Reaction pathway for the direct benzene hydroxylation by iron-oxo species

The direct benzene hydroxylation by an iron-oxo species is discussed from density-functional-theory (DFT) calculations. The proposed reaction pathway is FeO⁺ + C₆H₆ → OFe⁺(C₆H₆) → [TS1] → HO-Fe⁺-C₆H₅ → [TS2] → Fe⁺(C₆H₅OH) → Fe⁺ + C₆H₅OH, in which TS means transition state. This reaction is initiated by the formation of the reactant complex, OFe⁺(C₆H₆), exhibiting an η²-C₆H₆ binding mode; benzene C-H bonds are activated on this complex due to significant electron transfer from the benzene to the iron-oxo species. The reaction should proceed in a concerted manner, neither via the formation of radical species nor ionic intermediates. The reaction mechanism is quite similar to the two-step concerted mechanism that we have proposed originally for the direct methane hydroxylation by an iron-oxo species. The quartet potential energy surface affords a low-cost reaction pathway for the benzene hydroxylation, spin inversion being unimportant in contrast to the methane hydroxylation in which crossing between the sextet and quartet potential energy surfaces plays an important role. We suggest that our two-step concerted mechanism should be widely applicable to hydrocarbon hydroxylations catalyzed by transition-metal oxides if coordinatively unsaturated metal oxides are responsible for such important catalytic reactions.