An Ionic Liquid Consisting of Crown Ether – Coordinated Hydronium Cation and Amide Anion

Abstract

There are reports on ionic liquids (ILs) with strongly Lewis-acidic metal cations such as Li+ and K+ coordinated by ether-ligands to decrease charge density [1-2]. Crown ether 18-crown-6 (18C6) has a suitable cavity for encapsulation of K+ and hydronium ion (H3O+) [3-4]. Various compounds with H3O+ encapsulated by 18C6 ligand ([H3O+·18C6] cation) have been reported, the counteranions of which are such as hexafluorophosphate (PF6 –), tetrafluoroborate (BF4 –) and trifluoromethanesulfonate (TfO–; Tf = CF3SO2) ions [3-4]. However, their melting point are higher than 100 °C. Furthermore, as far as we know, nothing has been reported on [H3O+·18C6] complexes using bis(trifluoromethylsulfonyl)amide (Tf2N–) anion. Here, we report ILs that consist of [H3O+·18C6] cation or [H3O+· dicyclohexyl-18C6 (Dh18C6)] cation and an amide anion. The IL comprised of [H3O+·18C6] cation or [H3O+·Dh18C6] cation and Tf2N– anion was synthesized by adding equimolar amount of 18C6 or Dh18C6 and HTf2N in water, followed by vacuum-drying. In the obtained samples, a characteristic infrared band appeared at 2200 cm–1 that was not observed in pure 18C6, was attributable to the first harmonic of symmetric bending mode (Figure 1) [5]. The H2O content of the 18C6 complex and the Dh18C6 complex was estimated to be 3.29 wt% and 2.61 wt% from Karl-Fischer titration, in good agreement with the ideal value (3.20 wt% and 2.68 wt%). From elemental analysis, infrared spectra and Karl-Fischer titration, the composition of the obtained complexes were concluded as [H3O+·18C6][Tf2N–] and [H3O+·Dh18C6][Tf2N–] (Figure 2). 1H NMR spectrum of [H3O+·18C6][Tf2N–] contained a sharp singlet near 11 ppm, characteristic of H3O+ encapsulated by 18C6. The melting point of [H3O+·18C6][Tf2N–] was 66–68 °C and [H3O+·Dh18C6][Tf2N–] was a yellowish transparent liquid at room temperature (RT) (Figure 3). At 70 °C, the viscosity and conductivity of [H3O+·18C6][Tf2N–] were 39.5 mPa s and 1.9 mS cm–1, while those of [H3O+·Dh18C6][Tf2N–] were 33.8 mPa s and 1.4 mS cm–1. In linear sweep voltammetry, potential for hydrogen production by [H3O+·18C6][Tf2N–] was similar to those of general protic liquids. The crown ether-coordinated hydronium amide IL may have potential application as electrolyte materials toward a new class of fuel cells. [1] T. Tamura et al., Chem. Lett., 39, 753 (2010). [2] Y. Song, H. Jing, B. Li, and D. Bai, Chem. Eur. J., 17, 8731 (2011). [3] R. Chénevert et al., Can. J. Chem., 60, 853 (1982). [4] G. S. Heo and R. A. Bartsch, J. Org. Chem., 47, 3557 (1982). [5] R. Chénevert et al., Can. J. Chem., 62, 2293 (1984). Figure 1