Efficient Epoxidation of Styrene Derivatives by a Nonheme Iron(IV)-Oxo Complex via Proton-Coupled Electron Transfer with Triflic Acid

Article abstract of DOI:10.1021/acs.inorgchem.5b00504

Styrene derivatives are not oxidized by [(N4Py)Fe^IV(O)]²⁺ (N4Py = N,N-bis(2-pyridylmethyl)-N-bis(2-pyridyl)methylamine) in acetonitrile at 298 K, whereas epoxidation of styrene derivatives by the iron(IV)-oxo complex occurs efficiently in the presence of triflic acid (HOTf) via proton-coupled electron transfer (PCET) from styrene derivatives to the diprotonated species of [(N4Py)Fe^IV(O)]²⁺ with HOTf. Logarithms of the first-order rate constants of HOTf-promoted expoxidation of styrene derivatives with [(N4Py)Fe^IV(O)]²⁺ and PCET from electron donors to [(N4Py)Fe^IV(O)]²⁺ in the precursor complexes exhibit a remarkably unified correlation with the driving force of PCET in light of the Marcus theory of electron transfer when the differences in the formation constants of precursor complexes are taken into account. The same PCET driving force dependence is obtained for the first-order rate constants of HOTf-promoted oxygen atom transfer from thioanisols to [(N4Py)Fe^IV(O)]²⁺ and HOTf-promoted hydrogen atom transfer from toluene derivatives to [(N4Py)Fe^IV(O)]²⁺ in the precursor complexes. Thus, HOTf-promoted epoxidation of styrene derivatives by [(N4Py)Fe^IV(O)]²⁺ proceeds via the rate-determining electron transfer from styrene derivatives to the diprotonated species of [(N4Py)Fe^IV(O)]²⁺, as shown in the reactions of HOTf-promoted oxygen atom transfer from thioanisols to [(N4Py)Fe^IV(O)]²⁺ and HOTf-promoted hydrogen atom transfer from toluene derivatives to [(N4Py)Fe^IV(O)]²⁺.

Full text of DOI:10.1021/acs.inorgchem.5b00504

Article
pubs.acs.org/IC
Efficient Epoxidation of Styrene Derivatives by a Nonheme Iron(IV)-
Oxo Complex via Proton-Coupled Electron Transfer with Triflic Acid
†
‡
†,‡
,‡
,†,‡,§
Jiyun Park, Yong-Min Lee, Kei Ohkubo, Wonwoo Nam,* and Shunichi Fukuzumi*
†Department of Material and Life Science, Graduate School of Engineering, ALCA and SENTAN, Japan Science and Technology
Agency (JST), Osaka University, Yamadaoka 2-1, Suita, Osaka 565-0871, Japan
‡
Department of Chemistry and Nano Science, Ewha Womans University, Seoul 120-750, Korea
§
Faculty of Science and Technology, ALCA and SENTAN, Japan Science and Technology Agency (JST), Meijo University, Nagoya,
Aichi 468-8502, Japan
*
S Supporting Information
ABSTRACT: Styrene derivatives are not oxidized by [(N4Py)-
IV
2+
Fe (O)] (N4Py = N,N-bis(2-pyridylmethyl)-N-bis(2-pyridyl)-
methylamine) in acetonitrile at 298 K, whereas epoxidation of
styrene derivatives by the iron(IV)-oxo complex occurs efficiently
in the presence of triflic acid (HOTf) via proton-coupled electron
transfer (PCET) from styrene derivatives to the diprotonated
IV
2+
species of [(N4Py)Fe (O)] with HOTf. Logarithms of the first-
order rate constants of HOTf-promoted expoxidation of styrene
IV
2+
derivatives with [(N4Py)Fe (O)] and PCET from electron
IV
2+
donors to [(N4Py)Fe (O)] in the precursor complexes exhibit a
remarkably unified correlation with the driving force of PCET in
light of the Marcus theory of electron transfer when the differences
in the formation constants of precursor complexes are taken into
account. The same PCET driving force dependence is obtained for the first-order rate constants of HOTf-promoted oxygen
IV
2+
atom transfer from thioanisols to [(N4Py)Fe (O)] and HOTf-promoted hydrogen atom transfer from toluene derivatives to
IV
2+
[
(N4Py)Fe (O)] in the precursor complexes. Thus, HOTf-promoted epoxidation of styrene derivatives by [(N4Py)-
IV
2+
Fe (O)] proceeds via the rate-determining electron transfer from styrene derivatives to the diprotonated species of
IV
2+
IV
2+
[
(N4Py)Fe (O)] , as shown in the reactions of HOTf-promoted oxygen atom transfer from thioanisols to [(N4Py)Fe (O)]
IV
2+
and HOTf-promoted hydrogen atom transfer from toluene derivatives to [(N4Py)Fe (O)] .
