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.

本研究选用两类以吡唑基团作为连接臂的新型半刚性双吡啶双酰胺配体——N,N'-双(3-吡啶甲基胺)-1-氢吡唑（3-Hdpap）与N,N'-双(3-吡啶甲酰胺)-1-氢吡唑（3-Hdpyp），在溶剂热条件下与多金属氧酸盐（polymolybdates, POMos）及钴离子进行配位组装，成功得到四种多金属氧酸盐基金属有机配合物（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）与H2{Co(3-Hdpyp)[CrMo6(OH)5O19](H2O)2}·2H2O（4）。通过单晶X射线衍射、红外光谱及粉末X射线衍射对上述配合物完成结构表征。受所采用的多金属氧酸盐与配体种类差异影响，标题配合物中可观察到多种金属有机风车状构型，最终构筑得到多样化的多金属氧酸盐基骨架，包括二维（2D）网络（1）、一维（1D）超分子链（2~3）以及二维（2D）超分子网络（4）。采用三电极体系及对应修饰电极，对四种配合物的电化学性能进行系统研究：在1 A·g⁻¹的电流密度下，1~4修饰的玻碳电极的比电容值分别为1658.3、1026.5、647.8与832.2 F·g⁻¹。其中1修饰玻碳电极（1-GCE）展现出最优的赝电容性能，优于多数已报道的多金属氧酸盐基配合物；经1000次循环测试后，其电容保留率可达89%。此外，以配合物1本体修饰的碳糊电极作为电化学传感器，可用于六价铬（Cr(VI)）的检测，且表现出较低的检出限。