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), H-3[Cu-2(L)(2)(PMo12O40)] (1) and [Cu-2(H2L)(2)(beta-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 mu(4)-bridging ligands in complex 1 and linked the 1D [Cu(L)](n) metal-organic chains to form a 2D layered structure. The [beta-Mo8O26](4-) (Mo-8) anions adopted two diverse coordination modes in complex 2 and connected the 1D [Cu(H2L)](n)(2n+) 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 mu A mu M-1 and low detection limits of 0.16 and 0.33 mu M, which are below the maximum content of Cr(VI) in groundwater required by the WHO.