Brønsted Acid Scaling Relationships Enable Control Over Product Selectivity from O(2) Reduction with a Mononuclear Cobalt Porphyrin Catalyst

[Image: see text] 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 pK(a), while the Co(III/II) redox potential is independent of the acid pK(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 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 H(2)O.