Bifurcated hydrogen bonded supramolecular units in molecular complexes of picolines with chloranilic acid

Chloranilic acid (2,5-dichloro-3,6-dihydroxybenzoquinone, CA) readily forms molecular complexes with the three methylpyridine (picoline) isomers in both 1 : 1 and 1 : 2 ratios. In each case a predictable hydrogen-bonded supramolecular unit is formed consisting of a P:C:C:P or a P:C:P arrangement for each of the stoichiometric ratios, respectively. At least one proton is transferred from a chloranilic acid molecule to the nitrogen of the picoline in all the structures. The molecules are held together by a series of bifurcated hydrogen bonds in which a large degree of variation in the directionality can be observed. The proton depleted chloranilate ions show a delocalisation of charge across the entire molecule in the presence of highly bifurcated hydrogen bonds. The variation of the supramolecular units is discussed with particular reference to the degree of bifurcation in the hydrogen bonds.