Sequential and Selective Hydrogenation of the C_α−C_β and M−C_α 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 C_α−C_β double bond of the allenylidene ligand of 1. The reactions with phenol and t-butanol, which do not contain any β-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 C_β 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 (11), containing a metalated phosphine ligand. This reaction is a first-order process with activation parameters of ΔH^⧧ = 89.0 ± 6.3 kJ mol^-1 and ΔS^⧧ = −43.5 ± 9.6 J mol^-1 K^-1. The X-ray structures of 2 and 3 are also reported.