Crystal structure and Hirshfeld surface analysis of bis(2,6-diaminopyridinium) tetrachloridocobaltate(II)

The crystal structure of the title molecular salt features N—H⋯Cl and C—H⋯Cl hydrogen bonds and π–π interactions; Hirshfeld surface analysis and fingerprint plots are reported.


Chemical context
One of the best studied groups of organic-inorganic hybrid materials are the cobalt(II) halide compounds because of their important properties such as fluorescence and magnetism (Decaroli et al., 2015;Kurmoo, 2009). The coordination sphere of Co II is variable, leading to different geometries including octahedral, tetrahedral, square pyramidal, trigonal bipyramidal and square planar (Kurmoo, 2009). Pyridine as an organic heterocyclic molecule has various biological activities (Sellin, 1981;Davidson et al., 1988). As part of our studies in this area, the title compound, (C 5 H 8 N 3 ) 2 [CoCl 4 ] (I), has been investigated.

Structural commentary
The asymmetric unit of (I) is made up of one tetrachloridocobaltate, [CoCl 4 ] 2À , anion and two protonated 2,6-diaminopyridinium, (C 5 H 8 N 3 ) + , organic cations (Fig. 1). The geometry of the CoCl 4 anion is characterized by a range of Co-Cl bond length from 2.2595 (14) to 2.2795 (13) Å and Cl-Co-Cl angles varying from 106.44 (5) to 112.69 (5) , building a slightly distorted tetrahedron. These data are in agreement with those found in related compounds (Mghandef & Boughzala, 2015). The calculated average values of the distortion indice as described by Baur (1974) corresponding to the different lengths and angles in the CoCl 4 tetrahedra [ÁI(Co-Cl) = 0.004 and ÁI(Cl-Co-Cl) = 0.0019] show a slight distortion of the tetrahedra. The interanionic ClÁ Á ÁCl contact distances between the nearest neighbor tetrahedra are 3.986 (2) Å along the a axis and 3.889 (2) Å along the c axis ( Fig. 2), compared to a van der Waals contact distance of 3.50 Å . These contacts are sometimes associated with weak antiferromagnetic interactions (Shapiro et al., 2007), which decrease rapidly with increasing ClÁ Á ÁCl separation.
Pyridinium cations always possess an expanded angle of C-N-C in comparison with the parent pyridine (Ben Nasr et al., 2015). Thus, the observed angles in (I) of C1-N2-C5 and C6-N5-C10 are 124.2 (3) and 124.1 (3) , respectively, are wider than that in neutral pyridine (116.6 ), indicating that protonation takes place on the pyridine ring N2 and N5 atoms. Accordingly, within the cations, we note that the N-C and C-C distances range from 1.332 (5) to 1.393 (6) Å , while the C-C-C, N-C-N, C-C-N and N-C-C angles vary from 116.00 (4) to 126.50 (4) . The 2,6-diaminopyridinium units are essentially planar, with an r.m.s. deviation from the mean plane of 0.002 and 0.006 Å for the N2 and N5 species, respectively.

Figure 3
View of (I) towards the bc plane. The dotted lines indicate hydrogen bonds.
As can be seen from Fig. 5, the two nearest neighboring anti-parallel organic cations, which are not connected by hydrogen bonding, are stacked in a face-to-face mode. The centroid-centroid distance is 3.762 (5) Å , slightly less than 3.8 Å , which is the maximum value accepted forinteractions (Ben Hassen et al., 2017;Janiak, 2000).

Hirshfeld surface analysis
The Hirshfeld surface (Spackman & Jayatilaka, 2009) and the associated two-dimensional fingerprint plots were performed with CrystalExplorer (Wolff et al., 2012). The Hirshfeld surface of the title compound mapped over d norm is illustrated in Fig. 6. The red spots correspond to the HÁ Á ÁCl close contacts, which are due to the N-HÁ Á ÁCl hydrogen bonds. Similarly, the presence of HÁ Á ÁCl contacts (due to C-HÁ Á ÁCl hydrogen bonds) are indicated by a light-red color. The white areas correspond to the places where the distance separating neighboring atoms are close to the sum of the van der Waals radius of the considered atoms and indicate HÁ Á ÁH interactions. The bluish areas illustrate areas where neighboring atoms are too far apart for there to be interactions between them. In the shape-index map (Fig. 7), the adjacent red and blue triangle-like patches show concave regions that indicate stacking interactions (Bitzer et al., 2017).
The fingerprint plots of (I) (Fig. 8a) (Parkin et al., 2007;Rohl et al., 2008), reveal that the main intermolecular interactions with the highest percentage contributions are: HÁ Á ÁCl/ ClÁ Á ÁH (41.6%, Fig. 8b), HÁ Á ÁH (30.8%, Fig. 8c) and CÁ Á ÁH/ HÁ Á ÁC (11.3%, Fig. 8d). Fig. 9 shows the voids (Wolff et al., 2012) in the crystal structure of (I). These are based on the sum of spherical atomic electron densities at the appropriate nuclear positions (procrystal electron density). The crystal voids calculation (results under 0.002 a.u. isovalue) shows the void volume of title compound to be of the order of 172 Å 3 and surface area in the order of 648 Å 2 . With the porosity, the calculated void volume of (I) is 10%. There are no large cavities. We note that the electron-density isosurfaces are not completely closed around the components, but are open at those locations where interspecies approaches are found, e.g. N-HÁ Á ÁCl and C-HÁ Á ÁCl. View of the Hirshfeld surface of (I) mapped over d norm .

Figure 7
Hirshfeld surface mapped over shape-index, highlighting the regions involved instacking interactions.

Figure 5
stacking interactions between the neighboring aromatic organic cations in (I). The inorganic anions are shown as sticks for clarity.
(37%) was added dropwise to the mixture and the resulting blue solution was put aside for crystallization at room temperature. After two weeks, blue crystals of (I) were recovered.

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 2. All hydrogen atoms were found in a difference-Fourier map and refined isotropically.

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.