Two dialkylammonium salts of 2-amino-4-nitrobenzoic acid: crystal structures and Hirshfeld surface analysis

The structures of two ammonium salts of 2-amino-4-nitrobenzoic acid are described. Substantial hydrogen bonding leads to supramolecular layers in the [Me2NH2]+ salt and a three-dimensional architecture in the case of the [n-Bu2NH2]+ salt.


Structural commentary
The molecular structures of the constituents of (I) are shown in Fig. 1; the asymmetric unit comprises one cation and one anion. Confirmation of proton transfer during recrystallization of dimethylamine and 2-amino-4-nitrobenzoic acid is found in (i) the similarity of the C-O bond lengths [C7-O1, O2 = 1.2587 (17) and 1.2609 (16) Å , respectively] and (ii) the pattern of hydrogen bonding as discussed in Supramolecular features. The molecular structure of the cation is unremarkable with a C8-N3-C9 angle of 113.54 (11) . The anion features an intramolecular amino-N-HÁ Á ÁO(carboxylate) hydrogen bond (Table 1). Despite the presence of this interaction, there are small twists in the molecule as seen in the values of the C2-C1-C7-O2 and O3-N2-C4-C3 torsion angles of 169.51 (12) and 4.04 (19) , respectively. In terms of dihedral angles, the angles between the central ring and the carboxylate and nitro groups are 11.45 (13) and 3.71 (15)  The molecular structure of the constituents of (I), showing the atomlabelling scheme and displacement ellipsoids at the 50% probability level.

Figure 2
The molecular structure of the constituents of (II), showing the atomlabelling scheme and displacement ellipsoids at the 50% probability level. are in the same relative orientation with the dihedral angle between them being 7.9 (2) .
The asymmetric unit of (II) comprises two independent pairs of cations and anions. The molecular structures of these are shown Fig. 2. As for (I), the confirmation for proton transfer from acid to base is seen in the equivalence of the C-O [C7-O1, O2 = 1.262 (2) and 1.267 (3) Å , respectively and C14-O5, O6 = 1.269 (3) and 1.256 (3) Å , respectively] bond lengths and in the pattern of intermolecular interactions, see below. The C15-N5-C19 and C23-N6-C27 angles in the cations are 113.40 (19) and 112.99 (17) , respectively, i.e. similar to the comparable angle in (I). The cations adopt different conformations as seen in the relative orientations of the terminal methyl groups. For the N5-cation, this is quantified in the values of the C15-C16-C17-C18 and C19-C20-C21-C22 torsion angles of 171.9 (3) and 49.5 (3) , respectively, consistent with a (+)-antiperiplanar (+ap) and a (+)-synclinal (+sc) conformation, respectively. In the N6cation, each chain is +ap, i.e. with torsion angles of 173.0 (2) (C23-chain) and 176.0 (2) (C27-chain). The anions present similar conformations as in (I) and each features an intramolecular amino-N-HÁ Á ÁO(carboxylate) hydrogen bond, Table 2. However, there are some subtle differences between the anions in terms of the relationship between the central rings and terminal substituents. For the O1-anion, the angles between the central ring and the carboxylate and nitro groups are 12.73 (6) and 4.30 (10) , respectively, and the comparable angles for the O5-cation are 8.1 (4) and 12.6 (3) , respectively. The difference between (I) and (II) is that in the cations of (II), the terminal groups are con-rotatory, forming dihedral angles of 17.02 (8) and 19.0 (5) , respectively.

