Balakrishna R. Bhogala

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

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 ΔpKa values. We modify the ΔpKa (pKa conjugate acid of pyridine base – pKa carboxylic acid) rule in COOH–pyridine cocrystals: ΔpKa 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.

Ternary and quaternary co-crystals of 1,3-cis,5-cis-cyclohexanetricarboxylic acid and 4,4′-bipyridines

1,3cis,5cis-Cyclohexanetricarboxylic acid (H3CTA) was co-crystallized with 4,4′-bipyridine bases connected viamethylene and alkene chains to obtain ternary acid–base co-crystals, H3CTA·bipy1·bipy2. The hydrogen bond network and mode of interpenetration in co-crystal structures are discussed in relation to known (6,3) nets. The rectangular host container of composition H3CTA·bipy-eta·(bipy-bu)0.5 includes a variety of aromatic guest species in its 10 × 12 A channels. The ternary and quaternary co-crystals are characterized by X-ray diffraction to analyze the hydrogen bonding and molecular packing. Whereas the acid–pyridine O–H⋯N hydrogen bond sustains reliable supramolecular aggregation in multi-component crystal structures, the change of bipyridine base connector imparts structural diversity in these supramolecular structures. A self-assembly model based on helix and chain sub-units of acid·bipy1 being connected via the bipy2 spacer to give hexagonal and square network structures is proposed.

The rare 42.63.8 network and a chiral, trigonal net in crystal structures of 1,3,5-tris(4-pyridyl)benzenes

1,3,5-tris[4-Pyridyl(ethenyl)]benzene (1) functions as a four-connected node in the first organic example of 42.63.8 network (SrAl2 type) sustained by weak C–H⋯N hydrogen bonds. 1,3,5-tris[4-Pyridyl(ethyl)]benzene (2) crystallizes in a chiral, trigonal 3,6 network that is characteristic of octupolar NLO materials. These network topologies are atypical compared to the more common hexagonal packing of trigonal molecules.

Molecular networks. Design and serendipity

The supramolecular assembly of a 2D honeycomb network viacarboxylic acid–pyrimidinone-dimer synthon and the unexpected occurrence of 3D diamondoid net in the crystal structure of a dipyridyl urea hydrate illustrate the title theme.