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

The triplet delta-mechanism different from the previously reported ones, i.e., the pi-channel with the unoccupied pi(xz/yz)* (FeO) orbital and the sigma-channel involving the unoccupied alpha-spin Fe-sigma(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 (3)delta-channel from the sigma(C-H) orbital of the substrate to the final acceptor sigma(x2-y2)* orbital of the catalyst occurs through a complex mechanism, which is initiated by the original alpha-spin electron transfer from the pi* orbital of the catalyst to the sigma(x2-y2)* orbital, where the alpha-spin electron from the sigma(C-H) orbital of the substrate shifts to the just empty alpha-spin pi* orbital of the catalyst via the O-p(x/y) based pi(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 (3)sigma-channel, whereas the electron-rich SR- and OH-ligands proceed by the (3)delta-channel. With respect to reactivity, the (3)delta-pathway has a comparable barrier to the (3)pi and (5)pi-pathways, which may offer a new approach for the specific control of C-H bond activation by the iron(IV)-oxo species.