The proton-coupled electron transfer (PCET) mechanism for O–O bond cleavage in the oxygen reduction reaction (ORR) often mirrors O–O bond formation in the oxygen evolution reaction (OER) for both biological and chemical systems. For instance, Babcock first noted that the associative O2 bond forming chemistry in Photosystem II has remarkable mechanistic similarities to the dissociative O2 bond breaking chemistry in cytochrome c oxidase. The efficiency of associative and dissociative O–O bond activation depends on the critical coupling between the electron and the proton. When examining such PCET O2 ↔ H2O conversions, rarely can the PCET event of O–O bond activation be isolated. We report here that ORR catalyzed by manganese(II) porphyrins in the presence of Brønsted acids (HAs) does indeed allow the kinetics of O–O bond cleavage to be captured. Electrochemical kinetics data support a model where an oxygen bound to the reduced Mn(II) porphyrin platform is stabilized by a hydrogen bond to HA; in the case of the Mn hangman platforms, the HA is not needed as an internal hydrogen bond may be formed with the hanging acid group. The cleavage of the O–O bond of a subsequently formed Mn(III)-hydroperoxide is rate determining, as shown by foot-of-the wave analysis of catalytic cyclic voltammetric ORR profiles, thus allowing the kinetics for the PCET-induced O–O bond cleavage to be determined directly.

Proton coupled electron transfer bond cleavage of oxygen

255th National Meeting (2018)

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