Unveiling the interplay: comparing solvent extraction, structure, and stability constants in a phenanthroline diamide system with trivalent lanthanides

Abstract

Phenanthroline diamide ligands have shown promise as extractants for the selective separation of the An(III) from the Ln(III) in used nuclear fuel recycling. To enhance the effectiveness of these ligands, a fundamental understanding of their interactions with the full Ln(III) series is essential. In this study, the extraction behavior of the full Ln(III) series, including the typically unavailable Pm(III), was evaluated using the ligand N,N′,N,N′-tetraethylphenanthroline-2,9-diamide (TEtDAPhen). Solvent extraction experiments reveal a general trend of decreasing extraction efficiency from La(III) to Lu(III), which correlates inversely with the increasing thermodynamic stability constants of the corresponding Ln(TEtDAPhen)(NO₃)₃ complexes across the series. The stability constants suggest a lanthanide contraction effect, with stronger interactions with the increased charge density. Additionally, single-crystal X-ray diffraction analysis of the solid-state structures of Ln(III) complexes (excluding Pm(III)) provides valuable insight into the coordination environment and binding modes of the ligand. Together, these results help establish a structural relationship that informs the design and optimization of phenanthroline diamide ligands for An(III)/Ln(III) separations. The inclusion of Pm(III) in this study also offers a rare and comprehensive perspective across the entire lanthanide series.