Brønsted Acid Scaling Relationships Enable Control Over Product Selectivity from O2 Reduction with a Mononuclear Cobalt Porphyrin Catalyst

The selective reduction of O 2 , typically with the goal of forming H 2 O, represents a long-standing challenge in the field of catalysis. Macrocyclic transition-metal complexes, and cobalt porphyrins in particular, have been the focus of extensive study as catalysts for this reaction. Here, we show that the mononuclear Co-tetraarylporphyrin complex, Co(por OMe ) (por OMe = meso-tetra(4-methoxyphenyl)porphyrin), catalyzes either 2e - /2H + or 4e - /4H + reduction of O 2 with high selectivity simply by changing the identity of the Brønsted acid in dimethylformamide (DMF). The thermodynamic potentials for O 2 reduction to H 2 O 2 or H 2 O in DMF are determined and exhibit a Nernstian dependence on the acid p K a , while the Co III/II redox potential is independent of the acid p K a . The reaction product, H 2 O or H 2 O 2 , is defined by the relationship between the thermodynamic potential for O 2 reduction to H 2 O 2 and the Co III/II redox potential: selective H 2 O 2 formation is observed when the Co III/II potential is below the O 2 /H 2 O 2 potential, while H 2 O formation is observed when the Co III/II potential is above the O 2 /H 2 O 2 potential. Mechanistic studies reveal that the reactions generating H 2 O 2 and H 2 O exhibit different rate laws and catalyst resting states, and these differences are manifested as different slopes in linear free energy correlations between the log(rate) versus p K a and log(rate) versus effective overpotential for the reactions. This work shows how scaling relationships may be used to control product selectivity, and it provides a mechanistic basis for the pursuit of molecular catalysts that achieve low overpotential reduction of O 2 to H 2 O.