Ionic Hydrogenation of Ketones with Molybdenum Pentabenzylcyclopentadienyl Hydride Catalysts

A study of the reactivity of [MoCpBz(CO)3H] (CpBz = pentabenzylcyclopentadienyl) as a catalyst precursor for the ionic hydrogenation of 3-pentanone is presented. The complexes [MCpBz(CO)3OTf] (M = Mo (3), W (4)), [MoCpBz(CO)3(OCEt2)]BF4 (9), and [MoCpBz(CO)3(OCMe2)]BF4 (12) are described. 9+ and [MoCpBz(CO)3(OHCHEt2)]+ (8+) were identified by NMR under catalytic conditions. The reaction of [WCpBz(CO)3H] with triflic acid led to [WCpBz(CO)3(H)2]+ (7+), which was identified by NMR as a dihydride complex. The influence of steric and electronic effects on the catalyst activity is discussed on the basis of experimental results and a mechanism proposal resulting from DFT calculations. The proposed mechanism includes facile protonation of the acetone O atom promoted by a dihydride Mo(IV) complex, followed by slower hydride transfer from the metal center to the carbonyl C atom. The results indicate strong coordination of both the product (isopropyl alcohol) and the reactant (acetone), leading to some blocking of the catalyst active site. Regeneration of the catalyst active species is the rate-limiting step of the catalytic cycle. The steric bulk of the CpBz ligand increases the catalyst activity.