ENE YNE COUPLINGS AT A DITUNGSTEN CENTER TO GIVE ALKYLIDYNE HYDRIDO COMPLEXES SUPPORTED BY SILOXIDE LIGANDS

The ethyne-bridged compound W2(OSiMe2But)6(py)(mu-C2H2) (1) and its C- 13-labeled mu-(C2H2)-C-13 isotopomer (1) react in hydrocarbon solvent s at ca. -20-degrees-C with ethene to give HW2(OSiMe2But)6(mu-CCH=CHMe ) (2) (and 2) and with allene to give HW2(OSiMe2But)6[mu-CC(=CH2)(CH= CH2)] (3). Compounds 2 and 3 have been characterized by single-crystal X-ray crystallography, and the two are structurally related in having two five-coordinate W atoms united by a mu-alkylidyne ligand and one bridging siloxide. One tungsten is coordinated to three terminal silox ide ligands while the other is bonded to two terminal siloxides and a terminal hydride. The terminal hydride was not located crystallographi cally, but its presence is implicated by a vacant coordination site on tungsten and by H-1 NMR spectroscopy. At room temperature the compoun ds are fluxional and the hydride ligands see two time-averaged equival ent W nuclei. The W-W distances span the range 2.56-2.65 and are consi stent with a (W-W)10+ moiety supported by bridging alkylidyne ligands. Reactions employing the C-13-labeled compound 1 and C2D4 and allene reveal some interesting aspects about the formation of the bridging hy drocarbyl ligand. These reactions are compared to the chemistry of the related tert-butoxy-supported compound W2(OBut)6(py)(mu-C2H2). Crysta l data are the following: (i) HW2(OSiMe2But)6(mu-CCH=CHMe) at -172-deg rees-C: a = 14.037(2) angstrom, b = 18.453(3) angstrom, c = 11.681(2) angstrom, alpha = 91.20(1)degrees, beta = 106.63(1)degrees, gamma = 10 0.35(1)degrees, Z = 2, d(calcd) = 1.412 g cm-3, and space group P1BAR; and (ii) HW2(OSiMe2But)6[mu-CC(=CH2)(CH=CH2)] at -172-degrees-C: a = 14.211(8) angstrom, b = 18.296(11) angstrom, c = 11.724(6) angstrom, a lpha = 91.24(2)degrees, beta = 105.97(2)degrees, gamma = 99.98(2)degre es, Z = 2, d(calcd) = 1.41 g cm-1, and space group P1BAR.