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Unified Mechanism of Oxygen Atom Transfer and Hydrogen Atom Transfer Reactions with a Triflic Acid-Bound Nonheme Manganese(IV)–Oxo Complex via Outer-Sphere Electron Transfer Yong-Min Lee, Surin Kim, Kei Ohkubo, Kyung-Ha Kim, Wonwoo Nam, and Shunichi Fukuzumi J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.8b12935 • Publication Date (Web): 15 Jan 2019 Downloaded from http://pubs.acs.org on January 16, 2019

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Journal of the American Chemical Society

Yong-Min Lee,† Surin Kim,† Kei Ohkubo,‡ Kyung-Ha Kim,† Wonwoo Nam,*,†,¶ and Shunichi Fukuzumi*,†,§ †

Department of Chemistry and Nano Science, Ewha Womans University, Seoul 03760, Korea Institute for Advanced Co-Creation Studies, Open and Transdisciplinary Research Initiatives, Osaka University, Suita, Osaka 5650871, Japan ¶ State Key Laboratory for Oxo Synthesis and Selective Oxidation, Suzhou Research Institute of LICP, Lanzhou Institute of Chemical Physics (LICP), Chinese Academy of Sciences, Lanzhou 730000, China ‡

§

Faculty of Science and Engineering, Meijo University, SENTAN, Japan Science and Technology Agency (JST), Nagoya, Aichi 4680073, Japan ABSTRACT: Outer-sphere electron transfer from styrene, thioanisole and toluene derivatives to a triflic acid (HOTf)-bound nonheme Mn(IV)-oxo complex, [(N4Py)MnIV(O)]2+-(HOTf)2 (N4Py = N,N-bis(2-pyridylmethyl)-N-bis(2-pyridyl)methylamine), has been shown to be the rate-determining step of different types of redox reactions such as epoxidation, sulfoxidation and hydroxylation of styrene, thioanisole and toluene derivatives, respectively, by [(N4Py)MnIV(O)]2+-(HOTf)2. The rate constants of HOTf-promoted epoxidation of all styrene derivatives with [(N4Py)MnIV(O)]2+ and electron transfer from electron donors to [(N4Py)MnV(O)]2+ exhibit a remarkably unified correlation with the driving force of outer-sphere electron transfer in light of the Marcus theory of electron transfer. The same electron-transfer driving force dependence is observed in the oxygen atom transfer from [(N4Py)MnIV(O)]2+-(HOTf)2 to thioanisole derivatives as well as in the hydrogen atom transfer from toluene derivatives to [(N4Py)MnIV(O)]2+-(HOTf)2. Thus, mechanisms of oxygen atom transfer (epoxidation and sulfoxidation) reactions of styrene and thioanisole derivatives and hydrogen atom transfer (hydroxylation) reactions of toluene derivatives by [(N4Py)MnIV(O)]2+-(HOTf)2 have been unified for the first time as the same reaction pathway via outer-sphere electron transfer, followed by the fast bond-forming step, which exhibits the singly unified electron-transfer driving force dependence of the rate constants as outer-sphere electron-transfer reactions. In the case of the epoxidation of cis-stilbene by [(N4Py)MnIV(O)]2+-(HOTf)2, the isomerization of cis-stilbene radical cation to trans-stilbene radical cation occurs after outer-sphere electron transfer from cis-stilbene to [(N4Py)MnIV(O)]2+-(HOTf)2 to yield trans-stilbene oxide selectively, which is also taken as evidence for the occurrence of electron transfer in the acid-catalyzed epoxidation.

