2-Carboxyanilinium bromide monohydrate

The title compound, C7H8NO2 +·Br−·H2O, is isomorphous with 2-carboxyanilinium chloride monohydrate and contains an intramolecular N—H⋯O hydrogen bond, forming an S(6) motif. The main intermolecular interactions are of the N—H⋯O/Br and O—H⋯O/Br types. Hydrogen-bonding dimers are formed via the carboxyl groups and the uncoordinated water molecule, with centrosymmetric R 4 4(12) ring motifs, in tandem with centrosymmetric R 8 4(16) ring motifs formed by the cations and bromide anions. The hydrogen-bonded ring motifs intersect, forming chains with graph-set motif C 4 3(10) extending along the a axis. These form a two-dimensional hydrogen-bonded network in (101) which is extended along [010] through N—H⋯Br hydrogen bonds. Hydrophilic layers are generated at z = 0 and 1/2 which are sandwiched between alternate hydrophobic layers across z = 1/4 and 3/4.

The title compound, C 7 H 8 NO 2 + ÁBr À ÁH 2 O, is isomorphous with 2-carboxyanilinium chloride monohydrate and contains an intramolecular N-HÁ Á ÁO hydrogen bond, forming an S(6) motif. The main intermolecular interactions are of the N-HÁ Á ÁO/Br and O-HÁ Á ÁO/Br types. Hydrogen-bonding dimers are formed via the carboxyl groups and the uncoordinated water molecule, with centrosymmetric R 4 4 (12) ring motifs, in tandem with centrosymmetric R 8 4 (16) ring motifs formed by the cations and bromide anions. The hydrogen-bonded ring motifs intersect, forming chains with graph-set motif C 4 3 (10) extending along the a axis. These form a two-dimensional hydrogen-bonded network in (101) which is extended along [010] through N-HÁ Á ÁBr hydrogen bonds. Hydrophilic layers are generated at z = 0 and 1/2 which are sandwiched between alternate hydrophobic layers across z = 1/4 and 3/4.

2-Carboxyanilinium bromide monohydrate
V. Susindran, S. Athimoolam, S. A. Bahadur and B. Sridhar Comment Vitamin L, 2-aminobenzoic acid (anthranilic acid), is used as an intermediate for the production of dyes, pigments and saccharin, and its esters are used in preparing perfumes, pharmaceuticals and UV-absorber as well as corrosion inhibitors for metals and mould inhibitors in soya sauce. It is also known to be a specific precursor of the skimmianine and acronidine alkaloids (Prager & Skurray, 1968). Anthranilic acid and its derivatives are used as the preferred fluorescent labels for carbohydrate analysis, with very high sensitivity, and for specific labelling of the reducing mono-and oligosaccharides (Anumula, 1993(Anumula, , 1994. Generally, hydrdoxyl/amino-group-substituted benzoic acid derivatives have active bacteriostatic (e.g. p-aminibenzoic acid, a bacterial vitamin) and fragrant properties and are used in the pharmaceutical and perfume industry (Robinson, 1966).
2-Aminobenzoic acid occurs either as a positively or a negatively charged ion or as a neutral molecule (also as a zwitterion), depending on the environment and pH of the solution. The amine group can be protonated, R-NH 3 + , (Bahadur et al., 2007) and the carboxyl group can be deprotonated (forming R'-CO 2 -), where R and R' are residual moieties. One of the polymorphs of 2-aminobenzoic acid at low temperature occurs as a zwitterion in the solid state (Brown & Ehrenberg, 1985). In our study, anthranilic acid is observed as a protonated carboxyanilinium cation with a bromide anion and hydrogen-bonded water molecule. The present study was undertaken on the isomorphous bromide salt of 2-aminobenzoic acid,(I), to investigate their hydrogen-bonding interactions, aggregation patterns and crystalline packing of the molecules. Recently, an anthranilic acid salt with a chloride anion has been reported (Zaidi et al., 2008). 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 103 Å 3 larger than that of the chloride salt (Zaidi et al., 2008). The unhydrated form of 4-aminobenzoic acid -hydrobromic acid crystal was reported by Cinčić & Kaitner, 2008, with the focus on the hydrogen-bonding associations and crystal packing. The structure 2-(methoxycarbonyl)anilinium chloride monohydrate has also been reported (Ma et al., 2005).
The asymmetric unit of the title compound contains a 2-carboxyanilinium cation with a protonated amino group, a bromide anion and a hydrogen-bonded lattice water molecule (Fig. 1). Protonation of the cationic N atom is confirmed by the C-N bond length, 1.464 (3) Å . The asymmetric carboxyl C-O bond lengths (C1-O1 1.216 (3); C1-O2 1.307 (3) indicate the presence of an H atom on O2. The carboxyl group is essentially coplanar with the aromatic ring, with dihedral angle of 2.71 (1)°. However, twisting of the carboxyl plane from the aromatic ring plane is observed in many aminobenzoic acid complexes owing to extensive hydrogen bonding and packing interactions (Athimoolam & Natarajan, 2006).
As aminobenzoic acids have both donor and acceptor 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, through both the carboxylic acid and amine functional groups (Bernstein et al., 1995). The crystal packing and hydrogen bonding interactions are illustrated in Fig. 2 and hydrogen-bond parameters are listed in Table 2. All ammonium H atoms are involved in hydrogen bonds, two with two different bromide anions and the third with the carbonyl O atom of the same supplementary materials sup-2 molecule. A strong intramolecular N-H···O hydrogen bond with the graph-set S(6) motif (Fig. 3) is a characterestic feature in many anthranilic acid complexes (Bernstein et al., 1995).
The formation of a classical carboxyl-carboxyl dimer is another of the characteristic features found in most aminobenzoic acid complexes (Cambridge Structural Database, Version 5.29; Allen, 2002), but here the dimerization involves the solvent water molecule. The carboxyl O atom hydrogen bonds with neighbouring water O atom, which further interacts with an inversion-symmetry-related carbonyl O atom (Fig. 3). This generates R 4 4 (12) ring motifs about the inversion centers of the unit cell. Additional centrosymmetrically related hydrogen-bonded rings formed by cation-bromide interactions via N-H···Br and O-H···Br hydrogen bonds designated by the graph-set motif R 8 4 (16). These ring motifs are combined and form C 4 3 (10) chain motifs (Fig. 3) extending along a axis of the unit cell. These molecular aggregations form a two-dimensional sheet like structure stacked parallel to the (101) plane of the unit cell (Fig. 2). Further this two-dimensional network is extended to another direction [010] through an N-H···Br (-x + 1/2, -y + 3/2, -z) hydrogen bond. This leads to hydrophilic layers across z = 0 and 1/2 which are sandwiched between alternate hydrophobic layers across z=1/4 and 3/4, resulting from aromatic ring stacking. Even though the crystalline packing leads to the formation of two weak C-H···O hydrogen bonds [C3-H3···O2 # , C4-H4···O2 # ; symmetry code: (#) -x + 1/2,+y + 1/2,-z + 1/2], the extensive classical hydrogen bonds predominate. There are no significant C-H···π and π···π interactions.

Experimental
The title compound was crystallized at room temperature by the slow evaporation technique from aqueous solutions containing 2-aminobenzoic acid (anthranilic acid) with hydrobromic acid in a 1:1 stoichiometric ratio.