Polymolybdate-Based Complexes Containing Flexible Metal–Organic Windmills with Pseudocapacitive Activity and Electrochemical Sensing Performance

Two novel semirigid bis(pyridine)-bis(amide) ligands featuring a pyrazole group as a linker, N,N'-bis(3-picolylamine)-1-hydropyrazole (3-Hdpap), and N,N'-bis(3-pyridinamide)-1-hydropyrazole (3-Hdpyp) were used to combine with polymolybdates (POMos) and a cobalt ion under solvothermal conditions. Four different POMo-based metal-organic complexes (POMoOCs) [Co(3-Hdpap)(β-Mo₈O₂₆)_0.5(H₂O)] (1), [Co(3-H₂dpap)(TeMo₆O₂₄)_0.5(H₂O)₂] (2), H{Co(3-H₂dpap)[AlMo₆(OH)₅O₁₉](H₂O)₂} (3), and H₂{Co(3-Hdpyp)[CrMo₆(OH)₅O₁₉](H₂O)₂}·2H₂O (4) were obtained and structurally characterized by single-crystal X-ray diffraction, infrared spectroscopy, and powder X-ray diffraction. Various metal-organic windmill configurations can be observed in the title complexes due to different POMos and ligands, finally resulting in diverse POMo-based architectures, including a two-dimensional (2D) network (1), one-dimensional (1D) supramolecular chains (2-3), and a 2D supramolecular network (4). The electrochemical performances of four complexes were studied in the three-electrode system with their corresponding modified electrodes. At the current density of 1 A·g^-1, the specific capacitance values for the glassy carbon electrode modified by 1-4 were recorded as 1658.3, 1026.5, 647.8, and 832.2 F·g^-1, respectively. 1-GCE exhibits the best pseudocapacitive activity and is superior to most reported polyoxometalate-based complexes. After 1000 cycles, its capacitance retention rate reaches 89%. Furthermore, the carbon paste electrode bulk-modified by complex 1 serves as an electrochemical sensor for the detection of Cr(VI) and exhibits a low detection limit.