III
2+
INTRODUCTION
Fe (OOH)(CH COOH)] intermediate, which is promoted
3
■
by the protonation of the terminal oxygen atom of the
hydroperoxide by the coordinated carboxylic acid. Peracetic
acid used as an acid as well as an oxidant has also been reported
to promote olefin epoxidation. Perchloric acid and triflic acid
have been reported to remarkably promote oxidation of toluene
and thioanisole derivatives by [(N4Py)Fe (O)] .
ever, there has been no report on acid-promoted epoxidation of
olefins by nonheme iron(IV)-oxo complexes.
We report herein efficient epoxidation of styrene derivatives
by [(N4Py)Fe (O)] in the presence of triflic acid (HOTf) in
acetonitrile (MeCN) at 298 K, although [(N4Py)Fe (O)] is
not able to oxidize styrene derivatives efficiently without HOTf
under the same reaction conditions. The mechanism of HOTf-
promoted epoxidation of styrene derivatives is clarified by
comparing the reactivity of [(N4Py)Fe (O)] toward styrene
derivatives in the presence of HOTf with that of proton-
coupled electron transfer (PCET) from various electron donors
There are nonheme iron enzymes that can catalyze substrate
oxidations, similar to those observed in the oxidation reactions
by heme enzymes (e.g., cytochrome P450).
these enzymes, extensive efforts have been devoted to
developing biomimetic iron porphyrin catalysts and also to
12
1
−4
Inspired by
20
5
,6
IV
2+ 21−24
7
−14
How-
the design of new families of nonheme iron catalysts.
It has
been shown that the biomimetic iron compounds act as
efficient catalysts in various oxidation reactions, such as alkane
hydroxylation, olefin epoxidation, and the oxidation of
heteroatom compounds.
complexes catalyze not only epoxidation but also cis-
dihydroxylation of olefins.
dihydroxylation are in closely related transformations that have
been carried out by presumed HO-Fe (O) species.
Addition of acetic acid to nonheme iron-catalyzed olefin
oxidation reactions by H O resulted in an increase in both
catalytic activity and selectivity toward epoxidation.
acetic acid enhanced olefin epoxidation with [(TPA)Fe(OTf) ]
TPA = tris(2-pyridylmethyl)amine, OTf = triflate) has been
IV
2+
7
,9
In particular, nonheme iron
IV
2+
1
5,16
The olefin epoxidation and cis-
V
17
IV
2+
2
2
1
8,19
The
IV
2+
to [(N4Py)Fe (O)] in the presence of HOTf in light of the
2
−
(
V
2+
proposed to be mediated by [(TPA)Fe (O)(OOCCH )] ,
Received: March 6, 2015
3
generated from O−O bond heterolysis of the [(TPA)-
©
XXXX American Chemical Society
A
DOI: 10.1021/acs.inorgchem.5b00504
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Article
21−26
Marcus theory of electron transfer.
This is the first
Scheme 1
observation of the PCET pathway for epoxidation of styrene
derivatives with a nonheme iron(IV)-oxo complex.
EXPERIMENTAL SECTION
■
Materials. All chemicals, which were the best available purity, were
purchased from Aldrich Chemical Co. and Tokyo Chemical Industry
and 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.
Nonheme iron(II) complex [(N4Py)Fe (MeCN)](ClO ) and its
2
7
II
4
2
IV
2+
corresponding iron(IV)-oxo complex [(N4Py)Fe (O)] were
2
8,29
prepared by the literature methods.
Iodosylbenzene (PhIO) was
30
prepared by a literature method. Triflic acid was purchased from
Tokyo Chemical Industry.
unreacted should remain if 1 equiv of α-methylstyrene is
used (Supporting Information Figure S1). Styrene and p-
substituted styrene derivatives were also oxidized by [(N4Py)-
The detailed instrumentation and the experimental procedures of
kinetic studies and product analyses are given in the Supporting
Information.
