& Electron Transfer

Lewis Acid Coupled Electron Transfer of Metal–OxygenIntermediates

Shunichi Fukuzumi,*[a, b, c] Kei Ohkubo,[a, b] Yong-Min Lee,[a] and Wonwoo Nam*[a]

Chem. Eur. J. 2015, 21, 17548 – 17559 Ó 2015 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim17548

MinireviewDOI: 10.1002/chem.201502693

Abstract: Redox-inactive metal ions and Brønsted acids that

function as Lewis acids play pivotal roles in modulating theredox reactivity of metal–oxygen intermediates, such as

metal–oxo and metal–peroxo complexes. The mechanismsof the oxidative C¢H bond cleavage of toluene derivatives,sulfoxidation of thioanisole derivatives, and epoxidation ofstyrene derivatives by mononuclear nonheme iron(IV)–oxo

complexes in the presence of triflic acid (HOTf) and Sc(OTf)3

have been unified as rate-determining electron transfer cou-pled with binding of Lewis acids (HOTf and Sc(OTf)3) by

iron(III)–oxo complexes. All logarithms of the observedsecond-order rate constants of Lewis acid-promoted oxida-

tive C¢H bond cleavage, sulfoxidation, and epoxidation reac-tions of iron(IV)–oxo complexes exhibit remarkably unified

correlations with the driving forces of proton-coupled elec-

tron transfer (PCET) and metal ion-coupled electron transfer

(MCET) in light of the Marcus theory of electron transfer

when the differences in the formation constants of precursorcomplexes were taken into account. The binding of HOTf

and Sc(OTf)3 to the metal–oxo moiety has been confirmedfor MnIV–oxo complexes. The enhancement of the electron-transfer reactivity of metal–oxo complexes by binding ofLewis acids increases with increasing the Lewis acidity of

redox-inactive metal ions. Metal ions can also bind to mono-

nuclear nonheme iron(III)–peroxo complexes, resulting in ac-celeration of the electron-transfer reduction but deceleration

of the electron-transfer oxidation. Such a control on the re-activity of metal–oxygen intermediates by binding of Lewis

acids provides valuable insight into the role of Ca2 + in theoxidation of water to dioxygen by the oxygen-evolving com-

plex in photosystem II.

1. Introduction

Electron transfer accounts for the most important elementary

reactions in both chemical and biological processes, such asrespiration and photosynthesis, in which dioxygen is reduced

to water and water is oxidized to dioxygen, respectively.[1–5]

Redox-active metal–oxygen intermediates, such as metal–oxo

and metal–peroxo complexes, act as key intermediates for thecatalytic reduction of dioxygen and the oxidation of water and

organic substrates.[6–18] For example, a MnV–oxo intermediate is

proposed to be involved in water oxidation at the oxygen-evolving complex (OEC) in photosystem II (PS II), and FeIV–oxo

complexes are the key intermediates in cyctochrome c oxidase,cytochrome P450 and nonheme iron enzymes.[19–23]

In the OEC, Ca2+ acts as an essential cofactor in the manga-nese–calcium–oxygen cluster (Mn4CaO5), which is responsible

for water oxidation in PS II.[24–30] Among redox-inactive metal

ions, Sr2 + is the only surrogate that can restore the activity ofOEC after Ca2 + removal.[31–35] The similar size and Lewis acidity

of the Ca2 + and Sr2 + ions may be responsible for their abilityto function in OEC. However, the exact functional role of Ca2 +

ion has yet to be clarified.Redox-inactive metal ions, such as Ca2 + and Zn2 + , play pivo-

tal roles in controlling biological electron-transfer processes,

not only in water oxidation to produce O2 in OEC, but also inthe dismutation of O2C¢ in copper–zinc superoxide dismutase

(CuZn-SOD).[36–38] Binding of metal ions to electron acceptors

(A) results in enhancement of an electron-acceptor ability of A,because the one-electron reduction potential of A is shifted in

a positive direction (DEred) by the strong binding of metal ionsto the radical anion (AC¢) to produce AC¢¢Mn + . Thus, electron

transfer from electron donors (D) to A, which is thermodynami-cally infeasible in the absence of metal ions (DGet>0), would

be made possible in the presence of Mn + , provided that the

FDEred value is larger than DGet (DGet<FDEred ; Scheme 1).[39–41]

This is defined as metal ion-coupled electron transfer(MCET).[40, 41] Similarly, protonation of AC¢ by Brønsted acids en-hances electron transfer from D to A by the positive shift of

the reduction potential of A. This is well known as proton-cou-pled electron transfer (PCET).[42–46]

This Minireview focuses on the quantitative effects of redox-inactive metal ions and Brønsted acids on redox reactions of

redox-active metal–oxygen intermediates. First, MCET reduc-

tion of O2 is quantitatively analyzed based on the EPR spectraof O2C¢¢Mn + complexes. Then, the shifts of the one-electron re-

duction potentials (DEred) of metal–oxo complexes by bindingof redox-inactive metal ions and Brønsted acids, both of which

act as Lewis acids, are quantitatively utilized to unify the de-pendence of the rate constants of various redox reactions of

Scheme 1. Metal ion-coupled electron transfer.

[a] Prof. Dr. S. Fukuzumi, Dr. K. Ohkubo, Dr. Y.-M. Lee, Prof. Dr. W. NamDepartment of Chemistry and Nano ScienceEwha Womans University, Seoul 120-750 (Korea)E-mail : wwnam@ewha.ac.kr

[b] Prof. Dr. S. Fukuzumi, Dr. K. OhkuboDepartment of Material and Life Science, Graduate School of Engineering,Osaka UniversityALCA and SENTAN, Japan Science and Technology Agency (JST)Suita, Osaka, 565-0871 (Japan)E-mail : fukuzumi@chem.eng.osaka-u.ac.jp

[c] Prof. Dr. S. FukuzumiFaculty of Science and Engineering, Meijo UniversityALCA and SENTAN, Japan Science and Technology Agency (JST)Aichi, Nagoya, 468-0073 (Japan)

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metal–oxo complexes on the driving force of Lewis acid-cou-pled electron transfer in light of the Marcus theory of electron

transfer.[47] The effects of redox-inactive metal ions on theredox properties and the oxidation and reduction reactions of

metal–peroxo complexes have been discussed as well. Thus,observation of the effects of redox-inactive metal ions on the

redox reactions of both metal–oxo and metal–peroxo com-plexes may provide valuable insights into the actual role of

Ca2 + in the OEC of PS II.

