Two intermediates (2 and 3) are formed consecutively in the reaction of a diiron(III) complex [FeIII2(μ-O)(OH)(H2O)(TPA)2](ClO4)3 (TPA = tris(2-pyridylmethyl)amine, tris(picolyl)amine) with H2O2 in CH3CN at −40 °C. Low-temperature stopped-flow studies showed that both species are kinetically competent in oxidation of phosphines and phenols. The first intermediate (2) reacts with substrates very rapidly (second-order rate constants reach 105–106 M−1 s−1 for substituted triarylphosphines and 103–105 M−1 s−1 for substituted phenols), in keeping with a diiron(IV)–oxo formulation. The second intermediate (3), a mixed-valent Fe(III)Fe(IV) species, is more stable than 2, and reacts with substrates more slowly (second-order rate constants range from 150 to 550 M−1 s−1 for triaryl phosphineoxidation, and from 18 to 790 M−1 s−1 for phenoloxidation). Reaction rates increase with increasing electron donating abilities of substituents, indicating that both 2 and 3 act as electrophilic oxidants.
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