New plenary and lacunary polyoxotungstate structures assembled from nonatungstoarsenate(III) anions and uranyl cations

Abstract

A new structural family of heteropolytungstate complexes containing trans-dioxo uranium(VI) (uranyl) groups has been isolated in good yield by reaction of UO₂²⁺ with [NaAs₄W₄₀O₁₄₀]²⁷⁻ (W₄₀) in aqueous solution. The “lacunary” anion, [(UO₂)₃(H₂O)₅As₃W₂₉O₁₀₄]¹⁹⁻1 is formed by the reaction of W₄₀ with UO₂(MeCO₂)₂ at pH 7, and the crystal structure of (NH₄)₁₉[(UO₂)₃(H₂O)₅As₃W₂₉O₁₀₄]·28H₂O [orthorhombic, P2₁2₁2₁, a = 17.5626(2), b = 24.4929(2), c = 33.3019(3) Å] is reported. The “plenary” anion, [(UO₂)₃(H₂O)₆As₃W₃₀O₁₀₅]¹⁵⁻2 is formed by reaction of W₄₀ with UO₂(NO₃)₂ at pH 4, and the crystal structure of (NH₄)₁₅[(UO₂)₃(H₂O)₆As₃W₃₀O₁₀₅]·20H₂O [monoclinic, P2₁/c, a = 23.63520(10), b = 18.0646(2), c = 35.32640(10) Å, β = 107.46°] is reported. Both heteropolyanions contain three B-AsW₉O₃₃⁹⁻ groups linked by uranyl cations (pentagonal bipyramids) and three (or two) additional corner-sharing WO₆ octahedra cap the structure of 2 (1). A six-line ¹⁸³W NMR spectrum of 2 is consistent with an effective C_3v symmetry for the anion in solution. Addition of VO²⁺ to a solution of 1 yields the V-substituted anion (3) confirmed by a structure determination of (NH₄)₁₇[(UO₂)₃(H₂O)₆As₃W₂₉V^IVO₁₀₅]·44H₂O [orthorhombic, Pnma, a = 31.1821(4), b = 24.9672(3) Å, c = 20.3460(3) Å]. The cyclic voltammogram of 3 in 0.5 M sodium acetate buffer (pH 4.7) shows the V^IV/V couple at +0.26 V (E_pc) and +0.40 V (E_pa).