Four new dual-functional electro-catalysts formed from small molybdenum clusters and Cu-pyridyl complexes

By changing N-heterocyclic ligands in the same Mo₇/Cu/N-ligand reaction systems, four new organic-inorganic hybrids based on isopolymolybdates, [Cu₂(tpy)₂(β-Mo₈O₂₆)_0.5(γ-Mo₈O₂₆)_0.5]·0.25H₂O (1), [Cu₂(tpy)₂(H₂O)₂(β-Mo₈O₂₆)] (2), [Cu(bpy)(Mo₃O₁₀)]·H₂O (3), and [Cu(bpy)(H₂O)(β-Mo₈O₂₆)_0.5]_0.5 (4) (tpy = 2,2':6',2''-terpyridine and bpy = 2,6-bis(pyrazol-1-yl) pyridine), were prepared using hydrothermal methods at different pH values. X-ray structural analysis shows that compound 1 has a 1D {-β-[Mo₈O₂₆]-Cu₂-γ-[Mo₈O₂₆]}_n straight chain structure with mixed β-[Mo₈O₂₆] and γ-[Mo₈O₂₆] polyoxoanions; compound 2 possesses a 3D supramolecular structure based on [Cu(tpy)]²⁺ motifs and β-[Mo₈O₂₆] clusters; and compound 3 has a 1D chain structure built from [Cu(bpy)]²⁺ and [Mo₃O₁₀]^2- units. In compound 4, [β-Mo₈O₂₆] clusters are linked by [Cu(bpy)]²⁺ motifs to give rise to a 2D sheet structure including {(β-Mo₈O₂₆)₄Cu₄} 8-membered rings. Cyclic voltammograms of compounds 1-4 display discrepant dual-functional electro-catalytic activities toward the reduction of nitrite and the oxidation of ascorbic acid in acidic solution. Electrocatalytic tests indicate that the [Mo₃O₁₀]^2--based organic-inorganic hybrid exhibits better electro-catalytic performances than [Mo₈O₂₆]^4--type hybrids towards oxidation and reduction.