Bronsted acid-promoted C-H bond cleavage via electron transfer from toluene derivatives to a protonated nonheme iron(IV)-oxo complex with no kinetic isotope effect

Article abstract of DOI:10.1021/ja311662w

The reactivity of a nonheme iron(IV)-oxo complex, [(N4Py)Fe ^IV(O)]²⁺ (N4Py = N,N-bis(2-pyridylmethyl)-N-bis(2-pyridyl) methylamine), was markedly enhanced by perchloric acid (70% HClO₄) in the oxidation of toluene derivatives. Toluene, which has a high one-electron oxidation potential (E_ox = 2.20 V vs SCE), was oxidized by [(N4Py)Fe^IV(O)]²⁺ in the presence of HClO₄ in acetonitrile (MeCN) to yield a stoichiometric amount of benzyl alcohol, in which [(N4Py)Fe^IV(O)]²⁺ was reduced to [(N4Py)Fe ^III(OH₂)]³⁺. The second-order rate constant (k_obs) of the oxidation of toluene derivatives by [(N4Py)Fe ^IV(O)]²⁺ increased with increasing concentration of HClO₄, showing the first-order dependence on [HClO₄]. A significant kinetic isotope effect (KIE) was observed when mesitylene was replaced by mesitylene-d₁₂ in the oxidation with [(N4Py)Fe ^IV(O)]²⁺ in the absence of HClO₄ in MeCN at 298 K. The KIE value drastically decreased from KIE = 31 in the absence of HClO₄ to KIE = 1.0 with increasing concentration of HClO₄, accompanied by the large acceleration of the oxidation rate. The absence of KIE suggests that electron transfer from a toluene derivative to the protonated iron(IV)-oxo complex ([(N4Py)Fe^IV(OH)]³⁺) is the rate-determining step in the acid-promoted oxidation reaction. The detailed kinetic analysis in light of the Marcus theory of electron transfer has revealed that the acid-promoted C-H bond cleavage proceeds via the rate-determining electron transfer from toluene derivatives to [(N4Py)Fe^IV(OH)] ³⁺ through formation of strong precursor complexes between toluene derivatives and [(N4Py)Fe^IV(OH)]³⁺.

Full text of DOI:10.1021/ja311662w

Article
pubs.acs.org/JACS
Brønsted Acid-Promoted C−H Bond Cleavage via Electron Transfer
from Toluene Derivatives to a Protonated Nonheme Iron(IV)-Oxo
Complex with No Kinetic Isotope Effect
Jiyun Park,† Yong-Min Lee,‡ Wonwoo Nam,*,‡ and Shunichi Fukuzumi*,†,‡
†Department of Material and Life Science, Graduate School of Engineering, ALCA, Japan Science and Technology Agency, Osaka
University, Suita, Osaka 565-0871, Japan
‡Department of Bioinspired Science, Ewha Womans University, Seoul 120-750, Korea
S
* Supporting Information
ABSTRACT: The reactivity of a nonheme iron(IV)-oxo
complex, [(N4Py)FeIV(O)]2+ (N4Py = N,N-bis(2-pyridyl-
methyl)-N-bis(2-pyridyl)methylamine), was markedly en-
hanced by perchloric acid (70% HClO4) in the oxidation of
toluene derivatives. Toluene, which has a high one-electron
oxidation potential (Eox = 2.20 V vs SCE), was oxidized by
[(N4Py)FeIV(O)]2+ in the presence of HClO4 in acetonitrile
(MeCN) to yield a stoichiometric amount of benzyl alcohol, in
which [(N4Py)FeIV(O)]2+ was reduced to [(N4Py)-
FeIII(OH2)]3+. The second-order rate constant (kobs) of the oxidation of toluene derivatives by [(N4Py)FeIV(O)]2+ increased
with increasing concentration of HClO4, showing the first-order dependence on [HClO4]. A significant kinetic isotope effect
(KIE) was observed when mesitylene was replaced by mesitylene-d12 in the oxidation with [(N4Py)FeIV(O)]2+ in the absence of
HClO4 in MeCN at 298 K. The KIE value drastically decreased from KIE = 31 in the absence of HClO4 to KIE = 1.0 with
increasing concentration of HClO4, accompanied by the large acceleration of the oxidation rate. The absence of KIE suggests that
electron transfer from a toluene derivative to the protonated iron(IV)-oxo complex ([(N4Py)FeIV(OH)]3+) is the rate-
determining step in the acid-promoted oxidation reaction. The detailed kinetic analysis in light of the Marcus theory of electron
transfer has revealed that the acid-promoted C−H bond cleavage proceeds via the rate-determining electron transfer from
toluene derivatives to [(N4Py)FeIV(OH)]3+ through formation of strong precursor complexes between toluene derivatives and
[(N4Py)FeIV(OH)]3+.
ferrocene derivatives.18,19 With regard to C−H bond cleavage,
INTRODUCTION
■
nonheme iron(IV)-oxo complexes reported to date are capable
High-valent iron-oxo complexes with heme and nonheme
of activating weak C−H bonds of activated alkanes (e.g.,
ligands play pivotal roles as reactive intermediates in biological
and chemical oxidation reactions.1,2 Extensive efforts have been
alkylaromatics such as xanthene, 9,10-dihydroanthracene, and
fluorene).15 In the presence of Sc(OTf)3 (OTf− = triflate
devoted to synthesize various nonheme iron(IV)-oxo com-
anion), a nonheme iron(IV)-oxo complex becomes capable of
plexes, which were characterized by various spectroscopic
methods and X-ray crystallography.3−13 Over the past several
oxidizing benzyl alcohol derivatives with electron-donating
substituents via outer-sphere electron transfer from the benzyl
decades, the reactivities of the biomimetic iron(IV)-oxo
alcohol derivatives to a nonheme iron(IV)-oxo complex, which
complexes have been investigated in various oxidation reactions
is coupled with binding of Sc(OTf)3.20 In such an outer-sphere
such as oxygen atom transfer and C−H bond cleavage.7−17 It
has been demonstrated that the reactivities of iron(IV)-oxo
complexes are affected by supporting and axial ligands, solvents
electron-transfer pathway, the reactivity of nonheme iron(IV)-
oxo complexes is determined by the one-electron reduction
and other various reaction conditions.8−17 Understanding
potentials when no further enhancement of the reactivity is
possible because interactions between substrates and Sc3+ ion-
factors that control the reactivity of nonheme iron complexes
in oxidation reactions are indispensable in designing efficient
biomimetic catalysts with high reactivity and selectivity. One
bound iron(IV)-oxo complexes are prohibited due to the steric
effect of Sc3+ ion.21 In contrast to the case of Sc3+ ion-coupled
electron transfer reactions of nonheme iron(IV)-oxo com-
important factor that controls the reactivity of iron(IV)-oxo
plexes, it is possible to have strong interactions between
complexes is Brønsted acids, which can protonate iron(IV)-oxo
complexes to enhance the reacitivity.18
substrates and protonated nonhem iron(IV)-oxo complexes in
We have previously reported that Brønsted acids promote
oxygen atom transfer from a nonheme iron(IV)-oxo complex
and also electron transfer from one-electron reductants such as
Received: November 29, 2012
Published: March 25, 2013
© 2013 American Chemical Society
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proton-coupled electron-transfer (PCET) reactions.19 How-
ever, C−H bond cleavage via PCET from substrates to
nonheme iron(IV)-oxo complexes has yet to be reported.
