DOI: 10.1002/ijch.201900147

Generation and Electron-Transfer Reactivity of the Long-Lived Photoexcited State of a Manganese(IV)-Oxo-ScandiumNitrate ComplexNamita Sharma,[a] Yong-Min Lee,*[a, b] Wonwoo Nam,*[a, c] and Shunichi Fukuzumi*[a, d]

Abstract: Photoexcitation of a manganese(IV)-oxo-scandiumnitrate complex ([(Bn-TPEN)MnIV(O)]2+ � Sc(NO3)3) in asolvent mixture of trifluoroethanol and acetonitrile (v/v=1 :1) resulted in generation of the long-lived photoexcitedstate, which is detected by nanosecond laser transientabsorption measurements. The transient absorption maxi-mum (λmax) of the 2E excited state of [(Bn-TPEN)MnIV(O)]2+� Sc(NO3)3 is observed at 620 nm with lifetimes of 7.1 μs.The λmax value is blue-shifted and the lifetime becomeslonger as compared with the previously reported values of

λmax (640 nm) and lifetime (6.4 μs) of the 2E excited state ofthe 1 :2 complex between [(Bn-TPEN)MnIV(O)]2+ and Sc(OTf)3 ([(Bn-TPEN)Mn

IV(O)]2+ � (Sc(OTf)3)2). The electron-transfer reactivity of the 2E excited states of [(Bn-TPEN)MnIV

(O)]2+ � Sc(NO3)3 was similar to that of [(Bn-TPEN)MnIV

(O)]2+ � (Sc(OTf)3)2. The long lifetime and the high reactivityof the 2E excited state of [(Bn-TPEN)MnIV(O)]2+ � Sc(NO3)3provide an excellent photooxidant for oxidation of com-pounds, which would otherwise be impossible to beoxidized.

Keywords: manganese(IV)-oxo complex · scandium ion · photoexcited state · electron transfer

Introduction

Calcium ion (Ca2+) is indispensable for the function andstructural assembly of the oxygen evolving complex (OEC) ofphotosystem II (PSII), in which Ca2+ ion acts as a cofactor foroxygen evolution from water although Ca2+ ion is redox-inactive.[1–5] Ca2+ ion is proposed to act as a Lewis acid,modulating the redox reactivity of an Mn(V)-oxo and Mn(III)-peroxo intermediates, which are involved in the wateroxidation in the OEC in PSII.[6,7] However, the actual role ofCa2+ ion in the catalytic oxidation of water in the OEC has yetto be well clarified. Binding of redox-inactive metal ionsincluding Ca2+ ion to the oxo moiety of high-valentmanganese(IV)-oxo complexes is reported to result inenhancement of the redox reactivity.[8–16] Photoexcitation of aMnIV-oxo complex binding two scandium ions, [(Bn-TPEN)MnIV(O)]2+ � (Sc(OTf)3)2 (Bn-TPEN=N-benzyl-N,N’,N’-tris(2-pyridylmethyl)-1,2-diaminoethane)[12] is also reported toresult in the formation of a long-lived photoexcited state witha lifetime of 6.4 μs, which exhibits high redox reactivity,capable of hydroxylating benzene with water to producephenol.[17] Generation of such long-lived photoexcited state ofa manganese(IV)-oxo complex provides an excellent photo-oxidant for oxidation of organic substrates. However, there hasbeen no report on a long-lived photoexcited state of amanganese(IV)-oxo complex other than that of a 1 :2 complexbetween [(Bn-TPEN)MnIV(O)]2+ and Sc(OTf)3)2 (i. e., [(Bn-TPEN)MnIV(O)]2+ � (Sc(OTf)3)2).[18]

We report herein a long-lived photoexcited state of [(Bn-TPEN)MnIV(O)]2+ that forms a 1 :1 complex with Sc(NO3)3,which is found to exhibit a similar lifetime to the photoexcited

state of [(Bn-TPEN)MnIV(O)]2+� (Sc(OTf)3)2. The electron-transfer reactivity of the long-lived photoexcited state of [(Bn-TPEN)MnIV(O)]2+ � Sc(NO3)3 was examined by laser-inducedtransient absorption measurements and compared with that of[(Bn-TPEN)MnIV(O)]2+ � (Sc(OTf)3)2.

