O₂ reduction reaction by biologically relevant anionic ligand bound iron porphyrin complexes

Iron porphyrin complex with a covalently attached thiolate ligand and another with a covalently attached phenolate ligand has been synthesized. The thiolate bound complex shows spectroscopic features characteristic of P450, including the hallmark absorption spectrum of the CO adduct. Electrocatalytic O₂ reduction by this complex, which bears a terminal alkyne group, is investigated by both physiabsorbing on graphite surfaces (fast electron transfer rates) and covalent attachment to azide terminated self-assembled monolayer (physiologically relevant electron transfer rates) using the terminal alkyne group. Analysis of the steady state electrochemical kinetics reveals that this catalyst can selectively reduce O₂ to H₂O with a second-order k_cat. ∼10⁷ M^-1s^-1 at pH 7. The analogous phenolate bound iron porphyrin complex reduces O ₂ with a second-order rate constant of 10⁵ M^-1 s^-1 under the same conditions. The anionic ligand bound iron porphyrin complexes catalyze oxygen reduction reactions faster than any known synthetic heme porphyrin analogues. The kinetic parameters of O₂ reduction of the synthetic thiolate bound complex, which is devoid of any second sphere effects present in protein active sites, provide fundamental insight into the role of the protein environment in tuning the reactivity of thiolate bound iron porphyrin containing metalloenzymes.