Hydrogen Atom Transfer Reactions of Mononuclear Nonheme Metal

Article Cite This: Acc. Chem. Res. XXXX, XXX, XXX−XXX

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Hydrogen Atom Transfer Reactions of Mononuclear Nonheme Metal−Oxygen Intermediates Published as part of the Accounts of Chemical Research special issue “Hydrogen Atom Transfer”. Wonwoo Nam,*,†,‡ Yong-Min Lee,† and Shunichi Fukuzumi*,†,§ †

Department of Chemistry and Nano Science, Ewha Womans University, Seoul 03760, Korea School of Chemistry and Chemical Engineering, Shaanxi Normal University, Xi’an 710119, P. R. China § Graduate School of Science and Engineering, Meijo University, Nagoya, Aichi 468-8502, Japan Downloaded via DURHAM UNIV on September 4, 2018 at 22:00:25 (UTC). See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles.

‡

CONSPECTUS: Molecular oxygen (O2), the greenest oxidant, is kinetically stable in the oxidation of organic substrates due to its triplet ground state. In nature, O2 is reduced by two electrons with two protons to produce hydrogen peroxide (H2O2) and by four electrons with four protons to produce water (H2O) by oxidase and oxygenase metalloenzymes. In the process of the twoelectron/two-proton and four-electron/four-proton reduction of O2 by metalloenzymes and their model compounds, metal−oxygen intermediates, such as metal−superoxido, −peroxido, −hydroperoxido, and −oxido species, are generated depending on the numbers of electrons and protons involved in the O2 activation reactions. The one-electron reduction of metal−oxygen intermediates is coupled with the binding of one proton. Such a hydrogen atom transfer (HAT) is defined as proton-coupled electron transfer (PCET), and there is a mechanistic dichotomy whether HAT occurs via a concerted PCET pathway or stepwise pathways [i.e., electron transfer followed by proton transfer (ET/PT) or proton transfer followed by electron transfer (PT/ET)]. The metal−oxygen intermediates formed are oxidants that can abstract a hydrogen atom (H-atom) from substrate C−H bonds. The H-atom abstraction from substrate C−H bonds by the metal−oxygen intermediates can also occur via a concerted PCET or stepwise PCET pathways. In the PCET reactions, a proton can be provided not only by the substrate itself but also by an acid that is added to a reaction solution. This Account describes the reactivities of metal−oxygen intermediates, such as metal−superoxido, −peroxido, −hydroperoxido, and −oxido complexes, in HAT reactions, focusing on the mechanisms of PCET reactions of metal−oxygen intermediates and on the mechanistic dichotomy of concerted versus stepwise pathways. Recent developments in the reactivity studies of Cr−, Fe−, and Cu−superoxido complexes in H-atom and hydride transfer reactions are discussed. Reactivities of an iron(III)− hydroperoxido complex and an iron(III)−peroxido complex binding redox-inactive metal ions are also summarized briefly. Mononuclear nonheme iron(IV)− and manganese(IV)−oxido complexes have shown high reactivities in HAT reactions, and their chemistry in PCET reactions is discussed intensively. Acid-catalyzed HAT reactions of metal−oxygen intermediates are also discussed to demonstrate a unified driving force dependence of logarithm of the rate constants of acid-catalyzed oxidation of various substrates by an iron(IV)−oxido complex and that of PCET from one-electron donors to the iron(IV)−oxido complex. PCET reactions of metal−oxygen intermediates are shown to proceed via a concerted pathway (one-step HAT) or a stepwise ET/PT pathway depending on the ET and PCET driving forces (−ΔG). The boundary conditions between concerted versus stepwise PCET pathways are clarified to demonstrate a switchover of the mechanisms only by changing the reaction temperature in the boundary conditions. This Account summarizes recent developments in the HAT reactions by synthetic mononuclear nonheme metal−oxygen intermediates over the past 10 years. a MIII−superoxido complex, MIII(O2•−) (1 in Chart 1), in which O2•− (a Lewis base) binds to MIII (a Lewis acid). Addition of a hydrogen atom, composed of one proton and one electron, to [MIII(O 2•−)] affords a M(III)−hydroperoxido complex, MIII(O2H−) (3 in Chart 1; see the reaction pathway a in Chart 1), via simultaneous transfer of one proton and one electron. Alternatively, stepwise transfer of one electron (first) and one proton (second) or one proton (first) and one electron

1. INTRODUCTION Exquisite transition metal active sites in metalloenzymes have evolved for the purpose of activating molecular oxygen (O2) to generate metal−oxygen intermediates, such as metal−superoxido, −peroxido, −hydroperoxido, and −oxido species (see Chart 1), and utilizing the oxidative power of the metal−oxygen intermediates in the oxidation of organic substrates as well as the energy production and consumption in photosynthesis and respiration, respectively.1−6 Activation of O2 is initiated by the reaction of a transition metal(II) complex [MII] with O2 (Scheme 1).7 Electron transfer (ET) from MII to O2 generates © XXXX American Chemical Society

Received: June 22, 2018

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DOI: 10.1021/acs.accounts.8b00299 Acc. Chem. Res. XXXX, XXX, XXX−XXX

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Accounts of Chemical Research

dichotomy whether HAT from organic hydrogen donors (DH2) to hydrogen acceptors (A) occurs via a concerted PCET or a stepwise PCET.12−15 In the absence of any external proton(s), H-atom in the HAT reactions derives from DH2.12−15 However, ET from DH2 to A can be accelerated by external protons to produce DH2•+ and AH•, followed by HAT from DH2•+ to AH• to produce D, H+, and AH2.7 Thus, there is another mechanistic dichotomy whether HAT with H+ occurs via a one-step HAT or a stepwise ET/PT, in which ET is accelerated by the presence of H+.7 Such a mechanistic dichotomy in HAT reactions (concerted or stepwise) has yet to be systematically discussed for metal−oxygen intermediates. In this Account, we discuss the HAT reactions by metal−oxygen intermediates, such as metal− superoxido, −peroxido, −hydroperoxido, and −oxido complexes (see Chart 1), focusing on the mechanistic dichotomy in HAT reactions (Schemes 1 and 2).

