Syntheses and crystal structures of 4-(4-nitrophenyl)piperazin-1-ium benzoate monohydrate and 4-(4-nitrophenyl)piperazin-1-ium 2-carboxy-4,6-dinitrophenolate

Two new salts of the 4-(4-nitrophenyl)piperazin-1-ium cation have been prepared by co-crystallization with aromatic carboxylic acids. The supramolecular assembly in the benzoate salt, which crystallizes as a mono-hydrate, is two dimensional, while that in the 2-carboxy-4,6-dinitrophenolate salt is three dimensional.


Structural commentary
In each of compounds (I) and (II) (Figs. 1 and 2), the piperazine ring adopts a chair conformation, with the ring-puckering angle (Cremer & Pople, 1975) calculated for the atom sequence (N11/C12/C13/N14/C15/C16) close to the ideal value of zero (Boeyens, 1978): = 6.42 (11) for (I) and 8.75 (11) for (II). However, in (I), the nitrophenyl substituent occupies an equatorial site, whereas in (II) this substituent occupies an axial site. In each compound, the N-nitrophenyl unit shows the pattern of distances typical of 4-nitroaniline derivatives, namely both C-N distances are short for their types (Allen et al., 1987), while the nitro N-O distances are long for their type. In addition, the distances C141-C142 and C141-C146 lie in the range 1.4049 (16) to 1.4132 (15) Å whereas the remaining C-C distances for this ring are smaller, falling in the range 1.3764 (17) to 1.3881 (15) Å . These variations are most simply interpreted in terms of some 1,4-quinonoid type bond fixation, moderated by the high electronegativity of the nitro group, generally regarded as similar to that of a fluoro substituent (Huheey, 1966;Mullay, 1985).
In the anion of compound (II), the C21-O21 distance, 1.2788 (13) Å is more typical of those in ketones than those in phenols (Allen et al., 1987); the distances C21-C22 and C21-C26, 1.4394 (15) and 1.4340 (15) Å are longer than the remaining C-C distances in the ring, which are in the range 1.3747 (15) to 1.3869 (15). These observations, taken together, indicate that the negative charge in this anion is delocalized over atoms C22-C26 rather than being localized on atom O21 (see Scheme).

Supramolecular features
In each of compounds (I) and (II), the supramolecular assembly involves a combination of O-HÁ Á ÁO, N-HÁ Á ÁO and C-HÁ Á ÁO hydrogen bonds, augmented in the case of (I) by a single C-HÁ Á Á(arene) hydrogen bond: however, aromaticstacking interactions are absent from both structures.
The supramolecular assembly in (I) is di-periodic and the formation of the sheet structure is readily analysed in terms of two mono-periodic sub-structures (Ferguson et al., 1998a,b;Gregson et al., 2000). Within the selected asymmetric unit for (I) (Fig. 1), the ionic components are linked by an asymmetric bifurcated (three-centre) N-HÁ Á Á(O,O) hydrogen bond (Table 1), while the water molecule is linked to the anion by an O-HÁ Á ÁO hydrogen bond. In one of the two sub-structures, a combination of one two-centre N-HÁ Á ÁO hydrogen bond and a second O-HÁ Á ÁO hydrogen bond links these threecomponent aggregates (Fig. 1) into a chain of rings running parallel to the [100] direction ( Fig. 3) in which there are two different types of R 4 6 (12) ring (Bernstein et al., 1995), centred at (n, 0.5, 0.5) and (n + 0.5, 0.5, 0.5), respectively, where n represents an integer in each case. The second sub-structure, which includes the C-HÁ Á ÁO hydrogen bond (Table 1, Fig. 4), takes the form of another chain of rings in which R 4 6 (12) rings centred at (n + 0.5, n + 0.5, 0.5) alternate with R 2 4 (10) rings The molecular structure of (II), showing hydrogen bonds (drawn as dashed lines) within the selected asymmetric unit. Displacement ellipsoids are drawn at the 50% probability level.

Figure 1
The molecular structure of (I), showing hydrogen bonds (drawn as dashed lines) within the selected asymmetric unit. Displacement ellipsoids are drawn at the 50% probability level. Table 1 Hydrogen-bond geometry (Å , ) for (I).
Cg1 is the centroid of the C21-C26 ring. generates a sheet structure lying parallel to (001). The single C-HÁ Á Á(arene) hydrogen bond (Table 1) lies within this sheet, and so has no influence on the dimensionality of the assembly.
The supramolecular assembly for compound (II), by contrast, is tri-periodic (three dimensional) and, as for (I), the formation of the framework is readily analysed in terms of simple sub-structures. Within the selected asymmetric unit (Fig. 2), there is an intramolecular O-HÁ Á ÁO hydrogen bond in the anion, and the hydroxyl H atom plays no part in the supramolecular assembly. The two independent components are linked by a very asymmetric bifurcated N-HÁ Á Á(O,O) hydrogen bond (Table 2), and a two-centre N-HÁ Á ÁO hydrogen bond links these ion pairs into a chain of rings running parallel to the [010] direction (Fig. 5). There are four C-HÁ Á ÁO hydrogen bonds in the structure of (II) and that involving atom C145 (   Hydrogen bonds are drawn as dashed lines and, for the sake of clarity, the H atoms bonded to those C atoms that are not involved in the motif shown have been omitted.

