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.