Asymmetric Phenanthroline-Core Ligands with Pyrazole and Amide Groups for Selective Separation of Trivalent Americium from Europium.

Selective separation of trivalent minor actinides (An3+) from lanthanides (Ln3+) in high-level liquid waste is essential to reduce the long-term radiotoxicity of nuclear waste and enable advanced fuel cycles. Herein, the tetradentate chelators N-ethyl-N-(p-tolyl)-9-pyrazol-1-yl-1,10-phenanthroline-2-carboxamide (L1) and 9-(4-tert-butylpyrazol-1-yl)-N-ethyl-N-(p-tolyl)-1,10-phenanthroline-2-carboxamide (L2), two asymmetric chelators with a phenanthroline core and flanking pyrazole and amide groups, are presented as potential ligands for Am3+ separation from Ln3+ under acidic conditions. Benefiting from the "anomalous aryl strengthening" effect on amide groups and the high acid stability of C-N pyrazole groups, the ligands exhibit improved extraction performance for Am3+. Owing to the electron-donating effect of the substituent on the pyrazole ring, L2 exhibits better extraction performance than L1, achieving a distribution ratio for Am3+ (DAm) of 3.3 and a separation factor (SFAm/Eu) of 35.5 at 3.0 mol/L HNO3. Both ligands form 1:1 complexes with Am3+/Eu3+, which were comprehensively characterized through slope analysis, UV-vis titration, fluorescence titration, X-ray crystallography, and DFT simulations. Theoretical simulations confirm that the stronger covalent interactions between pyrazole N donors and Am3+ account for the observed selectivity. This work validates the construction of asymmetric chelators with varied pendant groups as a powerful strategy for performance tuning.