Ru(III)-NHC complexes with normal- and abnormal carbenes and their use as precursors for Ru(II)-NHC complexes

Abstract

Since late 90s, N-heterocyclic carbene (NHC) ligands have been a centre of attraction in the growing chemistry of transition metal complexes and their diverse applications [1–6] (Figure 1.1). These ligands have been considered as the potential alternatives and a superior choice over phosphines.[7–15] NHCs fall into the category of stable carbenes having a divalent carbon atom bound to at least one heteroatom within the heterocyclic ring. Easy accessibility, large variety of the structures with different substituents at N-alkyl wingtips and distinct electronic properties make these ligands multifunctional, cooperative, tunable and adaptable with transition metals.[1,4,16–19] Despite their exceptional stereo-electronic properties, free NHCs could not be isolated until 1991, when Arduengo and coworkers reported the isolation of 1,3-di(adamantyl)imidazol-2-ylidene (IAd) as the first stable free carbene.[20] It was recognized as the breakthrough in synthetic organic chemistry as its structure allowed the tailoring of the heterocyclic rings with respect to the electronic and steric properties, and the substituents attached at the wingtips, which can be fine-tuned for the desired applications.