The extraction behavior of several divalent metal cations (M 2+ ) in ionic liquid chelate extraction systems was investigated using several 1-alkyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imide ionic liquids ([Rmim][Tf2N]) as extraction solvent and thenoyltrifluoroacetone (Htta) as extractant. The behavior was compared with that using less hydrophobic 1-alkyl-3-methylimidazolium hexafluorophosphates ([Rmim][PF6]). The extracted species in the [Rmim][Tf2N] systems were neutral M(tta)2 for M = Cu and anionic M(tta)3 -for M = Mn, Co, Ni, Zn and Cd. Conversion of ionic liquid anion from PF6 -to more hydrophobic Tf2N -resulted in changing the species extracted for Ni 2+ from hydrated neutral complex to hydrophobic anionic one. Furthermore, the extractability for these metals was governed by the hydrophobicity of ionic liquid ions. Thus, in the ionic liquid chelate extraction system, selection of a suitable ionic liquid as extraction phase seems to be an important factor for enhancement of extraction selectivity.

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The mutual separation of trivalent lanthanoids is difficult due to their remarkable chemical similarity. Although the solvent extraction method is a best choice for the separation, light lanthanoids such as lanthanum (La 3+ ) generally have lower extractability than heavy ones such as lutetium (Lu 3+ ) because of their electrostatic nature or complexation ability. In other words, the selectively extractive separation of light lanthanoids from heavy ones is very difficult. Several researchers have investigated synergistic extraction systems using crown ethers including 18-crown-6 (18C6) and its analogs as size-recognizing synergistic reagents to enhance the extractability of light lanthanoids. 1 Especially, it is interesting that a synergistic extraction system using 2-thenoyltrifluoroacetone (Htta) and 18C6 as extractants and 1,2-dichloroethane having high polarity as an extraction solvent under the coexistence of hydrophobic perchlorate anion (ClO4 -) resulted in a uniquely high extractability of La 3+ as ion-pair La(tta)2(18C6)Recently, various novel extraction systems using several hydrophobic ionic liquids (ILs) as extraction phase have been investigated for the extraction of metal cations. 3,4Although neutral ligands have been mainly used in the systems including for lanthanoids, 5-7 ionic liquid chelate extraction system [8][9][10] in which an anionic (Brønsted acid-type) chelating agent such as Htta as extractant has superiority that chelates extracted into "non-volatile" IL can be back-extracted quantitatively by using an aqueous acid solution. Furthermore, in the system, metals can be extracted not only as neutral complexes but also as cationic or anionic ones by the ion-exchange process. Thus, the union of the ionic liquid chelate extraction system with the synergistic extraction system using a crown ether may result in the development of a novel selective extraction system for light lanthanoids.In this communication, we report on a fundamental study concerning a novel synergistic extraction system for a possible selective separation of light lanthanoids using an IL, 1-butyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imide ([bmim][Tf2N]), as the extraction solvent and Htta and 18C6 as extractants. As a result, a light lanthanoid, La 3+, was efficiently extracted as a cationic ternary complex by the cation-exchange process, whereas europium (Eu 3+) and Lu 3+ showed relatively low extractability without forming respective ternary complexes. ExperimentalChemicals were reagent-grade materials, and were used without further purification. Ionic liquid [bmim][Tf2N] was synthesized according to a reported procedure. 11 Distilled and deionized water was used throughout.An extraction study was performed as follows. In a centrifuge tube, an aliquot (1 cm 3 ) of [bmim][Tf2N] containing 1 ¥ 10 -2 mol dm -3 of Htta and 18C6 and 5 cm 3 of an aqueous phase containing 5 ¥ 10 -4 mol dm -3 of Ln 3+ (Ln = La, Eu or Lu), 1 ¥ 10 -1 mol dm -3 of lithium chloride and 1 ¥ 10 -2 mol dm -3 of buffer (acetic acid)...