Separation of lanthanides and actinides(iii) using tridentate benzimidazole, benzoxazole and benzothiazoleligands

Abstract

The ability of new hydrophobic tridentate ligands based on 2,6-bis(benzimidazol-2-yl)pyridine, 2,6-bis(benzoxazol-2-yl)pyridine and 2,6-bis(benzothiazol-2-yl)pyridine to selectively extract americium(III) from europium(III) was measured. The most promising ligand – 2,6-bis(benzoxazol-2-yl)-4-(2-decyl-1-tetradecyloxy)pyridine L⁹ was found to give separation factors (SF_Am/Eu) of up to 70 when used to extract cations from 0.02–0.10 M HNO₃ into TPH in synergy with 2-bromodecanoic acid. Six structures of lanthanide complexes with 2,6-bis(benzoxazol-2-yl)pyridine L⁶ were then determined to evaluate the types of species that are likely to be involved in the separation process. Three structural types were observed, namely [LnL⁶(NO₃)₃(H₂O)₂], 11-coordinate only for La, [LnL⁶(NO₃)₃ (CH₃CN)], 10-coordinate for Pr, Nd and Eu and [LnL⁶(NO₃)₃(H₂O)], L 10-coordinate for Eu and Gd. Quantum Mechanics calculations were carried out on the tridentate ligands to elucidate the conformational preferences of the ligands in the free state and protonated and diprotonated forms and to assess the electronic properties of the ligands for comparison with other terdentate ligands used in lanthanide/actinide separation processes.