Different packing motifs in the crystal structures of three molecular salts containing the 2-amino-5-carboxyanilinium cation: C7H9N2O2 +·Cl−, C7H9N2O2 +·Br− and C7H9N2O2 +·NO3 −·H2O

The title molecular salts containing the same cation accompanied by different simple anions show quite different intermolecular connectivities.

The syntheses and crystal structures of three molecular salts of protonated 3,4diaminobenzoic acid, viz. 2-amino-5-carboxyanilinium chloride, C 7 H 9 N 2 O 2 + Á-Cl À , (I), 2-amino-5-carboxyanilinium bromide, C 7 H 9 N 2 O 2 + ÁBr À , (II), and 2-amino-5-carboxyanilinium nitrate monohydrate, C 7 H 9 N 2 O 2 + ÁNO 3 À ÁH 2 O, (III), are described. The cation is protonated at the meta-N atom (with respect to the carboxy group) in each case. In the crystal of (I), carboxylic acid inversion dimers linked by pairwise O-HÁ Á ÁO hydrogen bonds are seen and each N-H group forms a hydrogen bond to a chloride ion to result in (100) undulating layers of chloride ions bridged by the inversion dimers into a three-dimensional network. The extended structure of (II) features O-HÁ Á ÁBr, N-HÁ Á ÁBr and N-HÁ Á ÁO hydrogen bonds: the last of these generates C(7) chains of cations. Overall, the packing in (II) features undulating (100) (001) sheets, and the nitrate ions and water molecules form undulating chains. Taken together, alternating (001) slabs of organic cations plus anions/water molecules result. Hirshfeld surfaces and fingerprint plots were generated to give further insight into the intermolecular interactions in these structures. The crystal used for the data collection of (II) was twinned by rotation about [100] in reciprocal space in a 0.4896 (15):0.5104 (15) ratio.

Chemical context
The benzoate anion, C 7 H 5 O 2 À is a classic ligand in coordination chemistry, with over 1500 crystal structures reported in the Cambridge Structural Database (version 5.40, updated to February 2020;Groom et al., 2016) for benzoate complexes of first-row transition metals alone. Functionalized benzoic acid derivatives add further structural variety: for example, -NH 2 substituents at the ortho, meta and/or para positions of the benzene ring can form or accept hydrogen bonds with respect to nearby acceptor or donor groups and/or bond as Lewis bases to another metal ion (i.e. as a 2 -N,O or 3 -N,O,O bridging ligand). It should be noted that the presence of amine groups allows for protonation and the possible formation of molecular salts with the aminobenzoic acid acting as the cation.
As part of our ongoing studies in this area (Khosa et al., 2015), we now describe the syntheses and structures of three molecular salts of protonated 3,4-diaminobenzoic acid, viz. C 7 H 9 N 2 O 2 ÁCl (I), C 7 H 9 N 2 O 2 ÁBr (II) and C 7 H 9 N 2 O 2 ÁNO 3 ÁH 2 O ISSN 2056-9890 (III). Hirshfeld surface analyses have been performed to gain further insight into the intermolecular interactions.

Structural commentary
The contents of the asymmetric units of (I) (Fig. 1), (II) (Fig. 2) and (III) (Fig. 3) confirm them to be molecular salts of 3,4diaminobenzoic acid (i.e. the C 7 H 9 N 2 O 2 + 2-amino-5-carboxyanilinium cation has been formed) and the appropriate strong acid (hydrochloric acid, hydrobromic acid and nitric acid, respectively); compound (III) also includes a water molecule of crystallization. The neutral organic molecule (C 7 H 8 N 2 O 2 ) is known to crystallize as a zwitterion (Rzaczyń ska et al., 2000) with nominal intramolecular proton transfer from the carboxylic acid to the meta-N atom and presumably exists in the same form in solution, thus the formal acid-base reaction to form the title salts involves proton transfer from the strong acid to the -CO 2 À carboxylate group of zwitterionic C 7 H 8 N 2 O 2 to form a -CO 2 H carboxylic acid group; atom N1 remains protonated, to result in the C 7 H 9 N 2 O 2 + cation. The preference for the meta -NH 2 group to be protonated in these salts compared to the para -NH 2 group can be rationalized in terms of the potential loss of conjugation of the para-N-atom lone pair of electrons with the carboxylic acid grouping via the benzene ring, i.e., a small contribution of a quinoid (C N + containing) resonance form to the structure (Lai & Marsh, 1967): the mean bond lengths for C1-C2, C3-C4, C4-C5 and C1-C6 (single bonds in the quinoid struc-ture) and C2-C3 and C5-C6 (double bonds) are 1.401/1.380, 1.400/1.381 and 1.403/1.373 Å for (I), (II) and (III), respectively (global averages = 1.401/1.378 Å ). These data compare very well to the equivalent values of 1.399/1.375 Å established over 50 years ago from Weissenberg data for p-aminobenzoic acid (Lai & Marsh, 1967).
This electronic effect is also no doubt reflected in the fact that the C4-N2 (para) bond in the title compounds is notably shorter than the C3-N1 (meta) bond [distances in (I) = 1.378 (2) and 1.4640 (19), respectively; (II) = 1.387 (6) and 1.468 (6); (III) = 1.386 (5) and 1.457 (5) Å ]. Even so, it may be noted that the bond-angle sums about N2 are 348.0, 340.4, and 339.2 for (I), (II) and (III), respectively, suggesting a tendency towards sp 3 hybridization (and presumably lone-pair localization) for the nitrogen atom in each case: it also correlates with the fact that N2 accepts a hydrogen bond in the crystal of (III) (vide infra).

