Development of Mixed O/S-Donor Imidazole Thione Ligands for Trivalent Lanthanide/Actinide Separations

Separation of lanthanides and actinides is difficult due to their very similar size and charge. Increasing selectivity for actinides over lanthanides is possible by incorporating a softer, more polarizable donor atom into a chelating ligand, since the softer donor atom is capable of greater orbital overlap with the 5f orbitals of the actinides vs the 4f orbitals of the lanthanides. Sulfur-containing imidazole thione chelating ligands have not been investigated for this purpose but possess a resonance structure that puts significant negative charge on the sulfur, enabling it to form strong metal-sulfur bonds compared to other sulfur-containing ligands such as thioethers and thiophenes. Addition of electron-withdrawing substituents on the imidazole nitrogens permits tuning of the ligand electronics. However, traditional basic conditions for synthesis of imidazole thiones do not work for the target acetate and propionate substituents, since the substituent arm is more readily deprotonated than the imidazole ring. Tang et al. reported a synthesis of imidazole thiones without the use of basic conditions that they proposed went through a radical addition of sulfur to imidazole. Investigation of this sulfur addition mechanism uncovered temperature and reagent concerns that led to the development of a new and simplified synthetic pathway for substituted imidazole thiones. The mechanism for sulfur addition to the imidazole ring is nucleophilic aromatic substitution rather than the radical mechanism reported by Tang et al. Understanding this new mechanism allows a significant broadening of the synthetic scope for these compounds. Utilizing our newly synthesized, mixed O/S-donor imidazole thione ligands, we performed liquid-liquid extractions, to determine their actinide vs lanthanide selectivity using 241Am and 152Eu. We compared our extraction results with these imidazole thione ligands with results from a commercially available thiophene ligand to determine the viability of the thione functional group for these applications. All the ligands demonstrated metal coordination, but significant selectivity was only observed for one of the tested thiones. To determine the radiolytic stability of imidazole thiones, 1H NMR spectroscopy was used to determine the concentration of intact thione in samples subjected to gamma irradiation. Increased radiolytic stability was observed for the imidazole thiones containing electron-withdrawing substituents. These studies demonstrate that tuning the electronics of these ligands can enhance radiolytic stability and impacts how the thione functional group can be further explored in the development of selective and radiolytically robust ligands for trivalent actinide and lanthanide separations.