Competition and selectivity in supramolecular synthesis: structural landscape around 1-(pyridylmethyl)-2,2'-biimidazoles.

Three isomeric forms of 1-(pyridylmethyl)-2,2'-biimidazole, A1-A3, have been synthesized and subjected to systematic co-crystallizations with selected hydrogen- and halogen-bond donors in order to explore the impact of electrostatics and geometry on the resulting supramolecular architectures. The solid-state supramolecular behavior of A1-A3 is largely consistent in halogen-bonded co-crystals. Only two types of primary interactions, the N-HN/NH-N homomeric hydrogen-bond interactions responsible for the pairing of biimidazole moieties and the IN(pyridine) halogen bonds responsible for the co-crystal formation and structure extension, are present in these systems. The co-crystallizations with hydrogen-bond donors (carboxylic acids), however, lead to multiple possible structural outcomes because of the presence of the biimidazole-acid N-HO[double bond, length as m-dash]C/NH-O heterosynthon that can compete with biimidazole-biimidazole N-HN/NH-N homosynthon. In addition, the somewhat unpredictable nature of proton transfer makes the hydrogen-bonded co-crystals structurally less consistent than their halogen-bonded counterparts.