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Although being investigated for more than 70 years by various methods, the uranyl(VI)–citrate (U(VI)–Cit) system still reveals interesting structural and spectroscopic features. The aqueous chemistry of the system is complex, as evidenced by the still controversial discussions on speciation and structures.

Upon complexation, a chiral center is induced in Cit’s central carbon, yielding two diastereomeric pairs of enantiomers, whereupon the 2:2 complexes exhibit syn- and anti-configured isomers. In fact, the combination of ¹⁷O-NMR (at natural abundance) and DFT calculations allowed an unambiguous discrimination. Hereby symmetry and especially deviation from regular pentagonal bipyramidal coordination geometry affect the O=U=O entities’ (O_yl) chemical shifts. In aqueous solution, the syn-isomer is favored, in contrast to the preferably crystallizing anti-isomer. Both isomers interconvert mutually with exchange rates of about 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.[1]

At circumneutral conditions, above 3 mM uranyl(VI) citrate concentrations, two trimeric 3:3 U(VI)–Cit complex molecules associate upon sandwiching a metal ion (Mⁿ⁺) as a O=U=O→Mⁿ⁺ Lewis acid–base adduct. The coordination shell formed by the six (inner) O_yl restricts the size of the enclosed Lewis acid. Among the studied metal ions, only Na⁺, Ca²⁺, and La³⁺ with ionic radii close to 1.0 Å match the formed cavity, while Li⁺ is too small, and K⁺ and Rb⁺ are too large. This cavity constitutes an anhydrous environment that shields the Mⁿ⁺ from the aqueous phase, causing the sandwiched Na⁺ and La³⁺ to resonate 25 ppm and 430 ppm up-field relative to their corresponding aquo ions, respectively.
Upon increasing Mⁿ⁺ charge/radius ratio, the sandwiched Lewis acids more and more withdraw electron density from those O_yl acting as Lewis base, and thus the latter sense a remarkable de-shielding. Increasing the charge of the sandwiched cation from +1 to +3, Δδ_O between signals of the coordinating O_yl and the outer O_yl increases remarkably (12, 69, and 93 ppm, respectively), as the O=U=O units become progressively polarized.[2]

Literature:

[1] J. Kretzschmar et al., Inorg. Chem. 2021, 60, 7998.
[2] J. Kretzschmar et al., Chem. Commun. 2020, 56, 13133.