Alkyne–carbaborane coupling at a molybdenum centre: crystal structure of [closo-3,3,3-(CO)3-8,3-{σ:η2-C(H)C(H)-SiMe3}-3,1,2-MoC2B9H10]

Abstract

In CH₂Cl₂ the complex [closo-1,2-Me₂-3-(η-PhC₂Ph)-3-(CO)-3-(PPh₃)-3,1,2-MoC₂B₉H₉] decomposed to yield a mixture of [closo-1,2-Me₂-3,3,3-(CO)₃-3-(PPh₃)-3,1,2-MoC₂B₉H₉] and [closo-1,2-Me₂-3,3-(CO)₂-3-(PPh₃)-8,3-{σ:η²-C(Ph)C(H)Ph}-3,1,2-MoC₂B₉H₈]. Protonation (HBF₄·Et₂O) of [NEt₄][closo-3-(η-C₃H₅)-3,3-(CO)₂-3,1,2-MoC₂B₉H₁₁], in CH₂Cl₂ at – 78 °C, in the presence of an excess of Me₃SiCCSiMe₃, afforded a chromatographically separable mixture of [closo-3,3-(η-Me₃SiC₂SiMe₃)₂-3-(CO)-3,1,2-MoC₂B₉H₁₁] and [closo-3,3,3-(CO)₃-8,3-{σ:η²-C(H)C(H)SiMe₃}-3,1,2-MoC₂B₉H₁₀]. The latter complex forms via the intermediacy of Me₃SiCCH, generated by HF cleavage of Me₃SiCCSiMe₃, and its structure was established by X-ray diffraction. The molybdenum atom is ligated on one side by three CO groups, and on the other by the open pentagonal face of the nido-1,2-C₂B₉ cage framework. The boron atom located in the β site with respect to the two carbons carries a vinyl substitutent C(H)C(H)SiMe₃, and this exopolyhedral group is η² co-ordinated to the molybdenum atom [Mo-C 2.43(1) and 2.55(1)Å]. Treatment of [N(PPh₃)₂][closo-1,2-Me₂3-(η-C₃H₅)-3,3-(CO)₂-3,1,2-MoC₂B₉H₉] and Me₃SiCCH with HBF₄·Et₂O gives initially the complex [closo-1,2-Me₂-3,3-(η-Me₃SiC₂H)₂-3-(CO)-3,1,2-MoC₂B₉H₉], which subsequently rearranges to [closo-1,2-Me₂-3-(η-Me₃SiC₂H)-3-(CO)-8,3-{σ:η²-C(H)C(H)SiMe₃}-3,1,2-MoC₂B₉H₈]. Use of Me₃SiCCD in this synthesis, combined with NMR studies, suggests that insertion of the alkyne into the cage B–H bond proceeds via the intermediacy of a molybdenum vinylidene species. The NMR data (¹H, ¹³C-{¹H}, ¹¹B-{¹H}, and ³¹P-{¹H}) for the new compounds are reported and discussed in relation to the structures proposed.