Crystal structure and Hirshfeld surface analysis of (2E,2′E)-1,1′-[selenobis(4,1-phenylene)]bis[3-(4-chlorophenyl)prop-2-en-1-one]

In the title organoselenium compound, the C—Se—C angle is 99.0 (2)°, with the dihedral angle between the planes of the attached benzene rings being 79.1 (3)°.

In the title compound, C 30 H 20 Cl 2 O 2 Se, the C-Se-C angle is 99.0 (2) , with the dihedral angle between the planes of the attached benzene rings being 79.1 (3) . The average endocyclic angles (Se-C-C) facing the Se atom are 122.1 (5) and 122.2 (5) . The Se atom is essentially coplanar with the attached benzene rings, deviating by 0.075 (1) and 0.091 (1) Å . In the two phenylene(4-chlorophenyl)prop-2-en-1-one units, the benzene rings are inclined to each other by 44.6 (3) and 7.8 (3) . In the crystal, the molecules stack up the a axis, forming layers parallel to the ac plane. There are no significant classical intermolecular interactions present. Hirshfeld surface analysis, two-dimensional fingerprint plots and the molecular electrostatic potential surface were used to analyse the crystal packing. The Hirshfeld surface analysis suggests that the most significant contributions to the crystal packing are by CÁ Á ÁH/HÁ Á ÁC contacts (17.7%).

Chemical context
During the last few years, organoselenium chemistry (Procter, 2001) has been the subject of constant scientific interest and organoselenium compounds have been used intensively as important reagents and intermediates in organic synthesis (Zade et al., 2005). Recently, various organoselenium compounds have attracted growing attention in medicine. Selenoproteins are very important for neuronal survival and function. It has been found that selenoprotein P may influence Alzheimer pathology (Bellinger et al., 2008). Furthermore, the potential of selenoproteins to protect against oxidative stress led to the expectation that selenium would be protective against type 2 diabetes, and indeed in the 1990s, selenium was shown to have antidiabetic and insulin mimetic effects (Steinbrenner et al., 2011). However, more recently, findings from observational epidemiological studies and randomized clinical trials have raised concern that high selenium exposure may lead to type 2 diabetes or insulin resistance at least in well-nourished populations (Stranges et al., 2010). In addition, molecules involving selenium are still efficient and encouraged in medicinal chemistry (Zhao et al., 2012). Moreover, organoselenium compounds are of considerable interest in academia, as anticancer (Zhu & Jiang, 2008), anti-oxidant (Anderson et al., 1996), anti-inflammatory and antiallergic agents (Abdel-Hafez, 2008), and in industry because of their involvement as key intermediates in the synthesis of pharmaceuticals (Woods et al., 1993), fine chemicals and polymers (Hellberg et al., 1997). Moreover, chalcone derivatives are notable for their excellent blue-light transmittance and good crystallizability; they also show considerable promise as organic nonlinear optical materials (Uchida et al., 1998). In continuation of our work on chalcone organoselenium derivatives, we report herein on the crystal structure of (2E,2 0 E)-1,1 0 -[selenobis(4,1-phenylene)]bis [3-(4-chlorophenyl)prop-2-en-1-one].
In the title compound, inner benzene rings A (atoms C1-C6) and C (C16-C21) (see Scheme) are inclined to each other by 79.1 (3) . This is similar to the same angle observed for the acetylphenyl derivative, viz. 87.08 (15)

Figure 3
A view of the Hirshfeld surface mapped over d norm in the colour range À0.0711 to 1.3645 a.u.

Figure 1
The molecular structure of the title compound, with the atom labelling and displacement ellipsoids drawn at the 50% probability level. different to that observed for the 4-nitrophenyl derivative, viz. 63.76 (10) .
In each phenylene-(4-chlorophenyl)prop-2-en-1-one unit, the C C has an E configuration. The C C bond lengths C8 C9 and C23 C24 are 1.317 (8) and 1.325 (8) Å , respectively, which confirms their double-bond character. Benzene rings A and B (C10-C15) of one unit are inclined to one another by 44.6 (3) , while rings C and D (C25-C30) of the other unit are almost coplanar, with a dihedral angle of 7.8 (3) . The outer benzene rings, B and D, are almost normal to one another, with a dihedral angle of 84.4 (3) .

