Coordination of Fluorine-Substituted 1,10-Phenanthroline Diphosphonates with Americium(III) and Lanthanides(III): Solvent Extraction, Complexation, XRD, and Theoretical Study

Abstract

Hybrid N,O-donor ligands based on 1,10-phenanthroline are a promising class of compounds for processing high-level waste. Here, we synthesized novel phenanthroline-based diphosphonates containing electron-withdrawing fluorine atoms in alkyl substituents. We studied their extraction properties for Am(III) and, for the first time, for the entire series of lanthanides(III). The extraction of nitric acid for these diphosphonates was also studied. It has been shown that replacing one hydrogen atom with a fluorine atom in the ethyl substituent of a diphosphonate leads to a decrease in the affinity of the ligands for metal cations. Replacing two hydrogen atoms leads to a decrease in the affinity of the ligands for both metal cations and protons. But such modifications led to a change in the nature of the extraction of Am(III) and Eu(III) from nitric acid solutions, and as a result, the diphosphonate with two substituted hydrogen atoms retains its extraction properties when extracted from 5 mol/L HNO₃. The complexation in solution was studied by using ultraviolet–visible (UV–vis) titration for Nd(NO₃)₃ and Eu(NO₃)₃. The study of complexation in solid form using SC-XRD revealed the formation of complexes of the composition LnL(NO₃)₃, as well as the possibility of the formation of hydrolyzed binuclear complexes [Eu(μ²,κ⁴-(RO)₂P(O)Phen(O)₂(OR))(NO₃)₂]₂. Density functional theory (DFT) calculations were performed to gain more insight into coordination properties and describe experimental data. It was shown that introducing fluorine atoms decreases the charge modules for both N_phen and O_P═O and decreases the protonation energy.