Oxygen Electroreduction on FeII and FeIII Coordinated to N₄ Chelates. Reversible Potentials for the Intermediate Steps from Quantum Theory

On the basis of spin-unrestricted hybrid gradient-corrected Becke, Lee, Yang, and Parr B3LYP density functional calculations and the reaction site models Fe(NH2)2(NH3)2 for FeII, and Fe(NH2)2(NH3)2OH for FeIII, FeII is predicted to be the active site for the four-electron reduction of oxygen by heat-treated iron macrocycles. It is favored over FeIII in the first step of the mechanism because of a site blocking effect: H2O bonds strongly to the FeIII site, blocking it against O2 adsorption, and it does not bond strongly to FeII. The stronger bonding of the product of the first reduction step, OOH, to FeII compared to FeIII also helps by contributing to a more positive reversible potential for its formation over FeII. Subsequent reduction steps have high reversible potentials over both centers, paralleling an earlier study of oxygen reduction over a single Pt site. However, the important difference compared to Pt is the hydrogen-bonding interaction between (OHOH) bonded to FeII and a nitrogen lone-pair orbital in the N4 chelate. This is in addition to the O lone-pair donation bond to the FeII center and is proposed to prevent hydrogen peroxide from leaving as a two-electron reduction product, as it was predicted to do over a single Pt site, and provides a path for reduction to water.