4.7 Article

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

Journal

INORGANIC CHEMISTRY
Volume 60, Issue 11, Pages 7998-8010

Publisher

AMER CHEMICAL SOC
DOI: 10.1021/acs.inorgchem.1c00522

Keywords

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Funding

  1. European Union's Horizon 2020 Research and Innovation Programme (CORI project) [847593]
  2. German Federal Ministry for Economic Affairs and Energy (BMWi) (GRaZ II project) [02E11860B]

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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.
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.

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