N‐Heterocyclic Carbenes: Generation under Mild Conditions and Formation of Group 8–10 Transition Metal Complexes Relevant to Catalysis

Abstract

Abstract 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 salts in liquid ammonia–the liquid ammonia route–is described. This method yields not only the known free monimeric 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‐dimethyl‐imidazoline‐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′‐dimethyldiimidazoline‐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 metalcarbene 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.