Guest Recognition Control Accompanied by Stepwise Gate Closing and Opening of a Macrocyclic Metallohost

Host–guest binding behavior of macrocyclic hosts is significantly influenced by the shapes and sizes of the hosts. In particular, closing/opening the apertures of the hosts controls the guest uptake/release. A post‐metalation modification method was used to achieve the open/close conversions. The starting open complex, [LCo 2 (pip) 4 ](OTf) 2 , was efficiently converted to the closed complex, [LCo 2 (hda) 2 ](OTf) 2 , which has a doubly bridged structure. The conversion of this closed complex to the open complex [LCo 2 (hda) 2 (OAc)] + was too slow to be completed, but this gate‐opening was dramatically accelerated by the addition of Na + . The Na + binding was also significantly enhanced by the gate‐opening, that is, conversion of [LCo 2 (hda) 2 ] 2+ to [LCo 2 (hda) 2 (OAc)] + .