Adventures in co-crystal land: High Z′, stoichiometric variations, polymorphism and phase transitions in the co-crystals of four liquid and solid cyclic carboxylic acids with the supramolecular reagent isonicotinamide

The supramolecular reagent isonicotinamide (isonico) was used to co-crystallize with three liquid carboxylic acids, cyclopropanecarboxylic acid (C₃A), cyclobutanecarboxylic acid (C₄A) and cyclopentanecarboxylic acid (C₅A), and a solid carboxylic acid, cyclohexanecarboxylic acid (C₆A). A total of six co-crystal structures resulted, which are an illustration of the co-crystal space observed in the ever-expanding research on co-crystallization of multiple molecular components. Co-crystal 1, (C₃A)·(isonico), has a crystal structure with one of the highest reported Z′ values. 12 acid and 12 isonico molecules combine in the asymmetric unit to give a Z′ = 12, all forming 1-D ribbons. Co-crystals 2a and 2b, both having a formula of (C ₄A)₂·(isonico), having, respectively, a Z′ of 1 and 2, illustrate both stoichiometric variation as well as polymorphism, resulting in different packing architectures. Co-crystals (C₅A) ·(isonico) 3 and (C₆A)·(isonico) 4 have isostructural structures at low temperature, and both show phase transitions above room temperature. For (C₆A)·(isonico), we determined the structure of both the low temperature phase, 4a, as well as a high temperature phase, 4b, by single crystal X-ray diffraction, and monitored the transition of 3 by variable temperature powder X-ray diffraction. In all of the compounds reported here, synthesized from two components that are not necessarily solids at ambient conditions and which we have called co-crystals, the primary hydrogen bonded interaction is the carboxylic acid⋯pyridine hydrogen bond.