Tubular polyoxoanion [(SeMo6O21)2(C2O4)3]10− and its transformations

Abstract

Self-assembly reactions in the Na₂MoO₄/SeO₂/H₂C₂O₄ system lead to the formation of a tubular [(SeMo₆O₂₁)₂(C₂O₄)₃]¹⁰⁻ hybrid polyanion, isolated and characterized by single crystal X-ray diffraction (SCXRD) as Na₉H[(SeMo₆O₂₁)₂(C₂O₄)₃]·NaNO₃·20.6H₂O (SeMo6-1). Dehydration of this compound leads to the formation of Na₁₀[(SeMo₆O₂₁)₂(C₂O₄)₃]·0.5NaNO₃·8H₂O (SeMo6-2). The electrochemical behavior of SeMo6-2 as a component of the paste electrode was studied by CV in CH₃CN solution. A concentrated aqueous solution of SeMo6-2 was studied using ⁷⁷Se and ⁹⁵Mo NMR techniques, which show extensive degradation of the complex. Addition of TBABr to the concentrated solution leads to a precipitate consisting of TBA salts of [Mo₆O₁₉]²⁻ and [β-Mo₈O₂₆Na₂(NO₃)₂]²⁻ anions. The latter was characterized by SCXRD as (TBA)₃[β-Mo₈O₂₆Na₂(NO₃)]·CH₃CN (Mo8-Na). This transformation shows the presence of [β-Mo₈O₂₆]⁴⁻ and [Mo₆O₁₉]²⁻ in the aqueous solution of SeMo6-2. The study of (NBu₄)₄[β-Mo₈O₂₆] and (NBu₄)₂[Mo₆O₁₉] solutions in CH₃CN by HPLC shows equilibration between [β-Mo₈O₂₆]⁴⁻ and [Mo₆O₁₉]²⁻.