Synthesis, Stability, and Crystal Structure of the (NMe2H2)9[(AsW9O33)2(NbO)3(H2O)]·44H2O Complex

Abstract

Citrate complexes of niobium(V) formed in the interaction of [Nb₆O₁₉]⁸⁻ hexaniobate with citric acid are used for transferring the {NbO}³⁺ niobium group to the pseudo-trilacunary [α-B-As^IIIW₉O₃₃]⁹⁻ anion with the formation of sandwich-type [(AsW₉O₃₃)₂(NbO)₃(H₂O)]⁹⁻ complexes. Slow evaporation of the reaction mixture in the presence of dimethylammonium chloride yields crystalline product (NMe₂H₂)₉[(AsW₉O₃₃)₂(NbO)₃(H₂O)]·44H₂O (1) whose structure is determined by single crystal XRD. When the (NBu₄)⁺ salt (obtained by the exchange reaction between 1 and tetrabutylammonium bromide) of [(AsW₉O₃₃)₂(NbO)₃(H₂O)]⁹⁻ is dissolved in DMF, As³⁺ is oxidized to As⁵⁺ with the formation of a mixture of the Keggin-type [AsW₁₁NbO₄₀]⁴⁻ and [AsW₁₀Nb₂O₄₀]⁵⁻ anions. In an aqueous solution 1 is oxidized by hydrogen peroxide with the formation of a mixture of Keggin anions with different numbers of niobium atoms in the structure.