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

Abstract

Abstract The water-soluble form of the Dawson-type trisubstituted tungstoheteropolyanion, B-α-P2W15Nb3O629−, 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 Na9 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 31P NMR 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 P2W15Nb3O629−-supported organometallic complexes. The effects of the cations (Li, K, Na, and Cs) on the P2W15Nb3O629− 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 polyoxoanion with the countercations and/or with hydrated waters by variable-temperature (VT) solid-state GHD/MAS 31P NMR spectroscopy.