Intra- and inter­molecular proton transfer in 2,6-diamino­pyridinium 4-hy­droxy­pyridin-1-ium-2,6-dicarboxyl­ate

Chelidamic acid (4-hy­droxy­pyridine-2,6-dicarb­oxy­lic acid) and 2,6-diamino­pyridine react to form the title salt, C(5)H(8)N(3) (+)·C(7)H(4)NO(5) (−); there are two formula units in the asymmetric unit. The pyridine N atom of 2,6-diamino­pyridine is protonated whereas chelidamic acid is deprotonated at both carboxyl­ate groups but protonated at the N atom; the reaction involves intra- and inter­molecular proton transfer. In the crystal, each 2,6-diamino­pyridinium cation participates in five strong N—H⋯O hydrogen bonds (including one bifurcated hydrogen bond). The crystal structure also features strong O—H⋯O hydrogen bonds between the chelidamate anions, leading to chains along the a axis.