FACILE AND QUANTITATIVE ACTIVATION OF DIHYDROGEN BY THE PHOSPHINIDENE-STABILIZED CLUSTER RU4(CO)13(MU-3-PPH) - SYNTHESIS AND SPECTROSCOPIC AND STRUCTURAL CHARACTERIZATION OF (MU-H)2RU4(CO)12(MU-3-PPH)

The phosphinidene-stabilized butterfly cluster Ru4(CO)13(mu3-PPh) (1) has a square pyramidal nido Ru4P skeleton with three ruthenium atoms and one phosphorus atom in a square face. The cluster 1 readily adds dihydrogen at room temperature and 1 atm pressure under photolytic conditions, affording (mu-H)2Ru4(CO)12(mu3-PPh) (2) quantitatively. The dihydride 2 is stable in the solid state and in solution under H-2. It has been fully characterized by IR and H-1, P-31, and C-13 NMR spectroscopy and by an accurate single-crystal X-ray structural analysis. Crystal data for 2: C18H7O12PRu4, triclinic, space group P1BAR, a = 10.059 (2) angstrom, b = 13.678 (3) angstrom, c = 18.093 (5) angstrom, alpha = 85.60 (2)-degrees, beta = 78.97 (2)-degrees, gamma = 88.65 (2)-degrees; with Z = 4, there are two independent molecules in the asymmetric unit. The structure was solved and refined to R = 2.77% (R(w) = 3.44%) on the basis of 9482 observed reflections (F greater-than-or-equal-to 6.0sigma(F)). The molecular structure of 2 is closely related to that of 1, with retention of the nido square pyramidal Ru4P framework. The two hydride ligands, located along the Ru(1)-Ru(3) and Ru(3)-Ru(4) basal edges, result in a slight increase in the length of the bridged Ru-Ru bonds and a modest overall expansion of the cluster core but no major change in geometry. Under CO and cyclohexene, 2 can be converted back to 1, with concomitant reduction of the olefin. In solution in hydrocarbon solvents and in the absence of H-2, 2 converts slowly but quantitatively to (mu-H)2Ru3(CO)9(mu3-PPh) and Ru3(CO)12. Variable-temperature C-13 NMR spectra have revealed three different dynamic processes involving localized CO exchanges. The implications of these reactions for small-molecule activation by main-group stabilized transition metal clusters are discussed.