Complexation of uranium(vi) with glutarimidoxioxime: thermodynamic and computational studies

Abstract

The complex formation between a cyclic ligand glutarimidoxioxime (denoted as HL^III in this paper) and UO₂²⁺ is studied by potentiometry and microcalorimetry. Glutarimidoxioxime (HL^III), together with glutarimidedioxime (H₂L^I) and glutardiamidoxime (H₂L^II), 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 H₂L^I and H₂L^II, HL^III forms a much weaker complex with UO₂²⁺, UO₂(L^III)⁺, and cannot effectively compete with the hydrolysis equilibria of UO₂²⁺ 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 UO₂(L^III)⁺. Differing from the tridentate or η²-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 UO₂(L^III)⁺. The bidentate coordination mode seems to be stabilized by the formation of a hydrogen bond between the carbonyl group of HL^III and a water molecule in the hydration sphere of UO₂²⁺.