Tuning Electron-Transfer Reactivity of a Chromium(III)−SuperoxoComplex Enabled by Calcium Ion and Other Redox-Inactive MetalIonsTarali Devi,† Yong-Min Lee,† Wonwoo Nam,*,†,‡ and Shunichi Fukuzumi*,†,§

†Department of Chemistry and Nano Science, Ewha Womans University, Seoul 03760, Korea‡School of Chemistry and Chemical Engineering, Shaanxi Normal University, Xi’an 710119, P. R. China§Faculty of Science and Engineering, Meijo University, Nagoya, Aichi 468-8502, Japan

*S Supporting Information

ABSTRACT: Calcium ion plays an indispensable role for wateroxidation by oxygen-evolving complex (OEC) composed of amanganese−oxo cluster (Mn4CaO5) in Photosystem II. In thiscontext, the effects of Ca2+ ion and other redox-inactive metal ionson the redox reactivity of high-valent metal−oxo and metal−peroxocomplexes have been studied extensively. Among metal−oxygenintermediates involved in interconversion between H2O and O2,however, the effects of Ca2+ ion and other redox-inactive metal ions(Mn+) on the redox reactivity of metal−superoxo complexes haveyet to be reported. Herein, we report that electron transfer (ET)from octamethylferrocene (Me8Fc) to a mononuclear nonhemeCr(III)−superoxo complex, [(Cl)(TMC)CrIII(O2)]

+ (1), occurs in the presence of redox-inactive metal ions (Mn+ = Ca2+,Mg2+, Y3+, Al3+, and Sc3+); in the absence of the redox-inactive metal ions, ET from Me8Fc to 1 does not occur. The second-order rate constants (ket) of ET from Me8Fc to 1 in the presence of a redox-inactive metal ion increased with increasingconcentration of Mn+ ([Mn+]), exhibiting a second-order dependence on [Mn+]: ket = kMCET[M

n+]2, where kMCET is the fourth-order rate constant of metal ion-coupled electron transfer (MCET). This means that two Mn+ ions are bound to the one-electron reduced species of 1. Such a binding of two Mn+ ions associated with the ET reduction of 1 resulted in a 92 mV positiveshift of the one-electron reduction potential of 1 (Ered) with increasing log([M

n+]). The log kMCET values increased linearly withthe increasing Lewis acidity of Mn+ (ΔE), which was determined from the g values of O2

•−−Mn+ complexes. The driving forcedependence of log ket of MCET from ferrocene derivatives to 1 in the presence of Mn+ has been well-evaluated in light of theMarcus theory of electron transfer.

■ INTRODUCTION

Redox-inactive metal ions that function as Lewis acids areessential cofactors in modulating the reactivity of metal−oxygen complexes and metalloenzymes, such as the oxygen-evolving complex (OEC) in Photosystem II, where Ca2+ ion inthe Mn4CaO5 cluster is indispensable for water oxidation.1−5

In the catalytic water oxidation in the OEC as well as in thereverse reaction, which is the four-electron reduction of O2 inrespiration, metal−oxygen complexes, such as metal−oxo,metal−peroxo, and metal−superoxo species, are involved asreactive intermediates.6−14 The effects of redox-inactive metalions, including Ca2+ ion, on the redox reactivity of high-valentmetal−oxo species, such as Fe(IV)−oxo and Mn(IV)−oxocomplexes, have been studied extensively.15−23 Similarly,calcium ion and other redox-inactive metal ions are shownto modulate the redox reactivity of metal−peroxo complexes,such as Fe(III)−peroxo complexes.24−26

Recently, metal−superoxo complexes have attracted increas-ing attention because metal−superoxo complexes have beeninvoked as reactive intermediates in C−H bond activation and

oxygen atom transfer reactions by nonheme iron (e.g.,isopenicillin N synthase, myo-inositol oxygenase, and cysteinedioxygenase) and copper (e.g., dopamine β-monooxygenaseand peptidylglycine-α-amidating monooxygenase) containingenzymes.27−30 Thus, a number of metal−superoxo complexeshave been successfully synthesized and characterized structur-ally and/or spectroscopically in biomimetic studies, and theirreactivities have been explored in a variety of oxidationreactions.31−44 However, the effects of redox-inactive metalions on the redox reactivity of metal−superoxo species haveyet to be explored.45,46 Thus, the role(s) of redox-inactivemetal ions in modulating the redox reactivity of metal−superoxo complexes remains elusive.We report herein a pivotal role of a series of redox-inactive

metal ions with different Lewis acidities, including Ca2+ ion, onthe redox reactivity of a mononuclear nonheme Cr(III)−superoxo complex, [(Cl)(TMC)CrIII(O2)]