Chart 1. Schematic Diagram Showing the Structures of Metal−Superoxido (1e and 1s for End-on and Side-on O2Binding, Respectively), Metal−Peroxido (2), Metal− Hydroperoxido (3), and High-Valent Metal−Oxido (4-(IV) and 4-(V) for the Oxidation States of 4+ and 5+, Respectively) Complexes and the HAT Reactions by the Metal−Oxygen Intermediates

Scheme 1. Pathways for Dioxygen Activation by Metal(II) Complex

2. PCET OF METAL−SUPEROXIDO COMPLEXES Metal−superoxido species have been invoked as reactive intermediates in C−H bond activation reactions by nonheme iron and copper enzymes.16,17 Several mononuclear nonheme metal−superoxido complexes have recently been synthesized, characterized spectroscopically and structurally, and investigated in HAT and OAT reactions.9,18−23 Metal−superoxido complexes (MO2•−) usually act as one-electron oxidants in abstracting H-atom from substrates, thereby forming their corresponding metal−hydroperoxido species.20,22,23 In the Hatom abstraction reactions, large kinetic isotope effect (KIE) values were observed frequently, such as a KIE value of 50 in the H-atom abstraction of 9,10-dihydroanthracene (DHA) by a mononuclear end-on Cr(III)−superoxido complex (1e in Chart 1), [CrIII(TMC)(O2)(Cl)]+ (TMC = 1,4,8,11-tetramethyl1,4,8,11-tetraazacyclotetradecane).22 More recently, we have reported a detailed mechanism of HAT from 10-methyl-9,10dihydroacridine (AcrH2) and its analogs to [CrIII(TMC)(O2)(Cl)]+ (eqs a−g in Scheme 3).24 First, HAT from AcrH2 to [CrIII(TMC)(O2)(Cl)]+ occurs to produce 10-methylacridinyl radical (AcrH•) and [CrIII(TMC)(OOH)(Cl)]+ (eq a), followed by fast ET from AcrH• to the hydroperoxide complex ([CrIII(TMC)(OOH)(Cl)]+) to produce 10-methylacridinium ion (AcrH+) and a reduced metal ion complex, [CrII(TMC)(OOH)(Cl)] (eq b). The subsequent heterolytic O−O bond cleavage of the putative [CrII(TMC)(OOH)(Cl)] species occurs rapidly to produce a Cr(IV)−oxido complex ([CrIV(TMC)(O)(Cl)]+) and OH− (eq b). Thus, the overall reaction is hydride transfer from AcrH2 to [CrIII(TMC)(O2)(Cl)]+. The OH− produced is added to AcrH+ to afford the adduct [AcrH(OH)] (eq c). Then, hydride transfer from AcrH(OH) to [CrIII(TMC)(O2)(Cl)]+ occurs via HAT, followed by ET to produce 10-methylacridone (AcrO), [CrIV(TMC)(O)(Cl)]+, and H2O (eq d). HAT from AcrH(OH) to [(Cl)(TMC)CrIV(O)]+ also occurs to produce AcrOH• and [CrIII(TMC)(OH)(Cl)]+ (eq e). Finally, HAT from AcrOH• to [CrIV(TMC)(O)(Cl)]+ occurs to produce AcrO and [CrIII(TMC)(OH)(Cl)]+ (eq f). The overall stoichiometry of the reaction of [CrIII(TMC)(O2)(Cl)]+ with AcrH2 is shown in eq g, where AcrH2 and [CrIII(TMC)(O2)(Cl)]+ act as a four-electron reductant and a three-electron oxidant, respectively. All the metal−oxygen intermediates, such as MIII(O2•−) = [CrIII(TMC)(O2)(Cl)]+, MIII(O2H−) = [CrIII(TMC)(OOH)(Cl)]+, and MIV(O2−) = [CrIV(TMC)(O)(Cl)]+ in Scheme 1 and MIII(OH−) = [CrIII(TMC)(OH)(Cl)]+ in Scheme 2, are involved in the mechanism of this three-

(second) occurs, being defined as ET/PT or PT/ET, respectively (Scheme 1). Thus, there is a mechanistic dichotomy whether hydrogen atom transfer (HAT) to [MII(O2•−)] occurs via a concerted proton-coupled electron transfer (PCET) (onestep HAT) or a stepwise PCET (ET/PT and PT/ET). The same mechanistic dichotomy holds for the further HAT to [MIII(O2H−)], which occurs via a concerted PCET or a stepwise PCET to produce a metal(IV)−oxido complex, MIV(O2−) (4 in Chart 1; see the reaction pathway b in Chart 1), and H2O through O−O bond cleavage as shown in Scheme 1, where electrons and protons are provided by NAD(P)H coenzymes in enzymatic reactions.7 The PCET reduction of MIV(O2−) to produce a M(III)−hydroxide complex, MIII(OH−), and the further PCET reduction of MIII(OH−) to MII(OH2) can occur via a concerted PCET or a stepwise PCET (ET/PT and PT/ ET), as shown in Scheme 2, where substrates are oxidized by Scheme 2. Pathways for the PCET Reduction of MIV(O2−)

PCET in enzymatic reactions.7 By combining Schemes 1 and 2, interconversion between H2O and O2 in the oxygen evolving complex in Photosystem II (PSII) and cytochrome c oxidase in respiration is made possible via metal−oxygen intermediates. Recently, various metal−oxygen intermediates have been synthesized and characterized spectroscopically and/or structurally in biomimetic studies. These synthetic metal−oxygen complexes have been investigated in various oxidation reactions, such as oxygen atom transfer (OAT) and HAT reactions.8−11 In the HAT reactions, metal−oxygen intermediates abstract hydrogen atom (H-atom) from substrate C−H bonds, and there have been several reports discussing the mechanistic B


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Accounts of Chemical Research Scheme 3. Mechanism of Oxidation of AcrH2 by a Cr(III)− Superoxido Complex

Figure 1. Plot of log kox for the oxidation of NADH analogs by [CrIII(TMC)(O2)(Cl)]2+ (1) in MeCN at 253 K vs log kox for the oxidation of a series of NADH analogs by p-chloranil in MeCN at 298 K; NADH analogs: 1-benzyl-1,4-dihydronicotinamide (BNAH), AcrH2, AcrD2, 9-methyl-10-methyl-9,10-dihydroacridine (AcrHMe), and 9-ethyl-10-methyl-9,10-dihydroacridine (AcrHEt). Reprinted with permission from ref 24. Copyright 2017 WILEY-VCH Verlag GmbH.

A copper(II)−superoxido complex, [CuII(DMM-tmpa)(O2)]+ (DMM-tmpa = tris((4-methoxy-3,5-dimethylpyridin-2yl)methyl)amine), reacts with a series of para-substituted 2,6-ditert-butylphenols (p-X-DTBPs) to afford 2,6-di-tert-butyl-1,4benzoquinone (DTBQ) in up to 50% yields.28 The observations of a significant KIE value of 11 and a linear correlation of logarithm of second-order rate constants (log k2) as compared to log k2 of HAT from p-X-DTBPs to cumylperoxyl radical indicate that HAT from p-XDTBP to [CuII(DMM-tmpa)(O2)]+ also proceeds via a concerted PCET pathway.28 Neither ET/PT nor PT/ET occurs judging from the dependence of log k2 on the one-electron oxidation potentials of p-X-DTBPs.28 A mononuclear side-on (η2) iron(III)−superoxido complex bearing a tetraamido macrocyclic ligand (TAML = 3,3,6,6,9,9hexamethyl-2,5,7,10-tetraoxo-3,5,6,7,9,10-hexahydro-2Hbenzo[e][1,4,7,10]tetraazacyclo-tridecine-1,4,8,11-tetraide), ([FeIII(TAML)(O2)]2−) (1s in Chart 1), which was characterized structurally and spectroscopically, undergoes HAT from p-DTBP to produce 2,2′-dihydroxy-3,3′,5,5′-tetra-tert-butylbiphenyl as a major product.29 A linear plot of the relative rates as a function of O−H bond dissociation energies (BDEs) of p-XDTBPs with a slope of −0.65 and the steric effect observed in the HAT reaction of 2,4-di-tert-butylphenol vs 2,6-di-tert-butylphenol indicate that HAT from p-X-DTBPs to [FeIII(TAML)(O2)]2− also proceeds via a concerted PCET, because the ET pathway would exhibit no steric effect.29 HAT reactions of psubstituted phenol derivatives with copper(III)−hydroxide complexes (LCuOH and NO2LCuOH, where L = N,N′-bis(2,6diisopropylphenyl)-2,6-pyridine-dicarboxamide and NO2L = N,N′-bis(2,6-diisopropyl-4-nitrophenyl)pyridine-2,6-dicarboxamide) were also reported to proceed via a concerted PCET pathway.30 In concerted PCET pathways described above, hydrogen donors (DH2) were the source of the proton in the reactions. However, protons can be provided by addition of an acid such as triflic acid (HOTf) in a PCET reaction of [CrIII(TMC)(O2)(Cl)]+ with an electron donor.31 No ET from [FeII(bpy)3]2+ to [CrIII(TMC)(O2)(Cl)]+ occurs, as indicated by the higher Eox value of [FeII(bpy)3]2+ (1.06 V vs SCE) than the Ered value (−0.52 V vs SCE).31 In the presence of HOTf, however, PCET from [FeII(bpy)3]2+ to [CrIII(TMC)(O2)(Cl)]+ occurs to

