Reactivity of Polyoxoniobates in Acidic Solution: Controllable Assembly and Disassembly Based on Niobium-Substituted Germanotungstates

The reactivity of polyoxoniobates has been studied in acidic solution by grafting niobium onto trivacant Keggin-type germanotungstates. Four niobium-containing compounds were obtained in the course of this study. Cs6.5K0.5[GeW9(NbO2)(3)O-37]center dot 6H(2)O (Cs6.5K0.5-1) synthesized by the reaction of K7H[Nb6O19] and A-alpha-Na-10[GeW(9)0(34)] in H2O2 solution is a tris(peroxoniobium)-substituted A-alpha-GeW9 derivative. Cs6.5K0.5[GeW9Nb3O40]center dot 10H(2)O (Cs6.5K0.5-2) is a peroxo-free compound obtained by eliminating the peroxo groups in 1. Monomers 1 and 2 as precursors can each afford two nanoscale POMs, dimer Cs-5[H15Ge2W18Nb8O88]center dot 18H(2)O (Cs-5-3) and tetramer Cs8K3H9[Ge4W36Nb16O166]center dot 27H(2)O (Cs8K3H9-4), through the formation of Nb-O-Nb bridges. Disassembly through the cleavage of Nb-O-Nb bonds from 4 to 2 and 1 was achieved by controlling the pH and by adding H2O2, respectively. The transition from 1 to 2 can be achieved by simply adding H2O2 to a solution of 1. All four compounds were characterized in the solid state by elemental analysis, infrared spectroscopy, thermogravimetry, and single-crystal X-ray diffraction. W-183 NMR analysis proved that the solid-state structures of polyanions 1-4 were retained after dissolution. Disassembly from 4 to 1 and 2 in solution was observed by W-183 NMR spectroscopy. The UV/Vis spectra of 1 at different pH confirmed that it is stable in the pH range of 0.1-14.0 at room temperature.