Bis(2-aminobenzimidazolium) sulfate monohydrate

The components of the title molecular salt are linked by N—H⋯O and O—H⋯O hydrogen bonds.


Structure description
2-Aminobenzimidazole has been used for the synthesis of a series of sulfur heterocycles such as 9H-3-thia-1,4a,9-triaza-fluorene-2,4-dithione (1): its potassium thiolate salt was used to prepare metal coordination compounds (Peñ a-Hueso et al., 2008), and is the precursor of the title compound. When compound 1 is dissolved in dimethyl sulfoxide and strong acids are added, instead of producing the protonated derivative, the thiadiazine ring breaks down, producing 2-aminobenzimidazolium sulfate (2): its crystal structural features are the subject of the present paper.
Compound 2 is formed by the transfer of two protons from sulfuric acid to the heterocycle: the crystal has two 2-aminobenzimidazolium cations, one sulfate anion and one water molecule in its asymmetric unit (Fig. 1). There is a small asymmetry in the S-O bond lengths of the SO 4 2ion from 1.4596 (16) to 1.4723 (15) Å , probably caused by the hydrogen bonds around the anion (Gagné & Hawthorne, 2018). Two benzimidazolium cations are stacked in a head-to-tail way, with a distance between C9 of one molecule and C18 of another of 3.441 (3) Å .
The sulfate ion accepts seven N-HÁ Á ÁO hydrogen bonds from four adjacent benzimidazolium cations and one O-HÁ Á ÁO link from a water molecule (Table 1, Fig. 2). The water molecule accepts one N-HÁ Á ÁO hydrogen bond and forms two O-HÁ Á ÁO links to two SO 4 2ions (Fig. 3). In the extended structure, the benzimidazolium cations form parallel ribbons propagating in the [010] direction (Fig. 4).

data reports
The first crystal structure of a 2-aminobenzimidazolium salt was reported with the nitrate anion (Bats et al., 1999) and a related structure with hydrogen sulfate as the counter-ion is also known (You et al., 2009).

Figure 1
The molecular structure of 2 showing displacement ellipsoids drawn at the 50% probability level

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 2.

Funding information
The authors thank Cinvestav for financial support.

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. data reports data-2 IUCrData (2022). 7, x220172 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 > 2sigma(F 2 ) is used only for calculating R-factors(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. The positions of all NH and OH hydrogen atoms were refined, and all CH were placed at ideal positions.

Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Å 2 )
x y z U iso */U eq C2 0.5660 (