Theoretical study of electronic structures of [(H2O)3(O-)Mn(μ-oxo)2Mn(OH2)4]q+ (q = 2 or 3) with Mn–O bond

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Molecular geometries and electronic structures of manganese binuclear complexes, [(H(2)O)(3)(O-)Mn(bis-mu-oxo-O(2))Mn(OH(2))(4)](q+) (q = 2 or 3), which are model complexes of the S(3) state in the Kok cycle of OEC, were examined using hybrid density functional theory and broken symmetry method. The complexes of q = 2 and 3, which have a Mn-O bond, correspond to those derived from Mn(2)(II,III) and Mn(2)(III,III) at the S(0) state of the Kok cycle. The conformers with the Mn-O bond axial to the Mn(2)O(2) core are lower in energy than those with the equatorial Mn-O bond. The equatorial Mn-O bonds are assigned as Mn(III)-O*(-) and Mn(IV)-O*(-) for M(2)(II,III) and M(2)(III,III) series, respectively. The axial Mn-O bonds are Mn(IV)=O(2-) for M(2)(II,III) and Mn(IV)-O*(-) for Mn(2)(III,III). The characters of Mn-O orbitals are pi-orbitals composed of d-p interactions. The conformer with the axial Mn(IV)=O(2-) derived from M(2)(II,III) at S(0) is thermochemically unstable, leading to proton transfer to give two OHs in the complex, while the axial Mn(IV)-O*(-) from Mn(2)(III,III) at S(0) is stable without the proton transfer. The magnetic interactions between two Mn ions and O were also examined by estimations of effective exchange integrals.