Crystal structure of dimethylformamidium bis(trifluoromethanesulfonyl)amide: an ionic liquid

The cation and anion of the title salt are linked by an O—H⋯N hydrogen bond and a C—H⋯O interaction, resulting in a high viscosity and a crystallization temperature slightly lower than ambient temperature.


Chemical context
A ionic liquid, also known as a liquid electrolyte, is a salt or an ion pair that remains in a liquid state below 373 K (Ghandi, 2014): such species extend the selection of solvents or media of chemical processes. The study of its solid-state structure can facilitate the exploration of other intermolecular forces of attraction besides electrostatic forces that govern the properties of these ionic liquids such as melting point, acidity, ion mobility, diffusion and viscosity. In this study we report the crystal structure of an organic liquid salt formed by a protontransfer reaction between bis(trifluoromethanesulfonyl)amine and dimethylformamide. This protic ionic liquid has been used as a solvent, an electrolyte and a substrate for electrocatalysis (Hou et al., 2014).

Supramolecular features
The ion pair features two hydrogen bonds (Table 1). One is between the acidic hydrogen atom attached to the formyl oxygen atom of the dimethylformamidium cation and the nitrogen atom of the bis(trifluoromethanesulfonyl)amide anion: the HÁ Á ÁN distance is 1.98 (3) Å . The other is a nonconventional C-HÁ Á ÁO hydrogen bond between the formyl hydrogen atom of the dimethylformamidium cation and one of the sulfoxide oxygen atoms of the anion (Desiraju, 1991). The C4-HÁ Á ÁO2 distance is 2.57 Å (Table 1). Together, these generate an R 2 2 (7) loop. A further very weak C-HÁ Á ÁO interaction links the ion pairs into an [001] chain.

Database survey
A CSD search (Web CSD version 1.1.1; May 4, 2016) found no structures that have the same ion pairing. Some structures feature the same bis(trifluoromethanesulfonyl)amide anion but different cations, which are usually metal complexes.

Synthesis and crystallization
A literature procedure was followed to synthesize [(DMF)H]NTf 2 (I) (Hou et al., 2014). Equimolar amounts of of dimethylformamide (17.6 mmol, 1.29 g) and bis(trifluoro-methanesulfonyl)amine (17.8 mmol, 5.0g) were mixed together after cooling each reagent to 238 K. The solution was stirred at room temperature until it formed a light-yellow viscous solution. The solution was then left to stand undisturbed at room temperature and colorless blocks of (I) were isolated.

Special details
Experimental. Absorption correction: SADABS-2014/5 (Bruker,2014/5) was used for absorption correction. wR2(int) was 0.0777 before and 0.0530 after correction. The Ratio of minimum to maximum transmission is 0.7795. The λ/2 correction factor is 0.00150. Geometry. All esds (except the esd in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell esds are taken into account individually in the estimation of esds in distances, angles and torsion angles; correlations between esds in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell esds is used for estimating esds involving l.s. planes. Refinement. Refinement of F 2 against ALL reflections. The weighted R-factor wR and goodness of fit S are based on F 2 , conventional R-factors R are based on F, with F set to zero for negative F 2 . The threshold expression of F 2 > 2sigma(F 2 ) is used only for calculating R-factors(gt) etc. and is not relevant to the choice of reflections for refinement. R-factors based on F 2 are statistically about twice as large as those based on F, and R-factors based on ALL data will be even larger.