As we recently reported in Angew. Chem. Int. Ed., 2008, 47, 8297-8301 (and highlighted on the front cover of that journal), the inexpensive formyl-pyridine and diaminobiphenyl disulfonate subcomponents shown below come together with iron(II) ions in water to form the tetrahedral cage shown below. Its crystal structure (further down), solved by Kari Rissanen, revealed the presence of an internal cavity 140 Å3 in volume. When this cage was prepared in the presence of cyclohexane, a host-guest complex was formed in which cyclohexane was tightly trapped within the hydrophobic cavity of the cage. Thus, despite cyclohexane's volatility, the solid complex did not lose guest under vacuum.

The guest may readily be liberated, however, upon addition of acid to the complex. This process is analogous to "unlocking" the cage in that the guest may be "relocked" within a regenerated cage upon addition of base. The cage may also be irreversibly destroyed upon addition of a different chemical signal, tris(ethylamino)amine, which breaks the cage open in forming a highly stable FeII chelate.

As we recently reported in Science, the same cage is capable of binding white phosphorus as a guest. Within the cage, ordinarily pyrophoric P4 molecules become air-stable! It is straightforward to extract the phosphorus through addition of the competing guest benzene, as shown below at left. Kari Rissanen again provided a crucial piece of this puzzle by solving the crystal structure of the P4-containing cage, shown below at right.

The cage does not stabilize its guest through hermetic exclusion of oxygen, but rather through a constrictive mechanism. The reaction of O2 with P4 would proceed through a transition state too large for the cage's cavity.

We are currently preparing larger cages, which may have relevance in the context of drug or fragrance delivery, by delivering a specific "payload" upon receipt of a specific chemical signal. The dynamic assembly of cages from subcomponents might also be used to trap harmful molecules, preventing hydrophobic toxins or chemical warfare agents from doing damage by isolating them from their environment.

Different sets of subcomponents might also create different cage structures, such as the cube and dodecahedron shown below!