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

The interaction of uranyl(VI) nitrate with a series of bis(2-hydroxyaryl)imine and bis(2-hydroxyaryl)amine derivatives (H₂L¹- H₂L⁷) 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 UO₂ ²⁺ 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 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 UO₂ ²⁺ 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 UO₂ ²⁺ relative to the more flexible Schiff bases H₂L¹ - H₂L⁵. A very significant synergistic enhancement of the extraction of UO₂ ²⁺ by H₂L¹‐H₂L⁴ and H₂L⁷ was observed in the presence of a 10-fold excess of 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 UO₂ ²⁺ over Eu³⁺ in all cases; separation of the uranyl ion from the rare earths is important in mineral processing;