The 2:2 complex of pyridine betaine with squaric acid studied by X-ray diffraction, FTIR, NMR and DTG methods

The 2:2 ionic crystals of pyridine betaine (PyB) with squaric acid (H2SQ) belong to monoclinic space group C2/c. Supramolecular structure of the crystals investigated is formed by the loss of one proton from every two squaric acid molecules. Pyridine betaines form a homoconjugated cation, [(PyB)2H]+, through a short, symmetric COO⋯H⋯OOC hydrogen bond of 2.463(2) Å. The hydrogen squarate anions are linked into a homoconjugated anion, [(HSQ)2H]−, by a short symmetric, non-linear O⋯H⋯O hydrogen bond of 2.453(1) Å, with the H-atom located on the twofold axis. The bis(hydrogen squarate)hydrogen anions are linked into a centrosymmetric cyclic dimer by two identical asymmetric OH⋯O hydrogen bonds of 2.536(2) Å. The (PyB)2H cation and cyclic dimer of hydrogen squarate anions are placed around two different systems of inversion centers in the unit cell. The FTIR spectrum is consistent with the X-ray results. The 13C chemical shift of the CO atom confirms the presence of the hydrogen squarate anion in the complex studied. The complex decomposed in three thermal stages.

► Pyridine betaines form a homoconjugated cation through the hydrogen bond of 2.463(2) Å.
► The hydrogen squarate anions form a homoconjugated anion through the hydrogen bond of 2.453(1) Å.
► The homoconjugated anions are linked into centrosymmetric cyclic dimer by the hydrogen bonds of 2.536(2) Å.
► Two broad bands in 2800–2200 and 1800–400 cm−1 in the FTIR spectrum are observed.