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 (N 4 H)(Cl) 2 ] + as a molecular electrocatalyst for the reduction of CO 2 to CO in wet acetonitrile. Operando UV–vis spectroelectrochemistry studies show that, under catalytic conditions, the resting state shifts from [Co I (N 4 H)] + to the carbonyl-bound species [Co I (N 4 H)(CO)] +, and cyclic voltammetry under CO atmosphere indicates even stronger binding of CO to the formal Co 0 species. This behavior indicates a self-moderation mechanism in which CO, the reaction product, reversibly binds to the catalytically active formal [Co 0 (N 4 H)] complex. Kinetic modeling of rotating disk voltammetry supports this interpretation, yielding an intrinsic catalytic rate constant of the order of 10 5 s –1 under CO 2 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 (N 4 H)(Cl) 2 ] + as a useful molecular platform for CO 2 reduction and provide design principles for next-generation cobalt catalysts that exploit reversible product binding.