IV
2+
Fe (O)] in the presence of HOTf to produce the
corresponding epoxides, which were not stable in the reaction
RESULTS AND DISCUSSION
32
■
solutions containing HOTf.
The rate of oxidation of α-methylstyrene by [(N4Py)-
Triflic Acid-Promoted Epoxidation of Styrene Deriv-
IV
2+
atives by [(N4Py)Fe (O)] . No oxidation of α-methylstyrene
IV
2+
Fe (O)] in the presence of HOTf (10 mM) in MeCN was
monitored by the change of the absorbance at 695 nm due to
IV
2+
(
6.3 mM) by [(N4Py)Fe (O)] (0.25 mM) occurred in
MeCN in 1 h at 298 K. However, addition of HOTf (10 mM)
IV
2+
[
(N4Py)Fe (O)] (Figure 1b), obeying first-order kinetics
IV
2+
to an MeCN solution of [(N4Py)Fe (O)] and α-
methylstyrene resulted in efficient oxidation of α-methylstyrene
with large excess α-methylstyrene and HOTf (Figure S4 in
Supporting Information). The pseudo-first-order rate constant
IV
2+
by [(N4Py)Fe (O)] within 200 s as shown in Figure 1,
(
kf) increased with increasing concentration of styrene ([S])
linearly up to 15 mM (Figure S5 in Supporting Information).
Similarly, the k values of other styrene derivatives were also
f
proportional to concentrations of styrene derivatives (Figure S6
in Supporting Information). The observed second-order rate
constants (k ) of epoxidation of styrene derivatives by
obs
IV
2+
[
(N4Py)Fe (O)] were determined from the slope in the
plot of k versus concentration of styrene derivatives with HOTf
f
(
10 mM) in MeCN at 298 K; kobs values are listed in Table 1.
IV
2+
The kobs value of styrene oxidation by [(N4Py)Fe (O)]
increased parabolically with increasing [HOTf] as given by eq 1
(
see also Figure 2a),
Figure 1. (a) Visible spectral changes observed in the oxidation of α-
kobs = k + [HOTf](k + k [HOTf])
(1)
0
1
2
IV
2+
methylstyrene (6.3 mM) by [(N4Py)Fe (O)] (0.25 mM) in the
presence of 10 mM of HOTf in MeCN at 298 K. (b) Time course
monitored by absorbance at 695 nm due to the decay of
where k , k , and k are rate constants corresponding to zero-,
0
1
2
first-, and second-order dependence on [HOTf], respectively.
IV
2+
[
(N4Py)Fe (O)] .
Similar dependence of k on [HOTf] was observed for other
f
styrene derivatives (Figures 2b−d). Because k is negligible, eq
0
where the absorption band at 695 nm due to [(N4Py)-
1 is rewritten by eq 2, which exhibits a linear correlation
IV
2+
Fe (O)] disappeared upon addition of HOTf. The oxidized
between k /[HOTf] and [HOTf] through the origin as
obs
1
products were identified as the epoxides by H NMR in the
shown in Figure 3.
reactions of α-methylstyrene and trans-stilbene with [(N4Py)-
IV
2+
kobs/[HOTf] = k + k [HOTf]
(2)
1
2
Fe (O)] in the presence of HOTf (Figures S1 and S2 in
31
Supporting Information, respectively). The yields of epoxides
Such a second-order dependence of kobs on [HOTf] was
reported to occur via proton-coupled electron transfer (PCET)
from various electron donors to the diprotonated species
of α-methylstyrene and trans-stilbene were 42 ± 4% and 45 ±
4
%, respectively.
The [(N4Py)Fe ] species was the metal product, which
III 3+
IV
4+ 24
(
[(N4Py)Fe (OH )] ).
2
was confirmed by EPR (Figure S3 in Supporting Information).
No EPR signal due to the high-spin Fe(III) species was
observed at the low magnetic field region. The formation of 2
Driving Force Dependence on Rate Constants. The
IV
2+
PCET reactivity of [(N4Py)Fe (O)] in styrene epoxidation
is compared with that in outer-sphere electron transfer
reactions in light of the Marcus theory of adiabatic
III 3+
equiv of [(N4Py)Fe ] has been confirmed by the EPR
2
5,33
quantification and titration (Supporting Information Figure
intermolecular electron transfer.
