HYDROGEN-TRANSFER CATALYTIC SYNERGISM IN BINUCLEAR COMPLEXES CONTAINING 2,2'-BIIMIDAZOLATE AS A BRIDGING LIGAND

The binuclear compounds [H(CO)(PPh3)2Ru(mu-bim)M(COD)] (bim = 2,2'-biimidazolate, COD = 1,5-cyclooctadiene, M = Rh, Ir) are more active catalysts for the reduction of cyclohexanone, styrene, and alpha,beta-unsaturated ketones (benzylideneacetophenone or benzylideneacetone), by hydrogen-transfer reactions from 2-propanol, than the mononuclear parent compounds [RuH(Hbim)(CO)(PPh3)2] and [M(Hbim)(COD)] (M = Rh, Ir). For the reduction of benzylideneacetone catalyzed by [H(CO)(PPh3)2Ru(mu-bim)Ir(COD)], the experimental kinetic data are in accordance with an expression of the form -d[benzylideneacetone]/dt = kappa[Ru-Ir][benzylideneacetone]. On the basis of these kinetic results and other experimental evidence, a mechanism for this reaction is proposed. The full catalytic cycle involves binuclear species. The hydrogen transfer proceeds via the ruthenium atom, while the iridium center acts as the core of a metal-ligand complex of variable electron density.