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.