Catalytic hydrogenation by triruthenium clusters: A mechanistic study with parahydrogen induced polarization.

The reactivity of the cluster family [Ru3(CO)12-x(L)x] (in which L=PMe3, PMe2Ph, PPh3 and PCy3, x=1-3) towards hydrogen is described. When x=2, three isomers of [Ru3(H)(-H)(CO)9(L)2] are formed, which differ in the arrangement of their equatorial phosphines. Kinetic studies reveal the presence of intra- and inter-isomer exchange processes with activation parameters and solvent effects indicating the involvement of ruthenium-ruthenium bond heterolysis and CO loss, respectively. When x=3, reaction with H2 proceeds to form identical products to those found with x=2, while when x=1 a single isomer of [Ru3(H)(-H)(CO)10(L)] is formed. Species [Ru3(H)(-H)(CO)9(L)2] have been shown to play a kinetically significant role in the hydrogenation of an alkyne substrate through initial CO loss, with rates of H2 transfer being explicitly determined for each isomer. A less significant secondary reaction involving loss of L yields a detectable product that contains both a pendant vinyl unit and a bridging hydride ligand. Competing pathways that involve fragmentation to form [Ru(H)2(CO)2(L)(alkyne)] are also observed and shown to be favoured by nonpolar solvents. Kinetic data reveal that catalysis based on [Ru3(CO)10(PPh3)2] is the most efficient although [Ru3(H)(-H)(CO)9(PMe3)2] corresponds to the most active of the detected intermediates.