes 215 bearing bis(triazolylidene) ligands with a chiral binol-derived backbone are active catalysts for the hydrosilylation of ketones as a process for the synthesis of alcohols (Scheme 75). Catalyst loadings as low as 0.2 mol % accomplish considerable conversion (400 TON) at room temperature, though asymmetric induction does not exceed 34% ee. 46 A related complex of Shi induces higher enantioselectivities; however, 2 mol % of catalyst is needed.220

Scheme 72

AJ


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Scheme 75

Scheme 77

The bis(triazolylidene) rhodium(I) complex 216 merges hydrosilylation with amide formation by catalyzing the reductive coupling of CO2 with secondary amines (Scheme 76).195 High catalytic conversions with up to 1000 TONs are accomplished in the presence of Ph2SiH2 and CO2 pressure (25 bar), affording a variety of N,N-disubstituted formamides. This bis(triazolylidene) complex also catalyzes the hydrosilylation of amides to yield the corresponding alkyl amines when using PhSiH3 as reducing agent (Scheme 77a). Combining these two processes leads to an attractive procedure for the reductive methylation of amines with CO2 in a two-step process (Scheme 77b).221 Complexes 217 and 218 catalyze the hydrothiolation of alkynes, with excellent selectivity toward the branched vinyl sulfides I after 24 h when K2CO3 is added (Scheme 78).143 Conversions up to 77% are achieved at 2 mol % catalyst loading, with complex 217b providing the highest selectivity toward the vinyl sulfide I. The rhodium(I) complex 64 containing a CNC pincer ligand with a carbazolide N-donor and two triazolylidenes catalyzes the hydrothiolation of alkynes using both terminal and internal alkynes (Scheme 79).144 Moreover, complex 64 is also an efficient catalyst for the homodimerization of alkynes, yielding enyne compounds that are on their own suitable for hydrothiolation. These reactivity patterns have been used to develop a cascade alkyne dimerization/hydrothiolation process to introduce significant complexity to simple substrates. The neutral triazolylidene rhodium(I) complexes 42 and 47 serve as catalysts in the hydroformylation of 1-octene (Scheme 80).136 This reaction is performed under 40 bar of a CO/H2 (1:1) mixture at 75 °C. As low as 0.08 mol % of catalysts is sufficient to induce full conversion within 8 h, transforming octene into a mixture of linear and branched aldehydes (2:1 linear/branched ratio), as well as internal octenes due to double-bond migration. No significant difference is imparted upon exchanging the phenyl substituent for a ferrocenyl group. 2.4.6. Triazolylidene Gold and Silver Catalysis. Triazolylidene gold complexes have been applied as catalyst precursors predominantly in cyclization reactions. For example, the triazolylidene gold complexes 219 together with AgBF4 as halide abstractor have been employed for the catalytic aldol condensation of benzaldehyde and isocyanoacetate to form

substituted oxazolines (Scheme 81).117 Turnover numbers of almost 10 000 and high turnover frequencies (TOF50 1000 h−1) are achieved with catalyst 219a. When the catalytic reaction is performed in the absence of silver salt but using other chloride scavengers instead such as KPF6, conversions and rates are much lower. Moreover, the corresponding triazolylidene silver complex shows very similar catalytic properties as the mixture of the triazolylidene gold complex activated with AgBF4.41 These investigations demonstrate that the presence of a silver(I) salt is essential for catalyst activation. Mechanistic work suggests that a retro-transmetalation involving carbene transfer from Au to Ag is relevant and that the catalytically active species is actually the triazolylidene Ag species. In agreement with such a model, chloride abstraction with other halide scavengers (MeOTf, KOTf, KPF6, KBF4) leads to a gold complex with only moderate activity.116 These triazolylidene gold complexes also catalyze the intramolecular cyclization of propynyl-functionalized benzamides to give 2-phenyl-oxazoline as a mixture of two tautomers (Scheme 82).115,193 Catalyst activation is imparted by Cu(OTf)2, which leads to 60% conversion vs 70% conversion with 5 mol % [Cu(OTf)2] and 1 mol % AgSbF6 (23 h). Bulky substituents in the triazolylidene ligand enhance catalytic activity, and highest conversions are accomplished with complexes 219h and 219a containing Dipp and Mes substituents (60% and 55% conversion after 4 h, respectively). The gold vinyl complex 221 has been isolated as a plausible late-stage intermediate of the catalytic process. A comparative study between phosphines versus triazolylidenes in Au(I) catalysis shows that gold complexes based on 2,4,6triarylphosphines are more active catalysts than species 219i. While full conversion is achieved with the phosphine derivative after 3 h in the presence of AgSbF6, more than 2 days are needed when catalyst 219i is employed.193 Similar internal cyclization reactions are promoted by the triazolylidene gold complex 219j containing a ferrocenyl substituent (Scheme 83).120 While the catalytic activity of this complex has been demonstrated in the synthesis of 2phenyl-oxazoline, this complex also efficiently accomplishes cyclizations yielding 2,4-substituted furan and phenol. Moreover, catalyst activation is performed by addition of acyl−

Scheme 76

AK


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Scheme 78

Scheme 79

Scheme 82

Scheme 80

Scheme 83

Scheme 81 ferrocenium salt as oxidizing agent rather than Ag(I) or Cu(II) additives. A related coupling is catalyzed by such triazolylidene gold complexes when unfunctionalized phenylacetylene is reacted with amines or hydrazines to yield the hydroamination and hydrohydrazination products, respectively.146,148 A mild cooperative effect in this reaction has been observed when using tetrametallic rather than monometallic triazolylidene gold complexes as catalyst precursors.201 Complexes of type 219 also catalyze different intramolecular cyclization reactions. For example, with complex 219k, allenes AL


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containing a remote primary alcohol afford dihydropyrane (Scheme 84) and enynes give cyclopentenes containing an

reached with 5 mol % catalyst loading in the presence of 5 mol % AgSbF6 (18 h, 25 °C) when 219k is used as precatalyst.118

Scheme 84

Scheme 86

exocyclic conjugated double bond (Scheme 85).119 The triazolylidene substituents have a negligible effect on the catalytic activity and selectivity, with the exception of complex 219c with two phenyl substituents, which is less productive,118 whereas the sulfonyl-containing triazolylidene complex 219o affords a cyclohexene isomer as minor cyclization product together with the cyclopentene II.122 When the reaction is performed in the presence of MeOH, the diene is intercepted and exocylic methoxy addition takes place. This methoxycyclization is very fast already at room temperature and is complete within 1 min when performed at a 5 mol % catalyst loading and upon activation with AgSbF6. Activity and selectivity are identical when using monometallic 219 or multimetallic complexes 222 and 223.139 Complexes 219 are catalyst precursors for the intermolecular allylic etherification (Scheme 86). Conversions up to 76% are

Triazolylidene gold complexes furthermore catalyze carbene insertion reactions. Thus, complex 219j affords ethers upon carbene insertion into O−H bonds of primary, secondary, and tertiary alcohols (Scheme 87a),119 while carbene insertion into benzene and styrene is less selective with complexes 219 and affords a mixture of substituted tropylidene and benzene due to carbene insertion into the C−C and C−H bond, respectively (Scheme 87b).116 Mechanistically, triazolylidene gold catalysis is most challenging. In particular, in reactions that require the activation with a silver(I) source, retro-transmetalation needs to be considered, and detailed investigations are required to identify whether the catalytically active species is indeed a triazolylidene gold unit. 2.4.7. Triazolylidene Copper Catalysis. Like many other copper systems, triazolylidene copper complexes 57, 58, 100, and 224 catalyze the [3 + 2] cycloaddition of a wide variety of alkynes and azides (Scheme 88).111,124,126,127,150,151,207 Mono-

Scheme 85

AM


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Scheme 87

Scheme 88

carbene complexes 100 tolerate a wide range of functional groups such as nitro, nitrile, ether, carbonyl, alcohol, and amine substituents. The catalysts are very active, and cyclizations are completed already at room temperature within a few hours when 0.5 mol % copper complex is used. Nitrile functional groups slow down the reaction and require up to 6 h, while ethynyl−pyridine accelerates the cycloaddition process.111 A loading of 100f as low as 0.05 mol % is sufficient for reaching full conversions in the cycloaddition of benzyl azide and phenyl acetylene within 2 h.150 Turnover numbers up to 16 400 have been reached with the bis(triazolylidene) copper complex 57d, demonstrating a better performance than the related neutral complexes 58 and 100.151 However, neutral complex 100b performs very well in the coupling of sterically hindered substrates such as the addition of Dipp-azide and Dippacetylene, albeit long reaction times are needed.127 The dimetallic species displays a decreased activity when compared to the monometallic complexes, especially when electron-poor aryl azides or sterically demanding alkynes are used as substrate.124 The ditopic triazolylidene copper complex 225 is an active catalyst for the hydroboration of trans-β-methylstyrene with bis(pinacolate)diborane, affording the boronate product in less than 15 min at a 0.25 mol % loading (TOF50 ca. 700 h−1; Scheme 89).123 This reaction requires the presence of NaOtBu and MeOH as additives. While the hydroboration also efficiently proceeds with styrene, yielding the anti-Markovnikov product, the conversion of cis-β-methylstyrene fails under the same conditions. Electron-withdrawing groups in the

Scheme 89

styrene substrate such as CF3 substituents considerably compromise the yield. Bis(triazolylidene) copper complexes 57 catalyze the hydrosilylation of cyclohexanone with Et3SiH (Scheme 90).149 However, temperatures of 70 °C and long reaction times (>10 h) are needed to obtain good conversions at 3 mol % catalyst loading. Further optimization of catalyst and Scheme 90

AN


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Scheme 91

Scheme 92

Scheme 93

spectator ligand.228 The piano-stool Fe(II) Cp complexes 71 and 72 have been used as catalyst precursors for the hydrosilylation of carbonyl compounds with PhSiH3 (Scheme 92).156 Monodentate triazolylidene complexes 71 display very similar catalytic activity, independent of the triazolylidene substituents. Maximum turnover frequencies of 14 400 h−1 are reached with complex 71c for the conversion of bromobenzaldehyde. In contrast, the cationic complex 72 containing a chelating pyridine donor has a significantly lower catalytic performance. Mechanistic studies support the formation of radical species as catalytic intermediates of this hydrosilylation reaction. Tricoordinate triazolylidene nickel Cp complexes 101 and 103 also catalyze the hydrosilylation of aldehydes (Scheme 92).173 In contrast to the analogous iron complexes, pyridyl chelation is beneficial and the pyridyl−triazolylidene complex 103z exhibits the highest catalytic activity of the nickel series,

conditions is required to be competitive with other Cu(NHC)based catalysts.222−227 Moreover, triazolylidene copper complexes 100 are catalysts for the carboxylation of benzoxazole and benzothiolate derivatives with CO2 to give, after treatment with MeI, the corresponding methyl ester (Scheme 91).125 Best results are obtained with the Dipp-substituted triazolylidene complex 100g in the presence of KOtBu, which affords the carboxylated benzoxazole in 91% yield after 3 h (5 mol % [Cu], 80 °C). The corresponding 2-imidazolylidene complex is slightly less active and reaches only 70% conversion under the same reaction conditions. 2.4.8. Catalysis with Triazolylidene Complexes of Fe, Co, Ni, and Mo. Apart from copper (see section 2.4.7), several other first-row transition metal complexes with triazolylidene ligands have been successfully used in catalytic transformations. Remarkably, these complexes all contain a Cp AO


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reaching TOFs up to 23 000 h −1 and 6000 TONs. Furthermore, hydrosilylation is highly selective toward aldehyde reduction, even in the presence of an excess of ketone. Remarkably, the related pyridyl−triazolylidene nickel complex 103aa is a considerably slower catalyst, indicating subtle electronic changes emerging from N- vs C-substitution on these triazolylidene ligand scaffolds. The N-bound pyridyl donor is more electron rich, which may induce stronger metal coordination and thus hinder substrate binding. The triazolylidene nickel complexes 101 and related bistriazolylidene species 103 have been used in the Suzuki− Miyaura cross-coupling.174 While high initial rates are accomplished with these complexes (TOFini > 1000 h−1), rapid catalyst deactivation has been noted due to the limited stability of the triazolylidene nickel complex toward (boronic) acids. Moreover, triazolylidene nickel complexes 101 catalyze the oxidation of n-octane with peroxide (Scheme 93).175 This reaction accomplishes up to 150 TONs, though with limited selectivity, affording a mixture of ketones, aldehydes, and alcohols. Cobalt complex 135 containing a pyridyl−triazolylidene chelate is an active catalyst for the electrochemical reduction of H+ from acetic acid and produces H2 with a maximum TOF of 400 s−1 (Scheme 94).130 A remarkably low overpotential (0.13 V) is required when using a glassy carbon electrode.

Scheme 96

with a tert-butoxide and a triflate ligand (40 °C), and around 100 °C for the other two complexes 226a and 227. 2.5. Other Applications

2.5.1. Photophysical Applications. NHC complexes have attractive optical properties and demonstrate potential for the preparation of new optical devices and organic lightemitting diodes (OLEDs).229,230 While this field of application is relatively unexplored with triazolylidene ligands, various complexes of Fe, Ru, Ir, Pd, Pt, and Au show promising photophysical properties. The heteroleptic iron(II) complex 69 comprised of two bis(triazolylidene) ligands and one bpy has been analyzed by static and ultrafast spectroscopy (Figure 21). The incorpo-

Scheme 94

Molybdenum complex 28 bearing a normal 1,2,3-triazolylidene ligand is an efficient catalyst for the epoxidation of alkenes using tert-butyl hydroperoxide (TBHP) as external oxidant in ionic liquids (Scheme 95).48 Essentially quantitative Scheme 95

Figure 21.

conversions are reached after 24 h at moderate temperatures (55 °C) and 1 mol % 28 in the ionic liquid [C8mim][NTf2] (1-methyl-3-octylimidazolium bis(trifluoromethanesulfonyl)imide). Under these conditions six consecutive catalytic cycles of 24 h do not lead to a significant drop of activity. The formation of a precipitate points toward the generation of a Mo−polyoxo salt as catalytically active species. The triazolylidene molybdenum complexes 226 are catalytically active in the ring-opening metathesis polymerization of dicyclopentadiene (Scheme 96).176 The complexes are all latent catalysts that are inactive at room temperature yet activate upon heating. The catalyst activation temperature is dependent on the ancillary ligands, lowest for complex 226b

ration of bis(triazolylidene) ligands destabilizes the MC state and thus increases the lifetime of the MLCT excited state, reaching a lifetime of 13 ps.154 The corresponding homoleptic iron(III) complex 70 with three bis(triazolylidene) ligands exhibits excellent light-emitting and photosensitizing properties.155 The excellent σ-donor and poor π-acceptor properties of the ligand stabilize the excited state considerably better than the bpy ligand, as demonstrated with the long charge transfer lifetime of 100 ps, as well as photoluminescence even at room temperature. The emission occurs from a long-lived doublet ligand-to-metal charge transfer (2LMCT) state, which is not common for transition metal complexes.155 Analogues of [Ru(bpy)3]2+ and of [Ru(terpy)2]2+ (terpy = 2,2′:6′,2′′-terpyridine) that contain one or two pyridyl donor groups replaced by triazolylidene units display interesting electro- and photochemical properties. The electrochemical impact of the triazolylidene ligand is demonstrated by a 300 mV lowered oxidation potential of the ruthenium center in complex 129 when compared to the parent bpy homologue AP


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Figure 22.

Figure 23. Emissive triazolylidene complexes of iridium, gold, and platinum.

Scheme 97

(Figure 22).105 The electron-acceptor sites remain centered on the bpy ligands and are unaffected, which causes the HOMO− LUMO gap to decrease by 0.3 eV. The triazolylidene complex 130 emits at λmax = 430 nm with an excited-state lifetime of 188 ns. The strong donor properties of the two triazolylidene ligands in complex 109 lead to a destabilization of the metalcentered triplet 3MC state, while the MLCT character is preserved. This situation gives rise to a lower energy emission at λmax = 640 nm with a remarkably long emission lifetime of 633 ns, which is 3 orders of magnitude longer than that of the parent [Ru(terpy)2]2+ complex.108 Modulation of the terpy ligand (109b−e) enhances the excited-state lifetime remarkably to a record-high 8 μs for 109e.109 These systems have been modulated to incorporate phosphonate groups onto the aryl substituent of the triazolylidene ligands and a carboxylate at the central pyridyl position of the terpy ligand. These three

functional groups provide a three-pronged attachment to TiO2, leading to very reliable immobilization for the generation of robust dye-sensitized solar cell (DSSC).36,231 Various iridium, platinum, and gold complexes are emissive. Complex 228 is derived from the well-known [Ir(ppy)2L2]+ family (ppy = 2-phenylpyridyl) and displays emission at λmax = 538 nm originating from LC excited states (Figure 23).65 The bis-tetrazolate ligand in 229 induces a blue shift in the emission spectra compared to complex 228 (λmax = 499 nm). Photoluminescence quantum yields are comparable to those of the archetypal complex [Ir-(ppy)2(bpy)]+ and related cyclometalated iridium(III) complexes with chloride ligands.232,233 Platinum acetylacetonate complexes 23 containing a cyclometalated phenyl−triazolylidene ligand emit in the λmax = 485 and 509 nm range (Figure 22).128 There is no obvious correlation between the electronic properties of the substituent AQ


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Methyl Transferase Mimicry. The triazolylidene iridium complex 235 offers a platform for the intramolecular transfer of a methylene group (Scheme 98).196 The process is related to the cobalamine-catalyzed methylation of cytosine and other substrates. Rather than using a methylsulfonium ion (Sadenosyl methionine) as alkyl source, this complex activates a pyridinium N−CH3 bond and delivers it as a CH2 group to a C(sp2) carbon, albeit in a stoichiometric process. Mechanistic work has identified the cyclometalated complex 236 as a critical intermediate to the acetate-mediated N−C bond cleavage process. Methylene transfer is followed by activation of the nitrile solvent, thus producing complex 237. Isotope labeling confirms that the methylene group is originating from the pyridinium N−CH3 group and is incorporated at the ortho position of the benzyl substituent.

