Sequential and selective hydrogenation of the C(alpha)-C(beta) and M-C(alpha) double bonds of an allenylidene ligand coordinated to osmium: new reaction patterns between an allenylidene complex and alcohols.

Complex [OsH(=C=C=CPh2)(CH3CN)2(PiPr3)2]BF4 (1) reacts with primary and secondary alcohols to give the corresponding dehydrogenated alcohols and the hydride-carbene derivative [OsH(=CHCH=CPh2)(CH3CN)2(PiPr3)2]BF4 (2), as a result of hydrogen transfer reactions from the alcohols to the Calpha-Cbeta double bond of the allenylidene ligand of 1. The reactions with phenol and t-butanol, which do not contain any beta-hydrogen, afford the alkoxy-hydride-carbyne complexes [OsH(OR)(CCH=CPh2)(CH3CN)(PiPr3)2]BF4 (R = Ph (3), tBu (4)), as a consequence of the 1,3-addition of the O-H bond of the alcohols to the metallic center and the Cbeta atom of the allenylidene of 1. On the basis of the reactions of 1 with these tertiary alcohols, deuterium labeling experiments, and DFT calculations, the mechanism of the hydrogenation is proposed. In acetonitrile under reflux, the Os-C double bond of 2 undergoes hydrogenation to give 1,1-diphenylpropene and [Os{CH2CH(CH3)PiPr2(CH3CN)3(PiPr3)]BF4 (11), containing a metalated phosphine ligand. This reaction is a first-order process with activation parameters of DeltaH = 89.0 +/- 6.3 kJ mol-1 and DeltaS = -43.5 +/- 9.6 J mol-1 K-1. The X-ray structures of 2 and 3 are also reported.