The extraction of both UO2(2+) and trivalent lanthanide and actinide ions (Am3+, Nd3+, Eu3+) by dialkylphosphoric or dialkylphosphinic acids from aqueous solutions into the ionic liquid, 1-decyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imide has been studied and compared to extractions into dodecane. Radiotracer partitioning measurements show comparable patterns of distribution ratios for both the ionic liquid/aqueous and dodecane/aqueous systems, and the limiting slopes at low acidity indicate the partitioning of neutral complexes in both solvent systems. The metal ion coordination environment, elucidated from EXAFS and UV-visible spectroscopy measurements, is equivalent in the ionic liquid and dodecane solutions with coordination of the uranyl cation by two hydrogen-bonded extractant dimers, and of the trivalent cations by three extractant dimers. This is the first definitive report of a system where both the biphasic extraction equilibria and metal coordination environment are the same in an ionic liquid and a molecular organic solvent.

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Hydrated lanthanide(III) chlorides, LnCl 3 ÁxH 2 O (Ln = La, Pr, Nd, Sm, Eu, Gd; x = 6-7) readily dissolve in the low melting ionic liquid 1-ethyl-3-methylimidazolium chloride ([C 2 mim]Cl) in an open vessel at 110 1C, and upon cooling crystallize as the anhydrous [C 2 mim] 3 [LnCl 6 ]. The crystal structures exhibit a face-centered packing arrangement of the [LnCl 6 ] 3À anions, with the cations located as slip aligned pairs in the void spaces which participate in hydrogen-bonding to chlorides. A second crystalline form of the Gd 3+ complex, GdCl 3 (OH 2 ) 4 Á2([C 2 mim]Cl), was isolated when the above reaction was conducted in a sealed system. For comparison, a third Gd 3+ compound was grown from the ionic liquid 1-butyl-3-methylimidazolium chloride ([C 4 mim]Cl) using the same unsealed conditions as above, and was found to be [C 4 mim] 3 [GdCl 6 ]. This compound exhibits a different packing arrangement to that observed for the [C 2 mim] + analogs. Based on these findings, ILs would appear to offer new crystallization process options based on their often high thermal stabilities and low to negligible vapor pressures.

The first definitive high-resolution single-crystal X-ray structure for the coordination of the 1-methylimidazole (Meimid) ligand to UO2(Ac)2 (Ac = CH3CO2) is reported. The crystal structure evidence is confirmed by IR, Raman, and UV-vis spectroscopic data. Direct participation of the nitrogen atom of the Meimid ligand in binding to the uranium center is confirmed. Structural analysis at the DFT (B3LYP) level of theory showed a conformational difference of the Meimid ligand in the free gas-phase complex versus the solid state due to small energetic differences and crystal packing effects. Energetic analysis at the MP2 level in the gas phase supported stronger Meimid binding over H2O binding to both UO2(Ac)2 and UO2(NO3)2. In addition, self-consistent reaction field COSMO calculations were used to assess the aqueous phase energetics of combination and displacement reactions involving H2O and Meimid ligands to UO2R2 (R = Ac, NO3). For both UO2(NO3)2 and UO2(Ac)2, the displacement of H2O by Meimid was predicted to be energetically favorable, consistent with experimental results that suggest Meimid may bind uranyl at physiological pH. Also, log(Knitrate/KAc) calculations supported experimental evidence that the binding stoichiometry of the Meimid ligand is dependent upon the nature of the reactant uranyl complex. These results clearly demonstrate that imidazole binds to uranyl and suggest that binding of histidine residues to uranyl could occur under normal biological conditions.