A new (monohydrate) form of 3,5-dicarboxy-anilinium nitrate: crystal structure and Hirshfeld surface analysis

The title compound, C8H8NO4
 +·NO3
 −·H2O, crystallizes in the same space group (P21/c) as the previously reported dihydrate form [Liang & Zhu (2010). Acta Cryst. E66, o667], but with two formula units per asymmetric unit instead of one. In the crystal, the components are linked into a three-dimensional network by classical intermolecular O—H...O and N—H...O hydrogen bonds and π–π stacking interactions. A Hirshfeld surface (HS) analysis indicated that the most important contributions to the crystal packing are from H...O/O...H (52.4%), H...H (13.9%) and C...C (11.2%) for one cation and H...O/O...H (46.3%), H...H (20%) and O...C/C...O (10.6%) for the other.


Chemical context
The amphoteric 5-aminoisophtalic acid (5-AIP) has a well known ability to form supramolecular assemblies with metal ions (Xin et al., 2021;Luo et al., 2011). As a result, it can operate like nodes similar to natural amino acids (Singh et al., 2019) (Fig. 1). In addition, 5-AIP may self-assemble as a result of many hydrogen-bonding patterns. It forms salts with a Brønsted acid or base and its structural characteristics enable it to take on a variety of ionic forms (Nath & Baruah, 2012;McGuire et al., 2016). Herein, we report on the synthesis and crystal structure of a new 3,5-dicarboxyanilinium nitrate hydrate, (I).

Structural commentary
Compound (I) differs from the previously reported crystal form of 3,5-dicarboxyanilinium nitrate (Liang & Zhu, 2010) by containing one water molecule per formula unit, instead of two. The asymmetric unit comprises two formula units, i.e., two 3,5-dicarboxylanilinium cations (A and B), two nitrate anions (A and B) and two water molecules (Fig. 2a). All bond distances and angles fall within normal ranges as compared to similar molecules (Wang & Zhang, 2006;Dobson & Gerkin, 1998;Nath & Baruah, 2012;Singh et al., 2019;Cai et al., 2020). The cations have similar conformations that differ mainly in the opposite orientations of one carboxylic group, as seen by the torsion angles C5-C4-C8-O3 of 6.0 (2) in cation A and À178.43 (18) in cation B. Mogul (Bruno et al., 2004) based on the Cambridge Structural Database (version 2022.2.0; Groom et al., 2016), indicated the single character of the C6-N1 bonds [1.457 (2) Å for A and 1.462 (2) Å for B], which have lengths close to the median of the 2198 found fragments of the same chemical nature. The C O double bonds in the carboxylic groups [1.202 (2) to 1.241 (2) Å ] are shorter than the C-O single bonds [1.285 (2) to 1.322 (2) Å , Table 1]. The planarity of the cations varies slightly: the dihedral angles between the carboxylic group planes (C1, O1, O2) and (C8, O3, O4) and the ring plane are 7.85 (9) and 5.90 (9) , respectively, in cation A, 5.93 (2) and 2.68 (2) in cation B; all non-hydrogen atoms are coplanar within 0.083 Å in cation A and 0.052 Å in B.

Hirshfeld surface analysis
In order to visualize and quantify the intermolecular interactions in compound (I), we carried out a Hirshfeld surface (HS) analysis (Spackman & Jayatilaka, 2009) using Crystal-Explorer21 (Spackman et al., 2021) and the associated twodimensional fingerprint plots (McKinnon et al., 2007) mapped in color with a normalized contact distance, d norm , varying from red through white to blue depending on the distances compared to the sum of the van der Waals radii. The Hirshfeld surfaces mapped over d norm , were calculated separately for cations A and B using a standard high surface resolution (Fig. 6a). The red spots correspond to contacts shorter than the van der Waals radii sum of the closest atoms and relate to the presence of O-HÁ Á ÁO and N-HÁ Á ÁO hydrogen bonds in the crystal structure, whereas the faint-red spots (highlighted by red circles for clarity) represent weaker C-HÁ Á ÁO interactions. The presence of characteristic red and blue triangles on the shape-index surface ( Fig. 6b) clearly suggest the presence ofinteractions between the neighboring organic cations and the curvedness plots (Fig. 6c) show flat surface patches characteristic of planar stacking. The overall two-dimensional fingerprint plot and those delineated into OÁ Á ÁH/HÁ Á ÁO, HÁ Á ÁH, CÁ Á ÁC, OÁ Á ÁC/CÁ Á ÁO, OÁ Á ÁO and CÁ Á ÁH/HÁ Á ÁC contacts for cations A and B are shown in Fig. 7 and their relative contributions to the HS are illustrated graphically in Fig. 8 Partial packing of (I) showing the ribbons formed by alternating R 6 6 (26) and R 8 8 (34) ring motifs.

Figure 5
Part of the crystal structure of (I) showing thestacking interactions, which appear parallel to the a axis.

Figure 6
The Hirshfeld surfaces of the organic cations A and B mapped over: (a) d norm in the range À0.7489 to 1.2298 a.u., (b) shape-index and (c) curvedness.

Database survey
The Cambridge Structural Database (Version 2022.2.0 updated to June 2022; Groom et al., 2016), was searched for structures with carboxyl-carboxyl R 2 2 (8) graph-set motifs using ConQuest (Bruno et al., 2002) for all searches, and filters were applied to ensure that only organic compounds and nondisordered molecules were included. In addition, the searches were also limited to structures with low R-factor values (R < 0.05). The results of the searches were analyzed using Mercury (CSD Version 2022.2.0; Macrae et al., 2020).
The geometries of O-HÁ Á ÁO hydrogen bonds from an analysis of 2883 crystal structures deposited in the CSD are illustrated in Fig. 9. The relationship between the HÁ Á ÁO distances and O-HÁ Á ÁO angles is shown as a two-dimensional plot, the OÁ Á ÁO distances being indicated by the color of the data points. The angle tends to increase as the OÁ Á ÁO and HÁ Á ÁO distances decrease. The greatest density of observed hydrogen bonds occurs in the range of 1.3-1.9 Å for the HÁ Á ÁO distance, 2.6-2.8 Å for the OÁ Á ÁO distances (indicated by green data points) and 160-180 for the O-HÁ Á ÁO angle.

Synthesis and crystallization
5-Aminoisophthalic acid (0.181 g, 1 mmol) dissolved in methanol (10 mL) was added under stirring to a methanolic solution of Er(NO 3 ) 3 Á5H 2 O (0.110 g, 0.25 mmol). After several minutes of stirring, a brighter orange precipitate appeared and was filtered. After slowly evaporating the filtrate over one week, colorless single crystals of the title compound suitable for X-ray diffraction analysis were isolated.

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.