Synthesis, crystal structure and Hirshfeld surface of bis(2-aminopyridinium) hexachloridostannate(IV)

The packing of the title molecular salt, in which the tin atom lies on a crystallographic inversion centre, is dominated by N—H⋯Cl hydrogen bonds.

In the title molecular salt, (C 5 H 7 N 2 ) 2 [SnCl 6 ], the cation is protonated at the pyridine N atom and the complete dianion is generated by a crystallographic centre of symmetry. In the crystal, N-HÁ Á ÁCl hydrogen bonds link the components into a three-dimensional network built up from the stacking of alternate cationic and anionic layers. The nature of the intermolecular interactions has been analysed in terms of the Hirshfeld surfaces of the cations and the anions. The thermal behaviour and the Raman spectrum of the title compound are reported.

Chemical context
So-called 'zero-dimensional' hybrid perovskites are characterized by a structure formed by isolated inorganic octahedra (or bioctahedra) and an organic cation (Cheng & Lin, 2010). They are easy to prepare through simple techniques (Mitzi, 2004) and they combine the properties of the various organic and inorganic compounds, i.e. the flexibility of the organic part, and the thermal stability and the rigidity of the inorganic part, in a single material, by cooperative effects, to obtain properties that are more than just the sum of the initial properties: an organic/inorganic 'synergy' is created. For example, in these hybrid materials, the organic part can have non-linear optical properties (Bi et al., 2008). Most of the physical properties come from the inorganic part, such as the electronic transport properties, the optical photoluminescence properties (Yangui et al., 2019), or even magnetic properties (Manser et al., 2016). As part of our studies in this area, we now describe the synthesis and structure of the title molecular salt, (I).
In the synthesis, the oxidation number of tin changes from +2 to +4 such that the resultant tin(IV) atom is hexacoordinated by chlorine atoms, generating a weakly distorted ISSN 2056-9890 octahedron in which the metal ion lies on a crystallographic inversion centre: the length of the Sn-Cl bonds varies from 2.4216 (4) to 2.4474 (5) Å . As for the Cl-Sn-Cl angles, the discrepancy of about AE 1 [89.109 (18)-90.805 (16) ] compared to the 90 value angle of a regular octahedron shows that the angular distortion is very small. These values are comparable to those of the same anion associated with other types of cations (BelhajSalah et al., 2018). The absence of larger distortions can probably be attributed to the fact that the hexachlorostannate(IV) anions are free, i.e. none of the chloride ions are bridging although they do accept N-HÁ Á ÁCl hydrogen bonds from the organic cations, which ensures charge balance.
In the pyridinium ring of the cation, the C-C bond lengths vary from 1.328 (3) to 1.405 (3) Å and the C-N bond lengths are 1.341 (3) Å and 1.344 (2) Å . The values of the C-C-C angles in the pyridinium ring vary from 118.9 (2) to 120.9 (2) whereas the C-N-C angle is 124.30 (18) : the larger angle can be attributed to the protonation of the N atom. These values are comparable with those of the same cation associated with other types of anions (Rao et al., 2011).

Supramolecular features
The special position of tin(IV) in the crystal of (I) gives rise to an alternation of cationic and anionic layers lying parallel to the (001) plane (Fig. 2a). The intermolecular interactions in (I) were analysed using PLATON (Spek, 2020), which shows that the structural cohesion in the crystalline structure of the compound (I) is ensured by N-HÁ Á ÁCl hydrogen bonds: Fig. 2a. The distances and the angles describing these interactions are presented in Table 1.

Figure 1
The molecular structure of the title compound with displacement ellipsoids drawn at the 30% probability level [Symmetry code: (i) Àx, Ày, Àz]. (Spackman & McKinnon, 2002) were calculated using the program Crystal Explorer 17 (Turner et al., 2017). The d norm representation mode was used in which red spots identify close contacts; in the white areas, the distance separating the neighboring atoms approaches the sum of the van der Waals radii of the concerned atoms whereas blue areas illustrate areas where neighbouring atoms are too far apart to interact significantly with each other. The presence of the adjacent red and blue triangles, obtained by using the shape index as a representation mode, demonstrates the presence ofand Y-XÁ Á Á type interactions. The Hirshfeld surface [ Fig. 3(a)] shows red spots corresponding to HÁ Á ÁCl/ClÁ Á ÁH close contacts, which are due to the N-HÁ Á ÁCl hydrogen bonds. The presence of the adjacent red and blue triangles in Fig. 3(b) demonstrates the presence of the Cg1Á Á ÁCg1 and Sn-Cl2Á Á ÁCg1 interactions. The contribution of different kinds of interatomic contacts to the Hirshfeld surfaces of the individual cations and anions is shown in the fingerprint plots in Fig. 4 and Fig. 5, respectively. These interactions are ensured by 47.3% of hydrogen bonds (HÁ Á ÁCl), 3.2% of Y-XÁ Á Á type (NÁ Á ÁCl and CÁ Á ÁCl), 6.6% of stacking type (CÁ Á ÁC and CÁ Á ÁN/NÁ Á ÁC), 15.6% of C-HÁ Á Á type (CÁ Á ÁH/HÁ Á ÁC), 6.2% of N-HÁ Á Á type (NÁ Á ÁH/ HÁ Á ÁN) and 21.1% of HÁ Á ÁH van der Waals interactions. The two-dimensional fingerprint analysis for the anionic moieties   Two-dimensional fingerprint plots for the cation of the title compound, and delineated into the principal contributions of HÁ Á ÁCl, CÁ Á ÁC, CÁ Á ÁN/ NÁ Á ÁC, NÁ Á ÁCl and CÁ Á ÁCl contacts. Other significant contacts are HÁ Á ÁH (21.1%), HÁ Á ÁC/CÁ Á ÁH (15.6%) and HÁ Á ÁN/NÁ Á ÁH (6.2%).
Crystalline cohesion in RIGDER and (I) is ensured by dipole-dipole interactions and hydrogen bonds of the N-HÁ Á ÁCl type with a slight difference in the donor-acceptor angles and distances of the two compounds. The different arrangement of the nitrogen atoms in the cation in RIGDER leads to much weakerstacking compared to (I): the centroid separations are 4.24 (1) and 3.552 (13) Å , respectively. We also notice a slight difference between the two compounds in the interaction percentages calculated by the Hirshfeld surface analysis (see Table S1 in the supporting information).

Thermal analysis
In order to investigate the thermal stability of (I), thermogravimetric analysis (DTA/TGA) was performed under an N 2 atmosphere at a heating rate of 10 C min À1 in the temperature range from 25 to 500 C. The thermogram of (I) (see Fig. S2 in the supporting information) shows that the compound loses 64.4% of its mass in the temperature range of 270-304 C. The mass loss can be attributed to the degradation of the organic entity and two chlorine atoms (Janiak & Blazejowski, 1990) to leave a reside of SnCl 4 .

Bis(2-aminopyridinium) hexachloridostannate(IV)
Crystal data 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.