Three-dimensional network in piperazine-1,4-diium–picrate–piperazine (1/2/1)

In the title compound, C4H12N2 2+·2C6H2N3O7 −·C4H10N2, the piperazine-1,4-diium cations and piperazine molecules lie on crystallographic inversion centres. In the crystal structure, intermolecular N—H⋯O and N—H⋯N hydrogen bonds link the components to form two-dimensional layers parallel to the (001) plane. These layers are, in turn, connected by weak intermolecular C—H⋯O hydrogen bonds and π–π stacking interactions [centroid–centroid distance between parallel aryl rings = 3.764 (2) Å, interplanar spacing = 3.500 (2) Å and ring offset = 1.387 (2) Å], forming a three-dimensional framework.


Comment
Studies of picric acid (abbr. PA, pKa = 0.38) have been carried out for many years due to its formation of salts which involve electrostatic forces, multiple hydrogen bond modes (e.g. Hundal et al., 1997;Szumna et al., 2000) and π-π stacking interactions (Colquhoun et al., 1986) which can improve the quality of the crystalline materials. Recently, picrate anion containing molecular adducts have also been reported frequently in order to probe the competition between various intermolecular forces in crystal engineering (Anitha et al., 2006a(Anitha et al., , 2006bVembu et al., 2003;Ma et al., 2005;Akutagawa et al., 2003, Kavitha et al., 2005Arnaud et al., 2007). As part of our study on molecular adducts involved with PA and piperazine (abbr. PP), we report here the molecular and supra-molecular structure of the title compound (I).
In (I), the asymmetric unit (atoms labelled without lower case suffixes in Fig.1) consists of one picrate anion, half a PA di-cation and half a neutral PA molecule. In the picrate anion, the nitro group at the 4-position is almost coplanar with the phenyl ring with a dihedral angle of only 4.1 (2)°, however, the nitro groups at the 2-and 6-positions are both significantly twisted out of the plane of the benzene ring, with dihedral angles of 45.2 (2)° and 21.7 (2)°, respectively. The rotations of the nitro groups at the 2-and 6-positions means that the picrate anion retains the approximate mirror symmetry which is also observed in the structure of a recently reported analog (Kavitha et al., 2006).

Experimental
All the reagents and solvents were used as obtained without further purification. 1:2 molar amount of anhydrous piperazine (0.2 mmol, 17.2 mg) and picric acid (0.4 mmol, 91.6 mg) were dissolved in 95% methanol (10 ml). The mixture was stirred for half an hour at ambient temperature and then filtered. The resulting yellow solution was kept in air for several days. Plate yellow crystals of (I) suitable for single-crystal X-ray diffraction analysis were grown by slow evaporation of the solution at the bottom of the vessel (yield: 45%, 56.7 mg, based on 2:1 organic salt; melting point: 512-514 K).
supplementary materials sup-2 Refinement H atoms bonded to C atoms were placed in calculated positions with C-H=0.93Å (aromatic), 0.97Å (methylene) and U iso (H) = 1.2U eq (both aromatic and methylene C). H atoms attached to N atoms were located from the difference maps with the N-H distances being refined freely and U iso (H) =1.2U eq (N).   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 > 2sigma(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 0.1576 (