Mechanistic borderline of one-step hydrogen atom transfer versus stepwise Sc3+-coupled electron transfer from benzyl alcohol derivatives to a non-heme iron(IV)-oxo complex

Article abstract of DOI:10.1021/ic3016723

The rate of oxidation of 2,5-dimethoxybenzyl alcohol (2,5-(MeO) ₂C₆H₃CH₂OH) by [Fe ^IV(O)(N4Py)]²⁺ (N4Py = N,N-bis(2-pyridylmethyl)-N-bis(2- pyridyl)methylamine) was enhanced significantly in the presence of Sc(OTf) ₃ (OTf^- = trifluoromethanesulfonate) in acetonitrile (e.g., 120-fold acceleration in the presence of Sc³⁺). Such a remarkable enhancement of the reactivity of [Fe^IV(O)(N4Py)] ²⁺ in the presence of Sc³⁺ was accompanied by the disappearance of a kinetic deuterium isotope effect. The radical cation of 2,5-(MeO)₂C₆H₃CH₂OH was detected in the course of the reaction in the presence of Sc³⁺. The dimerized alcohol and aldehyde were also produced in addition to the monomer aldehyde in the presence of Sc³⁺. These results indicate that the reaction mechanism is changed from one-step hydrogen atom transfer (HAT) from 2,5-(MeO)₂C₆H₃CH₂OH to [Fe ^IV(O)(N4Py)]²⁺ in the absence of Sc³⁺ to stepwise Sc³⁺-coupled electron transfer, followed by proton transfer in the presence of Sc³⁺. In contrast, neither acceleration of the rate nor the disappearance of the kinetic deuterium isotope effect was observed in the oxidation of benzyl alcohol (C₆H₅CH₂OH) by [Fe^IV(O)(N4Py)]²⁺ in the presence of Sc(OTf) ₃. Moreover, the rate constants determined in the oxidation of various benzyl alcohol derivatives by [Fe^IV(O)(N4Py)]²⁺ in the presence of Sc(OTf)₃ (10 mM) were compared with those of Sc ³⁺-coupled electron transfer from one-electron reductants to [Fe ^IV(O)(N4Py)]²⁺ at the same driving force of electron transfer. This comparison revealed that the borderline of the change in the mechanism from HAT to stepwise Sc³⁺-coupled electron transfer and proton transfer is dependent on the one-electron oxidation potential of benzyl alcohol derivatives (ca. 1.7 V vs SCE).

Full text of DOI:10.1021/ic3016723

Article
pubs.acs.org/IC
Mechanistic Borderline of One-Step Hydrogen Atom Transfer versus
Stepwise Sc3+-Coupled Electron Transfer from Benzyl Alcohol
Derivatives to a Non-Heme Iron(IV)-Oxo Complex
Yuma Morimoto,† Jiyun Park,† Tomoyoshi Suenobu,† Yong-Min Lee,‡ Wonwoo Nam,*,‡
and Shunichi Fukuzumi*,†,‡
†Department of Material and Life Science, Graduate School of Engineering, Osaka University, ALCA, Japan Science and Technology
Agency (JST), Suita, Osaka 565-0871, Japan
‡Department of Bioinspired Science, Ewha Womans University, Seoul 120-750, Korea
S
* Supporting Information
ABSTRACT: The rate of oxidation of 2,5-dimethoxybenzyl
alcohol (2,5-(MeO)2C6H3CH2OH) by [FeIV(O)(N4Py)]2+
(N4Py = N,N-bis(2-pyridylmethyl)-N-bis(2-pyridyl)-
methylamine) was enhanced significantly in the presence of
Sc(OTf)3 (OTf− = trifluoromethanesulfonate) in acetonitrile
(e.g., 120-fold acceleration in the presence of Sc3+). Such a
remarkable enhancement of the reactivity of [FeIV(O)-
(N4Py)]2+ in the presence of Sc3+ was accompanied by the
disappearance of a kinetic deuterium isotope effect. The radical
cation of 2,5-(MeO)2C6H3CH2OH was detected in the course
of the reaction in the presence of Sc3+. The dimerized alcohol
and aldehyde were also produced in addition to the monomer aldehyde in the presence of Sc3+. These results indicate that the
reaction mechanism is changed from one-step hydrogen atom transfer (HAT) from 2,5-(MeO)2C6H3CH2OH to
[FeIV(O)(N4Py)]2+ in the absence of Sc3+ to stepwise Sc3+-coupled electron transfer, followed by proton transfer in the
presence of Sc3+. In contrast, neither acceleration of the rate nor the disappearance of the kinetic deuterium isotope effect was
observed in the oxidation of benzyl alcohol (C6H5CH2OH) by [FeIV(O)(N4Py)]2+ in the presence of Sc(OTf)3. Moreover, the
rate constants determined in the oxidation of various benzyl alcohol derivatives by [FeIV(O)(N4Py)]2+ in the presence of
Sc(OTf)3 (10 mM) were compared with those of Sc3+-coupled electron transfer from one-electron reductants to
[FeIV(O)(N4Py)]2+ at the same driving force of electron transfer. This comparison revealed that the borderline of the change
in the mechanism from HAT to stepwise Sc3+-coupled electron transfer and proton transfer is dependent on the one-electron
oxidation potential of benzyl alcohol derivatives (ca. 1.7 V vs SCE).
In general, there are three possible reaction pathways in HAT
reactions of iron-oxo species (FeIV=O), as shown in Scheme 1.
Since a hydrogen atom consists of an electron and a proton, the
proposed mechanisms are (1) stepwise proton transfer
followed by electron transfer (PT/ET), (2) stepwise electron
transfer followed by proton transfer (ET/PT), and (3) one-step
HAT, in which an electron and a proton are transferred in a
concerted manner. In the case of iron-oxo species, one-step
INTRODUCTION
■
Homolytic C−H bond cleavage of organic compounds is one of
the fundamental reaction steps in various types of oxidation
processes both in synthetic and biological chemistry.1
Laboratory-scale syntheses and industrial oxidation processes
have utilized metal-oxo reagents (e.g., KMnO4) and a metal
oxide surface for the oxidation of organic substrates.2,3 In the
oxidative enzymes, heme and nonheme iron oxygenases
represented by cytochrome P450 and taurine/α-ketoglutarate
dioxygenase (TauD), respectively, high-valent iron-oxo species
have been regarded as reactive intermediates in their catalytic
cycles, where C−H bond is cleaved by oxometal species (M
O) and O−H bond is formed in the process of hydrogen atom
transfer (HAT) from substrate to oxometal species.4−6 In the
enzymatic oxidation reactions, for example, hydroxylation,
oxidation of alcohols, desaturation, cyclization, and chlorina-
tion, the initial step of those processes is widely believed to be
activation of C−H bonds via HAT from a substrate to high-
valent iron-oxo species.4−10
Scheme 1. Three Possible Reaction Pathways in HAT
Reactions of Iron-Oxo Species
Received: July 30, 2012
Published: September 6, 2012
© 2012 American Chemical Society
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prior to use.31 Iodosylbenzene (PhIO) was prepared by a literature
method.32 Nonheme iron(II) complex, [Fe(N4Py)](ClO4)2 (N4Py =
N,N-bis(2-pyridylmethyl)-N-bis(2-pyridyl)methylamine), and its iron-
(IV)-oxo complex, [FeIV(O)(N4Py)]2+, were prepared according to
the literature methods.15,16 For example, [FeIV(O)(N4Py)]2+ was
prepared by reacting [Fe(N4Py)](ClO4)2 (5.0 mM) with 1.2 equiv of
PhIO (6.0 mM) in MeCN at ambient temperature. The deuterated
compounds, phenyl [2H2]methanol (C6H5CD2OH) and 2,5-dimethox-
yphenyl [2H2]methanol (2,5-(MeO)2C6H3CD2OH), were prepared by
reduction of benzoic acid and 2,5-dimethoxybenzoic acid (2,5-
(MeO)2C6H3COOH) with LiAlD4 in ether.33
Spectral and Kinetic Measurements. Reactions of benzyl
alcohol and its derivatives with [FeIV(O)(N4Py)]2+ (1.0 × 10−4 M)
were examined by monitoring spectral changes in the presence of
various concentrations of benzyl alcohol or its derivatives (1.0 ×
10−3−2.0 × 10−1 M) in the absence and presence of Sc(OTf)3 in
deaerated MeCN at 298 K using a Hewlett-Packard 8453 photodiode-
array spectrophotometer and a quartz cuvette (path length =10 mm).
