The N-heterocyclic carbene chemistry of transition-metal carbonyl clusters

In the last decade, chemists have dedicated many efforts to investigate the coordination chemistry of N-heterocyclic carbenes (NHCs). Although most of that research activity has been devoted to mononuclear complexes, transition-metal carbonyl clusters have not escaped from these investigations. This critical review, which is focussed on the reactivity of NHCs (or their precursors) with transition-metal carbonyl clusters (mostly are of ruthenium and osmium) and on the transformations underwent by the NHC-containing species initially formed in those reactions, shows that the polynuclear character of these metallic compounds or, more precisely, the close proximity of one or more metal atoms to that which is or can be attached to the NHC ligand, is responsible for reactivity patterns that have no parallel in the NHC chemistry of mononuclear complexes (74 references).     

Bis-ligated Ti and Zr complexes of chelating N-heterocyclic carbenes

In this communication we report the synthesis of novel titanium and zirconium complexes ligated by bidentate “salicylaldimine-like” N-heterocyclic carbenes (NHC). Double addition of the NHC chelate to either TiCl₄(thf)₂ or ZrCl₄ forms bis-ligated organometallic fragments with a distorted octahedral geometry. These complexes are rare examples of group IV transition-metal NHC adducts. Preliminary catalytic tests demonstrate that in the presence of methylaluminoxane (MAO) these complexes are useful initiators for the polymerization of ethylene and the copolymerization of ethylene with norbornene and 1-octene.     

Steric nature of the bite angle. A closer and a broader look

The bite angle (ligand-metal-ligand angle) is known to greatly influence the activity of catalytically active transition-metal complexes towards bond activation. Here, we have computationally explored how and why the bite angle has such effects in a wide range of prototypical C-X bonds and palladium complexes, using relativistic density functional theory at ZORA-BLYP/TZ2P. Our model reactions cover the substrates H(3)C-X (with X = H, CH(3), Cl) and, among others, the model catalysts, Pd[PH(2)(CH(2))(n)PH(2)] (with n = 2-6) and Pd[PR(2)(CH(2))(n)PR(2)] (n = 2-4 and R = Me, Ph, t-Bu, Cl), Pd(PH(3))X(-) (X = Cl, Br, I), as well as palladium complexes of chelating and non-chelating N-heterocyclic carbenes. The purpose is to elaborate on an earlier finding that bite-angle effects have a predominantly (although not exclusively) steric nature: a smaller bite angle makes more room for coordinating a substrate by bending away the ligands. Indeed, the present results further consolidate this steric picture by revealing its occurrence in this broader range of model reactions and by identifying and quantifying the exact working mechanism through activation strain analyses.     

Synthesis, structures, and solution dynamics of palladium complexes of quinoline-functionalized N-heterocyclic carbenes

A new type of quinoline-functionalized palladium N-heterocyclic carbene (NHC) complexes has been synthesized via silver transmetallation. The quinoline moiety was either directly attached to the imidazole ring or linked to it by a methylene group. NHCs with a methylene linker tend to form trans biscarbene complexes in the reaction of Pd(COD)Cl2, while NHCs without any linker form chelating NHC-quinoline (NHC-N) complexes. These two types of carbenes also react with [Pd(allyl)Cl]2 to give monodentate NHC palladium eta(3)-allyl chlorides [Pd(NHC)(allyl)Cl]. Fluxionality in the NMR time scale was observed for most complexes, and the origin of their dynamic behaviors was discussed for each type of structure. For [Pd(NHC)(allyl)Cl] with a relatively small wing tip group of the NHC, the fluxionality (selective line-broadening of (1)H NMR signals) is caused by selective eta(3)-eta(1)-eta(3) allyl isomerization. For NHC with a bulkier (t)Bu group, a different line-broadening pattern was observed and was ascribed to partially hindered Pd-C(carbene) bond rotation. For cationic chelating complexes [Pd(NHC-N)(allyl)]BF4, the dynamic exchange process likely originates from a dissociative boat-to-boat inversion of 7-membered palladacycles. Activation parameters were measured for this process. Crystal structures were reported for representative complexes in each category.     

