A series of pH-dependent POM-based compounds as photocatalysts and electrochemical sensors

Abstract

By tuning pH, a Zn complex and five POM-based compounds decorated by a pyrazole derivative 1,1′-bis(3,5-dimethyl-1H-pyrazol-4-yl)methane (H2bdpm), [Ag4(H2bdpm)4(SiW12O40)]·4H2O (1), [Ag3(H2bdpm)3(PMo12O40)] (2), [Cu(H2bdpm)2(H2O)2(HPMo12O40)] (3), [Zn(H2bdpm)2(β-Mo8O26)0.5]·3H2O (4), [(ZnO)2(H2O)2(H2bdpm)] (5) and [(γ-Mo8O26)(H2bdpm)2]·2H4bdpm·4H2O (6), have been hydrothermally synthesized and characterized by single-crystal X-ray diffraction analysis, IR spectra and elemental analyses. In compound 1, the infinite wave-like metal–organic chains are linked by POM anions to form a 3D framework. In the compound 2, the H2bdpm ligands are linked by Ag+ ions to construct a chain with [PMo12O40]3− anions suspending up and down this chain by hydrogen bonding interactions. In compound 3, there is a mono-supporting [PMo12O40]3− anion with one [Cu(H2bdpm)2(H2O)2]2+ subunit. By tuning pH of [Mo7O24]6−/Zn2+/H2bdpm system with three values, three distinct structures 4–6 were constructed. In compound 4, two Zn ions and two H2bdpm form a bi-nuclear [Zn2(H2bdpm)2]4+ cycle. Adjacent cycles connect each other vertically to build a cycle-connecting-cycle chain. The discrete [β-Mo8O26]4− anions exist hydrogen bonding interactions with metal–organic chains. In compound 5, the cycle-like [(ZnO)4(H2O)4] subunits are linked by a pair of H2bdpm alternately to form a 1D chain. Compound 6 contains only a [γ-Mo8O26]4− anion functionalized by two H2bdpm through Mo-N bonds. The pH factor is essential for construction of this series. The 1-, 3- and 4-CPEs not only possess excellent multifunctional electrocatalytic activities but also can be used as electrochemical sensors for the detection of hydrogen peroxide, bromate and nitrite. Photocatalytic investigation indicates that compounds 1–4 and 6 are active for the degradation of methylene blue under UV irradiation.