Coordination Behaviors Between Aryl Diamide Amine Ligands and Trivalent Lanthanides: A Spectroscopic and Structural Study

Investigating the complexation of organic ligands with trivalent lanthanides (Ln(III)) is crucial in various fields, particularly in the separation of actinides from lanthanides for spent fuel reprocessing. While alkyl diamide amine ligands have been extensively studied, aryl-functionalized ligands are rarely reported. To understand the effect of substituents on the coordination mode of diamide amine ligands with Ln(III), two aryl-substituted diamide amine ligands, 2,2′-(p-tolylazanediyl)bis(N-ethyl-N-(p-tolyl)acetamide) (L1) and 2,2′-((2-ethylhexyl)azanediyl)bis(N-ethyl-N-(p-tolyl)acetamide) (L2), were synthesized and systematically studied. ¹H NMR titration showed that metal ion coordination affected the electron density of methylene groups near coordinating atoms more than that of aromatic rings, while fluorescence titration and single-crystal analysis confirmed 1 : 1 ligand-Ln(III) complexes. Compared to alkyl diamide amine ligands, aryl groups increased steric hindrance, enhancing torsional strain and preventing amine nitrogen from participating in complexation, leading to weaker coordination as only two carbonyl oxygen atoms coordinated with Ln(III) in a bidentate mode. This work provides insight into how substituents influence the complexation properties of diamide amine ligands with f-elements, aiding in the design of new ligands with improved performance.