electron reduction of [CrIII(TMC)(O2)(Cl)]+ by AcrH2 (eqs a−f), where there are four HAT steps (eqs a, d, e, and f).24 The rate-determining step of the three-electron reduction of [CrIII(TMC)(O2)(Cl)]+ by AcrH2 (eq g) was proposed to be the initial HAT from AcrH2 to [CrIII(TMC)(O2)(Cl)]+ (eq a), based on several experimental observations such as a large KIE value of 74.24 The rate constants of the oxidation of NADH analogs by [CrIII(TMC)(O2)(Cl)]+ are linearly correlated with those of the oxidation of the same NADH analogs by p-chloranil (Cl4Q), which acts as a two-electron oxidant (Figure 1).24,25 Such a linear correlation with the slope of unity indicates that HAT from NADH analogs to [CrIII(TMC)(O2)(Cl)]+ follows the same mechanism as HAT from NADH analogs to Cl4Q, which was reported to proceed via a concerted PCET.25 No ET from AcrH2 to [CrIII(TMC)(O2)(Cl)]+ should occur because of the much higher one-electron oxidation potential of AcrH2 (Eox = 0.81 V vs SCE)26 than the one-electron reduction potential of [CrIII(TMC)(O2)(Cl)]+ (Ered = −0.52 V vs SCE),23 when the Gibbs energy change of ET is largely positive (ΔGet = 1.34 eV). In enzymatic reactions, stereoselective hydride transfer from NADH to NO-bound heme was suggested from the crystal structure of P450nor in a complex with NADH.27 C


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Accounts of Chemical Research produce [FeIII(bpy)3]3+ and [CrIII(TMC)(H2O2)(Cl)]2+, which is in equilibrium (eq 1),

value would be independent of [HOTF]. It was confirmed that no protonation of [CrIII(TMC)(O2)(Cl)]+ by HOTf occurs, indicating that there is no stepwise PT/ET pathway either.31 Other metal−superoxide complexes, such as Fe(III)−superoxide and Cu(II)−superoxide complexes, may also undergo PCET reactions with acids, which have yet to be reported.

[Fe(bpy)3 ]2 + + [Cr III(TMC)(O2 )(Cl)]+ + 2H+ K et

XooY [Fe(bpy)3 ]3 + + [Cr III(TMC)(H 2O2 )(Cl)]2 +

(1)

as indicated by the ET titrations shown in Figure 2, where the concentration of remaining [FeII(bpy)3]2+ decreased with an

3. PCET OF METAL−PEROXIDO AND METAL−HYDROPEROXIDO COMPLEXES A mononuclear nonheme side-on (η2) iron(III)−peroxido complex, [FeIII(TMC)(O2)]+ (2 in Chart 1), undergoes no HAT reaction with alkylaromatic compounds even with weak C−H bonds such as xanthene (75.5 kcal mol−1) and DHA (77 kcal mol−1).32 The cyclic voltammograms of [FeIII(TMC)(O2)]+ exhibit a cathodic current peak (Epc) due to the oneelectron reduction at −0.43 V vs SCE with no corresponding anodic current peak (Epa) at the reverse scan, indicating that the one-electron reduction of [FeIII(TMC)(O2)]+ is irreversible.32 When redox-inactive metal ions, such as Sr2+, Ca2+, Zn2+, Lu3+, Y3+, and Sc3+, were added to [FeIII(TMC)(O2)]+, iron(III)− peroxido complexes binding the redox-active metal ions, FeIII(TMC)−(μ,η2:η2-O2)−Mn+, were formed, and the Epc values shifted in a positive direction with increasing the Lewis acidity of the redox-inactive metal ions.33 For example, in the case of Sc3+ ion, the Epc value shifted to 0.38 V vs SCE, and one additional cathodic current peak was observed at ∼0.06 V vs SCE, which was assigned as a cathodic current peak of [FeIV(TMC)(O)]2+ that was produced via the heterolytic O− O bond cleavage of FeII(TMC)−(μ,η2:η2-O2)−Sc3+.33 Thus, the oxidizing ability of the redox-inactive metal ion-bound iron(III)−peroxido complexes increases with increasing the Lewis acidity of metal ions bound to the iron−peroxido moiety.33 Addition of HClO4 to a solution of [FeIII(TMC)(O2)]+ resulted in the protonation of the peroxido ligand to produce an iron(III)−hydroperoxide complex, [FeIII(TMC)(O2H)]2+ (3 in Chart 1).32 In contrast to the case of [FeIII(TMC)(O2)]+, [FeIII(TMC)(O2H)]2+ is capable of abstracting H atoms from xanthene and DHA at −20 °C with rate constants of 8.1 × 10−1 and 2.4 × 10−1 M−1 s−1, respectively.32,34 The FeIII(TMC)− (μ,η2:η2-O2)−M3+ (M3+ = Sc3+, Y3+, Lu3+, and La3+) complexes can also abstract H atom from cyclohexadiene (CHD).35 Thus, binding of redox-inactive metal ions or proton to an iron(III)− peroxido complex enhances the oxidizing ability of the iron− peroxido complex. Similarly, Mn(III)− and Co(III)−peroxido complexes bearing TMC derivatives are not active in H atom abstraction reactions; however, their M(III)−hydroperoxido complexes, generated upon protonation of the M(III)−peroxido complexes, showed reactivity in H atom abstraction reactions.36−38 The HAT reactions of M(III)−hydroperoxido complexes with xanthene proceed via concerted PCET pathways where KIEs values of 3−5 were observed.38

Figure 2. Spectroscopic redox titrations at 520 nm for the disappearance of [Fe II (bpy)3 ] 2+ as a function of the initial concentration of [CrIII(TMC)(O2)(Cl)]+ added to an MeCN solution of [FeII(bpy)3]2+ and HOTf (blue circles, 1.5 mM; black circles, 2.5 mM; red circles, 3.0 mM) at 233 K. Inset shows the dependence of Ered of [CrIII(TMC)(O2)(Cl)]+ on log([HOTf]). Reprinted with permission from ref 31. Copyright 2018 American Chemical Society.

