A series of polyoxometalate compounds by tuning N sites and numbers of ligands: syntheses, characterization and electrochemical sensing, and photocatalytic and supercapacitor properties

Abstract

By using different morpholine derivatives with different N sites and numbers, we have synthesized seven compounds by a hydrothermal method, namely [Ag^I₂(HL1)][Cr(OH)₆Mo₆O₁₈]·H₂O (1), [Ag^I(H₂L)][Cr(OH)₆Mo₆O₁₈]·4H₂O (2), [Co^II(HL1)₂(H₂O)][Cr(OH)₆Mo₆O₁₈]₂·2H₂O (3), [Ni^II(HL2)(H₂O)][Cr(OH)₆Mo₆O₁₈]·4.5H₂O (4), [Co^II(HL2)(H₂O)][Cr(OH)₆Mo₆O₁₈]·4.5H₂O (5), [Ag^I(HL1)][(δ-Mo₈O₂₆)_0.5]·2H₂O (6) and [Co^II(HL3)₂(H₂O)₂](β-Mo₈O₂₆) (7) (L = 4-(2-aminoethyl)-morpholine, L1 = 4-[2-(5-pyridin-4-yl-[1,2,4]triazol-1-yl)-ethyl]-morpholine, L2 = 4-[2-(5-pyridin-2-yl-[1,2,4]triazol-1-yl)-ethyl]-morpholine, L3 = 4-[2-(5-pyrazin-2-yl-[1,2,4]triazol-1-yl)-ethyl]-morpholine, L = 4-[2-(dimethylamino)ethyl]morpholine). These compounds were characterized by single crystal X-ray diffraction, FT-IR and PXRD. Compound 1 exhibits a 3D framework constructed by 1D chains. Compounds 2 and 4–6 all show 2D structures, in which the 2D layer of 2 is only built by POM and transition metals (TMs). But in 4–6 both the POM and metal–organic subunits construct the layer structures. Compounds 3 and 6 are 1D structures, containing a POM-linked chain in 3 and only a metal–organic chain in 6. Compounds 1–7 can be used as bifunctional electrochemical sensors. They can detect not only NO₂⁻ anions, but also Cr(VI) and Fe(III) cations. For example, the response time of 1-CPE for Cr(VI) was 2.16 s and the detection limit was 4.3 × 10⁻³ M. Compound 4 also has certain capacitance performance. In addition, compounds 1–7 can be used as catalysts for the photodegradation of organic dyes and the reduction of Cr(VI).