Unified view of oxidative C-H bond cleavage and sulfoxidation by a nonheme iron(IV)-oxo complex via lewis acid-promoted electron transfer

Article abstract of DOI:10.1021/ic403124u

Oxidative C-H bond cleavage of toluene derivatives and sulfoxidation of thioanisole derivatives by a nonheme iron(IV)-oxo complex, [(N4Py)Fe ^IV(O)]²⁺ (N4Py = N,N-bis(2-pyridylmethyl)-N-bis(2-pyridyl) methylamine), were remarkably enhanced by the presence of triflic acid (HOTf) and Sc(OTf)₃ in acetonitrile at 298 K. All the logarithms of the observed second-order rate constants of both the oxidative C-H bond cleavage and sulfoxidation reactions exhibit remarkably unified correlations with the driving forces of proton-coupled electron transfer (PCET) and metal ion-coupled electron transfer (MCET) in light of the Marcus theory of electron transfer when the differences in the formation constants of precursor complexes between PCET and MCET were taken into account, respectively. Thus, the mechanisms of both the oxidative C-H bond cleavage of toluene derivatives and sulfoxidation of thioanisole derivatives by [(N4Py)Fe^IV(O)]²⁺ in the presence of HOTf and Sc(OTf)₃ have been unified as the rate-determining electron transfer, which is coupled with binding of [(N4Py)Fe^IV(O)]²⁺ by proton (PCET) and Sc(OTf)₃ (MCET). There was no deuterium kinetic isotope effect (KIE) on the oxidative C-H bond cleavage of toluene via the PCET pathway, whereas a large KIE value was observed with Sc(OTf)₃, which exhibited no acceleration of the oxidative C-H bond cleavage of toluene. When HOTf was replaced by DOTf, an inverse KIE (0.4) was observed for PCET from both toluene and [Ru ^II(bpy)₃]²⁺ (bpy =2,2′-bipyridine) to [(N4Py)Fe^IV(O)]²⁺. The PCET and MCET reactivities of [(N4Py)Fe^IV(O)]²⁺ with Bronsted acids and various metal triflates have also been unified as a single correlation with a quantitative measure of the Lewis acidity.

Full text of DOI:10.1021/ic403124u

Article
pubs.acs.org/IC
Unified View of Oxidative C−H Bond Cleavage and Sulfoxidation by a
Nonheme Iron(IV)−Oxo Complex via Lewis Acid-Promoted Electron
Transfer
Jiyun Park,† Yuma Morimoto,† 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 Chemistry and Nano Science, Department of Bioinspired Science, Ewha Womans University, Seoul 120-750, Korea
S
* Supporting Information
ABSTRACT: Oxidative C−H bond cleavage of toluene
derivatives and sulfoxidation of thioanisole derivatives by a
nonheme iron(IV)−oxo complex, [(N4Py)FeIV(O)]2+ (N4Py =
N,N-bis(2-pyridylmethyl)-N-bis(2-pyridyl)methylamine), were
remarkably enhanced by the presence of triflic acid (HOTf)
and Sc(OTf)3 in acetonitrile at 298 K. All the logarithms of the
observed second-order rate constants of both the oxidative C−H
bond cleavage and sulfoxidation reactions exhibit remarkably
unified correlations with the driving forces of proton-coupled
electron transfer (PCET) and metal ion-coupled electron transfer
(MCET) in light of the Marcus theory of electron transfer when
the differences in the formation constants of precursor complexes
between PCET and MCET were taken into account, respectively.
Thus, the mechanisms of both the oxidative C−H bond cleavage of toluene derivatives and sulfoxidation of thioanisole
derivatives by [(N4Py)FeIV(O)]2+ in the presence of HOTf and Sc(OTf)3 have been unified as the rate-determining electron
transfer, which is coupled with binding of [(N4Py)FeIV(O)]2+ by proton (PCET) and Sc(OTf)3 (MCET). There was no
deuterium kinetic isotope effect (KIE) on the oxidative C−H bond cleavage of toluene via the PCET pathway, whereas a large
KIE value was observed with Sc(OTf)3, which exhibited no acceleration of the oxidative C−H bond cleavage of toluene. When
HOTf was replaced by DOTf, an inverse KIE (0.4) was observed for PCET from both toluene and [RuII(bpy)3]2+ (bpy =2,2′-
bipyridine) to [(N4Py)FeIV(O)]2+. The PCET and MCET reactivities of [(N4Py)FeIV(O)]2+ with Brønsted acids and various
metal triflates have also been unified as a single correlation with a quantitative measure of the Lewis acidity.
the Marcus theory of electron transfer.26 The enhanced
INTRODUCTION
■
reactivity of [(N4Py)FeIV(O)]2+ in the oxidative C−H bond
High-valent heme and nonheme iron−oxo complexes have
cleavage of toluene derivatives and sulfoxidation of thioanisole
been investigated as key intermediate in various biological and
chemical oxidation reactions.1,2 Since the first crystal structure
derivatives by acids is suggested to result from the change in the
reaction mechanism from the direct hydrogen atom transfer
and oxygen atom transfer to electron transfer pathways coupled
with binding of acids to [(N4Py)FeIV(O)]2+, respectively. If this
is true, all the rate constants of oxidative C−H bond cleavage
and sulfoxidation reactions as well as electron-transfer reactions
in the absence and presence of acids would be correlated by a
unified fashion to the corresponding driving force of electron
transfer in light of the Marcus theory of electron transfer.
However, such unified understanding of the reactivity of
nonheme iron(IV)−oxo complexes in various oxidation
reactions has yet to be made. In addition, unified understanding
of effects of various acids (Brønsted and Lewis acids) on the
of mononuclear nonheme iron(IV)−oxo was reported in 2003,
the reactivity of synthetic nonheme iron(IV)−oxo complexes
with various substrates have been extensively studied.3−17
Reactivities of nonheme iron(IV)−oxo complexes in various
oxidation reactions, such as oxidative C−H bond cleavage and
oxygen atom transfer, are significantly affected not only by axial
ligand and solvents,3−21 but also by binding of metal ions
(Lewis acids) and Brønsted acids to the oxo moiety of
nonheme iron(IV)−oxo complexes.22−26 The binding of
Sc(OTf)3 to a nonheme iron(IV) complex was confirmed by
the X-ray crystal structure of Sc(OTf)3-bound [(TMC)-
FeIV(O)]2+ (TMC = 1,4,8,11-tetramethyl-1,4,8,11-tetraazacy-
clotetradecane).23,24 The electron transfer (ET) reactivity of a
nonheme iron(IV)−oxo complex ([(N4Py)FeIV(O)]2+) with
electron donors was enhanced by binding of protons and metal
ions, and the enhanced reactivity was well analyzed in light of
Received: December 21, 2013
Published: March 7, 2014
© 2014 American Chemical Society
3618
dx.doi.org/10.1021/ic403124u | Inorg. Chem. 2014, 53, 3618−3628

