Separation and Complexation of Trivalent Actinides and Lanthanides by Two Novel Asymmetric N,O-Hybrid Pyridyl Ligands: A Combination of Phosphoryl and Triazinyl Groups

Two novel asymmetric hard-soft combined ligands, diphenyl(6-(5,9,9-trimethyl-5,6,7,8-tetrahydro-5,8-methanobenzo-[1,2,4]triazin-3-yl)pyridin-2-yl)phosphine oxide (Ph₂-MTP) and butylphenyl(6-(5,9,9-trimethyl-5,6,7,8-tetrahydro-5,8-methanobenzo-[1,2,4]triazin-3-yl)pyridin-2-yl)phosphine oxide (BuPh-MTP), were designed based on the combination of the nature of phosphoryl and triazinyl groups for the selective extraction of trivalent minor actinides from lanthanides. The synthesis of these two ligands and their solvent extraction and complexation behaviors with Am(III) and typical lanthanides were investigated using UV-vis and time-resolved fluorescence spectrophotometry, ¹H/³¹P NMR spectrometry, single-crystal X-ray diffraction, and DFT calculation methods. Solvent extraction experiments showed that both the ligands had strong extraction ability and high selectivity toward Am(III) over Eu(III) from the highly acidic HNO₃ solution. The separation factors (SF_Am/Eu) of these two ligands ranged from 17 to 26, with the concentrations of HNO₃ increasing from 1.0 to 4.0 M. Slope analysis showed that the 3:1 ligand/metal complex was the prevailing species formed during extraction. The formation of the 3:1 ratio of the species of these two ligands with lanthanides was also identified by UV-vis spectrophotometry and single crystallography methods. The stability constants for the formation of the 1:3 complexes of Ph₂-MTP and BuPh-MTP with Nd(III) were determined as 7.06 ± 0.015 and 6.67 ± 0.007, respectively. The geometric structures of the 1:3 complexes were clearly illustrated using the single-crystal X-ray diffraction technique and DFT theoretical calculation. This work provides an effective strategy to design new asymmetric hard-soft mixed actinide extractants by combining two different functional groups in one ligand, and the interaction mechanism between the functional groups and metal ions needs to be further investigated.