Crystal structure of a second triclinic polymorph of 2-methylpyridinium picrate

The title molecular salt, C6H8N+·C6H2N3O7 − (systematic name: 2-methylpyridinium 2,4,6-trinitrophenolate), crystallizes with two cations and two anions in the asymmetric unit. In the crystal, the cations are linked to the anions via bifurcated N—H⋯(O,O) hydrogen bonds, generating R 1 2(6) graph-set motifs. Numerous C—H⋯O hydrogen bonds are observed between these cation–anion pairs, which result in a three-dimensional network. In addition, weak aromatic π–π stacking between the 2-methylpyridinium rings [inter-centroid distance = 3.8334 (19) Å] and very weak stacking [inter-centroid distance = 4.0281 (16) Å] between inversion-related pairs of picrate anions is observed. The title salt is a second triclinic polymorph of the structure (also with Z′ = 2) reported earlier [Anita et al. (2006). Acta Cryst. C62, o567–o570; Chan et al. (2014 ▸). CrystEngComm, 16, 4508–4538]. In the title compound, the cations and anions display a chequerboard arrangement when viewed down [100], whereas in the first polymorph, (010) layers of alternating cations and anions are apparent in a [100] view. It is interesting that the unit-cell lengths are almost identical for the two polymorphs, although the inter-axial angles are quite different.

The title molecular salt, C 6 H 8 N + ÁC 6 H 2 N 3 O 7 À (systematic name: 2-methylpyridinium 2,4,6-trinitrophenolate), crystallizes with two cations and two anions in the asymmetric unit. In the crystal, the cations are linked to the anions via bifurcated N-HÁ Á Á(O,O) hydrogen bonds, generating R 1 2 (6) graph-set motifs. Numerous C-HÁ Á ÁO hydrogen bonds are observed between these cation-anion pairs, which result in a three-dimensional network. In addition, weak aromaticstacking between the 2-methylpyridinium rings [inter-centroid distance = 3.8334 (19) Å ] and very weak stacking [intercentroid distance = 4.0281 (16) Å ] between inversion-related pairs of picrate anions is observed. The title salt is a second triclinic polymorph of the structure (also with Z 0 = 2) reported earlier [Anita et al. (2006). Acta Cryst. C62, o567-o570;Chan et al. (2014). CrystEngComm,16,. In the title compound, the cations and anions display a chequerboard arrangement when viewed down [100], whereas in the first polymorph, (010) layers of alternating cations and anions are apparent in a [100] view. It is interesting that the unit-cell lengths are almost identical for the two polymorphs, although the inter-axial angles are quite different.
Keywords: crystal structure; polymorphism; 2-methylpyridinium picrate; 3-methylpyridinium picrate; 2,4,6-trinitrophenolate.   Table 1 Hydrogen-bond geometry (Å , ). Symmetry codes: (i) x þ 1; y; z; (ii) x À 1; y; z; (iii) x þ 1; y þ 1; z; (iv) x À 1; y À 1; z. supporting information . E71, o848-o849 [doi:10.1107/S205698901501912X] Crystal structure of a second triclinic polymorph of 2-methylpyridinium picrate Jeganathan Gomathi and Doraisamyraja Kalaivani S1. Comment Previous attempt in our laboratories to synthesize carbon-bonded anionic sigma complex with two heterocyclic moieties (substituted imidazole and pyridine) from the ethanolic solution containing 2-chloro-1,3,5-trinitrobenzene, hydantoin and 3-methylpyridine yielded 3-methylpyridinium picrate, a triclinic polymorph (Gomathi & Kalaivani, 2015). In the present work, a similar attempt with 2-methylpyridine instead of 3-methylpyridine in the reaction mixture, yielded 2-methylpyridinium picrate which crystallizes in the triclinic system with space group P1. unambigously reflects the absence of π-bonding between the aromatic rings of anion and cation and supports the fact that the main contributing factor of the formation of the product is proton-transfer reaction. Protonation of the nitrogen atom is further evidenced from the values of the C-N bond distances. N-H···O hydrogen bonding is noticed between the cation and anion parts of two molecules of asymmetric unit and the bifurcation at N-H forming N-H···O hydrogen bonds with the oxygen atoms of phenolate and nitro group results in R 1 2 (6) ring motif and this sort of linkage is highly responsible for the stability of the molecule. Along with this ring motif, other ring motifs such as R 2 2 (7), R 3 3 (13) and R 4 3 (19) are also stabilizing the crystal system. The nitro group involved in forming R 1 2 (6) ring motif bends only slightly from the plane of

S2. Experimental
2-Chloro-1,3,5-trinitrobenzene [2.56 g (0.01 mol)] was dissolved in 30 ml of rectified spirit and mixed with hydantoin [1.00 g (0.01 mol)] in 20 ml of the same solvent. After mixing of these two solutions, 3 ml of 2-methylpyridine (0.03 mol) was added and the solution was heated to 318 K. The solution was stirred at this temperature with the help of magnetic stirrer for 5 h. The solution was cooled to room temperature and then filtered carefully. The clear maroon-red colour solution obtained was allowed to evaporate slowly maintaining the temperature at 293 K. After a period of six weeks, maroon-red coloured crystals formed from the solution. The crystals were filtered, powdered and washed with 30 ml of dry ether and recrystallized from rectified spirit. Instead of the expected carbon-bonded anionic sigma complex with hydantoin, crystals of 2-methylpyridinium picrate were obtained (yield: 70%; m.p.: 423 K).

S3. Refinement
Crystal data, data collection and structure refinement details are summarized.

Figure 1
ORTEP view of the title molecular salt with displacement ellipsoids drawn at 40% probability.

Figure 2
A partial view of the crystal packing diagram of the title molecular salt (hydrogen bonds and π-π stacking are shown as dotted lines).