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 (H 2 O) 2 ] 4 [KBW 12 O 40 ]·9H 2 O ( 1 ), [Al(apca) 2 (H 2 O) 2 ] 8 [SiW 12 O 40 ] 2 ·13H 2 O ( 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 [BW 12 O 40 ] 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 (H 2 O) 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.