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.
Department of Chemistry, Johns Hopkins University, 3400 North Charles Street, Baltimore, Maryland 21218, USA.
Rupam
R
Sarma
Alfredo M
AM
Angeles-Boza
David W
DW
Brinkley
Justine P
JP
Roth
D004578:Electron Spin Resonance Spectroscopy; D007501:Iron / Q000737:chemistry*; D010100:Oxygen / Q000737:chemistry*; D013816:Thermodynamics; D014867:Water / Q000737:chemistry*
D014867:Water; D007501:Iron; D010100:Oxygen