The driving force
S3). Acetonitrile may be coordinated as the sixth ligand to
(
−ΔG ) of PCET is obtained from the difference between
et
III 3+
[
(N4Py)Fe ] . The stoichiometry of the reaction of styrene
the one-electron oxidation potentials (E ) of electron donors
ox
IV
2+
derivatives with [(N4Py)Fe (O)] in the presence of HOTf
to yield epoxides is given by Scheme 1, where α-methylstyrene
is taken as an example and the maximum yield based on
and the one-electron reduction potential (E ) of [(N4Py)-
red
IV
2+
Fe (O)] in the presence of HOTf (eq 3)
−ΔG = e(E − Eox)
IV
2+
(3)
[
(N4Py)Fe (O)] was 50%; 50% of α-methylstyrene
et
red
B
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Inorganic Chemistry
Article
Table 1. One-Electron Oxidation Potentials of Styrene Derivatives, Formation Constants (K) of Complexes between Styrene
IV
2+
Derivatives and [(N4Py)Fe (O)] , and Second-Order Rate Constants (k ) in Oxidation of Styrene Derivatives by
[
obs
IV
2+
(N4Py)Fe (O)] in the Presence of HOTf (10 mM) in MeCN at 298 K
a
K, M−1
kobs, M−1 s−1
no.
styrene derivative
Eox (V vs SCE)
1
2
3
4
5
trans-stilbene
1,1-diphenylethene
α-methylstyrene
3-methylstyrene
styrene
1.41
1.73
1.82
1.86
1.88
(1.5 ± 0.5) × 102
(9.5 ± 0.5) × 10
(9.8 ± 0.5) × 10
(6.7 ± 0.4) × 10
(3.8 ± 1.0) × 10
(4.0 ± 0.2) × 102
(1.5 ± 0.2) × 10
3.3 ± 0.3
−
1
2
(5.0 ± 0.3) × 10
(4.3 ± 0.3) × 10
−
a
One-electron oxidation potentials were determined by the second-harmonic alternating current voltammetry (SHACV) measurements at scan rate
−1
of 4 mV s using a Pt working electrode (see Figure S7 in Supporting Information).
Figure 2. Plots of second-order rate constants (kobs) vs concentrations
of HOTf for oxidation of styrene derivatives [(a) styrene, (b) α-
Figure 3. Plots of kobs/[HOTf] vs [HOTf] for epoxidation of styrene
derivatives [(a) styrene, (b) α-methylstyrene, (c) 1,1-dipheynylethene,
and (d) trans-stilbene] by [(N4Py)Fe (O)] (0.25 mM) in the
methylstyrene, (c) 1,1-diphenylethylene, and (d) trans-stilbene] by
IV
2+
[
2
(N4Py)Fe (O)] (0.25 mM) in the presence of HOTf in MeCN at
98 K.
IV
2+
presence of HOTf (10 mM) in MeCN at 298 K.
where e is the elementary charge. The Eox values of styrene
derivatives were determined by SHACV measurements (Figure
S7 in Supporting Information).
(reactions of 1−5) are significantly larger than the log k
et
values of outer-sphere electron transfer reactions of [(N4Py)-
IV
2+
Fe (O)] in the presence of 10 mM HOTf (see solid line in
The driving force dependence of log k of electron transfer
et
Figure 4). For example, the kobs value of trans-stilbene (1) is 2
from outer-sphere one-electron reductants (coordinatively
orders magnitude larger than the corresponding k value. The
et
IV
2+
saturated metal complexes) to [(N4Py)Fe (O)] in the
absence and presence of 10 mM of HOTf (reactions of 18−28
in Figure 4) is well-fitted by the Marcus equation of outer-
log kobs values of oxygen atom transfer from thioanisoles to
IV
2+
24
[
(N4Py)Fe (O)] (reactions of 6−10) and hydrogen atom
IV 2+
transfer from toluene derivatives to [(N4Py)Fe (O)]
reactions of 11−17) in the presence of 10 mM HOTf are
2
4,25
24
sphere intermolecular electron transfer (eq 4),
(
2
also significantly larger than the Marcus line (solid line in
Figure 4) for outer-sphere electron transfer reactions. Such
deviation from the Marcus line may result from the difference
in the K values of precursor complexes, because the stronger
k = Z exp[−(λ/4)(1 + ΔG /λ) /k T]
(4)
et
et
B
where Z is the frequency factor (Z = k TK/h where k is the
B
B
Boltzmann constant, T is absolute temperature, K is the
formation constant of the precursor complex, and h is the
Planck constant), using the same value of reorganization energy
IV
4+
interaction of [(N4Py)Fe (OH )] with organic substrates
2
(styrene derivatives, thioanisoles, and toluene derivatives) is
expected as compared with that of outer-sphere one-electron
23,25
of electron transfer (λ = 2.74 eV).
