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

The title crystal (systematic name: 2,3,5,6-tetra­bromo­benzene-1,4-dicarbonitrile), C₈Br₄N₂, is the first bromo analog in a study of cyano-halo (C≡N⋯X) non-bonded contacts in crystals of halogenated di­cyano­benzenes. The complete mol­ecule is generated by a crystallographic center of symmetry. In the extended structure, each Br atom accepts one C≡N⋯Br inter­action, and each N atom is bis­ected by two. This contact network forms a nearly planar sheet structure propagating in the (01) plane, similar to that reported in hexa­methyl­benzene co-crystals of the tetra­chloro analog.

1. Chemical context

The title crystal is part of a study of solid-state C≡N⋯X (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≡N⋯F contacts do not form (Bond et al., 2001), C≡N⋯Cl contacts form in isolation or as inversion dimers (Pink et al., 2000), and C≡N⋯Br and ⋯I contacts form networks (Noland et al., 2018). Contact strength tends to increase with the polarizability of the halogen atom (Desiraju & Harlow, 1989).

The crystal structures of neat (i.e. not co-crystals, no solvent included in the crystal) halogenated terephthalodi­nitriles have followed this trend. The crystal of 2,3,5,6-tetra­fluoro­terephthalodi­nitrile (F4TN) does not contain any C≡N⋯F contacts, with mol­ecules adopting a sawtooth formation (Fig. 1a; Hirshfeld, 1984), similar to a crystal of penta­fluoro­benzo­nitrile (Bond et al., 2001). The crystal of the tetra­chloro analog (Cl4TN) contains one C≡N⋯Cl contact per N atom, forming staggered C₂²(14) chains (Britton, 1981b; Fig. 1b). In co-crystals of Cl4TN with anthracene (Britton, 2005b), phenanthrene, or pyrene (Britton, 2005a), no C≡N⋯Cl contacts are found. However, Cl4TN and the corresponding ortho- and meta-di­cyano isomers each form co-crystals with hexa­methyl­benzene wherein C≡N⋯Cl-based sheets occur, in alternating layers with sheets of hexa­methyl­benzene (Britton, 2002). No crystals involving the title compound (Br4TN) have been reported previously.

Figure 1 Packing in the crystals of (a) F4TN, viewed along [50]; (b) Cl4TN, viewed along [10]. The dashed blue lines represent short contacts.

2. 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.

Figure 2 The mol­ecular structure of Br4TN, with atom labeling and displacement ellipsoids at the 50% probability level. Unlabeled atoms are generated by the (x, −y + 1, z), (−x + 1, y, −z + 2), and (−x + 1, −y + 1, −z + 2) symmetry operations.

3. Supra­molecular features

C≡N⋯Br contacts are the most prominent packing feature (Table 1). The length of these contacts is similar to 3.064 (4) Å, the mean C≡N⋯Br distance found in crystals of 2,4,6-tri­bromo­benzo­nitrile (Br3BN; Noland et al., 2018). The N and ortho-Br atoms of Br3BN form a contact network similar to a half-mol­ecule of Br4TN. Pairs of these contacts form centrosymmetric R₂²(10) rings (Fig. 3). Each mol­ecule of Br4TN participates in four such rings, generating a nearly planar sheet structure that is similar to Cl4TN layers in the Cl4TN-hexa­methyl­benzene co-crystal (Britton, 2002). In Br4TN, adjacent sheets stack roughly along [70], and the [001] translation relates mol­ecules in neighboring sheets.

Table 1
Contact geometry for Br4TN (Å, °).

C≡N⋯Br	C≡N	N⋯Br	C≡N⋯Br
C1≡N1⋯Br1^i	1.139 (5)	3.015 (2)	135.48 (5)
Symmetry code: (i) −x + , −y + , −z + 1.

Figure 3 The nearly planar sheet structure in a crystal of Br4TN, viewed along 01. The dashed blue lines represent short contacts.

