Organic–inorganic hybrid mixed-halide ZnII and CdII tetrahalometallates with the 2-methylimidazo[1,5-a]pyridinium cation

The three isomorphous hybrid salts are assembled from discrete 2-methylimidazo[1,5-a]pyridinium cations and ZnII or CdII tetrahalometallate anions that show disorder involving partial substitution of Br by Cl and Cl by I in the [CdBr2.42Cl1.58]2–, [CdCl3.90I0.10]2– and [ZnCl3.19I0.81]2– anions.

Three isomorphous 0-D hybrid salts, namely, 2-methylimidazo[1,5-a]pyridinium trichloridoiodidozincate(II), (C 8 (III), are assembled from discrete 2-methylimidazo[1,5-a]pyridinium cations, L + , and mixed-halide tetrahalometallate anions. In the three structures, there are two crystallographically non-equivalent cations that were modelled as being rotationally disordered by 180 . In the lattices of the three compounds, a disordered state exists involving partial substitution of Cl by I for sites 2-4 in (I), Br by Cl for all four sites in (II) and Cl by I for site 2 in (III). In the solid state, the organic and inorganic sheets alternate parallel to the bc plane in a pseudo-layered arrangement. In the organic layer, pairs of centrosymmetically related trans-oriented cations form -bonded chains. The adjacent tetrahalometallate anions in the inorganic layer show no connectivity with the shortest MÁ Á ÁM separations being greater than 7 Å . A variety of C-HÁ Á ÁX-M (X = Cl, Br, I) contacts between the organic and inorganic counterparts provide additional structural stabilization. The title structures are isomorphous with the previously reported structures of the chloride analogues, [L]

Chemical context
Hybrid organic-inorganic halide salts have proven to be promising materials for optoelectronic applications spanning light-emitting diodes (LED), lasers, photodetectors and solar cells (Manser et al., 2016;Dou et al., 2014;Stranks et al., 2015). The versatile photophysical properties of these materials are combined with low-temperature solution processability and the tunability of their electronic and crystal structures via chemical composition modification. This research field has been mostly dominated by Pb-and Sn-based hybrid halide perovskites due to their prominent semiconducting properties and large optical absorption. However, water permeability in air and the low thermal stability of these perovskite systems limit their industrial manufacturing (Leijtens et al., 2015). The instability issues have been largely related to the volatility of small organic cations. The introduction of larger organic cations that also lower the dimensionality of a 3-D MX 6 (X = halide ion) octahedral halometallate network is expected to improve the air, moisture and thermal stability of the hybrid metal halides (Leblanc et al., 2019).
The selective combination of organic and inorganic components to incorporate other metal polyhedra and connectivity directly impacts the properties exhibited by the organic-inorganic halide materials. Hybrid halometallates containing group 12 (IIB) elements have been of increasing research interest in this respect (Yangui et al., 2019). Based on the combined experimental and computational results, (CH 3 NH 3 ) 2 CdX 4 (X = Cl, Br, I) and related compounds were found to be potential candidates for broadband white-light emitting phosphors and self-activated scintillators (Roccanova et al., 2017). Engineering hybrid halometallate salts through mixing halogen elements is a recent new strategy that allows fine-tuning of the electronic structure and optoelectronic properties depending on the anionic speciation and ratio (Askar et al., 2018;Rogers et al., 2019).
Recently, we have developed a successful synthetic procedure towards organic-inorganic hybrid halometallates with imidazo[1,5-a]pyridinium-based counter-ions (Buvaylo et al., 2015;Vassylyeva et al., 2020). The latter represent an important class of fused nitrogen-containing bicyclic systems owing to their biological activity and potential applications in materials chemistry. They show strong fluorescence intensity and high quantum yield (Yagishita et al., 2018). The 2-methylimidazo[1,5-a] pyridinium cation, L + , has been synthesized from the oxidative cyclocondensation of equimolar amounts of formaldehyde, methylamine hydrochloride and 2-pyridinecarbaldehyde in an aqueous solution. The incorporation of L + in the metal chloride structure reduced the dimensionality of the PbCl 2 3-D perovskite framework to a 1-D stepwise chloroplumbate(II) wire arrangement in [L] n [PbCl 3 ] n1 and produced [L] 2 [MCl 4 ] (M = Zn, Cd) hybrid salts with tetrahedral anions (Vassilyeva et al., 2020(Vassilyeva et al., , 2021. The three compounds exhibited intense sky blue-light photoluminescence in the solid state.
In this work, we have explored the possibility of preparing the Br and I analogues of [L] 2 [MCl 4 ] hybrids in an attempt to induce changes of the dimensionality in the resulting structures. In the synthesis, a combination of ZnO and NH 4 I was used instead of ZnCl 2 , while cadmium chloride was replaced with the corresponding bromide or iodide. This approach appeared to be only partially successful because of the competing Cl À anions from the dissociation of the HCl adduct of methylamine. Herein, we report the preparations, crystal structures and spectroscopic characterization of three isomorphous 0-D hybrid salts [L]

