ALKYNE CARBABORANE COUPLING AT A MOLYBDENUM CENTER - CRYSTAL-STRUCTURE OF [CLOSO-3,3,3-(CO)3-8,3-(SIGMA-ETA-2-C(H)=C(H)-SIME3)-3,1,2-MOC2B9H10]

In CH2Cl2 the complex [closo-1,2-Me2-3-(eta-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-{sigma:eta2-C(Ph)=C(H)Ph}-3,1,2-MoC2B9H8]. Protonation (HBF4.Et2O) of [NEt4][closo-3-(eta-C3H5)-3,3-(CO)2-3,1,2-MoC2B9H11], in CH2Cl2 at -78-degrees-C, in the presence of an excess of Me3SiC=CSiMe3, afforded a chromatographically separable mixture of [closo-3,3-(eta-Me3SiC2SiMe3)2-3-(CO)-3,1,2-MoC2B9H11] and [closo-3,3,3-(CO)3-8,3-{sigma:eta2-C(H)=C(H)SiMe3}-3,1,2-MoC2B9H10]. The latter complex forms via the intermediacy of Me3SiC=CH, generated by HF cleavage of Me3SiC=CSiMe3, 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 beta site with respect to the two carbons carries a vinyl substituent C(H)=C(H)SiMe3, and this exopolyhedral group is eta2 co-ordinated to the molybdenum atom [Mo-C 2.43(1) and 2.55(1) angstrom]. Treatment of [N(PPh3)2][closo-1,2-Me2-3-(eta-C3H5)-3,3-(CO)-3,1,2-MoC2B9H9] and Me3SiC=CH with HBF4.Et2O gives initially the complex [closo-1,2-Me2-3,3-(eta-Me3SiC2H)2-3-(CO)-3,1,2-MoC2B9H9], which subsequently rearranges to [closo-1,2-Me2-3-(eta-Me3Si-C2H)-3-(CO)-8,3-{sigma:eta2-C(H)=C(H)SiMe3}-3,1,2-MoC2B9H8]. Use of Me3SiC=CD 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 (H-1, C-13-{H-1}, B-11-{H-1}, and P-31-{H-1}) for the new compounds are reported and discussed in relation to the structures proposed.