A Clathrate Uncertainty Principle

The crystal engineering design of new clathrate hosts that employ intermolecular attractive forces much weaker than hydrogen bonding and coordination complexation remains problematic. One approach has been to prepare a series of V-shaped molecules with C2symmetry, a small degree of conformational freedom, and protruding halogen atoms. This design has been highly successful (95%), but little prediction of the exact packing in the potential clathrate crystals is possible. This situation arises because of competition between combinations of different types of weak intermolecular host-guest and host-host attractions in their search for the lowest energy structure. Uncharacteristically, the dibromodiquinoline host 6 proved to be rather consistent in its molecular assembly across nine inclusion structures. The extent of packing prediction therefore was probed by exploring the behaviour of its positional isomer 10. This new substance behaved very differently from its molecular cousin 6 and gave a range of different crystal forms. The X-ray structures of 10, (10)2.(acetone), (10).(pyridine) and (10)2.(dioxane) are described here. Therefore a type of clathrate uncertainty principle has been reached, wherein prediction of clathrate formation has been maximised but at the cost of minimising packing prediction.