Ammonium·18-crown-6 bis(trifluoromethylsulfonyl)amide

We report synthesis and characterization of an ammonium-based molten salt, ammonium bis(trifluoromethylsulfonyl)amide-18-crown-6 (1/1), i.e. [NH₄⁺･18C6][Tf₂N⁻] (Tf = SO₂CF₃). Raman spectra shows [NH₄⁺･18C6][Tf₂N⁻] consists of NH₄⁺ ion encapsulated by 18C6 and Tf₂N⁻ anion. The melting point of [NH₄⁺･18C6][Tf₂N⁻] was around 100°C. At 140°C, the viscosity of [NH₄⁺･18C6][Tf₂N⁻] was 14.7 mPa s, the conductivity was 8.0 mS cm⁻¹, and the density was 1.23 g cm⁻³. These properties were comparable to those of common ionic liquids

Suppression of Fast Proton Conduction by Dilution of a Hydronium Solvate Ionic Liquid: Localization of Ligand Exchange

A dilution effect on the proton conduction of a hydronium solvate ionic liquid [H₃O⁺centerdot18C6]Tf₂N, which consists of hydronium ion (H₃O⁺), 18-crown-6-ether ligand (18C6), and bis[(trifluoromethyl)sulfonyl]amide anion (Tf₂N⁻; Tf = CF₃SO₂), has been studied. When [H₃O⁺･18C6]Tf₂N was diluted using equimolar 18C6 solvent, the distinctive fast proton conduction in [H₃O⁺･18C6]Tf₂N was suppressed in stark contrast to the case of common protic ionic liquids. Nuclear magnetic resonance spectroscopy showed that the fast exchange between free 18C6 molecules and coordinated ones, suggesting that the added solvent had induced a local proton exchange rather than a cooperative proton relay

Proton conduction in hydronium solvate ionic liquids affected by ligand shape

We investigated the ligand dependence of the proton conduction of hydronium solvate ionic liquids (ILs), consisting of a hydronium ion (H₃O⁺), polyether ligands, and a bis[(trifluoromethyl)sulfonyl]amide anion (Tf₂N⁻; Tf = CF₃SO₂). The ligands were changed from previously reported 18-crown-6 (18C6) to other cyclic or acyclic polyethers, namely, dicyclohexano-18-crown-6 (Dh18C6), benzo-18-crown-6 (B18C6) and pentaethylene glycol dimethyl ether (G5). Pulsed-field gradient spin echo nuclear magnetic resonance results revealed that the protons of H₃O⁺ move faster than those of cyclic 18C6-based ligands but as fast as those of acyclic G5 ligands. Based on these results and density functional theory calculations, we propose that the coordination of a cyclic ether ligand to the H₃O⁺ ion is essential for fast proton conduction in hydronium solvate ILs. Our results attract special interest for many electro- and bio-chemical applications such as electrolyte systems for fuel cells and artificial ion channels for biological cells

Glyme-Lithium Bis(trifluoromethylsulfonyl)amide Super-concentrated Electrolytes: Salt Addition to Solvate Ionic Liquids Lowers Ionicity but Liberates Lithium Ions

Solvate ionic liquids (ILs) such as binary equimolar mixtures of glymes (ethyleneglycol-dimethylether or CH₃(OCH₂CH₂)nOCH₃) and lithium bis(trifluoromethylsulfonyl)amide (LiTf₂N; Tf = SO₂CF₃) are known to show identical self-diffusion coefficients for glymes and Li⁺ ions. Here, we report that the addition of LiTf₂N to the solvate ILs drastically changes their electrolyte properties. When the lithium salts are added to give the super-concentrated electrolytes with [O]/[Li⁺] = 3 (molar ratio of ether oxygen to Li⁺), ligand exchange or hopping conduction of Li⁺ takes place for triglyme (G3; n = 3) and tetraglyme (G4; n = 4). In addition, the Li⁺ transference number tLi⁺(EC), electrochemically measured under anion blocking conditions, increases about 3–6 times compared with the solvate ILs. Consequently, segmental motion of glymes apparently affects the transport properties even for the shorter G3 in the super-concentrated region. The relationship between the coordination structure and the transport properties are also discussed as a function of ionicity, the extent of the contribution of self-diffusion to the actual ion conduction. Plots vs ionicity demonstrate that a clear line can be drawn between the solvate ILs and the super-concentrated electrolytes