Two new supramolecular compounds with reversible skeleton structure changes during dehydration and rehydration: Synthesis and characterization

Abstract

Two novel supramolecular compounds of Keggin-type polyoxometalate (POM), [Cr(apca)2(H2O)2]4[KBW12O40]·9H2O (1), [Al(apca)2(H2O)2]8[SiW12O40]2·13H2O (2); apca = 3-aminopyrazine-2-carboxylic acid), were synthesized in aqueous solution and characterized by single-crystal X-ray diffraction technique, elemental analysis, TG analyses, IR, X-ray powder diffraction and fluorescence spectroscopy. In compound 1 [BW12O40]5− anion acting as octadentate ligand coordinates to two K ions through weak interaction, forming an infinite anionic chain extending along c–axis. The chains locate in the channels formed by stacking of the complex cations [M(apca)2(H2O)2]+. Hydrogen bonds combine the chains, cations and lattice water molecules forming a 3D framework. Besides the hydrogen bonds there exists π-π interaction between the complex cations. Compound 2 has a similar structure except without the K+ ion. They show a high proton conductivity of 1.70 × 10−3 S cm−1 and 9.30 × 10−3 S cm−1 at 77° C under 97% RH, respectively. Compound 1-2 showed reversible skeleton structure changes upon dehydration and rehydration, which had been established and monitored by X-ray powder diffraction.