2. Binding of Redox-Inactive Metal Ions to O2C¢¢

Binding of redox-inactive metal ions to superoxide anion (O2C¢)

has been confirmed by EPR spectroscopy. The EPR spectrum ofO2C¢ in frozen media at 77 K shows anisotropic signals with

gzz = 2.072 and gxx = gyy = 2.009.[48, 49] In solution, the EPR spec-

trum of O2C¢ cannot be detected owing to fast relaxationcaused by the orbital degeneracy. Binding of redox-inactive

metal ions, such as Sc3+ , to O2C¢ results in the splitting of thedegenerate p* orbitals, enabling observaton of the EPR spec-

trum of the O2C¢¢Sc(HMPA)3 + complex (HMPA = hexamethyl-phosphoric triamide) in solution, as shown in Figure 1.[50, 51] O2C¢

was produced by photoinduced electron transfer from the di-

meric 1-benzyl-1,4-dihydronicotinamide [(BNA)2] , which can actas a unique two-electron donor,[52] to O2 enriched in 17O in ace-

tonitrile. The eight-line isotropic spectrum at the center due tothe superhyperfine coupling of O2C¢ with the 7:2 nuclear spin

of the scandium nucleus [coupling constant, a(Sc) = 3.82 G]clearly indicates the binding of Sc3 + to O2C¢ .[50] The O2C¢ com-

plex is relatively stable in the presence of HMPA and the EPR

spectrum can be detected even at 60 8C under light irradiation,although it disappeared completely in 20 min after cutting off

the light.[50] The two inequivalent a(17O) values (21 and 14 G;Figure 1) indicate “end-on” coordination rather than “side-on”

in the O2C¢¢Sc(HMPA)33+ complex in which the electron spin is

more localized at the terminal oxygen.[50] Without the HMPA

ligand, the O2C¢¢Sc3 + complex becomes unstable due to themuch faster disproportionation of the O2C¢¢Sc(HMPA)3

3+ com-

Shunichi Fukuzumi earned a Bachelor’sdegree and PhD. degree at Tokyo Institute ofTechnology in 1973 and 1978, respectively.After working as a postdoctoral fellow (1978–1981) at Indiana University in the USA, hejoined the Department of Applied Chemistryat Osaka University, as an Assistant Professorin 1981 and was promoted to Full Professorin 1994. His research interests are electrontransfer chemistry all fields of chemistry. He isnow the leader of an ALCA (Advanced CarbonTechnology Research and Development) proj-ect that started in 2011, a Professor Emeritusat Osaka University, a Professor at EwhaWomans University, and a Designated Professor at Meijo University.

Kei Ohkubo earned his Ph.D. degree fromGraduate School of Engineering, Osaka Uni-versity in 2001. He was working as a JSPSFellow and JST Research Fellow at Osaka Uni-versity (2001–2005) and a Designated Associ-ate Professor at Osaka University (2005–2014). He has been a Designated Professor atOsaka University since 2014 and a Special Ap-pointment Professor at Ewha Womans Univer-sity since 2015.

Yong-Min Lee received his Ph.D. degree in In-organic Chemistry from Pusan National Uni-versity, Korea, under the direction of ProfessorSung-Nak Choi in 1999. Then, he joined theMagnetic Resonance Center (CERM) at Univer-sity of Florence, Italy, as a Postdoctoral fellowand Researcher under the direction of Profes-sors Ivano Bertini and Claudio Luchinat(1999–2005). He is now a Special Appoint-ment Professor at Ewha Womans University.

Wonwoo Nam received his B.S. (Honors)degree in Chemistry from California State Uni-versity, Los Angeles, and his Ph.D. degree inInorganic Chemistry from UCLA under the di-rection of Professor Joan S. Valentine in 1990.After one year’s postdoctoral experience atUCLA, he became an Assistant Professor atHong Ik University in 1991. He moved toEwha Womans University in 1994, where he iscurrently a Distinguished Professor. His cur-rent research focuses on dioxygen activation,water oxidation, and sensors for metal ions inbioinorganic chemistry.

Figure 1. a) EPR spectrum of the O2C¢¢Sc(HMPA)33+ complex after irradiation

of an 17O (40 %) oxygen-saturated propionitrile solution containing (BNA)2,Sc(OTf)3, and HMPA with a high-pressure mercury lamp at room tempera-ture; b) computer-simulated spectrum with g = 2.0165, a(Sc) = 3.82 G,a1(17O) = 21 G, a2(17O) = 14 G, DHmsl = 3.5 G. Figure 1 was taken with permis-sion from reference [50].

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plex.[50] The a(Sc) value (4.26 G) without HMPA is larger thanthat with HMPA (3.82 G) because of the stronger interaction

between O2C¢ and Sc3 + without the HMPA ligand.[50] Thus, thestrength of the binding of Sc3+ to O2C¢ can be controlled by

the ligand of Sc3 + (see below).The clear eight-line isotropic ESR spectrum (g = 2.0218) was

also observed for the O2C¢¢Lu(HMPA)3 + complex due to the su-perhyperfine coupling of O2C¢ with the 7/2 nuclear spin of theLu nucleus [a(Lu) = 8.58 G].[50] The ESR spectrum of the O2C¢¢Y(HMPA)3+ complex (g = 2.0202) was also detected at roomtemperature, although the superhyperfine splitting due to the1:2 nuclear spin of the Y nucleus was not resolved.[50]

The gzz value of the EPR spectrum of the O2C¢¢Mn + complex

varies depending on the binding strength of Mn + to O2C¢ ,[53]

because the deviation of the gzz value from the free spin value

(ge = 2.0023) is caused by the spin–orbit coupling and the

energy splitting of pg levels due to the binding of Mn + to O2C¢

(DE), in accordance with Equation (1),[54]

DE ¼ 2 lsoc=ðgzz¢geÞ ð1Þ

where lsoc is the spin–orbit coupling constant (0.014 eV).[55] The

DE value, commonly in the range 0.3–1.0 eV, increases in theorder of monovalent cations (M+) <divalent cations (M2 +) <

trivalent cations (M3 +).[50] The DE value also increases with a de-

crease in the ionic radius when the oxidation state of themetal ion is the same. Scandium(III), with the smallest ionic

radius among the trivalent metal cations, provides the largestDE value, which indicates that Sc3 + ion is the strongest Lewis

acid. The calculated O¢O distance decreases in the order ofO2C¢ (1.343 æ) >O2C¢¢Li+ (1.309 æ) > O2C¢¢Mg2 + (1.297 æ) >

O2C¢¢Sc3 + (1.211 æ) as the DE value increases.[50] The DE value

can be used as a quantitative measure of the Lewis acidity ofnot only metal cations with different counter anions but also

neutral Lewis acids, such as organotin compounds.[56, 57] In thecase of Sc3 + , the Lewis acidity decreases in the order of

