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

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 = SbIII or TeIV) 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.