increase in [HOTf].31 The ET equilibrium constants (Ket) in eq 1 were determined by global fitting of plots in Figure 2. The Ered values of [CrIII(TMC)(O2)(Cl)]+ with HOTf (Figure 2, inset) were determined from the Ket values and the Eox value of [FeII(bpy)3]2+ using the Nernst equation (eq 2).31 Ered = Eox + (2.3RT /F )log Ket III

(2) +

+

The dependence of Ered of [Cr (TMC)(O2)(Cl)] on [H ] is given by the Nernst equation (eq 3), 0 Ered = Ered + (2.3RT /F )log(K [H+]2 )

(3)

where K is the equilibrium constant of the diprotonation of the one-electron reduced species of [CrIII(TMC)(O2)(Cl)]+.31 A linear correlation of Ered vs log[HOTf] with a slope of 93 mV/ log[HOTf] (inset of Figure 2) agrees with eq 3, since the expected slope from eq 3 is 2 × (2.3RT/F) at 233 K = 93 mV/ log[HOTf] is the same as the observed value.31 The Ered value of [CrIII(TMC)(O2)(Cl)]+ shifted significantly in the positive direction from −0.52 V vs SCE without HOTf to 1.12 V vs SCE with HOTf (2.5 mM).31 The second-order rate constant (ket) of PCET from [FeII(bpy)3]2+ to [CrIII(TMC)(O2)(Cl)]+ increased with increasing [HOTf], exhibiting a second-order dependence with respect to [HOTf] (eq 4).31 Such a second-order ket = k 2[HOTf]2

4. PCET OF MONONUCLEAR NONHEME IRON(IV)−OXIDO COMPLEXES Since the first crystal structure of a mononuclear nonheme iron(IV)−oxido complex, [FeIV(TMC)(O)]2+ (4 in Chart 1), was reported in 2003,39 a large number (∼80) of iron(IV)− oxido complexes bearing nonheme ligands were synthesized, spectroscopically and structurally characterized, and investigated in various oxidation reactions.8,9,40 One notable example is the C−H bond activation of alkanes by the synthetic iron(IV)−oxido complexes; some of them are capable of

(4)

dependence of ket on [HOTf] indicates that PCET occurs in aconcerted manner where two protons are consumed to produce H2O2 that is bound to the CrIII complex (eq 1), because the binding of two protons stabilizes the peroxido complex thermodynamically more than that of one proton.31 If ET occurs first, followed by a fast PT (stepwise ET/PT), the ket D


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well fitted as a function of the driving force of PCET by the Marcus equation of the adiabatic outer-sphere intramolecular ET as given by eq 5.47 The driving force dependence of kET in the

activating the C−H bonds of cyclohexane (BDEC−H ∼99.3 kcal/ mol).41−43 The reactivity of the iron(IV)−oxido complexes was also shown to be influenced significantly by supporting and axial ligands, spin states of the iron(IV) ion, and proton and redoxinactive metal ions in the C−H bond activation reactions.8,40 As we have discussed in the case of a chromium(III)− superoxido complex, [CrIII(TMC)(O2)(Cl)]+, the Ered value of a nonheme iron(IV)−oxido complex, [FeIV(N4Py)(O)]2+ (N4Py = N,N-bis(2-pyridylmethyl)-N-bis(2-pyridyl)methylamine), with HOTf or Sc(OTf)3 shifted in a positive direction according to eq 3, where [H+] can be replaced by [HOTf] or [Sc(OTf)3], indicating that the ET reduction of [FeIV(N4Py)(O)]2+ is coupled with binding of two protons or two Sc3+ ions to [FeIII(N4Py)(O)]+.44−46 The second-order rate constants (ket) of PCET from [RuII(bpy)3]2+ to [FeIV(N4Py)(O)]2+ increased with increasing [HOTf], exhibiting a secondorder dependence with respect to [HOTf].46 When HOTf is replaced by the deuterated acid (DOTf), the ket value of PCET from [RuII(bpy)3]2+ to [FeIV(N4Py)(O)]2+ with DOTf in MeCN at 298 K is larger than that with the same concentration of HOTf.46 The rate constant of the oxidation of toluene by [FeIV(N4Py)(O)]2+ with HOTf also increased with increasing [HOTf], and it also exhibited a second-order dependence with respect to [HOTf] as the case of ET from [RuII(bpy)3]2+ to [FeIV(N4Py)(O)]2+ with HOTf.46 The KIE value of toluene vs toluene-d8 decreased with increasing [HOTf] from 31 without HOTf to reach KIE = 1.0 at [HOTf] > 50 mM.46 Such a drastic change in KIE from 31 to 1.0 indicates that the rate-determining step in the oxidation of toluene by [FeIV(N4Py)(O)]2+ is changed from concerted PCET (one-step HAT) in the absence of HOTf to ET in the presence of HOTf.46 A general mechanism for PCET from substrates (S = toluene and thioanisole derivatives) to [FeIV(N4Py)(O)]2+ with HOTf, as well as MCET (metal ion-coupled electron transfer)7 from S to [FeIV(N4Py)(O)]2+ with Sc(OTf)3 is shown in Scheme 4.46

kET = (kBT /h)exp[− (λ /4)(1 + ΔGet /λ)/(kBT )]

(5)

oxidation ofstyrene, thioanisole, and toluene derivatives by [FeIV(N4Py)(O)]2+ with HOTf (10 mM) in MeCN at 298 K is remarkably unified with that of kET of PCET from various electron donors to [FeIV(N4Py)(O)]2+ using the same value of ET reorganization energy (λ = 2.74 eV) in the exergonic region (ΔGet < 0) as shown in Figure 3, providing clear evidence for the

Figure 3. Driving force (−ΔGet) dependence of log kET for oxidation of styrene and toluene derivatives [(1) trans-stilbene, (2) 1,1diphenylethene, (3) α-methylstyrene, (4) 3-methylstyrene, (5) styrene, (6) p-Me-thioanisole, (7) thioanisole, (8) p-Cl-thioanisole, (9) p-Brthioanisole, (10) p-CN-thioanisole, (11) hexamethylbenzene, (12) 1,2,3,4,5-pentamethylbenzene, (13) 1,2,4,5-tetramethylbenzene, (14) 1,2,4-trimethylbenzene, (15) 1,4-dimethylbenzene, (16) 1,3,5-trimethylbenzene, and (17) toluene] by [FeIV(N4Py)(O)]2+ and log kET for PCET from various electron donors {coordinatively saturated metal complexes; (18) [FeII(Ph2Phen)3]2+, (19) [FeII(bpy)3]2+, (20) [Ru II (4,4′-Me 2 Phen) 3 ] 2+ , (21) [Ru II (5,5′-Me 2 Phen) 3 ] 2+ , (22) [FeII(ClPhen)3]2+, and (23) [RuII(bpy)3]2+} to [FeIV(N4Py)(O)]2+ in the presence of HOTf (10 mM) in MeCN at 298 K. The black circles show the driving force dependence of log kET of electron transfer from electron donors [(24) decamethylferrocene, (25) octamethylferrocene, (26) 1,1′-dimethylferrocene, (27) n-amylferrocene, and (28) ferrocene] to [FeIV(N4Py)(O)]2+ in the absence of HOTf in MeCN at 298 K. Reprinted with permission from ref 48. Copyright 2015 American Chemical Society.

