Hydrogen Atom Transfer Reactions of Mononuclear NonhemeMetal−Oxygen IntermediatesPublished 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

CONSPECTUS: Molecular oxygen (O2), the greenest oxidant, is kineticallystable in the oxidation of organic substrates due to its triplet ground state. Innature, O2 is reduced by two electrons with two protons to produce hydrogenperoxide (H2O2) and by four electrons with four protons to produce water(H2O) by oxidase and oxygenase metalloenzymes. In the process of the two-electron/two-proton and four-electron/four-proton reduction of O2 bymetalloenzymes 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 inthe O2 activation reactions. The one-electron reduction of metal−oxygenintermediates is coupled with the binding of one proton. Such a hydrogenatom transfer (HAT) is defined as proton-coupled electron transfer (PCET), and there is a mechanistic dichotomy whetherHAT 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 canabstract a hydrogen atom (H-atom) from substrate C−H bonds. The H-atom abstraction from substrate C−H bonds by themetal−oxygen intermediates can also occur via a concerted PCET or stepwise PCET pathways. In the PCET reactions, a protoncan 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 andon 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, andtheir chemistry in PCET reactions is discussed intensively. Acid-catalyzed HAT reactions of metal−oxygen intermediates arealso discussed to demonstrate a unified driving force dependence of logarithm of the rate constants of acid-catalyzed oxidationof various substrates by an iron(IV)−oxido complex and that of PCET from one-electron donors to the iron(IV)−oxidocomplex. PCET reactions of metal−oxygen intermediates are shown to proceed via a concerted pathway (one-step HAT) or astepwise ET/PT pathway depending on the ET and PCET driving forces (−ΔG). The boundary conditions between concertedversus stepwise PCET pathways are clarified to demonstrate a switchover of the mechanisms only by changing the reactiontemperature in the boundary conditions. This Account summarizes recent developments in the HAT reactions by syntheticmononuclear nonheme metal−oxygen intermediates over the past 10 years.

1. INTRODUCTION

Exquisite transition metal active sites in metalloenzymes haveevolved for the purpose of activating molecular oxygen (O2) togenerate metal−oxygen intermediates, such as metal−super-oxido, −peroxido, −hydroperoxido, and −oxido species (seeChart 1), and utilizing the oxidative power of the metal−oxygenintermediates in the oxidation of organic substrates as well as theenergy production and consumption in photosynthesis andrespiration, respectively.1−6 Activation of O2 is initiated by thereaction of a transition metal(II) complex [MII] with O2

(Scheme 1).7 Electron transfer (ET) from MII to O2 generates

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(O2

•−)] affords a M(III)−hydroperoxido complex,MIII(O2H

−) (3 in Chart 1; see the reaction pathway a inChart 1), via simultaneous transfer of one proton and oneelectron. Alternatively, stepwise transfer of one electron (first)and one proton (second) or one proton (first) and one electron

Received: June 22, 2018Published: September 4, 2018

Article

pubs.acs.org/accountsCite This: Acc. Chem. Res. 2018, 51, 2014−2022

© 2018 American Chemical Society 2014 DOI: 10.1021/acs.accounts.8b00299Acc. Chem. Res. 2018, 51, 2014−2022

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(second) occurs, being defined as ET/PT or PT/ET,respectively (Scheme 1). Thus, there is a mechanistic dichotomywhether hydrogen atom transfer (HAT) to [MII(O2

•−)] occursvia a concerted proton-coupled electron transfer (PCET) (one-step HAT) or a stepwise PCET (ET/PT and PT/ET). Thesame mechanistic dichotomy holds for the further HAT to[MIII(O2H

−)], which occurs via a concerted PCET or a stepwisePCET to produce a metal(IV)−oxido complex, MIV(O2−) (4 inChart 1; see the reaction pathway b in Chart 1), and H2Othrough O−O bond cleavage as shown in Scheme 1, whereelectrons and protons are provided by NAD(P)H coenzymes inenzymatic reactions.7 The PCET reduction of MIV(O2−) toproduce a M(III)−hydroxide complex, MIII(OH−), and thefurther PCET reduction of MIII(OH−) to MII(OH2) can occurvia a concerted PCET or a stepwise PCET (ET/PT and PT/ET), as shown in Scheme 2, where substrates are oxidized by

