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Compartmentalization of the aqueous space within a cell is necessary for life. In similar fashion to the nanometer-scale compartments in living systems, synthetic water-soluble coordination cages (WSCCs) can isolate guest molecules and host chemical transformations. Such cages thus show promise in biological, medical, environmental, and industrial domains. This review highlights examples of three-dimensional synthetic WSCCs, offering perspectives so as to enhance their design and applications. Strategies are presented that address key challenges for the preparation of coordination cages that are soluble and stable in water. The peculiarities of guest binding in aqueous media are examined, highlighting amplified binding in water, changing guest properties, and the recognition of specific molecular targets. The properties of WSCC hosts associated with biomedical applications, and their use as vessels to carry out chemical reactions in water, are also presented. These examples sketch a blueprint for the preparation of new metal–organic containers for use in aqueous solution, as well as guidelines for the engineering of new applications in water.

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 ...

Separation technology is central to industries as diverse as petroleum, pharmaceuticals, mining, and life sciences. Metal-organic cages, a class of molecular containers formed via coordination-driven self-assembly, show great promise as separation agents. Precise control of the shape, size, and functionalization of cage cavities enables them to selectively bind and distinguish a wide scope of physicochemically similar substances in solution. Extensive research has thus been performed involving separations of high value targets with coordination cages, ranging from gases and liquids to compounds dissolved in solution. Enantiopure capsules also show great potential for the separation of chiral molecules. The use of crystalline cages as absorbents, or the incorporation of cages into polymer membranes, could increase the selectivity and efficiency of separation processes. This review covers recent progress in using metal-organic cages to achieve separations, with discussion of the many methods of using them in this context. Challenges and potential future developments are also discussed.Compared to metal-organic frameworks (MOFs), a class of solid, porous coordination-based materials that have been widely used for separation, [29][30][31][32] an advantage of coordination cages is their tailorable solubilities. 33,34 Cages in solution, in contrast to solid MOFs, can deform or de-ligate so as to encapsulate a guest much larger than the cage pores. Cages can also be incorporated into polymers, [35][36][37][38] as fillers, to prepare mixed-matrix membranes for performing separations. [39][40][41][42] The solubility of these capsules in different solvents enables them, in contrast to MOFs, to form homogeneous mixtures with polymer matrices, avoiding agglomeration or precipitation of fillers during membrane formation, which may lead to poor separation performance.This review summarizes recent advances in using metal-organic cages for separations, with emphasis on their multiple modes of use, ranging from cages in solution and in the solid, to polymer membranes and separation columns with cages incorporated. Cages in solution for extractionCoordination cages have been explored as liquid-phase extractants to extract target molecules from an immiscible liquid phase (liquid-liquid extraction) or a solid phase (solid-liquid extraction).These strategies require binding affinities that are strong enough to pull target molecules from the initial phase into another. Cages bring guests into solvents in which the guests are not normally soluble; the cages are thus useful liquid-phase extractants. Successful application of this extraction strategy requires cages that are stable in different solvents. The high stability of a host-guest complex after extraction can impede extractant recycling and cargo recovery, however stimuli-responsive guest release can be used to overcome these challenges. [43][44][45][46][47] Liquid-liquid extractionThe Nitschke group recently reported Fe II 4L4 tetrahedron 1 (Figure 1A), prepared from an azaphosp...

A variety of different three-dimensional metal-organic container molecules have recently been prepared using subcomponent self-assembly, which relies upon metal template effects to generate complex structures from simple molecular precursors and metal salts. Many of these structures have well defined internal pockets, allowing guest species to be bound and the chemical reactivity of these guests to be modified. Such host molecules have potential applications ranging from the protection of sensitive chemical species to the separation and purification of substrates as diverse as gases, gold compounds, and fullerenes.