Crystal structure and Hirshfeld surface analysis of hexyl 1-hexyl-2-oxo-1,2-dihydroquinoline-4-carboxylate

The dihydroquinoline unit is slightly twisted and the hexyl groups extend out on either side. In the crystal, C—H⋯O hydrogen bonds form chains of molecules extending along the b-axis direction, which are paired up by slipped π-stacking interactions. The ends of the hexyl groups from neighbouring chains are in contact but do not intercalate.

The asymmetric unit of the title compound, C 22 H 31 NO 3 , comprises of one molecule. The molecule is not planar, with the carboxylate ester group inclined by 33.47 (4) to the heterocyclic ring. Individual molecules are linked by aromatic C-HÁ Á ÁO carbonyl hydrogen bonds into chains running parallel to [001]. Slippedstacking interactions between quinoline moieties link these chains into layers extending parallel to (100). Hirshfeld surface analysis, twodimensional fingerprint plots and molecular electrostatic potential surfaces were used to quantify the intermolecular interactions present in the crystal, indicating that the most important contributions for the crystal packing are from HÁ Á ÁH (72%), OÁ Á ÁH/HÁ Á ÁO (14.5%) and CÁ Á ÁH/HÁ Á ÁC (5.6%) interactions.
In view of the biological importance of quinoline, and in a continuation of our research work devoted to the syntheses and crystal structures of quinoline derivatives (Bouzian et al., 2019a,b), we report herein on the molecular and crystal structures of hexyl 1-hexyl-2-oxo-1,2-dihydroquinoline-4carboxylate, (I), which was prepared by reacting ethyl 6-chloro-2-oxo-1,2-dihydroquinoline-4-carboxylate with 1-bromohexane in the presence of a catalytic quantity of tetran-butylammonium bromide. Intermolecular interactions were quantified by Hirshfeld surface analysis.

Structural commentary
The molecule of (I) is shown in Fig. 1. It is non-planar, with the carboxyl ester group inclined by 33.47 (4) to the heterocyclic ring (r.m.s. deviation of the ten atoms = 0.0174 Å ). The hexyl chain attached to N1 is twisted out of this plane by 14.2 (2) whereas the hexyl chain attached to O1 is twisted by 23.1 (2) from this plane.

Supramolecular features
In the crystal, C4-H4Á Á ÁO1 hydrogen bonds between the phenyl ring and the carbonyl group of an adjacent molecule lead to the formation of chains running parallel to [001] ( Table 1, Fig. 2). These chains are connected in pairs along [010] through slippedstacking interactions between inversion-related dihydroquinoline moieties [Cg1Á Á ÁCg2 i = 3.5472 (9) Å with a slippage of 0.957 Å ; Cg1 and Cg2 are the centroids of the N1/C6/C1/C9/C8/C7 and C1-C6 rings; symmetry code: (i) 1 À x, Ày, 1 À z] (Figs. 2, 3). This way, (100) layers with a width corresponding to the length of the a axis are formed. Unlike the packing features of similar molecules, the hexyl chains are not oriented in parallel. This is possibly a consequence of thestacking interactions, which result in a 'crossed' orientation of neighbouring hexyl groups (Fig. 3).

Figure 2
The crystal packing viewed along [010], with C-HÁ Á ÁO hydrogen bonds andstacking interactions indicated by black and orange dashed lines, respectively.

Figure 3
The crystal packing viewed along [001], withstacking interactions indicated by orange dashed lines.

Figure 1
The title molecule with displacement ellipsoids drawn at the 50% probability level. different colours, representing short or long contacts and further the relative strength of the interaction. The generated Hirshfeld surface mapped over d norm is shown in Fig. 4a. A view of the three-dimensional Hirshfeld surface of the title compound plotted over electrostatic potential, highlighting the C-HÁ Á ÁO contacts, is given in Fig. 4b. As revealed by the two-dimensional fingerprint plots (Fig. 5), the crystal packing is dominated by HÁ Á ÁH contacts, representing van der Waals interactions (72% contribution to the overall surface), followed by OÁ Á ÁH and CÁ Á ÁH interactions, which contribute with 14.5% and 5.6%, respectively. The contributions of the CÁ Á ÁC (5.4%), CÁ Á ÁO (0.8%), CÁ Á ÁN (0.7%) and NÁ Á ÁH (0.6%) interactions are less significant.

Synthesis and crystallization
A mixture of 2-oxo-1,2-dihydroquinoline-4-carboxylic acid (0.5 g, 2.6 mmol), K 2 CO 3 (0.73 g, 5.29 mmol), 1-bromohexane (0.66 g, 4 mmol) and tetra-n-butylammonium bromide as catalyst in DMF (25 ml) was stirred at room temperature for 48 h. The solution was filtered by suction, and the solvent was removed under reduced pressure. The residue was chromatographed on a silica-gel column using hexane and ethyl acetate (v/v, 95/5) as eluents to afford (I). Single crystals were obtained by slow evaporation of an ethanolic solution.

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 2. All H atoms were located in difference-Fourier maps and were refined freely.

Funding information
The support of NSF-MRI grant No. 1228232 for the purchase of the diffractometer and Tulane University for support of the Tulane Crystallography Laboratory are gratefully acknowledged.

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. Refinement. Refinement of F 2 against ALL reflections. The weighted R-factor wR and goodness of fit S are based on F 2 , conventional R-factors R are based on F, with F set to zero for negative F 2 . The threshold expression of F 2 > 2sigma(F 2 ) is used only for calculating R-factors(gt) etc. and is not relevant to the choice of reflections for refinement. R-factors based on F 2 are statistically about twice as large as those based on F, and R-factors based on ALL data will be even larger.