Complexation of\nActinyl Ions with DHOA and TBP in\nan Ionic Liquid and a Molecular Solvent: How Similar or Different\nAre They?

Complexation thermodynamics of uranyl ions with well-known\nreprocessing\nligands like tributyl phosphate (TBP) and dihexyl octanamide (DHOA)\nwas studied in an ionic liquid (IL) versus a molecular solvent. Whereas\n1-butyl-3-methylimidazolium bis(trifluoromethyl sulfonyl)imide (Bumim·Tf<sub>2</sub>N) was used as an IL due to its favorable viscosity, acetonitrile\nwas the choice of molecular solvent due to its poor coordinating nature.\nOptical spectroscopy studies revealed that UO<sub>2</sub><sup>2+</sup> ions formed species of the types ML<sub>1</sub> and ML<sub>2</sub> with both TBP and DHOA, in a stepwise manner. The formation constants\n(log β) of UO<sub>2</sub><sup>2+</sup> with DHOA in the IL were\ndetermined as 3.58 ± 0.03 and 6.54 ± 0.10, while those with\nTBP were obtained as 3.42 ± 0.05 and 6.38 ± 0.08, respectively.\nIn the case of acetonitrile, on the other hand, these β values\nwere significantly lower than those observed in the IL medium, which\nwas supported by the DFT calculations. Calorimetric titrations of\nthe UO<sub>2</sub><sup>2+</sup> ions with DHOA and TBP confirmed that\nthe complex formation reactions were thermodynamically more favored\nin the ionic liquid medium than the molecular solvent. The nature\nof binding through FTIR investigations and DFT calculations suggested\nthat the complexes formed in the IL medium were cationic in nature\nof the types [UO<sub>2</sub>(H<sub>2</sub>O)<sub>3</sub>(TBP)<sub>2</sub>]<sup>2+</sup> and [UO<sub>2</sub>(H<sub>2</sub>O)<sub>3</sub>(DHOA)<sub>2</sub>]<sup>2+</sup>, but neutral complexes were formed\nin the molecular solvent of the types [UO<sub>2</sub>(NO<sub>3</sub>)<sub>2</sub>(TBP)<sub>2</sub>] and [UO<sub>2</sub>(NO<sub>3</sub>)<sub>2</sub>(DHOA)<sub>2</sub>].