Hydrogen bonding in the inner-salt zwitterion and in two different charged forms of 5,6-bis(2-pyridyl)pyrazine-2,3-dicarboxylic acid.

5,6-Bis(2-pyridyl)pyrazine-2,3-dicarboxylic acid exists as an inner-salt zwitterion, 3-carboxy-5-(2-pyridinio)-6-(2-pyridyl)pyrazine-2-carboxylate, (Ia), C(16)H(10)N(4)O(4). The adjacent pyridine and pyridinium rings are almost coplanar due to the presence of an intramolecular hydrogen bond involving the pyridine N atom and the NH H atom of the pyridinium group. In the crystal of (Ia), symmetry-related molecules are hydrogen bonded via the carboxylic acid OH group and one of the carboxylate O atoms to form a polymer, which exhibits a channel-type structure. In the HCl, HClO(4) and HPF(6) salts, 6-carboxy-5-carboxylatopyrazine-2,3-diyldi-2-pyridinium chloride 2.25-hydrate, (II), C(16)H(11)N(4)O(4)(+).Cl(-).2.25H(2)O, 6-carboxy-5-carboxylatopyrazine-2,3-diyldi-2-pyridinium perchlorate trihydrate, (IIIa), C(16)H(11)N(4)O(4)(+).ClO(4)(-).3H(2)O, and 6-carboxy-5-carboxylatopyrazine-2,3-diyldi-2-pyridinium hexafluorophosphate trihydrate, (IIIb), C(16)H(11)N(4)O(4)(+).PF(6)(-).3H(2)O, both pyridine rings are protonated. In the perchlorate form, and in the isomorphous hexafluorophosphate form, the molecule possesses C(2) symmetry, with has a symmetrical intramolecular hydrogen bond involving the adjacent carboxylate and carboxylic acid substituents. In the crystals of the chloride and perchlorate (or hexafluorophosphate) salts, hydrogen-bonded polymers are formed which are three-dimensional and one-dimensional, respectively.