4. Database survey

A search of the Cambridge Structural Database (CSD, Version 5.40, November 2018; Groom et al., 2016) found six additional reports similar to Br4TN. For F4TN, a co-crystal with 9-acetyl­anthracene (Wang et al., 2018), and an η²-complex with tungsten(II) (Kiplinger et al., 1997) are both given; these contain no C≡N⋯F contacts. Neat crystals are reported for the ortho- (Britton, 1981c) and meta-di­cyano (Hu et al., 2004) isomers of Cl4TN, and 2,4,6-tri­chloro­tri­cyano­benzene (Britton, 1981a).

5. Synthesis and crystallization

2,3,5,6-Tetra­bromo­terephthaldi­amide (Br4TA), adapted from the work of Schäfer 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 Na₂CO₃ 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%). A trace of ammonium chloride could not be removed, based on the MS results. M.p. 627 K (lit. 615 K; Knobloch & Ramirez, 1975); ¹H NMR (500 MHz, DMSO-d₆; 2 conformers obs.) δ 8.085 (s, 2H, both), 7.936 (s, 2H, minor), 7.889 (s, 2H, major); ¹³C NMR (126 MHz, DMSO-d₆) δ 166.8 (2C), 143.4 (2C), 122.2 (4C); IR (KBr, cm⁻¹) 3292, 3158, 2966, 2907, 2853, 1679, 1427, 1315, 1287, 1252, 1114, 1089, 866; MS (ESI, m/z) [M+³⁵Cl]⁻ calculated for C₈H₄⁷⁹Br₂⁸¹Br₂N₂O₂ 514.6660, found 514.6672.

2,3,5,6-Tetra­bromo­terephthalodi­nitrile (Br4TN), adapted from the work of Schäfer et al. (2017) (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; ¹³C NMR (126 MHz, DMSO-d₆) δ 129.6 (4C, C3), 123.5 (2C, C2), 116.0 (2C, C1); IR (KBr, cm⁻¹) 2236, 1364, 1330, 1293, 1229, 1156, 1121, 732; MS (EI, m/z) [M]⁺ calculated for C₈⁷⁹Br₂⁸¹Br₂N₂ 443.6749, found 443.6764.

Figure 4 The synthesis of Br4TN via amination of 2,3,5,6-tetra­bromo­terephthalic acid, followed by dehydration.

Crystallization: A solution of Br4TN (150 mg) in bis­(2-meth­oxy­eth­yl) ether (10 mL) at 425 K was cooled by 30 K h⁻¹ until a precipitate began to form. The temperature was held for 1 h, and then cooled by 10 K h⁻¹ 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.

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.

6. Refinement

Crystal data, data collection and structure refinement details are summarized in Table 2

Table 2
Experimental details

Crystal data
Chemical formula	C8Br4N2
Mr	443.74
Crystal system, space group	Monoclinic, C2/m
Temperature (K)	100
a, b, c (Å)	7.8500 (6), 9.8330 (8), 6.7540 (6)
β (°)	90.202 (4)
V (Å^3)	521.33 (7)
Z	2
Radiation type	Mo Kα
μ (mm^−1)	15.40
Crystal size (mm)	0.15 × 0.06 × 0.03
Data collection
Diffractometer	Bruker VENTURE PHOTON-II area detector
Absorption correction	Multi-scan (SADABS; Sheldrick, 1996)
Tmin, Tmax	0.253, 0.494
No. of measured, independent and observed [I > 2σ(I)] reflections	3324, 837, 741
Rint	0.063
(sin θ/λ)max (Å^−1)	0.715
Refinement
R[F^2 > 2σ(F^2)], wR(F^2), S	0.029, 0.072, 1.04
No. of reflections	837
No. of parameters	38
Δρmax, Δρmin (e Å^−3)	1.09, −1.01
Computer programs: APEX2 and SAINT (Bruker, 2012), SHELXT2014/5 (Sheldrick, 2015a), SHELXL2018/3 (Sheldrick, 2015b), Mercury (Macrae et al., 2008) and publCIF (Westrip, 2010).

Supporting information

Computing details

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).

