structure and reactivity of the first-row d-block metal...

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Cite this: Dalton Trans., 2019, 48,9469

Received 2nd April 2019,Accepted 1st May 2019

DOI: 10.1039/c9dt01402k

rsc.li/dalton

Structure and reactivity of the first-row d-blockmetal-superoxo complexes

Shunichi Fukuzumi, *a,b Yong-Min Lee *a,c and Wonwoo Nam *a,d

In the first-row of d-block metals, ten elements are included, such as scandium (Sc, 3d1), titanium (Ti,

3d2), vanadium (V, 3d3), chromium (Cr, 3d54s1), manganese (Mn, 3d5), iron (Fe, 3d6), cobalt (Co, 3d7),

nickel (Ni, 3d8), copper (Cu, 3d104s1) and zinc (Zn, 3d10). The synthesis, characterization, and reactivity of

first-row d-block metal-superoxo complexes are discussed together with the structures of the end-on

(η1) and side-on (η2) metal-superoxo complexes in this review article. Electron transfer from electrondonors to O2 is enhanced by binding of Sc

3+ to produce an end-on type Sc(III)-superoxo complex. Metal-

superoxo complexes such as Ti(IV)-superoxo, oxovanadium(V)-superoxo, Cr(III)-superoxo, Fe(III)-superoxo,

Co(III)-superoxo, Ni(III)-superoxo and Cu(II)-superoxo species generally undergo hydrogen atom transfer

reactions. A Cr(III)-superoxo complex undergoes not only hydrogen atom transfer but also oxygen atom

transfer reactions. In the presence of protons (e.g., trifluoromethanesulfonic acid, HOTf), much enhanced

acid catalysis was observed in oxygen atom transfer reactions from a nonheme Cr(III)-superoxo complex,

[(Cl)(TMC)CrIII(O2)]+, to thioanisole. The enhanced reactivity of [(Cl)(TMC)CrIII(O2)]

+ by HOTf results from

proton-coupled electron transfer (PCET) from electron donors, including thioanisole, to [(Cl)(TMC)

CrIII(O2)]+. A manganese(IV)-superoxo complex plays a very important role in thermal and photoinduced

dioxygen activation by a Mn(III) corrolazine complex. A metal-superoxide complex using the last element

in the first-row of transition metals, that is a Zn(II)-superoxide complex, is produced to accelerate the

reduction of O2•− in a SOD (superoxide dismutase) model.

1. Introduction

The first step in the dioxygen activation cycle of metalloen-zymes is the binding of dioxygen to metal centre by electrontransfer from the metal centre to O2, forming metal-superoxocomplexes, in which the nature of metal ions as well asligands modulates the redox reactivity of the superoxideion.1–15 Thus, metal-superoxo complexes play crucial roles askey intermediates in various biological redox reactions ofheme enzymes [e.g., cytochrome c oxidases (COX) and cyto-chrome P450], nonheme iron enzymes [e.g., isopenicillin Nsynthase (IPNS), myo-inositol oxygenase (MIOX), and cysteinedioxygenase (CDO)] and copper enzymes [e.g., dopamineβ-monooxygenase (DβM) and peptidylglycine-α-amidating

monooxygenase (PHM)].1–15 The first-row d-block metals, suchas Mn, Fe, Co, Ni, Cu and Zn, are used in various metalloen-zymes. Among these transition metals, an X-ray diffractionstructure of a cobalt(III)-superoxo complex supported by bzacenand pyridine ligands (bzacen = N,N′-ethylene-bis(benzoylaceto-niminide) was reported for the first time in 1972.16 Theobserved O–O distance of 126(4) pm is comparable to thesuperoxide ion value (128 pm) and is therefore in accord withthe Co(III)–O2

•− formulation as proposed from EPR studies.17

The Co–O–O bond angle of 126(2)° observed for the end-ontype coordination of O2

•− agrees with that predicted by Paulingin 1948,18,19 and differs from the alternative sideways coordi-nation postulated by Griffith.20 Since then, there have so farbeen many papers and reviews on the structure and reactivity ofthe first-row d-block metal-superoxo complexes, mainly focusingon Fe, Co and Cu.21–34 However, no research studies on thestructure and reactivity of metal-superoxo complexes coveringall the ten first-row metals from Sc to Zn have yet to be reported.

This review focuses on the structure and reactivity of metal-superoxo complexes covering all the first-row d-block metalsfrom Sc to Zn. Firstly, the structure and reactivity of Sc3+–O2

•−

complexes are discussed, including how Sc3+–O2•− complexes

are produced and characterized. Then, the structure and redoxreactivity of the other d-block metal-superoxo complexes are

aDepartment of Chemistry and Nano Science, Ewha Womans University, Seoul

03760, Korea. E-mail: [email protected], [email protected],

[email protected] School of Science and Technology, Meijo University, Nagoya, Aichi 468-

8502, JapancResearch Institute for Basic Sciences, Ewha Womans University, Seoul 03760, KoreadState Key Laboratory for Oxo Synthesis and Selective Oxidation, Suzhou Research

Institute of LICP, Lanzhou Institute of Chemical Physics (LICP), Chinese Academy of

Sciences, Lanzhou 730000, China

This journal is © The Royal Society of Chemistry 2019 Dalton Trans., 2019, 48, 9469–9489 | 9469

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discussed in the order of the periodic table, such as Sc, Ti, V,Cr, Mn, Fe, Co, Ni, Cu and Zn.

2. Scandium-superoxo complexes

Superoxide ions (O2•−) can be produced by photoinduced elec-

tron transfer (ET) from dimeric 1-benzyl-1,4-dihydronicotin-amide [(BNA)2] to O2 as shown in Scheme 1,

35 where the photo-induced ET is followed by a facile cleavage of the C–C bondof the BNA dimer radical cation to produce theN-benzylnicotinamide radical (BNA•) and BNA+.36,37 Then, ETfrom BNA• to O2 occurs rapidly to produce BNA

+ and O2•−

(Scheme 1), because the one-electron oxidation potential ofBNA• (Eox vs. SCE = −1.08 V)38 is more negative than the one-electron reduction potential of O2 (Ered vs. SCE = −0.87 V)39 inacetonitrile (MeCN).35 Thus, two equivalents of O2

•− are pro-duced by photoinduced ET from (BNA)2. In the presence ofscandium triflate (Sc(OTf)3), Sc

3+ is bound to O2•− to produce

the Sc3+–O2•− complex, which becomes more stable in the pres-

ence of three equivalents of hexamethylphosphoric triamide(HMPA) ligand.35 Thus, the EPR spectrum of the (HMPA)3Sc

3+–

O2•− complex was observed even at 60 °C in propionitrile, exhi-

biting the clear eight lines due to the superhyperfine couplingof O2

•− with the Sc nucleus (I = 7/2, aSc = 3.82 G).35

When dioxygen is enriched in 17O2, two sets of six lines dueto the hyperfine splitting of two inequivalent 17O atoms (I =5/2) are observed as shown in Fig. 1, although the centrelines are overlapped by the strong eight-line signal of(HMPA)3Sc

3+–16O2•−.35 The two inequivalent a(17O) values are

determined to be 21 and 14 G by comparison of the observedsignal (Fig. 1a) with the computer simulation lines (Fig. 1b).35

Such inequivalent a(17O) values clearly indicate an “end-on”coordination of the O2

•− ligand to the Sc3+ centre in the(HMPA)3Sc

3+–O–O•− complex, where the unpaired electron ismore localized at the terminal oxygen.35

A Sc3+-superoxo (ScO22+) complex is reported to act as an

intermediate for the catalytic two-electron reduction of O2 bydecamethylferrocene (Fc*) with mononuclear copper com-plexes, [(tmpa)CuII(CH3CN)](ClO4)2 (tmpa = tris(2-pyridyl-methyl)amine) and [(BzQ)CuII(H2O)2](ClO4)2 (BzQ = bis(2-quinolinylmethyl)benzylamine), in the presence of Sc(OTf)3 inacetone.40 It was confirmed that the one-electron oxidation ofScO2

+ afforded Sc3+–O2•− that was characterized by EPR

(vide supra).40 The catalytic cycle is shown in Scheme 2, where ETfrom Fc* to [LCuII]2+ produces the Cu(II)-superoxo ([LCuIIO2]

+)complex, which is replaced by the Sc3+-superoxo (ScO2

2+)complex, followed by ET from Fc* to ScO2

2+ to produce theSc3+-peroxide (ScO2

+) complex and regenerate [LCuII]2+. The re-placement of [LCuII]2+ in [LCuIIO2]

+ by Sc3+ to yield ScO22+

results from the stronger Lewis acidity of Sc3+ that binds withO2

•− much stronger than [LCuIIO2]+. The stronger the Lewis

acidity of metal ions, the stronger the binding of metal ions toO2

•−.41–43

(HMPA)3Sc3+–O–O•− acts as a one-electron reductant to

reduce p-benzoquinone derivatives (X-Q) to produce(HMPA)3Sc

3+-semiquinone radical anion (X-Q•−) complexes.44

The number of (HMPA)3Sc3+ ion binding to semiquinone

radical anions is changed depending on the type of X-Q.44

In the case of p-benzoquinone derivatives with electron-with-drawing substituents (Cl2Q, Cl4Q and F4Q), ET from(HMPA)3Sc

3+–O2•− to X-Q occurs, followed by rapid binding

of (HMPA)3Sc3+ to X-Q•− (Scheme 3), when the ET rate con-

stant exhibited no dependence on the concentration of

Shunichi Fukuzumi

Shunichi Fukuzumi received B.S.,M.S. and Ph.D. degrees inChemical Engineering andApplied Chemistry at the TokyoInstitute of Technology in 1973,1975 and 1978, respectively. Heworked as a postdoctoralresearcher at Indiana Universityin the USA from 1978 to 1981. In1981, he became an AssistantProfessor at Osaka Universitywhere he was promoted to a FullProfessor in 1994. He wasselected as a Distinguished

Professor in 2013 and retired from Osaka University in 2015. Hehas studied electron transfer chemistry, particularly bioinspiredartificial photosynthesis, metal-ion-coupled electron transfer(MCET) and redox catalysis. He is now a Distinguished Professorof Ewha Womans University in the Republic of Korea, a ProfessorEmeritus of Osaka University and a Designated Professor of MeijoUniversity in Japan.

Yong-Min Lee

Yong-Min Lee received Bachelor,Master and Ph.D. degrees inChemistry at Pusan NationalUniversity, Republic of Korea in1990, 1995 and 1999, respect-ively, under the supervision ofProfessor Sung-Nak Choi. Then,he joined the Centro di Ricercadi Risonanze Magnetiche(CERM) at Università degli Studidi Firenze (Italy) as a postdoc-toral researcher under the super-vision of Professors Ivano Bertiniand Claudio Luchinat from 1999

to 2005. Then, he moved to the Centre for Biomimetic Systems atEwha Womans University, as a Research Professor (2006–2009).He is now a Special Appointment Professor at Ewha WomansUniversity from 2009.

