Redox-active metal−organic nanocages are of interest for many applications, but the development of cages with extensive redox activity is often hindered by their limited stability and solubility across multiple charge states. This report reveals that these properties can be tuned for cages with redox-active walls by incorporating additional redox activity into the linkers. In particular, new +12 charged triangular nanoprisms 1a,b were formed from three electroactive tetrakis(3pyridyl)porphyrin walls linked by six [(TMEDA)Pt] 2+ (for 1a) or [(2,2′-bipy)Pt] 2+ (for 1b) vertices, the latter of which are also electroactive. Thus, 1b exhibits extensive redox activity, consisting of two porphyrin-centered (x3) and two 2,2′-bipy-centered (x6) reductions that provide reversible access to +12, +9, +3, 0, and −6 charge states, whereas 1a undergoes only two, porphyrin-centered (x3) reversible reductions. Comparisons of 1a and 1b (and monomeric control compounds) by cyclic voltammetry and UV−vis−NIR spectroelectrochemistry show that the redox-activity of the linkers in 1b lowers the second reduction potential of the porphyrins by 100 mV and improves the stability and solubility of this structure under highly reducing conditions (e.g., −2.25 V vs Fc +/0 in MeCN). These findings reveal new principles for controlling the properties of highly electroactive molecular nanostructures. Anion exchange rates (≫10 3 s −1 ) were also probed, showing that the narrow apertures (≤3 Å van der Waals width) of 1a,b do not impede the loss/gain of PF 6 − anions during redox processes.

Nanocages with porphyrin walls are common, but studies of such structures hosting redox-active metals are rare. Pt2+-linked M6L3 nanoprisms with cobalt-porphyrin walls were prepared and their redox properties were evaluated...

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 1a 12+ and 1b 12+ to zwitterionic states that rapidly uptake organometallic cations Cp* 2 Co + and Cp 2 Co + , respectively. Cp* 2 Co + binds strongly (K a = 1.3 × 10 3 M −1 ) in the neutral state 1a 0 of host 1a 12+ , which has its three porphyrin walls doubly reduced and its six (bipy)Pt 2+ linkers singly reduced (bipy = 2,2′bipyridine). The less-reduced states of the host 1a 3+ and 1a 9+ also bind Cp* 2 Co + , though with lower affinities. The smaller Cp 2 Co + cation binds strongly (K a = 1.7 × 10 3 M −1 ) in the 3e − reduced state 1b 9+ of the (tmeda)Pt 2+ -linked host 1b 12+ (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* 2 Co + @1a 12+ and Cp 2 Co + @1b 12+ 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 6 − anions kinetically stabilize Cp* 2 Co + @1a 12+ through electrostatic interactions and by influencing conformational changes of the host that open and close its apertures. However, when Cp* 2 Co + @1a 12+ 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 1a 12+ . This finding shows that metastable host−guest complexes can respond to subtler stimuli than those required to induce guest release from thermodynamically favorable complexes.

Nanocages with porphyrin walls are common, but studies of such structures hosting redox-active metals are rare. Pt-linked M6L3 nanoprisms with cobalt-porphyrin walls were prepared and their redox properties were evaluated electrochemically and chemically, leading to the first time that cobalt-porphyrin nanocages have been characterized in Co(I), Co(II), and Co(III) states.

Frost diagrams provide convenient illustrations of the aqueous reduction potentials and thermodynamic tendencies of different oxidation states of an element. Undergraduate textbooks often describe the lowest point on a Frost diagram as the most stable oxidation state of the element, but this interpretation is misleading because the thermodynamic stability of each oxidation state depends on the specific redox conditions in solution (i.e., the potential applied by the environment or an electrode). Further confusion is caused by the widespread use of different, contradictory conventions for labeling the y-axis of these diagrams as either nE°or −nE°, among other possibilities. To aid in clarifying these common points of confusion, we introduce a series of interactive Frost diagrams that illustrate the conditional dependence of the relative stabilities of each oxidation state of an element. We include instructor's notes for using these interactive diagrams and a written activity for students to complete using these diagrams.