Dioxygen cleavage and methane activation on diiron enzyme models: A theoretical study

Two possible mechanisms for dioxygen cleavage on non-heme diiron enzyme models are studied with an approximate molecular orbital method, the extended Huckel method. Diiron peroxo model complexes with different μ-η¹:η¹-O₂ and μ-η²:η²-O₂ binding modes are distorted to corresponding dioxo complexes along an assumed O-O bond cleavage reaction coordinate. Fragment molecular orbital (FMO) and Walsh diagram analyses clarify the bonding and orbital interactions. While the π(g)* orbitals of O₂ are initially occupied by two electrons, in the first dioxygen binding step two other electrons are effectively transferred from the 't(2g)' block to O₂ to form O₂^2-. To cleave the dioxygen O-O bond, it is necessary further to fill the σ(u)* orbital (high lying and unoccupied in the peroxide). The computations suggest that the μ-ν¹:ν¹-O₂ mode is more effective for electron transfer from the d-block orbitals to the σ(u)*. Our calculations indicate that a C(3v)- or D(2d)-distorted methane can be activated if a coordinatively unsaturated iron, which has been proposed to exist in the diamond Fe₂(μ- O)₂ core of intermediate Q of methane monooxygenase, is generated. The complex is suggested to include a five-coordinate carbon species with an Fe- CH₄ bond. We propose possible concerted reaction pathways for the conversion of methane to methanol on the supposed diiron active site of methane monooxygenase. Inversion at a five-coordinate carbon species is suggested to reasonably occur in an initially formed complex of methane and a model of intermediate Q, leading to inversion of stereochemistry at a labeled carbon center.