We report herein remarkable acceleration effects of Brønsted
acid (i.e., HClO4) on C−H bond cleavage of toluene
derivatives by a nonheme iron(IV)-oxo complex, [(N4Py)-
FeIV(O)]2+ (N4Py = N,N-bis(2-pyridylmethyl)-N-bis(2-
pyridyl)methylamine), via electron transfer from toluene
derivatives to a protonated iron(IV)-oxo complex, [(N4Py)-
FeIV(OH)]3+. Strong precursor complexes were formed
between toluene derivatives and [(N4Py)FeIV(OH)]3+ prior
to electron transfer. Although C−H bond cleavage of
mesitylene with [(N4Py)FeIV(O)]2+ without HClO4 is sluggish
to exhibit a significant deuterium kinetic isotope effect (KIE =
31), the rate of C−H bond cleavage with [(N4Py)FeIV(O)]2+ is
103−fold faster in the presence of HClO4 (10 mM) to show no
KIE (KIE = 1.0). The rate constants of C−H bond cleavage of
toluene derivatives with [(N4Py)FeIV(O)]2+ and [(N4Py)-
FeIV(OH)]3+ are quantitatively compared with those of outer-
sphere electron-transfer reactions from coordinatively saturated
metal complexes (one-electron reductants) to [(N4Py)-
FeIV(OH)]3+ in light of the Marcus theory of electron transfer.
The present study provides valuable mechanistic insights into
the switch of the C−H bond cleavage mechanism from
hydrogen atom transfer from toluene derivatives to [(N4Py)-
FeIV(O)]2+ in the absence of HClO4 to electron transfer from
toluene derivatives to [(N4Py)FeIV(OH)]3+ in the presence of
HClO4.
further oxidation of pentamethylbenzyl alcohol was observed in
this study.
In the presence of HClO4 (10 mM), the one-electron
reduction potential of [(N4Py)FeIV(O)]2+ is reported to be
shifted to a positive direction from 0.51 V vs SCE to 1.43 V vs
SCE,18,19,25 which is much more positive than the one-electron
oxidation potential of [(N4Py)FeII(MeCN)]2+ (Eox = 1.00 V vs
SCE).26 Thus, [(N4Py)FeII(MeCN)]2+ is oxidized by [(N4Py)-
FeIV(O)]2+ in the presence of HClO4 to produce [(N4Py)-
FeIII]3+ (Figure S1 in SI).
Rates of oxidation of toluene derivatives by [(N4Py)-
FeIV(O)]2+ were remarkably enhanced by the presence of
HClO4. A typical example is shown in Figure 1, where the
RESULTS AND DISCUSSION
■
Brønsted Acid-Promoted C−H Bond Cleavage of
Toluene Derivatives by [(N4Py)FeIV(O)]2+ with HClO4.
Although oxidation of hexamethylbenzene by [(N4Py)-
FeIV(O)]2+ in acetonitrile (MeCN) is sluggish, the reaction
with HClO4 (10 mM) occurred efficiently to afford
pentamethylbenzyl alcohol as the sole oxidized product, when
[(N4Py)FeIV(O)]2+ was reduced to yield [(N4Py)FeIII]3+ as
indicated by the EPR spectrum [Figure S1 in Supporting
Information (SI)]. The stoichiometry of the oxidation of
hexamethylbenzene by [(N4Py)FeIV(O)]2+ with HClO4 is
given by eq 1. In such a case the observed 50% yield of
Figure 1. Visible spectral changes observed in the reaction of
[(N4Py)FeIV(O)]2+ (0.25 mM) with 1,2,4,5-Tetramethylbenzene
(TMB) ((a) 25 mM and (b) 6.3 mM) in the absence (a) and
presence (b) of HClO4 (10 mM) in MeCN at 298 K (left panel).
Right panels show time courses monitored at 695 nm due to the decay
of [(N4Py)FeIV(O)]2+.
absorption band at 695 nm due to [(N4Py)FeIV(O)]2+ decays
in the course of oxidation of 1,2,4,5-tetramethylbenzene
(TMB) by [(N4Py)FeIV(O)]2+. The decay rate of [(N4Py)-
FeIV(O)]2+ becomes much faster in the presence of HClO4 (10
mM) (Figure 1a vs b).
Decay rates of [(N4Py)FeIV(O)]2+ with large excess of TMB
in the absence and presence of large excess HClO4 obeyed
pseudo-first-order kinetics. The pseudo-first-order rate constant
increased proportionally with increasing TMB concentration
(see Figure S3 in SI). The second-order rate constant (kobs) was
determined from the slope of the linear plot of the pseudo-first-
order rate constant vs TMB concentration. The observed
second-order rate constant (kobs) increased linearly with
increasing HClO4 concentration (Figure 2). The kobs values
of other toluene derivatives also exhibited linear correlations
with HClO4 concentration as given by eq 2, where k0 is the rate
pantametheylbenzyl alcohol (Figure S2 in SI) indicates the
quantitative conversion because [(N4Py)FeIV(O)]2+ in the
presence of HClO4 acts as a one-electron oxidant without
oxygen in contrast to the case in the absence of HClO4.15,22−24
No further oxidation of pantametheylbenzyl alcohol was
observed. The one-electron oxidation potential of pentam-
ethylbenzyl alcohol in the presence of an acid in MeCN is
higher than that of hexamethylbenzene (1.49 V vs SCE).20
Thus, PCET from petamethylbenzyl alcohol to [(N4Py)-
FeIV(O)]2+ is expected to be slower than that from
hexamethylbenzene as discussed later. In addition, much excess
hexamethybenzene relative to [(N4Py)FeIV(O)]2+ was em-
ployed to analyze the product. This is the reason why no
kobs = k0 + k1[HClO ]
(2)
4
constant in the absence of HClO4. A similar correlation
between the rate constant of PCET from the excited state of
[Ru(bpy)3]2+ to aromatic carbonyl compounds and [HClO4]
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Table 1. The kobs values in the presence of HClO4 (10 mM)
relative to those in the absence of HClO4 increase with
increasing the number of methyl groups on benzene, and the
enhancement is as large as 103-fold in the case of
hexamethylbenzene.
In the absence of HClO4, C−H bond cleavage of toluene
derivatives with [(N4Py)FeIV(O)]2+ proceeds via the rate-
determining hydrogen atom transfer from toluene derivatives to
[(N4Py)FeIV(O)]2+. It was evidenced by observation of a large
deuterium kinetic isotope effect (KIE) as shown in Figure 3a,
Figure 2. Plots of kobs vs HClO4 concentration in oxidation of (a)
hexamethylbenzene, (b) 1,2,3,4,5-pentamethylbenzene, (c) 1,2,4,5-
tetramethylbenzene and (d) 1,3,5-trimethylbenzene) with [(N4Py)-
FeIV(O)]2+ in the presence of HClO4 in MeCN at 298 K.
has been established by using 70 wt % HClO4 in MeCN.27 The
PCET rate constants are known to decrease significantly by the
addition of water at constant [HClO4].28 Although the kobs
values for oxidation of 1,2,4,5-tetramethylbenzene with
[FeIV(O)(N4Py)]2+ in the presence of 10 mM and 20 mM
of HClO4 (70%) in MeCN at 298 K decrease significantly with
increasing added H2O concentration, the ratio of kobs with 20
mM vs 10 mM remains constant (two) irrespective of added
H2O concentration (Figure S4 in SI). This indicates that the
acidity of HClO4 (70%) decreases with increasing added H2O
but that the kobs values are proportional to concentration of
HClO4 (70%) irrespective of added H2O concentration. If we
could use HClO4 (100%), the acidity would be much higher.
However, HClO4 without H2O may explode. Thus, we have
used HClO4 (70%) in this study to utilize the high acidity
safely. We have also confirmed that the kobs values of oxidation
of 1,2,4,5-tetramethylbenzene with [FeIV(O)(N4Py)]2+ both in
the absence and presence or 10 mM of HClO4 remain constant
irrespective of concentration of tetra-n-butylammonium per-
chlorate (TBAP) as shown in Figure S5 in SI. This indicates
that the ionic strength does not affect the kobs values.
