Brønsted Acid Scaling Relationships Enable ControlOver Product Selectivity from O2 Reduction with a MononuclearCobalt Porphyrin Catalyst

摘要

The selective reduction of O2, 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 O2 with high selectivity simply by changing the identity of the Brønsted acid in dimethylformamide (DMF). The thermodynamic potentials for O2 reduction to H2O2 or H2O in DMF are determined and exhibit a Nernstian dependence on the acid pKa, while the Co^III/II redox potential is independent of the acid pKa. The reaction product, H2O or H2O2, is defined by the relationship between the thermodynamic potential for O2 reduction to H2O2 and the Co^III/II redox potential: selective H2O₂ formation is observed when theCo^III/II potential is below the O₂/H₂O₂ potential, while H₂O formation is observed when the Co^III/II potential is above the O₂/H₂O₂ potential.Mechanistic studies reveal that the reactions generating H₂O₂ and H₂O exhibit different rate laws andcatalyst resting states, and these differences are manifested as differentslopes in linear free energy correlations between the log(rate) versuspK_a and log(rate) versus effective overpotentialfor the reactions. This work shows how scaling relationships may beused to control product selectivity, and it provides a mechanisticbasis for the pursuit of molecular catalysts that achieve low overpotentialreduction of O₂ to H₂O.