Crystal structure of 4-(2-methoxyphenyl)piperazin-1-ium 3,5-dintrosalicylate

The different intra and intermolecular hydrogen-bonding interactions in the crystal structure of the title salt are discussed.

As a continuation of our earlier study on the crystal structure and supramolecular analysis of a monohydrated 1:1 adduct of bis(piperazine-1,4-diium), 3,5-dinitro-2-oxidobenzoate and piperazine, we have now investigated the crystal structure of 1-(2-methoxyphenyl) piperazinium 3,5-dinitrosalicylate (I). In this study, the crystal structure, Hirshfeld surface (HS) analysis, structural features and various intermolecular interactions that exist in the title protonated salt are reported.
In the DNSA molecule, deprotonation of the -COOH group (pK COOH = 2.2) is easier than that of the phenolic -OH group (pK OH = 6.8). 62 carboxylate moiety structures (COO À ) and 70 phenolate anion structures (O À ) were found in a search of the Cambridge Structural Database (CSD, Version 5.43, update of March 2020;Groom et al., 2016), which is perhaps unexpected because the number of crystal structures containing phenolate anions is larger than those containing carboxylate anions. These conflicting results may suggest that the formation of protonated salts of the DNSA molecule with phenolate ions is favoured by the thermodynamic stability and the intermolecular interactions between the phenolate anion and counter-ions in the respective crystal structures. The crystal structure of (I) suggests that the title salt was formed by deprotonation of the phenolic group in the DNSA molecule. In order to better understand the deprotonation of the phenolic group in DNSA molecule, the H-atom electron density in the difference-Fourier electron-density maps was calculated as they can yield additional insight into the proton-transfer behaviour. From Fig. 2, the electron density associated with atom H6 is shown to be smeared out between the O6 and O7 atoms, with the maximum lying closer to O6 atom than O7. It suggests that the H6A atom is attached to the carboxylic acid group and that deprotonation occurs through the phenolic group. As a result, the strong intramolecular O6-H6AÁ Á ÁO7 hydrogen bond formed. The interatomic distance between the phenolate oxygen atom, O7, and the O6 atom in the carboxylic acid group is 2.448 (2) Å , which also indicates that the strong intramolecular hydrogen bond between the O6 and O7 atoms. Similar types of intramolecular hydrogen bonds were observed in salicylic acid with a distance of 2.62 Å (Woiń ska et al., 2016; Montis & Hursthouse et al., 2012) and in other proton-transfer salts of DNSA in the range 2.409-2.540 Å (Smith et al., 1995(Smith et al., , 1996(Smith et al., , 1997(Smith et al., , 2000(Smith et al., , 2001a(Smith et al., ,b,c,d,e, 2002(Smith et al., , 2006  Difference-Fourier electron-density map showing the electron density associated with the H atom involved in the O6-H6Á Á ÁO7 hydrogen bond.

Figure 1
The molecular structure of the title molecular salt, (I), showing the atomlabelling scheme. Displacement ellipsoids are drawn at the 50% probability level.

Figure 6
Overall packing diagram for the title salt (I) into a layered structure propagating parallel to the b axis (Fig. 5). Of the above three N-HÁ Á ÁO interactions [N3-H3AÁ Á Á(O5,O6), and N3-H3BÁ Á ÁO7], the N3-H3BÁ Á ÁO7 interaction is stronger [DÁ Á ÁA = 2.787 (3) Å ] than the other two, which is due to the fact that two charged components are involved in this interaction, i.e. the phenolate O7 atom in DNSA À1 and the protonated N3-H3B unit in 2MeOPP +1 . All of the above interactions facilitate the arrangement of the DNSA 1À ions in a layered molecular structure. The top and bottom sides of the DNSA 1À layers are stabilized by the two adjacent cationic layers. As a result, a sandwich-like arrangement is observed. Briefly, the layered DNSA 1À units form the core with the top and bottom sides of the cation layers arranged facing. An overall packing diagram is shown Fig. 6.

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., , 2004Spackman & Jayatilaka, 2009) of the title protonated salt and generate two-dimensional fingerprint plots (full and decomposed, 2D-FP; Spackman & McKinnon, 2002) in order to investigate and quantify the different intermolecular interactions. Distinct colours and intensities 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. Two views of the HS mapped with d norm in the range À0.6295 to 1.3240 a.u. (front and back) are shown in Fig. 7. Bright red spots on the surface near O2, O3, O4A, O7, O6, H10B and H3B suggest that these atoms participate in hydrogen-bonding interactions (see Table 1). No significant pattern of convex blue and concave red triangles are observed in the shapeindex (SI) diagram, indicating the absence of -stacking interactions in the title salt. The 2D-FP plots show the relative contributions of the various non-covalent contacts (Fig. 8 (Fig. 8).

Database survey
A search of the Cambridge Structural Database (CSD, Version 5.43, update of March 2020; Groom et al., 2016) using Conquest (Bruno et al., 2002) for 1-(2-methoxyphenyl)piperazine gave 111 hits, of which seven hits were for the protonated piperazinium unit. In particular, the crystal structure of 1-(2-methoxyphenyl) piperazin-4-ium picrate, which like the title compound has a phenolate anion, has been reported (CSD refcode NEBGIK; Verdonk et al., 1997). In the case of the DNSA molecule, 21 hits were observed for neutral DNSA molecules and 65 and 71 hits for DNSA carboxylate and DNSA phenolate, respectively.

Synthesis and crystallization
The title protonated salt was synthesized using 1-(2-methoxyphenyl)piperazine (Sigma Aldrich, 99%) and 3,5-dinitrosalicylic acid (Merck India, 99.5%) in an equimolar ratio. The stoichiometrically (1 mmol) weighed starting materials were completely dissolved in 50 mL of methanol at room temperature and stirred continuously for 3 h. The homogeneous solution was filtered using Whatmann filter paper and placed in a dust-free atmosphere, and allowed to evaporate   slowly at room temperature. A suitable single crystal was harvested after a growth period of 25 days.

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 2. The amine H atoms and O-bound H atoms were located in a difference-Fourier map and refined freely along with their isotropic displacement parameters. Cbound H atoms were included in calculated positions and treated as riding atoms [C-H = 0.93-0.98 Å , with U iso (H) = 1.2U eq (C)].

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 Occ. (