Synthesis and crystal structures of N-H, N-phenyl and N-benzyl-2-(4-hexyloxyphenyl)benzimidazoles

N-H, N-phenyl and N-benzyl-2-(4-hexyloxyphenyl)benzimidazoles were prepared and studied by 1H NMR and single-crystal X-ray analysis. The unsubstituted benzimidazole forms intermolecular N—H⋯N bonds while in the crystal structures of the other two compounds, the molecules are assembled only through π–π and C—H⋯π interactions.


Chemical context
2-Arylbenzimidazoles have attracted considerable attention as biologically active compounds (Vasava et al., 2020). They are also used as ligands in constructing cyclometalated iridium(III) and ruthenium(II) complexes for organic lightemitting diodes and photosensitizers in dye-sensitized solar cells Lavrova et al., 2020). For the latter application, the aryl unit of these ligands should contain -electron-donating substituents to increase the lightharvesting characteristics of the corresponding organometallic complexes (Aghazada & Nazeeruddin, 2018;Bezzubov et al., 2014Bezzubov et al., , 2016. In addition, long aliphatic chains in the ligands are preferable to diminish aggregation of the complexes on the semiconductor surface (Hagfeldt et al., 2010). In line with this, we synthesized 2-(4-hexyloxyphenyl)-1H-benzimidazole (1) and its N-phenyl and N-benzyl analogues (2 and 3, respectively) and studied their crystal structures. ISSN 2056-9890

Structural commentary
In all three structures, the organic molecules occupy general positions and contain identical benzimidazole and 4-hexyloxyphenyl units and different . The benzimidazole systems are essentially flat while the alkoxyaryl rings are inclined to them with dihedral angles of 35.02 (17), 31.46 (4) and 38.67 (6) for 1, 2 and 3, respectively. Although the N-phenyl ring is expected to exert a larger steric pressure in 2 as compared with 3, its rotation by 68.92 (4) along the N2-C8 bond seems to reduce the steric hindrance in the molecule and results in the smallest interplanar angle between the aryl and imidazole moieties in the series. In the structures of 1 and 2, the hydrocarbon chains crystallize in the common trans zigzag conformation, while in the structure of 3 the chain adopts a gauche conformation about the C23-C24 bond.
In the 1 H NMR spectra of 1-3, a similar set of high-field multiplets assigned to protons of the aliphatic chain was observed. In contrast, the NMR pattern corresponding to the aromatic protons in the substances becomes more complex when going from 1 to 2 and 3. In the aromatic part of 1 H NMR spectrum of 1, there are four individual resolved multiplets corresponding to the symmetric benzimidazole part, assuming rapid exchange of the N-H proton on the NMR time scale. Phenyl or benzyl substituents at the nitrogen atom decrease the symmetry of the benzimidazole moiety, which results in the appearance of additional signals that are highly overlapped and make the spectra of 2 and 3 difficult to interpret.

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 2. In the structures of 1 and 3, hydrogen atoms were placed in calculated positions and refined using a riding model [C-H = 0.94-0.97 Å with U iso (H) = 1.2-1.5U eq (C)]. In the structure of 1, the N-H hydrogen atom was located from a difference electron-density map and refined using a riding model [N-H = 0.88 Å with U iso (H) = 1.2U eq (N)]. Hydrogen atoms in the structure of 2 were located from difference electron-density maps and were refined freely.

2-(4-Hexyloxyphenyl)-1H-benzimidazole (3)
Crystal data  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.

2-(4-Hexyloxyphenyl)-1-phenyl-1H-benzimidazole (2)
Crystal data 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.

Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Å 2 )
x y z U iso */U eq O1 0.41962 (10) 0.79543 (5)  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.