New alkyne bridged mixed-metal clusters and studies of their activity for catalytic hydrosilylation of alkynes

The homologous series of compounds MPt2(CO)5(PPh3)2(PhC2Ph), M=Fe (1), M=Ru (3) and M=Os (4) were obtained in the yields 55%, 24% and 30% from the reactions of Pt(PPh3)2(PhC2Ph) with Fe(CO)5, Ru3(CO)10, and Os(CO)5, respectively. Each of the products was characterized by IR, elemental analysis and a single crystal X-ray diffraction analysis. A second mixed-metal cluster product, Ru2Pt(CO)7(PPh3)2(PhC2Ph) (2) (45% yield) was also obtained from the reaction that yielded 3. In fact, it is obtained in a higher yield than that of 3. Compounds 1, 3, and 4 are isostructural and are comprised of a MPt2 (M=Fe, Ru or Os) triangular metal cluster containing a triply bridging diphenylacetylene ligand. The structure of compound 2 is similar to that of 1, 3, and 4, but contains two ruthenium and one platinum atom in the triangular cluster. One coproduct Os(CO)2(PPh3)2(PhC2Ph) (5) (25% yield) was obtained from the reaction of Os(CO)5 with Pt(PPh3)2(PhC2Ph). Compound 5 contains only one metal atom with an approximately trigonal bipyramidal coordination having the two phosphine ligands in the axial positions and the alkyne in an equatorial site. The reaction of 2 with H2 in refluxing hexane afforded the tetranuclear complex H2Ru2Pt2(CO)8(PPh3)2 (6) (11% yield) that contains an Ru2Pt2 tetrahedral shaped cluster with two bridging hydride ligands. The ability of the solutions of compounds 1–6 and Pt(PPh3)2(PhC2Ph) to produce catalytic hydrosilylation of diphenylacetylene by triethylsilane to yield (E)-[(1,2-diphenyl)ethenyl]triethylsilane at 30°C, and 1,4-bis(trimethylsilyl)butadiyne with triethylsilane to yield (E)-2-triethylsilyl-1,4-bis(trimethylsilyl)-1-buten-3-yne at 60°C was investigated. Pt(PPh3)2(PhC2Ph) was the best catalyst for these reactions. Compound 5 is completely inactive. A combination of evidence suggests that the catalytic activity exhibited by the mixed-metal cluster complexes is produced principally by fragmentation products which are almost certainly mononuclear platinum complexes.