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Million-fold activation of the [Fe(2)(micro-O)(2)] diamond core for C-H bond cleavage.

ABSTRACT: In biological systems, the cleavage of strong C-H bonds is often carried out by iron centres-such as that of methane monooxygenase in methane hydroxylation-through dioxygen activation mechanisms. High valent species with [Fe(2)(micro-O)(2)] diamond cores are thought to act as the oxidizing moieties, but the synthesis of complexes that cleave strong C-H bonds efficiently has remained a challenge. We report here the conversion of a synthetic complex with a valence-delocalized [Fe(3.5)(micro-O)(2)Fe(3.5)](3+) diamond core (1) into a complex with a valence-localized [HO-Fe(III)-O-Fe(IV)=O](2+) open core (4), which cleaves C-H bonds over a million-fold faster. This activity enhancement results from three factors: the formation of a terminal oxoiron(iv) moiety, the conversion of the low-spin (S = 1) Fe(IV)=O centre to a high-spin (S = 2) centre, and the concentration of the oxidizing capability to the active terminal oxoiron(iv) moiety. This suggests that similar isomerization strategies might be used by nonhaem diiron enzymes.

SUBMITTER: Xue G

PROVIDER: S-EPMC2859466 | biostudies-literature | 2010 May

REPOSITORIES: biostudies-literature

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Million-fold activation of the [Fe(2)(micro-O)(2)] diamond core for C-H bond cleavage.

Xue Genqiang G De Hont Raymond R Münck Eckard E Que Lawrence L

Nature chemistry 20100321 5

In biological systems, the cleavage of strong C-H bonds is often carried out by iron centres-such as that of methane monooxygenase in methane hydroxylation-through dioxygen activation mechanisms. High valent species with [Fe(2)(micro-O)(2)] diamond cores are thought to act as the oxidizing moieties, but the synthesis of complexes that cleave strong C-H bonds efficiently has remained a challenge. We report here the conversion of a synthetic complex with a valence-delocalized [Fe(3.5)(micro-O)(2)F ...[more]

PMID: 20414242

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Project description:An [Fe(IV)(2)(μ-O)(2)] diamond core structure has been postulated for intermediate Q of soluble methane monooxygenase (sMMO-Q), the oxidant responsible for cleaving the strong C-H bond of methane and its hydroxylation. By extension, analogous species may be involved in the mechanisms of related diiron hydroxylases and desaturases. Because of the paucity of well-defined synthetic examples, there are few, if any, mechanistic studies on the oxidation of hydrocarbon substrates by complexes with high-valent [Fe(2)(μ-O)(2)] cores. We report here that water or alcohol substrates can activate synthetic [Fe(III)Fe(IV)(μ-O)(2)] complexes supported by tetradentate tris(pyridyl-2-methyl)amine ligands (1 and 2) by several orders of magnitude for C-H bond oxidation. On the basis of detailed kinetic studies, it is postulated that the activation results from Lewis base attack on the [Fe(III)Fe(IV)(μ-O)(2)] core, resulting in the formation of a more reactive species with a [X-Fe(III)-O-Fe(IV)═O] ring-opened structure (1-X, 2-X, X = OH(-) or OR(-)). Treatment of 2 with methoxide at -80 °C forms the 2-methoxide adduct in high yield, which is characterized by an S = 1/2 EPR signal indicative of an antiferromagnetically coupled [S = 5/2 Fe(III)/S = 2 Fe(IV)] pair. Even at this low temperature, the complex undergoes facile intramolecular C-H bond cleavage to generate formaldehyde, showing that the terminal high-spin Fe(IV)═O unit is capable of oxidizing a C-H bond as strong as 96 kcal mol(-1). This intramolecular oxidation of the methoxide ligand can in fact be competitive with intermolecular oxidation of triphenylmethane, which has a much weaker C-H bond (D(C-H) 81 kcal mol(-1)). The activation of the [Fe(III)Fe(IV)(μ-O)(2)] core is dramatically illustrated by the oxidation of 9,10-dihydroanthracene by 2-methoxide, which has a second-order rate constant that is 3.6 × 10(7)-fold larger than that for the parent diamond core complex 2. These observations provide strong support for the DFT-based notion that an S = 2 Fe(IV)═O unit is much more reactive at H-atom abstraction than its S = 1 counterpart and suggest that core isomerization could be a viable strategy for the [Fe(IV)(2)(μ-O)(2)] diamond core of sMMO-Q to selectively attack the strong C-H bond of methane in the presence of weaker C-H bonds of amino acid residues that define the diiron active site pocket.

Dataset Information

Million-fold activation of the [Fe(2)(micro-O)(2)] diamond core for C-H bond cleavage.

Publications

Million-fold activation of the [Fe(2)(micro-O)(2)] diamond core for C-H bond cleavage.

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