Encapsulation of Anion/Cation in the Central Cavity of Tetrameric Polyoxometalate, Composed of Four Trititanium(IV)-Substituted α-Dawson Subunits, Initiated by Protonation/Deprotonation of the Bridging Oxygen Atoms on the Intramolecular Surface

Preparation and structural characterization of a novel polyoxometalate (POM), [(P2W15Ti3O60.5)4(NH4)]35-1, i.e., an encapsulated NH4+ cation species in the central cavity of a tetramer (called the Dawson tetramer) constituted by trititanium(IV)-substituted α-Dawson POM substructure, are described. POM 1 was synthesized by several different methods and unequivocally characterized by complete elemental analysis, thermogravimetric and differential thermal analysis (TG/DTA), FTIR spectroscopy, solution (15N{1H}, 31P, 183W) NMR spectroscopy, and X-ray crystallography. First, POM 1 was synthesized by a reaction of NH4Cl in aqueous solution with a precursor, which was derived by thermal treatment of a monomeric triperoxotitanium(IV)-substituted Dawson POM, [α-1,2,3-P2W15(TiO2)3O56(OH)3]9-2, for 3 h in an electric furnace at 200 °C. The encapsulated NH4+ cation in 1 was confirmed by 15N{1H} NMR measurement and X-ray crystallography. As another synthesis of 1, a direct exchange of the Cl– anion encapsulated in [{α...