A 2:1 co-crystal of 3,5-dibromo-4-cyanobenzoic acid and anthracene

The title cyano acid forms a honeycomb-like sheet structure, six molecules circumscribing anthracene, with carboxy hydrogen-bonded dimers linked by half of the possible (10) CN⋯Br rings.

The title co-crystal, C 8 H 3 Br 2 NO 2 Á0.5C 14 H 10 , was self-assembled from a 2:1 mixture of the components in slowly evaporating dichloromethane. The molecules adopt a sheet structure parallel to (112) in which carboxy hydrogen-bonded dimers and anthracene molecules stagger in both dimensions. Within the sheets, six individual cyano acid molecules surround each anthracene molecule. Cyano acid molecules form one of the two possible R 2 2 (10) rings between neighboring cyano and bromo groups. Compared to the dichloro analog [Britton (2012). J. Chem. Crystallogr. 42,[851][852][853][854][855], the dihedral angle between the best-fit planes of acid and anthracene molecules has decreased from 7.1 to 0.9 (2) . Doyle Britton (1930-2015 published roughly 30 crystallographic articles on solid-phase cyano-halo interactions from variously substituted halobenzonitriles and isocyanides. (a) The honeycomb-like structure envisioned by Doyle Britton. (b) A 2:1 co-crystal of 3,5-dichloro-4-cyanobenzoic acid with anthracene, viewed along 281 (Britton, 2012). Magenta dashed lines represent short contacts. Britton postulated that 3,5-dichloro-4-cyanobenzoic acid might assemble into a honeycomb-like sheet structure (Fig. 1a) via a combination of carboxy hydrogen-bond dimerization and CNÁ Á ÁCl short contacts. In 2012, he found that the cyano acid molecules alone do not pack in this way, but slow evaporation of mixtures containing naphthalene or anthracene afforded 2:1 acid:hydrocarbon co-crystals roughly matching his proposed structure (Britton, 2012). However, no CNÁ Á ÁCl contacts were observed (Fig. 1b). Anthracene was the better fit, although it was too large to allow the ideal molecular arrangement. There is no obvious substitute for anthracene or naphthalene. Thus, we have prepared anthracene co-crystals with the dibromo analog in hopes that the larger Br bond and contact radii might close the CNÁ Á ÁX gaps observed with Cl.

Structural commentary
The benzene (C2-C5/C7/C8) and anthracene (C9-C15 and symmetry equivalents, Fig. 2) ring systems are nearly planar. The mean deviation of atoms from the planes of best fit are 0.0074 (17) Å and 0.0041 (14) Å , respectively, both of which are comparable to the corresponding values in the dichloro crystal. However, the dihedral angle between the carboxy group (O1-C1-O2) and the benzene ring is 3.2 (4) , compared with 7.2 in the dichloro analog.

Supramolecular features
The dihedral angle between the benzene and anthracene planes is 0.9 (2) , which is much lower than 7.1 of the dichloro analog. As expected, R 2 2 (8) carboxy hydrogen-bonded dimers are observed (Table 1); these are located on an inversion center. R 2 2 (10) rings form about another inversion center based on C6 N1Á Á ÁBr2 contacts (Table 2); however, the corresponding N1Á Á ÁBr1 contacts are not observed (Fig. 3). Instead, 3.5534 (5) Å Br1Á Á ÁBr1 contacts form, slightly closer than the 3.70 Å non-bonded contact diameter of Br (Rowland & Taylor, 1996). In the title co-crystal, two corners of the anthracene molecule contact the cyano acid network (Fig. 3), whereas all four corners made contact in the Cl analog (Fig. 1b). Overall, substitution of Cl atoms with Br atoms has facilitated the formation of half of the envisioned CNÁ Á ÁX short contacts and also improved the coplanarity of the acid and hydrocarbon molecules, but anthracene is slightly too large to allow the ideal arrangement of cyano acid molecules. It is possible that upon substitution of Br atoms with I atoms, the improvements would continue and the envisioned sheet structure might occur. This possibility is currently being studied in our laboratory.

