Phosphine-Amido Complexes of Ruthenium and Mechanistic Implications for Ketone Transfer Hydrogenation Catalysis

The air-sensitive phosphine-anilido complexes [RuR(eta(6)-P-cymene)(P,N-Ph(2)PAr(-))] (R = H, Et; Ar(-) = o-C(6)H(4)NMe(-)) have been prepared. While the precursor [RuCl(eta(6)-p-cymene)(P,N-Ph(2)PAr(-))] is a moderately active ketone transfer hydrogenation catalyst under basic conditions, the hydrido derivative is much less active, ruling out the possibility of an inner-sphere mechanism during catalysis. The possibility of an alternate mechanism, related to the Meerwein-Ponndorf-Verley-Oppenauer (MPVO) pathway, is discussed. While attempts to isolate intermediate alkoxo derivatives demonstrate their propensity toward beta-hydride elimination to afford the hydrido complex, the ethyl derivative is remarkably stable, even in refluxing benzene, providing an interesting contrast between the labilities of the Ru-O and Ru-C bonds.