Characterization of a High-Spin Non-Heme Fe^III–OOH Intermediate and Its Quantitative Conversion to an Fe^IV═O Complex

Abstract

We have generated a high-spin FeIII–OOH complex supported by tetramethylcyclam via protonation of its conjugate base and characterized it in detail by various spectroscopic methods. This FeIII–OOH species converts quantitatively to an FeIV=O complex via O–O bond cleavage, which represents the first example of such a conversion. This conversion is promoted by two factors: the strong FeIII–OOH bond that inhibits Fe–O bond lysis and the addition of protons that facilitate O–O bond cleavage. This example provides a synthetic precedent for how O–O bond cleavage of high-spin iron(III)-peroxo intermediates of nonheme iron enzymes may be promoted.

A mononuclear nonheme iron(iii)–peroxo complex binding redox-inactive metal ions

“…Thus, the O-O stretching frequencies of 2 -Sc and 2 -Y indicate that the O 2 unit in 2 -Sc and 2 -Y retains the peroxo character, O 2 2− . Further, the O–O stretching frequencies of 2 -Sc (807 cm −1 ) and 2 -Y (799 cm −1 ) are smaller than that of 1 (825 cm −1 ), 10,11 whereas the Fe–O stretching frequencies of 2 -Sc (543 cm −1 ) and 2 -Y (540 cm −1 ) are higher than that of 1 (487 cm −1 ) 10,11 (Fig.1b). Furthermore, these values are significantly smaller than the Fe–O (658 cm −1 ) and O–O (868 cm −1 ) stretches in a high-spin end-on Fe(III)-hydroperoxo complex bearing the TMC ligand, [(TMC)Fe III (OOH)] 2+ ( 3 ).…”

The biology and chemistry of high-valent iron–oxo and iron–nitrido complexes

“…The missing link connecting the mechanistic steps of the above studies was the experimental demonstration of O-O bond cleavage in a hydroperoxoiron(III) complex to yield the corresponding high-valent iron-oxo species. This link has recently been identified independently by the groups of Li et al 56 and Cho et al, who generated a high-spin Fe III -OOH complex supported by the Me 4 cy ligand via protonation of the side-on peroxoiron(III) conjugate base 56,57 . This hydroperoxo complex was shown to convert quantitatively to the corresponding S = 1 oxoiron(IV) complex either through acid-mediated O-O bond heterolysis, followed by the reduction of the transient oxoiron(V) intermediate 56 46, also reacts with O 2 to yield an oxoiron(III) intermediate, which is proposed to derive from the reduction of an initially formed oxoiron(IV) species.…”

“…This link has recently been identified independently by the groups of Li et al 56 and Cho et al, who generated a high-spin Fe III -OOH complex supported by the Me 4 cy ligand via protonation of the side-on peroxoiron(III) conjugate base 56,57 . This hydroperoxo complex was shown to convert quantitatively to the corresponding S = 1 oxoiron(IV) complex either through acid-mediated O-O bond heterolysis, followed by the reduction of the transient oxoiron(V) intermediate 56 46, also reacts with O 2 to yield an oxoiron(III) intermediate, which is proposed to derive from the reduction of an initially formed oxoiron(IV) species. Although the oxoiron(IV) species on the way to the generation of the oxoiron(III) complex could not be trapped, it has recently been synthesized by the one-electron oxidation of the preformed [(H 3 buea)Fe III (Fig.…”

Use of EPR Spectroscopy to Unravel Reaction Mechanisms in (Catalytic) Bond Activation Reactions: Some Selected Examples

“…For the models that work with peroxides etc. as oxidizing agents, the reactive high-valent oxo species have sometimes been detected as mentioned above [64,67]. However, all these detections were at very low temperatures, which are not relevant for the catalytic reactions.…”

“…The model complexes that have been generated to mimic oxygenase reactivity usually only work catalytically for C-H oxygenation when a stronger oxidizing agent than O 2 is used (peroxide, PhIO and so on) [64][65][66][67][68][69]. This remains a drawback in such modelling studies, and one that will possibly be the focus of future biomimetic models.…”