A covalent organic nanocapsule has been prepared by linking together the ligands of a metallocapsule and subsequently removing its metal centres
The hollows within molecular cages, which enable the encapsulation of guest species, are particularly attractive for catalysis and recognition applications. Most approaches to the synthesis of such cages have focused on supramolecular self-assembly, relying on reversible interactions such as hydrogen bonding or metal coordination. Organic capsules held together covalently are more stable but also more difficult to prepare. Now, Shuichi Hiraoka, Mitsuhiko Shionoya and colleagues at the University of Tokyo have prepared1 a covalent organic capsule using its metallated counterpart as a template.
Previously, the researchers had created a capsule that formed an octahedral cage by coordinating palladium centres (acting as vertices) with triangular ligands (acting as faces). They have now designed ligands bearing three alkyl chains ending with olefin groups that can be joined to hold the capsule together on subsequent removal of the metal vertices. On assembly of the metallated cage, pairs of unlinked olefin chain-ends are brought close together — allowing their crosslinking by metathesis and forming covalent chains between adjacent faces. In a final step, the remaining intrachain olefin functions were reduced, leading to a covalent organic capsule with flexible alkyl linkers between its faces.
The researchers were also able to prepare a bigger, polycationic capsule — in which the flexible linking chains were fully expanded — by functionalizing the ligands post-synthesis to create positively charged, and thus electrostatically repelling, nitrogen atoms at their corners.
Hiraoka, S., Yamauchi, Y., Arakane, R. & Shionoya, M. Template-directed synthesis of a covalent organic capsule based on a 3 nm-sized metallocapsule. J. Am. Chem. Soc. 10.1021/ja903324r (2009).