Experimental Section

Materials. Commercially available chemicals were used with-out further purification unless otherwise indicated. Benzenederivatives, such as benzene, toluene, ethylbenzene, m-xylene,mesitylene, 1,2,4,5-tetramethylbenzene (durene), naphthaleneand scandium(III) triflate (Sc(OTf)3), and scandium(III) nitrate

[a] N. Sharma, Y.-M. Lee, W. Nam, S. FukuzumiDepartment of Chemistry and Nano Science, Ewha WomansUniversity, Seoul 03760, KoreaE-mail: fukuzumi@chem.eng.osaka-u.ac.jp

yomlee@ewha.ac.krwwnam@ewha.ac.kr

[b] Y.-M. LeeResearch Institute for Basic Sciences, Ewha Womans University,Seoul 03760, Korea

[c] W. NamState Key Laboratory for Oxo Synthesis and Selective Oxidation,Suzhou Research Institute of LICP, Lanzhou Institute of ChemicalPhysics (LICP), Chinese Academy of Sciences, Lanzhou 730000,China

[d] S. FukuzumiFaculty of Science and Engineering, Meijo University, Nagoya,Aichi 468-8502, Japan

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(Sc(NO3)3) were purchased from Aldrich Chemical Co. andTokyo Chemical Industry Co., Ltd. and used as received.Solvents were dried according to published procedures anddistilled under Argon prior to use.[19] Iodosylbenzene (PhIO)was prepared by literature method.[20] MnII(CF3SO3)2 ·2CH3CNwas prepared by literature method.[21] N-benzyl-N,N’,N’-tris(2-pyridylmethyl)-1,2-diaminoethane (Bn-TPEN) ligand and[(Bn-TPEN)MnII]2+ were synthesized according to the liter-ature methods.[11,12] [(Bn-TPEN)MnIV(O)]2+ was generated bythe reaction of [(Bn-TPEN)MnII]2+ with PhIO.[11,12] C60 used asa reference compound for determination of the quantum yieldof the excited state of [(Bn-TPEN)MnIV(O)]2+ � Sc(NO3)3 waspurchased from Tokyo Chemical Industry Co., Ltd. and usedas received.

Instrumentation. Nanosecond time-resolved transient ab-sorption measurements were performed using the laser systemprovided by UNISOKU Co., Ltd. A mixture solution(CF3CH2OH/CH3CN v/v=1 :1) in a quartz cell (1.0 cm ×1.0 cm) was excited by a Nd:YAG laser (Continuum SLII-10,4–6 ns fwhm, λex = 355 nm, 80 mJ pulse� 1, 10 Hz). The ratesof electron transfer were monitored by continuous exposure toa xenon lamp for visible region as a probe light and aphotomultiplier tube (Hamamatsu 2949) as a detector. UV-visspectra were recorded on Hewlett Packard 8453 diode arrayspectrophotometer equipped with a UNISOKU ScientificInstruments Cryostat USP-203 A. X-band electron paramag-netic resonance (EPR) spectra were taken at 77 K using aJEOL X-band spectrometer (JES-FA100). The experimentalparameters for EPR measurements by JES-FA100 were asfollows: microwave frequency=9.028 GHz, microwavepower=1.0 mW, modulation amplitude=1.0 mT, modulationfrequency=100 kHz and time constant=0.03 s.

