In situ mechanistic investigation of O2 reduction by synthetic and bio-synthetic iron porphyrins : factors affecting rate and selectivity

Orateur : Abhishek DEY (Department of Inorganic Chemistry, Indian Association for the Cultivation of Science, Kolkata, India)

The mechanism of electrochemical O2 reduction by iron porphyrin complexes by SERRS-RDE show that FeII, FeIII-OOH and FeIV=O species accumulate when the system is under steady state. Thus O2 binding to FeII, O-O bond cleavage and a proton coupled electron transfer to FeIV=O are slower relative to the other steps in the catalytic cycle. Analysis of H/D isotope effects on the kinetics of 2e-/2H+ and 4e-/4H+ reduction of O2 suggest that the protonation of this FeIII-OOH species (pKa measured to be 5.5-6.5) leads to the release of the partially reduced oxygen species, H2O2, which is detrimental for these complexes. In nature, facile heterolytic O-O bond cleavage is affected by a) push effect of a cysteine ligand (cytochrome P450), b) pull effect of a basic distal residue (peroxidase) and c) distal metal (cytochrome c oxidase). Similarly, the hydrolysis of a FeIII-OOH species to release H2O2 is avoided by 2nd sphere residues regulating the site of protonation (e.g. Threonine in cytochrome P450). These factors are introduced in and around the iron porphyrin in a series of synthetic and biosynthetic systems and myoglobin based models of CcO. Emulating the structure function correlations established in natural systems in these artificial systems lets enhance rates of O2 reduction by two orders of magnitude relative to previously reported systems and offers selective 4e-/4H+ reduction over a pH range of 2-9 ! SERRS-RDE data on these systems show that the effects outlined above affects the rates of O-O bond cleavage and FeIV=O reduction differently. Depending on the modification introduced either O-O cleavage or reduction of FeIV=O and in some cases even O2 binding to FeII is found to be the rate determining step in O2 reduction.

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