Perspective Dalton Transactions

9470 | Dalton Trans., 2019, 48, 9469–9489 This journal is © The Royal Society of Chemistry 2019

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. View Article Online

https://doi.org/10.1039/c9dt01402k

(HMPA)3Sc3+.44 In the case of p-benzoquinone derivatives

with electron donating substituents (Me2Q and MeQ), ETfrom (HMPA)3Sc

3+–O2•− to X-Q is coupled with the binding

of two (HMPA)3Sc3+ molecules to X-Q•− (Scheme 4), when

Scheme 1 Photoinduced generation of O2•− via photoinduced ET from

(BNA)2 to O2 and binding of Sc3+ to O2

•−.35

Fig. 1 (a) EPR spectrum detected by photoirradiation of an 17O (40%)oxygen-saturated propionitrile solution containing (BNA)2 (6.9 × 10

−3 M),Sc(OTf)3 (8.1 × 10

−2 M) and HMPA (2.5 × 10−1 M) using a high pressuremercury lamp at 298 K. (b) Computer simulation spectrum using theparameters, g = 2.0165, a(Sc) = 3.82 G, a(17O1) = 21 G, a(17O2) = 14 G andΔHmsl = 3.5 G. Reprinted with permission from ref. 35. Copyright 1999,American Chemical Society.

Wonwoo Nam

Wonwoo Nam received his B.S.(honours) degree in Chemistryfrom California State University,Los Angeles, and his Ph.D.degree in Inorganic Chemistryfrom the University of California,Los Angeles (UCLA) in the USAunder the supervision ofProfessor Joan S. Valentine in1990. After working as a post-doctoral researcher at the UCLAfor one year, he became anAssistant Professor at HongikUniversity in 1991. In 1994, he

moved to Ewha Womans University, where he is currently aDistinguished Professor. His research interests are on O2 acti-vation, water oxidation, metal–oxygen intermediates, such asmetal-oxo, metal-superoxo, metal-peroxo and metal-hydroperoxospecies, and important roles of metal ions in bioinorganicchemistry.

Scheme 2 Catalytic cycle of the 2e− reduction of O2 by Fc* with Sc3+

and [LCuII]2+ complexes.

Scheme 3 ET from (HMPA)3Sc3+–O–O•− to Cl4Q, followed by binding

of Sc(HMPA)33+ to Cl4Q

•−.

Scheme 4 ET from (HMPA)3Sc3+–O–O•− to Q, coupled by binding of

two (HMPA)3Sc3+ molecules to Q•−.

Dalton Transactions Perspective


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the ET rate constant exhibited second-order dependence onthe concentration of (HMPA)3Sc

3+ in addition to first-orderdependence as observed for ET from (HMPA)3Sc

3+–O2•−

to Q.44

3. Titanium(IV)-superoxo complexes

Two titanium ozonide complexes, OTiIV(η2-O2)(η2-O3) andOTiIV(η2-O2)(η1-O3), were produced via the reactions of Tiatoms with O2 in solid Ar.

45 Their geometric structures andchemical bondings were clarified via matrix isolation infrared(IR) absorption spectroscopy and density functional theory(DFT) calculations (Fig. 2).45 The formation of an end-on-bonded OTiIV(η2-O2)(η1-O3) complex is accompanied by thedecay of a side-on-bonded OTiIV(η2-O2)(η2-O3) complex undervisible light (λ = 532 nm) irradiation and this is reversed uponannealing.45

The light yellow Ti(IV)-superoxo (TiIV–O2•−) complex was

prepared by the reaction of H2O2 (50%) on Ti(OR)4 in anhy-drous methanol at 298 K [eqn (1)]. The TiIV–O2

•− complexwas characterised by EPR, FTIR, Raman, X-ray diffraction(XRD), thermogravimetric/differential thermal analysis (TG/DTA), and elemental analysis.46 The TiIV–O2

•− catalystefficiently catalysed the oxidation of primary amines by H2O2to the corresponding nitro derivatives in high yields.46 Thecatalytic cycle is started by hydrogen atom transfer from theamine to the TiIV(O2

•−) species to generate the transientRNH• radical, which is further oxidized by H2O2 to yield thenitro derivatives.46 The TiIV(O2

•−) species also catalysed theoxidative esterification of aldehydes with alkylarenes or alco-hols to afford the corresponding benzyl and alkyl esters inexcellent yields.47 The TiIV–O2

•− species were detected by

EPR in the treatment of titanium silicate-1 with H2O2 in thegas phase.48

ð1Þ

4. Vanadium(V)-superoxo complexes

Vanadium(V)-superoxo species are believed to be involved inmany vanadium-catalysed oxidation reactions.49–51 An oxo-peroxovanadium(V) complex, [V(L-N4Me2)(O)(O2)]

+ (1: seeScheme 5 for the molecular structure of L-N4Me2), wasobtained from the reaction of the dioxovanadium(V) complex,[V(L-N4Me2)(O)2]

+ (2), with H2O2 (30% aqueous) in MeCN(Scheme 5).52 The side-on coordination of the peroxo group tothe V(V) centre in 1 was characterized by X-ray crystal structureand 1H NMR and IR spectroscopy.52 The electrochemical oxi-dation of [V(L-N4Me2)(O)(O2)]

+ was performed by applying apotential of 1.63 V vs. SCE at −30 °C in the cavity of an EPRspectrometer. The solution EPR spectrum (Fig. 3a) is com-posed of a signal at an isotropic g value (giso = 2.0119), whichis split into eight lines with a hyperfine coupling constant of2.50 × 10−4 cm−1 due to the vanadium nucleus (I = 7/2).52 Thisresult indicates that one electron is obtained from a molecularorbital on the peroxo ligand of [V(L-N4Me2)(O)(O2)]

+ (1) duringelectrochemical oxidation to produce the oxovanadium(V)-superoxo species, [V(L-N4Me2)(O)(O2

•−)]2+ (3), as shown inScheme 5.52

When the temperature of an MeCN solution of 3 was raisedto higher than −30 °C, an ESR spectrum corresponding to the

Fig. 2 Optimized structures of the OTi(η2-O2)(η2-O3) and OTi(η2-O2)(η1-O3) complexes (bond distances in Å and bond angles in degrees) onthe left, the electron density ρ contour maps in the middle, and thedensity gradient maps ∇2ρ on the right (green lines denote regions ofelectronic charge connection, and black lines denote regions of elec-tronic charge depletion). Reprinted with permission from ref. 45.Copyright 2007, American Chemical Society.

Scheme 5 Formation of a vanadium(V)-peroxo complex ([VV(L-N4Me2)(O)(O2)]

+) and electrochemical reduction to produce the [VV(L-N4Me2(O)(O2

•−)]2+ species. Reprinted with permission from ref. 52. Copyright2001, WILEY-VCH Verlag GmbH.



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vanadyl(IV) complex, [VIV(L-N4Me2)(MeCN)(O)](ClO4)2 (4-(ClO4)2), exhibited eight lines centred at giso = 1.977 with ahyperfine coupling constant of 92.8 × 10−4 cm−1 (Fig. 3b).52 4was produced by the release of O2 via intramolecular ET fromthe superoxo moiety to the V(V) centre of 3.52 4 was preparedindependently from the vanadyl(IV) complex, [VIV(L-N4Me2)(ClO)](ClO4), by treatment with AgClO4 (Scheme 5).

52 A quanti-tative conversion of 4 to 1 was observed with O2 using an anhy-drous MeCN–THF mixture (v/v = 1 : 1) as a solvent(Scheme 5).52 Since the one-electron reduction potential of 3 isrelatively high (1.63 V vs. SCE), the high oxidising ability of 3 isexpected.52

The addition of non-redox metal ions such as Al3+ actingas a Lewis acid can facilitate dioxygen activation by anoxovanadium(IV) complex, [VIV(O)(Cl)(TPA)]PF6 (TPA = tris-[(2-pyridyl)methyl]-amine), leading to efficient hydrogen atomtransfer from cyclohexadiene at ambient temperature. In theabsence of a Lewis acid, hydrogen transfer from cyclohexa-diene to the oxovanadium(IV) complex occurred much moreslowly.53 The acceleration effect of Al3+ results from the dis-sociation of Cl− from [VIV(O)(Cl)(TPA)]PF6 to generate avacant site for O2 binding to produce the vanadium(V)-super-oxo species, which may be stabilized by binding Al3+

(Scheme 6).53 The generated oxovanadium(V)-superoxospecies undergo hydrogen atom transfer from cyclohexadieneto produce a cyclohexadienyl radical and oxovanadium(V)-hydroperoxide, which reacts with H2O to produce H2O2 andoxovanadium(V)-hydroxo species (Scheme 6).53 Hydrogenatom transfer from the cyclohexadienyl radical to theoxovanadium(V)-hydroxo species affords the formation ofbenzene and the regeneration of the oxovanadium(IV)complex (Scheme 6).53

5. Chromium(III)-superoxocomplexes

The reaction of Cr2+ with O2 yields a chromium(III)-superoxoion, Cr(O2

•−)2+, in an aqueous solution.54 Unlike most of theother transition metal–oxygen adducts, Cr(O2

•−)2+ can behandled at room temperature even under air-freeconditions.54a,55 The X-ray crystal structure of a Cr(III)-superoxocomplex, [CrIII(O2

•−)(TptBu,Me)(pz′H)]BARF (TptBu,Me = hydrotris(3-tert-butyl-5-methylpyrazolyl)borate, pz′H = 3-tert-butyl-5-methylpyrazole and BARF = tetrakis(3,5-bis(trifluoromethyl)phenyl)borate), which was generated by the reaction of[CrII(TptBu,Me)(pz′H)]BARF with O2, is shown in Fig. 4, wherethe two Cr–O distances 1.861(4) and 1.903(4) Å are essentiallyidentical in the side-on coordination of the O2

•− ligand.56 TheO–O bond length of 1.327(5) Å is consistent with the superoxocategory.56 The solid-state IR spectrum of [CrIII(O2

•−)(TptBu,Me)(pz′H)]BARF exhibited an O–O stretching vibration at1072 cm−1, which was shifted to 1007 cm−1 when 16O2 wasreplaced by 18O2.

56 Antiferromagnetic coupling between theCrIII ion (d3, S = 3/2) and the coordinated O2

•− (S = 1/2) gavethe effective magnetic moment of the CrIII-superoxo complex(μeff (295 K) = 2.8(1)μB).

56

Fig. 3 (a) EPR spectrum of [VV(L-N4Me2(O)(O2•−)]2+ generated in elec-

trolysis of [VV(L-N4Me2)(O)(O2)]+ in MeCN containing 0.10 M Et4NClO4

with an applied potential of 1.63 V vs. SCE at −30 °C in the EPR cavityand (b) after raising temperature to room temperature. Reprinted withpermission from ref. 52. Copyright 2001, WILEY-VCH Verlag GmbH.