PCET in enzymatic reactions.7 By combining Schemes 1 and 2,interconversion between H2O and O2 in the oxygen evolvingcomplex in Photosystem II (PSII) and cytochrome c oxidase inrespiration is made possible via metal−oxygen intermediates.Recently, various metal−oxygen intermediates have been

synthesized and characterized spectroscopically and/or struc-turally in biomimetic studies. These synthetic metal−oxygencomplexes have been investigated in various oxidation reactions,such as oxygen atom transfer (OAT) and HAT reactions.8−11 Inthe HAT reactions, metal−oxygen intermediates abstracthydrogen atom (H-atom) from substrate C−H bonds, andthere have been several reports discussing the mechanistic

dichotomy whether HAT from organic hydrogen donors (DH2)to hydrogen acceptors (A) occurs via a concerted PCET or astepwise PCET.12−15 In the absence of any external proton(s),H-atom in the HAT reactions derives fromDH2.

12−15 However,ET from DH2 to A can be accelerated by external protons toproduce 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 ora stepwise ET/PT, in which ET is accelerated by the presence ofH+.7 Such a mechanistic dichotomy in HAT reactions(concerted or stepwise) has yet to be systematically discussedfor metal−oxygen intermediates. In this Account, we discuss theHAT reactions by metal−oxygen intermediates, such as metal−superoxido, −peroxido, −hydroperoxido, and −oxido com-plexes (see Chart 1), focusing on the mechanistic dichotomy inHAT reactions (Schemes 1 and 2).

2. PCET OF METAL−SUPEROXIDO COMPLEXESMetal−superoxido species have been invoked as reactiveintermediates in C−H bond activation reactions by nonhemeiron and copper enzymes.16,17 Several mononuclear nonhememetal−superoxido complexes have recently been synthesized,characterized spectroscopically and structurally, and inves-tigated in HAT and OAT reactions.9,18−23 Metal−superoxidocomplexes (MO2

•−) usually act as one-electron oxidants inabstracting H-atom from substrates, thereby forming theircorresponding metal−hydroperoxido species.20,22,23 In the H-atom abstraction reactions, large kinetic isotope effect (KIE)values were observed frequently, such as a KIE value of 50 in theH-atom abstraction of 9,10-dihydroanthracene (DHA) by amononuclear end-on Cr(III)−superoxido complex (1e in Chart1), [CrIII(TMC)(O2)(Cl)]

+ (TMC = 1,4,8,11-tetramethyl-1,4,8,11-tetraazacyclotetradecane).22 More recently, we havereported a detailed mechanism of HAT from 10-methyl-9,10-dihydroacridine (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-methylacridinylradical (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-methylacridiniumion (AcrH+) and a reduced metal ion complex, [CrII(TMC)-(OOH)(Cl)] (eq b). The subsequent heterolytic O−O bondcleavage of the putative [CrII(TMC)(OOH)(Cl)] speciesoccurs rapidly to produce a Cr(IV)−oxido complex([CrIV(TMC)(O)(Cl)]+) and OH− (eq b). Thus, the overallreaction is hydride transfer from AcrH2 to [CrIII(TMC)(O2)-(Cl)]+. The OH− produced is added to AcrH+ to afford theadduct [AcrH(OH)] (eq c). Then, hydride transfer fromAcrH(OH) to [CrIII(TMC)(O2)(Cl)]

+ occurs via HAT,followed by ET to produce 10-methylacridone (AcrO),[CrIV(TMC)(O)(Cl)]+, and H2O (eq d). HAT from AcrH-(OH) to [(Cl)(TMC)CrIV(O)]+ also occurs to produceAcrOH• and [CrIII(TMC)(OH)(Cl)]+ (eq e). Finally, HATfrom AcrOH• to [CrIV(TMC)(O)(Cl)]+ occurs to produceAcrO and [CrIII(TMC)(OH)(Cl)]+ (eq f). The overallstoichiometry of the reaction of [CrIII(TMC)(O2)(Cl)]

+ withAcrH2 is shown in eq g, where AcrH2 and [CrIII(TMC)(O2)-(Cl)]+ act as a four-electron reductant and a three-electronoxidant, respectively. All the metal−oxygen intermediates, suchas 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-

