Reductive Head-to-Head Coupling of Phenylacetylenes in the Coordination Sphere of an Osmium-Polyhydride

Complex OsH₆(PⁱPr₃)₂ releases H₂ at 50 °C. The resulting tetrahydride OsH₄(PⁱPr₃)₂ promotes head-to-head reductive dimerization of phenylacetylenes to give the 1,4-dibranched-butenediyl derivatives OsH₂{η⁴-[C₄H₄R₂]}-(PⁱPr₃)₂ (R = C₆H₅, C₆H₄-CF₃, C₆H₄-NMe₂). DFT calculations suggest that the formation of these compounds proceeds via five-coordinate unsaturated bis-(alkenyl)-osmium-(II)-(Kubas-type dihydrogen) intermediates, which evolve by alkenyl coupling and H-H cleavage of the dihydrogen. The reactions are sensitive to temperature and the amount of alkyne used. At higher temperatures and excess alkyne, the reductive coupling is accompanied by two dehydrogenation reactions, one at the metal center and the other involving an isopropyl substituent of a phosphine. As a result, mixtures of the dihydrides and Os-{η⁴-[C₄H₄R₂]}-(PⁱPr₃)-{η²-C,C;κ¹-P-[(CH₂CMe)-PⁱPr₂]}-(PⁱPr₃) (R = H, CF₃, NMe₂) are formed. Both families react with H₂ to regenerate OsH₆(PⁱPr₃)₂ and release the corresponding 1,4-diarylbutane. According to these reactions, 1,4-diarylbutanes have been obtained in approximately 20% yield, by stirring phenylacetylenes with 5 mol % of OsH₆(PⁱPr₃)₂, in toluene, under 1 atm of H₂.