Salts and co-crystals of chloranilic acid with organic bases: is it possible to predict a salt formation?

Abstract

On supramolecular chemistry grounds proton donors (chloranilic acid) and acceptors (organic bases) were combined to generate multi-component hydrogen bonded systems. Two steps of deprotonation of chloranilic acid, leading to the monoanion and dianion, are coupled by a single or double electron delocalization, respectively. The dissociation of chloranilic acid is accompanied by proton transfer from the acid to an organic base if their ΔpKa value is large enough to support salt formation. A series of salts with organic bases such as dimethylamine, 2-methylpyridine, 2,6-dimethyl-4-amino-pyrimidine, and R,S-tryptophane were prepared and their crystal structures determined. The co-crystals of neutral chloranilic acid with 5,6-phenantroline were also obtained. In the crystal packing of these multicomponent systems, the predominant interactions are hydrogen bonds. According to the topology of the hydrogen bonds, six distinctive motifs are recognized. The basic structural units are either dimeric monoanions of chloranilic acid or monomeric dianions, and monomers of the neutral acid. Due to the presence of protonated bases, the dominant interaction is the N+–H⋯O hydrogen bond whereas a negative charge of an acceptor from the chloranilate anion is delocalised. The stereochemical parameters of the bases influence the topology leading to the formation of discrete motifs or chains.