Gallic acid pyridine monosolvate

In the title compound (systenatic name: 3,4,5-trihydroxybenzoic acid pyridine monosolvate), C5H5N·C7H6O5, the gallic acid molecule is essentially planar (r.m.s deviation = 0.0766 Å for non-H atoms) and is linked to the pyridine molecule by an O—H⋯N hydrogen bond. An intramolecular O—H⋯O hydrogen bond occurs in the gallic acid molecule. The gallic acid and pyridine mean planes make a dihedral angle 12.6 (3)°. Intermolecular O—H⋯O and O—H⋯N hydrogen bonding involving the hydroxy and carboxyl groups and the pyridine molecule, and π–π interactions between inversion-related pyridines [centroid–centroid distance = 3.459 (6) Å] and between pyridine and benzene rings [centroid–centroid distance = 3.548 (6) Å], lead to a three-dimensional network in the crystal.


Comment
Gallic acid, a dietary polyphenol, is widely distributed in many edible and medicinal plants. It can exist as a single molecule or as a structural unit of hydrolysable tannins. It has been found to show strong pharmacological activities including antioxidant (Souza, et al. 2011), antiviral (Ozcelik, et al., 2011) and antitumor properties (Liu, et al., 2011). This compound contains two of the most common functional groups in natural products, e.g. carboxylic acid and phenolic groups. Crystal engineering studies have revealed interesting polymorphism. Four polymorphs of the monohydrate of gallic acid with three space groups (P 2 1 /c, P 2/n, and P 1), and an anhydrous form with space group C 2/c have been reported (Clarke et al., 2011). We report herein the pyridine monosolvate of gallic acid.
The gallic acid molecule is essentially planar. The mean deviation of the benzene ring is 0.0030 Å, which is similar to that in gallic acid monohydrate (0.0028 Å), and its dihedral angle with the plane of the carboxyl group is 9.8 (3) °, which is larger than that in gallic acid monohydrate (2.9°) (Jiang, et al., 2000). The gallic acid and pyridine molecules make a dihedral angle of 12.8 (4) °. The bond distances are all normal.
Within the asymmetric unit, the gallic acid molecule and pyridine molecule are linked through hydrogen bond O4-H···N1.

Experimental
The title compound was extracted from the whole plant of Polygonum chinense L. The dried plant material (5 kg) was powdered and extracted with 95% ethanol at room temperature to afford the crude extract, which was suspended in distilled water and partitioned with petroleum ether, ethyl acetate and n-butanol. The n-butanol fraction (100g) was subjected to macroporous resin, reverse phase silica gel chromatography to give compound I (21 mg), which was recrystallized in pyridine to afford the monopyridine solvate of gallic acid.

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. Refinement of F 2 against ALL reflections. The weighted R-factor wR and goodness of fit S are based on F 2 , conventional R-factors R are based on F, with F set to zero for negative F 2 . The threshold expression of F 2 > 2sigma(F 2 ) is used only for calculating R-factors(gt) etc. and is not relevant to the choice of reflections for refinement. R-factors based on F 2 are statistically about twice as large as those based on F, and R-factors based on ALL data will be even larger.

Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Å 2 )
x y z U iso */U eq O1 0.2507 (