Highly Efficient Extraction of Trivalent f-Cations Using Several N-Pivot Tripodal Diglycolamide Ligands in an Ionic Liquid: The Role of Ligand Structure on Metal Ion Complexation

Complexation of trivalent lanthanides and actinides was studied in a room temperature ionic liquid (RTIL) using five N-pivot tripodal diglycolamide ligands (L_I–L_V) having different structural features, viz. spacer lengths and substituents. The nature of the metal/Ligand complex changed with the substitution of N atom near the tripodal platform from ML with L_III and L_IV to ML₂ with L_I, L_II and L_V (where M = Eu³⁺/Nd³⁺/Am³⁺). With 0.2 mmol/L ligands, the distribution ratio of Am³⁺ at 1 m HNO₃ followed the order: L_I (210) > L_II (42) > L_III (2.1) > L_V (0.7) > L_IV (0.2). The formation constants between Nd³⁺ and the ligands (log β_ML,ML2) followed an identical order, viz. L_I > L_II > L_III > L_V > L_IV. The extraction of Am³⁺ or Eu³⁺ with all the ligands followed a cation-exchange extraction mechanism, which is in sharp contrast to the results obtained in a molecular solvent (n-dodecane), where a solvation extraction mechanism prevalied. Luminescence studies on the Eu³⁺ extract in RTIL confimed the absence of water molecules in the inner coordination sphere of the complex. EXAFS studies indicated that all three DGA arms were binding to the Eu³⁺ cation.