4,4’-(Ethene-1,2-diyl)dipyridinium bis(2-hydroxy-3-methoxybenzoate)

In the title molecular salt, the components are linked by N—H⋯O and C—H⋯O hydrogen bonds.


Structure description
2-Hydroxy-3-methoxybenzoic acid (o-vanillic acid, C 7 H 8 O 4 ) is similar in nature to its isomeric counterpart 4-hydroxy-3-methoxybenzoic acid (p-vanillic acid), with the exception of the hydroxyl-group positioning. Much like its counterpart, o-vanillic acid is being investigated for its medicinal benefits, such as its anti-allergic inflammatory effects (Kim et al., 2017). Despite its potential usage for medicinal purposes, there is a significant lack of structural data on this compound and its salts. As such it is beneficial to study the solid-state forms of o-vanillic acid and its salts to better understand its interactions. To achieve this, bipyridine ethylene (C 12 H 10 N 2 ) was selected due to its demonstrated ability to form both simple and complex hydrogen-bonded networks (MacGillivray et al., 2000;Wang et al., 2007). In addition, as the ÁpK a value between o-vanillic acid (pK a = 2.5) and bipyridine ethylene (pK a = 5.5) is approximately 3, the observed salt formation can reasonably be expected due to the acid-base crystalline complexes ÁpK a rule (Cruz-Cabeza, 2012).

Figure 2
A trimolecular unit of the synthesized salt consisting of two o-vanillate anions and one bipyridinium ethylene cation. Hydrogen-bonding interactions are shown as red dashed lines with distances displayed between interacting heteroatoms.

Figure 4
Top down view of the -stacked trimolecular units. Offset units are shown in different colors.

Figure 5
A single layer of the side by side trimolecular unit interactions are shown. Hydrogen-bonding interactions are shown as dashed blue lines; C-HÁ Á ÁO type hydrogen-bonding interactions are shown as solid green lines.

Figure 1
The asymmetric unit of the title molecular salt showing 50% displacement ellipsoids.

Figure 3
Side on view of the -stacked trimolecular units. Offset units are shown in different colors. Distances, in Å , are shown between interacting ring centroids.

Synthesis and crystallization
A 1:2 molar ratio of bipyridine ethylene (182.2 mg, 1 mmol) and o-vanillic acid (336.2 mg, 2 mmol) were dissolved into a vial of excess methanol. The resulting solution was vortexed for 30 s at 3,000 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.

data-1
IUCrData ( 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. H atoms attached to heteroatoms were freely refined isotropically. H atoms connected to carbon atoms were placed geometrically (C-H = 0.95 Å) and refined as riding atoms with U iso (H) = 1.2U eq (C).