Nonheme iron enzymes generate powerful and versatile oxidants that perform a wide range of oxidation reactions, including the functionalization of inert C−H bonds, which is a major challenge for chemists. The oxidative abilities of these enzymes have inspired bioinorganic chemists to design synthetic models to mimic their ability to perform some of the most difficult oxidation reactions and study the mechanisms of such transformations. Iron‐oxygen intermediates like iron(III)‐hydroperoxo and high‐valent iron‐oxo species have been trapped and identified in investigations of these bio‐inspired catalytic systems, with the latter proposed to be the active oxidant for most of these systems. In this Review, we highlight the recent spectroscopic and mechanistic advances that have shed light on the various pathways that can be accessed by bio‐inspired nonheme iron systems to form the high‐valent iron‐oxo intermediates.
Protons play essential roles in natural systems in controlling O−O bond cleavage of peroxoiron(III) species to give rise to the high-valent iron oxidants that carry out the desired transformations. Herein, we report kinetic and mechanistic evidence that acids can control the mode of O− O bond cleavage for a nonheme S = 1/2 Fe III −OOH species [(BnTPEN)Fe III (OOH)] 2+ (2, BnTPEN = N-benzyl-N,N′,N′tris(2-pyridylmethyl)-1,2-diaminoethane). Addition of acids having pK a values of >8.5 in CH 3 CN results in O−O bond homolysis, leading to the formation of hydroxyl radicals that give rise to alcohol/ketone (A/K) ratios of around 1 in the oxidation of cyclohexane. However, the introduction of acids with pK a values of <8.5 elicits a different outcome, namely the achievement of A/K ratios of as high as 9, the observation of rapid and catalytic hydroxylation of cyclohexane, and a million-fold acceleration in the decay rate of the Fe III −OOH intermediate at −40 °C. These results implicate the generation of a highly reactive Fe V O species via proton-assisted O−O bond heterolysis of the Fe III −OOH intermediate, which is unprecedented for nonheme iron complexes supported by neutral pentadentate ligands and serves as a nonheme analogue for heme enzyme compounds I.
The Fe(TPA) (TPA = tris(pyridyl-2-methyl)amine) class of non-haem Fe catalysts is proposed to carry out selective hydrocarbon oxidations through the generation of high-valent iron species. Using ambient mass spectrometry, we obtain direct evidence for the formation of an Fe(O)(OH) species under catalytic conditions. In addition, O-labelling suggests that this Fe(O)(OH) species serves as the active oxidant in hydrocarbon oxidation catalysis.
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