Figure 3. (a) Plots of first-order rate constants vs concentration of
mesitylene in oxidation of mesitylene (black circle) and mesitylene-d12
(red circle) with [(N4Py)FeIV(O)]2+ (0.25 mM) in the absence of
HClO4 in MeCN at 298 K. (b) Plot of KIE vs concentration of HClO4
in MeCN at 298 K. Red numbers show the KIE values obtained
experimentally.
where the rate constant of mesitylene (1,3,5-trimethylbenzene)
is about 30 times larger than that of fully deuterated mesitylene
(KIE = 31). Such a large KIE value suggests that the hydrogen
atom transfer occurs via tunneling.29−32
The observed second-order rate constants (kobs) of C−H
bond cleavage of toluene derivatives with [(N4Py)FeIV(O)]2+
in the absence and presence of HClO4 (10 mM) are listed in
Table 1. One-Electron Oxidation Potentials (Eox) of Toluene Derivatives and Second-Order Rate Constants of the C−H Bond
Cleavage by [(N4Py)FeIV(O)]2+ in the Presence of HClO4 (10 mM) in MeCN at 298 K
kobs, M−1 s−1
a
toluene derivative
Eox (vs SCE, V)
without HClO4
with HClO4 (10 mM)
hexamethylbenzene
1,2,3,4,5-pentamehtylbenzene
1,2,4,5-tetramethylbenzene
1,2,4-trimethylbenzene
1,4-dimethylbenzene
1,3,5-trimethylbenzene
toluene
1.49
1.58
1.63
1.79
1.93
1.98
2.20
(5.1 0.2) × 10−2
(1.0 0.1) × 10−2
(9.8 0.4) × 10−3
(5.5 0.2) × 10−3
(4.0 0.1) × 10−3
(3.7 0.1) × 10−3
(1.5 0.1) × 10−4
(5.7 0.2) × 10
(1.2 0.1) × 10
3.3 0.2
(1.9 0.1) × 10−1
(1.6 0.1) × 10−2
(1.6 0.2) × 10−2
(5.3 0.2) × 10−4
a
Taken from ref 33.
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In the presence of HClO4, the kobs value of oxidation of
mesitylene with [(N4Py)FeIV(O)]2+ increased with increasing
concentration of HClO4 (Figure 2d). However, the KIE value
decreased significantly with increasing concentration of HClO4
as shown in Figure 3b, where the large KIE value of 31 in the
absence of HClO4 is changed to no KIE (KIE = 1.0) in the
presence of large concentrations of HClO4 (>10 mM). The
absence of KIE in the presence of HClO4 (>10 mM) indicates
the change of the mechanism from a rate-determining
hydrogen atom transfer pathway in the absence of HClO4
(eq 3) to an electron-transfer pathway from toluene derivatives
to the monoprotonated iron(IV)-oxo complex ([(N4Py)-
FeIV(OH)]3+) in the presence of HClO4 (eq 4).
and [(N4Py)FeIII]3+ to be consistent with the stoichiometry in
eq 1. The more detailed mechanism of C−H bond cleavage of
toluene derivatives with [(N4Py)FeIV(O)]2+ in the presence of
HClO4 is discussed after comparison of the reactivity with that
of outer-sphere PCET reactions of [(N4Py)FeIV(O)]2+ (vide
infra).
Outer-Sphere PCET Reactions of [(N4Py)FeIV(O)]2+. In
order to compare the reactivity of C−H bond cleavage
reactions, which proceed via PCET of toluene derivatives
with [(N4Py)FeIV(OH)]3+, the reactivity of outer-sphere
PCET reactions of [(N4Py)FeIV(OH)]3+ was examined using
coordinatively saturated metal complexes, [FeII(Me2bpy)3]2+
(Me2bpy = 4,4′-dimethyl-2,2′-bipyridine), [RuII(Me2bpy)3]2+,
[FeII(Clphen)3]2+ (Clphen = 5-chloro-1,10-phenanthroline)
and [RuII(Clphen)3]2+, as electron donors.19 In contrast to
the case of PCET reactions of [(N4Py)FeIV(OH)]3+ with
toluene derivatives as shown in Figure 2, where the second-
order rate constants (kobs) show linear correlations with
[HClO4], the kobs values of PCET reactions of [(N4Py)-
FeIV(OH)]3+ with coordinatively saturated metal complexes
show the first-order dependence on [HClO4] at lower
concentrations that changes to the second-order dependence
on [HClO4] at higher concentrations (Figure 5). Such mixture
The change in the KIE values in the presence of HClO4 was
also observed in the oxidation reaction of toluene vs toluene-d8
as shown in Figure 4. The large KIE value (31) in the absence
Figure 4. Plot of KIE vs concentration of HClO4 in oxidation reaction
of toluene and toluene-d8 with [(N4Py)FeIV(O)]2+ (0.25 mM) in
MeCN at 298 K. Red numbers show the KIE values obtained
experimentally.
of HClO4 decreased with increasing concentration of HClO4.
In this case, however, a small KIE value (2.0) was observed in
the presence of large excess HClO4. This indicates that an
hydrogen atom transfer pathway (eq 3) is still competing with
an electron-transfer pathway from toluene to the monoproto-
nated iron(IV)-oxo complex ([(N4Py)FeIV(OH)]3+) in the
presence of HClO4 (eq 4) under the conditions in Figure 4.
Figure 5. Plots of ket vs concentration of HClO4 in PCET from (a)
[FeII(Me2bpy)3](PF6)2, (b) [RuII(Me2bpy)3](PF6)2, (c)
[FeII(Clphen)3-(PF6)2 and (d) [RuII(Clphen)3](PF6)2 to [(N4Py)-
FeIV(O)]2+ in the presence of HClO4 (70%) in MeCN at 298 K.
of the first- and second-order dependence of kobs on [HClO4] is
given by eq 5,
kobs = k′0 + k′1[HClO ] + k′2[HClO ]2
(5)
4
4
where k′0, k′1 and k′2 are the rate constants for the zero-, first-
and second-order dependence on [HClO4], respectively.
Because the k′0 value in the absence of HClO4 is negligible
as compared with k′1 and k′2, eq 5 is rewritten by eq 6,
In the presence of HClO4, PCET from toluene derivatives to
[(N4Py)FeIV(OH)]3+ may become much faster than the
hydrogen atom transfer reaction. Radical cations of toluene
derivatives are known to undergo rapid deprotonation,
producing benzyl radical derivatives, which may react with
[(N4Py)FeIV(O)]2+ with H+ to yield benzyl alcohol derivatives
k
obs/[HClO ] = k′1 + k′2[HClO ]
(6)
4
4
which predicts a linear correlation between kobs/[HClO4] vs
[HClO4]. Linear plots of kobs/[HClO4] vs [HClO4] are shown
in Supporting Information (Figure S6). The first- and second-
order dependence of kobs on [HClO4] suggests that PCET
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occurs from the metal complexes not only to the monoproto-
nated iron(IV)-oxo complex ([(N4Py)FeIV(OH)]3+) but also
to the diprotonated iron(IV)-oxo complex ([(N4Py)-
FeIV(OH2)]4+), both of which exist in equilibrium with
[(N4Py)FeIV(O)]2+ as shown in Scheme 1.18 It should be
Scheme 1
Figure 6. Plots of log kobs for C−H bond cleavage of toluene
derivatives ((1) hexamethylbenzene, (2) 1,2,3,4,5-pentamethylben-
zene, (3) 1,2,4,5-tetramethylbenzene, (4) 1,2,4-trimethylbenzene, (5)
1,2-dimethylbenzene, (6) 1,3,5-trimethylbenzene, and (7) toluene) by
[(N4Py)FeIV(O)]2+ in the presence of HClO4 (70%, 10 mM) in
MeCN at 298 K vs the driving force of electron transfer [−ΔG = e(Ered
− Eox)] from toluene derivatives to [(N4Py)FeIV(O)]2+ in the absence
(green closed circles) and presence of HClO4 (10 mM) (red closed
circles). The black closed circles show the driving force dependence of
the rate constants (log ket) of PCET from one-electron reductants ((8)
[FeII(Me2bpy)3](PF6)2, (9) [RuII(Me2bpy)3](PF6)2, (10)
[FeII(Clphen)3](PF6)2 and (11) [RuII(Clphen)3](PF6)2) to [(N4Py)-
FeIV(O)]2+ in the presence of HClO4 (10 mM) in MeCN at 298 K
(see Table S1 in SI).19 The blue closed circles show the driving force
dependence of the rate constants (log ket) of electron transfer from
one-electron reductants ((12) decamethylferrocene, (13) octamethyl-
ferrocene, (14) 1.1′-dimethylferrocene, (15) n-amylferrocene and (16)
ferrocene) to [(N4Py)FeIV(O)]2+ in the absence of HClO4 in MeCN
at 298 K.25 The black line is drawn using eq 9 with λ = 2.74 eV. The
red line is drawn in parallel with the black line.
noted that concentrations of [(N4Py)FeIV(OH)]3+ and
[(N4Py)FeIV(OH2)]4+ were too small to be detected by the
change in absorption spectrum of [(N4Py)FeIV(O)]2+ in the
presence of large excess HClO4 in MeCN at 298 K. This is the
reason for the dependence of kobs on [HClO4] (eq 2) without
exhibiting any saturation behavior.