Kinetic Measurements. All the reactions were run in a1.0 cm quartz cuvette and followed by monitoring transientabsorption spectral changes (excited at 355 nm) of the reactionsolutions of [(Bn-TPEN)MnIV(O)]2+ � Sc(NO3)3 (0.50 mM) inthe presence of benzene (100–500 mM) in CF3CH2OH/CH3CN(v/v=1 :1) at 298 K. The same procedure was used forspectral measurements for oxidation of other benzene deriva-tives. Second-order rate constants were determined underpseudo-first-order conditions (i. e., [substrate]/[[(Bn-TPEN)MnIV(O)]2+ � Sc(NO3)3] >10) by fitting the changes in transientabsorbance for the decay of peaks due to the excited state of[(Bn-TPEN)MnIV(O)]2+ � Sc(NO3)3 in the oxidation reactionsof substrates (0.10–500 mM) in CF3CH2OH/CH3CN (v/v=1 :1) at 298 K. Electron transfer reactions of substrates, suchas benzene, toluene, ethylbenzene, m-xylene, mesitylene anddurene, to the 2E excited state of [(Bn-TPEN)MnIV(O)]2+ � Sc(NO3)3 were monitored by the decay of the transientabsorption bands at 620 nm.

Results and Discussion

Binding of Scandium Nitrate to an Mn(IV)-Oxo Complex.Scandium ion-bound [(Bn-TPEN)MnIV(O)]2+ complex was

generated by addition of Sc(OTf)3 to a trifluoroethanol/actonitrile (TFE/MeCN; v/v=1 :1) mixture solution of [(Bn-TPEN)MnIV(O)]2+ as reported previously.[11,12] Addition of upto two equiv. of Sc(OTf)3 resulted in the blue shift of theabsorption band of [(Bn-TPEN)MnIV(O)]2+ (λmax=1020 nm)with an isosbestic point at 900 nm to the absorption band atλmax=740 nm.[12] Further addition of Sc(OTf)3 resulted in amore blue shift to the absorption band at 690 nm.[12] No furtherspectral change was observed by addition of more than nineequiv. of Sc(OTf)3. Such stepwise spectral change indicatesbinding of one and two Sc(OTf)3 molecules to [(Bn-TPEN)MnIV(O)]2+ to produce the 1 :1 (i. e., [(Bn-TPEN)MnIV(O)]2+ � Sc(OTf)3) and 1 :2 (i. e., [(Bn-TPEN)MnIV(O)]2+ � (Sc(OTf)3)2)complexes, respectively.[12]

Addition of three equiv. of tetrabutylammonium nitrate(TBANO3, 1.5 mM) to a TFE/MeCN (v/v=1 :1) mixturesolution of [(Bn-TPEN)MnIV(O)]2+ � (Sc(OTf)3)2 results in redshift of the absorption band from 690 nm to 720 nm as shownin Figure 1, which is similar to that due to the 1 :1 [(Bn-TPEN)MnIV(O)]2+ � Sc(OTf)3 complex. The conversion from1 :2 [(Bn-TPEN)MnIV(O)]2+ � (Sc(OTf)3)2 complex to the 1 :1[(Bn-TPEN)MnIV(O)]2+ � Sc(NO3)3 complex by addition ofthree equiv. of NO3� [Equation (1)] results from the strongerbinding of NO3� than that of OTf� because of the strongernucleophilicity of NO3� than that of OTf� .[22]

½ðBnTPENÞMnIVðOÞ�2þ� ðScðOTfÞ3Þ2 þ 3NO3� !

½ðBnTPENÞMnIVðOÞ�2þ� ScðNO3Þ3 þ ScðOTfÞ3 þ 3OTf�(1)

The formation of the 1 :1 [(Bn-TPEN)MnIV(O)]2+ � Sc(NO3)3complex was confirmed independently by addition of Sc

Figure 1. UV-vis spectral changes showing the conversion from [(Bn-TPEN)MnIV(O)]2+ � (Sc(OTf)3)2 (0.50 mM, black line) to [(Bn-TPEN)MnIV(O)]2+ � Sc(NO3)3 (0.50 mM, red line) by addition of tetrabuty-lammonium nitrate (TBANO3, 0.50 mM each addition) to a TFE/MeCN (v/v=1 :1) solution of [(Bn-TPEN)MnIV(O)]2+ � (Sc(OTf)3)2(0.50 mM) at 273 K.

