Even just within catenanes and rotaxanes there is scope for a huge amount of structural variation dependent upon the number of components, the manner in which they are interlocked, and their chemical structure. The literature on MIMs is vast; reviews detailing synthetic strategies based on anions and metal cations are indicated in the relevant sections below, with alternative template motifs covered here. Perhaps unsurprisingly, given their critical role in the development of high-yielding synthetic methods to access MIMs, 210 the binding of transition metal ions has been explored extensively (Fig. ). The mechanically-enforced proximity of coordinating units on sub-components induces a "mechanical chelate" effect (originally termed the catenand effect by Sauvage). In this manner, MIM ligands have been used to stabilise oxidation states or enforce unusual coordination environments of metal ions 213 and for selective detection. Like transition metal ions, anions have been used to template the formation of MIMs. As such, various interlocked molecules have been engineered to possess units capable of interacting with anions (Fig. ) 216 through non-covalent interactions. In this manner, MIMs have been synthesised that demonstrate binding of various anions, including preferences between halides, 217 selective binding of polyatomic anions such as NO3 -, 218 SO4 2-, 219 and encapsulation in aqueous media. Whilst most commonly employed for the binding of charged species, encapsulation of small neutral molecules by MIMs has been reported, albeit often serendipitously (Fig. ). Stoddart and co-workers have developed stepwise syntheses of both oligo [n]catenanes and rotacatenanes that proceed through the (transient) formation of a pseudorotacatenane, i.e. an unstoppered axle component bound between the components of a catenane via π-π and hydrogen bonding interactions, the solid-state structure of one of which (9695) was reported. Loeb and co-workers reported the binding of a dibenzo-24crown-8 macrocycle between the components of a [3]catenanes through π-π and CH-π interactions (9798). It is possible to specifically engineer recognition components within the sub-components of MIMs for binding to small-molecule guests (Fig. ). Smithrud and co-workers, for example, have designed rotaxane hosts incorporating macrocyclic recognition units into the axle components, with additional host-guest interactions from the macrocyclic components (e.g. 99 in Fig. ), for the intracellular delivery of various guests. Niemeyer and co-workers synthesised [2]catenane 100 incorporating chiral BINOL-phosphate units within each of the macrocycles and demonstrated its ability to bind diamine guests in DMSO, observing stereoselective binding of chiral guests; Beer and co-workers showed binding of dicarboxylates within a [3]rotaxane, also reporting chiral discrimination. 225 A [3]rotaxane with porphyrin units attached to the two macrocycles was shown by Morin and co-workers to bind fullerenes in a tweezer-like fashion. 226 Fig. MIMs are well suited to the binding of small, charged guests such as Cu(I), Cl-and I-due to the mechanically-enforced proximity of interacting motifs. Less well explored is the binding of neutral guest molecules through interactions between the mechanically interlocked components.