Scheme 4. Mechanism of Oxidation of Toluene and Thioanisole by [FeIV(N4Py)(O)]2+ in the Presence of Mn+(OTf)n

rate-determining PCET in acid-catalyzed oxidation of those substrates by [FeIV(N4Py)(O)]2+.47,48 The deviation from the Marcus line in the endergonic region (ΔGet > 0) suggests that the one-step OAT and HAT pathways become more favorable as compared with the PCET pathway.47,48

Oxidation of S by [FeIV(N4Py)(O)]2+ with HOTf or Sc(OTf)3 is initiated by PCET or MCET from S to [FeIV(N4Py)(O)]2+ binding two molecules of HOTf or Sc(OTf)3, following the formation of precursor complexes (Scheme 4).46 The ratedetermining PCET or MCET may be followed by rapid transfer of O•− from [FeIII(N4Py)(O)]2+−(HOTf)2 or [FeIII(N4Py)(O)]2+−(Sc(OTf)3)2 to S•+ to produce the oxygenated product (SO = benzyl alcohol and sulfoxide derivatives) and [FeII(N4Py)]2+ (Scheme 4).46 The first-order (unimolecular) rate constants (kET) of PCET from various electron donors to [Fe IV (N4Py)(O) 2+ − (HOTf)2]) in the precursor complexes in Scheme 4 can be

5. PCET OF MONONUCLEAR NONHEME MANGANESE(IV)−OXIDO COMPLEXES HAT reactions from substrates to metal−oxygen intermediates generally proceed via concerted PCET rather than stepwise ET/ PT or PT/ET, since the concerted pathway is thermodynamically more favorable as compared with the stepwise pathway (vide supra). However, when ET is thermodynamically feasible, ET from substrates to metal−oxygen intermediates may occur first, followed by proton transfer (ET/PT pathway). Such an ET/PT pathway was reported for hydride transfer from AcrH2 E


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Accounts of Chemical Research to [MnIV(Bn-TPEN)(O)]2+ (Bn-TPEN = N-benzyl-N,N′,N′tris(2-pyridylmethyl)ethane-1,2-diamine).49,50 When AcrH2 was replaced by AcrD2, the KIE was determined to be 1.0 ± 0.1, which indicates that ET from AcrH2 to [MnIV(BnTPEN)(O)]2+ is the rate-determining step, followed by fast PT and ET from AcrH2•+ to [MnIII(Bn-TPEN)(O)]2+.50 This is in sharp contrast to the hydride transfer from AcrH2 to [FeIV(Bn-TPEN)(O)]2+, in which a large KIE value of 18 was observed25 when ET from AcrH2 (Eox = 0.81 V vs SCE) to [FeIV(Bn-TPEN)(O)]2+ (Ered = 0.49 V vs SCE)51 is endergonic. Thus, HAT from AcrH2 to [FeIV(Bn-TPEN)(O)]2+ proceeds via a concerted PCET pathway. In contrast, ET from AcrH2 to [MnIV(Bn-TPEN)(O)]2+ (Ered = 0.78 V vs SCE)52 is only slightly endergonic, when ET is the rate-determining step, followed by rapid PT and ET, where R = H (Scheme 5).

Figure 4. Eyring plots of ln(kobs/T) against 1/T obtained in the reactions of [MnIV(N4Py)(O)]2+ with mesitylene (blue circles) and mesitylene-d12 (green circles) and [MnIV(N4Py)(O)]2+−(HOTf)2 with mesitylene (red circles) and mesitylene-d12 (black circles) at various temperatures (243−293 K). Reprinted with permission from ref 54. Copyright 2016 WILEY-VCH Verlag GmbH.

Scheme 5. Mechanism of Hydride Transfer from AcrHR to [MnIV(Bn-TPEN)(O)]2+

Hydroxylation of mesitylene and mesitylene-d12 by a mononuclear manganese(IV)−oxido complex ([MnIV(N4Py)(O)]2+) and a triflic acid-bound manganese(IV)−oxido complex, [Mn IV (N4Py)(O)]2+ −(HOTf) 2 , 53 proceeds in CF3CH2OH/CH3CN (v/v = 1:1) at various temperatures (eq 6).54 The Eyring plots of the second-order rate constants of the

[MnIV(N4Py)(O)]2+.54 The ΔS⧧ value of ET is close to zero or negative, being much less negative than that of HAT; the transition state of which is more ordered to afford the largely negative ΔS⧧ value. In such a case, ET is energetically more favored at higher temperatures. This is the reason why the mechanism changes from ET at 293 K with a KIE of 1.0 to HAT at temperatures lower than 263 K with a KIE of 2.9. The observation of the different mechanisms by changing only temperature indicates that the two mechanisms are distinguishable and competing without the continuous change of the mechanism, but switchable by changing only reaction temperature.54 More recently, high-valent cobalt(IV)−oxido complexes bearing TAML and 13-TMC (13-TMC = 1,4,7,10-tetramethyl-1,4,7,10-tetraazacyclotridecane) ligands were synthesized and characterized using various spectroscopic techniques.55,56 Interestingly, the Co(IV)−oxido complexes showed reactivities in oxidation reactions, such as C−H bond activation, sulfoxidation, olefin epoxidation, and intermetal OAT reactions.55,56 The Co(IV)−oxido complexes are competent oxidants in HAT reactions, providing us with a good opportunity to investigate detailed mechanisms for the HAT reactions by high-valent Co−oxido complexes.

hydroxylationof mesitylene and mesitylene-d 1 2 by [MnIV(N4Py)(O)]2+ and [MnIV(N4Py)(O)]2+−(HOTf)2 are shown in Figure 4.54 At 293 K, the rate constant of the hydroxylation of mesitylene by [MnIV(N4Py)(O)]2+−HOTf)2 is the same as that of mesitylene-d12 to afford a KIE of 1.0, indicating that the HAT reaction proceeds via ET from mesitylene to [MnIV(N4Py)(O)]2+−(HOTf)2 at 293 K.54 Interestingly, when the reaction temperature is lowered to less than 263 K in the reactions of [MnIV(N4Py)(O)]2+−(HOTf)2 with mesitylene and mesitylene-d12, however, the mechanism changes from ET to HAT with a KIE of 2.9.54 Such a switchover of the reaction mechanism from ET to HAT is shown to occur by changing only temperature in the boundary region between ET and HAT pathways when the driving force of ET from toluene derivatives to [MnIV(N4Py)(O)]2+−(HOTf)2 is around −0.5 eV. In the case of [MnIV(N4Py)(O)]2+, the KIE remains to be 3.2 in the temperature range of 243−293 K.54 The ΔH⧧ value of [MnIV(N4Py)(O)]2+−(HOTf)2 is larger than that of [MnIV(N4Py)(O)]2+, whereas the absolute ΔS⧧ value of [MnIV(N4Py)(O)]2+−(HOTf)2 is much smaller than that of

