The chemical environment inside nanometre-scale cages typically differs from that in bulk solution. This effect is manifest in proteins, which contain pockets that carry out a host of tasks that would be impossible in bulk water. Taking the lead from natural systems, chemists have demonstrated several examples in which they could run a chemical reaction only if the reagents were enclosed in nanometre-scale cages. Adding to this arsenal, Rizzuto et al. now show that structurally rigid metal–organic coordinated cages can be designed to host molecules that undergo unique chemistry.
In a specific example, zinc(ii) amino-terminated porphyrins are linked together through zinc(ii) anions with a phenanthroline linker to give a cuboctahedral metal–organic complex with an internal volume of about 3,000 Å. At the centre of the three pairs of faces of the cuboid, there is a Zn(ii) atom that can be itself used as a secondary coordination centre for a guest molecule. If the guest molecule is a Co(ii) porphyrin, Rizzuto et al. observe a decrease of the oxidation potential and the formation of Co(iii); if the guest molecule is a high-spin Ni(ii) porphyrin, they observe a high-spin to low-spin transformation. Neither of the two reactions would occur outside the cage.