Date Published: September 01, 2016
Publisher: International Union of Crystallography
Author(s): Allan Jay P. Cardenas, Molly O’Hagan.
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.
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 proton-transfer 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 ▸).
The asymmetric unit consists of one bis(trifluoromethanesulfonyl)amide anion and one dimethylformamidium cation (Fig. 1 ▸): when the components were mixed, the acidic N—H proton of HNTf2 was transfered to the formyl group of dimethylformamide. The dimethylformamidium C4—O5 and N2—C4 bond lengths are 1.2983 (16) and 1.2888 (15) Å respectively, which reflect the delocalization of charge via π-electrons. The N2—C4—O5 angle does not deviate from the expected 120° of an sp2-hybridized carbon atom [120.37 (11)°]. The bis(trifluoromethanesulfonyl)amide anion features S1—N1 and S2—N1 bond distances of 1.6035 (11) and 1.5947 (11) Å, respectively.
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 H⋯N distance is 1.98 (3) Å. The other is a non-conventional C—H⋯O 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—H⋯O2 distance is 2.57 Å (Table 1 ▸). Together, these generate an (7) loop. A further very weak C—H⋯O interaction links the ion pairs into an  chain.
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.
A literature procedure was followed to synthesize [(DMF)H]NTf2 (I) (Hou et al., 2014 ▸). Equimolar amounts of of dimethylformamide (17.6 mmol, 1.29 g) and bis(trifluoromethanesulfonyl)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.
Crystal data, data collection and structure refinement details are summarized in Table 2 ▸. H atoms were positioned geometrically and allowed to ride on their parent atoms: C—H = 0.93–0.96 Å with Uiso(H) = 1.5Ueq(C-methyl) and 1.2Ueq(C) for other H atoms. The methyl groups were refined as rotating groups.