Redox processes involving hydrocarbylmetal (N-heterocyclic carbene) complexes and associated imidazolium salts: ramifications for catalysis

The comparison of nucleophilic heterocyclic carbenes with phosphines and the premise that they are phosphine analogues is a useful but limited concept. Electronically and sterically NHC’s are different. These differences lead to unique chemical behavior where the NHC can act as a spectator ligand or as a participative ligand—finishing up in the products in altered form. Here, we review the redox processes undergone by the NHC/imidazolium salt couple in conjunction with Group 10 metals. The often facile reductive elimination reaction of hydrocarbylM-carbene complexes, yielding 2-substituted imidazolium salt and M(0) is examined, with factors influencing the reaction reviewed in some detail. Conversely, the related imidazolium salts (the important imidazolium based ionic liquids (IL’s)) may generate carbene complexes by oxidative addition to low valent metal centres. The significance of these reactions for the application of carbene complexes in catalysis, and for the use of imidazolium based ionic liquids as solvents is manifest.