Cationic abnormal N-heterocyclic carbene ruthenium complexes as suitable precursors for the synthesis of heterobimetallic compounds.

Abstract

The cationic abnormal N-heterocyclic carbene complex [Ru(OAc)(dppe)(PC)]Br (2b) (PC = bidentate phosphine-carbene ligand) has been obtained by treatment of the neutral derivative RuBr(OAc)(PPh3)(PC) (2a) with dppe in THF. Reaction of 2b with Ag2O afforded the heterobimetallic complex Ru(OAc)(dppe)(PC)AgBr (2c) which can be easily transmetallated with [Ir(cod)Cl]2 giving Ru(OAc)(dppe)(PC)IrCl(cod) (2d). Similarly, [{Ru(OAc)(PC)(PC')}2Ag][AgBr2] (1b) reacts with [Ir(cod)Cl]2 with formation of Ru(OAc)(PC)(PC')IrCl(cod) (1e). All complexes were characterized by NMR in solution and by single crystal X-ray diffraction studies in solid state. For the cationic complexes the 1H NMR resonance of the carbene-proton is considerably down-field shifted (δ > 9 ppm) with respect to that of the neutral derivatives. Cyclic voltammetry and differential pulse voltammetry studies show that the Ru-centered redox process occurs at lower potential (ΔE > 100 mV) after transmetallation with iridium, suggesting an interaction of the two metals along the π-system of the heterocycle ligand. These complexes display catalytic activity in transfer hydrogenation of acetophenone in 2-propanol with a significant influence of Ir and Ag centers on the Ru moiety. The synthesis of the imidazolyl-based N-heterocyclic dicarbene Ru-Ir complexes, which entails the easy metallation of the reactive cationic abnormal N-heterocyclic carbene Ru complexes with Ag2O, represents a suitable pathway for the preparation of heterometallic ruthenium complexes.