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 chloroand 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 salts 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 can 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′-dimethyldi imidazoline-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 bonds. 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.