A major pathway in the reaction between Fe(II) and H 2 O 2 at pH 6-7 in non-coordinating buffers exhibits inverse kinetic dependence on [H + ] and leads to oxidation of dimethyl sulfoxide (DMSO) to dimethyl sulfone (DMSO 2 ). This step regenerates Fe(II) and makes the oxidation of DMSO catalytic, a finding that strongly supports Fe(IV) as a Fenton intermediate at near-neutral pH. This Fe(IV) is a less efficient oxidant for DMSO at pH 6-7 than is (H 2 O) 5 FeO 2+ , generated by ozone oxidation of Fe(H 2 O) 6 2+ , in acidic solutions. Large concentrations of DMSO are needed to achieve significant turnover numbers at pH $ 6 owing to the rapid competing reaction between Fe(II) and Fe(IV) that leads to irreversible loss of the catalyst. At pH 6 and #0.02 mM Fe(II), the ratio of apparent rate constants for the reactions of Fe(IV) with DMSO and with Fe(II) is $10 4 . The results at pH 6-7 stand in stark contrast with those reported previously in acidic solutions where the Fenton reaction generates hydroxyl radicals. Under those conditions, DMSO is oxidized stoichiometrically to methylsulfinic acid and ethane. This path still plays a role (1-10%) at pH 6-7.

Figure 1. Mössbauer spectra of a sample at 4.2 K containing 250 mm Z, prepared by the rapid freeze-quench technique (see Supporting Information). The solid lines (red for Z, representing ca. 50 % of total Fe) are simulations based on Equation (2). The contribution from [Fe(H 2 O) 6 ] 3+ is shown in blue. We found the following parameters for Z: D = 9.7(7) cm À1 , A x /g n b n = A y / g n b n = À20.3(3) T, DE Q = À0.33(3) mm s À1 , d = 0.38(2) mm s À1. For the 8.0-T spectrum, the theoretical curves for Z and [Fe(H 2 O) 6 ] 3+ were added (black).

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Oxidation of cyclobutanol by aqueous Fe(IV) generates cyclobutanone in approximately 70% yield. In addition to this two-electron process, a smaller fraction of the reaction takes place by a one-electron process, believed to yield ring-opened products. A series of aliphatic alcohols, aldehydes, and ethers also react in parallel hydrogen atom and hydride transfer reactions, but acetone and acetonitrile react by hydrogen atom transfer only. Precise rate constants for each pathway for a number of substrates were obtained from a combination of detailed kinetics and product studies and kinetic simulations. Solvent kinetic isotope effect for the self-decay of Fe(IV), kH2O/kD2O = 2.8, is consistent with hydrogen atom abstraction from water.

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The aqueous iron(IV) ion, Fe(IV)(aq)O(2+), generated from O(3) and Fe(aq)(2+), reacts rapidly with various oxygen atom acceptors (sulfoxides, a water-soluble triarylphosphine, and a thiolatocobalt complex). In each case, Fe(IV)(aq)O(2+) is reduced to Fe(aq)(2+), and the substrate is oxidized to a product expected for oxygen atom transfer. Competition methods were used to determine the kinetics of these reactions, some of which have rate constants in excess of 10(7) M(-1) s(-1). Oxidation of dimethyl sulfoxide (DMSO) has k = 1.26 x 10(5) M(-1) s(-1) and shows no deuterium kinetic isotope effect, k(DMSO-d(6)) = 1.23 x 10(5) M(-1) s(-1). The Fe(IV)(aq)O(2+)/sulfoxide reaction is the product-forming step in a very efficient Fe(aq)(2+)-catalyzed oxidation of sulfoxides by ozone. This catalytic cycle, combined with labeling experiments in H(2)(18)O, was used to determine the rate constant for the oxo-group exchange between Fe(IV)(aq)O(2+) and solvent water under acidic conditions, k(exch) = 1.4 x 10(3) s(-1).

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