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, C5H8N3 +·C7H4NO5 −; 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.

Chelidamic acid (4-hydroxypyridine-2,6-dicarboxylic acid) and 2,6-diaminopyridine react to form the title salt, C 5 H 8 N 3 + ÁC 7 H 4 NO 5 À ; 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-HÁ Á ÁO hydrogen bonds (including one bifurcated hydrogen bond). The crystal structure also features strong O-HÁ Á ÁO hydrogen bonds between the chelidamate anions, leading to chains along the a axis.
Chelidamic acid can exist in two tautomeric forms Ia and Ib. A search of the Cambridge Structural Database (CSD, Version 5.33 of November 2011, plus two updates; Allen, 2002) yielded no hits for chelidamic acid as neutral 4-hydroxypyridine or 4-pyridone tautomer. The structure of chelidamic acid monohydrate has been found to be zwitterionic [refcode KIXCUP (Hall et al., 2000)] and two crystal structures involve chelidamic acid in coordination complexes (refcodes FEZHEY and FEZHIC). A recent study of three pseudopolymorphs of chelidamic acid has been carried out by Tutughamiarso et al. (2012). In general the different forms of chelidamic acid depend on the pK a values (Norkus et al., 2003). In this study chelidamic acid is doubly deprotonated. The first proton transfer is assumed to be intramolecular (Hall et al., 2000) while the second deprotonation is suggested to occur intermolecular with 2,6-diaminopyridine as proton acceptor. Compound II is known to be reactive in the presence of dicarboxylic acid anhydrides (Schmid & Mann, 1954) but a reaction with dicarboxylic acids without activation is not expected in this case. In the crystal structure of the title compound two one-dimensional hydrogen-bond networks are observed, connecting symmetry-equivalent chelidamates (generated by an a glide plane) via O-H···O chains, whilst those fragments are twisted approximately by 60° with respect to each other (Fig. 2). When comparing the symmetry-independent chelidamates with each other, a slight difference in planarity is noticeable. The plane stretching over all non-hydrogen atoms in the N1 or N1′ unit shows a a-axis in a zigzag alignment (Fig. 4).

Experimental
Chelidamic acid and 2,6-diaminopyridine are commercially available. Chelidamic acid was utilized without purification while 2,6-diaminopyridine had to be sublimed before use. A small amount of each compound was dissolved separately in approximately 15 drops of dimethyl sulfoxide (DMSO) before they were combined in a flask and set aside at room temperature. From the green-yellow mixture, block shaped crystals were obtained after several weeks.

Refinement
Due to the absence of anomalous scatterers, 2351 Friedel pairs were merged. All H atoms were initially located by   Four planes generated by chelidamates each gliding along the a-axis with at least four fragments. The planes are stretched over the defined N1 and O5 atoms (red or green) or N1′ and O5′ atoms (yellow or blue) whilst two planes are made up of symmetric chelidamates, which enclose angles of approximately 60°. Crystal packing of the title compound viewed along the a-axis. Red dashed lines indicate the hydrogen-bond network which interlinks two chelidamates and two 2,6-diaminopyridinium cations to an entity that glides along the a-axis.

Special details
Geometry. All e.s.d.'s (except the e.s.d. in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell e.s.d.'s are taken into account individually in the estimation of e.s.d.'s in distances, angles and torsion angles; correlations between e.s.d.'s in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell e.s.d.'s is used for estimating e.s.d.'s 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 > σ(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.