Although the uranyl(VI)-malate system has been repeatedly studied, there are still open questions regarding 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 spectroscoy (TRLFS), isothermal titration calorimetry (ITC), ultraviolet-visible spectroscopy (UV-vis), complemented by density functional theory calculations (DFT). In acidic solution (pH 1.5 – 5.5), covering metal excess through ligand excess, two dinuclear complexes of 2:1 and 2:2 U(VI):malate stoichiometry form virtually exclusively. 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 the 2:2 complexes, malate features a (κ3O,O’,O’’) coordination motif by 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 quantitites (ΔG, ΔH and ΔS) as well as formation constants (log β) of both complexes were determined by calorimetric titrations and TRLFS. 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 predominating, while mononuclear species exist only in very acidic and/or very dilute solution. This study provides new data which complement and expand the understanding of both structure and thermodynamics of these complexes

Although the uranyl(VI)-malate system has been repeatedly studied, there are still open questions regarding 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 spectroscoy (TRLFS), isothermal titration calorimetry (ITC), ultraviolet-visible spectroscopy (UV-vis), complemented by density functional theory calculations (DFT). In acidic solution (pH 1.5 – 5.5), covering metal excess through ligand excess, two dinuclear complexes of 2:1 and 2:2 U(VI):malate stoichiometry form virtually exclusively. 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 the 2:2 complexes, malate features a (κ3O,O’,O’’) coordination motif by 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 quantitites (ΔG, ΔH and ΔS) as well as formation constants (log β) of both complexes were determined by calorimetric titrations and TRLFS. 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 predominating, while mononuclear species exist only in very acidic and/or very dilute solution. This study provides new data which complement and expand the understanding of both structure and thermodynamics of these complexes