2,3-Dimethyl-1H-imidazol-3-ium benzenesulfonate–1,2-dimethyl-1H-imidazole co-crystal

The title compound exists with one protonated imidazolium ring, one neutral imidazole ring, and a benzenesulfonate anion in the asymmetric unit. The imidazole rings are held together through hydrogen bonding via a protonated nitrogen on the ring.

In the title co-crystal, C 5 H 9 N 2 + ÁC 6 H 5 O 3 S À ÁC 5 H 8 N 2 , the two 1,2-dimethylimidazole rings exist as partially protonated moieties in the asymmetric unit as a two-part disordered unit wherein the acidic hydrogen atom is bound to each ring. The two imidazolium cations share a strong hydrogen bond via the acidic hydrogen atom, which is disordered between two positions, being bonded to the first versus second imidazole ring in a 0.33 (2) to 0.67 (2) ratio. A benzene sulfonate anion is present for charge balance and interacts with the aromatic H atoms on both imidazole rings as well as with the methyl groups on the rings.

Structure description
The title compound ( Fig. 1) crystallizes with two 1,2-dimethylimidazolium cations in the asymmetric unit. The two imidazole rings are each partially protonated, wherein the acidic hydrogen atom is bound between the two N atoms of the aromatic ring in a 0.33 (2) to 0.67 (2) ratio. Hydrogen bonding appears to the dominant intermolecular interaction with each molecule or ion exhibiting interactions (Fig. 2). For instance, the shortest hydrogen bonds are N-HÁ Á ÁN links between the imidazolium rings with HÁ Á ÁN = 1.83 (8) and 1.90 (8) Å . This bonding arises from the disordered hydrogen atom, which appears to be shared between the two rings. Further, cation-anion C-HÁ Á ÁO interactions occur between the aromatic H atoms and the sulfonate O atoms. Finally, there are anion-anion interactions wherein O atoms of the sulfonate group interact with hydrogens on the benzene rings. A summary of the distances for the hydrogen bonds is found in Table 1.
For a related structure with a chloride anion, see Kelley et al. (2013).

Figure 3
Proposed mechanism leading to the formation of the crystallized product reported herein.

Figure 1
The title compound shown with 50% probability ellipsoids. Only the major component is shown.

Figure 2
Packing diagram of the title compound viewed down the (010) plane showing a layered network of ion pairs held together through hydrogen interactions. Both parts of the disorder are shown. 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. H atoms attached to carbon atoms were positioned geometrically and constrained to ride on their parent atoms. C-H bond distances were constrained to 0.95 Å for aromatic and alkene C-H moieties, and to 0.98 Å for CH 3 moieties, respectively. The N-H proton hydrogen bonding between atoms N1 and N3 was found to be disordered and was refined as split between two positions. The H atoms were assigned as bonded to a planar (sp 2 hybridized) N atom, respectively with fixed bond angles and torsion angles, but the N-H bond distances were allowed to refine to account for asymmetry induced by charge and hydrogen bonding (AFIX 44 command). N-H distances refined to 0.83 (5) for N1 -H1 and to 0.89 (2) for N3-H3, occupancies refined to 0.33 (2) for H1 and 0.67 (2) for H3. Methyl CH 3 were allowed to rotate but not to tip to best fit the experimental electron density. U iso (H) values were set to a multiple of U eq (C/N) with 1.5 for CH 3 and 1.2 for C-H and NH + , respectively.