Accurate prediction of the rate constant of a chemical reaction is one of the central subjects in molecular reaction dynamics.1-3 Various theories have been developed for this purpose such as the transition state theories and the methods based on classical or quantum dynamics calculation.1–3 However, accurate theoretical calculation of a rate constant is an extremely difficult task, one of the main reason being its dramatic change with the changes in the energy content of the system and the reaction critical energy. Solvation of molecules has further precluded the accurate prediction of the rate constant of a chemical reaction in solution.4 Only one exception is the rate constant of an outer-sphere electron transfer reaction, which can be well predicted by using the Marcus theory of electron transfer (eq 1),

the reorganization energy of electron transfer, and ΔGet is the free energy change of electron transfer.5 The driving force of outersphere electron transfer (–ΔGet) is obtained from the one-electron oxidation potential (Eox) of an electron donor and the one-electron reduction potential (Ered) of an electron acceptor (eq 2), –ΔGet = e(Ered – Eox)

(2)

(1)

where e is the elementary charge and the electrostatic interaction is neglected in a polar solvent.5 Thus, the rate constant of outersphere electron transfer from any electron donor to an electron acceptor can be well predicted from the Eox value of an electron donor and the Ered value of an electron acceptor, provided that the λ value remains the same in a series of electron donors and acceptors.6-18 The Marcus theory of electron transfer (eq 1) has been applied not only to electron transfer but also to hydrogen atom transfer reactions.6-19

where Z is the frequency factor, which is kBTK/h (kB is the Boltzmann constant, T is absolute temperature, K is the formation constant of the precursor complex and h is the Planck constant), λ is

There are many examples for various organic transformations, which proceed via the rate-limiting outer-sphere electron-transfer processes.20-25 In such cases, the rate constants of different types of

ket = Z exp[−(λ/4)(1 + ΔGet/λ )2/(kBT)]

1 ACS Paragon Plus Environment

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reactions can be well predicted from the Eox and Ered values by using eq 1, provided that the λ value remains the same in the series of substrates. The λ value is desired to be large in order to predict a wide range of the rate constants before the rate constant reaches the diffusion-limited value at the large driving force of electron transfer. Metal-oxygen complexes normally afford large λ values of electron transfer because of the large change in the metal-oxygen bond distance upon the electron-transfer reduction.26-33 However, it has been quite difficult to obtain a unified driving force dependence of the rate constants of different types of organic transformations because of the difference in the formation constants of the precursor complexes from those in inner-sphere electron transfer, in which the interaction in the precursor complexes is significantly larger than that in outer-sphere electron-transfer reactions.34-37 Thus, there has so far been no singly unified dependence of the secondorder rate constants of different types of intermolecular redox reactions. We report herein efficient epoxidation of styrene derivatives by a triflic acid (HOTf)-bound nonheme Mn(IV)-oxo complex, [(N4Py)MnIV(O)]2+-(HOTf)2 (N4Py = N,N-bis(2-pyridylmethyl)-N-bis(2-pyridyl)methylamine),33 in trifluoroethanol/acetonitrile (CF3CH2OH/CH3CN: v/v = 1:1), which proceeds via outer-sphere electron transfer from styrene derivatives to [(N4Py)MnIV(O)]2+-(HOTf)2. Dependence of the rate constants of the epoxidation of styrene derivatives by [(N4Py)MnIV(O)]2+(HOTf)2 on the driving force of outer-sphere electron transfer from styrene derivatives to [(N4Py)MnIV(O)]2+-(HOTf)2 agrees with that of outer-sphere electron transfer from one-electron donors to [(N4Py)MnIV(O)]2+-(HOTf)2 in accordance with the Marcus equation (eq 1). This provides an excellent opportunity to obtain a remarkably unified electron-transfer driving force dependence of not only epoxidation of styrene derivatives but also sulfoxidation of thioanisole derivatives and hydroxylation of toluene derivatives by [(N4Py)MnIV(O)]2+-(HOTf)2, which were reported previously, but yet to be combined.33,38

Triflic Acid-Catalyzed Epoxidation of Styrene Derivatives by [(N4Py)MnIV(O)]2+. A mononuclear high-valent MnIV(O) complex, [(N4Py)MnIV(O)]2+ (1), was generated by using iodosylbenzene (PhIO) in CF3CH2OH/CH3CN (TFE/MeCN, v/v = 1:1) at 273 K as reported previously.32,33 Addition of trifluoromethanesulfonic acid (HOTf) to a solution of 1 at 273 K afforded a reddish brown solution, resulting in the disappearance of the absorption band at 940 nm due to 1, accompanied by appearance of a new absorption band at 550 nm (shoulder, ε = 540 M–1 cm–1) due to [(N4Py)MnIV(O)]2+-(HOTf)2. (2).33 Binding of two HOTf molecules to the oxo moiety of [(N4Py)MnIV(O)]2+ has been confirmed by Extended X-ray absorption fine structure (EXAFS) data.33 IV

