Theoretical modeling of the mechanism of dioxygen activation and evolution by tetranuclear manganese complexes

Oxygen evolution mechanisms at polynuclear manganese centers of the type present in Photosystem II (PSII) are studied using a combination of molecular mechanics and extended Hückel (EH) computational techniques. The energies of two complexes containing peroxo groups bound at a dinuclear ('dimer') subset of tetranuclear ('dimer-of-dimers') Mn aggregates are minimized using the universal force field (UFF) technique. The resulting geometric parameters are in reasonable agreement with those of known Mn peroxo structures. The total energy variation resulting from the structural distortions that accompany the change of the OO bond order from 0.01 (no bond) to 1.0 indicates lower 'oxygen activation' barriers compared with previous models. Mn bound Cl is found to reduce the net charge on the metal to which it is attached. The equilibrium geometries of the peroxo bound tetranuclear models reveal asymmetric coordination of the peroxo oxygen (MnOOMn torsion angles of approx. 60°) consistent with ground state (triplet) oxygen release by PSII. Removal of the two Mn ions which are not directly bound to the peroxide ligand is found to disfavor the oxygen-oxygen coupling by 0.3 eV. Based on the above results a water oxidation mechanism is proposed.