1-Chloro-4-[2-(4-chlorophenyl)ethyl]benzene and its bromo analogue: crystal structure, Hirshfeld surface analysis and computational chemistry

Two independent molecules comprise the asymmetric unit of the chloro compound, each disposed about a centre of inversion. Each molecule approximates mirror symmetry. By contrast, the bromo compound is significantly twisted [dihedral angle between the benzene rings = 59.29 (11)° cf. 0° for the chloro-containing molecules].

The conformational differences between the molecules in (I) and (II) are highlighted in the overlay diagram shown in Fig. 2.

Supramolecular features
In the crystal of (I), the main point of contact between the independent molecules comprising the asymmetric unit are of the type benzene-C-HÁ Á Á(benzene),     The molecular structures of (a) the Cl1A-containing molecule of (I), (b) the Cl1B-containing molecule of (I) and (c) the molecule of (II) showing the atom-labelling scheme and displacement ellipsoids at the 70% probability level. Unlabelled atoms in (a) and (b) are related by the symmetry operations 1 À x, 1 À y, 1 À z and 1 2 À x, 3 2 À y, 1 À z, respectively. Table 1 Hydrogen-bond geometry (Å , ) for (I).

Hirshfeld surface analysis
The Hirshfeld surface calculations for (I) and (II) were performed in accord with established procedures (Tan et al., 2019) with the aid of Crystal Explorer (Turner et al., 2017) to determine the influence of weak intermolecular interactions upon the molecular packing in the absence of conventional hydrogen bonds.
In the crystal of (I), with two independent molecules, labelled A and B, disposed about a centre of inversion the presence of faint-red spots near the benzene-C2A, C3A and H5B atoms in the images of Hirshfeld surfaces mapped over d norm in Fig. 5 represent C-HÁ Á Á contacts, Tables 1 and 3. The diminutive red spot viewed near the benzene-C5B atom in  Table 2 Hydrogen-bond geometry (Å , ) for (II).

Figure 4
Molecular packing in (II): a view of the unit-cell contents shown in projection down the a axis, highlighting C-HÁ Á Á and BrÁ Á ÁBr contacts as purple and orange dashed lines, respectively.

Figure 5
Views of the Hirshfeld surfaces for (I) mapped over d norm for ( contact, Table 3. Also, the presence of diminutive red spots near the terminal chlorine atoms of both independent molecules in Fig. 5 are due to the formation of short interatomic ClÁ Á ÁCl contacts, Table 3. In the crystal of (II), the bright-red spots near the bromine atoms on the Hirshfeld surfaces mapped over d norm in Fig. 6 indicate interatomic BrÁ Á ÁBr contacts, Table 3, whereas those near the benzene-C2 and H6 atoms in Fig. 6(b) indicate short interatomic C-HÁ Á Á interactions, Table 3. The presence of faint-red spots near the benzene-C13, C14 and H3 atoms in Fig. 6(a) also reflect the presence of C-HÁ Á Á contacts, Table 3.
From the views of Hirshfeld surfaces mapped over the calculated electrostatic potentials in Figs. 7(a) and (b) for the independent molecules of (I) highlight the small deviations from putative mirror symmetry through the slight differences in the blue and red regions around the atoms of their surfaces corresponding, respectively, to positive and negative potentials. For (II), Fig.7(c), the donors and acceptors of the C-HÁ Á Á interactions are viewed as blue bumps and light-red concave regions. Further, the donors and acceptors of the C-HÁ Á Á contacts for each of (I) and (II) are also illustrated through black dotted lines on the Hirshfeld surfaces mapped with shape-index properties in Fig Table 3 Summary of short interatomic contacts (Å ) in (I) a .

Contact
Distance Symmetry operation The overall two-dimensional fingerprint plot for the independent molecules A and B as well as entire (I) are shown in Fig. 9(a), and those delineated into HÁ Á ÁH, CÁ Á ÁH/HÁ Á ÁC, ClÁ Á ÁH/HÁ Á ÁCl and ClÁ Á ÁCl contacts are illustrated in Fig. 9(b)-(e), respectively. The quantitative summary of percentage contributions from the different interatomic contacts to the respective Hirshfeld surfaces of A, B and (I) are presented in Table 4. Some qualitative differences in the fingerprint plots are evident for molecules A and B, confirming their distinct packing interactions. The complementary pair of forceps-like tips at d e + d i $2.3 Å in the fingerprint plots delineated into HÁ Á ÁH contacts for A and B in Fig. 9(b) represent the short interatomic HÁ Á ÁH contact, Table 3, which merge to form a pair of tips in the overall plot for (I). The fingerprint plots delineated into CÁ Á ÁH/HÁ Á ÁC contacts for molecules A and B in Fig. 9(c) exhibit the clearest distinction between the interatomic contacts formed by the molecules through the asymmetric distribution of points. The complementary distribution of points in the acceptor and donor regions of the plots for A and B, respectively, with the peaks at d e + d i $2.7 Å , are due to the formation of short interatomic CÁ Á ÁH/HÁ Á ÁC contacts between the benzene-C2A, C3A and H5B atoms, Table 3. Similar short interatomic contacts between benzene-C5B and H2B atoms of B results in forceps-like tips at d e + d i $2.7 Å in the acceptor region of the plot whereas it is merged within the tip of previously mentioned contact in the donor region. However, the respective plot for an overall structure is symmetric owing to the merging of the asymmetric distribution of points. The significant and quite similar contributions from ClÁ Á ÁH/HÁ Á ÁCl contacts to the Hirshfeld surfaces of A, B and overall (I), Fig. 9(d), have very little influence on the molecular packing due to their interatomic distances being equal to or greater than the sum of their van der Waals radii.
The linear distribution of points beginning from d e + d i $3.3 and 3.4 Å , Fig. 9(e), in the ClÁ Á ÁCl delineated plots for A and B, respectively, indicate the presence of ClÁ Á ÁCl interactions. The small contribution from CÁ Á ÁC contacts to the Hirshfeld surface of (I) has a negligible effect on the packing.    Table 4. The short interatomic HÁ Á ÁH contact between symmetry-related ethylene-H8B atoms is viewed as a single peak at d e + d i $2.2 Å in Fig. 10(b). In Fig. 10(c), delineated into CÁ Á ÁH/ HÁ Á ÁC contacts, Table 3, the forceps-like tips at d e + d i $2.6 Å reflect the significant C-HÁ Á Á contacts in the molecular packing. The contribution of BrÁ Á ÁH/HÁ Á ÁBr contacts to the Hirshfeld surface of (II), Fig. 10(d), have very little influence on the packing due to their interatomic distances being around the sum of their van der Waals radii. The short interatomic BrÁ Á ÁBr contacts in (II) are viewed as a thin, linear distribution of points initiating from d e + d i $3.5 Å , Fig. 10(e). As for (I), the small contribution from CÁ Á ÁC contacts to the Hirshfeld surface of (II) has a negligible effect in the crystal.

