Structural effects of benzene-centered tripodal diglycolamides on extraction of trivalent f-cations into a room temperature ionic liquid

A series of three benzene functionalized tripodal diglycolamide (DGA) extractants (abbreviated as Bz-T-DGA, and termed as L_I, L_II and L_III) were studied for their extraction abilities for Eu³⁺ and Am³⁺ ions. Extraction efficiencies of these ligands are distinctly dependent on: (i) the nature of alkyl substituents on amidic N atoms of DGA, (ii) alkyl chain length that connects the DGA moeties with the benzene ring, and (iii) the medium of extraction. Substitution of protons with iso-pentyl groups on DGA nitrogen atoms enhanced the ligand extraction efficiency many folds. At the same time, increasing the spacer atoms between the benzene platform and the DGA units reduced the ligand extraction efficiency. The distribution ratio of Eu³⁺ was higher than that of Am³⁺ with all the three ligands, but the separation factor between the two elements was less than 2. Solvent extraction mechanism was entirely different when the organic solvent changed from n-dodecane (a molecular solvent) to ionic liquid. DFT calculations revealed that the solvent extraction abilities of these ligands were primarily goverened by their complexation abilities for trivalent f-cations.