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(NO3)3 to a TFE/MeCN (v/v=1 :1) solution of [(Bn-TPEN)MnIV(O)]2+ (Figure 2a). Addition of large excess Sc(NO3)3resulted in no further change of the absorption spectrum. Thisindicates that [(Bn-TPEN)MnIV(O)]2+ forms only the 1 :1complex with Sc(NO3)3 [Equation (2)].

½ðBn-TPENÞMnIVðOÞ�2þ þ ScðNO3Þ3K! �

½ðBn-TPENÞMnIVðOÞ�2þ� ScðNO3Þ3(2)

The formation constant of [(Bn-TPEN)MnIV(O)]2+ � Sc(NO3)3(K) is given by Equation (3),

K ¼ ½1�=½½MnIVðOÞ�2þ�½ScðNO3Þ3� (3)

where [1]= [[(Bn-TPEN)MnIV(O)]2+ � Sc(NO3)3] and K is theformation constant of 1. Equation (3) can be rewritten byEquation (4), where [MnIV(O)]0 and [Sc(NO3)3]0 are the initialconcentrations of [(Bn-TPEN)MnIV(O)]2+ and Sc(NO3)3, re-spectively.

1=K ¼ ð½MnIVðOÞ�0� ½1�Þð½ScðNO3Þ3�0 � ½1�Þ=½1�

¼ ð½MnIVðOÞ�0=½1�� 1Þð½ScðNO3Þ3�0� ½1�Þ(4)

Equation (5) is derived from Equation (4), where α= [1]/[MnIV(O)]0.

ða� 1� 1Þ� 1 ¼ Kð½ScðNO3Þ3�0� ½1�Þ (5)

A plot of (α� 1� 1)� 1 vs. [Sc(NO3)3]0� [1] is shown in Figure 2b,which exhibits a linear correlation, confirming the 1 :1complex formation in Equation (5). The K value wasdetermined from the slope of the linear correlation (α� 1� 1)� 1vs. [Sc(NO3)3]0� [1] to be 1.9×103 M� 1 at 298 K.

The cold-spray ionization time-of-flight mass (CSI-MS)spectrum of [(Bn-TPEN)MnIV(O)]2+ � Sc(NO3)3 confirmedbinding of Sc(NO3)3 to [(Bn-TPEN)MnIV(O)]2+, exhibiting ionpeaks at m/z=787.4 and 874.4, which shift to 789.4 and 876.4when PhI18O was used to generate [(Bn-TPEN)MnIV(18O)]2+(Figure 3). An X-band EPR spectrum of [(Bn-TPEN)MnIV(O)]2+ � Sc(NO3)3 exhibits a signal that is characteristic of S=3/2 MnIV as the case of [(Bn-TPEN)MnIV(O)]2+ (Figure 4).[12]

Figure 2. (a) UV-vis spectral changes observed in the formation of[(Bn-TPEN)MnIV(O)]2+� Sc(NO3)3 upon addition of Sc(NO3)3 (0–4.0 mM) to a TFE/MeCN (v/v=1 :1) solution of [(Bn-TPEN)MnIV

(O)]2+ at 273 K. Inset shows the plot of concentration of [(Bn-TPEN)MnIV(O)]2+ � Sc(NO3)3 produced upon addition of Sc(NO3)3 to [(Bn-TPEN)MnIV(O)]2+ in TFE/MeCN (v/v=1 :1) at 273 K vs. initialconcentration of Sc(NO3)3. (b) Plot of (α� 1� 1)� 1 vs. [Sc(NO3)3]0� [1],(where α= [1]/[MnIV(O)]0 and 1= [[(Bn-TPEN)MnIV(O)]2+ � Sc(NO3)3]) to determine the formation constant of [(Bn-TPEN)Mn

IV

(O)]2+ � Sc(NO3)3 by addition of Sc(NO3)3 (0–4.0 mM) to [(Bn-TPEN)MnIV(O)]2+ (0.50 mM) in TFE/MeCN (v/v=1 :1) at 273 K.

