Aperture‐Controlled Cs + Uptake/Release Kinetics and Extremely Short Cs + –π Contact in Open‐Cage Cobalt(III) Metallocryptands

ABSTRACT Cage‐shaped molecules with an internal cavity provide platforms for molecular recognition. However, systematic control over guest exchange kinetics remains limited. Here, we report a series of cobalt(III) metallocryptands in which guest uptake/release rates are tuned by steric modulation of the cage apertures. The open‐cage metallocryptands LCo 3 A 6 , bearing monoamine ligands (A = MeNH 2 , EtNH 2 , PrNH 2 ) of different steric bulk, were synthesized and their Cs + uptake/release behavior was investigated. The guest exchange rates depend strongly on the coordinated amine ligands, spanning timescales from 6.5 s to 24 min, which are significantly faster than those of the corresponding closed‐cage analogue, LCo 3 (hda) 3 (hda = 1,6‐hexanediamine). X‐ray crystallographic analyses confirmed that the Cs + ion is completely encapsulated within the cavity to form robust cation‐in‐cation structures, despite the tricationic nature of the cryptand host. These analyses further revealed unusually short Cs + –π distances in the range of 2.98–3.04 Å, significantly shorter than the previously reported range of 3.3–3.6 Å, due to geometrical constraints imposed by the rigid metallocryptand framework with a 6.0 Å π–π spacing. Structural comparisons demonstrated that the steric bulk of the amine ligands modulates the cage aperture sizes, accounting for the observed kinetic trends.