Uptake, Trapping, and Release of Organometallic Cations in 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 affinity on/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⁺ or Cp₂Co⁺. Cp*₂Co⁺ binds strongly (K_a = 1.3 x 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. The less-reduced states of the host 1a³⁺ and 1a⁹⁺ also bind Cp*₂Co⁺, though with lower affinities. The smaller Cp₂Co⁺ cation binds strongly (K_a = 1.7 x 10³ M^-1) in the 3 e⁻ reduced state 1b⁹⁺ of (tmeda)Pt²⁺ linked host 1b¹²⁺. 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-fold even though neither Fc⁺ nor Fc have any competing affinity for 1a¹²⁺. This finding shows that metastable host-guest complexes can respond to subtler stimuli than are required to induce guest release from thermodynamically favorable complexes.