N-heterocyclic carbene ligands NHCs

The case studies below will provide selected examples of calorimetric studies carried out on metal-ligand exchange reactions of phosphorous-based ligands (such as phosphines, phosphite, A -pyrrolyl-phosphines) and N-heterocyclic carbene ligands (NHCs) as well as addition reactions relevant to catalytic transformations, involving metals such as ruthenium, rhodium,platinum,and molybdenum. ... 

The novel complexes of N-heterocyclic carbene ligands (NHC), /ac-[Re(CO)3(N C)X] (N C = l-phenyl-3-(2-pyridyl)imidazole or 1-quinoli-nyl-3-(2-pyridyl)imidazole X = Cl or Br), were found to undergo a solvent-dependent photochemical CO dissociation under excitation to their lowest excited state. The key step of the mechanism is the formation of a photo-chemically active tricarbonyl solvato-complex, following the dissociation of the CO in trans to the strongly cr-donatingNHC moiety. The photoinduced CO dissociation of c-[Re(CO)3(NHC)L] complexes, upon excitation at 370 nm, has been proven by the change in both IR and NMR spectra, as well as by the red shift in the emission profile after photolysis. Photochemical studies suggest that Re-C bond cleaves from a MLCT-type excited state. ... 

N-Heterocyclic carbene ligands (NHCs) are, in molecular chemistry, strongly associated with ruthenium since the work of Grubbs on metathesis. However, these ligands had not been used for the stabilization of NPs. It was, therefore, of interest, after a comprehensive study of phosphine coordination on NPs, to study the interaction between RuNPs and NHCs [35]. The RuNPs were prepared by the decomposition of [Ru(COD)(COT)] in pentane, under dihydrogen atmosphere (3 bar) at room temperature, and in the presence of two different carbenes as stabilizing agents, namely, l,3-bis(2,6-diisopropylphenyl)imidazol-2-ylidene (IPr) and MA -di(ferf-butyl)imidazol-2-ylidene (I Bu), shown in Fig. 31.6. 

Others have investigated the kinetics of amination reactions mediated by catalyst systems employing the new electron-rich monodentate ligands. In particular, Hartwig has shown that for catalysis by a 1 1 palladium to Xn tert-butyl)phosphine system, a mechanism in which oxidative addition of aryl chlorides follows coordination of base to the palladium competes with the standard nonanionic pathway. Finally, Caddick, Cloke, and coworkers have studied amination reactions of aryl chlorides performed by palladium complexes of N-heterocyclic carbene ligands. They found the rate to be limited by the oxidative addition step, which occurs first through the dissociation of an NHC ligand. 

A series of three phosphorescent mononuclear (NHC)-Cu(I) complexes has been investigated. Their photophysical properties were found to be largely controlled by NHC (N-heterocyclic carbenes) ligand chromo-phores. Modification of this ligand leads to emission colour tuning over 200 nm range, and emission quantum yields of 0.16-0.80 in the solid state. These complexes offer high quantum efficiencies, a short emission decay time, and tunable emission color. 

Several Cu(I) complexes with N-heterocyclic carbene ligands have been described as CuAAC catalysts at elevated temperature in organic solvents, under heterogeneous aqueous conditions (when both reactants are not soluble in water), and under neat conditions [75]. These catalyst show high activity under the solvent-free conditions, achieving turnover numbers as high as 20 000. However, their activity in solution-phase reactions is significantly lower than that of other catalytic systems (for example, a stoichiometric reaction of the isolated copper(I) acetylide/NHC complex with benzhydryl azide required 12 h to obtain 65% yield of the product [76], whereas under standard solution conditions even a catalytic reaction would proceed to completion within 1 h). 

The large potential unveiled by the introduction of the NHC ligands motivated chemists to expand the NHC toolbox in order to achieve more stable and active catalysts. The following sections account for only a brief survey of some interesting and useful modifications of N-heterocyclic carbene ligands in ruthenium metathesis complexes, illustrated with their activity in various metathesis reactions using standard substrates (Scheme 11.3). 

N-HeterocycUc Carbene ligands IV-heterocyclic carbene ligands (NHC, named Cb in the following) were introduced by Harmann via the precatalyst PdljlCb) to activate poorly reactive aryl bromides and chlorides in Heck reactions [13,3h, k]. NHCs are indeed strong a-donor [3k] that make Pd complexes more reactive in the oxidative addition of aryl halides. The Heck reaction is accelCTated upon addition of hydrazine, a reducing agent. This establishes that a Pd is the active species in the oxidative addition [13]. 

Scheme 23.51. Wacker oxidation of alkenes with t-BuOOH mediated by a Pd(II) catalyst bearing a NHC (N heterocyclic carbene ligand) leading to ketones and aldehydes.

Particularly interesting new work includes the development of remote and abnormal NHCs (see Chapter 5 for further details). Also, NHCs have not only been used as ligands in transition metal chemistry but also as nucleophilic organocatalysts. In an extension of this concept, Lavallo and Grubbs presented recently the first organometallic transformation catalyzed by NHCs. This illustrate nicely that almost 20 years after Arduengo s first report on an N-heterocyclic carbene, new NHCs as well as new applications for these interesting and versatile molecules are still and will be for some time the subject of intensive research. 

The first efficient catalytic applications of N-heterocyclic carbene ligands with late transition metals were reported for rhodium. Since the mid 1990s, NHC-rhodium chemistry has been extensively studied and may be comparable to palladium or nickel in terms of the number of catalytic applications. Outside the context of catalysis, NHC-Rh complexes have featured prominently in studies probing the electronic properties of NHCs, usually quantified by comparison of the pco frequencies in the IR spectra of [LRh(CO)2Cl] complexes. ... 

A year later, Hermann and co-workers showed that these Corriu-Kumada cross-coupling were efficiently obtained under NHC-Ni catalysis with aryl fluorides as starting materials. They showed that an in situ generated species worked similarly if not better than a pre-formed [(NHC)2Ni] complex. This observation suggested that the catalytic active species would be a zero-valent nickel coordinated with only one N-heterocyclic carbene ligand. The formation of a 12-electrons complex would be evidently favored in an in situ process. Both catalysts were active with electron-rich or electron-poor fluoroarenes as well as with congested organometallic species (Equation (10.12)). 

Ray, L., Shaikh, M.M. and Ghosh, P. (2008) Shorter Argentophilic Interaction than Aurophilic Interaction in a Pair of Dimeric (NHC)MC1 2 (M=Ag, Au) Complexes Supported over a N/O-Functionalized N-Heterocyclic Carbene (NHC) Ligand. Inorganic Chemistry, 47, 230-240. 

Utilizing more reactive discrete palladium-N-heterocyclic carbene (NHC) complexes (for example, Pd(carb)2) or in situ generated palladium/imidazolium salt complexes (1 mol% ligand A), Caddick and coworkers were able to extend the rapid amination protocols described above to electron-rich aryl chlorides (Scheme 6.61) [128],... 

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