Complexation of uranium(VI) with glutarimidoxioxime: Thermodynamic and computational studies

The complex formation between a cyclic ligand glutarimidoxioxime (denoted as HL in this paper) and UO is studied by potentiometry and microcalorimetry. Glutarimidoxioxime (HL), together with glutarimidedioxime (HL) and glutardiamidoxime (HL), belongs to a family of amidoxime derivatives with prospective applications as binding agents for the recovery of uranium from seawater. An optimized procedure of synthesis that leads to the preparation of glutarimidoxioxime in the absence of other amidoxime byproducts is described in this paper. Speciation models based on the thermodynamic results from this study indicate that, compared with HL and HL, HL forms a much weaker complex with UO, UO(L), and cannot effectively compete with the hydrolysis equilibria of UO under neutral or alkaline conditions. DFT computations, taking into account the solvation by including discrete hydration water molecules and bulk solvent effects, were performed to evaluate the structures and energies of the possible isomers of UO(L). Differing from the tridentate or η-coordination modes previously found in the U(vi) complexes with amidoxime-related ligands, a bidentate mode, involving the oxygen of the oxime group and the nitrogen of the imino group, is found to be the most probable mode in UO(L). The bidentate coordination mode seems to be stabilized by the formation of a hydrogen bond between the carbonyl group of HL and a water molecule in the hydration sphere of UO.