Crystal structures and Hirshfeld analysis of 4,6-dibromoindolenine and its quaternized salt

In the crystal, molecules of 4,6-dibromoindolenine are linked by C—Br⋯π halogen bonds, forming zigzag chains propagating in the [001] direction. The molecules of the salt form layers parallel to the (010) plane where they are linked by C—H⋯Br hydrogen bonds C—Br⋯Br and C—Br⋯I halogen bonds.

4,6-Dibromo-2,3,3-trimethyl-3H-indole, C 11 H 11 Br 2 N, exists as a neutral molecule in the asymmetric unit. The asymmetric unit of 4,6-dibromo-2,3,3trimethyl-3H-indol-1-ium iodide, C 12 H 14 Br 2 N + ÁI À , contains one organic cation and one iodine anion. The positive charge is localized on the quaternized nitrogen atom. In the crystal, molecules of 4,6-dibromoindolenine are linked by C-BrÁ Á Á halogen bonds, forming zigzag chains propagating in the [001] direction. The molecules of the salt form layers parallel to the (010) plane where they are linked by C-HÁ Á ÁBr hydrogen bonds, C-BrÁ Á ÁBr and C-BrÁ Á ÁI halogen bonds. The Hirshfeld surface analysis and two dimensional fingerprint plots were used to analyse the intermolecular contacts present in both crystals.
In this work, we carried out an X-ray diffraction and Hirshfeld surface analysis of 4,6-dibromoindolenine (1) and its quaternized salt (2), crystals of which were obtained by sequential synthesis starting from 3,5-dibromoaniline (3) by its diazotization with nitrosylsulfuric acid in sulfuric acid ISSN 2056-9890 followed by reduction of the diazonium salt 4 with tin(II) chloride. The resulting 3,5-dibromophenylhydrazine was refluxed with 3-methyl-2-butanone in acetic acid to give 4,6dibromoindolenine, 1, which after N-alkylation with the excess of iodomethane in benzene solution forms crystals of the quaternized indolium salt 2 (Fig. 1).

Structural commentary
In the crystal, 4,6-dibromo-2,3,3-trimethyl-3H-indole, 1, exists as one neutral molecule in the asymmetric unit (Fig. 2). The quaternized molecule 2 exists as a salt with an iodine anion in the crystal phase (Fig. 2). All atoms of the quaternized cation, with exception of the C9 atom and the hydrogen atoms of the C10H 3 and C11H 3 methyl groups are located in a special position relative to the symmetry plane. In compound 2, the positive charge is localized on the nitrogen atom, which is caused by its quaternization. The N1-C11 bond is shortened to 1.460 (10) Å in comparison with the mean value of 1.485 Å for an N-Csp 3 bond (Burgi & Dunitz, 1994). An analysis of the bond lengths in both structures showed that they are typical of those in similar compounds (Seiler et al., 2018;Connell et al., 2014;Holliman et al., 2009;Bellê tete et al., 1993).

Supramolecular features
In the crystal, molecules of 1 form zigzag chains in the [001] direction as a result of the formation of intermolecular C3-Br1Á Á ÁN1() and C3-Br1Á Á ÁC8() halogen bonds (Table 1, Fig. 3). Neighbouring chains are linked by weak C11-H11CÁ Á ÁN1 hydrogen bonds (Table 1). It should be noted that only one of the bromine atoms participates in these interactions. The presence of the iodide anion in compound 2 leads to the complete involvement of both bromine atoms in the formation of intermolecular interactions. As a result, molecules of 2 form chains in the [100] direction as a result of the C2-H2Á Á ÁBr2 hydrogen bond and C5-Br2Á Á ÁBr1 halogen bond (Table 2, Fig. 4). Neighbouring chains are connected through the bridged iodide anion by the strong C3-Br1Á Á ÁI1 halogen bond and C11-HÁ Á ÁI1 hydrogen bond. Layers parallel to the (010) plane can be recognized as a structural motif in the structure of 2 (Table 2).  Table 1 Hydrogen-bond geometry (Å , ) for 1. Symmetry codes: (i) Àx þ 1 2 ; Ày þ 1; z þ 1 2 ; (ii) x þ 1 2 ; Ày þ 3 2 ; Àz þ 1.

Figure 1
Synthesis of the title compounds 1 and 2.

Figure 2
Molecular structure of compounds 1 and 2 with the atom labelling. Displacement ellipsoids are drawn at the 50% probability level.

Figure 4
The chains (left) and layers (right) in the crystal of compound 2.

Figure 3
Zigzag chains in the crystal of compound 1.
over d norm (Figs. 5-7) were generated. The molecular Hirshfeld surfaces were obtained using a standard (high) surface resolution with the three-dimensional d norm surfaces mapped over a fixed colour scale of À0.1256 (red) to 1.401 (blue). The areas in red on the d norm -mapped Hirshfeld surfaces (Fig. 5) correspond to contacts that are shorter than van der Waals radii sum of the closest atoms. As can be seen in Fig. 5, short contacts in 1 are present at the nitrogen and Br1 atoms. In 2, the areas of short contacts are located at both the bromine atoms, the iodine atom and the hydrogen atoms of the methyl groups (

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 3. H atoms were included in calculated positions and treated as riding on their parent C atom: C-H = 0.93-0.98 Å with U iso (H) = 1.5U eq (C-methyl) or 1.2U eq (C) for all other H atoms.  SHELXT2014/5 (Sheldrick, 2015b); program(s) used to refine structure: SHELXL2016/6 (Sheldrick, 2015a); molecular graphics: OLEX2 (Dolomanov et al., 2009); software used to prepare material for publication: OLEX2 (Dolomanov et al., 2009).  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.

Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Å 2 )
x y z U iso */U eq Br1 0.24163 (11) 0.35819 (9) 0.83683 (7) 0.0630 (4)  where P = (F o 2 + 2F c 2 )/3 (Δ/σ) max = 0.001 Δρ max = 0.86 e Å −3 Δρ min = −0.91 e Å −3 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.

Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Å 2 )
x y z U iso */U eq Occ. (