trans-2,5-Dimethylpiperazine-1,4-diium bis(perchlorate) dihydrate: crystal structure and Hirshfeld surface analysis

The extended structure consists of infinite [010] chains linked by Ow—H⋯O (w = water) hydrogen bonds. These chains are cross-linked by the dications via N—H⋯Ow and weak C—H⋯O hydrogen bonds, thus forming a three-dimensional supramolecular network. Three-dimensional Hirshfeld surface analysis and two-dimensional fingerprint maps reveal that the structure is dominated by H⋯O/O⋯H and H⋯H contacts.

The asymmetric unit of the title hydrated molecular salt, C 6 H 16 N 2 2+ Á-2ClO 4 À Á2H 2 O, contains a half dication (completed by inversion symmetry), a perchlorate anion and a water molecule. The extended structure consists of infinite chains of formula [(ClO 4 )H 2 O] n nÀ ions extending along the b axis linked by O w -HÁ Á ÁO (w = water) hydrogen bonds. These chains are cross-linked by the dications via N-HÁ Á ÁO w and weak C-HÁ Á ÁO hydrogen bonds, thus forming a three-dimensional supramolecular network. Three-dimensional Hirshfeld surface analysis and two-dimensional fingerprint maps reveal that the structure is dominated by HÁ Á ÁO/OÁ Á ÁH and HÁ Á ÁH contacts.

Chemical context
Piperazine (C 4 H 10 N 2 ) and its derivatives are a family of strongly basic amines able to form dications, in which all of the N-H bonds are generally active in hydrogen-bond formation. They are used in pharmacology and found in biologically active compounds across a number of different therapeutic areas, displaying antibacterial, antifungal, antimalarial, antipsychotic, antidepressant and antitumor activity (Brockunier et al., 2004;Bogatcheva et al., 2006).
In this work, as part of our studies in this area, we report the preparation and structural investigation of a new hydrated perchlorate salt, C 6 H 16 N 2 2+ Á2ClO 4 À Á2H 2 O (I).

Hirshfeld surface analysis
The three-dimensional Hirshfeld surfaces and two-dimensional fingerprint plots of (I) were prepared using Crystal-Explorer (Wolff et al., 2012) and are shown in Figs. 4 and 5, respectively. The interaction between N-H and oxygen atoms can be seen in the Hirshfeld surface as the bright-red area in Fig. 4 (labeled a). The light-red spots are due to O w -HÁ Á ÁO interactions (labeled b). For the salt, OÁ Á ÁH/HÁ Á ÁO contacts, which are attributed to N-HÁ Á ÁO w and O w -HÁ Á ÁO hydrogen-bonding interactions, appear as two sharp symmetric spikes in the two-dimensional fingerprint maps. They have the most significant contribution to the total Hirshfeld surfaces. The HÁ Á ÁH contacts appear in the middle of the scattered points in the two-dimensional fingerprint maps. For further information on Hirshfeld surfaces, see: Spackman & McKinnon (2002) and Spackman & Jayatilaka (2009).

Synthesis and crystallization
The title compound was prepared from an alcoholic solution containing trans-2,5-dimethylpiparazine (0.  Projection of (I) along the b axis. The H-atoms not involved in hydrogen bonding are omitted.
99%, Aldrich) dissolved in ethanol (20 ml) and perchloric acid HClO 4 (0.2 g, 2 mmol, purity 96%, Aldrich) with a molar ratio of 1:2. This mixture was stirred for 1 h. After a week of evaporation at room temperature, colorless single crystals of suitable dimensions for crystallographic study were formed, and were isolated by filtration and washed with a small amount of distilled water. The crystals can be stable for months under normal conditions of temperature and humidity.

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 2. All H atoms were located in a difference map but were placed geometrically and refined using a riding model, with C-H = 0.96 Å (methyl), or 0.98 Å (methine), N-H = 0.90 Å (NH 2 ) with U iso (H) = 1.2U eq (C or N). The H atoms of the water molecule were refined with a distance restraint of O-H = 0.85 (1) Å using DFIX and DANG commands (Sheldrick, 2015) with U iso (H) = 1.5U eq (O).   Hirshfeld surface around the constituents of (I) coloured according to d norm . The surfaces are shown as transparent to allow visualization of the orientation and conformation of the functional groups. Data collection: APEX2 (Bruker, 2014); cell refinement: SAINT (Bruker, 2014); data reduction: SAINT (Bruker, 2014); program(s) used to solve structure: SIR97 (Altomare et al., 1999); program(s) used to refine structure: SHELXL2014/7 (Sheldrick, 2015); molecular graphics: ORTEP-3 for Windows (Farrugia, 2012); software used to prepare material for publication: WinGX publication routines (Farrugia, 2012).

trans-2,5-Dimethylpiperazine-1,4-diium bis(perchlorate) dihydrate
Crystal data 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.