Self-Assembled Coordination Cages and Organic Capsules as Catalytic Supramolecular Reaction Vessels

Abstract

Host-guest chemistry has undergone an enormous development since the discovery of cyclodextrins more than 100 years ago which has culminated in the preparation of many artificial host molecules that are not only capable of encapsulating a variety of guests but also of promoting reactions inside their cavities. As the environment dramatically influences the behavior of chemical systems, recent years have seen increased interest in the use of the shielded inner phases of synthetic hosts to stabilize reactive species, shift equilibria, or achieve otherwise unfavorable conformations of guest species. Confinement inside hosts has been used to lower the symmetry of guests, thereby creating new means to control the outcomes of asymmetric reactions in the same way that biological systems make extensive use of tailored microenvironments to promote stereospecific reactions by destabilizing the ground state and stabilizing certain transition state geometries. This chapter will focus on the use of self-assembled coordination cages and organic capsules as homogeneous catalytic supramolecular reaction vessels. Modulation of the cavity environment and binding selectivity is relatively easily achieved because small changes to the geometries of building blocks can lead to much larger changes in the structures and properties of the hollow polyhedral coordination cages formed upon self-assembly. As the reaction medium influences the binding of the reactants and products in subtle but important ways, control over host solubility through host framework charge and substituent effects provides further means to control guest binding strengths, selectivity, and dynamics, and thereby a possible way to overcome product inhibition which is often encountered in supramolecular catalysis. A review will be provided over unusual selectivity observed in reactions carried out in metal-organic capsules as a result of structural constraints. Similarly, rate enhancements in bimolecular reactions due to an increase in effective molarity and stabilization of the transition state as well as transformations carried out under unusual conditions—for example, the acid catalyzed hydrolysis of orthoformates under neutral or basic conditions—will be discussed in this chapter. Furthermore, self-assembled coordination cages based on chiral ligands are of particular interest because they provide an asymmetric microenvironment for promoting stereoselective reactions by purely non-covalent interactions. Particular emphasis will be laid on the hydrolysis of organophosphorus species: As an example of the author’s work, the catalytic degradation of the insecticide dichlorvos by a [Fe4L6]8+ cage molecule will be presented, and this report will also include the up-to-date unpublished results obtained from experiments with other organophosphorus insecticides.