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. Cs(6.5)K(0.5)[GeW(9)(NbO(2))(3)O(37)]·6H(2)O (Cs(6.5)K(0.5)-1) synthesized by the reaction of K(7)H[Nb(6)O(19)] and A-α-Na(10)[GeW(9)0(34)] in H(2)O(2) solution is a tris(peroxoniobium)-substituted A-α-GeW(9) derivative. Cs(6.5)K(0.5)[GeW(9)Nb(3)O(40)]·10H(2)O (Cs(6.5)K(0.5)-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)[H(15)Ge(2)W(18)Nb(8)O(88)]·18H(2)O (Cs(5)-3) and tetramer Cs(8)K(3)H(9)[Ge(4)W(36)Nb(16)O(166)]·27H(2)O (Cs(8)K(3)H(9)-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 H(2)O(2), respectively. The transition from 1 to 2 can be achieved by simply adding H(2)O(2) 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. (183)W 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 (183)W 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.