The role of bis(trifluoromethanesulfonyl)imide (Tf2N−) anions in the ionic liquid–water distribution systems of Eu(III) chelates with 2-thenoyltrifluoroacetone (Htta) was investigated by liquid–liquid distribution and time-resolved laser-induced fluorescence spectroscopy (TRLFS). The extraction constants of neutral Eu(tta)3 and anionic Eu(tta)4− chelates in 1-alkyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imide ([Cnmim][Tf2N]) were determined by analyzing the distribution equilibrium. The effect of the ionic liquids on the distribution constant of the neutral Eu(tta)3 chelate was evaluated by the regular solution theory. The distribution constant of Eu(tta)3 in [Cnmim][Tf2N] was increased dramatically by the solvation effects of Eu(tta)3 in [Cnmim][Tf2N]. TRLFS for [Eu(tta)3(H2O)3] synthesized revealed that the Eu(tta)3 chelate was almost completely dehydrated in a series of [Cnmim][Tf2N] (n = 2–10). The Eu(tta)3 chelate exists as di- or tri-hydrates in 1-ethyl-3-methylimidazolium perchlorate ([C2mim][ClO4]) containing 20 mol dm−3 water, whereas mono-hydrated chelate was formed in [C2mim][Tf2N, ClO4] in the presence of 0.50 mol dm−3 Tf2N− and 20 mol dm−3 water. These results show that the coordinated water molecules of [Eu(tta)3(H2O)3] were replaced by the Tf2N− anions. In fact, an anionic adduct, [Eu(tta)3(Tf2N)]−, was observed by electrospray ionization mass spectrometry in the presence of [C4mim][Tf2N].

Distribution constants of 2-thenoyltrifluoroacetone (Htta) and its Ni(II)and Cu(II) chelates between 1-alkyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imide (CnmimTf2N; n = 4, 6, and 8) as ionic liquid (IL) and an aqueous phase were determined. The enol fraction of Htta in ILs was spectrophotometrically measured to calculate the distribution constant of the enol form (KD,HE) of Htta. The KD,HE values in ILs were evaluated by comparing those in various molecular solvents such as alkanes, aromatic hydrocarbons, chlorohydrocarbons, ethers, ketones, and esters previously reported on the basis of the regular solution theory (RST). It was elucidated that the IL solutions of Htta (enol) can be taken as apparently regular solutions as expected in the organic solvents. On the other hand, the effect of ILs on the distribution constant of metal(II) chelates (KD,M) was fairly complicated. The Cu(tta)2-IL solutions behaved like the alkane and aromatic hydrocarbon solutions but the Ni(tta)2-IL (C4mimTf2N) like ether and ketone solutions. In the Ni(II) case, some specific interactions between the Ni(II) chelate and IL was suggested. Finally, the solubility parameters of ILs were calculated using KD,HE by RST and were in good agreement with the literature values obtained by the enthalpy of vaporization.

The extraction constant and the two-phase stability constant (KD,Mβ3) of tris(2-thenoyltrifluoroacetonato)europium(III) between 1-butyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imide ([C4mim][Tf2N]) as an ionic liquid and an aqueous phase were determined by considering the extraction equilibria including anionic tetrakis(2-thenoyltrifluoroacetonato)europate(III). Specific solute–solvent interactions between the neutral Eu(III) chelate and [C4mim][Tf2N] molecules were revealed from the relationships between the distribution constant of the enol form of 2-thenoyltrifluoroacetone (Htta) as a proton chelate and the distribution constant (KD,M) of the neutral Eu(III) chelate because the [C4mim][Tf2N] system gave the high KD,Mβ3 value compared with those in conventional molecular solvents such as benzene and 1,2-dichloroethane. The coordination environment of Eu3+ in the neutral Eu(III) chelate in [C4mim][Tf2N] was investigated by time-resolved laser-induced fluorescence spectroscopy and infrared absorption spectroscopy. Both methods consistently indicated that not only the Eu(III) chelate extracted but also Eu(tta)3(H2O)3 synthesized as a solid crystal were almost completely dehydrated in [C4mim][Tf2N] saturated with water. Consequently, the higher KD,M or extractability of the neutral Eu(III) chelate in the [C4mim][Tf2N] system can be ascribed to the dehydration of the Eu(III) chelate, which is caused by the specific solvation with [C4mim][Tf2N] molecules.Graphical abstractThe distribution equilibrium of Eu(III) between an ionic liquid, 1-butyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imide ([C4mim][Tf2N]), and an aqueous phase with 2-thenoyltrifluoroacetone (Htta) was studied to determine the two-phase stability constant. Furthermore, the coordination environment of Eu3+ in the neutral Eu(III) chelate in the [C4mim][Tf2N] system was investigated by time-resolved laser-induced fluorescence spectroscopy and infrared absorption spectroscopy. Not only the neutral Eu(tta)3 chelate extracted but also Eu(tta)3(H2O)3 synthesized as a solid crystal were almost completely dehydrated in [C4mim][Tf2N] saturated with water.Download full-size imageHighlights► Two-phase stability constant of the Eu(III) chelate in ionic liquid was determined. ► Solute–solvent interactions between the Eu(III) chelate and ionic liquid were revealed. ► The Eu(III) chelate was completely dehydrated in ionic liquid saturated with water. ► The Eu(III) chelate can be solvated or coordinated with ionic liquid molecules. ► Specific solvation by ionic liquid molecules caused higher extractability of Eu(III).