Homogeneous catalytic O ₂ reduction to water by a cytochrome c oxidase model with trapping of intermediates and mechanistic insights

An efficient and selective four-electron plus four-proton (4e ^-?4H ⁺) reduction of O ₂ to water by decamethylferrocene and trifluoroacetic acid can be catalyzed by a synthetic analog of the heme a ₃?Cu _B site in cytochrome c oxidase ( ⁶LFeCu) or its Cu-free version ( ⁶LFe) in acetone. A detailed mechanistic-kinetic study on the homogeneous catalytic system reveals spectroscopically detectable intermediates and that the rate-determining step changes from the O ₂-binding process at 25 °C room temperature (RT) to the O-O bond cleavage of a newly observed Fe ^III-OOH species at lower temperature (-60 °C). At RT, the rate of O ₂-binding to ⁶LFeCu is significantly faster than that for ⁶LFe, whereas the rates of the O-O bond cleavage of the Fe ^III-OOH species observed (-60 ° C) with either the ⁶LFeCu or ⁶LFe catalyst are nearly the same. Thus, the role of the Cu ion is to assist the heme and lead to faster O ₂-binding at RT. However, the proximate Cu ion has no effect on the O-O bond cleavage of the Fe ^III-OOH species at low temperature.