Discrete metallosupramolecular systems are often macrocyclic or cage-like architectures with an accessible internal cavity. Guest molecules can reside within these cavities and much of the interest in these systems is derived from these fascinating host-guest interactions. A range of potential applications stem from the ability of these metallosupramolecular architectures to encapsulate guests. These applications include catalysis or acting as molecular reaction flasks, the molecular scavenging of pollutants, storage of reactive species, and drug delivery. Multicavity metallosupramolecular architectures combine the ability of large hollow assemblies to bind multiple guests concurrently with the binding specificity associated with small cages. A variety of different approaches to generating separate compartments within a single metallosupramolecular assembly have emerged. These include interpenetrated cages, cages with polytopic ligands that have a long backbone, and molecules that have two or more clefts. This review examines these approaches, and highlights key contributions to the field.

Two new ferrocene-containing [Pd2(LFc)4]4+(X−)4 (where X− = BF4− or SbF6−) self-assembled cages were synthesised from the known, rotationally flexible, 1,1′-bis(3-pyridylethynyl)ferrocene ligand (LFc). The cages were shown to bind neutral and anionic guest molecules and displayed reversible redox activity.

The need for effective CO capture systems remains high, and due to their tunability, metallosupramolecular architectures are an attractive option for gas sorption. While the use of extended metal organic frameworks for gas adsorption has been extensively explored, the exploitation of discrete metallocage architectures to bind gases remains in its infancy. Herein the solid state gas adsorption properties of a series of [Pd (L) ] lantern shaped coordination cages (L = variants of 2,6-bis(pyridin-3-ylethynyl)pyridine), which had solvent accessible internal cavities suitable for gas binding, have been investigated. The cages showed little interaction with dinitrogen gas but were able to take up CO . The best performing cage reversibly sorbed 1.4 mol CO per mol cage at 298 K, and 2.3 mol CO per mol cage at 258 K (1 bar). The enthalpy of binding was calculated to be 25-35 kJ mol , across the number of equivalents bound, while DFT calculations on the CO binding in the cage gave ΔE for the cage-CO interaction of 23-28 kJ mol , across the same range. DFT modelling suggested that the binding mode is a hydrogen bond between the carbonyl oxygen of CO and the internally directed hydrogen atoms of the cage.

The cavities of metallosupramolecular cages can be used to mimic the central spaces of naturally occurring proteins and bind a wide variety of molecular guests. A range of potential applications have arisen from this capacity for host-guest chemistry. However, to truly harness the opportunities thus afforded, methodologies to controllably allow the release and reuptake of guests from the cavities of metallosupramolecular cages are required. Methods to accomplish this have centered upon reversibly altering the character of either the guest or host. This minireview outlines the current approaches used to carry out the binding and release of guests from metallosupramolecular hosts using important examples from the field.