Figure 3. CSI-MS spectrum of [(Bn-TPEN)MnIV(O)]2+ � Sc(NO3)3 inTFE/MeCN (v/v=1 :1) at 298 K. The peaks at m/z=787.4 and 874.4correspond to [MnIV(16O)(Bn-TPEN)(Sc(NO3)3)(NO3)]

+ (calcd. m/z=787.5) and [MnIV(16O)(Bn-TPEN)(Sc(NO3)3)(CF3SO3)]

+ (calcd.m/z=874.5), respectively. Insets show the isotope distributionpatterns of [(Bn-TPEN)MnIV(16O)]2+ � Sc(NO3)3 (black lines) and[(Bn-TPEN)MnIV(18O)]2+ � Sc(NO3)3 (red lines), respectively.

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It was recently reported that Sc(OTf)3 underwent rapidhydrolysis by residual water in MeCN (ca. 0.5–3 mM H2O) togenerate HOTf under the conditions used for catalyticoxidations with H2O2 and a Mn(II) complex [Mn2(μ-O)3(tmtacn)2](PF6)2 (tmtacn=N,N’,N’’-trimethyl-1,4,7-triazacyclo-nonane).[23] It should be noted that [(Bn-TPEN)MnIV(O)]2+�(Sc(OTf)3)2 (λmax=690 nm) is clearly different from a HOTf-bound complex, [(Bn-TPEN)MnIV(O)]2+ � (HOTf)2 (λmax=560 nm) in TFE/MeCN (v/v=1 :1).[24] In addition, a CSI-MSspectrum of [(Bn-TPEN)MnIV(O)]2+ � Sc(NO3)3 (Figure 3)clearly indicates the binding of Sc(NO3)3 rather than HNO3.

Reactivity of Photoexcited State of [(Bn-TPEN)MnIV(O)]2+ � Sc(NO3)3. Although [(Bn-TPEN)MnIV(O)]2+ exhib-ited no long-lived transient absorption, nanosecond laserexcitation of a deaerated TFE/MeCN (v/v=1 :1) of [(Bn-TPEN)MnIV(O)]2+ � Sc(NO3)3 resulted in the formation of thelong-lived excited state, which has an absorption band atλmax=620 nm (Figure 5a). From the decay time profile ofabsorbance at 620 nm, the lifetime of the photoexcited state of[(Bn-TPEN)MnIV(O)]2+ � Sc(NO3)3 at 298 K was determinedto be τ=7.1 μs (Figure 5b), which is similar to the lifetime(τ=6.4 μs) due to the doublet 2E photoexcited state of [(Bn-TPEN)MnIV(O)]2+ � (Sc(OTf)3)2 reported previously.[17] Thislong-lived excited state results from the doublet 2E photo-excited state because of the spin forbidden decay to the quartetground state as reported for Mn4+-doped compounds.[25]

The 2E excited state was produced via an extremely rapidintersystem crossing (ISC) process from the quartet 4E excitedstate. The quantum yield (Φ) of the 2E excited state of [(Bn-TPEN)MnIV(O)]2+ � Sc(NO3)3 was determined to be 0.96 usingthe triplet excited state of C60 (3C60*: λmax=750 nm, ɛ=1.8×104 M� 1 cm� 1 in benzene; �T=0.98)[32,33] as a reference andnaphthalene radical cation (λmax=685 nm, ɛ=6.8×103 M� 1 cm� 1)[34,35] as the product of electron transfer formnaphthalene to the 2E excited state of [(Bn-TPEN)MnIV(O)]2+� Sc(NO3)3 (vide infra, see Figure 6 and ExperimentalSection). First, the initial concentration of naphthalene radical

