Separation and Complexation of Lanthanides with an Acidic Phenanthroline Carboxamide Ligand: Extraction, Spectroscopy, and Crystallography

The separation of lanthanides (Ln(III)) is a challenging task because of their similar physiochemical properties. In this work, the extraction and complexation behavior of Ln(III) with a saponified phenanthroline carboxamide ligand 9-(N,N-dioctylcarbamoyl)-1,10-phenanthroline-2-sodium carboxylate (NaDOAPA) was investigated. The saponified ligand NaDOAPA showed a considerable ability to separate heavy lanthanides from light and middle lanthanides in acidic solutions (2 ≤ pH ≤ 5), with an average separation factor of 2.5 for the adjacent heavy lanthanides. The complexation mechanism between Ln(III) and NaDOAPA was thoroughly investigated by 1H NMR titration, ultraviolet–visible (UV–vis) spectrophotometric titration, and fluorescence spectrophotometric titration. The coordination chemistry studies indicated that NaDOAPA could complex with Ln(III) with 1:1 and 1:2 stoichiometries. The increasing stability constants (log β) further suggested a stronger affinity of the ligand for heavy lanthanides, consistent with solvent extraction results. The Ln(III) complexes with different stoichiometries were characterized by crystallography to give a molecular-level explanation of the extraction and complexation mechanism. This work provides a promising choice for the separation of heavy lanthanides. The thorough study of the extraction and complexation of lanthanides will also facilitate the application of the ligand in practical processes.