Unsymmetrical Amide-lactam-phenanthroline (ALPhen) Extractant for Selective Separation of Actinides and Lanthanides

The phenanthroline diamide (DAPhen) extractants demonstrate remarkable selectivity in trivalent actinide/lanthanide separation. To enhance the extraction performance of DAPhen derivatives, partial lactamization was strategically introduced into the architecture. Two unsymmetrical extractants possessing flexible amide and rigid lactam functional groups were synthesized: N,12-diethyl-9-methyl-13-oxo-N-(p-tolyl)-12,13-dihydroquinolino[3,4-b][1,10]phenanthroline-2-carboxamide (EtTol–EtTol-ALPhen) and N,N-dibutyl-12-ethyl-9-methyl-13-oxo-12,13-dihydroquinolino[3,4-b][1,10]phenanthroline-2-carboxamide (But2-EtTol-ALPhen). The ligand EtTol–EtTol-ALPhen achieved superior Am(III)/Eu(III) selectivity (SFAm/Eu > 100, 2.0 M HNO3) and favorable Am(III) distribution ratios (DAm ≈ 10, 2.0–4.0 M HNO3) in n-octanol. The coordination behavior was explored using 1H NMR titration, ESI-MS, and UV-vis spectrophotometric titration, revealing 1:1 and 1:2 metal-to-ligand binding stoichiometries. Single-crystal X-ray diffraction analysis confirmed the formation of ML (NO3)3 and ML2(H2O)23+ species in the NO3– and ClO4– systems, respectively. Furthermore, density functional theory (DFT) calculations offered theoretical insights into ligand–metal interactions from the electronic structure and thermodynamics. This work balances extraction capability and solubility through preorganization optimization, offering a reference for future extractant design.