Crystal structure of imidazo[1,5-a]pyridinium-based hybrid salt (C13H12N3)2[MnCl4]

The replacement of ZnII with MnII ions in the hybrid structure, which also changed the space group from orthorhombic Pbca with Z = 8 to monoclinic P21/c with Z = 4, quenched the fluorescence emission of the hybrid material.

A new organic-inorganic hybrid salt [L] 2 [MnCl 4 ] (I) where L + is the 2-methyl-3-(pyridin-2-yl)imidazo[1,5-a]pyridinium cation, is built of discrete organic cations and tetrachloridomanganate(II) anions. The L + cation was formed in situ in the oxidative cyclocondensation of 2-pyridinecarbaldehyde and CH 3 NH 2 ÁHCl in methanol. The structure was refined as a two-component twin using PLATON (Spek, 2020) to de-twin the data. The twin law (À1 0 0 0 À 1 0 0.5 0 1) was applied in the refinement where the twin component fraction refined to 0.155 (1). The compound crystallizes in the space group P2 1 /c with two crystallographically non-equivalent cations in the asymmetric unit, which possess similar structural conformations. The fused pyridinium and imidazolium rings of the cations are virtually coplanar [dihedral angles are 0.89 (18) and 0.78 (17) ]; the pendant pyridyl rings are twisted by 36.83 (14) and 36.14 (13) with respect to the planes of the remaining atoms of the cations. The tetrahedral MnCl 4 2anion is slightly distorted with the Mn-Cl distances falling in the range 2.3469 (10)-2.3941 (9) Å . The distortion value of 0.044 relative to the ideal tetrahedron was obtained by continuous shape measurement (CShM) analysis. In the crystal, the cations and anions form separate stacks propagating along the a-axis direction. The organic cations display weakstacking. The anions, which are stacked identically one above the other, demonstrate loose packing; the minimum MnÁ Á ÁMn separation in the cation stack is approximately 7.49 Å . The investigation of the fluorescent properties of a powdered sample of (I) showed no emission. X-band EPR data for (I) at 293 and 77 K revealed broad fine structure signals, indicating moderate zero-field splitting.

Chemical context
Salts comprised of organic cations (A) and halometallate anions are a highly promising class of compounds within the more general domain of organic-inorganic hybrid materials. Hybrid salts A 2 [MHal 4 ] based on tetrahedral anions of divalent transition metal ions (M = Zn, Mn, Co, Fe, Cd) can exhibit thermochromism (Kelley et al., 2015) and multiferroic properties (Kapustianyk et al., 2015) as well as acting as molecular switchable dielectrics (Ji et al., 2018) and ionic liquids (Miao et al., 2011). Monovalent organic cations, where size, shape and electronic structure can be varied over wide limits, are a valuable tool for introducing useful properties into the hybrid structure. Heterocycles with the imidazo[1,5-a]pyridine skeleton have been identified as highly emissive fluorophores that render them suitable for optoelectronic devices (Hutt et al., 2012;Yagishita et al., 2018). Incorporation of the imidazo[1,5-a]pyridinium moiety in the hybrid structure is ISSN 2056-9890 expected to extend the applications of the organic material, and also address such issues as mechanical properties, chemical resistance, thermal stability, etc., that limit the applicability of pure organics.
The investigation of the fluorescent properties of a powdered sample of (I) at room temperature under experimental conditions similar to those for [L] 2 [ZnCl 4 ] showed no emission. The replacement of Zn II with Mn II ions in the hybrid structure, which also changed the space group from orthorhombic Pbca with Z = 8 to monoclinic P2 1 /c with Z = 4, quenched the fluorescence emission.  (Buvaylo et al., 2015;Vassilyeva et al., 2019a,b).

Structural commentary
The tetrahedral MnCl 4 2ion is slightly distorted. The Mn-Cl distances fall in the range 2.3469 (10)-2.3941 (9) Å and the Cl-Mn-Cl angles vary from 107.60 (3) to 112.95 (4) ( Table 1). The maximum differences in the lengths and angles are 0.047 Å and 5.35 , respectively. The distortion value of 0.044 relative to the ideal tetrahedron obtained by the continuous shape measurement (CShM) analysis using the SHAPE 2.1 program (Casanova et al., 2005) supports a low degree of deformation.

Supramolecular features
In the crystal, the cations and anions form separate stacks propagating along the a-axis direction (Fig. 2). The alternating L1 and L2 organic cations display offsetstacking between the six-membered rings of the fused cores with the ringcentroid distances of 3.556 (2) and 4.0410 (2) Å . The aromatic stacking between the neighbouring pendant pyridyl rings of L1 and L2, which are twisted with respect to each other by 19.41 (17) is also weak [the ring-centroid separations are 3.724 (2) and 3.956 (2)

Figure 1
The molecular structure and principal labelling of [L] 2 [MnCl 4 ] (I) with displacement ellipsoids drawn at the 50% probability level.
of 3.79 Å is larger than double the chloride ionic radius (3.62 Å ; Shannon, 1976). As a consequence, the minimum MnÁ Á ÁMn separation in the cation stack is approximately 7.49 Å . Classical hydrogen-bonding interactions are absent in (I). There are five C-HÁ Á ÁCl contacts between the cations and adjacent MnCl 4 2anions shorter than the van der Waals contact limit of 2.95 Å ( Table 2). The closest cation-anion distance (C24-H24Á Á ÁCl3-Mn) is 2.64 Å . The imidazo[1,5-a]pyridinium core can be modified with various substituents on the aromatic rings and N(CH 3 ) atom. Crystal structures of ten organic salts with L + derivatives as cations but no organic-inorganic hybrids or metal complexes are known. UREYIA (Tü rkyılmaz et al., 2011) and YIHFEB (Mitra et al., 2007), which bear ethylimidazolium and chlorophenyl substituents, respectively, instead of the methyl group in L + are the most closely related. The lack of the methyl group turns L + into a neutral molecule L 0 that acts as a 2 (N,N) chelate ligand, forming the molecular Mn II complex [Mn{S 2 P(OEt) 2 } 2 (L 0 )] (Á lvarez et al., 2012).

