Active catalyst for methane hydroxylation by an iridium-oxo complex

Motivated by a recent study on [IrV(η5-C5H5){ppy}(O)]2+ (ppy is 2-phenylpryridine), which is able to cleave the C-H bond of decalin but not of methane due to a high activation energy of 25.7 kcal/mol (Zhou, M. et al. Cp∗ iridium Precatalysts for Selective C-H Oxidation via Direct Oxygen Insertion: A Joint Experimental/Computational Study. ACS Catal. 2012, 2, 208 218), in the present study, we perform density functional theory (DFT) calculations on another kind of an Ir-oxo complex, namely, [IrV(η5-C5Me5){bpy(COOH)2}(O)]2+ or OC, where bpy(COOH)2 is 2,2′-bipyridine-4,4′ dicarboxylic acid, to assess its ability to hydroxylate methane to methanol. OC is formed by light-induced oxidation of the Ir-H2O complex to generate electricity. By hydroxylating methane to methanol and inserting H2O, OC can regenerate the Ir-H2O complex and thus create a catalytic loop similar to a fuel cell. Herein, by utilizing intrinsic bond orbital (IBO) analysis, we show detailed mechanisms of how OC efficiently cleaves the strong C-H bond of methane to form methanol in three possible spin states, namely, the triplet, open-shell singlet, and closed-shell singlet states. In the triplet state, the C-H bond cleavage proceeds with an activation energy of only 11.3 kcal/mol via a hydrogen atom transfer mechanism, where an intermediate species involving •CH3 and Ir-OH• is formed in the same spin state, although a spin change to the open-shell singlet state is likely to occur. A subsequent HO-CH3 recombination occurs without a barrier through direct radical coupling in the open-shell singlet state. In contrast, the C-H bond cleavage in the closed-shell singlet state occurs via a hydride transfer (or oxygen insertion) mechanism, where methanol is directly formed without any intermediate states. Although this reaction mechanism requires a lower C-H activation energy (5.6 kcal/mol), OC in the closed-shell singlet state is less stable by 11.9 kcal/mol than that in the triplet state. These results provide a theoretical prediction of an alternative promising catalyst for the direct conversion of methane to methanol and the methane fuel cell.