Hydrazinium 2-amino-4-nitrobenzoate dihydrate: crystal structure and Hirshfeld surface analysis

In the title salt dihydrate, the conrotatory relationship between the carboxylate and nitro groups of the anion leads to a dihedral angle between them of 26.73 (15)°. Substantial charge-assisted water-O—H⋯O(carboxylate) hydrogen bonding leads to supramolecular zigzag chains. These are connected into a three-dimensional architecture by N—H⋯O and N—H⋯N hydrogen bonds.

In the anion of the title salt hydrate, H 5 N 2 + ÁC 7 H 5 N 2 O 4 À Á2H 2 O, the carboxylate and nitro groups lie out of the plane of the benzene ring to which they are bound [dihedral angles = 18.80 (10) and 8.04 (9) , respectively], and as these groups are conrotatory, the dihedral angle between them is 26.73 (15) . An intramolecular amino-N-HÁ Á ÁO(carboxylate) hydrogen bond is noted. The main feature of the crystal packing is the formation of a supramolecular chain along the b axis, with a zigzag topology, sustained by charge-assisted water-O-HÁ Á ÁO(carboxylate) hydrogen bonds and comprising alternating twelve-membered {Á Á ÁOCOÁ Á ÁHOH} 2 and eight-membered {Á Á ÁOÁ Á ÁHOH} 2 synthons. Each ammonium-N-H atom forms a charge-assisted hydrogen bond to a water molecule and, in addition, one of these forms a hydrogen bond with a nitro-O atom. The amine-N-H atoms form hydrogen bonds to carboxylate-O and water-O atoms, and the amine N atom accepts a hydrogen bond from an amino-H atom. The hydrogen bonds lead to a three-dimensional architecture. An analysis of the Hirshfeld surface highlights the major contribution of OÁ Á ÁH/ HÁ Á ÁO hydrogen bonding to the overall surface, i.e. 46.8%, compared with HÁ Á ÁH contacts (32.4%).

Chemical context
The present structure determination of the title salt dihydrate, [NH 2 NH 3 ][O 2 C 6 H 4 NO 2 -4]Á2H 2 O (I), is a continuation of ongoing structural studies of the relatively unexplored chemistry of 2-amino-4-nitrobenzoic acid. This acid carries several groups capable of hydrogen bonding, viz. carboxylic/ carboxylate, amino and even nitro, and is anticipated to form crystals with significant hydrogen-bonding interactions, in both its neutral and deprotonated forms. Beyond the structure determination of several polymorphs of the parent structure (Wardell & Tiekink, 2011;) and its 1:1 co-crystal with bis(pyridin-2-yl)methanone and 2:1 co-crystal with 2-amino-4-nitrobenzoic acid (Wardell & Tiekink, 2011), all other investigations have been of deprotonated forms of the acid. Thus, the anion has been found coordinating in the carboxylate-O, amino-N mode towards Pb II in the coordination polymer catena-[bis( 2 -2-amino-4-nitrobenzoato)lead(II)] (Chen & Huang, 2009), with the remaining literature structures being salts. These are either alkali metal salts, i.e. Na + , K + (Smith, 2013), Rb + (Smith, 2014a) and Cs + (Smith & Wermuth, 2011), or are ammonium salts, as discussed below. Herein, the crystal and molecular structures of (I) are described along with an evaluation of its Hirshfeld surface.