Computational chemistry
The pairwise interaction energies between the molecules in the crystals of (I) and (II) were calculated by summing up four energy components, being electrostatic (E ele ), polarization

Figure 11
A comparison of the energy frameworks composed of (a) electrostatic potential force, (b) dispersion force and (c) total energy for cluster about a reference molecule of A and B of (I), and for (II). The energy frameworks were adjusted to the same scale factor of 80 with a cut-off value of 2 kJ mol À1 within 4 Â 4 Â 4 unit cells.
(E pol ), dispersion (E dis ) and exchange-repulsion (E rep ) (Turner et al., 2017). These energies were obtained by using the wave functions calculated at the B3LYP/6-31G(d,p) level theory for (I) and the HF/STO-3G level theory for (II). The individual energy components as well as total interaction energy relative to reference molecule within molecular clusters out to 3.8 Å .
The nature and strength of the energies for the key identified intermolecular interactions are quantitatively summarized in Table 5. Dispersive components are dominant as conventional hydrogen bonding is not possible. The significant contributions from the C-HÁ Á Á interaction and short interatomic CÁ Á ÁH/HÁ Á ÁC contacts in the crystal of (I) are evident from Table 5. Also notable, are the negligible energies associated with the ClÁ Á ÁCl contacts due to the dominance of repulsive contributions. With respect to (II), it is evident from the comparison of the dispersive component as well as total energies for the different interactions that the strength of interactions in the crystal depend upon distance between the respective molecules. The short BrÁ Á ÁBr contacts in (II) also have very small interaction energies.
The magnitudes of intermolecular energies are represented graphically in the energy frameworks of Fig. 11. Here, the supramolecular architecture of each crystal is viewed through the cylinders joining the centroids of molecular pairs. The red (E ele ), green (E disp ) and blue (E tot ) colour scheme represent the specified energy components. The radii of the cylinders are proportional to the magnitude of interaction energies which are adjusted with a cut-off value of 2 kJ mol À1 within 4 Â 4 Â 4 unit cells. The energy frameworks constructed for the clusters about the independent molecules A and B of (I) as well as that for (II) also indicate the distinct mode of supramolecular association around the molecules in the molecular packing. The small effect of the electrostatic components and the significant influence of the dispersive components are clearly evident from the energy frameworks shown in Fig. 11.

Database survey
There are only four halo-substituted 1,2-bis(phenyl)ethylene derivatives in the literature. The key structural parameters for these are summarized in Table 6. Only one literature structure is not disposed about a centre of inversion, namely the nonsymmetric, mixed-halo structure (4-Br,2,6-F 2 C 6 H 2 )-CH 2 CH 2 C 6 H 4 Br-4 (Galá n et al., 2016). Generally, the central C e -C e (e = ethylene) bonds are long in these compounds with the exception being the pentabromo derivative, C 6 Br 5 CH 2 CH 2 C 6 Br 5 (Kö ppen et al., 2007).

Synthesis and crystallization
Tri(4-chlorobenzyl)tin chloride was prepared by direct synthesis using tin powder (Merck) and 4-chlorobenzyl chloride (Sigma-Aldrich) in water (Sisido et al., 1961). Tri(4chlorobenzyl)tin chloride (5.3 g, 10 mmol) was dissolved in 95% ethanol (150 ml) and to this was added dropwise 10% sodium hydroxide solution (4 ml). The resulting solution was heated for 1 h. After cooling, the white tri(4-chlorobenzyl)tin hydroxide was filtered off and the filtrate was evaporated slowly to obtain a colourless crystalline solid which was identified crystallographically as (I). Yield: 0.28 g (0.11%). The bromo analogue was similarly obtained as a side-product from the base hydrolysis of tri(4-bromobenzyl)tin bromide. Tri(4-bromobenzyltin) bromide was prepared from the reaction of tin powder (Sigma-Aldrich) and 4-bromobenzyl bromide (Merck) in water (Sisido et al., 1961). Tri(4-bromobenzyl)tin bromide (7.0 g, 10 mmol) was dissolved in 95% ethanol (150 ml) and to this was added 10% sodium hydroxide solution (4 ml). The resulting precipitation was heated for 1 h. After cooling, the yellow tri(4-bromobenzyl)tin hydroxide was filtered off and the filtrate was evaporated slowly to obtain a yellow crystalline solid which was identified crystallographically as (II). Yield: 0.25 g (0.07%)

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.

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. Owing to poor agreement, the (1 1 1) reflection was omitted from the final cycles of refinement.