Crystal structure of piperazine-1,4-diium bis(4-aminobenzenesulfonate)

The asymmetric unit of the title salt, C4H12N2 2+·2C6H6NO3S−, consists of half a piperazindiium dication, located about an inversion centre, and a 4-aminobenzenesulfonate anion. The piperazine ring adopts a chair conformation. In the crystal, the cations and anions are linked via N—H⋯O and C—H⋯O hydrogen bonds, forming a three-dimensional framework. Within the framework there are C—H⋯π interactions and the N—H⋯O hydrogen bonds result in the formation of R 4 4(22) and R 3 4(13) ring motifs.

Data collection: APEX2 (Bruker, 2008); cell refinement: SAINT (Bruker, 2008); data reduction: SAINT; program(s) used to solve structure: SHELXS97 (Sheldrick, 2008); program(s) used to refine structure: SHELXL97 (Sheldrick, 2008); molecular graphics: ORTEP-3 for Windows (Farrugia, 2012)  tuberculosis (Eswaran et al., 2010), antitumor (Chou et al., 2010), anticancer (Chen et al., 2004) and antiviral (Shingalapur et al., 2009) agents. The piperazine nucleus is capable of binding to multiple receptors with high affinity and therefore piperazine has been classified as a privileged structure. In the last decade, a number of piperazine derivatives have been synthesized and evaluated for their cytotoxic activity (Shchekotikhin et al., 2005). The piperazine nucleus has been classified as a privileged structure and is frequently found in biologically active compounds across a number of different therapeutic areas (Faist et al., 2012). Some of these therapeutic areas include antimicrobial, anti-tubercular, anticonvulsant, antidepressant, anti-inflammatory, cytotoxic, antimalarial, antiarrhythmic, antioxidant and antiviral activities etc. possessed by the compounds having piperazine nucleus (Kulig et al., 2007). In view of the above said importance,the crystal structure of the title compound has been determined by crystallographic methods.
The molecular structure of the title salt is shown in Fig. 1. The crystallographic inversion centered piperazine ring adopts a chair conformation. The bond lengths N2-C7 and C4-S1 are comparable with the values observed in the related structure piperazine-1,4-diium naphthalene-1,5-disulfonate (Wei, 2011). In the anions atom S1 deviates from the benzene ring plane by −0.076 (1) Å. There is a short non-hydrogen contact involving atoms N2···O3 [2.764 Å] at x, y, z.
In the crystal, the N1-H1A···O2 and N1-H1B···O1 hydrogen bonds form an infinite chain leads to the formation of an R 4 4 (22) ring motif (Table 1 and Fig. 2). Similarly, the N2-H2···O hydrogen bonds in the molecular structure results in the formation of an R 3 4 (13) ring motif. These two motifs combine to form a hydrogen-bonded molecular ribbons running along b axis (Table 1 and Fig. 3). A C-H···π interaction is also observed involving atom C6 in the benzene ring of the anion and the centroid of another anion ring with an H···centroid distance of 2.92 Å ( Table 1). The molecular structure is stabilized by strong N-H···O hydrogen bonds which form infinite one dimensional chains. These various interactions result finally in the formation of a three-dimensional framework structure (Table 1 and Fig. 4).

S2. Synthesis and crystallization
The title compound was synthesized by slow evaporation at room temperature of an aqueous mixture of piperazine (1.43 g) and sulfanilic acid (2.88 g). Colourless transparent crystals were obtained in a period of 7 days. Single crystals suitable for X-ray diffraction studies were obtained by slow evaporation of a solution in ethyl acetate at room temperature.

S3. Refinement
Crystal data, data collection and structure refinement details are summarized in  The molecular structure of the title salt, with atom labelling. Displacement ellipsoids are drawn at the 30% probability level. The unlabelled atoms of the cation are related to the labelled atoms by inversion symmetry (-x + 2, − y, − z + 1).

Figure 2
A partial view of the crystal packing of the title salt, viewed along the a axis. Hydrogen-bonded chains (dashed lines) run along the a and c axes (see Table 1).

Figure 3
Crystal packing of the title salt, viewed along the b axis, illustrating the formation of the hydrogen-bonded (dashed lines) molecular ribbons running along the b axis direction (see Table 1). For the sake of clarity, H atoms not involved in hydrogen bonds have been omitted.

Figure 4
A view along the a axis of the crystal packing of the title salt. The hydrogen bonds are shown as dashed lines (Table 1), and H atoms not involved in these interactions have been omitted for clarity.

Piperazine-1,4-diium bis(4-aminobenzenesulfonate)
Crystal data Δρ min = −0.41 e Å −3 Extinction correction: SHELXL97 (Sheldrick, 2008), Fc * =kFc[1+0.001xFc 2 λ 3 /sin(2θ)] -1/4 Extinction coefficient: 0.0332 (17) Special details Geometry. All e.s.d.'s (except the e.s.d. in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell e.s.d.'s are taken into account individually in the estimation of e.s.d.'s in distances, angles and torsion angles; correlations between e.s.d.'s in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell e.s.d.'s is used for estimating e.s.d.'s involving l.s. planes. Refinement. Refinement of F 2 against ALL reflections. The weighted R-factor wR and goodness of fit S are based on F 2 , conventional R-factors R are based on F, with F set to zero for negative F 2 . The threshold expression of F 2 > σ(F 2 ) is used only for calculating R-factors(gt) etc. and is not relevant to the choice of reflections for refinement. R-factors based on F 2 are statistically about twice as large as those based on F, and R-factors based on ALL data will be even larger.