Scheme 6 Catalytic mechanism for desaturation of cyclohexadiene tobenzene by O2 with [V

IV(O)(Cl)(TPA)]PF6. Reprinted with permission fromref. 53. Copyright 2017, American Chemical Society.

Fig. 4 ChemDraw and X-ray structures of a chromium(III)-superoxocomplex, [TptBu,MeCrIII(pz’H)(O2)]BARF. Reprinted with permission fromref. 56. Copyright 2002, WILEY-VCH Verlag GmbH.



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A chromium(II) complex, [(Cl)(TMC)CrII]+ (TMC = 1,4,8,11-tetramethyl-1,4,8,11-tetraazacyclotetradecane) (Fig. 5a), reactswith O2 to yield an end-on chromium(III)-superoxo complex,[(Cl)(TMC)CrIII(O2

•−)]+ (Fig. 5b),57 where the O–O bond length(1.231(6) Å) of the end-on Cr(III)-superoxo complex is shorterthan that of the side-on Cr(III)-superoxo complex (1.327 Å inFig. 4b).56–58 The resonance Raman (rRaman) spectrum of[(Cl)(TMC)CrIII(O2)]

+ exhibited an O–O stretching vibrationband at 1170 cm−1, which shifts to 1104 cm−1 when 18O2 wasused instead of 16O2.

57 This value is comparable to thosereported for other end-on Cr(III)-superoxo complexes, such as[(H2O)5Cr

III(O2•−)]2+ (1166 cm−1)59 and [(H2O)(cyclam)

CrIII(O2•−)]2+ (1134/1145 (doublet) cm−1),59 but higher than

that of the side-on Cr(III)-superoxo complex (1072 cm−1),56

agreeing with the shorter O–O bond distance of the end-onCr(III)-superoxo complex compared to that of the side-on Cr(III)-superoxo complex.56,57

The structural and vibration data of [(Cl)(TMC)CrIII(O2)]+

are listed in Table 1 together with the data of other metal-superoxo complexes (vide infra).

The Cr(III)-superoxo complex ([(Cl)(TMC)CrIII(O2•−)]+) acts as

an unusual three electron oxidant in the oxidation of anNADH analogue, 1-benzyl-1,4-dihydronicotinamide (BNAH), asshown in eqn (2),

ð2Þ

where the 3 : 2 stoichiometry entails the removal of six elec-trons from BNAH by 2 molecules of [(Cl)(TMC)CrIII(O2

•−)]+ (athree electron oxidant) to produce BNA+ and [(Cl)(TMC)CrIII(OH)]+.60 The oxidation of BNAH by [(Cl)(TMC)CrIII(O2

•−)]+

is started by hydride transfer from BNAH to [(Cl)(TMC)CrIII(O2

•−)]+ to produce BNA+ and [(Cl)(TMC)CrII(OOH)][eqn (3)],60 followed by the O–O bond heterolysis of [(Cl)(TMC)CrII(OOH)] to produce the Cr(IV)-oxo complex, [(Cl)(TMC)CrIV(O)]+ [eqn (4)]. Then, hydrogen atom transfer from BNAH

to [(Cl)(TMC)CrIV(O)]+ occurs to produce BNA• and [(Cl)(TMC)CrIII(OH)]+ [eqn (5)]. This is followed by fast ET from BNA•,which is a strong electron donor (Eox = −1.1 V vs. SCE),38 to[(Cl)(TMC)CrIV(O)]+ to produce BNA+ and [(Cl)(TMC)CrIII(O)]+

[eqn (6)] that reacts with H2O to produce [(Cl)(TMC)CrIII(OH)]+

and OH− [eqn (7)].60 The overall 3 : 2 stoichiometry of the reac-tion of BNAH and [(Cl)(TMC)CrIII(O2

•−)]+ in eqn (2) is obtainedby summing up equations of (3) × 2, (4) × 2, (5), (6) and (7).60

BNAHþ ½ðClÞðTMCÞCrIIIðO2Þ�þ ! BNAþ þ ½ðClÞðTMCÞCrIIðOOHÞ�ð3Þ

½ðClÞðTMCÞCrIIðOOHÞ� ! ½ðClÞðTMCÞCrIVðOÞ�þ þ OH� ð4Þ

BNAHþ ½ðClÞðTMCÞCrIVðOÞ�þ ! BNA• þ ½ðClÞðTMCÞCrIIIðOHÞ�þð5Þ

BNA• þ ½ðClÞðTMCÞCrIVðOÞ�þ ! BNAþ þ ½ðClÞðTMCÞCrIIIðOÞ�þð6Þ

½ðClÞðTMCÞCrIIIðOÞ�þ þH2O! ½ðClÞðTMCÞCrIIIðOHÞ�þ þ OH�ð7Þ

When BNAH was replaced by an acid-stable NADH ana-logue, 10-methyl-9,10-dihydroacridine (AcrH2), four-electronoxidation of AcrH2 by [(Cl)(TMC)Cr

III(O2•−)]+ occurred to yield

10-methylacridone (AcrvO), which is the four-electron oxi-dized product of AcrH2,

61,62 and [(Cl)(TMC)CrIII(OH)]+ [eqn(8)].60 The 3 : 4 stoichiometry entails the removal of twelve elec-trons from AcrH2 by four molecules of [(Cl)(TMC)Cr

III(O2•−)]+

(a three electron oxidant) to produce AcrvO and [(Cl)(TMC)CrIII(OH)]+ [eqn (8)].60 The 18O-labeling experiments confirmedthat the oxygen atom in AcrvO was derived from [(Cl)(TMC)CrIII(18O2

•−)]+, which was synthesized by reacting [(Cl)(TMC)CrII]+ with 18O2.

63,64

ð8Þ

In the case of BNAH, hydride transfer from AcrH2 to[(Cl)(TMC)CrIII(O2

•−)]+ also occurs to produce AcrH+ and[(Cl)(TMC)CrII(OOH)] [eqn (9)].60 The Cr(II)-hydroperoxo complex([(Cl)(TMC)CrII(OOH)]) undergoes the O–O bond heterolysis toproduce the Cr(IV)-oxo complex, [(Cl)(TMC)CrIV(O)]+, and OH−

[eqn (4)]. Then, OH− is added to AcrH+ to produce AcrH(OH),61–63,65 which transfers an hydride ion to [(Cl)(TMC)CrIII(O2

•−)]+ to produce AcrvO, [(Cl)(TMC)CrIV(O)]+ and H2O[eqn (11)].60 This is followed by hydrogen atom transfer fromAcrH2 to [(Cl)(TMC)Cr

IV(O)]+ to produce AcrOH• and[(Cl)(TMC)CrIII(OH)]+ [eqn (12)].60 Finally, ET from AcrOH• to[(Cl)(TMC)CrIV(O)]+ occurs to yield AcrvO and [(Cl)(TMC)CrIII(OH)]+ [eqn (13)].60 The overall 3 : 4 stoichiometry of the

Fig. 5 X-ray structures of (a) chromium(II) complex ([(Cl)(TMC)CrII]+) and(b) chromium(III)-superoxo species ([(Cl)(TMC)CrIII(O2)]

+). Reprinted withpermission from ref. 57. Copyright 2010, American Chemical Society.



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reaction of AcrH2 and [(Cl)(TMC)CrIII(O2

•−)]+ in eqn (8) isobtained by summing up 3 × [eqn (9) + eqn (4)] + eqn (10) +eqn (11) + 2 × [eqn (12) + eqn (13)].60

AcrH2 þ ½ðClÞðTMCÞCrIIIðO2Þ�þ ! AcrHþ þ ½ðClÞðTMCÞCrIIðOOHÞ�ð9Þ

ð10Þ

ð11Þ

AcrHðOHÞ þ ½ðClÞðTMCÞCrIVðOÞ�þ

! AcrOH• þ ½ðClÞðTMCÞCrIIIðOHÞ�þ ð12Þ

AcrOH• þ ½ðClÞðTMCÞCrIVðOÞ�þ ! AcrvOþ ½ðClÞðTMCÞCrIIIðOHÞ�þð13Þ

The rate of hydride transfer from AcrH2 (large excess) to[(Cl)(TMC)CrIII(O2

•−)]+ [eqn (9)], which is the rate-determiningstep in the overall reaction [eqn (8)], obeyed the pseudo-first-order kinetics, and the observed second-order rate constant(k2) was determined to be 2.0 M

−1 s−1 at 253 K from the slopeof the plot of the pseudo-first-order rate constant (k1) vs. con-centration of AcrH2. A very large deuterium kinetic isotopeeffect (KIE = 74) was observed at 253 K in the oxidation ofAcrH2 and a deuterated compound (AcrD2) by [(Cl)(TMC)CrIII(O2)]

+, indicating the involvement of hydrogen atom tun-nelling in the hydride-transfer reaction.66,67

A good linear correlation between the log k2 values ofhydride transfer from NADH analogues to [(Cl)(TMC)CrIII(O2

•−)]+ and those of hydride transfer from the sameNADH analogues to p-chloranil (Cl4Q) was obtained as shownin Fig. 6.68,69 Such a linear correlation indicates that hydridetransfer from NADH analogues to [(Cl)(TMC)CrIII(O2

•−)]+

occurs via a concerted proton-coupled electron transfer(PCET), followed by a rapid ET as reported for hydride transferfrom NADH analogues to Cl4Q.

68,69

[(Cl)(TMC)CrIII(O2•−)]+ can also oxidise thioanisole via

direct oxygen atom transfer to yield methyl phenyl sulfoxideand [(Cl)(TMC)CrIV(O)]+ [eqn (14)].63 The observed second-order rate constant of sulfoxidation of thioanisole by [(Cl)(TMC)CrIII(O2

•−)]+ (kox) in the absence of HOTf in MeCN wasdetermined to be 3.6 × 10−4 M−1 s−1 at 233 K, where the reac-tion rate was very slow.70 However, the addition of HOTf (1equiv.) to an MeCN solution of [(Cl)(TMC)CrIII(O2

•−)]+ andthioanisole resulted in 104-fold enhancement to afford the rateconstant of 3.5(3) M−1 s−1 at 233 K.70 Thus, HOTf acts as aremarkable acid-catalyst to accelerate the sulfoxidation ofthioanisole by [(Cl)(TMC)CrIII(O2

•−)]+ [eqn (14)]. The kox valueof sulfoxidation of p-methoxythioanisole by [(Cl)(TMC)CrIII(O2)]

+ increased with increasing concentration of HOTf,exhibiting the second-order dependence on [HOTf] (Fig. 7).70

ð14Þ

As indicated by the negative one-electron reduction poten-tial of [(Cl)(TMC)CrIII(O2)]

+ (Ered vs. SCE = −0.52 V), no electrontransfer from [Fe(bpy)3]

2+ (Eox vs. SCE = 1.06 V) to [(Cl)(TMC)CrIII(O2

•−)]+ occurred because the electron transfer is highly

Fig. 6 Plot of log k2 for hydride-transfer reactions of Cr(III)-superoxocomplex ([(Cl)(TMC)CrIII(O2)]

+) and NADH analogues in MeCN at 253 Kversus log k2 for hydride-transfer reactions of p-chloranil (Cl4Q) and thesame NADH analogues in MeCN at 253 K. Reprinted with permissionfrom ref. 60. Copyright 2017, WILEY-VCH Verlag GmbH.

