2-Nitroanilinium bromide

The title compound, C6H7N2O2 +·Br−, is isomorphous with 2-nitroanilinium chloride and contains an characteristic intramolecular N—H⋯O hydrogen bond, forming an S(6) motif. Intermolecular N—H⋯Br hydrogen bonds occur in the crystal structure. Two zigzag chains of C 2 1(4) motifs extend along the b-axis direction. These primary chain motifs intersect like a double helix structure, leading to R 6 3(12) ring motifs, which are arranged in tandem along the b axis. Hence, hydrophilic layers are generated at z = 1/4 and 3/4, which are sandwiched between alternate hydrophobic layers across z = 0 and 1/2.

The title compound, C 6 H 7 N 2 O 2 + ÁBr À , is isomorphous with 2nitroanilinium chloride and contains an characteristic intramolecular N-HÁ Á ÁO hydrogen bond, forming an S(6) motif. Intermolecular N-HÁ Á ÁBr hydrogen bonds occur in the crystal structure. Two zigzag chains of C 2 1 (4) motifs extend along the b-axis direction. These primary chain motifs intersect like a double helix structure, leading to R 6 3 (12) ring motifs, which are arranged in tandem along the b axis. Hence, hydrophilic layers are generated at z = 1/4 and 3/4, which are sandwiched between alternate hydrophobic layers across z = 0 and 1/2.

Comment
Intermolecular forces play an essential role in the formation of supramolecular systems which are useful for definite social applications. In which, the phenomenon of hydrogen bond has its importance in the areas of molecular recognition, crystal engineering research and supramolecular chemistry. Their strength and directionality is responsible for crystal packing and entire molecular arrays (Desiraju, 1989). 2-nitroaniline extists in three phases, viz., α-& β-polymorphs (Herbstein, 1965) & γ-polymorph (Dhaneshwar et al., 1978). As a special attention, a non-proton transfer adduct of 2-nitroaniline with picric acid as 2:1 complex is reported by Saminathan & Sivakumar, 2007. Based on the above fact, we are interested on the specificity of recognition of nitroaniline with other inorganic/organic acids. Hence, the present work is attempted here.
The asymmetric part of the title compound, (I), contains one 2-nitroanilinium cation and a bromide anion (Fig. 1). The title compound is an isomorphous of 2-nitroanilinium chloride reported by Ploug-Sørensen & Andersen, 1983. There is only a quantitative change in the crystallographic parameters owing to the size of the anion; the unit cell volume in (I) is about 67 Å 3 larger than that of the chloride salt (Ploug-Sørensen & Andersen, 1983). This present study was undertaken to investigate the hydrogen-bonding interactions with the concept of graph-set motifs, aggregation patterns and crystalline packing of the molecules. The protonation of the N site of the cation is evident from the elongated C-N bond distance. The plane of the nitro group (-NO 2 ) is twisted out from the plane of the aromatic ring with an angle of 26.9 (2)°. Especially, the O atom which is involved in the intramolecular hydrogen bond is moved more away from the aromatic plane (0.565 (3) Å) than that of the other O atom (0.377 (4) Å) which is not participating in any hydrogen bonding interaction.
As nitroanilines have both donor (amine) and acceptor (nitro) sites for hydrogen bonding interactions, they have proved to be versatile reagents for structure extension by linear (chain C motifs) and cyclic (ring R motifs) hydrogen-bonding associations. In the present crystal structure, the molecular aggregations are stabilized through intricate three dimensional hydrogen bonding network ( Fig. 2; Table 1). A characteristic intramolecular N-H···O hydrogen bond, forming an S(6) motif, is observed in the cation (Fig. 1). The other intermolecular hydrogen bonds are only N-H···Br type. Two zigzag chains of C 2 1 (4)motifs are extending along b-axis of the unit cell. These primary chain motifs intersect like a double helix structure leading to ring R 6 3 (12) motifs. These ring motifs are arranged in tandem along b-axis. Hence, hydrophilic layers are generated at z = 1/4 and 3/4 which are sandwiched between alternate hydrophobic layers across z = 0 and 1/2.

Experimental
The title compound was crystallized from an aqueous mixture containing 2-nitroaniline and hydrobromic acid in the stoichiometric ratio of 1:1 at room temperature by slow evaporation technique.
supplementary materials sup-2 Refinement All the H atoms except the atoms involved in hydrogen bonds were positioned geometrically and refined using a riding model, with C-H = 0.93 Å and U iso (H) = 1.2 U eq (parent atom). H atoms involved in hydrogen bonds were located from differential fourier map and refined isotropically. Fig. 1. The molecular structure of the title compound (I) with the numbering scheme for the atoms and 50% probability displacement ellipsoids. H bonds are drawn as dashed lines.   (14) 0.0255 (14) Geometric parameters (Å, °)