R and the emission, though the wide range indicates that these complexes provide an excellent scaffold for generating complexes with fluorescence at a specific wavelength. Emission lifetimes are around 15 μs and quantum yields remarkably high. For example, complex 23a with a phenyl substituent gives a quantum yield of 65%, substantially higher than that of analogous 2-imidazolylidene- or 1,2,4-triazolylidene-based complexes (7% and 30%, respectively).234,235 Gold complexes 230 and 231 emit around 500 nm with excited-state lifetimes in the order of several microseconds (Figure 23).121 Both the emission intensity and the lifetime strongly decrease in the presence of oxygen, which is commensurate with emission originating from a triplet state. Palladium complex 232 is emissive because of the bodipy substituent in the backbone of the cyclometalated triazolylidene ligand (Scheme 97).69 Displacement of acridine from complex 232, obtained upon cleavage of the dimeric complex 76 (cf. Scheme 18), by 4-(dimethylamino)pyridine (DMAP) affords complex 233 and results in a reduced luminescence intensity because the released acridine acts as a quencher. Hence, the fluorescence intensity serves as a probe for this ligand substitution at the metal center. 2.5.2. Biological and Bioinspired Applications. Anticancer Activity. The cytotoxicity of triazolylidene ruthenium and osmium complexes 211 and 234 containing a variety of substituents on the triazole scaffold has been evaluated against two human tumor cell lines (A2780 and A2780R) and a nontumor cell line (HEK), Figure 24.103 The most active

3. COMPLEXES WITH MESOIONIC IMIDAZOLYLIDENE LIGANDS 3.1. General Aspects

Since the first discovery of C4/5-imidazolylidene bonding to iridium in 2001,9 a wide range of organometallic complexes with this bonding motif has been reported. Various metalation routes have been discovered, sometimes rather serendipitously, though converging to a refined model that enables now the deliberate synthesis of such mesoionic/abnormal imidazolylidene (aNHC) complexes. Initial efforts toward directing metal coordination to the imidazolylidene C4 or C5 site have included direct C−H bond activation and oxidative addition, and these methods have been covered in the previous review on this type of ligands.19 Here, only more recent results will be compiled. In addition, several new routes have become available since the appearance of the previous account, specifically transmetalation, the free base route involving deprotonation of the imidazolium at the C4/5 position, isomerization of C2-imidazolylidenes upon metalation, and the synthesis of mesoionic imidazolylidenes directly within the metal coordination sphere. Great strides have also been made in metalating imidazolium salts twice to give complexes of type AQ (Figure 25). Originally, these systems have been denoted

Figure 24.

complexes contain a dodecyl substituent on the triazolylidene ligand and reach anticancer activities in the submicromolar range. With this ligand both the Ru and the Os complexes are equally active. Of note, the activitiy toward carcinogenic cells is about 1 order of magnitude higher than that toward the healthy HEK cell line. Recent work on silver(I) and gold(I) complexes containing a triazolylidene with metallocenyl wingtip groups showed moderate cytotoxicity effects against breast cancer and colon carcinoma cell lines (IC50 values > 25 μM).236 No selectivity toward tumor cells was noted as nontumorigenic cells were affected at comparable levels.

Figure 25. Carbanionic NHC ligands which may formally be denoted as mesoionic carbenes.

N-heterocyclic dicarbene complexes, though obviously only one bonding site is carbenic while the other one is formally carbanionic, and these systems are therefore better termed

Scheme 98

AR


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carbanionic NHC ligands.237−240 On the basis of the degree of N-heteroatom stabilization, the more carbenic bonding is assumed to be the metal bond with C2, and therefore, these species are not further considered in this section.

Scheme 100

3.2. Synthesis of Mesoionic Imidazolylidene Metal Complexes

3.2.1. Transmetalation. Transmetalation through a Carbene Silver Intermediate. Silver complexes of 4imidazolylidenes are considerably less stable than those with Arduengo-type carbenes or triazolylidenes. Moreover, imidazolium salts with an alkyl substituent at the C2 position are readily oxidized by silver(I) and lead to 2- rather than 4imidazolylidene complexes.241 Hence, alkyl groups are not reliable protecting groups for the C2 position, and transmetalation with silver intermediates has focused on imidazolium precursors with an aryl substituent at C2. On the basis of these concepts, complex 238 has been prepared by reaction of the corresponding imidazolium salt with Ag2O (Figure 26).242

imidazolylidene palladium complexes 247 and 248. Albeit in a low yield, a putative transmetalation agent 246 has also been isolated and characterized by NMR spectroscopy and singlecrystal X-ray diffraction (Scheme 101).245 Addition of a small amount of pyridine to 247 cleanly yields Pd−PEPPSI-type complexes 249. These complexes can also be prepared in a one-pot reaction by using the silver transmetalation strategy. Transmetalation through Other Intermediates. The copper(I) NHC transfer methodology has emerged as a powerful alternative to NHC transfer from silver and has been demonstrated for Arduengo-type 2-imidazolylidenes, triazolylidenes (see section 2.2.1), cyclic (alkyl)(amino)carbenes (cAACs), and recently also 4-imidazolylidenes (aNHCs). Complex 250 reacts with [Pd(Cl)2(NCPh)2], [Au(SMe2)Cl], [Ir(COD)Cl]2, and [Rh(CO)2Cl]2 to give the aNHC complexes 251, 252, 253, and 254 respectively (Scheme 102).133 Treatment of the tris(aryl)-substituted imidazolium salt 245 with NaHBEt3 yields the boron adduct 255, which serves as an imidazolylidene transfer agent and yields complex 256 upon reaction with [Co{N(SiMe3)2}2] (Scheme 103).246 3.2.2. Free Carbene Route. Even though calculations have suggested a very low stability of 4-imidazolylidenes compared to their normal 2-imidazolylidene analogues, Bertrand and co-workers have succeeded in isolating and crystallizing the free 4-imidazolylidene 258a without any stabilizing additive (Scheme 104).247 This carbene is stable at room temperature and is conveniently prepared by deprotonation of the corresponding imidazolium salt 257a with KHMDS. Notably, using a strong lithium base does not form the free base but the Li-adduct instead. The free carbene readily coordinates to gold and affords the 4-imidazolylidene gold(I) complex 259a. Stabilization of the free carbene requires aryl substituents on the imidazolium nitrogen nuclei. Variation of the C-bound aryl groups in the imidazlium salt 257 indicates that the stability of the corresponding free carbene 258 strongly depends on the electronic properties of the aryl group. Electron-withdrawing substituents stabilize the free carbene, whereas electrondonating substituents do not impart sufficient stability, and metal complexation requires an in situ deprotonation− metalation protocol to form the iridium(I) carbonyl complexes 260 containing a mesoionic imidazolylidene ligand. The variability of substituents in the imidazolylidene C5 position is an attractive feature for tailoring specific properties (Scheme 105).248 The demonstration that the free mesoionic imidazolylidene 258a is stable at room temperature has been a defining point in

Figure 26.

It constitutes a rare example of an isolated silver complex of a mesoionic imidazolylidene and has been characterized by NMR spectroscopy and elemental analysis. Generally, silver carbene complexes are formed in situ and used directly for transmetalation. For example, imidazolium salt 239 affords the air-stable 5-imidazolylidene gold(I) complexes 240 through intermediate formation of the silver adduct and subsequent transmetalation with [Au(SMe2)Cl] (Scheme 99).243 Scheme 99

Similarly, the methyl salt of imidazo[1,2-a]pyridine 241 affords ruthenium(II), iridium(III), and palladium(II) complexes 242−244 with a mesoionic imidazolylidene-type ligand via in situ formation of the silver carbene intermediate (Scheme 100).244 Metalation takes place exclusively at the position proximal to the alkylated nitrogen. The limited stability of complexes 242−244, especially toward acids, may be a direct consequence of the absence of a protecting bulky substituents adjacent to the carbene site, which may be improved upon introduction of supporting functional groups as N-substituents. Reaction of the tris(aryl)-substituted imidazolium salt with Ag2O and subsequent transmetalation with PdCl2 gives the 4AS


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Scheme 101

group metals. Complexes with iron(II),249 nickel(II),250 copper(I),251 gold(I),242 zinc(II),252 aluminum(III),252 silicon(II),253 silicon(IV),254 germanium(II),254 and tin(II)253 are accessible from the free mesoionic imidazolylidene (complexes 261−270, Figure 27). The same protocol has also been applied for the in situ generation of a free mesoionic imidazolylidene from the tris(aryl)-substituted imidazolium salt 245 and subsequent metal coordination, which affords the aNHC copper(I) complex 271 (Scheme 106).255 A similar procedure provides access to aNHC gold(I) complexes.256 3.2.3. Isomerization from Free 2-Imidazolylidene Ligands. A metalation methodology that is conceptually related to the direct metalation method described above is the isomerization of 2-imidazolylidene ligands and 2-imidazolylidene-containing metal complexes into their mesoionic analogues, producing 4-imidazolylidene metal complexes. For example, reaction of the free 2-imidazolylidene 272a with [Pt(COD)Me2] leads to the formation of the platinum(II) complex 273 containing one imidazolylidene ligand bound mesoionically (Scheme 107).257 The activation of the C4/C5− H bond is remarkable since DFT calculations indicate a ca. 20 kcal mol−1 higher stability of the C2-metalated imidazolylidene compared to the C4-metalated isomer.258 In addition, the acidity of the proton attached to C2 (pKa = 21−23)8 is about 6 orders of magnitude higher than that of the C4/C5-bound proton (pKa = 30−31).259 This large difference suggests that metalation at the C4 position of imidazolium salts is not governed by thermodynamic parameters such as acidity. Instead, the bulkiness of the N-substituents is a decisive factor, and large substituents such as Dipp, adamantyl, and tBu promote C4 bonding of imidazolylidenes, an effect that is further pronounced when the metal precursor contains sterically demanding ancillary ligands. Similar isomerization has been observed when reacting the free 2-imidazolylidene 272a with a tin(II) precursor containing a bulky aryl ligand (Scheme 108).260 Upon abstraction of the chloride ligand, the cationic dimeric complex 274 containing a 4-imidazolylidene ligand is obtained. Likewise, metalation of the free carbene 272b with [Sn(OTf)2] affords the dicarbene complex 275 featuring one C2-bound and one C4-bound imidazolylidene ligand (Scheme 109).261 This reactivity is reminiscent to the reaction of the nickel(II) complex 276 containing ligand 272b (Scheme 110).262 Upon addition of a second equivalent of the carbene ligand, coordination takes place through the C4 position to yield the nickel species 277 containing both a C2- and a C4-bound imidazolylidene.

Scheme 102

Scheme 103

Scheme 104

Scheme 105

the organometallic chemistry of 4-imidazolylidenes. With access to free carbenes, metalation procedures conveniently involve just addition of a metal precursor to form the corresponding aNHC metal complex. This procedure has been demonstrated with a wide variety of transition and main AT


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Figure 27.

Scheme 106

Scheme 110

Scheme 107

Scheme 111

Scheme 108

eventually yields complex 279 as the final product, presumably via a migratory CO deinsertion process. While the above reactivity pattern suggests that 2- to 4imidazolylidene isomerization occurs with the free ligand, experiments with hard metals point to an isomerization process after complexation. For example, the three-coordinate iron and cobalt complexes 280 containing a 2-imidazolylidene ligand undergo a thermal isomerization to yield the 4-imidazolylidene analogue 281 (Scheme 112).264,265 A similar isomerization has Scheme 112

Scheme 109

been observed with AlMe3 bound to 2-imidazolylidene ligand 272a266,267 and a [ZnMe] fragment bound to two 2imidazolylidene ligands 272b.268 Considering the lability of the M−CNHC bond with these rather hard metals (see section 3.3), isomerization of 280 to 281 is also plausible via initial metal dissociation and C4−H migration to the C2 position to form the mesoionic 4-imidazolylidene ligand and subsequent trapping of this species by metal recoordination.

A different reaction pathway is shown in the metalation of the free 2-imidazolylidenes 272 with the osmium cluster [Os3(CO)12]. Initial NHC-induced activation of a carbonyl ligand forms the cluster 278 containing an acyl ligand (Scheme 111).263 This product is sufficiently stable for isolation and complete characterization, though gradual loss of a CO ligand AU


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no such intramolecular C−H bond breaking is observed when the carbene binds normally through the C2 carbon.271 Reaction of RhCl3 with the trimethylene-linked diimidazolium salts 287 containing methyl-protected C2 sites yields, in the presence of NaOAc, the tridentate dicarbene complexes 288 and after column chromatography the dimetallic complexes 289 (Scheme 115).272,273 Formation of 288 implies a triple C−H bond activation process including activation of the alkyl C−H bond in the linker in addition to the expected heterocyclic C−H bond activation. This reaction outcome has been compared to the reactivity of the related imidazolium salt 287e, in which the imidazolium C2 positions are not protected and hence available for metal coordination as Arduengo-type carbenes. Upon exposure of 287e to identical conditions as the diimidazolium salts 287a− d, a mixture of products is obtained, including a complex that features a bidentate-coordinated ligand with two C2-bound imidazolylidene ligands and two tridentate biscarbene complexes. In one of these complexes, one imidazolylidene is C4 bound to the rhodium center. This outcome together with time-dependent monitoring of the reaction indicate that the mesoionic imidazolylidene promotes the aliphatic C−H bond activation considerably more than classic 2-imidazolylidenes. Calculations further suggest that the rate-limiting step is not the C(sp3)−H bond activation process but iodide dissociation and formation of an agostic intermediate, which is supported by the stronger donor properties of 4- vs 2-imidazolylidenes. The dimeric structure of the rhodium(dicarbene) complexes 290 is readily cleaved in the presence of donor groups, such as diphosphines and diimines, resulting in the formation of dicarbene complexes with diphosphine ligands 291274 or with diimine ligands 292−294 (Scheme 116).275 Metalation of a pyridinium−imidazolium system reveals remarkably diverse reactivity patterns of the imidazolium ligand site, including C−C bond-making and -breaking processes, C−N bond cleavage, and C−H bond activation.276 For example, rhodation of the pyridinium-functionalized 2methylimidazolium salt 295 with [Rh(COD)Cl]2 yields the 2imidazolylidene rhodium complex 296 as a result of imidazolium C(sp2)−C(sp3) bond activation (Scheme 117). The same complex is obtained when starting from the ligand precursor containing a hydrogen at the imidazolium C2 position. While a CH3 group at the imidazolium C2 position generally promotes the formation of mesoionic 4-imidazolylidene complexes,277−281 the Cimi−CH3 bond cleavage to from 296 may be related to oxidative cleavage as observed with

An extended version of this isomerization process is shown in the transformation of complex 282 containing a [M(CO)5] (M = Cr, W) unit linked to the NHC ligand via a SiCl2 fragment (Scheme 113).269 In the presence of CsOH as a mild base, the SiCl2 group eliminates and ligand 272b isomerizes to the C4-bonding mode to produce complex 283. Scheme 113

3.2.4. Direct Imidazolium Metalation. Historically, the direct C−H bond activation of imidazolium salts has been the first methodology to synthesize mesoionic imidazolylidene metal complexes. Also, it is still the most frequently applied process. While this method is potentially applicable to a wide range of ligand precursors, substitution of the C2-bound proton with a blocking alkyl or aryl group often provides more reliable access to mesoionic 4-imidazolylidene complexes. Because of the large body of work accomplished thus far, this section is organized according to metal centers Reaction with Rhodium. Metalacycles of different size are obtained from the reaction of the phosphine-tethered 2methylimidazolium salt 284 with [Rh(COD)(OR)]2, yielding either complex 285 from aliphatic C(sp3)−H bond activation or complex 286 with a mesoionic imidazolylidene ligand from aromatic C(sp2)−H bond activation (Scheme 114).270 The Scheme 114

selectivity is triggered by the steric bulk of the alkoxide ligand in the rhodium precursor salt, with small OR groups (R = Me) favoring the formation of the mesoionic imidazolylidene complex 286. Abnormal C4 bonding of N-heterocyclic carbenes effectively modulates the electron density at rhodium and allows for the selective cleavage of an unactivated C(sp3)−H bond, whereas Scheme 115

AV


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Scheme 116

Scheme 117

to iridium. For example, metalation of the imidazolium salt 272b·HCl featuring bulky Dipp wingtip groups with [Ir(COE)2Cl]2 (COE = cyclooctene) affords the iridium(I) complex 301 by dehydrogenation of one of the isopropyl groups in the N-substituent and chelate formation that relieves the steric congestion around the iridium center (Scheme 119).283 Complex 302 containing both a 2- and a 4-

silver oxide,19 supporting the notion that a CH3 group can be an unreliable C2-protecting group for various metals. Reaction with Iridium. The first mesoionic 4-imidazolium complex has been accomplished upon reaction of the 2picolylimidazolium salt 297 with the iridium polyhydride [IrH5 (PPh3 ) 2 ]. 9 The ratio of 4- vs 2-imidazolylidene coordination has been demonstrated to depend on various factors including the imidazolium anion, the solvent, and the steric demand of the N-substituent of the imidazolium unit, either promoting or disfavoring C−H···X hydrogen bonding as a key driver for C2-imidazolylidene bond formation. Similar metalation is also achieved when changing the metal precursor to the iridium dihydride 298 (Scheme 118).282 The selectivity of metalation is solvent dependent with reactions in THF favoring complexes 299a,b containing a mesoionic 4imidazolylidene ligand, while C2-imidazolylidene bonding and formation of complexes 300a,b is preferred in CH2Cl2. Steric overcrowding around the iridium coordination sphere is another major factor for inducing 4-imidazolylidene bonding

Scheme 119

imidazolylidene ligand is obtained as a minor byproduct (6% yield) after hydrogenation of complex 301 and recrystallization from benzene. Steric congestion imposed by the bulky adamantyl groups in ligand precursor 303 also leads to mixed C2/C4 bonding of the tridentate pincer-type ligand in complex 304 upon base-promoted direct metalation with [Ir(COD)Cl]2 (Scheme 120).284 No isomerization has been noted when exchanging the spectator ligands to the dihydride complex 305.

