Much enhanced acid catalysis was observed in oxygen atom transfer (OAT) reactions by a mononuclear nonheme Cr(III)-superoxo complex, [(Cl)(TMC)Cr(O)] (1), in the presence of triflic acid. In the acid-catalyzed reactions, the reactivity of 1 in OAT of thioanisole was enhanced significantly, showing more than 10-fold acceleration in rate. Electron transfer (ET) from electron donors to 1 also occurred only in the presence of HOTf. The enhanced reactivity of 1 by HOTf was explained by proton-coupled electron transfer from electron donors, such as ferrocene, to 1 in light of the Marcus theory of ET. The present study reports for the first time the dramatic proton effect on the chemical properties of metal-superoxo species.

Calcium ion plays an indispensable role for water oxidation by oxygen-evolving complex (OEC) composed of a manganese–oxo cluster (Mn4CaO5) in Photosystem II. In this context, the effects of Ca2+ ion and other redox-inactive metal ions on the redox reactivity of high-valent metal–oxo and metal–peroxo complexes have been studied extensively. Among metal–oxygen intermediates involved in interconversion between H2O and O2, however, the effects of Ca2+ ion and other redox-inactive metal ions (M n+) on the redox reactivity of metal–superoxo complexes have yet to be reported. Herein, we report that electron transfer (ET) from octamethylferrocene (Me8Fc) to a mononuclear nonheme Cr­(III)–superoxo complex, [(Cl)­(TMC)­CrIII(O2)]+ (1), occurs in the presence of redox-inactive metal ions (M n+ = Ca2+, Mg2+, Y3+, Al3+, and Sc3+); in the absence of the redox-inactive metal ions, ET from Me8Fc to 1 does not occur. The second-order rate constants (k et) of ET from Me8Fc to 1 in the presence of a redox-inactive metal ion increased with increasing concentration of M n+ ([M n+]), exhibiting a second-order dependence on [M n+]: k et = k MCET[M n+]2, where k MCET is the fourth-order rate constant of metal ion-coupled electron transfer (MCET). This means that two M n+ ions are bound to the one-electron reduced species of 1. Such a binding of two M n+ ions associated with the ET reduction of 1 resulted in a 92 mV positive shift of the one-electron reduction potential of 1 (E red) with increasing log­([M n+]). The log k MCET values increased linearly with the increasing Lewis acidity of M n+ (ΔE), which was determined from the g values of O2 •––M n+ complexes. The driving force dependence of log k et of MCET from ferrocene derivatives to 1 in the presence of M n+ has been well-evaluated in light of the Marcus theory of electron transfer.

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Metal-superoxo species are involved in a variety of enzymatic oxidation reactions, and multi-electron oxidation of substrates is frequently observed in those enzymatic reactions. A Cr -superoxo complex, [Cr (O )(TMC)(Cl)] (1; TMC=1,4,8,11-tetramethyl-1,4,8,11-tetraazacyclotetradecane), is described that acts as a novel three-electron oxidant in the oxidation of dihydronicotinamide adenine dinucleotide (NADH) analogues. In the reactions of 1 with NADH analogues, a Cr -oxo complex, [Cr (O)(TMC)(Cl)] (2), is formed by a heterolytic O-O bond cleavage of a putative Cr -hydroperoxo complex, [Cr (OOH)(TMC)(Cl)], which is generated by hydride transfer from NADH analogues to 1. The comparison of the reactivity of NADH analogues with 1 and p-chloranil (Cl Q) indicates that oxidation of NADH analogues by 1 proceeds by proton-coupled electron transfer with a very large tunneling effect (for example, with a kinetic isotope effect of 470 at 233 K), followed by rapid electron transfer.

Nitrate reductases (NRs) are molybdoenzymes that reduce nitrate (NO3−) to nitrite (NO2−) in both mammals and plants. In mammals, the salival microbes take part in the generation of the NO2−...