Iron(IV)-oxo species are powerful oxidants that are involved as intermediates in iron-catalyzed oxidations of organic substrates. This includes their role in enzymatic oxidations which provided strong incentive to generate, characterize and explore the chemistry of novel Fe(IV) compounds from the perspective or reactivity and mechanism. The simplest iron(IV) complexes that have been prepared in solution are aqueous Fe(IV)-oxo ions. Even though these species can be conveniently generated from Fe(II) precursors and oxygen atom donors, studies of the chemistry of aqueous iron(IV) are difficult because of their short lifetime, high reactivity, and sensitivity to the surroundings. Herein we describe how changes in the pH, coordinating ligands and solvent can lead to dramatic changes in the lifetime and chemistry of aqueous Fe(IV)-oxo complexes.

pH-Induced Mechanistic Changeover From Hydroxyl Radicals to Iron(IV) in the Fenton Reaction

A major pathway in the reaction between Fe(II) and H2O2 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 (DMSO2). 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 (H2O)5FeO2+, generated by ozone oxidation of Fe(H2O)62+, 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 ~104. The results at pH 6-7 stand in stark contrast with those reported previously in acidic solutions where Fenton reaction generates hydroxyl radicals. Under those conditions, DMSO is oxidized stoichiometrically to methylsulfinic acid and ethane. This path still plays a minor role (1-10%) at pH 6-7.

Fe(II) Catalysis in Oxidation of Hydrocarbons with Ozone in Acetonitrile

Oxidation of alcohols, ethers, and sulfoxides by ozone in acetonitrile is catalyzed by sub-millimolar concentrations of Fe(CH3CN)62+. The catalyst provides both rate acceleration and greater selectivity toward the less oxidized product. For example, Fe(CH3CN)62+-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(CH3CN)62+ and ozone, 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, kcat = (8±1)  104 M-1 s-1, and that for (C2H5)2O is (5±0.5)  104 M-1 s-1. In the absence of substrate, Fe(CH3CN)62+ reacts with O3 with k5 = (9.3±0.3)  104 M-1 s-1. The similarity between the rate constants k5 and kcat strongly argues for Fe(CH3CN)62+/O3 reaction as rate determining in catalytic oxidation. The active oxidant produced in Fe(CH3CN)62+/O3 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.

Electron Transfer Reactivity of Aqueous Iron(IV) oxo Complex

The reactivity of FeaqIVO2+, generated in the reaction of Feaq2+ and ozone at pH 1, toward various inorganic complexes and some organic substrates, including ferrocene derivatives, Ni(II) macrocyclic tetraamines complexes, polypyridyl complexes of Os(II), Fe(II) and Ru(II), phenothiazines, HABTS-, Na3IrCl6, CoII(dmgBF2)2 and Ce(ClO4)3 with reduction potentials ranging from 0.52 to 1.7 V has been studied at room temperature. All substrates have shown to react with FeaqIVO2+ quantitatively producing the 1e oxidation product except for the phenothiazines and polypyridyl complexes of Fe(II) and Ru(II). Phenothiazines reacted through oxygen atom transfer to produce sulfoxides while the reactions with polypyridyl complexes of Fe(II) and Ru(II) were complicated and showed no Fe(III) or Ru(III) formation. The obtained second order rate constants of these reactions are within 104 - 108 M-1 s-1 with no straightforward relation to reduction potentials. Among all the substrates, Os(phen)32+ seems to react through outer-sphere ET. In addition, the no dependence of Os(phen)32+ reactivity on acid concentration (0.05 - 0.2 M) indicates no prior protonation of the FeaqIVO2+, which is consistent with stepwise electron-transfer followed by proton transfer. Our results suggest that the FeaqIVO2+/FeaqIIIO+ potential is not much lower than that for Os(phen)33+/ Os(phen)32+ couple (0.84 V vs. NHE).