DFT insight into axial ligand effects on electronic structure and mechanistic reactivity of oxoiron(IV) porphyrin

A series of DFT studies on the epoxidation reactions of olefins by oxoiron(IV) porphyrin cation radical complexes are performed in this work, to elucidate the axial ligand effects on the electronic feature and reaction mechanism in detail. We analyzed the molecular orbitals, spin populations, and Mulliken charges along the intrinsic reaction coordinate route. From the findings, we confirmed that the interaction between the axial ligand and the oxoiron(IV) porphyrin is strong and the initial changes in the electronic structures occur early during the reaction, which further enhances the reactivity toward olefin epoxidation. More importantly, the patterns of the electron transfer from olefin to oxoiron(IV) porphyrin were impacted by the axial ligand. The pattern of successive electron transfer from FeO to porphyrin and then from C=C to FeO for oxoiron(IV) porphyrin in case of fluorine and acetate axial ligands, whereas the pattern of electron transfer occurs from C=C to porphyrin for oxoiron(IV) porphyrin in case of chlorine and nitrate axial ligands during the epoxidation reaction of the olefins. We also determined the intersystem crossing between the quartet and sextet spin states occurring at the second transition state (TS2) by the analysis of the two-dimensional potential energy surface.