Kinetic measurements for 2,5-(MeO)2C6H3CH2OH in the presence of
Sc3+ were performed on a UNISOKU RSP-601 stopped-flow
spectrometer equipped with a MOS-type highly sensitive photodiode
array or a Hewlett-Packard 8453 photodiode array spectrophotometer
at 298 K. Rates of reactions of benzyl alcohol and its derivatives with
[FeIV(O)(N4Py)]2+ were monitored by a decrease in the absorption
band due to [FeIV(O)(N4Py)]2+ (λmax = 695 nm) in the absence and
presence of Sc(OTf)3 in MeCN. The concentration of benzyl alcohol
or its derivatives was maintained to be more than 10-fold excess of
[FeIV(O)(N4Py)]2+ to keep pseudo-first-order conditions. Pseudo-
first-order rate constants were determined by a least-squares fit of the
first-order plot of time course of spectral change. Reactions of 3,4,5-
(MeO)3C6H2CH2OH with [FeIV(O)(N4Py)]2+ in the presence of Sc3+
were performed in the presence of 0.10 mM of [FeIV(O)(N4Py)]2+,
0.050 mM of 3,4,5-(MeO)3C6H2CH2OH, and excess amount of Sc3+
(5.0−20 mM).
Electrochemical Measurements. Measurements of cyclic
voltammetry (CV) and second harmonic AC voltammetry
(SHACV) were performed at 298 K using a BAS 630B electrochemical
analyzer in deaerated MeCN containing 0.10 M TBAPF6 as a
supporting electrolyte at 298 K. A conventional three-electrode cell
was used with a platinum working electrode and a platinum wire as a
counter electrode. The measured potentials were recorded with
respect to Ag/AgNO3 (1.0 × 10−2 M). The one-electron oxidation
potential values (Eox) (vs Ag/AgNO3) were converted to those vs SCE
by adding 0.29 V.34 All electrochemical measurements were carried out
under an Ar atmosphere.
EPR Measurements. Electron paramagnetic resonance (EPR)
detection of iron(III) complexes and radical cation of 2,5-
(MeO)2C6H3CH2OH was performed as follows: Typically, a MeCN
solution of [FeIV(O)(N4Py)]2+ (1.0 × 10−3 M) in the absence and
presence of Sc(OTf)3 (1.0 × 10−2 M) in an EPR cell (3.0 mm i.d.) was
purged with N2 for 5 min. Then, deaerated benzyl alcohol or 2,5-
(MeO)2C6H3CH2OH solution (5.0 × 10−2 M) was added to the
solution. The EPR spectra of the radical cation of 2,5-
(MeO)2C6H3CH2OH and iron(III) complexes were recorded on a
JEOL JES-RE1XE spectrometer at 243 and 77 K, respectively. The
magnitude of modulation was chosen to optimize the resolution and
signal-to-noise (S/N) ratio of the observed spectra under non-
saturating microwave power conditions. The g value was calibrated
using an Mn2+ marker (g = 2.034, 1.981). Computer simulation of the
EPR spectra was carried out by using Calleo EPR version 1.2 (Calleo
Scientific Publisher) on a personal computer.
HAT can be generally regarded as concerted proton−electron
transfer (CPET), because an electron and a proton are
transferred simultaneously to the metal center and the oxo
moiety, respectively.11 Tremendous efforts have so far been
devoted to elucidate the mechanism of HAT reactions of iron-
oxo species by employing model complexes bearing heme and
nonheme ligands in the field of bioinorganic chemistry, where
the transfer of an electron and a proton proceeds in a concerted
maner.11−19 There are some cases where C−H bond activation
by iron-oxo species undergoes via an ET/PT pathway when
electron-rich substrates such as N,N-dimethylaniline derivatives
are used as substrates.20 In the case of HAT reactions from
substrates to triplet excited states of photosensitizers, the
mechanistic borderline of one-step HAT vs ET/PT pathways
has been clarified by changing the electron donor ability of
hydrogen donors as well as the electron acceptor ability of
hydrogen acceptors.21 The borderline of one-step hydride-
transfer vs ET/PT pathways has also been discussed for Sc3+-
promoted hydride transfer from an NADH analogue to a p-
benzoquinone derivative.22,23 There are also studies on the
mechanistic change from one-step HAT to ET/PT with metal-
centered oxidants.24,25 However, the mechanistic borderline of
one-step HAT vs ET/PT pathways in C−H bond cleavage by
iron-oxo species has yet to be clarified because of the lack of
systematic studies.
We report herein one example of the switch of the reaction
pathway from a one-step HAT pathway in C−H bond
activation of benzyl alcohol derivatives (X-C6H4CH2OH)
with a nonheme iron(IV)-oxo complex, [FeIV(O)(N4Py)]2+
(N4Py = N,N-bis(2-pyridylmethyl)-N-bis(2-pyridyl)-
methylamine),15,16 to a stepwise ET/PT pathway by addition
of Sc3+. We have recently reported that one-electron reduction
of [FeIV(O)(N4Py)]2+ is accelerated by the addition of Lewis
acids such as Ca2+, Mg2+, and Zn2+, and so forth.26,27 Electron-
acceptability of [FeIV(O)(N4Py)] is enhanced by the much
stronger binding of Sc3+ to [FeIII(O)(N4Py)] than [FeIV(O)-
(N4Py)]. We have chosen Sc3+ in this work, because Sc3+ has
the largest acceleration effect among examined metal ions. In
the presence of 10 mM of Sc3+, the reduction potential of the
iron(IV)-oxo complex (Ered) was shifted from 0.51 V vs SCE to
the positive direction up to 1.19 V.26−30 Such a change in the
Ered value of [FeIV(O)(N4Py)]2+ by the presence of Sc3+ and
well-determined reduction potential provides an excellent
opportunity to scrutinize the borderline of one-step HAT vs
ET followed by subsequent PT steps in C−H bond activation
by metal-oxo species in a systematic manner.