Figure 1. (a) Visible spectral changes observed in the epoxidation of cis-stilbene (20 mM) by [(N4Py)MnIV(O)]2+ (1, 0.50 mM) in TFE/MeCN (v/v = 1:1) at 273 K. Inset shows time profile of absorbance at 940 nm due to 1. (b) Plot of the pseudo-first-order rate constant vs concentration of cis-stilbene. Scheme 1. Catalytic Cycle for the Oxidation of cis-Stilbene by PhIO with [(N4Py)MnII]2+

Table 1. Product Yields of Epoxidation of cis- and trans-Stilbene by [(N4Py)MnIV(O)]2+ (1) and [(N4Py)MnIV(O)]2+-(HOTf)2 (2) under Stoichiometric and Catalytic Conditions substrate

cis-stilbene 1

trans-stilbene

2+

Epoxidation of cis-stilbene by [(N4Py)Mn (O)] (1) occurred slowly as shown in Figure 1, where the absorption band at 940 nm due to 1 disappeared with an isosbestic point, accompanied by appearance of the absorption band at 450 nm due to [(N4Py)MnII]2+. The yield of cis-stilbene oxide was 85(5)%. No isomerized product (i.e., trans-stilbene oxide) was produced (see Table 1 and Experimental Section for products analysis). The reduced product of 1 was [(N4Py)MnII]2+ as revealed by EPR and ESI-MS (Supporting Information (SI), Figures S1 and S2). Thus, the stoichiometry of the reaction is given by eq 3.

cis-stilbene 2

trans-stilbene

product

yield(%) kinetic a

catalytic b

cis-stilbene oxide

85(5)

83(5)

trans-stilbene oxide

ND

ND

c

ND c

cis-stilbene oxide

ND


89(4)

93(4)

cis-stilbene oxide

ND c

-


91(4)

-

cis-stilbene oxide

ND c

-


93(5)

-

a

In the oxidation of cis-stilbene (20 mM) and trans-stilbene (20 mM), 0.50 mM of MnIV(O) was used in TFE/MeCN (v/v = 1:1) at 273 K. b [MnII] : [PhIO] : [substrate] = 1.0 mM : 20 mM : 400 mM for cis-stilbene and 40 mM for trans-stilbene in TFE/MeCN (v/v = 1:1) at 273 K. c ND: Not detected.


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Figure 3. Dependence of the second-order rate constant (kox) for epoxidation of cis-stilbene by [(N4Py)MnIV(O)]2+ (2; 0.50 mM) on [HOTf] (0 – 50 mM) in TFE/MeCN (v/v = 1:1) at 273 K. Inset shows plot of 1/kox against [HOTf]–2. Table 2. One-Electron Oxidation Potentials (Eox) of Styrene Derivatives and Second-Order Rate Constants (kox) of Epoxidation Reactions by 1 and 2 in the Absence and Presence of HOTf (30 mM) in TFE/MeCN (v/v = 1 : 1) at 273 K electron donor

Figure 2. (a) Visible spectral changes observed in the epoxidation of cis-stilbene (10 mM) by [(N4Py)MnIV(O)]2+-(HOTf)2 (2, 0.50 mM) in the presence of HOTf (50 mM) in TFE/MeCN (v/v = 1:1) at 273 K. Inset shows time profile of absorbance at 550 nm due to 2. (b) Plot of the pseudo-first-order rate constant vs concentration of cis-stilbene.

