Characterization of the High-Spin Co(II) Intermediate Species of the O₂-Evolving Co₄O₄ Cubic Molecules

An artificial oxygen-evolving complex (OEC) contains a tetranuclear Co^III₄O₄ cubic cluster ligated with acetate and pyridine molecules, a light-activated Ru(bpy)₃²⁺ moiety (bpy = 2,2'-bipyridine), and the sacrificial electron acceptor S₂O₈^2- side. A recent EPR investigation of this system showed the formation of the high-spin Co(II) ion under visible-illumination conditions. It has been supported that this center originates from the cluster and is involved in the oxygen-evolving process. The present report is focused on the further characterization of the high-spin d⁷ configuration of the Co(II) species. The measurement of the EPR spectrum at a wider magnetic field range in comparison to that reported recently shows the presence of an additional signal that contributes to the spectrum of the Co(II) center. The theoretical simulation of this spectrum reveals that an isotropic g value and considerably small zero-field splitting parameters describe the high-spin Co(II) ion in a unique way, which supports a tetrahedral crystal field symmetry. On the basis of the spin-Hamiltonian parameters, the looping transitions that lead to the experimental EPR signals are determined. Additionally, a possible role of the symmetry of the Co(II) species and a proposed model that explains its formation during the O₂-evolving process of the Co₄O₄ cubic molecules are discussed.