Reactivity under confinement often differs greatly from reactivity in the bulk. Metal-organic cages (MOCs) are a class of discrete, solution-processable container molecules encompassing well-defined nanospaces, which can be rapidly constructed in modular fashion via self-assembly. Supramolecular chemists have created an extensive library of MOCs and demonstrated their ability to serve as molecular flasks, with cavities tailored to bind guests of interest. In this review, we cover selected examples of the encapsulation and relative stabilisation of reactive species within MOCs, from early reports to the most recent developments. Most reactive species are not inherently unstable; but they persist only as long as they do not encounter a partner with whom they can react. MOCs can prevent or reduce the rate of this deletrious reactivity through acting as a shield and providing a physical barrier between an encapsulated reactive guest and other system components regularly encountered in the bulk environment, including air, water, solvent, light, another molecule of itself, or a co-reactant. Thus, MOCs can extend the lifetime of these short-lived reactive species enhancing their study, or allowing for different reactivity to be explored. Examples have been segregated based on the nature of stabilisation (i.e., with what partner a reaction has been prevented). We believe this analysis will help provide more nuanced understanding of what types of highly reactive species can be tolerated within a dynamic MOC system to enable MOCs to find use in a wider variety of real-world applications.