Figure 3
The molecular structure of (III) showing 50% displacement ellipsoids. The O-HÁ Á ÁO hydrogen bonds are indicated by double-dashed lines.

Supramolecular features
In the crystal of (I), the cations are connected into carboxylic acid inversion dimers via pairwise O-HÁ Á ÁO hydrogen bonds (Table 1), thereby generating classical R 2 2 (8) loops. All five N-H groups link to a nearby chloride ion: the HÁ Á ÁCl contacts from the protonated -N1H 3 + moiety (mean = 2.35 Å ) are substantially shorter than those arising from the unprotonated -N2H 2 group (mean = 2.70 Å ). As a result, the chloride ion accepts five N-HÁ Á ÁCl bonds from four cations (one cation bonds from both N1 and N2) in an irregular geometry (Fig. 4). The overall packing for (I) results in corrugated (100) sheets of chloride ions bridged by the carboxylic acid dimers into a three-dimensional supramolecular network (Fig. 5).
Rather than carboxylic acid inversion dimers, the packing for (II) features O-HÁ Á ÁBr hydrogen bonds as well as N-HÁ Á ÁBr and N-HÁ Á ÁO contacts ( Table 2). The bromide ion ( Fig. 6) is five-coordinated in an irregular geometry by four N-HÁ Á ÁBr and one O-HÁ Á ÁBr link arising from five different cations. The N1-H2NÁ Á ÁO2 interaction from the protonated -NH 3 + group to the C O bond of the carboxylic acid generates [010] C(7) chains of cations, with adjacent ions in the chain related by the 2 1 screw axis. When all the hydrogen bonds are considered together, the packing for (II) can be described as a three-dimensional supramolecular network of undulating (100) sheets of bromide ions alternating with the organic cations (Fig. 7). A notably short C-HÁ Á ÁO interaction (HÁ Á ÁO = 2.23 Å ), which reinforces the C(7) chain of cations, is also observed.

Figure 6
Environment of the bromide ion in the structure of (II) with N-HÁ Á ÁBr and O-HÁ Á ÁBr hydrogen bonds indicated by double-dashed lines. Symmetry codes: (i) x, y, z À 1; (ii) 2 À x, 1 À y, 1 À z; (iii) 2 À x, 1 2 + y, bond ( Fig. 8) from three cations and two water molecules. As in (II), an N1-H2NÁ Á ÁO2 hydrogen bond in (III) generates C(7) chains of cations propagating in [010] with adjacent ions related by the screw axis but an N1-H3NÁ Á ÁN2 interaction also occurs; by itself it leads to [100] C(5) chains with adjacent ions related by translation; together, (001) hydrogen-bonded sheets of cations arise. When the nitrate ion and water molecules are taken together, undulating hydrogen-bonded chains propagating in the [010] direction arise. Collectively, the packing in (III) (Fig. 9) can be described as alternating (001) slabs of nitrate anions + water molecules and organic cations arising from a three-dimensional supramolecular network of hydrogen bonds. The three structures feature weak aromaticstacking (Table 4). In each case, infinite stacks of molecules, with considerable slippage between adjacent benzene rings, arise: these stacks propagate in the [001], [001] and [100] directions for (I), (II) and (III), respectively. A crystallographic c-glide generates the stacks in (I) and (II), whereas in (III) adjacent molecules are related by simple translation.

