Analysis of an alternative to the H-atom abstraction mechanism in methane C–H bond activation by nonheme iron(iv)-oxo oxidants

Abstract

The triplet δ-mechanism different from the previously reported ones, i.e., the π-channel with the unoccupied π^*_xz/yz (FeO) orbital and the σ-channel involving the unoccupied α-spin Fe-σ^*_z2 orbital, has been theoretically described for the methane hydroxylation by [Fe^IV = O(TMC)(SR)]⁺ and its derivative [Fe^IV = O(TMC)(OH)]⁺ complex for the first time, and we have undertaken a detailed DFT study on the nature of this state by probing its geometry, electronic property and reactivity in comparison to all other possibilities. DFT calculations indicate that the electron transfer for the ³δ-channel from the σ_C–H orbital of the substrate to the final acceptor σ^*_x2−y2 orbital of the catalyst occurs through a complex mechanism, which is initiated by the original α-spin electron transfer from the π* orbital of the catalyst to the σ^*_x2−y2 orbital, where the α-spin electron from the σ_C–H orbital of the substrate shifts to the just empty α-spin π* orbital of the catalyst via the O-p_x/y based π^*_xz/yz-orbital concomitantly. It is also found that the electron-donating ability of the axial ligand could influence the reaction channels, evident by the distinction that the electron-deficient F⁻ and CF₃CO₂⁻ ligands react via the ³σ-channel, whereas the electron-rich SR⁻ and OH⁻ ligands proceed by the ³δ-channel. With respect to reactivity, the ³δ-pathway has a comparable barrier to the ³π and ⁵π-pathways, which may offer a new approach for the specific control of C–H bond activation by the iron(IV)-oxo species.