Bronsted 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 H2O, 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 Bronsted acid in dimethylformamide (DMF). The thermodynamic potentials for O-2 reduction to H(2)O(2 )or H2O in DMF are determined and exhibit a Nernstian dependence on the acid pK(a), while the Co-III/II redox potential is independent of the acid pK(a). The reaction product, H2O or H2O2, is defined by the relationship between the thermodynamic potential for O-2 reduction to H2O2 and the Co-III/II redox potential: selective H2O2 formation is observed when the Co-III/II potential is below the O-2/H2O2 potential, while H2O formation is observed when the Co-III/II potential is above the O-2/H2O2 potential. Mechanistic studies reveal that the reactions generating H(2)O(2 )and H2O 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 pK(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 H2O .