A Transient, Highly Reactive Fe(IV)═O Species Revealed Through the Interference by O₂ in the Activation of Organic Peracids by [(N4Py)Fe(II)]²⁺

Biomimetic models of high-valent species relevant to those formed by the activation of O₂ by nonheme iron enzymes are essential for understanding reactivity. In synthetic complexes, oxidants such as peroxides and peroxyacids rather than O₂, are used to generate these species. However, although O₂ is not the terminal oxidant in these models, its presence in reaction mixtures can negatively impact the outcome of catalytic reactions. In this report, the origin of this impact is elucidated using the reaction of the nonheme iron complex [(N4Py)Fe(II)(CH₃CN)]²⁺ (<b>1</b>, N4Py = <i>N,N</i>-bis(2-pyridylmethyl)-<i>N</i>-bis(2-pyridyl)methylamine) with phenyl peracetic acid. We show that the speciation of the catalyst is sensitive to changes in composition by monitoring reaction progress using multiple <i>operando</i> spectroscopic techniques (UV/vis, Raman, FTIR, luminescence spectroscopy) concurrently to track changes in concentrations of the iron complexes, organic compounds, and gases. We elucidate the fundamental role played by molecular oxygen in the observed progress of the reactions, affecting the product distribution as expected, but also driving the system toward the accumulation of Fe(IV)=O species by scavenging intermediate benzyl radicals. These reactions influence the operation of a Fe(II)/Fe(IV) catalytic cycle with the peracid. An unexpected outcome of the study is that the data strongly indicate the transient formation of a highly reactive iron species capable of oxidizing organic substrates (e.g., methanol to methanal) within the solvent cage. We show that it is this species that enables an Fe(II)/Fe(IV) catalytic cycle. These findings shed light on differences in the catalytic performance of biomimetic nonheme iron complexes compared to the enzymes that inspire them.