Mechanistic Studies of the Hydrogenation of Alkynes with Os3(CO)10(μ-H)2 Using para-Hydrogen as a Probe

Abstract

The reactions of 1-pentyne and acetylene with Os3(CO)10(μ-H)2 in the presence of para-H2 at 3−4 atm and 50−60 °C 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 Os3(CO)10(μ-H)2, only the σ-π-vinyl complex Os3(CO)10(μ-η2-CHCHR)(μ-H) (A R = H or n-Pr) goes on to yield alkene and the starting Os3(CO)10(μ-H)2. The μ-η2-acetylido complex Os3(CO)10(μ-η2-CCR)(μ-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 H2. The μ3-η2-alkyne complex Os3(CO)9(m-CO)(μ3-η2-HCCR) (C) is formed when Os3(CO)10(CH3CN)2 reacts with the subject alkynes in the presence of para-H2, but does not react with para-hydrogen. An unstable α-β-unsatu...