EXPERIMENTAL SECTION
■
Materials. Benzyl alcohol (C6H5CH2OH), pentamethylbenzyl
alcohol (Me5C6CH2OH), 2,5-dimethoxybenzyl alcohol (2,5-
(MeO)2C6H3CH2OH), 2,5-dimethoxybenzoic acid, and scandium(III)
trifluoromethanesulfonate (Sc(OTf)3) were purchased from Aldrich
Chemicals Co. p-Chlorobenzyl alcohol (p-ClC6H4CH2OH), p-
methylbenzyl alcohol (p-MeC6H4CH2OH), 3,5-dimethoxybenzyl
alcohol (3,5-(MeO)2C6H3CH2OH), 3,4,5-trimethoxybenzyl alcohol
(3,4,5-(MeO)3C6H2CH2OH), and benzoic acid were obtained from
Tokyo Chemical Industry Co., Ltd.; p-nitrobenzyl alcohol (p-
NO2 C6 H4 CH2 OH) and p-methoxybenzyl alcohol (p-
MeOC6H4CH2OH) were obtained from Wako Pure Chemical
Industries, Ltd. and lithium aluminum deuteride (LiAlD4) was
purchased from CIL, Inc. The commercially available compounds
used in this study were the best available purity and used without
further purification unless otherwise noted. Acetonitrile (MeCN) was
dried according to the literature procedures and distilled under Ar
NMR Measurements. 1H NMR spectra were recorded on a JEOL
JMN-AL-300 NMR spectrometer at room temperature. To obtain
clear NMR signal by removing inorganic products, reaction solutions
with Sc3+ were treated with alumina column before measurement. The
yields of the oxidation products were determined based on the peak of
iodobenzene (between 7.0 and 8.0 ppm), which is a product of the
reaction of [FeII(N4Py)]2+ with iodosylbenzene to form [FeIV(O)-
(N4Py)]2+.
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Figure 1. Spectral changes observed in the oxidation of benzyl alcohol (50 mM) by [FeIV(O)(N4Py)]2+ (0.1 mM) in MeCN at 298 K in the absence
(a) and presence (b) of Sc3+ (20 mM).
absence and presence of Sc3+ (10 mM) (Supporting
Calculations. Density functional theory (DFT) calculations on the
properties of molecules were performed with the UB3LYP density-
Information, Figures S2a and S2b). The electrospray ionization
functional and the 6-311G++(d,p) basis set.35 All calculations were
mass spectrometry (ESI-MS) spectrum of the reaction solution
performed using Gaussian 09, revision A.02.36 Graphical outputs of the
performed in the absence of Sc3+ shows peaks at m/z 539.1 and
computational results were generated with the Gauss View software
629.0, whose mass and isotope distribution indicate formation
of [FeIII(OH)(N4Py)]2+ and [FeIII(OCH2Ph)(N4Py)]2+, re-
spectively (Supporting Information, Figures S3a and S3b). The
formation of the iron(III) species was supported by taking EPR
spectra of the reaction solution (Supporting Information,
Figure S4a).38 In the presence of Sc3+, the ESI-MS spectrum of
the resulting solution of the reaction shows peak at m/z 572.4,
which indicates formation of [FeII(N4Py)(OTf)]+ (calcd. m/z
= 572.4) (Supporting Information, Figure S3c). The EPR
spectrum, however, indicates that the major product of the
reaction is not iron(II) but iron(III) species, [FeIII(N4Py)-
(NCMe)]3+ with orthogonal signals around g = 2.5 and 1.7
(Supporting Information, Figures S4b and S4c). The iron(III)
complex might not be detected by ESI-MS probably because of
the high one-electron-reduction potential of [FeIII(N4Py)-
(NCMe)]3+ (Ered = 1.0 V vs SCE) and the occurrence of the
one-electron reduction from the iron(III) to iron(II) species
under the ESI-MS condition.39 To confirm the oxidation state
of the iron complex in the resulting solution in both the
absence and the presence of Sc3+, a one-electron donor,
ferrocene (Fc) was added into the solutions and one equiv of
ferrocenium ion (Fc+) to [FeIV(O)(N4Py)]2+ was produced
with the concurrent formation of [FeII(N4Py)(NCMe)] in
both cases (Supporting Information, Figures S5a and S5b).
This indicates that [FeIV(O)(N4Py)]2+ acts as a one-electron
oxidant rather than a two-electron oxidant. It should be noted
that the [FeIII(OH)(N4Py)]2+ produced during the reaction is
not so reactive to oxidize C6H5CHOH•.
program (ver. 3.09) developed by Semichem, Inc.37
RESULTS AND DISCUSSION
■
Oxidation of Benzyl Alcohol by [FeIV(O)(N4Py)]2+.
Oxidation of benzyl alcohol by the iron(IV)-oxo complex was
examined both in the absence and presence of Sc3+ at 298 K in
MeCN. Addition of excess amount of benzyl alcohol (50 mM)
to the solution of [FeIV(O)(N4Py)]2+ (0.10 mM) caused the
spectral change with a clean isosbestic point in both the
absence and the presence of Sc3+ (20 mM) as shown in Figures
1a and 1b, respectively. The decay of the characteristic
absorption band due to [FeIV(O)(N4Py)]2+ (695 nm) was
accelerated with increase in concentration of benzyl alcohol in
both the absence and the presence of Sc3+ (Supporting
Information, Figures S1a and S1b), obeying pseudo-first-order
kinetics. The pseudo-first-order rate constants (kobs) increased
proportionally with an increase in concentration of benzyl
alcohol, and the second-order rate constant (kox) was
determined from the linear correlation to be 9.9 × 10−2 M−1
s−1 in the absence of Sc3+, which was nearly the same as the
value (1.1 × 10−1 M−1 s−1) determined in the presence of Sc3+
(20 mM) as shown in Supporting Information, Figures S1c and
S1d, respectively. In addition, the kobs values were constant
irrespective of the change in concentration of Sc3+ (Figure 2).
