Role of the hydroxo group in the coordination of citric acid toward trivalent americium

The molecular characterization based on multi-technique approach has led to major highlights on revealing the coordination environment of americium (Am) surrounded by citric acid (H3CitH). The structure of the different complexes at pH 1 and 3 are described. These characterizations are made possible by the comparison of the americium-citric acid system with the americium-tricarballylic acid (one analogue of the citric acid without the alpha-hydroxo group). The structural analyses (Vis spectrophotometry, NMR, EXAFS, TRLFS and capillary electrophoresis) were carried out after the establishment of the speciation distribution diagrams so that the complex percentages in solution are known, allowing to take into account the species repartition for structural analysis data treatment. With this combination of means, it was proved for the 1:1 complex that the hydroxo group is counter intuitively deprotonated and coordinated to the Am(III) at pH 1 as well as two carboxylate functions, whereas at pH 3 the hydroxo is not coordinated and stays protonated allowing the three carboxylate functions to coordinate the metallic cation. Therefore, the hydroxo group affects the Am complexation differently depending on the pHs: the complexation is favored by inductive effect at pH 3, and by direct coordination at pH 1.