Table 1 Binding mode, O–O bond distance and O–O bond stretching frequencies of metal-superoxo complexes

Metal-superoxo complex Binding mode O–O (Å) ν(O–O)a (cm−1) Ref.

[CrIII(O2)(TptBu,Me)(pz′H)]+ η2 1.327(5) 1072 (1007) 56

[CrIII(O2)(Cl)(TMC)]+ η1 1.231(6) 1170 (1104) 57

[MnIII(O2•−)(L)(H2O)]

2+ η1 1.249(4) 1124 (1035) 75[FeIII(O2)(N-Meimid)TpivPP] η1 1.23(8) 1150 (1074) 86[(TAML)FeIII(O2

•−)]2− η2 1.323(3) 1260 (1183) 90[CoIII(η1-O2•−)(LOiPr)(TpMe2)] η1 1.301(5) 1168 (1090) 100[NiII(O2

•−)(L)] (L = β-diketiminato anion) η2 1.347(2) 971 (919) 134[CuII(O2)[HB(3-

tBu-5-iPrpz)3]] η2 1.22(3) 1112 (1060) 150

a ν(18O–18O) in parentheses.



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endergonic (ΔGet = +1.57 eV).70 In the presence of HOTf,however, electron transfer from [Fe(bpy)3]

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

•−)]+ was made possible to yield [Fe(bpy)3]3+ and [(Cl)

(TMC)CrIII(H2O2)]+ [eqn (15)].70 The Ered value of [(Cl)(TMC)

CrIII(O2•−)]+ in the presence of HOTf (2.5 mM) was determined

to be 1.12 V vs. SCE, which is by 1.64 V more positive than thevalue without HOTf (−0.52 V vs. SCE).70

½FeðbpyÞ3�2þ½ðClÞðTMCÞCrIIIðO2Þ�þ þ 2Hþ

! Ket ½FeðbpyÞ3�3þ þ ½ðClÞðTMCÞCrIIIðH2OÞ�2þ

ð15Þ

Remarkable acid catalysis in sulfoxidation of p-methoxy-thioanisole results from binding of two protons to the super-oxo moiety of [(Cl)(TMC)CrIII(O2

•−)]+ to produce [(Cl)(TMC)CrIII(O2

•−)]+ − (H+)2 (Scheme 7).70 The protonation equilibriumlies far to the left, when the concentration of [(Cl)(TMC)

CrIII(O2•−)]+ − (H+)2 is proportional to [HOTf]2.70 Although

no electron transfer occurs from p-methoxythioanisole to[(Cl)(TMC)CrIII(O2

•−)]+ in the absence of HOTf, electrontransfer from p-methoxythioanisole to [(Cl)(TMC)CrIII(O2

•−)]+ −(H+)2 occurs to produce a p-methoxythioanisole radical cationand [(Cl)(TMC)CrIII(H2O2)]

+, followed by O•− transfer from[(Cl)(TMC)CrIII(H2O2)]

+ to the p-methoxythioanisole radicalcation to yield p-methoxyphenyl methyl sulfoxide and[(Cl)(TMC)CrIV(O)]+, accompanied by the regeneration of twoprotons, indicating that no HOTf is consumed in the reaction(Scheme 7).70 Thus, HOTf acts as an efficient acid-catalystrather than a reactant for the sulfoxidation of p-methoxythio-anisole by [(Cl)(TMC)CrIII(O2

•−)]+ via PCET. Similar catalyticeffects of acids were reported for sulfoxidation by non-hemeiron(IV)-oxo complexes via PCET.71–74

6. Manganese(III)-superoxocomplexes

The reaction of [H4L][PF6]4 (H4L = 5,11,17,23-tetrakis-(tri-methylammonium)-25,26,27,28-tetrahydroxy-calix[4]arene)with four equivalents of Mn(OAc)2·4H2O and O2 in air affordeda Mn(III)-superoxo complex, [MnIII(O2

•−)(L)(H2O)](PF6)2, whichcontains a bowl-shaped cationic D4d structure and a linearend-on Mn(III)-O2 structure as shown by the X-ray crystal struc-ture in Fig. 8.75 The rRaman spectrum (λex = 632.8 nm) of[MnIII(O2

•−)(L)(H2O)]2+ showed the O–O stretching vibration at

1124 cm−1, which was shifted to 1035 cm−1 by 18O-labelling.The ν(O–O) vibration of the linear end-on Mn(III)-superoxocomplex is in-between those of the side-on Cr(III)-superoxocomplex (1072 cm−1)56 and the end-on Cr(III)-superoxocomplex (1170 cm−1) (Table 1).57 The magnetic moment for[MnIII(O2

•−)(L)(H2O)]2+ was determined to be 5.8μB, which indi-

cates a high-spin state (S = 5/2) of the Mn(III) species with theO2

•− ligand.75 Inclusion of the water solvation effect wasrequired to describe the geometric structure of the linear end-on [MnIII(O2

•−)(L)(H2O)]2+ complex from a theoretical model.76

Fig. 7 Plot of kox versus [HOTf] for the oxidation of para-MeO-thio-anisole (0.50 mM) by Cr(III)-superoxo complex ([(Cl)(TMC)CrIII(O2)]

+;0.50 mM) in the presence of HOTf (0–3.0 mM) at 233 K. Inset shows thesecond-order dependence of kox versus [HOTf]2. Reprinted with per-mission from ref. 70. Copyright 2018, American Chemical Society.

Scheme 7 Proposed reaction mechanism of acid-catalysed oxidationof p-methoxythioanisole by [(Cl)(TMC)CrIII(O2

•−)]+. Reprinted with per-mission from ref. 70. Copyright 2018, American Chemical Society.

Fig. 8 X-ray crystal structure of a mononuclear Mn(III)-superoxocomplex, [MnIII(O2

•−)(L)(H2O)]2+. Reprinted with permission from ref. 75.

Copyright 2011, Royal Society of Chemistry.



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The Mn(III)-superoxo complex exhibited good reactivity andselectivity in the catalytic oxidation of alkenes with O2 and iso-butyraldehyde under mild reaction conditions.75

A manganese(IV)-superoxo complex plays a very importantrole in photoinduced dioxygen activation by a Mn(III) corrol-azine complex, [MnIII(TBP8Cz)].

77 Femtosecond laser flashphotolysis of [MnIII(TBP8Cz)] resulted in the formation of atripquintet excited state (5T1), followed by the rapid intersys-tem crossing (ISC) to a tripseptet excited state (7T1), whichreacts with O2 via a diffusion-limited rate constant to generatethe putative manganese(IV)-superoxo species, [MnIV(O2

•−)-(TBP8Cz)].

77 The MnIV-superoxo complex abstracts a hydrogenatom from toluene to produce a benzyl radical and the MnIV-hydroperoxo (MnIV–OOH) complex, both of which further reactvia the O–O bond cleavage to yield benzyl alcohol and theMnV-oxo complex, [MnV(O)(TBP8Cz)].

77,78

In the presence of HOTf (1 equiv.), [MnIII(TBP8Cz)] can actas a photoredox catalyst for the oxygenation of hexamethyl-benzene (HMB) by O2 as shown in Scheme 8,

79 where[MnIII(TBP8Cz)] is protonated by HOTf to produce the mono-protonated complex, [MnIII(TBP8Cz(H))(OTf)]. Photoexcitationof [MnIII(TBP8Cz(H))(OTf)] affords the tripquintet state (

5T1),followed by rapid ISC to produce the tripseptet excited state(7T1). ET from

7T1 to O2 occurs to produce the MnIV-superoxo

complex, [MnIV(O2•−)(TBP8Cz(H))(OTf)], followed by hydrogen

atom transfer from HMB to produce the MnIV-hydroperoxocomplex, [MnIV(OOH)(TBP8Cz(H))(OTf)], and pentamethyl-benzyl radical species, competing with the back ET to regener-ate the ground state [MnIII(TBP8Cz)] and O2.Pentamethylbenzyl radical species reacts with [MnIV(OOH)(TBP8Cz(H))(OTf)] via O–O bond cleavage to produce penta-methylbenzyl alcohol and [MnIV(O)(TBP8Cz(H)

•+)(OTf)], whichis produced by the protonation of [MnV(O)(TBP8Cz)] via intra-molecular ET from the TBP8Cz ligand to the Mn

V(O) moiety.79

HMB can also be thermally oxidised by [MnIV(O)(TBP8Cz(H)•+)

(OTf)] via PCET to produce pentamethylbenzyl alcohol,accompanied by the regeneration of [MnIII(TBP8Cz(H))(OTf)] tocomplete the catalytic cycle (Scheme 8).79 In the presence ofexcess HOTf, however, [MnIII(TBP8Cz(H))(OTf)] is further pro-tonated to produce the diprotonated complex, [MnIII(TBP8Cz(H)2)(OTf)(H2O)](OTf). The

7T1 excited state of [MnIII(TBP8Cz

(H)2)(OTf)(H2O)](OTf) exhibited no reactivity towards O2 toproduce the MnIV-superoxo complex, because the one-electronoxidation potential of [MnIII(TBP8Cz(H)2)(OTf)(H2O)](OTf) ismuch higher than that of [MnIII(TBP8Cz(H))(OTf)], indicatingthat ET from the 7T1 excited state to O2 may be too endergonicto occur.80

Manganese porphyrins [MnIII(Porp)], [MnIII(TMP)(OH)](TMP = 5,10,15,20-tetrakis-(2,4,6-trimethylphenyl)porphinatodianion) and [MnIII(TPFPP)(CH3COO)] (TPFPP = 5,10,15,20-tetrakis(pentafluorophenyl)porphyrinato dianion), also act aseffective photocatalysts for photodriven oxygenation of10-methyl-9,10-dihydroacridine by O2 via photoinduced ETfrom MnIII(Porp) to O2 to produce Mn

IV(O2•−)(Porp) species.62

Manganese(III)-superoxo complexes are often proposed asthe first intermediates formed upon reaction of Mn(II) withO2.

