Mechanisms of concurrent hydride migration processes in a triruthenium cluster capped by a phenylphosphinidene (PPh) ligand

Two methods were used to synthesise [Ru-3(mu-H)(2)(mu(3)-PPh)-(CO)(7) (mu-dppm)] (3) (dppm = Ph2PCH2PPh2), the subject of this paper. Treatment of [Ru-3(CO)(10)(mu-dppm)] (1) with phenylphosphane in refluxing THF gave both [Ru-3(mu-H)(2)(mu-PHPh)-(CO)(8)(mu-dppm)] (2) and [Ru-3(mu-H)(2)(mu(3)-PPh)(CO)(7)(mu-dppm)] (3). Cluster 2 converts to 3 in refluxing THF. Alternatively the phenylphosphinidene cluster [Ru-3(mu-H)(2)(mu(3)-PPh)(CO)(9)] (4), prepared by the reported method of treating [Ru-3(CO)(12)] with phenylphosphane, reacts with dppm to produce cluster 3. The single-crystal X-ray structures of 2 and 3 are reported. Hydride mobility in [Ru-3(mu-H)(2)(mu(3)-PPh)(CO)(7)(mu-dppm)] (3) was analysed by variable-temperature H-1 and P-31(H-1) NMR methods. The variations in the spectra with temperature could not be interpreted by a single process, several of which were explored and which gave inadequately matching computed and experimental spectra. However, the spectra were successfully analysed by two concurrent processes, both involving the migration of hydride ligands between Ru-Ru edges. The faster process leads to the exchange of the nonequivalent phosphorus nuclei but not hydride exchange, whereas the hydrides are also exchanged in the slower process. Both processes require hydride ligand migration from one Ru-Ru edge to a vacant one. The hydride ligand bridging the same pair of ruthenium atoms as the dppm ligand is the slower to migrate.