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Electrochemical Formation of Fe V (O) and Mechanism of Its Reaction with Water During O−O Bond Formation
Author(s) -
Pattanayak Santanu,
Chowdhury Debarati Roy,
Garai Bikash,
Singh Kundan K.,
Paul Amit,
Dhar Basab B.,
Gupta Sayam Sen
Publication year - 2017
Publication title -
chemistry – a european journal
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.687
H-Index - 242
eISSN - 1521-3765
pISSN - 0947-6539
DOI - 10.1002/chem.201605061
Subject(s) - overpotential , chemistry , electrochemistry , redox , catalysis , kinetic isotope effect , aqueous solution , inorganic chemistry , tafel equation , electron transfer , nucleophile , reaction mechanism , organic chemistry , electrode , physics , deuterium , quantum mechanics
A detailed electrochemical investigation of a series of iron complexes (biuret‐modified tetraamido iron macrocycles Fe III ‐bTAML), including the first electrochemical generation of Fe V (O), and demonstration of their efficacy as homogeneous catalysts for electrochemical water oxidation (WO) in aqueous medium are reported. Spectroelectrochemical and mass spectral studies indicated Fe V (O) as the active oxidant, formed due to two redox transitions, which were assigned as Fe IV (O)/Fe III (OH 2 ) and Fe V (O)/Fe IV (O). The spectral properties of both of these high‐valent iron oxo species perfectly match those of their chemically synthesised versions, which were thoroughly characterised by several spectroscopic techniques. The O−O bond‐formation step occurs by nucleophilic attack of H 2 O on Fe V (O). A kinetic isotope effect of 3.2 indicates an atom–proton transfer (APT) mechanism. The reaction of chemically synthesised Fe V (O) in CH 3 CN and water was directly probed by electrochemistry and was found to be first‐order in water. The p K a value of the buffer base plays a critical role in the rate‐determining step by increasing the reaction rate several‐fold. The electronic effect on redox potential, WO rates, and onset overpotential was studied by employing a series of iron complexes. The catalytic activity was enhanced by the presence of electron‐withdrawing groups on the bTAML framework. Changing the substituents from OMe to NO 2 resulted in an eightfold increase in reaction rate, while the overpotential increased threefold.