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

The reactivity of the cluster family [Ru-3(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 [Ru-3(H)(mu-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 H-2 proceeds to form identical products to those found with x = 2, while when x = 1 a single isomer of [Ru-3(H)(mu-H)(CO)(10)(L)] is formed. Species [Ru-3(H)(mu-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 H-2 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 [Ru-3(CO)(10)(PPh3)(2)] is the most efficient although [Ru-3(H)(mu-H)(CO)(9)(PMe3)(2)] corresponds to the most active of the detected intermediates.