Figure 5
Part of the crystal structure of compound (II), showing the formation of a hydrogen-bonded chain of rings running parallel to [010]. Hydrogen bonds are drawn as dashed lines and, for the sake of clarity, the H atoms bonded to C atoms have all been omitted.

Figure 3
Part of the crystal structure of compound (I) showing the formation of a chain of hydrogen-bonded rings running parallel to the [100] direction. Hydrogen bonds are drawn as dashed lines and, for the sake of clarity, the H atoms bonded to C atoms have all been omitted. Table 2 Hydrogen-bond geometry (Å , ) for (II).  (Fig. 6). The two C-HÁ Á ÁO hydrogen bonds involving atoms C12 and C16 link inversion-related pairs of cations into a centrosymmetric motif containing R 2 2 (8) rings (Fig. 7), and the aggregates of this type are further linked by the final C-HÁ Á ÁO hydrogen bond, that involves atom C146, to form a complex chain of rings running parallel to the [001] direction (Fig. 8). The combination of hydrogen-bonded chains parallel to [010], [001] and [101] generates a three-dimensional network. We also note a fairly short nitro-nitro contact, 2.823 (4) Å , between atom O142 at (x, y, z) and atom N24 at (1 + x, y, 1 + z): this probably represents a dipolar attraction between negatively charged O and positively charged N atoms.
Part of the crystal structure of compound (II) showing the linkage of an inversion-related pair of cations by two independent C-HÁ Á ÁO hydrogen bonds, drawn as dashed lines. For the sake of clarity, the anions, the H atoms bonded to those C atoms that are not involved in the motif shown, and the unit-cell outline have been omitted. The atoms marked with an asterisk (*) are at the symmetry position (1 À x, 1 À y, 2 À z).

Figure 6
Part of the crystal structure of compound (II) showing the formation of a chain of hydrogen-bonded rings running parallel to the [101] direction. Hydrogen bonds are drawn as dashed lines and, for the sake of clarity, the H atoms bonded to those C atoms that are not involved in the motif shown have been omitted.

Figure 8
Part of the crystal structure of compound (II) showing the formation of a chain of hydrogen-bonded rings running parallel to the [001] direction. Hydrogen bonds are drawn as dashed lines and, for the sake of clarity, the H atoms bonded to those C atoms that are not involved in the motif shown have been omitted.

Database survey
The first structure report on a salt of N-(4-nitrophenyl)piperazine concerned the chloride salt, which crystallizes as a monohydrate (Lu, 2007); despite the presence of hydrogen bonds of N-HÁ Á ÁO, N-HÁ Á ÁCl and O-HÁ Á ÁCl types, the supramolecular assembly is only mono-periodic. The structures of six salts of N-(4-nitrophenyl)piperazine with aromatic carboxylic acids have recently been reported (Mahesha et al., 2022): in all but one of these, the supramolecular assembly is mono-periodic, although it is di-periodic in the 4-ethoxybenzoate salt. This may be contrasted with the triperiodic assembly found here for compound (II).

Synthesis and crystallization
For the preparation of compounds (I) and (II), a solution of N-(4-nitrophenyl)piperazine (100 mg, 0.483 mmol) in methanol (10 ml) was mixed with a solution of either benzoic acid (59 mg, 0.483 mmol) for (I) or 3,5-dinitrosalicylic acid (110 mg, 0.483 mmol) for (II) in methanol/ethyl acetate (1:1 v/v, 20 ml). The solutions of the base and the corresponding acid were mixed, stirred at ambient temperature for 15 min, and then set aside to crystallize at ambient temperature and in the presence of air. After one week, crystals suitable for single-crystal X-ray diffraction were collected by filtration and dried in air: compound (I), pale yellow, m.p. 410-413 K; compound (II), orange, m.p. 446-448 K.

Refinement
Crystal data, data collection and refinement details are summarized in Table 3. All H atoms were located in difference maps. The H atoms bonded to C atoms were then treated as riding atoms in geometrically idealized positions with C-H distances of 0.95 Å (aromatic) or 0.99 Å (CH 2 ), and with U iso (H) = 1.2U eq (C). For the H atoms bonded to N or O atoms, the atomic coordinates were refined with U iso (H) = 1.2U eq (N) or 1.5U eq (O), giving the N-H and O-H distances shown in Tables 1 and 2.

Funding information
APEX3 (Bruker, 2016); program(s) used to solve structure: SHELXT2014/5 (Sheldrick, 2015a); program(s) used to refine structure: SHELXL2014 (Sheldrick, 2015b); molecular graphics: PLATON (Spek, 2020); software used to prepare material for publication: SHELXL2014 (Sheldrick, 2015b) and PLATON (Spek, 2020). 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 N11 0.22281 (16)  C16 Hydrogen-bond geometry (Å, º) Cg1 is the centroid of the C21-C26 ring.  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 N11 0.30883 (