Co-crystals of tetrakis-1,2,3,4-(4′-carboxyphenyl)cyclobutane with dipyridyl spacers: design and serendipity

Tetrakis-1,2,3,4-(4′-carboxyphenyl)cyclobutane (TCCB), a tetracarboxylic acid, has been employed for making co-crystals with linear dipyridyl spacers molecules like 4,4′-bipyridine (4,4′-bpy), 1,2-bis(4′-pyridyl)ethane (4,4′-bpethane), trans-1,2-bis(4′-pyridyl)ethylene (4,4′-bpe) and 1,4-bis(4′-pyridyl)-2,3-diaza-1,3-butadiene (4,4′-bpdb). In the case of 4,4′-bpy, a 2 : 1 co-crystal was obtained with TCCB having a three dimensional 5-fold interpenetrated dmp network. The diagonal–diagonal interpenetrated isostructural (4,4)-connected 2D networks were obtained in 1 : 1 co-crystals of TCCB with 4,4′-bpe and 4,4′-bpdb. The (4,4)-connected nets in the 1 : 1 co-crystal of TCCB with 4,4′-bpethane were found to stack parallel instead of interpenetrating. 1 : 1 co-crystals were always obtained in the last three cases regardless of molar ratio during co-crystallization, indicating the influence of kinetic factors. The structural diversity and similarities in this series of co-crystals in the context of composition variation and solvent interference are discussed. The serendipitous formation of (4,4)-connected networks is critically compared with designability of the system in presence of synthon competition.