OS(CO)(4)(ETA(2)-C(2)ME(2))-PROMOTED COUPLING OF ALKYNES AND CO - FORMATION OF (ETA(4)-C(4)ME(2)R(2)CO)OS(CO)(3) (R=ME, ET, PR-N) AND CATALYTIC ACTIVITY OF (ETA(4)-C(4)R(4)CO)OS(CO)(3) (R=ME, PH)

Low-temperature photolysis of Os(CO)(5) in the presence of excess 2-butyne gives Os(CO)(4)(eta(2)-C(2)Me(2)) (1) in moderate yield. Complex 1 undergoes facile alkyne-CO coupling with an additional alkyne ligand under mild thermal activation to give the cyclopentadienone-containing species (eta(4)-C(4)Me(2)R(2)CO)Os(CO)(3) (R = Me (2a), Et (2d), Pr-n(2e)). isolation of the alkyne-carbonyl complex is not necessary to effect the transformation; in situ generated M(CO)(4)(eta(2)-C(2)R(2)) provides a convenient method for the synthesis of (eta(4)-C(4)R(4)CO)M(CO)(3) (M = Ru, Os; R = Me, Et, Pr-n). On the basis of these observations, the proposed mechanisms for the formation of (eta(4)-C(4)R(4)CO)M(CO)(3) complexes are discussed and evaluated. The solid-state structure of 2d was determined and compared to the tetraphenyl analogue, (eta(4)-C(4)Ph(4)CO)Os(CO)(3) (4). Compound 2d crystallizes in the monoclinic space group P2(1)/c with a = 8.336(1) Angstrom, b = 15.334(2) Angstrom, c 12.849(2) Angstrom, beta = 108.78(2)degrees, Z = 4, R = 0.040, and R(w) = 0.046. The use of 2a and 4 as catalyst precursors for both Tishchenko and hydrogenation reactions was investigated and compared to the analogous ruthenium complex (eta(4)-C(4)Ph(4)CO)Ru(CO)(3).