Sc(OTf)3> (TTP)ScCl (TPP2¢= dianion of tetraphenylporphyr-in)> (HMPA)3Sc(OTf)3. The fluorescence maxima (lmax) of com-

plexes with various Lewis acids can also be used as a quantita-

tive measure of Lewis acidity and there is a linear correlationbetween the DE values and the Dhnmax values of 10-methylacri-

done–Lewis acid complexes.[58]

Binding of Lewis acids to O2C¢ makes electron transfer from

(TPP)Co to O2 possible,[53, 59] and the second-order rate constantof Lewis acid-coupled electron transfer increased linearly withincreasing concentrations of Lewis acids.[53] The rates of reduc-

tion of O2 by MnII and FeII complexes were also reported to beenhanced by the presence of redox-inactive metal ions, which

act as Lewis acids.[60, 61] There is a remarkable linear correlationbetween the logarithm of the rate constant of Lewis acid-cou-

pled electron transfer (log ket) from (TPP)Co to O2 in MeCN and

DE of the O2C¢¢Lewis acid complexes (Figure 2).[56] The linearcorrelation spans a range of nearly 107 in the rate constant.

The slope of the linear correlation between log ket and DE isobtained as 14.0, which is close to the value of 1/(2.3 kBT) (=

16.9, where kB is the Boltzmann constant and T = 298 K).[56] Thisindicates that the variation of DE is well reflected in the differ-

ence in the activation free energy for the Lewis acid-coupled

electron transfer from (TPP)Co to O2. The stronger the bindingof Lewis acids with O2 is, the larger the promoting effects of

Lewis acids becomes. Thus, DE can be regarded as the binding

energy in O2C¢¢Lewis acid complexes.

3. Metal Ion-Coupled Electron Transfer ofMetal–Oxo Complexes

3.1. Iron(IV)–oxo complexes

The electron-transfer reduction of an iron(IV)–oxo complex,[(N4Py)FeIV(O)]2 + [N4Py = N,N-bis(2-pyridylmethyl)-N-bis(2-pyri-

dyl)methylamine], is accelerated by binding of redox-inactivemetal ions (Mn +) to the corresponding iron(III)–oxo complex,

as shown in the case of the electron-transfer reduction of O2 inFigure 3.[62] The one-electron reduction potential of [(N4Py)-

FeIV(O)]2 + in MeCN (Ered = 0.51 V vs. SCE)[63] was shifted to a posi-

tive direction by binding of Sc(OTf)3 to [(N4Py)FeIII(O)]+ .[62] TheEred values in the presence of Sc(OTf)3 were determined from

the equilibrium constants of electron transfer with variouselectron donors.[62] The Ered values increase linearly with in-creasing logarithm of concentration of Sc(OTf)3 (log [Sc3 +] ;Figure 3), where the slope is 118 mV/log [Sc3 +] .[62] This indicates

that the one-electron reduction of [(N4Py)FeIV(O)]2 + is accom-panied by binding of one or two Sc3 + ions to [(N4Py)FeIII(O)]+ ,in accordance with the Nernst equation [Equation (2)] ,

Ered ¼ E0redþ ð2:3 RT=FÞlogð1þ K 1½Sc3þ¤ þ K 1K 2½Sc3þ¤2Þ ð2Þ

where E0red is the one-electron reduction potential without

Sc3 + , and K1 and K2 are the formation constants of 1:1 and 1:2complexes of [(N4Py)FeIII(O)]+ with one Sc3 + and two Sc3 +

ions, respectively (Scheme 2).[62]

The observed second-order rate constant (kobs) of electrontransfer from electron donors to [(N4Py)FeIV(O)]2 + in the pres-

ence of Sc(OTf)3 in MeCN increased with a first-order depend-ence on [Sc3 +] at low concentrations, changing to a second-

Figure 2. Plot of log ket vs. DE in MCET from (TPP)Co to O2 in MeCN at 298 K.Triflate or perchlorate metal ion salts:*; organotin compounds and scandi-um complexes:*. Figure 2 was taken with permission from reference [56].

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order dependence at high concentrations, as given by Equa-

tion (3).[62]

kobs ¼ k1½Sc3þ¤ þ k2½Sc3þ¤2 ð3Þ

The k1 and k2 values were determined for MCET from ferroceneto [(N4Py)FeIV(O)]2 + in the presence of various metal ions.[62]

Both log k1 and log k2 exhibit good linear correlations with theDE values of metal ions [Equation (1)] , as shown in Figure 4.[62]

Thus, the stronger Lewis acidity of the metal ion is, the largerthe rate constants (k1 and k2) of MCET from ferrocene to

[(N4Py)FeIV(O)]2 + become.

The binding of a redox-inactive metal ion to a high-valentmetal–oxo complex was reported as the X-ray crystal structure

of Sc3 +-bound [(TMC)FeIV(O)]2 + (TMC = 1,4,8,11-tetramethyl-1,4,8,11-tetra-azacyclotetradecane),[64] which was initially formu-lated as the neutral [(TMC)(FeIVOScIII)(OTf)4(OH)] (Figure 5).[65]

However, density functional theory (DFT) calculations per-formed later suggested that the formulation was

[(TMC)FeIIIOScIII(OTf)4(OH2)] ,[66] which was supported by Mçssba-uer and X-ray absorption spectroscopies of the crystals as well

as electrospray ionization mass spectrometry and EPR spectros-

copy.[67] It may be suggested that the FeIV(O) complex is re-duced to the Sc3 +-bound FeIII(O) species during the crystalliza-

tion in the presence of ether. However, the source of one elec-tron for the reduction of the FeIV(O) complex to the FeIII(O)

complex has yet to be clarified. In any case, the strong bindingof Sc3 + ion to the iron–oxo moiety of the FeIII(O) complex re-

sults in enhancement of the two-electron reduction of the

FeIV(O) complex to the FeII complex.[65]

3.2. Manganese(IV)–oxo complexes

In contrast to the case of iron(IV)–oxo complexes, Sc(OTf)3

binds directly to manganese(IV)–oxo complexes, such as[(N4Py)MnIV(O)]2 + and [(Bn-TPEN)MnIV(O)]2 + (Bn-TPEN = N-

benzyl-N,N’,N’-tris(2-pyridylmethyl)-1,2-diaminoethane).[68.69] Ad-dition of up to two equivalents of Sc(OTf)3 to a solution of[(Bn-TPEN)MnIV(O)]2+ in a solvent mixture of CF3CH2OH/CH3CNresulted in a blue shift of the absorption band at lmax =

