Ionic Liquid Extraction of Metal(II, III) Complexes with Macrocyclic and Chelating Ligands

The ionic liquid (IL) extraction of various metal(II, III) complexes with macrocyclic and anionic chelating ligands has been investigated to clarify the solvent effect of ILs and to demonstrate the specificity and superiority of the IL extraction systems over conventional ones. The extraction equilibrium of Eu (III) with 2-thenoyltrifluoroacetone (Htta) in 1-alkyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imide ([C(n)mim] [Tf2N]) was studied. Specific solute-solvent interactions between Eu(tta)(3) and [C(n)mim] [Tf2N] were revealed by evaluating the distribution constant and laser-induced fluorescence spectroscopy. The coordination of Tf2N- to Eu(tta)(3) results in higher extractability in [C(n)mim] [Tf2N]. The IL synergistic extraction with Htta and a macrocyclic ligand including 18-crown-6 and cis-dicyclohexano-18-crown-6 was developed. The extractability of lighter lanthanides(III) was remarkably enhanced by a synergistic effect, which depends on size-fitting into the cavity of crown ethers. A macrocyclic ligand (H-2 beta DA18C6) composed of diaza-18-crown-6 (DA18C6) and two 4-acyl-5-pyrazolones was synthesized for the IL extraction of Sr(II). The extraction performance of H-2 beta DA18C6 was significantly enhanced only in [C(n)mim] [Tf2N] by the intramolecular cooperative effect. A shortening of the average Sr-O distance in the Sr(beta DA18C6) complex in IL was revealed from extended X-ray absorption fine structure spectroscopy. Furthermore, the DA18C6 derivative having two 5-chloro-8-quinolinol moieties indicated high performance for the extraction and fluorescence detection of Cd(II).