+ (1, TMC =

Received: October 13, 2019Published: December 4, 2019

Article

pubs.acs.org/JACSCite This: J. Am. Chem. Soc. 2020, 142, 365−372

© 2019 American Chemical Society 365 DOI: 10.1021/jacs.9b11014J. Am. Chem. Soc. 2020, 142, 365−372

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1,4,8,11-tetramethyl-1,4,8,11-tetraazacyclotetradecane), towardelectron-transfer (ET) reactions using ferrocene derivatives,such as octamethylferrocene (Me8Fc), as electron donors; suchan ET assisted by redox-inactive metal ions is referred to asmetal ion-coupled electron transfer (MCET).47,48 Themechanism of MCET from Me8Fc to 1 in the presence ofLewis acid metal ions has been clarified based on kinetic andthermodynamic investigations. The effect of redox-inactivemetal ions on the reaction rates of MCET from electrondonors to 1 is well-analyzed in light of the Marcus theory ofelectron transfer. The logarithm of the rate constants of MCETwith different metal ions is linearly correlated with aquantitative measure of the Lewis acidity of metal ions. Tothe best of our knowledge, this study reports for the first timethe fundamental MCET properties of metal−superoxocomplexes, providing an excellent opportunity to comparethe MCET properties of metal−oxygen intermediates,including metal−oxo and metal−peroxo complexes reportedpreviously.15−26

■ RESULTS AND DISCUSSIONEffects of Redox-Inactive Metal Ions (Mn+) on MCET

from Me8Fc to a Cr(III)−Superoxo Complex. TheCr(III)−superoxo complex, [(Cl)(TMC)CrIII(O2)]

+ (1), wasprepared and characterized according to the reported literatureprocedures.49−51 Interestingly, the absorption spectrum of 1remains the same upon addition of redox-inactive metal ions(Mn+: Ca2+, Mg2+, Y3+, Al3+, and Sc3+; Lewis acidity (ΔE) ofthe redox-inactive metal ions is in the order of Ca2+ < Mg2+ <Y3+ < Al3+ < Sc3+)19,52,53 to the solution of 1 in acetonitrile(MeCN) at 233 K (Supporting Information, Figure S1),indicating no binding of the redox-inactive metal ions to thesuperoxo moiety of the Cr(III)−superoxo complex. Notably,ET from a one-electron donor (Me8Fc; Eox vs saturatedcalomel electrode (SCE) = −0.04 V) to 1 occurs in thepresence of redox-inactive metal ions, such as Ca2+ ion, inMeCN at 233 K (Figure 1a), which leads to the generation ofMe8Fc

+ at λmax = 750 nm; no reaction occurs upon addition ofMe8Fc to the solution of 1 in the absence of the redox-inactivemetal ions.When Ca2+ ion was replaced by Sc3+ ion, which is the

strongest Lewis acid metal ion (i.e., Mn+ (ΔE): Ca2+ (0.58 eV)≪ Sc3+ (1.0 eV); ΔE is the quantitative measure of the Lewisacidity of metal ions, which was determined from the g valuesof O2

•−−Mn+ complexes),52,53 remarkable enhancement of therate of ET from Me8Fc to 1 was observed in the presence ofSc3+ ion (Figure 1b).The kinetic measurements for the determination of the

second-order rate constant (ket) of MCET reaction wasperformed under the second-order reaction conditions withrespect to 1 and Me8Fc in the presence of Ca2+ ion in MeCNat 233 K. The second-order rate constant value of 0.21(2) M−1

s−1 was obtained in the MCET reaction from Me8Fc to 1 in thepresence of 2.5 mM Ca(OTf)2 in MeCN at 233 K (SupportingInformation, Table S1 and Figure S2a). This result was alsoconfirmed under the pseudo-first-order conditions (SupportingInformation, Figure S2b). The effect of the concentration ofCa2+ ion, [Ca2+], on the MCET reaction rate was alsoexamined by varying the concentration of Ca2+ ion. The rate ofMCET from Me8Fc to 1 increases with an increase in [Ca2+](Figure 1a, inset). Using the same concentration of Me8Fc and1 in the presence of varying concentrations of Ca2+ ion, therate obeyed the second-order kinetics, and the second-order

rate constants (ket) in the presence of various concentrations ofCa2+ ion were determined from the second-order plots(Supporting Information, Table S1 and Figures S2 and S3).It was observed that the ket value increased with an increase in[Ca2+], exhibiting the second-order dependence on [Ca2+], asgiven by eq 1 (Figure 2).

