Preparation of Preyssler-type Phosphotungstate with One Central Potassium Cation and Potassium Cation Migration into the Preyssler Molecule to form Di-Potassium-Encapsulated Derivative

A mono-potassium cation-encapsulated Preyssler-type phosphotungstate, [P5W30O110K]14− (1), was prepared as a potassium salt, K14[P5W30O110K] (1a), by heating mono-bismuth- or mono-calcium-encapsulat ed Prey ssler-type p hosphot ungstates (K12[P5W30O110Bi(H2O)] or K13[P5W30O110Ca(H2O)]) in acetate buffer. Characterization of the potassium salt 1a by single-crystal X-ray structure analysis, 31P and 183W nuclear magnetic resonance (NMR) spectroscopy, Fourier transform infrared spectroscopy, high-resolution electrospray ionization mass spectroscopy, and elemental analysis revealed that one potassium cation is encapsulated in the central cavity of the Preyssler-type phosphotungstate molecule with a formal D5h symmetry. Density functional theory calculations have confirmed that the potassium cation prefers the central position of the cavity over a side position, in which no water molecules are coordinated to the encapsulated potassium cation. 31P NMR and cyclic voltammetry analyses revealed the rapid protonation−deprotonation of the oxygens in the cavity compared to that of other Preyssler-type compounds. Heating of 1a in the solid state afforded a di-K+-encapsulated compound, K13[P5W30O110K2](2a), indicating that a potassium counter-cation is introduced in one of the side cavities, concomitantly displacing the internal potassium ion from the center to a second side cavity, thus providing a new method to encapsulate an additional cation in Preyssler compounds.M.S. is grateful for the A-STEP program of the Japanese Science and Technology Agency (JST), and Furukawa Foundation for the Promotion of Technology. X.L. thanks the Spanish Ministry of Science and Innovation (MICINN) (project CTQ2011-29054-C02-01/BQU), the DGR of the Generalitat de Catalunya (grant no. 2014SGR199), and the XRQTC. This work was also supported by the Center for Functional Nano Oxide at Hiroshima University. M.N.K.W. thanks the Indonesian Endowment Fund for Education (LPDP), Ministry of Finance, Republik Indonesia, for a Ph.D. scholarship