cation produced by electron transfer from naphthalene(1.0 mM) to the 2E excited state of [(Bn-TPEN)MnIV(O)]2+� Sc(NO3)3 at 1.0 μs after the laser excitation was determinedto be 0.0078/6.80×103=1.15×10� 6 M (Figures 6a and 6b).Under the present experimental conditions, no π-dimer radicacation of naphthalene was produced in the presence ofnaphthalene (1.0 mM) in TFE/MeCN (v/v=1 :1).[35,36] Becausethe quenching ratio of the 2E excited state of [(Bn-TPEN)MnIV(O)]2+ � Sc(NO3)3 by naphthalene (1.0 mM) was determinedfrom the quenching constant (ketτ=5.5×109×7.1×10� 6=3.9×104 M� 1) to be 3.9×104×1.0×10� 3/(1+3.9×104×1.0×10� 3)=0.975, the concentration of the 2E excited state of [(Bn-TPEN)MnIV(O)]2+ � Sc(NO3)3 was determined to be 1.15×10� 6/0.975=1.18×10� 6 M. On the other hand, the concen-tration of 3C60* produced at 1.0 μs after laser excitation of C60(0.20 mM) at 355 nm in benzene was determined to be 0.0308/1.80×104=1.71×10� 6 M (Figure 6d). Because the absorbanceof C60 at 355 nm is 0.80 (Figure 6c), the incident light intensity

Figure 4. EPR spectrum of [(Bn-TPEN)MnIV(O)]2+ � Sc(NO3)3(1.0 mM) in TFE/MeCN (v/v=1 :1) recorded at 5 K.

Figure 5. (a) Transient absorption spectral changes of the 2E excitedstate of [(Bn-TPEN)MnIV(O)]2+ � Sc(NO3)3 (0.50 mM) upon nano-second laser excitation at 355 nm in TFE/MeCN (v/v=1 :1) at 298 K.(b) Time profile of absorbance monitored at 620 nm.

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was determined to be 1.71×10� 6/(1–10� 0.80)/0.98=2.08×10� 6einstein dm� 3. The incident light intensity absorbed by [(Bn-TPEN)MnIV(O)]2+ � Sc(NO3)3 at 355 nm was determined to be2.08×10� 6× (1–10� 0.39)=1.23×10� 6 einstein dm� 3, becausethe absorbance of [(Bn-TPEN)MnIV(O)]2+ � Sc(NO3)3 at355 nm was 0.39 (Figure 6c). Finally, the quantum yield (Φ)of the 2E excited state of [(Bn-TPEN)MnIV(O)]2+ � Sc(NO3)3was determined to be 1.18×10� 6/1.23×10� 6=0.96�0.10, inwhich the experimental error is �10%. It was confirmed thatvirtually the same Φ value of the 2E excited state of [(Bn-TPEN)MnIV(O)]2+ � Sc(NO3)3 was obtained using a largerconcentration of naphthalene (2.0 mM).

When benzene was added to a deaerated TFE/MeCN (v/v=1 :1) solution of [(Bn-TPEN)MnIV(O)]2+ � Sc(NO3)3, thedecay of absorbance at 620 nm due to the 2E excited state of[(Bn-TPEN)MnIV(O)]2+ � Sc(NO3)3 was accelerated by elec-tron transfer from benzene to the 2E excited state of [(Bn-TPEN)MnIV(O)]2+ � Sc(NO3)3 and the decay rate of absorbanceat 620 nm increased with increasing concentration of benzene.The first-order decay rate constant increased linearly withincreasing concentration of benzene (Figure 7a). The rateconstant of electron transfer from benzene to the 2E excited

state of [(Bn-TPEN)MnIV(O)]2+ � Sc(NO3)3 was determinedfrom the slope of the linear plot of kobs vs. concentration ofbenzene to be (1.0�0.1)×106 M� 1 s� 1 at 298 K. The first-orderdecay rate constants also increased linearly with increasingconcentration of toluene (Figure 7b). The rate constant ofelectron transfer from toluene to the 2E excited state of [(Bn-TPEN)MnIV(O)]2+ � Sc(NO3)3 was determined from the slopeof the linear plot of kobs vs. concentration of toluene to be(5.1�0.4)×107 M� 1 s� 1 at 298 K. Similarly, the rate constantsof electron transfer (ket) from other benzene derivatives suchas ethylbenzene, m-xylene, mesitylene, and durene were alsodetermined from the slopes of the linear plots of kobs vs.concentrations of benzene derivatives as listed in Table 1.

