Crystal structure of (2E)-1-(4-ethoxyphenyl)-3-(4-fluorophenyl)prop-2-en-1-one

The title molecule is nearly planar with a slight structural bend involving the carbonyl group. The molecules pack in the crystal by intermolecular C—H⋯O/F hydrogen bonding, π–π stacking, and H–π interactions.


Chemical context
Chalcones are a group of 1,3-diaryl-2-propen-1-one compounds that have been found to exhibit a wide variety of biological activity including anticancer, antimicrobial and antiinflammatory properties (Sahu et al., 2012). Chalcones are also important starting materials for the synthesis of several pharmacologically interesting classes of heterocyclic compounds such as isoxazoles, pyrazolines and pyrazoles (Kamal et al., 2019). In our research involving the synthesis of chalcone derivatives, we have synthesized and obtained an X-ray structure for the title compound, C 17 H 15 FO 2 , 2(E)-1-(4ethoxyphenyl)-3-(4-fluorophenyl)-2-propen-1-one.

Structural commentary
This chalcone has aromatic rings with substitutions in the 4 position on both ends of the molecule, where the phenyl on the alkene is fluorinated, and the phenyl on the carbonyl contains an ethoxide (Fig. 1). Both phenyl rings are inclined towards the same side of the molecule thanks to the E geometry of the chalcone's alkene. The compound is a heavily -conjugated structure that is nearly planar. To measure the deviation from planarity, three torsion angles were examined. The angles involving the aromatic rings are nearly identical with little bend, where the torsion between the C8-C7 and C5-C4 bonds is À1.2 (4) , and the torsion between the C8-C9 and C15-C10 bonds is 1.2 (3) . However, the torsion angle of the chalcone between the O1-C9 and C7-C8 bonds is 12.0 (4) , indicating a break in planarity. This single deviation causes a slight concave bend in the molecule. The title compound crystallized as a racemic mixture in the space group Pca2 1 ; thus, a clockwise and anticlockwise torsion of the chalcone are present with a 1:1 ratio in the unit cell.
There are several other chalcones with a comparable 4 and 4 0 set of substitutions that are summarized in Table 1 from a CSD database search. If the halogen (-X) is maintained as a fluorine, the other substituent (-R) varies as either a methyl, hydroxyl, methoxy, or ethoxy group. Examination of the three torsion angles described above suggests that there is a trend in the degree of distortion from planarity, with an order of methyl, methoxy, ethoxy, to hydroxy by increasing planarity. While there are no direct examples that contain a halogen and an ethoxy, we felt comparison of our compound to the nearest chloro-and bromo-substituted compounds was warranted.
The closest examples are a bromo/methoxy and a chloro/ methoxy 4,4 0 -substituted chalcone. Both cases are more distorted from planar than our fluoro/ethoxy chalcone. Lastly, we found a set of chalcones with an ethoxy substituent, where there are chlorine atoms in the 2 and 3 position of the respective phenyl ring. Both of these cases are more planar than our chalcone.
All torsions were measured in Mercury (v2020.2.0; Macrae et al., 2020), with the exception of the torsion angles from this work, which were calculated using the CONF command of SHELXL2018/3 (Sheldrick, 2015b

Figure 2
Packing of the title compound viewed along the a axis. Hydrogen bonds and H-bonds are shown as blue lines.

Figure 3
Packing of the title compound viewed along the b axis. Hydrogen bonds and H-bonds are shown as blue lines.

Figure 4
Packing of the title compound viewed along the c axis. Hydrogen bonds and H-bonds are shown as blue lines.
connect each molecule to three of its neighbors. The first is between the C3-H3 bond and an adjacent F1 atom, the second pairs the C5-H5 bond and a nearby O1 atom, and the final two involve the C14-H14 and C16-H16A bonds with a neighboring O1 atom. Given the extent of conjugated bonds throughout this molecule,stacking is present between adjacent molecules along the a axis [centroid-centroid distance = 4.240 Å ], with alternating molecules related by the a glide plane of the Pca2 1 space group; this orients these molecules such that adjacent molecules are mirror images of one another with opposing chalcone bond torsions. Lastly, there are H-interactions present between H17A and the aromatic ring comprised of C1-C6, as well as between H17C and this same ring on another molecule, forming a chain of interactions that parallel the a axis.
In comparison to the other chalcones described in Table 1, our structure packs in a unique space group Pca2 1 , where many others pack in Pbca or P1. Common themes that appear among these structures includestacking and hydrogen bonding to the carbonyl oxygen. However, it is interesting to note that the chloro/methoxy and bromo/methoxy analogs pack withstacking where the molecules are mirror images from a plane that is colinear with the molecular mean plane, rather than images related to a plane that is orthogonal to the molecular mean plane as observed in our structure. Also, in the case where the -R substituent is a hydroxide, hydrogen bonding between the hydroxyl group and and the carbonyl oxygen dominates the packing.

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 3. Hydrogen atoms were generated using a riding model with geometric constraints and refined isotropically. Aromatic C-H distances are 0.95 Å , methylene C-H distances are 0.99 Å , and methyl C-H distances are 0.98 Å . U iso (H) was 1.2 times U eq (C) for aromatic and methylene hydrogen atoms, and 1.5 times U eq (C) for methyl hydrogen atoms. There is minor whole molecule disorder visible in the residual peaks that was not refined since these peaks are rather small (< 0.26 e Å À3 ) and there was little improvement in the model.

(2E)-1-(4-Ethoxyphenyl)-3-(4-fluorophenyl)prop-2-en-1-one
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. Refinement. There is a minor disordered component visible in the residual peaks that is not refined, since there is little improvement in the model.