Comparison of Reactivity of C−H Bond Cleavage via
PCET vs Outer-Sphere PCET. We now compare the reactivity
of PCET of [(N4Py)FeIV(O)]2+ in C−H bond cleavage vs
outer-sphere electron-transfer in light of the Marcus theory of
adiabatic electron transfer.34,35 The driving forces (−ΔGet) of
PCET are obtained from the one-electron oxidation potentials
(Eox) of electron donors (toluene derivatives and coordinatively
saturated metal complexes) and the one-electron reduction
potentials (Ered) of [(N4Py)FeIV(O)]2+ in the presence of
HClO4 as given by eq 7, where e is the elementary charge. The
Ered values are shifted to a positive direction with increasing
concentration of HClO4 according to the Nernst equation (eq
8),19 where K1 and K2 are the binding constants of HClO4 to
produce the monoprotonated and diprotonated iron(III)-oxo
complexes, respectively.18
ket = Zexp[−(λ/4)(1 + ΔGet/λ)2/kBT]
(9)
which corresponds to (kBTK/h; kB is the Borzmann constant, T
is absolute temperature, K is the formation constant of the
precursor complex and h is the Planck constant) and λ is the
reorganization energy of electron transfer. The Z value of outer-
sphere electron-transfer reactions of coordinatively saturated
metal complexes is normally taken as 1.0 × 1011 M−1 s−1.36−39
This result 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. The fitting of the data of outer-sphere PCET from the
metal complexes to [(N4Py)FeIV(OH)]3+ affords the λ value of
2.74 eV (black circles in Figure 6).When the ket values of the
outer-sphere electron-transfer reactions (coordinatively satu-
rated metal complexes) are compared with those of toluene
derivatives in the absence of HClO4, the latter values (green
circles in Figure 6) are much larger than those expected from
the outer-sphere electron transfer because the hydrogen atom
transfer pathway involves much larger interactions between
toluene derivatives and [(N4Py)FeIV(O)]2+.
In the presence of HClO4 (10 mM), however, the log kobs
values of oxidation of toluene derivatives with [(N4Py)-
FeIV(O)]2+ (red circles in Figure 6) exhibit a parallel
relationship with the driving force dependence of log ket on
−ΔGet (black circles in Figure 6) although the kobs values are
always three-orders of magnitude larger than the ket values. The
exception is the point No. 7 (toluene), which is deviated from
the red line. The reason will be discussed later. The parallel
−ΔGet = e(Ered − Eox)
(7)
Ered = E0 + RTln(K1[HClO ] + K1K2[HClO ]2)
(8)
red
4
4
Because the Ered values vary depending on [HClO4] (eq 8)
and the one-electron reduction potential of [FeIV(O)(N4Py)]2+
was determined in the presence of 10 mM HClO4 in MeCN at
298 K previously,19 we have chosen the conditions of 10 mM of
HClO4 (70%) to compare the driving force dependence of
logarithm of the rate constants (ket) of PCET reactions of
[(N4Py)FeIV(O)]2+ with coordinatively saturated metal com-
plexes and those (kobs) with toluene derivatives. At this
concentration, electron transfer from electron donors to the
monoprotonated iron(IV)-oxo complex is the major pathway as
compared with the electron transfer to the diprotonated
ion(IV)-oxo complex. Figure 6 shows the driving force
(−ΔGet) dependence of log ket of PCET of outer-sphere
reductants (coordinatively saturated metal complexes) and log
kobs of PCET of toluene derivatives to [(N4Py)FeIV(OH)]3+.
In the case of outer-sphere PCET from the coordinatively
saturated metal complexes to [(N4Py)FeIV(OH)]3+ in the
presence of HClO4 (10 mM) in MeCN at 298 K, the driving
force dependence of log ket is well fitted by the Marcus
equation of outer-sphere electron transfer (eq 9),34 where Z is
frequency factor,
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driving force dependence of log kobs with log ket in Figure 6
suggests that oxidation of toluene derivatives with [(N4Py)-
FeIV(O)]2+ in the presence of HClO4 proceeds via PCET from
toluene derivatives to [(N4Py)FeIV(OH)]3+. In such a case, the
difference in the kobs and ket values at the same driving force
may result from the difference in the K values of precursor
complexes (vide infra).
In order to determine the equilibrium constants of precursor
complexes prior to PCET from toluene derivatives to
[(N4Py)FeIV(OH)]3+, the dependence of pseudo-first-order
constants (kf) on concentrations of toluene derivatives was
examined using larger concentrations. An example of depend-
ence of kf on concentration of a toluene derivative is shown in
Figure 7, where the kf value increases with increasing
Figure 8. Plot of ln K vs 1/T in temperature in the reaction of
[(N4Py)FeIV(O)]2+ (0.25 mM) with 1,2,4,-trimethylbenzene in the
presence of 10 mM of HClO4 in MeCN. The ΔH and ΔS values can
be determined by van’t Hoff equation, ln K = −ΔH/RT + ΔS/R.
generally three-order of magnitude larger than the value (0.020
M−1) usually used for outer-sphere electron-transfer reactions.
Formation of such strong precursor complexes has also been
reported for toluene derivatives with photoexcited quinones in
photoinduced electron-transfer reactions.41 The charge-transfer
interactions may be responsible for the binding between
toluene derivatives and photoexcited quinones. There are also
many examples for formation of intermediate charge-transfer
complexes in organic and inorganic redox reactions.42−50
Now that K values are determined, comparison of the log kET
values (first-order rate constants in the precursor complexes)
are made between electron transfer reactions from coordina-
tively saturated metal complexes and toluene derivatives to
[(N4Py)FeIV(OH)]3+ by using average K values of toluene
derivatives (83 M−1) and the value (0.020 M−1) of outer-sphere
electron-transfer reactions. The two separate correlations in
Figure 6 are now unified as a single correlation between log kET
vs −ΔGet in Figure 9. Such a unified correlation together with
the absence of KIE in Figure 3b strongly indicates that C−H
bond cleavage of toluene derivatives by [(N4Py)FeIV(O)]2+
with HClO4 proceeds via PCET from toluene derivatives to
[(N4Py)FeIV(OH)]3+ through strong precursor complexes
formed between toluene derivatives and [(N4Py)FeIV(OH)]3+
as shown in Scheme 2.50 The formation of strong precursor
complexes prior to electron transfer in Scheme 2 suggests
occurrence of an inner-sphere electron transfer pathway rather
than an outer-sphere pathway.51,52 However, the single and
unified correlation between log kET an ΔGet in Figure 9, which
is well fitted by the Marcus equation (eq 9), indicates that
electron transfer from toluene derivatives to [(N4Py)-
FeIV(OH)]3+ in the precursor complexes occurs in an outer-
sphere pathway as the case of outer-sphere reductants
(coordinatively saturated metal complexes).52 The absence of
the second-order dependence of kobs on [HClO4] in PCET
from toluene derivatives may result from the much smaller
interaction between toluene derivatives and the diprotonated
iron(IV)-oxo complex ([(N4Py)FeIV(OH2)]4+) due to the
steric effect of second proton bound to the Fe(IV)-oxo
complex.53 A slight deviation from the unified line is observed
for No. 7 (toluene) in Figure 9 as well as in Figure 6, because
toluene may be on the borderline between the hydrogen atom
transfer and PCET.
