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 <b>1a¹²⁺</b> and <b>1b¹²⁺</b> to zwitterionic states that rapidly uptake organometallic cations Cp*₂Co⁺ or Cp₂Co⁺. Cp*₂Co⁺ binds strongly (<i>K</i>_a = 1.3 x 10³ M⁻¹) in the neutral state <b>1a⁰</b> of host <b>1a¹²⁺</b>, which has its three porphyrin walls doubly reduced and its six (bipy)Pt²⁺ linkers singly reduced. The less-reduced states of the host <b>1a³⁺</b> and <b>1a⁹⁺</b> also bind Cp*₂Co⁺, though with lower affinities. The smaller Cp₂Co⁺ cation binds strongly (<i>K</i>_a = 1.7 x 10³ M^-1) in the 3 e⁻ reduced state <b>1b⁹⁺</b> of (tmeda)Pt²⁺ linked host <b>1b¹²⁺</b>. Upon reoxidation of the hosts with Ag⁺, the guests become trapped to provide unprecedented metastable cation-in-cation complexes <b>Cp*₂Co⁺@1a¹²⁺ </b>and <b>Cp₂Co⁺@1b¹²⁺</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 PF₆⁻ anions kinetically stabilize <b>Cp*₂Co⁺@1a¹²⁺ </b>through electrostatic interactions and by influencing conformational changes of the host that open and close its apertures. However, when <b>Cp*₂Co⁺@1a¹²⁺ </b>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 <b>1a¹²⁺</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.