N-heterocyclic carbenes([+]): Generation under mild conditions and formation of group 8-10 transition metal complexes relevant to catalysis

Stable mono- and dicarbene adducts of late transition metals are readily accessible either by reaction of imidazolium salts with metal complexes bearing basic ligands, or by the cleavage of chloro- and acetato-bridged dinuclear metal complexes with the free carbenes (e.g., 1,3-dimethylimidazoline-2-ylidene). A general novel method for the deprotonation of N-substituted azolium sails in liquid ammonia-the liquid ammonia route-is described. This method yields not only the known free monomeric 1,3-dimethylimidazoline-2-ylidene in quantitative yield, but also otherwise hardly accessible derivatives. For example imidazoline-2-ylidenes with linear, branched cyclic, heteroatom-substituted and even chiral hydrocarbon residues call be obtained. The nucleophilic behaviour of 1,3-dimethylimidazoline-2-ylidene is reported and compared with that of other donor ligands. Novel carbene complexes of Ru-II, Rh-I, Pd-II, Os-II and Ir-I are presented. Reaction of the potentially chelating ligand 1,1'-(1,2-ethylene)-3,3'-dimethgldiimidazoline-2,2'-diylidene with [(COD)RhCl](2) yields a dinuclear complex. in which two rhodium centres are linked by the dicarbene bridge. Four single-crystal X-ray diffraction structures of new metal-carbene complexes (Rh, Os) are reported. N-Heterocyclic carbene complexes of Group 8-10 transition metals are both thermally and chemically very stable. They do not show the typical reactivity of metal-carbon double bunds. For a number of reasons, these complexes must be regarded as donor adducts of the Lewis-basic imidazoline-2-ylidene ligand and the Lewis-acidic organometallic fragment.