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

Abstract

Molecular geometries and electronic structures of manganese binuclear complexes, [(H₂O)₃(O–)Mn(bis-μ-oxo-O₂)Mn(OH₂)₄]^q+ (q = 2 or 3), which are model complexes of the S₃ 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 Mn₂(II,III) and Mn₂(III,III) at the S₀ state of the Kok cycle. The conformers with the Mn–O bond axial to the Mn₂O₂ 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 M₂(II,III) and M₂(III,III) series, respectively. The axial Mn–O bonds are Mn(IV)O²⁻ for M₂(II,III) and Mn(IV)–O˙⁻ for Mn₂(III,III). The characters of Mn–O orbitals are π-orbitals composed of d–p interactions. The conformer with the axial Mn(IV)O²⁻ derived from M₂(II,III) at S₀ is thermochemically unstable, leading to proton transfer to give two OHs in the complex, while the axial Mn(IV)–O˙⁻ from Mn₂(III,III) at S₀ is stable without the proton transfer. The magnetic interactions between two Mn ions and O were also examined by estimations of effective exchange integrals.