LOCAL-DENSITY FUNCTIONAL-STUDY OF OXOIRON(IV) PORPHYRIN COMPLEXES ANDTHEIR ONE-ELECTRON OXIDIZED DERIVATIVES - AXIAL LIGAND EFFECTS

A systematic study of the electronic structure of models for the activ e sites of heme enzymes such as peroxidases and cytochromes P450 has b een carried out for high-valent transition states of their catalytic c ycles, namely, compound I and compound II for peroxidases, as a functi on of the second axial ligand. The investigation is based on molecular orbital calculations in local density approximation and comprises fiv e-coordinated oxoiron-(IV) porphyrin as well as the corresponding six- coordinated species with chlorine, imidazole, and H3CS- as axial ligan ds. In all cases, the ground state of compound II is obtained as the f erryl (t(2g))(4) configuration (3d(xy))(2)(3d(xz),3d(xy))(2) with tota l spin S = 1 distributed between the iron and oxygen atom in a ratio o f 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 th e oxoiron and the radical spin are calculated, and the influence of th e position and orientation of the axial ligand on J is investigated. T he results are correlated with the available experimental data.