Chart 1. Schematic Diagram Showing the Structures ofMetal−Superoxido (1e and 1s for End-on and Side-on O2-Binding, 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 theMetal−Oxygen Intermediates

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

Scheme 2. Pathways for the PCET Reduction of MIV(O2−)

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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 bethe initial HAT from AcrH2 to [Cr

III(TMC)(O2)(Cl)]+ (eq a),

based on several experimental observations such as a large KIEvalue of 74.24 The rate constants of the oxidation of NADHanalogs by [CrIII(TMC)(O2)(Cl)]

+ are linearly correlated withthose 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 thatHAT from NADH analogs to [CrIII(TMC)(O2)(Cl)]

+ followsthe same mechanism as HAT from NADH analogs to Cl4Q,which was reported to proceed via a concerted PCET.25 No ETfromAcrH2 to [Cr

III(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 theGibbs energy change of ET is largely positive (ΔGet = 1.34 eV).In enzymatic reactions, stereoselective hydride transfer fromNADH to NO-bound heme was suggested from the crystalstructure of P450nor in a complex with NADH.27

A copper(II)−superoxido complex, [CuII(DMM-tmpa)-(O2)]

+ (DMM-tmpa = tris((4-methoxy-3,5-dimethylpyridin-2-yl)methyl)amine), reacts with a series of para-substituted 2,6-di-tert-butylphenols (p-X-DTBPs) to afford 2,6-di-tert-butyl-1,4-benzoquinone (DTBQ) in up to 50% yields.28 The observationsof a significant KIE value of 11 and a linear correlation oflogarithm of second-order rate constants (log k2) as compared tolog k2 of HAT from p-X-DTBPs to cumylperoxyl radical indicatethat HAT from p-XDTBP to [CuII(DMM-tmpa)(O2)]

+ alsoproceeds via a concerted PCET pathway.28 Neither ET/PT norPT/ET occurs judging from the dependence of log k2 on theone-electron oxidation potentials of p-X-DTBPs.28

A mononuclear side-on (η2) iron(III)−superoxido complexbearing a tetraamido macrocyclic ligand (TAML = 3,3,6,6,9,9-hexamethyl-2,5,7,10-tetraoxo-3,5,6,7,9,10-hexahydro-2H-benzo[e][1,4,7,10]tetraazacyclo-tridecine-1,4,8,11-tetraide),([FeIII(TAML)(O2)]

2−) (1s in Chart 1), which was charac-terized structurally and spectroscopically, undergoes HAT fromp-DTBP to produce 2,2′-dihydroxy-3,3′,5,5′-tetra-tert-butylbi-phenyl as a major product.29 A linear plot of the relative rates as afunction of O−H bond dissociation energies (BDEs) of p-X-DTBPs with a slope of−0.65 and the steric effect observed in theHAT reaction of 2,4-di-tert-butylphenol vs 2,6-di-tert-butylphe-nol indicate that HAT from p-X-DTBPs to [FeIII(TAML)-(O2)]

2− also proceeds via a concerted PCET, because the ETpathway would exhibit no steric effect.29 HAT reactions of p-substituted phenol derivatives with copper(III)−hydroxidecomplexes (LCuOH and NO2LCuOH, where L = N,N′-bis(2,6-diisopropylphenyl)-2,6-pyridine-dicarboxamide and NO2L =N,N′-bis(2,6-diisopropyl-4-nitrophenyl)pyridine-2,6-dicarbox-amide) were also reported to proceed via a concerted PCETpathway.30

In concerted PCET pathways described above, hydrogendonors (DH2) were the source of the proton in the reactions.However, protons can be provided by addition of an acid such astriflic 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 Eoxvalue 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, PCETfrom [FeII(bpy)3]

2+ to [CrIII(TMC)(O2)(Cl)]+ occurs to

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 theoxidation of a series of NADH analogs by p-chloranil in MeCN at 298K; 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 withpermission from ref 24. Copyright 2017 WILEY-VCH Verlag GmbH.