= [ ]+k k Caet MCET2 2

(1)

Similarly, the MCET reactions from Me8Fc to 1 wereinvestigated in the presence of other redox-inactive metal ions(Mn+), such as Mg2+, Y3+, Al3+, and Sc3+; the Lewis acidity(ΔE) increased in the order of Ca2+ < Mg2+ < Y3+ < Al3+ <

Figure 1. (a) UV−visible spectral changes observed in the ETreaction from Me8Fc (0.50 mM) to 1 (0.50 mM) in the presence ofCa(OTf)2 (2.5 mM) in MeCN at 233 K. Inset shows the time profilemonitored at 750 nm due to the formation of Me8Fc

+ in the presenceof Ca(OTf)2 [2.5 mM (black), 5.0 mM (blue), 10 mM (green), and15 mM (red)]. (b) UV−visible spectral changes observed in the ETreaction from Me8Fc (0.50 mM) to 1 (0.50 mM) in the presence ofSc(OTf)3 (2.5 mM) in MeCN at 233 K. Inset shows the time profilemonitored at 750 nm due to the formation of Me8Fc

+ in the presenceof Sc(OTf)3 [1.0 mM (black), 2.5 mM (blue), 4.0 mM (green), and5.0 mM (red)].

Figure 2. Dependence of ket on [Ca2+] for the ET reaction fromMe8Fc (0.50 mM) to 1 (0.50 mM) with Ca2+ (0−15 mM) in MeCNat 233 K. Inset shows the plot of ket vs [Ca

2+]2.

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366

Sc3+.19,52,53 The second-order rate constants (ket) of MCETreactions from Me8Fc to 1 in the presence of Mn+ ions (2.5mM) were determined under the second-order reactionconditions with respect to 1 and Me8Fc in MeCN at 233 K.Interestingly, the ket value increased with an increase in Lewisacidity (ΔE) of redox-inactive metal ions (SupportingInformation, Table S1). The dependence of the rate constantsof MCET reactions on the concentrations ([Mn+]) of Mg2+,Y3+, Al3+, and Sc3+ was also determined by varying theconcentrations of the respective metal ions, [Mn+]. The ketvalue increased with an increase in the corresponding metal ionconcentrations, exhibiting a second-order dependence on[Mn+] as given by eq 1, where [Ca2+] is replaced by [Mn+](Supporting Information, Table S1 and Figure S4). Thefourth-order rate constants (kMCET) of MCET from Me8Fc to1 in the presence of different metal ions (Mn+) weredetermined from the dependence of ket on the square of[Mn+] (i.e., [Mn+]2) (Figure 3 and Supporting Information,

Table S2). A plot of log kMCET versus the quantitative measureof Lewis acidity (ΔE) of Mn+ is shown in Figure 4, where thelog kMCET values are linearly correlated with the ΔE values.Thus, the stronger the Lewis acidity of metal ions is, the largerthe rate constant (kMCET) of MCET from Me8Fc to 1 becomes.The stoichiometry of MCET from Me8Fc to 1 in the

presence of Mn+ (e.g., Sc3+ and Y3+) was established by

performing UV−vis spectral titration experiments, as shown inthe Supporting Information, Figures S5 and S6. From thespectral titration experiments, it was confirmed that 2 equiv ofSc3+ ion was required to generate 1 equiv of Me8Fc

+ in MeCNat 233 K. Thus, the stoichiometry of the MCET reaction fromMe8Fc to 1 in the presence of Sc3+ ion in MeCN at 233 K isgiven by eq 2.

