ROLE OF AXIAL LIGAND IN THE ELECTRONIC-STRUCTURE OF MODEL COMPOUND-I COMPLEXES

A quantum chemical study of the two low-lying quartet states of seven model compound I iron-porphyrin complexes with varying axial ligands has been carried out using the INDO method. The varying axial ligands included in this study are five that are models for those in the intact enzymes, imidazole and imidazolate (model peroxidase HRP and CCP), CH3CONH2 (Gln175 mutant of CCP), PhO-1 (catalase), CH3S-1 (P450), and two that have been used in biomimetics of these enzymes: Cl-1 (hemin) and PhS-1 (model P450s). The purpose of these studies was to determine the role of the axial ligands in determining (i) the relative energies of the two nearly degenerate quartet electronic states of compound I, involved either as an a1u or a2u porphyrin-pi cation radical and (ii) the electron and spin distributions in the a1u and a2u radical cations of compound I. For most of the model complexes, including both HRP-I and CAT-I, a moderate effect of the axial ligand on the relative energy of these two states was observed and the a1u radical cation was found to be the ground state. The energy order of these two radical cations, however, was reversed in the P450-I model complexes, indicating an association of the unique property of the Fe=O bond breaking with an a2u radical cation. The symmetry-allowed overlap between the Fe=O and 3a2u orbitals may lower the activation energy for the Fe=O bond cleavage in P450-I. However, the calculated electronic and spin properties, including the unpaired spin and net charge on the oxygen and the Fe=O bond overlap density, important determinants of the reactivity of this complex in the ligand Fe=O region, are very similar for all complexes and in both cation states.