Bis(hydroxylamino)triazines: High Selectivity and Hydrolytic Stability of Hydroxylamine-Based Ligands for Uranyl Compared to Vanadium(V) and Iron(III)

Abstract

The development of ligands with high selectivity and affinity for uranium is critical in the extraction of uranium from human body, radioactive waste, and seawater. A scientific challenge is the improvement of the selectivity of chelators for uranium over other heavy metals, including iron and vanadium. Flat ligands with hard donor atoms that satisfy the geometric and electronic requirements of the U^VIO₂²⁺ exhibit high selectivity for the uranyl moiety. The bis(hydroxylamino)(triazine) ligand, 2,6-bis[hydroxy(methyl)amino]-4-morpholino-1,3,5-triazine (H₂bihyat), a strong binder for hard metal ions (Fe^III, Ti^IV, V^V, and Mo^VI), reacted with [U^VIO₂(NO₃)₂(H₂O)₂]·4H₂O in aqueous solution and resulted in the isolation of the complexes [U^VIO₂(bihyat)(H₂O)], [U^VIO₂(bihyat)₂]^2-, and {[U^VIO₂(bihyat)(μ-OH)]}₂^2-. These three species are in equilibrium in aqueous solution, and their abundance varies with the concentration of H₂bihyat and the pH. Reaction of H₂bihyat with [U^VIO₂(NO₃)₂(H₂O)₂]·4H₂O in CH₃CN gave the trinuclear complex [U^VI₃O₆(bihyat)₂(μ-bihyat)₂]^2-, which is the major species in organic solvents. The dynamics between the U^VIO₂²⁺ and the free ligand H₂bihyat in aqueous and dimethyl sulfoxide solutions; the metal binding ability of the H₂bihyat over pyridine-2,6-dicarboxylic acid (H₂dipic) or glutarimidedioxime for U^VIO₂²⁺, and the selectivity of the H₂bihyat to bind U^VIO₂²⁺ in comparison to V^VO₄^3- and Fe^III in either U^VIO₂²⁺/V^VO₄^3- or U^VIO₂²⁺/Fe^III solutions were examined by NMR and UV-vis spectroscopies. The results revealed that H₂bihyat is a superior ligand for U^VIO₂²⁺ with high selectivity compared to Fe^III and V^VO₄^3-, which increases at higher pHs. Thus, this type of ligand might find applications in the extraction of uranium from the sea and its removal from the environment and the human body.