Supramolecular synthons in crystal engineering. 3. Solid state architecture and synthon robustness in some 2,3-dicyano-5,6-dichloro-1,4-dialkoxybenzenes

Supramolecular synthons in crystals are structural units formed with intermolecular interactions. The assembly of an organic crystal is not just a geometrical interlinking with synthons but is often the consequence of compromise or mutual interference between adjacent synthons. Accordingly, synthon interference can be a major problem in systematic crystal engineering especially if it results in a lack of correspondence between molecular and crystal structures. Conversely, synthon robustness or the capacity to withstand interference is an essential attribute in crystal structure design. These ideas are illustrated with reference to the structural chemistry of a series of 1,4-disubstituted 2,3-dicyano-5,6-dichlorobenzenes and related compounds. The supramolecular synthons in this structural family are constructed with polarization-induced C≡ N···Cl interactions. Synthon interference is analyzed in terms of the trade-off between these interactions on the one hand and hydrophobic and CH···N interactions on the other. Topological relationships between these C≡ N···Cl synthons and others containing stronger hydrogen bond interactions lead to similarities in the networks formed in the crystal structures of compounds, which at the molecular level, are widely disparate