81,82 For example, a manganese(IV)-superoxo complex is pro-posed as a putative intermediate for an O2-activation with[MnIII(TPFC)] (TPFC = 5,10,15-tris(pentafluorophenyl)corrolatotrianion) as shown in Scheme 9, where ET from [MnIII(TPFC)]with an axially coordinated OH− (pathway a) to O2 occurs toproduce a putative Mn(IV)-superoxo complex (pathway b),which abstracts a hydrogen atom from hydrogen donor mole-cules such as tetrahydrofuran and cyclohexene to generate theMn(IV)-hydroperoxo complex (pathway c).83 The O–O bondhomolysis of the Mn(IV)-hydroperoxo complex affords theMn(V)-oxo complex (pathway d), which can be produced by thereaction of [MnIII(TPFC)] with PhIO (pathway h).83 In the pres-ence of excess OH−, the Mn(IV)-peroxo complex can be formedeither from the reaction of the Mn(V)-oxo complex and OH−

(pathway e) or from the deprotonation of the Mn(IV)-hydro-

Scheme 8 Mechanism of acid-catalysed oxygenation of HMB by[MnIV(O2

•−)(TBP8Cz(H))(OTf)]. Reprinted with permission from ref. 79.Copyright 2015, American Chemical Society.

Scheme 9 Mechanism of O2-activation and O–O bond formation reac-tions with [MnIII(corrole)]. Reprinted with permission from ref. 83.Copyright 2017, American Chemical Society.



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peroxo species (pathway f or i).83 The addition of a proton tothe Mn(IV)-peroxo complex regenerates the Mn(IV)-hydroperoxocomplex (pathway g).83

7. Iron(III)-superoxo complexes

Iron(III)-superoxo intermediates are known not only in hemeenzymes but also in the nonheme mononuclear FeII enzymefamily.84 An Fe(III)–O2

•− intermediate was detected by the reac-tion of FeII-containing homoprotocatechuate 2,3-dioxygenasewith mutation of the active site His200 to Asn (H200N).84 AnFe(III)–O2

•− intermediate was also produced under illuminationto an O2-saturated acetonitrile solution of Fe(bpmcn)Cl2(bpmcn = (1S,2S)-N,N′-dimethyl-N,N′-bis(2-pyridinylmethyl)cyclohexane-1,2-diamine).85 It was also reported that dioxygenbound with [FeII(N-Meimid)(α,α,α,α-o-TpivPP)] (N-Meimid =N-methyl imidazole and α,α,α,α-o-TpivPP = meso-tetra(α,α,α,α-o-pivalamidephenyl)porphinato dianion) reversibly intoluene.86 The X-ray crystal structure of the O2 complex,[FeII(O2)(N-Meimid)(α,α,α,α-o-TpivPP)], showed four pivalamidogroups on one side of the porphyrin to generate a hydro-phobic pocket of 5.4 Å depth, which encloses “end-on” coordi-nation of O2 with a bent Fe–O–O bond.

86 The O–O stretchingvibrations were observed as doublets at 1150 and 1155 cm−1

for 16O2 and 1074 and 1079 cm−1 for 18O2.

87 These values werecomparable to those reported for an end-on Cr(III)-superoxocomplex, [Cr(O2

•−)(cyclam)(H2O)]2+ (1134 and 1145 cm−1).59

Cryoreduction of oxy-hemoglobin (oxy-GMH3) from Glyceradibranchiata, oxy-ferrous octaethyl porphyrin, and oxy-ferrouscomplex of the heme model (cyclidene complex) resulted inthe formation of the ferrous-superoxo complexes with nearlyunit spin density localized on a superoxo moiety.88 The end-oncoordination of O2

•− to a low-spin ferrous ion is supported bytheir g tensors and 17O hyperfine couplings, which are charac-teristic of the superoxide ion coordinated to a diamagneticmetal ion. Upon annealing to T > 150 K, the ferrous-superoxospecies were converted to peroxo/hydroperoxo-ferricintermediates.88

The one-electron reduction of the Fe(III)-superoxo specieswas also carried out by γ-ray irradiation of the sample at 77 Kin MeCN/2-MeTHF (v/v 1 : 4; 2-MeTHF = 2-methyl-tetrahydrofuran) as shown in Scheme 10.89 The γ-ray irradiatedsample exhibited an Fe–O stretching vibration at 459 cm−1 thatshifted to 435 cm−1 by 18O-labelling (16–18Δν = −24 cm−1).Thus, the iron–oxygen stretching (νFe–O) frequency of theferrous-superoxo is significantly lower than that of the ferric-superoxo complex observed at 579 cm−1.89 The O–O stretchingvibration of the ferrous-superoxo complex could not beobserved as this mode is known to be difficult to detect.89

A side-on iron(III)-superoxo non-heme complex ([(TAML)FeIII(O2

•−)]2−) was successfully synthesized by the reaction ofan iron(III) complex bearing a tetraamido macrocyclic ligand(TAML) with solid potassium superoxide (KO2) in the presenceof 2.2.2-cryptand (6 equiv.) in CH3CN at 5 °C.

90 The X-raycrystal structure of [(TAML)FeIII(O2

•−)]2− showed two crystallo-

graphically independent mononuclear side-on 1 : 1 iron com-plexes of O2 with O–O bond lengths (O1–O2: 1.323(3) Å andO7–O8: 1.306(7) Å) (Fig. 9), which are significantly shorterthan those of Fe(III)-peroxo species, such as [FeIII(TMC)(O2)]

+

(1.463(6) Å)91 and naphthalene dioxygenase (ca. 1.45 Å),92 butsimilar to that of Fe(II)-superoxo species found in homoproto-catechuate 2,3-dioxygenase (1.34 Å).93 The infrared spectrumof [(TAML)FeIII(O2

•−)]2−, which was recorded in CH3CN at−40 °C, exhibits an O–O stretching at 1260 cm−1, which shiftsto 1183 cm−1 on substitution of 16O with 18O. The observed16–18Δν value of 77 cm−1 agrees within an experimental errorwith the calculated value of 72 cm−1.90

The [(TAML)FeIII(O2•−)]2− complex can abstract a hydrogen

atom from 2,4-di-tert-butyl phenol to yield 2,2′-dihydroxy-3,3′,5,5′-tetra-tert-butylbiphenyl, which is a C–C bond couplingproduct, as a major product (75% yield based on the initialconcentration of [(TAML)FeIII(O2

•−)]2−).90 The logarithm of therate constants (log k2) of hydrogen atom transfer from para-substituted 2,6-di-tert-butylphenols (p-X-2,6-t-Bu2-C6H2OH; X =OMe, Me, H, CN) to [(TAML)FeIII(O2

•−)]2− is linearly correlatedwith the O–H bond dissociation energies (BDEs) of p-X-2,6-t-Bu2-C6H2OH.

90 A linear Hammett plot of log k2 vs. σp+ was

obtained with a negative slope (ρ) of −2.5, indicating that[(TAML)FeIII(O2

•−)]2− acts as an electrophile for the hydrogenatom transfer reactions.90 Singha and Dey recently reportedthat an iron(III)-superoxo porphyrin with a covalently attachedhydroquinol group underwent hydrogen atom abstractionfrom the hydroquinol group to produce the ferric hydroperox-ide, which performed the second hydrogen abstraction togenerate the ferryl species.94

Scheme 10 Conversion from FeIII-superoxo porphyrin to FeII-superoxoporphyrin by one-electron reduction (R = mesityl group). Reprinted withpermission from ref. 89. Copyright 2015, Royal Society of Chemistry.

Fig. 9 X-ray crystal structures of [(TAML)FeIII(O2•−)]2−, showing the two

crystallographically independent moieties. Reprinted with permissionfrom ref. 90. Copyright 2014, Springer Nature.



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The FeIII-superoxo complex ([(TAML)FeIII(O2•−)]2−) also acts

as a nucleophile in the reaction with 2-phenylpropionaldehyde(2-PPA) to yield acetophenone as the deformylated product(90% yield based on the initial concentration of [(TAML)FeIII(O2

•−)]2−),90 as reported for other nucleophilic reactions ofmetal-peroxo complexes.95–98 The nucleophilic reaction of[(TAML)FeIII(O2

•−)]2− with para-substituted benzaldehydederivatives (p-Y-C6H4CHO; Y = OMe, Me, H, Cl) afforded theHammett plot with a positive ρ value of 1.4 in contrast to theelectrophilic hydrogen atom transfer reactions (vide supra).90

The [(TAML)FeIII(O2•−)]2− complex can also oxidise nitric

oxide (NO) to yield [(TAML)FeIII(NO3)]2− and NO2

− via for-mation of Fe(IIII)-peroxynitrite species ([(TAML)FeIII(O–O–NvO−)]2−), which undergo O–O bond homolysis, followed byrebound of the resulting Fe(IV)-oxo species, [(TAML)FeIV(O)]2−,and NO2

•.99

A high-spin mononuclear iron(II) complex supported by afive-azole donor set, [FeII(LPh)(TpMe2)], is reported to react withO2 to yield the corresponding mononuclear non-heme iron(III)-superoxo species, [FeIII(O2

•−)(LPh)(TpMe2)], in THF at −60 °C asshown in Scheme 11.100 The rRaman spectrum of [FeIII(O2

•−)(LPh)(TpMe2)] exhibited an O–O bond stretching vibration at1168 cm−1, which shifts to 1090 cm−1 on substitution of 16Owith 18O.100 The 16–18Δν value of 78 cm−1 is consistent with thecalculated value of 72 cm−1. The ν(O–O) value of 1168 cm−1 isnearly the same as that of the end-on Cr(III)-superoxo complex,[(Cl)(TMC)CrII(O2

•−)]+, with ν(O–O) of 1170 cm−1.57 Thus, thesuperoxo ligand of [FeIII(O2

•−)(LPh)(TpMe2)] is suggested to becoordinated to the sixth site of the Fe(III) centre with an end-onmode.100 A somewhat smaller ν(O–O) value (1125 cm−1) wasreported for a Fe(III)-superoxo complex, [FeIII(O2

•−)(BDPP)](H2BDPP = 2,6-bis(((S)-2-(diphenylhydroxymethyl)-1-pyrrolidi-

nyl)methyl)-pyridine), in which an end-on coordination ofO2

•− to the Fe(III) centre is also suggested.101 A similar ν(O–O)value (1120 cm−1) was recently reported for an Fe(III)-superoxocomplex, [FeIII(O2

•−)TpMe2(2-ATP)] (2-ATP = 2-aminothiopheno-late).102 DFT calculations suggest the lowest-energy quintetstructure as shown in Fig. 10, where the O2

•− ligand forms ahydrogen bond with the –NH2 donor of 2-ATP, as indicated bythe O⋯H distance of 1.81 Å and also by the smaller Fe–O–Oangle of 122°.102

The [FeIII(O2•−)(LPh)(TpMe2)] complex exhibited a hydrogen

atom abstraction ability from substrates having a weak X–Hbond, where X = O or N; BDE of X–H < 72.6 kcal mol−1, suchas 2-hydroxy-2-azaadamantane (AZADOL) and phenylhydra-zine.100 Hydrogen atom transfer from AZADOL to [FeIII(O2

•−)(LPh)(TpMe2)] afforded an iron(III)-hydroperoxo complex,[FeIII(OOH)(LPh)-(TpMe2)] (Scheme 11).100 The [FeIII(O2

•−)(TpMe2)(2-ATP)] complex can also abstract hydrogen atomsfrom 9,10-dihydroanthracene (DHA) to yield anthracene.102

When DHA-d4 was used as a substrate, a deuterium kineticisotope effect (KIE = 7) was observed, indicating that C–Hbond cleavage is the rate-determining step.102

An alkyl thiolate-ligated iron(II) complex can also react withO2 to form an Fe(III)-superoxo intermediate, [Fe

III(O2•−)-

(S2Me2N3(Pr,Pr))], in THF at −73 °C.103 The rRaman spectrum

of [FeIII(O2•−)(S2

Me2N3(Pr,Pr))] showed O–O stretchingvibrations at 1093 and 1122 cm−1 that shift to 1022 cm−1 when16O2 was replaced by

18O2.103 The DFT calculated structure of

[FeIII(O2•−)(S2

Me2N3(Pr,Pr))] contains an O2 moiety cis to one ofthe thiolate sulphurs with an O–O bond length of 1.289 Å.103 Acalculated O–O stretching vibration of 1154 cm−1 is consistentwith a ferric superoxo complex.103 The frontier orbitals of[FeIII(O2

•−)(S2Me2N3(Pr,Pr))] as shown in Fig. 11 contain two

unpaired electrons of opposite spin; one on the superoxo π*(O–O) orbital and the other on the Fe(dxy) orbital.