Figure 3
The molecular packing in (I), showing (a) supramolecular chain comprising anions only, orientated along the c axis and sustained by amino-N-HÁ Á ÁO(carboxylate) interactions shown as orange dashed lines, (b) detail of the 12-membered {Á Á ÁHNHÁ Á ÁOCO} 2 synthon with ammonium-N-HÁ Á ÁO(carboxylate) hydrogen bonds shown as blue dashed lines and (c) a view of the unit-cell contents in projection down the c axis. In part (b), all but the CO 2 groups of the two central benzoate residues have been removed for clarity.
OÁ Á Á(arene) and methyl-C-HÁ Á ÁO(nitro) contacts. The nitro-O4 atom is crucial in the formation of these contacts, being the donor and acceptor, respectively, Table 1, Fig. 3c. The crystal of (II) features extensive N-HÁ Á ÁO hydrogen bonding, Table 2. The anions assemble into four-ion aggregates as a result of charge-assisted amino-N-HÁ Á ÁO(carboxylate) hydrogen bonding. For the O1-anion, the carboxylate-O atom not participating in the intramolecular amino-N-HÁ Á ÁO interaction forms an intermolecular amino-N-HÁ Á ÁO interaction. However, for the O5-anion, the carboxylate-O atom participating in the intramolecular amino-N-HÁ Á ÁO interaction also forms the intermolecular amino-N-HÁ Á ÁO contact, as illustrated in Fig. 4a. The result of this self-assembly is a centrosymmetric, 20-membered {Á Á ÁHNHÁ Á ÁOCOÁ Á ÁHNHÁ Á ÁO} 2 ring which encompasses two {Á Á ÁHNC 3 O} loops formed by the intramolecular amino-N-HÁ Á ÁO(carboxylate) hydrogen bonds. Each of the cations associates with two anions in a very similar fashion to that in (I), in that the H atoms of the N5-ammonium cation bridge two O1-anions over a centre of inversion to form a centrosymmetric, 12-membered {Á Á ÁHNHÁ Á ÁOCO} 2 synthon, Fig. 4b. The N6-ammonium H atoms form similar bridges but with the O5-anion. The result is the formation of a three-dimensional architecture, Fig. 4c.

Hirshfeld surface analysis
Hirshfeld surface analysis for (I) and (II) was carried out as described previously (Cardoso et al., 2016)     the Hirshfeld surface for (I) mapped over d norm in the range À0.3 to + 1.8 au shown in Fig. 5a and b, the bright-red spots appearing near the amino-H2N, ammonium-H3N and H4N, and carboxylate-O1 and O2 atoms represent donors and acceptors of the dominating hydrogen bonds; they are viewed as blue and red regions on Hirshfeld surfaces mapped over electrostatic potential in the range À0.24 to + 0.31 au in Fig. 5c and correspond to positive and negative potentials, respectively. The faint-red spots at the methyl-H8C and nitro-O4 atoms in Fig. 5b are due to the presence of comparatively weak C-HÁ Á ÁO interactions. Also from Fig. 5c, it is evident that the electrostatic coulombic interaction between the dimethylammonium and 2-amino-4-nitrobenzoate species results in a cation-anion pair through a C-HÁ Á Á contact between methyl-H9C and the benzene (C1-C6) ring, as highlighted by the dotted bond. The immediate environment about the ion-pair within the Hirshfeld surface mapped over d norm mediated by the above interactions is illustrated in Fig. 6    Views of Hirshfeld surfaces mapped over d norm for (II), showing (a) ionpair 1 in the range À0.2 to + 1.8 au, (b) ion-pair 2 in the range À0.2 to + 1.6 au and (c) ion-pair 2 in the range À0.1 to + 1.6 au.
In the crystal of the dibutylammonium salt, (II), each of the two independent pairs of cations and anions are connected by charge-assisted ammonium-N-HÁ Á ÁO(carboxylate) hydrogen bonds. The Hirshfeld surfaces for each of the independent pairs, hereafter referred as ion-pair 1 (involving the N4-cation and O1-anion) and ion-pair 2 (involving the N3-cation and O5-anion), were generated as well that for the entire structure of (II). The Hirshfeld surfaces mapped over the electrostatic potential for the ion-pairs are shown in Fig. 7.
Views of Hirshfeld surfaces mapped over d norm , in the ranges À0.2 to +1.8 au for ion-pair 1, Fig. 8a, À0.1 to +1.6 au for ion-pair 2, Fig. 8b, and in order to reveal more detail (redspots) on the surface, À0.1 to +1.6, for ion-pair 2, Fig. 8c. The bright-red spots appearing near amino-H2N and H4N, ammonium-H6N and H8N, and carboxylate-O1, O2, O5 and O6 atoms indicate donors and acceptors of charge-assisted N-HÁ Á ÁO hydrogen bonds between the respective ion-pairs. The short interatomic OÁ Á ÁH contact between the amino-H2N and nitro-O8 atoms, Table 3, is evident from the faint-red spots at the N1, Fig. 8a, and nitro-O8 atoms, Fig. 8c. The faint-red spots present in Fig. 8b near atoms N4, C11, C13 and O6 of ion-pair 2 indicate their participation in short interatomic contacts in the crystal, Table 3. As the intermolecular C-HÁ Á ÁO interactions involving the butyl-C19-and C20-H atoms of ion-pair 2 are very weak compared to the above, they only appear as very faint spots in Fig. 8c; the C27-H27AÁ Á ÁO4 interaction is even weaker than these, showing no spots even at the lower d norm range. The immediate environments about the ion-pairs within d norm mapped Hirshfeld surface mediated by N-HÁ Á ÁO hydrogen-bonding interactions are illustrated in Fig. 9.
The fingerprint plot delineated into OÁ Á ÁH/HÁ Á ÁO contacts for (I), Fig. 10c, shows that these contacts make the most significant contribution, i.e. almost half (49.4%), to the Hirshfeld surface. This may be due to salt formation through electrostatic interactions resulting in only a few hydrogen atoms being available on the surface to form interatomic HÁ Á ÁH and other contacts. This is also reflected in a comparatively low contribution from HÁ Á ÁH contacts to the Table 3 Summary of short interatomic contacts (Å ) in (I) and (II).