Structural commentary
The molecular structures of the constituents of (I) are shown in Fig. 1; the asymmetric unit comprises one hydrazinium cation, one 2-amino-4-nitrobenzoate anion and two water molecules of crystallization. In all-organic structures, when protonated in crystals, hydrazine is ten times more likely to be present as a mono-protonated hydrazinium cation rather than in its the diprotonated form, i.e. hydrazine-1,2-diium di-cation (Groom et al., 2016); when non-organic structures are also considered, this ratio increases to 20:1. The confirmation of the mono-protonation in (I) is found in the pattern of intermolecular interactions, in particular in the observation that the amine-N4 atom accepts a hydrogen bond (see below). The N3-N4 bond length in (I) is 1.4492 (15) Å . The assignment of deprotonation of 2-amino-4-nitrobenzoic acid during cocrystallization is readily adduced in the near equivalence of the carboxylate C-O bond lengths, i.e. C7-O1 = 1.2579 (15) and C7-O2 = 1.2746 (15) Å . While there is an intramolecular amino-N-HÁ Á ÁO(carboxylate) hydrogen bond, Table 1, a significant twist of the carboxylate group with respect to the benzene ring to which it is connected is noted, as evidenced in the value of the C2-C1-C7-O1 torsion angle of 18.83 (17) . With respect to the nitro group, this is also twisted but, to a lesser extent: the O3-N2-C4-C3 torsion angle is 7.53 (16) . The terminal groups are conrotatory, forming a dihedral angle of 26.73 (15) .

Supramolecular features
As expected from the chemical composition of (I), there are a number of conventional hydrogen-bonding interactions in the crystal, involving all possible hydrogen-bond donors and acceptors, Table 1. These sustain a three-dimensional architecture. A view of the interactions involving the hydrazinium cation is shown in Fig. 2a. Each of the ammonium-N3-H atoms forms a charge-assisted hydrogen bond to a water molecule, with the HN4 atom also forming a hydrogen bond to a nitro-O4 atom indicating that the HN4 atom is bifurcated [i.e.: N-HÁ Á Á(O,O)]. The amine-N4-H atoms form a hydrogen bond to a carboxylate-O1 atom and to a water molecule and at the same time accept a hydrogen bond from an amino-H atom, this being the only N-HÁ Á ÁN hydrogen bond in the structure; the second amino-H atom forms an intramolecular hydrogen bond with the carboxylate-O1 atom, as mentioned above. Each of the water-H atoms forms a charge-assisted hydrogen bond with a carboxylate-O atom, leading to a zigzag supramolecular chain aligned along the b axis, as shown in Fig. 2b. The chain comprises alternating twelve-membered {Á Á ÁOCOÁ Á ÁHOH} 2 and eight-membered {Á Á ÁOÁ Á ÁHOH} 2 synthons. As shown in Fig. 2c, two of the ammonium-N3-H atoms bridge water molecules in the chain shown in Fig. 2b to form a non-symmetric, eight-membered {Á Á ÁHNHÁ Á ÁOHÁ Á ÁOÁ Á ÁHO} synthon while the amine-H atoms provide a second bridge between water-and carboxylate-O atoms to form a ten-membered {Á Á ÁHNHÁ Á ÁOHÁ Á ÁOÁ Á ÁHOHÁ Á ÁO} synthon. Further hydrogen bonds to water molecules leads to the formation of additional synthons, i.e. ten-membered {Á Á ÁHNNHÁ Á ÁO} 2 and eightmembered {Á Á ÁHNHÁ Á ÁO} 2 . A view of the unit-cell contents is shown in Fig. 2d. In addition to the above, (phenyl)-(phenyl) interactions are noted between inversion-related rings with the inter-centroid separation being 3.6190 (8) Å [symmetry operation 1 À x, Ày, 1 À z]. The molecular structures of the asymmetric unit of (I), showing displacement ellipsoids at the 70% probability level. Table 1 Hydrogen-bond geometry (Å , ). (14) 175 (2) 4. Hirshfeld surface analysis

