Ionic liquid binary mixtures: how different factors contribute to determine their effect on the reactivity

Abstract

We studied how mixing ionic liquids affected the rate of the Diels Alder reaction between 9-anthracenemethanol and N-ethylmaleimide. The mixtures considered spanned the whole compositional range and differed for the anion or the cation. These comprised mixtures of the ionic liquid 1-benzyl-3-butyl-imidazolium bis-(trifluoromethanesulfonyl)imide ([Bzbim][NTf₂]) with 1-(2,3,4,5,6-pentafluorophenyl)-3-butyl-imidazolium bis-(trifluoromethanesulfonyl)imide ([Bz(F₅)bim][NTf₂]) or 1-benzyl-3-butyl-imidazolium tetrafluoroborate ([Bzbim][BF₄]). Moreover we studied the reaction in a set of mixtures containing an aliphatic and an aromatic cation, namely [Bzbim⁺] and N-butyltriethylammonium ([bEt₃N⁺]) sharing the [NTf₂⁻] anion. In this latter case we carried out RLS measurements, UV-Vis spectra of the solvatochromic probe Nile Red and ¹H NMR measurements to gain insights into how the structural organization of the mixtures changes as a function of composition. Altogether, these results show a concomitant effect of solvent viscosity and structural organization on the rate of the process. Finally, no obvious correlation was found between the kinetic trend and the polarity of the mixtures, as expressed by solvatochromic parameters.