Systematic construction of progressively larger capsules from a fivefold linking pyrrole-based subcomponent

Biological encapsulants, such as viral capsids and ferritin protein cages, use many identical subunits to tile the surface of a polyhedron. Inspired by these natural systems, synthetic chemists have prepared artificial nanocages with well-defined shapes and cavities. Rational control over the self-assembly of discrete, nanometre-scale, hollow coordination cages composed of simple components remains challenging as a result of the entropic costs associated with binding many subunits together, difficulties in the error-correction processes associated with assembly and increasing surface energy as their size grows. Here we demonstrate the construction of nanocages of increasing size derived from a single pentatopic pyrrole-based subcomponent. Reasoned shifts in the preferred coordination number of the metal ions used, along with the denticity and steric hindrance of the ligands, enabled the generation of progressively larger cages. These structural changes of the cages are reminiscent of the differences in the folding of proteins caused by minor variations in their amino acid sequences; understanding how they affect capsule structure and thus cavity size may help to elucidate the construction principles for larger and functional capsules, capable of binding and carrying large biomolecules as cargoes. Controlling the self-assembly of large coordination cages is challenging owing to entropic costs and difficulties in error correction. Now an array of large cages prepared by the rational design of alterations that allow for the tuning of the dihedral angle between pentagonal subunits is reported.