Single-Step Hydride Transfer from CpMo(CO)₂(PPh₃)H to Protonated Ketones

The only alcohol complex formed during the ionic hydrogenation of acetone by CpMo(CO) 2 (PPh 3 )H ( 1b ) and CF 3 SO 3 H in acetonitrile is trans -CpMo(CO) 2 (PPh 3 )( i -PrOH) + ( 4 ); on standing the coordinated isopropyl alcohol of 4 is replaced by solvent, forming trans -CpMo(CO) 2 (PPh 3 )(CH 3 CN) + ( 2b ). Treatment of 1b in the same solvent with CF 3 SO 3 H alone yields the more stable cis acetonitrile complex 3b . The final products from treating an acetonitrile solution of acetone and 1b with CF 3 SO 3 H thus include both 2b and 3b . The trans stereochemistry of 4 implies that the ionic hydrogenation of acetone by 1b involves a single-step hydride transfer rather than separate e - and H • transfers. The equilibrium constant for the protonation of acetone by CF 3 SO 3 H in CH 3 CN at 25 °C is 2.4 × 10 - 2, and the rate constant for hydride transfer from 1b to Me 2 COH + is 12 300 M - 1 s - 1 under the same conditions. Other ketones undergo ionic hydrogenation more slowly.