Polyoxometalates decorated with metal-organic moieties as new molecular photo- and electro-catalysts

Three new polyoxometalate-based organic-inorganic hybrid compounds (POIHCs), [Cu(PBI)2(H2O)][{Cu(PBI)(OH)(H2O)}{PW12O40}]·5H2O (1), [Cu(PBI)2(H2O)][{Cu(PBI)2(H2O)}{SiW12O40}]·4H2O (2) and [Cu4(HPO4)(PO4)(H2O)2(PBI)4][PMo12O40]·H2O (3) (PBI =2-(pyridin-2-yl)-1H-benzo[d]imidazole), have been synthesized under hydrothermal conditions. All three compounds were characterized by elemental analysis, IR spectroscopy, thermogravimetric analysis, powder X-ray diffraction and single-crystal X-ray diffraction. In 1 and 2, the Keggin-type polyoxometalate (POM) units are decorated by the {Cu/PBI} metal-organic fragments, together with additional {Cu/PBI} isolated cations, which further stack into 3-D supramolecular frameworks with porous structural features. Compound 3 consists of a rare cationic cluster [Cu4(HPO4)(PO4)(H2O)2(PBI)4]3+ and the Keggin-type POM anion, forming a 3-D supramolecular framework with limited interspaces for the entry of guest molecules. Comparable photocatalytic properties of 13 were investigated by the use of methylene blue (MB) and Rhodamine B (RhB) degradation under UV light. All compounds showed good catalytic activity for the degradation of MB in contrast to RhB. The possible catalytic mechanism was discussed based on the size of dye molecules, the low adsorption ability and crystal packing mode of the as-synthesized compounds. Additionally, the electrochemical properties of 13 have also been investigated, exhibiting good electrocatalytic performance for nitrite reduction.