Similar yield of cis-stilbene oxide product (83(5)% based on PhIO concentration used) was also obtained in the catalytic oxidation of cis-stilbene (400 mM) by [(N4Py)MnII]2+ (1.0 mM) with PhIO (20 mM), because [(N4Py)MnIV(O)]2+ intermediate was produced by the reaction of [(N4Py)MnII]2+ with PhIO (see Scheme 1 and Table 1). When cis-stilbene was replaced by transstilbene, only trans-stilbene oxide was obtained without formation of any isomerized product (see Table 1). The rates of epoxidation of cis-stilbene (large excess) by 1 were determined by monitoring a decrease in absorbance at 940 nm due to 1 (Figure 1a, inset), obeying first-order kinetics. The pseudofirst-order rate constant was proportional to concentration of cisstilbene (Figure 1b) and the second-order rate constant (kox) was determined from the slope of a linear plot of the pseudo-first-order rate constant vs concentration of cis-stilbene to be 1.2(1)  10–2 M–1 s–1 at 273 K (see also Table 2 and SI, Figure S3 for the rate constants of other styrene derivatives). In the presence of excess HOTf (50 mM), 1 was converted to [(N4Py)MnIV(O)]2+-(HOTf)2 (2),33 which was much more reactive than 1 for the epoxidation of cis-stilbene as shown in Figure 2a, where the absorbance at 550 nm due to 2 decayed much more rapidly than the case of 1. Interestingly, trans-stilbene oxide with 91(4)% yield was obtained as a sole product in the oxidation of cis-stilbene by 2, indicating that no cis-stilbene oxide was detected (Table 1). The pseudo-first-order rate constant was proportional to concentration of cis-stilbene (Figure 2b; see also SI, Figure S4 for other

kox, M–1 s–1

Eox,a V vs SCE

1

2 –3

7.9(6) × 103

4-MeO-styrene

1.34

5.0(4) × 10

trans-stilbene

1.44

1.6(1) × 10–2

1.4(1) × 103

cis-stilbene

1.58

1.2(1) × 10–2

1.3(1) × 102

4-Me-styrene

1.77

5.7(5) × 10–4

1.1(1)

styrene

2.02

3.3(3) × 10

–4

9.5(7) × 10–3

3-Cl-styrene

2.18

1.9(2) × 10–4

6.1(5) × 10–3

2,6-di-Cl-styrene

2.38

9.2(8) × 10–5

2.4(2) × 10–3

a

One-electron oxidation potentials in TFE/MeCN (v/v = 1:1) were determined by the differential pulse voltammetry (DPV) and/or the secondharmonic alternating current voltammetry (SHACV) at scan rate of 4 mV s–1 using Pt working electrode.

styrene derivatives). Thus, there is no indication of formation of strong precursor complexes of cis-stilbene with [(N4Py)MnIV(O)]2+-(HOTf)2. The second-order rate constant (kox) was determined from the slope to be 1.3(1)  102 M–1 s–1 at 273 K, which is 1.1 × 104 times larger than the value of 1 without HOTf. When the concentration of HOTf used was more than 20 mM, the kox value remained constant (1.3(1) × 102 M–1 s–1) with increasing concentration of HOTf (Figure 3), because there is no further HOTf binding to the MnIV(O) moiety. From the plot of 1/kox vs [HOTf]–2 as shown in the inset of Figure 3, the formation constant (K) of [(N4Py)MnIV(O)]2+ with two molecules of HOTf was determined to be 1.6(2) × 105 M–2, which is identical to that obtained in the oxidation of thioanisole with 2 reported previously.33 Similarly, the kox values of epoxidation of other styrene derivatives by 2 were determined as listed in Table 2. The results in Figures 2 and 3 for the epoxidation of styrene derivatives by [(N4Py)MnIV(O)]2+-(HOTf)2 show sharp contrast to the epoxidation of styrene derivatives by the corresponding iron(IV)-oxo complex [(N4Py)FeIV(O)]2+ in the presence of HOTf, in which the substrate complex of [(N4Py)FeIV(O)]2+ was formed prior to the epoxidation (the formation constant of trans-



