Linear diamine and formylpyridine subcomponents form a tetrahedral cage with iron(II) in water (see scheme). This cage traps hydrophobic guests with high specificity within a rigid cavity, isolating them from the aqueous environment. The cage may be broken, releasing the guest, upon the addition of a triamine. It may also be unlocked by adding acid, allowing the guest to be reversibly released until base is added, relocking it within.
Mal, P., Schultz, D., Beyeh, N. K., Rissanen, K., & Nitschke, J. (2008). An Unlockable–Relockable Iron Cage by Subcomponent Self-Assembly. Angewandte Chemie (International Edition) , 47(43), 8297–8301. https://doi.org/10.1002/anie.200803066