Crystal structure of bis(3-carboxy-1-methylpyridinium) octabromide

The structure of the title compound is composed of rare Z-shaped octabromide anions embedded within N-methylnicotinic acid cations.

One of the most common applications of polybromide anions is in halogenation reactions.They are typically accessible in stable solid bulk form or as liquids with no measurable vapor pressure, depending on the organic cation.Thus, they can be handled much more easily then elemental liquid bromine (Sonnenberg et al., 2020).Polybromides, for example [HMIM][Br 9 ] where HMIM = 1-hexyl-3-methylimidazolium, have also been shown to form room-temperature ionic liquids, which can potentially be applied as a liquid electrode (Haller et al., 2013).Moreover, the use of the tribromide anion in the [Br 3 ] À /Br À redox pair as a mediator in dye-sensitized solar cells has been reported to be an efficient alternative to the frequently used [I 3 ] À /I À system (Kakiage et al., 2013).Poly-bromides have also been applied in zinc/bromine redox-flow batteries (Naresh et al., 2022).
In the present communication, we report a new polybromide compound containing a Z-shaped octabromide anion, 2(C 7 H 8 NO 2 ) + [Br 8 ] 2-, and report its synthesis, crystal structure and Hirshfeld surface analysis.

Hirshfeld surface analysis
Hirshfeld surface analysis and two-dimensional fingerprint plots of the title compound were generated using Crystal Explorer (Spackman et al., 2021).
The graphical representation of the Hirshfeld surface of the 3-carboxy-1-methylpyridinium cation reveals the presence of a rather strong O-H� � �O hydrogen bond with a neighboring organic cation, as shown in bright red (d norm plot, Fig. 5a), and the presence of weak C-H� � �Br contacts between the organic cation and the octabromide anion (d norm plot, Fig. 5b-d) as well as �� � �Br interactions between the 3-carboxy-1-methylpyridinium and the fragment of polybromide anions located above the aromatic ring (shape-index plot, Fig. 5e).The contributions of selected weak interactions to the crystal packing are shown as two-dimensional Hirshfeld surface fingerprint plots in Fig. 6.The strongest contribution is from Br� � �H interactions (38.2%) with the next major contributions from O� � �H (20.4%) and Br� � �C (13.0%).

Figure 4
The �� � �anion interactions in the title compound.The graphical representation of the Hirshfeld surface of the octabromide anion is given in Fig. 7 (d norm plot).The most prominent interaction is observed with a neighboring [Br 8 ] 2- anion and is shown in red.Observed contacts with the organic cation are significantly weaker and are shown in colors from light pink to white.The fingerprint plots for the octabromide anion are given in Fig. 8.Here the highest contributions are observed for Br� � �H (70.0%) and Br� � �C (15.3%) contacts.Other types of interaction make significantly smaller contribution to the crystal packing, viz.Br� � �O (7.7%), Br� � �Br (4.9%) and Br� � �N (2.2%).

Database survey
A search of the tribromide moiety in the Cambridge Crystal Database (CSD version 5.43, last update March 2022;Groom et al., 2016) revealed 327 crystal structures, while only 28 of them containing four Br atoms connected in a row.The closest analogues to the title compound containing Z-shaped octabromide anions were found to be REKBAK (Robertson et al., 1997), ICOVUS (Fromm et al., 2006), RAQGIB (Wolff et al., 2011) and PAQSAE (Sonnenberg et al., 2017).

Synthesis and crystallization
0.5 mmol of N-methylnicotinamide was mixed with 2 ml of HBr (48% wt ) and left to evaporate.After three days, red crystals appeared in the mixture.They were separated and kept in the mother solution prior to the diffraction measurement.

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 2. Aromatic H atoms were positioned geometrically and refined with riding coordinates [U iso (H) = 1.2U eq (C)].Methyl H atoms were positioned geometrically and were allowed to ride on C atoms and rotate around the N-C bond [U iso (H) = 1.5U eq (C)].The H atom of the carboxyl group was found from a difference-Fourier map and was refined with a fixed distance of d(O-H) = 0.85 A ˚and with U iso (H) = 1.5U eq (O).Computer programs: CrysAlis PRO (Rigaku OD, 2021), SHELXT (Sheldrick, 2015), OLEX2.refine(Bourhis et al., 2015) and OLEX2 (Dolomanov et al., 2009).

Figure 1 A
Figure 1A fragment of the crystal structure of title compound showing the atomlabeling scheme.Displacement ellipsoids are drawn at the 50% probability level.[Symmetry code: (i) À x, À y, -z.] Figure 3 The crystal structure of the title compound in a view along the b axis showing infinite chains of anions.Hydrogen bonds between organic cations are shown as black dashed lines.Br� � �Br contacts between [Br 8 ] 2- anions are shown as red dashed lines.

Figure 5
Figure 5 Hirshfeld surface of the 3-carboxy-1-methylpyridinium cation plotted over d norm (a-d) or shape index (e).The neighboring atoms are shown in ball-and-stick mode for clarity.The surface regions with the strongest intermolecular interactions are shown in red.

Figure 6
Figure 6 Hirshfeld surface fingerprint plot for 3-carboxy-1-methylpyridinium showing overall (100%), Br� � �H, O� � �H and Br� � �C contributions.The d e and d i values are the distances to the closest external and internal atoms, respectively, from a given point to the Hirshfeld surface.

Figure 7
Figure 7 Hirshfeld surface of the octabromide anion plotted over d norm .The surface regions with the strongest intermolecular interactions are shown in red.

Table 2
Experimental details.