Semi-empirical AM1 and PM3 calculations of five- and six-coordinate oxo iron (IV) porphyrin complexes

The geometrical structures and electronic states of the reactive oxo species of iron (III) porphyrin chloride and its meso-tetraphenyl derivatives proposed as a synthetic model of oxidases were examined by using semi-empirical AM1 and PM3 parametrizations. The spin population for oxo-iron (IV) porphyrin π-cation radical (quartet, S=3/2) and its one-electron reduction species (triplet, S=1) were reproduced by both calculation methods irrespective of the presence of chloride ion as a sixth ligand. Mostly planar structures were obtained for iron porphyrins and their meso-tetraphenyl derivatives with four phenyl rings perpendicular to the porphyrin ring except when bromine atoms were introduced into β-positions. AM1 parametrization gave more appropriate orbital energies and spin population for both of the quartet and triplet oxo-iron (IV) species than PM3 and the electron-withdrawing phenyl groups in meso-positions were found to decrease the energy level of dπ (Fe)-pπ (O, Cl) orbitals being involved in the porphyrin-catalyzed oxidation.