Sequential and selective hydrogenation of the Cα-Cβ and M-Cα double bonds of an allenylidene ligand coordinated to osmium:: New reaction patterns between an allenylidene complex and alcohols

Complex [OsH(CCCPh2)(CH3CN)(2)((PPr3)-Pr-i)(2)]BF4 (1) reacts with primary and secondary alcohols to give the corresponding dehydrogenated alcohols and the hydride-carbene derivative [OsH(CHCHCPh2)(CH3CN)(2)((PPr3)-Pr-i)(2)]BF4 (2), as a result of hydrogen transfer reactions from the alcohols to the C-alpha-C-beta 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)(CCHCPh2)(CH3CN)((PPr3)-Pr-i)(2)]BF4 (R = Ph (3), Bu-t (4), as a consequence of the 1,3-addition of the O-H bond of the alcohols to the metallic center and the C-beta 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 Os{CH2CH(CH3)PPr2(CH3CN)(3)((PPr3)-Pr-i)]BF4 (11), containing a metalated phosphine ligand. This reaction is a first-order process with activation parameters of Delta H = 89.0 +/- 6.3 kJ mol(-1) and Delta S = -43.5 +/- 9.6 J mol(-1) K-1. The X-ray structures of 2 and 3 are also reported.