103

The [FeIII(O2•−)(S2

Me2N3(Pr,Pr))] complex can also abstracthydrogen atoms from 1,4-cyclohexadiene (CHD : BDE =76 kcal mol−1) with the deuterium kinetic isotope effect(KIE = 4.8) and also from THF (BDE = 92 kcal mol−1) to producean Fe(III)-hydroperoxo complex, [FeIII(OOH)(S2

Me2N3(Pr,Pr))].103

Scheme 11 Formation of a non-heme iron(III)-superoxo complex,[FeIII(O2

•−)(LPh)(TpMe2)], and the hydrogen atom transfer reaction with2-hydroxy-2-azaadamantane to produce [FeIII(OOH)(LPh)(TpMe2)]species. Reprinted with permission from ref. 100. Copyright 2015,WILEY-VCH Verlag GmbH.

Fig. 10 DFT-optimized structure of [FeIII(O2•−)(TpMe2)(2-ATP)] with a

quintet state (S = 2). Black and green texts provide interatomic distancesin Å and Mulliken spin populations, respectively. Reprinted with per-mission from ref. 102. Copyright 2018, Royal Society of Chemistry.



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8. Cobalt(III)-superoxo complexes

Mononuclear and dinuclear cobalt(III)-superoxide complexeshave been known for decades.104–106 It was reported earlierthat cobalt(II) complexes such as cyanocob(II)alamin (vitaminB12r) and cobalt(II) porphyrins reacted with O2 to produceCo(III)-superoxo complexes.18,107,108 The end-on coordinationof O2

•− to a Co(III) complex, [CoIII(bzacen)(pyridine)] (bzacen =N,N′-ethylene-bis(benzoylacetoniminide)), was shown by theX-ray crystal structure of [CoIII(O2

•−)(bzacen)(pyridine)].17

Single crystal EPR of the 17O enriched superoxide complexwith vitamin B12r gave the g tensor and the

17O hyperfinetensors of B12rO2, which are coaxial with the largest g principalaxis along the Oα–Oβ bond, whereas one of the principal axesof the 59Co hyperfine tensor is oriented approximately alongthe corrin normal.109,110 The Co–O–O moiety was bent with abond angle of 111°.109 A total spin density on the O2 of ρO2 wasdetermined to be 0.7 ± 0.1.109 The X-ray crystal structure ofB12rO2 confirmed that the dioxygen molecule is attached to themetal centre in a bent end-on mode at the β-face of the cobal-amin molecules.111

As the case of a high-spin iron(II) complex supported by afive-azole donor set ([FeII(LPh)(TpMe2)]) (vide supra; Scheme 11),cobalt(II) complexes, [CoII(LX)(TpMe2)] (X = Ph and OiPr),reacted with O2 reversibly to produce the end-on Co(III)-super-oxo complex, [CoIII(η1-O2•−)(LOiPr)(TpMe2)] (Scheme 12), wherean O–O bond length of 1.301(5) Å is typical of the superoxocomplex.100,112 The O2 binding affinity of the five azole-sup-ported cobalt centres is controlled by the structural and elec-tronic properties of the ligand substituent groups located inthe secondary coordination sphere.112

Dinuclear cofacial cobalt(II) porphyrin μ-superoxo com-plexes were generated by the reactions of cofacial dicobalt(II)porphyrins with O2 in the presence of a bulky base (1-tert-butyl-5-phenylimidazole) and the subsequent one-electron oxi-dation of the resulting peroxo species by iodine.113 The super-hyperfine structure due to two equivalent 59Co nuclei wasobserved at room temperature in the EPR spectra of the

μ-superoxo species.113 The peroxo-bridged dinuclear cofacialCo(III) complexes act as key intermediates for the catalytic four-electron reduction of O2 by one-electron reductants such asferrocene in the presence of an acid such as HClO4 in benzo-nitrile via the O–O bond cleavage of the dinuclear peroxospecies to produce the high-valent cobalt(IV)-oxo complex thatundergoes further reduction by ferrocene to regenerate dinuc-lear cobalt(III) complexes.113–115

In contrast to dinuclear cobalt complexes that catalyse thefour-electron reduction of O2 (vide supra), mononuclear cobaltcomplexes act as efficient catalysts for two-electron reductionof O2 to H2O2 via cobalt(III)-superoxo complexes.

113,116–123 Inthe case of cobalt(III) corrole complexes, such as (TPFCor)CoIII

(TPFCor = 5,10,15-tris( pentafluorophenyl)corrole),(F5PhMes2Cor)Co

III (F5PhMes2Cor = 10-penta-fluorophenyl-5,15-dimesitylcorrole), (Mes3Cor)Co

III (Mes3Cor = 5,10,15-trismesityl-corrole) and (tpfcBr8)Co

III (tpfcBr8 =2,3,7,8,12,13,17,18-octabromo-5,10,15-tris(pentafluorophenyl)-corrole), the Co(III)/(IV) couple is responsible for the catalytictwo-electron reduction of O2.

124 The best catalytic performancewas obtained for [(tpfcBr8)Co

III], which exhibited the onsetpotential as positive as 0.81 V vs. RHE.125 Cobalt(II) corrolesare known to react with O2 to produce the end-on cobalt(III)-superoxo corroles, which are well characterised by rRaman andEPR spectroscopy, together with DFT analyses.126

A mononuclear cobalt(II) complex, [CoII(Pytacn)(CH3CN)2]2+,

is also reported to react with O2 to form initially the mono-nuclear cobalt(III)-superoxo complex, followed by stabilisationwith another molecule of [CoII(Pytacn)(CH3CN)2]

2+ to affordthe peroxo-bridged dicobalt(III) complex (Scheme 13).127 TheCW X-band EPR spectrum of [CoIII(O2

•−)(Pytacn)(CH3CN)2]2+

recorded in acetone at 100 K exhibited an axial S = 1/2 signal,which is assigned to a CoIII-superoxo species based on thecomparison of the corresponding EPR parameters (A∥ = 15.9 Gdue to 59Co (I = 7/2) nucleus, g∥ = 2.0808, g⊥ = 1.99) with those

Fig. 11 Singly occupied molecular orbitals (SOMO) of [FeIII(O2•−)

(S2Me2N3(Pr,Pr))]. Reprinted with permission from ref. 103. Copyright

2019, American Chemical Society.

Scheme 12 Formation of cobalt(III)-superoxo complexes, [CoIII(η1-O2•−)(LX)(TpMe2)] (X = Ph and OiPr), and X-ray crystal structure of [CoIII(η1-O2

•−)(LOiPr)(TpMe2)]. Reprinted with permission from ref. 100. Copyright2015, WILEY-VCH Verlag GmbH.



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reported for cobalt(III)-superoxo complexes.128 A dithiolate-ligated cobalt(III)-superoxo complex, CoIII(O2)(Me3TACN)(S2SiMe2), was also produced by the reaction of Co

II(Me3TACN)(S2SiMe2) with O2 and characterized structurally by X-rayabsorption spectroscopy (XAS) and spectroscopically by elec-tron paramagnetic resonance (EPR) and resonance Raman (rR)spectroscopies.129

A CoII triazacorrole complex, [CoII(TBP8Cz)(py)]− (TBP8Cz =

octa(4-tert-butylphenyl)corrolazine), produced by the one-elec-tron reduction of [CoIII(TBP8Cz)(py)2], also reacts with O2 rever-sibly to generate the Co(III)-superoxo complex, [CoIII(O2

•−)(TBP8Cz)(py)]

−, in CH2Cl2/py (Scheme 14).130,131 At 240 K > T >

200 K, a clear eight-line signal with Aiso(59Co) = 40 MHz (14 G)

and giso = 2.026 was observed, whereas the EPR spectrum dis-appeared at T > 240 K (Fig. 12), indicating a shift to the rightin the equilibrium between [CoIII(O2

•−)(TBP8Cz)(py)]− and

{[CoII(TBP8Cz)(py)]− + O2}.

131

The [CoIII(O2•−)(TBP8Cz)(py)]

− complex can abstract ahydrogen atom from TEMPOH (2,2,6,6-tetramethylpiperidin-1-ol, BDE(O–H) = 72.1 kcal mol−1), phenylhydrazine (BDE(N–H)= 75.0 kcal mol−1) and diphenylhydrazine (BDE(N–H) =71.7 kcal mol−1) to produce a cobalt(III)-hydroperoxo complexthat rapidly loses hydroperoxide via displacement by theexcess pyridine (Scheme 15).130 The [CoIII(O2

•−)(BDPP)]complex, which contains a low-spin cobalt(III) ion bound to asuperoxo ligand, can also abstract a hydrogen atom from

TEMPOH to form a structurally characterized cobalt(III)-hydro-peroxo complex, [CoIII(OOH)(BDPP)], and TEMPO•.132

Besides cobalt(III)-superoxo complexes, a mononuclearcobalt(II)-superoxo complex, (Et4N)[Co

II(O2•−)(L•2−)] (L3− = (N(o-

PhNC(O)iPr)2)3−), was reported to be produced by the reaction

of a bimetallic (Et4N)2[CoII2 (L)2] complex with two equivalentsof O2 (Fig. 13).

133 The cobalt(II)–cyanide complex,(Et4N)2[Co

II(CN)(L)], was also produced by the reaction of a bi-metallic (Et4N)2[Co2(L)2] complex with two equivalents of CN

−.The X-ray crystal structure of (Et4N)2[Co

II(CN)(L)] is shown inFig. 13.133 Magnetic measurements indicate [CoII(CN)(L)]2− tobe a high-spin CoII-cyano species (S = 3/2), whereas IR, EXAFS,and EPR spectroscopies indicate [Co(O2)(L)]

− to be an end-onsuperoxo complex with an S = 1/2 ground state.133 X-ray spec-troscopy and calculations indicate that [Co(O2)(L)]

− features ahigh-spin CoII centre. The S = 1/2 spin state results from theCo electrons coupled to both O2

•− and the aminyl radical onredox noninnocent L•2−.133

Scheme 14 Formation of [CoIII(O2•−)(TBP8Cz)(py)]

− by O2-activation.Reprinted with permission from ref. 130. Copyright 2019, Royal Societyof Chemistry.

