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, [LCo2(pip)4](OTf)2, was efficiently converted to the closed complex, [LCo2(hda)2](OTf)2, which has a doubly bridged structure. The conversion of this closed complex to the open complex [LCo2(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 [LCo2(hda)2]2+ to [LCo2(hda)2(OAc)]+. [ABSTRACT FROM AUTHOR]