Structural characterization of a ternary Fe(III)-U(VI)-citrate complex

Abstract

Summary Citric acid, a naturally occurring hydroxycarboxylic acid, forms bidentate, tridentate, dinuclear, or polymeric species, depending on the metal. In the presence of iron and uranium a ternary Fe: U: citric acid complex is formed. We determined the molecular structure of the ternary complex in both the aqueous and solid phase. Fourier transform infrared spectroscopy (FTIR) showed the presence of uni- and bi-dentate bonding of the citric acid to the metals, and the involvement of the α-hydroxyl group. Analysis of molecular fragments generated from time-of-flight secondary ion mass spectroscopy (TOF-SIMS) confirmed the bonding of terminal carboxylate groups of citric acid to uranium and iron. Extended X-ray absorption fine structure (EXAFS) analysis revealed the dinuclear nature of the Fe bonding to citrate and mononuclear uranium species with citrate. Coordination of a sodium atom to the iron was also noted. The proposed empirical formula for the complex is [NaFe2(μ-O)(μ-citrato)(OH)2(H2O)(C6H4O7)bis(UO2)(C6H5O7)]7-. The structure consists of a dinuclear ferric ion core coordinated through an oxy (μ-O) and a carboxylate (μ-citrato) group of citric acid. The mononuclear uranyl ions are coordinated in bidentate fashion to the central carboxylate groups of citric acid and tridentate coordinated to citric acid.