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

The selective reduction of O, typically with the goal of forming HO, 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) (por = meso-tetra(4-methoxyphenyl)porphyrin), catalyzes either 2e/2H or 4e/4H reduction of O with high selectivity simply by changing the identity of the Brønsted acid in dimethylformamide (DMF). The thermodynamic potentials for O reduction to HO or HO in DMF are determined and exhibit a Nernstian dependence on the acid p , while the Co redox potential is independent of the acid p . The reaction product, HO or HO, is defined by the relationship between the thermodynamic potential for O reduction to HO and the Co redox potential: selective HO formation is observed when the Co potential is the O/HO potential, while HO formation is observed when the Co potential is the O/HO potential. Mechanistic studies reveal that the reactions generating HO and HO 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 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 to HO.