Phenanthroline-diamide ligands have been reported in the selective separation of actinides over Eu(III); on the contrary, relevant basic coordination chemistry studies are still limited, and extraction under actual application conditions is rarely involved. In this work, N,N′-diethyl-N,N′-ditolyl-2,9-diamide-1,10-phenanthroline [Et-Tol-DAPhen (L)] was applied to explore the coordination performance of lanthanides in simulative high-level liquid waste. For the first time, cascade countercurrent extraction was conducted with Et-Tol-DAPhen as the extractant, which reveals the periodic tendency of the extraction efficiency of lanthanides to decrease gradually as the atomic number increases. Comparison of elements with similar radii verifies the hypothesis that the increase in the atomic number leads to a decrease in the ionic radius, thus reducing the coordination and extraction capacity of ligands. Slope analysis, electrospray ionization mass spectrometry, and ultraviolet−visible titration results show that the ligand forms 1:1 and 1:2 complexes with lanthanides and the coordination ability follows the tendency of extraction efficiency, and the first crystal structures of Lns(III) with a phenanthroline-diamide ligand, i.e., [LaL(NO 3 ) 3 (H 2 O)] and [LaL 2 (NO 3 ) 2 ][(NO 3 )], were obtained, which confirms the conclusions described above. This work promises to enhance our comprehension of the chemical properties of Lns(III) and offer new clues for the design and synthesis of novel separation ligands.

The solvent extraction, complexing ability, and basicity of tetradentate N-donor 2,9-bis(5,5,8,8-tetramethyl-5,6,7,8-tetrahydro-1,2,4-benzotriazin-3-yl)-1,10-phenanthroline (CyMe4-BT- Phen) and its derivatives functionalized by Br, hydroxyphenyl, nitryl were discussed and compared. It was demonstrated that four BTPhen ligands are able to selectively extract Am(lll) over Eu(lll). It was notable that the distribution ratio of 5-nitryl-CyMe4-BTPhen for Eu(lll) was suppressed under 0.02, which was much lower compared to DEu(lll) = 1 by CyMe4-BTPhen. The analysis of the effect of the substituent on the affinity to lanthanides was conducted by UV/vis and fluorescence spectroscopic titration. The stability constants of various ligands with Eu(lll) were obtained by fitting titration curve. Additionally, the basicity of various ligands was determined to be 3.1 ± 0.1, 2.3 ± 0.2, 0.9 ± 0.2, 0.5 ± 0.1 by NMR in the media of CD3OD with the addition of DClO4. The basicity of ligands follows the order of L1 > L2 > L3 > L4, indicating the tendency of protonation decreases with the electron-withdrawing ability increase.

The complexation of uranium­(VI) [U­(VI)] with dipicolinic acid was revisited with respect to the missing protonated complex species existing in acidic solutions. For the first time, the presence of the protonated complex, UO2HL+, in aqueous solutions was confirmed and the stability constant was determined by fluorescence spectroscopy. Considering the protonated species, which was missing in previous investigations, the overestimated stability constants of the two known complexes, UO2L and UO2L2 2–, were carefully reevaluated with potentiometry using N-(2-hydroxyethyl)­ethylenediamine-N,N′,N′-triacetic acid as a competing ligand. While a single crystal of the solid compound UO2(HL)2(H2O)4 with two monoprotonated HL– bonding UO2 2+ in a tridentate mode was successfully grown from aqueous solutions, the corresponding complex species UO2(HL)2(aq) could not yet be clearly identified and characterized.