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produce [FeIII(bpy)3]3+ and [CrIII(TMC)(H2O2)(Cl)]

2+, whichis in equilibrium (eq 1),

Fe(bpy) Cr (TMC)(O )(Cl) 2H

Fe(bpy) Cr (TMC)(H O )(Cl)K

32 III

2

33 III

2 22et

X Yoo

[ ] + [ ] +

[ ] + [ ]

+ + +

+ +(1)

as indicated by the ET titrations shown in Figure 2, where theconcentration of remaining [FeII(bpy)3]

2+ decreased with an

increase in [HOTf].31 The ET equilibrium constants (Ket) in eq1 were determined by global fitting of plots in Figure 2. The Eredvalues 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

E E RT F K(2.3 / )logred ox et= + (2)

The dependence of Ered of [CrIII(TMC)(O2)(Cl)]

+ on [H+] isgiven by the Nernst equation (eq 3),

E E RT F K(2.3 / )log( H )red red0 2= + [ ]+

(3)

where K is the equilibrium constant of the diprotonation of theone-electron reduced species of [CrIII(TMC)(O2)(Cl)]

+.31 Alinear correlation of Ered vs log[HOTf] with a slope of 93 mV/log[HOTf] (inset of Figure 2) agrees with eq 3, since theexpected 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 positivedirection from −0.52 V vs SCE without HOTf to 1.12 V vs SCEwith 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 dependencewith respect to [HOTf] (eq 4).31 Such a second-order

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

value would be independent of [HOTF]. It was confirmed thatno 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)−super-oxide and Cu(II)−superoxide complexes, may also undergoPCET reactions with acids, which have yet to be reported.

3. PCET OF METAL−PEROXIDO ANDMETAL−HYDROPEROXIDO COMPLEXES

A mononuclear nonheme side-on (η2) iron(III)−peroxidocomplex, [FeIII(TMC)(O2)]

+ (2 in Chart 1), undergoes noHAT reaction with alkylaromatic compounds even with weakC−H bonds such as xanthene (75.5 kcal mol−1) and DHA (77kcal mol−1).32 The cyclic voltammograms of [FeIII(TMC)-(O2)]

+ exhibit a cathodic current peak (Epc) due to the one-electron reduction at −0.43 V vs SCE with no correspondinganodic current peak (Epa) at the reverse scan, indicating that theone-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 Epcvalues shifted in a positive direction with increasing the Lewisacidity of the redox-inactive metal ions.33 For example, in thecase of Sc3+ ion, the Epc value shifted to 0.38 V vs SCE, and oneadditional cathodic current peak was observed at ∼0.06 V vsSCE, which was assigned as a cathodic current peak of[FeIV(TMC)(O)]2+ that was produced via the heterolytic O−Obond cleavage of FeII(TMC)−(μ,η2:η2-O2)−Sc3+.33 Thus, theoxidizing ability of the redox-inactive metal ion-bound iron-(III)−peroxido complexes increases with increasing the Lewisacidity 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 producean 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+) complexescan 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)−peroxidocomplexes bearing TMC derivatives are not active in H atomabstraction reactions; however, their M(III)−hydroperoxidocomplexes, generated upon protonation of theM(III)−peroxidocomplexes, showed reactivity in H atom abstraction reac-tions.36−38 The HAT reactions of M(III)−hydroperoxidocomplexes with xanthene proceed via concerted PCETpathways where KIEs values of 3−5 were observed.38

4. PCET OF MONONUCLEAR NONHEMEIRON(IV)−OXIDO COMPLEXES

Since the first crystal structure of a mononuclear nonhemeiron(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 inves-tigated in various oxidation reactions.8,9,40 One notable exampleis the C−H bond activation of alkanes by the syntheticiron(IV)−oxido complexes; some of them are capable of

Figure 2. Spectroscopic redox titrations at 520 nm for thedisappearance of [FeII(bpy)3]

2+ as a function of the initialconcentration of [CrIII(TMC)(O2)(Cl)]

+ added to an MeCN solutionof [FeII(bpy)3]

2+ and HOTf (blue circles, 1.5 mM; black circles, 2.5mM; red circles, 3.0 mM) at 233 K. Inset shows the dependence of Eredof [CrIII(TMC)(O2)(Cl)]

+ on log([HOTf]). Reprinted with permis-sion from ref 31. Copyright 2018 American Chemical Society.

k k HOTfet 22= [ ] (4)

