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-encapsulated Preyssler-type phosphotungstates (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 D_5h 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.