Role of Tf2N− anions in the ionic liquid–water distribution of europium(iii) chelates

Abstract

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 ([C_nmim][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 [C_nmim][Tf₂N] was increased dramatically by the solvation effects of Eu(tta)₃ in [C_nmim][Tf₂N]. TRLFS for [Eu(tta)₃(H₂O)₃] synthesized revealed that the Eu(tta)₃ chelate was almost completely dehydrated in a series of [C_nmim][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].