1020 nm to that at lmax = 740 nm with an approximate isosbes-tic point at 900 nm (Figure 6 a).[69] Further addition of Sc(OTf)3

resulted in an even larger blue-shift to give the absorption

band at lmax = 690 nm (Figure 6 b).[69] Such stepwise spectralchanges indicate the binding of one and two Sc3 + ions to

[(Bn-TPEN)MnIV(O)]2+ to produce MnIV(O)¢(Sc3+)1 and MnIV(O)¢(Sc3 +)2 complexes, respectively.[69] The formation constants of

the [(Bn-TPEN)MnIV(O)]2+¢(Sc3 +)1 and [(Bn-TPEN)¢MnIV(O)]2 +¢(Sc3 +)2 complexes were determined from the spectral titrationsto be 4.0 Õ 103 and 1.2 Õ 103 m¢1, respectively.[69] The formation

constant of [(N4Py)MnIV(O)]2+¢(Sc3 +)2 complex was also deter-mined to be 6.1 Õ 103 m¢1.[69]

The extended X-ray absorption fine structure (EXAFS) spec-troscopy results indicated that on going from no Sc3 + binding

Scheme 2. Metal ion-coupled electron-transfer reduction of [(N4Py)FeIV(O)]2 + .Scheme 2 was taken with permission from reference [62]. Figure 5. ORTEP representation of the Sc3 +-bound nonheme iron–oxo com-

plex, which was initially formulated as [(TMC)FeIV(O)¢Sc(OTf)4(OH)],[65] butlater corrected as [(TMC)FeIII(O)¢Sc(OTf)4(OH2)] .[67] Thermal ellipsoids are de-picted at the 30 % probability. Figure 5 was taken with permission from ref-erence [65].

Figure 4. Plots of log k1 (red circles) and log k2 (blue squares) of MCET fromferrocene to [(N4Py)FeIV(O)]2 + in the presence of various metal ions in MeCNat 298 K vs. DE (quantitative measure of Lewis acidity of metal ions). Figure4 was taken with permission from reference [62].

Figure 3. Dependence of Ered of [(N4Py)FeIV(O)]2 + in MeCN at 298 K onlog[Sc3 +] . The Ered values were derived from equilibrium constants (Ket) ofthe electron transfer from electron donors to [(N4Py)FeIV(O)]2 + in the pres-ence of Sc(OTf)3 in MeCN at 298 K. Blue circles, [RuII(4,4’-Me2bpy)3]2 + ; redsquares, [RuII(bpy)3]2 +). Figure 3 was taken with permission from reference[62].

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in [(Bn-TPEN)MnIV(O)]2 + to binding one and two Sc3 + ions, the

Mn=O bond elongates from 1.69 to 1.74(2) æ, which is consis-tent with a weakening of the Mn=O bond.[69] The EXAFS data

also revealed a short Mn¢Sc distance (3.45(10) æ), whichclearly indicates that Sc3 + ions bind to the MnIV(O) moiety in

both [(Bn-TPEN)MnIV(O)]2+¢(Sc3 +)1 and [(Bn-TPEN)MnIV(O)]2 +¢(Sc3 +)2.[69] DFT calculations of the MnIV(O)¢(Sc3 +)2 complex sug-gested that one Sc3+ ion binds directly to the Mn-O moiety of

the MnIV(O) complex but the second Sc3 + ion is located at thesecondary coordination sphere (Figure 7).[69]

The Ered values of MnIV(O) complexes, [(Bn-TPEN)MnIV(O)]2 +

and [(N4Py)MnIV(O)]2 + , in the presence of various concentra-

tions of Sc(OTf)3 were determined from the Ket values obtained

by the electron-transfer titrations with [RuII(bpy)3]2 + (Eox =

1.24 V vs. SCE) and [RuII(5-Cl-phen)3]2+ (5-Cl-phen = 5-chloro-

1,10-phenanthroline; Eox = 1.36 V vs. SCE).[69] The Ered values of[(Bn-TPEN)MnIV(O)]2 + and [(N4Py)MnIV(O)]2 + were shifted from

0.78 and 0.80 V vs. SCE[70] in the absence of Sc(OTf)3 to 1.28and 1.31 V vs. SCE in the presence of two equivalents (for Bn-TPEN) or one equivalent (for N4Py) of Sc(OTf)3, which approxi-

mately correspond to the Ered values of [(Bn-TPEN)MnIV(O)]2 +¢(Sc3 +)1 and [(N4Py)MnIV(O)]2 +¢(Sc3 +)1, respectively.[69] The Ered

values increase with increasing concentration of Sc(OTf)3 toreach constant values with more than six equivalents of

Sc(OTf)3 (Figure 8).[69] This is consistent with the UV/Vis spectral

change owing to formation of the [MnIV(O)]2 +¢(Sc3 +)2 complex.The saturated Ered values of 1.36 and 1.42 V vs. SCE in the pres-

ence of large excess Sc(OTf)3 correspond to those of [(Bn-TPEN)MnIV(O)]2 +¢(Sc3 +)2 and [(N4Py)MnIV(O)]2 +¢(Sc3 +)2, respec-

tively.[69] The large difference in the Ered values between

[MnIV(O)]2+ and [MnIV(O)]2 +¢Sc3 + , indicating that the bindingof Sc3 + to [MnIII(O)]+ is much stronger than that to [MnIV(O)]2 + ,

as expected from the increased basicity of the oxo moiety in[MnIII(O)]+ . The smaller difference in Ered between [MnIV(O)2 +]¢(Sc3 +)1 and [MnIV(O)]2 +¢(Sc3 +)2 results from the binding ofsecond Sc3+ ion at the secondary coordination sphere rather

at the oxo moiety (see below).[69]

3.3. Cobalt(IV)–oxo complexes

Although high-valent cobalt–oxo complexes remain elusive,

binding of redox-inactive metal ions has been shown to stabi-

lize a cobalt(IV)–oxo complex (Scheme 3).[71] The cobalt(III)complex, [(TAML)CoIII]¢ (TAML = tetraamido macrocyclic ligand),

is readily oxidized by cerium ammonium nitrate (CAN) in ace-tone to afford the [(TAML)CoIV(O)-Ce]2 + complex.[71] The reac-

tion of [(TAML)CoIII]¢ with iodosylbenzene (PhIO) in the pres-ence of Sc(OTf)3 also generated the [(TAML)CoIV(O)¢Sc]2+ com-plex, which is stabilized by binding of Sc3 + ion to the metal–oxo moiety.[71] Theoretical calculations predict that the binding

of Lewis acidic metal ions (Mn +) to the cobalt–oxo core in-creases the electrophilicity of the oxygen atom, resulting inthe redox tautomerism of a highly unstable [(TAML)CoIII(OC)]2¢

species to a more stable [(TAML)CoIV(O)(Mn +)] core.[71] The CoIV

oxidation state of [(TAML)CoIV(O)¢Sc]2 + was confirmed by EPR

and XAS measurements. It should be noted that the Co¢O dis-tance of 1.67 æ for [(TAML)CoIV(O)¢Sc]2 + determined by EXAFS

is significantly shorter than the Co¢O distances of 1.79 æ and

1.85 æ determined for [(a-P2W17O61CoIV)2O]14¢ by X-ray diffrac-tion[72] and [(TMG3tren)CoIV(O)(Sc)(OTf)3]]2 + (TMG3tren = tris[2-

(N-tetramethylguanidyl)ethyl]amine) by EXAFS.[73] The latterspecies was later suggested to contain a CoIII(OH) unit rather

than a CoIV(O) unit,[74] and further investigation is needed toclarify the nature of the Co¢O(H) moiety.

