A concerted mechanism is proposed for the conversion of methane to methanol on intermediate Q of soluble methane monooxygenase (sMMO), the active species of which is considered to involve an Fe2(μ-0)2 diamond core. A hybrid density functional theory (DFT) method is used. Methane is highly activated on the dinuclear iron complex through the formation of the Q(CH4) complex, in which a four-coordinate iron plays a central role in the bonding interactions between intermediate Q and methane. An H atom abstraction via a four-centered transition state and a methyl migration via a three-centered transition state successively occur within the complex, leading to the formation of product methanol. The reaction pathway for the methane hydroxylation on the diiron complex follows the mechanism for the gas-phase reaction by the bare FeO+ complex described in a previous paper (K. Yoshizawa, Y. Shiota, and T. Yamabe, Chem. Eur. J., 3, 1160 (1997)). Our mechanism for the methane hydroxylation by sMMO is against a radical mechanism which has been widely believed to play a role in hydrocarbon hydroxylations by cytochrome P450.
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