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.

The kinetics of oxidation of organic and inorganic reductants by aqueous iron(IV) ions, Fe(IV)(H2O)5O(2+) (hereafter Fe(IV)aqO(2+)), are reported. The substrates examined include several water-soluble ferrocenes, hexachloroiridate(III), polypyridyl complexes M(NN)3(2+) (M = Os, Fe and Ru; NN = phenanthroline, bipyridine and derivatives), HABTS(-)/ABTS(2-), phenothiazines, Co(II)(dmgBF2)2, macrocyclic nickel(II) complexes, and aqueous cerium(III). Most of the reductants were oxidized cleanly to the corresponding one-electron oxidation products, with the exception of phenothiazines which produced the corresponding oxides in a single-step reaction, and polypyridyl complexes of Fe(II) and Ru(II) that generated ligand-modified products. Fe(IV)aqO(2+) oxidizes even Ce(III) (E(0) in 1 M HClO4 = 1.7 V) with a rate constant greater than 10(4) M(-1) s(-1). In 0.10 M aqueous HClO4 at 25 °C, the reactions of Os(phen)3(2+) (k = 2.5 × 10(5) M(-1) s(-1)), IrCl6(3-) (1.6 × 10(6)), ABTS(2-) (4.7 × 10(7)), and Fe(cp)(C5H4CH2OH) (6.4 × 10(7)) appear to take place by outer sphere electron transfer (OSET). The rate constants for the oxidation of Os(phen)3(2+) and of ferrocenes remained unchanged in the acidity range 0.05 < [H(+)] < 0.10 M, ruling out prior protonation of Fe(IV)aqO(2+) and further supporting the OSET assignment. A fit to Marcus cross-relation yielded a composite parameter (log k22 + E(0)Fe/0.059) = 17.2 ± 0.8, where k22 and E(0)Fe are the self-exchange rate constant and reduction potential, respectively, for the Fe(IV)aqO(2+)/Fe(III)aqO(+) couple. Comparison with literature work suggests k22 < 10(-5) M(-1) s(-1) and thus E(0)(Fe(IV)aqO(2+)/Fe(III)aqO(+)) > 1.3 V. For proton-coupled electron transfer, the reduction potential is estimated at E(0) (Fe(IV)aqO(2+), H(+)/Fe(III)aqOH(2+)) ≥ 1.95 V.

Oxidation of alcohols, ethers, and sulfoxides by ozone in acetonitrile is catalyzed by sub-millimolar concentrations of Fe(CH 3 CN) 6 2+ . The catalyst provides both rate acceleration and greater selectivity toward the less oxidized products. For example, Fe(CH 3 CN) 6 2+ -catalyzed oxidation of benzyl alcohol yields benzaldehyde almost exclusively (>95%) whereas uncatalyzed reaction generates a 1:1 mixture of benzaldehyde and benzoic acid. Similarly, aliphatic alcohols are oxidized to aldehydes/ketones, cyclobutanol to cyclobutanone, and diethyl ether to a 1:1 mixture of ethanol and acetaldehyde. The kinetics of oxidation of alcohols and diethyl ether are first order in [Fe(CH 3 CN) 6 2+ ] and [O 3 ], and independent of [Substrate] at concentrations greater than ~5 mM. In this regime, the rate constant for all of the alcohols is approximately the same, k cat = (8±1) × 10 4 M -1 s -1 , and that for (C 2 H 5 ) 2 O is (5±0.5) × 10 4 M -1 s -1 . In the absence of substrate, Fe(CH 3 CN) 6 2+ reacts with O 3 with k Fe = (9.3±0.3) × 10 4 M -1 s -1 . The similarity between the rate constants k Fe and k cat strongly argues for Fe(CH 3 CN) 6 2+ /O 3 reaction as rate determining in catalytic oxidation. The active oxidant produced in Fe(CH 3 CN) 6 2+ /O 3 reaction is suggested to be an Fe(IV) species in analogy with a related intermediate in aqueous solutions.This assignment is supported by the similarity in kinetic isotope effects and relative reactivities of the two species toward substrates.