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The synthesis, Eu complexation, and solvent extraction of Am and Eu from nitric acid solutions by tetradentate phenanthroline-derived bis(pyrazole) (BPPhen) ligands were described. By using meta-nitrobenzotrifluoride as diluent, BPPhen ligands in combination with 2-bromohexanoic acid extracted Am and Eu with remarkably high efficiency, excellent selectivity, and fast extraction kinetics. Stripping posed no issues. The ligands also showed excellent hydrolytic stability and acid tolerance. 2-Bromohexanoic anion neutralized the charge and increased the lipophilicity of the extracted ion pair. The extraction conformed to a cation exchange model. Slope analysis demonstrated the extraction of 1:2 metal/ligand complexes. Analyses by electrospray ionization mass spectrometry, time-resolved laser-induced fluorescence spectroscopy, Raman, and Fourier transform infrared techniques indicated that the composition of the extracted species is [Eu(nOct-BPPhen)(HO)]. The formation of 1:2 complexes was also confirmed by UV-vis spectroscopic titration and microcalorimetric titration methods. Meanwhile, the stability constants ( K) and the thermodynamic parameters (Δ H, Δ S, Δ G) for the complexation of Eu with nOct-BPPhen were presented too.

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1,3-Bis[3-(2-pyridyl)pyrazol-1-yl]propane (Bippp) and 1,2-bis[3-(2-pyridyl)pyrazyl-1-methyl]benzene (Dbnpp), the pyridylpyrazole based tetradentate ligands, were synthesized and characterized by MS, NMR, and FT-IR. The solvent extraction and complexation behaviors of Am(III) and Eu(III) with the ligands were investigated experimentally and theoretically. In the presence of 2-bromohexanoic acid, the two ligands can effectively extract Am(III) over Eu(III) and other rare earth(III) metals (RE(III)) in HNO3 solution with the separation factors (SFAm/RE) ranging from 15 to 60. Slope analyses showed that both Am(III) and Eu(III) were extracted as monosolvated species, which agrees well with the results observed from X-ray crystallography and MS analyses. The stability constants (log K) obtained from UV-vis titration for Eu(III) complexes with Bippp and Dbnpp are 4.75 ± 0.03 and 4.45 ± 0.04, respectively. Both UV-vis titration and solvent extraction studies indicated that Bippp had stronger affinity for Eu(III) than Dbnpp, which is confirmed by density functional theory (DFT) calculations. DFT calculations revealed that the AmL(NO3)3 (L = Bippp and Dbnpp) complexes are thermodynamically more stable in water than their Eu(III) analogues, which is caused by greater covalency of the Am-N than Eu-N bonds. Theoretical studies gave an insight into the nature of the M(III)-ligand bonding interactions.

To further reveal the extraction model for selective separation of trivalent actinides over lanthanides by dithiophosphinic acids (DPAHs), five representative DPAH ligands with different substituent groups have been synthesized, and their extraction and complexation behaviors toward Am3+/Eu3+ have been investigated both experimentally and theoretically. The introduction of electron-withdrawing group −CF3 into DPAH ligands is beneficial to their extractability among the five ligands. Slope analyses show that both Am3+ and Eu3+ were extracted as tetra-associated species with DPAH ligands. In addition, the results obtained from luminescence spectroscopy, Raman spectroscopy, and ESI-MS suggest that all of the five DPAHs coordinate with Eu3+ mainly in the form of ML3(HL)­(H2O) (L represents deprotonated DPAH). Density functional theory (DFT) calculations on the thermodynamic parameters illustrate that the extractability of DPAHs is dominated by the deprotonation property of these ligands. Meanwhile, molecular orbital analysis indicates that the unoccupied valence orbitals of Am3+ display a stronger affinity to the sulfur lone electron pair than those of Eu3+, which should be one of the key factors contributing to the excellent selectivity of Am3+ over Eu3+ by DPAH ligands.

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To clarify the structure-property relationships of pyridylpyrazole ligands and provide guidance for the design of new and more efficient ligands for the selective extraction of actinides over lanthanides, a series of alkyl-substituted pyridylpyrazole ligands with different branched chains at different positions of the pyrazole ring were synthesized. Extraction experiments showed that the pyridylpyrazole ligands exhibited good selective extraction abilities for Am III ions, and the steric effects of the branched chain had a significant impact on the distribution ratios of Am III and Eu III ions as well as the separation factor. Moreover, both slope analyses and UV/Vis spectrometry titra- [a]