Nonheme Iron Catalyst Selectively Activates Oxygen to Hydrogen Peroxide

Iron complexes are known for their excellent reactivity toward the oxygen reduction reaction (ORR), which proceeds via two possible pathways: a two-electron/two-proton (2e^-/2H⁺) process to form hydrogen peroxide or a four-electron/four-proton (4e^-/4H⁺) process to form water. Developing catalysts that enable selective oxygen reduction remains a challenge. Inspired by heme-based systems, we designed two iron complexes incorporating secondary coordination sphere interactions to investigate their influence on the ORR selectivity. The complexes, [Py₂Py-(afa^Cy)₂Fe]-OTf₂ and [N-(afa^Cy)₃Fe]-OTf₂, were evaluated for their catalytic activity using decamethylferrocene as the reductant, with reaction progress monitored via absorbance spectroscopy. [Py₂Py-(afa^Cy)₂Fe]-OTf₂ exhibited a selectivity profile comparable to iron porphyrin but with a slower kinetic rate, likely due to the steric hindrance from ligand functionalization. [N-(afa^Cy)₃Fe]-OTf₂ demonstrated exceptional selectivity toward the 2e^-/2H⁺ pathway, a rare observation for nonheme iron complexes. Kinetic measurements revealed that the catalytic reaction with [N-(afa^Cy)₃Fe]-OTf₂ follows second-order kinetics with a rate constant of 81 mM^-1 s^-1. We propose that the rate-determining step involves electron transfer from decamethylferrocene to the hydroperoxo iron-(III) complex, occurring through a stepwise proton transfer/electron transfer (PTET) or electron transfer/proton transfer (ETPT) process, followed by hydrogen peroxide dissociation.