A series of polyoxometalate compounds: syntheses, characterization, electrochemical sensing and photocatalytic properties†

The degradation of organic dyes under visible light is an effective method to reduce environmental pollution. The key to implementing this method is to develop good photocatalysts. Under hydrothermal conditions, four polyoxometalate-based compounds were prepared, namely, [Co₂(TPTP)₄(H₂O)₄(SiMo₁₂O₄₀)]·4H₂O (1), [Ni₂(TPTP)₄(H₂O)₄(SiMo₁₂O₄₀)]·2.5H₂O (2), {[Cu(H₂TPTP)(TPTP)(H₂O)₃]₂(SiMo₁₂O₄₀)₂}·7.5H₂O (3), and {Cu₂(TPTP)₂(β-Mo₈O₂₆)_1/2}·H₂O (4) (TPTP = 4-[2-(5-pyridyl-4-yl-1,2,4-triazol-1-yl)-methyl]-pyran). Compounds 1 and 2 are isostructural, showing a two dimensional (2D) metal–organic layer. The Keggin anions are discrete between adjacent layers. Compound 3 contains a bi-supporting Keggin anion and a discrete anion. The TPTP ligands in 4 were linked by Cu atoms to form ladder like chains, which are connected by β-Mo₈O₂₆⁴⁻ anions to construct a 2D layer. The compounds can be used as photocatalysts. They have good separation efficiency of photogenerated electron–hole pairs. When compounds 1 and 2 were used as photoelectrocatalysts, they exhibited good photoelectrocatalytic activities. The compounds also exhibit excellent electrochemical performance and can be used as electrochemical sensors for the selective recognition of NO₂⁻ and Cr(VI).