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Redox potential and C-H bond cleaving properties of a nonheme Fe(IV)=O complex in aqueous solution.

ABSTRACT: High-valent iron-oxo intermediates have been identified as the key oxidants in the catalytic cycles of many nonheme enzymes. Among the large number of synthetic Fe(IV)=O complexes characterized to date, [Fe(IV)(O)(N4Py)](2+) (1) exhibits the unique combination of thermodynamic stability, allowing its structural characterization by X-ray crystallography, and oxidative reactivity sufficient to cleave C-H bonds as strong as those in cyclohexane (D(C-H) = 99.3 kcal mol(-1)). However, its redox properties are not yet well understood. In this work, the effect of protons on the redox properties of 1 has been investigated electrochemically in nonaqueous and aqueous solutions. While the cyclic voltammetry of 1 in CH(3)CN is complicated by coupling of several chemical and redox processes, the Fe(IV/III) couple is reversible in aqueous solution with E(1/2) = +0.41 V versus SCE at pH 4 and involves the transfer of one electron and one proton to give the Fe(III)-OH species. This is in fact the first example of reversible electrochemistry to be observed for this family of nonheme oxoiron (IV) complexes. C-H bond oxidations by 1 have been studied in H(2)O and found to have reaction rates that depend on the C-H bond strength but not on the solvent. Furthermore, our electrochemical results have allowed a D(O-H) value of 78(2) kcal mol(-1) to be calculated for the Fe(III)-OH unit derived from 1. Interestingly, although this D(O-H) value is 6-11 kcal mol(-1) lower than those corresponding to oxidants such as [Fe(IV)(O)(TMP)] (TMP = tetramesitylporphinate), [Ru(IV)(O)(bpy)(2)(py)](2+) (bpy = bipyridine, py = pyridine), and the tert-butylperoxyl radical, the oxidation of dihydroanthracene by 1 occurs at a rate comparable to rates for these other oxidants. This comparison suggests that the nonheme N4Py ligand environment confers a kinetic advantage over the others that enhances the C-H bond cleavage ability of 1.

SUBMITTER: Wang D

PROVIDER: S-EPMC2887765 | biostudies-literature | 2010 Jun

REPOSITORIES: biostudies-literature

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Redox potential and C-H bond cleaving properties of a nonheme Fe(IV)=O complex in aqueous solution.

Wang Dong D Zhang Mo M Bühlmann Philippe P Que Lawrence L

Journal of the American Chemical Society 20100601 22

High-valent iron-oxo intermediates have been identified as the key oxidants in the catalytic cycles of many nonheme enzymes. Among the large number of synthetic Fe(IV)=O complexes characterized to date, [Fe(IV)(O)(N4Py)](2+) (1) exhibits the unique combination of thermodynamic stability, allowing its structural characterization by X-ray crystallography, and oxidative reactivity sufficient to cleave C-H bonds as strong as those in cyclohexane (D(C-H) = 99.3 kcal mol(-1)). However, its redox prope ...[more]

PMID: 20476758

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Project description:Redox-inactive metal ions that function as Lewis acids play pivotal roles in modulating reactivities of oxygen-containing metal complexes in a variety of biological and biomimetic reactions, including dioxygen activation/formation and functionalization of organic substrates. Mononuclear nonheme iron(III)-peroxo species are invoked as active oxygen intermediates in the catalytic cycles of dioxygen activation by nonheme iron enzymes and their biomimetic compounds. Here, we report mononuclear nonheme iron(III)-peroxo complexes binding redox-inactive metal ions, [(TMC)FeIII(O2)]+-M3+ (M3+ = Sc3+ and Y3+; TMC = 1,4,8,11-tetramethyl-1,4,8,11-tetraazacyclotetradecane), which are characterized spectroscopically as a 'side-on' iron(III)-peroxo complex binding a redox-inactive metal ion, (TMC)FeIII-(?,?2:?2-O2)-M3+ (2-M). While an iron(III)-peroxo complex, [(TMC)FeIII(O2)]+, does not react with electron donors (e.g., ferrocene), one-electron reduction of the iron(III)-peroxo complexes binding redox-inactive metal ions occurs readily upon addition of electron donors, resulting in the generation of an iron(IV)-oxo complex, [(TMC)FeIV(O)]2+ (4), via heterolytic O-O bond cleavage of the peroxide ligand. The rates of the conversion of 2-M to 4 are found to depend on the Lewis acidity of the redox-inactive metal ions and the oxidation potential of the electron donors. We have also determined the fundamental electron-transfer properties of 2-M, such as the reduction potential and the reorganization energy in electron-transfer reaction. Based on the results presented herein, we have proposed a mechanism for the reactions of 2-M and electron donors; the reduction of 2-M to the reduced species, (TMC)FeII-(O2)-M3+ (2'-M), is the rate-determining step, followed by heterolytic O-O bond cleavage of the reduced species to form 4. The present results provide a biomimetic example demonstrating that redox-inactive metal ions bound to an iron(III)-peroxo intermediate play a significant role in activating the peroxide O-O bond to form a high-valent iron(IV)-oxo species.

Dataset Information

Redox potential and C-H bond cleaving properties of a nonheme Fe(IV)=O complex in aqueous solution.

Publications

Redox potential and C-H bond cleaving properties of a nonheme Fe(IV)=O complex in aqueous solution.

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