Crystal structure and Hirshfeld surface analysis of a conformationally unsymmetrical bischalcone: (1E,4E)-1,5-bis(4-bromophenyl)penta-1,4-dien-3-one

The title bischalcone shows s-trans and s-cis conformations for its C=C—C=O bonds.

In the title bischalcone, C 17 H 12 Br 2 O, the olefinic double bonds are almost coplanar with their attached 4-bromophenyl rings [torsion angles = À10.2 (4) and À6.2 (4) ], while the carbonyl double bond is in an s-trans conformation with with respect to one of the C C bonds and an s-cis conformation with respect to the other [C C-C O = 160.7 (3) and À15.2 (4) , respectively]. The dihedral angle between the 4-bromophenyl rings is 51.56 (2) . In the crystal, molecules are linked into a zigzag chain propagating along [001] by weak C-HÁ Á Á interactions. The conformations of related bischalcones are surveyed and a Hirshfeld surface analysis is used to investigate and quantify the intermolecular contacts.

Chemical context
Dibenzalacetone, or bischalcone, [(1E,4E)-1,5-diphenylpenta-1,4-dien-3-one] was first prepared by the base-catalyzed Aldol condensation of benzaldehyde and acetone (Conard & Morris, 1932): it results in a highly conjugated system involving the ,-unsaturated pentadienone (-C C-(C O)-C C-) moiety. Bischalcones have a number of uses including antiinflammatory (Mahapatra et al., 2017) and anti-oxidant (Pandey & Syed, 2009) agents. Different bischalcones consist of two benzene rings substituted with different types of functional groups (electron donor or acceptor) bonded to the ends of the central ,-unsaturated ketone which provides good configuration for the transfer of intramolecular charge (Fun et al., 2011). In a continuation of our ongoing studies on the non-linear optical properties of various chalcone derivatives (Sim et al., 2017;Kwong et al., 2018), we report herein the synthesis, structure determination and Hirshfeld surface analysis of the title compound (I).

Supramolecular features
No classical hydrogen bonding is possible in (I) and in the crystal, molecules are linked by C-HÁ Á Á interactions (Table 1): the first of these results in a phenyl-phenyl T-shaped geometry via C1-H1AÁ Á ÁCg1 i (Fig. 3a). The C14-H14AÁ Á ÁCg2 ii (Fig. 3b) interactions lead to a zigzag chain along the c-axis direction.

Database survey
A survey of the Cambridge Structural Database (CSD, version 5.40, last update February 2019; (Groom et al., 2016)) using (1E,4E)-1,5-diphenylpenta-1,4-dien-3-one as the main skeleton revealed the presence of 27 structures containing a similar bischalcone moiety to the title compound but with different substituents on the terminal phenyl rings. The different substituents (R 1 and R 2 ) together with the torsion angles of the penta-4,4-dien-3-one connecting bridge are compiled in Table 2 General chemical diagram showing torsion angles, 1 , 2 , 3 and 4 in the title compound. Table 1 Hydrogen-bond geometry (Å , ).

Figure 3
A partial packing diagram of the title compound, with (a) C1-H1AÁ Á Á and (b) C14-H14AÁ Á Á interactions (dotted lines). Hydrogen atoms not involved in these interactions have been omitted for clarity.

Hirshfeld surface analysis
The Hirshfeld surfaces mapped with normalized contact distance d norm and the two-dimensional fingerprint plots for (I) were generated using CrystalExplorer17.5 (Turner et al., 2017). The darkest red spots on the Hirshfeld surface mapped with d norm (Fig. 4a) correspond to the C14-H14AÁ Á ÁCg2 ii interaction. Even through the C1-H1AÁ Á ÁCg1 i interaction is not visible in the d norm surface mapping, this interaction can be seen as a unique pattern of a red 'circle' on the shape-index surface mapping (Fig. 4b). Besides the C-HÁ Á Á interactions, the d norm surface mapping indicated a short contact between atom O1 and C5 with a distance of 0.06 Å shorter than the sum of the van der Waals radii of O and C atoms (3.22 Å ; Fig. 5a). Together with this short contact, another weak C7-H7AÁ Á ÁO1 interaction was also revealed as light spots on the d norm surface (Fig. 5b).
As illustrated in Fig. 6, the corresponding fingerprint plots for ( The Hirshfeld surface mapped with (a) d norm and (b) shape-index for the title compound showing the C-HÁ Á Á interactions.

Figure 5
The Hirshfeld surface mapped with d norm showing (a) the C5Á Á ÁO1 short contact and (b) the weak C7-H7AÁ Á ÁO1 interaction.

Figure 6
The two-dimensional fingerprint plots of the title compound for different intermolecular contacts and their percentage contributions to the Hirshfeld surface. d e and d i are the distances from the Hirshfeld surface to the nearest atom interior and exterior, respectively, to the surface. the most populated contacts and contribute 34.1% to the total intermolecular contacts, followed by HÁ Á ÁH (22.1%), HÁ Á ÁBr/ BrÁ Á ÁH (20.4%) and HÁ Á ÁO/OÁ Á ÁH (9.2%) contacts (Fig. 6). As the C-HÁ Á Á bonds are the main interaction in the crystal, the most populated HÁ Á ÁC/CÁ Á ÁH contacts appear as two symmetrical narrow wings at diagonal axes d e + d i ' 2.7 Å (Fig. 6b). The HÁ Á ÁH contacts appear in the central region of the fingerprint plots with d e = d i = 2.4 Å (Fig. 6c). With the presence of relatively larger bromine atoms in the structure, the HÁ Á ÁBr/BrÁ Á ÁH contacts appear as symmetrical broad wing at diagonal axes of d e + d i ' 3.0 Å (Fig. 6d). Two symmetric spikes in the fingerprint plots with a short spike at d e + d i ' 2.7 Å represent the HÁ Á ÁO/OÁ Á ÁH contacts (Fig. 6e), indicating the presence of the weak C7-H7AÁ Á ÁO1 interaction. The percentage contributions for other contacts are less than 15% in the Hirshfeld surface mapping.

Synthesis and crystallization
A mixture of 4-bromobenzaldehyde (4.9 g, 12.5 mmol) and acetone (0.363 g, 6.25 mmol) dissolved in absolute ethanol (30 ml) was slowly added to an aqueous solution of potassium hydroxide (4.0 g in 20 ml water). The mixture was vigorously stirred at room temperature for two h and then 20 ml chilled water was added. The resulting yellow precipitate was recovered by vacuum filtration and washed with cold water (100 ml). The crude product was recrystallized from absolute ethanol solution as yellow blocks.

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 3. C-bound H atoms were positioned geometrically (C-H = 0.93 Å ) and refined using a riding model with U iso (H) = 1.5U eq (C).

(1E,4E)-1,5-Bis(4-bromophenyl)penta-1,4-dien-3-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.