The fascinating field of molecular capsules has recently begun to see the creation of structures that, medicated by the encapsulation of guest molecules within their central cavity, are able to change the properties or reactivity of the substrate. The current capsule designs are however, prone to exchange of either part or whole ligands. This exchange or the capsule's subsequent disassembly can lead to loss of the cavity or modification of their external properties, and is a barrier to their more widespread application, a problem this work seeks to address by creating more a robust capsule structure. This thesis presents the design, synthesis and properties displayed by three novel capsules. All the species presented share a similar supramolecular tetrahedral structure, but their properties deviate significantly, showing either switchable behaviour, spin-crossover or a novel synthetic route to a kinetically inert structure. Improvements in the design have led to a final capsule that is water-soluble, robust, non-toxic and has been shown to encapsulate a range of guests. Chapter 1 includes an overview of the types of capsule constructed in literature and their possible application. The fundamental properties of these capsules are identified, with emphasis given to a discussion of mechanisms underlying their encapsulation phenomena. Chapter 2 describes efforts to construct a tetrahedral capsule based on iron(II) and an oxime ligand. While the use of an oxime motif achieved the aim of preventing exchange of the external groups, the capsule also displayed the surprising property of possessing a solvent responsive assembly-disassembly process. This potentially provides a basis for 'on-demand' encapsulation by being able to choose when to have hydrophobic cavity available for guests. Chapter 3 details the synthesis of a tetrahedral capsule containing iron (II) coordinated by a pyridyl-triazole bonding motif. the spin-crossover properties of the complex were initially demonstrated in the solid state, however, when in solution the capsule displayed the unusual ability of spin-crossover mediated structural rearrangement. Chapter 4 demonstrates the synthesis of a robust capsule. The synthetic route shown alleviates the problems surrounding the construction of inert species in a self-assembly process. Based around a cobalt (III) cation, the stability of the capsule to carious conditions is examined and its host-guest chemistry is explored, revealing some insights into the encapsulation behaviour of this structure.