Figure 7. Plot of concentration of 1,2,4-trimethylbenzene vs first-order
rate constant (kf) in the reaction of [(N4Py)FeIV(O)]2+ (0.25 mM)
with 1,2,4-trimethylbenzene in the presence of 10 mM of HClO4 in
MeCN at 298 K.
concentration of 1,2,4-trimethylbenzene to approach a constant
value. Such a saturation behavior of kf on concentration of a
toluene derivative is given by eq 10, where kET is the rate
constant in the precursor complex, K is the formation constant
of the precursor complex, and [S] is concentration of a
substrate.40 Equation 10 is rewritten by eq 11, which predicts a
linear correlation between kf−1 vs [S]−1. Other toluene
derivatives show a similar behavior.
kf = kETK[S]/(1 + K[S])
(10)
−1
kf −1 = (kETK[S])−1 + kET
(11)
The kET and K values were determined from linear plot of
kf−1 vs [S]−1 (Figures S8 and S9 in SI). The slope and intercept
to be (4.5
0.2) × 10−3 s−1 and (3.6
0.2) × 10 M−1,
respectively. The kET and K values were also determined at
various temperatures. The heat of formation and entropy of the
precursor complex (ΔH and ΔS) were determined to be ΔH =
−2.4 kcal mol−1 and ΔS = −17 cal K−1 mol−1 from the van’t
Hoff plot as shown in Figure 8 (see also Figure S10 in SI). The
K, ΔH and ΔS values of another toluene derivative in
temperature were also determined and their values are listed
in Table 2. The large K and −ΔH values in Table 2 indicate
that PCET proceeds via precursor complexes in which
interactions between toluene derivatives and [(N4Py)-
FeIV(OH)]3+ are much stronger than the case of coordinatively
saturated metal complexes.
As shown in Table 2, the K values of precursor complexes in
PCET from toluene derivatives to [(N4Py)FeIV(OH)]3+ are
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Table 2. Formation Constants and Activation Parameters in Temperature in Oxidation Reaction of Toluene Derivatives, 1,2,4-
Trimethylbenzene and 1,2,4,5-Tetramethylbenzene, by [(N4Py)FeIV(O)]2+ in the Presence of HClO4 (10 mM) in MeCN
formation constant (K, M−1
)
toluene derivative
303 K
298 K
293 K
1,2,4-trimethylbenzene
1,2,4,5-tetramethylbenzene
toluene derivative
(4.4 0.2) × 10
(6.5 0.3) × 10
(3.6 0.2) × 10
(3.0 0.2) × 10
(1.6 0.2) × 102
(1.3 0.2) × 102
a
a
a
ΔH (kcal mol−1
)
ΔS (cal K−1 mol−1
)
ΔG (eV)
1,2,4-trimethylbenzene
−1.6 ( 0.1)
−2.4 ( 0.2)
−15 ( 1)
−17 ( 1)
0.19 ( 0.01)
0.22 ( 0.01)
1,2,4,5-tetramethylbenzene
a
All of the values calculated by van’t Hoff equation, ln K = −ΔH/RT + ΔS/R.
CONCLUSION
■
C−H bond cleavage of toluene derivatives by a nonheme
iron(IV)-oxo complex, [(N4Py)FeIV(O)]2+, is remarkably
enhanced by Brønsted acid (HClO4) to yield benzyl alcohol
derivatives and [(N4Py)FeIII(OH2)]3+ in MeCN at 298 K. Even
toluene, which has high oxidation potential (2.20 V vs SCE),
can be oxidized efficiently by [(N4Py)FeIV(O)]2+ in the
presence of HClO4. Such a remarkable acceleration of C−H
bond cleavage of toluene derivatives with [(N4Py)FeIV(O)]2+
in the presence of HClO4 results from the change in the
reaction mechanism from direct hydrogen atom transfer from
toluene derivatives to [(N4Py)FeIV(O)]2+ in the absence of
HClO4 to PCET from toluene derivatives to [(N4Py)-
FeIV(OH)]3+ in the presence of HClO4 via strong precursor
complexes formed between toluene derivatives and [(N4Py)-
FeIV(OH)]3+ as indicated by the unified correlation of log kET
vs the PCET deriving force (−ΔGet) in light of the Marcus
theory of outer-sphere electron transfer and the absence of KIE.
In the case of toluene, which is the least reactive, the hydrogen
atom transfer pathway competes with the PCET pathway to
exhibit a small KIE (2.0), and the kobs value is a bit larger than
that expected from the outer-sphere electron transfer (no. 7 in
Figure 6).
Figure 9. Plots of log kET for C−H cleavage of toluene derivatives by
[(N4Py)FeIV(O)]2+ in the presence of HClO4 (70%, 10 mM) in
MeCN at 298 K vs the driving force of electron transfer [−ΔG = e(Ered
− Eox)] from toluene derivatives to [(N4Py)FeIV(O)]2+ in the
presence of HClO4 (10 mM) (red closed circles). The black closed
circles show the driving force dependence of the rate constants (log
kET) of PCET from one-electron reductants to [(N4Py)FeIV(O)]2+ in
the presence of HClO4 (10 mM) in MeCN at 298 K (kET = ket/K, K =
0.020). The red line is drawn based on eq 9 with λ = 2.74 eV.19 The
blue closed circles show the driving force dependence of the rate
constants (log kET) of electron transfer from one-electron reductants
to [(N4Py)FeIV(O)]2+ in the absence of HClO4 in MeCN at 298 K.25
The numberings of substrates denote those used in Figure 6.
EXPERIMENTAL SECTION
■
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,
such as acetonitrile (MeCN) and diethyl ether, were dried according
to the literature procedures and distilled under Ar prior to use.54
Nonheme iron(II) complex [(N4Py)FeII(MeCN)](ClO4)2 and its
corresponding iron(IV)-oxo [(N4Py)FeIV(O)]2+ were prepared by the
literature methods.15,55 Iodosylbenzene (PhIO) was prepared by the
literature method.56 Perchloric acid (70 wt % in H2O) was purchased
from Sigma Aldrich Chemical Co.
Scheme 2
Caution: Perchlorate salts are potentially explosive and should be
handled with care.
Kinetic Studies. Kinetic measurements were performed on a
Hewlett-Packard 8453 photodiode-array spectrophotometer using a 10
mm quartz cuvette (10 mm path length) at 298 K. C−H bond
cleavage of toluene derivatives by [(N4Py)FeIV(O)]2+ was examined
by monitoring the spectral change due to [(N4Py)FeIV(O)]2+ (2.5 ×
10−5 M) with various concentrations of alkylbenzenes (2.5 × 10−3
−
1.0 × 10−1 M) in the absence and presence of HClO4 in MeCN at 298
K. Rates of C−H bond cleavage of toluene derivatives by
[(N4Py)FeIV(O)]2+ were monitored by the decay of the absorption
band at 695 nm due to [(N4Py)FeIV(O)]2+ (λmax = 695 nm) in the
absence and presence of HClO4 in MeCN. The concentration of
toluene derivatives was maintained at least more than 10-fold excess of
[(N4Py)FeIV(O)]2+ to attain pseudofirst-order conditions. First-order
fitting of the kinetic data allowed us to determine the pseudofirst-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
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Article
confirmed that the rate constants derived from at least five
independent measurements agreed within an experimental error of
5%. The pseudofirst-order rate constants increased proportionally
with increase in concentrations of substrates, from which second-order
rate constants were determined.
(2) (a) Sono, M.; Roach, M. P.; Coulter, E. D.; Dawson, J. H. Chem.
Rev. 1996, 96, 2841. (b) Watanabe, Y. J. Biol. Inorg. Chem. 2001, 6,
846. (c) Jung, C. Biochim. Biophys. Acta 2011, 1814, 46.
(3) Rohde, J.-U.; In, J.-H.; Lim, M. H.; Brennessel, W. W.; Bukowski,
M. R.; Stubna, A.; Munck, E.; Nam, W.; Que, L., Jr. Science 2003, 299,
̈
Product Analysis. Typically, hexamethylbenzene (2.0 × 10−2 M)
was added to an MeCN solution (0.50 mL) containing [(N4Py)-
FeIV(O)]2+ (4.0 × 10−3 M) in the presence of HClO4 (10 mM) in a
vial. The reaction was complete within 5 min under these conditions.
