Electrochemical characterization of oxygen reduction by FeII[ethylenediaminetetraacetate]

The kinetics of the reduction Of O-2 by Fe(II)EDTA is evaluated using electrochemical methods and shows a promising approach to the study of oxygen activation by metal complexes. Beneficial to this method is the use of stable Fe(III)EDTA, which can be electro-reduced to generate air sensitive Fe(II/III)EDTA. The Fe(II/III)EDTA oxidation states act as an electron mediator between the electrode and O-2(aq). At a sweep rate of 5 mV/s, cyclic voltammeteric waves indicate an EC' type mechanism. The kinetics Of O-2 reduction by Fe(II)EDTA is evident by the EC' current and is dependent on pH with a maximum at pH 3. This rate quickly decreases below pH 3 and above 8 and is nearly constant between pH 4-8. This behavior is strongly correlated with the Fe(II)EDTA species distribution diagram indicating that the protonated Fe(II)EDTA-H complex is the optimal species for O-2 reduction. Applying computer simulation to representative cyclic voltammograms provides rate constants for the reaction steps involved in O-2 reduction by Fe(II)EDTA. The mechanistic steps evaluated consist of (1) the binding Of O-2 to Fe(II)EDTA, (2) the reduction of bound O-2 by Fe(II)EDTA to produce O-2(.-), and (3) further reduction of O-2(.-) by Fe(II)EDTA to produce H2O2. We report a rate constant of 1.07 x 10(4) M-1 S-1 for step (1), 6.23 x 10(9) s(-1) for step (2), and 1.00 X 10(4) M-1 S-1 for step (3) at pH 3. The data is consistent to literature values obtained by stopped-flow techniques. (c) 2007 Elsevier B.V. All rights reserved.