Long-range electrostatic effects from intramolecular Lewis acid binding influence the redox properties of cobalt–porphyrin complexes

Abstract

A Co^II–porphyrin complex (1) with an appended aza-crown ether for Lewis acid (LA) binding was synthesized and characterized. NMR spectroscopy and electrochemistry show that cationic group I and II LAs (i.e., Li⁺, Na⁺, K⁺, Ca²⁺, Sr²⁺, and Ba²⁺) bind to the aza-crown ether group of 1. The binding constant for Li⁺ is comparable to that observed for a free aza-crown ether. LA binding causes an anodic shift in the Co^II/Co^I couple of between 10 and 40 mV and also impacts the Co^III/Co^II couple. The magnitude of the anodic shift of the Co^II/Co^I couple varies linearly with the strength of the LA as determined by the pK_a of the corresponding metal–aqua complex, with dications giving larger shifts than monocations. The extent of the anodic shift of the Co^II/Co^I couple also increases as the ionic strength of the solution decreases. This is consistent with electric field effects being responsible for the changes in the redox properties of 1 upon LA binding and provides a novel method to tune the reduction potential. Density functional theory calculations indicate that the bound LA is 5.6 to 6.8 Å away from the Co^II ion, demonstrating that long-range electrostatic effects, which do not involve changes to the primary coordination sphere, are responsible for the variations in redox chemistry. Compound 1 was investigated as a CO₂ reduction electrocatalyst and shows high activity but rapid decomposition.