Mechanistic study of methanol oxidation by Ru^IV-oxo complexes

The catalytic conversion of methanol to formaldehyde by three kinds of non-porphyrin Ru complexes, Ru^IVO(TPA) (TPA = tris(2-pyridylmethyl)amine) (1a), Ru^IVO(6-COO-TPA) (6-COOTPA = 2-(6-carboxyl-pyridyl)methyl-bis(2-pyridylmethyl)amine) (1b), and Ru^IVO(N4Py) (N4Py = N,N-bis(2-pyridyl-methyl)-N-bis(2-pyridyl)methylamine) (1c), is discussed by using density functional theory (DFT) calculations. There are two possible reaction pathways for the oxidation of methanol to formaldehyde with respect to the first hydrogen abstraction from the methyl group (path 1) and the hydroxyl group (path 2). Path 1 and path 2 involve the hydroxymethyl radical (•CH₂OH) and the methoxyl radical (CH₃O•), respectively, as an intermediate. DFT calculations demonstrate that the two pathways are energetically comparable in the reactions by the three Ru^IV-oxo complexes. The reactions with 1a and 1c are initiated by the C-H bond dissociation with activation barriers of 22.2 and 21.4 kcal/mol, respectively, while the reaction with 1b is initiated by the O-H bond dissociation with an activation barrier of 18.1 kcal/mol. However, the calculations showed that the rate-determining step is the H-atom abstraction from the CH₃ group of methanol in all the pathways. These results are in good agreement with kinetic analysis of the reactions by the Ru^IV-oxo complexes, being useful for considering the mechanism of methanol oxidation.