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, Co, Ni, Zn, and Cd, 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 ...

Improved Acid Resistance of a Metal–Organic Cage Enables Cargo Release and Exchange between Hosts

“…The strategy for improving the acid resistance of coordination cages developed herein may well allow other functional capsules to be constructed, in turn permitting more complex tasks to be carried out within systems of capsules. [12] Applications of such systems may include cage-mediated catalytic processes, [3a, 18] wherein the functioning of different cages may be turned on and off without impacting other parts of the system; such processes might be built into complex catalytic relays. [19] Related systems could also show utility in the selective sequential release of multiple drugs, as well as for new methods of chemical purification involving selective guest uptake and release.…”

“…[9] Such stimuli-responsive hosts, [10] capable of trapping and releasing guests in a controlled manner, are finding new uses. [11] Systems consisting of multiple hosts and guests together allow for complex functions to be designed, [12] such as functional mimicry of multi-enzyme systems. [13] Within such a system, cargo delivery from the cavity of one molecular container to another would imitate the sequential transformations in the synthesis of natural products, [14] whereby the intermediate product from one enzymatic transformation becomes the substrate of another enzyme.…”

“…We hypothesized that di(2-pyridyl)ketone might be employed in place of 2-formylpyridine during the construction of cages by subcomponent self-assembly. [12] Such new cages might exhibit enhanced acid stability due to the free basic pyridyl units at their corners. This concept was realized through the synthesis of tetrahedron 1 (Figure 1 a), which was shown to be capable of cargo release and exchange between the cavities of 1 and its analogue 2, incorporating 2formylpyridine residues, using acid and base as chemical stimuli.…”

Multicomponent Porphyrin‐Based Tetragonal Prismatic Metallacages and their Photophysical Properties

“…[7] Nitschke and coworkers reported the formation of tetrahedral cages via multicomponent self-assembly of metal ions (Fe(II), Zn(II), etc), 2-formylpyridine derivatives and diamine or triamine moieties. [8] Stang et. al.…”

Dynamic Open Coordination Cage from Nonsymmetrical Imidazole–Pyridine Ditopic Ligands for Turn‐On/Off Anion Binding

“…Despite these difficulties,t here are advantages for selfassembly of nonsymmetrical building blocks,i ncluding al ack of symmetry in building components that result in an ondense packed structure with ar ich space inside that is available for open-cage host-guest binding. [5][6][7] Moreover,the resulting coordination cage may be highly dynamic,enabling turn-on/off binding through stimuli-responsive structural transitions of the host cage.S uch controlled guest uptake and release is of potential interest but remains af ormidable challenge in host-guest chemistry. [8][9][10][11][12] Herein, we propose anew nonconventional ligand design for creating adynamic open coordination cage from nonsymmetrical building components.T he present work focuses on an M 2 L 4 -type coordination cage,which remains predominant and is easy to design with ditopic N-donor ligands (L) and as quare-planar coordination metal (M = Pd II or Pt II ).…”