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Scheme 2. Proposed Mechanism of Epoxidation of cis-Stilbene by [(N4Py)MnIV(O)]2+-(HOTf)2 via Electron Transfer and cis-trans Isomerization

Figure 4. Electron-transfer driving force (–ΔGet) dependence of log kox for epoxidation of styrene derivatives [(1) 4-MeO-styrene, (2) transstilbene, (3) cis-stilbene, (4) 4-Me-styrene, (5) styrene, (6) 3-Cl-styrene and (7) 2,6-di-Cl-styrene], sulfoxidation of thioanisole derivatives [(8) 4-MeO-thioanisole, (9)4-Me-thioanisole, (10) thioanisole, (11) 4-F-thioanisole, (12) 4-Br-thioanisole and (13) 4-CN-thioanisole]33 and hydroxylation of toluene derivatives [(14) hexamethylbenzene, (15) pentamethylbenzene, (16) 1,2,4,5-tetramethylbenzene and (17) mesitylene]38 by 1 (black circles) and 2 (red circles) and log ket for electron transfer from various electron donors [coordinatively saturated metal complexes; (18) [RuII(Me2-bpy)3]2+, (19) [RuII(bpy)3]2+, (20) [RuII(5-Cl-phen)3]2+, (21) [RuII(5-Br-bpy)3]2+ and (22) [RuII(5NO2-phen)3]2+] to 2 (blue circles)33 in TFE/MeCN (1:1 v/v) at 273 K. The blue squares show the driving force dependence of the rate constants (log ket) of electron transfer from electron donors [(23) ferrocene, (24) bromoferrocene, (25) acetylferrocene and (26) dibromoferrocene] to 1 in TFE/MeCN (1:1 v/v) at 273 K.32b

stilbene was 150 M–1).29,39 In addition, the second-order rate constants of epoxidation of styrene derivatives by [(N4Py)FeIV(O)]2+ increased parabolically with increasing concentration of HOTf in sharp contrast to the case in Figure 3.29 The binding of two HOTf molecules to the oxo moiety of [(N4Py)MnIV(O)]2+ (1) prohibited the substrate binding due to the steric effect, whereas no binding of HOTf to[(N4Py)FeIV(O)]2+ allowed relatively strong binding of substrates. Driving Force Dependence of Rate Constants. The rate constants of epoxidation of styrene derivatives with 1 and 2 are compared with those of outer-sphere electron transfer from one-electron donors to 1 and 2 in light of the Marcus theory of adiabatic electron transfer (eq 1).5 The driving forces (–ΔGet) of outersphere electron transfer are obtained from the one-electron oxidation potentials (Eox) of electron donors and the one-electron reduction potentials (Ered) of 1 (0.80 V vs SCE)32b and 2 (1.65 V vs SCE)33 using eq 2. The Ered values of styrene derivatives in TFE/MeCN (v/v = 1:1) were determined by SHACV measurements (Table 2; SI, Figure S5). The electron-transfer driving force dependence of log ket of outer-sphere electron transfer from coordinatively saturated metal complexes to 1 and 2 in Figure 4 is well fitted by the Marcus equation of outer-sphere electron transfer (eq 1)5 using the same reorganization energy of electron transfer (λ = 2.23 eV).32b,33

