Coordination\nof Fluorine-Substituted 1,10-Phenanthroline\nDiphosphonates with Americium(III) and Lanthanides(III): Solvent Extraction,\nComplexation, XRD, and Theoretical Study

Hybrid N,O-donor ligands based on 1,10-phenanthroline\nare a promising\nclass of compounds for processing high-level waste. Here, we synthesized\nnovel phenanthroline-based diphosphonates containing electron-withdrawing\nfluorine atoms in alkyl substituents. We studied their extraction\nproperties for Am(III) and, for the first time, for the entire series\nof lanthanides(III). The extraction of nitric acid for these diphosphonates\nwas also studied. It has been shown that replacing one hydrogen atom\nwith a fluorine atom in the ethyl substituent of a diphosphonate leads\nto a decrease in the affinity of the ligands for metal cations. Replacing\ntwo hydrogen atoms leads to a decrease in the affinity of the ligands\nfor both metal cations and protons. But such modifications led to\na change in the nature of the extraction of Am(III) and Eu(III) from\nnitric acid solutions, and as a result, the diphosphonate with two\nsubstituted hydrogen atoms retains its extraction properties when\nextracted from 5 mol/L HNO<sub>3</sub>. The complexation in solution\nwas studied by using ultraviolet–visible (UV–vis) titration\nfor Nd(NO<sub>3</sub>)<sub>3</sub> and Eu(NO<sub>3</sub>)<sub>3</sub>. The study of complexation in solid form using SC-XRD revealed the\nformation of complexes of the composition <b>LnL(NO</b><sub><b>3</b></sub><b>)</b><sub><b>3</b></sub>, as well\nas the possibility of the formation of hydrolyzed binuclear complexes\n[Eu(μ<sup>2</sup>,κ<sup>4</sup>-(RO)<sub>2</sub>P(O)Phen(O)<sub>2</sub>(OR))(NO<sub>3</sub>)<sub>2</sub>]<sub>2</sub>. Density functional\ntheory (DFT) calculations were performed to gain more insight into\ncoordination properties and describe experimental data. It was shown\nthat introducing fluorine atoms decreases the charge modules for both\nN<sub>phen</sub> and O<sub>PO</sub> and decreases the protonation\nenergy.