Role of ligand structure and basicity on the extraction of uranyl isoxazolonate adducts

The extraction behavior of uranium(VI) from dilute nitric acid was investigated employing 3-phenyl-4-acetyl-5-isoxazolone (HPAI) as the chelating ligand and several amides with varying degree of branching such as di-2-ethylhexylacetamide (D2EHAA), di-2-ethylhexylpropanamide (D2EHPrA), di-2-ethylhexylisobutyramide (D2EHiBA), and di-2-ethylhexylpivalamide (D2EHPvA) as neutral donors in toluene medium. The two-phase extraction constant (log K-ex) for the binary species UO2(PBI)(2)H2O was found to be 0.66. The overall equilibrium constants (log K) for the ternary species of the type UO2(PAI)(2)B are estimated to be 5.29 (D2EHAA), 5.02 (D2EHPrA), 4.86 (D2EHiBA), and 4.28 (D2EHPvA), respectively, which vary linearly with the basicity of the amides. The role of steric factors in the co-ordination of the amides to the uranyl beta-diketones and the adduct formation in the case of the ternary complexes was explained on the basis of molecular modeling.