+ [ ] +

→ + [ ]−

+ +

+ − +

Me Fc (Cl)(TMC)Cr (O ) 2Sc

Me Fc (Cl)(TMC)Cr (O ) (Sc )8

III2

3

8III

22 3

2 (2)

The product analysis after the MCET reaction from Me8Fc to1 in the presence of Mn+ was performed by cold-sprayionization mass spectrometry (CSI-MS) and electron para-magnetic resonance (EPR), revealing the generation of a CrIII

species as an inorganic product (Supporting Information,Figure S7). The formation of the peroxo species was confirmedby 1H NMR (Supporting Information, Figure S8). Iodometrictitration was also performed to confirm the formation of theCr(III)−peroxo complex in the electron-transfer reduction of1 by octamethylferrocene in the presence of Mn+, showing thatthe rate of the oxidation of I− by a Cr(III)−peroxo−metal ioncomplex to produce I3

− increased with increasing the Lewisacidity of metal ion (Supporting Information, Figure S9). Thelatter result indicates that the Lewis acidity of bound metalions controls the oxidizing ability of the Cr(III)−peroxo−metal ion complex.

MCET Equilibrium Constants Depending on [Mn+].When Me8Fc was replaced by 1,1′-dibromoferrocene (Br2Fc;Eox vs SCE = 0.71 V), which is a weaker reductant than Me8Fc(Eox vs SCE = −0.04 V), the MCET oxidation of Br2Fc by 1 inthe presence of Sc3+ ion (2.5 mM) was in equilibrium inMeCN at 233 K. The MCET equilibrium constant (Ket) in eq3 was then determined by global fitting of plots of redoxtitrations, as shown in Figure 5a.

X Yoo

+ [ ] +

+ [ ]−

+ +

+ − +

Br Fc (Cl)(TMC)Cr (O ) 2Sc

Br Fc (Cl)(TMC)Cr (O ) (Sc )K

2III

23

2III

22 3

2et

(3)

The one-electron reduction potentials of 1 (Ered) at variousconcentrations of Sc3+ were also determined from the Ketvalues (Supporting Information, Table S3 and Figure S10) andthe Eox value of Br2Fc using the Nernst equation (eq 4). Thedependence of one-electron reduction potential of 1 (Ered) onSc3+ ion concentration is given by the Nernst equation (eq5),54,55

Figure 3. Dependence of ket on [Mn+]2 for MCET from Me8Fc (0.50mM) to 1 (0.50 mM) in the presence of (a) Mg(ClO4)2, (b)Y(OTf)3, (c) Al(OTf)3, and (d) Sc(OTf)3 in MeCN at 233 K.

Figure 4. Plot of log kMCET vs Lewis acidity of the redox-inactivemetal ions (ΔE).

Figure 5. (a) Plots of concentration of Br2Fc+ produced in electron

transfer from Br2Fc to 1 (0.50 mM) in the presence of Sc(OTf)3 (1.0mM, green circles; 1.5 mM, blue circles; 2.0 mM, black circles; 2.5mM, red circles) in MeCN at 233 K against initial concentration ofBr2Fc, [Br2Fc]0. (b) Dependence of Ered of 1 on log([Sc(OTf)3]).

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= +E E RT F K(2.3 / ) logred ox et (4)

= + + [ ]+E E RT F K(2.3 / ) log(1 Sc )red red0

red3 2

(5)

where Kred is the equilibrium constant of the two Sc3+ ionsbinding to the one-electron reduced species of 1. A plot of Eredversus log([Sc(OTf)3]) is shown in Figure 5b, which exhibits alinear correlation with a slope of 92 mV/log([Sc(OTf)3]),indicating that the one-electron reduction of 1 is accompaniedby binding of two Sc3+ ions to the one-electron reduced speciesof 1. The expected slope is 93 mV/log([Sc(OTf)3]) because of2 × (2.3RT/F) = 93 mV at 233 K, which is the same as theobserved value. When Sc(TOf)3 was replaced by Y(OTf)3 (2.5mM), which is a weaker Lewis acid than Sc3+, electron transferfrom Fc (Eox vs SCE = 0.37 V) to 1 was in equilibrium. TheMCET equilibrium constants (Ket) in the presence of variousconcentrations of Y(OTf)3 were determined by global fitting ofplots of redox titrations (Supporting Information, Figure S11).The Ered values of 1 in the presence of different Y(OTf)3concentrations were then determined by the Nernst equation(eq 4) (see Table 1 and Supporting Information, Table S4).

