Ascend 400 spectrometer

¹H NMR spectra were recorded at 400 MHz on a Bruker Ascend 400 spectrometer (Bruker Daltonics Inc., Billerica, MA, USA) with tetramethylsilane as the internal standard. The Fourier transform infrared (FTIR) spectroscopy was performed on a Nicolet iS5FTIR spectrometer equipped with an attenuated total reflectance (ATR) accessory. The samples were first mixed with dried KBr before analysis and the spectrum of each sample was obtained in the range of 4000–500 cm⁻¹. The surface morphology of hydrogels was observed with a scanning electron microscope JSM-6490LV (JEOL, Tokyo, Japan). The XPS measurements were conducted on an ESCALAB 250Xi spectrometer. An atomic absorption spectrometer (AAS) was applied for the determination of the metal ions in the aqueous medium.

The purity of all the compounds tested was above 95%. The synthesis of ATC (compound 1) and compounds 2 to 5 has been described elsewhere (38 ). The synthesis of compounds 6 to 9 is described in the Supplementary Materials. For synthesis, all reactions were carried out using commercially available starting materials (Sigma-Aldrich, Schnelldorf, Germany; BioFine International Inc., Vancouver, Canada; Fluorochem, Hadfield, UK) and solvents without further purification. Column chromatography was performed with an automated Isolera One high-performance flash chromatography system (Biotage, Uppsala, Sweden) using a 0.1-mm path length flow cell ultraviolet (UV) detector/recorder module (fixed wavelength, 254 nm). Analytical thin-layer chromatography was carried out using 0.2-mm silica gel plates (silica gel 60, F₂₅₄; Merck KGaA, Darmstadt, Germany). NMR spectra (¹H NMR and ¹³C NMR) were recorded on a Bruker Ascend 400 spectrometer (Bruker Corporation, Billerica, MA, USA). ¹H NMR was measured at 400 MHz, and ¹³C NMR was measured at 100 MHz. High-resolution mass spectra (HRMS) were measured on a Waters Synapt G2 mass spectrometer (Waters Corporation, Milford, MA, USA) and reported for the molecular ions [M + H]⁺.

The synthesis processes of the intermediates and target compounds were supervised through thin-layer chromatography (TLC). Using an XT-4 binocular microscope (Beijing Tech Instrument Co., Beijing, China), the melting points were measured. ¹³C and ¹H NMR spectra were obtained by a Bruker Ascend–400 spectrometer (Bruker, Karlsruhe, Germany). The HRMS data were acquired by a Thermo Scientific Q Exactive (Thermo Scientific, Waltham, MA, USA).
General Procedures for the Preparation of Compounds 1–26. The different oxadiazole intermediates 1a–2a were synthesized according to known methods [23 (link)]. Different acids containing aromatic groups were added to SOCl₂ (5–8 mL) and refluxed for 5–8 h to obtain intermediates 1b–17b. Next, the triethylamine (3.8 mmol) and intermediates 1a–2a (2.5 mmol) were added to CH₂Cl₂ in ice–water bath conditions. The intermediates 1b–17b were added and stirred for 2.5–5 h. Then, the intermediates 1c–26c were obtained by silica gel column chromatography. Finally, the intermediates 1c–26c (1.5 mmol), (NH₄)₆Mo₇O₂₄·4H₂O (0.3 mmol), and hydrogen peroxide solution (30%, 15 mmol) were admixed and stirred at 25 °C for 3–8 h. The saturated ice saltwater was added to the wash, and the target compounds 1–26 were obtained with a yield of 52–89% (Figure 4). The details of compounds 1–26 can be found in the Supplementary Materials.

To make the ¹³C and ¹H NMR spectra, a Bruker Ultrashield 300 MHz spectrometer (Bruker, Billerica, MA, USA) was used and the ³¹P and ²⁹Si spectra were recorded on a Bruker Ascend 400 spectrometer (Bruker, Billerica, MA, USA). A CDCl₃ solvent was used to make all spectra.