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.

Mononuclear nonheme high-spin (S=2) iron(IV)-oxo species have been identified as the key intermediates responsible for the C-H bond activation of organic substrates in nonheme iron enzymatic reactions. Herein we report that the C-H bond activation of hydrocarbons by a synthetic mononuclear nonheme high-spin (S=2) iron(IV)-oxo complex occurs through an oxygen non-rebound mechanism, as previously demonstrated in the C-H bond activation by nonheme intermediate (S=1) iron(IV)-oxo complexes. We also report that C-H bond activation is preferred over C=C epoxidation in the oxidation of cyclohexene by the nonheme high-spin (HS) and intermediate-spin (IS) iron(IV)-oxo complexes, whereas the C=C double bond epoxidation becomes a preferred pathway in the oxidation of deuterated cyclohexene by the nonheme HS and IS iron(IV)-oxo complexes. In the epoxidation of styrene derivatives, the HS and IS iron(IV) oxo complexes are found to have similar electrophilic characters.

Redox-inactive metal ions play important roles in tuning chemical properties of metal–oxygen intermediates. Herein we report the effect of water molecules on the redox properties of a nonheme iron(III)–peroxo complex binding redox-inactive metal ions. The coordination of two water molecules to a Zn2+ ion in (TMC)FeIII-(O2)-Zn(CF3SO3)2 (1-Zn2+) decreases the Lewis acidity of the Zn2+ ion, resulting in the decrease of the one-electron oxidation and reduction potentials of 1-Zn2+. This further changes the reactivities of 1-Zn2+ in oxidation and reduction reactions; no reaction occurred upon addition of an oxidant (e.g., cerium(IV) ammonium nitrate (CAN)) to 1-Zn2+, whereas 1-Zn2+ coordinating two water molecules, (TMC)FeIII-(O2)-Zn(CF3SO3)2-(OH2)2 [1-Zn2+-(OH2)2], releases the O2 unit in the oxidation reaction. In the reduction reactions, 1-Zn2+ was converted to its corresponding iron(IV)–oxo species upon addition of a reductant (e.g., a ferrocene derivative), whereas such a reaction occurred at a much slower rate in the case of 1-Zn2+-(OH2)2. The present results provide the first biomimetic example showing that water molecules at the active sites of metalloenzymes may participate in tuning the redox properties of metal–oxygen intermediates.

A series of Ir complexes has been developed as multifunctional photocatalysts for CO2 reduction to give HCO2H selectively. The catalytic activities and photophysical properties vary widely across the series, and...

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Mononuclear nonheme high-spin (S = 2) iron(IV)oxos pecies have been identified as the key intermediates responsible for the CÀHbond activation of organic substrates in nonheme iron enzymatic reactions.Herein we report that the C À Hb ond activation of hydrocarbons by as ynthetic mononuclear nonheme high-spin (S = 2) iron(IV)-oxo complex occurs through an oxygen non-rebound mechanism, as previously demonstrated in the CÀHb ond activation by nonheme intermediate (S = 1) iron(IV)-oxoc omplexes.W e also report that CÀHb ond activation is preferred over C=C epoxidation in the oxidation of cyclohexene by the nonheme high-spin (HS) and intermediate-spin (IS) iron(IV)-oxo complexes,w hereas the C = Cd ouble bond epoxidation becomes apreferred pathway in the oxidation of deuterated cyclohexene by the nonheme HS and IS iron(IV)-oxo complexes.I nt he epoxidation of styrene derivatives,the HS and IS iron(IV) oxo complexes are found to have similar electrophilic characters.