Dehydrogenation, disproportionation and transfer hydrogenation reactions of formic acid catalyzed by molybdenum hydride compounds

Abstract

The cyclopentadienyl molybdenum hydride compounds, Cp^RMo(PMe₃)_3−x(CO)_xH (Cp^R = Cp, Cp*; x = 0, 1, 2 or 3), are catalysts for the dehydrogenation of formic acid, with the most active catalysts having the composition Cp^RMo(PMe₃)₂(CO)H. The mechanism of the catalytic cycle is proposed to involve (i) protonation of the molybdenum hydride complex, (ii) elimination of H₂ and coordination of formate, and (iii) decarboxylation of the formate ligand to regenerate the hydride species. NMR spectroscopy indicates that the nature of the resting state depends on the composition of the catalyst. For example, (i) the resting states for the CpMo(CO)₃H and CpMo(PMe₃)(CO)₂H systems are the hydride complexes themselves, (ii) the resting state for the CpMo(PMe₃)₃H system is the protonated species [CpMo(PMe₃)₃H₂]⁺, and (iii) the resting state for the CpMo(PMe₃)₂(CO)H system is the formate complex, CpMo(PMe₃)₂(CO)(κ¹-O₂CH), in the presence of a high concentration of formic acid, but CpMo(PMe₃)₂(CO)H when the concentration of acid is low. While CO₂ and H₂ are the principal products of the catalytic reaction induced by Cp^RMo(PMe₃)_3−x(CO)_xH, methanol and methyl formate are also observed. The generation of methanol is a consequence of disproportionation of formic acid, while methyl formate is a product of subsequent esterification. The disproportionation of formic acid is a manifestation of a transfer hydrogenation reaction, which may also be applied to the reduction of aldehydes and ketones. Thus, CpMo(CO)₃H also catalyzes the reduction of a variety of ketones and aldehydes to alcohols by formic acid, via a mechanism that involves ionic hydrogenation.