Product analysis of the oxidation of benzyl alcohol (1.0 mM)
by [FeIV(O)(N4Py)]2+ (1.0 mM) revealed the formation of
benzaldehyde with 50% yield as a sole product both in the
The rate of the oxidation of PhCD2OH in the presence of
Sc3+ (20 mM) in MeCN is significantly slower than that of
PhCH2OH as shown in Figure 3, giving a deuterium kinetic
isotope effect (KIE) of 7.2 at 298 K.40 As reported previously in
the oxidation of benzyl alcohol by [FeIV(O)(N4Py)]2+ in the
absence of Sc3+,17 such a large KIE value clearly indicates that
the rate-determining step (r.d.s.) of the reaction is HAT from
benzyl alcohol to [FeIV(O)(N4Py)]2+ in both the absence and
the presence of Sc3+. The subsequent HAT from C6H5CHOH•
to [FeIV(O)(N4Py)]2+ to yield C6H5CHO and [FeIII(OH)-
(N4Py)]2+ occurs with a much faster rate than the initial HAT
because of the weaker C−H bond in the radical species.41 This
reaction mechanism is summarized in Scheme 2. In the
presence of benzyl alcohol, [FeIII(OH)(N4Py)]2+ is converted
to [FeIII(OCH2C6H5)(N4Py)]2+ and H2O. No acceleration of
the initial HAT was observed in the presence of Sc3+.
Figure 2. Dependence of the pseudo-first-order rate constant (kobs
)
determined in the oxidation of PhCH2OH (1.0 × 102 mM) by
[FeIV(O)(N4Py)]2+ (0.10 mM) on concentration of Sc3+.
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respectively. Such a dependence of electron-transfer rate
constants on [Sc3+] was reported previously for metal ion-
coupled electron transfer from one-electron reductant to
[FeIV(O)(N4Py)]2+.27 For instance, the dependence of
second-order rate constant of electron transfer (ket) from
[FeII(bpy)3]2+ to [FeIV(O)(N4Py)]2+ on [Sc3+] is shown in
Figure 5 (black squares).
To determine the KIE value, the oxidation rate of a
deuterated substrate (2,5-(MeO)2C6H3CD2OH) by [FeIV(O)-
(N4Py)]2+ was also examined in both the absence and the
presence of Sc3+. In the absence of Sc3+, the oxidation rate of
2,5-(MeO)2C6H3CD2OH (1.0 × 102 mM) by [FeIV(O)-
(N4Py)]2+ (0.060 mM) was significantly slower than that of
2,5-(MeO)2C6H3CH2OH by [FeIV(O)(N4Py)]2+ under the
same conditions as employed in the oxidation of benzyl alcohol
(Figure 6a). Second-order rate constants (kH and kD) were
determined from the slopes of the plots of pseudo-first order
rate constants of 2,5-(MeO)2C6H3CH2OH (kH,obs) and (2,5-
(MeO)2C6H3CD2OH (kD,obs) vs [2,5-(MeO)2C6H3CH2OH]
and [2,5-(MeO)2C6H3CD2OH], respectively (red and blue
circles in Figure 6c), respectively. The KIE value was
determined to be 14 for the reaction performed at 298 K.
This clearly indicates that HAT is the rate-determining step as
the case of oxidation of benzyl alcohol by [FeIV(O)(N4Py)]2+
in Scheme 2.12b,16,42−45
Figure 3. Time profiles of decay at 695 nm due to [FeIV(O)(N4Py)]2+
(1.0 mM) in the oxidation of PhCH2OH (2.4 × 102 mM) (red) and
PhCD2OH (2.4 × 102 mM) (blue) in the presence of Sc3+ (20 mM) in
MeCN at 298 K. Inset shows the first-order plots of the spectral
changes.
Scheme 2. Proposed Mechanism of Benzyl Alcohol
Oxidation by [FeIV(O)(N4Py)]2+
In sharp contrast to the case of the absence of Sc3+, no KIE
was observed in the presence of Sc3+ (Figures 6b and 6c). The
disappearance of KIE by the presence of Sc3+ indicates a
mechanistic change from HAT to a process in which C−H
bond cleavage is not involved in the rate-determining step,
which is most likely to be electron transfer (vide infra). Indeed,
new transient absorption bands around 430 and 455 nm, which
were not observed in the absence of Sc3+ in Figure 4a, were
detected in the presence of Sc3+ (Figure 4b). These absorption
bands agree with those observed in the one-electron oxidation
of 2,5-(MeO)2C6H3CH2OH by a strong one-electron oxidant
such as CAN, as shown in Figure 4b (green line).46 This
indicates the occurrence of electron transfer from 2,5-
(MeO)2C6H3CH2OH to [FeIV(O)(N4Py)]2+ to produce a
radical cation of 2,5-(MeO)2C6H3CH2OH (i.e., 2,5-(MeO)2-
C6H3CH2OH•+). The formation of the radical cation was
confirmed by EPR measurements. Figure 7a shows an EPR
spectrum of 2,5-(MeO)2C6H3CH2OH•+ produced in the
oxidation of 2,5-(MeO)2C6H3CH2OH by [FeIV(O)(N4Py)]2+
in the presence of Sc3+. The hyperfine coupling constants
obtained in the computer simulation spectrum (Figure 7b) are
in a reasonable agreement with those calculated by the DFT
method (Figure 7c). The same EPR signal was observed in the
one-electron oxidation of 2,5-(MeO)2C6H3CH2OH by
[RuIII(bpy)3]3+ (Supporting Information, Figure S7a).
Oxidation of 2,5-(MeO)2C6H3CH2OH with [FeIV(O)-
(N4Py)]2+. Different from the benzyl alcohol oxidation shown
in Scheme 2, the rate of the oxidation of 2,5-
(MeO)2C6H3CH2OH by [FeIV(O)(N4Py)]2+ was significantly
accelerated by the presence of Sc3+ (Figure 4b), as compared to
the rate determined in the absence of Sc3+ (Figure 4a).
