Crystal structures of 2,6-dibromo-4-methylbenzonitrile and 2,6-dibromo-4-methylphenyl isocyanide

The title crystals are isomorphous, with tetrameric Br⋯Br contacts as the principal packing interaction. No CN⋯Br or NC⋯Br contacts are observed.


Chemical context
As 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 2,6dihalobenzonitrile 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.  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 2,6dihaloaryl 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 commentary
RNC 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, hence the reflected ellipsoid orientations between RCN and RNC. For RCN, the mean deviation from the plane of best fit for the benzene ring (C1-C4) is 0.002 (3) Å . For RNC, this deviation (C11-C14) is 0.001 (2) Å . These planes are roughly parallel to (332).

Supramolecular features
The methyl group is sufficiently bulky to prevent planar ribbons or inversion dimers of the types found in the tribromo analogs. Instead, neighboring molecules of RCN and RNC adopt a mutually inclined arrangement. The inclination between best-fit planes for adjacent molecules of RCN is 38.3 (3) , and 41.0 (2) for RNC. This molecular arrangement prevents CNÁ Á ÁBr and NCÁ Á ÁBr contacts, but is probably affected by the formation of R 4 4 (4) rings of BrÁ Á ÁBr contacts (Table 1). Each Br atom participates both as a donor (narrow C-BrÁ Á ÁBr angle) and an acceptor (wide C-BrÁ Á ÁBr angle). Each molecule participates in two such R 4 4 (4) rings, forming R 4 4 (24) rings. The result is a tetragonally puckered sheet structure parallel to (001) (Fig. 3). This is similar to the sheet structure reported for 2,6-dibromobenzonitrile , although without the methyl group, the sheets were nearly planar. As future work, we plan to find whether this packing motif changes when the Br atoms are replaced with I atoms.

Synthesis and crystallization
The synthesis of RCN and RNC is shown in Fig. 4.

Figure 4
The synthesis of RCN and RNC.
Crystallization: RCN and RNC crystals were grown by slow evaporation of dichloromethane solutions under ambient conditions. Crystals were collected by suction filtration when a small portion of the original solvent remained, and then they were washed with pentane.

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 2. 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 or 0.98 Å ) and refined as riding atoms with U iso (H) set to 1.2U eq (C) for aryl H atoms and 1.5U eq (C) for methyl H atoms. Because the molecules lie across mirror planes, the methyl H atoms are disordered across two sets of sites with 1:1 occupancy.

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. ( 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.