Density functional theoretical study of oxo(porphyrinato)iron(IV) complexes, models of peroxidase compounds I and II

Local density functional (LDF) calculations, including geometry optimization, have been carried out on oxo(porphyrinato)iron(IV), PFeO, and the corresponding cation, [PFeO]⁺, which have been chosen as simple models of peroxidase compounds II and I, respectively. In the optimized structure of PFeO, the Fe-O distance was 1.622 Å and the iron atom was positioned 0.215 Å above the plane of the four porphyrin nitrogens. The harmonic Fe-O stretching frequencies of PFeO and [PFeO]⁺ were 934 and 964 cm^-1, respectively. A three-body (P-Fe-O) vibrational analysis revealed negligible coupling between the Fe-O stretch and the displacement of the iron atom out of the porphyrin plane. In both PFeO and [PFeO]⁺, two unpaired spins, corresponding to a (π*)² configuration, were cleanly localized on the ferryl moiety, being divided among Fe and O in the ratio 1.2:0.8. The third unpaired spin of [PFeO]⁺ was distributed over the porphyrin ring as an "A_2u"-type cation radical. Overall, these LDF results represent good agreement between first-principles theory and experiment. Spin-restricted Hartree-Fock theory is known to provide a poor description of both the porphyrin ligand and the ferryl group. The CASSCF method provides a good description of the ferryl group but is computationally unwieldy for large molecules such as hemes. Density functional theory appears to provide an expedient solution to the problem of several configurations with significant contributions to the wave functions of ferryl intermediates and is a practical theoretical tool for studying ferryl species.