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

Abstract

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 (Na-DOAPA) 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 1 H 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.