Crystal structure and Hirshfeld surface analysis of 2,6-diiodo-4-nitrotoluene and 2,4,6-tribromotoluene

The crystal structures of DIN and TBT were determined by X-ray diffraction at 150 K. In the crystal of DINT, molecules are linked via short N—O⋯I contacts, forming chains along [100]. In TBT, molecules are linked by C—H⋯Br hydrogen bonds, forming chains along [010].

The title compounds, 2,6-diiodo-4-nitrotoluene (DINT, C 7 H 5 I 2 NO 2 ) and 2,4,6tribromotoluene (TBT, C 7 H 5 Br 3 ,), are trisubstituted toluene molecules. Both molecules are planar, only the H atoms of the methyl group, and the nitro group in DINT, deviate significantly from the plane of the benzene ring. In the crystals of both compounds, molecules stack in columns up the shortest crystallographic axis, viz. the a axis in DINT and the b axis in TBT. In the crystal of DINT, molecules are linked via short N-OÁ Á ÁI contacts, forming chains along [100]. In TBT, molecules are linked by C-HÁ Á ÁBr hydrogen bonds, forming chains along [010]. Hirshfeld surface analysis was used to explore the intermolecular contacts in the crystals of both DINT and TBT.

Chemical context
In order to understand the methyl radical behaviour of benzene molecules substituted by halogens and methyl groups, we have studied a number of halogenomesitylenes, such as triiodomesitylene (TIM; Boudjada et al., 2001), trichloromesitylene (TCM; Tazi et al., 1995), tribromomesitylene (TBM; Boudjada et al., 1999) and dibromomesitylene (DBM; Hernandez et al., 2003). In the solid state of these halogenomethyl-benzene (HMB) compounds, the steric hindrance between the methyl group and the halogen atoms results in small out-of -plane deformations of the heavy atoms. In spite of the small amplitudes of the deformation, the impact on the rotational potential of the methyl group is considerable because of the contribution of the neighbouring halogen atoms on the methyl groups as observed in these planar structures. This study has now been extended to understand and identify the methyl-group behaviour of halogeno-toluene molecules, and we report herein on the crystal and molecular structures of the title compounds, 2,6-diiodo-4-nitrotoluene (DINT; systematic name: 1,3-diiodo-2-methyl-5-nitrobenzene) and 2,4,6-tribromotoluene (TBT; systematic name: 1,3,5-tribromo-2-methyl-benzene). Hirshfeld surface analysis was used to explore the intermolecular contacts in the crystals of both compounds.

Structural commentary
The molecular structure of DINT is illustrated in Fig. 1, and that of TBT in Fig. 2. The structures of the title compounds are ISSN 2056-9890 compared with those of the dichloronitrotoluene (DCNT; Medjroubi et al., 2017) and dibromonitrotoluene (DBNT; Medjroubi et al., 2016) analogues, illustrated in Fig. 3.
In DBNT, the methyl group exhibits rotational disorder about the C ar -C me axis. As in DCNT, the methyl group of DINT does not present any disorder. Hence, no significant steric hindrance of the methyl group by the ortho halogen atoms is observed. The longest bond lengths C ar -C ar are adjacent to the C ar -C me bond with an average value of 1.405 (4) Å . The bond-length values are close to those found in the literature. Moreover, in the crystal structure of DINT, the methyl group has a C-H bond perpendicular to the mean plane with a torsion angle C2-C1-C7-H7A = 90.0 , which has already previously been reported in the literature. The intermolecular N1-OÁ Á ÁI [3.115 (3) Å ] interactions are competing to ensure cohesion in the crystal, see Fig. 4. Atom I2 bonded to the atom C6, with the C1-C6-I2 angle of 121.7 (3) being greater than the angle C5-C6-I2 [116.1 (2) ] located on the other side of the C6-I2 bond (Fig. 1). The aromatic ring is planar in DINT. The methyl C atom, the I atoms and the N atom of the nitro substituent, lie extremely close to the plane of the benzene ring; the deviations are 0.001 (1) Å for the methyl C atom, 0.097 (3) and À0.097 (3) Å for the two I atoms, À0.011 (2) Å for the nitro N atom, whereas for the oxygen atoms, which are located on either side of the mean plane, the deviations are 0.293 (3) and À0.283 (3) Å . In the crystal of DCNT, molecules are linked by weak C-HÁ Á ÁO and C-HÁ Á ÁCl hydrogen bonds, forming layers parallel to the ab plane Fig. 5 (Medjroubi et al., 2017).
The molecular structure of TBT is illustrated in Fig. 2. The structural study did not reveal any disorder and the intramolecular interaction ensuring the cohesion in the crystal is C-BrÁ Á Á H7B (2.61 Å ) as seen in Fig. 6  The molecular structure of DINT with the atom labelling and displacement ellipsoids drawn at the 50% probability level.

