Two 2D Polyoxometalate-Based Metal-Organic Complexes Constructed from a New Flexible N-Donor Ligand with High Capacitance Performances and Low Detection Limits of Cr(VI)

By designing and using a new flexible bis(pyrimidine)-bis(amide) ligand H₂L H₂L=N,N′-bis(4-pyrimidinecarboxamido)-1,3-propane], two new polyoxometalate (POM)-based metal-organic complexes (MOCs), H₃Cu₂(L)₂(PMo₁₂O₄₀)] ( 1 ) and Cu₂(H₂L)₂(β-Mo₈O₂₆)] ( 2 ), were synthesized under solvothermal and hydrothermal conditions, respectively. In complexes 1 and 2 , metal-organic units and POM anions are linked together to form two distinct 2D structures. The PMo₁₂O₄₀]³⁻ (PMo₁₂) anions were used as μ₄-bridging ligands in complex 1 and linked the 1D Cu(L)]_n metal-organic chains to form a 2D layered structure. The β-Mo₈O₂₆]⁴⁻ (Mo₈) anions adopted two diverse coordination modes in complex 2 and connected the 1D Cu(H₂L)]_n²ⁿ⁺ metal-organic chains to generate a 2D grid structure. Complexes 1 – 2 can serve as electrode materials of supercapacitor and show large specific capacitances, up to 1065 and 956 F g⁻¹ at current density of 2 A g⁻¹, respectively, which surpass the parent POM and most of the previous reported POM-based electrode materials, thus demonstrating the important role of introducing metal-organic units in improving capacitive performances. Besides, the electrocatalytic redox activities of complexes 1 – 2 were also studied, both of them can be used as electrochemical sensors to detect Cr(VI) ions. They possess high sensitivity of 0.537 and 0.455 μA μM⁻¹ and low detection limits of 0.16 and 0.33 μM, which are below the maximum content of Cr(VI) in groundwater required by the WHO.