Supramolecular features
In the crystal, molecules stack up the a axis, forming layers parallel to the ac plane (Fig. 2). There are no significant classical intermolecular interactions present (PLATON; Spek, 2009). The shortest atom-atom contacts in the crystal (Figs. 3 and 4) are given in Table 1 and are discussed in x4 (Hirshfeld surface analysis).   A view of the Hirshfeld surface plotted over the calculated electrostatic potential energy in the range À0.0489 to 0.0448 a.u.

Hirshfeld surface analysis
Insight into the intermolecular interactions in the crystal were obtained from an analysis of the Hirshfeld surface (Spackman & Jayatilaka, 2009) and the two-dimensional fingerprint plots (McKinnon et al., 2007). The program CrystalExplorer (Turner et al., 2017) was used to generate both the Hirshfeld surfaces, mapped over d norm , and the electrostatic potential for the title compound. The function d norm is a ratio enclosing the distances of any surface point to the nearest interior (d i ) and exterior (d e ) atom and the van der Waals (vdW) radii of the atoms. The function d norm will be equal to zero when intermolecular distances are close to the van der Waals contacts. They are indicated by a white colour on the Hirshfeld surface, while contacts longer than the sum of the vdW radii with positive d norm values are coloured blue.
The analysis of the Hirshfeld surface (HS) mapped over d norm is shown in Fig. 4. The HÁ Á ÁO contacts between the corresponding donor and acceptor atoms are visualized as bright-red spots on the side (zone 4) of the Hirshfeld surface ( Fig. 4). Three other red spots exist, corresponding to the CÁ Á ÁSe, ClÁ Á ÁCl and CÁ Á ÁO contacts, viz. zones 1, 2 and 3, respectively (Fig. 4). These contacts are considered to be the strongest when comparing them to the sum of the vdW radii [ Table 1; calculated using Mercury (Macrae et al., 2008)].
A view of the molecular electrostatic potential using the 6-31G(d) basis set with the density functional theory (DFT) method for the title compound is shown in Fig. 5. The HÁ Á ÁO donors and acceptors are shown as blue and red areas around the atoms related with positive (hydrogen-bond donors) and negative (hydrogen-bond acceptors) electrostatic potentials, respectively.
The full two-dimensional fingerprint plot for the title compound is given in Fig. 6(a). Those for the most significant contacts contributing to the HS are given in Fig. 6 Table 2.
A contribution of 36.0% was found for the HÁ Á ÁH contacts (Fig. 6b), representing the largest contribution, and is displayed on the fingerprint plots by a pair of very short spikes at d e + d i = 2.3 Å ; the vdW radius for this interaction is 2.18 Å , which means it is a weak interaction.
The plot of OÁ Á ÁH/HÁ Á ÁO contacts between H atoms located inside the Hirshfeld surface and oxygen from outside and vice versa is shown in Fig. 6(d). These contacts account for 11.5% and are characterized by two symmetrical peaks with d e + d i = 2.5 Å ; this reveals the presence of strong OÁ Á ÁH contacts.

Database survey
In the title compound (Fig. 1), the C-Se-C angle is 99.0 (2) , similar to the value observed in five of the compounds mentioned above, viz. 100.03 (15)

Synthesis and crystallization
The title compound was prepared according to a method proposed by Mechehoud et al. (2010). 2-Chloro-1-(4-chlorophenyl)ethan-1-one (ClC 8 H 6 COCl; 36.5 mmol) and anhydrous aluminium chloride (5 g, 37.5 mmol, 3 equiv.) were taken up in dry methylene chloride (100 ml). The reaction mixture was cooled to 273-278 K, protected from atmospheric moisture and stirred continuously for 15 min. A solution of diphenyl selenide (3 g, 1.87 mmol) in CH 2 Cl 2 was added dropwise over a period of 5 min. The reaction mixture was allowed to reach room temperature gradually and then stirred at this temperature overnight. The solution was then washed with ice water-HCl (80 ml) and extracted with CH 2 Cl 2 . The organic layer was separated and dried (Na 2 SO 4 ). Removal of  the solvent under reduced pressure afforded the crude product, which was recrystallized from petroleum ether to yield 4.2 g of the title compound. Yellow single crystals suitable for X-ray diffraction analysis were obtained by recrystallization from CH 2 Cl 2 .

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. Refined as a 2-component inversion twin.