Crystal structure and Hirshfeld surface analysis of the hydrated 2:1 adduct of piperazine-1,4-diium 3,5-dinitro-2-oxidobenzoate and piperazine

In the title adduct, two 3,5-dinitrosalicylic acid molecules in the dianionic (DNSA2−) form, two protonated piperazine-1,4-diium cations (PIP2+) and a neutral piperazine molecule (PIP) along with two water molecules are found in the asymmetric unit. The crystal structure of the title adduct is reported, and hydrogen-bonding interactions are discussed.


Structural commentary
The title compound crystallizes in the triclinic space group P1 with Z = 1 with the asymmetric unit comprising two DNSA 2À ions, two protonated piperazine-1,4-diium cations, and a neutral piperazine molecule along with two water molecules and having the formula 2C 4 H 12 N 2 2+ Á2C 7 H 2 N 2 O 7 2À Á-C 4 H 10 N 2 Á2H 2 O. The atom-numbering scheme and molecular structure of (I) are shown in Fig. 1. The distance between the phenolate oxygen atom, O7, and the carboxylate oxygen atom, O6, in the anion is 2.770 (2) Å and is comparable to that found earlier reported dianionic salts (2.735-2.912 Å ). This, together with the absence of a locateable H atom between these oxygen atoms (O6 and O7) is good evidence for the existence of the dianion in this adduct. One of the nitro groups (N1, O1, and O2) and the phenolate oxygen atom, O7 are coplanar with the mean plane of the phenyl ring, while the second nitro group (N2, O3, and O4) and the carboxylate group (C7, O5, and O6) are slightly twisted from the above plane. These twists are measured by the dihedral angles between the mean plane of the phenyl ring and those of the second nitro and carboxylate groups of 19.4 (3) and 24.4 (3) , respectively, and by the C2-C3-N2-O4 and C6-C5-C7-O6 torsion angles of 161.1 (2) and 156.6 (2) , respectively. These slight twists of the nitro and carboxylate groups are due to the differences in the intermolecular hydrogen-bonding patterns in which the N2/O3/O4 and C7/O6/O7 groups participate as compared to the N1/O1/O2 group.
The piperazine rings in the piperazine-1,4-diium cations and the neutral piperazine molecule in (I) adopt chair conformations with puckering parameters (Cremer & Pople, 1975) Additionally, we carried out a structural overlay study of the DNSA 2À units in the di-anionic salts found in (I), EGUVAD (Smith et al., 2002), ROFLIJ (Gao et al., 2014), ZONBIP  and XEBFAM (Smith et al., 2006) using the six carbon atoms in the phenyl ring in DNSA 2À as the basis. The DNSA 2À units in all five structures overlay quite well with one another. The maximum r.m.s.d. observed between any molecular pair is 0.0095 Å (for ROFLIJ and ZONBIP). However, the slight rotation of the nitro and carboxylate groups in the DNSA 2À unit (Fig. 2) may well be due to the oxygen atoms in these functional groups participating in different intermolecular interactions in their crystal structures as noted above.

Hirshfeld surface analysis
Crystal Explorer 17.5 (Turner et al., 2017) was used to calculate the Hirshfeld surfaces (HS; McKinnon et al., 1998McKinnon et al., , 2004 of the title adduct and generate two-dimensional fingerprint plots (full and decomposed, 2D-FP; Spackman & McKinnon, 2002;Spackman & Jayatilaka, 2009). The HS and 2D-FP were used to provide additional information and to quantify the intermolecular interactions using distinct colours and intensities to indicate short and long contacts, as well as the relative contribution of the different interactions in the solid-state (Venkatesan et al., 2015(Venkatesan et al., , 2016. The HS is plotted over d norm in the range À0.7438 to 1.3459 a.u. and two views (front and back) of the HS are shown in Fig. 6. Bright-red spots on the HS confirm the existence of hydrogen-bonding contacts in the [198][199][200][201][202] research communications Table 1 Hydrogen-bond geometry (Å , ).

Figure 4
Part of the crystal structure of (I) showing the molecular chain, formed by O8-H8CÁ Á ÁN4 and N5-H5AÁ Á ÁO8 hydrogen bonds, which propagates parallel to the b axis.  (a) Part of the crystal structure of (I) showing the R 2 2 (8) and R 2 1 (6) motifs formed by intermolecular N-HÁ Á ÁO and C-HÁ Á ÁO hydrogen bonds (see Table 1), which link the neighbouring anionic unit (DNSA 2À ) and cationic moiety (PIP 2+ ) into a molecular chain, which propagates parallel to the c axis. (b) Part of the crystal structure of (I) showing the sheet-like architecture.
crystal structure. The 2D FP plots show that the relative contributions of the various non-covalent contacts (Fig. 7). OÁ Á ÁH contacts contribute most (50.2%) to the crystal packing while the second significant contact is HÁ Á ÁH, which contributes 36.2%. The relative contributions of CÁ Á ÁO, CÁ Á ÁH, NÁ Á ÁH, CÁ Á ÁN and CÁ Á ÁC contacts are 4.6%, 2.9%, 2.7%, 1.7% and 1.0%, respectively. In the XEBFAM structure, the relative contributions of OÁ Á ÁH, HÁ Á ÁH, CÁ Á ÁO, CÁ Á ÁH, NÁ Á ÁH, CÁ Á ÁN and CÁ Á ÁC contacts are 49.7%, 37.6%, 3.6%, 2.7%, 1.3%, 0.7%, and 2.1%, respectively. The relative contribution of various interatomic contacts in XEBFAM and the title adduct (I) are similar, even though the compounds have different compositions as discussed earlier.  Kumar et al., 1999). For monoanions of DNSA, a total of 62 structures containing the carboxylate (COO À ) moiety and 70 containing the phenolate anion (O À ) were found. As mentioned earlier, the removal of the carboxylic acid proton is expected to be easier than the removal of the proton from the phenolic -OH group in DNSA so it is somewhat surprising that the number of crystal structures containing phenolate ions is larger than those containing carboxylate ions. These seemingly conflicting results may suggest that the formation and stability of the salts with phenolate ions of the DNSA moiety is governed by intermolecular interactions in the crystal. However, it has been pointed out (Fá bry, 2018), that since the monoanions generally contain a hydrogen atom bridging between the the carboxylate and phenolate oxygen atoms, how one formulates the anion (carboxylate or phenolate) depends critically on how this hydrogen atom is treated in the refinement so that some of the reported phenolate structures may actually be carboxylates.

Database survey
As

Synthesis and crystallization
The title adduct was synthesized from 3,5-dinitrosalicylic acid (1 mmol, 228 mg) and piperazine (5 mmol, 426 mg, 0.5 mL) Crystal packing of (I), (a) viewed along the a axis and (b) viewed along the c axis.

Figure 6
Two different views of the Hirshfeld surface of (I).
dissolved in 50 mL of methanol and stirred well for 6 h. The homogeneous solution was filtered and the solution was allowed to evaporate slowly at room temperature. Red blocklike crystals suitable for single X-ray diffraction were harvested after a growth period of 10 days.

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.