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

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.

Partial Text

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 [001] 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.