1,2-Bis(pyridin-4-yl)ethene–4-hydroxy-3-methoxybenzoic acid (1/1)

A co-crystal consisting of a 1:1 ratio of p-vanillic acid and bipyridine ethylene was synthesized. A series of O—H⋯N interactions connect the molecules into twisting wires, which are cross-linked through van der Waals interactions.


Structure description
4-Hydroxy-3-methoxybenzoic acid, C 8 H 8 O 4 , known commonly as p-vanillic acid, is used as a flavoring agent and naturally found in a variety of fruits and edible plants (Ingole et al., 2021). In addition, p-vanillic acid is currently being investigated for its inflammatory pain-inhibiting properties (Calixto-Campos et al., 2015). Despite the prevalence of the molecule in our foods and its potential medicinal benefits, structural information on vanillic acid is sparse with few crystal structures being reported thus far. As such it is crucial to expand the number of structures containing vanillic acid in order to better understand the non-covalent interactions involving this molecule. Bipyridine ethylene (C 12 H 10 N 2 ; BPyE) was selected as a suitable coformer for the present study because of its ability to form both simple and complex hydrogen-bonded networks with organic acids (Delori et al., 2013;Bhattacharya et al., 2013).
When p-vanillic acid is combined with BPyE in a 1:1 molar ratio, the resulting 1:1 cocrystal possesses monoclinic (P2 1 /c) symmetry at 90 K. The vanillic acid has two distinct O-HÁ Á ÁN-type hydrogen-bonding interactions ( Table 1); one of these involves the carboxylic acid group and a BPyE N atom acceptor and resulting in a 2.6295 (12) Å distance between heteroatoms (Fig. 1). The other hydrogen bond occurs between the para-position hydroxyl group and the other pyridine N atom of a BPyE molecule resulting in a 2.6868 (13) Å distance between heteroatoms (Fig. 2)

Synthesis and crystallization
A 1:1 molar ratio of bipyridine ethylene (182.2 mg, 1 mmol) and p-vanillic acid (168.1 mg, 1 mmol) was added to a 25 ml scintillation vial to which methanol was added until both compounds dissolved (approximately 20 ml). The resulting solution was vortexed for 30 s at 3000 rpm on a VWR Mini Vortexer MV I. The solution was then stored in the dark uncapped to allow for crystal formation while the solvent slowly evaporated.

Refinement
Crystal data, data collection, and structure refinement details are summarized in Table 2.

Figure 1
A bimolecular unit consisting of p-vanillic acid and BPyE with the hydrogen bond depicted as a blue dashed line. The BPyE molecule illustrated is generated by the symmetry operation x -1, y, z from the asymmetric molecule. Table 1 Hydrogen-bond geometry (Å , ).

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. The O-bound H atoms were located in difference maps and their positions were freely refined. The C-bound H atoms were placed geometrically (C-H = 0.95-0.98 Å) and refined as riding atoms with U iso (H) = 1.2U eq (C) or 1.5U eq (methyl C).