Uranyl(VI) binding by bis(2-hydroxyaryl)diimine and bis(2-hydroxyaryl)diamine ligand derivatives. Synthetic, X-ray, DFT and solvent extraction studies

Abstract

The interaction of uranyl(VI) nitrate with a series of bis(2-hydroxyaryl)imine (H2L1–H2L5) and bis(2-hydroxyaryl)amine (H2L8, H2L9) derivatives incorporating 1,3-dimethylenebenzene or 1,3-dimethylenecyclohexane bridges between nitrogen sites is reported. Crystalline complexes of type [UO2(H2L)(NO3)2] (where H2L is H2L1–H2L4) were isolated from methanol. X-ray structures of the complexes of H2L1, H2L2 and H2L4 show that each of these neutral ligands bind to their respective UO22+ centres in a bidentate fashion in which coordination only occurs via each ligand’s hydroxy functions. Two bidentate nitrate anions complete the metal’s coordination sphere in each complex to yield hexagonal bipyramidal coordination geometries. A density functional theory (DFT) investigation of [UO2(H2L1)(NO3)2] in a simulated methanol environment is in accord with this complex maintaining its solid state conformation in solution. Solvent extraction experiments (water/chloroform) employing H2L1–H2L7 in the organic phase and uranyl(VI) nitrate in the aqueous phase showed that both amine derivatives, H2L8 and H2L9, yielded enhanced extraction of UO22+ over the corresponding imine derivatives, H2L1 and H2L2. These results were further compared with those obtained for the corresponding Schiff bases incorporating 1,2-phenylene and 1,2-cyclohexane bridged ligands, H2L6 and H2L7; these more rigid systems also yielded enhanced extraction of UO22+ relative to the more flexible Schiff bases H2L1–H2L5. A very significant synergistic enhancement of the extraction of UO22+ by H2L1–H2L4 and H2L7 was observed in the presence of a 10-fold excess of n-octanoic acid; the influence of pH on extraction efficiency was also investigated. A parallel set of experiments employing H2L1–H2L9 as extractants for europium(III) nitrate indicated a clear uptake preference for UO22+ over Eu3+ in all cases; separation of the uranyl ion from the rare earths is an important objective in mineral processing.