Tape and layer structures in cocrystals of some di- and tricarboxylic acids with 4,4′-bipyridines and isonicotinamide. From binary to ternary cocrystals

Abstract

Linear, zigzag tapes and flat, corrugated sheet structures are described in binary cocrystals 1–9 of some di- and tricarboxylic acids with 4,4′-bipyridine bases and isonicotinamide. Carboxylic acid⋯pyridine O–H⋯N, its proton transfer form N⁺–H⋯O⁻, carboxamide dimer N–H⋯O, and extended motifs are present in these crystal structures. Our results show modular expansion of carboxylic acid dimer in neutral cocrystals with pyridine type bases. Even as the structures of these aggregates can be understood from the molecular constituents in a straightforward manner, the nature of acid⋯pyridine synthon, i.e. neutral or ionic, is difficult to anticipate from their ΔpK_a values. We modify the ΔpK_a (pK_a conjugate acid of pyridine base – pK_a carboxylic acid) rule in COOH–pyridine cocrystals: ΔpK_a < 0 results in neutral O–H⋯N hydrogen bond; 0 < ΔpK_a < 3.75 have an intermediate O–H⋯N/N⁺–H⋯O⁻ H bond character; and ΔpK_a > 3.75 results in proton migration to the ionic interaction N⁺–H⋯O⁻. Proton transfer results in the assembly of a supramolecular 3-connected node of cyclohexane tricarboxylate and these anions form chair cyclohexane or parquet grid sheets in 10 and 11. The length of bipyridinium cation plays a role in the overall structure. A novel approach to three-component adducts is crystallization of 1,3cis,5cis-cyclohexane tricarboxylic acid with 4,4′-bipyridine and 4,4′-bipyridine-N-oxide bases of different strengths, which results in a ternary cocrystal 12 sustained via O–H⋯N and O–H⋯O⁻ hydrogen bonds.