The dependence of the logarithm of the rate constants ofelectron transfer (log ket) from benzene derivatives to the 2Eexcited state of [(Bn-TPEN)MnIV(O)]2+ � Sc(NO3)3 on the one-electron oxidation potentials of benzene derivatives (Eox) isshown in Figure 8, where log ket exhibits typical dependenceof the rate constant for photoinduced electron transfer; the logket value increases with a decrease in Eox to reach a diffusionlimited value, as expressed by the Marcus equation of electrontransfer [Equation (6)],[26–28]

Figure 6. (a) Transient absorption spectral change of the 2E excited state of [(Bn-TPEN)MnIV(O)]2+ � Sc(NO3)3 (0.50 mM,) in the presence ofnaphthalene (2.0 mM, red closed circles) observed at 1.0 μs after nanosecond laser excitation at 355 nm in TFE/MeCN (v/v=1 :1) at 298 K.(b) Time profile of absorbance monitored at 685 nm due to naphthalene radical cation produced by electron transfer from naphthalene to the2E excited state of [(Bn-TPEN)MnIV(O)]2+ � Sc(NO3)3 observed at 1.0 μs after nanosecond laser excitation at 355 nm in TFE/MeCN (v/v=1 :1)at 298 K. (c) UV-vis absorption spectra of C60 (0.20 mM, black line), and [(Bn-TPEN)Mn

IV(O)]2+ � Sc(NO3)3 (0.20 mM, red line) withabsorbance at 355 nm. (d) Time profile of absorbance monitored at 750 nm due to the triplet excited state of fullerene (3C60*) at 1.0 μs afternanosecond laser excitation at 355 nm in argon-saturated benzene at 298 K.

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1=ket ¼ 1=kdiff þ 1=ðZexp½ð� l=4Þð1þ DGet=lÞ2=ðkBTÞ�Þ(6)

where λ is the reorganization energy of electron transfer, kdiff isthe diffusion rate constant, Z is the collision frequency, whichis taken as 1011 M� 1 s� 1, kB is the Boltzmann constant, T is theabsolute temperature, and kdiff=1.0×1010 M� 1 s� 1.[26–28] TheGibbs energy change of electron transfer (ΔGet) is given byEquation (7),

DGet ¼ eðEox� EredÞ (7)

where e is the elementary charge, Ered and Eox are the one-electron reduction potential of an electron acceptor and theone-electron oxidation potential of an electron donors. Thebest fit line in Figure 8 gives the Ered value of the 2E excitedstate of [(Bn-TPEN)MnIV(O)]2+ � Sc(NO3)3 ([(Bn-TPEN)MnIV(O)]2+ � Sc(NO3)3*)=2.1(1) V with λ=0.53(4) eV. The Ered

Figure 7. Plots of kobs vs. concentration of electron donors [(a) benzene (100–400 mM), (b) toluene (5.0–20 mM), (c) ethylbenzene (1.0–4.0 mM), (d) m-xylene (0.1–0.4 mM), (e) mesitylene (0.1–0.4 mM), and (f) durene (0.1–0.4 mM)] for electron transfer from electron donors tothe 2E excited state of (Bn-TPEN)MnIV(O)]2+ � Sc(NO3)3 (0.50 mM) in TFE/MeCN (v/v=1 :1) at 298 K. The concentration of the

2E excitedstate of [(Bn-TPEN)MnIV(O)]2+ � Sc(NO3)3 (0.50 mM) was determined to be 1.18×10

� 3 mM from the concentration of naphthalene radicalcation produced by electron transfer from naphthalene to the 2E excited state of [(Bn-TPEN)MnIV(O)]2+ � Sc(NO3)3. The smallest substrateconcentration used is 0.10 mM, which is much larger than the concentration of the 2E excited state of [(Bn-TPEN)MnIV(O)]2+ � Sc(NO3)3, whenthe pseudo-first-order conditions are fulfilled.

