Crystal structure of 2-oxo-2H-chromen-7-yl 4-fluorobenzoate

The structure of a coumarin ester stabilized by C—H⋯O hydrogen bonds and C=O⋯π and π–π stacking interactions has been studied by X-ray diffraction, Hirshfeld surface analysis and quantum chemical calculations.

In the title compound, C 16 H 9 FO 4 , (I), the benzene ring is oriented at an acute angle of 59.03 (15) relative to the coumarin plane (r.m.s deviation = 0.009 Å ). This conformation of (I) is stabilized by an intramolecular C-HÁ Á ÁO hydrogen bond, which closes a five-membering ring. In the crystal, molecules of (I) form infinite zigzag chains along the b-axis direction, linked by C-HÁ Á ÁO hydrogen bonds. Furthermore, the crystal structure is supported bystacking interactions between neighbouring pyrone and benzene or coumarin rings [centroid-centroid distances in the range 3.5758 (18)-3.6115 (16) Å ], as well as C OÁ Á Á interactions [OÁ Á Ácentroid distances in the range 3.266 (3)-3.567 (3) Å ]. The theoretical data for (I) obtained from quantum chemical calculations are in good agreement with the observed structure, although the calculated C-O-C-C torsion angle between the coumarin fragment and the benzene ring (73.7 ) is somewhat larger than the experimental value [63.4 (4) ]. Hirshfeld surface analysis has been used to confirm and quantify the supramolecular interactions.

Chemical context
Coumarins and their derivatives constitute one of the major classes of naturally occurring compounds and interest in their chemistry continues unabated because of their usefulness as biologically active agents. They also form the core of several molecules of pharmaceutical importance. Coumarin and its derivatives have been reported to serve as anti-bacterial (Basanagouda et al., 2009), anti-oxidant (Vuković et al., 2010) and anti-inflammatory agents (Emmanuel-Giota et al., 2001). In view of their importance and as a continuation of our work on the crystal structure analysis of coumarin derivatives Oué draogo et al., 2018), we report herein the synthesis, crystal structure, geometry optimization and Hirshfeld surface analysis of the title coumarin derivative (I).

Structural commentary
The molecular structure of (I) is illustrated in Fig. 1. In the structure, an S(5) ring motif arises from the intramolecular C16-H16Á Á ÁO3 hydrogen bond (Table 1), and generates a pseudo bicyclic ring system (Fig. 1). The coumarin fragment is planar (r.m.s deviation = 0.009 Å ) and oriented at an acute ISSN 2056-9890 angle of 59.03 (15) with respect to the C11-C16 benzene ring, while the hydrogen-bonded five-membered ring [r.m.s deviation = 0.007 Å ] forms dihedral angles of 59.23 (13) and 0.59 (18) , respectively, with the coumarin ring system and the benzene ring. These dihedral angles suggest that the fivemembered hydrogen-bonded and C11-C16 benzene rings are coplanar. An inspection of the bond lengths shows that there is a slight asymmetry of the electronic distribution around the pyrone ring: the C2-C3 [1.332 (5) Å ] and C1-C2 [1.451 (5) Å ] bond lengths are shorter and longer, respectively, than those expected for a C ar -C ar bond. This suggests that the electron density is preferentially located in the C3-C2 bond of the pyrone ring, as seen in other coumarin derivatives (Gomes et al., 2016;Ziki et al., 2016).

Figure 2
Part of the crystal packing of (I) showing the formation of an infinite C(4) chain along the b-axis. Dashed lines indicate hydrogen bonds. H atoms not involved in hydrogen-bonding interactions have been omitted for clarity.

Figure 3
A view of the crystal packing showing C1 O2Á Á Á andstacking interactions (dashed lines). The yellow dots are ring centroids.

Figure 1
The molecular structure of (I), along with the atomic numbering scheme. Displacement ellipsoids are drawn at the 50% probability level. H atoms are shown as spheres of arbitrary radius. The intramolecular hydrogen bond is indicated by a dashed line.