Scheme 118

AW


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Scheme 120

Metalation of the C2-methylated pyridyl−imidazolium salt 306 with [IrCp*Cl2]2 induces cyclometalation via C(sp3)−H bond activation and formation of the ylidic complex 308 under basic conditions or via heterocyclic C(sp2)−H bond activation yielding the 4-imidazolylidene complex 307 if the reaction is performed in the absence of a base (Scheme 121).171 It is

be initiated by iridium coordination to the imidazole at the C4 position and subsequent hydrolysis of the heterocycle. Reaction with Palladium. In an attempt to compare the electronic impact of 4- vs 2-imidazolylidene bonding, palladium complexes 314 and 315 with a sterically essentially identical ligand set have been prepared (Scheme 124).286

Scheme 121

Scheme 124

worth noting that ylidic complex formation is also observed when starting from the 3-pyridinium analogue of the imidazolium salt 306, suggesting a direct C(sp3)−H bond activation with less relevance of preceding N-coordination. However, metalation of the 4-pyridinium imidazolium salt 295 with [IrCp*Cl2]2 leads to Cimi−CH3 bond cleavage and formation of complex 309 containing a 2-imidazolylidene ligand (Scheme 122).276 This reaction profile is identical to that described for rhodium (cf. Scheme 117). Scheme 122

Metalation of the polymethylated imidazolium salt 313 is accomplished by a thermally induced direct C−H bond activation using [Pd(OAc)2]. The presence of KCl leads to the formation of a chloro-bridged dimer 316, presumably because KCl reacts with the palladium precursor to form a palladate which is not reactive in the direct metalation. This direct palladation protocol has been extended to the palladation of the related diimidazolium salt 317 containing sulfonyl groups,

An entirely different reaction trajectory is observed when the imidazolium salt 310 containing a 4-pyridinium substituent is reacted with [IrCp*Cl2]2 (Scheme 123).285 In addition to the expected 2-imidazolylidene iridium(III) complex 312, the dimetallic species 311 is formed as a minor product. This complex is the result of an imidazole ring opening, which may Scheme 123

AX


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Scheme 125

Scheme 126

Scheme 127

Scheme 128

imidazo[1,2-a]pyridinium salt 322 containing an amide functional group in the N-substiuent (Scheme 127).290 Palladation with [Pd(OAc)2] in the presence of pyridine generates the mesoionic imidazolylidene complex 323a−c if a secondary amide is present and the C,N-bidentate chelating mesoionic carbene complex 323d from a primary amide ligand precursor. Direct palladation with [Pd(OAc)2] is a reliable procedure for the metalation of imidazolium salts. Accordingly, imidazolium salt 324.HI produces the dimeric palladium complex 325, which is cleaved into monomeric 4-imidazolylidene species either by addition of 3-chloropyridine affording complex (327) or by transmetalation of an imidazolylidene silver complex (326, Scheme 128).291

which yields the water-soluble bis(aNHC) palladium(II) complex 318 (Scheme 125).287 Palladation of the 2-phenyl-protected imidazolium salt 245 with PdCl2 in the presence of K2CO3 as a base and with pyridine as solvent affords the palladium complex 319 featuring a 4-imidazolylidene ligand (Scheme 126a).245 Application of the same method with the imidazo[1,2a]pyridinium iodide salt 320 yields the palladium complex 321 containing a mesoionic imidazolylidene-type ligand (Scheme 126b).288,289 Due to the blocking aryl substituent, palladation is directed toward the distal site of the alkylated nitrogen, complementary to the selectivity observed in an analogous ligand lacking the aryl substituent (cf. Scheme 100). Selective palladation of the site proximal to the alkylated nitrogen is also achieved when starting from the functionalized AY


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When the substituents are too bulky as in 257.H(HX2), cyclometalation involving the proximal phenyl group takes place and yields the palladium complex 328 (Scheme 129).292

The mixed (NHC)/(aNHC) palladium complex 332e and aNHC palladium complex 333e have been prepared by direct palladation of 331e using [Pd(OAc)2] and PdCl2 as metal precursors, respectively (Scheme 131).294,295 The C−H bond activation is directed to the C4 rather than the C2 position presumably due to the steric congestion imparted by the bulky aryl and tert-butyl groups, which effectively shield the imidazolium C2 position and would lead to a highly congested palladium coordination sphere if both carbenes were C2 bound. Similar steric arguments also provide a rationale for the formation of the thiolato imidazolylidene complexes 335 and 336 from palladation of the C2-unprotected imidazolium salt 334 (Scheme 132).296 The dimeric complex 336 features one 2-imidazolylidene and one mesoionic 4-imidazolylidene ligand.

Scheme 129

Scheme 132 Similar cyclometalation occurs in 329 when the substituent of the proximal nitrogen is a phenyl group, as this group is activated for electrophilic CPh−H bond activation, yielding complex 330 (Scheme 130).293 Scheme 130 Palladation of the polyfunctional C2-protonated imidazolium salt 337 occurs selectively at the C4 position when the reaction is performed with [Pd(OAc)2] in the presence of NaOAc as a base to obtain complex 339, while metalation with PdCl2/K2CO3 yields the C2-metalated analogue 340 selectively (Scheme 133).297 The selective formation of mesoionic imidazolylidene complex 339 has been rationalized by a Scheme 131

AZ


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Scheme 133

Scheme 134

chelate-assisted concerted metalation−deprotonation pathway with acetate as an intramolecular ligand and base. When starting from the analogous ligand 338 containing a methyl group at the imidazolium C2 position, exclusive formation of complex 341 containing a mesoionic imidazolylidene as part of the tridentate chelate takes place. The selectivity of C2 vs C4 imidazolium metalation in the pyrazole bridged bis(imidazolium) salt 342 with [Pd(OAc)2] also decisively depends on the reaction conditions. In the absence of NH4OAc complex 343 containing a mesoionic imidazolylidene is formed selectively, while addition of NH4OAc yields exclusively 344 with 2-imidazolylidene ligand sites (Scheme 134).298 The potentially tetradentate ligand 345 produces a mixture of products 346−348 upon metalation with K2[PdCl4] in the presence of a base (Scheme 135).299 Irrespective of the Nsubstituent (R = Me, Ph), complex 348 containing two mesoionic imidazolylidene donor sites is the major product. Interestingly, however, the ligand precursor with the sterically less demanding Me substituent affords less of the C2-bound imidazolylidene products than the ligand with phenyl substituents. Also, metalation with NiCl2 only induces C2 bonding, with no mesoionic carbene formation observed.299 Reaction with Ruthenium. Chelation-assisted reaction of the phosphine-tethered imidazolium salt 349 with [Ru(CO)2(PPh3)3] induces imidazolium C4−H bond activation at room temperature and formation of the mesoionic

Scheme 135

imidazolylidene complex 350 (Scheme 136).300 This reactivity mirrors earlier results from metalation of this ligand with an iridium(I) precursor, which is directed to the C4 position due to steric congestion.301,302 The same ligand precursor 349 also undergoes ruthenation with the ruthenium(II) precursor [Ru(OAc)2(PPh3)3] to obtain 351,303 though higher temperBA


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Scheme 136

A rational approach to a mesoionic 4-imidazolylidene ruthenium complex is provided when starting from ligand precursor 357 which also contains a potentially chelating pyridyl moiety (Scheme 139).306 Reaction of 357 with

atures (THF, reflux) are required than when using the ruthenium(0) precursor. Steric factors are also relevant in the ruthenation of diimidazolium salts. Ruthenation of the methylene-bridged diimidazolium salt 352 with [RuCl2(PPh3)3] affords complex 353, which contains two bis(carbene) ligands with one NHC bonding as mesoionic 4-imidazolylidene while the other three NHC ligand sites display the expected 2-imidazolylidene coordination mode (Scheme 137).304 The complex is in an all-

Scheme 139

Scheme 137

[Ru(COD)Cl2]n in the presence of LiHMDS induces chelate formation, and after bromide addition, complex 358 is obtained in high yields. Complex 358 is highly interesting as it combines a strongly bound mesoionic carbene as well as a labile COD ligand in the ruthenium coordination sphere. Reaction of the C2-blocked naphthyridine-substituted imidazole 359 with [Ru2(CO)4(CH3CN)6][BF4]2 affords the mononuclear cyclometalated complex 360, where the metal− metal bond is cleaved with a concomitant increase in metal oxidation state from RuI to RuII (Scheme 140).307 The

cis configuration. Replacement of the mesityl substituents by nbutyl groups leads to a complex with only C2-bound imidazolylidene ligand sites, and the four NHC ligands are in an equatorial arrangement, hence supporting the notion that C4−H bond activation is sterically promoted. Similarly, the lutidine-bridged bis(imidazolium) salt 354 affords the mixed 2/4-imidazolylidene CNC pincer ruthenium complex 355 upon ruthenation with [RuHCl(CO)(PPh3)3] (Scheme 138).305 Addition of LiBr during the metalation process suppresses C4−H bond activation and gives the pincer ruthenium analogue 356 comprised of only C2-bound imidazolylidene ligand sites.

Scheme 140

molecular structure of 360 reveals a hexanuclear cluster with circular topology. The asymmetric unit is comprised of a cyclometalated ligand that chelates through the C5-imidazole carbon and a naphthyridine nitrogen. The imidazole N3 coordinates to a neighboring ruthenium center which results in an octahedral geometry around ruthenium and a carbenic ligand. Reaction with Other Metals. Platinum(II) complexes 362 comprised of a mesoionic diimidazolylidene ligand are obtained by microwave-assisted double C−H bond activation of the C2-protected diimidazolium salt 361 with cis[PtMe2(DMSO)2] (Scheme 141).308 Oxidative addition of Cl2 of Br2 yields the corresponding platinum(IV) complexes 363. Direct C−H bond activation mediated by Cu2O has been used in analogy to Ag2O-mediated metalation (see section 3.2.1) to prepare the 4-imidazolylidene copper complex 365

Scheme 138

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Scheme 141

(Scheme 142).111 This procedure is more efficient, cleaner, and less sensitive than the synthesis via a free carbene (cf.

368 and 370 oxidatively add to [Pd(dba)2] to yield the corresponding mesoionic imidazolylidene palladium complexes 369 and 371, respectively (Scheme 145).310 The imidazolium precursor salt 370 furthermore contains open coordination sites that are palladated through a transmetalation at the imidazolium position. Subsequent oxidative addition yields the bimetallic palladium species 372 that is comprised of a carbanionic ligating site toward one metal and a carbenic donor to the other one. Similar oxidative addition of 4-iodo-imidazolium salts 373 to [Pd(PPh3)4] proceeds also in the absence of a supporting chelating donor group and affords complexes 374 (Scheme 146).311 Oxidative addition to a broader range of metals has been demonstrated with the fused heterocyclic azolium salt 375 (Scheme 147).307 Reaction with a ruthenium(II), rhodium(II), and palladium(II) precursor gives the corresponding complexes 376, 377, and 378, respectively, containing a chelatestabilized mesoionic imidazolylidene ligand. 3.2.6. Mesoionic Carbene Formation in the Metal Coordination Sphere. Hashmi and co-workers developed an elegant method to assemble the heterocycle of mesoionic imidazolylidene ligands directly within the metal coordination sphere. Coupling of gold(I)-bound isonitrile 379 with the difunctionalized amine 380 yields in a single step the mesoionic imidazolylidene gold complex 381 (Scheme 148).312,313 The synthetic protocol is very robust and allows a range of aliphatic and aromatic groups to be incorporated as N-substituents, including very bulky groups as well as chiral substituents. When the coupling of the isonitrile complex is performed with the α-methylated amine 382, mesoionic imidazolylidene gold(I) complexes 383 are obtained that have the C5 position methylated. 3.2.7. Other Methods. Demetalation of Ditopic Carbanionic Carbenes. While doubly metalated imidazolylidene complexes such as 384 presumably have only little mesoionic carbene character, quenching of the metal bound to C2 offers a strategy toward mesoionic imidazolylidene complexes. This approach has been validated with the Ga/Li dimetallic complex 384. Alkylation with MeOTf or protonation with MeOH affords the mesoionic imidazolylidene gallium complex 385 or 386, respectively (Scheme 149). The lithium center is also stripped off in the presence of a free carbene such as IMes (272c).314 Alkylation/Protonation of 4-Imidazolyl Metal Complexes. Mesoionic imidazolylidene platinum complexes are accessible starting from the imidazolyl complex 387. Protonation or alkylation of the unsubstituted nitrogen yields the corresponding mesoionic imidazolylidene complexes 388 and 389, respectively (Scheme 150).315,316 Likewise, a chelating thiocarbamate group has been successfully inserted by reacting the imidazolyl ligand with CS2, which affords complex 390. Uranium and other rare-earth metal complexes 392−395 containing a mesoionic imidazolylidene ligand have been

Scheme 142

Scheme 106). In contrast to the analogous metalation of triazolium salts, imidazolylidene complex formation occurs under conventional heating and microwave irradiation is not needed. Direct metalation of imidazolium salts is also accomplished with metal precursors that contain a strong amide base. For example, [Co{N(SiMe3)2}2] induces C−H bond activation in the imidazolium salt 245 and affords the bis(carbene) cobalt(II) complex 366 with two mesoionic 4-imidazolylidene ligands (Scheme 143).246 Likewise, [Ta(NMe2)5] activates the Scheme 143

imidazolium C4−H bond even in C2-protonated imidazolium salts such as 272c·HCl to produce complex 367 (Scheme 144),309 presumably as a result of the large steric demand of the NMe2 ligands around tantalum. Scheme 144

3.2.5. Oxidative Addition. Oxidative addition of 4-haloimidazolium salts to low-valent metal centers constitutes a procedure for the selective metalation of the mesoionic C4 position. This procedure is complementary to the more often applied strategy of blocking the imidazolium C2 position and relies on activating one position rather than deactivating another one. Oxidative addition involves, in particular, palladium(0) precursors due to their established reactivity in oxidative transformations. Several 4-iodo-imidazolium salts BC


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Scheme 145

Scheme 146

Scheme 149

prepared from the N-heterocyclic olefin 391 (Scheme 151).317 Metalation of this precursor ligand involves a formal tautomerization, with migration of the C4-bound proton to the olefinic residue.

Scheme 147

3.3. Properties and Reactivity

3.3.1. Electronic and Steric Properties. A variety of techniques have been employed to characterize the electronic and steric properties of mesoionic 4-imidazolylidenes, including TEP values via IR analysis of carbonyl complexes, NMR spectroscopy using 13C and 31P NMR probes, X-ray Scheme 148

BD


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substituents at positions adjacent to the carbenic site. Gusev recently reported a theoretical model that takes into account the electron-donating properties of NHC ligands as well as the steric repulsiveness, which provides a more comprehensive perspective on the bonding of these ligands to transition metal centers.318 While these studies show an excellent correlation between experimental and calculated values, the method is entirely DFT based and may therefore have predictive quality. NMR Spectroscopy. The donor strength of 4-imidazolylidene ligands has been determined by 13C NMR spectroscopy (cf. section 2.3.1) and 31P NMR spectroscopy, which provide a relative scale for the ligand donor ability. For example, the deshielding of the 13C resonance in the 4-imidazolylidene complex 396 (δC 181.9) reveals stronger donor properties than 2-imidazolylidene analogues, which feature resonance frequencies in the 176.6−180.1 ppm range (e.g., 397 δC 179.0; Figure 29).319 Similarly, complexes trans-[Pd(PPh3)2I(L)] show a 31P

Scheme 150

Scheme 151

diffraction and photoelectron spectroscopy, as well as theoretical considerations by computational chemistry. These properties have been reviewed for palladium complexes specifically.25 Tolman Electronic Parameter (TEP). A variety of iridium(I) dicarbonyl complexes with 4-imidazolylidene ligands of type [Ir(CO)2Cl(aNHC)] have been prepared to determine the TEP value of 4-imidazolylidenes through the average CO vibration energy. For example, analysis of νav(CO) in the 4imidazolylidene iridium complexes 260a−l reveals TEP values ranging from 2036.5 (260a) to 2043.0 cm−1 (260i; Figure 28),248 which is considerably lower than the 2049−2052 cm−1

Figure 29. Palladium complexes containing 2- and 4-imidazolylidene ligands used for comparative assessment of ligand donor strength.

NMR signal that is diagnostic for the donor properties of the ligand L. Even though this is a cis influence, the resonance shifts characteristically from δP 18.76 to 20.48 in complexes 399 and 398 upon moving the coordination mode from C2 to C4 in isosteric imidazolylidene ligands (Figure 29).320 This deshielding is associated with an enhanced paramagnetic contribution to the isotropic shielding constant. Obviously, both NMR probes may be sensitive to steric factors. X-ray Photoelectron Spectroscopy (XPS). XPS has been used to determine the bonding energies of the palladium 3d electrons as a function of the carbene bonding mode in palladium diimidazolylidene palladium complexes 314a and 315a (Figure 29).286 Electron dissociation from the palladium center in 314a coordinated by mesoionic imidazolylidenes occurs at 0.8 eV lower energy than when the imidazolylidene ligands are coordinated through C2, implying a substantially higher electron density at the palladium center coordinated by mesoionic imidazolylidene ligands. The 0.8 eV energy difference is commensurate with an effective change of oxidation state by almost one full unit. While XPS perhaps provides the most direct technique to observe the ligandinduced electron density at the palladium center, analyses are far from being routine and require specific instrumentation. Calculations. In silico analyses predict that the mesoionic imidazolylidene 258a (cf. Scheme 104) is 14.1 kcal mol−1 less stable than its isomeric normal NHC with the phenyl group bound to C5 instead of C2.247 The HOMO (−4.403 eV) is a σ-type lone-pair orbital at C5, and the HOMO−1 (−4.879 eV)

Figure 28. Iridium dicarbonyl complexes 260 used for TEP value determination for 4-imidazolylidene ligands.

range determined for Arduengo-type 2-imidazolylidenes. These low TEP values clearly indicate that 4-imidazolylidenes, even when bearing electron-withdrawing substituents, are significantly stronger electron donors than their C2-bound counterparts. From this series it also emerges that the C2-substituent only modestly affects the donor properties, while the group bound to C5 has a strong influence and hence allows for tailoring the TEP efficiently. Such an effect is in agreement with only partial π electron delocalization and the relevance of a NCN amidinium fragment and a vinylanionic metal coordination unit.183 Moreover, stereoelectronic effects may affect the CO vibration energies, especially when modulating BE


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is a C5−C4 π-bonding orbital, which exhibits antibonding conjugation with the π orbital of the phenyl substituent at C4. These molecular orbitals are much higher in energy than those of the isomeric 2-imidazolylidene (−5.000 and −5.279 eV, respectively), indicating that mesoionic 4-imidazolylidenes are more basic than the corresponding 2-imidazolylidenes, which is also confirmed by a higher proton affinity. Structural features. Analysis of M−Ccarbene bond lengths for almost 50 palladium complexes containing a 4-imidazolylidene ligand reveals an average Pd−C bond length of 1.98 Å, with extremes at 1.93 and 2.04 Å. While these bond lengths are in the same range as related 2-imidazolylidene complexes, diverging results have been obtained for complexes with other metals. In gold(I) complexes, 4-imidazolylidene ligands lead to a longer Au−C bond than 2-imidazolylidenes, even though the mesoionic carbenes are predicted by DFT to form stronger bonds.243 In contrast, the Ir−C and Cu−C bond lengths are identical within esd’s in 2- and 4-imidazolylidene complexes,244,251 while in cobalt(III) complexes, mesoionic 4imidazolylidene complexes feature a shorter M−C bond than the analogous 2-imidazolylidene complexes.265 Similarly, the bond between the imidazolylidene ligand and the group 14 metals (Ge, Si, Sn) is shorter when the imidazolylidene is mesoionic.254,321 Hence, the M−C bond does not provide any trend that reflects the distinct differences of 2- vs mesoionic 4imidazolylidene ligands. The trans influence constitutes a better parameter for assessing the impact of mesoionic imidazolylidene ligands. While stereoelectronic effects often preclude the comparison of different types of ligands, complexes 314a and 315a are isostructural and differences can therefore be attributed to bonding differences (Figure 29).286 These two complexes reveal identical bond lengths and angles except for the Pd−Cl bonds, which are considerably shorter in 315a than in complex 314a with mesoionic imidazolylidene ligands (2.35 vs 2.40 Å), in agreement with a considerably stronger trans influence of 4imidazolylidenes in comparison to their C2-bound analogues. This trend is absent when comparing the corresponding iodide complexes (Pd−I 2.68 and 2.67 Å), which has been rationalized with enhanced steric repulsion between the large iodide and the proximal methyl group of the carbene ligand. 3.3.2. Reactivity. The trans effect of 4-imidazolylidene ligands has been remarkably little investigated. A platinum complex containing a chelating bis(4-imidazolylidene) ligand undergoes rapid ligand exchange of the ancillary DMSO ligand with a kinetic exchange rate of 0.050(±2) s−1.308 The corresponding complex with 2-imidazolylidene ligands is inert and does not show any ligand exchange activity, while related C,N-bidentate ligands such as phenyl−pyridine or pyridylidene−pyridine show faster exchange rates and hence a higher trans effect. The strong basicity of 4-imidazolylidenes and the ensuing high electron density at the metal center impart specific reactivity patterns. For example, 4-imidazolylidene complexes are considerably less stable under acidic conditions, and both platinum and palladium complexes with 4-imidazolylidene ligands readily undergo protonolysis in the presence of strong acids, whereas the corresponding 2-imidazolylidene analogues are robust under these conditions.286,308 With weak acids, the 4-imidazolylidene complexes are apparently robust. However, when using D+ instead of H+ as reagent, rapid isotope exchange at the C5 position is observed, which supports a vinyl-type bonding site in these 4-imidazolylidene ligands.171

