Cooperative catalysis of a trinuclear ruthenium(II) complex in transfer hydrogenation of ketones by formic acid

A novel TPA derivative (TPA = tris(2-pyridylmethyl)amine) having two 1,10-phenanthroline (phen) moieties via amide linkage was synthesized and this ligand reacted with [Ru(hmb)Cl2]2 (hmb: hexamethylbenzene) to give a trinuclear Ru(II) complex, [RuCl(TPA-{phenRuCl(hmb)}2-H+)](PF6)2 (1-Cl), in a moderate yield. The complex involves a deprotonated and oxygen-coordinated amide linkage, which exhibits reversible protonation–deprotonation equilibrium. The chlorido complex was converted to be an aqua complex, [Ru(H2O)(TPA-{phenRu(H2O)2(hmb)}2-H+)](SO4)5/2 (1-H2O), by the reaction of 1-Cl with Ag2SO4 in H2O. Transfer hydrogenation of ketones was examined by using 1-Cl as a catalyst and HCOONa as a hydride source in H2O/CH3OH (1:1 v/v) at 50 °C under Ar. The time-course of the transfer hydrogenation of cyclohexanone to give cyclohexanol revealed that 1-Cl showed a cooperative effect on the catalytic reactivity as compared with that of mononuclear [RuCl(hmb)(phen)] (3-Cl) and [RuCl((1-Naph)2-TPA)]PF6 in H2O/CH3OH (1:2 v/v) under the same conditions. The detailed kinetic study has revealed that the catalytic transfer hydrogenation proceeds via the formato complex, which interacts with a substrate rather than via the hydrido complex. The two Ru centers placed at close proximity in 1-H2O enhanced the interaction of the formato complex with a substrate, resulting in an increase in the catalytic reactivity as compared with the mononuclear complex.