Bis(2-methyl-1H-benzimidazol-3-ium) naphthalene-1,5-disulfonate

The asymmetric unit of the title compound, 2C8H9N2 +·C10H6O6S2 2−, contains a 2-methylbenzimidazolium cation and one half of a naphthalene-1,5-disulfonate anion. The formula unit is generated by an inversion center. In the crystal, N—H⋯O hydrogen bonds link the components into chains along [001]. In addition, weak C—H⋯O hydrogen bonds and weak C—H⋯π interactions are observed. The methyl H atoms were refined as disordered over two sets of sites with equal occupancy.

The asymmetric unit of the title compound, 2C 8 H 9 N 2 + Á-C 10 H 6 O 6 S 2 2À , contains a 2-methylbenzimidazolium cation and one half of a naphthalene-1,5-disulfonate anion. The formula unit is generated by an inversion center. In the crystal, N-HÁ Á ÁO hydrogen bonds link the components into chains along [001]. In addition, weak C-HÁ Á ÁO hydrogen bonds and weak C-HÁ Á Á interactions are observed. The methyl H atoms were refined as disordered over two sets of sites with equal occupancy.
Recently the main focus for sulfonic acids has been in crystal engineering via hydrogen bonded assembly of sulfonic acid and organic base (Zhang et al., 2005). As an extension of our study concentrating on hydrogen bonded assemblies of organic acids and organic bases (Jin et al., 2012) herein we report the crystal structure of the title compound (I).
The molecular structure of (I) is shown in Fig. 1. The anion lies across an inversion center. In the crystal, N-H···O hydrogen bonds link the components into chains along [001] (Fig. 2). In addition, weak C-H···O hydrogen bonds and weak C-H···π interactions are observed.
Experimental 2-Methyl-1H-benzimidazole (24.0 mg, 0.20 mmol) was dissolved in 10 ml of methanol, and naphthalene-1,5-disulfonic acid tetrahydrate (36 mg, 0.1 mmol) was added. The solution was stirred for 1 h, and then filtered into a test tube. The solution was left standing at room temperature for about one week whereupon colorless block crystals were obtained.

Refinement
All H atoms were positioned geometrically with C-H = 0.93-0.96 Å, N-H = 0.86 Å and constrained to ride on their parent atoms with U iso (H) = 1.2U eq (C,N). The methyl H atoms were refined as disordered over six sites with equal occupancy.