Mechanistic Studies of the Hydrogenation of Alkynes with Os3(CO)10H2 using para-hydrogen as a probe

The reactions of 1-pentyne and acetylene with Os-3(CO)(10)(mu-H)(2) in the presence of para-H-2 at 3-4 atm and 50-60 degreesC have been studied. An attempt is made to understand the nature of the intermediates in the catalytic hydrogenation process and the mechanistic relationship of the various organometallic clusters formed to each other and to the overall hydrogenation process. It was found that of the four products usually formed by the reaction of terminal alkynes with Os-3(CO)(10)(mu-H)(2), only the sigma-pi-vinyl complex Os-3(CO)(10)(mu-eta(2)-CH=CHR)(mu-H) (A R = H or n-Pr) goes on to yield alkene and the starting Os-3(CO)(10)(mu-H)(2). The mu-eta(2)-acetylido complex Os-3(CO)(10)(mu-eta(2)-CCR)(mu-H) (B), a product that results from the loss of hydrogen, is not formed under these conditions, as might be expected, in the presence of excess-H-2. The mu(3)-eta(2)-alkyne complex Os-3(CO)(9)(m-CO)(mu(3)-eta(2)-HCCR) (C) is formed when Os-3(CO)(10)(CH3CN)(2) reacts with the subject alkynes in the presence of para-H-2, but does not react with parahydrogen. An unstable alpha-beta-unsaturated aldehyde complex Os-3(CO)(9)(mu(3)-eta(4)-RC=CC(O)H)(mu-H) (E) is detected, which is thought to go on to form the mu(3)-eta(4)-metallacyclohexadienone cluster Os-3(CO)(9)(mu(3)-eta(4)-(RC=CH)(2)C(O)) (D), after loss of hydrogen and addition of a second mole of alkyne. Product E is also detected when Os-3(CO)(10)(CH3CN)(2) is reacted with I-pentyne or acetylene in the presence of para-H-2. The observation of this complex represents an example of direct detection of C-CO coupling in a transient intermediate on a trimetallic site. Labeling experiments with (CO)-C-13 and deuterio-1-pentyne elucidate the origin of E, and the observation of hyperpolarized intermediates using para-H-2 is shown to be a valuable tool for elucidating the entire manifold of reactions,