Face-to-face π-stacking in the multicomponent crystals of chloranilic acid, alkali hydrogenchloranilates, and water

The multicomponent crystals of chloranilic acid (H2CA) and its alkali salts of monochloranilate trihydrate [RbHCA·0.5H2CA·3H2O and CsHCA·0.5H2CA·3H2O] and cocrystals with urea [NaHCA·2(NH2)2CO and Cs2CA·(NH2)2CO] were prepared and their crystal structures determined. In the crystal structures of RbHCA·0.5H2CA·3H2O and CsHCA·0.5H2CA·3H2O, molecules of neutral chloranilic acid and its monoanion form stacks of quinoid rings in a face-to-face fashion (the sequence ⋯HCA−⋯H2CA⋯HCA−⋯); within the triplet very short centroid–centroid separation distances (<3.30 Å) with the offset of 0.27 Å are observed. In an infinite stack of triplets, interactions between two monoanions are characterised by the centroid–centroid separation distances of ∼3.50 Å and offset of 1.5 Å. The two novel cocrystals with urea reveal anion stacks similar to those of aromatic rings. The Cambridge Structural Database search was used to spot preferences of chloranilic acid and its anions for π-stacking.