Contact
Distance Symmetry operation The immediate environment about reference ion-pairs within Hirshfeld surfaces mapped over d norm showing N-HÁ Á ÁO hydrogen bonding in (II), showing (a) ion-pair 1 and (b) ion-pair 2.
Hirshfeld surface, Fig. 10b and Table 4. A pair of long spikes with tips at d e + d i $1.8 Å in Fig. 10c is the result of chargeassisted N-HÁ Á ÁO hydrogen bonds,  Fig. 10e, is the result of a short interatomic contact, Table 3, and an intra-ion-pair methyl-C-HÁ Á Á interaction within the cation-anion pair.
In the structure of (II), the most significant contribution to the Hirshfeld surface is from HÁ Á ÁH contacts, an observation clearly related to the hydrogen-rich n-butyl side chains in the cations, cf. (I). This is also reflected through the appearance of green points in the fingerprint plot delineated into HÁ Á ÁH contacts, Fig. 10b, and in the nearly same percentage contribution from these contacts in the plots for each ion-pair and overall Hirshfeld surface, Table 4. A pair of small peaks at d e + d i $2.2 Å in Fig. 10b is the result of short interatomic HÁ Á ÁH contacts in the crystal, Table 3.
A pair of long spikes with the tips at d e + d i $1.8 Å in the fingerprint delineated into OÁ Á ÁH/HÁ Á ÁO contacts, Fig. 10c, are a result of the N-HÁ Á ÁO hydrogen bonds. A pair of regions comprising aligned green points in the plot beginning at d e + d i $2.7 Å are due to short interatomic OÁ Á ÁH/HÁ Á ÁO contacts present in the structure, Table 3. The distinct shapes in the fingerprint plots delineated into CÁ Á ÁH/HÁ Á ÁC contacts for ion-pairs 1 and 2, Fig. 10e, and their different percentage contributions to the respective Hirshfeld surfaces, Table 4, reflect the different conformations of the butyl chains in the cations; the small tips at d e + d i $2.9 Å in the overall plot indicate short interatomic CÁ Á ÁH/HÁ Á ÁC contacts, Table 3.

Database survey
As indicated in the Chemical context, a good number of ammonium salts of anions derived from 2-amino-4-nitrobenzoic acid have been described in the crystallographic literature. Salient geometric data for these are collated in Table 5. The consistent feature of the 2-amino-4-nitrobenzoate anions is deprotonation of the original carboxylic acid. Most of the dianions are relatively close to being planar with the outlier structures being the salts with [NH 4 ] + (Smith, 2014b), with a dihedral angle of 26.4 (3) between the C 6 ring and the carboxylate group, and (II) with a dihedral angle of 12.6 (3) between the the nitro group and the C 6 ring. The greatest twist between the carboxylate and nitro substituents in any of the anions included in Table 3 is 24.1 (4) , which also occurs in the aforementioned ammonium salt (Smith, 2014b). Table 5 Geometric data ( ) for ammonium salts of 2-amino-4-nitrobenzoate.

Synthesis and crystallization
The salts were isolated from the very similar reaction conditions. A solution of the respective R 2 NH amine (0.1 mmol) in EtOH (5 ml) and 4-nitroanthranilic acid (0.1 mmol) in EtOH (10 ml) were mixed and left at room temperature. The yellow blocks of (I) and orange blocks of (II), which had formed after 4 days, were collected and used as such in the structure determinations.

(I) Dimethylazanium 2-amino-4-nitrobenzoate
Crystal data 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 O1 0.09438 (14