Uptake, Trapping, and Release of Organometallic Cations by Redox-Active Cationic Hosts

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 this affinity on or off. Herein, we achieve this effect
by reducing porphyrin-walled cationic nanoprisms 1a12+ and 1b12+ to zwitterionic states that rapidly uptake organometallic cations
Cp*2Co+ and Cp2Co+, respectively.
Cp*2Co+ binds strongly (Ka = 1.3 × 103 M–1) in the
neutral state 1a0 of host 1a12+, which has its three porphyrin
walls doubly reduced and its six (bipy)­Pt2+ linkers singly
reduced (bipy = 2,2′-bipyridine). The less-reduced states of
the host 1a3+ and 1a9+ also bind Cp*2Co+, though with lower affinities. The smaller Cp2Co+ cation binds strongly (Ka = 1.7
× 103 M–1) in the 3e– reduced state 1b9+ of the (tmeda)­Pt2+-linked host 1b12+ (tmeda
= N,N,N′,N′-tetramethylethylenediamine). Upon reoxidation
of the hosts with Ag+, the guests become trapped to provide
unprecedented metastable cation-in-cation complexes Cp*2Co+@1a12+ and Cp2Co+@1b12+ 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 PF6– anions kinetically
stabilize Cp*2Co+@1a12+ through electrostatic interactions and by influencing conformational
changes of the host that open and close its apertures. However, when Cp*2Co+@1a12+ was prepared using
ferrocenium (Fc+) instead of Ag+ to reoxidize
the host, dissociation was accelerated >200× even though neither
Fc+ nor Fc have any observable affinity for 1a12+. This finding shows that metastable host–guest
complexes can respond to subtler stimuli than those required to induce
guest release from thermodynamically favorable complexes.