Oxygen Transfer Involving Non-Heme Iron. The reaction of (EDTA)Fe(III) with m-Chloroperbenzoic Acid

Article abstract of DOI:10.1021/ja00278a021

Oxygen transfer from percarboxylic acids to (ethylenediaminotetraacetato)iron(III) ((EDTA)Fe(III)) has been investigated (solvent CH3OH; T 30 deg C), using 2,4,6-tri-tert-butylphenol (TBPH) as a trapping reagent for the reactive iron-oxo intermediate formed upon oxygen transfer.In the presence of TBPH the percarboxylic acid peroxo bond undergoes 100 percent heterolytic cleavage during oxygen transfer to (EDTA)Fe(III), as demonstrated with phenylperacetic acid.This requires the reaction to involve the transfer of an oxene or oxygen atom to the iron(III) moiety of the (EDTA)Fe(III) catalyst to form what may be formally written as (EDTA)Fe(V)O.Since the so-constituted oxidant oxidized PhCH2COH by 1e^- transfer to provide (PhCH2)2 at a rate greater than the oxidation of carboxylate ligand, the electron deficiency of the oxidant must reside at the oxygen to a greater extent than on iron.The structure of the oxidant is, therefore, best represented as something between (EDTA)Fe(V)O and one in which the iron(III) species stabilizes the oxygen atom by d-orbital overlap ((EDTA)Fe(III)O) species.The kinetics of the reaction of a percarboxylic acid with (EDTA)Fe(III) were studied by using m-chloroperbenzoic acid (MCPBA).The reaction was monitored by following the appearance of 2,4,6-tri-tert-butylphenoxyl radical (TPB*).Oxygen transfer was shown to be rate limiting from the first-order dependence upon and zero-order dependence upon .At constant and , the pseudo-first-order rate constants, k_obsd, for the appearance of TBP* increases linearly with increase in <(EDTA)Fe(III)> and reaches a limiting value at high (EDTA)Fe(III) concentrations.Oxidation of TBPH to TBP* is shown to compete with methanol solvent oxidation to formaldehyde and the oxidative coupling of MCPBA with TBPH to provide a peroxidic component, OXO.The percentage yields of TBP*, CH2O, and OXO are independent of the concentration of (EDTA)Fe(III), provided the concentrations of MCPBA and TBPH are kept constant.However, increasing the initial concentration of MCPBA increases the concentration of OXO, and, correspondingly, the yield of the other two products decreases.Similarly, the yield of phenoxyl radical increases with increase in .The initial addition of formaldehyde retards the rate of the oxidation reactions.A kinetically competent series of reactions which allow the calculation of experimental values of k_obsd and the yields of products have been arrived at by computer simulation (Scheme III).Comparison of the second-order rate constants (in methanol) for oxygen transfer from MCPBA to (tetraphenylporphyrinato)iron(III) chloride ((TPP)Fe(III)Cl) and to (EDTA)Fe(III) establishes the former to be but 45-fold a better acceptor.