6. SUMMARY As represented by Schemes 1 and 2 with Chart 1, the mechanistic details on reactions of mononuclear nonheme metal−oxygen intermediates, such as metal−superoxido, −hydroperoxido, and −oxido complexes, with respect to the kinetics and thermodynamics of electron transfer or protonation (or Lewis-acid activation) are of great interest and of considerable fundamental importance in the activation of O2 by metal ions in biology or in stoichiometric or catalytic reactions involving the oxygenation or oxidative dehydrogenation of organic substrates. We have shown that HAT from AcrH2 to a Cr(III)−superoxido complex and Fe(IV)−oxido complexes proceeds via a concerted PCET pathway when the ET pathway is endergonic (ΔGet > 0). In contrast, HAT from AcrH2 to a Mn(IV)−oxido complex proceeds via a ratedetermining ET pathway, followed by fast PT when the ET pathway is thermodynamically feasible (ΔGet ≈ 0). We have also shown that HAT from toluene derivatives to an Fe(IV)−oxido complex, [FeIV(N4Py)(O)]2+, is accelerated by addition of acids; the HAT occurs via PCET from toluene derivatives to a F


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Accounts of Chemical Research HOTf-bound Fe(IV)−oxido complex, [FeIV(N4Py)(O)]2+− (HOTf)2. The driving force dependence of the logarithm of the first-order rate constant of acid-catalyzed oxidation of various substrates including HAT in the precursor complexes (i.e., log kET) is remarkably unified with that of kET of PCET from various one-electron donors to [FeIV(N4Py)(O)]2+ in the absence and presence of HOTf. Another interesting observation is the switchover of the reaction mechanism from a concerted PCET pathway (one-step HAT) to an ET pathway in the oxidation of mesitylene by an acid-bound manganese(IV)−oxido complex, [MnIV(N4Py)(O)]2+−(HOTf)2, by changing only temperature in the boundary region between one-step HAT and ET pathways. Another interesting and important aspect that should be addressed in the near future is the oxygen rebound vs oxygen nonrebound mechanisms in the HAT reactions by heme and nonheme metal−oxygen intermediates;57 we hope to understand factors (and the role of factors) that determine the oxygen rebound vs oxygen nonrebound pathways in the HAT reactions.

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(2) Que, L., Jr. 60 Years of Dioxygen Activation. J. Biol. Inorg. Chem. 2017, 22, 171−173. (3) Huang, X.; Groves, J. T. Oxygen Activation and Radical Transformations in Heme Proteins and Metalloporphyrins. Chem. Rev. 2018, 118, 2491−2553. (4) Cox, N.; Pantazis, D. A.; Neese, F.; Lubitz, W. Biological Water Oxidation. Acc. Chem. Res. 2013, 46, 1588−1596. (5) Meyer, T. J.; Sheridan, M. V.; Sherman, B. D. Mechanisms of Molecular Water Oxidation in Solution and on Oxide Surfaces. Chem. Soc. Rev. 2017, 46, 6148−6169. (6) Mano, N.; de Poulpiquet, A. O2 Reduction in Enzymatic Biofuel Cells. Chem. Rev. 2018, 118, 2392−2468. (7) Fukuzumi, S.; Ohkubo, K.; Lee, Y.-M.; Nam, W. Lewis Acid Coupled Electron Transfer of Metal−Oxygen Intermediates. Chem. Eur. J. 2015, 21, 17548−17559. (8) Nam, W.; Lee, Y.-M.; Fukuzumi, S. Tuning Reactivity and Mechanism in Oxidation Reactions by Mononuclear Nonheme Iron(IV)-Oxo Complexes. Acc. Chem. Res. 2014, 47, 1146−1154. (9) Nam, W. Synthetic Mononuclear Nonheme Iron−Oxygen Intermediates. Acc. Chem. Res. 2015, 48, 2415−2423. (10) Neu, H. M.; Baglia, R. A.; Goldberg, D. P. A Balancing Act: Stability versus Reactivity of Mn(O) Complexes. Acc. Chem. Res. 2015, 48, 2754−2764. (11) Hong, S.; Lee, Y.-M.; Ray, K.; Nam, W. Dioxygen Activation Chemistry by Synthetic Mononuclear Nonheme Iron, Copper and Chromium Complexes. Coord. Chem. Rev. 2017, 334, 25−42. (12) Fukuzumi, S.; Koumitsu, S.; Hironaka, K.; Tanaka, T. Energetic Comparison between Photoinduced Electron-Transfer Reactions from NADH Model Compounds to Organic and Inorganic Oxidants and Hydride-Transfer Reactions from NADH Model Compounds to pBenzoquinone Derivatives. J. Am. Chem. Soc. 1987, 109, 305−316. (13) Fukuzumi, S.; Ohkubo, K.; Tokuda, Y.; Suenobu, T. Hydride Transfer from 9-Substituted 10-Methyl-9,10-dihydroacridines to Hydride Acceptors via Charge-Transfer Complexes and Sequential Electron-Proton-Electron Transfer. A Negative Temperature Dependence of the Rates. J. Am. Chem. Soc. 2000, 122, 4286−4294. (14) Galli, C.; Gentili, P.; Lanzalunga, O. Hydrogen Abstraction and Electron Transfer with Aminoxyl Radicals: Synthetic and Mechanistic Issues. Angew. Chem., Int. Ed. 2008, 47, 4790−4796. (15) Litwinienko, G.; Ingold, K. U. Solvent Effects on the Rates and Mechanisms of Reaction of Phenols with Free Radicals. Acc. Chem. Res. 2007, 40, 222−230. (16) Bollinger, J. M., Jr.; Krebs, C. Enzymatic C-H Activation by Metal-Superoxo Intermediates. Curr. Opin. Chem. Biol. 2007, 11, 151− 158. (17) Quist, D. A.; Diaz, D. E.; Liu, J. J.; Karlin, K. D. Activation of Dioxygen by Copper Metalloproteins and Insights from Model Complexes. J. Biol. Inorg. Chem. 2017, 22, 253−288. (18) Cho, J.; Sarangi, R.; Nam, W. Mononuclear Metal-O2 Complexes Bearing Macrocyclic N-Tetramethylated Cyclam Ligands. Acc. Chem. Res. 2012, 45, 1321−1330. (19) Würtele, C.; Gaoutchenova, E.; Harms, K.; Holthausen, M. C.; Sundermeyer, J.; Schindler, S. Crystallographic Characterization of a Synthetic 1:1 End-On Copper Dioxygen Adduct Complex. Angew. Chem., Int. Ed. 2006, 45, 3867−3869. (20) Chiang, C.-W.; Kleespies, S. T.; Stout, H. D.; Meier, K. K.; Li, P.Y.; Bominaar, E. L.; Que, L., Jr.; Münck, E.; Lee, W.-Z. Characterization of a Paramagnetic Mononuclear Nonheme Iron-Superoxo Complex. J. Am. Chem. Soc. 2014, 136, 10846−10849. (21) Oddon, F.; Chiba, Y.; Nakazawa, J.; Ohta, T.; Ogura, T.; Hikichi, S. Characterization of Mononuclear Non-heme Iron(III)-Superoxo Complex with a Five-Azole Ligand Set. Angew. Chem., Int. Ed. 2015, 54, 7336−7339. (22) Cho, J.; Woo, J.; Nam, W. An “End-On” Chromium(III)Superoxo Complex: Crystallographic and Spectroscopic Characterization and Reactivity in C−H Bond Activation of Hydrocarbons. J. Am. Chem. Soc. 2010, 132, 5958−5959. (23) Cho, J.; Woo, J.; Han, J. E.; Kubo, M.; Ogura, T.; Nam, W. Chromium(V)-oxo and Chromium(III)-superoxo Complexes Bearing

