Crystal structure and UV spectra of a 1,2-disubstituted benzimidazolium chloride

1-(2-Hydroxybenzyl)-2-(2-hydroxyphenyl)-1H-benzimidazol-3-ium chloride, C20H17N2O2 +·Cl−, is prepared by reaction of salicylaldehyde with o-phenylenediamine in the presence of trimethylsilyl chloride acting as a source of HCl. As a result of steric hindrance, the cation in the crystal is far from planar: the benzimidazole ring system makes dihedral angles of 55.49 (9) and 81.36 (8)° with the planes of the phenolic groups. The crystal packing is dominated by the O—H⋯Cl and N—H⋯Cl hydrogen bonds. The title compound exhibits two transitions in the UV region, which are revealed in the solid state and solution spectra as an absorption maximum at 280 nm and a shoulder at 320 nm. According to the results of TD–DFT calculation, both transitions have a π–π* nature and the molecular orbitals involved in these transitions are mostly localized on the benzimidazole ring system and on the phenyl ring attached to it at the 2-position.

1-(2-Hydroxybenzyl)-2-(2-hydroxyphenyl)-1H-benzimidazol-3-ium chloride, C 20 H 17 N 2 O 2 + ÁCl À , was prepared by reaction of salicylaldehyde with o-phenylenediamine in the presence of trimethylsilyl chloride acting as a source of HCl. As a result of steric hindrance, the cation in the crystal is far from planar: the benzimidazole ring system makes dihedral angles of 55.49 (9) and 81.36 (8) with the planes of the phenolic groups. The crystal packing is dominated by O-HÁ Á ÁCl and N-HÁ Á ÁCl hydrogen bonds, which link the cations and anions into four-membered rings and then into chains along [100]. The title compound exhibits two transitions in the UV region, which are revealed in the solid state and solution spectra as an absorption maximum at 280 nm and a shoulder at 320 nm. According to the results of TD-DFT calculation, both transitions have a -* nature and the molecular orbitals involved in these transitions are mostly localized on the benzimidazole ring system and on the phenyl ring attached to it at the 2-position.

Chemical context
Benzimidazole derivatives are well known to exhibit antibacterial, antimalarial and anti-inflammatory properties (Keri et al., 2015;Carvalho et al., 2011). Besides this, 1,2-disubstituted benzimidazoles are used as intermediates in synthesis of dyes and pigments (Carvalho et al., 2011). Some substituted benzimidazoles, e.g. 2-(2 0 -hydroxyphenyl)benzimidazole and its derivatives, are strongly fluorescent and show dual emission due to the excited state proton transfer (Douhal et al., 1994). In the solid state, these compounds exhibit fluorescence, which is governed by their polymorphism and steric effects (Konoshima et al., 2012;Benelhadj et al., 2013;Shida et al., 2013). Thus, this class of compounds is considered for applications in fluorescence imaging and optoelectronics (Zhao et al., 2011). Benzimidazolium salts attract attention due to their non-linear optical properties (Sun et al., 2011;Wang et al., 2011). 2-(2 0 -Hydroxyphenyl)benzimidazole, which is a member of this class of compounds, exhibits rotamerism (Ríos Vazquez et al., 2008). In this work, the crystal structure of 1-(2hydroxybenzyl)-2-(2-hydroxyphenyl)-1H-benzimidazol-3-ium chloride and its UV spectra have been reported. DFT calculations were carried out to study the geometry and electronic transitions.

Structural commentary
All bond lengths and bond angles are within the ranges reported for similar structures (Ha, 2012). The asymmetric unit, consisting of a 1-(2-hydroxybenzyl)-2-(2-hydroxyphen- ISSN 2056-9890 yl)-1H-benzo[d]imidazol-3-ium cation and a chloride anion, is presented in Fig. 1. As a result of steric hindrance, the cation is far from planar: the benzimidazole ring system makes dihedral angles of 55.49 (9) and 81.36 (8) with the planes of phenolic groups immediately attached to it at position 2 and linked via the methylene bridge to position 1, respectively. The deviation from planarity in the 2-(2-hydroxyphenyl)benzimidazolium skeleton is larger than in the reported similar structures (Al-Douh et al., 2009b;Wang et al., 2011).

Supramolecular features
In the crystal, each cation forms three hydrogen bonds, two O-HÁ Á ÁCl and one N-HÁ Á ÁCl (Table 1), to chloride anions. As a result of these interactions, the cations and anions form ribbons along [100], which consist of centrosymmetric four-membered rings each formed by two cations and two anions in the R 2 4 (16) and R 2 4 (20) manner, as shown in Fig. 2. Some weak contacts C-HÁ Á ÁO, C-HÁ Á ÁCl and C-HÁ Á Á are also present (Table 1).

