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.