Hydride Alkenylcarbyne Osmium Complexes versus Cyclopentadienyl Type Half-Sandwich Ruthenium Derivatives

The dihydride complex OsH2Cl2(PiPr3)2 (1) reacts with 2-methyl-1-hexen-3-yne and 2,4-dimethyl-1,3-pentadiene to give the hydride alkenylcarbyne derivatives OsHCl2{CC(Me)CHR}(PiPr3)2 (R = nPr (2), iPr (3)), which have been characterized by X-ray diffraction analysis. DFT calculations (B3PW91) suggest that the enyne is initially hydrogenated to afford a conjugated diene. The latter evolves into the hydride alkenylcarbyne derivative by means of two hydrogen migrations. The first migration is a 1,4-hydrogen shift within the diene (from the terminal CH2 group to the internal double bond) which takes place through the metal center. The second migration is a 1,2-hydrogen shift from the terminal CH2 group to the osmium atom. In contrast to the case for 1, the ruthenium counterpart RuH2Cl2(PiPr3)2 (16) reacts with 2-methyl-1-hexen-3-yne to give a complex mixture of compounds, from which the derivatives Ru(η5-C5HR1R2R3R4)Cl(PiPr3) (17; R1 = C(CH3)CH2, R2 = Et, R3 = nPr, R4 = Me) and RuCl2{C(Et)CHCMe2}(PiPr3)2 (18, traces) are isolated. Both 17 and 18 have been also characterized by X-ray diffraction analysis. DFT calculations (B3PW91) on the formation of 17 suggest that in the ruthenium case the hydrogenation of the enyne leads to an alkenylcarbene intermediate, which reacts with a second enyne molecule to afford the tetrasubstituted cyclopentadienyl group.