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

• Published in: Chemistry : a European journal Vol. 16; no. 45; pp. 13435 - 13442
• Main Authors: Li, Shu-Jun, Liu, Shu-Xia, Li, Cong-Cong, Ma, Feng-Ji, Liang, Da-Dong, Zhang, Wei, Tan, Rui-Kang, Zhang, Yuan-Yuan, Xu, Lin
• Format: Journal Article
• Language: English
• Published: Weinheim WILEY-VCH Verlag 03-12-2010; WILEY‐VCH Verlag; Wiley Subscription Services, Inc
• Subjects: Bridges (structures); Chemistry; cluster compounds; Derivatives; Disassembly; Dismantling; Dissolution; Monomers; Nanostructure; nanostructures; niobium; polyoxometalates; reactivity; Spectrum analysis
• ISSN: 0947-6539; 1521-3765
• DOI: 10.1002/chem.201001963

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)3O37]⋅6H2O (Cs6.5K0.5‐1) synthesized by the reaction of K7H[Nb6O19] and A‐α‐Na10[GeW9034] in H2O2 solution is a tris(peroxoniobium)‐substituted A‐α‐GeW9 derivative. Cs6.5K0.5[GeW9Nb3O40]⋅ 10H2O (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 Cs5[H15Ge2W18Nb8O88]⋅18H2O (Cs5‐3) and tetramer Cs8K3H9[Ge4W36Nb16O166]⋅ 27H2O (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. 183W 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 183W 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.
Nanoscale polyoxometalates: The reactivity of polyoxoniobates has been studied in acidic solution by grafting niobium onto trivacant Keggin‐type germanotungstates. Monomer 1 or 2 afforded two nanoscale polyoxometalates, dimer 3 and tetramer 4, through the formation of NbONb bridges. Disassembly based on the cleavage of NbONb bonds was achieved by controlling the pH or by adding H2O2 (see figure).