Under basic conditions, isotope exchange involves the C2bound methyl group. With silver(I) ions as softer electrophiles than H+, adduct formation has been observed, which results in a short Pd···Ag interaction.286 The high nucleophilicity of the metal when coordinated to 4-imidazolylidene ligands also predisposes the complexes to undergo oxidative addition reactions. Platinum(II) complex 362 reacts with Br2 or PhICl2 to give the corresponding platinum(IV) complexes 363, which are stable and have been isolated and characterized (Scheme 141, section 3.2.4).308 In contrast, the corresponding palladium complexes are reactive and undergo a rapid reductive elimination, which includes either reductive C−Cl elimination or C−C bond formation, depending on whether the imidazolylidene contains a substituent at the C5 position or not. These reactivity patterns are distinct and have not been observed with isostructural analogues that feature 2-imidazolylidene ligands. Hence, the considerably stronger donor properties of 4imidazolylidenes compared to their Arduengo-type 2-imidazolylidene ligands imparts distinct reactivity patterns and stabilization of high-valent metal centers, which are attractive characteristics also for catalytic applications (see section 3.4). The adducts 265 and 266 are thermally stable on heating in toluene up to 75 and 110 °C, respectively (cf. Figure 26, section 3.2.2).252 The stability of the adducts is remarkable when considering the thermal instability of the free carbene.247 DFT calculations and natural bond orbital analysis suggest that both the Zn−carbene and the Al−carbene bonds are covalent single bonds, with about 85% contribution from the more electronegative carbene carbon, with an overall charge transfer of of 0.241 and 0.258 electrons from the 4-imidazolylidene component to the organozinc and organoaluminum fragments, respectively. Subtle electronic effects arising from C4 bonding of imidazolylidenes to rhodium(III) trigger a swap of a chelating 2-pyridyl donor group from N-coordination to C3-coordinated ligand via a roll-over C−H bond activation.322 In the presence of an internal alkyne, this rollover pathway leads to C−C bond formation, affording a tricyclic product. 3.4. Catalytic Applications

3.4.1. Imidazolylidene Palladium Catalysis. The application of mesoionic imidazolylidene ligands in palladium-catalyzed cross-coupling reactions has been largely motivated by the beneficial effect that strong donor ligands are expected to have on the rate-limiting oxidative addition of aryl chlorides to a palladium(0) intermediate. The strong σdonor properties of 4-imidazolylidene are particularly attractive for these purposes, and some work along these lines has been reviewed recently.19,323 A theoretical study on a specific bis(imidazolylidene) system has predicted little energetic differences for catalytic cycles involving either 2- or 4imidazolylidene spectator ligands,324 though the assumption of two carbene ligands bound to the palladium center throughout the full catalytic cycle may be questionable, as the most active catalytic species are monocarbene palladium complexes containing a (hemi)labile “throw-away” ligand. Such a setup is expected to enhance catalyst activation, i.e., metal reduction to form the critical palladium(0) species, while the strong donor properties of 4-imidazolylidenes facilitate oxidative C−X bond addition. Suzuki−Miyaura Cross-Coupling. Complexes 325, 326, and 327 are active catalysts in the Suzuki−Miyaura crossBF


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complex 314 suggest that formation of the carbene palladium(0) intermediate is critical. The mixed phosphine 4-imidazolylidene-ligated palladium center in complexes 374a−c shows moderate performance in Suzuki−Miyaura cross-coupling reactions and achieves up to 94% yield in the coupling of aryl bromides with phenylbronic acid after 24 h (1 mol % complex, 100 °C).311 Rather atypically, bromoanisole containing a deactivated C−Br bond is converted slightly better than bromobenzene with all three complexes. The catalytic activity is not correlated with the donor ability of the 4-imidazolylidene, and complex 374b with a phosphine functional group at C5 performs best. Compounds 247, 249a, and 249b with a C5-unsubstituted 4-imidazolylidene induce the coupling of phenylboronic acid to a variety of aryl halides and at room temperature.245 The dimeric complex 247 outperforms the monomeric PEPPSItype analogues 249 and leads to quantitative conversions of bromo- and iodoaryls within 2 h at 3 mol % catalyst loading, while 4-chlorotoluene was converted less well with up to 69% conversion after 24 h. Other Cross-Coupling Reactions. Complexes 321 efficiently catalyze the Sonogashira cross-coupling of aryl bromides and iodides with a variety of terminal alkynes in air in a mixed aqueous medium at 90 °C and in the absence of a copper source (Scheme 153).288,289 With aryl iodides, quantitative conversions are accomplished with 2 mol % precatalysts 321 within 1 h, while aryl bromides require a higher loading (4 mol %) and longer reaction times of 3 h to reach the same conversion level under these Cu-free conditions. These complexes are inactive toward aryl chlorides substrates. Complex 330 containing a cyclometalated 4-imidazolylidene ligand converts arylboronic acids with electron-deficient olefins at 1 mol % catalyst loading in an oxidative Heck coupling (Scheme 154).293 Reaction times of 12 h are required when the reaction is performed at ambient temperature and in water to produce trans-β-arylated products exclusively. This catalyst also efficiently couples electron-rich olefins selectively to the βarylated product without any detectable α-arylated isomers. Complex 330 is highly selective toward oxidative Heck coupling processes, and even in the presence of 4-iodotoluene, the coupling of 4-tert-butylphenylboronic acid and methyl acrylate is preferred over Heck or Suzuki cross-coupling reactions. Olefin Transformations. Palladium complex 314c containing a chelating bis(aNHC) ligand is a useful catalyst precursor for the hydrogenation of cyclooctene and reaches full conversion in about 8 h under mild conditions (1 mol % catalyst, EtOH, RT, 1 atm H2; Scheme 155). In contrast, only low conversions are noted when the palladium is bound to a normal dicarbene ligand.286 The high catalytic activity of complex 314c is rationalized by the easier catalyst activation

coupling of bromobenzene with arylboronic acids (Scheme 152).291 Conversions reach up to 82% within 24 h for Scheme 152

reactions with 2.5 mol % catalyst loading performed at 80 °C. While this performance is far from competitive with highly active palladium catalysts, it is worth noting that the 2imidazolylidene analogues 400 and 401 reach considerably lower conversion (maximum 52%) under identical conditions. Rather remarkably, complex 326 containing two 4-imidazolylidene ligands exhibits reaction rates comparable to the PEPPSI-type monocarbene complex 327, while analogue 400 is much less active. This reactivity may point to a lower stability of the Pd−C bond in 4-imidazolylidene complexes and a similar active species emerging from 326 and 327 via dissociation of 4-imidazolylidene and pyridine, respectively, whereas 2-imidazolylidene dissociation is less favored. Complexes 328 convert aryl chlorides at room temperature in the presence of a base (NaOMe or Cs2CO3).292 The catalyst remains active for 10 successive catalytic runs, and catalyst loadings down to 0.01 mol % preserve catalytic fitness, producing up to 9500 TONs within 6 h. DFT calculations performed on a model complex with Me rather than Dipp groups as N-substituents suggest facile, base-assisted cleavage of the halide bridge followed by oxidative Ph−Cl addition to give a palladium(IV) intermediate rather than a classic palladium(0)/(II) cycle.325 No experimental evidence has been obtained to support the involvement of high-valent palladium complexes in this reaction, and numerous observations such as the inactivity of bis(carbene) palladium Scheme 153

BG


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Scheme 154

Scheme 155

Scheme 157

due to the increased electron density at the palladium center, which is a direct consequence of the exceptionally strong donor ability of the 4-imidazolylidene ligands. In the presence of H2, oxidative addition and reductive elimination sequences similar to the reaction patterns developed for 314a,b with Cl2 (see section 3.3.2) are assumed as a specific catalyst activation mode that is enabled only with C4-bound imidazolylidene ligands. This activation leads initially to [PdH4]2− and eventually to colloidal palladium as the active catalyst for olefin hydrogenation. A combination of GC and DLS studies indeed indicates that particles in the submicrometer dimension are the catalytically active species.214 Complex 332e containing a 2- and 4-imidazolylidene ligand shows excellent catalytic activity in the polymerization of norbornene when activated with MAO, affording up to 2.3 × 107 g of polynorbornene per mole of Pd an hour with as little as 0.25 μmol of catalyst (Scheme 156).326 This activity is similar to analogues containing 2-imidazolylidenes with less bulky N-substituents. Aerobic Oxidation. Complex 318 catalyzes the oxidation of 1-phenylethanol using molecular oxygen as the oxidant (Scheme 157).287 With 0.25 mol % palladium complex and 2 bar O2 at 100 °C (water solvent), a mere 14% conversion is

achieved after 15 h. In contrast, the analogous palladium complex with 2-imidazolylidene ligands accomplishes full conversion under otherwise identical conditions. 3.4.2. Imidazolylidene Iridium Catalysis. Hydrogenation Transfer Reactions. Iridium complex 402 containing a mesoionic 4-imidazolylidene with a phosphine chelate is an active catalyst for the transfer hydrogenation of ketones and enones (Scheme 158).327 The reduction of methyl aryl ketones containing both electron-withdrawing and electron-donating groups by iPrOH with 0.1 mol % loading of 402 affords the corresponding alcohols in high isolated yield. Complex 402 is considerably more active than the C2-bound imidazolylidene analogue. With α,β-unsaturated ketones and aldehydes, both the CC and the CO double bonds are reduced, resulting in the formation of the saturated alcohol. The transfer hydrogenation activity of complex 402 has been used in a hydrogen-borrowing process leading to the overall dehydrative C−C coupling reactions between primary aliphatic or benzylic alcohols and different 1-arylethanols (Scheme 158).327 Aryl groups with electron-withdrawing or electrondonating groups are converted with 402 to the corresponding β-alkylated products in high yields and selectivities. For example, benzylation of 1-phenylethanol with benzyl alcohol afforded the alcohol product in 92% isolated yield without any ketone byproduct. In contrast, iridium analogues with 2imidazolylidene ligands showed lower selectivity in the same transformation (91:9 alcohol/ketone ratio), suggesting that mesoionic 4-imidazolylidene complexes store the in-situformed H2 better than their normal analogues. The dehydrogenation half-cycle of hydrogen transfer reactions is also catalyzed by complex 402 in the absence of an acceptor substrate (Scheme 158).327 Acceptor-free dehydrogenation of benzyl alcohol produces benzyl benzoate in moderate yield and selectivity when using 2 mol % catalyst and Cs2CO3 as base, conditions that induce a 52% conversion within 48 h.

Scheme 156

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Scheme 158

Complex 403 is also an active catalyst for water oxidation mediated by CAN as sacrificial oxidant (Scheme 160).171 Oxygen evolution, monitored at different concentrations of iridium complex, reaches the theoretical limit of oxygen production, corresponding to about 8000 TONs under dilute conditions (25 μM complex), which compares well with some of the most active systems known to date.82,88,328−330 Kinetic analysis indicates a high initial rate of oxygen evolution (TOFmax = 0.4 s−1), which is about twice as high when compared to related complexes containing triazolylidene instead of 4-imidazolylidene ligand. This higher activity of 403 may be a direct consequence of the enhanced donor properties of the 4-imidazolylidene ligand and the ensuing better stabilization of high-valent iridium states on the catalytic cycle. 3.4.3. Imidazolylidene Rhodium Catalysis. Si−H Bond Activation. Complex 288c catalyzes the hydrolytic oxidation of PhMe2SiH to silanol and siloxane and concomitant formation of H2 with full conversion within 2 h at catalyst loadings of 1 mol % (Scheme 161).273 In dry MeNO2, the siloxane product is formed exclusively, while changing the solvent to wet THF affords the silanol selectively. When extraneous H2O is replaced by ROH, silyl ethers are cleanly formed, thus providing a mild route toward the protection of alcohols, with the generation of H2 as the only byproduct. Remarkably, ketones are not affected by these conditions, despite the propensity of many transition metals to catalyze hydrosilylation reactions. Hydrosilylation takes place, however, when alkynes are employed as substrates. Further expansion of the Si−H bond activation to dihydrosilanes affords silicones and polysilyl ethers through mild hydrolytic oxidation (Scheme 161).273 For example, using 1,4-bis(dimethylhydrosilyl)benzene as substrate, polymerization occurs readily in the presence of 288c and H2O. Likewise, the dihydrosilane H2SiMePh is converted into silicone. Transfer Hydrogenation. Rhodium(III) complexes 290 (cf. Scheme 116) containing a chelating bis(aNHC) ligand are

Alkane dehydrogenation as a most challenging version of transfer hydrogenation is catalyzed by complex 305 and requires tert-butylethene as H2 acceptor (Scheme 159).284 At Scheme 159

200 °C, the in-situ-generated dihydride complex 305 dehydrogenates cyclooctane with a moderate 3.5 TONs within 10 h. The low catalytic activity has been attributed in part to the poor solubility of complex 305 in the reaction mixture. Other Redox Reactions. The dicationic 4-imidazolylidene complex 403 displays high activity in the catalytic hydrosilylation of acetophenone with triethylsilane as the reducing agent (Scheme 160).171 With a 1 mol % catalyst loading, these Scheme 160

complexes require less than 5 min to reach completion, whereas hydrosilylation with 2-imidazolylidene analogues proceeds slower. The catalytic hydrosilylation of ketones proceeds with turnover frequencies as high as 6000 h−1. Of note, the corresponding monocationic complex containing an Ir−Cl instead of an Ir−OH2 site is inactive. Scheme 161

BI


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Scheme 162

essentially identical for complex 355 with a mesoionic 4imidazolylidene ligand and complex 356 with 2-imidazolylidenes only. At a substrate/catalyst ratio of 200, quantitative yields are obtained within 4 h. In contrast, the analogous PNP pincer ruthenium complex containing phosphine rather than imidazolylidene donor groups is inactive in this reaction.332 Miscellaneous Reactions. Complex 353 catalyzes the thermally induced addition of carboxylic acids to terminal alkynes (Scheme 165).304 While the reaction is relatively slow

catalysts for the transfer hydrogenation of different ketones. Replacement of the two MeCN ligands in 404 by substituted bipyridine or phenanthroline ligands (292a, 293) does not enhance catalytic activity (TOF50 300 h−1), though complex 292a undergoes a reversible color change from yellow to purple upon formation of the catalytically active species and hence provides a diagnostic probe for the catalytic on/off state of the complexes (Scheme 162).331 Introduction of diphosphine ligands yields complex cis/trans-291, which is a considerably more active catalyst that reaches TOFs of 1000 h−1 and up to 4000 TONs.274 Optimization of the catalytic conditions allowed transfer hydrogenation to be run with only 1 mol % base instead of the often used 10 mol %. 3.4.4. Imidazolylidene Ruthenium Catalysis. Hydrogenation Catalysis. The ruthenium complex 351 containing a 4-imidazolylidene ligand with a chelating phosphine is a very efficient catalyst for the transfer hydrogenation of ketones to alcohols with 2-propanol as hydrogen donor and 2 mol % NaOiPr (Scheme 163).303 With as little as 0.05 mol % complex

Scheme 165

Scheme 163

and requires 24 h to reach completion at a 2 mol % catalyst loading, high regioselectivity toward product I from antiMarkovnikov addition with Z-selectivity is achieved, with minor quantities of the E-olefin II and product III from Markovinkov addition. For example, benzoic acid addition to phenylacetylene produces a 86:6:10 ratio of the three addition products I, II, and III. The corresponding analogue of 353 with only C2-bound imidazolylidene, which exists in the ruthenium(III) oxidation state, achieves slightly lower conversion and more importantly no E/Z-selectivity (50:50:0 product ratio). The diverging selectivity has been attributed to the different steric impact of the C2- vs C4-bound imidazolylidene, especially with respect to the positioning of the mesityl wingtip group and the ensuing (de)shielding of the metal coordination sphere. The ruthenium(II) complex 358 is an excellent catalyst for the highly selective oxidation of olefinic double bonds to carbonyls using NaIO4 as terminal oxidant (Scheme 166).306 With 1 mol % catalyst loading, double-bond cleavage occurs at room temperature and within 30 min, providing quantitatively aldehydes and ketones. The catalyst is highly selective and tolerates a large variety of functional groups such as

351, full conversion is achieved. The rate and productivity is substantially increased by addition of benzylamine or ethylenediamine, which generates complex 405. With this complex, TOFs as high as 140 000 h −1 are achieved for the hydrogenation of acetophenone. The CNC pincer ruthenium complexes 355 and 356 hydrogenate esters at high H2 pressure (50 bar) and using KOMe as a base (Scheme 164).305 The activity of the ruthenium center is not significantly affected by the binding mode of the imidazolylidene, and the catalytic performance is Scheme 164

BJ


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Scheme 166

Scheme 168

Scheme 169

carbohydrates and amino acids and does not lead to overoxidation of the aldehyde products. With alkyne substrates, triple-bond oxidation takes place and yields αdiketones. 3.4.5. Imidazolylidene Catalysis with Miscellaneous Metals. Gold. Cyclization reactions of propargyl-functionalized furanes such as I are catalyzed by 4-imidazolylidene gold complexes 381 and 383 and afford the corresponding phenol II (Scheme 167).312 At catalyst loadings of 0.25 mol % 340 TON are accomplished when the catalyst is activated with AgNTf2. Caution is needed with the activation of gold carbene complexes using silver. The achieved TONs are appreciable, though about five times lower than the best catalyst known to date.333 Complexes 240 are catalytically active in the hydration of alkynes (Scheme 168).243 The best results for the hydration of phenylacetylene are achieved when combining equimolar quantities of the gold complex 240a and AgSbF6, which affords acetophenone in 91% yield after 11 h. In comparison, the sterically similar C2-bound imidazolylidene gold analogue accomplishes this hydration quantitatively in less than 2 h under the same catalytic conditions. This diverging performance may be a consequence of the different donor properties of the imidazolylidene isomers to the gold center or due to different reactivity and stability toward silver during catalyst activation. Copper. The abnormal N-heterocyclic carbene 365 is a versatile and highly efficient catalyst for the Huisgen 1,3dipolar cycloaddition of azides and alkynes at room temperature (Scheme 169).251,111 The click reaction of benzyl azide with phenylacetylene under solvent-free conditions affords quantitatively the corresponding triazole within 20 min when using 1 mol % of complex 365. A wide variety of alkynes undergo this 365-catalyzed cycloaddition including electronrich, electron-deficient, and functionalized alkynes containing halides, amines, amides, acids, pyridines, and other substituents. Lowering the catalyst loadings to 0.005 mol % results in 19 800 TONs at room temperature. Sterically hindered azides and alkynes such as Dipp-CCH and Dipp-N3 are

efficiently coupled and also internal alkyne substrates such as 3-hexyne undergo cycloaddition, demonstrating the versatility of 365 in click chemistry. Comparison with a sterically similar triazolylidene copper complex in several model reactions reveals essentially identical activity, while an analogue with a cyclic (amino)(alkyl)carbene ligand leads to lower productivity and achieves only about 50% conversion at the time when complex 365 reaches completion.111 Both complexes, 365 and its triazolylidene homologue, are activated much faster than the benchmark complex [CuCl(SIMes)] containing a 2-imidazolinylidene ligand.334 Aluminum and Zinc. Zinc complex 265 and aluminum complex 266, both containing 258a as the 4-imidazolylidene ligand, catalyze the lactone ring-opening polymerization reaction (Scheme 170).252 Complex 265 induces controlled polymerization of several different lactones at room temperature within 15 min at a monomer/catalyst ratio of 200:1, which may be complex or free base carbene catalyzed.335,336 The organoaluminum adduct 266 is considerably less active and requires 100 °C and 8 h for for polymerizing rac-lactide (100:1 monomer/catalyst ratio). The controlled nature of polymerization has been utilized for the synthesis of triblock copolymer. Interestingly, the free-base imidazolylidene 258a (see Scheme 104, section 3.2.2) acts itself as an organocatalyst for this lactone ring-opening polymerization and shows identical activity to the Zn complex.337 The cationic complex 406 containing a C2- and C4-bound imidazolylidene ligand hydrosilylates CO2 (1.5 atm.) in the presence of HSi(OEt)3 to afford the corresponding silylformate selectively (Scheme 171).268 When using 15 equiv of HSi(OEt)3 with respect to 406, high conversions are reached at 90 °C, albeit after long reaction times (60 h). Cobalt. The cobalt complex 366 is active in catalyzing Kumada cross-coupling reactions using various aryl bromides