In the presence of Sc3+, an intermediate with two absorption
maxima at 430 and 455 nm appeared although it is not
observed in the absence of Sc3+ (vide infra). The decay of the
absorption band at 695 nm due to [FeIV(O)(N4Py)]2+ in both
the absence and the presence of Sc3+ obeyed first-order kinetics
(Supporting Information, Figures S6a and S6c).16 The pseudo-
first order rate constant (kobs) increased linearly with increasing
concentration of 2,5-(MeO)2C6H3CH2OH, and the second-
order rate constant (kox) was determined by plotting kobs vs
[2,5-(MeO)2C6H3CH2OH]. The dependence of kox on [Sc3+]
is shown in Figure 5 (red circles), where kobs values increased
with increasing concentration of Sc3+, exhibiting both a first-
order dependence and a second-order dependence on [Sc3+] as
given by eq 1, where k0 is the rate constant in the absence of
Sc3+, k1 and k2 are the rate constants exhibiting the first-order
and second-order dependences on [Sc3+]. The first-order and
second-order dependence of kox on [Sc3+] is shown by a linear
plot of (kox − k0)/[Sc3+] vs [Sc3+] as shown in Supporting
Information, Figure S7, where the intercept and the slope
correspond to the contribution of the first-order and second-
order dependence,
The decay of the EPR signal of 2,5-(MeO)2C6H3CH2OH•+
produced in the oxidation of 2,5-(MeO)2C6H3CH2OH by
[FeIV(O)(N4Py)]2+ in the presence of Sc3+ obeyed second-
order kinetics (Figure 8a).47 The second-order rate constant
was determined to be 2.3 × 103 M−1 s−1 from the linear second-
order plot (Figure 8b) and the initial concentration of 2,5-
(MeO)2C6H3CH2OH, which was determined by the double
integration of the EPR spectrum by comparing with an
authentic radical sample of 2,2-diphenyl-2-picrylhydrazyl
(DPPH). The decay of the EPR signal of 2,5-
(MeO)2C6H3CH2OH•+ produced in the one-electron oxidation
of 2,5-(MeO)2C6H3CH2OH by [RuIII(bpy)3]3+ also obeyed
second-order kinetics (Supporting Information, Figure S8b),
kox = k0 + k1[Sc3+] + k2[Sc3+]2
(1)
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Figure 4. (a) Spectral changes observed in the oxidation of 2,5-(MeO)2C6H3CH2OH (20 mM) by [FeIV(O)(N4Py)]2+ (0.060 mM) in the absence
of Sc3+ in deaerated MeCN. Right panel shows time courses monitored at 695 nm (blue) and 450 nm (red). (b) Spectral changes observed in the
oxidation of 2,5-(MeO)2C6H3CH2OH (2.5 mM) by [FeIV(O)(N4Py)]2+ (0.060 mM) in the presence of Sc3+ (10 mM) (left panel; 4 s (blue), 30 s
(black), and 120 s (red) after mixing) in deaerated MeCN. Green line indicates the reference spectrum of 2,5-(MeO)2C6H3CH2OH radical cation
produced by oxidizing 2,5-(MeO)2C6H3CH2OH with (NH3)2[CeIV(NO3)6] (CAN). Right panel shows time courses monitored at 695 nm (blue)
and 420 nm due to 2,5-(MeO)2C6H3CH2OH radical cation (red).
aldehyde, 2,5-(MeO)2C6H3CHO, with 50% yield as the case of
benzyl alcohol oxidation (Supporting Information, Figure
S9).48 The major inorganic products were determined to be
[FeIII(OH)(N4Py)]2+ and [FeIII(OCH2C6H3(OMe)2)-
(N4Py)]2+ by ESI-MS and EPR spectroscopy (Supporting
Information, Figures S10 and S11). Titration of the resulting
solution by Fc resulted in formation of Fc+ with the same
concentration as the initial concentration of [FeIV(O)-
(N4Py)]2+ to produce [FeII(N4Py)(NCMe)]2+ (Supporting
Information, Figure S12). This indicates that [FeIV(O)-
(N4Py)]2+ acts as a one-electron oxidant rather than two-
electron oxidant in the oxidation of 2,5-(MeO)2C6H3CH2OH
as the case of PhCH2OH oxidation.
Figure 5. Plots of the second-order rate constant (kox) vs Sc3+
In the presence of Sc3+ (10 mM), however, the oxidation of
concentration in the oxidaiton of 2,5-(MeO)2C6H3CH2OH by
2,5-(MeO)2C6H3CH2OH by [FeIV(O)(N4Py)]2+ (1.0 mM)
resulted in the oxidative coupling to yield 2,2′,5,5′-tetrame-
thoxybiphenylyl-4,4′-dimethanol as a major product (31% yield,
0.31 mM) together with the further oxidized product 2,2′,5,5′-
tetramethoxybiphenylyl-4,4′-dicarbaldehyde (1.0% yield, 0.010
mM) as well as the corresponding aldehyde (2,5-
(MeO)2C6H3CHO) only in 10% yield (0.10 mM) as shown
in Scheme 3 with each products’ yield and the oxidation
equivalent (see Supporting Information, Figure S8b).48 The
ESI-MS and EPR spectrum of the resulting solution and
titration of the resulting solution by Fc indicate that the
inorganic products are iron(III) species and [FeIV(O)-
(N4Py)]2+ acts as a one-electron oxidant in the presence of
Sc3+ as is the case in the absence of Sc3+ (Supporting
Information, Figures S10 and S11).49 The overall oxidation
[FeIV(O)(N4Py)]2+ (red circles) and the second-order rate constant
(ket) vs Sc3+ concentration in the Sc3+-coupled ET from [Fe(bpy)3]2+
to [FeIV(O)(N4Py)]2+ (black squares) in deaerated MeCN at 298 K.
indicating that bimolecular reactions of 2,5-
(MeO)2C6H3CH2OH•+ are responsible for the decay of 2,5-
(MeO)2C6H3CH2OH•+.
Product analyses of the oxidation of 2,5-
(MeO)2C6H3CH2OH by [FeIV(O)(N4Py)]2+ in the absence
and presence of Sc3+ were performed by the same method for
1
the benzyl alcohol oxidation. H NMR spectra of the reaction
products obtained in the oxidation of 2,5-(MeO)2C6H3CH2OH
(1.0 mM) by one equiv of [FeIV(O)(N4Py)]2+ (1.0 mM) in the
absence of Sc3+ in CD3CN revealed that 2,5-
(MeO)2C6H3CH2OH was converted to the corresponding
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Figure 6. (a) Time courses of the absorption spectral changes observed at 695 nm due to [FeIV(O)(N4Py)]2+ (0.060 mM) in the oxidation of 2,5-
(MeO)2C6H3CH2OH (1.0 × 102 mM) (red) and 2,5-(MeO)2C6H3CD2OH (1.0 × 102 mM) (blue) in the absence of Sc3+. Inset shows first-order
plots of the absorption change at 695 nm. (b) Time courses of the absorption spectral changes observed at 695 nm due to [FeIV(O)(N4Py)]2+
(0.060 mM) in the oxidation of 2,5-(MeO)2C6H3CH2OH (2.5 mM) (red) and 2,5-(MeO)2C6H3CD2OH (2.5 mM) (blue) in the presence of Sc3+
(2.5 mM). Inset shows first-order plots of the absorption change at 695 nm. (c) Plots of kH,obs (red) and kD,obs (blue) vs [2,5-(MeO)2C6H3CH2OH]
and [2,5-(MeO)2C6H3CD2OH] in the absence (circles) and presence (squares) of Sc3+ (2.5 mM), respectively.
Figure 7. (a) X-band EPR spectrum of 2,5-(MeO)2C6H3CH2OH•+ produced by electron-transfer oxidation of 2,5-(MeO)2C6H3CH2OH (1.0 mM)
by [FeIV(O)(N4Py)]2+ (1.0 mM) in the presence of Sc3+ (10 mM) in deaerated MeCN at 298 K. (b) The computer simulation spectrum. (c) DFT
optimized structure of 2,5-(MeO)2C6H3CH2OH•+ with hyperfine coupling constants together with the calculated values given in parentheses.
efficiency was determined to be 88% by counting the number of
electrons oxidized by [FeIV(O)(N4Py)]2+ (31 × 2 + 10 × 2 + 1
× 6 = 88). Because oxidative coupling products were obtained
by the oxidation of 2,5-(MeO)2C6H3CH2OH with one-electron
oxidant CAN,50−52 formation of the dimer products, together
with the detection of the radical cation intermediate by
absorption and EPR spectra in the oxidation of 2,5-
(MeO)2C6H3CH2OH by [FeIV(O)(N4Py)]2+ in the presence
of Sc3+ indicates the change of the reaction pathway from the
one-step HAT in the absence of Sc3+ (step a shown in Scheme
4) to the Sc3+-coupled ET pathway in the presence of Sc3+
(step b shown in Scheme 4).53,54 This is consistent with the
bimolecular decay kinetics of 2,5-(MeO)2C6H3CH2OH•+ in
Figure 8.
