Unified Mechanism of Oxygen Atom Transfer and Hydrogen Atom Transfer Reactions with a Triflic Acid-Bound Nonheme Manganese(IV)–Oxo Complex via Outer-Sphere Electron Transfer

Abstract

Outer-sphere electron transfer from styrene, thioanisole, and toluene derivatives to a triflic acid (HOTf)-bound nonheme Mn­(IV)–oxo complex, [(N4Py)­MnIV(O)]2+–(HOTf)2 (N4Py = N,N-bis­(2-pyridylmethyl)-N-bis­(2-pyridyl)­methylamine), has been shown to be the rate-determining step of different types of redox reactions such as epoxidation, sulfoxidation, and hydroxylation of styrene, thioanisole, and toluene derivatives, respectively, by [(N4Py)­MnIV(O)]2+–(HOTf)2. The rate constants of HOTf-promoted epoxidation of all styrene derivatives with [(N4Py)­MnIV(O)]2+ and electron transfer from electron donors to [(N4Py)­MnV(O)]2+ exhibit a remarkably unified correlation with the driving force of outer-sphere electron transfer in light of the Marcus theory of electron transfer. The same electron-transfer driving force dependence is observed in the oxygen atom transfer from [(N4Py)­MnIV(O)]2+–(HOTf)2 to thioanisole derivatives as well as in the hydrogen atom transfer from toluene derivatives to [(N4Py)­MnIV(O)]2+–(HOTf)2. Thus, mechanisms of oxygen atom transfer (epoxidation and sulfoxidation) reactions of styrene and thioanisole derivatives and hydrogen atom transfer (hydroxylation) reactions of toluene derivatives by [(N4Py)­MnIV(O)]2+–(HOTf)2 have been unified for the first time as the same reaction pathway via outer-sphere electron transfer, followed by the fast bond-forming step, which exhibits the singly unified electron-transfer driving force dependence of the rate constants as outer-sphere electron-transfer reactions. In the case of the epoxidation of cis-stilbene by [(N4Py)­MnIV(O)]2+–(HOTf)2, the isomerization of cis-stilbene radical cation to trans-stilbene radical cation occurs after outer-sphere electron transfer from cis-stilbene to [(N4Py)­MnIV(O)]2+–(HOTf)2 to yield trans-stilbene oxide selectively, which is also taken as evidence for the occurrence of electron transfer in the acid-catalyzed epoxidation.

Recognition of Surface Oxygen Intermediates on NiFe Oxyhydroxide Oxygen-Evolving Catalysts by Homogeneous Oxidation Reactivity

“…We took inspiration from the well-studied reactivities in homogeneous catalytic oxidation and utilized typical reactivities of oxygen atom transfer (OAT) and hydrogen atom transfer (HAT). OAT probes, often including phosphines, thioethers, and alkenes, can specifically target oxygen-terminated structures for abstracting oxygen atoms. − HAT probes, often C–H-based molecules, can specifically target oxygen-associated high-valent metal species, such as metal oxo and metal peroxo species, for donating H atoms to generate the corresponding radical. , These reactivities can widely exist on the oxygen intermediates formed during OER and we anticipate that probes with different reactivity can behave divergently, allowing for the differentiation and identification of oxygen intermediates. To be compatible with in situ experiments, the reactive probes should also demonstrate high solubility and chemical stability in the electrolyte.…”

“…We took inspiration from the well-studied reactivities in homogeneous catalytic oxidation and utilized typical reactions of oxygen atom transfer (OAT) and hydrogen atom transfer (HAT). OAT probes can specifically target oxygen-terminated structures for abstracting oxygen atoms, often including phosphines, thioethers and alkenes [33][34][35] . HAT probes, often C-H-based molecules, can specifically target oxygen-associated high-valent metal species, such as metal oxo and metal peroxo species, for donating H atoms to generate the corresponding radical [36][37] .…”

Enhanced Redox Reactivity of a Nonheme Iron(V)–Oxo Complex Binding Proton

“…Protons are known to play important roles in tuning redox potentials and reactivities of metalloenzymes and metal catalysts in oxidation reactions, such as in water oxidation, substrate oxidation, dioxygen reduction, and proton reduction. − ,− In this context, Brønsted acids, such as perchloric acid (HClO 4 ) and triflic acid (HOTf), and Lewis acids, including redox-inactive metal ions, have been shown to enhance the electron transfer (ET) and oxidizing capability of high-valent nonheme metal­(IV)–oxo complexes via a proton-coupled electron-transfer (PCET) mechanism. − For example, binding of a redox-inactive metal ion (e.g., Sc 3+ ion) to the oxo ligand of a nonheme iron­(IV)–oxo complex has shown dramatic enhancement of the rates of ET reactions. , In addition, binding of Lewis acidic metal ions or triflic acid to the oxo moiety of nonheme Mn­(IV)–oxo complexes resulted in the large acceleration of rates of ET and OAT reactions. − It was also shown that the Brønsted and Lewis acids increased the catalytic reactivity of metal catalysts in oxidation reactions. − In heme models, Karlin and co-workers reported an elegant result demonstrating that the binding of 2,6-lutidinium triflate to an iron­(IV)–oxo porphyrin complex via hydrogen bonding resulted in a significant enhancement of the oxidizing capability of the iron­(IV)–oxo porphyrin complex along with a positive shift of the one-electron reduction potential by >0.89 V. , In the case of a Mn­(V)–oxo TAML complex, a Lewis acidic metal ion (e.g., Sc 3+ ion) was shown to bind to the TAML ligand, which resulted in an enhancement of the oxidizing power of the Mn­(V)–oxo complex with a positive shift (0.70 V) of the one-electron reduction potential . However, to the best of our knowledge, the effects of proton and metal ion on the chemical properties of nonheme iron­(V)–oxo complexes have never been explored previously.…”

Generation and Electron‐Transfer Reactivity of the Long‐Lived Photoexcited State of a Manganese(IV)‐Oxo‐Scandium Nitrate Complex

“…The LMCT energy of [(Bn‐TPEN)Mn IV (O)] 2+ ([(Bn‐TPEN .+ )Mn III (O)] 2+ ) is expected to decrease by increasing the number of binding molecules of Sc(OTf) 3 from one to two, because the electron accepting ability of the Mn IV (O) moiety increases with an increase in the number of binding molecules of Sc(OTf) 3 from one to two. The λ value of 0.53 eV is much smaller than that of the ground state of Mn IV (O) complexes ( λ =2.2–2.3 eV) [12,29] because of the ligand centered electron transfer to the ligand‐to‐metal charge transfer (LMCT) excited state of [(Bn‐TPEN)Mn IV (O)] 2+ −Sc(NO 3 ) 3 as compared with the metal‐centered electron transfer to the ground state of Mn IV (O) complexes. Sc(NO 3 ) 3 is known to act as a strong Lewis acid even in an aqueous solution [30,31] …”