Functional Organic Zeolite Analogues

Abstract

There is much current interest in organic solid hosts, whose guest-binding properties are reminiscent of traditional inorganic zeolites. The basic design strategy is to assemble organic and metal-ion building blocks into a network by using directional intermolecular interactions such as hydrogen bonding and coordination. The network structures can be controlled by the geometrical and topological properties of the building blocks. When free from interpenetration, the resulting networks afford cavities or channels (occupying, in some cases, as much as 60-70% of the total volume) capable of selective guest binding. Some coordination and multiply hydrogen-bonded networks are robust enough to withstand the removal of included guests, thus sustaining as large as 10 Angstrom guest-free channels. Less robust hydrogen-bonded networks undergo a transition to more dense structures upon guest removal; they are, however, flexible enough to readsorb the guests and restore the single-crystal structures of the host-guest adducts. Guest exchange also occurs, during which crystallinity is retained. Another important consequence of host-guest complexation is activation of trapped guests. Facilitated intracavity reactions, coupled with dynamic guest-binding behaviours (exchange of products and reactants as guests), suggest a potential use of the present type of microporous organic and metal-organic solids as catalysts. This is in fact demonstrated for the Diels-Alder and related ene reactions. The present stage of functional organic zeolite analogues and the problems and prospects associated therewith are discussed from both static and dynamic viewpoints.