Figure 7. DFT-optimized structure of [(Bn-TPEN)MnIV(O)-[Sc(OTf)3]2]2+ , calcu-lated at the B3LYP/LACVP level. The Mn¢O bond lengths of [MnIV(O)]2+ ,[MnIV(O)]2 +¢Sc3 + , and [MnIV(O)]2 +¢(Sc3 +)2 were calculated to be 1.68, 1.75,and 1.75 æ, respectively. Figure 7 was taken with permission from reference[69].

Figure 8. Dependence of Ered of [(N4Py)MnIV(O)]2 + (blue circles) and [(Bn-TPEN)MnIV(O)]2 + (red circles) on [Sc3 +] in CF3CH2OH/CH3CN (1:1 v/v) at 273 K.The Ered values were determined from the equilibrium constants (Ket) be-tween one electron donors and [(N4Py)MnIV(O)]2 + . Figure 8 was taken withpermission from reference [69] .

Figure 6. a) UV/Vis spectral changes showing the conversion from [(Bn-TPEN)MnIV(O)]2 + (red line) to [(Bn-TPEN)MnIV(O)]2 +¢(Sc3 +)1 (green line) uponaddition of incremental amounts of Sc3 + (from 0.0 to 2.0 mm) in the titrationexperiment; b) UV/Vis spectral changes showing the conversion from [(Bn-TPEN)MnIV(O)]2 +¢(Sc3+)1 (green line) to [(Bn-TPEN)MnIV(O)]2 +¢(Sc3+)2 (blueline) upon addition of Sc3 + ions (from 0 to 12 mm). Figure 6 was taken withpermission from reference [69] .

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4. Proton-Coupled Electron Transfer of Metal–Oxo Complexes

As in the case of Sc(OTf)3 (see above), the Ered value of [(N4Py)-FeIV(O)]2 + was shifted in a positive direction by binding of triflic

acid (HOTf) to [(N4Py)FeIII(O)]+ .[75] The dependence of Ered of

[(N4Py)FeIV(O)]2 + on log [HOTf] and log [Sc(OTf)3] is shown inFigure 9, where the slopes are determined to be 113�6 mV

for HOTf and 118�8 mV for Sc(OTf)3, in agreement with Equa-tion (2) ;[75] either one or two HOTf molecules are bound to

[(N4Py)FeIII(O)]+ . The binding of HOTf molecules to [(N4Py)-FeIII(O)]+ is stronger than that of Sc(OTf)3 molecules, resulting

in the more positive Ered values of [(N4Py)FeIV(O)]2 + in the pres-

ence of HOTf than those in the presence of the same concen-tration of Sc(OTf)3 (Figure 9).[75]

When HOTf was replaced by the deuterated acid (DOTf), theket value of PCET from [RuII(bpy)3]2 + to [(N4Py)FeIV(O)]2 + with

DOTf in MeCN at 298 K was larger than that with the sameconcentration of HOTf (Figure 10).[75] Both the ket values with

DOTf and HOTf increased with increasing concentration of

DOTf and HOTf, in accordance with Equation (3), where [Sc3+]is replaced by [DOTf] and [HOTf] .[75] The kinetic isotope effect

(KIE) value (kobs(HOTf)/kobs(DOTf)) was determined to be 0.25�0.05 at HOTf (DOTf) = 2.5 mm, which increases to a constant

value of 0.40�0.04 at higher concentrations of HOTf (DOTf).[75]

Such inverse KIEs indicate that binding of protons to the Fe–

oxo group to produce an O¢H (or O¢D) bond is involved in

the rate-determining step of PCET from [RuII(bpy)3]2 + to

[(N4Py)FeIV(O)]2 + , because an in-verse KIE results from a larger

zero-point energy difference be-tween the O¢H and O¢D bonds

in the transition state relative tothe ground state.[76, 77] Thus, the

O¢H bonds in the reduced spe-cies (e.g. , [(N4Py)FeIII(OH)]2 + and

[(N4Py)FeIII(OH2)]3 +) are much stronger than those in [(N4Py)-

FeIV(OH)]3+ and [(N4Py)FeIV(OH2)]4 + , respectively. The inverseKIEs clearly indicate that electron transfer is coupled with the

binding of one and two HOTf molecules to [(N4Py)FeIII(O)]+ .In contrast to [(N4Py)FeIV(O)]2+ , two HOTf molecules are direct-

ly bound to [(N4Py)MnIV(O)]2+ to produce [(N4Py)MnIV(O)]2+¢(HOTf)2, as evidenced by UV/Vis spectra (Figure 11), where the

absorption change at 550 nm, due to [(N4Py)MnIV(O)]2 +¢(HOTf)2, exhibits a sigmoidal curvature (inset of Figure 11).[78]

The EXAFS data of [(N4Py)MnIV(O)]2+¢(HOTf)2 indicate an elon-

gated Mn¢O bond of 1.74 æ as compared with 1.67 æ for theMn¢O bond in [(N4Py)MnIV(O)]2+ .[78] The Mn¢O bond length in[(N4Py)MnIV(O)]2 +¢(HOTf)2 is nearly identical to that in the[(N4Py)MnIV(O)]2 + complex binding two Sc(OTf)3 molecules.[68, 69]

The Ered value of [(N4Py)MnIV(O)]2 + increased with increasingconcentration of HOTf to reach a constant value of 1.65 V vs.SCE at more than 30 mm of HOTf at 273 K (Figure 12).[78] The

saturated Ered value of 1.65 V vs. SCE in the presence of a largeexcess of HOTf corresponds to that of [(N4Py)MnIV(O)]2 +¢(HOTf)2. The large difference (DEred = 0.85 V) in the Ered valuesbetween [MnIV(O)]2 + and [MnIV(O)]2+¢(HOTf)2 indicates that the

binding of HOTf to [MnIII(O)]+ is much stronger than that to

[MnIV(O)]2+ , as expected from the increased basicity of the oxomoiety in [MnIII(O)]+

.[78] Thus, the HOTf-bound MnIV(O) complex

acts as the strongest oxidant among the nonheme Mn–oxocomplexes bearing the same ligand system.Figure 9. Dependence of Ered of [(N4Py)FeIV(O)]2 + on log [HOTf] (red circles)

and log [Sc(OTf)3] (black circles) in deaerated MeCN at 298 K. Black and redlines are fitted by Equation (2), where Sc3 + is replaced by HOTf. Figure 9 wastaken with permission from reference [75].

