Catalytic reaction of methanol with a series of ruthenium(II) complexes and the mechanism of the formation of acetic acid from methanol alone

Abstract

The catalytic abilities of a series of ruthenium(II) complexes containing zero, one and two SnCl₃^– ligands, [RuCl₂{P(OMe)₃}₄]1, [RuCl(SnCl₃){P(OMe)₃}₄]2 and [Ru(SnCl₃)₂{P(OMe)₃}₃]3, have been compared in the reaction of methanol to form acetic acid (and/or methyl acetate due to esterification), as well as their reactions with the possible intermediates (formaldehyde, methyl formate) in the overall reaction. It was found that the formation of acetic acid from methanol occurred only with 3, which also converted paraformaldehyde or methyl formate into acetic acid. Complex 1 showed only a catalytic activity for the Tischenko-type dimerization (2HCHO → HCO₂Me), and 2 exhibited an intermediate character, being able to catalyse the two reactions (2HCHO → HCO₂Me, HCO₂Me → MeCO₂H) but unable to react with methanol. Based on kinetic results for the reaction of methanol with 3, a possible reaction pathway is proposed where methyl formate and acetic acid are formed from formaldehyde competitively sharing a common reaction path. For the isomerization of methyl formate as a substrate a separate reaction path is suggested, where the Ru^II–Sn^II bimetallic centre of 2 and 3 converts the co-ordinated HCO₂Me into a five-membered acetate bridge.