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (W.N.). *E-mail: [email protected] (S.F.). ORCID

Wonwoo Nam: 0000-0001-8592-4867 Yong-Min Lee: 0000-0002-5553-1453 Shunichi Fukuzumi: 0000-0002-3559-4107 Author Contributions

The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes

The authors declare no competing financial interest. Biographies Wonwoo Nam received his Ph.D. degree in Inorganic Chemistry from UCLA in 1990. He is currently a Distinguished Professor of Ewha Womans University in Seoul, Korea. Yong-Min Lee received his Ph.D. degree in Inorganic Chemistry from Pusan National University in Korea in 1999. He is a Special Appointment Professor at Ewha Womans University since 2009. Shunichi Fukuzumi earned Ph.D. degree in applied chemistry at Tokyo Institute of Technology in 1978. He has been a Full Professor of Osaka University from 1994 to 2015. He is now a Distinguished Professor at Ewha Womans University.

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ACKNOWLEDGMENTS The authors gratefully acknowledge the contributions of their collaborators and co-workers mentioned in the cited references and financial support by the NRF of Korea through CRI (NRF2012R1A3A2048842 to W.N.), GRL (NRF-2010-00353 to W.N.), and Basic Science Research Program (2017R1D1A1B03029982 to Y.M.L. and 2017R1D1A1B03032615 to S.F.) and by a SENTAN project from JST and JSPS KAKENHI (Grant Numbers 16H02268 to S.F.) from MEXT, Japan.

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REFERENCES

(1) Nam, W. Dioxygen Activation by Metalloenzymes and Models. Acc. Chem. Res. 2007, 40, 465. G


Article


(40) Puri, M.; Que, L., Jr. Toward the Synthesis of More Reactive S = 2 Non-Heme Oxoiron(IV) Complexes. Acc. Chem. Res. 2015, 48, 2443− 2452. (41) Kaizer, J.; Klinker, E. J.; Oh, N. Y.; Rohde, J.-U.; Song, W. J.; Stubna, A.; Kim, J.; Münck, E.; Nam, W.; Que, L., Jr. Nonheme FeIVO Complexes That Can Oxidize the C-H Bonds of Cyclohexane at Room Temperature. J. Am. Chem. Soc. 2004, 126, 472−473. (42) Seo, M. S.; Kim, N. H.; Cho, K.-B.; So, J. E.; Park, S. K.; Clémancey, M.; Garcia-Serres, R.; Latour, J.-M.; Shaik, S.; Nam, W. A Mononuclear Nonheme Iron(IV)-Oxo Complex Which Is More Reactive than Cytochrome P450 Model Compound I. Chem. Sci. 2011, 2, 1039−1045. (43) Monte Pérez, I.; Engelmann, X.; Lee, Y.-M.; Yoo, M.; Kumaran, E.; Farquhar, E. R.; Bill, E.; England, J.; Nam, W.; Swart, M.; Ray, K. A Highly Reactive Oxoiron(IV) Complex Supported by a Bioinspired N3O Macrocylic Ligand. Angew. Chem., Int. Ed. 2017, 56, 14384− 14388. (44) Morimoto, Y.; Park, J.; Suenobu, T.; Lee, Y.-M.; Nam, W.; Fukuzumi, S. Mechanistic Borderline between One-Step Hydrogen Atom Transfer versus Stepwise Sc3+-Coupled Electron Transfer and Proton Transfer from Benzyl Alcohol Derivatives to a Non-Heme Iron(IV)-Oxo Complex. Inorg. Chem. 2012, 51, 10025−10036. (45) Park, J.; Lee, Y.-M.; Nam, W.; Fukuzumi, S. Brønsted AcidPromoted C-H Bond Activation via Electron Transfer from Toluene Derivatives to a Protonated Non-Heme Iron(IV)-Oxo Complex with No Kinetic Isotope Effect. J. Am. Chem. Soc. 2013, 135, 5052−5061. (46) Park, J.; Morimoto, Y.; Lee, Y.-M.; Nam, W.; Fukuzumi, S. Unified View of Oxidative C-H Bond Cleavage and Sulfoxidation by a Nonheme Iron(IV)-Oxo Complex via Lewis Acid-Promoted Electron Transfer. Inorg. Chem. 2014, 53, 3618−3628. (47) Marcus, R. A. Chemical and Electrochemical Electron-Transfer Theory. Annu. Rev. Phys. Chem. 1964, 15, 155−196. (48) Park, J.; Lee, Y.-M.; Ohkubo, K.; Nam, W.; Fukuzumi, S. Efficient Epoxidation of Styrene Derivatives by a Nonheme Iron(IV)-Oxo Complex via Proton-Coupled Electron Transfer with Triflic Acid. Inorg. Chem. 2015, 54, 5806−5812. (49) Wu, X.; Seo, M. S.; Davis, K. M.; Lee, Y.-M.; Chen, J.; Cho, K.-B.; Pushkar, Y. N.; Nam, W. A Highly Reactive Mononuclear Non-Heme Manganese(IV)-Oxo Complex That Can Active the Strong C-H Bonds of Alkanes. J. Am. Chem. Soc. 2011, 133, 20088−20091. (50) Yoon, H.; Lee, Y.-M.; Nam, W.; Fukuzumi, S. Hydride Transfer from NADH Analogues to a Nonheme Manganese(IV)-Oxo Complex via Rate-Determining Electron Transfer. Chem. Commun. 2014, 50, 12944−12946. (51) Lee, Y.-M.; Kotani, H.; Suenobu, T.; Nam, W.; Fukuzumi, S. Fundamental Electron-Transfer Properties of Non-heme Oxoiron(IV) Complexes. J. Am. Chem. Soc. 2008, 130, 434−435. (52) Yoon, H.; Morimoto, Y.; Lee, Y.-M.; Nam, W.; Fukuzumi, S. Electron-Transfer Properties of a Nonheme Manganese(IV)−Oxo Complex Acting as a Stronger One-Electron Oxidant than the Iron(IV)−Oxo Analogue. Chem. Commun. 2012, 48, 11187−11189. (53) Chen, J.; Yoon, H.; Lee, Y.-M.; Seo, M. S.; Sarangi, R.; Fukuzumi, S.; Nam, W. Tuning the Reactivity of Mononuclear Nonheme Manganese(IV)-Oxo Complexes by Triflic Acid. Chem. Sci. 2015, 6, 3624−3632. (54) Jung, J.; Kim, S.; Lee, Y.-M.; Nam, W.; Fukuzumi, S. Switchover of the Mechanism between Electron Transfer and Hydrogen-Atom Transfer for a Protonated Manganese(IV)-Oxo Complex by Changing Only the Reaction Temperature. Angew. Chem., Int. Ed. 2016, 55, 7450−7454. (55) Hong, S.; Pfaff, F. F.; Kwon, E.; Wang, Y.; Seo, M.-S.; Bill, E.; Ray, K.; Nam, W. Spectroscopic Capture and Reactivity of a Low-Spin Cobalt(IV)-Oxo Complex Stabilized by Binding Redox-Inactive Metal Ions. Angew. Chem., Int. Ed. 2014, 53, 10403−10407. (56) Wang, B.; Lee, Y.-M.; Tcho, W.-Y.; Tussupbayev, S.; Kim, S.-T.; Kim, Y.; Seo, M. S.; Cho, K.-B.; Dede, Y.; Keegan, B. C.; Ogura, T.; Kim, S. H.; Ohta, T.; Baik, M.-H.; Ray, K.; Shearer, J.; Nam, W. Synthesis and Reactivity of a Mononuclear Non-Haem Cobalt(IV)-Oxo Complex. Nat. Commun. 2017, 8, 14839.

