Dimeric and Trimeric Uranyl(VI)-Citrate Complexes in Aqueous Solution

This research addresses a subject discussed controversially for almost 70 years. The interactions between the uranyl(VI) ion, U(VI), and citric acid, H₃Cit, were examined using a multi-method approach comprising nuclear magnetic resonance (NMR), ultraviolet-visible (UV-vis), attenuated total reflectance Fourier-transform infrared (ATR FT-IR), and extended X-ray absorption fine-structure (EXAFS) spectroscopies as well as density functional theory (DFT) calculations. Combining ¹⁷O NMR spectroscopy and DFT calculation provided an unambiguous decision on complex configurations, evidencing for the first time that the dimeric complex, (UO₂)₂(HCit_-H)₂^2-, exists as two diastereomers with the syn-isomer in aqueous solution strongly favored over the anti-isomer. Both isomers interconvert mutually with exchange rates of ∼30 s^-1 at -6 °C and ∼249 s^-1 at 60 °C in acidic solution corresponding to an activation barrier of about 24 kJ mol^-1. Upon increasing the pH value, ternary dimeric mono- and bis-hydroxo as well as trimeric complexes form, that is, (UO₂)₂(HCit_-H)₂(OH)^3-, (UO₂)₂(HCit_-H)₂(OH)₂^4-, (UO₂)₃(O)(Cit_-H)₃^8-, and (UO₂)₃(O)(OH)(Cit_-H)₂^5-, respectively. Stability constants were determined for all dimeric and trimeric species, with log β° = -(8.6 ± 0.2) for the 3:3 species being unprecedented. Additionally, in the 6:6 sandwich complex, formed from two units of 3:3 species, the ¹⁷O NMR resonance of the trinuclear uranyl(VI) core bridging μ₃-O is shown for the first time. Species distribution calculations suggest that the characterized polynuclear U(VI)-citrate species do not significantly increase uranium(VI) mobility in the environment. Furthermore, we revise the misconceptions in the aqueous U(VI)-citric acid solution chemistry, that is, structures proposed and repeatedly taken up, and outline generalized isostructural considerations to provide a basis for future U(VI) complexation studies.