Synthesis and Characterization of Water-Soluble, All-Inorganic Composition, Dawson-Type Trisubstituted Heteropolytungstates. Effect of Alkali Metal Countercations (Li, Na, K, and Cs) on the P2W15Nb3O629- Polyoxoanion

Article abstract of DOI:10.1246/bcsj.70.1369

The water-soluble form of the Dawson-type trisubstituted tungstoheteropolyanion, B-α-P₂W₁₅Nb₃ O₆₂^9-was synthesized as its nona alkali-metal salts of Li, K, and Cs. Analytically pure compounds, obtained as homogeneous colorless solids, had nonahydrate for the nonalithium (Li₉) salt, tetrahydrate for the nonasodium (Na₉) salt, trihydrate for the nonapotassium (K₉) salt and trihydrate for the nonacaesium (Cs₉) salt, respectively, after each was dried overnight at room temperature under 10^-3 - 10^-4 Torr. TG/DTA analyses show that they can be made with 25 water for the Li₉ salt, 23 water for the Nag salt, 9 water for the K₉ salt, and 8 water for the Cs₉ salt under atmospheric conditions. A compositional characterization was accomplished by complete elemental analyses and TG/DTA, and structural characterization was achieved by a combination of FT-IR and variable-temperature solid-state ³¹PNMR and by both ³¹P and ¹⁸³W NMR measurements at room temperature in D₂O and DMSO-d₆ solutions. The amount of hydrated waters, solubility in solvents and ease of crystallization were significantly influenced by the alkali-metal countercations. These cation-dependent properties are useful for selecting polyoxoanion precursor favorable to the crystallization of P₂W₁₅Nb₃O₆₂^9- -supported organometallic complexes. The effects of the cations (Li, K, Na, and Cs) on the P₂W₁₅Nb₃O₆₂^9- were observed as different hydrated structures of the polyoxoanion and its cation-dependent thermal stability in the solid state by a combination of TG/DTA and FT-IR measurements, and also as a spectral change of the downfiled ³¹P signal due to interactions of the polyoxo- anion with the countercations and/or with hydrated waters by variable-temperature (VT) solid-state GHD/MAS ³¹P NMR spectroscopy.