Products formed in the oxidation reactions of toluene derivatives by
[(N4Py)FeIV(O)]2+, which were carried out in the presence of HClO4
1037.
(4) (a) de Visser, S. P.; Rohde, J.-U.; Lee, Y.-M.; Cho, J.; Nam, W.
Coord. Chem. Rev. 2013, 257, 381. (b) McDonald, A. R.; Que, L., Jr.
Coord. Chem. Rev. 2013, 257, 414.
(5) (a) Meunier, B.; de Visser, S. P.; Shaik, S. Chem. Rev. 2004, 104,
3947. (b) Shaik, S.; Cohen, S.; Wang, Y.; Chen, H.; Kumar, D.; Thiel,
W. Chem. Rev. 2010, 110, 949. (c) Abu-Omar, M. M.; Loaiza, A.;
Hontzeas, N. Chem. Rev. 2005, 105, 2227. (d) Denisov, I. G.; Makris,
T. M.; Sligar, S. G.; Schlichting, I. Chem. Rev. 2005, 105, 2253.
(e) Ortiz de Montellano, P. R. Chem. Rev. 2010, 110, 932.
(6) (a) Groves, J. T.; Shalyaev, K.; Lee, J. In The Porphyrin
Handbook; Kadish, K. M., Smith, K. M., Guilard, R., Eds.; Elsevier
Science: New York, 2000, Vol. 4, pp 17−40. (b) Watanabe, Y. In The
Porphyrin Handbook; Kadish, K. M., Smith, K. M., Guilard, R., Eds.;
Elsevier Science: New York, 2000; Vol. 4, pp 97−117. (c) Groves, J. T.
J. Inorg. Biochem. 2006, 100, 434.
(7) (a) Gunay, A.; Theopold, K. H. Chem. Rev. 2010, 110, 1060.
(b) Che, C.-M.; Lo, V. K.-Y.; Zhou, C.-Y.; Huang, J.-S. Chem. Soc. Rev.
2011, 40, 1950. (c) Costas, M. Coord. Chem. Rev. 2011, 255, 2912.
(8) (a) Cho, K.; Leeladee, P.; McGown, A. J.; DeBeer, S.; Goldberg,
D. P. J. Am. Chem. Soc. 2012, 134, 7392. (b) Cho, K.-B.; Chen, H.;
Janardanan, D.; de Visser, S. P.; Shaik, S.; Nam, W. Chem. Commun.
2012, 48, 2189. (c) Seo, M. S.; Kim, N. H.; Cho, K.-B.; So, J. E.; Park,
S. K.; Clemancey, M.; Garcia-Serres, R.; Latour, J.-M.; Shaik, S.; Nam,
W. Chem. Sci. 2011, 2, 1039. (d) Borovik, A. S. Chem. Soc. Rev. 2011,
40, 1870. (e) Janardanan, D.; Wang, Y.; Schyman, P.; Que, L., Jr.;
Shaik, S. Angew. Chem., Int. Ed. 2010, 49, 3342.
1
under Ar atmosphere in MeCN-d3 at 298 K, were analyzed by H
NMR. Quantitative analyses were made on the basis of comparison of
1H NMR spectral integration between products and their authentic
samples. In the case of hexamethylbenzene, pentamethyl benzyl
alcohol was obtained as a product with 50% of yield (based on the
intermediate generated) in the presence of HClO4 (10 mM). In the
cases of other toluene derivatives, (methyl)nbenzene, (meth-
yl)n‑1benzyl alcohols were obtained in quantitative amounts17,31 by
1H NMR, as the case of hexamethylbenzene (HMB).
Instrumentation. UV−vis spectra were recorded on a Hewlett-
Packard 8453 photodiode-array spectrophotometer. X-band EPR
spectra were taken at 5 K using a X-band Bruker EMX-plus
spectrometer equipped with a dual mode cavity (ER 4116DM). Low
temperatures were achieved and controlled with an Oxford Instru-
ments ESR900 liquid He quartz cryostat with an Oxford Instruments
ITC503 temperature and gas flow controller. The experimental
parameters for EPR spectra were as follows: Microwave frequency =
9.648 GHz, microwave power = 1 mW and modulation amplitude =
1
10 G. H NMR spectra were measured with Bruker model digital
AVANCE III 400 FT-NMR spectrometer. GC-MS spectra were
monitored using a Shimadzu GC-17A gas chromatograph and
Shimadzu MS-QP5000 mass spectrometer.
(9) (a) Krebs, C.; Fujimori, D. G.; Walsh, C. T.; Bollinger, J. M., Jr.
Acc. Chem. Res. 2007, 40, 484. (b) Que, L., Jr. Acc. Chem. Res. 2007, 40,
493. (c) Nam, W. Acc. Chem. Res. 2007, 40, 522. (d) Borovik, A. S. Acc.
Chem. Res. 2005, 38, 54. (e) Shaik, S.; Lai, W.; Chen, H.; Wang, Y. Acc.
Chem. Res. 2010, 43, 1154.
(10) Hong, S.; Lee, Y.-M.; Cho, K.-B.; Sundaravel, K.; Cho, J.; Kim,
M. J.; Shin, W.; Nam, W. J. Am. Chem. Soc. 2011, 133, 11876.
(11) Tang, H.; Guan, J.; Zhang, L.; Liu, H.; Huang, X. Phys. Chem.
Chem. Phys. 2012, 14, 12863.
ASSOCIATED CONTENT
■
S
* Supporting Information
Table S1 and Figures S1−S10. This material is available free of
charge via the Internet at http://pubs.acs.org.
AUTHOR INFORMATION
■
Corresponding Author
fukuzumi@chem.eng.osaka-u.ac.jp; wwnam@ewha.ac.kr
(12) Kumar, D.; Sastry, G. N.; de Visser, S. P. J. Phys. Chem. B 2012,
116, 718.
(13) (a) Groves, J. T.; McClusky, G. A. J. Am. Chem. Soc. 1976, 98,
859. (b) Ortiz de Montellano, P. R.; Stearns, R. A. J. Am. Chem. Soc.
Notes
The authors declare no competing financial interest.
1987, 109, 3415. (c) Schooneboom, J. C.; Cohen, S.; Lin, H.; Shaik, S.;
̈
Thiel, W. J. Am. Chem. Soc. 2004, 126, 4017. (d) Hirao, H.; Kumar, D.;
Que, L., Jr.; Shaik, S. J. Am. Chem. Soc. 2006, 128, 8590.
ACKNOWLEDGMENTS
■
The research at OU was supported by Grant-in-Aid (No.
19205019 to S.F.) and Global COE program, “the Global
Education and Research Center for Bio-Environmental
Chemistry” from the Ministry of Education, Culture, Sports,
Science and Technology, Japan (to S.F.). The research at EWU
was supported by NRF/MEST of Korea through CRI (to
W.N.), GRL (2010-00353) (to W.N.), 2011 KRICT OASIS
project (to W.N.), and WCU (R31-2008-000-10010-0) (to
W.N. and S.F.). J.P. gratefully acknowledges support from JSPS
by Grant-in-Aid for JSPS fellowship for young scientists.
(14) Chiavarino, B.; Cipollini, R.; Crestoni, M. E.; Fornarini, S.;
Fornarini, S.; Lapi, A. J. Am. Chem. Soc. 2008, 130, 3208.
(15) Kaizer, J.; Klinker, E. J.; Oh, N. Y.; Rohde, J.-U.; Song, W. J.;
Stubna, A.; Kim, J.; Munck, E.; Nam, W.; Que, L., Jr. J. Am. Chem. Soc.
̈
2004, 126, 472.
(16) (a) Park, M. J.; Lee, J.; Suh, Y.; Kim, J.; Nam, W. J. Am. Chem.
