QUANTUM-CHEMICAL STUDY OF THE 4-ELECTRON CATALYTIC-OXIDATION OF WATER TO DIOXYGEN IN THE PRESENCE OF DINUCLEAR AND TETRANUCLEAR HYDROXIDE COMPLEXES OF COBALT(III) AND IRON(III) - INTERMEDIATES OF THE CATALYTIC CYCLE AND THEIR RELATIVE ENERGIES

Quantum-chemical calculations have been performed to elucidate possible intermediates and their relative energies in the closed cycle of the catalytic oxidation of water to dioxygen by one-electron oxidants. Di- and tetranuclear hydroxocomplexes of Co(III) and Fe(III) served as the simplest prototypes of artificial catalysts for the reaction under study. The predicted intermediates of the consecutive one-electron oxidation of the reaction site accompanied by its deprotonation appear to be analogs of well-known chemical structures of the type of transition metal complexes with dioxygen ligands. It is found that during the process only one metal ion undergoes the redox transformations, changing its valence state from IV to II, while the second metal ion serves mainly to stabilize the intermediates resulting from the oxidized water molecule. Thus, it confirms the well-known hypothesis at least on the dinuclear nature of the ''oxygen evolving reaction centre''. Release of a dioxygen molecule most probably occurs after the third oxidation of the initial form of the reaction site. The calculated energies of intermediates participating in the catalytic cycle are in reasonable agreement with available experimental data.