The dependence of Ered on log([Y(OTf)3]) exhibited a linearcorrelation with a slope of 94 mV/log([Y(OTf)3]) (Support-ing Information, Figure S12), which also indicates that theone-electron reduction of 1 is accompanied by binding of twoY3+ ions to the one-electron reduced species of 1 (vide supra).As discussed earlier, the Ered (vs SCE) value of 1 is

significantly shifted to the positive direction from −0.52 V inthe absence of Sc(OTf)3 to 0.78 V in the presence of Sc(OTf)3(2.5 mM). The change in Ered of 1 was also investigated in thepresence of other metal ions, such as Ca2+, Mg2+, and Al3+. TheEred value of 1 in the presence of Ca(OTf)2 (2.5 mM) wasdetermined by performing the MCET titration withN,N,N′,N′-tetramethyl-p-phenylenediamine (TMPD, Eox vsSCE = 0.107 V)56 as an electron donor. In the presence ofCa(OTf)2 (2.5 mM), an efficient MCET from TMPD to 1occurred in MeCN at 233 K, generating an absorption band atλmax = 615 nm (ε = 20 000 M−1 cm−1) due to the formation ofTMPD•+ (Supporting Information, Figure S13).57 The MCETfrom TMPD to 1 in the presence of Ca(OTf)2 was found to bein equilibrium, where the concentration of TMPD•+ producedincreased with increasing the initial concentration of TMPD([TMPD]0), as shown in Figure 6a. The equilibrium constant(Ket) was determined to be 1.4 at 233 K by fitting the plot of[TMPD•+] vs [TMPD]0 with the use of the ET equilibriumequations (eqs 8−10 in the Experimental Section). The Ered(vs SCE) value of 1 in the presence of Ca(OTf)2 (2.5 mM)was determined to be 0.113 V from the Ket value of 1.4 and the

Eox value of TMPD (0.107 V) using the Nernst equation (eq4).54,55

The change in Ered of 1 was further investigated in thepresence of Mg(ClO4)2 and Al(OTf)3 using TMPD (Eox vsSCE = 0.107 V) and Br2Fc (Eox vs SCE = 0.71 V) as efficientelectron donors, respectively. The MCET oxidation of thesuitable electron donors (TMPD and Br2Fc) by 1 in thepresence of Mn+ (2.5 mM; Mn+ = Mg2+ and Al3+) was inequilibrium in MeCN at 233 K, respectively. The MCETequilibrium constants (Ket) were then determined by perform-ing ET titration experiments from the suitable electron donorsto 1 in the presence of the respective metal ions (SupportingInformation, Figures S14 and S15). Thus, the equilibriumconstants (Ket) were determined to be 7.6 and 17 for the casesof Mg2+ with TMPD and Al3+ with Br2Fc, respectively, at 233K by following the ET equilibrium equations (eqs 8−10 in theExperimental Section; Table 1). Then, the Ered (vs SCE) valuesof 1 in the presence of Mn+ (2.5 mM; Mn+ = Mg2+ and Al3+)were determined to be 0.147 and 0.767 V, respectively (Table1), as determined from the Ket values and the Eox values of thesuitable electron donors using the Nernst equation (eq 4).54,55

The Ered value of 1 increases with an increase in Lewis acidityof redox-inactive metal ions, tuning Ered (vs SCE) of 1 from0.113 V for Ca2+ to 0.780 V for Sc3+. A plot of Ered of 1determined in the presence of different metal ions (Mn+) vsLewis acidity of Mn+ (ΔE) is shown in Figure 7. The Eredvalues of 1 determined in the presence of different metal ionsare linearly correlated to the quantitative measure of Lewisacidity (ΔE), suggesting that the higher the Lewis acidity ofthe redox-inactive metal ions is, the more positive is the shiftobserved in Ered of 1.

Table 1. Electron-Transfer Equilibrium Constants (Ket) of 1with Different Electron Donors in the Presence of VariousMetal Ions (Mn+; 2.5 mM) and One-Electron ReductionPotentials (Ered) of 1 Determined from the Ket Values andthe Eox Values of Electron Donors Using the NernstEquation (eq 4)

Mn+ electron donor Ket Ered, V vs SCE

Sc3+ Br2Fc 34 0.780Al3+ Br2Fc 17 0.767Y3+ Fc 36 0.442Mg2+ TMPD 7.6 0.147Ca2+ TMPD 1.4 0.113

Figure 6. (a) Spectroscopic redox titrations using absorbance at 615nm due to TMPD•+ as a function of the initial concentration ofTMPD ([TMPD]0) added to a MeCN solution of 1 (0.050 mM) inthe presence of Ca(OTf)2 (2.5 mM) in MeCN at 233 K. (b) Plot of(α−1 − 1)−1 vs ([TMPD]0/α[1]0 − 1) to determine the equilibriumconstant (Ket) in the MCET from TMPD to 1 upon addition ofTMPD (0.0−0.15 mM) into a MeCN solution of 1 (0.050 mM) inthe presence of Ca(OTf)2 (2.5 mM) at 233 K.