Soc. 2006, 128, 2630. (b) Sastri, C. V.; Lee, J.; Oh, K.; Lee, Y. J.; Lee,
J.; Jackson, T. A.; Ray, K.; Hirao, H.; Shin, W.; Halfen, J. A.; Kim, J.;
Que, L., Jr.; Shaik, S.; Nam, W. Proc. Natl. Acad. Sci. U. S. A. 2007, 104,
19181. (c) Jeong, Y. J.; Kang, Y.; Han, A.-R.; Lee, Y.-M.; Kotani, H.;
Fukuzumi, S.; Nam, W. Angew. Chem., Int. Ed. 2008, 47, 7321. (d) Lee,
Y.-M.; Dhuri, S. N.; Sawant, S. C.; Cho, J.; Kubo, M.; Ogura, T.;
Fukuzumi, S.; Nam, W. Angew. Chem., Int. Ed. 2009, 48, 1803.
(17) (a) Lee, Y.-M.; Hong, S.; Morimoto, Y.; Shin, W.; Fukuzumi, S.;
Nam, W. J. Am. Chem. Soc. 2010, 132, 10668. (b) Wilson, S. A.; Chen,
REFERENCES
■
(1) (a) Ortiz de Montellano, P. R. Cytochrome P450: Structure,
Mechanism, and Biochemistry, 3rd ed.; Kluwer Academic/Plenum
Publishers: New York, 2005. (b) Meunier, B., Ed. Biomimetic
Oxidations Catalyzed by Transition Metal Complexes; Imperial College
Press: London, 2000. (c) The Ubiquitous Role of Cytochrome P450
Proteins In Metal Ions in Life Sciences; Sigel, A., Sigel, H., Sigel, R. K. O.,
Eds.; John Wiley & Sons Ltd: Chichester, England, 2007; Vol. 3.
́
J.; Hong, S.; Lee, Y.-M.; Clemancey, M.; Garcia-Serres, R.; Nomura,
T.; Ogura, T.; Latour, J.-M.; Hedman, B.; Hodfson, K. O.; Nam, W.;
Solomon, E. I. J. Am. Chem. Soc. 2012, 134, 11791.
(18) Fukuzumi, S. Coord. Chem. Rev. 2013, DOI: 10.1016/
j.ccr.2012.07.021.
5059
dx.doi.org/10.1021/ja311662w | J. Am. Chem. Soc. 2013, 135, 5052−5061

Journal of the American Chemical Society
Article
(19) Park, J.; Morimoto, Y.; Lee, Y.-M.; Nam, W.; Fukuzumi, S. J.
Am. Chem. Soc. 2012, 134, 3903.
(20) Morimoto, Y.; Park, J.; Suenobu, T.; Lee, Y.-M.; Nam, W.;
(39) (a) Fukuzumi, S.; Nakanishi, I.; Tanaka, K.; Suenobu, T.;
Tabard, A.; Guilard, R.; Van Caemelbecke, E.; Kadish, K. M. J. Am.
Chem. Soc. 1999, 121, 785. (b) Fukuzumi, S.; Mochizuki, S.; Tanaka,
T. Inorg. Chem. 1989, 28, 2459.
Fukuzumi, S. Inorg. Chem. 2012, 51, 10025.
(21) (a) Fukuzumi, S.; Morimoto, Y.; Kotani, H.; Naumov, P.; Lee,
Y.-M.; Nam, W. Nature Chem. 2010, 2, 756. (b) Morimoto, Y.; Kotani,
H.; Park, J.; Lee, Y.-M.; Nam, W.; Fukuzumi, S. J. Am. Chem. Soc.
2011, 133, 403. (c) Park, J.; Morimoto, Y.; Lee, Y.-M.; Nam, W.;
Fukuzumi, S. J. Am. Chem. Soc. 2011, 133, 5236. (d) Park, J.;
Morimoto, Y.; Lee, Y.-M.; You, Y.; Nam, W.; Fukuzumi, S. Inorg.
Chem. 2011, 50, 11612.
(22) Bond dissociation energy (BDE) of acetonitrile is reported to be
96 1 kcal mol−1,23 which is significantly higher than those of toluene
derivatives (e.g., 88.5 kcal mol−1).24 In addition, the oxidation
potential of acetonitrile is much higher than those of toluene
derivatives. Indeed [(N4Py)FeIV(O)]2+ is stable in the absence and
presence of HClO4 in acetonitrile.
(40) The saturation behavior could be interpreted as an acid-
substrate complexation. However, it was confirmed that no
protonation of TMB occurred in the presence of HClO4 up to 20
mM (see Figure S7 in SI). Thus, the observed saturation behavior
results from a strong precursor complex rather than the protonation of
the substrate.
(41) Hubig, S. M.; Kochi, J. K. J. Am. Chem. Soc. 1999, 121, 1688.
(42) (a) Fukuzumi, S.; Mochida, K.; Kochi, J. K. J. Am. Chem. Soc.
1979, 101, 5961. (b) Fukuzumi, S.; Kochi, J. K. J. Am. Chem. Soc. 1980,
102, 2141. (c) Fukuzumi, S.; Kochi, J. K. J. Am. Chem. Soc. 1981, 103,
2783. (d) Fukuzumi, S.; Kochi, J. K. J. Am. Chem. Soc. 1981, 103, 7240.
(e) Fukuzumi, S.; Kochi, J. K. J. Am. Chem. Soc. 1982, 104, 7599.
(43) (a) Park, J. S.; Karnas, E.; Ohkubo, K.; Chen, P.; Kadish, K. M.;
Fukuzumi, S.; Bielawski, C. W.; Hudnall, T. W.; Lynch, V. M.; Sessler,
J. L. Science 2010, 329, 1324. (b) Fukuzumi, S.; Ohkubo, K.;
Kawashima, Y.; Kim, D. S.; Park, J.-S.; Jana, A.; Lynch, V. M.; Kim, D.-
H.; Sessler, J. L. J. Am. Chem. Soc. 2011, 133, 15938.
(23) Menon, A. S.; Henry, D. J.; Bally, T.; Radom, L. Org. Biomol.
Chem. 2011, 9, 3636.
(24) Kojima, T.; Nakayama, K.; Ikemura, K.; Ogura, T.; Fukuzumi, S.
J. Am. Chem. Soc. 2011, 133, 11692.
(25) Lee, Y.-M.; Kotani, H.; Suenobu, T.; Nam, W.; Fukuzumi, S. J.
Am. Chem. Soc. 2008, 130, 434.
(26) (a) Roelfes, G.; Vrajmasu, V.; Chen, K.; Ho, R. Y. N.; Rohde, J.-
U.; Zondervan, C.; la Crois, R. M.; Schudde, E. P.; Lutz, M.; Spek, A.
(44) (a) Fukuzumi, S.; Kochi, J. K. Tetrahedron 1982, 38, 1035.
(b) Kim, J. H.; Lindeman, S. V.; Kochi, J. K. J. Am. Chem. Soc. 2001,
123, 4951. (c) Fukuzumi, S.; Koumitsu, S.; Hironaka, K.; Tanaka, T. J.
Am. Chem. Soc. 1987, 109, 305. (d) Fukuzumi, S.; Nishizawa, N.;
Tanaka, T. J. Org. Chem. 1984, 49, 3571.
(45) (a) Zaman, K. M.; Yamamoto, S.; Nishimura, N.; Maruta, J.;
Fukuzumi, S. J. Am. Chem. Soc. 1994, 116, 12099. (b) Zhu, X.-Q.;
Zhang, J.-Y.; Cheng, J.-P. J. Org. Chem. 2006, 71, 7007. (c) Fukuzumi,
S.; Endo, Y.; Imahori, H. J. Am. Chem. Soc. 2002, 124, 10974. (d) Zhu,
X.-Q.; Li, X.-T.; Han, S.-H.; Mei, L.-R. J. Org. Chem. 2012, 77, 4774.
(46) (a) Fukuzumi, S.; Ohkubo, K.; Tokuda, Y.; Suenobu, T. J. Am.
Chem. Soc. 2000, 122, 4286. (b) Lu, Y.; Zhao, Y.; Handoo, K. L.;
Parker, V. D. Org. Biomol. Chem. 2003, 1, 173. (c) Ohkubo, K.;
Fukuzumi, S. J. Phys. Chem. A 2005, 109, 1105. (d) Le Magueres, P.;
Lindeman, S. V.; Kochi, J. K. Organometallics 2001, 20, 115.
