Synthesis, crystal structure and Hirshfeld surface analysis of the hybrid salt bis(2-methylimidazo[1,5-a]pyridin-2-ium) tetrachloridomanganate(II)

The 0-D hybrid salt (C8H9N2)2[MnCl4] with a pseudo-layered arrangement of the organic and inorganic sheets is isomorphous with the Zn and Cd analogues. According to the Hirshfeld surface analysis, non-conventional C—H⋯Cl—Mn hydrogen bonding is predominant in the crystal packing.

The 0-D hybrid salt bis(2-methylimidazo[1,5-a]pyridin-2-ium) tetrachloridomanganate(II), (C 8 H 9 N 2 ) 2 [MnCl 4 ] or [L] 2 [MnCl 4 ], consists of discrete L + cations and tetrachloridomanganate(II) anions. The fused heterocyclic rings in the two crystallographically non-equivalent monovalent organic cations are almost coplanar; the bond lengths are as expected. The tetrahedral MnCl 4 2dianion is slightly distorted with the Mn-Cl bond lengths varying from 2.3577 (7) to 2.3777 (7) Å and the Cl-Mn-Cl angles falling in the range 105.81 (3)-115.23 (3) . In the crystal, the compound demonstrates a pseudolayered arrangement of separate organic and inorganic sheets alternating parallel to the bc plane. In the organic layer, pairs of centrosymmetrically related trans-oriented L + cations are -stacked. Neighboring MnCl 4 2dianions in the inorganic sheet show no connectivity, with the minimal MnÁ Á ÁMn distance exceeding 7 Å . The Hirshfeld surface analysis revealed the prevalence of the non-conventional C-HÁ Á ÁCl-Mn hydrogen bonding in the crystal packing.

Chemical context
Hybrid metal halides combining organic cations and inorganic anions are the focus of research attention as novel lightemitting materials because their photoluminescence properties are conveniently tunable by engineering their organic and inorganic components (Saparov & Mitzi, 2016). These materials have potential uses in light-emitting diodes (LEDs), solar cells, and photodetectors as well as in laser technology (Li et al., 2021). The Pb element in this family, however, prevents these materials from being used in commercial settings (Gan et al., 2021). Therefore, the development of leadfree hybrid metal halides is of particular interest. Environmentally safe organic-inorganic manganese(II) halides have been shown to exhibit potent luminescence arising from d-d transitions, making them promising for use in X-ray scintillators, sensors, and optical devices (Kumar Das et al., 2022).

Structural commentary
The organic-inorganic hybrid salt I crystallizes in the triclinic space group P1 and is isomorphous with the [L] 2 [ZnCl 4 ] (GOTHAB; Vassilyeva et al., 2020) and [L] 2 [CdCl 4 ] (GOTJAD; Vassilyeva et al., 2021) analogues as well as the sister mixed-halide Zn II and Cd II tetrahalometalates with the L + cation involving partial substitution of bromide by chloride and chloride by iodide ions (NOTZAA01, NOVSEZ01 and NOVSOJ01; Vassilyeva et al., 2022). Compound I is composed of discrete L + cations and tetrahedral MnCl 4 2anions ( Fig. 1). In the asymmetric unit, there are two crystallographically non-equivalent cations (N22, N23A and N12, N13A) with similar structural configurations, which are very close to those of the isomorphous hybrid salts. In the fused cores, the imidazolium rings show C-N/C bond lengths in the range 1.332 (3)-1.405 (3) Å ; the pyridinium rings have normal bond distances; the nitrogen atoms are planar, with a total sum of three angles of 360 . The five-and six-membered rings in the cations are almost coplanar, showing dihedral angles between them of less than 2 [0.61 (N22, N23A) and 1.46 (N12, N13A)].
The geometry of the slightly distorted tetrahedral MnCl 4

Supramolecular features
In the crystal of I, there is a pseudo-layered arrangement of the organic and inorganic sheets alternating parallel to the bc plane (Fig. 2). The a-axis length  Table 1 Selected geometric parameters (Å , ).

Hirshfeld surface analysis
The Hirshfeld surface mapped over d norm and fingerprint plots for I were generated using CrystalExplorer (Version 21.5; Spackman et al., 2021). The red spots on the Hirshfeld surface indicate close hydrogen-bond donor and acceptor contacts, while the white and blue areas represent van der Waals and longer contacts, respectively. The bright-red spots are found near chlorine atoms involved in C-HÁ Á ÁCl hydrogen-bonding interactions between organic cations and MnCl 4 2anions (Fig. 3). In the fingerprint plots (Fig. 4), those are associated with sharp spikes of 54.8% of the surface area. The next highest contributions to the surface contacts come from the HÁ Á ÁH (31%), HÁ Á ÁC (6%) and CÁ Á ÁC (2.5%) interactions, whereas other X i Á Á ÁX d contacts (X = H, N, C, Mn) cover less than 6% (Fig. 4). These numbers show that non-conventional hydrogen bonding predominates in the crystal packing of I, but that C-HÁ Á Á andinteractions also make an appreciable contribution.

Database survey
Compound I is a new member of the family of salts with imidazo[1,5-a]pyridinium-based cations. More than 50 structures of the compounds including such cations are found in the Cambridge Structural Database (CSD, Version 5.42; Groom et al., 2016) with 24 halometalates (M = Mn, Co, Fe, Ni, Cu, Zn, Cd, Pb and Sn) contributed by our research team. Another large group comprises organic salts with substituted L + cations and inorganic anions such as perchlorate or hexafluorophosphate. NAKNET (Mishra et al., 2005) and DIWYEP (Kriechbaum et al., 2014) with bulky methylphenyl and dimethylphenyl substituents, respectively, instead of the methyl group in L + are close analogues. A limited amount of the main-group metal halides with imidazo[1,5-a]pyridiniumbased cations are known. The proligand bearing a 6-methylpyridin-2-yl substituent in place of the methyl group in L + (SOHPUC; Samanta et al., 2014) was reported to stabilize both Au I and Au III ions, enabling the mixed-valence hybrid salt with [AuCl 2 ] À and [AuCl 4 ] À anions (SUWVIR; Nandy et al., 2016). In the reaction with mercury(II) acetate, a similar ligand that lacked a methyl group, produced an Hg II -Nheterocyclic carbene complex of virtually linear geometry [C carbene -Hg-C carbene = 176.56 (17)      The ubiquitous tetrachloride anion is found in more than 200 structures stored in the CSD. The average Mn-Cl distance of 2.37 Å in I is comparable to those found in the database for other salts containing isolated MnCl 4 2tetrahedral anions (the range of average Mn-Cl distances for this anion is 2.27-2.42 Å ).

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 nonhydrogen atoms.