Crystal structures of isotypic poly[bis(benzimidazolium) [tetra-μ-iodido-stannate(II)]] and poly[bis(5,6-difluorobenzimidazolium) [tetra-μ-iodido-stannate(II)]]

The bicyclic aromatic benzimidazolium cation stabilizes the layered perovskite structure comprising inorganic {[SnI4]2−}n sheets. A difluoro-substitution of the organic cation demonstrates the structural versatility of the new approach.


Chemical context
The title compounds, (1) and (2), belong to an extensive family of materials exhibiting a perovskite-type structure, which can vary with respect to the dimensionality of its extended inorganic framework, ranging from two-dimensional, [MX 4 ] n 2nÀ , to three-dimensional, [MX 3 ] n nÀ (Mitzi, 1999(Mitzi, , 2001(Mitzi, , 2004Mitzi et al., 2001;Zhengtao et al., 2003a,b). The former case is exemplified by anionic [MX 4 ] n 2nÀ sheets (M = divalent metal ion; X = halogen) of corner-sharing MX 6 octahedra, which are separated by bilayers of organic cations. ISSN 1600-5368 For most reported layered perovskites, these organic molecules are terminated with one or two protonated primary amine groups. Thereby, the ammonium head(s) form N-HÁ Á ÁX hydrogen bonds to any of the bridging and terminal halogen atoms in the inorganic layers (Mitzi et al., 2002;Mercier et al., 2004;Sourisseau et al., 2007;Pradeesh et al., 2013). In the actual case, however, as a novel aspect, the bicyclic aromatic benzimidazole unit is introduced as an organic part. There are numerous general examples of benzimidazole acting as a neutral ligand (Keene et al., 2010) and similarly in its protonated form (Mouchaham et al., 2010). In this context, the present study explicitly demonstrates that benzimidazolium cations and corresponding derivatives can stabilize the layered perovskite structure as well, while fitting perfectly into the 'footprint' provided by the inorganic framework. This observation bears importance since the extent of the in-and out-of-plane angular distortions, twisting and buckling of the anionic sheets, is largely determined by the relative charge density, steric requirements and hydrogenbonding pattern of the organic cations (Knutson & Martin, 2005;Takahashi et al., 2007). These distortions correlate with the band gaps of the perovskite-type semiconductors. It is interesting to note that perovskite-based solar cells have recently been catapulted to the cutting edge of thin-film photovoltaic research (Hao et al., 2014;Marchioro et al., 2014). Consequently, the chemical variability which comes with the imidazolium cation, especially the range of possible substitutions on its molecular skeleton, gives an additional structural diversity to this class of compounds. As a case in point, consider the difluoro-substituted compound (2) which renders not only modified van der Waals interactions for the organic bilayers, but also tailors the 'chemistry' of the crystal surfaces.

Structural commentary
Compounds (1) and (2) are isostructural. Their asymmetric units, Figs. 1 and 2, consist of an Sn 2+ cation situated on a twofold rotation axis (Wyckoff position 4e), three iodine atoms [one in a general position, one on an inversion centre (4a) and one on a twofold rotation axis (4e)] and a benzimidazolium or 5,6-difluorobenzimidazolium cation, respectively. The main building blocks of the structure are cornersharing [SnI 6 ] octahedra, which form planar sheets with formula {[SnI 4 ] 2À } n which extend parallel to (100). The negative charge of these layers is compensated by the organic cations, which are on both sides of the layer, attached by strong hydrogen-bonding and Coulombic interactions (Figs. 3 and 4). This structural motif can be regarded as an A-B-A layer system, where A represents the aromatic cation and B the tin iodide layer. The coherence between organic bilayers along [100] is mainly achieved through van der Waals interactions. The Sn-I bond lengths for (1) range from 3.0626 (3)   The main building units of (1), showing atom labeling and displacement ellipsoids drawn at the 50% probability level. [Symmetry codes: (i) x, y + 1, z; (ii) Àx, y, Àz + 1 2 .]
There is no out-of-plane distortion of the inorganic sheet. The arrangement of the aromatic cations is mainly determined through the direction of N-HÁ Á ÁI hydrogen bonds to the apical iodine atoms (Tables 3 and 4

