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)]+ .