An area of supramolecular chemistry that has recently been growing in popularity is the synthesis of metal-ligand cages. These are most commonly comprised of organic ligands and transition metal ions. Cage complexes often take the form of geometric polyhedra such as tetrahedra and octahedra, where the ligands act as the edges or faces and the metals serve as the vertices. Because these complexes have a polyhedral design, there is a central cavity in the cage, and this has been exploited for guest binding. Many types of guests have been bound inside supramolecular cage complexes, and binding is dependent on the environment inside the cage. Encapsulated guests can also react, either with themselves or with a coencapsulated guest.
Host-guest chemistry is often inspired by the selectivity of enzymes for their substrates, and the development of biomimetic systems is one of the goals for these assemblies. While encapsulation has been effective for promoting reactions and accessing novel reactivity, reactions are limited to those that require no additional reagents such as cycloadditions. No functional groups are present to direct reactivity, and reactivity is promoted by an increased effective concentration of guest molecules in the confined interior of a host. Introducing functional groups to cages can mimic the side chains of amino acids that are present in enzyme active sites, and this strategy can lead to more specialized, truly biomimetic reactivity.
This work presents a series of supramolecular cage complexes with varied geometries and assembly properties. Endohedral functionality could be introduced to a series of palladium-pyridine "paddle-wheel" complexes, and functional groups were able to influence assembly. Bidentate hydroxamic acid ligands were shown to complex bismuth and praseodymium, and attempts toward endohedrally-functionalizing these assemblies are discussed. Variable assembly was observed in complexes with 9-coordinate bismuth, and assembly was primarily dependent on metal concentration. Lanthanide complexes were also synthesized, and cooperative, thermodynamic control of selectivity between metal ions was observed. Fluorene-based ligands were combined with lanthanide ions, and complexes differing only in endohedral functionality narcissistically self-sorted despite identical ligand geometry and metal coordinating motif.
