Quinone-annulated N-heterocyclic carbene-transition-metal complexes:: Observation of π-backbonding using FT-IR spectroscopy and cyclic voltammetry

A new N-heterocyclic carbene architecture comprising a 1,4-naphthoquinone annulated to 1,3-dimesitylimidazolylidene (NpQ-NHC) was synthesized in two high yielding steps from commercially available starting materials. The free NpQ-NHC was characterized (solution and solid-state) and was used to synthesize various Rh and Ag complexes that ranged in π-electron density. Enabled by the quinone moiety, the π-systems of these complexes were analyzed using infrared spectroscopy and cyclic voltammetry. In contrast to previous reports, π-backbonding was found to be non-negligible and was directly influenced by the metal's electronic character. Copyright © 2006 American Chemical Society.