Redox-inactive metal ions are one of the most important co-factors involved in dioxygen activation and formation reactions by metalloenzymes. In this study, we have shown that the logarithm of the rate constants of electron-transfer and C-H bond activation reactions by nonheme iron(III)-peroxo complexes binding redox-inactive metal ions, [(TMC)Fe (O )] -M (M =Sc , Y , Lu , and La ), increases linearly with the increase of the Lewis acidity of the redox-inactive metal ions (ΔE), which is determined from the g values of EPR spectra of O -M complexes. In contrast, the logarithm of the rate constants of the [(TMC)Fe (O )] -M complexes in nucleophilic reactions with aldehydes decreases linearly as the ΔE value increases. Thus, the Lewis acidity of the redox-inactive metal ions bound to the mononuclear nonheme iron(III)-peroxo complex modulates the reactivity of the [(TMC)Fe (O )] -M complexes in electron-transfer, electrophilic, and nucleophilic reactions.

Mononuclear nonheme iron(III)-hydroperoxo intermediates play key roles in biological oxidation reactions. In the present study, we report the highly intriguing reactivity of a nonheme iron(III)−hydroperoxo complex, [(TMC)Fe III (OOH)] 2+ (1), in the deformylation of aldehydes, such as 2-phenylpropionaldehyde (2-PPA) and its derivatives; that is, the reaction pathway of the aldehyde deformylation by 1 varies depending on reaction conditions, such as temperature and substrate. At temperature above 248 K, the aldehyde deformylation occurs predominantly via a nucleophilic addition (NA) pathway. However, as the reaction temperature is lowered, the reaction pathway changes to a hydrogen atom transfer (HAT) pathway. Interestingly, the reaction rate becomes independent of temperature below 233 K with a huge kinetic isotope effect (KIE) value of 93 at 203 K, suggesting that the HAT reaction results from tunneling. In contrast, reactions with a deuterated 2-PPA at the αposition and 2-methyl-2-phenylpropionaldehyde proceed exclusively via a NA pathway irrespective of the reaction temperature. We conclude that the bifurcation pathways between NA and HAT result from the tunneling effect in the HAT reaction by 1. To the best of our knowledge, this study reports the first example showing that tunneling plays a significant role in the activation of substrate C−H bonds by a mononuclear nonheme iron(III)−hydroperoxo complex.

The spin states (S = 1 and S = 2) of nonheme FeIVO intermediates are believed to play an important role in determining their chemical properties in enzymatic and biomimetic reactions.