Piperazine-1,4-diium bis(2,4,5-tricarboxybenzoate) dihydrate

In the title hydrated salt, C4H12N2 2+·2C10H5O8 −·2H2O, the piperazinediium cation, lying about an inversion center, adopts a chair conformation. The benzene ring of the anion makes dihedral angles of 25.17 (8)° with the carboxylate group and angles of 8.50 (7), 20.07 (7) and 80.86 (8)° with the three carboxylic acid groups. In the crystal, the cations, anions and water molecules are connected by O—H⋯O and N—H⋯O hydrogen bonds into double layers parallel to (110).

In the title hydrated salt, C 4 H 12 N 2 2+ Á2C 10 H 5 O 8 À Á2H 2 O, the piperazinediium cation, lying about an inversion center, adopts a chair conformation. The benzene ring of the anion makes dihedral angles of 25.17 (8) with the carboxylate group and angles of 8.50 (7), 20.07 (7) and 80.86 (8) with the three carboxylic acid groups. In the crystal, the cations, anions and water molecules are connected by O-HÁ Á ÁO and N-HÁ Á ÁO hydrogen bonds into double layers parallel to (110).
Piperazinediium cations and water molecules are involved in building this supramolecular structure through N-H···O and O-H···O bonds. Besides hydrogen bonding, the weak aromatic π-π stacking interactions between aromatic rings of H3btcmolecules could contribute for further stabilization of this layered supramolecular crystal structure. As shown in Fig. 3, the π-π stacking interactions between H3btc-(intercentroid separation of 3.7954 Å) were found to be in agreement with the reported values (Janiak, 2000). The packing diagram of the title compound is shown in Fig.4.

Refinement
Refinement on F2 against ALL reflections. The weighted R-factor wR2 and goodness of fit S were based on F2 and conventional R-factors R were based on F, with F set to zero for negative F2. The threshold expression of F2 > 2sig(F2) was used only for calculating R factors(gt) etc. and was not relevant to the choice of reflections for refinement. R-factors based on F2 were statistically about twice as large as those based on F and R-factors based on ALL data will be even larger.  Brandenburg, 2007).

Figure 1
The view of the title compound showing the atom-labelling scheme. Displacement ellipsoids are drawn at the 50% probability level.  Crystal packing viewed along a axis. Hydrogen atoms are omitted for clarity.

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.

Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Å 2 )
x y z U iso */U eq C1 0.06597 (