Hirshfeld surface analysis
To evaluate the effect of close range interactions and compare their significance, Hirshfeld surface analysis (Spackman & Jayatilaka, 2009;Soman et al., 2014) has been performed and its results are presented in Fig. 3. Four red spots on the Hirshfeld surface indicate short contacts. All close interactions are mediated by Cl À anions. The HÁ Á ÁH and CÁ Á ÁH interactions are associated with 46% and 26% surface area, respectively. The contributions of the ClÁ Á ÁH (15%) and OÁ Á ÁH (6%) interactions are smaller, but significant for the crystal architecture.

Figure 2
Packing diagram highlighting the hydrogen-bonding interactions.

Figure 1
ORTEP diagram of the title compound with displacement ellipsoids drawn at the 50% probability level.
observed in the crystal structure within the range of 0.04 Å ( Table 2). The largest distinction between the calculated and crystallographic geometries is related to the twist of the phenolic group attached to the benzimidazole ring system at position 2: in the crystal, the corresponding torsion angles are by 7-14 nearer to 180 than the calculated values (Table 2). This could be due to the hydrogen-bonding and C-HÁ Á Á interactions. The ionic nature of the optimized cation-anion pair is reflected in the large calculated dipole moment of 18.05 D. The time-dependent DFT (TD-DFT) calculation was performed on the crystal geometry at the same level of theory as for geometry optimization.

UV spectra
The solid-state diffuse reflectance spectrum was measured with a Shimadzu-3600 spectrophotometer fitted with an MPC-3100 sample compartment. For that, the crystals were crushed to powder and mixed with BaSO 4 to a final concentration of 5% (v/v). The Kubelka-Munk transformation (Kubelka & Munk, 1931) was applied to the reflectance data. The spectrum of methanol solution was measured with JASCO V530 spectrophotometer. The solid-state spectrum closely resembles the spectrum of the solution, thus indicating that the geometry and electronic structure of the cation did not change in moving from solid state to solution. In the UV region, the title compound exhibits an absorption maximum at 280 nm and a shoulder around 320 nm (Fig. 4a). The absorption maximum at 280 nm is typical of benzimidazole (Hirayama, 1967), and the 320 nm shoulder is typical of benzimidazole derivatives (Mosquera et al., 1996;Konoshima et al., 2012

Figure 3
Hirshfeld surface of the ionic pair mapped with normalized contact distances (d norm ) indicated by red spots. Positions of close contacts are highlighted by red arrows. (Shida et al., 2013). The positions and intensities of calculated transitions agree well with the experimental data (Fig. 4a, Table 3). The transition at 277 nm is found to have the -* nature. The associated molecular orbitals (HOMO-5 and LUMO) are spread over benzimidazole and 2-phenyl group (Fig. 4b, Table 3). On the other hand, HOMO-3 is localized on 2-phenyl group, making the transitions at 356 nm partially charge-transfer in nature. Halide derivatives (CIQQOJ, NEGRIB) have also been reported (Fang et al., 2007;Yang et al., 2006). A search for protonated molecules containing the 1-benzyl-2-phenyl-1Hbenzo[d]imidazol-3-ium skeleton gave 11 hits, three of which being closely related to this work are reported in the same article (EBOHOU, EBOHUA and EBOJAI; Wang et al., 2011).

Synthesis and crystallization
Salicylaldehyde (SD Fine Chemicals, Mumbai, India), ophenylenediamine (Sigma-Aldrich, USA) and trimethylsilyl chloride (Sigma-Aldrich, USA) were used as received. The title compound was synthesized by the reaction of o-phenylenediamine (1 g) with salicylaldehyde (1:2 mole ratio) in double distilled water at 363 K using trimethylsilyl chloride as catalyst (1:1 molar ratio with respect to o-phenylenediamine) for 8-10 h (Wan et al., 2009). The reaction mixture was cooled to room temperature, and the white precipitate was filtered off, washed with water, dried by pressing against filter paper and allowed to dry at ambient conditions over a few days. Unexpectedly, the product turned out to be a salt, not a neutral compound, as prescribed by the literature synthetic procedure. It was crystallized from a solution in acetonitrile/ methanol mixture (15:85) in a refrigerator and then at room temperature. The resulting plate-shaped crystals were used for single crystal XRD measurements. Even after repeated attempts with crude and recrystallized samples, a clean 1 H NMR spectrum, which is an indication of rotamerism in solution, was not obtained. For the spectroscopic study, the parent solvent was decanted and then the crystals were washed with diethyl ether and finally air dried.