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) . Chemical contextDoyle Britton published roughly 30 crystallographic articles on solid-phase cyano-halo interactions from variously substituted halobenzonitriles and isocyanides. 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 commentaryThe 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 featuresThe 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 ...
In the title crystals, C 8 H 5 Br 2 N, which are isomorphous, the steric bulk of the methyl group causes neighboring molecules to become mutually inclined. This prevents the formation of planar or nearly planar sheets, which were observed in the trichloro and tribromo analogs. Instead of CN/NCÁ Á ÁBr contacts, tetrameric BrÁ Á ÁBr contacts are observed. These contacts form tetragonally puckered sheets parallel to (001). The CN/NC and methyl groups are grouped at the peaks and troughs. Both molecules lie across crystallographic mirror planes; thus, the methyl H atoms are disordered over two sets of sites with equal occupancy. The title nitrile is a redetermination. The refinement converged at R[F 2 > 2(F 2 )] = 0.020, whereas the original determination [Gleason & Britton, (1976). Cryst. Struct. Commun. 5, 229-232] had R = 0.112. Chemical contextAs part of an ongoing study of cyano-halo short contacts, the para-Br atom of 2,4,6-tribromobenzonitrile (van Rij & Britton, 1972) was replaced by a methyl group (Gleason & Britton, 1976), giving 2,6-dibromo-4-methylbenzonitrile (RCN). The methyl group was bulky enough to disrupt the planar sheet structure that was observed in the tribromo nitrile. As of the most recent update of the Cambridge Structural Database (CSD; Version 5.37, Feb 2017;Groom et al., 2016), RCN remains the only example of a 2,6-dihalobenzonitrile with a methyl group at the 4-position. Most of the examples with polyatomic 4-substituents are fluorinated benzonitriles, with applications including tuning the fluoride affinity of phosphoranes (Solyntjes et al., 2016), study of magnetostructural correlation (Thomson et al., 2012), and use as metal ligands (Díaz-Á lvarez et al., 2006). The chlorinated and brominated entries are either bis(carbonitriles) [(I), Fig. 1; Britton, 1981;Hirshfeld, 1984;van Rij & Britton, 1981] or 4-carboxy analogs [(II);Britton, 2012;Noland et al., 2017]. All of these 4-substituents have stronger interactions than a methyl group, and exhibit different packing motifs than RCN. Contextual compounds.The comparison of corresponding nitriles and isocyanides is a rare opportunity to explore the subtle differences between molecules that are both isomeric and isoelectronic. In the 2,6-dihaloaryl series, there are only three prior examples in the CSD. The trichloro and tribromo pairs [(III);Pink et al., 2000;Britton et al., 2016] are polytypic, and the pentafluoro pair [(IV), Fig. 1; Bond et al., 2001;Lentz & Preugschat, 1993] is isomorphous. The question arose as to whether RCN and its isocyanide (2,6-dibromo-4-methylphenyl isocyanide, RNC) would be isomorphous, polytypic, or polymorphic. A single crystal of RNC and a redetermination of RCN are presented. Structural commentaryRNC and the redetermination of RCN are isomorphous with the original RCN structure (Gleason & Britton, 1976). The molecular structures of RCN ( Fig. 2a) and RNC ( Fig. 2b) are nearly planar. The two crystals described herein were pseudoenantiomorphic, roughly being enantiomorphs with swapped cyano C and N atoms, he...
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