A Contrasting Effect of Acid in Electron Transfer, Oxygen Atom Transfer, and Hydrogen Atom Transfer Reactions of a Nickel(III) Complex

There have been many examples of the accelerating effects of acids in electron transfer (ET), oxygen atom transfer (OAT), and hydrogen atom transfer (HAT) reactions. Herein, we report a contrasting effect of acids in the ET, OAT, and HAT reactions of a nickel(III) complex, [NiIII(PaPy3*)]2+ (1) in acetone/CH3CN (v/v 19:1). 1 was synthesized by reacting [NiII(PaPy3*)]+ (2) with magic blue or iodosylbenzene in the absence or presence of triflic acid (HOTf), respectively. Sulfoxidation of thioanisole by 1 and H2O occurred in the presence of HOTf, and the reaction rate increased proportionally with increasing concentration of HOTf ([HOTf]). The rate of ET from diacetylferrocene to 1 also increased linearly with increasing [HOTf]. In contrast, HAT from 9,10-dihydroanthracene (DHA) to 1 slowed down with increasing [HOTf], exhibiting an inversely proportional relation to [HOTf]. The accelerating effect of HOTf in the ET and OAT reactions was ascribed to the binding of H+ to the PaPy3* ligand of 2; the one-electron reduction potential (Ered) of 1 was positively shifted with increasing [HOTf]. Such a positive shift in the Ered value resulted in accelerating the ET and OAT reactions that proceeded via the rate-determining ET step. On the other hand, the decelerating effect of HOTf on HAT from DHA to 1 resulted from the inhibition of proton transfer from DHA•+ to 2 due to the binding of H+ to the PaPy3* ligand of 2. The ET reactions of 1 in the absence and presence of HOTf were well analyzed in light of the Marcus theory of ET in comparison with the HAT reactions

Mechanistic Insights into the C–H Bond Activation of Hydrocarbons by Chromium(IV) Oxo and Chromium(III) Superoxo Complexes

The mechanism of the C–H bond activation of hydrocarbons by a nonheme chromium­(IV) oxo complex bearing an N-methylated tetraazamacrocyclic cyclam (TMC) ligand, [Cr^IV(O)­(TMC)­(Cl)]⁺ (<b>2</b>), has been investigated experimentally and theoretically. In experimental studies, reaction rates of <b>2</b> with substrates having weak C–H bonds were found to depend on the concentration and bond dissociation energies of the substrates. A large kinetic isotope effect value of 60 was determined in the oxidation of dihydroanthracene (DHA) and deuterated DHA by <b>2</b>. These results led us to propose that the C–H bond activation reaction occurs via a H-atom abstraction mechanism, in which H-atom abstraction of substrates by <b>2</b> is the rate-determining step. In addition, formation of a chromium­(III) hydroxo complex, [Cr^III(OH)­(TMC)­(Cl)]⁺ (<b>3</b>), was observed as a decomposed product of <b>2</b> in the C–H bond activation reaction. The Cr^IIIOH product was characterized unambiguously with various spectroscopic methods and X-ray crystallography. Density functional theory (DFT) calculations support the experimental observations that the C–H bond activation by <b>2</b> does not occur via the conventional H-atom-abstraction/oxygen-rebound mechanism and that <b>3</b> is the product formed in this C–H bond activation reaction. DFT calculations also propose that <b>2</b> may have some Cr^IIIO^•– character. The oxidizing power of <b>2</b> was then compared with that of a chromium­(III) superoxo complex bearing the identical TMC ligand, [Cr^III(O₂)­(TMC)­(Cl)]⁺ (<b>1</b>), in the C–H bond activation reaction. By performing reactions of <b>1</b> and <b>2</b> with substrates under identical conditions, we were able to demonstrate that the reactivity of <b>2</b> is slightly greater than that of <b>1</b>. DFT calculations again support this experimental observation, showing that the rate-limiting barrier for the reaction with <b>2</b> is slightly lower than that of <b>1</b>

Mechanistic Insights into the C–H Bond Activation of Hydrocarbons by Chromium(IV) Oxo and Chromium(III) Superoxo Complexes

The mechanism of the C–H bond activation of hydrocarbons by a nonheme chromium­(IV) oxo complex bearing an N-methylated tetraazamacrocyclic cyclam (TMC) ligand, [Cr^IV(O)­(TMC)­(Cl)]⁺ (<b>2</b>), has been investigated experimentally and theoretically. In experimental studies, reaction rates of <b>2</b> with substrates having weak C–H bonds were found to depend on the concentration and bond dissociation energies of the substrates. A large kinetic isotope effect value of 60 was determined in the oxidation of dihydroanthracene (DHA) and deuterated DHA by <b>2</b>. These results led us to propose that the C–H bond activation reaction occurs via a H-atom abstraction mechanism, in which H-atom abstraction of substrates by <b>2</b> is the rate-determining step. In addition, formation of a chromium­(III) hydroxo complex, [Cr^III(OH)­(TMC)­(Cl)]⁺ (<b>3</b>), was observed as a decomposed product of <b>2</b> in the C–H bond activation reaction. The Cr^IIIOH product was characterized unambiguously with various spectroscopic methods and X-ray crystallography. Density functional theory (DFT) calculations support the experimental observations that the C–H bond activation by <b>2</b> does not occur via the conventional H-atom-abstraction/oxygen-rebound mechanism and that <b>3</b> is the product formed in this C–H bond activation reaction. DFT calculations also propose that <b>2</b> may have some Cr^IIIO^•– character. The oxidizing power of <b>2</b> was then compared with that of a chromium­(III) superoxo complex bearing the identical TMC ligand, [Cr^III(O₂)­(TMC)­(Cl)]⁺ (<b>1</b>), in the C–H bond activation reaction. By performing reactions of <b>1</b> and <b>2</b> with substrates under identical conditions, we were able to demonstrate that the reactivity of <b>2</b> is slightly greater than that of <b>1</b>. DFT calculations again support this experimental observation, showing that the rate-limiting barrier for the reaction with <b>2</b> is slightly lower than that of <b>1</b>

