Organic–inorganic hybrid hexachloridostannate(IV) with 2-methylimidazo[1,5-a]pyridin-2-ium cation

The asymmetric unit of the title compound, (C8H9N2)2[SnCl6], contains one cation in a general position and one-half of the dianion situated on an inversion centre. The octahedral SnCl6 2– dianion is almost undistorted. The crystal structure can be seen as an arrangement of alternating organic and inorganic layers with little support from C—H⋯Cl—Sn contacts.


Chemical context
Organic-inorganic hybrid perovskites that combine discrete organic cations and rigid metal halide architectures have been considered promising materials for diverse optoelectronic applications: solar cells, light-emitting diodes, photodetectors, spintronics (Gan et al., 2021;Li et al., 2021). Most of the materials reported to date are based on Pb II , Sb III , Bi III and Cd II halides (Saparov & Mitzi, 2016), whose widespread application is restrained by the potential toxicity. Being in the same main group of metal atoms that Pb belongs to, Sn forms hybrid halide perovskites with similar electronic properties, which are more friendly to the environment. At the same time, the aforementioned hybrid systems suffer from high water permeability and low thermal stability, the issues being largely related to the volatility of small organic cations (Leijtens et al., 2015). The stability of hybrid perovskites can be improved by introducing larger organic cations and lowering the dimensionality of the octahedral halometallate frameworks (Zhang et al., 2016;Leblanc et al., 2019). Moreover, functional organic cations are a valuable tool for introducing useful properties into the hybrid structure. For example, the use of the photoactive zwitterion viologen N,N 0 -4,4 0 -bipyridiniodipropionate (CV) afforded the formation of the covalently bonded pillared layered bromoplumbate, [Pb 3 Br 6 (CV)] n , showing high thermal stability in air and a remarkable increase of capacitance after photoinduced electron transfer (Sun et al., 2019). Mono-periodic hybrid lead halides incorporating optically active protonated 1,3-bis(4-pyridyl)-propane cations exhibit dual-light emissions combined of higher energy blue and lower energy yellow light spectra, which were attributed to the individual contributions of the organic and inorganic components (Sun et al., 2021).
Multiple advantages of the organic-inorganic hybrid materials inspire the huge appeal in exploring other kinds of low-dimensional metal halide compounds templated by functional aromatic cations. Fine-tuning of the electronic structure and optoelectronic properties of the metal halide hybrids, which depend, among other things, on the anionic speciation and halogen ratio, can be achieved by mixing halide ligands in self-assembled organic-inorganic systems (Rogers et al., 2019;Askar et al., 2018).
Pursuing our research on hybrid halometalates incorporating substituted imidazo[1,5-a]pyridinium cations (Buvaylo et al., 2015;Vassilyeva et al., 2019;2020;, we attempted the synthesis of a hybrid tin mixed halide with 2-methylimidazo[1,5-a]pyridinium, L + , a product of the oxidative cyclocondensation between 2-pyridinecarbaldehyde (2-PCA), formaldehyde and CH 3 NH 2 . One necessary component of the reaction is acid, which is introduced as a hydrohalide adduct of the amine (Vassilyeva et al., 2020). Following the method of preparation used to obtain mixed-halide Zn II and Cd II tetrahalometalates (Cl/I, Br/Cl) with L + (Vassilyeva et al., 2022), SnCl 2 Á2H 2 O was reacted with the solution of L + formed in situ using 2-PCA, formaldehyde and CH 3 NH 2 ÁHBr. The isolated product was crystallographically characterized as [L] 2 [SnCl 6 ], (I); the detrimental oxidation of Sn II to Sn IV appeared unavoidable leading to the formation of ubiquitous hexachloridostannate(IV) dianion. Herein, the synthesis, structural analysis and spectroscopic characterization of (I) are reported.

Structural commentary
The title hybrid salt, with formula (C 8 H 9 N 2 ) 2 [SnCl 6 ], crystallizes in the monoclinic space group P2 1 /n. The asymmetric unit consists of an Sn 0.5 Cl 3 fragment (Sn site symmetry 1) and 2-methylimidazo[1,5-a]pyridinium cation, as shown in Fig. 1 Vassilyeva et al., 2021). Bond lengths in the pyridinium ring of the fused core are as expected; the C-N/C bond distances in the imidazolium entity fall in the range 1.337 (5)-1.401 (5) Å ; N2 and N3A atoms are planar with the sums of three angles being equal to 360 . The almost coplanar five-and six-membered rings in the cation show the dihedral angle between them of 1.6 (2) . The octahedral SnCl 6 2dianion in (I) is almost undistorted with the Sn-Cl distances varying from 2.4255 (9) to 2.4881 (8) Å and the cis Cl-Sn-Cl angles approaching 90 (Table 1). The geometric parameters of the dianion are normal and comparable to those of similar structure types.

Figure 2
Projection of the crystal packing of (I) on the bc plane showing organic and inorganic sheets alternating parallel to the (101) plane.
3.530 (2) Å (Fig. 3). The pairs further form -bonded chains with a distance of 3.713 (2) Å between neighbouring pyridinium ring centroids. In the anion sheet, loose packing of SnCl 6 2dianions that are identically stacked one above the other with the shortest Sn-ClÁ Á ÁCl-Sn distance being 4.4433 (12) Å , results in a closest separation of 7.7926 (1) Å between the metal atoms. The hybrid salt lacks classical hydrogen-bonding interactions but shows a variety of C-HÁ Á ÁCl-Sn contacts between the organic and inorganic counterparts (Table 2), a feature common to hybrid chlorometalates with nitrogencontaining aromatic cations (Coleman et al., 2013). Most of these contacts are longer than the van der Waals contact limit of 2.85 Å (Cl) (Mantina et al., 2009) and can be considered a result of crystal packing.  (Mishra, et al., 2005) and hexafluorophosphate DIWYEP (Kriechbaum, et al., 2014), which bear methylphenyl and dimethylphenyl substituents, respectively, in place of the methyl group in L + are the most closely related. ] 2dianions, the latter are mostly highly symmetrical being associated with special positions. The structures including organic counterparts can be seen as an arrangement of alternating organic and inorganic layers supported by hydrogen bonds of the N-HÁ Á ÁCl type in the case of protonated N-containing cations. An organic-inorganic hybrid compound with the structure most similar to that of the title compound is, for example, monoclinic bis[1-(prop-2-en-1-yl)-1H-imidazol-3-ium] hexachloridostannate(IV), in space group P2 1 /n, with layers formed by isolated [SnCl 6 ] 2octahedra and (C 6 H 9 N 2 ) + organic cations, which propagate along the a-axis direction at y = 0 and y = 1/2 (Ferjani, 2020).

Figure 3
Fragment of the -stacked chain built of pairs of L + cations of (I

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 3. All hydrogen atoms were included in calculated positions and refined using a riding model with isotropic displacement parameters based on those of the parent atom (C-H = 0.95 Å , U iso (H) = 1.2U eq C for CH, C-H = 0.98 Å , U iso (H) = 1.5U eq C for CH 3 ). Anisotropic displacement parameters were employed for the non-hydrogen atoms.   (Rigaku OD, 2016); cell refinement: CrysAlis PRO (Rigaku OD, 2016); data reduction:

Bis(2-methylimidazo[1,5-a]pyridin-2-ium) hexachloridostannate(IV)
Crystal data (C 8  where P = (F o 2 + 2F c 2 )/3 (Δ/σ) max < 0.001 Δρ max = 1.54 e Å −3 Δρ min = −1.15 e Å −3 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 )