Figure 2
The molecular structure of TBT with the atom labelling and displacement ellipsoids drawn at the 50% probability level.

Figure 3
The molecular structures of DCNT and DBNT, with displacement ellipsoids drawn at the 50% probability level.

Figure 4
A view along the a and b axes of the crystal packing of DINT. conformation produces a significant steric effect between the hydrogen atoms of the methyl group and atom Br2 bonded to atom C6 with an angle C1-C6-Br3 = 119.8 (2) , which is clearly greater than the angle C5-C6-Br3 = 116.5 (2) located on the other side of the C6-Br1 bond. This is the same case for the exocyclic angles C6-C1-C7 = 121.9 (3) and C2-C1-C7 = 123.6 (3) located on either side of the C1-C7 bond. As found in DINT and DCNT, the longest C ar -C ar bond lengths are adjacent to the C ar -C me bond (Fig. 2). The methyl C atom C7 is displaced from the benzene ring by À0.005 (1) Å , while the Br atoms lie almost in the plane of the benzene ring [deviations are 0.003 (3) Å for atoms Br1 and Br3 and À0.012 (3) Å for atom Br2]. The CH 3 group presents an eclipsed C-H bond with the mean plane of the molecule. A difference of 2 is found between the exocyclic angles C ar -C ar -C me , which explains the importance of the interaction of this eclipsed bond with atom Br3. The endocyclic angle in front of the methyl group is equal to 116.6 (3) for DINT, 115.7 (2) for DCNT (Medjroubi et al., 2017), 114.7 (3) for DBNT (Medjroubi et al., 2016) and 114.5 (3) for TBT. The variation of this angle is very sensitive to the substitution of the halogen atoms surrounding the methyl groups of these different compounds.

Analysis of the Hirshfeld surfaces of DINT and TBT
Additional insight into the intermolecular interactions was obtained from analysis of the Hirshfeld surface (HS) (Spackman & Jayatilaka, 2009) and the two-dimensional fingerprint plots (McKinnon et al., 2007). The program Crys-talExplorer (Turner et al., 2017) was used to generate Hirshfeld surfaces mapped over d norm and the electrostatic potential for compounds TBT and DINT. 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 electrostatic potentials were research communications Acta Cryst. (2020). E76, 1391-1397 Medjroubi et al. C 7 H 5 I 2 NO 2 and C 7 H 5 Br 3 1393 Table 1 Hydrogen bonds (Å , ) for (I) and (II).

Figure 6
A view along the b axes of the crystal packing of 2,4,6-tribromotoluene, DBNT.

Figure 5
A view along the a axes of the crystal packing of 2,6-dichloro-4nitrotoluene, DCNT.  (Medjroubi et al., 2017) and TBT are given in Table 2. The Hirshfeld surface (HS) mapped over the electrostatic potential for DINT in the range [À0.071 to +0.041], is shown in Fig. 7a where the red and blue regions represent negative and positive electrostatic potentials respectively. The Hirshfeld surface mapped over d norm is depicted in Fig. 7b. The HS mapped over shape-index and curvedness are shown in Fig. 7c and 7d, respectively. The crystal environment about a DINT molecule is illustrated in Fig. 8: the interactions are shown on the Hirshfeld surfaces with short contacts indicated in red. The twodimensional fingerprint plots for all contacts are illustrated in Fig. 9a. The IÁ Á ÁO/OÁ Á ÁI interaction ensures the cohesion of the crystal with a contribution of 16% of all the interactions appearing in bright-red spots on the Hirshfeld surfaces mapped over d norm . The fingerprint consists of two spikes with the ends at d e + d i ' 3.1 Å , Fig. 9c. The fingerprint of the CÁ Á ÁH/HÁ Á ÁC interaction consists of two symmetrical peaks with a d e + d i ' 2.8 Å , Fig. 9f and almost equal to the sum of van der Waals radii. It is represented on the HS mapped with the shape-index property in the form of a pale-red spot, Fig. 7c. The IÁ Á ÁI interaction is less important than OÁ Á ÁI/IÁ Á ÁO; however, it does contribute 4.8% to the total interactions, illustrating the equality of the IÁ Á ÁI distance to the sum of van der Waals radii. The fingerprint is composed of a single peak in the form of a needle with d e + d i ' 3.8 Å , Fig. 9h. The absence ofstacking interactions is consistent with the low contributions of CÁ Á ÁC contacts to the Hirshfeld surface ( Table 2). The Hirshfeld surface analysis (Fig. 10b)  Hirshfeld surface for DINT mapped over (a) calculated electrostatic potential, (b) d norm , (c) shape-index and (d) curvedness.

Figure 8
A view of the Hirshfeld surface of DINT mapped over d norm , with interactions shown as dashed lines.