The Z value of outer-sphere electron-transfer reactions is normally taken as 1.0 × 1011 M−1 s−1.40,41 This indicates that the K value of outer-sphere electron-transfer reactions is as small as 0.020 M−1, because there is little interaction in the precursor complex for outer-sphere electron-transfer reactions of coordinatively saturated metal complexes. The log kox values of epoxidation of styrene derivatives by 1 (black circles in Figure 4; nos. 1 – 7) are significantly larger than the log ket values of outer-sphere electron transfer from electron donors (i.e., ferrocene derivatives and coordinatively saturated metal complexes) to 1 and 2 (blue solid line in Figure 4). Thus, the epoxidation of styrene derivatives by 1 without HOTf proceeds via the direct oxygen atom transfer from 1 to styrene derivatives. In contrast to 1, the log kox values of epoxidation of styrene derivatives by 2 (red circles in Figure 4; nos. 1 – 7) agree well with the log ket values of outer-sphere electron transfer from electron donors (i.e. ferrocene derivatives and coordinatively saturated metal complexes) to 1 and 2 (blue solid line in Figure 4). Thus, the epoxidation of styrene derivatives by 2 proceeds via outer-sphere electron transfer from styrene derivatives to 2 to produce the radical cations of styrene derivatives. The outer-sphere electron-transfer pathway of 2 may result from the steric effects of two HOTf molecules that bind to the oxo moiety of 2, precluding inner-sphere interaction with styrene derivatives as the case of 1. In the case of cis-stilbene, outer-sphere electron transfer from cis-stilbene to 2 produces cis-stilbene radical cation and [MnIII(O)]+-(HOTf)2 (Scheme 2). The radical cation of cis-stilbene is well known to be isomerized to the radical cation of transstilbene.42,43 However, the rate constant of isomerization from cisstilbene radical cation to trans-stilbene radical cation has yet to be determined.43 We have detected cis-stilbene radical cation produced by electron transfer from the acridinyl radical moiety of the electron transfer state of 9-mesityl-10-methylacdiridnium ion,


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Journal of the American Chemical Society Scheme 4. Unified Mechanism of the Oxidation of Substrate (S) by [(N4Py)MnIV(O)]2+-(HOTf)2 via Electron Transfer

Figure 5. Nanosecond laser-induced transient absorption spectra obtained upon photoexcitation (λex = 355 nm) of an Ar-saturated TFE/MeCN solution containing cis-stilbene (10 mM) and 9-mesityl10-methylacridinium ion at 298 K. Inset shows time profile of absorbance at 850 nm due to the conversion from cis-stilbene radical cation to trnas-stilbene radical cation. Scheme 3. Proposed Mechanism of the Epoxidation of Styrene by [(N4Py)MnIV(O)]2+-(HOTf)2 via Electron Transfer

much faster than the electron transfer. In such a case, no radical cations (electron transfer products) are observed in the oxidation reactions.

which was produced by intramolecular photoinduced electron transfer from the mesityl moiety to the acridinium moiety,44 to cisstilbene. The observed transient absorption spectra are shown in Figure 5, where the transient absorption band at 510 nm and 800 nm (broad) due to cis-stilbene radical cation decreased, accompanied by an increase in the absorption band at 470 nm and 750 nm due to trans-stilbene radical cation. The rate constant of isomerization from cis-stilbene radical cation to trans-stilbene radical cation was determined from the decay in absorbance at 850 nm (Figure 5, inset) to be 1.1 × 106 s–1. The rate-determining outer-sphere electron transfer from styrene derivatives to 2 is followed by formal O•– transfer from [(N4Py)MnIII(O)]+–(HOTf)2 to styrene radical cations to produce the corresponding styrene oxide derivatives and the MnII complex (Scheme 3). In the case of cis-stilbene, the isomerization from cis-stilbene radical cation to trans-stilbenze radical cation with the rate constant of 1.1 × 106 s–1 (vide supra) may occur more rapidly than the formal O•– transfer from [(N4Py)MnIII(O)]+– (HOTf)2 to cis-stilbene radical cation. Thus, trans-stilbene oxide was obtained as a sole product selectively in the oxidation of cisstilbene by 2. It should be noted that the rate-determining step is electron transfer when the subsequent step after electron transfer is

