The effects of pH on the molecular distribution of water soluble ruthenium(II) hydrides and its consequences on the selectivity of the catalytic hydrogenation of unsaturated aldehydes

The effect of pH on the formation and equilibrium distribution of the water soluble ruthenium hydrides [HRuCl(TPPMS)(2)](2), [HRuCl(TPPMS)(3)] and [H2Ru(TPPMS)(4)] (TPPMS=(3-sulfonatophenyl)diphenylphosphine sodium salt) was studied in aqueous solution by pH-potentiometric and H-1 and P-31 NMR methods. Depending on the pH, [RuCl2(TPPMS)(2)](2) and its hydride-derivatives hydrolyse extensively, giving rise to formation of hydroxo-ruthenium complexes. It was established that at pH less than or equal to 3.3 the dominant ruthenium(II) species was [HRuCl(TPPMS)(3)], while at pH greater than or equal to 7 it was [H2Ru(TPPMS)(4)]. While [HRuCl(TPPMS)(3)] catalyzed the slow, selective hydrogenation of the C=C bond in trans-cinnamaldehyde, [H2Ru(TPPMS)(4)] was found an active and selective catalyst for C=O reduction. Consequently, the selectivity of the hydrogenation of trans-cinnamaldehyde could be completely inverted by minor changes in the solution pH, shifting the equilibrium between [HRuCl(TPPMS)(3)] and [H2Ru(TPPMS)(4)]. (C) 1998 Elsevier Science B.V. All rights reserved.