a Macrocyclic TMC Ligand in Hydrogen Atom Abstraction Reactions. Chem. Sci. 2011, 2, 2057−2062. (24) Devi, T.; Lee, Y.-M.; Jung, J.; Sankaralingam, M.; Nam, W.; Fukuzumi, S. A Chromium(III)-Superoxo Complex as a ThreeElectron Oxidant with a Large Tunneling Effect in Multi-Electron Oxidation of NADH Analogues. Angew. Chem., Int. Ed. 2017, 56, 3510− 3515. (25) Fukuzumi, S.; Kotani, H.; Lee, Y.-M.; Nam, W. Sequential Electron-Transfer and Proton-Transfer Pathways in Hydride-Transfer Reactions from Dihydronicotinamide Adenine Dinucleotide Analogues to Non-heme Oxoiron(IV) Complexes and p-Chloranil. Detection of Radical Cations of NADH Analogues in Acid-Promoted HydrideTransfer Reactions. J. Am. Chem. Soc. 2008, 130, 15134−15142. (26) Fukuzumi, S.; Tokuda, Y.; Kitano, T.; Okamoto, T.; Otera, J. Electron-Transfer Oxidation of 9-Substituted 10-Methyl-9,10-dihydroacridines. Cleavage of C-H vs C-C Bond of the Radical Cations. J. Am. Chem. Soc. 1993, 115, 8960−8968. (27) Oshima, R.; Fushinobu, S.; Su, F.; Zhang, L.; Takaya, N.; Shoun, H. Structural evidence for direct hydride transfer from NADH to cytochrome P450nor. J. Mol. Biol. 2004, 342, 207−217. (28) Lee, J. Y.; Peterson, R. L.; Ohkubo, K.; Garcia-Bosch, I.; Himes, R. A.; Woertink, J.; Moore, C. D.; Solomon, E. I.; Fukuzumi, S.; Karlin, K. D. Mechanistic Insights into the Oxidation of Substituted Phenols via Hydrogen Atom Abstraction by a Cupric-Superoxo Complex. J. Am. Chem. Soc. 2014, 136, 9925−9937. (29) Hong, S.; Sutherlin, K. D.; Park, J.; Kwon, E.; Siegler, M. A.; Solomon, E. I.; Nam, W. Crystallographic and spectroscopic characterization and reactivities of a mononuclear non-haem iron(III)-superoxo complex. Nat. Commun. 2014, 5, 5440. (30) Dhar, D.; Yee, G. M.; Markle, T. F.; Mayer, J. M.; Tolman, W. B. Reactivity of the copper(III)-hydroxide unit with phenols. Chem. Sci. 2017, 8, 1075−1085. (31) Devi, T.; Lee, Y.-M.; Nam, W.; Fukuzumi, S. Remarkable Acid Catalysis in Proton-Coupled Electron-Transfer Reactions of a Chromium(III)-Superoxo Complex. J. Am. Chem. Soc. 2018, 140, 8372−8375. (32) Cho, J.; Jeon, S.; Wilson, S. A.; Liu, L. V.; Kang, E. A.; Braymer, J. J.; Lim, M. H.; Hedman, B.; Hodgson, K. O.; Valentine, J. S.; Solomon, E. I.; Nam, W. Structure and Reactivity of a Mononuclear Non-Haem Iron(III)−Peroxo Complex. Nature 2011, 478, 502−505. (33) Bang, S.; Lee, Y.-M.; Hong, S.; Cho, K.-B.; Nishida, Y.; Seo, M. S.; Sarangi, R.; Fukuzumi, S.; Nam, W. Redox-Inactive Metal Ions Modulate the Reactivity and Oxygen Release of Mononuclear NonHaem Iron(III)−Peroxo Complexes. Nat. Chem. 2014, 6, 934−940. (34) Liu, L. V.; Hong, S.; Cho, J.; Nam, W.; Solomon, E. I. Comparison of High-Spin and Low-Spin Nonheme FeIII−OOH Complexes in O−O Bond Homolysis and H-Atom Abstraction Reactivities. J. Am. Chem. Soc. 2013, 135, 3286−3299. (35) Bae, S. H.; Lee, Y.-M.; Fukuzumi, S.; Nam, W. Fine Control of the Redox Reactivity of a Nonheme Iron(III)−Peroxo Complex by Binding Redox-Inactive Metal Ions. Angew. Chem., Int. Ed. 2017, 56, 801−805. (36) So, H.; Park, Y. J.; Cho, K.-B.; Lee, Y.-M.; Seo, M. S.; Cho, J.; Sarangi, R.; Nam, W. Spectroscopic Characterization and Reactivity Studies of a Mononuclear Nonheme Mn(III)-Hydroperoxo Complex. J. Am. Chem. Soc. 2014, 136, 12229−12232. (37) Sankaralingam, M.; Lee, Y.-M.; Jeon, S. H.; Seo, M. S.; Cho, K.B.; Nam, W. A Mononuclear Manganese(III)-Hydroperoxo Complex: Synthesis by Activating Dioxygen and Reactivity in Electrophilic and Nucleophilic Reactions. Chem. Commun. 2018, 54, 1209−1212. (38) Tcho, W.-Y.; Wang, B.; Lee, Y.-M.; Cho, K.-B.; Shearer, J.; Nam, W. A Mononuclear Nonheme Cobalt(III)-Hydroperoxo Complex with an Amphoteric Reactivity in Electrophilic and Nucleophilic Oxidative Reactions. Dalton Trans. 2016, 45, 14511−14515. (39) Rohde, J.-U.; In, J.-H.; Lim, M. H.; Brennessel, W. W.; Bukowski, M. R.; Stubna, A.; Münck, E.; Nam, W.; Que, L., Jr. Crystallographic and Spectroscopic Characterization of a Nonheme Fe(IV)O Complex. Science 2003, 299, 1037−1039. H


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Accounts of Chemical Research (57) Cho, K.-B.; Hirao, H.; Shaik, S.; Nam, W. To Rebound or Dissociate? This Is the Mechanistic Question in C-H Hydroxylation by Heme and Nonheme Metal-Oxo Complexes. Chem. Soc. Rev. 2016, 45, 1197−1210.

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Hydrogen Atom Transfer Reactions of Mononuclear Nonheme Metal

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