The log kox values of oxygen atom transfer from 2 to thioanisoles (red circles in Figure 4; nos. 9 – 13) and hydrogen atom transfer from toluene derivatives to 2 (red circles in Figure 4; nos. 14 – 17) also agree with those of outer-sphere electron transfer from electron donors (i.e., ferrocene derivatives and coordinatively saturated metal complexes) to 1 and 2 (blue solid line in Figure 4).33,38 Thus, the sulfoxidation of thioanisole and hydroxylation of toluene derivatives by 2 also proceed via the rate-determining outer-sphere electron transfer from thioanisole and toluene derivatives to 2. The unified mechanism of epoxidation of styrene derivatives, sulfoxidation of thioanisole derivatives and hydroxylation of toluene derivatives via outer-sphere electron transfer is shown in Scheme 4. The rate-determining outer-sphere electron transfer from substrates (S) to 2 is followed by rapid formal O•– transfer from [(N4Py)MnIII(O)]+–(HOTf)2 to substrate radical cations (for oxygen atom transfer reactions) or by rapid proton transfer from substrate radical cations to [(N4Py)MnIII(O)]+–(HOTf)2 and then, rapid oxygen-rebound from [(N4Py)MnIII(OH)]2+–(HOTf)2 to substrate radicals (for hydrogen atom transfer reactions) to produce the corresponding oxygenated products (SO) and the MnII complex (Scheme 4).

Not only epoxidation of styrene derivatives by [(N4Py)MnIV(O)]2+-(HOTf)2 (2) but also sulfoxidation of thioanisole derivatives and hydroxylation of toluene derivatives by 2 have been shown to proceed via the rate-limiting outer-sphere electron transfer reactions, which exhibit the same electron-transfer driving force dependence of the rate constants in light of the Marcus theory of outer-sphere electron transfer (Figure 4). Outersphere electron transfer from cis-stilbene to 2 is followed by isomerization of cis-stilbene radical cation to trans-stilbene radical cation and the subsequent formal O•– transfer to produce trans-stilbene oxide selectively. The present study has paved a way to obtain singly unified dependence of the second-order rate constants of different types of intermolecular reactions between electron donors



and acceptors on the driving force of outer-sphere electron transfer from electron donors to acceptors, enabling us to predict the rate constants accurately, provided that the interaction between electron donors and acceptors are sterically hindered.

Materials. All chemicals, which were the best available purity, were purchased from Aldrich Chemical Co. and Tokyo Chemical Industry, used without further purification unless otherwise noted. Solvents were dried according to the literature procedures and distilled under Ar prior to use.45 Iodosylbenzene (PhIO)46 and manganese(II) triflate salt (MnII(OTf)2·2CH3CN)47 were prepared according to the literature methods. N4Py ligand was prepared as reported previously.48 [(N4Py)MnII]2+ and [(N4Py)MnIV(O)]2+ complexes were also prepared as reported previously.32,33 Instrumentation. UV-vis spectra were recorded on a Hewlett Packard 8453 photodiode-array spectrophotometer equipped with a UNISOKU Scientific Instruments Cryostat USP-203A or on a UNISOKU RSP-601 stopped-flow spectrometer equipped with a MOS-type highly sensitive photodiode-array. Electrospray ionization mass spectra (ESI MS) were collected on a Thermo Finnigan (San Jose, CA, USA) LCQTM Advantage MAX quadrupole ion trap instrument, by infusing samples directly into the source using a manual method. Xband EPR spectra were recorded at 5 K using X-band Bruker EMX-plus spectrometer equipped with a dual mode cavity (ER 4116DM). Low temperature was achieved and controlled with an Oxford Instruments ESR900 liquid He quartz cryostat with an Oxford Instruments ITC503 temperature and gas flow controller. EPR spectra were recorded under non-saturating microwave power conditions. The magnitude of modulation was chosen to optimize the resolution and the signal-to-noise (S/N) ratio of the observed spectra. The experimental parameters for EPR measurement were as follows: Microwave frequency = 9.647 GHz, microwave power = 1.0 mW, modulation amplitude = 10 G, gain = 1.0 × 104, modulation frequency = 100 kHz, time constant = 40.96 ms, and conversion time = 81.00 ms. 1H NMR spectra were measured with a Bruker model digital AVANCE III 400 FT-NMR spectrometer. Electrochemical measurements, such cyclic voltammetry (CV), differential pulse voltammetry (DPV) and second-harmonic alternating current voltammetry (SHACV),49 were performed on a CHI630B electrochemical analyzer (CH Instruments, Inc.) in CH3CN containing 0.10 M Bu4NPF6 (TBAPF6) as a supporting electrolyte. A conventional three-electrode cell was used with a platinum working electrode (surface area of 0.30 mm2), a platinum wire as a counter electrode, and an Ag/Ag+ electrode as a reference electrode. The platinum working electrode was routinely polished with BAS polishing alumina suspension and rinsed with acetone and acetonitrile before use. The measured potentials were recorded with respect to an Ag/Ag+ (0.010 M) reference electrode. All potentials (vs. Ag/Ag+) were converted to values vs. SCE by adding 0.29 V.50 All electrochemical measurements were performed under an Ar atmosphere. Kinetic Studies. Kinetic measurements were performed on a Hewlett Packard 8453 photodiode-array spectrophotometer equipped with a UNISOKU Scientific Instruments Cryostat USP-203A or on a UNISOKU RSP-601 stopped-flow spectrometer equipped with a MOS-type highly sensitive photodiode-array at 273 K. Rates of epoxidation of styrene derivatives (5.0 × 10–3 – 1.6 M) by [(N4Py)MnIV(O)]2+ (5.0 × 10–4 M) were examined by monitoring spectral changes at 940 nm due to [(N4Py)MnIV(O)]2+ and 550 nm due to [(N4Py)MnIV(O)]2+-(HOTf)2 in the absence and presence of HOTf in CF3CH2OH/CH3CN (v/v = 1:1) at 273 K. The concentration of styrene derivatives was maintained at least more than 10-fold excess of [(N4Py)MnIV(O)]2+ to attain pseudo-first-order conditions. First-order fitting of the kinetic data allowed us to determine the

