Effect of Alkyl Substituent on the Amide of Bis-lactam-1,10-phenanthroline Ligands for Lanthanide Extraction

Abstract

2,9-Diamide-1,10-phenanthroline (DAPhen) and bis-lactam-1,10-phenanthroline (BLPhen) ligands have been demonstrated to extract light lanthanides through size-specific coordination cavities selectively. Given that the alkyl chain has a substantial impact on the extraction performance of DAPhen, elucidating the role of the substituent chain for BLPhen is of great importance. In this study, four BLPhen ligands with varying alkyl substituents were synthesized to investigate the impact of alkyl chain length and branching on the extraction and coordination behavior toward trivalent lanthanides. Extraction studies demonstrate that the extraction trend for lanthanides was primarily governed by the size of the coordination cavity of BLPhen, and remained unchanged upon the introduction of variations in alkyl chain length or branching. However, ligands with shorter straight-chain alkyl groups (L1) exhibited optimal extraction efficiency and selectivity for light lanthanides, while increasing the alkyl chain length or incorporating branching led to a decline in performance. Slope analyses indicate variable metal-to-ligand stoichiometries for light, middle, and heavy lanthanide elements. UV–vis titration and ESI-MS analysis in homogeneous solution confirmed uniform 1:1 complexation for all metal-ligand systems. X-ray diffraction single-crystal structure provided clear coordination information, showing that coordination by the rigid ligand occurred without the need for conformational rotation, thereby minimizing energetic costs and enhancing complex stability. Overall, this study highlights the role of alkyl substituent geometry in modulating the coordination strength and selectivity of BLPhen ligands, providing a structural foundation for the rational design of highly efficient and preorganized extractants for rare earth separation.