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Driving Force Dependence of MCET Rate Constants.The driving force dependence of the rate constants (ket) ofMCET from one-electron donors to 1 in the presence ofdifferent redox-inactive metal ions has been evaluated in lightof the Marcus theory of adiabatic outer-sphere electron transferas given by eq 6,

λ λ= [− + Δ ]k Z G k Texp ( /4)(1 / ) /( )et2

B (6)

where Z is the frequency factor that is normally taken as 1011

M−1 s−1, which corresponds to kBTK/h (kB is the Boltzmannconstant, T is the absolute temperature, K is the formationconstant of the precursor complex, which is normally taken as0.020 M−1, and h is the Planck constant), ΔGet is the freeenergy change of MCET, and λ is the reorganization energy ofMCET.58−60 The driving force dependence of ket is shown inFigure 8, where log ket values of MCET from electron donors

to 1 with Mn+ (2.5 mM) at 233 K (Table 2) are plotted againstthe driving force (−ΔGet) of MCET. The −ΔGet values weredetermined from the difference between the one-electronoxidation potentials (Eox vs SCE) of electron donors and theone-electron reduction potential of [(Cl)(TMC)CrIII(O2)]

+

(Ered vs SCE) with Mn+, as given by eq 7,

−Δ = −G e E E( )et red ox (7)

where e is the elementary charge. The driving forcedependence of ket values of MCET from ferrocene derivativesto 1 with M2+ (2.5 mM; M2+ = Ca2+ and Mg2+) is well-fittedwith the Marcus line for PCET from ferrocene derivatives to 1in the presence of HOTf (2.5 mM) in MeCN at 233 K usingthe Marcus equation (eq 6) with the best fit λ value of 2.32eV.51 However, the driving force dependence of ket values ofMCET from ferrocene derivatives to 1 in the presence of M3+

(2.5 mM; M3+ = Y3+, Al3+, and Sc3+) is fitted with somewhathigher reorganization energy with the best-fit λ value of 2.66eV. The smaller reorganization energy of metal-ion coupledelectron-transfer reduction of a Cr(III)−superoxo complex inthe presence of Mg2+ and Ca2+ may result from the weakerbinding of the divalent metal ions (e.g., Mg2+ and Ca2+) due tothe weaker Lewis acidity as compared with a trivalent metal ion(e.g., Sc3+). Thus, the reorganization energy of MCET variesfrom 2.32 to 2.66 eV, depending on the Lewis acidity of theredox-inactive metal ion.

■ CONCLUSIONWe have shown in this study that metal ion-coupled electrontransfer (MCET) from one-electron donors to a Cr(III)−superoxo complex (1) occurs in the presence of redox-inactivemetal ions (Mn+). The rate constants of MCET exhibit thesecond-order dependence with the increasing concentration ofmetal ions, suggesting involvement of two metal ions duringthe MCET process, which is also supported by thethermodynamic studies. The logarithm of the fourth-orderrate constant, kMCET, of MCET shows a good linearrelationship with the Lewis acidity of the metal ions (ΔE);the higher the Lewis acidity of Mn+ is, the faster the rate ofMCET from one-electron donors to 1 becomes. Such a changein the reactivity of 1 in the presence of Lewis acid metal ionscan be described by the change in the one-electron reductionpotential (Ered vs SCE) of 1 toward the positive direction withan increase in the Lewis acidity of Mn+. Thus, the present studyreports for the first time the dependence of the driving force ofthe rate constants of MCET from various electron donors to 1in the presence of various redox-inactive metal ions in light ofthe Marcus theory of electron transfer. From the Marcus plot,it was observed that the reorganization energy of electrontransfer from one-electron donor to 1 varies depending on theLewis acidity of the metal ions between 2.32 and 2.66 eV,providing valuable insights into the MCET reactions of metal−superoxo species.

Figure 7. Plot of Ered (vs SCE) of 1 determined in the presence ofdifferent metal ions ([Mn+]) vs quantitative measure of Lewis acidityof metal ions (ΔE).