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pseudo-first-order rate constants. The first-order plots were linear for three or more half-lives with the correlation coefficient ρ > 0.999. In each case, it was confirmed that the rate constants derived from at least five independent measurements agreed within an experimental error of 5%. The pseudo-first-order rate constants increased proportionally with increase in concentrations of substrates, from which second-order rate constants were determined. Time-resolved transient absorption measurements. Nanosecond time-resolved transient absorption measurements were carried out using the laser system provided by UNISOKU Co., Ltd. Measurements of nanosecond transient absorption spectra were performed according to the following procedure. A deaerated solution containing a sample in a quartz cell (1 cm  1 cm) was excited by a Nd:YAG laser (Continuum SLII-10, 4-6 ns fwhm, λex = 355 nm, 5mJ/pulse). The photodynamics were monitored by continuous exposure to a xenon lamp (150 W) as a probe light and a photomultiplier tube (Hamamatsu 2949) as a detector. The solution was oxygenated by nitrogen purging for 15 min prior to measurements. Product Analysis. Typically, styrene derivatives (20 mM) was added to a solution containing [(N4Py)MnIV(O)]2+ (0.50 mM) in the presence of HOTf (30 mM) in CF3CH2OH/CH3CN (v/v = 1:1) at 273 K. Products formed in the oxidation reactions of styrene derivatives by [(N4Py)MnIV(O)]2+ were analyzed by 1H NMR. Quantitative analyses were made on the basis of comparison of 1H NMR spectral integration between products and their authentic samples. For cis- and trans-stilbenes, product yields were determined by HPLC equipped with SunFire C18 5μm column (4.6 mm × 25 cm).

Supporting Information. Figures S1 – S5. This material is available free of charge via the Internet at http://pubs.acs.org.

[email protected]; [email protected]

This work was supported by the NRF of Korea through CRI (NRF2012R1A3A2048842 to W.N), GRL (NRF-2010-00353 to W.N.), and Basic Science Research Program (2017R1D1A1B03029982 to Y.-M.L. and 2017R1D1A1B03032615 to S.F.). This work was also supported by Grants-in-Aid (nos. 16H02268 to S.F. and 18H04650, 17H03010, 16K13964 to K.O.) from the Ministry of Education, Culture, Sports, Science and Technology (MEXT), SENTAN project (to S.F.) from Japan Science and Technology Agency (JST).

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