The complex Pt3Ru6(CO)(20)(mu(3)-PhC(2)Ph)(mu(3)-H)(mu-H) (1a) has been found to be an effective catalyst for the hydrogenation of diphenylacetylene to (Z)-stilbene at 50 degrees C and 1 atm of hydrogen. The turnover frequency (TOF) of 47(1) h(-1) was observed at a catalyst concentration of 10(-4) M and alkyne concentration of 10(-2) M. Kinetic studies of this reaction have revealed the rate expression, R = k[1a][PhC(2)Ph]p(H-2)/p(Co)(1 + k'[PhC(2)Ph]). Measurement of the rates as a function of temperature has allowed determination of the activation parameters, Delta H* = 24.9(6) kcal/mol and Delta S* = 1.4(9) eu. The complex Pt3Ru6(CO)(20)(mu(3)-TolC(2)Tol)(mu(3)-H)(mu-H)(mu-H) (1b) was also prepared. The TOF for the hydrogenation of TolC(2)Tol to (Z)-Tol(H)C=C(H)Tol by 1b is 30(1) h(-1). At high alkyne/1a ratios (1000/1), side reactions become important after a few hundred turnovers and the catalyst is transformed into species that have low catalytic activity. Four of these complexes have been isolated and characterized. Three of these are the new complexes Ru6Pt3(CO)(18)(mu(3)-eta(6)-PhC(2)H(4)Ph)(mu(3)-H)(4) (2a), Ru6Pt3(CO)(18)(eta(6)-PhCH(2)CH(2)Ph)(mu(3)-H)(4) (3a), and Ru6Pt3(CO)(15)(mu(3)-PhC(2)Ph)(3)(mu-H)(6) (4), and one is the previously reported compound Ru6Pt3(CO)(14)(mu(3)-Ph(2)C(2))(3) (5). The tolyl analog of 3a, RU(6)Pt(3)(CO)(18)(eta(6)-TolCH(2)CH(2)Tol)(mu(3)-H)(4) (3b), was also prepared. Complexes 2a, 3b, and 4 were characterized by single crystal X-ray crystallographic analyses. The molecular structures of 2a and 3b were found to consist of three triangular groups of three metal atoms arranged in a face-shared bioctahedral structure. The central triangle is pure platinum and the outer triangles are pure ruthenium as also found in the compound 1a. Both compounds contain diarylethane ligands which are pi-coordinated to the cluster through one of the aryl rings. In 2a, the coordinated ring is bridging three metal atoms, while in 3b, it is coordinated to only one metal atom. Compounds 3a,b are believed to be isostructural. The structure of 4 consists of a raftlike structure of three ruthenium and three platinum atoms. The central triangular group contains the three platinum atoms. The three remaining ruthenium atoms bridge Ru-Pt edges of the raft, and all three lie on one side of the Pt3Ru3 plane producing an overall C-3 symmetry of the cluster. Each of the three Ru2Pt triangles contains a triply bridging PhC(2)Ph ligand and one edge bridging hydride ligand. The remaining three hydride ligands bridge Pt-Ru edges of the Pt3Ru3 raft. The catalytic activity of a variety of related platinum and ruthenium complexes toward hydrogenation of PhC(2)Ph was tested under conditions identical to those used for 1a,b, but none are as active as compounds 1a,b. A mechanism for the catalysis based on complex 1a is proposed that begins with dissociation of a CO ligand from the cluster. This is followed by hydrogen activation and alkyne addition steps. The latter is believed to induce the transfer of the hydride ligands to the coordinated alkyne in the slow step of the reaction. It is suggested that the unusually high activity of the catalysis is produced in part by a synergistic interplay of the small molecule activations occurring at the platinum and ruthenium layers of the metal. Crystal data. For 2a: space group = P2(1)/n, a 16.648(3) Angstrom, b = 14.065(3) Angstrom, c = 17.558(3) Angstrom, beta = 92.78(1)degrees, Z = 4, 2625 reflections, R = 0.033. For 3b: space group = P2(1)/n, a = 17.192(3) Angstrom, b = 12.126(3) Angstrom, c = 22.649(4) A, beta = 111.98(1)degrees, Z = 4, 3023 reflections, R = 0.030. For 4: space group = P2(1)/n, a = 13.567(2) Angstrom, b = 36.282(9) Angstrom, c = 13.711(2) Angstrom, Z = 4, 4178 reflections, R = 0.049.
