Molybdenum Carbonyl Complexes in the Solvent-Free Catalytic Hydrogenation of Ketones

The heterodifunctional ligand Li[η5-C5H4(CH2)2PR2] (R = Ph, Cy, and tBu) reacts with
Mo(CO)3(diglyme) to give the molybdenum anion complex Li{Mo(CO)3[η5-C5H4(CH2)2PR2]}.
Protonation with HOAc gives the metal hydride complexes HMo(CO)2[η5:η1-C5H4(CH2)2PR2],
in which the phosphine and cyclopentadienyl ligands are linked by a two-carbon bridge.
Crystal structures of HMo(CO)2[η5:η1-C5H4(CH2)2PR2] with all three R groups (R = Ph, Cy,
and tBu) are reported. Syntheses of the C3-bridged complex, HMo(CO)2[η5:η1-C4H5(CH2)3PPh2], and a W analogue, HW(CO)3[η5-C5H4(CH2)2PtBu2], were carried out by analogous
routes. Hydride transfer to Ph3C+BAr‘4- [Ar‘ = 3,5-bis(trifluoromethyl)phenyl] from the
catalyst precursors HMo(CO)2[η5:η1-C5H4(CH2)2PR2] leads to homogeneous catalysts for
hydrogenation of ketones, with the best performance being found for R = Cy. Protonation of
HMo(CO)2[η5:η1-C5H4(CH2)2PR2] by HOTf leads to metal triflate complexes (TfO)Mo(CO)2[η5:η1-C5H4(CH2)2PR2], which are used in ketone hydrogenation. Compared to the previously
prepared complexes that did not have the phosphine and Cp linked together, these new
complexes provide catalysts that have much longer lifetimes (up to about 500 turnovers)
and higher thermal stability. Solvent-free ketone hydrogenation can be carried out with
these complexes at catalyst loadings as low as 0.1 mol %.