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.