Systematic construction of progressively larger capsules from a 5-fold linking 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 an extensive series of 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 still poses considerable challenges 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 a family of nanocages of increasing size derived from a single simple pentatopic pyrrole-based subcomponent. Reasoned shifts in the preferred coordination number of the metal ions employed, along with the denticity and steric hindrance of the ligands, enabled the generation of progressively larger cages, incorporating more subunits. These structural changes of the cages through these ‘mutations’ are reminiscent of differences in the folding of proteins caused by minor variations in their amino acid sequences; understanding how they impact capsule structure and thus cavity size may help to elucidate construction principles for still larger, more complex and functional capsules, capable of binding and carrying large biomolecules as cargoes.