Figure 8. Driving force (−ΔGet) dependence of rate constants (logket) of PCET from ferrocene derivatives [(1) 1,1′-dibromoferrocene,(2) bromoferrocene, (3) ferrocene, and (4) 1,1′-dimethylferrocene]to 1 in the presence of HOTf (2.5 mM; red closed circles) in MeCNat 233 K51 and that of MCET from ferrocene derivatives [(4) 1,1′-dimethylferrocene, (5) octamethylferrocene, and (6) decamethylfer-rocene] in the presence of Mn+ [2.5 mM; Sc(OTf)3 (orange opentriangles), Al(OTf)3 (pink open triangles), Y(OTf)3 (black opentriangles), Mg(ClO4)2 (blue closed circles), and Ca(OTf)2 (greenclosed circles)] to 1 in MeCN at 233 K.

Table 2. One-Electron Oxidation Potentials (Eox) ofElectron Donors and Second-Order Rate Constants (ket) ofMCET from Electron Donors to 1 in the Presence of MetalIons (Mn+; 2.5 mM) in MeCN at 233 K

Mn+ electron donor Eox vs SCE, V ket, M−1 s−1

Sc3+ Me10Fc −0.08 9.4(7) × 103

Me8Fc −0.04 7.6(7) × 103

Me2Fc 0.26 2.1(2) × 102

Al3+ Me10Fc −0.08 3.6(3) × 103

Me2Fc −0.04 6.0(5) × 10Y3+ Me10Fc −0.08 2.5(2) × 102

Me8Fc −0.04 4.0(3) × 10Mg2+ Me10Fc −0.08 1.3(1) × 10

Me8Fc −0.04 9.0(8) × 10−1

Ca2+ Me10Fc −0.08 8.9(8)Me8Fc −0.04 2.1(2) × 10−1

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■ EXPERIMENTAL SECTIONMaterials. Commercially available chemicals were used without

further purification unless otherwise indicated. Solvents were driedaccording to published procedures and distilled under an Aratmosphere prior to use.61 Ferrocene derivatives were purchasedfrom Aldrich Chemical Co. and used as received. Metal ion salts, suchas Sc(OTf)3, Al(OTf)3, Y(OTf)3, Mg(ClO4)2, and Ca(OTf)2, werepurchased from Aldrich Chemical Co. or Tokyo Chemical IndustryCo. and used as received. The chromium complexes, [(Cl)(TMC)-CrII]Cl and [(Cl)(TMC)CrIII(O2)]Cl (1), were synthesized accord-ing to the literature methods.49−51

Instrumentation. UV−vis spectra were recorded on a Hewlett-Packard 8453 diode array spectrophotometer equipped with anUNISOKU Scientific Instruments Cryostat USP-203A for low-temperature experiments or on an UNISOKU RSP-601 stopped-flow spectrometer equipped with a MOS-type highly sensitivephotodiode array. Cold spray ionization mass (CSI-MS) spectrawere collected on a JMS-T100CS (JEOL) mass spectrometerequipped with a CSI source (conditions: needle voltage = 2.2 kV,orifice 1 current = 50−500 nA, orifice 1 voltage = 0−20 V, ring lensvoltage = 10 V, ion source temperature = 5 °C, spray temperature =−40 °C). X-band CW-EPR spectra were recorded at 5 K using an X-band Bruker EMX-Plus spectrometer equipped with a dual-modecavity (ER 4116DM) (experimental parameters: microwave frequency= 9.647 GHz, microwave power = 1.0 mW, modulation amplitude =10 G, gain = 1 × 104, modulation frequency = 100 kHz, time constant= 40.96 ms, conversion time = 81.00 ms). Low temperatures wereachieved and controlled with an Oxford Instruments ESR900 liquidHe quartz cryostat with an Oxford Instruments ITC503 temperatureand gas-flow controller.Kinetic Measurements. Kinetic measurements for MCET from

one-electron donors to [(Cl)(TMC)CrIII(O2)]+ (1) in the presence

of different redox-inactive metal ions (Mn+) were performed on aUNISOKU RSP-601 stopped-flow spectrometer in MeCN at 233 K.Rates of MCET from one-electron donors (ferrocene derivatives) to 1were monitored by formation of absorption bands due to ferroceniumcation derivatives generated after MCET from one-electron donors to1 in the presence of different Mn+ ions in MeCN at 233 K. All kineticmeasurements were performed under the second-order conditions([electron donor] = [1]) unless otherwise noted for pseudo-first-order conditions. All kinetic measurements were run at least intriplicate, and the data reported represent the average of thesereactions.Spectral Redox Titrations of MCET Equilibria. Redox titration

