Influence of Aqueous Phase Acidity on Ln(III) Coordination by N,N,N',N'-Tetraoctyldiglycolamide

This study highlights the importance of combining distribution ratio measurements with multiple spectroscopic techniques to provide a more comprehensive understanding of organic phase Ln coordination chemistry. Solvent extraction investigations with N,N,N',N'-tetraoctyldiglycolamide (TODGA) in n-heptane reveal the sensitivity of Ln complexation to the HNO₃ concentration. Distribution ratio measurements in tandem with UV-Vis demonstrated that increasing the concentration of HNO₃ above 0.5 M with a constant NO₃^- of 1 M increases the number of coordinating TODGA molecules, from a 1:2 to a 1:3 Ln:TODGA complex. At each concentration of HNO₃ considered herein (from 0.01 to 1 M), Eu lifetime analysis demonstrated no evidence of H₂O coordination. Results from Fourier transform infrared investigations suggest the presence of inner-sphere NO₃^- under low concentrations of HNO₃ when the 1:2 Ln:TODGA complex is present. Increasing the HNO₃ concentration above 0.5 M increases the propensity for outer-sphere interactions by removing the coordinated NO₃^- and saturating the Ln coordination sphere with three TODGA molecules, resulting in the well-established cationic, trischelate homoleptic [Ln(TODGA)₃]³⁺ complex. This work demonstrates the importance in considering the NO₃^- source for solvent extraction systems. In particular, for systems with an affinity for outer-sphere interactions with molar concentrations of HNO₃, changing the NO₃^- source can change the inner-sphere coordination of the Ln complex, which, in turn, affects the separation efficacy.