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activating the C−Hbonds of cyclohexane (BDEC−H∼99.3 kcal/mol).41−43 The reactivity of the iron(IV)−oxido complexes wasalso shown to be influenced significantly by supporting and axialligands, spin states of the iron(IV) ion, and proton and redox-inactive 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 ofa 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 positivedirection 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 ortwo Sc3+ ions to [FeIII(N4Py)(O)]+.44−46 The second-order rateconstants (ket) of PCET from [RuII(bpy)3]

2+ to [FeIV(N4Py)-(O)]2+ increased with increasing [HOTf], exhibiting a second-order dependence with respect to [HOTf].46 When HOTf isreplaced by the deuterated acid (DOTf), the ket value of PCETfrom [RuII(bpy)3]

2+ to [FeIV(N4Py)(O)]2+ with DOTf inMeCN at 298 K is larger than that with the same concentrationof HOTf.46

The rate constant of the oxidation of toluene by [FeIV(N4Py)-(O)]2+ with HOTf also increased with increasing [HOTf], andit 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-d8decreased with increasing [HOTf] from 31 without HOTf toreach KIE = 1.0 at [HOTf] > 50 mM.46 Such a drastic change inKIE from 31 to 1.0 indicates that the rate-determining step in theoxidation of toluene by [FeIV(N4Py)(O)]2+ is changed fromconcerted PCET (one-stepHAT) in the absence of HOTf to ETin the presence of HOTf.46

A general mechanism for PCET from substrates (S = tolueneand thioanisole derivatives) to [FeIV(N4Py)(O)]2+ with HOTf,as well as MCET (metal ion-coupled electron transfer)7 from Sto [FeIV(N4Py)(O)]2+ with Sc(OTf)3 is shown in Scheme 4.46

Oxidation of S by [FeIV(N4Py)(O)]2+ with HOTf or Sc(OTf)3is initiated by PCET or MCET from S to [FeIV(N4Py)(O)]2+

binding two molecules of HOTf or Sc(OTf)3, following theformation of precursor complexes (Scheme 4).46 The rate-determining PCET or MCET may be followed by rapid transferof 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 PCETfrom various electron donors to [FeIV(N4Py)(O)2+−(HOTf)2]) in the precursor complexes in Scheme 4 can be

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

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

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

5. PCET OF MONONUCLEAR NONHEMEMANGANESE(IV)−OXIDO COMPLEXES

HAT reactions from substrates to metal−oxygen intermediatesgenerally proceed via concerted PCET rather than stepwise ET/PT or PT/ET, since the concerted pathway is thermodynami-cally more favorable as compared with the stepwise pathway(vide supra). However, when ET is thermodynamically feasible,ET from substrates to metal−oxygen intermediates may occurfirst, followed by proton transfer (ET/PT pathway). Such anET/PT pathway was reported for hydride transfer from AcrH2

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

k k T h G k T( / )exp ( /4)(1 / )/( )ET B et Bλ λ= [− + Δ ] (5)

Figure 3.Driving force (−ΔGet) dependence of log kET for oxidation ofstyrene and toluene derivatives [(1) trans-stilbene, (2) 1,1-diphenylethene, (3) α-methylstyrene, (4) 3-methylstyrene, (5) styrene,(6) p-Me-thioanisole, (7) thioanisole, (8) p-Cl-thioanisole, (9) p-Br-thioanisole, (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-trime-thylbenzene, and (17) toluene] by [FeIV(N4Py)(O)]2+ and log kET forPCET from various electron donors {coordinatively saturated metalcomplexes; (18) [FeII(Ph2Phen)3]

2+, (19) [FeII(bpy)3]2+, (20)

[RuII(4,4′-Me2Phen)3]2+, (21) [RuII(5,5′-Me2Phen)3]

2+, (22)[FeII(ClPhen)3]

2+, and (23) [RuII(bpy)3]2+} to [FeIV(N4Py)(O)]2+

in the presence of HOTf (10mM) inMeCN at 298 K. The black circlesshow the driving force dependence of log kET of electron transfer fromelectron donors [(24) decamethylferrocene, (25) octamethylferrocene,(26) 1,1′-dimethylferrocene, (27) n-amylferrocene, and (28) ferro-cene] to [FeIV(N4Py)(O)]2+ in the absence of HOTf in MeCN at 298K. Reprinted with permission from ref 48. Copyright 2015 AmericanChemical Society.