Heterocyclic carbenes: synthesis and coordination chemistry

The chemistry of heterocyclic carbenes has experienced a rapid development over the last years. In addition to the imidazolin-2-ylidenes, a large number of cyclic diaminocarbenes with different ring sizes have been described. Aside from diaminocarbenes, P-heterocyclic carbenes, and derivatives with only one, or even no heteroatom within the carbene ring are known. New methods for the synthesis of complexes with N-heterocyclic carbene ligands such as the oxidative addition or the metal atom template controlled cyclization of beta-functionalized isocyanides have been developed recently. This review summarizes the new developments regarding the synthesis of N-heterocyclic carbenes and their metal complexes.     

含吡啶鎓阳离子的氮杂环卡宾Pd(Ⅱ)配合物的合成、结构及催化性能

合成了四个含吡啶鎓阳离子的氮杂环卡宾钯(II)三碘配合物. 通过核磁共振谱和元素分析对它们进行了表征. 同时利用X射线单晶衍射确定了其中两个配合物的晶体结构. 研究表明该系列配合物对Suzuki-Miyaura偶联具有催化活性.     

Theoretical investigations of the reactivities of saturated five-membered ring N-heterocyclic carbenes with heavier group 14 elements

The potential energy surfaces for the chemical reactions of group 14 carbenes have been studied using density functional theory (B3LYP/LANL2DZ). Five saturated five-membered-ring N-heterocyclic carbene Dipp[upper bond 1 start]N(CH(2))(2)N(Dipp)E[upper bond 1 end]: (five-ring-E:) species, where E = C, Si, Ge, Sn and Pb, have been chosen as model reactants in this work. Also, four kinds of chemical reactions; addition of water, methane insertion, alkene cycloaddition and dimerization, have been used to study the chemical reactivities of these group 14 carbenes. The present theoretical investigations suggest that the relative carbenic reactivity decreases in the order: C > Si > Ge > Sn > Pb. That is, the heavier the group 14 atom (E), the more stable is the carbene towards chemical reactions. This may be the reason that there have been many instances reported of the synthesis and characterization of stable group 14 five-membered-ring N-heterocyclic carbene species with various alkyl protecting substituents at room temperature. Furthermore, the singlet-triplet energy splitting of the five-ring-E:, as described in the configuration mixing model attributed to the work of Pross and Shaik, can be used as a diagnostic tool to predict their reactivities. The results obtained allow a number of predictions to be made.     

Sterically demanding, bioxazoline-derived N-heterocyclic carbene ligands with restricted flexibility for catalysis

A unique family of N-heterocyclic carbenes derived from bioxazolines (IBiox) suitable for application in transition-metal catalysis is described. The ligands are electron rich, sterically demanding, and have restricted flexibility. Their usefulness has been demonstrated in the Suzuki-Miyaura cross-coupling of sterically hindered aryl chlorides and boronic acids. For the first time, tetraortho-substituted biaryls with methyl and larger ortho-substituents have been synthesized from aryl chlorides using the Suzuki-Miyaura method.     

N-杂环卡宾及其金属络合物的性质和合成方法

由于N-杂环卡宾结构的多样性，以及它们的金属络合物良好的稳定性和催化活性，近年来受到了人们的广泛关注。本文对N-杂环卡宾及其金属络合物的性质和合成方法进行了综述。参考文献30篇。     

Palladium catalysts for aerobic oxidative kinetic resolution of secondary alcohols based on mechanistic insight

The oxidative kinetic resolution of secondary alcohols has been accomplished using 1:1 complexes of PdCl(2) and N-heterocyclic carbenes. In these reactions, both achiral and chiral carbene ligands are used in conjunction with the chiral base (-)-sparteine. A general synthesis of 1:1 PdCl(2)-carbene complexes has been developed and is amenable to a wide range of carbene ligands. The potential of these complexes in aerobic oxidations is highlighted by the use of a chiral Pd(II) complex and the chiral base (-)-sparteine to enhance the kinetic resolution of a racemic alcohol. [reaction--see text]     

Nanoclusters in ionic liquids: evidence for N-heterocyclic carbene formation from imidazolium-based ionic liquids detected by (2)H NMR

The mystery of how 1,3-substituted imidazolium-based ionic liquids (ILs) can provide high stabilization for transition-metal(0) nanoclusters, that is, in the absence of the usual strongly coordinating anions, has been probed. 2H NMR product and kinetic studies of 1,3-substituted imidazolium ILs under D2 reveal that nanocluster-catalyzed H/D exchange occurs at the 2- (as well as at the 4-, 5-, and 8-) C-H positions of the imidazolium cation. The results (i) provide compelling evidence that N-heterocyclic carbene formation and ligation of nanoclusters is occurring in ILs; and (ii) argue that N-heterocyclic carbenes merit further investigation as heretofore unappreciated stabilizers of transition-metal nanoclusters.     

