A Metallo-molecular Cage That Can Close the Apertures with Coordination Bonds.

We designed a novel tricobalt(III) metallo-molecular cage in which its apertures can be closed with bifunctional ligands via coordination bonds. A closed-cage complex was easily formed by ligand exchange of its open-cage analogue. Guest uptake/release by the open-cage complex was sufficiently fast to quickly reach guest-binding equilibrium, while that by the closed-cage complex was extremely slow and took about 1 week to reach equilibrium. The guest uptake was retarded by at least 2000 times by introduction of the bifunctional ligands at the cage apertures.