Structural study of a bistriazole-based Cd(II) coordination polymer and its application in highly effective detection of TNP and Fe3+ in water and in catalytic CO2 cycloaddition

A new coordination polymer, namely, [Cd(H₂tdpt)(H₂O)]·5H₂O (Cd-CP-1, H₄tdpt = 3-(triazol-5-yl)-5-(3,5-dicarboxy phenyl)triazole) was synthesized under solvothermal conditions and structurally characterized. X-ray single crystal diffraction revealed that Cd-CP-1 exhibited a one-dimensional (1D) ladder-shaped double chain structure, and the chains were further associated into a 3D architecture through π-π stacking and interchain hydrogen bonding (O-H···O) interactions. Cd-CP-1 not only exhibited outstanding chemical and pH stability but also possessed excellent fluorescence properties, which could function as a fluorescent sensor for the selective detection of 2,4,6-trinitrophenol (TNP) and Fe³⁺ in water with low detection limits, and short sensing response times (18 s for TNP, 12 s for Fe³⁺). Moreover, the possible sensing mechanisms were analyzed in detail by using powder X-ray diffraction (PXRD), ultraviolet-visible (UV-vis) absorption spectra, X-ray photoelectron spectra (XPS) and density functional theory (DFT). In addition, the activated Cd-CP-1 (Cd-CP-1a) featured uncoordinated N_triazole and O_carboxylate atoms acting as Lewis base sites and unsaturated Cd²⁺ ions serving as Lewis acid sites, which can enhance the activation of CO₂ and epoxides, respectively. As a result, Cd-CP-1a exhibited high catalytic activity for the cycloaddition reaction of CO₂ with epoxide into cyclic carbonates under mild conditions (1 atm CO₂, 70°C) in the absence of solvent. Moreover, Cd-CP-1 could be reused for at least five cycles without an obvious decrease in catalytic activity. This work provides useful information on the applications of coordination polymers in contaminant detection and CO₂ cycloaddition.