IR and EPR spectroscopy measurements
The IR spectrum of (I) (Vassilyeva et al., 2019a;Buvaylo et al., 2015) and shows a distinctive pattern that can be considered characteristic of L + (see supporting information). It includes intense absorption in the aromatic C-H stretching region (3136-3012 cm À1 ) with several narrow peaks, weak bands below 3000 cm À1 due to alkyl -C-H stretching, sharp bands of medium intensity at 1650, 1586, 1516, 1470 and 1422 cm À1 associated with heterocyclic ring stretching, a very strong band at 780 cm À1 and two less strong absorptions in the out-ofplane C-H bending region 800-600 cm À1 (peaks at 742 and 664 cm À1 ). The remarkable feature of the spectrum is a gap in absorbance from 1650 to 1586 cm À1 . The electronic structure of (I) was probed through X-band EPR spectroscopy at room temperature (r.t.) and 77 K. The EPR spectra of the neat powder sample are temperaturedependent ( Fig. 3). At both temperatures, they are dominated by a strong line at 3500 G (g eff $2) flanked by broad fine  Table 2 Hydrogen-bond geometry (Å , ). Symmetry codes: (i) Àx þ 1; y À 1 2 ; Àz þ 1 2 ; (ii) x þ 1; Ày þ 1 2 ; z þ 1 2 .
Figure 3  structure signals at approximately 900, 2200, 5000 and 6000 G (77 K; g eff $7.97, 3.20, 1.42 and 1.16, respectively). The outer lines indicate zero-field splitting (ZFS) of the spin states for the high-spin d 5 metal ion. As expected for neat-powder EPR spectra, the 55 Mn hyperfine structure due to the coupling of the unpaired electron spins with the I = 5 2 55 Mn nucleus is not resolved. The intermolecular dipole-dipole interactions and the D-strain (D is the axial ZFS parameter) broaden the lines, thus preventing observation of the hyperfine structure in the spectra of neat powders (Duboc et al., 2010). Computer simulation of the 77 K spectrum performed with the program SPIN (S > 1 2 ; Ozarowski, 2019) yielded axial and rhombic ZFS parameters D of 0.062 cm À1 and E close to D/3, respectively. The highest field lines at $6000 and 5000 G result from a mixture of the Z and Y transitions | 3 2 > ! | 5 2 > and | 1 2 > ! | 3 2 >, respectively.
For high-spin Mn II complexes, a very small anisotropy of the Zeeman interaction leads to g values close to 2, and the shape of the spectra depends on the ZFS terms only (Pilbrow, 1990). ZFS is highly sensitive to the coordination environment of the metal ion and if all bonds in the MnCl 4 tetrahedron are equal, one may expect only a strong and broadened single resonance line at g = 2 recorded in the EPR spectra. Indeed, one isotropic line with an unchanged linewidth of about 100 mT and a g-value of 2.0039 was observed in the X-band EPR spectrum of the organic-inorganic hybrid [(CH 3 ) 4 N] 2 -MnCl 4 from 400 down to 20 K (Kö ksal et al., 1999). The EPR spectra of another hybrid, [(C 2 H 5 ) 4 N] 2 MnCl 4 , also consist of a broadened line with the isotropic g-value of 2.001 (3) in the temperature range 170-300 K (Ostrowski & Ciżman, 2008). The appearance of fine structure in the spectrum of (I) needs further study that requires the high-field/high-frequency EPR spectroscopy experiments to be undertaken at lower temperatures (Gagnon et al., 2019).

Synthesis and crystallization
2-PCA (0.38 ml, 4 mmol) was stirred with CH 3 NH 2 ÁHCl (0.27 g, 4 mmol) in 20 ml of methanol in a 50 ml conical flask at room temperature for half an hour. The resultant yellow solution was left in open air overnight and used as the ligand without further purification. Dry MnCl 2 Á4H 2 O (0.40 g, 2 mmol) was added to the solution of the ligand (which had turned olive) and the mixture was magnetically stirred under mild heating for 20 min to ensure dissolution of the metal salt. The resulting solution was filtered and left to cool at r.t. Colourless needles of (I) suitable for X-ray analysis were deposited next day. They were filtered off, washed with diethyl ether and finally dried in air. More product was obtained upon slow evaporation in air of the mother liquor.

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 3. The structure was refined as a twocomponent twin using PLATON (Spek, 2020) to de-twin the data. The twin law (À1 0 0 0 À 1 0 0.5 0 1) was applied in the refinement where the twin component fraction refined to 0.155 (1). Anisotropic displacement parameters were employed for the non-hydrogen atoms. All hydrogen atoms were added at calculated positions and refined by use of 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 ).

Bis[2-methyl-3-(pyridin-2-yl)imidazo[1,5-a]pyridinium] tetrachloridomanganate(II)
Crystal data (C 13  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. Refined as a 2-component twin using PLATON to de-twin the data.