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

Kretzschmar, J.; Tsushima, S.; Lucks, C.; Jäckel, E.; Meyer, R.; Steudtner, R.; Müller, K.; Roßberg, A.; Schmeide, K.; Brendler, V.

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 by a multi-method approach comprising NMR, UV-Vis, ATR FT-IR, and EXAFS spectroscopies as well as DFT calculations. Combining ¹⁷O NMR and DFT calculation allowed an unambiguous decision on complex configurations, evidencing for the first time that the dimeric complex, (UO₂)₂(HCit–H)₂², 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 s¹ at −6 °C and ~249 s¹ at 60 °C in acidic solution, corresponding to an activation barrier of about 24 kJ mol¹. Upon increasing pH, ternary dimeric mono- and bis-hydroxo as well as trimeric complexes form, i.e. (UO₂)₂(HCit–H)₂(OH)³, (UO₂)₂(HCit–H)₂(OH)₂⁴, (UO₂)₃(O)(Cit–H)₃⁸, and (UO₂)₃(O)(OH)(Cit–H)₂⁵, 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 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 uranium(VI)-citate species do not significantly increase uranium(VI) mobility in the environment. Furthermore, we revise the misconceptions in aqueous U(VI) citric acid solution chemistry, i.e. structures proposed and repeatedly taken up, and outline generalized isostructural considerations to provide a basis for future uranium(VI) complexation studies.

Keywords: uranium; uranyl; U(VI); citric acid; citrate; molecular structure; NMR; ATR FT-IR; UV-Vis; EXAFS; spectroscopy; ternary complex; uranium hydrolysis; stability constant; isomerization; NMR dynamics

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Publ.-Id: 28980