Table 4
Aromaticstacking interactions in the title compounds.
All interactions involve the C1-C6 benzene rings. CgÁ Á ÁCg is the centroidcentroid separation, is the dihedral angle between the ring planes.
The fingerprint plot for the cation in (I) (Fig. 10) of outward (i.e. non-reciprocal) contacts shows three prominent features: the spike ending at (d i , d e ) = ($0.76, $1.36 Å ) and extending backwards corresponds to the short intermolecular HÁ Á ÁCl contacts associated with the N-HÁ Á ÁCl hydrogen bonds. The pronounced (0.65, 1.00 Å ) feature equates with the HÁ Á ÁO (donor) contact of the O-HÁ Á ÁO hydrogen bond and that at (1.00, 0.65 Å ) is associated with the HÁ Á ÁO (acceptor) contact. The fingerprint plot for the cation in (II) (Fig. 11) shows the equivalent three spikes ending at (0.76, 1.45), (0.72, 1.08) and (1.06, 0.72 Å ): the greater value of d e for the first of these presumably reflects the larger size of the bromide ion in (II) compared to the chloride ion in (I). The fingerprint plot for the cation in (III) (Fig. 12) naturally lacks the HÁ Á ÁX (X = Cl, Br) features and has a more symmetric appearance, with the spike at (0.68, 1.02) equating to HÁ Á ÁO (donor) and that at (1.08, 0.74 Å ) equating to the OÁ Á ÁH (acceptor) contact. The HÁ Á ÁN (donor) contact is just perceptible as a shoulder-like feature terminating at (0.94, 1.30 Å ) but mostly superimposed on the tail of the HÁ Á ÁO spike. The 'wing' like fingerprint plot for the research communications Acta Cryst. (2020). E76, 527-533 Mukombiwa and Harrison C 7 H 9 N 2 O 2 + ÁX, X = Cl À , Br À and NO 3 À ÁH 2 O 531 Figure 11 Hirshfeld fingerprint plot for the C 7 H 9 N 2 O 2 + cation in (II).

Figure 10
Hirshfeld fingerprint plot for the C 7 H 9 N 2 O 2 + cation in (I).

Figure 12
Hirshfeld fingerprint plot for the C 7 H 9 N 2 O 2 + cation in (III).
chloride ion in (I) (Fig. 13) looks radically different to that of the cation (Fig. 10) although the end-point at (1.38, 0.77 Å ) of the sweeping feature corresponds well with the HÁ Á ÁCl contact for the cation. The fingerprint plot for the bromide ion in (II) (Fig. 14) with its sweeping feature terminating at (1.45, 0.77 Å ) shows similar correspondence with the HÁ Á ÁBr spike for the cation in (II) (Fig. 11).  , 2000). Interestingly, the neutral, non-zwitterionic form of C 7 H 8 N 2 O 2 has been co-crystallized with an organo-rhenium compound and other species (DONDUH; Davies et al., 2014). In VODWIU, the -CO 2 À group accepts several N-HÁ Á ÁO hydrogen bonds while in DONDUH pairwise carboxylic-acid inversion dimers are formed. The different possible structures of the organic species are shown in Fig. 15.

Synthesis and crystallization
Equimolar mixtures of 3,4-diaminobenzoic acid and hydrochloric acid (I), hydrobromic acid (II) and nitric acid (III) dissolved in water were decanted into petri dishes at room temperature and brown plates of (I), pale-brown laths of (II) and colourless slabs of (III) formed as the water evaporated over the course of a few days. The IR spectra for 3,4-diaminobenzoic acid, (I), (II) and (III) are available as supporting information.

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 6. For each structure, the N-bound H atoms were located in difference maps and their positions were freely refined. The C-bound H atoms were geometrically placed (C-H = 0.95 Å ) and refined as riding atoms. The water 532 Mukombiwa and Harrison C 7 H 9 N 2 O 2 + ÁX, X = Cl À , Br À and NO 3 À ÁH 2 O Acta Cryst. (2020). E76, 527-533 research communications Figure 14 Hirshfeld fingerprint plot for the bromide anion in (II).

Figure 13
Hirshfeld fingerprint plot for the chloride anion in (I).

Figure 15
Different structures based on 3,4-diaminobenzoic acid: (a) neutral C 7 H 8 N 2 O 2 molecule as found in DONDUH; (b) zwitterion in VODWIU; (c) quinoid resonance form of the C 7 H 9 N 2 O 2 + cation in the title compounds (see text and compare scheme 1); (d) C 7 H 7 N 2 O 2 À anion as found in the metal complexes noted in the text.

2-Amino-5-carboxyanilinium chloride (I)
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.