Selective separation and complexation of trivalent actinides and lanthanides using an unsymmetric pyridine-derived triazinyl and amide extractant†

Unsymmetric N-heterocyclic extractants have been proven to have good application prospects in the separation of trivalent actinides over lanthanides from highly active liquid waste. In this article, a novel unsymmetric pyridine-based extractant functionalized with triazinyl and amide groups (N-ethyl-N-(p-tolyl)-6-(5,9,9-trimethyl-5,6,7,8-tetrahydro-5,8-methanobenzo[e][1,2,4]triazin-3-yl)picolinamide (Et-Tol-CA-ATP)) was designed and used for the extraction and complexation towards trivalent lanthanide and actinide ions. The ligand Et-Tol-CA-ATP exhibited moderate extraction ability but high selectivity towards Am(III) over Eu(III) (SF_{Am(III)/Eu(III)} = 16.9) under highly acidic HNO₃ conditions. The complexation mechanism and extraction behaviors of Et-Tol-CA-ATP for Am(III) and typical lanthanides were thoroughly investigated by NMR spectroscopy, UV-vis spectrophotometry, time-resolved laser fluorescence spectroscopy (TRLFS), high-resolution electrospray ionization mass spectrometry (HR-ESI-MS) analysis and DFT calculations. The stability constants for the formation of 1 : 1 and 1 : 2 Nd(III) complexes with Et-Tol-CA-ATP in CH₃CN were determined as 2.26 ± 0.02 and 4.38 ± 0.01, respectively. The geometric structures of the 1 : 1 and 1 : 2 complexes of Am(III) and Eu(III) coordinated with Et-Tol-CA-ATP and their differences in the bonding nature leading to the extraction selectivity were further illustrated using DFT calculations. This work describes the successful synthesis of a new unsymmetric pyridine-based triazinyl and amide extractant to selectively separate Am(III) over Eu(III) from highly acidic HNO₃ solutions, providing an alternative strategy for developing some new unsymmetric extractants for the separation of trivalent minor actinides.