Dioxygen Activation and Catalytic Aerobic Oxidation by a Mononuclear Nonheme Iron(II) Complex

Abstract

We have used dioxygen, not artificial oxidants such as peracids, iodosylarenes, and hydroperoxides, in the generation of a mononuclear nonheme oxoiron(IV) complex, [Fe(IV)(TMC)(O)]2+ (TMC = 1,4,8,11-tetramethyl-1,4,8,11-tetraazacyclotetradecane), from its corresponding Fe(II) complex, [Fe(TMC)(CF3SO3)2]. The formation of oxoiron(IV) species by activating dioxygen was markedly dependent on iron(II) complexes and solvents, and this observation was interpreted with the electronic effect of iron(II) complexes on dioxygen activation to form oxoiron(IV) species. A catalytic aerobic oxidation of organic substrates was demonstrated in the presence of the [Fe(TMC)]2+ complex. By carrying out 18O-labeled water experiment, we were able to conclude that the oxidation of organic substrates was mediated by an oxoiron(IV) intermediate, not by a radical type of autoxidation process.

High-Valent Iron(IV)–Oxo Complexes of Heme and Non-Heme Ligands in Oxygenation Reactions

“…15,21,54 Since then, a handful of non-heme iron(IV)-oxo complexes have been synthesized using macrocyclic tetradentate N4, tripodal tetradentate N4, and pentadentate N5 and N4S ligands ( Figure 3 for ligand structures). 5,8,[14][15][16][17][18][19][20][21][22][23][24][25][26][27][28]55 The structural analysis of the intermediates by X-ray crystallography for [(TMC)Fe…”

“…19 While two-electron oxidation of Fe(II) to the Fe(IV)-oxo species was proposed in the reactions of single-oxygen atom donors (reaction a), 15,16 Fe(III)-OOR species was homolytically cleaved to form Fe(IV)-O species in the reactions of hydroperoxides (reaction b). 53,57 In the case of O 2 activation (reaction c), we found that the structures of iron(II) complexes and solvents (e.g., alcohols) are important factors in generating iron(IV)-oxo species by activating O 2 .…”

“…27 More recently, we have shown that non-heme iron(IV)-oxo complexes, [(TPA)Fe , are capable of oxygenating sulfides to the corresponding sulfoxides (Figure 4, S-oxidation). 18,19,52 In the sulfide oxidation, the relative reactivities of the iron-oxo species were in the following order:…”

A Two‐State Reactivity Rationale for Counterintuitive Axial Ligand Effects on the CH Activation Reactivity of Nonheme FeIVO Oxidants

“…[13][14][15] Besides structural and spectroscopic characterization, a wealth of reactivity data is rapidly accumulating that demonstrates the versatility of these reagents, which can perform a variety of transformations: alkane oxidation, [16][17][18] alcohol oxidation, [19,20] olefin epoxidation, [17,18,21] oxygen transfer to dialkylsulfides and trialkylphosphines, [13,18,[21][22][23][24][25][26] and hydrogen abstraction from dihydroanthracene. [26][27][28] Interestingly, these complexes exhibit unusual kinetic isotope effect (KIE) patterns in H-abstraction reactions, ranging from near-classical KIE values of 10 for some reactions [26] to nonclassical values of 50-60 in others.…”

Reactivity of Molecular Dioxygen towards a Series of Isostructural Dichloroiron(III) Complexes with Tripodal Tetraamine Ligands: General Access to μ‐Oxodiiron(III) Complexes and Effect of α‐Fluorination on the Reaction Kinetics

“…[28] A few reports describe oxygen-atom transfer reactions from dioxygen to triphenylphosphine, or oxidation of benzylic alcohols to aldehydes by using iron(II) complexes of cyclic amines, or even ironA C H T U N G T R E N N U N G (III) derivatives of tetraamido cyclic ligands, for which a m-oxodiiron(IV) species was characterized. [29,30] In these cases, the postulated metal-oxo based mechanism must at least share two common steps: 1) coordination of dioxygen to the metal center and 2) formation of potentially reactive intermediates such as peroxidic or high-valent oxoiron species that might react with substrates if present in the medium.…”