Fig. 12 EPR spectra of [CoIII(O2•−)(TBP8Cz)(py)]

− in fluid pyridine/EtOH(v/v 1 : 1) solution at various temperatures (160–239 K). Reprinted withpermission from ref. 131. Copyright 2004, American Chemical Society.

Scheme 15 Mechanism for the reaction between the CoIII(O2•−)

complex and H-atom donors. Reprinted with permission from ref. 130.Copyright 2019, Royal Society of Chemistry.

Scheme 13 Reversible reaction of Co(II) with O2 to form CoIII-superoxo

and CoIII-peroxo-CoIII species depending on temperature. Reprintedwith permission from ref. 127. Copyright 2017, Royal Society ofChemistry.



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9. Nickel(II)-superoxo complexes

The first nickel(II)-superoxo complex, which was structurallycharacterized by X-ray diffraction analysis, was prepared by thereaction of the nickel(I) precursor [(NiI(β-diketiminato))2-(μ–η3:η3-C6H5Me)] with dry O2 in toluene and the structure isshown in Fig. 14, where O2

•− is coordinated to the Ni(II) centrein a side-on fashion.134 The O–O bond length of 1.347(2) Å istypical of the superoxo species as compared with those ofperoxo ligands, which are longer than 1.40 Å.135 The 16O–16Ostretching vibration at 971 cm−1 (ν(18O–18O) = 919 cm−1) deter-mined by IR spectroscopy is also typical of the superoxospecies.134 The low O–O bond stretching vibration may resultfrom the side-on coordination as compared with the value ofan end-on Ni(II)-superoxo complex (vide infra). The EPR spec-trum of [NiII(O2

•−)(L)] (L = β-diketiminato) in frozen toluene at

50 K showed a signal at g = 2.138, 2.116, and 2.067, and theaverage gavg value of 2.107 agrees with the effective magneticmoment determined for a solid sample in the temperaturerange of 20–300 K (μeff = 1.8 B.M. that corresponds to gavg =2.08).135 The [NiII(O2

•−)(L)] complex can oxygenate PPh3 toproduce Ph3PvO. The [Ni

II(O2•−)(L)] complex can also abstract

a hydrogen atom from 2,4,6-tri-tert-butylphenol, demonstrat-ing the dioxygenase-like reactivity.135

A side-on Ni(II)-superoxo complex was also reported for[NiII(O2

•−)(PhTtAd)] (PhTtAd = phenyltris((1-adamantylthio)-methyl)borate), which showed a rhombic EPR spectrum at g =2.24, 2.19 and 2.01, containing a five-coordinate NiII centre.136

There was no 17O hyperfine broadening of the EPR spectrum,indicating that the unpaired electron in O2

•− is coupled anti-ferromagnetically with that in the Ni dx2−y2 orbital and theremaining unpaired electron resides primarily in the Ni dz2orbital.136

The first end-on Ni(II)-superoxo complex was reported for[NiII(O2

•−)(14-TMC)](OTf) (14-TMC = 1,4,8,11-tetramethyl-1,4,8,11-tetraazacyclotetradecane), which was generated by thereaction of [NiI(14-TMC)](OTf) with excess O2 in THF at−78 °C.137 The end-on coordination of O2•− to the Ni(II) centreis indicated by the extended X-ray absorption fine structure(EXAFS) data, which exhibited an O/N scatterer at 1.98 Å andfour N/O scatterers at 2.17 Å.137 The DFT calculations indicatethat the end-on low-spin state (S = 1/2) is much more favouredover both the end-on high-spin (S = 3/2) and side-on low-spinstates.137 The [NiII(O2

•−)(14-TMC)]+ complex as well as the[NiII(O2

•−)(L)] complex as shown in Fig. 14 can oxygenate PPh3to Ph3PvO and it can also abstract a hydrogen atom fromxanthene and cyclohexadiene.137

When 14-TMC was replaced by 12-TMC (= 1,4,7,10-tetra-methyl-1,4,7,10-tetraazacyclododecane), a Ni(III)-peroxocomplex was formed with the 12-TMC ligand as shown inFig. 15.138,139 The X-ray crystal structure of [NiIII(O2)(12-TMC)](ClO4)·CH3CN showed the mononuclear side-on 1 : 1 nickelcomplex with the O2 moiety in a distorted octahedral geome-try.138 The O–O bond length (1.386(4) Å) of the Ni(III)-peroxocomplex, [NiIII(O2

2−)(12-TMC)]+, is longer than that of theNi(II)-superoxo complex, [NiII(O2

•−)(14-TMC)]+.138 The observedO–O stretching frequency of [NiIII(O2

2−)(12-TMC)]+

Fig. 13 Formation of a cobalt(II)-superoxo complex, (Et4N)[CoII(O2

•−)(L•2−)], and a cobalt(II)-cyano species (Et4N)2[Co(CN)(L)] by the reactionof (Et4N)2[Co2(L)2] with 2 equiv. of O2 and CN

−, respectively. Reprintedwith permission from ref. 133. Copyright 2016, American ChemicalSociety.

Fig. 14 X-ray crystal structure of nickel(II)-superoxo complex, [NiII(O2•−)

(L)] (L = β-diketiminato anion). Reprinted with permission from ref. 134.Copyright 2008, WILEY-VCH Verlag GmbH.

Fig. 15 The macrocyclic ring-size effect of TMC ligands on the gene-ration of Ni–O2 species. Reprinted with permission from ref. 139.Copyright 2013, Royal Society of Chemistry.



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(1002 cm−1)138 is significantly lower than that of [NiII(O2•−)

(14-TMC)]+ (1131 cm−1).137 The nickel(III)-peroxo complex([NiIII(O2

2−)(12-TMC)]+) is not reactive in electrophilic reac-tions, but is capable of conducting nucleophilic reactions.138

When 13-TMC (= 1,4,7,10-tetramethyl-1,4,7,10-tetraaza-cyclotridecane) was employed as a ligand for generation of theNi–O2 complex, both the nickel(II)-superoxo complex([NiII(O2

•−)(13-TMC)]+) and the nickel(III)-peroxo complex([NiIII(O2

2−)(13-TMC)]+) were formed in the reaction of [NiII(13-TMC)(CH3CN)]

2+ and H2O2 in the presence of tetramethyl-ammonium hydroxide (TMAH) and triethylamine (TEA),respectively.139 The superoxo ligand in [NiII(O2

•−)(13-TMC)]+ isbound in an end-on fashion to the nickel(II) centre, whereasthe peroxo ligand in [NiIII(O2

2−)(13-TMC)]+ is bound in a side-on fashion to the nickel(III) centre.139 The observed O–Ostretching frequency [ν(O–O)] of [NiII(O2

•−)(13-TMC)]+ at1130 cm−1 is virtually the same as that reported for [NiII(O2

•−)(14-TMC)]+ (1131 cm−1).139 The ν(O–O) value of [NiIII(O2

2−)(13-TMC)]+ (1008 cm−1) is lower than those of Ni(II)-superoxocomplexes, such as [NiII(O2

•−)(13-TMC)]+ (1130 cm−1) and[NiII(O2

•−)(14-TMC)]+ (1131 cm−1),137–139 but similar to that ofthe Ni(III)-peroxo complex, [NiIII(O2

2−)(12-TMC)]+

(1002 cm−1).138 The [NiII(O2•−)(13-TMC)]+ complex exhibited

electrophilic reactivity, whereas the [NiIII(O22−)(13-TMC)]+

complex showed nucleophilic reactivity.139

A side-on nickel(II)-superoxo species was also produced bythe reaction of nickel(I) dispersed inside the nanopores of theZSM-5 zeolite with O2.

140 The side-on η2-coordination of O2•−

to the Ni(II) centre was indicated by detailed analysis of theEPR spectra of both 16O2 and

17O2 species. The computersimulations of the spectra and relativistic DFT calculations ofthe EPR signatures with the g and A(17O) tensors (gxx = 2.0635,gyy = 2.0884, gzz = 2.1675; |Axx| ≈ 10 G, |Ayy| = 56.7 G, |Azz| ≈ 13G) indicate a mixed metalloradical with two supporting oxygendonor ligands and even triangular spin-density redistributionwithin the η2-{NiO2} unit.140 This shows sharp contrast to thecase of the side-on Ni(II)-superoxo complex ([NiII(O2

•−)(PhTtAd)]), which shows no 17O hyperfine when the unpairedelectron resides primarily in the Ni dz2 orbital (vide supra).

136

A pyrazolate-based dinickel(II) dihydride complex [KL(Ni–H)2] reacted with O2 to produce the μ-1,2-peroxo Ni(II) complex[KLNi2(O2

2−)], which reacted further with excess O2 to affordthe μ-1,2-superoxo dinickel(II) complex ([LNi2(O2•−)]).141 TheX-ray crystal structure of [LNi2(O2

•−)] is shown in Fig. 16, wherethe O–O bond distance is typical of a superoxo ligand (1.326(2)Å) that is significantly shorter than that of the peroxo ligand(1.465(2) Å) in the corresponding μ-1,2-peroxo dinickel(II)complex.141 A Raman spectrum of [LNi2(O2

•−)] exhibited theO–O stretching frequency at 1007 cm−1, which was shifted to951 cm−1 when 16O2 was replaced by

18O2 (Δν(18O2–16O2) =−56 cm−1).141 This is in the range that is typical of superoxocomplexes and has significantly higher energy than the O–Ostretches of the peroxo complex ([KLNi2(O2

2−)]: 680 cm−1 andΔν(18O2–16O2) = −40 cm−1).141 The one-electron reductionpotential of the μ-1,2-superoxo dinickel(II) complex([LNi2(O2

•−)]) was determined by the cyclic voltammogram,

which showed the reversible redox couple at E1/2 = −1.22 V vs.Fc+/Fc in THF.141

10. Copper(II)-superoxo complexes

Mononuclear CuII-superoxo complexes are the first intermedi-ates produced at several Cu protein active sites, such as pepti-dylglycine α-hydroxylating mono-oxygenase, dopamineβ-monooxygenase, Cu/Zn superoxide dismutase and copperamine oxidase.10,34,142,143 CuII-Superoxo complexes are also thefirst intermediates for the catalytic O2 reduction.

144–148 Thefirst copper(II)-superoxo species, which were characterizedstructurally by X-ray diffraction analysis, were formed by react-ing a copper(I) species with a sterically demanding ligand, HB(3-tBu-5-iPrpz)3, to prevent the dimerization.

