Crystal structures of three hydrogen-bonded 1:2 compounds of chloranilic acid with 2-pyridone, 3-hydroxypyridine and 4-hyroxypyridine

The crystal structures of the 1:2 compounds of chloranilic acid (systematic name: 2,5-dichloro-3,6-dihydroxy-1,4-benzoquinone) with 2-pyridone, 3-hydroxypyridine and 4-hyroxypyridine, namely, bis(2-pyridone) chloranilic acid, 2C₅H₅NO·C₆H₂Cl₂O₄, (I), bis(3-hydroxypyridinium) chloranilate, 2C₅H₆NO⁺·- C₆Cl₂O₄ 2^-, (II), and bis(4-hydroxypyridinium) chloranilate, 2C₅H₆NO+·^- C₆Cl₂O₄ 2^-, (III), have been determined at 120 K. In the crystal of (I), the base molecule is in the lactam form and no acid-base interaction involving H-atom transfer is observed. The acid molecule lies on an inversion centre and the asymmetric unit consists of one half-molecule of chloranilic acid and one 2-pyridone molecule, which are linked via a short O-H· · ·O hydrogen bond. 2-Pyridone molecules form a head-to-head dimer via a pair of N-H· · ·O hydrogen bonds, resulting in a tape structure along [201]. In the crystals of (II) and (III), acid-base interactions involving H-atom transfer are observed and the divalent cations lie on an inversion centre. The asymmetric unit of (II) consists of one half of a chloranilate anion and one 3-hydroxypyridinium cation, while that of (III) comprises two independent halves of anions and two 4-hydroxypyridinium cations. The primary intermolecular interaction in (II) is a bifurcated O-H· · ·(O,O) hydrogen bond between the cation and the anion. The hydrogen-bonded units are further linked via N-H· · ·O hydrogen bonds, forming a layer parallel to the bc plane. In (III), one anion is surrounded by four cations via O-H· · ·O and C-H· · ·O hydrogen bonds, while the other is surrounded by four cations via N-H· · ·O and C-H· · ·Cl hydrogen bonds. These interactions link the cations and the anions into a layer parallel to (301).