The complexation and solvent extraction properties of a phenanthroline diamide extractant for trivalent actinide and lanthanide ions

Abstract

The study of trivalent actinide (An(III)) interactions with nitrogen and oxygen donor (N,O-donor) extractants is highly relevant to efforts aimed at closing the nuclear fuel cycle. Despite their importance, extraction of transplutonium actinides remains underexplored. A popular N,O-donor extractant, phenanthroline diamide (DAPhen), has incurred interest for f-element separations due to numerous positive qualities such as molar acid stability, a pre-organized binding mode, and tunable amide functionalities. Solvent extraction experiments of TEtDAPhen in nitrobenzene demonstrated selectivity for multiple An(III) (An = Am, Cm, Bk, Cf) metals over Eu(III). Surprisingly, extraction efficiency did not follow periodic trends. Extraction was most effective for Am(III), followed by Cf(III) ≈ Bk(III), and lowest for Cm(III). To provide additional insight into the extraction process, coordination chemistry studies were conducted between Am(III) and numerous Ln(III) cations (Pr(III), Nd(III), Tb(III), Lu(III)) with TEtDAPhen. Lanthanide speciation and stability constants were measured using UV-Visible spectroscopy which showed increasing stability constants from Nd(III)–Gd(III) with consistent one-to-one metal-to-ligand stoichiometry. Single crystal X-ray crystallography studies of M(TEtDAPhen)(NO₃)₃ complexes confirmed these findings. Collectively, these results highlight the potential of DAPhen-based extractants for selective An(III) separations and contribute to a deeper understanding of f-element coordination behavior.