Role of bis(trifluoromethanesulfonyl)imide anions in the ionic liquid extraction ofeuropium(III) chelates
H. Okamura1), N. Aoyagi2), K. Shimojo1), H. Naganawa1) and H. Imura3)
1) : Interfacial reaction-field chemistry Gr., JAEA 2): Radiochemistry Gr., JAEA3): Kanazawa Univ.

decrease in water concentration. These observations prove theligand displacement between water and Tf2N− in Eu(tta)3 in ILs.
Figure 2(b) shows the effect of the Tf2N− concentration in1,2-dichloroethane on the hydration number. Almost completedehydration of [Eu(tta)3(H2O)3] was observed more than 3.4 ×10−2 M Tf2N− (1.0 vol% [C4mim][Tf2N]). This result reveals
that the coordination ability of Tf2N− to Eu(tta)3 is stronger thanthat of water. A stable adduct species, [Eu(tta)3(Tf2N)]−, wasfound by electrospray ionization mass spectrometry in thepresence of [C4mim][Tf2N].
This study revealed that, in the IL extraction system, thehydrophobic metal chelate is effectively formed by thecoordination of Tf2N−, resulting in the enhancement in theextractability of the neutral chelates. The knowledge obtained inthis study can lead to the further progress in the development ofIL extraction systems of metal ions.

CF3O

Intensity / a.u.

[Eu(tta)3(H2O)3] in [C4mim][Tf2N]

OO

S
F3C

[Eu(tta)3(D2O)3] in [C4mim][Tf2N]

H2O
H2O
H2O
OEu

SO O
S CF3

[Eu(tta)3(H2O)3]

3
2550 600

Fig.1

H3C

1650 700

750

N + N

0

CnH2n+1F3COO
S N S CF3OO[Cnmim][Tf2N]

Fluorescence decay for [Eu(tta)3(H2O)3] in [C4mim][Tf2N]
saturated with H2O (red) and D2O (blue) following pulsed
394 nm excitation. Concentration of [Eu(tta)3(H2O)3] was 1.0
× 10−4 M.
44(a)Hydration number

Hydration number

Liquid–liquid extraction is one of the separation methods
based on the distribution of solutes between two immisciblesolvents. Recently, ionic liquids (ILs) defined as roomtemperature molten salts have been widely investigated as novel
alternatives to conventional organic solvents. Application of ILs
as extraction media in liquid–liquid extraction has many
advantages [1]. In the IL system, not only neutral species but
also charged species can be extracted by ion exchange via thecationic or anionic constituent of the ILs [2]. These uniquemechanisms often lead to an enhancement in the extractabilityof a particular metal ion. However, the cause of the extraction
enhancement except the ion-exchange has yet to be clarified.
Time-resolved laser-induced fluorescence spectroscopy(TRLFS) has been widely used as an analytical technique for thespeciation of lanthanides(III). The fluorescence lifetime gives
the information of the hydration state in the first coordination
sphere of the metal ion. The number of coordinated watermolecules can be determined by the fluorescence decay
constants in H2O- and D2O-containing solvents.
In this study, the extraction of Eu(III) with 2thenoyltrifluoroacetone (Htta) in 1-alkyl-3-methylimidazolium
wasbis(trifluoromethanesulfonyl)imide([Cnmim][Tf2N])
investigated to clarify the role of the Tf2N− anions in the IL
extraction system [3]. The hydration state of neutral Eu(tta)3 in aseries of ILs was examined by TRLFS.
In the [Cnmim][Tf2N] systems, Eu(III) was extracted as theneutral Eu(tta)3 and the anionic Eu(tta)4− chelates. The
extraction constants of Eu(tta)3 (Kex,3) in [Cnmim][Tf2N] arehigher than those in conventional organic solvents. The twophase stability constant of Eu(tta)3 was calculated from Kex,3,and was evaluated with the aid of the regular solution theory.
The values against the solubility parameters in [Cnmim][Tf2N]
and in oxygen-containing organic solvents such as ketones,
ethers, and esters show a large positive deviation from the linear
relationship obtained for nonpolar or inert solvents. Thisdeviation demonstrates the existence of specific solute–solvent
interactions between Eu(tta)3 and [Cnmim][Tf2N], i.e., innerand/or outer-sphere solvation effects.
To clarify such specific solvation effect, the hydration state ofEu(tta)3 in [Cnmim][Tf2N] was examined by TRLFS using solid[Eu(tta)3(H2O)3] synthesized (Fig. 1). The [Eu(tta)3(H2O)3]complex was almost completely dehydrated in the series of[Cnmim][Tf2N] saturated with water, whereas the hydrated
chelate was observed in organic solvents. It was, therefore,plausible that the Tf2N− anions could coordinate to Eu(tta)3 andreplace the coordinated water molecules of [Eu(tta)3(H2O)3].
Figure 2(a) shows the hydration number as a function ofwater concentration in 1-ethyl-3-methylimidazolium perchlorate([C2mim][ClO4]) and [C2mim][Tf2N, ClO4]. The Eu(tta)3
chelate exists as di- or tri-hydrates in [C2mim][ClO4] containing
20 M water, indicating that the ClO4− anion does not give rise tothe dehydration of Eu(tta)3. In contrast, mono-hydrated Eu(tta)3
chelate was formed in [C2mim][Tf2N, ClO4] containing 0.50 M
Tf2N− and 20 M water. This means that the Tf2N− anions
displaced the coordinated water molecules of [Eu(tta)3(H2O)3].
The hydration number in [C2mim][Tf2N, ClO4] decreased with a321005

1015[water] / M

2025(b)32100.0

0.5

1.0

1.5

2.0-[Tf2N ] / M

Fig.2 (a) Effect of water concentration in [C2mim][ClO4] (●) and[C2mim][Tf2N, ClO4] (●) and (b) effect of Tf2N− concentrationin 1,2-dichloroethane (●) on the hydration number of Eu(tta)3.
(a) Tf2N− concentration in [C2mim][Tf2N, ClO4] was 0.50 M
and (b) the solution was saturated with water. Concentrationof [Eu(tta)3(H2O)3] was 1.0 × 10−4 M.

References[1] H. Okamura et al., Anal. Chem. 84, 9332 (2012).
[2] H. Okamura et al., Polyhedron 31, 748 (2012).
[3] H. Okamura et al., RSC Adv. 7, 7610 (2017).