A new structural family of heteropolytungstate lacunary complexes with the uranyl, UO22+, cation

Abstract

The reaction of UO₂²⁺ with the trivacant lacunary polyoxometalate anions, [SbW₉O₃₃]⁹⁻ and [TeW₉O₃₃]⁸⁻, yields the novel isostructural complexes [(UO₂)₂(H₂O)₂(SbW₉O₃₃)₂]¹⁴⁻ (1) and [(UO₂)₂(H₂O)₂(TeW₉O₃₃)₂]¹²⁻ (2), respectively. The complex anions contain two [XW₉O₃₃]ⁿ⁻ (X = Sb^III or Te^IV) anions linked by two UO₂²⁺ cations. Each uranyl moiety bonds to two unsaturated oxygen atoms of each lacunary anion in the complex. Each [XW₉O₃₃]ⁿ⁻ anion has six unsaturated oxygen atoms meaning that in 1 and 2 each [XW₉O₃₃]ⁿ⁻ anion has two unsaturated oxygen atoms which remain uncoordinated to uranium with the result being the formation of an ‘open’ sandwich structure. The fact that a third UO₂²⁺ cation is not coordinated to form a ‘closed’ sandwich structure (as is observed for first row d-block transition metals) is attributed to the steric hindrance of the axial ‘yl’ oxygen atoms of the uranyl group. The products, prepared as NH₄⁺ salts, have been characterised by single crystal X-ray diffraction, elemental analysis, TGA analysis, IR, Raman and UV/vis spectroscopy, which indicate that the O donor atoms of the lacunary heteropolytungstate anions are strongly coordinating to U(VI) in the equatorial plane, weakening the uranyl U–O axial bonds.