Crystal structure, Hirshfeld surface analysis and DFT study of 1-ethyl-3-phenyl-1,2-dihydroquinoxalin-2-one

The dihydroquinoxaline moiety in the title compound is not planar. In the crystal, C—H⋯O hydrogen bonds form helical chains about the crystallographic 21 axes. The chains pack with normal van der Waals contacts.


Chemical context
Nitrogen-based structures have attracted attention in recent years because of their interesting properties in structural and inorganic chemistry (Chkirate et al., 2019;2020a,b). The family of nitrogenous drugs, particularly those containing the quinoxaline moiety, is important in medicinal chemistry because of their wide range of pharmacological activities, which include anticancer, anti-inflammatory, antibacterial, antituberculosis, anti-glycation, anti-analgesic and antifungal properties, and for their antioxidant potential. In particular, quinoxalin-2-one derivatives are active anti-tumor agents with tyrosine kinase receptor inhibition properties (Galal et al., 2014). They can also selectively antagonize the glycoprotein in cancer cells (Sun et al., 2009). Quinoxalin-2-one derivatives are also potential antagonist ligands for imaging the A2A adenosine receptor by positron emission tomography (PET) (Holschbach et al., 2005). Given the wide range of therapeutic applications for such compounds, we have already reported a route for the preparation of quinoxalin-2-one derivatives using N-alkylation reactions carried out with di-halogenated carbon chains (Benzeid et al., 2011); a similar approach yielded the title compound, C 16 H 14 N 2 O, (I). In addition to the synthesis, we also report the molecular and crystal structure along with a Hirshfeld surface analysis and a density functional theory (DFT) computational study carried out at the B3LYP/6-311 G(d,p) level. ISSN 2056-9890

Supramolecular features
In the crystal, helical chains about the crystallographic 2 1 axes are formed by C9-H9BÁ Á ÁO1 hydrogen bonds (

Figure 2
Packing view along the a-axis direction with C-HÁ Á ÁO hydrogen bonds shown as dashed lines.

Figure 3
Packing view along the b-axis direction with C-HÁ Á ÁO hydrogen bonds shown as dashed lines.

Figure 1
The title molecule with the atom-labelling scheme and 50% probability ellipsoids.

DFT calculations
The optimized structure of (I) in the gas phase was calculated by density functional theory (DFT) using a standard B3LYP functional and the 6-311 G(d,p) basis-set (Becke, 1993) as implemented in GAUSSIAN 09 (Frisch et al., 2009). The theoretical and experimental results related to bond lengths and angles are in good agreement (Table 2). Calculated numerical values for (I) including electronegativity (), hardness (), ionization potential (I), dipole moment (), electron affinity (A), electrophilicity (!) and softness () are collated in Table 3. The electron transition from the HOMO to the LUMO energy level is shown in Fig. 6. View of the three-dimensional Hirshfeld surface of the title compound, plotted over d norm .

Figure 5
The full two-dimensional fingerprint plots for the title compound  Table 2 Comparison of selected (X-ray and DFT) bond lengths and angles (Å , ).    (4) , which is approximately the same as in IDOSUR but more tilted than in NIBXEE.

Synthesis and crystallization
To a solution of 3-phenylquinoxalin-2(1H)-one (0.7 g, 0.0032 mol) in N,N-dimethylformamide (20 ml) were added bromoethane (0.48 ml), potassium carbonate K 2 CO 3 (0.5g, 0.004 mol) and a catalytic quantity of tetra-n-butylammonium bromide. The reaction mixture was stirred at room temperature for 24 h. The solution was filtered and the solvent removed under reduced pressure. The residue thus obtained was separated by chromatography on a silica gel column using a hexane/ethyl acetate 9:1 mixture as eluent. The solid obtained was recrystallized from ethanol solution to afford colourless plates of the title compound (yield: 85%).

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 4. Hydrogen atoms were included as riding contributions in idealized positions (C-H = 0.95-0.99 Å ) with U iso (H) = 1.2U eq (C) or 1.5U eq (C-methyl).

Special details
Experimental. The diffraction data were obtained from 3 sets of 400 frames, each of width 0.5° in ω, collected at φ = 0.00, 90.00 and 180.00° and 2 sets of 800 frames, each of width 0.45° in φ, collected at ω = -30.00 and 210.00°. The scan time was 10 sec/frame. 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.