Acid-Promoted Homogeneous Hydrogenation of Alkenes Catalyzed by the Ruthenium−Hydride Complex (PCy₃)₂(CO)(Cl)RuH:  Evidence for the Formation of 14-Electron Species from the Selective Entrapment of the Phosphine Ligand

The addition of 1.0 equiv of HBF 4 ·OEt 2 led to a ca. 2−3-fold increase in the catalyst activity of (PCy 3 ) 2 (CO)(Cl)RuH ( 1a ) toward the hydrogenation of alkenes. The stoichiometric reaction of 1a with HBF 4 ·OEt 2 produced a 1:3.5 mixture of the new ruthenium−hydride species 2 and Cy 3 PH + BF 4 - . The catalyst activity of the isolated 2 /Cy 3 PH + mixture was found to be similar to 1a /HBF 4 ·OEt 2 . The complex 2 slowly decomposed in C 6 H 6 solution to give a novel tetrameric complex 3 . The treatment of 1a with HBF 4 ·OEt 2 in CH 3 CN led to the selective formation of the monophosphine species 4, which was converted to the stable complex 5 upon treatment with excess PPh 3 . The reaction of 1a with a weak acid, HSiCl 3, led to the formation of the anionic silyl complex 7 . The structures of 3, 5, and 7 were determined by X-ray crystallography. The formation of η 2 -H 2 complex 8 was observed by NMR at low temperature. These results suggested that the 14-electron species [(PCy 3 )(CO)RuHCl], generated from the selective entrapment of the phosphine ligand, is the key species involved in the catalytic hydrogenation reaction.