Chain-Length-Selective Aperture Closure of a Cobalt(III) Tris(Salen) Cryptophane With Bridging Diamines

Precise geometric matching between a bridging ligand and its target binding sites provides a powerful yet underexplored mechanism for selective structural control in supramolecular host systems. Here we demonstrate that the aperture of a cobalt(III) tris(salen) cryptophane can be selectively closed by bridging diamines whose chain length is geometrically compatible with the Co⋯Co distance within the cage framework. Systematic variation of the diamine chain length revealed pronounced chain-length selectivity, with efficient formation of a single closed-cage complex, LCo₃(diamine)₃(OTf)₃, observed only for diamines of appropriate length. Diamines that were too short produced mixtures of incompletely bridged species, whereas longer diamines gave additional products containing diamines with monodentate coordination. Furthermore, the same closed-cage complexes were generated by ligand exchange from a preformed open-cage complex bearing monodentate methylamine ligands, demonstrating that aperture closure can also be achieved through dynamic structural transformation. These results establish geometric compatibility between bridging ligands and metal coordination sites as a design principle for chain-length-selective aperture closure in cryptophane-based hosts. Moreover, access to the same closed-cage complexes through ligand exchange provides a strategy for the dynamic modulation of molecular host architectures.