Two new polytypes of 2,4,6-tribromobenzonitrile

Two new polymorphs of 2,4,6-tribromobenzonitrile have been found. Together with the known polymorph, they are polytypic. One new polytype is isostructural with the previously reported crystal structure of 1,3,5-tribromo-2-isocyanobenzene.


Chemical context
The reported structures of 2,4,6-tribromobenzonitrile (RCN, Figs. 1 and 2; Carter & Britton, 1972) and 1,3,5-tribromo-2isocyanobenzene (RNC, Figs. 1 and 3; Carter et al., 1977) have two-dimensional layers of similarly arranged molecules, but the packing of adjacent layers is distinctly different. At the time, no explanation was offered. It was puzzling, given that the two compounds are isoelectronic, isosteric, and the principal intermolecular interactions, C NÁ Á ÁBr and N CÁ Á ÁBr, are similar. Recent reports of polytype organic structures, such as picryl bromide (Parrish et al., 2008) and 5,6-dimethylbenzofurazan 1-oxide (Britton et al., 2012) led to the idea that RCN and RNC might occur as polytypes. Earlier, Bredig (1930) had determined the space group and unit cell of RCN with the same results as Carter & Britton. Bredig was trying to follow up on the goniometer studies of Jaeger (1909), but while he found the same a:b ratio as Jaeger in the RCN unit cell, he found a different b:c ratio.
Accordingly, a search was made for polytypes of RCN, and to a lesser extent, of RNC. Four different structures were identified. RCN-I is the original Z = 2 structure of RCN; RCN-II is a new Z = 8 polytype; RCN-III is a new Z = 12 polytype. No RNC counterparts to RCN-I or RCN-III were observed.

Structural commentary
Molecules of RCN and RNC are nearly planar. The average distance of atoms from the plane of best fit is 0.025 Å in RCN-I. For RCN-II, the average distances are 0.037 and 0.010 Å , for the (N27) and (N37) molecules, respectively. In RNC-II, the molecules are slightly more distorted, with average deviations of 0.043 and 0.017 Å for the (N127) and (N137) molecules, respectively. For RCN-III, the average distances are 0.009, 0.018, and 0.032 Å for the (N47), (N57), and (N67) molecules, respectively.
The bond lengths in RCN and RNC are generally similar (Fig. 4). They are also similar to the mean bond distances reported for bonds of each type (Allen et al., 1987). The N research communications Figure 1 Synthesis of RCN and RNC.

Figure 2
Molecular structures, with atom labeling, of RCN-I viewed along [111]; ; . Displacement ellipsoids are drawn at the 50% probability level. In discussion, molecules are named by their respective nitrogen atoms. Each molecule lies across a crystallographic mirror plane.

Figure 3
Molecular structure, with atom labeling, of RNC-II viewed along [120]. Displacement ellipsoids are drawn at the 50% probability level. Each molecule lies across a crystallographic mirror plane.

Figure 4
Selected bond lengths (Å ) in RCN and RNC, averaged across all polytypes. The data shown in parentheses are the mean distances for each bond type reported by Allen et al. (1987). atom in RNC is displaced toward the aryl ring compared to the literature distances for aryl isocyanides. Fig. 5 shows a two-dimensional layer of RCN-I. All of the structures are composed of similar layers. Adjacent molecules are associated through C NÁ Á ÁBr interactions, arranged in R 2 2 (10) rings (Etter, 1990;Bernstein et al., 1995). The CNÁ Á ÁBr distances in these rings range between 3.053 and 3.077 Å (Table 1); these distances can be compared with the NÁ Á ÁBr van der Waals distance of 3.40 Å (Bondi, 1964;Rowland & Taylor, 1996). Each layer in RCN-II is composed of alternating (N27) and (N37) molecules. RCN-III contains two layers of alternating (N47) and (N57) molecules for each layer composed entirely of (N67) molecules. Adjacent pairs of layers show translational or pseudotranslational, or pseudocentric stacking (Fig. 6). RCN-I shows translational stacking between all adjacent layers (Fig. 7). In RCN-II, alternating pairs of layers show pseudocentric and pseudotranslational stacking (Fig. 8). In RCN-III, each layer of (N67) molecules pseudotranslationally overlaps both neighboring (N47/N57) layers, while pairs of adjacent (N47/N57) layers, every third pair of layers, overlap pseudocentrically ( Fig. 9      The NCÁ Á ÁBr contact distances in RNC-II are a smaller percentage of the van der Waals distance, 3.63 Å , versus corresponding atoms in RCN-II. The contacts in RNC-II occur at slightly wider angles than those in RCN-II (Table 1).

Supramolecular features
In RCN-II, the planes of best fit of the two different molecules are inclined by 6.5 to each other; in RNC-II this inclination is 7.5 . In RCN-III, the relative inclination of planes of (N47) and (N57) molecules is 7.0 . These two planes are approximately bisected by the planes of (N67) molecules.
Layers of the type observed in RCN were reported in 2,6dibromo entries with Cl, Br, or I at the 4-position. Other entries exhibit short contacts between the cyano-or isocyanogroup and one ortho-halogen atom of an intralayer molecule, with various interlayer contacts. Polymorphs are only reported for 2,4,6-trichlorobenzonitrile; those are not polytypic.
Expanding the search to include organometallic complexes found three more entries, with the cyano N or isocyano C atom ligating gallium (trifluorobenzonitrile; Tang