Local Density Functional Study of Oxoiron(IV) Porphyrin Complexes and Their One-Electron Oxidized Derivatives. Axial Ligand Effects

Abstract

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 (t2g)4 configuration (3dxy)2(3dxz,3dxy)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 a1u or a2u 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...