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.