149,150 The X-raycrystal structure of [HB(3-tBu-5-iPrpz)3]Cu

II(O2•−) is shown in

Fig. 17, where O2•− is bound to the Cu(II) centre in a side-on

fashion with the O–O bond distance of 1.22 Å. The O–O bondstretching frequency at 1112 cm−1 observed in the IR spectrum

Fig. 16 (a) X-ray crystal structure of a pyrazolate-based μ-1,2-superoxodinickel(II) complex ([(L)Ni2(O2

•−)]). (b) Front view of the structure of[LNi2(O2

•−)] (iPr groups and hydrogen atoms are omitted for clarity).Reprinted with permission from ref. 141. Copyright 2018, AmericanChemical Society.

Fig. 17 X-ray structure of [HB(3-tBu-5-iPrpz)3]CuII(O2

•−). Reprinted withpermission from ref. 150. Copyright 2003, American Chemical Society.



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of [HB(3-tBu-5-iPrpz)3]CuII(O2

•−) shifted to 1060 cm−1 when16O2 was replaced by

18O2 (Δν(18O2–16O2) = −52 cm−1).150 Theground state is a singlet due to the covalent interactionbetween the superoxide π*σ and Cu dxy orbitals.150

The X-ray crystal structure of the Cu–O2 complex in theenzyme peptidylglycine-α hydroxylating mono-oxygenase (PHM)was successfully obtained as shown in Fig. 18, where the CuBsite reveals a four-coordinate distorted tetrahedral (Td) geometryand O2

•− binds to the CuB centre with an end-on η1 geometry(O–O distance = 1.23 Å and the CuB–O–O angle = 110°).

151 Thisgeometry is compatible with the O2 molecule or O2

•− speciesbound to the copper centre, but not with Cu-peroxo species,which have typical O–O bond lengths of >1.4 Å.152–155

Cu(II)-Superoxo complexes are capable of abstracting ahydrogen atom to cleave phenol O–H bonds and weak C–Hbonds.156–163 The hydrogen abstracting reactivity of Cu(II)-superoxo complexes is enhanced by hydrogen-bonding moi-eties in a series of TMPA-based Cu(L) (L = TMPA, BA, F5BA andMPPA; see the structures A–D in Fig. 19) complexes, whichinhibit the formation of the corresponding binuclear trans-μ-1,2-peroxo[{CuII(L)}2(O22−)]2+ complex, in favour of mono-copper [CuII(O2

•−)(L)]+ species.164 [CuII(O2•−)(TMPA)]+ (A), which

has no H-bonding moiety, exhibited the lowest O–O stretchingfrequency at 1119 cm−1 with Δν(18O2–16O2) = −61 cm−1 andthe ν(O–O) values increased with enhancing H-bonding abilitywith an increase in the N–H dipoles in order of B–D.164

[CuII(O2•−)(TMPA)]+ (A) exhibited no reactivity toward

4-methoxyphenol with BDE(O–H) = 87.6 kcal mol−1 in DMSOat −135 °C.164 In contrast, [CuII(O2•−)(BA)]+ (B) exhibited slug-gish reactivity, which took >6 h for completion. [CuII(O2

•−)(F5BA)]

+ (C) reacted much faster than [CuII(O2•−)(BA)]+ (B).

[CuII(O2•−)(MPPA)]+ (D) exhibited the highest reactivity with

the second-order rate constant of 2.33 × 10−2 M−1 s−1. Thehydrogen bonding is proposed to occur to the proximalO-atom in [CuII(O2

•−)(L)]+ complexes.164

A mononuclear copper(II)-(end-on)superoxide complex sup-ported by a N-[(2-pyridyl)methyl]-1,5-diazacyclooctane triden-tate ligand was recently reported to induce a catalytic C–C

bond formation reaction between carbonyl compounds (sub-strate) and the solvent molecule (acetone), giving β-hydroxy-ketones (aldol).165

Besides mononuclear copper-superoxo complexes (videsupra), a dinuclear Cu(II)-superoxo complex was prepared bythe reaction of a mixed-valence phenolate-bridged Cu(I)Cu(II)complex, [(UN-O−)CuICuII]2+ (UN-O− = phenol-containing binu-cleating ligand), with O2 in a reversible manner, under cryo-genic conditions.166 The dinuclear Cu(II)-peroxo complex,[(UN-O−)CuII2 (O2

2−)]+, was produced by the reversible reactionof the dicopper(I) precursor complex [(UNO−)CuI2]

+ with O2.166

A standard reduction potential for the Cu(II)-superoxo/Cu(II)-peroxo pair was determined to be E° vs. SCE = +130 mV.166 Akinetic study using a stopped-flow technique revealed anouter-sphere ET process with a total reorganization energy (λ)of 1.1 eV between the superoxo and peroxo complexes, whichwas evaluated in light of the Marcus theory of ET.166

A pyrazolate-based μ-1,2-peroxo dicopper(II) complex under-goes clean one-electron oxidation at a low potential (−0.22 vs.SCE) to produce the μ-1,2-superoxo dicopper(II) complex,which was characterized by the O–O bond stretching vibrationfrequency at 1070 cm−1 with −59 cm−1 of Δν(18O2–16O2).167

The μ-1,2-superoxo dicopper(II) complex can abstract hydrogenatoms from weak X–H bonds such as TEMPO-H to produce thehydroperoxide complex.167

The formation of a mixed-valence Cu(II)–Cu(I)-superoxocomplex, [CuII(O2

•−)CuI]2+ (λmax = 685–740 nm), was madepossible upon femtosecond laser photoexcitation of an end-ontrans-μ-1,2-peroxodicopper(II) complex [(tmpa)2CuII2 (O2

2−)]2+

(λmax = 525 and 600 nm), followed by fast intramolecular elec-tron transfer to yield an “O2-caged” dicopper(I) adduct, CuI2–O2, and a barrierless stepwise back ET to regenerate[(tmpa)2CuII2 (O2

2−)]2+.168 Femtosecond laser excitation of side-on μ–η2:η2-peroxodicopper(II) complexes, [(N5)CuII2 (O2

2−)]2+ and[(N3)CuII2 (O2

2−)]2+, also resulted in the generation of [CuII(O2•−)

CuI]2+, but followed by O2 release to produce the CuI2complexes.168,169

Fig. 18 X-ray crystal structure of the binding site of O2 (red rod) to theCuB centre (green ball) in an end-on manner. Reprinted with permissionfrom ref. 151. Copyright 2004, the American Association for theAdvancement of Science.

Fig. 19 (Left panel) [CuII(O2•−)(L)]+ complexes without (A) and with

internal hydrogen-bonding substituents (B–D) used in this study. (Rightpanel) Resonance Raman spectra (λex = 413.1 nm) of complexes A–D(blue for [CuII(16O2

•−)(L)]+ and red for [CuII(18O2•−)(L)]+) in frozen

2-methyltetrahydrofuran. Reprinted with permission from ref. 164.Copyright 2018, American Chemical Society.



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11. Zinc(II)-superoxo complexes

The important role of Zn2+ ions in accelerating both the oxi-dation and reduction of O2

•− has been indicated by an imid-azolate-bridged CuII–ZnII heterodinuclear complex containinga dinucleating Hbdpi ligand (Hbdpi = 4,5-bis(di(2-pyridyl-methyl)aminomethyl)imidazole), which is well-characterizedas a SOD model.170 A large positive shift (0.21 V) in the one-electron reduction potential (Ered) of the copper(II) moiety ofthe CuII–ZnII complex is observed as compared to the corres-ponding mononuclear CuII complexes without Zn2+ ions.170

Such a positive shift of the Ered value of the CuII–ZnII complex

results from the electron-withdrawing effect of the imidazo-late-bound Zn2+ ion, which leads to a decrease in the electrondensity on the copper ion.170 Thus, an important role of theZn2+ ion in the imidazolate-bridged CuII–ZnII complex is toaccelerate an outer-sphere electron transfer from O2

•− toproduce the CuI–ZnII complex, when the free energy change ofelectron transfer becomes thermodynamically more favourableas compared to that without Zn2+ ions as shown in Scheme 16,where ET from the CuI–ZnII complex to O2

•− is also acceleratedby binding of O2

•− to the Zn2+ centre.170 The binding of O2•− to

the Zn2+ centre of [ZnII(MeIm(Me)2)]2+ was confirmed by the

reaction of the [ZnII(MeIm(Me)2)]2+ complex with the O2

•−

anion to produce [ZnII(O2•−)(MeIm(Me)2)]

2+ (Scheme 17),which was detected by the EPR spectrum.171

12. Conclusions

Superoxide anions (O2•−) bind with metal complexes of the

first row d-block elements to produce metal-superoxo com-plexes. Side-on (η2) or end-on (η1) binding of the superoxideligand to the metal centres has been observed depending onthe types of metals and ligands. Binding of metal ions to O2

•−

results in the enhancement of the radical reactivity of metal-superoxo complexes, which generally undergo hydrogen-atomabstraction reactions. An end-on Cr(III)-superoxo complexundergoes not only hydrogen-atom abstraction reactions butalso oxygen atom transfer reactions. The sulfoxidation of thio-anisole by a Cr(III)-superoxo complex is much enhanced in thepresence of HOTf by the PCET pathway from thioanisole to aCr(III)-superoxo complex. ET from one-electron donors to theCr(III)-superoxo complex is much enhanced by PCET. Hydrogenatom transfer from phenol derivatives to the Cr(III)-superoxocomplex is also much enhanced in the presence of HOTf bythe PCET pathway. The introduction of hydrogen bonding moi-eties into Cu(II)-superoxo complexes resulted in the enhance-ment of the hydrogen atom transfer (HAT) reaction probablyvia PCET processes. Such PCET processes may also be appliedto other metal-superoxo complexes although the effects ofacids on the reactivity of metal-superoxo complexes have yet tobe studied well. Not only PCET pathways but also metal ion-coupled electron transfer (MCET) pathways of other metalsuperoxo complexes to produce dinuclear metal-superoxo com-plexes may be exploited further to provide new insights intometal–oxygen chemistry.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

We are grateful to the collaborators and co-workers whosenames are presented in the references for their contributionsto the work described herein. Financial support for the workdescribed herein was provided by the JSPS KAKENHI (GrantNumbers 16H02268 to S. F.) from MEXT, Japan and by theNRF of Korea through CRI (NRF-2012R1A3A2048842 to W. N.),GRL (NRF-2010-00353 to W. N.), and the Basic ScienceResearch Program (2017R1D1A1B03029982 to Y. M. L. and2017R1D1A1B03032615 to S. F.).

Notes and references

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Scheme 16 Proposed catalytic cycle of the imidazolate-bridged CuII–ZnII heterodinuclear SOD model complex.170

Scheme 17 Reaction of [ZnII(MeIm(Me)2)(MeCN)]2+ with O2

•− toproduce [ZnII(O2

•−)(MeIm(Me)2)]2+ complex.171



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structure and reactivity of the first-row d-block metal...

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