Design of Clathrate Compounds that Use Only Weak Intermolecular Attractions

Intermolecular attractive forces that are considerably weaker than hydrogen bonding and coordination complexation may be used in the design of new molecules that function as host molecules in the solid-state. Known literature examples of accidentally discovered hosts (clathrands), which do not involve strong interactions in their crystals, are identified and discussed. Their molecular symmetry and supramolecular interactions are analysed in order to identify structural features that facilitate and promote molecular inclusion. The solid-state properties of a family of designed compounds that embody these principles are then described. Prediction of their inclusion behaviour was 95% successful and a wide variety of crystal packing arrangements were encountered. This is an inevitable consequence of competition between many different molecular interactions of comparable energy during the crystallisation process. The lowest energy combination of these host-host and host-guest associations generates the observed outcome. One consequence of this behaviour is that detailed prediction of a new clathrate crystal packing arrangement is extremely difficult. However, a second consequence is that crystal structure analysis provides a rich source of information about weak intermolecular forces and new supramolecular synthons that previously had remained hidden.