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

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 ⁺ , Ca ²⁺ , Sr ²⁺ , and Ba ²⁺ ) 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 p K _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 CO ₂ reduction electrocatalyst and shows high activity but rapid decomposition.
Competing Interests: There are no conflicts to declare.
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