2,3,5,6-Tetrabromobenzene-1,4-dinitrile

Crystal data

C8Br4N2	Dx = 2.827 Mg m^−3
Mr = 443.74	Melting point: 603 K
Monoclinic, C2/m	Mo Kα radiation, λ = 0.71073 Å
a = 7.8500 (6) Å	Cell parameters from 2993 reflections
b = 9.8330 (8) Å	θ = 3.0–30.4°
c = 6.7540 (6) Å	µ = 15.40 mm^−1
β = 90.202 (4)°	T = 100 K
V = 521.33 (7) Å^3	Prism, colourless
Z = 2	0.15 × 0.06 × 0.03 mm
F(000) = 404

Data collection

Bruker VENTURE PHOTON-II area detector diffractometer	741 reflections with I > 2σ(I)
Radiation source: micro-focus	Rint = 0.063
φ and ω scans	θmax = 30.6°, θmin = 3.0°
Absorption correction: multi-scan (SADABS; Sheldrick, 1996)	h = −11→9
Tmin = 0.253, Tmax = 0.494	k = −13→14
3324 measured reflections	l = −9→9
837 independent reflections

Refinement

Refinement on F^2	Primary atom site location: dual
Least-squares matrix: full	w = 1/[σ^2(Fo^2) + (0.0175P)^2 + 0.7909P] where P = (Fo^2 + 2Fc^2)/3
R[F^2 > 2σ(F^2)] = 0.029	(Δ/σ)max < 0.001
wR(F^2) = 0.072	Δρmax = 1.09 e Å^−3
S = 1.04	Δρmin = −1.01 e Å^−3
837 reflections	Extinction correction: SHELXL2018/3 (Sheldrick 2015b), Fc^*=kFc[1+0.001xFc^2λ^3/sin(2θ)]^-1/4
38 parameters	Extinction coefficient: 0.0029 (10)
0 restraints

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 (Ų)

	x	y	z	Uiso*/Ueq
Br1	0.40138 (3)	0.78605 (3)	0.77786 (4)	0.01672 (15)
N1	0.2553 (5)	0.500000	0.4857 (5)	0.0219 (7)
C1	0.3260 (5)	0.500000	0.6333 (6)	0.0158 (7)
C2	0.4158 (5)	0.500000	0.8209 (6)	0.0148 (7)
C3	0.4576 (3)	0.6237 (3)	0.9090 (4)	0.0142 (5)

Atomic displacement parameters (Ų)

	U^11	U^22	U^33	U^12	U^13	U^23
Br1	0.0186 (2)	0.0174 (2)	0.0142 (2)	0.00120 (9)	0.00004 (12)	0.00249 (9)
N1	0.0253 (19)	0.0198 (18)	0.0204 (19)	0.000	−0.0045 (15)	0.000
C1	0.0190 (19)	0.0140 (19)	0.0144 (18)	0.000	0.0032 (14)	0.000
C2	0.0125 (17)	0.021 (2)	0.0113 (16)	0.000	0.0035 (13)	0.000
C3	0.0138 (12)	0.0151 (14)	0.0136 (13)	0.0007 (9)	0.0026 (10)	0.0018 (10)

Geometric parameters (Å, º)

Br1—C3	1.878 (3)	C2—C3	1.393 (3)
N1—C1	1.139 (5)	C2—C3^i	1.393 (3)
C1—C2	1.448 (5)	C3—C3^ii	1.395 (5)
N1—C1—C2	180.0 (4)	C2—C3—C3^ii	119.17 (17)
C3—C2—C3^i	121.7 (3)	C2—C3—Br1	119.1 (2)
C3—C2—C1	119.17 (17)	C3^ii—C3—Br1	121.75 (8)
C3^i—C2—C1	119.17 (17)
C3^i—C2—C3—C3^ii	−0.5 (6)	C3^i—C2—C3—Br1	178.56 (19)
C1—C2—C3—C3^ii	179.2 (3)	C1—C2—C3—Br1	−1.7 (4)

Symmetry codes: (i) x, −y+1, z; (ii) −x+1, y, −z+2.