Scheme 167

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Scheme 170

Scheme 171

Figure 30. Ruthenium complexes with 4- and 2-imidazolylidene ligands with antioxidant potential.

and phenylmagnesium bromide (Scheme 172).246 Biaryls are produced in up to 79% yield, with dehalohydrogenation to form Ar−H as the most competing side reaction. 4Iodotoluene exclusively yields toluene but no cross-coupling product, whereas 4-chlorotoluene reacts sluggishly and yields mixtures of products. Likewise, aryl bromides bearing sterically demanding substituents or an electron-releasing group react via dehalogenation predominantly.

noma (SW620), and small-cell lung carcinoma (NCI-H1688; Figure 31).297 Palladium complex 340b with a 2-imidazolyli-

3.5. Other Applications

Some 4-imidazolylidene complexes have been used for biomedical applications. The oxidant activity of the ruthenium(II) complex 407 containing a 4-imidazolylidene ligand has been evaluated together with its C2-bound analogue Ru(II) complexes 408 (Figure 30) and compared to [RuCl2(cym)]2 and [RuCl2(dmso)4].338 Investigations in vitro as well as in zebrafish (Danio rerio) as an in vivo model reveal a dual behavior for both NHC ruthenium complexes 407 and 408, as they act as antioxidants at low concentrations yet show prooxidant capacity at higher concentrations. Zebrafish embryos exposed to the ruthenium(II) complexes under several different conditions (0 or 24 h after fertilization, with or without the chorion) have been assessed by various parameters, such as viability, edema, heart rate, blood coagulation, pigmentation, scoliosis, malformation, and hatching. In general, zebrafish embryos are not harmed by exposure to either of the two ruthenium(II) complexes independent of the experimental conditions. Several toxicity profiles are observed depending upon the chemical structure of the compound in question, with the mesoionic carbene complex 407 outperforming the 2-imidazolylidene analogue 408 in cytotoxicity. Their characteristic properties as pro-oxidant and antioxidant agents together with their biosafety suggests a great potential of these complexes for biomedical applications as antitumor or neuroprotective drugs. The chelated palladium complexes 339 and 340 display anticarcinogenic behavior against three types of human tumor cells including ovarian cancer (TOV21G), colon adenocarci-

Figure 31. Palladium complexes used for anticancer activity.

dene ligand site is the most active compound against all three cell lines and exhibits IC50 values as low as 6 μM. In contrast, complex 339a with a mesoionic 4-imidazolylidene ligand does not show any inhibition of TOV21G cell growth, demonstrating that the inhibition activity of these complexes is strongly structure dependent.

4. COMPLEXES WITH OTHER 5-MEMBERED N-HETEROCYCLIC CARBENE LIGANDS 4.1. Pyrazolylidene Complexes

4.1.1. Metalation Procedures. The chemistry of pyrazolylidenes has been reviewed previously,339,25 and electronic and steric parameters have been assessed experimentally318,319 and theoretically.189 Pyrazolylidene complexes are formally normal when metalated at C3 and are abnormal and remote if metalated at C4, which leads to very strong donor properties. Metalation procedures applicable for the formation of pyrazolylidene metal complexes include (i) C−H bond activation (direct metalation), (ii) oxidative addition, (iii) alkylation or protonation of the corresponding anionic pyrazolyl ligand, (iv) transmetalation from carbene silver

Scheme 172

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Scheme 173

intermediates, and (v) cycloaddition to Fischer carbene complexes.19 Here only recent developments are collated, highlighting specific aspects imparted by these ligands. Direct Metalation. Palladation of the indazolium salt 409 with [Pd(OAc)2] generates the dimeric 3-indazolylidene palladium complex 410 with bridging iodide ligands (Scheme 173).340 The dimeric structure is readily cleaved in the presence of PPh3, leading to complex 411. Subsequent iodide abstraction with Ag(TFA) yields the indazolylidene palladium complex 412 (TFA = trifluoroacetate, CF3COO−) with better solubility properties than its iodide analogue. Transmetalation. Due to the low stability of silver pyrazolylidene complexes, transmetalation procedures are generally performed in situ. For example, treatment of the pyrazolium salt 413 with Ag2O followed by the addition of [PdCl2(PPh3)2] yields, without isolation of the silver carbene intermediate, the 3-pyrazolylidene palladium complex 414 (Scheme 174).341

The olefin ligand in complex 417 is readily displaced by CO to yield complex 418. Oxidative Addition. Reaction of the iodo-pyrazolium salts 419 with [Pd2(dba)3] in the presence of pyridine affords complexes 420a−c featuring the pyridine and pyrazolylidene ligands in mutual trans arrangement (Scheme 176).342 When the reaction is performed in the presence of PPh3 a variety of products is obtained depending on the nature of the 3,5 substituents in pseudo-ortho position. With methyl groups, the two phosphine ligands are mutually trans oriented (421a), while larger phenyl groups lead to a cis/trans mixture of 421b. With sterically more demanding iPr substituents, only one phosphine coordinates, producing the dimeric complex 422 with bridging iodide ligands. In the absence of a coordinating ligand, oxidative addition of 4-iodopyrazolium salts 419 to [Pd2(dba)3] yields the iodido-bridged Pd(II) dimers 423.343 This oxidative addition protocol is also suitable for the metalation of bis(pyrazolium) salts. The linker length between the two pyrazolium salts dictates the topology and controls the nuclearity of the resulting complexes. Oxidative addition of 424b with a tris-methylene linker to palladium(0) produces the dinuclear complex 425 with two independent palladium centers, whereas precursor 424a with a short methylene linker generates the tetranuclear rectangle 426 with two bridging dicarbene ligands and iodide-bridged metal centers (Scheme 177).344 Cycloadditions to Fischer Carbene Complexes. Reaction of the bis(dimethylamino)allenylidene complex 427 with freshly prepared diethyldiazomethane, Et2C−N2, at room temperature proceeds slowly to afford the pyrazolylidene complex 428 and the η1-butatriene complex 429 as two major product fractions (Scheme 178).345 Complex 428 results from cycloaddition of Et2C−N2 to the Cα−Cβ bond of the allenylidene ligand, presumably initiated by nucleophilic attack of the diazoalkane at Cα of 427. Similar cyclization reactions have been observed in the reaction of allenylidene

Scheme 174

Similar transmetalation of the indazolium salt 409 using an in situ protocol for the formation of the silver carbene intermediate and subsequent transmetalation with [PdBr2(CH3CN)2], [AuCl(SMe2)] or [Rh(COD)Cl]2 affords the indazolylidene palladium(II), gold(I), and rhodium(I) complexes 415, 416, and 417, respectively (Scheme 175).340 Scheme 175

BM


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Scheme 176

Scheme 177

= 2066 cm−1),347 which is counterintuitive as benzannulation generally reduces the electron density of the heterocycle. According to 13C NMR spectroscopy, the 3-indazolylidene ligand is a stronger donor than Arduengo-type NHCs but slightly weaker than pyrazolylidene.319 The discrepancy between the two methods is quite significant. In agreement with the NMR spectroscopic evaluation, an X-ray diffraction analysis of complex 411 (see Scheme 173) indicates that indazolylidene exerts a slightly stronger trans influence than PPh3 as deduced from the slightly longer Pd−I bond trans to the indazolylidene ligand. The reactivity of the Pd−C bond of pyrazolin-4-ylidene and indazolylidene dimers 423 and 410, respectively, has been probed by reaction with aliphatic and aromatic isocyanides (Scheme 179). 343 Treatment of complex 423a with isocyanides leads to insertion of the isocyanide into the Pd− Cpyrazolylidene bond, generating the dimer 430, bearing novel betain-type C-imino ligands. In contrast, analogous reaction with the indazolylidene palladium complex dimer 410 does not lead to an insertion process and the isocyanide behaves as a neutral donor in complex 431, positioned trans to the indazolylidene coordination site. A similar reactivity was observed with benzimidazolylidene complexes. This reactivity difference may be attributed to the superior donor ability of 4-

Scheme 178

pentacarbonyl complexes with ynamines.346 In THF solution, complex 428 slowly eliminates N2 and rearranges to 429, which limits the utility of this method to generate pyrazolylidene complexes. 4.1.2. Properties and Reactivity. The strong donor properties of pyrazolylidene ligands have been assessed previously.19 The donor properties of indazolylidene ligands have been evaluated by IR spectroscopy of the rhodium dicarbonyl complex as well as by NMR spectroscopy of an indazolylidene palladium complex and using the carbenic chemical shift of a trans-positioned benzimidazolylidene as reporter group. Consideration of the average CO vibration frequency of the rhodium carbonyl complex 418 (see Scheme 175, νav(CO) = 2030 cm−1) suggests that 3-indazolylidenes are stronger donors than the analogous pyrazolylidenes (νav(CO) BN


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cross-coupling reactions, while complexes 420 with a pyridine ligand are generally less active.

Scheme 179

4.2. Oxazolylidene and Thiazolylidene Complexes

Mesoionic oxazolylidenes and thiazolylidenes are related to imidazolylidenes, with the difference that the two heteroatoms are distinct, and hence, the C4 and C5 positions become distinguishable. Due to the lack of a substituted N−R unit, handles for tailoring the solubility are more difficult to insert. Complexes containing 5-thiazolylidene ligands have been prepared by Bertrand via deprotonation of a series of 2,3,4triaryl-susbtituted thiazolium salts 432 with LiHMDS in the presence of [PdCl(allyl)]2, [Au(tht)Cl], or [Rh(COD)Cl]2 affording complexes 433, 434, and 435, respectively (Scheme 181).348 Scheme 181

pyrazolylidenes or to a weaker Pd−C bond compared to 3indazolinylidenes, which is comprised of a carbene that is better stabilized by an adjacent nitrogen atom. 4.1.3. Catalytic Applications. The 4-pyrazolylidene palladium complexes 420 and 421 are catalyst precursors for Suzuki−Miyaura and Mizoroki−Heck cross-coupling reactions (Scheme 180).342 Suzuki−Miyaura cross-coupling reactions Scheme 180

Ligand exchange from COD in complex 435 to CO in complex 436 enables the determination of the average CO vibration frequency for evaluation of the donor properties. The νav(CO) = 2034 cm−1 indicates that the electron-donor properties of 5-thiazolylidenes are slightly superior to those of Arduengo-type NHCs (νav(CO) = 2039−2041 cm−1)184 and similar to cyclic (amino)(alkyl)carbenes (νav(CO) = 2036 cm−1)349 but inferior to those of other mesoionic carbenes (νav(CO) = 2016−2025 cm−1).350 Thiazolium triflates 437a−c containing a bromide in the 2, 4, or 5 position oxidatively add to [M(PPh3)4] (M = Ni, Pd) to obtain normal 2-thiazolylidene complexes 438 and 439 as well as their mesoionic 4- and 5-thiazolylidene analogues 440−442 (Scheme 182).351 While the ca. 40 ppm downfield shift of the Scheme 182 performed in water and with 1 mol % catalyst loading gives moderate to good yields (31−100%) for the coupling of activated aryl bromides with phenylboronic acid at ambient temperature and within 19 h. However, none of the four complexes shows acceptable activity toward the activation of more difficult substrates such as 4-chlorobenzaldehyde, reaching 50% conversion or less. Mizoroki−Heck olefination of aryl halides with tert-butyl acrylate occurs with 1 mol % catalyst loading in DMF. Complexes 420 and 421 convert 4-bromobenzaldehyde in essentially quantitative yields at 120 °C within 5 h (Scheme 180).342 Coupling reactions with 4-bromoacetophenone are much slower and afford moderate conversions (31%−85%) even after extending the reaction time to 22 h. With deactivated substrates such as 4-bromoanisole and 4chlorobenzaldehyde, yields were low even at elevated temperatures (140 °C) and with addition of Bu4NBr. Complex 421b with a 3,5-diphenyl substitution pattern shows overall the best performance in both Suzuki−Miyaura and Mizoroki−Heck

carbenic nucleus in the 13C NMR spectrum suggests considerable carbene-type behavior, solid-state analysis by Xray diffraction does not support a strong carbenic character and points to a retention of the immonium character as in the precursor thiazolium salt. The thiazolylidene palladium complexes 438, 440, and 442 are catalytically active in the Suzuki−Miyaura coupling of phenylboronic acid with bromo-acetophenone (Scheme 183). Under inert conditions and with a catalyst loading of 0.1 mol %, these thiazolylidene palladium complexes achieve conversions up to 83%. No substantial difference is noted upon changing the ligand bonding mode from 2- to 4- to 5BO


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behavior is in line with the higher electronegativity of oxygen compared to nitrogen and sulfur. Ynamide 449 can therefore act as a synthetic ligand equivalent of mesoionic 4-oxazolylidenes. Similar ring closure has been accomplished to form 1,3-dithiol-5-ylidenes. Even though strictly speaking this ligand class falls not within the scope of this review, its relationship with the thiazolylidenes and the elegant synthesis warrant its inclusion. Specifically, deprotonation of the dithiolium salt 454 leads to the formation of ethynylcarbamodithiolate 455. The addition of trifluoromethanesulfonic acid to 455 induces ring closure to afford the dithiolium triflate salt 454 (Scheme 186).354 This protoninduced cyclization of 455 to 454 has prompted the replacement of H+ with a gold(I) complex. Reaction of 455 with [Au(tht)Cl] cleanly affords the mesoionic carbene gold(I) complex 456. These results identify the ene−yne 455 as a ligand equivalent of 1,3-dithiol-5-ylidene. Different metals induce ring closure, and mesoionic carbene complexes 457, 458, and 459 form upon addition of [PdCl(allyl)]2, [RuCl2(cym)]2, and [Rh(COD)Cl]2, respectively. The corresponding rhodium(I) dicarbonyl complex 460 reveals CO vibrations with νav(CO) = 2030.8 cm−1, indicating that the dithiolylidene ligand is less strongly donating than mesoionic thiazolylidenes yet in a similar range as mesoionic oxazolylidene ligands (see above).

Scheme 183

thiazolylidene. The complexes are slightly less active than related pyridylidene compounds (see section 5.4). A series of new 2,3,5-triaryl-substituted oxazolium and thiazolium salts is readily accessible by a Tf2O-mediated intramolecular cyclization. Deprotonation of the corresponding oxazolium and thiazolium salts 443 or 444 by KHMDS in the presence of [Rh(COD)Cl]2 leads to the formation of the 4oxazolylidene rhodium complexes 445 and to the 4thiazolylidene analogues 446 (Scheme 184).352 To evaluate Scheme 184

4.3. Isoxazolylidene Complexes

Mesoionic 4-isoxazolylidene complexes are related to pyrazolylidenes and have been synthesized via oxidative addition of the iodo-isoxazolium salt 461 to [Pd(dba)2] in the presence of pyridine, which affords complex 462 as an air-stable, offwhite solid (Scheme 187).320 When [Pd(PPh3)4] is used as the palladium(0) source for the oxidative addition, the isoxazolylidene complex 463 is obtained.355 The 31P NMR chemical shift of this complex has been compared to structurally related mesoionic complexes and demonstrates a linear relationship with the donor properties as deduced from IR spectroscopic CO stretch frequency measurements, hence providing a convenient probe for assessing the carbene donor ability. Interestingly, the donor ability also correlates with the catalytic activity of these different mesoionic carbene complexes in Suzuki−Miyaura cross-coupling of 4-bromobenzaldehyde with phenylboronic acid.320 While the correlation is worth noting, the overall activity of these complexes in cross-coupling catalysis is rather low, requiring 19 h at 1 mol % catalyst loading for reactions at room temperature. Conversion of 4chloroacetophenone is essentially complete within 2 h when the reaction is performed at 140 °C.

the electron-donating ability of these new mesoionic carbene ligands, complexes 445 and 446 have been treated with excess carbon monoxide, which affords the rhodium dicarbonyl species 447 and 448, respectively. The average CO vibration frequency of these complexes (νav(CO) = 2026−2028 cm−1 for 447 and 2021−2022 cm−1 for 448) suggests that 4thiazolylidene ligands are stronger donors than 4-oxazolylidenes. Metal complexes 450−452 with 4-oxazolylidene ligands are obtained by ring closure of ynamide 449 induced by different transition metals such as [PdCl(allyl)]2, [Au(tht)Cl], and [Rh(COD)Cl] 2 (Scheme 185). 353 The corresponding Scheme 185

4.4. Tetrazolylidene Complexes

Complexes containing tetrazolylidene complexes only partially fit within the theme of this review, as tetrazolylidenes are extensively stabilized by heteroatoms. However, mesoionic tetrazolylidenes exist and have historical relevance, since 1,3disubstituted tetrazolylidenes were the first compounds that to be termed mesoionic N-heterocyclic carbenes.356 The reaction of tetrazolium salt 464 with [Rh(COD)Cl]2 in the presence of a mild base affords a mixture of two products, namely, the neutral monotetrazolylidene rhodium(I) complex 465 and the cationic bis(tetrazolylidene) rhodium(I) complexes 466 (Scheme 188).357 Bond lengths and angles are not different from analogous complexes with Arduengo-type 2imidazolylidene ligands. These complexes catalyze the decarbonylative addition reaction of benzoyl chloride to

rhodium(I) dicarbonyl chloride complex 453, obtained in the presence of CO, reveals an average CO vibration frequency νav(CO) = 2035.3 cm−1, which is considerably higher than the related complex 447 containg a 4-oxazolylidene ligand with three aryl substituents (νav(CO) = 2027 cm−1) and also higher than related thiazolylidene and 4-imidazolylidene ligands. This BP