Hirshfeld surface analysis
Molecular Hirshfeld surfaces and the associated two-dimensional fingerprint plots of (I) were calculated using a standard (high) surface resolution with the the three-dimensional d norm surfaces mapped over a fixed colour scale of À0.26 (red) to 1.20 Å (blue) with the program CrystalExplorer 3.1 (Wolff et al., 2012). The analysis of intermolecular interactions through the mapping of three-dimensional d norm surfaces is permitted by the contact distances d i and d e from the Hirshfeld surface to the nearest atom inside and outside, respectively. In (I), the surface mapped over d norm highlights several red spots showing distances shorter than the sum of the van der Waals radii. These dominant interactions correspond to intermolecular C-HÁ Á ÁO hydrogen bonds, C8Á Á ÁC5 (1 + x, y, z), OÁ Á Á andstacking interactions between the surface and the neighbouring environment. The mapping also shows white or pale-red spots with distances almost equal to the sum of the van der Waals radii and blue regions with distances longer than the sum of the van der Waals radii. The surfaces are shown as transparent to allow visualization of the molecule (Fig. 4). In the shape-index map (À0.99 to 1 Å ) (Fig. 5), the adjacent red and blue triangle-like patches show concave regions that indicatestacking interactions (Bitzer et al., 2017). Furthermore, the 2D fingerprint plots (FP), decomposed to highlight particular close contacts of atom pairs and the contributions from different contacts, are provided in Fig. 6. The red spots in the middle of the surface appearing near d e = d i = 1.8-2.0 Å correspond to close CÁ Á ÁC interplanar contacts. These contacts, which comprise 10.1% of the total Hirshfeld surface area, are related tointeractions ( Fig. 6a) as predicted by the X-ray study. The most significant contribution to the Hirshfeld surface (27.7%) is from HÁ Á ÁO/ OÁ Á ÁH contacts, which appear on the left-side as blue spikes with the tip at d e + d i = 2.4 Å , top and bottom (Fig. 6b). As expected in organic compounds, the HÁ Á ÁH contacts are important with a 24.5% contribution to Hirshfeld surface; these appear in the central region of the FP with a central blue tip spike at d e = d i = 1.10 Å (Fig. 6c) whereas the FÁ Á ÁH/HÁ Á ÁF contacts with a contribution to the Hirshfeld surface of 11.4% are indicated by the distribution of points around a pair of wings at d e + d i ' 2.6 Å (Fig. 6d). The CÁ Á ÁH/HÁ Á ÁC plot (16.2%) reveals information on the intermolecular hydrogen bonds (Fig. 6e). Other visible spots in the Hirshfeld surfaces indicate the CÁ Á ÁO/OÁ Á ÁC, OÁ Á ÁO, FÁ Á ÁF and CÁ Á ÁF/FÁ Á ÁC contacts, which contribute only 6.6, 1.3, 1.2 and 1.1%, respectively ( Fig. 6f-6i). Hirshfeld surface mapped over shape-index highlighting the regions involved instacking interactions. Cg(I) and Cg(J) are centroids of rings I and J; CgI_Perp is the perpendicular distance of Cg(I) on ring J and slippage is the distance between Cg(I) and the perpendicular projection of Cg(J) on ring I.

Figure 4
A view of the Hirshfeld surface for (I) with the three-dimensional d norm surfaces mapped over a fixed colour scale of À0.26 (red) to 1.20 Å (blue).

Theoretical calculations
The geometry optimization of (I) was performed using the density functional theory (DFT) method with a 6-311 ++ G(d,p) basis set. The crystal structure in the solid state was used as the starting structure for the calculations. The DFT calculations were performed with the GAUSSIAN09 program package (Frisch et al., 2013). The resulting geometrical parameters are compared with those obtained from the X-ray crystallographic study, showing a good agreement for the bond lengths and bond angles with r.m.s. deviations of 0.017 Å and 1.06 , respectively (see Supplementary Tables S1 and S2). In addition, an inspection of the calculated torsion angles shows that the coumarin fragment and the C11-C16 benzene ring are coplanar (see Supplementary Table S3), which is in good agreement with the experimental results, although the calculated C10-O3-C7-C8 torsion angle between them (73.7 ) is somewhat larger than the observed value [63.4 (4) ].

Synthesis and crystallization
To a solution of 4-fluorobenzoyl chloride (6.17 mmol; 0.98 g) in dried tetrahydrofuran (40 mL) was added dried triethylamine (3 molar equivalents; 2.6 mL) and 7-hydroxycoumarin (6.17 mmol; 1 g) by small portions over 30 min. The mixture was then refluxed for 4 h and poured into 40 mL of chloroform. The solution was acidified with diluted hydrochloric acid until the pH was 2-3. The organic layer was extracted, washed with water to neutrality, dried over MgSO 4 . The resulting precipitate (crude product) was filtered off with suction, washed with petroleum ether and recrystallized from acetone.

Figure 6
Decomposed two-dimensional fingerprint plots for (I). Various short contacts and their relative contributions are indicated.

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.