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to [MnIV(Bn-TPEN)(O)]2+ (Bn-TPEN = N-benzyl-N,N′,N′-tris(2-pyridylmethyl)ethane-1,2-diamine).49,50 When AcrH2was replaced by AcrD2, the KIE was determined to be 1.0 ±0.1, which indicates that ET from AcrH2 to [MnIV(Bn-TPEN)(O)]2+ is the rate-determining step, followed by fastPT and ET from AcrH2

•+ to [MnIII(Bn-TPEN)(O)]2+.50 This isin sharp contrast to the hydride transfer from AcrH2 to[FeIV(Bn-TPEN)(O)]2+, in which a large KIE value of 18 wasobserved25 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+ proceedsvia a concerted PCET pathway. In contrast, ET from AcrH2 to[MnIV(Bn-TPEN)(O)]2+ (Ered = 0.78 V vs SCE)52 is onlyslightly endergonic, when ET is the rate-determining step,followed by rapid PT and ET, where R = H (Scheme 5).

Hydroxylation of mesitylene and mesitylene-d12 by amononuclear manganese(IV)−oxido complex ([MnIV(N4Py)-(O)]2+) and a triflic acid-bound manganese(IV)−oxidocomplex, [MnIV(N4Py)(O)]2+−(HOTf)2,

53 proceeds inCF3CH2OH/CH3CN (v/v = 1:1) at various temperatures (eq6).54 The Eyring plots of the second-order rate constants of the

hydroxylationof mesitylene and mesitylene-d12 by[MnIV(N4Py)(O)]2+ and [MnIV(N4Py)(O)]2+−(HOTf)2 areshown in Figure 4.54 At 293 K, the rate constant of thehydroxylation of mesitylene by [MnIV(N4Py)(O)]2+−HOTf)2is the same as that of mesitylene-d12 to afford a KIE of 1.0,indicating that the HAT reaction proceeds via ET frommesitylene to [MnIV(N4Py)(O)]2+−(HOTf)2 at 293 K.54

Interestingly, when the reaction temperature is lowered to lessthan 263 K in the reactions of [MnIV(N4Py)(O)]2+−(HOTf)2with mesitylene and mesitylene-d12, however, the mechanismchanges from ET to HAT with a KIE of 2.9.54 Such a switchoverof the reaction mechanism from ET to HAT is shown to occurby changing only temperature in the boundary region betweenET and HAT pathways when the driving force of ET fromtoluene derivatives to [MnIV(N4Py)(O)]2+−(HOTf)2 is around−0.5 eV. In the case of [MnIV(N4Py)(O)]2+, the KIE remains tobe 3.2 in the temperature range of 243−293 K.54 TheΔH⧧ valueof [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

[MnIV(N4Py)(O)]2+.54 The ΔS⧧ value of ET is close to zero ornegative, being much less negative than that of HAT; thetransition state of which is more ordered to afford the largelynegative ΔS⧧ value. In such a case, ET is energetically morefavored at higher temperatures. This is the reason why themechanism changes from ET at 293 K with a KIE of 1.0 to HATat temperatures lower than 263 K with a KIE of 2.9. Theobservation of the different mechanisms by changing onlytemperature indicates that the two mechanisms are distinguish-able and competing without the continuous change of themechanism, but switchable by changing only reaction temper-ature.54

More recently, high-valent cobalt(IV)−oxido complexesbearing TAML and 13-TMC (13-TMC = 1,4,7,10-tetrameth-yl-1,4,7,10-tetraazacyclotridecane) ligands were synthesized andcharacterized using various spectroscopic techniques.55,56

Interestingly, the Co(IV)−oxido complexes showed reactivitiesin oxidation reactions, such as C−H bond activation,sulfoxidation, olefin epoxidation, and intermetal OAT reac-tions.55,56 The Co(IV)−oxido complexes are competentoxidants in HAT reactions, providing us with a goodopportunity to investigate detailed mechanisms for the HATreactions by high-valent Co−oxido complexes.

