The host-guest chemistry of metal-organic nanocages is
typically driven by thermodynamically favorable interactions with their guests,
such that uptake and release of guests can be controlled by switching affinity
on/off. Herein, we achieve this effect by reducing porphyrin-walled cationic
nanoprisms <b>1a<sup>12+</sup></b> and <b>1b<sup>12+</sup></b> to zwitterionic
states that rapidly uptake organometallic cations Cp*<sub>2</sub>Co<sup>+</sup>
or Cp<sub>2</sub>Co<sup>+</sup>. Cp*<sub>2</sub>Co<sup>+</sup> binds strongly (<i>K</i><sub>a</sub>
= 1.3 x 10<sup>3</sup> M<sup>−1</sup>)
in the neutral state <b>1a<sup>0</sup></b> of host <b>1a<sup>12+</sup></b>,
which has its three porphyrin walls doubly reduced and its six (bipy)Pt<sup>2+</sup>
linkers singly reduced. The less-reduced states of the host <b>1a<sup>3+</sup></b>
and <b>1a<sup>9+</sup></b> also bind Cp*<sub>2</sub>Co<sup>+</sup>, though with
lower affinities. The smaller Cp<sub>2</sub>Co<sup>+</sup> cation binds
strongly (<i>K</i><sub>a</sub> = 1.7 x 10<sup>3</sup> M<sup>-1</sup>) in the 3
e<sup>−</sup> reduced state <b>1b<sup>9+</sup></b>
of (tmeda)Pt<sup>2+</sup> linked host <b>1b<sup>12+</sup></b>. Upon reoxidation
of the hosts with Ag<sup>+</sup>, the guests become trapped to provide
unprecedented metastable cation-in-cation complexes <b>Cp*<sub>2</sub>Co<sup>+</sup>@1a<sup>12+</sup>
</b>and <b>Cp<sub>2</sub>Co<sup>+</sup>@1b<sup>12+</sup></b> that persist for
>1 month. Thus, dramatic kinetic effects reveal a way to confine the guests
in thermodynamically unfavorable environments. Experimental and DFT studies
indicate that PF<sub>6</sub><sup>−</sup>
anions kinetically stabilize <b>Cp*<sub>2</sub>Co<sup>+</sup>@1a<sup>12+</sup> </b>through
electrostatic interactions and by influencing conformational changes of the
host that open and close its apertures. However, when <b>Cp*<sub>2</sub>Co<sup>+</sup>@1a<sup>12+</sup>
</b>was prepared using ferrocenium (Fc<sup>+</sup>) instead of Ag<sup>+</sup>
to reoxidize the host, dissociation was accelerated >200-fold even though neither
Fc<sup>+</sup> nor Fc have any competing affinity for <b>1a<sup>12+</sup></b>.
This finding shows that metastable host-guest complexes can respond to subtler
stimuli than are required to induce guest release from thermodynamically
favorable complexes.