The Z value of outer-sphere intermolecular electron transfer
24
reductants (coordinatively saturated metal complexes). This
1
1
−1 −1 34−37
reactions is normally taken as 1.0 × 10
M
s .
This
was confirmed by examining the dependence of the pseudo-
indicates that the K value of outer-sphere electron transfer
first-order rate constants (k ) on concentrations of styrene
f
−1
reactions is ca. 0.020 M , because there is little interaction in
the precursor complexes of outer-sphere one-electron reduc-
tants (coordinatively saturated metal complexes).
derivatives in the large concentration range (vide infra).
The dependence of k values of oxidation of styrene by
f
IV
2+
[(N4Py)Fe (O)] in the presence of HOTf (10 mM) on
concentration of styrene exhibits a saturation behavior as
shown in Figure 5. Such a saturation behavior is expressed by
The log k values of epoxidation of styrene derivatives by
(N4Py)Fe (O)] in the presence of 10 mM HOTf
obs
IV
2+
[
C
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Inorganic Chemistry
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Figure 6. Plot of 1/k vs 1/[styrene] for oxidation of styrene by
f
IV
2+
[
2
(N4Py)Fe (O)] in the presence of HOTf (10 mM) in MeCN at
−1
−1
98 K. The slope [(k K) ] and intercept (k
) were determined
ET
ET
2
to be 1.5 × 10 M s and 5.5 × 10 s, respectively.
Figure 4. Driving force (−ΔG ) dependence of log k for oxidation
et
obs
of styrene and toluene derivatives [(1) trans-stilbene, (2) 1,1-
diphenylethene, (3) α-methylstyrene, (4) 3-methylstyrene, (5)
styrene, (6) p-Me-thioanisole, (7) thioanisole, (8) p-Cl-thioanisole,
in Table 1 together with the one-electron oxidation potentials
of styrene derivatives (E , see Figure S7 in Supporting
ox
39
Information). Both the kET and K values increase with
decreasing the Eox values of styrene derivatives, because both
electron transfer and charge transfer become thermodynami-
cally more feasible.
(
(
(
9) p-Br-thioanisole, (10) p-CN-thioanisole, (11) hexamethylbenzene,
12) 1,2,3,4,5-pentamethylbenzene, (13) 1,2,4,5-tetramethylbenzene,
14) 1,2,4-trimethylbenzene, (15) 1,4-dimethylbenzene, (16) 1,3,5-
IV
2+
trimethylbenzene, and (17) toluene] by [(N4Py)Fe (O)] and log
ket for PCET from various electron donors {coordinatively saturated
metal complexes; (18) [(Ph Phen) Fe ] , (19) [(bpy) Fe ] , (20)
[
[
k = k K[S]/(1 + K[S])
(5)
(6)
f
ET
II 2+
II 2+
2
3
3
II 2+
II 2+
kf = (k K) [S] + kET−1
−1
−1
−1
(4,4′-Me Phen) Ru ] , (21) [(5,5′-Me Phen) Ru ] , (22)
2
3
2
3
ET
II 2+ II 2+ IV 2+
(ClPhen) Fe ] , and (23) [(bpy) Ru ] } to [(N4Py)Fe (O)]
3
3
in the presence of HOTf (10 mM) in MeCN at 298 K (see Table S1
in Supporting Information). The black circles show the driving force
The first-order (unimolecular) rate constants (k ) of PCET
ET
from various electron donors to the diprotonated species
IV
2+
dependence of the rate constants (log k ) of electron transfer from
et
([(N4Py)Fe (O) -(HOTf) ]) in the precursor complexes can
2
electron donors [(24) decamethylferrocene, (25) octamethylferrocene,
be well-fitted as a function of the driving force of PCET by the
(
26) 1.1′-dimethylferrocene, (27) n-amylferrocene, and (28) ferro-
Marcus equation of the adiabatic outer-sphere intramolecular
IV 2+
cene] to [(N4Py)Fe (O)] in the absence of HOTf in MeCN at 298
K.
