Trimeric uranyl(VI)–citrate forms Na⁺, Ca²⁺, and La³⁺ sandwich complexes in aqueous solution

Basile et al. (Chem. Commun., 2015, 51, 5306–5309) showed that a sodium ion is sandwiched by the uranyl(VI) oxygen atoms of two 3:3 uranyl(VI)–citrate complex molecules in single-crystals. By means of NMR spectroscopy supported by DFT calculation we provide unambiguous evidence for this complex to persist in aqueous solution above a critical concentration of 3 mM uranyl citrate. Unprecedented Ca²⁺ and La³⁺ coordination by a bis-(η³-uranyl(VI)-oxo) motif advances the understanding of uranium’s aqueous chemistry. As determined from ¹⁷O NMR, Ca²⁺ and especially La³⁺ cause strong O=U=O polarization which opens up new ways for uranyl(VI)-oxygen activation and functionalization.