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XAS and DFT investigation of mononuclear cobalt(III) peroxo complexes: electronic control of the geometric structure in CoO2 versus NiO2 systems.

ABSTRACT: The geometric and electronic structures of two mononuclear [(L)CoO2](+) complexes, [(12-TMC)CoO2](ClO4) (1) and [(14-TMC)CoO2](ClO4) (2), have been evaluated using Co K-edge X-ray absorption spectroscopy (XAS) and extended X-ray absorption fine structure (EXAFS) and correlated with density functional theory (DFT) calculations to evaluate the differences in the geometric and electronic structures due to changes in the TMC chelate ring size. Co K-edge XAS shows that both 1 and 2 are Co(III) species. Co K-edge EXAFS data show that both 1 and 2 are side-on O2-bound cobalt(III) peroxide complexes. A combination of EXAFS and DFT calculations reveals that while the constrained 12-TMC ring in 1 allows for side-on O2 binding to the Co center with ease, the 14-TMC chelate in 2 has to undergo significant distortion of the ring to overcome steric hindrance posed by the four cis-methyl groups of the chelate to allow side-on O2 binding to the Co center. The Ni analogue of 2, [(14-TMC)NiO2](+), has been shown to form an end-on-bound nickel(II) superoxide species. The electronic and geometric factors that determine the different electronic structures of 2 and [(14-TMC)NiO2](+) are evaluated using DFT calculations. The results show that while the sterics of the cis-14-TMC chelate contribute to the geometry of O2 binding and result in an end-on-bound Ni(II)O2(-) complex in [(14-TMC)NiO2](+), the higher thermodynamic driving force for oxidation of Co(II) overcomes this steric constraint, resulting in stabilization of a side-on-bound Co(III)O2(2-) electronic structure in 2.

SUBMITTER: Sarangi R

PROVIDER: S-EPMC3130071 | biostudies-literature | 2011 Jan

REPOSITORIES: biostudies-literature

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XAS and DFT investigation of mononuclear cobalt(III) peroxo complexes: electronic control of the geometric structure in CoO2 versus NiO2 systems.

Sarangi Ritimukta R Cho Jaeheung J Nam Wonwoo W Solomon Edward I EI

Inorganic chemistry 20101213 2

The geometric and electronic structures of two mononuclear [(L)CoO2](+) complexes, [(12-TMC)CoO2](ClO4) (1) and [(14-TMC)CoO2](ClO4) (2), have been evaluated using Co K-edge X-ray absorption spectroscopy (XAS) and extended X-ray absorption fine structure (EXAFS) and correlated with density functional theory (DFT) calculations to evaluate the differences in the geometric and electronic structures due to changes in the TMC chelate ring size. Co K-edge XAS shows that both 1 and 2 are Co(III) specie ...[more]

PMID: 21142119

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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

XAS and DFT investigation of mononuclear cobalt(III) peroxo complexes: electronic control of the geometric structure in CoO2 versus NiO2 systems.

Publications

XAS and DFT investigation of mononuclear cobalt(III) peroxo complexes: electronic control of the geometric structure in CoO2 versus NiO2 systems.

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