Selective Am(III)/Eu(III) Separation by C–N Coupling Phenanthroline-Derived Bis(pyrazole) Ligands

The separation of trivalent actinides (An(III)) from lanthanides (Ln(III)) is a critical challenge in nuclear waste management, as it enables the reduction of long-term radiotoxicity and supports advanced fuel cycle strategies. Herein, we report a series of tetradentate N-donor ligands (L1-L4) derived from a straightforward C-N coupling reaction between 2,9-dichloro-1,10-phenanthroline (Phen) and pyrazole (Pz) derivatives, i.e., 2,9-bis(1H-pyrazol-1-yl)-1,10-phenanthroline (L1), 2,9-bis(4-ethyl-1H-pyrazol-1-yl)-1,10-phenanthroline (L2), 2,9-bis(4-butyl-1H-pyrazol-1-yl)-1,10-phenanthroline (L3), and 2,9-bis(3-butyl-1H-pyrazol-1-yl)-1,10-phenanthroline (L4). These ligands exhibit exceptional Am(III) extraction efficiency and selectivity, achieving separation factors (SF_Am/Eu) exceeding 220 in highly acidic HNO₃ media. Slope analysis and electrospray ionization mass spectrometry (ESI-MS) confirmed a 1:1 M/L stoichiometry for the extracted species. Systematic UV-vis titration and single-crystal X-ray diffraction studies revealed an analogous 1:1 coordination structure in solution and solid states. The stability constants for Eu(III) followed the trend of L3 (5.92 ± 0.08) > L2 (5.75 ± 0.11) > L1 (5.56 ± 0.06) > L4 (5.31 ± 0.02), reflecting the interplay of inductive effects and steric hindrance imposed by alkyl substituents on the pyrazole moieties of the ligands. This work highlights the potential of C-N-coupled bis(pyrazole)phenanthroline ligands as highly selective extractants for An(III)/Ln(III) separation under acidic conditions.