Proton affinities of N-heterocyclic carbene super bases

The gas-phase proton affinity of the N-heterocyclic carbene, 1-ethyl-3-methylimidazol-2-ylidene, was determined to be 251.3 +/- 4 kcal/mol using the kinetic method, a value which makes the carbene one of the strongest bases reported thus far. Density functional theory calculations have been carried out at the B3LYP/6-31+G(d) level to compare the high experimental value with that estimated theoretically. Experimental results also show that two other N-heterocyclic carbenes with larger substituents have even higher proton affinities. [structure: see text]     

Carbohydrate-containing N-heterocyclic carbene complexes

Novel carbohydrate bearing imidazolium salts have been synthesized and used for the in situ generation of the corresponding N-heterocyclic carbenes. These compounds were successfully used as catalysts of the conjugate umpolung of cinnamaldehyde to form γ-butyrolactones. In addition, silver and palladium complexes of these N-heterocyclic carbenes were synthesized and structurally characterized.     

N-heterocyclic carbenes as organocatalysts

Organocatalyzed reactions represent an attractive alternative to metal-catalyzed processes notably because of their lower cost and benign environmental impact in comparison to organometallic catalysis. In this context, N-heterocyclic carbenes (NHCs) have been studied for their ability to promote primarily the benzoin condensation. Lately, dramatic progress in understanding their intrinsic properties and in their synthesis have made them available to organic chemists. This has resulted in a tremendous increase of their scope and in a true explosion of the number of papers reporting NHC-catalyzed reactions. Here, we highlight the ever-increasing number of reactions that can be promoted by N-heterocyclic carbenes.     

Air-stable, convenient to handle Pd based PEPPSI (pyridine enhanced precatalyst preparation, stabilization and initiation) themed precatalysts of N/O-functionalized N-heterocyclic carbenes and its utility in Suzuki-Miyaura cross-coupling reaction

Several new air-stable, convenient to handle and easily synthesized Pd based PEPPSI (Pyridine Enhanced Precatalyst Preparation, Stabilization and Initiation) type precatalysts supported over N/O-functionalized N-heterocyclic carbenes (NHC) namely, trans-[1-(benzyl)-3-(N-t-butylacetamido)imidazol-2-ylidene]Pd(pyridine)Cl2 (), trans-[1-(2-hydroxy-cyclohexyl)-3-(benzyl)imidazol-2-ylidene]Pd(pyridine)Cl2 () and trans-[1-(o-methoxybenzyl)-3-(t-butyl)imidazol-2-ylidene]Pd(pyridine)Br2 (), have been designed. Specifically, the Pd-NHC complexes, , and , were conveniently synthesized from their respective imidazolium halide salts by the reaction with PdCl2 in pyridine in presence of K2CO3 as a base. A new imidazolium chloride salt, 1-(benzyl)-3-(N-t-butylacetamido)imidazolium chloride () was synthesized by the alkylation reaction of benzyl imidazole with N-t-butyl-2-chloroacetamide. The molecular structures of the imidazolium chloride salt, , and the Pd-NHC complexes, , and , have been determined by X-ray diffraction studies. The density functional theory studies of the , and complexes were carried out to in order to gain insight about their structure, bonding and the electronic properties. The nature of the NHC-metal bond in these complexes was examined using Charge Decomposition Analysis (CDA), which revealed that the N-heterocyclic carbene ligands are effective sigma-donors. In addition, the catalysis studies revealed that the Pd-NHC complexes, , and , are effective catalysts for the Suzuki-Miyaura type C-C cross-coupling reactions.     