; Figs. 3 and 4).
There is no N-HÁ Á ÁI bridging contact smaller than the sum of the respective van der Waals radii (H: 1.2, I: 1.98 Å ; Bondi, 1964). This is in contrast to primary ammonium cations, which form hydrogen bonds to both apical and bridging iodine atoms. The shortest HÁ Á ÁI bridging distance is C3-H3Á Á ÁI2 with 3.12 Å for (1)  The crystal packing of compound (1) viewed along [010]. N-HÁ Á ÁI hydrogen bonds are shown as dashed lines.

Figure 5
View along the a* axis of a tin iodide layer of (2). For clarity, the atoms are represented as spheres with uniform sizes selected for each atom type. of 3.786 Å for (1) [(2): 3.730 Å ] (Fig. 6). The shortest contact distances between the organic bilayers for both compounds are close to the sums of the van der Waals radii [C8Á Á ÁH6 i 2.801 Å in (1) and F8Á Á ÁH9 ii 2.557 Å in (2); (i): 1 2 À x, À 1 2 + y, 1 2 À z; (ii): 1 2 À x, 1 2 À y, Àz]. The larger size of the fluorine atom in comparison to the hydrogen atom is reflected in a larger A-B-A layer spacing of 14.407 Å for (2) compared to 13.950 Å for (1).

Database survey
In the Cambridge Structural Database (Version 5.35, last update November 2013; Allen, 2002) no structures of compounds containing a (benz)imidazolium cation for layered perovskites are listed, making the two examples presented herein the only ones reported so far.

Synthesis and crystallization
Compound (1) was synthesized and crystallized by a solvothermal method using a mixture of tin(II) iodide and benzimidazole in a 1:2 molar ratio. In a 50 ml round-bottom flask, 4 ml concentrated HI (57 wt. %, stabilized with hypophosphorous acid) was mixed with 2 mmol (0.236 g) benzimidazole. After stirring for one minute, this solution was added to a sample flask containing 1 mmol (0.372 g) tin(II) iodide. The reaction flask was put in a 23 ml Teflon container. The reaction was conducted at 363 K for ten h after which the

Figure 6
View along the a* axis of a double layer of tin iodide and the organic cations of (2). For clarity, the [SnI 6 ] octahedra are shown as polyhedra, the atoms of the organic cations are represented as spheres with uniform sizes selected for each atom type.

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 5. The N-H hydrogen atoms were located in difference Fourier maps and were freely refined. The C-bound hydrogen atoms were included in calculated positions and treated as riding atoms with C-H = 0.95 Å . The isotropic displacement parameters of all H atoms were constrained to 1.2U eq of their parent atoms. The crystal of compound (2) was a non-merohedral twin. The two twin components were related by a twofold rotation about the c* axis. The data from both twin components were integrated to give 8236 and 7625 non-overlapped reflections for twin components 1 and 2, respectively, plus 13836 overlapping reflections from both twin components. Symmetry-equivalent reflections were merged. The major twin fraction, component 1, refined to 0.6870 (12).

1) Poly[bis(benzimidazolium) [tetra-µ-iodido-stannate(II)]]
Crystal data (C 7    where P = (F o 2 + 2F c 2 )/3 (Δ/σ) max = 0.001 Δρ max = 1.95 e Å −3 Δρ min = −1.74 e Å −3 Special details Geometry. All e.s.d.'s (except the e.s.d. in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell e.s.d.'s are taken into account individually in the estimation of e.s.d.'s in distances, angles and torsion angles; correlations between e.s.d.'s in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell e.s.d.'s is used for estimating e.s.d.'s involving l.s. planes. Refinement. Refined as a 2-component twin.