Mn(III)-Iodosylarene Porphyrins as an Active Oxidant in Oxidation Reactions: Synthesis, Characterization, and Reactivity Studies

Mn­(III)-iodosylarene porphyrin adducts, [Mn­(III)­(ArIO)­(Porp)]⁺, were synthesized by reacting electron-deficient Mn­(III) porphyrin complexes with iodosylarene (ArIO) at −60 °C and characterized using various spectroscopic methods. The [Mn­(III)­(ArIO)­(Porp)]⁺ species were then investigated in the epoxidation of olefins under stoichiometric conditions. In the epoxidation of olefins by the Mn­(III)-iodosylarene porphyrin species, epoxide was formed as the sole product with high chemoselectivities and stereoselectivities. For example, cyclohexene oxide was formed exclusively with trace amounts of allylic oxidation products; <i>cis</i>- and <i>trans</i>-stilbenes were oxidized to the corresponding <i>cis</i>- and <i>trans</i>-stilbene oxides, respectively. In the catalytic epoxidation of cyclohexene by an electron-deficient Mn­(III) porphyrin complex and ^sPhIO at low temperature (e.g., −60 °C), the Mn­(III)-iodosylarene porphyrin species was evidenced as the active oxidant that effects the olefin epoxidation to give epoxide as the product. However, at high temperature (e.g., 0 °C) or in the case of using an electron-rich manganese­(III) porphyrin catalyst, allylic oxidation products, along with cyclohexene oxide, were yielded, indicating that the active oxidant(s) was not the Mn­(III)-iodosylarene adduct but probably high-valent Mn-oxo species in the catalytic reactions. We also report the conversion of the Mn­(III)-iodosylarene porphyrins to high-valent Mn-oxo porphyrins under various conditions, such as at high temperature, with electron-rich porphyrin ligand, and in the presence of base (OH^–). The present study reports the first example of spectroscopically well-characterized Mn­(III)-iodosylarene porphyrin species being an active oxidant in the stoichiometric and catalytic oxidation reactions. Other aspects, such as one oxidant versus multiple oxidants debate, also were discussed

A Mononuclear Non-Heme Manganese(IV)–Oxo Complex Binding Redox-Inactive Metal Ions

Redox-inactive metal ions play pivotal roles in regulating the reactivities of high-valent metal–oxo species in a variety of enzymatic and chemical reactions. A mononuclear non-heme Mn­(IV)–oxo complex bearing a pentadentate N₅ ligand has been synthesized and used in the synthesis of a Mn­(IV)–oxo complex binding scandium ions. The Mn­(IV)–oxo complexes were characterized with various spectroscopic methods. The reactivities of the Mn­(IV)–oxo complex are markedly influenced by binding of Sc³⁺ ions in oxidation reactions, such as a ∼2200-fold increase in the rate of oxidation of thioanisole (i.e., oxygen atom transfer) but a ∼180-fold decrease in the rate of C–H bond activation of 1,4-cyclohexadiene (i.e., hydrogen atom transfer). The present results provide the first example of a non-heme Mn­(IV)–oxo complex binding redox-inactive metal ions that shows a contrasting effect of the redox-inactive metal ions on the reactivities of metal–oxo species in the oxygen atom transfer and hydrogen atom transfer reactions

A Mononuclear Non-Heme Manganese(IV)–Oxo Complex Binding Redox-Inactive Metal Ions

Redox-inactive metal ions play pivotal roles in regulating the reactivities of high-valent metal–oxo species in a variety of enzymatic and chemical reactions. A mononuclear non-heme Mn­(IV)–oxo complex bearing a pentadentate N₅ ligand has been synthesized and used in the synthesis of a Mn­(IV)–oxo complex binding scandium ions. The Mn­(IV)–oxo complexes were characterized with various spectroscopic methods. The reactivities of the Mn­(IV)–oxo complex are markedly influenced by binding of Sc³⁺ ions in oxidation reactions, such as a ∼2200-fold increase in the rate of oxidation of thioanisole (i.e., oxygen atom transfer) but a ∼180-fold decrease in the rate of C–H bond activation of 1,4-cyclohexadiene (i.e., hydrogen atom transfer). The present results provide the first example of a non-heme Mn­(IV)–oxo complex binding redox-inactive metal ions that shows a contrasting effect of the redox-inactive metal ions on the reactivities of metal–oxo species in the oxygen atom transfer and hydrogen atom transfer reactions