Figure 10. Plots of kobs vs. [HOTf] (black circles) and [DOTf] (red circles) foroxidation of toluene by [(N4Py)FeIV(O)]2 + in the presence of HOTf and DOTfin MeCN at 298 K, respectively. Figure 10 was taken with permission fromreference [75].

Scheme 3. Oxidation of a cobalt(III) complex to a cobalt(IV)–oxo-metal ion complexes by CAN/H2O or PhIO/Mn + .Scheme 3 was taken with permission from reference [71] .

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5. Unified Mechanism of Lewis Acid-PromotedOxidation of Substrates by Metal–Oxo Com-plexes

Not only the rates of electron-transfer reactions of [(N4Py)-

FeIV(O)]2 + but also the rates of the oxidative C¢H bond cleav-age of toluene derivatives, benzyl alcohol derivatives, sulfoxi-dation of thioanisole derivatives, and epoxidation of styrene

derivatives with [(N4Py)FeIV(O)]2 + were remarkably enhancedby the presence of perchloric acid (HClO4), HOTf, or Sc(OTf)3 in

acetonitrile at 298 K.[75, 79–85] The enhancement of electron-trans-fer reactions is ascribed to the positive shifts of Ered values of

[(N4Py)FeIV(O)]2 + in the presence of Sc(OTf)3 and HOTf

(Figure 9).[75] The driving-force dependence of log ket of elec-tron transfer from coordinatively saturated metal complexes

(electron donors) to [(N4Py)FeIV(O)]2 + in the absence and pres-ence of 10 mm of HOTf (black line in Figure 13) is well fitted

by the Marcus equation of outer-sphere electron transfer[Equation (4)] ,[47]

ket ¼ Zexp½¢ðl=4Þð1þ DGet=lÞ2=kBT ¤ ð4Þ

where Z is the frequency factor, which is kBTK/h (kB is the Boltz-

mann constant, T is absolute temperature, K is the formationconstant of the precursor complex of electron transfer, and h is

the Planck constant), using the same value of reorganization

energy of electron transfer (l= 2.74 eV). The Z value of outer-sphere electron-transfer reactions is taken as 1.0 Õ

1011 m¢1 s¢1.[86, 87] The log kobs values of oxidation of toluene andthioanisole derivatives by [(N4Py)FeIV(O)]2 + in the absence of

an acid (Figure 13; nos. 1–12, black) are much larger thanthose predicted by Equation (4) with l= 2.74 eV (black line in

Figure 13), because the oxidation of toluene and thioanisole

derivatives by [(N4Py)FeIV(O)]2 + in the absence of an acid pro-ceeds by hydrogen atom transfer (HAT) and oxygen atom

transfer (OAT), which require significant interactions in thetransition state as compared with outer-sphere electron-trans-

fer processes.[75] In contrast to the results without acid, log kobs

values of oxidation of toluene and thioanisole derivatives by

[(N4Py)FeIV(O)]2 + in the presence of HOTf and Sc(OTf)3 are par-

allel with those of outer-sphere electron-transfer reactions of[(N4Py)FeIV(O)]2 + .[75] Such parallel relations suggest that the oxi-

dation of toluene and thioanisole derivatives by [(N4Py)-FeIV(O)]2 + in the presence of HOTf and Sc(OTf)3 proceeds by

the rate-determining electron-transfer steps.[75] The larger log

Figure 12. Dependence of Ered of [(N4Py)MnIV(O)]2 + on the concentration ofHOTf (0–80 mm) in CF3CH2OH/CH3CN (1:1 v/v) at 273 K. Figure 12 was takenwith permission from reference [78].

Figure 11. UV/Vis spectral changes observed in the titration of [(N4Py)MnIV-(O)]2 + (black bold line) with HOTf. HOTf (0–20 mm) was added incrementallyinto the solution of [(N4Py)MnIV(O)]2 + (0.50 mm) in CF3CH2OH/CH3CN (1:1 v/v) at 273 K. Inset shows the spectral titration monitored at 550 nm (blue cir-cles) due to the formation of [(N4Py)MnIV(O)]2+¢(HOTf)2. Figure 11 was takenwith permission from reference [78].

Figure 13. Plots of log kobs for oxidation of toluene and thioanisole deriva-tives (1. hexamethylbenzene; 2. 1,2,3,4,5-pentamethylbenzene; 3. 1,2,4,5-tet-ramethylbenzene; 4. 1,2,4-trimethylbenzene; 5. 1,4-dimethylbenzene;6. 1,3,5-trimethylbenzene; 7. toluene; 8. p-Me-thioanisole; 9. thioanisole;10. p-Cl-thioanisole; 11. p-Br-thioanisole; 12. p-CN-thioanisole) by [(N4Py)-FeIV(O)]2 + in the absence (black) and presence of 10 mm HOTF (red) andSc(OTf)3 (blue) in MeCN at 298 K vs. the driving force of electron transfer[¢DGet = e(Ered¢Eox)] from toluene derivatives (squares) and thioanisole deriv-atives (triangles) to [(N4Py)FeIV(O)]2 + in the presence of HOTf (red) andSc(OTf)3 (blue). The red and blue circles show the driving-force dependenceof the rate constants (log ket) of electron transfer from electron donors(13. [FeII(Ph2phen)3]2+ ; 14. [FeII(bpy)3]2 + ; 15. [RuII(4,4’-Me2phen)3]2 + ;16. [RuII(5,5’-Me2phen)3]2 + ; 17. [FeII(Clphen)3]2+ ; 18. [RuII(bpy)3]2 +) to [(N4Py)-FeIV(O)]2 + in the presence of HOTf (10 mm) and Sc(OTf)3 (10 mm) in MeCN at298 K. The black line is drawn using Equation (4) with l = 2.74 eV. The blackcircles show the driving-force dependence of the rate constants (log ket) ofelectron transfer from electron donors (19. decamethylferrocene; 20. octa-methylferrocene; 21. 1.1’-dimethylferrocene; 22. n-amylferrocene and(23. ferrocene) to [(N4Py)FeIV(O)]2+ in the absence of acids in MeCN at 298 K.Figure 13 was taken with permission from reference [75].

