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.
The cage has also been used as a supramolecular protecting group in the Diels-Alder reaction between furan and maleimide. Encapsulation of furan in the cage prevents the Diels-Alder reaction, while subsequent addition of a competing guest will initiate the reaction.
A self-assembled M4L6 cage, incorporating small amounts of enantiopure subcomponents at the peripheries, forms predominantly with a one-handed twist at all metal centers. Strong stereochemical coupling between metal centers in the cage amplifies energy differences between the ΔΔΔΔ and ΛΛΛΛ diastereomers as compared to analogous mononuclear metal complexes.
Recently we described a synthetic chemical system that is capable of structural reconstitution on receipt of one anionic signal (perchlorate) to create a tight binding pocket for another anion (chloride). The complex, barrel-like structure of the chloride receptor is templated by five perchlorate anions. This second-order templation phenomenon allows chemical networks to be envisaged that express more complex responses to chemical signals than is currently feasible.
