Encapsulation of Two Potassium Cations in Preyssler-Type Phosphotungstates: Preparation, Structural Characterization, Thermal Stability, Activity as an Acid Catalyst, and HAADF-STEM Images

Dipotassium cation (K⁺)-encapsulated Preyssler-type phosphotungstate, [P₅W₃₀O₁₁₀K₂]^13‑, was prepared by heating monobismuth (Bi³⁺)-encapsulated Preyssler-type phosphotungstate, [P₅W₃₀O₁₁₀Bi­(H₂O)]^12‑, in acetate buffer in the presence of an excess amount of potassium cations. Characterization of the isolated potassium salt, K₁₃[P₅W₃₀O₁₁₀K₂] (1a), and its acid form, H₁₃[P₅W₃₀O₁₁₀K₂] (1b), by single crystal X-ray structure analysis, ³¹P and ¹⁸³W nuclear magnetic resonance (NMR), Fourier transform infrared (FT-IR) spectroscopy, cyclic voltammetry (CV), high-resolution electrospray ionization mass spectroscopy (HR-ESI-MS), and elemental analysis revealed that two potassium cations are encapsulated in the Preyssler-type phosphotungstate molecule with formal D_5h symmetry, which is the first example of a Preyssler-type compound with two encapsulated cations. Incorporation of two potassium cations enhances the thermal stability of the potassium salt, and the acid form shows catalytic activity for hydration of ethyl acetate. Packing of the Preyssler-type molecules was observed by high-resolution high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM).