Crystal structure, Hirshfeld and electronic transition analysis of 2-[(1H-benzimidazol-1-yl)methyl]benzoic acid

In the title compound, the benzimidazole ring system is inclined to the the benzene ring by 78.04 (10)°. The crystal structure features O—H⋯N and C—H⋯O hydrogen bonding and C—H⋯π and π–π interactions.


Chemical context
Benzimidazole is a naturally ocurring compound, being present in vitamin B 12 (Crofts et al., 2014) and may also be synthesized from benzoic acid and o-phenylenediamine in presence of an excess of acid. Benzimidazole and its derivatives show biological activities such as antibacterial, antifungal (Yadav et al., 2015), antimicrobial (Shruthi et al., 2016), and anticancer (Kalalbandi et al., 2015). Cyanobenzyl compounds are used as intermediates in the synthesis of species that possess significant pharmaceutical properties. Compounds having carboxylic acid as a functional group have shown chelating properties and thus have potential applications in the field of biology. Such groups are also helpful in building metal-organic frameworks that usually form supramolecular networks due to extensive hydrogen bonding and weak interactions. For example,imidazol-1-yl)methyl]benzoic acid has been used to construct coordination polymers with different metal ions (Ahmad et al., 2013). Herein, we report the title compound, 2-[(1H-benzimidazol-1yl)methyl]benzoic acid, which was synthesized by a condensation reaction of benzimidazole and 2-(bromomethyl) benzonitrile in acetonitrile followed by a hydrolysis process.

Hirshfeld surface analysis
A Hirshfeld surface analysis was performed and the twodimensional fingerprint plots generated (McKinnon et al., 2007;Spackman & Jayatilaka et al., 2009) using Crystal-Explorer17 (Turner et al., 2017). The Hirshfeld surface mapped over d norm , colour-mapped from red (shorter distance than the sum of van der Waals radii) through white to blue  Table 1 Hydrogen-bond geometry (Å , ).

Figure 2
View of the crystal packing along the a axis, showing O-HÁ Á ÁN and C-HÁ Á ÁO hydrogen-bonding interactions forming a one-dimensional chain.

Figure 3
The hydrogen bonding and C-HÁ Á Á andinteractions form zigzag chains, giving a supramolecular structure along the bc plane.

Figure 1
Asymmetric unit of title compound, with atom labelling and displacement ellipsoids are drawn at the 50% probability level.
(longer distance than the sum of the van der Waals radii). The principal weak interactions are clearly visible. The surface coverage corresponding to O-HÁ Á ÁN and C-HÁ Á ÁO interactions are 9% and 11.8%, respectively. The dark-red spot indicates significant hydrogen bonding. The two-dimensional finger plots are given in Fig

Electronic transition analysis
Electro-conducting materials synthesized by conjugated organic compounds show promising electronic properties due to the availability of delocalized electrons, except for semiconducting materials such as TiO 2 , ZnO and other metal oxide nano-materials, which are electro-conducting in themselves (Odziomek et al., 2017). The electronic properties of organic compounds depend on the electronic transition between the highest occupied molecular orbital (HOMO) or valence band and lowest occupied molecular orbital (LUMO) or conduction band. In a simple method, the energy band gap (Eg) of organic molecule is determined by a Tauc plot from the absorption spectra ( max = 245 nm, in this case). The band gap energy, Eg = 4.6 eV, of the title compound is very large (Fig. 5). This large band gap arises due to high -conjugation or polarization in the title molecule system. The title molecule could be useful for developing or enhancing the organic electronic properties of conducting materials such as metal-organic frameworks.

Synthesis and crystallization
In an equimolar ratio, benzimidazole (2 g, 16.9 mmol) and dry K 2 CO 3 (4.66 g, 33.85 mmol) were mixed in a round-bottom flask in acetonitrile (MeCN, 60 ml) under an inert atmosphere. The mixture was then allowed to stirred for 60 min at 363 K then treated with 2-(bromomethyl) benzonitrile (3.31 g, 16.9 mmol), and the resulting solution refluxed for 24 h. After completion of this step, the solution was allowed to cool to room temperature and the mixture was poured slowly onto  ice-water (100 ml) under constant stirring. A greenish muddy crystalline precipitate was obtained and it was left to stand at 293 K for two days. After two days, a crystalline powder of 2- [(1H-benzo[d]imidazol-1-yl)methyl]benzonitrile was obtained (Ahmad et al., 2013).
The title compound was synthesized by hydrolysis of 2- [(1H-benzo[d]imidazol-1-yl)methyl]benzonitrile, 2 g being mixed with 20 equimolar of potassium hydroxide (6.86 g, 8.58 mmol) in water. The solution was refluxed at 373 K for 36 h, the resultant solution was then allowed to cool at room temperature and then poured onto ice-water, and after that acidified using 6 N HCl for protonation. The protonated solution was kept for slow evaporation. After two weeks, paleyellow cubic crystals were obtained in good yield, which were suitable for data collection. The reaction scheme is shown in Fig. 6.