Sequestration of Am³⁺ and Eu³⁺ into ionic liquid containing Aza-macrocycle based multiple-diglycolamide ligands: Extraction, complexation, luminescence and DFT studies

Extraction of Am³⁺ and Eu³⁺ was carried out into a room temperature ionic liquid, C₈mim⋅Tf₂N, using two multiple-diglycolamide extractants with three or four diglycolamide arms tethered to either a triaza- or tetraazamacrocyclic ligand termed as TAM-3-DGA and TAM-4-DGA, respectively. The metal ion extraction was highly efficient as only 5 × 10^-4 M solutions of TAM-3-DGA resulted in D values of 680 and 845 for Am³⁺ and Eu³⁺, respectively, at pH 2.0, while the respective values with a five times lower concentration of TAM-4-DGA were 142 and 296, which is probably the best reported thus far. Though Eu³⁺ ion was preferentially extracted over Am³⁺, the selectivity was not as good as reported in molecular diluent-based extraction systems. The extracted species contained two units of the extractant and did not contain any nitrate ion suggesting a cation-exchange mechanism. Studies on the temperature effect on metal ion extraction were carried out to determine the thermodynamic parameters. Luminescence studies of the extracts were also performed which suggested strong complex formation with no inner-sphere water molecules. To understand the ligand coordination to Eu³⁺ and Am³⁺ and other bonding parameters, DFT studies were carried out.