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kobs values than the log ket values of outer-sphere electron-transfer reactions result from the larger K values of precursor

complexes formed prior to electron transfer, because the stron-ger interaction with toluene and thioanisole derivatives is ex-

pected as compared with that with coordinatively saturatedmetal complexes. This was confirmed by the driving-force de-

pendence of logarithm of the first-order rate constants (log kET)

of electron transfer in the precursor complexes, which weredetermined from the dependence of the pseudo-first-order

rate constants on concentrations of substrates (Figure 14).[75]

The unified correlations of driving-force dependence oflog kET of oxidation of all kinds of substrates by [(N4Py)-FeIV(O)]2 + (Figure 14) indicate that oxidations of toluene and

thioanisole derivatives by [(N4Py)FeIV(O)]2 + in the presence ofHOTf and Sc(OTf)3 proceeds via rate-determining PCET and

MCET from substrates to [(N4Py)FeIV(O)]2 + , following formation

of the precursor complexes as shown in Scheme 4, providedthat the free energy change of electron transfer is smaller than

0.5 eV.[75] When the free energy change of electron transfer islarger than 0.5 eV, HAT and OAT processes become predomi-

nant.[75]

The rate-determining PCET and MCET of toluene derivatives

are followed by rapid proton transfer and oxygen rebound to

produce the corresponding benzyl alcohol derivatives and[(N4Py)FeII]2 + , which is oxidized by [(N4Py)FeIV(O)]2 + to [(N4Py)-

FeIII]3 + in the presence of HOTf and Sc(OTf)3.[75] In the case of

oxidation of thioanisole derivatives and styrene derivatives by

[(N4Py)FeIV(O)]2 + in the presence of HOTf and Sc(OTf)3, thePCET and MCET are followed by rapid OC¢ transfer to produce

the corresponding sulfoxide and epoxide derivatives and[(N4Py)FeII]2 + (Scheme 4), which is also oxidized by [(N4Py)-

FeIV(O)]2 + to [(N4Py)FeIII]2 + in the presence of HOTf andSc(OTf)3.[75, 83] In the case of [(N4Py)FeIII]3 + , H2O is likely coordi-

nated to FeIII.

In the case of oxidation of N,N-dimethylaniline (DMA) by[(N4Py)FeIV(O)]2 + , competition between DMA and [(N4Py)-

FeIV(O)]2 + for the binding of Sc3 + ion results in overall en-hancement of electron transfer from DMA to [(N4Py)FeIV(O)]2 + ,

leading to the formation of DMA dimer radical cation (TMBC+ ,tetramethylbenzidine radical cation),[88] in contrast to demethy-

lation in the absence of an acid.[89–91]

6. Lewis Acid-Coupled Electron Transfer ofMetal–Peroxo Complexes

The reaction of [(TMC)FeII]2 + with in situ-formed O2C¢¢Mn + spe-

cies affords the iron(III)–peroxo-metal ion complex([(TMC)FeIII(O2)]+¢Mn +).[92, 93] The [(TMC)FeIII(m,h2 :h2-O2)]+¢Mn +

complexes were also synthesized by the reaction of[(TMC)FeIII(O2)]+ with redox-inactive metal ions.[92, 93] The ab-

sorption maxima due to the ligand to metal charge transfer(LMCT) transition of [(TMC)FeIII(O2)]+¢Mn + vary depending onthe Lewis acidity of metal ions (Figure 15) and the hnmax valueincreased linearly with increasing Lewis acidity of Mn + (DE),

which was determined from the gzz values of O2C¢¢Mn + com-plexes [Equation (1)] as shown in Figure 16.[93] It is of interestto note that the hnmax value of [(TMC)FeIII(O2)]+¢Ca2 + is similar

to that of [(TMC)FeIII(O2)]+¢Sr2 + .[93] The EXAFS data, togetherwith DFT calculations, indicate that the optimized Fe¢M2 + dis-

tances and the Fe-O-M2 + angles in [(TMC)FeIII(O2)]+¢Ca2+ and[(TMC)FeIII(O2)]+¢Sr2 + are 4.11 æ and 4.34 æ and 1408 and 1428,

respectively.[93]

The electron-transfer reduction of [(TMC)FeIII(O2)]+¢Mn + toundergo the O¢O bond cleavage was found to be accelerated

by increasing the Lewis acidity of Mn + , but this led to decelera-tion of the electron-transfer oxidation of [(TMC)FeIII(O2)]+¢Mn +

to release O2.[93] Thus, it was proposed that, in addition to therole(s) of the Ca2+ (or Sr2 +) ion in facilitating the O¢O bond

Figure 14. Plots of log kET for C¢H bond cleavage of toluene derivatives andsulfoxidation of thioanisole derivatives (1. hexamethylbenzene; 2. 1,2,3,4,5-pentamethylbenzene; 3. 1,2,4,5-tetramethylbenzene; 4. 1,2,4-trimethylben-zene; 5. 1,4-dimethylbenzene; 6. 1,3,5-trimethylbenzene; 7. toluene; 8. p-Me-thioanisole; 9. thioanisole; 10. p-Cl-thioanisole; 11. p-Br-thioanisole; 12. p-CN-thioanisole) by [(N4Py)FeIV(O)]2 + in the absence (black) and presence ofacids (10 mm), HOTf (red) and Sc(OTf)3 (blue), in MeCN at 298 K vs. the driv-ing force of electron transfer [¢DGet = e(Ered¢Eox)] from toluene derivatives(squares) and thioanisole derivatives (triangles) to [(N4Py)FeIV(O)]2+ in thepresence of HOTf (red) and Sc(OTf)3 (blue). The red and blue circles showthe driving-force dependence of the rate constants (log ket) of electron trans-fer from electron donors (13. [FeII(Ph2phen)3]2 + ; 14. [FeII(bpy)3]2 + ;15. [RuII(4,4’-Me2phen)3]2 + ; 16. [RuII(5,5’-Me2phen)3]2 + ; 17. [FeII(Clphen)3]2 + ;18. [RuII(bpy)3]2 +) to [(N4Py)FeIV(O)]2 + in the presence of acids (10 mm), HOTfand Sc(OTf)3, in MeCN at 298 K, respectively. The black circles show the driv-ing-force dependence of the rate constants (log ket) of electron transfer fromelectron donors (19. decamethylferrocene; 20. octamethylferrocene; 21. 1.1’-dimethylferrocene; 22. n-amylferrocene; 23. ferrocene) to [(N4Py)FeIV(O)]2+ inthe absence of acids in MeCN at 298 K. Figure 14 was taken with permissionfrom reference [75].

