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

Abstract

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 R12(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.

Keywords

• crystal structure
• polymorphism
• 2-methylpyridinium picrate
• 3-methylpyridinium picrate
• 2,4,6-trinitrophenolate