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Scheme 186

Scheme 187

Scheme 189

Scheme 190

Scheme 188

N4 heteroatom, thus providing normal nonmesoionic tetrazolylidene species. Direct metalation of tetrazolium salt 471 with Na2[Cr2(CO)10] yields the mesoionic tetrazolylidene chromium complex 472 (Scheme 191).360 Likewise, reaction of the tetrazolium chloride 471 with [Ir(COD)(OMe)]2 containing an internal methoxy base gives the tetrazolylidene complex 473a and upon saturation with CO the corresponding carbonyl complex 473b.

ethynylbenzene (Scheme 188). The mono(tetrazolylidene) rhodium complex 465 is as effective as [Rh(COD)Cl]2 and gives chloro-Z-stilbene in 61% yield when using a 10 mol % catalyst loading. In contrast, the bis(tetrazolylidene) complex 466 is less reactive, resulting in only 22% yield. This lower activity may be due to enhanced steric crowding around the rhodium center, a lower electron density in the cationic complex, or a tighter bonding of the tetrazolylidene in complex 466. An alternative methodology toward tetrazolylidene complexes consists of forming first the tetrazolato complex, followed by alkylation of one of the nitrogen atoms. For example, N-alkylation of the bis(tetrazolato) complex 467 with Meerwein salt [Me3O]BF4 under inert atmosphere affords the corresponding mesoionic tetrazolylidene nickel complex 468, which induces a 10 ppm downfield shift of the metal-bound carbon resonance in 13C NMR spectroscopy (Scheme 189).358 Similar alkylation of tetrazolato complexes has been demonstrated with different gold complexes obtained from lithiation of tetrazole 469 followed by transmetalation with [Au(C6F5)(tht)], [AuCl(PPh3)], or [AuCl(tht)] (Scheme 190).359 Subsequent alkylation with methyl triflate afforded neutral complex 470a and the cationic mono- and bis(tetrazolylidene) gold(I) complexes 470b and 470c, respectively. In contrast to the nickel complexes 467, alkylation of these gold(I) complexes occurs selectively on the nucleophilic

Scheme 191

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Transmetalation is also a viable route to form mesoionic tetrazolylidene complexes. The tetrazolylidene silver complex generated from the tetrazolium chloride 473 and Ag2O promotes tetrazolylidene transfer to the molybdenum center in the presence of [Mo(CO)3ClCp] and yields complex 474.360 Similar direct metalation also proceeds when starting from the 1,4-disubstituted tetrazolium salt 475, which produces the normal nonmesoionic tetrazolylidene chromium complex upon reaction with K3[Cr2(CO)6(μ-OH)3], and the iridium complexes 476a and 476b in analogy to 473 upon reaction with [Ir(COD)(OMe)]2 followed by exposure to CO (Scheme 192).360

Scheme 193

Scheme 192

type compound. For example, reaction with S8 yields the 2pyridinethione 479, and addition of [AuCl(SMe2)] affords the pyridylidene gold complex 480. Pyridylidene ligands offer fundamentally interesting perspectives. Depending on the position of metalation, normal AR, remote and mesoionic AS, and remote and nonmesoionic carbene AT can be formed (Figure 32). Due to the

Comparison of the mesoionic 1,3-disubstituted tetrazolylidene complexes with their normal 1,4-disubstituted analogues reveals only limited stability for the nonmesoionic carbene complex since the ligand is prone to release N2 from the backbone.360 Spectroscopic, crystallographic, and computational studies indicate distinct differences arising from the change in substitution pattern. Both NMR and IR spectroscopies suggest donor properties of the mesoionic tetrazolylidene ligand comparable to a bulky Arduengo-type carbene (νav(CO) = 2022 cm−1 and TEP = 2048.8 cm−1 for 473b), while the normal tetrazolylidene in 476b has a donor strength similar to P(iPr)3 (νav(CO) = 2032.5 cm−1 and TEP = 2057.7 cm−1). Interestingly, the metal−carbon bond distances are shortened in the tetrazolylidene chromium complex 472 and its normal analogue when compared to related imidazolylidene or pyrazolylidene complexes. According to computational studies, this bond contraction is a consequence of steric rather than electronic factors.

Figure 32.

stabilization of the carbenic site with only one (remote) heteroatom, pyridylidenes are generally stronger σ-donors and better π-acceptors than Arduengo-type imidazolylidenes.84 Despite these aspects, pyridine-derived carbenes have received much less attention, not only because of the low stability of the free pyridylidene but also because of the limited procedures available for the metalation of the precursor pyridinium salts. As a consequence, the application potential of pyridylidene complexes has been much less explored so far compared to that of five-membered N-heterocyclic carbenes. 5.2. Metalation Procedures

Transmetalation and metal coordination to a free carbene base, either formed in situ or isolated, have emerged as the most attractive and versatile metalation methods in the organometallic chemistry of N-heterocyclic carbenes. Both of these methods are essentially unknown19 in pyridylidene chemistry. The most prolific routes for the formation of pyridylidene metal complexes include C−H bond activation and oxidative addition, though other methodologies such as the decarboxylation may become increasingly attractive if suitable ligand precursors are synthetically accessible. 5.2.1. C−H Bond Activation. Tautomerization of MetalBound Pyridine. Carmona and co-workers demonstrated that the tris(pyrazolyl)borate iridium complex 481 containing a labile N2 ligand reacts with 2-substituted pyridines to afford a series of protic 2-pyridylidene complexes 482 (Scheme 194).365,366 Tautomerization rather than direct C−H bond activation is supported by deuterium-labeling experiments and by modification of the pyrazolyl substituents. Using the 3mesityl pyrazolyl analogue 483 in the reaction with pyridine produces a 1:1 mixture of the C- and N-bound isomers 484 and 485 (Scheme 195).367 This reactivity also affords pyridylidene complexes derived from terpyridine, revealing

5. COMPLEXES WITH PYRIDYLIDENES AND RELATED LIGANDS 5.1. General Aspects

Even though pyridylidene ligands have been known since 1974,361 the chemistry of pyridine-derived carbenes is considerably less developed than that of imidazolylidenes and triazolylidenes. This fact may be attributed to the higher electron density of the pyridine heterocycle compared to imidazole and triazole due to the presence of only one heteroatom. This lower heteroatom count imparts much less stabilization of the free carbene. As a consequence, isolation of a free pyridylidene has not been accomplished yet, and until now, free pyridylidenes have been characterized only in the gas phase by mass spectrometry.362 Support for the existence of a free pyridylidene in the bulk has been obtained from the basepromoted generation of the sterically hindered 1,3,5-triaryl-2pyridylidene 478 from the corresponding pyridinium salt 477 (Scheme 193).363 While its analysis has been limited to decomposition products364 and does not allow one to distinguish a free base from a lithium adduct, trapping reactions are consistent with the intermediacy of a carbeneBR


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Scheme 194

Scheme 197

formation of the pyridylidene complexes 492−494 upon reaction of the hydride iridium(III) complex 491 with 2methylpyridine and (substituted) quinolines (Scheme 198).372 Similarly, reaction of the tris(pyrazolyl)borate osmium precursor 495 with 2-methylpyridine generates the pyridylidene osmium(II) complex 496 (Scheme 199).373 Cyclometalation. Cyclometalation of pyridinium derivatives has been successfully utilized for the preparation of 2-, 3-, and 4-pyridylidene complexes with various transition metals.19 Early work has explored the pyridine-assisted cyclometalation of pyridyl−pyridinium precursors, which leads to N,Cbidentate coordinating pyridine−pyridylidene ligands.19 Within this theme, 1,9-phenanthroline undergoes cyclometalation with [Ir(COD)Cl]2 to afford complex 497 (Scheme 200).374 In this complex, the pyridylidene ligand features a N-bound proton that is strongly hydrogen bonded to the chloride anion. Anion substitution from Cl− to BArF4− or another noncoordinating anion breaks up this hydrogen bond and produces the pyridylidene iridium(I) complex 498, which is in dynamic equilibrium with the iridium(III) hydride complex 499 via a 1,3-hydrogen migration. Temperature-dependent NMR spectroscopy reveals a negligible free energy difference for the two species ΔG°298 = −0.09 kcal mol−1 with a very small enthalpy change (ΔH° = −1.3 kcal mol−1) for the interconversion of 498a to 499a. The equilibrium is sensitive to the type of phosphine, with stronger donating phosphines favoring the iridum(III) complex 499 while electron-poor phosphines shift the equilibrium to the pyridylidene iridium(I) species 498. The equilibrium constant correlates linearly with the Hammett σP values. DFT studies of the metalation process suggest an initial C− H oxidative addition to give a five-coordinate iridium(III) hydride intermediate.375 Calculations reveal two plausible pathways for the subsequent 1,3-shift of the iridium-bound hydrogen to the pyridyl nitrogen including either β-H insertion or a water-assisted proton transfer with the latter more favored energetically. The benzimidazole−pyridinium salt 500 is comprised of a similar imine donor for inducing cyclometalation with [IrCp*Cl2]2 (Scheme 201).376 The formed pyridylidene− benzimidazole iridium(III) complex 501 contains a remote NH site that undergoes reversible deprotonation to yield complex 502 with an anionic pyridylidene−benzimidazolate

Scheme 195

an unusual coordination mode of this ligand. Reaction of the tris(pyrazole) iridium complex 486 containing a diene ligand with terpyridine gives the mono- and bis-carbene iridium(III) complexes 487 and 488 (Scheme 196).368,369 During this process, the formally neutral diene ligand is converted into a κ2-C,C’-bidentate and formally dianionic dialkyl ligand. This ligand is not inert, and when reacting with a 2-SiMe3-pyridine instead of terpyridine, the dialkyl ligand reacts with the desilyated pyridylidene and forms a new C−N bond.370 Experimental and theoretical investigations into the mechanism of the C−H bond activation indicate that the tautomerization from N- to C-coordination of 2-substituted pyridines is reversible and involves all aromatic C−H bonds apart from the sterically hindered C3−H bond and most likely proceeds through a complex-assisted σ-bond metathesis (σCAM) involving a phenyl ligand of 481.371 The product from C6−H bond activation is the most stable isomer and leads to the formation of pyridylidene complex 482 exclusively. When 482 is heated to 90 °C in C6D6, replacement of the phenyl ligands by C6D5 groups takes place with concomitant deuterium incorporation at the pyridylidene nitrogen exclusively.371 This observation indicates that thermal elimination of benzene from 482 generates a coordinatively unsaturated pyridyl complex 489, which cleaves the C−D bond of C6D6 to afford the deuterated analogue 490 (Scheme 197). This tautomerization methodology has been extended to different iridium precursors and to different tris(pyrazolate) metal systems. Esteruelas and co-workers have shown the Scheme 196

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Scheme 198

Scheme 199

Scheme 201

chelate. This reactivity therefore constitutes a molecular coordination switch that is acid−base controlled by pH input. Apart from these examples, pyridylidene complex formation has focused on the formation of C,C-chelating systems during the last 10 years. With these chelates, the initial metal bonding in close proximity to the C−H bond occurs through a kinetically more stable M−C bond.19 Due to the soft character of the anchoring C-donor, this cyclometalation has naturally concentrated on soft platinum group metals, specifically rhodium, iridium, and platinum. In 2010 we showed that metalation of the pyridiniumfunctionalized triazolium salt 144a with [IrCp*Cl2]2 generates the pyridylidene−triazolylidene iridium complex 145a as the major product and the pyridyl−ylide complex 146 as minor

component (cf. Scheme 38), Scheme 202.84 Both chelating carbene ligands in 145a are mesoionic. The metalation is initiated with Ag2O, which forms the triazolylidene silver complex that undergoes smooth transmetalation with iridium. Subsequent C(sp2)−H bond activation of the pyridinium site is favored if the triazolylidene N-substituent is deactivated

Scheme 200

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Scheme 202

Scheme 203

toward cyclometalation.73 Deactivation is effective if the benzylic substituent contains electron-withdrawing functional groups or if the N- substituent is an alkyl group, which produces the 3-pyridylidene complexes 145b−e selectively.73,87 Computational analyses are in agreement with an electrophilic Cpyr−H bond activation mechanism, commensurate with the functional group dependence observed experimentally. Colbran et al. disclosed a methodology to use imidazolylidenes as anchors to form pyridylidene complexes through cyclometalation. Reaction of the imidazolium−pyridinium salt 503 with [Rh(COD)Cl]2 in the presence of various carboxylate bases yields the rhodium(III) complexes 504 containing a chelating imidazolylidene−pyridylidene ligand (Scheme 203).377 Considering the low electron density in the nicotine−amide-derived pyridinium heterocycle and the established reactivity of imidazolium salts with rhodium(I) precursors under mild basic conditions, a plausible mechanistic scenario includes initial rhodium(I) binding to the imidazolylidene, followed by cyclometalation by oxidative addition. Steric shielding then directs the metalation to take place selectively at the pyridylidene C2 position. Inspired by these results, a series of imidazolium− pyridinium salts has been metalated with iridium and rhodium complexes which has unveiled some remarkable selectivity patterns276 Reaction of the pyridinium salt 505 containing the imidazolium substituent in the 3 position with [M(COD)Cl]2 (M = Rh, Ir) at elevated temperatures affords the corresponding pyridylidene−imidazolylidene rhodium(III) and iridium(III) complexes 506 and 507, respectively, containing a Cimi,Cpyr-bidentate chelating dicarbene ligand (Scheme 204). The pyridylidene unit coordinates exclusively via C4, in contrast to the selective C2 bonding observed with the nicotinamide-derived analogue (cf. complex 504, Scheme 203), and induces remote carbene bonding. Similar coordination has also been observed with ligands containing butyl instead of methyl groups bound to the nitrogen centers.

Scheme 204

Remarkably, when the diene in the metal precursor is changed from COD to norbornadiene (NBD), metalation of 505 under otherwise identical conditions yields the rhodium(III) complex 508 with a C2-bound pyridylidene ligand (Scheme 204), suggesting that steric factors control the selectivity of pyridinium C−H bond activation. Reaction of the analogous pyridinium salt 310 containing the imidazolium group in the 4 position reacts similarly to 505 with [M(COD)Cl]2 (M = Rh, Ir) and yields analogous Cimi,Cpyr-chelated iridium(III) and rhodium(III) complexes 296 and 309, respectively (Scheme 205).276 No selectivity aspects arise with this pyridinium substitution pattern, and mesoionic pyridylidene complexes form exclusively. Metalation Scheme 205

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Scheme 206

Scheme 207

rearrangement of complex 512 has been observed, and instead, formation of a tetramethylfulvene intermediate has been proposed. Rhodation of salt 509 containing a methyl-protected imidazolium salt generates complex 515 (Scheme 206). Formation of complex 515 implies a C(sp3)−H bond activation similar to that observed with complex 511. Moreover, a 3,6-cycloaddition of the [RhCp*] unit has taken place, resulting in dearomatization of the pyridine heterocycle and coordination of a unique quaternary carbon to the rhodium center. Metalation of the pyridinium imidazolium salt 505 has also been accomplished with [PtCl2(dmso)2] under basic conditions to yield the 2-pyridylidene platinum complexes 516 and 517 containing a chelating imidazolylidene ligand (Scheme 207).380 The reaction is thermally induced and reveals diverging stability of the alkyl−Npyr bond. While the methyl group is stable and leads to the dialkylated bis(carbene) complex 516, the iPr substituent is reactive and is completely substituted by a proton as a consequence of a Hofmann elimination process,381 which generates the protic pyridylidene exclusively. The pyridyl-bound ethyl group displays intermediate stability and produces a mixture of 516 and 517, with a distribution that is highly dependent on the reaction temperature. Increasing the temperature to 150 °C promotes the Hofmann elimination and provides complex 517 predominantly. Notably, the imidazole-bound alkyl group is absolutely stable, and no dealkylation has been observed. In all product complexes, the chloride ligands are readily displaced with a variety of N,N-bidentate coordinating bis(pyrazolate) ligands (see also section 4.4). Base-Assisted C−H Bond Activation. Complexes with 2pyridylidene ligands are accessible by the reaction of readily available N-methylpyridinium salts 518 with [Ru3(CO)12] in the presence of a strong base such as KHMDS (Scheme 208).382,383 While this metalation procedure may involve the

of the pyridinium−imidazolium salts 505 and 310 entails direct C−H bond activation without the assistance of any base and involves metal +1 to +3 oxidation. Control experiments indicate that the COD ligand does not act as dihydrogen acceptor. A plausible mechanistic model therefore may involve (reversible) oxidative Cimi−H activation and subsequent Cpyr− H bond activation by the metal−hydride intermediate with concomitant release of H2. No hydride intermediate has been detected when monitoring the reaction, which may point to a short lifetime and a high reactivity of this species. Similar pathways have been discussed in related oxidative addition of azolium salts to platinum(0) and palladium(0).378,379 Metalation of the pyridinium−imidazolium salts 505 and 509 with iridium(III) or rhodium(III) precursors in the presence of a base and KI induces a remarkably diverse range of coordination modes of the formed ligands. Metalation of these salts with [IrCp*Cl2]2 leads to the clean formation of the remote pyridylidene complexes 510 and 511, respectively (Scheme 206).285 The latter originates from exocyclic imidazolium C−H bond activation, a process that has been observed also with low-valent metal precursors (see section 3.2.4). However, replacement of the iridium precursor by its rhodium analogue, [RhCp*Cl2]2, completely changes the course of the reaction. Rhodation of salt 505 yields the three different complexes 512−514 (Scheme 206) which are separable by column chromatography. Complexes 512 and 513 feature a 4- and 2-pyridylidene ligand, respectively. The 2:1 ratio of these two complexes indicates a preference for remote pyridylidene formation, though the 2:1 selectivity suggests only a small bias. The third product complex 514 is comprised of a macrocyclic ligand in which the pyridinium ring is coupled to the Cp* unit. When considering the connectivity pattern, it seems likely that complex 514 is derived from the pyridylidene complex 512 as a direct consequence of the high basicity of the pyridylidene ligand. However, no thermal BV


