Functional Capsules via Subcomponent Self-Assembly

Conspectus Coordination-driven self-assembly can produce large, symmetrical, hollow cages that are synthetically easy to access. The functions provided by these aesthetically attractive structures provide a driving force for their development, enabling practical applications. For instance, cages have provided new methods of molecular recognition, chirality sensing, separations, stabilization of reactive species, and catalysis. We have fruitfully employed subcomponent self-assembly to prepare metal–organic capsules from simple building blocks via the simultaneous formation of dynamic coordinative (N→metal) and covalent (NC) bonds. Design strategies employ multidentate pyridyl–imine ligands to define either the edges or the faces of polyhedral structures. Octahedral metal ions, such as FeII, CoII, NiII, ZnII, and CdII, constitute the vertices. The generality of this technique has enabled the preparation of capsules with diverse three-dimensional structures. This Account highlights how fundamental investigations into the host–guest chemistry of capsules prepared through subcomponent self-assembly have led to the design of useful functions and new applications. We start by discussing simple host–guest systems involving a single capsule and continue to systems that include multiple capsules and guests, whose interactions give rise to complex functional behavior. Many of the capsules presented herein bind varied neutral guests, including aromatic or aliphatic molecules, biomolecules, and fullerenes. Binding selectivity is influenced by solvent effects, weak non-covalent interactions between hosts and guests, and the size, shape, flexibility, and degree of surface enclosure of the inner spaces of the capsules. Some hosts are able to adaptively rearrange structurally or express a different ratio of cage diastereomers to optimize the guest binding ability of the system. In other cases the bound guest can be either protected from degradation or catalytically transformed through encapsulation. Other capsules bind anions, most often in organic solvents and occasionally in water. Complexation is usually driven by a combination of electrostatic interactions, hydrogen bonding, and coordination to additional metal centers. Anion binding can also induce cage diastereomeric reconfiguration in a similar manner to some neutral guests, illustrating the general ability of subcomponent self-assembled capsules to respond to stimuli due to their dynamic nature. Capsules have been