Synthesis, crystal structure, and Hirshfeld surface analysis of 1,3-dihydro-2 H -benzimidazol-2-iminium 3-carboxy-4-hydroxybenzenesulfonate

The asymmetric unit of the title salt, C7H8N3
 +·C7H5O6S−, comprises two 1,3-dihydro-2H-benzimidazol-2-iminium cations and two 2-hydroxy-5-sulfobenzoate anions (Z′ = 2). In the crystal, the molecules interact through N—H...O, O—H...O hydrogen bonds and C—O...π contacts. The hydrogen-bonding interactions lead to the formation of layers parallel to (\overline{1}01). Hirshfeld surface analysis revealed that H...H contacts contribute to most of the crystal packing with 38.9%, followed by H...O contacts with 36.2%.

5-Sulfosalicylic acid is a particularly strong organic acid, which is capable of protonating N-containing heterocycles and other Lewis bases (Muthiah et al., 2003) and thus can form structures with a variety of supramolecular arrangements.
The present work was undertaken as part of our research program aimed at further understanding hydrogen-bonding interactions involving 2-aminobenzimidazole and 5-sulfosalicylic acid.Here, we report the synthesis, crystal structure, and Hirshfeld surface analysis of the new organic salt 1,3-dihydro-2H-benzimidazol-2-iminium 2-hydroxy-5-sulfobenzoate, C 7 H 8 N 3

Supramolecular features
In the crystal, the N atoms of the imidazolium cation form intermolecular N-H� � �O hydrogen bonds with oxygen atoms of the sulfate group of the two hydroxybenzoate anions.
Moreover, O-H� � �O interactions between the carboxy group and one of the sulfonate O atoms are present (Fig. 2, Table 2).Additionally, �-� interactions between the aromatic rings with centroid-to-centroid distances between 3.5094 (9) and 3.9824 (10) A ˚(numerical details are given in ESI Table S2) as well as C14 O5� � �Cg4(x, 1 2 À y, À 1 2 + z) interactions of 3.7089 (19) A ˚are present (Fig. 3).All of the above contacts contribute to the tri-periodic packing of the molecular entities in the crystal.

Hirshfeld surface analysis
In order to quantify the intermolecular interactions in the title salt, the Hirshfeld surface (HS) (Spackman & Jayatilaka, 2009) was analysed and the associated two-dimensional fingerprint plots (McKinnon et al., 2007)

Synthesis and crystallization
All reagents for synthesis and analysis were commercially available and purchased from Sigma Aldrich and used as received without further purification.Sulfosalicylic acid and 2-aminobenzimidazole were reacted in a molar ratio of 1:1, using a solution of 0.133 g of 2aminobenzimidazole in 5 ml of ethanol that was added dropwise to 0.228 g of the acid dissolved in 5 ml of ethanol.The mixture was stirred on a magnetic stirrer for 4 h.During the reaction time, the color of the solution changed from transparent to light brown.The solution was left for 2 weeks at room temperature for crystal growth.The formed crystals were filtered off and washed several times with ethanol to remove impurities.

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.

Figure 2
Figure 2 Crystal packing in a view along the b axis.Intermolecular N-H� � �O and O-H� � �O interactions are shown as light-blue dashed lines and intramolecular O-H� � �O interactions as dark-blue dashed lines.

Fig. 4 .
White surface areas indicate contacts with distances equal to the sum of van der Waals radii, whereas red and blue colors denote distances shorter or longer than the sum of the van der Waals radii.The red spots clearly visible in Fig.4emphasize the importance of classical hydrogen-bonding interactions in the title salt.The twodimensional fingerprint plot for all contacts is depicted in Fig.5a.H� � �H contacts are responsible for the largest contribution (38.9%) to the Hirshfeld surface (Fig.5b).Besides these contacts, H� � �O/O� � �H (36.2%),C� � �C (10.2%),H� � �C/C� � �H (5.1%) and O� � �C/C� � �O (3.7%) interactions contribute significantly to the total Hirshfeld surface; their decomposed fingerprint plots are shown in Fig.5c-f.The contributions of further contacts are only minor and amount to N� � �C/C� � �N (2.6%), O� � �O (2.1%) and N� � �H/H� � �N (1.1%).

Figure 4 HS
Figure 4 HS plotted over d norm (a) along the a axis, (b) along the b axis and (c) along the c axis.

Figure 5
Figure 5 Two-dimensional fingerprint plots for (a) all interactions and (b)-(i) individual interatomic contacts.

Table 3
Experimental details.
Computer programs: CrysAlis PRO