D-HÁ
The Hirshfeld surface analysis of (I) provides additional insight into its molecular packing and was performed in accord with a recent study of related ammonium salts . The Hirshfeld surface mapped over electrostatic potential in Fig. 3 highlights the positive potential (blue region) around the hydrazinium cation and the negative potential (red) about the carboxylate-oxygen atoms of the nitrobenzoate anion. The numerous bright-, diminutive-and faint-red spots appearing on the Hirshfeld surface mapped over d norm in Fig. 4 are indicative of the variety of intermolecular interactions in the crystal. The pair of chargeassisted water-O-HÁ Á ÁO(carboxylate) hydrogen bonds between the water-O-H2W and -H4W atoms and carboxylate-O1 and -O2 atoms are evident through the brightred spots appearing near the respective donor and acceptor atoms, Fig. 4a. The donors of these interactions appear as light-blue spots near the water O-H atoms and the acceptors as red regions surrounding carboxylate-O1 and -O2 atoms on the Hirshfeld surface mapped over electrostatic potential in Fig. 3. The two pairs of bright-red spots near each water-O1W and -O2W atoms, and near the hydrazinium-H3N, H4N, H5N and H7N atoms in Fig. 4b are indicative of the hydrazinium-N-HÁ Á ÁO(water) hydrogen bonds. In the same way, the amine-N4-H6NÁ Á ÁO1 hydrogen bond is also viewed as a pair of bright-red spots near these atoms in Fig. 4b Two views of the Hirshfeld surface for (I) mapped over the electrostatic potential over the range À0.214 to +0.341 au; the red and blue regions represent negative and positive electrostatic potentials, respectively. HÁ Á ÁO hydrogen bonds compared to those just described, is viewed as the diminutive red spot in Fig. 4a. The presence of faint-red spots near the phenyl-C2-C4 atoms in Fig. 4b indicate their participation in edge-to-edge overlap with a symmetry-related phenyl ring, as seen in the short interatomic CÁ Á ÁC contacts listed in Table 2. In addition to above intermolecular interactions, the crystal also features short interatomic CÁ Á ÁO/OÁ Á ÁC and NÁ Á ÁO/OÁ Á ÁN contacts,   (15) x, 1 + y, z H5Á Á ÁO4 2.61 1 À x, À1 À y, 1 À z H2NÁ Á ÁO4 2.652 (15) x, 1 + y, z N1Á Á ÁO4 3.0205 (15) x, 1 + y, z C7Á Á ÁO3 3.1231 (17) -x, Ày, 1 À z C2Á Á ÁC4 3.2936 (19) -x, Ày, 1 À z C3Á Á ÁC3 3.3235 (19) -x, Ày, 1 À z Figure 4 Two views of the Hirshfeld surface for (I) mapped over d norm over the range À0.352 to 1.156 au.   Fig. 5a and 5b, respectively. The overall two-dimensional fingerprint plot for (I) and those delineated (McKinnon et al., 2007) into OÁ Á ÁH/HÁ Á ÁO, HÁ Á ÁH, CÁ Á ÁC, CÁ Á ÁH/HÁ Á ÁC, CÁ Á ÁO/OÁ Á ÁC and NÁ Á ÁO/OÁ Á ÁN contacts are illustrated in Fig. 6a-g, respectively; their relative contributions to the Hirshfeld surfaces are summarized in Table 3. It is important to note that the most significant contribution to the Hirshfeld surface in (I) comes from OÁ Á ÁH/ HÁ Á ÁO contacts, i.e. 46.8%, due to the involvement of all the acidic hydrogen atoms in hydrogen bonds, mainly to oxygen, many of which are charge-assisted. Reflecting this dominance, sharp spikes are evident in the fingerprint plot delineated into OÁ Á ÁH/HÁ Á ÁO contacts shown in Fig. 6b. The pair of green spikes have their tips at d e + d i $1.9 Å and extend linearly up to d e + d i $2.3 Å . The points merged within the plot up to d e + d i $2.7 Å indicate the presence of short interatomic OÁ Á ÁH/ HÁ Á ÁO contacts, Table 2. The extensive hydrogen bonding is the cause of the relatively small percentage contribution to the Hirshfeld surface from HÁ Á ÁH contacts, i.e. 32.4%, Fig. 6c, as relatively few hydrogen atoms are available to form interatomic contacts. The pair of tips at d e + d i $2.3 Å in the mirror-reflected saw-tooth distribution are due to short interatomic HÁ Á ÁH contacts involving water-and hydraziniumhydrogen atoms, Table 2. The distributions of points in the fingerprint plot delineated into CÁ Á ÁC contacts, shown in Fig. 6d, represents twostacking interactions. In the first of these, the symmetry-related phenyl rings have a face-to-face overlap to give the arrow-like distribution in lower (d e , d i ) region at around d e = d i = 1.6 Å . This interaction is also seen as the flat region appearing about the phenyl ring on the Hirshfeld surface mapped over curvedness, shown in Fig. 7. The otherstacking interaction involves edge-to-edge overlap through short interatomic CÁ Á ÁC contacts involving the C2-C4 atoms, Fig. 4b and Table 2, and is viewed as the arrow-like distribution of points around d e = d i = 1.8 Å , i.e. adjacent to first arrow-like distribution. Even though CÁ Á ÁH/ HÁ Á ÁC contacts have a significant contribution to the Hirshfeld surface, i.e. 5.9%, as seen from the fingerprint plot in Fig. 6e, the interatomic separations are much greater than sum of their van der Waals radii and hence do not appear to have influence on the molecular packing. The presence of short interatomic CÁ Á ÁO/OÁ Á ÁC and NÁ Á ÁO/OÁ Á ÁN contacts in the crystal, Table 2, is also evident from the small but significant contributions of 3.3 and 1.3%, respectively, to the Hirshfeld surfaces and appear as pairs of forceps-like tips, Fig. 6f, and conical tips, Fig. 6g, at d e + d i $3.1 Å in their respective fingerprint plots. The small contributions from the other interatomic OÁ Á ÁO, CÁ Á ÁN/NÁ Á ÁC, NÁ Á ÁN and NÁ Á ÁH/HÁ Á ÁN contacts listed in Table 2 have a negligible effect on the packing in the crystal.

