Mechanistic study on dehydrogenation of methanol with [RuCl 2(PR 3) 3]-type catalyst in homogeneous solutions

Abstract

Catalytic dehydrogenation of methanol has been investigated in homogeneous solutions with a series of Ru(II) complexes, [RuCl 2(P( p-C 6H 4X) 3) 3] (X = H (1), Me (2), F (3), OMe (4)) and [RuCl 2(PMePh 2) 3] (5). In the gas phase, hydrogen was formed selectively (> 99.5%), and formaldehyde, methylal (formaldehyde dimethyl acetal) and methyl formate were found in the liquid phase with satisfactory stoichiometry to the formed hydrogen. The reaction was retarded by the extra addition of free phosphine, suggesting the presence of pre-equilibrium dissociation of phosphine ligand. Kinetic analyses from this viewpoint explained well the dependence of rate on the concentration of catalyst (saturation curve). The order of evaluated pre-equilibrium constant (1 ≈ 2 > 5) is in accord with the general idea that the dissociation of phosphine ligand is controlled principally by steric bulk of ligands. The order of rate (3 > 1 > 2 > 4) for 1–4, possessing the same cone angle of phosphine ligand, correlated clearly with basicity of phosphines. The results are interpreted in terms of the mechanism of rate-determining β-hydrogen abstraction in the Ru-OCH 3 intermediate.