Crystal and molecular structure of 2-methyl-1,4-phenylene bis(3,5-dibromobenzoate)

Molecules of the aryl diester, 2-methyl-1,4-phenylene bis(3,5-dibromobenzoate), crystallized out from the melt (m.p. = 502 K/DSC). The crystal structure consists of a C—H⋯Br hydrogen-bonded network and weaker, offset π–π interactions.


Chemical context
Inverse vulcanization (InV) polymerization is an important solvent-less process for the synthesis of elastomeric materials from elemental sulfur and thermally stable organic co-monomers, both of which are often found as waste products of the chemical industry (Chung et al., 2013;Karunarathna et al., 2020).Recently, aryl halide co-monomers, including the title aryl diester, were investigated for un-catalyzed InV chemistry, and shown to react via a radical aryl sulfur polymerization (RASP) mechanism at temperatures > 493 K (Karunarathna et al., 2020;Thiounn et al., 2020).An advantage of the title aryl diester as a co-monomer for InV reactions is reflected by its conjugated aromaticity and attendant exceptional thermal stability (T d = 563 K/TGA).Further, a more recent study (Lauer et al., 2024) demonstrated that successful InV reactions could be carried out at temperatures as low as 463 K, using the title aryl diester co-monomer in conjunction with a dithiocarbamate (DTC) catalyst.The catalyzed reaction data were significant because they provided evidence for the possible involvement of anionic sulfur intermediates and expanded the possible scope of the InV reactions to more thermally sensitive co-monomers (Lauer et al., 2024).

Structural commentary
The aryl diester compound, 2-methyl-1,4-phenylene bis(3,5dibromobenzoate), crystallizes in the space group P1 with one half molecule per asymmetric unit.Molecules lie on crystallographic inversion centers that impose disorder of the methyl group (C11H 3 ) and an H atom (H10) across the central benzene ring (Fig. 1).The two 3,5-dibromobenzoate end groups are attached to the central benzene ring in an anti fashion, with the planes of the 3,5-dibromobenzoate rings inclined at a dihedral angle of 54.53 (9) � with respect to the plane of the central benzene ring (Fig. 1).The ester groups are nearly co-planar with their conjugated 3,5-dibromophenyl rings, making a dihedral angle of only 8.21 (11) � , but inclined at a dihedral angle of 62.58 (10) � with respect to the central benzene ring (Fig. 1).This compares well to the structure of the related 1,4-phenylene dibenzoate, with the end group rings and ester groups tipped with respect to the central 1,4-benzene ring at dihedral angles of 55.29 (8) and 60.31 (9) � , respectively, and the ester groups with their conjugated end group rings tipped at only a small dihedral angle of 5.94 (8) � (Ganaie et al., 2016).

Supramolecular features
Intermolecular contacts of the title aryl diester involve hydrogen-bonding and weaker ring �-� interactions.Complementary end-to-end hydrogen bonding C5-H5� � �Br1 [3.12 (2) A ˚, Table 1] between the 3,5-dibromophenyl groups forms chains of aryl diester molecules that run parallel to [011] (Fig. 2).The planes between the 3,5-dibromophenyl rings on adjacent molecules in the chains are offset, giving a stair-step pattern of aryl diester molecular links in the chains (Fig. 2).Complementary C3-H3� � �Br1 interactions [3.02 (1) A ˚, Table 1] extend along [100] and result in shorter side-to-side hydrogen bonding that cross-links the end-to-end chains forming a tri-periodic network (Fig. 2).This arrangement places the Br2 and H7 atoms in positions that point towards the central benzene rings of adjacent molecules and that are free of C-H� � �Br hydrogen-bonding interactions (Fig. 2).The network of C-H� � �Br hydrogen bonds between Br1 and H3/H5 leaves a side-to-side packing of molecules along [100] with all rings on adjacent molecules oriented parallel (Fig. 3).Weak �-� interactions are evident between these parallel rings with centroid-to-centroid (Cg� � �Cg) distances of 3.8875 (1) A ˚, but with their centroids shifted by 1.726 A ˚(3,5dibromophenyl rings) and 1.905A ˚(central benzene rings).In the crystal structure of the related 1,4-phenylene dibenzoate, three C-H� � �� interactions and one displaced �-� interaction between the peripheral rings [Cg� � �Cg distance = 3.9590 (10) A ˚] were noted (Ganaie et al., 2016).The presence of Br and the attendant network of stronger C-H� � �Br hydrogen bonds in the title aryl diester structure precludes

Synthesis and crystallization
The synthesis of 2-methyl-1,4-phenylene bis(3,5-dibromobenzoate) was carried out using a modified Steglich esterification procedure and was previously published (Lauer et al., 2024;Jordan et al., 2021).M.p. = 502 K/DSC, T d = 563 K/ TGA).A crystalline sample was obtained by melting a sample of a white powder of 2-methyl-1,4-phenylene bis(3,5-dibromobenzoate) in a glass vial on a hot plate.The melt was allowed to cool to room temperature, forming a crystalline solid.Crystals suitable for single crystal X-ray diffraction were obtained by cutting into the solidified crystalline melt sample.

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 2.All hydrogen atoms, except H3, H5 and H10, were placed using a riding model with their positions constrained relative to their parent C atom using the appropriate HFIX command in SHELXL (Sheldrick, 2015b).Hydrogen atoms involved in C-H� � �Br hydrogen-bonding, H3 and H5, as well as H10 were placed from the electrondensity map, and their C-H distances restrained (DFIX, C-H range 0.94-0.95A ˚) at 0.95 A ˚with U iso (H) = 1.2U eq (C).Electron density corresponding to the disordered methyl group (C11) and H atom (H10) positions was obvious in the electron-density map.The occupancies of disordered atoms, H10 and C11, were set to 0.5, and H atoms attached to C11 (H11A, H11B, and H11C) were placed using a riding model (HFIX 137).(Sheldrick, 2015a), SHELXL (Sheldrick, 2015b), and OLEX2 (Dolomanov et al., 2009).T min = 0.543, T max = 0.998 9500 measured reflections 1892 independent reflections 1848 reflections with I > 2σ(I)

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.

Figure 1
Figure 1Molecular structure of 2-methyl-1,4-phenylene bis(3,5-dibromobenzoate), depicting the anti-position of the 3,5-dibromobenzoate end groups.The methyl group is shown in both positions, disordered across the central benzene ring in space group P1.Displacement ellipsoids are shown at the 50% probability level; non-labeled atoms are generated by symmetry operation 1 À x, 2 À y, À z.

Table 2
Experimental details.