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value of [(Bn-TPEN)MnIV(O)]2+ � Sc(NO3)3* is the same asthat of [(Bn-TPEN)MnIV(O)]2+ � (Sc(OTf)3)2* (Ered vs. SCE=2.1 V), probably because a decrease in the Ered value at theground state may be canceled out by an increase in theexcitation energy of [(Bn-TPEN)MnIV(O)]2+� Sc(NO3)3 ascompared with that of [(Bn-TPEN)MnIV(O)]2+ � (Sc(OTf)3)2*.The LMCT energy of [(Bn-TPEN)MnIV(O)]2+ ([(Bn-TPEN*+)MnIII(O)]2+) is expected to decrease by increasing the numberof binding molecules of Sc(OTf)3 from one to two, becausethe electron accepting ability of the MnIV(O) moiety increaseswith an increase in the number of binding molecules of Sc(OTf)3 from one to two. The λ value of 0.53 eV is muchsmaller than that of the ground state of MnIV(O) complexes(λ=2.2–2.3 eV)[12,29] because of the ligand centered electrontransfer to the ligand-to-metal charge transfer (LMCT) excitedstate of [(Bn-TPEN)MnIV(O)]2+ � Sc(NO3)3 as compared withthe metal-centered electron transfer to the ground state of MnIV

(O) complexes. Sc(NO3)3 is known to act as a strong Lewisacid even in an aqueous solution.[30,31]

Conclusion

Photoexcitation of a MnIV(O) complex binding one Sc(NO3)3molecule ([(Bn-TPEN)MnIV(O)]2+ � Sc(NO3)3) affords a long-lived 2E excited state with a lifetime of 7.1 μs at 298 K viaintersystem crossing from the quartet 4E excited state. Electrontransfer from benzene and its derivatives to the long-liveddoublet 2E excited state of [(Bn-TPEN)MnIV(O)]2+ � Sc(NO3)3occurs and the electron-transfer driving force dependence ofthe logarithm of the rate constants of electron transfer is fittedby the Marcus equation for outer-sphere electron transfer toafford a small reorganization energy of electron transfer (λ=0.53 eV), because of the ligand-centered electron transfer tothe ligand-to-metal charge transfer (LMCT) excited state of[(Bn-TPEN)MnIV(O)]2+ � Sc(NO3)3 as compared with themetal-centered electron transfer to the ground state of MnIV(O)complexes. This study provides a new photooxidant that has along-lived photoexcited state of a redox-inactive metal ion-bound high-valent metal-oxo complex and high oxidizingcapability.

Acknowledgement

This work was supported by NRF of Korea through CRI(NRF-2012R1A3A2048842 to W.N), GRL (NRF-2010-00353to W.N.), Basic Science Research Program(2017R1D1A1B03029982 to Y.M.L. and2017R1D1A1B03032615 to S.F.), and Grants-in-Aid (no.16H02268 to S.F.) from the Ministry of Education, Culture,Sports, Science and Technology (MEXT).

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Figure 8. Plots of log ket of photoinduced electron transfer fromelectron donors [(1) benzene, (2) toluene, (3) ethylbenzene, (4) m-xylene, (5) mesitylene, (6) benzyl alcohol, (7) durene, and (8)naphthalene] to the 2E excited state of [(Bn-TPEN)MnIV(O)]2+� (Sc(OTf)3)2 (black circles) and [(Bn-TPEN)Mn

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Manuscript received: November 20, 2019Revised manuscript received: January 11, 2020

Version of record online: January 31, 2020

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