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, H(3)Cit, 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 (ONMR)-O-17 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 similar to 30 s(-1) at -6 degrees C and similar to 249 s(-1) at 60 degrees 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 beta degrees = -(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 O-17 NMR resonance of the trinuclear uranyl(VI) core bridging mu(3)-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.

Automated summary

This research investigates the interactions between the uranyl(VI) ion and citric acid using multiple spectroscopic techniques and computational methods, revealing the existence of two diastereomers of a dimeric complex and their interconversion in acidic solution. Furthermore, ternary dimeric and trimeric complexes were identified at higher pH values. The study also challenges previous misconceptions in the aqueous U(VI)-citric acid solution chemistry and provides a basis for future studies on U(VI) complexation.