Crystal structure of 1,4-bis(3-ammoniopropyl)piperazine-1,4-diium bis[dichromate(VI)]

The organic-inorganic title salt contains a cation with a chair conformation of the piperazine ring and an eclipsed dichromate anion. The entities are linked by N—H⋯O and C—H⋯O hydrogen bonds into a three-dimensional network structure.


Chemical context
Chromium is usually found in trivalent and hexavalent oxidation states in soil, ground water and seawater (Cespó n- Romero et al., 1996). Trivalent chromium is an essential element in mammals for maintaining efficient glucose, lipid and protein metabolism. On the other hand, hexavalent chromium is toxic and recognized as a carcinogen to humans and wildlife. Hence the dichromate ion is environmentally important due to its high toxicity (Yusof & Malek, 2009) and its use in many industrial processes (Goyal et al., 2003). Recently, the reactions between hexaureachromium(III) and inorganic oxoanions (such as Cr 2 O 7 2À or CrO 4 2À ) in aqueous solution have been investigated (Moon et al., 2015). Numerous piperazine derivatives have shown a wide spectrum of biological activities, viz. antibacterial (Foroumadi et al., 2007), antifungal (Upadhayaya et al., 2004), anticancer , antiparasitic (Cunico et al., 2009), antihistamine (Smits et al., 2008) or antidepressive activities (Becker et al., 2006). Antidiabetic, anti-inflammatory, antitubercular, antimalarial, anticonvulsant, antipyretic, antitumor, anthelmintic and analgesic activities (Gan et al., 2009a,b;Willems & Ilzerman, 2010) have also been found to be caused by this versatile moiety. In view of these important properties, we have undertaken the synthesis and X-ray diffraction study of the title compound.

Structural commentary
The molecular entities of the title compound, consisting of a centrosymmetric 1,4-bis(3-ammoniopropyl)piperazinediium cation and a dichromate anion, are shown in Fig. 1. In the cation, the central piperazine ring (N1/C1/C2/N1 i /C1 i /C2 i ; for symmetry operators, see Fig. 1) is substituted at the two N atoms by two ammoniopropyl moieties. The piperazine ring adopts a chair conformation, as is evident from the puckering parameters: Q = 0.599 (2) Å , = 180.0 and ' = 0 . Atoms N1 and N1 i are on opposite sides of the C1/C1 i /C2/C2 i plane and are both displaced from it by 0.2446 (19) Å . The chair conformation of the cation in the title structure is very similar to those of the same cation in the crystal structures of the 2-hydroxybenzoate (Cukrowski et al., 2012), the nitrate (Junk & Smith, 2005) and the tetrahydrogenpentaborate (Jiang et al., 2009) salts, despite the differences in the size and shape of the anions in the various structures. The tetrahedral CrO 4 groups in the anion of the title structure are fused together by a common O atom (O8) and are in an almost eclipsed conformation (Brandon & Brown, 1968

Figure 2
The packing of the molecular entities in the crystal structure of the title salt.
[010], forming a layered arrangement parallel to (001). N-HÁ Á ÁO hydrogen bonds (Table 1) between the cations, involving both primary and tertiary ammonium groups, and the anions lead to a three-dimensional network structure (Figs. 2 and 3). Additional C-HÁ Á ÁO interactions consolidate this arrangement.

Synthesis and crystallization
Potassium dichromate and 1,4-bis(3-aminopropyl)piperazine (PDBP) were mixed in a molar ratio of 2:1 in water. Potassium dichromate was first dissolved in Millipore water of 18.2 MÁcm resistivity. Then the amount of PDBP was slowly added to the solution together with a few drops of concentrated hydrochloric acid and the mixture stirred for 18 h. The solution was then filtered twice with Wattmann filter paper and poured into petri dishes to evaporate at room temperature for several days. Recrystallization from water improved the quality of the material and increased the size of the crystals (maximum crystal size 5Â3Â2 mm 3 after 35 d). A specimen was cleaved for the present structure determination.

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 2. All hydrogen atoms were placed geometrically and refined using a riding model: N-H = 0.89 Å for the primary ammonium group with U iso (H) = 1.5U eq (N); N-H = 0.98 Å for the tertiary ammonium group with U iso (H) = 1.2U eq (N); C-H = 0.97 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 )