The Influence of Redox-Active Linkers on the Stability and Physical Properties of a Highly Electroactive Porphyrin Nanoprism

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(3-pyridyl)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 (≫103 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 PF6- anions during redox processes.