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Figure 8. (a) Time course of decay of [2,5-(MeO)2C6H3CH2OH•+] observed by EPR measured at 243 K in deaerated MeCN. (b) Second-order
plot of the time course of decay in EPR signal of 2,5-(MeO)2C6H3CH2OH•+.
Scheme 3. Proposed Oxidation Pathways for Oxidation of 2,5-(MeO)2C6H3CH2OH by [FeIV(O)(N4Py)]2+ in the Presence of
Sc3+
Scheme 4. Reaction Pathway for Oxidation of 2,5-(MeO)2C6H3CH2OH by [FeIV(O)(N4Py)]2+ in the Absence and Presence of
Sc3+
The Sc3+-coupled electron-transfer pathway is further
confirmed by the first-order and second-order dependence of
kox of the oxidation of 2,5-(MeO)2C6H3CH2OH by [FeIV(O)-
(N4Py)]2+ in the presence of Sc3+ on [Sc3+], which is quite
similar to that observed Sc3+-coupled ET from [Fe(bpy)3]2+
(one-electron reductant) to [FeIV(O)(N4Py)]2+ on [Sc3+]
(Figure 5) and the absence of KIE.
benzyl alcohol derivatives (see Supporting Information, Table
S1 for the Eox values of benzyl alcohol derivatives). To explore
the borderline between one-step HAT and Sc3+-coupled ET
pathways, we investigated the dependence of the second-order
rate constants for the oxidation of a series of benzyl alcohol
derivatives on [Sc3+]. The results are shown in Figures 9a−9g.
The kox values for the oxidation of benzyl alcohol derivatives
with relatively high Eox values, p-NO2C6H4CH2OH,
C6H5CH2OH, and p-ClC6H4CH2OH, were the same as those
in the absence of Sc3+ with increasing concentration of Sc3+
(Figures 9a−9c), whereas the kox values for the oxidation of
Effect of Sc3+ on the Oxidation Rate of Benzyl Alcohol
Derivatives by [FeIV(O)(N4Py)]2+. The change in the reaction
pathway from one-step HAT to Sc3+-coupled ET may be
determined by the one-electron oxidation potentials (Eox) of
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Figure 9. Plots of kox vs [Sc3+] in the reaction of [FeIV(O)(N4Py)]2+ (0.10 mM) and benzyl alcohol derivatives in the presence of Sc3+ in deaerated
MeCN at 298 K: (a) p-NO2C6H4CH2OH (25 mM), (b) p-ClC6H4CH2OH (50 mM), (c) p-MeC6H4CH2OH (25 mM), (d) p-MeOC6H4CH2OH
(50 mM), (e) Me5C6CH2OH (25 mM), (f) 3,5-(MeO)2C6H3CH2OH (10 mM), and (g) 3,4,5-(MeO)3C6H2CH2OH (0.050 mM).
benzyl alcohol derivatives with lower oxidation potentials, p-
MeC6H4CH2OH, p-MeOC6H4CH2OH, Me5C6CH2OH, 3,5-
(MeO)2C6H3CH2OH, 3,4,5-(MeO)3C6H2CH2OH, and 2,5-
(MeO)2C6H3CH2OH, increased with increasing concentration
of Sc3+ (Figures 9d−9g) as is the case for the oxidation of 2,5-
(MeO)2C6H3CH2OH (Figure 5). Although no radical cation
intermediates were observed in the oxidation of p-
MeOC6H4CH2OH, Me5C6CH2OH, 3,5-(MeO)2C6H3CH2OH,
and 3,4,5-(MeO)3-C6H2CH2OH in the presence of Sc3+ (10
mM), the acceleration may be attributed to Sc3+-coupled ET.
The −ΔGet and kox values of the oxidation of a series of benzyl
alcohol derivatives by [FeIV(O)(N4Py)]2+ in the absence and
presence of Sc3+ (10 mM) are summarized in Table 1. The
−ΔGet values (in eV) were determined from the Eox values of
benzyl alcohol derivatives for the one-electron oxidation to give
the corresponding radical cations and the Ered values of
[FeIV(O)(N4Py)]2+ in the absence and the presence of Sc3+ (10
mM) by eq 2, where e is the elementary charge.55
Table 1. Driving Force for ET (−ΔGet) and Second-Order
Rate Constants (kox) for Oxidation of Benzyl Alcohol
Derivatives with [FeIV(O)(N4Py)]2+ in the Absence and
Presence of Sc3+ (10 mM) in MeCN at 298 K
−ΔGet, eV
kox, M−1 s−1
Sc3+
(0 mM)
Sc3+
(10 mM)
Sc3+
(0 mM)
Sc3+
(10 mM)
entry substituent
1
2
3
4
5
6
7
8
p-NO2
p-H
−2.37
−1.82
−1.74
−1.54
−1.14
−1.07
−0.98
−0.71
−1.69
−1.14
−1.04
−0.86
−0.61
−0.59
−0.30
−0.23
1.1 × 10−1
9.9 × 10−2
1.0 × 10−1
1.4 × 10−1
3.5 × 10−2
1.4 × 10−1
1.2 × 10−1
1.5 × 10−1
1.3 × 10−1
9.9 × 10−2
1.1 × 10−1
1.7 × 10−1
5.6 × 10−2
2.5 × 10−1
3.8 × 10−1
4.0 × 10
p-Cl
p-Me
Me5
p-MeO
3,5-(MeO)2
3,4,5-
(MeO)3
9
2,5-(MeO)2
−0.69
−0.14
2.0 × 10−1
1.3 × 10
−ΔGet = e(Ered − Eox)
(2)
Figure 10 shows plots of log kox of oxidation of benzyl
alcohols by [FeIV(O)(N4Py)]2+ in the absence and presence of
Sc3+ (10 mM) and log ket of electron transfer from one-electron
reductants to [FeIV(O)(N4Py)]2+ in the presence of Sc3+ (10
mM) vs driving force of ET (−ΔGet). In the absence of Sc3+,
the −ΔGet values are largely negative (endergonic) and the log
kox values are rather independent of the −ΔGet values (blue
points in Figure 10). The log kox values in the absence of Sc3+
are much larger than those predicted by the extrapolated line of
electron transfer, indicating that the oxidation of benzyl alcohol
derivatives by [FeIV(O)(N4Py)]2+ undergoes via one-step HAT
rather than via electron transfer. This is also supported by the
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borderline of the mechanism change between one-step HAT
and Sc3+-coupled ET. The results at a different concentration of
Sc3+ (1.0 mM) are presented in Supporting Information, Figure
S14. Although the best fit λ value (2.33 eV) for the driving
force dependence of log ket with 1.0 mM Sc3+ becomes larger
than the value with 10 mM Sc3+ (2.27 eV) and therefore the
plot looks somewhat different, the conclusion on the
mechanism change between one-step HAT and Sc3+-coupled
ET remains virtually the same.