N-heterocyclic carbenes with inorganic backbones: electronic structures and ligand properties

The electronic structures of known N-heterocyclic carbenes (NHCs) with boron, nitrogen, and phosphorus backbones are examined using quantum chemical methods and compared to the experimental results and to the computational data obtained for a classical carbon analogue, imidazol-2-ylidene. The sigma-donor and pi-acceptor abilities of the studied NHCs in selected transition-metal complexes are evaluated using a variety of approaches such as energy and charge decomposition analysis, as well as calculated acidity constants and carbonyl stretching frequencies. The study shows that the introduction of selected heteroatoms into the NHC backbone generally leads to stronger metal-carbene bonds and therefore improves the ligand properties of these systems. The backdonation of pi-electron density from the metal to the ligand is found to be strong in complexes of the studied NHCs with group 11 metals, where it constitutes up to nearly 35% of the total orbital interaction energy. The ligand properties of the aluminum analogues of some of the reported NHCs with boron backbones are also assessed.     

Dual reactivity of N-heterocyclic carbenes towards copper(II) salts

Complexes of N-heterocyclic carbenes (NHCs) with copper(II) halogenides are unstable. Upon formation, these complexes decompose to give haloamidinium salts. Contrastingly, O-substituted copper(II) NHC complexes are fairly stable. A series of new five-, six- and seven-membered ring NHC complexes of Cu(OAc)(2) have been synthesised and characterised in the solid state.     

Influence of anellation in N-heterocyclic carbenes: novel quinoxaline-anellated NHCs trapped as transition metal complexes

The synthesis, NMR-, and crystal structure data of novel electron-deficient quinoxaline anellated imidazol-2-ylidene precursors and complexes thereof are reported and compared with related less electron-withdrawing or non-anellated N-heterocyclic carbenes and complexes to illustrate anellation effects.     

Diaminocarbene- and Fischer-carbene complexes of palladium and nickel by oxidative insertion: preparation, structure, and catalytic activity

Oxidative insertion of [Pd(PPh3)4] or [Ni(cod)2]/PPh3 into the C-Cl bond of various 2-chloroimidazolinium- and other -amidinium salts affords metal-diaminocarbene complexes in good to excellent yields. This procedure is complementary to existing methodology in which the central metal does not change its oxidation state, and therefore allows to incorporate carbene fragments that are difficult to access otherwise. The preparation of a variety of achiral as well as enantiomerically pure, chiral metal-NHC complexes (NHC = N-heterocyclic carbene) and metal complexes with acyclic diaminocarbene ligands illustrates this aspect. Furthermore it is shown that oxidative insertion also paves a way to prototype Fischer carbenes of Pd(II). Since the required starting materials are readily available from urea- or thiourea derivatives, this novel approach allows for substantial structural variations of the ligand backbone. The catalytic performance of the resulting library of nickel- and palladium-carbene complexes has been evaluated by applications to prototype Suzuki-, Heck-, and Kumada-Corriu cross-coupling reactions as well as Buchwald-Hartwig aminations. It was found that even Fischer carbenes show appreciable catalytic activity. Moreover, representative examples of all types of neutral and cationic metal-carbene complexes formed in this study have been characterized by X-ray crystallography.     

Uncovering Nitrosyl Reactivity at N-Heterocyclic Carbene Center

N-heterocyclic carbenes (NHCs) have garnered much attention due to their unique properties, such as strong σ-donating and π-accepting abilities, as well as their transition-metal-like reactivity toward small molecules. In 2015, we discovered that NHCs can react with nitric oxide (NO) gas to form radical adducts that resemble transition metal nitrosyl complexes. To elucidate the analogy between NHC and transition metal NO adducts, here we have undertaken a systematic investigation of the electron- and proton-transfer chemistry of [NHC−NO]⋅ (N-heterocyclic carbene nitric oxide radical) compounds. We have accessed a suite of compounds, comprised of [NHC−NO]⁺, [NHC−NO]⁻, [NHC−NOH]⁰, and [NHC−NHOH]⁺ species. In particular, [NHC−NO]⁻ was isolated as potassium and lithium ion adducts. Most interestingly, a monomeric potassium [NHC−NO]⁻ compound was isolated with the assistance of 18-crown-6, which is the first instance of a monomeric alkali N-oxyl compound to the best of our knowledge. Our results demonstrate that [NHC−NO]⋅ exhibits redox behavior broadly similar to metal nitrosyl complexes, which opens up more possibilities for utilizing NHCs to build on the known reactivity of metal complexes.