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Unusual cation··· cation interactions in pure and mixed Zundel salts of 4H-[1,2,3] triazolo [4,5-c][1,2,5] oxadiazole 5-oxide
Three new salts of 4H-[1,2,3]triazolo[4,5-c][1,2,5]oxadiazole 5-oxide containing the Zundel (H5O2+) cation were obtained, including a binary salt and two mixed isostructural salts with potassium and ammonium cations. Their structures were determined using single-crystal X-ray diffraction and analyzed by periodic PBE0-D3/POB-TZVP calculations, employing geometrical descriptors and topological analysis of the electron density. The anion shows consistent characteristics across all three salts, retaining approximate C2v symmetry in geometry, bond critical point properties, and atomic charges. Formation of the anion increases the aromaticity of the triazole ring and contributes to the acidity of the H-form and the stability of the deprotonated form. In contrast, the Zundel cations exhibit different geometries, being closer to cis in the binary salt and trans in the mixed salts, although the O···O distance remains comparable, as do the atomic charges and the topological description of the bonding interactions. DFT calculations reveal that the cation is asymmetric in all cases, and that the degree of asymmetry depends on the nature of the second cation in the mixed salts. Atomic-charge analysis indicates that the positive charge in the Zundel cation is concentrated on the central hydrogen atom, enabling unusual bonding cation···cation interactions formed by the terminal atoms, as evidenced by the presence of bond critical points between the atoms of the cations. Thus, besides typical cation···anion and π-stacking anion···anion contacts, the salts feature direct cation···cation interactions. In the mixed salts these interactions lead to the formation of cationic chains with relatively strong bonds, whereas in the binary salt cationic chains of a different geometry are also present but exhibit much lower, van der Waals-type interaction energies.