A series of iron porphyrins that vary only in the distal H-bonding network exhibit turnover frequencies (TOFs) ranging from 1.0 s⁻¹ to 10³ s⁻¹.

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 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 O₂ reduction to H₂O₂ and the Co^III/II redox potential: selective H₂O₂ formation is observed when the Co^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 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 O₂ to H₂O.

A synthetic mimic of Fe–Fe hydrogenase (H₂ase) is reported which bears a terminal alkyne group in the ligand. Using a terminal azide bearing organic linkers, this complex could be covalently attached to various electrode surfaces (e.g. edge plane graphite, reduced graphene oxide, etc.). The electrocatalytic hydrogen evolution (HER) efficiency of these constructs is investigated and the results show that the EPG–H₂ase mimic construct is able to produce H₂ from acidic water efficiently with over 90% selectivity.