Structures and thermodynamics of dinuclear species formed in a uranyl(VI)-malic acid system: a multi-technique...

Although uranyl(VI)-malate systems have been repeatedly studied, there are still open questions regarding their structures, stoichiometry, and thermodynamic key parameters. We therefore examined the interactions between the uranyl(VI) ion, U(VI), and malic acid, H₂Mal, using a multi-technique approach performing nuclear magnetic resonance spectroscopy (NMR), time-resolved laser-induced fluorescence spectroscopy (TRLFS), isothermal titration calorimetry (ITC), and ultraviolet-visible spectroscopy (UV-vis), complemented by density functional theory (DFT) calculations. In acidic solution (pH 1.5-5.5), by covering metal excess through ligand excess, two dinuclear complexes of 2 : 1 and 2 : 2 U(VI) : malate stoichiometry form predominantly. This species distribution is mainly influenced by the metal-to-ligand ratio given in solution. DFT and NMR confirmed that the 2 : 1 U(VI) malate complex involves a bridging hydroxo ligand (µ₂-OH). In both the 2 : 1 and 2 : 2 complexes, malate features a (κ³O,O',O″) coordination motif with two carboxylate groups and the alkoxylato group including bridging between two U(VI). In the 2 : 2 complex, changing the ligands' relative orientation yields two geometric isomers. Thermodynamic quantities (ΔG, ΔH, and ΔS) and formation constants (log β) of both complexes were determined by calorimetric titrations and TRLFS. The formation of the 2 : 1 and 2 : 2 species is endothermic and entropy-driven, with log β of 17.1 ± 0.1 and 37.7 ± 0.1, respectively. Notably, even for U(VI) concentrations as low as 10 µM, dinuclear species are predominant, while mononuclear species exist only in very acidic and/or very dilute solutions. This study provides new data which complement and expand the understanding of both structures and thermodynamics of these complexes.