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removed with Me3SiOTf, which affords the related dicationic solvento complexes. Oxidative addition of the 2-, 3-, and 4-chloropyridinium salts 529 to nickel or palladium gives complexes 530 or 531, respectively (Scheme 211).387 These complexes allow for a systematic comparison of the different pyridylidene isomers by X-ray structure and 13C NMR spectroscopic analysis. Of note, formation of the mesoionic pyridylidene complex 530b requires significantly longer reaction times (3 days vs 17 h for 530a or 530c). The dichloro-substituted pyridinium salt 532 undergoes selective oxidative addition of the remote C−Cl bond and results in 4-pyridylidene palladium and nickel complexes 533 and 534 (Scheme 212).387 While the shielding effect of the methyl group is small, it may be sufficiently large to favor oxidative addition at C4. In the absence of any shielding, the 2-pyridylidene ligand appears to be more stable (cf. complex 484, Scheme 195). Amine substituents have been introduced into the pyridinium framework to investigate the electronic modularity of the pyridylidene ligand. Oxidative addition of the o-amino-pchloro-pyridinium salt 535 and the o-chloro-p-amino analogue 538 to palladium(0) or nickel(0) affords the formally cationic carbene complexes 536/537 and 539/540 (Scheme 213).294 Backdonation from the amine is not sufficiently high to effectively hinder rotation about the Cpy−NR2 bond according to NMR spectroscopic analysis. Chelating di(pyridylidene) complexes are obtained from the reaction of the monochlorinated 3,3′-dipyridinium salt 541 (Scheme 214).388,389 Oxidative addition to [Pd(PPh3)4] gives the monopyridylidene complex 542, which readily undergoes carbonate-promoted cyclometalation to yield the di(pyridylidene)palladium(II) complexes 543. This reaction sequence is conveniently performed also in a one-pot procedure involving the palladium(0) precursor and the carbonate base simultaneously. Addition of acetic acid promotes an anion exchange reaction and replaces the chelating carbonate ligand by a bridging acetate ligand, thus leading to the bimetallic complex 544. The neutral di(pyridylidene) ligand in complexes 543 and 544 is an analogue of 2,2′-bipyridine and 1,10-phenanthroline, though much stronger electron donation is expected for the Cdonor system. This stronger donation is demonstrated by electrochemical experiments, where complex 544 displays two irreversible oxidation waves at +0.51 and +0.97 V (vs Fc/Fc+). Analogues of complex 544 containing a 2,2′-bipyridine and a 1,10-phenanthroline ligand instead of the di(pyridylidene) ligand did not reveal any oxidation process up to +2.0 V, suggesting that the di(pyridylidene) ligand stabilizes higher metal oxidation states substantially better than related N,N’bidentate ligands. Also, DFT calculations suggest that the

Scheme 208

generation of a free carbene, no evidence for such an intermediate is available, and the steric protection of a putative free carbene would be considerably lower than in orthoarylated systems (cf. 478, Scheme 193). The close proximity of a metal center to the ortho-C−H bond of the ligand in a trinuclear cluster and the irreversibility of the CO elimination have been put forward as triggers for the orthometalation of pyridylidenes and the subsequent face-capping coordination of the heterocycle in complex 519. This mechanistic model may explain the formation of clusters such as 521 and another trinuclear cluster having a face-capping NHC-derived ligand with a CH unit adjacent to the carbenic carbon.384 5.2.2. Oxidative Addition. The oxidative addition of halopyridinium salts to low-valent metal centers constitutes another strategy toward the synthesis of pyridylidene complexes that is complementary to the C−H bond activation methods.385 Oxidative addition of the aldehyde-functionalized 2-chloropyridinium 522 to [Pd2(dba)3] gives the 2-pyridylidene palladium(II) complex 523 (Scheme 209).386 Subsequent condensation of complex 523 with chiral amines such as (R)-methylbenzylamine or (R)-(1-naphthyl)-ethylamine yields complexes 524 and 525, respectively, containing a chiral C,Nbidentate chelating pyridylidene imine ligand. Inversion of this metalation−functionalization sequence has been demonstrated with the functionalized 3-chloropyridinium salt 526. Condensation with p-anisidine gives the iminefunctionalized pyridinium salt 527. Subsequent oxidative addition of 527 to [Pd2(dba)3] cleanly yields the desired palladium chloride complex 528 (Scheme 210).386 The metalbound halides of complexes 524, 525, and 528 are readily Scheme 209

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Scheme 210

such species has been identified so far because of their low stability.389 High oxidation state metal centers are stabilized however by the bulky pyridylidene ligand in 480 (cf. Scheme 193). The gold(I) center in this complex is oxidized with PhICl2 to form the corresponding pyridylidene Au(III) complex 545 (Scheme 215).390

Scheme 211

Scheme 215 Scheme 212

5.2.3. Miscellaneous Methods. Quaternization of Pyridyl Complexes. Oxidative addition of the 2-bromopyridines 546a−c containing a S- or N-donor group to [Pd(dba)2] affords dimetallic pyridyl palladium complexes 547 (Scheme 216).391 Subsequent protonation of 547 with HBr leads to the

Scheme 213

Scheme 216

reversible formation of the protic monometallic pyridylidene complexes 548. Remarkably, N-methylation of the 2bromopyridines inhibits ligand chelation, and oxidative addition instead yields a one-dimensional polymer. This method has been extended to more remote pyridylidene formation within the metal coordination sphere. The cyclometalated 1,5-, 1,6-, and 1,7-phenanthroline

di(pyridylidene) may be useful for stabilizing high metal oxidation states such as a Pd(III) configuration; however, no Scheme 214

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complexes 549, 550, and 551 are converted into remote pyridylidene complexes 552, 553, and 554, respectively, upon protonation of the noncoordinating nitrogen center (Scheme 217).392 Notably, the pyridylidene ligand in complex 554 is

Scheme 219

Scheme 217

gold(III) complex 562 upon metal-mediated sulfination of the alkyne in the precursor 561 in the presence of AuCl3 (Scheme 220).395,396 Scheme 220

formally mesoionic. According to pH-dependent electronic absorption spectra, protonation of 551 triggers profound changes of the electronic structure upon pyridylidene formation, while the changes are much smaller in the more remote systems 549 and 550. Pyridylidene Formation in the Metal Coordination Sphere. Aldol-type condensation of the Fischer carbene complex 555 with the methyl adduct of MeCN gives the modified Fischer carbene complex 556 featuring an amine and an activated methyl ether residue (Scheme 218).393 This

An oxidative cyclization takes place when the innermethylated N-confused porphyrin 563 is reacted with Re2(CO)10 (Scheme 221).397 Cyclization is unselective and occurs with either of the two adjacent pyrrole units to produce the 2-pyridylidene Re(I) complexes 564 and 565. Pyridinium Decarboxylation. An attractive procedure for the synthesis of 2-pyridylidene complexes has been disclosed by Conejero and co-workers. Reaction of the zwitterionic pyridinium 2-carboxylate 566 with [Ir(COD)Cl]2 at elevated temperatures generates the 2-pyridylidene complex 567 with concomitant release of CO2 (Scheme 222).398 This thermal decarboxylation method is versatile and provides access to a variety of 2-pyridylidene metal complexes including rhodium(I), platinum(II), and gold(I) 568−570, similar to the previously established decarboxylation of imidazolium salts.38

Scheme 218

carbene complex undergoes a cycloaddition with alkynes to yield the remote 4-pyridylidene complexes 557 with a 2,5substitution pattern. This method has high versatility potential if variation in the nitrile and the alkyne coupling agent is tolerated. A related intramolecular cyclization takes place upon reaction of the alkynyl−imine 558, yielding the mesoionic 3isoquinolylidene silver complexes 559 and 560 (Scheme 219).394 These complexes are intermediates en route to fluorinated isoquinolines, which are obtained upon substituting the silver center with F+. Evidence for fast pyridylidene ligand exchange at silver has been obtained, in agreement with the lability of other carbene silver intermediates. Selectivity toward either the monocarbene complex 559 or the cationic bis(carbene) analogue 560 is controlled by the availability of pyridyl−oxazoline (Pyox) as a scavenger for silver. Hence, high Pyox concentration favors the formation of the bis(isoquinolylidene) complex 560. Interestingly, the accessibility of pure silver complexes may provide a route for accessing pyridylidene-type complexes via transmetalation. This intramolecular cyclization method is versatile and has also been applied to the synthesis of the thiopyridylidene

5.3. Donor Properties and Reactivity

5.3.1. Donor Properties. Various structural parameters have been used to characterize the donor properties of pyridylidene ligands, including analysis of the M−Cpyr bond length, analysis of the trans influence, and bond length analysis within the pyridylidene heterocycle. Analysis of the M−Cpyr bond length requires an unsupported pyridylidene ligand in order to avoid values to be biased by chelate effects. Palladium complexes 530a−c offer interesting insights as they are comprised of a monodentate N-methyl-pyridylidene ligand that is palladium-bound via C2, C3, or C4 (cf. Scheme 211).387 In these complexes the more remote the stabilizing nitrogen is positioned, the shorter the Pd−Cpyr bond is, as the Pd−C distance decreases from 2.002(3) Å in 530a to 1.996(7) Å in 530b to 1.979(7) Å in 530c.399 The analogous nickel complexes do not display the same trend, and the Ni−Cpyr bond lengths are essentially identical within esds for all three isomers. BY


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Scheme 221

section 2.3), while considerable reactivity differences have been noted between mesoionic and nonmesoionic imidazolylidenes. Classic assessments of donor properties have been limited for pyridylidene ligands as the required complexes for CO vibration measurements to determine Tolman electronic parameters (low-valent Ni, Rh, or Ir complexes) are much more difficult to access than with other NHCs. Ligand exchange from COD to CO in complex 567 (cf. Scheme 222) reveals an average CO stretch vibration of about 2020 cm−1, which is comparable to the donor properties of mesoionic triazolylidenes yet weaker than 4-imidazolylidene ligands (see above).398 Likewise, electrochemical analyses for the determination of Lever electronic parameters have been elusive, as pyridylidene complexes with redox-active metals (Ru, Fe, and Cu, for example) are rare. Early work on [Ru(terpy)2]2+ analogues featuring one pyridyl ligand replaced by either a 3or a 4-pyridylidene unit indicate stronger donor properties of the 3-pyridylidene system due the 50 mV lower oxidation potential of the ruthenium center when compared to the 4pyridylidene analogue.400 A rationale for the stronger donation of the mesoionic 3-pyridylidene system may be deduced from analysis of the bonding situation within the pyridylidene heterocycle. While 2- and 4-pyridylidene isomers 530a and 530c feature distinct bond length alterations and indicate considerable double-bond localization, the C−C bond lengths in the mesoionic 3-pyridylidene palladium complex 530b lack any characteristic pattern. Bond lengths in 2- and 4pyridylidene complexes are highly reminiscent to related pyridones and implicate a considerable degree of CE double bond (E = O or Pd), whereas bond lengths in the mesoionic 3pyridylidene complex resemble those of pyridyl-derived ligands. Hence, significant π backbonding from the palladium center to the carbene is only postulated for normal pyridylidene ligands, while the abnormal pyridylidene ligand is essentially a strong σ-donor ligand. This conclusion is corroborated by palladium complexes with pyridylidene ligands comprised of a hydrogen rather than a methyl group at nitrogen.401 The acidity of this hydrogen decreases in the series 3-pyridylidene > 4-pyridylidene > 2-pyridylidene. Computational analysis suggests that the π-orbital term of the Pd−C bond is not affected by changes in the bonding mode of the pyridylidene ligand, but the bond strength is altered.402 5.3.2. Stability toward Acids and Bases. Even though quantitative investigations into the stability of pyridylidene complexes under acidic and basic conditions are missing, the robustness of the M−Cpyr bond can be estimated qualitatively from specific applications. For example, the protic pyridylidene palladium complexes 544 and 548 are formed under acidic conditions (cf. Schemes 214 and 216, respectively), suggesting

Scheme 222

The inverse trend is noted when analyzing the Pd−Cl and Ni−Cl bond lengths trans to the pyridylidene ligand, which increases along the series 2-pyridylidene < 3-pyridylidene < 4pyridylidene, suggesting that the bond strength and the trans influence correlate in this triad.387 The trans influence of pyridylidene ligands is further revealed by bond length comparison in mixed pyridylidene−imidazolylidene complexes 296 and 507b containing a 3- and 4-pyridylidene ligand, respectively (Figure 33).276 In both complexes the M−NMeCN

Figure 33. Complexes 296 and 507b with 3- and 4-pyridylidene donors, respectively, and a chelating 2-imidazolylidene for relative assessment of the carbene trans influence.

bond trans to the pyridylidene ligand is considerably longer than the bond trans to the 2-imidazolylidene, indicating a markedly larger trans influence of pyridylidenes compared to Arduengo-type imidazolylidenes. Comparison of the trans influence of the 3- vs 4-pyridylidene ligand is hampered by the fact that the two complexes contain a different metal center. When using the imidazolylidene trans influence as a reference, the modulation of the M−N bond length is almost proportional for both complexes (1.5% and 1.7% bond lengthening for 3- and 4-pyridylidene complex, respectively). These bond length analyses therefore lend further support to the notion that the structural features do not depend on the mesoionic vs nonmesoionic character of the carbene. Similar conclusions have been drawn for triazolylidene ligands (cf. BZ


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Scheme 223

[Tp*IrPh2(NCMe)] without the pyridyl ligand does not react with water under the same conditions. In addition to the activation of these polar substrates, which benefit from the polarity difference of the Lewis acidic iridium center and the Lewis basic nitrogen site, also apolar substrates are effectively activated (Scheme 223).403,404 Addition of alkenes such as ethylene or propylene to the pyridylidene iridium complex 482 yields the metalacyclic complex 576 with an alkylated pyridylidene ligand. Acetylene addition produces initially the vinylidene intermediate 577, which reacts with adventitious water to form the acyl complex 578 containing a protic pyridylidene ligand. Under strictly anhydrous reactions, intermediate 577 undergoes a cyclization which is related to the pyridylidene-mediated activation of the diene ligand in 579.370 The added ring strain in the final product 579 is evidently compensated by the energy gain due to formation of the iridium−vinyl bond and the pyridylidene formation. All of these reactivities not only show the high activity of the pyridylidene complex 482 and its equilibrium-related counterpart 489 but also demonstrate the substantial energy gain upon pyridylidene formation from the pyridyl precursor. With O2 as another small molecule, bond activation with complex 482 leads to two products, complex 580 from oxygen bond cleavage to produce a metalated pyridine-N-oxide as well as complex 581 from an oxidative dehydrogenation leading to C−N bond formation (Scheme 224).406 Both products are

considerable stability of the bond under these conditions. Likewise, the ruthenium complexes 552−554 are acid stable (cf. Scheme 217). Moreover, acid−base switching between the iridium complexes 501 and 502 indicates robust metal bonding in a mildly acidic and basic environment (cf. Scheme 201). Moreover, complex 145 is a robust water oxidation catalyst in the presence of CAN (cf. section 2.4.4), which requires stability under harsh conditions (water, pH ≈ 1, strongly oxidizing conditions). The stability of the Ir−Cpyr bond in this complex is obviously further supported through chelation of the triazolylidene unit. 5.3.3. Stoichiometric Bond Activation. The reversible benzene C−H bond activation of the pyridylidene iridium complex 482 has been suggested to proceed through benzene dissociation, involving an equilibrium with a lower coordinate pyridyl iridium complex 489 (cf. Scheme 197). While 489 has not been isolated yet, its relevance has been supported by an alternative pathway and through the elimination of H2O from the pyridylidene complex 571 containing a metal-bound hydroxide ligand (Scheme 223).371 Moreover, this coordinatively unsaturated species has been successfully trapped as the carbonyl species 573 upon pressurizing a solution of 482 with CO.403,404 Complex 489 provides a metal−ligand bifunctional platform constituted of an underligated iridium center and a basic pyridyl unit. Such a configuration is reminiscent of frustrated Lewis pairs, and the close proximity of the Lewis acidic and basic site offers high versatility for the activation of strong bonds.403,404 For example, dihydrogen is heterolytically cleaved and yields the pyridylidene iridium hydride complex 572 (Scheme 223). Upon addition of CO to this complex, the carbonyl species 573 is formed, which suggests that H2 activation is reversible, possibly via a η-H2 intermediate. The bifunctional platform also efficiently activates CO2 to form the carbamate complex 574, which constitutes a rare example of a formally acylated pyridylidene system. The bifunctional complex also activates a mixture of MeCN and H2O, which results in the formation of the acetamidato complex 575. The hydration of the bound MeCN ligand is promoted by the Bronsted basic pyridyl ligand,405 because in control experiments using complex

Scheme 224

formed independently, and their relative ratio is dependent on the temperature and to a lesser extent on the pyridyl substituent. A similar oxidative C−N bond formation is observed when the two phenyl groups in the iridium precursor 482 are substituted by a ethylphenyl chelate, though with this precursor the benzylic C(sp3)−H bond is activated rather than CA


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Scheme 225

Scheme 226

Scheme 227

activity is identical for all three isomers, yet final conversions slightly vary. Complex 530b containing a mesoionic pyridylidene ligand deactivates first, which may hint to a lower stability of this complex under catalytic conditions. The iridium(III) complex 145 with a triazolylidenepyridylidene chelate is a precursor for efficient water oxidation catalysis (see section 2.4.4).84 The structurally related pyridylidene−benzimidazolate iridium(III) complex 502 is an active catalyst for the hydrogenation of primary and secondary imines under ambient H2 pressure, reaching TOFs up to 15.8 h−1 (Scheme 228).376 The protonated analogue 501 is more

the phenyl C(sp2)−H bond in 482 and oxygen incorporation into the C(sp2)−C(sp3) bond is a major pathway.407 Acetylene activation has also been observed with the related tris(pyrazolyl)borate osmium complex 496 containing a pyridylidene ligand and a labile acetone in the coordination sphere (Scheme 225).373 Acetylene coordination forms the vinylidene complex 582, which is deprotonated with KOtBu either at the vinylidene ligand to yield the acetylide complex 583 or at the pyridylidene ligand to produce the pyridyl vinylidene osmium complex 584. These two species are in equilibrium, though the latter undergoes a rearrangement involving C- to N-coordination of the pyridyl ligand and C−C bond formation between the originally carbenic carbon and the central vinylidene carbon, yielding the strained metala-azacyclobutane 585 with an N-coordinated heterocycle. The reactivity is remarkably diverging from that observed with the iridium analogue (cf. 577, Scheme 223) and involves C−C rather than C−N bond formation within the metal coordination sphere.

Scheme 228

5.4. Catalytic Applications

Catalytic applications are relatively limited and include only a handful of studies in addition to those compiled in the previous review.19 Complexes 547 catalyze the Mizoroki− Heck arylation of styrene with aryl bromides using 0.2 mol % catalyst loading at 140 °C and achieve full conversion after 6 h (Scheme 226).391 Mechanistic studies involving inter alia mercury poisoning experiments support a heterogeneous mode of action, including the loss of the heterocyclic ligand from the metal coordination sphere. Complexes 530 display catalytic activity in the Suzuki− Miyaura cross-coupling between phenylboronic acid and bromoacetophenone that is slightly better than that of [Pd(PPh3)4] as benchmark catalyst (Scheme 227).387 Initial

than 1 order of magnitude less active (TOF ≈ 1 h−1). Hence, this catalysis is switched on and off efficiently by addition of base and acid, respectively, resulting in a pH-switchable hydrogenation catalyst for several cycles. Obviously, acid/base cycling is limited due to the increasing buffer strength of the solution. The pyridylidene gold(I) complex 480 shows catalytic activity in the oxidative C−H arylation of isoxazoles with arylsilanes (Scheme 229).390 At a 5 mol % catalyst loading, yields were moderate at 14−55% over 18 h, perhaps because iodosobenzoic acid and camphorsulfonic acid as additives compromise the catalyst integrity. Nonetheless, this direct C− CB


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external quantum efficiency up to 12.5% and luminescence and power efficiencies up to 44.0 cd A−1 and 28.0 lm W−1, respectively. These properties are very promising for further development of photoactive materials based on pyridylidene platinum structures.