Figure 2
The molecular structures of the components of the title co-crystal, with atom labeling and displacement ellipsoids at the 50% probability level.

Figure 3
The sheet structure observed in the title co-crystal, viewed along [011].

Synthesis and crystallization
Methyl 4-amino-3,5-dibromobenzoate (V): Bromine (3.9 mL) and then pyridine (5.7 mL) were added dropwise to ice-cold methanol (35 mL). This mixture was added dropwise to a solution of methyl 4-aminobenzoate [(IV), commercially available, Fig. 5] in methanol (50 mL). The resulting mixture was refluxed for 4 h and then cooled to room temperature. The methanol was removed on a rotary evaporator. Dichloromethane (50 mL) and water (50 mL) were added. Aliquots (5 mL) of Na 2 CO 3 solution (aq., sat.) were added until the aqueous phase remained slightly alkaline after 10 min. The organic phase was separated and then washed with Na 2 S 2 O 3 solution (aq., sat., 25 mL), water (25 mL), brine (25 mL), and was then concentrated on a rotary evaporator. The resulting brown residue was recrystallized from ethyl acetate, giving colorless needles ( Methyl 3,5-dibromo-4-cyanobenzoate (VI), adapted from the work of Toya et al. (1992): Cyanide suspension: NaCN (680 mg) and CuCN (480 mg) and water (40 mL) were combined in a 400 mL beaker. After the solids dissolved, NaHCO 3 (6.5 g) was added. The resulting suspension was cooled in an ice bath. Diazotization: Dibromo ester [(V), 720 mg] was ground in a mortar and then combined with acetic acid (2.6 mL) in a round-bottomed flask. H 2 SO 4 (0.6 mL) was added dropwise over 1 min, followed by a solution of NaNO 2 (313 mg) in water (1.5 mL) over 30 min.. During the course of the additions, the reaction mixture was gradually warmed in an oil bath to 315 K. Cyanation: When no more starting material remained, as indicated by TLC, the diazotization mixture was removed from the heat and then added dropwise to the cyanide suspension. The ice bath was removed. The cyanation mixture was stirred overnight and then extracted with dichloromethane (3 Â 20 mL). The combined organic portions were washed with water (20 mL), brine (20 mL), dried with Na 2 SO 4 , filtered, and then concentrated on a rotary evaporator. The resulting brown residue was separated by column chromatography. The desired fraction (R f = 0.34 in 3:1 hexane:ethyl acetate on SiO 2 ) was concentrated on a rotary evaporator, giving a tan powder (681 mg, 92%  (128 mg), and pyridine (10 mL) were combined in a round-bottomed flask. The resulting mixture was refluxed for 24 h and then cooled to room temperature. Chloroform (25 mL), water (25 mL), and hydrochloric acid (12 M, 25 mL) were added. After being stirred for 10 min, the resulting mixture was separated by suction filtration, giving a light-brown powder (217 mg, 99%). The three-step synthesis of the title cyano acid (VII).
Crystallization: 3,5-Dibromo-4-cyanobenzoic acid (100 mg) and anthracene (29 mg) were dissolved in dichloromethane (25 mL) in a loosely covered beaker. Most of the solvent was allowed to evaporate gradually over 3 d. The resulting colorless or pale-orange plate-shaped crystals were collected after decantation and then washed with several drops of ice-cold 1:3 dichloromethane:pentane.

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 3. A direct-methods solution was calculated, followed by full-matrix least squares/difference-Fourier cycles. All H atoms were placed in calculated positions (C-H = 0.95 Å , O-H = 0.84 Å ) and refined as riding atoms with U iso (H) set to 1.2U eq (C) and 1.5U eq (O).   program(s) used to solve structure: SHELXS97 (Sheldrick 2015a); program(s) used to refine structure: SHELXL2014 (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.