(47) Kochi, J. K. Acc. Chem. Res. 1992, 25, 39.
L.; Hage, R.; Feringa, B. L.; Munck, E.; Que, L., Jr. Inorg. Chem. 2003,
̈
42, 2639. (b) Hong, S.; Lee, Y.-M.; Shin, W.; Fukuzumi, S.; Nam, W. J.
Am. Chem. Soc. 2009, 131, 13910.
(27) Fukuzumi, S.; Ishikawa, K.; Hironaka, K.; Tanaka, T. J. Chem.
Soc., Perkin Trans. 2 1987, 751.
(28) (a) Fukuzumi, S.; Kuroda, S. Res. Chem. Int. 1999, 25, 789.
(b) Fukuzumi, S.; Chiba, M.; Ishikawa, M.; Ishikawa, K.; Tanaka, T. J.
Chem. Soc., Perkin Trans. 2 1989, 1417. (c) Fukuzumi, S.; Ishikawa, M.;
Tanaka, T. Tetrahedron 1986, 42, 1021. (d) Fukuzumi, S.; Chiba, M.;
Tanaka, T. Chem. Lett. 1989, 31.
(29) Kwart, H. Acc. Chem. Res. 1982, 15, 401.
(30) (a) Price, J. C.; Barr, E. W.; Glass, T. E.; Krebs, C.; Bollinger, J.
M., Jr. J. Am. Chem. Soc. 2003, 125, 13008. (b) Wu, A.; Mayer, J. M. J.
Am. Chem. Soc. 2008, 130, 14745.
(48) Goodwin, J. A.; Wilson, L. J.; Stanbury, D. M.; Scott, R. A. Inorg.
Chem. 1988, 28, 42.
(31) (a) Kohen, A.; Klinman, J. P. Acc. Chem. Res. 1998, 31, 397.
(b) Knapp, M. J.; Rickert, K.; Klinman, J. P. J. Am. Chem. Soc. 2002,
124, 3865. (c) Klinman, J. P. Biochim. Biophys. Acta, Bioenergetics 2006,
1757, 981. (d) McCusker, K. P.; Kilnman, J. P. J. Am. Chem. Soc. 2010,
132, 5114.
(49) (a) Rosokha, S. V.; Kochi, J. K Acc. Chem. Res. 2008, 41, 641.
(b) Hubig, S. M.; Kochi, J. K. J. Am. Chem. Soc. 1999, 121, 1688.
(c) Gaede, W.; van Eldik, R. Inorg. Chem. 1994, 33, 2204.
(50) The saturation behavior of kf vs concentration of a substrate
may result from the competition between deprotonation of [(N4Py)
FeIV(OH)]3+ and the reaction with the substrate. In such a case, the
rate-determining step at the saturated would be the protonation of
[(N4Py)FeIV(O)]2+, when the rate constant would be independent of
the PCET driving force. The results in Figure 9, which exhibit a single
correlation between log kET vs −ΔGet, clearly indicate that the
saturation behavior in Figure 7 does not result from the change in the
rate-determining step to the protonation of [(N4Py)FeIV(O)]2+.
(51) For the classical definition of inner-sphere electron transfer, see:
(a) Taube, H.; Myers, H. J.; Rich, R. L. J. Am. Chem. Soc. 1953, 75,
4118. (b) Taube, H. Science 1984, 226, 1028.
(32) Fukuzumi, S.; Kobayashi, T.; Suenobu, T. J. Am. Chem. Soc.
2010, 132, 1496.
(33) Fukuzumi, S.; Ohkubo, K.; Suenobu, T.; Kato, K.; Fujitsuka, M.;
Ito, O. J. Am. Chem. Soc. 2001, 123, 8459.
(34) (a) Marcus, R. A. Annu. Rev. Phys. Chem. 1964, 15, 155.
(b) Marcus, R. A. Discuss. Faraday Soc. 1960, 29, 129. (c) Marcus, R.
A. Angew. Chem., Int. Ed. Engl. 1993, 32, 1111.
(35) (a) Cannon, R. D. Electron-Transfer Reactions; Butterworth:
London, 1980. (b) Eberson, L. Adv. Phys. Org. Chem. 1982, 18, 79.
(c) Silverstein, T. P. J. Chem. Educ. 2012, 89, 1159.
(36) (a) Sutin, N. Acc. Chem. Res. 1968, 1, 225. (b) Sutin, N. Adv.
Chem. Phys. 1999, 106, 7. (c) Chou, M.; Creutz, C.; Sutin, N. J. Am.
Chem. Soc. 1977, 99, 5615. (d) Keeney, L.; Hynes, M. J. Dalton Trans.
2005, 133.
(37) (a) Fukuzumi, S.; Honda, T.; Kojima, T. Coord. Chem. Rev.
2012, 256, 2488. (b) Fukuzumi, S.; Karlin, K. D. Coord. Chem. Rev.
2012, 257, 187. (c) Tahsini, L.; Kotani, H.; Lee, Y.-M.; Cho, J.; Nam,
W.; Karlin, K. D.; Fukuzumi, S. Chem.−Eur. J. 2012, 18, 1084.
(d) Yoon, H.; Morimoto, Y.; Lee, Y.-M.; Nam, W.; Fukuzumi, S.
Chem. Commun. 2012, 48, 11187.
(38) (a) Takai, A.; Gros, C. P.; Barbe, J.-M.; Guilard, R.; Fukuzumi,
S. Chem.−Eur. J. 2009, 15, 3110. (b) Nakanishi, T.; Ohkubo, K.;
Kojima, T.; Fukuzumi, S. J. Am. Chem. Soc. 2009, 131, 577.
(c) Murakami, M.; Ohkubo, K.; Fukuzumi, S. Chem.−Eur. J. 2010,
16, 7820.
(52) For more generalized distinction between the inner-sphere and
outer-sphere electron-transfer pathways, see: (a) Rosokha, S. V.;
Kochi, J. K. Acc. Chem. Res. 2008, 41, 641. (b) Fukuzumi, S.; Wong, C.
L.; Kochi, J. K. J. Am. Chem. Soc. 1980, 102, 2928. (c) Fukuzumi, S.;
Kochi, J. K. Bull. Chem. Soc. Jpn. 1983, 56, 969.
(53) The assumption that all reactivities at 10 mM HClO4 are
dominated by the monoprotonated species ([(N4Py)FeIV(OH)]3+) is
valid for oxidation of toluene derivatives with [FeIV(O)(N4Py)]2+ as
indicated by linear correlations between the kobs values and HClO4
concentration in Figure 2. In the case of PCET from inorganic one-
electron reductants to [FeIV(O)(N4Py)]2+, however, the diprotonated
species ([(N4Py)FeIV(OH2)]4+) may also be involved as indicated by
the contribution of the second-order dependence of kobs on [HClO4]
in Figure 5. At 10 mM HClO4, there is some contribution of [(N4Py)
FeIV(OH2)]4+ in PCET from inorganic one-electron reductants to
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Article
[FeIV(O)(N4Py)]2+. This may be the reason why the kET values of
PCET oxidation of toluene derivatives (red points) are somewhat
smaller than the Marcus line in Figure 9. Nevertheless, the assumption
that all reactivities at 10 mM HClO4 are dominated by the
monoprotonated species ([(N4Py)FeIV(OH)]3+) is good enough to
obtain the largely unified correlation in Figure 9.
(54) Armarego, W. L. F.; Chai, C. L. L. Purification of Laboratory
Chemicals, 6th ed.; Pergamon Press: Oxford, 2009.
(55) Lubben, M.; Meetsma, A.; Wilkinson, E. C.; Feringa, B.; Que, L.,
Jr. Angew. Chem., Int. Ed. 1995, 34, 1512.
(56) Organic Syntheses; Saltzman, H., Sharefkin, J. G., Eds.; Wiley:
New York, 1973; Vol. V, p 658.
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