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 = 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.