Tetrameric, Tri-Titanium(IV)-Substituted Polyoxometalates with an α-Dawson Substructure as Soluble Metal Oxide Analogues. Synthesis and Molecular Structure of Three Giant “Tetrapods” Encapsulating Different Anions (Br−, I−, and NO3−)

Abstract

Abstract Preparation and structural characterization of giant “tetrapod”-shaped polyoxometalates (POMs) with approximately Td symmetry, [{α-P2W15Ti3O59(OH)3}4{μ3-Ti(H2O)3}4X]21− (X=Br− (1a), I− (2a), and NO3− (3a)), consisting of four tri-titanium(IV)-substituted α-Dawson subunits, four octahedral Ti(H2O)3 bridging groups and one encapsulated anion X, are described. Sodium salts of three giant “tetrapod”-shaped POMs encapsulating different anions 1–3 were prepared as analytically pure crystals in 1:5 molar-ratio reactions in water of the tri-lacunary Dawson POM with the in situ-generated “TiX4” (X=Br−, I−, and NO3−), which were prepared by the reactions of Ti(SO4)2 with BaX2 in aqueous solution [Note: The polyoxoanion is represented by 1a, 2a, 3a, …, and the whole formula, which contains counter ions, polyoxoanion, hydrated waters, and so on, is represented by 1, 2, 3, …]. The compounds were characterized by using elemental analysis, thermogravimetric and differential thermal analyses (TG/DTA), solid-state and solution IR, solid-state and solution 31P NMR spectroscopy, and X-ray crystallography. Molecular structures of 1a–3a are analogous to the previously prepared POM with X=Cl− (4a). The fact that encapsulation of the different anions (Cl−, Br−, I−, and NO3−) was achieved in the central cavity of the giant “tetrapod”-shaped POM shows the cationic character of the central framework constituted by the protonated Ti–O–Ti bonds, i.e., the Ti–OH–Ti bonds. The solid-state and solution 31P NMR measurements showed that 1a–3a were stable in the solid-state and in DMSO, but in water, they partially decomposed. These properties of 1a–3a in water are in contrast to that of 4a.