Functional Capsules via Subcomponent Self-Assembly

Abstract

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 Fe^II, Co^II, Ni^II, Zn^II, and Cd^II, 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 developed as supramolecular extractants for the selective removal of anions from water and as channels for transporting anions through planar lipid bilayers and into vesicles. Different capsules may work together, allowing for functions more complex than those achievable within single host-guest systems. Incorporation of stimuli-responsive capsules into multicage systems allows individual capsules within the network to be addressed and may allow signals to be passed between network members. We first present strategies to achieve selective guest binding and controlled guest release using mixtures of capsules with varied affinities for guests and different stabilities toward external stimuli. We then discuss strategies to separate capsules with encapsulated cargos via selective phase transfer, where the solvent affinities of capsules change as a result of anion exchange or post-assembly modification. The knowledge gained from these multicage systems may lead to the design of synthetic systems that can perform complex tasks in biomimetic fashion, paving the way for new supramolecular technologies to address practical problems.