Proton-Rich POM-Type K₁₂Mo₈O₂₀(HPO₄)₈(PO₄)Cl with Ion-Exchange Capabilities

A proton-rich POM-type molybdenum phosphate K₁₂Mo₈O₂₀(HPO₄)₈(PO₄)Cl was successfully obtained. It crystallizes in a noncentrosymmetric tetragonal space group of P-4 (No. 81) with the unit cell parameters of a = 9.6580(4) Å, c = 14.2607(10) Å, and Z = 1. The occurrence and positions of the light element H in the structure are inferred from single-crystal X-ray diffraction and confirmed by DFT calculations. The hydrogen atoms are found to form hydroxyl bonds with O atoms from P(2)O₄ and P(3)O₄ constituting the [Mo₄P₄O₂₆H₄]^4- layers but are only weakly bound to the isolated P(3)O₄ group through hydrogen bonds. The title compound presents a POM-type framework of corrugated [Mo₄P₄O₂₆H₄]^4- layers with four K⁺ ions and mixed ions (K₄Cl³⁺ and isolated PO₄^3-) orderly imbedding in the interlayer spaces with distances of 5.0396 (1) and 5.5966 (3) Å, respectively. The proton-rich nature and the structure feature were further verified by a series of experiments including ¹H, ⁷Li, and ³¹P MAS NMR spectra, IR spectroscopy, and thermal analysis. Moreover, the weak bonding and large interlayer spaces make K⁺ and H⁺ ions susceptible to exchange with ions of Cs⁺, Ba²⁺, Zn²⁺, Pb²⁺, Cu²⁺, and Ni²⁺ commonly presented in chemical pollutants or nuclear wastes. In addition, the title compound shows a small second-harmonic generation signal, consistent with its noncentrosymmetric structure.