Refinement
Crystal data, data collection and structure refinement details are summarized in  site occupancies refined to 0.855 (17) and its complement for both cations in (I), 0.73 (2) and its complement for cation 1 and 0.75 (2) and its complement for cation 2 in (II), and 0.72 (3) and its complement for cation 1 and 0.81 (3) and its complement for cation 2 in (III). In compound (I), the halide atom sites 2, 3 and 4 were modelled as being part Cl and part I, with Cl site occupancies refined to 0.3034 (15), 0.9489 (12) and 0.9343 (12), respectively, with the I site occupancies being the complements. The halide atom sites in compound (II) were modelled as being part Br and part Cl with the Br occupancy for sites 1-4 refined to 0.417 (2), 0.857 (2), 0.558 (2) and 0.590 (2) with the Cl occupancies being the complements.
Cd-X bond lengths of the disordered atoms were restrained to ideal values. The halide atom site 2 in (III) was modelled as being part Cl and part I, with Cl site occupancies refined to 0.9008 (15) with the I site occupancies being its complement.
Cd-X bond lengths of the disordered atoms were restrained to ideal values. The coordinates of the halogens were refined to be independent for all three structures. 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.

Funding information
Funding for this research was provided by: Ministry of Education and Science of Ukraine (grant No. 22BP037-13). program(s) used to refine structure: SHELXL2017 (Sheldrick, 2015b); molecular graphics: Mercury (Macrae et al., 2020); software used to prepare material for publication: WinGX (Farrugia, 2012).

Bis(2-methylimidazo[1,5-a]pyridinium) trichloridoiodidozincate(II), (I)
Crystal data (C 8  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. Refinement. The halogen sites 2,3,4 were modelled as being part Cl and part I, with Cl site occupancies refined to 0.3034 (15), 0.9489 (12) and 0.9343 (12) respectively with the I site occupancies being the complements. The cations were modelled as being rotationally disordered by 180 degrees. The site occupancies refined to 0.855 (17) and its complement for both cations after independent refinement showed insignificant differences in the values for the two cations.

Bis(2-methylimidazo[1,5-a]pyridinium) dibromidodichloridozincate(II) (II)
Crystal data (C 8 H 9 N 2 ) 2 [CdBr 2.42 Cl 1.58 ] M r = 628.14 Triclinic, P1 a = 9.5172 (5)  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. Refinement. The halide atom sites were modelled as being part Br and part Cl with site occupancies refined to 0.417 (2), 0.857 (2), 0.558 (2) and 0.590 (2) for the Br occupancy for sites 1-4 with the Cl occupancies being the complements. Cd-X bond lengths of the disordered atoms were restrained to ideal values. The cations were modelled as being rotationally disordered by 180 degrees. The site occupancies refined to 0.73 (2) and its complement for cation 1 and 0.75 (2) and its complement for cation 2. Three reflections with very poor agreement were omitted from the refinement.

Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Å 2 )
x y z U iso */U eq Occ. (

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. Refinement. The halogen site 2 was modelled as being part Cl and part I, with Cl site occupancies refined to 0.9008 (15) with the I site occupancies being its complement. Cd-X bond lengths of the disordered atoms were restrained to ideal values. The cations were modelled as being rotationally disordered by 180 degrees. The site occupancies refined to 0.72 (3) and its complement for cation 1 and 0.81 (3) and its complement for cation 2.