Local density functional study of oxoiron(IV) porphyrin complexes and their one-electron oxidized derivatives. Axial ligand effects

A systematic study of the electronic structure of models for the active sites of heme enzymes such as peroxidases and cytochromes P450 has been carried out for high-valent transition states of their catalytic cycles, namely, compound I and compound II for peroxidases, as a function of the second axial ligand. The investigation is based on molecular orbital calculations in local density approximation and comprises five-coordinated oxoiron-(IV) porphyrin as well as the corresponding six-coordinated species with chlorine, imidazole, and H₃CS^- as axial ligands. In all cases, the ground state of compound II is obtained as the ferryl (t_2g)⁴ configuration (3d_xy)²(3d_xz,3d_xy)² with total spin S = 1 distributed between the iron and oxygen atom in a ratio of approximately 60/40. Different electronic states of compound I with the radical located in the a_1u or a_2u orbitals of the porphyrin or in the lone-pair orbital of the axial ligand are discussed in detail. The corresponding Heisenberg exchange coupling constants J between the oxoiron and the radical spin are calculated, and the influence of the position and orientation of the axial ligand on J is investigated. The results are correlated with the available experimental data.