of MCET from various concentrations of N,N,N′,N′-tetramethyl-p-phenylenediamine (TMPD) to 1 in the presence of Ca(OTf)2 wasexamined in MeCN at 233 K using a Hewlett-Packard 8453photodiode-array spectrometer with a quartz cuvette (path length =1.0 cm). Typically, an MeCN solution of TMPD (0.00−0.15 mM)was added to an MeCN solution containing 1 (0.050 mM) in thepresence of Ca(OTf)2 (2.5 mM). The concentration of TMPD•+ wasdetermined from the absorption band at λ = 615 nm (ε = 2.0 × 104

M−1 cm−1).57

The equation used to determine Ket in the MCET reaction in thepresence of Ca(OTf)2 (2.50 mM) was derived according to ourprevious work.62 The equilibrium constant (Ket) in Figure 6b isexpressed by eq 8, from which eq 9 is derived, where [CrIII(O2

2−)] =[TMPD•+], [TMPD]0 = [TMPD] + [TMPD•+], [1]0 = [1] +[CrIII(O2

2−)]; and [1]0 and [TMPD]0 are the initial concentrations of1 and TMPD, respectively.

=[ ][[ ] ]

[ ][ ]

·+ − +K

1TMPD Cr (O )

TMPDet

III22

(8)

Equation 10 is derived from eq 9, where α = [TMPD•+]/[1]0.

ikjjjj

y{zzzzikjjjj

y{zzzz

=[ ] − [ ] [ ] − [ ]

[ ]

=[ ][ ]

−[ ]

[ ]−

·+ ·+

·+

·+ ·+

K1

1

1 ( TMPD TMPD )( TMPD )TMPD

TMPDTMPD

1TMPD

1

et

0 02

0 0

(9)

ikjjjjj

y{zzzzzα

α− =

[ ][ ]

−− − K1

( 1)TMPD

11 1et

0

0 (10)

The Ket value of 1.4 was determined from the slope of the linear plotbetween (α−1 − 1)−1 and ([TMPD]0/α[1]0 − 1), as shown in Figure6b.

The Nernst equation is given by eq 11, where [Cox] = [1] and[Cred] = [CrIII(O2

2−)] + [CrIII(O22−)−(Sc3+)2] = [CrIII(O2

2−)](1 +Kred[Sc

3+]2).

= + {[ ] [ ]}E E RT F C C(2.3 / ) log /red red0

ox red (11)

Equation 11 is rewritten by eq 12, where E0red = Ered0 + (2.3RT/F)

log{[1]/[CrIII(O22−)]}.

= + + [ ]+E E RT F K(2.3 / ) log(1 Sc )red red0

red3 2 (12)

■ ASSOCIATED CONTENT*S Supporting InformationThe Supporting Information is available free of charge athttps://pubs.acs.org/doi/10.1021/jacs.9b11014.

Experimental details, rate constants, equilibrium con-stants, UV−vis spectra, dependence of rate constants onconcentrations, CSI-MS spectrum, 1H NMR spectra,and absorption spectra (PDF)

■ AUTHOR INFORMATIONCorresponding Authors*wwnam@ewha.ac.kr*fukuzumi@chem.eng.osaka-u.ac.jpORCIDTarali Devi: 0000-0002-3460-3286Yong-Min Lee: 0000-0002-5553-1453Wonwoo Nam: 0000-0001-8592-4867Shunichi Fukuzumi: 0000-0002-3559-4107NotesThe authors declare no competing financial interest.

■ ACKNOWLEDGMENTSThis work was supported by the NRF of Korea through CRI(NRF-2012R1A3A2048842 to W.N.), GRL (NRF-2010-00353t o W .N . ) , B a s i c S c i e n c e R e s e a r c h P r o g r am( 2 0 1 7 R 1 D 1 A 1 B 0 3 0 2 9 9 8 2 t o Y . - M . L . a n d2017R1D1A1B03032615 to S.F.), and the Grants-in-Aid (no.16H02268 to S.F.) from MEXT.

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