Crystal structure and DFT study of 2-(pyren-1-yl)-1H-benzimidazole

The title compound was prepared from an equimolar mixuture of o-phenylenediamine and pyrene-1-carboxaldehyde. We report herein on its crystal structure and a density functional theory (DFT) study.


Chemical context
Benzimidazoles, which are analogues of imidazole contained in histidine, are an important class of biologically active compounds (Collman et al., 1973). In addition, they are excellent organic ligands of many metal ions (Sundberg & Martin, 1974). The pyrene unit is one of the most commonly used fluorophores due to its strong luminescence and chemical stability (Aoki et al., 1991;Nishizawa et al., 1999;van der Veen et al., 2000). Another interesting feature of the pyrene unit is the interaction between the pyrene aromatic rings in the crystal packing, which can permit the formation of highly ordered molecular aggregates in the solid state by architecturally controlled self-assembly (Desiraju & Gavezzotti, 1989;Munakata et al., 1994). Pyrene is a commonly used fluorophore due to its unusual fluorescence properties, viz. intense fluorescence signals and vibronic band dependence with the media (Karpovich & Blanchard, 1995), and has been used in fluorescence sensors (Bell & Hext, 2004) and excimer formation (Lodeiro et al., 2006). As a result of these particular properties and because of its chemical stability, it is also employed as a probe for solid-state studies and polymer association (Seixas de Melo et al., 2003).
The title compound was prepared from an equimolar mixuture of 1:1 o-phenylenediamine and pyrene-1-carbaldehyde. Synthesis and characterization of many benzimidazole- ISSN 2056-9890 ring-containing compounds have been reported (Yan et al., 2009;Hallett et al., 2012;Xia et al., 2014;Dhanalakshmi et al., 2014;Guo et al., 2015;Song et al., 2010), but very few compounds have been structurally characterized. Previously, Zhao et al. (2016) reported on the synthesis of 2-(pyren-1yl)benzimidazole, used as a fluorescent probe for the detection of iron(III) ions in aqueous solution, but gave no structural details of the compound. The present work is part of an ongoing structural study of pyrene-ring-system derivatives (Faizi & Prisyazhnaya, 2015). The results of the calculations by density functional theory (DFT) on (I), carried out at the B3LYP/6-311G(d,p) level, are compared with the experimentally determined molecular structure in the solid state.

Figure 2
A view of the N-HÁ Á ÁN hydrogen-bonded column (dashed lines; Table 1) in the crystal of compound (I), propagating along the b-axis direction.

Figure 3
A view along the b axis of the crystal packing of compound (I). The C-HÁ Á Á interactions are illustrated by dashed lines (Table 1).

Figure 1
The molecular structure of compound (I), with the atom labelling. Displacement ellipsoids are drawn at the 40% probability level.

DFT study
The DFT quantum-chemical calculations were performed at the B3LYP/6-311G(d,p) level (Becke, 1993), as implemented in GAUSSIAN09 (Frisch et al., 2009). DFT structure optimization of (I) was performed starting from the X-ray geometry and the values compared with experimental values (see Table 2). From these results we can conclude that basis set 6-311G(d,p) is well suited in its approach to the experimental data.
The DFT study of (I) shows that the HOMO and LUMO are localized in the plane extending from the whole pyrene ring to the benzimidazole ring. The electron distribution of the HOMO-1, HOMO, LUMO and the LUMO+1 energy levels are shown in Fig. 4. The molecular orbital of HOMO contains both and character, whereas HOMO-1 is dominated by orbital density. The LUMO is mainly composed of density, while LUMO+1 has both and character and electronic density. The HOMO-LUMO gap was found to be 0.273 a.u. and the frontier molecular orbital energies, E HOMO and E LUMO , were À0.20083 and À0.07230 a.u., respectively.

Synthesis and crystallization
Pyrene-1-carbaldehyde (0.2306 g, 1.0 mmol) was added to a 50 ml round-bottomed flask containing 10 ml of CH 2 Cl 2 . Then a 10 ml CH 2 Cl 2 solution containing 0.1080 g (1.0 mmol) o-phenylenediamine was added dropwise over a period of 30 min with stirring. The mixture was stirred at room temperature for 48 h. The solvent was then evaporated and the residue purified by aluminium oxide gel-column chromatography using CH 2 Cl 2 as the eluent to obtain a pale-yellow powder of (I) (yield 0.2311 g, 72.6%). Colourless prismatic crystals were obtained by slow evaporation of a solution of (I) from methanol.

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 3. The N-bound H atoms were located in a difference Fourier map and refined with U iso (H) = 1.2U eq (N). The C-bound H atoms were included in calculated  Table 2 Comparison of selected geometric data for (I) (Å , ) from X-ray and calculated (DFT) data.

Figure 4
Electron distribution of the HOMO-1, HOMO, LUMO and LUMO+1 energy levels for compound (I).

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.