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1 2007

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Co-crystals of 2,4,6-Trihydroxybenzoic Acid with Aromatic Diazaheterocycles – Crystallographic Studies

Jankowski W., Kadzewski A., Gdaniec M.

Polish Journal of Chemistry

2007

Vol. 81, nr 5-6

1095-1108

Binary co-crystals of 2,4,6-trihydroxybenzoic acid with pyrazine (1:1), quinoxaline (1:1) and phenazine (1:2) were prepared and their structure determined by X-ray crystallography. In the complex with pyrazine the carboxylic group of the acid is in the syn configuration and is connected via strong hydrogen bond to the N atom of the heterocyclic base. In the complex with phenazine the carboxylic group adopts the anti form and the acid molecule binds, via the phenolic groups, to two molecules of the base, in a manner typical for interaction of resorcinol with azaheterocycles. In the complex with quinoxaline the asymmetric part of the unit cell consists of two pairs of acid-base hydrogen- bonded units. In the first pair, 2,4,6-trihydroxybenzoic acid binds to quinoxaline via the carboxylic group, which is in the syn form. In the second pair, proton is transferred from the carboxylic group to the heterocyclic base, and the ions join via N-H+źźźO–(carboxylate) interaction. In both units acidic hydrogen atom is significantly shifted towards the center of hydrogen bond. Hydrogen bonds between the carboxyl group and the pyrazine and quinoxaline N-atoms are linear and short [OźźźN distances in the range 2.570(2)–2.630(2) capital A, ring].