It should be noted that it is rather unusual to apply the
Marcus theory to conditional rate constants that depend on the
concentration of Sc3+. Nevertheless we can explain the
dependence of the rate constant on the concentration of Sc3+
using the Marcus theory as follows.58 Under the conditions
such that λ ≪ −ΔGet in Figure 10, the ket value is estimated by
Figure 10. Plots of log kox for oxidation of benzyl alcohol derivatives
(numbers refer to compounds in Table 1) by [FeIV(O)(N4Py)]2+ in
MeCN at 298 K vs −ΔGet for electron transfer from benzyl alcohol
derivatives to [FeIV(O)(N4Py)]2+ in the absence (blue) of Sc3+ and
the presence (red) of Sc3+ (10 mM). Black points are plots of log ket of
electron transfer from one-electron reductants (a: [RuII(bpy)3]2+, b:
[FeII(Clphen)3]2+, c: [RuII(Me2bpy)3]2+, d: [FeII(bpy)3]2+, e:
[FeII(Ph2phen)3]2+) to [FeIV(N4Py)(O)]2+ in the presence of 10
mM of Sc3+ in MeCN at 298 K vs −ΔGet. The blue and red parts
correspond to HAT and ET pathways, respectively. The solid lines are
drawn using eq 4 with λ values of 1.93 and 2.27 eV.
the Marcus cross relationship as shown in eq 4,57 where kDex
,
k
Aex, and Ket are the rate
ket = (kDex Aex
k
Ket)1/2
(4)
constant of electron exchange between electron donor (D) and
the radical cation, the rate constant of electron exchange
between electron acceptor (A) and the one-electron reduced
species, and the equilibrium constants of the electron transfer
from D to A. The kDex value is constant independent of
concentration of Sc3+, whereas kAex and Ket are expected to
increase in proportion to [Sc3+]2 at high concentrations of Sc3+
as given by eqs 5 and 6, respectively, where kAex0 and Ket0 are
the proportional constants. In such a case, the dependence of
ket on
KIE observed in the oxidations of C6H5CD2OH and 2,5-
(MeO)2C6H3CD2OH in the absence of Sc3+ (Figure 3).
In sharp contrast to the case in the absence of Sc3+, log kox
values of benzyl alcohol derivatives, which are accelerated by
the presence of Sc3+ (10 mM) (entries 7−9), show similar
driving force dependence to that of log ket of electron transfer
from a series of one-electron reductants to [FeIV(O)(N4Py)]2+
in the presence Sc3+ (10 mM).56 Both log kox and log ket values
increase with increasing ET driving force (−ΔGet values). The
ET driving force dependence of ket and kox is well fitted by the
Marcus equation of outer-sphere electron transfer (eq 3),
where Z is the frequency factor (1 × 1011 M−1 s−1) and λ is the
reorganization energy of electron transfer.57,58 The dependence
of log kox on −ΔGet clearly indicates the
kAex = kAex0[Sc3+]2
(5)
Ket = Ket0[Sc3+]2
(6)
[Sc3+] is given by eq 7, which agrees with the results in Figure
5.
ket = (kDex Aex0
Ket0)1/2[Sc3+]2
k
(7)
The dependence of log k1 and log k2 of Sc3+-coupled ET on
−ΔGet in the absence of Sc3+ is shown in Supporting
Information, Figure S15, where a roughly parallel relationship
between k1 and k2 is observed. This plot for benzyl alcohol
derivatives is not shown because the k1 and k2 values of benzyl
alcohol derivatives close to the region of one-step HAT could
not be obtained accurately because of the large contribution of
k0.
log kox = log Z − λ(1 + ΔGet/λ)2/(2.3 × 4kBT)
(3)
occurrence of Sc3+-coupled ET from benzyl alcohol derivatives
to [FeIV(O)(N4Py)]2+. Although the −ΔGet values for entry
7−9 are negative (endergonic), the Sc3+-coupled ET may be
followed by fast subsequent reaction (pathway b in Scheme 4).
The λ values for electron transfer from one-electron reductants
and benzyl alcohol derivatives to [FeIV(O)(N4Py)]2+ in the
presence of 10 mM of Sc3+ are determined to be 2.27 and 1.93
eV by fitting plots of entries a−e and 7−9 in red in Figure 10
with eq 3, respectively.
CONCLUSION
■
In this study, we have demonstrated the change of the rate-
determining step in the oxidation of benzyl alcohol derivatives
by [FeIV(O)(N4Py)]2+ from one-step HAT to Sc3+-coupled ET
depending on the one-electron oxidation potentials of benzyl
alcohol derivatives. The change in the reaction mechanism is
initiated by acceleration of ET by Sc3+ while HAT is not
accelerated by Sc3+ at all.61 Such a change in the reaction
pathways by the presence of Sc3+ has been clearly shown by the
disappearance of KIE in the presence of Sc3+ in contrast to a
large KIE value observed in the absence of Sc3+, when
formation of the radical cation of a benzyl alcohol derivative
was detected as the initial product of Sc3+-coupled ET from the
substrate to [FeIV(O)(N4Py)]2+, leading to the dimerized
product as a major product in contrast to the corresponding
aldehyde obtained as the sole product in the absence of Sc3+.
On the other hand, the kox values of benzyl alcohol
derivatives are independent of the ET driving force even in
the presence of Sc3+ (10 mM) when −ΔGet is smaller than
−0.5 eV (entries 1−4 in Table 1). This indicates that the
oxidation of those benzyl alcohol derivatives proceeds via one-
step HAT, which is an energetically much more favorable
pathway than the highly endergonic Sc3+-coupled ET. The rates
of oxidation of benzyl alcohol derivatives (entries 5 and 6 in
Table 1) are slightly accelerated by the presence of Sc3+ (10
mM) (1.8 and 1.6 times respectively), and the log kox values are
on the borderline of the mechanism change between one-step
HAT and Sc3+-coupled ET.59,60
The comparison of the driving force dependence of kox and
ket in Figure 10 has provided a clear view with regard to the
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The mechanistic borderline between one-step HAT and Sc3+-
coupled ET has been found to be determined by the ET driving
force from the substrate to [FeIV(O)(N4Py)]2+ with the
borderline of −ΔGet ≈ −0.5 eV. The C−H bond is cleaved via
HAT when −ΔGet is more negative than −0.5 eV, whereas
Sc3+-coupled ET becomes a predominant pathway when −ΔGet
is more positive than −0.5 eV. In other words, Sc3+-coupled ET
occurs when Eox of substrate is more negative than 1.7 V. This
study provides the first example for the change in the
mechanism of oxidation of substrates by a high-valent metal-
oxo complex from one-step HAT to ET that is accelerated by
Sc3+ depending on the ET driving force. The oxidation reaction
takes place even when −ΔGet is negative, that is, ET is
endergonic, indicating the ET process is coupled with the
following proton transfer. It is of interest to note that the
borderline between one-step HAT and Sc3+-coupled ET is at
the ET driving force (ΔGet) of about −0.5 eV, which is similar
to that reported previously for the borderline between one-step
oxygen atom transfer and Sc3+-coupled ET.57 This type of
switching in reaction pathway from HAT to ET depending on
the −ΔGet value would generally appear in the reaction systems
where high-valent oxometal species are employed as an oxidant
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ASSOCIATED CONTENT
* Supporting Information
Further details are given in Table S1 and Figures S1−S13. This
material is available free of charge via the Internet at http://
pubs.acs.org.
