Crystal structures of three functionalized chalcones: 4′-dimethylamino-3-nitrochalcone, 3-dimethylamino-3′-nitrochalcone and 3′-nitrochalcone

The structure of three functionalized chalcones (1,3-diarylprop-2-en-1-ones), containing combinations of nitro and dimethylamino functional groups, are presented.


Chemical context
Chalcones, 1,3-diarylprop-2-en-1-ones, are a group of organic molecules containing two aromatic rings joined by an enone backbone. Chalcones are studied for a range of medicinal purposes, with many reviews published on their biological applicability (see, for example, Rammohan et al., 2020;Zhuang et al., 2017;Singh et al., 2014).
A range of chalcones, functionalized on either aromatic ring, can be readily synthesized via an aldol condensation reaction (Mandge et al., 2007). Altering the functional groups on the chalcone structure has been shown to yield a variety of useful properties, including changes in colour and fluorescent properties (Ibnaouf et al., 2018).
Hm7m (Fig. 1b) crystallizes in space group P2 1 /n with a single molecule in the asymmetric unit. The combination of torsion angles along the long-axis of the molecule means that although the backbone remains relatively planar [C4-C10-C2 = 2.86 (6) ], the 1-and 3-rings are twisted with respect to each other, with a twist angle of 13.80 (8) between the planes of the rings.
Hm1-crystallizes in space group P2 1 /c and contains two molecules in the asymmetric unit. One of the molecules (1) is almost planar, with a twist angle of only 1.88 (8) between the planes of the 1-and 3-rings of the molecule. The second molecule (2) is less planar with È 1 = À164.6 (2) and È 2 = À172 (9) , leading to a twist angle of 12.85 (8) between the planes of the 1-and 3-rings. There is a stacking interaction between the aryl rings of the two molecules in the asymmetric unit, with a centroid-to-centroid distance of 3.82782 (17) Å (Fig. 1c).
In each of the molecular structures, the functionalized group in the meta-position sits on the same side of the molecule as the carbonyl oxygen group (1-ring: C6, 3-ring: C12). This is likely due to the optimization of hydrogen-bonding motifs in the crystal structures.

Supramolecular features
Although being in a different space group, the crystal structure of Gp8m is very similar to that of a previously reported chalcone 3 0 -nitro,4-dimethylaminochalcone (Rosli et al., 2007). This may be expected, as the only difference between these molecules is that the functional groups are on opposite rings. Within the crystal structure, chains of molecules form down the long axis of the molecule via short contacts between the dimethylamino and nitro groups ( Fig. 2a; C17-H17BÁ Á ÁO3 iii ). These molecules form stacks parallel to the b axis, with alternate molecules the opposite way around such that the nitro group sits above the 1-ring of the adjacent molecule. The final 3D structure is completed by a linking of the stacks via C-HÁ Á ÁO cyclic hydrogen bonding (C3-H3Á Á ÁO1 i , C5-  Table 1 Torsion and ring angles ( ) describing the planarity of molecules in each crystal structure.

Figure 1
Displacement ellipsoid plots showing the asymmetric units of the solved crystal structures (a) Gp8m, (b) Hm7m and (c) Hm1-. Displacement ellipsoids are shown at the 50% probability level. The stacking interaction between the 1-and 3-rings of the molecules in the asymmetric unit of Hm1-is highlighted.
Within the crystal structure of Hm7m, sheets are formed in the plane of the aromatic rings of the molecule. Within the plane, pairs of inverted molecules form via cyclic hydrogen bonding between the dimethylamino and nitro groups, offset in the short axis of the molecule (C16-H16BÁ Á ÁO2 ii ). The pairs of molecules then form sheets via a trifurcated hydrogenbonding interaction involving the nitro group (C15-H15Á Á ÁO3 i , C15-H2Á Á ÁO3 i , C15-H9Á Á ÁO3 i ). These sheets make up the 3D structure via a stacking interaction, where the nitro group of one molecule sits over the 1-ring of another (Fig. 3). Numerical details of the hydrogen-bond geometry and symmetry codes are given in Table 3.
The crystal structure of Hm1-contains two molecules in the asymmetric unit cell, which differ slightly in their planarity. Sheets of molecules form via the same interactions as in Hm7m; however, the pairs of molecules form between different independent molecules, meaning they are not directly related by an inversion centre. Furthermore, the absence of the dimethylamino group means that the molecules are shifted relatively along the long axis of the molecule, forming hydrogen bonds that utilize the carbonyl oxygen (Fig. 4a). The stacking interactions that make up the 3D structure of Hm1-are more complex than those in Hm7m, and are highlighted in Fig. 4b. Molecule 1 forms a direct stack with a symmetrically equivalent molecule, with an inversion centre relating the molecules. There is a half stack that forms between the 1-ring of molecule 1 and the 3-ring of molecule 2, which sit at approximately 90 to each other. Finally, molecule 2 forms a half stack with a symmetrically equivalent molecule, where the 1-ring of each molecule sits on top of the other.  Table 2 Hydrogen-bond geometry (Å , ) for Gp8m. Symmetry codes: (i) Àx þ 1; Ày þ 1; Àz; (ii) x; Ày þ 3 2 ; z þ 1 2 ; (iii) x À 1; y; z.

Figure 3
Bonding motifs present in the crystal structure of Hm7m. Two sheets of molecules are highlighted, coloured in black and light blue for contrast.
Numerical details of the hydrogen-bond geometry are given in Table 4.

Database survey
A survey of the Cambridge Structural Database (CSD, version 5.41, last update March 2020; Groom et al., 2016) revealed 38 structures of chalcones functionalized with either nitro or dimethylamino-groups in either the meta or para positions of the 1-or 3-ring. None of the structures contain chalcones with a dimethylamino group on the 1-ring, as in Gp8m. However, there are 14 structures of chalcones substituted with a dimethylamino group on the 3-ring, likely owing to their fluorescent properties (Jiang et al., 1994;Tomasch et al., 2012). 17 of the 29 structures that contain nitro ring substitutions contain the bonding motif between the nitro group and the region between H15, H2 and H9, as observed in Hm7m and Hm1-. This is likely caused by the optimization of electrostatic interactions, as highlighted by the electrostatic potentials in Fig. 5. The layered motif in Hm7m is the same as that present in the structure of 3 0 -nitro-3,5-dimethoxychalcone (Qiu & Yang, 2006). The planes of molecules seen in Hm1-are similar to those seen in the structure of 4 0 -nitrochalcone (BUDXOO; Jing, 2009).

Synthesis and crystallization
Each of the functionalized chalcones was synthesized via an aldol condensation reaction between a suitably functionalized benzaldehyde and acetophenone. While syntheses were not specifically reported for Gp8m and Hm7m, the first reports for Hm1-appeared in 1929and 1935(Dilthey et al., 1929Weygand et al., 1935).
Crystals of Hm1-and Gp8m suitable for structural solution via single crystal X-ray diffraction were produced via evaporation of an ethanol solution of concentration 10 mg mL À1 . Crystals of Gp8m appeared as fine yellow needles and Hm1-as colourless block-like crystals. Each single crystal was mounted onto a glass capillary using paraffin oil.

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.

1-(3-Nitrophenyl)-3-phenylprop-2-en-1-one (Hm1-)
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.