-({[(Pyridin-1-ium-2-ylmeth­yl)carbamo­yl]form­amido}­meth­yl)pyridin-1-ium bis­­(3,5-di­carb­­oxy­benzoate): crystal structure and Hirshfeld surface analysis

The asymmetric unit of the title salt, C14H16N4O22+·2C9H5O6−, comprises half a dication, being located about a centre of inversion, and one anion, in a general position. The central C4N2O2 group of atoms in the dication are almost planar (r.m.s. deviation = 0.009 Å), and the carbonyl groups lie in an anti disposition to enable the formation of intra­molecular amide-N—H⋯O(carbon­yl) hydrogen bonds. To a first approximation, the pyridinium and amide N atoms lie to the same side of the mol­ecule [Npy—C—C—Namide torsion angle = 34.8 (2)°], and the anti pyridinium rings are approximately perpendicular to the central part of the mol­ecule [dihedral angle = 68.21 (8)°]. In the anion, one carboxyl­ate group is almost coplanar with the ring to which it is connected [Cben—Cben—Cq—O torsion angle = 2.0 (3)°], whereas the other carboxyl­ate and carb­oxy­lic acid groups are twisted out of the plane [torsion angles = 16.4 (3) and 15.3 (3)°, respectively]. In the crystal, anions assemble into layers parallel to (10-4) via hy­droxy-O—H⋯O(carbon­yl) and charge-assisted hy­droxy-O—H⋯O(carboxyl­ate) hydrogen bonds. The dications are linked into supra­molecular tapes by amide-N—H⋯O(amide) hydrogen bonds, and thread through the voids in the anionic layers, being connected by charge-assisted pyridinium-N—O(carboxyl­ate) hydrogen bonds, so that a three-dimensional architecture ensues. An analysis of the Hirshfeld surface points to the importance of O—H⋯O hydrogen bonding in the crystal structure.