A good proton-conductive polyoxometalate complex {[Co(H2O)8] [H (H2O)3] (HINO)4 (PW12O40) }n: synthesis, crystal structure and properties

Abstract

One organic/inorganic hybrid complex {[Co(H2O)8] [H(H2O)3] (HINO)4 (PW12O40)}n, was constructed by a self-assembly of H+(H2O)3 clusters, [Co (H2O)8]2+ clusters, [PW12O40]3- anions and isonicotinic acid N-oxide (HINO). It was characterized by X-ray diffraction analysis, elemental analysis, IR, TG, UV, XRD, proton conductivity analysis. In the synthetic procedure of title complex, a mixture of result heteropolyacid cobalt salts CoHPW12O40 • nH2O (90 mg, 0.03 mmol) and HINO (17 mg, 0.12 mmol) was dissolved in enough acetonitrile/water (1:1, V/V) to form a homogeneous solution. Finally, the solution was filtered and the solvent left to evaporate at room temperature. A week later, light red crystals appeared and were collected and dried in air after quickly being washed with water. Single-crystal X-ray diffraction analysis at 293 K showed that complex 1 crystallizes in orthorhombic crystal system, Pnnm, and there was exactly a three-dimensional (3D) hydrogen-bonded network with large one-dimensional (1D) channels, in which [PW12O40]3- anions just filled in the 1D channels and self-assembled into poly-Keggin-anion chains. The result of the UV spectra indicates that the saturated aqueous concentration of 1 is ca. 10-5 mol/L and it is insoluble in water. Thermogravimetric analysis shows no weight loss in the temperature range of 20~100°C, indicating that all water molecules in the structure are not easily lost below 100°C. The powder X-ray diffraction data exhibits that although crystallized water molecules are lost at 200°C, the evacuated complex has the same structure as the single-crystal of 1. Moreover, the powder sample after the proton-conductive measurement has the same supramolecular framework as that of complex 1. Interestingly, the title complex was characterized by a satisfactory ionic conductivity (1 × 10-3~2 × 10-3 S • cm-1) in the temperature range of 85 to 100°C only. The mechanism of proton conduction of the title complex is, expected to be similar to that of the vehicle mechanism, that is, the direct diffusion of additional protons with water molecules (Grotthuss mechanism). © 2013 Shanghai Institute of Organic Chemistry, Chinese Academy of Sciences.