Salts and ionic cocrystals simultaneously comprising N-oxide and carboxylic acid functional groups constitute a very fertile ground for the investigation of various proton transfer phenomena. This is because such compounds combine two types of proton transfer: that is, inter- and intramolecular hydrogen bonding in acid–base systems. To this end, a series of novel salts based on pyridine-2,5-dicarboxylic acid N-oxide (H2pydco) as an organic acid and 2,4,6-triamino-1,3,5-triazine (tata), 2-aminopyrimidine (2a-pym), 2-amino-6-methylpyridine (2a-6mpy), 1,10-phenanthroline (phen), and 9-aminoacridine (9a-acr) as organic bases have been synthesized and characterized by elemental analyses, infrared spectroscopy, and single-crystal X-ray diffraction: (Htata)+(Hpydco)− (1), (H2a-pym)+(Hpydco)− (2), (H2a-6mpy)+(Hpydco)− (3A,B), [(Hphen)+(Hpydco)−](H2pydco) (4), and [(H9a-acr)+(Hpydco)−]·EtOH (5). The unit cells of 3A and 3B differ slightly; however, 3A crystallizes in a chiral orthorhombic space group P212121, while 3B crystallizes in the achiral space group P21/n. In both cases, the asymmetric unit comprises one cation and one anion. The influence of different organic cations on the packing of Hpydco– in the crystal lattice is studied. The most important feature of these crystals is the presence of extensive O–H···O, N–H···O, N–H···N, and C–H···O hydrogen bond networks, which form base-dependent supramolecular synthons: 1, 2, and 3A,B comprise an α-aminopyridinium moiety, and all involve the hydrogen-bonded motif R22(8) with the Hpydco– anion. Compounds 4 and 5, which lack the α-aminopyridinium moiety, reveal different hydrogen-bonding patterns. The interaction energies of each individual hydrogen bond have been estimated using the quantum theory of “atoms-in-molecules”, which led us to the identification of the energetically favorable antielectrostatic N–H···N hydrogen bonds (stabilization energy of 4.0 kcal/mol) between positively charged melaminium species in 1. It has been also established that charge-assisted hydrogen bonding does not always offer an energetic advantage over “noncharged” hydrogen bonds. With the use of Hirshfeld surface (HS) analysis we have also explored the influence of the protonation state of pydco species on the composition of contact contributions, as well as established specific properties of their 2D fingerprint plots. Finally, a comment is provided on the applicability of HS analysis for the exploration of polymorphs featuring intramolecular proton transfer.

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