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.