Synthesis and uranyl(VI) extraction performance of a calix[4]pyrrole-tetrahydroxamic acid receptor

The contamination of water by uranium poses a serious threat to ecosystems and human health, creating a need for efficient and selective remediation strategies. Supramolecular materials, with their pre-organized structures, offer a promising route for uranium removal. Phenoxycalix[4]pyrroles (PCP) are well-known supramolecular scaffolds capable of selective metal binding, making them attractive candidates for designing uranium extractants. Here, we report the design and synthesis of PCP HA, a phenoxycalix[4]pyrrole scaffold functionalized with four hydroxamic acid (HA) groups, and evaluate its uranium(VI) extraction potential. PCP HA was synthesized from its ester precursor (PCP E) via hydroxyaminolysis using KOH, achieving a 95% yield. Its structure was confirmed by ¹H NMR, ¹³C NMR, and HRMS. The uranium(VI) extraction efficiency of PCP HA was evaluated by solid-liquid extraction experiments, using uranyl acetate as the uranium source, with measurements performed by gamma spectroscopy. PCP HA demonstrated good performance, removing up to 95% of uranyl(VI) from aqueous solutions (1 mM) at acidic pH, likely due to the strong coordination provided by its hydroxamic acid groups. Further studies revealed that the extraction efficiency also depends on the ligand-to-metal molar ratio. These findings establish PCP HA as a promising supramolecular material for the removal of uranyl from aqueous media.