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 Hückel method. Diiron peroxo model complexes with different μ-η 1:η 1 -O 2 and μ-η 2:η 2 -O 2 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 O 2 are initially occupied by two electrons, in the first dioxygen binding step two other electrons are effectively transferred from the “t 2g ” block to O 2 to form O 2 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-η 1:η 1 -O 2 mode is more effective for electron transfer from the d-block orbitals to the σ u *. Our calculations indicate that a C 3 v - or D 2 d -distorted methane can be activated if a coordinatively unsaturated iron, which has been proposed to exist in the diamond Fe 2 (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−CH 4 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.