6. SUMMARYAs represented by Schemes 1 and 2 with Chart 1, themechanistic details on reactions of mononuclear nonhememetal−oxygen intermediates, such as metal−superoxido,−hydroperoxido, and −oxido complexes, with respect to thekinetics and thermodynamics of electron transfer or protonation(or Lewis-acid activation) are of great interest and ofconsiderable fundamental importance in the activation of O2by metal ions in biology or in stoichiometric or catalyticreactions involving the oxygenation or oxidative dehydrogen-ation of organic substrates. We have shown that HAT fromAcrH2 to a Cr(III)−superoxido complex and Fe(IV)−oxidocomplexes proceeds via a concerted PCET pathway when theET pathway is endergonic (ΔGet > 0). In contrast, HAT fromAcrH2 to a Mn(IV)−oxido complex proceeds via a rate-determining ET pathway, followed by fast PT when the ETpathway is thermodynamically feasible (ΔGet≈ 0).We have alsoshown that HAT from toluene derivatives to an Fe(IV)−oxidocomplex, [FeIV(N4Py)(O)]2+, is accelerated by addition ofacids; the HAT occurs via PCET from toluene derivatives to a

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

Figure 4. Eyring plots of ln(kobs/T) against 1/T obtained in thereactions of [MnIV(N4Py)(O)]2+ with mesitylene (blue circles) andmesitylene-d12 (green circles) and [MnIV(N4Py)(O)]2+−(HOTf)2with mesitylene (red circles) and mesitylene-d12 (black circles) atvarious temperatures (243−293K). Reprinted with permission from ref54. Copyright 2016 WILEY-VCH Verlag GmbH.

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HOTf-bound Fe(IV)−oxido complex, [FeIV(N4Py)(O)]2+−(HOTf)2. The driving force dependence of the logarithm of thefirst-order rate constant of acid-catalyzed oxidation of varioussubstrates including HAT in the precursor complexes (i.e., logkET) is remarkably unified with that of kET of PCET from variousone-electron donors to [FeIV(N4Py)(O)]2+ in the absence andpresence of HOTf. Another interesting observation is theswitchover of the reaction mechanism from a concerted PCETpathway (one-step HAT) to an ET pathway in the oxidation ofmesitylene by an acid-bound manganese(IV)−oxido complex,[MnIV(N4Py)(O)]2+−(HOTf)2, by changing only temperaturein the boundary region between one-step HAT and ETpathways. Another interesting and important aspect that shouldbe addressed in the near future is the oxygen rebound vs oxygennonrebound mechanisms in the HAT reactions by heme andnonheme metal−oxygen intermediates;57 we hope to under-stand factors (and the role of factors) that determine the oxygenrebound vs oxygen nonrebound pathways in the HAT reactions.

■ AUTHOR INFORMATIONCorresponding Authors

*E-mail: wwnam@ewha.ac.kr (W.N.).*E-mail: fukuzumi@chem.eng.osaka-u.ac.jp (S.F.).ORCID

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

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

The authors declare no competing financial interest.

Biographies

Wonwoo Nam received his Ph.D. degree in Inorganic Chemistry fromUCLA in 1990. He is currently a Distinguished Professor of EwhaWomans University in Seoul, Korea.

Yong-Min Lee received his Ph.D. degree in Inorganic Chemistry fromPusan National University in Korea in 1999. He is a SpecialAppointment Professor at Ewha Womans University since 2009.

Shunichi Fukuzumi earned Ph.D. degree in applied chemistry atTokyo Institute of Technology in 1978. He has been a Full Professor ofOsaka University from 1994 to 2015. He is now a DistinguishedProfessor at Ewha Womans University.

■ ACKNOWLEDGMENTSThe authors gratefully acknowledge the contributions of theircollaborators and co-workers mentioned in the cited referencesand financial support by the NRF of Korea through CRI (NRF-2012R1A3A2048842 to W.N.), GRL (NRF-2010-00353 toW .N . ) , a nd Ba s i c S c i e n c e Re s e a r c h P r o g r am( 2 0 1 7 R 1 D 1 A 1 B 0 3 0 2 9 9 8 2 t o Y . M . L . a n d2017R1D1A1B03032615 to S.F.) and by a SENTAN projectfrom JST and JSPS KAKENHI (Grant Numbers 16H02268 toS.F.) from MEXT, Japan.

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