Figure 10
Hirshfeld surface for DCNT mapped over (a) calculated electrostatic potential, (b) d norm , (c) shape-index and (d) curvedness.  trostatic potential surface (Fig. 10a) show the intermolecular interactions between different units in the crystalline environment of DCNT. The HS mapped over shape-index and curvedness are shown in Fig. 10c and d, respectively. The twodimensional fingerprint plots for all contacts are illustrated in Fig. 11. The contributions of the major intermolecular contacts in the title compound are ClÁ Á ÁH/HÁ Á ÁCl (26.8%), OÁ Á ÁH/ HÁ Á ÁO (26.1%), and HÁ Á ÁH (10.6%) (Fig. 11a). Other contacts (e.g. CÁ Á ÁC, HÁ Á ÁC/CÁ Á ÁH, ClÁ Á ÁCl, ClÁ Á ÁC/CÁ Á ÁCl, ClÁ Á ÁO/ OÁ Á ÁCl) make contributions of less than 7.5% to the HS. The graph shown in Fig. 11b represents the one-third of all the intermolecular contacts. All fingerprint points are located at distances with d i equal to or greater than van der Waals distances, d e + d i ' 1.27 Å , reflecting a zero tendency to form this intermolecular contact (ClÁ Á ÁH). The graph shown in Fig. 11c (OÁ Á ÁH/HÁ Á ÁO) shows the contact between the oxygen atoms inside the surface and the hydrogen atoms outside the surface, d e + d i ' 2.35 Å , and has two symmetrical points at the top, bottom left and right. The graph shown in Fig. 11d (HÁ Á ÁH; 10.6%) shows the two-dimensional fingerprint of the (d i , d e ) points associated with hydrogen atoms, which has two symmetrical wings on the left and right with d e + d i ' 2.3 Å .
The electrostatic potentials were mapped on the Hirshfeld surface using the STO-3G basis/Hartree-Fock level of theory over the range [À0.016, 0.036] for TBT (Fig. 12). Atoms H7A, H7B and H7C of the methyl group and H3, H5 as donors, and the bromine atom acceptors, are also evident in Fig. 12a. The Hirshfeld surface mapped over d norm is depicted in Fig. 12b. The overall 2D fingerprint plot is presented in Fig. 13a. The halogen-halogen (BrÁ Á ÁBr) interaction contributes 17.4% to the HS and ensures the cohesion of the crystal and dictates the intermolecular stacking. The short intramolecular HÁ Á ÁBr/ BrÁ Á ÁH contact between C3-H7B and Br1 (H7BÁ Á ÁBr1 = 2.6 Å ) can be recognized from the two neighbouring blue      regions on the surface mapped with electrostatic potential in Fig. 14c,d. The two-dimensional fingerprint plot delineated into BrÁ Á ÁH/HÁ Á ÁBr has two peaks pointing to the pairs d e + d i ' 3.0 Å , slightly lower than or equal to the sum of van der Waals radii, Fig. 14b. These correspond to a 42.7% contribution to the Hirshfeld surface, and reflect the presence of intermolecular C7-H7CÁ Á ÁBr3 interactions. The interatomic HÁ Á ÁH contacts at distances greater than their van der Waals separation appear as scattered points in the greater part of the fingerprint plot Fig. 14c. The presence of C-HÁ Á Á and H-CÁ Á Á [or p-p ?]stacking interactions between the TBTrings is also apparent from the appearance of red and blue triangle pairs on the Hirshfeld surface mapped with shape-index property identified with arrows in Fig. 12c. The immediate environment about each molecule highlighting close contacts to the Hirshfeld surface by neighboring molecules is shown in Fig. 13. The relative contributions to the overall surface are given in Table 2.  (Medjroubi et al., 2016), there are two independent molecules per asymmetric unit and the methyl group H atoms are positionally disordered, as found for DBM (Hernandez et al., 2003). While in the compounds DINT and DCNT (Medjroubi et al., 2017), there is only one molecule in the asymmetric unit and no disorder is observed for the methyl group H atoms. In the molecule of DINT, a C m -H (m = methyl) bond is perpendicular to the mean plane of the molecule, as found for triiodomesitylene (TIM; Boudjada et al., 2001). The nitro group is inclined to the benzene ring by 16.72 (1) in DINT, compared with 9.8 (3) in DCNT, and 2.5 (5) and 5.9 (4) in DBNT. In the molecule of TBT, the CH 3 group presents an eclipsed C-H bond with the mean plane of the molecule. This also applies to DCNT (Medjroubi et al., 2017), which does not present any disorder, as was also found in the case of tribromomesitylene TBM (Boudjada et al., 1999). In 2,6-dihalogeno-4-nitrotoluene, as in the title compounds, the cohesion of the crystal is ensured by interactions of the type C-HÁ Á Áhalogen and C-halogenÁ Á Áhalogen.