24
3
8
electron transfer as given by eq 7.
2
kET = (k T/h)exp[−(λ/4)(1 + ΔG /λ) /k T]
(7)
B
et
B
The driving force dependence of k of oxidation of styrene
ET
IV
2+
derivatives by [(N4Py)Fe (O)] in the presence of 10 mM
HOTf in MeCN at 298 K is remarkably unified with that of k
ET
IV
2+
of PCET from various electron donors to [(N4Py)Fe (O)]
using the same value of reorganization energy of electron
2
3,24
transfer (λ = 2.74 eV) as shown in Figure 7.
The unified driving force dependence of kET in Figure 7
strongly indicates that the rate-determining step in the
IV
2+
epoxidation of styrene derivatives by [(N4Py)Fe (O) -
HOTf) ]] in the presence of HOTf (10 mM) in MeCN is
(
2
definitely PCET from styrene derivatives to the diprotonated
species ([(N4Py)Fe (O) -(HOTf) ]) as shown in Scheme 2,
IV
2+
2
which has been proposed for oxidation of toluene and
IV
2+
23,24
thioanisole derivatives by [(N4Py)Fe (O) -(HOTf) ].
Figure 5. Plot of first-order rate constant (k ) vs concentration of
styrene in the reaction of [(N4Py)Fe (O)] (0.25 mM) with styrene
in the presence of 10 mM of HOTf in MeCN at 298 K.
2
f
IV
2+
The formation constants of precursor complexes of [(N4Py)-
Fe (OH )] with styrene derivatives prior to electron transfer
IV
4+
2
in Table 1 are similar to those reported for toluene and
24
eq 5, resulting from formation of the precursor complex with
thioanisole derivatives. The rate-determining PCET may be
•
−
III
3+
the formation constant K prior to electron transfer (k ) as
followed by rapid O transfer from [(N4Py)Fe (OH )] to
ET
2
2
4
II 2+
reported previously. The kET and K values are obtained from
styrene radical cation to produce [(N4Py)Fe ] and styrene
−
1
−1
the intercept and slope of a linear plot of kf versus [S] (eq
epoxide by releasing two protons (Scheme 2). Because the
corresponding epoxide was obtained without radical coupling
products, the produced radical in Scheme 2 is caged to react
−3
−1
−1
6
) as shown in Figure 6 to be 1.8 × 10
s
and 37 M ,
respectively. Similarly, the kET and K values were determined
III
3+
II 2+
for oxidation of various styrene derivatives by [(N4Py)-
with [(N4Py)Fe (OH )] in the cage. [(N4Py)Fe ] must
2
IV
2+
IV
2+
Fe (O)] in the presence of HOTf (10 mM) in MeCN at
98 K (Figures S8 − S11 in Supporting Information) as listed
be rapidly oxidized by [(N4Py)Fe (O)] with two protons to
produce 2 equiv of [(N4Py)Fe ] and H O, because the one-
III 3+
2
2
D
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starting the reaction using styrene oxide instead of styrene
(
Figure S12 in Supporting Information). Because the E value
ox
of styrene oxide (1.75 V vs SCE, Figure S13 in Supporting
Information) is lower than the Eox value of styrene, the
IV
2+
oxidation of styrene oxide by [(N4Py)Fe (O)] in the
presence of HOTf may also proceed via the rate-determining
PCET pathway (pink circle in Figure 7; Figure S14 in
40
Supporting Information).
When styrene (Eox = 1.88 V vs SCE) was replaced by
cyclohexene (Eox = 2.30 V vs SCE, Figure S15 in Supporting
Information), no acceleration of the rate of oxidation of
IV
2+
cyclohexene by [(N4Py)Fe (O)] was observed by the
presence of HOTf in MeCN at 298 K as shown in Figure 8.
