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 H 3 CS - 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.