A Self-Moderation Mechanism in CO ₂ Electroreduction Catalyzed by a Cobalt Macrocyclic Complex

We present a detailed mechanistic study of the tetra-aza-macrocyclic Schiff base cobalt complex [Co^III(N₄H)(Cl)₂]⁺ as a molecular electrocatalyst for the reduction of CO₂ to CO in wet acetonitrile. Operando UV-vis spectroelectrochemistry studies show that, under catalytic conditions, the resting state shifts from [Co^I(N₄H)]⁺ to the carbonyl-bound species [Co^I(N₄H)(CO)]⁺, and cyclic voltammetry under CO atmosphere indicates even stronger binding of CO to the formal Co⁰ species. This behavior indicates a self-moderation mechanism in which CO, the reaction product, reversibly binds to the catalytically active formal [Co⁰(N₄H)] complex. Kinetic modeling of rotating disk voltammetry supports this interpretation, yielding an intrinsic catalytic rate constant of the order of 10⁵ s^-1 under CO₂ atmosphere. This reversible CO binding attenuates catalytic activity by product inhibition yet does not induce irreversible deactivation, distinguishing it from classical catalyst poisoning. While this self-regulated behavior lowers the efficiency of CO production, it may favor subsequent activation of CO. These findings highlight [Co^III(N₄H)(Cl)₂]⁺ as a useful molecular platform for CO₂ reduction and provide design principles for next-generation cobalt catalysts that exploit reversible product binding.