Structure and reactivity of a mononuclear non-haem iron(III)–peroxo complex

Abstract

Oxygen-containing mononuclear iron species—iron(III)–peroxo, iron(III)–hydroperoxo and iron(IV)–oxo—are key intermediates in the catalytic activation of dioxygen by iron-containing metalloenzymes1–7. It has been difficult to generate synthetic analogues of these three active iron–oxygen species in identical host complexes, which is necessary to elucidate changes to the structure of the iron centre during catalysis and the factors that control their chemical reactivities with substrates. Here we report the high-resolution crystal structure of a mononuclear non-haem side-on iron(III)–peroxo complex, [Fe(III)(TMC)(OO)]+. We also report a series of chemical reactions in which this iron(III)–peroxo complex is cleanly converted to the iron(III)–hydroperoxo complex, [Fe(III)(TMC)(OOH)]2+, via a short-lived intermediate on protonation. This iron(III)–hydroperoxo complex then cleanly converts to the ferryl complex, [Fe(IV)(TMC)(O)]2+, via homolytic O–O bond cleavage of the iron(III)–hydroperoxo species. All three of these iron species—the three most biologically relevant iron–oxygen intermediates—have been spectroscopically characterized; we note that they have been obtained using a simple macrocyclic ligand. We have performed relative reactivity studies on these three iron species which reveal that the iron(III)–hydroperoxo complex is the most reactive of the three in the deformylation of aldehydes and that it has a similar reactivity to the iron(IV)–oxo complex in C–H bond activation of alkylaromatics. These reactivity results demonstrate that iron(III)–hydroperoxo species are viable oxidants in both nucleophilic and electrophilic reactions by iron-containing enzymes.

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.…”

“…Labelling experiments with 57 Fe as well as 15 However, in a subsequent in-depth spectroscopic and theoretical work, Aliaga-Alcalde et al 82 concluded that [(cy-ac)Fe V (N)] + has an unusual orbitally degenerate S = 1/2 ground state. Initially, the Fe-N stretch in [(cy-ac)Fe V (N)] + could not be identified in the infrared vibrational spectrum.…”

“…These high-valent iron-oxo intermediates in biology have been primarily characterized by 57 Fe Mössbauer spectroscopy 19, as it serves as a local probe of the iron centre. Mössbauer isomer shifts (δ) are directly related to the electron density at the iron nucleus and, therefore, are often used as a probe of the 'oxidation state' of the metal.…”

Biomimetic Conversion of Glucose to Organic Acid Facilitated by Metalloporphyrin under Mild Conditions

“…Under alkaline conditions, the iron(III)-peroxo species, acknowledged as nucleophilic oxidant intermediates, are subjected to production by the reaction of H2O2 with iron porphyrins and non-heme iron complexes (Cho et al 2011;Franke et al 2012). Thus, it is reasonable to assume that the metalloporphyrins in glucose oxidation might be responsible for the formation of the metalo-peroxo species, (TSPPMO2 -), by the addition of H2O2 generated in situ.…”

Synthesis and Characterization of the Peroxo Complexes of Uranium (VI) Containing Organic Acids and Amine Bases Ligands

“…They are widely used in stoichiometric as well as catalytic oxidation in organic and biochemistry [3], for example in the oxidation of thioanisole [4], methyl benzenes [5], tertiary amines, alkenes, alcohols [6], bromide [7] and also in olefin epoxodations [8]. Peroxo complexes of uranium are attracting interest as oxidants in organic synthesis [9].…”