3-(1-Methyl-3-imidazolio)propanesulfonate: a precursor to a Brønsted acid ionic liquid

The title compound, C7H12N2O3S, is a zwitterion precursor to a Brønsted acid ionic liquid with potential as an acid catalyst. The C—N—C—C torsion angle of 100.05 (8)° allows the positively charged imidazolium head group and the negatively charged sulfonate group to interact with neighboring zwitterions, forming a C—H⋯O hydrogen-bonding network; the shortest among these interactions is 2.9512 (9) Å. The C—H⋯O interactions can be described by graph-set notation as two R 2 2(16) and one R 2 2(5) hydrogen-bonded rings.

The title compound, C 7 H 12 N 2 O 3 S, is a zwitterion precursor to a Brønsted acid ionic liquid with potential as an acid catalyst. The C-N-C-C torsion angle of 100.05 (8) allows the positively charged imidazolium head group and the negatively charged sulfonate group to interact with neighboring zwitterions, forming a C-HÁ Á ÁO hydrogen-bonding network; the shortest among these interactions is 2.9512 (9) Å . The C-HÁ Á ÁO interactions can be described by graph-set notation as two R 2 2 (16) and one R 2 2 (5) hydrogen-bonded rings.
Portions of this work were funded by the Office of Naval Research and the US Naval Academy Research Foundation. Any opinions, findings, and conclusion or recommendations expressed in this material are those of the author(s) and do not necessarily reflect the views of the US Navy. Ionic liquids (ILs) have proven to be highly versatile materials with an ever expanding suite of chemical applications. An application that has recently shown great promise is the use of functionalized ILs as Brønsted acid catalysts for organic reactions (Cole et al., 2002). These IL catalysts are most commonly prepared by the reaction of 1-methylimidazolium-3-alkyl sulfonate zwitterion with an acid that has a pKa low enough to protonate the sulfonate group (Yoshizawa et al., 2001;Cole et al., 2002). The activity of the IL (e.g. the effectiveness of proton transfer) is significantly impacted by the structure and interactions of the zwitterion. It has been shown that the local structure of ILs is often conserved on transition from the solid state to the liquid state (Triolo et al., 2006;Henderson et al., 2007;Reichert et al., 2007). Thus, a structural analysis of the zwitterion, 1-methylimidaolium-3-propanesulfonate (I) might provide valuable insight into the activity of the Brønsted acid IL catalyst.

Structure Reports Online
The asymmetric unit of the title compound is presented in Figure 1. The dominant intermolecular interactions are Coulombic in nature and are through the charged centers of the zwitterion: the imidazolium ring and the sulfonate group (Fig. 2).
The negative charged sulfonate group is surrounded by four imidazolium head groups forming six close contacts (Table 1).
The interactions of the imidazolium ring hydrogen atoms with the sulfonate group establish two three-dimensional R22 (16) rings. The packing along the b axis (Fig. 3) shows the zwitterions arranged in columns along the c axis. The head-to-tail orientation maximizes the polar interaction and minimizes cation-cation and anion-anion repulsions.

Experimental
Compound I was synthesized following the procedure for similar zwitterionic compounds published by (Yoshizawa et al., 2001). 1,3-Propane sultone (25 g, 0.122 mol) was added dropwise to a solution of 1-methylimidazole (10 g, 0.122 mol) in acetone (40 ml) and stirred, then cooled on an ice bath overnight. A white precipitate was recovered from the reaction solution through filtration and washing with acetone. The product was then dried under vacuum giving a white solid (m.p. 482 K). A colourless crystal suitable for single crystal X-ray diffraction was retrieved from the dried product.

Refinement
(type here to add refinement details) Figures   Fig. 1. The thermal ellipsiod plot of the asymmetric unit of (I). The displacement ellipsiods are shown at the 50% probability level.

Special details
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 > σ(F 2 ) is used only for calculating Rfactors(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.