Scheme 229

6. CONCLUSIONS The chemistry of mesoionic N-heterocyclic carbenes has progressed substantially over the past decade. In particular, the synthesis and application of mesoionic triazolylidene complexes has thrived since the appearance of the previous review, and both fundamental properties as well as applications, predominantly in catalysis, demonstrate the great potential of this new subclass of mesoionic carbenes. Specific reactivity patterns such as the demethylation observed upon generating free carbene bases are unique to triazolylidenes and have not been observed with other NHC systems. The synthetic accessibility of the ligand precursor through either click chemistry or by cycloaddition between 1,3-diaza-2-azoniallene salts and alkynes or alkyne subrogates is very versatile and provides access to a huge variety of functional groups for incorporation onto the triazolylidene scaffold. Considering this variety, it is remarkable that hybrid systems that explore the functionality of the click partners are still relatively scarce. Also, the functional group tolerance of click chemistry gives access to ligand systems that are not easily accessible with related NHC subclasses, as specific functional groups can be installed in very close proximity to the metal−carbene fragment. The series of 4,5-bimetalated triazolylidene complexes is an example of such functionalization in close proximity to the metal coordination sphere. Mesoionic NHC ligands have become interesting spectator ligands for a wide variety of mid and late transition metals, even though their utilization in stabilizing first-row transition metals has only gained momentum during recent years. With the emergence of novel metalation procedures, mesoionic carbenes are likely to become attractive ligands for earthabundant metals. Similarly, main group metals, lanthanides, and actinides, which have rarely been coordinated to mesoionic NHCs so far, may benefit from novel methods. More challenging will be the bonding to early transition metals to a large extent because of the hard−soft mismatch of early transition metal Lewis acids and the soft carbon bonding site of carbene ligands. Numerous exciting catalytic applications have been disclosed with a variety of mesoionic carbene metal complexes, which encompass classical catalytic reactions (such as palladium-

H activation is attractive as it eliminates the need for substrate prefunctionalization. 5.5. Other Applications

The terpyridyl iridium complex 487 and the dinuclear system 488 are both emissive (see Scheme 196).369 Their absorption and emissive properties are very similar, suggesting only little interactions between the two metal centers in complex 488. The absorptions at higher energies correspond with the excitation maxima in the steady-state excitation spectra. The red emission observed for both complexes is therefore suggested to originate from a metal to pyridyl or metal to pyrazolyl charge transfer [3MLCT]. The platinum(II) complexes 586, 587, and 588 have been obtained by chloride displacement from the parent pyridylidene−imidazolylidene platinum dichloride 516 in the presence of NaOAc (Figure 34, cf. Scheme 207).380 These bis(pyrazolate) analogues are nonemissive in solution but show bright emission in the crystalline state, which is red shifted upon changing the morphology to powder. Such a shift is consistent with the formation of highly aggregated species. Remarkably, both the observed and the radiative lifetimes are reduced to the submicrosecond range, which may be related to the increased intermolecular Pt···Pt interaction and an enhanced MMLCT character of this transition in the powder. The long Pt···Pt distances and consequential solid-state stacking of 587a and 588b have been successfully reproduced by hybrid DFT computation. Accordingly, the crystalline forms may be considered as monomers because TD-DFT emission data of monomers agree with the experimentally determined emissions between 462 and 475 nm of the polycrystalline solids observed. Computational data on a model dimer complex further suggest that the experimentally observed low-energy emissions measured in the powder form are derived from dimers with relatively short Pt···Pt distances, which gives rise to 3MMLCT transitions.380 Complexes 587a and 588a−c show strong solid-state emission. Multilayered OLEDs fabricated from complexes 588a and 588b show excellent device performance with an

Figure 34. CC


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AUTHOR INFORMATION

catalzyed cross-coupling or copper-catalyzed click reactions), as well as challenging bond activation reactions. In particular, redox reactions are promoted by mesoionic carbene ligands, and remarkable progress has been accomplished in hydrogen transfer reactions (dehydrogenation, transfer hydrogenation, hydrosilylation) as well as in water oxidation. The excellent catalytic performance of mesoionic carbene complexes in redox catalysis may be due to numerous effects imparted by the ligand, including the strong bonding of the mesoionic carbene to the metal center, the high basicity of the ligand and the ensuing high electron density at the coordinated metal center, as well as the potential of the ligand to play a noninnocent role, for example, for the stabilization of reactive radical species or as transient proton reservoir. Such proton bonding may occur via different mechanisms, including, for example, coordination to the unsubstituted N2 site in mesoionic triazolylidenes or to the C5 position of mesoionic 4-imidazolylidenes as supported by deuteration experiments. These reactivity patterns are promoted by the highly dipolar structure of mesoionic carbenes, which is represented by a carbanionic metal bonding site that is charge balanced by a relatively remote iminium cation. Such pronounced charge separation is absent in normal Arduengo-type NHCs and may be a key factor for the unique reactivity patterns observed with mesoionic carbene ligands. Moreover, the dipolar structure also imparts high solubility of mesoionic carbene complexes in polar solvents, and many complexes are in fact water soluble without the need of additional functionalization with sulfonate or carboxylate groups. In addition, the pronounced dipolar configuration of mesoionic carbene ligands greatly facilitates solvolysis of metal−halide bonds, a process that is driven by competitive ion pairing of the halide with the remote iminium cationic fragment in this class of ligands. Such M−X bond cleavage enhances the carbanionic character of the carbenic donor site and may constitute a relevant process for biomedical as well as catalytic applications. Numerous studies have demonstrated also that the M−C bond in mesoionic carbene complexes is labile, and a variety of complexes including Cu, Ag, Au, Ru, Pd, and Ni have in fact been used for transmetalation. Accordingly, caution is needed when assuming a kinetically inert coordination of the carbene to the metal center. Depending on the additives, carbene dissociation from the metal center may be a critical process, e.g., for catalyst activation. Mechanistic studies therefore need to probe the reliable coordination of the (mesoionic) carbene to the metal center, which may not be trivial in most cases. Of course, chelation provides an attractive methodology to support the M−C bond and to avoid dissociation, which is a particularly attractive concept for materials chemistry such as the formation of optically or electronically switchable materials. Other applications actually rely on the gradual dissociation of the metal center, such as biomedical application of mesoionic carbene complexes as anticarcinogenic agents. Considering the flexibility of the ligand system, countless variations and combinations of orthogonal functionalities are conceivable for being incorporated to the mesoionic scaffold, both for enhancing activity as well as for disclosing new areas of application, which will undoubtedly keep up the fascination of this field.

Corresponding Author

*E-mail: [email protected]. ORCID

Á ngela Vivancos: 0000-0001-9375-8002 Martin Albrecht: 0000-0001-7403-2329 Notes

The authors declare no competing financial interest. Biographies Á ngela Vivancos completed her Ph.D. degree at the Institute of Chemical Research (University of Sevilla−CSIC) in 2015 under the supervision of Prof. Margarita Paneque and Prof. Manuel L. Poveda. In the same year she started working as a postdoctoral researcher in the group of Prof. Martin Albrecht at the University College Dublin, where she worked on mesoionic N-heterocyclic carbenes. Within the same research group, she continued her postdoctoral experience at the University of Bern during 2016−2017. As part of the COST CARISMA Action, she spent 3 months in the Leibniz Institute for Catalysis (Rostock, Germany), in the group of Prof. Matthias Beller, where she studied catalytic (de)hydrogenation reactions mediated by triazolylidene iridium complexes. In November 2017 she joined the University of Murcia (Spain), where her focus is on the synthesis and photochemical application of platinum complexes with Prof. Pablo González-Herrero. Recently, she obtained a Saavedra Fajardo postdoctoral fellow from the Seneca Foundation to continue working in the organometallic chemistry group at the University of Murcia. Candela Segarra studied Chemistry at the Universitat Jaume I, Castellon, Spain, and received her Ph.D. degree there in 2014 under the supervision of Prof. Eduardo Peris. Her Ph.D. work focused on the development of homogeneous catalysts for green processes and nonconventional NHC-based complexes. In the same year she joined Prof. Martin Albrecht’s group for her first postdoctoral position at the University College Dublin, Ireland, where she obtained a MSCA individual fellowship from the European Commission. After this she moved to the University of Bern, Switzerland, within the same research group to continue her postdoctoral studies based on mesoionic NHCs and related N-mesoionic ligands. Currently, she is working as a postdoctoral fellow at the Instituto de Tecnologiá ́ ́ and Quimica in Valencia, Spain, in the groups of Dr. Urbano Diaz Prof. Fernando Rey within the MULTI2HYCAT project, a Research and Innovation program funded by the European Commission. Martin Albrecht studied Chemistry at the University of Bern and earned his Ph.D. degree in Organometallic Chemistry under the supervison of Gerard van Koten at Utrecht University in 2000. After postdoctoral work on N-heterocyclic carbene chemistry with Bob Crabtree at Yale and 1 year with Ciba SC (Basel, Switzerland), he started independent research as an Alfred Werner Assistant Professor at the University of Fribourg (Switzerland). In 2009, he joined the faculty at University College Dublin as Full Professor and returned in 2015 to his alma mater. His research generally revolves around the ligand-induced control and expansion of the reactivity of metal centers, both for catalysis and for materials application. He is fascinated by new ligand classes and has been particularly intrigued by mesoionic N-heterocyclic carbenes and recently also by their N-donor analogues. He has been privileged to have continuously been leading a dedicated and inspiring research team. His work has been recognized amongst others by a Humboldt research award, by the Catalysis Society of South Africa, and by the European Research Council through an ERC consolidator grant. CD


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(20) Crowley, J. D.; Lee, A.-L.; Kilpin, K. J. 1,3,4-Trisubtituted-1,2,3Triazol-5-Ylidene “Click” Carbene Ligands: Synthesis, Catalysis and Self-Assembly. Aust. J. Chem. 2011, 64, 1118−1132. (21) Donnelly, K. F.; Petronilho, A.; Albrecht, M. Application of 1,2,3-Triazolylidenes as Versatile NHC-Type Ligands: Synthesis, Properties, and Application in Catalysis and Beyond. Chem. Commun. 2013, 49, 1145−1159. (22) Aizpurua, J. M.; Fratila, R. M.; Monasterio, Z.; Perez-Esnaola, N.; Andreieff, E.; Irastorza, A.; Sagartzazu-Aizpurua, M. Triazolium Cations: From The “click” pool to Multipurpose Applications. New J. Chem. 2014, 38, 474−480. (23) Schulze, B.; Schubert, U. S. Beyond Click Chemistry Supramolecular Interactions of 1,2,3-Triazoles. Chem. Soc. Rev. 2014, 43, 2522−2571. (24) Schweinfurth, D.; Hettmanczyk, L.; Suntrup, L.; Sarkar, B. Metal Complexes of Click-Derived Triazoles and Mesoionic Carbenes: Electron Transfer, Photochemistry, Magnetic Bistability, and Catalysis. Z. Anorg. Allg. Chem. 2017, 643, 554−584. (25) Poulain, A.; Iglesias, M.; Albrecht, M. Abnormal NHC Palladium Complexes: Synthesis, Structure, and Reactivity. Curr. Org. Chem. 2011, 15, 3325−3336. (26) Sarkar, B.; Suntrup, L. Illuminating Iron: Mesoionic Carbenes as Privileged Ligands in Photochemistry. Angew. Chem., Int. Ed. 2017, 56, 8938−8940. (27) Meldal, M.; Tornøe, C. W. Cu-Catalyzed Azide - Alkyne Cycloaddition. Chem. Rev. 2008, 108, 2952−3015. (28) Bock, V. D.; Hiemstra, H.; Van Maarseveen, J. H. Cu ICatalyzed Alkyne-Azide “click” cycloadditions from a Mechanistic and Synthetic Perspective. Eur. J. Org. Chem. 2006, 2006, 51−68. (29) Kolb, H. C.; Finn, M. G.; Sharpless, K. B. Click Chemistry: Diveverse Chemical Funtion from a Few Good Reactions. Angew. Chem., Int. Ed. 2001, 40, 2004−2021. (30) Zhang, L.; Chen, X.; Xue, P.; Sun, H. H. Y.; Williams, I. D.; Sharpless, K. B.; Fokin, V. V.; Jia, G. Ruthenium-Catalyzed Cycloaddition of Alkynes and Organic Azides. J. Am. Chem. Soc. 2005, 127, 15998−15999. (31) Kwok, S. W.; Fotsing, J. R.; Fraser, R. J.; Rodionov, V. O.; Fokin, V. V. Transition-Metal-Free Catalytic Synthesis of 1,5-Diaryl1,2,3-Triazoles. Org. Lett. 2010, 12, 4217−4219. (32) Bouffard, J.; Keitz, B. K.; Tonner, R.; Guisado-Barrios, G.; Frenking, G.; Grubbs, R. H.; Bertrand, G.; Lavallo, V.; GuisadoBarrios, G.; Frenking, G.; et al. Synthesis of Highly Stable 1,3-Diaryl1H-1,2,3-Triazol-5-Ylidenes and Their Applications in RutheniumCatalyzed Olefin Metathesis. Organometallics 2011, 30, 2617−2627. (33) Schaper, L.-A.; Ö fele, K.; Kadyrov, R.; Bechlars, B.; Drees, M.; Cokoja, M.; Herrmann, W. A.; Kühn, F. E. N-Heterocyclic Carbenes via Abstraction of Ammonia: “normal” Carbenes with “abnormal” Character. Chem. Commun. 2012, 48, 3857−3859. (34) Á lvarez, C. M.; García-Escudero, L. A.; García-Rodríguez, R.; Miguel, D. Beyond Click Chemistry: Spontaneous C-Triazolyl Transfer from Copper to Rhenium and Transformation into Mesoionic C-Triazolylidene Carbene. Chem. Commun. 2012, 48, 7209−7211. (35) Maity, R.; van der Meer, M.; Hohloch, S.; Sarkar, B. Di- and Trinuclear Iridium(III) Complexes with Poly-Mesoionic Carbenes Synthesized through Selective Base-Dependent Metalation. Organometallics 2015, 34, 3090−3096. (36) Sinn, S.; Schulze, B.; Friebe, C.; Brown, D. G.; Jäger, M.; Altuntaş, E.; Kübel, J.; Guntner, O.; Berlinguette, C. P.; Dietzek, B.; Schubert, U. S. Physicochemical Analysis of Ruthenium(II) Sensitizers of 1,2,3-Triazole-Derived Mesoionic Carbene and Cyclometalating Ligands. Inorg. Chem. 2014, 53, 2083−2095. (37) Furstner, A.; Seidel, G.; Kremzow, D.; Lehmann, C. W. Preparation of Metal - Imidazolidin-2-Ylidene Complexes by Oxidative Addition. Organometallics 2003, 22, 907−909. (38) Voutchkova, A. M.; Appelhans, L. N.; Chianese, A. R.; Crabtree, R. H. Disubstituted Imidazolium-2-Carboxylates as Efficient Precursors to N-Heterocyclic Carbene Complexes of Rh, Ru, Ir, and Pd. J. Am. Chem. Soc. 2005, 127, 17624−17625.

ACKNOWLEDGMENTS We acknowledge financial support for our work in this area from the European Research Council (ERC CoG 615653), the Swiss National Science Foundation (200021_162868), and the H2020 program (MCSA 6618005 to C.S.). We particularly thank the many co-workers, past and present, for their dedicated work and the many friends, collaborators, and colleagues for countless inspiring discussions and contributions to continuously advance this exciting area of chemistry. REFERENCES (1) Igau, A.; Grutzmacher, H.; Baceiredo, A.; Bertrand, G. Analogous α,α’-Bis-Carbenoid Triply Bonded Species: Synthesis of a Stable λ3-Phosphinocarbene-λ5-Phosphaacetylene. J. Am. Chem. Soc. 1988, 110, 6463−6466. (2) Arduengo, A. J.; Harlow, R. L.; Kline, M. A Stable Crystalline Carbene. J. Am. Chem. Soc. 1991, 113, 361−363. (3) Arduengo, A. J.; Bertrand, G. Carbenes Introduction. Chem. Rev. 2009, 109, 3209−3210. (4) In N-Heterocyclic Carbenes: From Laboratory Curiosities to Efficient Synthetic Tools; Diez-Gonzalez, S., Ed.; Catalysis Series; The Royal Society of Chemistry: Cambridge, UK, 2011. (5) In N-Heterocyclic Carbenes: From Laboratory Curiosities to Efficient Synthetic Tools, 2nd ed.; Diez-Gonzalez, S., Ed.; Catalysis Series; The Royal Society of Chemistry: Cambridge, UK, 2017. (6) In N-Heterocyclic Carbenes in Transition Metal Catalysis and Organocatalysis; Cazin, C. S. J., Ed.; Springer: Dordrecht, Netherlands, 2011. (7) Alder, R. W.; Allen, P. R.; Williams, S. J. Stable Carbenes as Strong Bases. J. Chem. Soc., Chem. Commun. 1995, 1267. (8) Magill, A. M.; Cavell, K. J.; Yates, B. F. Basicity of Nucleophilic Carbenes in Aqueous and Nonaqueous Solvents - Theoretical Predictions. J. Am. Chem. Soc. 2004, 126, 8717−8724. (9) Gründemann, S.; Kovacevic, A.; Albrecht, M.; Faller, J. W.; Crabtree, R. H. Abnormal Binding in a Carbene Complex Formed from an Imidazolium Salt and a Metal Hydride Complex. Chem. Commun. 2001, 2274−2275. (10) Albrecht, M. C4-Bound Imidazolylidenes: From Curiosities to High-Impact Carbene Ligands. Chem. Commun. 2008, 3601−3610. (11) McNaught, A. D.; Wilkinson, A. IUPAC Compendium of Chemical Terminology, 2nd ed.; Blackwell Scientific Publications: Oxford, 1997. (12) Crabtree, R. H. Abnormal, Mesoionic and Remote NHeterocyclic Carbene Complexes. Coord. Chem. Rev. 2013, 257, 755−766. (13) Albrecht, M. Normal and Abnormal N-Heterocyclic Carbene Ligands. Similarities and Differences of Mesoionic C-Donor Complexes. Adv. Organomet. Chem. 2014, 62, 111−159. (14) Confusingly, some reports use the term MIC as a synonym for 1,2,3-triazolylidenes, though it should be obvious from the IUPAC definition that MICs include a much broader range of NHCs than triazolylidenes only and not all triazolylidenes are mesoionic. (15) Vignolle, J.; Cattoën, X.; Bourissou, D. Stable Noncyclic Singlet Carbenes. Chem. Rev. 2009, 109, 3333−3384. (16) Soleilhavoup, M.; Bertrand, G. Cyclic (Alkyl)(amino)carbenes (CAACs): Stable Carbenes on the Rise. Acc. Chem. Res. 2015, 48, 256−266. (17) Melaimi, M.; Soleilhavoup, M.; Bertrand, G. Stable Cyclic Carbenes and Related Species beyond Diaminocarbenes. Angew. Chem., Int. Ed. 2010, 49, 8810−8849. (18) Ofele, K.; Tosh, E.; Taubmann, C.; Herrmann, W. A. Carbocyclic Carbene Metal Complexes. Chem. Rev. 2009, 109, 3408−3444. (19) Schuster, O.; Yang, L.; Raubenheimer, H. G.; Albrecht, M. Beyond Conventional N-Heterocyclic Carbenes: Abnormal, Remote, and Other Classes of NHC Ligands with Reduced Heteroatom Stabilization. Chem. Rev. 2009, 109, 3445−3478. CE


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