Scheme 4. Unified mechanism of oxidation of toluene and thioanisole deriv-atives by [(N4Py)FeIV(O)]2 + in the presence of HOTf and Sc(OTf)3. Scheme 4was taken with permission from reference [75] .

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making process in the water oxidation reaction, another possi-ble role of the Ca2 + (or Sr2 +) ion is to release O2 by controlling

the redox potential of the manganese–peroxo (or –hydroper-oxo) species in the OEC.[93] The stronger Lewis acid that bindsto the MnV(O) complex in OEC may facilitate the O¢O bond

formation with water but decelerate the oxidation of the re-sulting MnIII–peroxo (or –hydroperoxo) complex.[93] This hy-

pothesis, derived from the study on [(TMC)FeIII(O2)]+¢Mn + (seeabove), may explain why nature does not choose a stronger

Lewis acidic redox-inactive metal ion, such as Zn2 + , but insteadchooses Ca2 + in the oxidation of water to evolve O2 and why

Sr2 + , which has virtually the same Lewis acidity as Ca2 + , is theonly surrogate that can replace Ca2+ for the reactivity of theOEC in PS II.[94]

In another reaction, [(TMC)FeIII(O2)]+¢Sc3 + was suggested asan intermediate for Sc3 + ion-promoted O2 activation by

[(TMC)FeII]2 + with BPh4¢ to produce [(TMC)FeIV(O)]2 + .[95] It was

proposed that electron transfer from [(TMC)FeII]2+ to O2 was

followed by Sc3+ ion-promoted electron transfer from BPh4¢ to

the superoxo complex ([(TMC)FeIII(O2)]2 +) to produce[(TMC)FeIII(O2)]+¢Sc3 + , which was further reduced to yield

[(TMC)FeIV(O)]2 + via O¢O bond cleavage.[95] Sc3+ could be re-placed by Brønsted acids for the formation of [(TMC)FeIV(O)]2 +

from [(TMC)FeII]2 + with O2 and BPh4¢ .[96] However, the actual

mechanism was later clarified to be an autocatalytic radical

chain reaction (Scheme 5) rather than direct O2 activation by

[(TMC)FeII]2 + .[97] The initiation is started by Sc3 + ion-coupledelectron transfer from BPh4

¢ to [(TMC)FeIV(O)]2+ to produce

phenyl radical (PhC). The chain propagation step is composed

of the addition of O2 to PhC and Sc3 + ion-coupled electrontransfer from BPh4

¢ to the resulting phenylperoxyl radical

(PhOOC) to produce the PhOO¢¢Sc3 + complex, accompaniedby regeneration of PhC (propagation step). The PhOO¢¢Sc3 +

complex reacts with [(TMC)FeII]2 + to produce [(TMC)FeIV(O)]2 +

and PhOH after the reaction with residual water.[97] Such radical

chain autoxidation reactions of iron(II) complexes by O2 have

also been reported for formation of iron(IV)–oxo complexesfrom the reactions of iron(II) complexes with hydrogen sources

and O2.[98, 99]

Sc3 + ion has also been reported to bind to copper(II)–

peroxo complexes in the Sc3+ ion-promoted catalytic two-elec-tron reduction of O2 by decamethylferrocene (Fc*).[100] The

Sc3 +-bound peroxo complex, [(tmpa)CuII(O2)¢Sc(OTf)3] , (tmpa =

tris(2-pyridylmethyl)amine) was formed by addition of oneequivalent of Sc(OTf)3 to an acetone solution of a trans-m-1,2-

peroxodicopper complex, [(tmpa)CuII(O2)CuII(tmpa)]2 + .[100] WhenSc(OTf)3 was replaced by HOTf, [(tmpa)CuII(OOH)]+ was further

reduced by the PCET reduction, acting as an intermediate forthe catalytic four-electron reduction of O2 by Fc*.[101, 102]

7. Conclusion

Redox-inactive metal ions and Brønsted acids, which act asLewis acids, bind to O2C¢ , metal–oxo and metal–peroxo com-plexes, thereby tuning the redox reactivity of the metal–oxygen intermediates. The quantitative measure of the Lewis

acidity of metal ions with different counter anions and ligandsis obtained from the gzz values of EPR spectra of O2C¢-metal ioncomplexes. The driving-force dependence of rate constants of

proton-coupled electron transfer (PCET) and metal ion-coupledelectron transfer (MCET) of metal–oxo complexes has been

evaluated quantitatively in light of the Marcus theory of elec-tron transfer, enabling the unification of the mechanisms of

Lewis acid-promoted oxidation of substrates by metal–oxo

complexes. Metal ions can also bind to iron(III)–peroxo com-plexes to accelerate the electron-transfer reduction but decel-

erate the electron-transfer oxidation. Such reactivity control ofmetal–oxygen intermediates by binding of Lewis acids pro-

vides valuable mechanistic insights into the actual role of Ca2 +

ion on water oxidation at the OEC in PS II.

Figure 16. Plot of hnmax of [(TMC)FeIII(O2)]+¢Mn + complexes vs. Lewis acidityof metal ions (DE). Figure 16 was adapt with permission from reference [93].

Figure 15. UV/Vis spectra of [(TMC)FeIII(O2)]¢Mn + complexes [Mn + = Sc3+

(pink), Y3 + (cyan), Lu3 + (green), Zn2 + (blue), Ca2 + (red), Sr2 + (black)] obtainedin the reaction of the isolated [(TMC)FeIII(O2)](OTf) (0.50 mm) with redox-inac-tive metal ions (Mn +) in MeCN at ¢20 8C: lmax = 535 nm (Sc3+), lmax = 560 nm(Y3+), lmax = 570 nm (Lu3 +), lmax = 650 nm (Zn2 +), lmax = 670 nm (Ca2 +) andlmax = 710 nm (Sr2 +). Figure 15 was taken with permission from reference[93].

Scheme 5. Sc3 +-coupled electron-transfer chain reactions for formation of[(TMC)FeIV(O)]2 + in reaction of [(TMC)FeII]2+ with O2 and BPh4

¢ . Scheme 5was adapted with permission from reference [97].

Chem. Eur. J. 2015, 21, 17548 – 17559 www.chemeurj.org Ó 2015 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim17557

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Acknowledgements

This work was supported by Grants-in-Aid (Nos. 26620154 and

26288037 to K.O.) from the Ministry of Education, Culture,Sports, Science and Technology (MEXT), by an ALCA project from

JST, Japan (to S.F.) and by the NRF of Korea through CRI (NRF-

2012R1A3A2048842 to W.N.) and GRL (NRF-2010-00353 to W.N.).

Keywords: electron transfer · Lewis acids · metal–oxo

complexes · metal–peroxo complexes · superoxides

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