Redox active mixed-valence hexamanganese double-cubane complexes supported by tetravanadates

Abstract

A mixed-valence hexamanganese complex, [Mn^III₄Mn^IV₂O₆(V₄O₁₃)₂(MeCO₂)₂]⁶⁻ [1(MeCO₂)], has been synthesized by the reaction of a dimanganese precursor, [Mn₂(V₅O₁₅)₂]⁶⁻ (Mn2V10), with (n-Bu₄N)MnO₄ and Mn(MeCO₂)₂ in acetonitrile. Compound 1(MeCO₂) possesses a face-sharing double-cubane {Mn^III₄Mn^IV₂O₆} core that is sandwiched by two [V₄O₁₃]⁶⁻ units with two bridging acetate groups. Several benzoate derivatives, [Mn₆O₆(V₄O₁₃)₂(RCO₂)]⁶⁻ [1(RCO₂), R = Ph, p-ClPh, p-MePh, p-NO₂Ph], are also synthesized to study the substituent effects and the redox potentials were well-correlated with Hammett constants. In complex 1(MeCO₂), four manganese atoms at the edges are assigned as being in a trivalent state and two manganese atoms at the center are assigned as being in a tetravalent state from X-ray crystallography. One electron oxidation product, [Mn^III₃Mn^IV₃(V₄O₁₃)₂(RCO₂)₂]⁵⁻, is also isolated for the benzoate derivatives of [2(RCO₂), R = p-MePh, o-MePh]. The arrangements of Mn(III) and Mn(IV) sites depending on the oxidation state of the hexamanganese core and type of functional group on the acetate group were investigated by a crystallographic study along with a magnetic susceptibility study.