Homogeneous Hydrogenation of Alkenes Catalyzed by the Ruthenium−Hydride Complex (PCy3)2(CO)(Cl)RuH: Spectroscopic Observation of the Ruthenium−Ethyl and Ruthenium−Ethylene−Hydride Intermediate Species

The ruthenium−hydride complex (PCy 3 ) 2 (CO)(Cl)RuH ( 1a ) was found to catalyze the hydrogenation of terminal and cyclic alkenes. For example, the reaction of 1-hexene with 4 atm of H 2 in the presence of 1a (1-hexene: 1a = 8300:1) produced the hydrogenation product 2 (TON = 12 000 h - 1 ). The treatment of 1a with excess ethylene led to the observation of both the ruthenium−ethyl and ruthenium−ethylene−hydride species 4a and 5a, respectively. The thermodynamic parameters for the equilibria among 1a, 4a, and 5a were estimated from the VT NMR data. These results are consistent with a monohydride mechanistic pathway.