Database survey
In the Chemical context section above, it was indicated that in the crystallographic literature there are several ammonium salts of 2-amino-4-nitrobenzoate anions. The ammonium cations range from the simple ammonium cation (Smith, 2014b) to R 2 NH 2 , i.e. R = Me, n-Bu , Cy (Smith et al., 2004) and R 2 = (CH 2 CH 2 ) 2 O (Smith & Lynch, 2016). More exotic examples of ammonium cations are found with [(H 2 N) 2 C NH 2 ] + , i.e. guanidinium (Smith et al., 2007) and the dication, [H 3 NCH 2 CH 2 NH 3 ] 2+ (Smith et al., 2002). Key geometric data for these are collated in Table 4. From these data it is apparent that the dihedral angle formed between the the carboxylate group and benzene ring in (I) is at the upper end of structures included in Table 4, and in the same way, the angle between the nitro group and benzene ring is in the upper range of comparable angles. Given that the relationship between the carboxylate and nitro groups in (I) is conrotatory, the dihedral angle between these groups in (I), at 26.73 (14) , is the greatest among the series.

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 5. Carbon-bound H-atoms were placed in calculated positions (C-H = 0.95-0.99 Å ) and were included in the refinement in the riding-model approximation, with U iso (H) set to 1.2U eq (C). The O-and N-bound H atoms were located from difference maps, but refined with O-H = 0.84AE0.01 Å and U iso (H) = 1.5U eq (O), and with N-H = 0.86-0.88AE0.01 Å and U iso (H) = 1.2U eq (N), respectively. Owing to poor agreement, two reflections, i.e. (202) and (212) (Farrugia, 2012) and DIAMOND (Brandenburg, 2006); software used to prepare material for publication: publCIF (Westrip, 2010). where P = (F o 2 + 2F c 2 )/3 (Δ/σ) max < 0.001 Δρ max = 0.29 e Å −3 Δρ min = −0.30 e Å −3 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.