Studies of the Di-iron(VI) Intermediate in Ferrate-Dependent Oxygen Evolution from Water

Molecular oxygen is produced from water via the following reaction of potassium ferrate (K(2)FeO(4)) in acidic solution: 4[H(3)Fe(VI)O(4)](+) + 8H(3)O(+) → 4Fe(3+) + 3O(2) + 18H(2)O. This study focuses upon the mechanism by which the O-O bond is formed. Stopped-flow kinetics at variable acidities in H(2)O and D(2)O are used to complement the analysis of competitive oxygen-18 kinetic isotope effects ((18)O KIEs) upon consumption of natural abundance water. The derived (18)O KIEs provide insights concerning the identity of the transition state. Water attack (WA) and oxo-coupling (OC) transition states were evaluated for various reactions of monomeric and dimeric ferrates using a calibrated density functional theory protocol. Vibrational frequencies from optimized isotopic structures are used here to predict (18)O KIEs for comparison to experimental values determined using an established competitive isotope-fractionation method. The high level of agreement between experimental and theoretic isotope effects points to an intramolecular OC mechanism within a di-iron(VI) intermediate, consistent with the analysis of the reaction kinetics. Alternative mechanisms are excluded based on insurmountably high free energy barriers and disagreement with calculated (18)O KIEs.