■
S
AUTHOR INFORMATION
Corresponding Author
*E-mail: fukuzumi@chem.eng.osaka-u.ac.jp (S.F.), wwnam@
ewha.ac.kr (W.N.).
■
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This work at OU was supported by a Grant-in-Aid (No
20108010 to S.F.) and a Global COE program, “the Global
Education and Research Center for Bio-Environmental
Chemistry” from MEXT, Japan (to S.F.), the work at EWU
was supported by NRF/MEST of Korea through CRI (to
W.N.), GRL (2010-00353) (to W.N.), and WCU (R31-2008-
000-10010-0) (to W.N. and S.F.). Y.M. appreciates support
from the Global COE program of Osaka University and Grant-
in-Aid for JSPS fellowship for young scientists.
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iron(IV) and iron(II) complexes as a reason why [FeIV(O)(N4Py)]2+
acts as a one-electron oxidant rather than a two-electron oxidant is
unlikely under the present reaction conditions.
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reaction was started by mixing two reactant solutions, which is about 5
min before starting EPR measurements, suggests that the radical cation
is produced quantitatively.
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(48) Yield = [Product]/[2,5-(MeO)2C6H3CH2OH]0 × 100 where
[2,5-(MeO)2C6H3CH2OH]0 is the initial concentration of 2,5-
(MeO)2C6H3CH2OH.
(49) We could not observe the Sc3+-bound complexes by ESI-mass
except for [FeIII(N4Py)(NCMe)]3+ which may be due to weak Fe-O
bond of the Sc3+-bound complex. However, the EPR spectrum of
resulting solution shown in Supporting Information, Figure S11b
agreed with the spectrum of the Sc3+-bound complex reported in our
previous study.27a
(27) (a) Morimoto, M.; Kotani, H.; Park, J.; Lee, Y.-M.; Nam, W.;
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see: Fukuzumi, S.; Morimoto, Y.; Kotani, H.; Naumov, P.; Lee, Y.-M.;
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electron transfer. See: (a) Fukuzumi, S.; Ohkubo, K. Chem.Eur. J.
2000, 6, 4532. (b) Fukuzumi, S.; Ohkubo, K. J. Am. Chem. Soc. 2002,
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(51) The major product in the oxidation of 2,5-
(MeO)2C6H3CH2OH by CAN is not a dimer of aldehyde but
diquinone, 4,4′-dimethoxy-[bi-1,4-cyclohexadien-1-yl]-3,3′,6,6′-te-
trone.50
(30) For the effect of Brønsted acid on the reactivity of the iron(IV)-
oxo complex, see: Park, J.; Morimoto, Y.; Lee, Y.-M.; Nam., W.;
Fukuzumi, S. J. Am. Chem. Soc. 2012, 134, 3903.
(31) Armarego, W. L. F.; Chai, C. L. L. Purification of Laboratory
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Wiley: New York, 1973; p 658.
(52) In the oxidation of benzyl alcohol by [Fe(O)(N4Py)]2+ both in
the presence and absence of Sc3+, we cannot detect dimerized products
as shown in Supporting Information, Figure S2. This may be because
the hydrogen-abstracted radicals by [Fe(O)(N4Py)]2+ can not
produce dimerized products not like radical cations of substrates.
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(53) In the oxidation of 2,5-(MeO)2C6H3CH2OH by [RuIII(bpy)3]2+,
no aldehyde formation was observed as a product. This indicates that
[FeIII(O)(N4Py)]2+−Sc3+ works as a base to accept a proton released
from 2,5-(MeO)2C6H3CH2OH•+ to produce 2,5-(MeO)2C6H3CHO.
(54) The pseudo-first-order rate constants were proportional to
concentrations of substrates without exhibiting no intercepts as shown
in Figure 6. This indicates that the rate-determining step is the Sc3+-
coupled electron transfer followed by subsequent reactions which are
faster than the back electron-transfer reaction.
(55) Some of benzyl alcohol derivatives interact with Sc3+. This
interaction resulted in the positive shifts of the one-electron oxidation
potentials (Supporting Information, Table S1).
(56) The observed rate constant (kox) consists of three rate
constants, i.e., k0, k1, and k2 (eq 1). Under the conditions in Figure
5 ([Sc3+] = 10 mM), k2 is the main component. Thus, kox in Figure 10
virtually corresponds to the rate constant for elementary step electron
transfer to [FeIV(O)(N4Py)]2+-(Sc3+)2.27a
(57) (a) Marcus, R. A. Annu. Rev. Phys. Chem. 1964, 15, 155.
(b) Marcus, R. A. Angew. Chem., Int. Ed. Engl. 1993, 32, 1111.
(58) The Marcus equation can be applied at different concentrations
of Sc3+, when the driving force and reorganization energy of electron
transfer are changed as discussed previously on metal ion-coupled
electron transfer; see: Okamoto, K.; Imahori, H.; Fukuzumi, S. J. Am.
Chem. Soc. 2003, 125, 7014.
(59) For the use of the Marcus plot to clarify the reaction
mechanisms of [FeIV(O)(N4Py)]2+, see: (a) Park, J.; Morimoto, Y.;
Lee, Y.-M.; Nam, W.; Fukuzumi, S. J. Am. Chem. Soc. 2011, 133, 5236.
(b) Park, J.; Morimoto, Y.; Lee, Y.-M.; You, Y.; Nam, W.; Fukuzumi, S.
Inorg. Chem. 2011, 50, 11612.
(60) No clear correlation between log kox values and bond
dissociation energies (BDE(C−H)) of C−H bonds at the benzyl
position of a series of benzyl alcohol derivatives calculated with DFT
excludes the possibility that the reaction pathway is controlled by
BDE(C−H) (Supporting Information, Figure S13).
(61) The binding of Sc3+ to [FeIV(O)(N4Py)]2+ may increase the
oxidizing ability. However, the direct hydrogen atom transfer reaction
may be prohibited by the steric effect of Sc3+. In contrast to this,
electron transfer is generally insensitive to the steric effect because
outer-sphere electron transfer requires little interaction between
electron donor and acceptor molecules. This is the reason why ET
is accelerated whereas no acceleration of HAT occurs by the addition
of Sc3+.
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