Figure 7. Driving force (−ΔG ) dependence of log k for oxidation
et
obs
of styrene and toluene derivatives [(1) trans-stilbene, (2) 1,1-
diphenylethene, (3) α-methylstyrene, (4) 3-methylstyrene, (5)
styrene, (6) p-Me-thioanisole, (7) thioanisole, (8) p-Cl-thioanisole,
(
(
(
9) p-Br-thioanisole, (10) p-CN-thioanisole, (11) hexamethylbenzene,
12) 1,2,3,4,5-pentamethylbenzene, (13) 1,2,4,5-tetramethylbenzene,
14) 1,2,4-trimethylbenzene, (15) 1,4-dimethylbenzene, (16) 1,3,5-
IV
2+
trimethylbenzene, and (17) toluene] by [(N4Py)Fe (O)] and log
kET for PCET from various electron donors {coordinatively saturated
II 2+
II 2+
metal complexes; (18) [(Ph Phen) Fe ] , (19) [(bpy) Fe ] , (20)
2
3
3
Figure 8. Plot of second-order rate constants (kobs’s) for oxidation of
II 2+
II 2+
[
[
(4,4′-Me Phen) Ru ] , (21) [(5,5′-Me Phen) Ru ] , (22)
IV
2+
2
3
2
3
cyclohexene by [(N4Py)Fe (O)] (0.25 mM) in the presence of
II 2+
II 2+
IV
2+
(ClPhen) Fe ] , and (23) [(bpy) Ru ] } to [(N4Py)Fe (O)]
3
3
HOTf (10 mM) in MeCN at 298 K vs concentrations of HOTf.
in the presence of HOTf (10 mM) in MeCN at 298 K (Tables S1 and
S2 in Supporting Information). The black circles show the driving
In this case no epoxide was formed, and instead the allylic
force dependence of the rate constants (log k ’s) of electron transfer
et
41
oxidation occurred as reported previously. The k values of
from electron donors [(24) decamethylferrocene, (25) octamethylfer-
f
IV
2+
oxidation of cyclohexene by [(N4Py)Fe (O)] in the
presence of HOTf (10 mM) also show saturation behavior
with very high concentration of cyclohexene (Figure S16 in
Supporting Information). The K value was determined as 7
rocene, (26) 1.1′-dimethylferrocene, (27) n-amylferrocene, and (28)
IV
2+
ferrocene] to [(N4Py)Fe (O)] in the absence of HOTf in MeCN at
38
2
98 K. The pink and purple rectangles show the driving force
(
−ΔG ) dependence of log k for oxidation of (29) styrene oxide
et
obs
IV
2+
and (30) cyclohexene by [(N4Py)Fe (O)] in the presence of HOTf
10 mM) in MeCN at 298 K.
(
±2), which is a quite small value compared with other styrene
(
derivatives. Since the driving forces of PCET from styrene and
cyclohexene to [(N4Py)Fe (O)] in the presence of 10 mM
IV
2+
Scheme 2. Unified Mechanism of HOTf-Promoted
HOTf are −ΔG = −0.30 and −0.75 eV, respectively, the
et
Oxidation of Styrene, Thioanisole, and Toluene derivatives
energetic limit for the PCET pathway may be located around
IV
2+
by [(N4Py)Fe (O)]
24
−
ΔG > −0.30 eV. The allylic oxidation of cyclohexene by
et
IV 2+
[
(N4Py)Fe (O)] may occur via hydrogen atom transfer as
indicated by the large KIE value. In sharp contrast to the
PCERT pathway, the hydrogen atom transfer is retarded when
41
IV
2+
[
(N4Py)Fe (O)] is protonated by HOTf because of the
steric effect of HOTf.
CONCLUSION
We have shown that the protonation of [(N4Py)Fe (O)]
■
IV
2+
with HOTf increases the reactivity of the iron(IV)-oxo species
that allows the occurrence of the epoxidation of styrene
derivatives via PCET from styrene derivatives to [(N4Py)-
IV
4+
Fe (OH )] , as demonstrated in the case of C−H bond
2
cleavage of toluene derivatives and sulfoxidation of thioanisole
IV
2+
derivatives by [(N4Py)Fe (O)] with HOTf. Such a drastic
IV
2+
enhancement of the reactivity of [(N4Py)Fe (O)] with
HOTf via PCET pathways may open a new strategy for
efficient oxidation of various substrates by high-valent metal-
oxo complexes. Moreover, a unified PCET driving force
dependence shown in Figure 7 makes it possible to evaluate
PCET reactivity of high-valent metal-oxo complexes toward
substrate oxidation quantitatively.
