title: Intra- and inter-molecular proton transfer in 2,6-diamino-pyridinium 4-hy-droxy-pyridin-1-ium-2,6-dicarboxyl-ate.
abstract: Chelidamic acid (4-hy-droxy-pyridine-2,6-dicarb-oxy-lic acid) and 2,6-diamino-pyridine 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-diamino-pyridine is protonated whereas chelidamic acid is deprotonated at both carboxyl-ate groups but protonated at the N atom; the reaction involves intra- and inter-molecular proton transfer. In the crystal, each 2,6-diamino-pyridinium 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.
authors.affiliation: Institut für Organische Chemie und Chemische Biologie, Goethe-Universität Frankfurt, Max-von-Laue-Strasse 7, 60438 Frankfurt am Main,  Germany.
authors.forename: Quoc-Cuong
authors.initials: QC
authors.lastname: Ton
authors.forename: Michael
authors.initials: M
authors.lastname: Bolte