Crystal structure of 2,3,5,6-tetrabromoterephthalonitrile

The title compound is the first bromo analog in a study of cyano–halo (C≡N⋯X) non-bonded contacts in crystals of halogenated dicyanobenzenes. Each Br atom accepts one C≡N⋯Br non-bonded contact, and each N atom is bisected by two, forming a nearly planar sheet structure.


Chemical context
The title crystal is part of a study of solid-state C NÁ Á ÁX (X = F, Cl, Br, I) non-bonded contacts in substituted benzonitriles. The question is whether these contacts will form for a given nitrile, and whether they are isolated or extended to create ribbon or sheet structures in their crystals. The prevailing trend is that C NÁ Á ÁF contacts do not form (Bond et al., 2001), C NÁ Á ÁCl contacts form in isolation or as inversion dimers (Pink et al., 2000), and C NÁ Á ÁBr and Á Á ÁI contacts form networks (Noland et al., 2018). Contact strength tends to increase with the polarizability of the halogen atom (Desiraju & Harlow, 1989).

Structural commentary
In the crystal of Br4TN, the molecules lie about an inversion center and a vertical mirror plane, and are almost planar (Fig. 2). The ring C2/C3 atoms have r.m.s. deviations of 0.002 (2) Å from the plane of best fit. The Br1 and N1 atoms deviate from this plane by 0.038 (4) and 0.026 (9) Å , respectively. This distortion is chair-like, with adjacent ring positions bent to opposite sides of the best-fit plane.

Synthesis and crystallization
2,3,5,6-Tetrabromoterephthaldiamide (Br4TA), adapted from the work of Schä fer et al. (2017): Tetrabromoterephthalic acid (4.01 g; Sigma-Aldrich, Inc., No. 524441) and thionyl chloride (24 mL) were combined in a round-bottomed flask. The resulting mixture was refluxed for 3 h, and then cooled to ambient temperature. The thionyl chloride was removed under reduced pressure. The resulting white solid was dissolved in 1,4-dioxane (60 mL). An ammonium hydroxide solution (15 M, 50 mL) was added and then the mixture was stirred for 18 h. Water (50 mL) and an Na 2 CO 3 solution (2 M, 50 mL) were added, and then the mixture was stirred for 24 h. A precipitate was collected by suction filtration, and then washed with water, giving a white powder (5.71 g, 71%  (Fig. 4): A portion of Br4TA (515 mg) and phosphorus oxychloride (16 mL) were combined in a round-bottomed flask. The resulting mixture was refluxed for 24 h, then cooled to ambient temperature, and then poured into ice-water (200 mL). This mixture was stirred until the ice melted, then a precipitate was collected by suction filtration, and then washed with water, giving a white powder (342 mg, 72%). M.p. 603 K; 13 C NMR (126 MHz, DMSO-d 6 ) 129.6 (4C, C3), 123.5 (2C, C2), 116.0 (2C, C1); IR (KBr, cm À1 ) 2236, 1364,1330,1293,1229,1156,1121,732 Crystallization: A solution of Br4TN (150 mg) in bis(2methoxyethyl) ether (10 mL) at 425 K was cooled by 30 K h À1 until a precipitate began to form. The temperature was held for 1 h, and then cooled by 10 K h À1 to ambient temperature. After 24 h, colorless, highly twinned, prismatic crystals were collected by decantation and then washed with methanol. A monocrystalline tip similar to the one indicated in Fig. 5 was harvested for X-ray diffraction.

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 2 Figure 5 A confocal micrograph showing two colorless crystals of Br4TN. The apparent yellow colour is caused by the lighting. The blurry portions are out of the focal plane toward the viewer. A prismatic tip similar to the one indicated by the red arrow was used for X-ray diffraction.

Figure 4
The synthesis of Br4TN via amination of 2,3,5,6-tetrabromoterephthalic acid, followed by dehydration.  Data collection: APEX2 (Bruker, 2012); cell refinement: SAINT (Bruker, 2012); data reduction: SAINT (Bruker, 2012); program(s) used to solve structure: SHELXT2014/5 (Sheldrick, 2015a); program(s) used to refine structure: SHELXL2018/3 (Sheldrick, 2015b); molecular graphics: Mercury (Macrae et al., 2008); software used to prepare material for publication: publCIF (Westrip, 2010). 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.