IV
2+
electron reduction potential of [(N4Py)Fe (O)] in the
presence of 10 mM HOTf (Ered = 1.55 V vs SCE) is much
more positive than the one-electron oxidation potential of
II 2+
24
[
(N4Py)Fe ] (E = 1.00 V vs SCE). The styrene oxide may
be further oxidized by [(N4Py)Fe (O)] in the presence of
HOTf to produce benzaldehyde, which was confirmed by
ox
IV
2+
E
DOI: 10.1021/acs.inorgchem.5b00504
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
(
7) (a) Meunier, B. Chem. Rev. 1992, 92, 1411. (b) Nam, W. In
ASSOCIATED CONTENT
Supporting Information
Tables S1−S2 and Figures S1−S16. The Supporting
Information is available free of charge on the ACS Publications
website at DOI: 10.1021/acs.inorgchem.5b00504.
■
Comprehensive Coordination Chemistry II; Que, L., Jr., Tolman, W., Vol.
Eds.; McCleverty, J. A., Meyer, T. J., Series Eds.; Elsevier: San Diego,
*
S
2
2
004; Vol. 8, pp 281−307. (c) Costas, M. Coord. Chem. Rev. 2011,
55, 2912.
(
8) 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,
037.
(9) (a) Nam, W. Acc. Chem. Res. 2007, 40, 522. (b) Que, L., Jr. Acc.
Chem. Res. 2007, 40, 493. (c) Comba, P.; Rajaraman, G. Inorg. Chem.
008, 47, 78.
10) Wang, D.; Ray, K.; Collins, M. J.; Farquhar, E. R.; Frisch, J. R.;
Gomez, L.; Jackson, T. A.; Kerscher, M.; Waleska, A.; Comba, P.;
Costas, M.; Que, L., Jr. Chem. Sci. 2013, 4, 282.
11) (a) England, J.; Martinho, M.; Farquhar, E. R.; Frisch, J. R.;
Bominaar, E. L.; Munck, E.; Que, L., Jr. Angew. Chem., Int. Ed. 2009,
8, 3622. (b) Bigi, J. P.; Harman, W. H.; Lassalle-Kaiser, B.; Robles, D.
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̈
AUTHOR INFORMATION
Corresponding Authors
E-mail: wwnam@ewha.ac.kr.
E-mail: fukuzumi@chem.eng.osaka-u.ac.jp.
1
■
*
2
(
*
Notes
́
The authors declare no competing financial interest.
(
ACKNOWLEDGMENTS
■
̈
4
The research at OU and MU was supported by ALCA and
SENTAN projects from the Japan Science and Technology
Agency (JST), Japan (to S.F.), and Grants-in-Aid (Nos.
6620154 and 26288037) from the MEXT, Japan (to K.O.).
The research at EWU was supported by NRF/MEST of Korea
through CRI (NRF-2012R1A3A2048842 to W.N.), GRL
NRF-2010-00353) (to W.N.), and WCU (R31-2008-000-
0010-0) (to W.N. and S.F.). J.P. gratefully acknowledges
support from JSPS by Grant-in-Aid for JSPS fellowship for
young scientists.
2
(
2
Ray, K.; Hirao, H.; Shin, W.; Halfen, J. A.; Kim, J.; Que, L., Jr.; Shaik,
S.; Nam, W. Proc. Nat. Acad. Sci. U.S.A. 2007, 104, 19181. (b) Zhou,
Y.; Shan, X.; Mas-Balleste, R.; Bukowski, M. R.; Stubna, A.;
Chakrabarti, M.; Slominski, L.; Halfen, J. A.; Mu
Angew. Chem., Int. Ed. 2008, 47, 1896.
(13) Sahu, S.; Quesne, M. G.; Davies, C. G.; Du
Burmazovic, I.; Siegler, M. A.; Jameson, G. N. L.; de Visser, S. P.;
Goldberg, D. P. J. Am. Chem. Soc. 2014, 136, 13542.
(
1
̈
nck, E.; Que, L., Jr.
̈
rr, M.; Ivanovic-
́
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DOI: 10.1021/acs.inorgchem.5b00504
Inorg. Chem. XXXX, XXX, XXX−XXX