Are fancy acidic or neutral ligands really needed for synergism in ionic liquids? A comparative study of lanthanoid extraction in CHCl3 and an ionic liquid

Abstract

The study of trivalent lanthanoid (La, Nd, Eu, Ho and Lu) extraction by two individual ligands, one acidic such as a pyrazolone derivative, 3-methyl-1-phenyl-4-(4-phenylbenzoyl)-pyrazol-5-one (HL), and one neutral such as 5,11,17,23-tetra-tert-butyl-25,26,27,28-tetrakis-(dimethylphosphinoylmethoxy)calix[4]arene (S_IV), was performed in CHCl₃ and in one ionic liquid, IL 1-methyl-1,3-butylimidazolium-bis(trifluoromethanesulfonyl)imide ([C₁C₄im][Tf₂N]), in order to establish and examine the impact of the diluent on individual and synergistic extraction processes. Chloride aqueous medium and constant ionic strength μ = 0.1 M were set in both cases. The parameters of the extraction processes were determined. Comparison between the two diluents was made on the basis of (i) composition of the extracted species: LnL₃·HL in CHCl₃vs. LnL₃ in IL with HL as a single extractant (L⁻ represents the HL anion), and LnL₃·S_IV in the presence of the phosphorus-containing calix[4]arene in both diluents; and (ii) values of the equilibrium constants and the synergistic enhancements; the separation factors between metals were evaluated as well. The IL medium appears far superior to the molecular solvent in terms of individual and overall synergistic extraction efficiencies. However, the huge differences in extraction efficiencies observed in CHCl₃ for chemically different ligands are erased in the IL medium, thus modifying the lever arms on which chemists should rely in order to obtain even better extracting systems in ionic liquid media.