Inorganic Chemistry
Article
were maintained to be in excess more than 10-fold of that of
[(N4Py)FeIV(O)]2+.
reactivity of nonheme iron(IV)−oxo complexes has not been
made, either.
First-order fitting of the kinetic data allowed us to determine the
pseudo-first-order rate constants. The first-order plots were linear for
three or more half-lives with the correlation coefficient ρ > 0.999. In
each case, it was confirmed that the rate constants derived from at least
five independent measurements agreed within an experimental error of
5%. The pseudo-first-order rate constants increased proportionally
with increase in concentrations of substrates, from which second-order
rate constants were determined.
Spectral Redox Titration. ET from an electron donor
([RuII(NO2phen)3](PF6)2 (2.5 × 10−4−1.0 × 10−3 M)) to [(N4Py)-
FeIV(O)]2+ (2.5 × 10−4 M) was examined by the spectral change in the
presence of HOTf (1.0 × 10−2 M) in MeCN at 298 K using a Hewlett-
Packard 8453 photodiode array spectrophotometer with a quartz
cuvette (path length = 10 mm).
We report herein a unified view on the remarkable
enhancement of reactivity of [(N4Py)FeIV(O)]2+ by triflic
acid (HOTf) as well as Sc(OTf)3 in oxidative C−H bond
cleavage and oxygen atom transfer reactions as well as electron
transfer from various one-electron donors to [(N4Py)-
FeIV(O)]2+ in light of the Marcus theory of electron transfer.
All the rate constants of oxidative C−H bond cleavage of
toluene derivatives, sulfoxidation of thioanisole derivatives and
electron transfer from various one-electron donors to [(N4Py)-
FeIV(O)]2+ in the presence of HOTf and Sc(OTf)3 are well
correlated by a unified fashion with the driving forces of
proton-coupled electron transfer (PCET) and metal ion
(Sc(OTf)3)-coupled electron transfer (MCET), respectively.
In this paper, PCET and MCET are defined as electron-transfer
reactions, which occur by binding of protons and metal ions to
electron acceptors, respectively.27 The enhanced reactivities of
[(N4Py)FeIV(O)]2+ with HOTf and Mn+(OTf)n are also well
correlated with a quantitative measure of the Lewis acidity of
acids, which we reported previously.23b
Product Analysis. Typically, hexamethylbenzene and thioanisole
(2.0 × 10−2 M) were added to an MeCN solution containing
[(N4Py)FeIV(O)]2+ (4.0 × 10−3 M) in the presence of HOTf and
Sc(OTf)3 (1.0 × 10−2 M) in a vial. Products formed in the oxidation
reactions of toluene derivatives by [(N4Py)FeIV(O)]2+, which were
carried out in the presence of acids under Ar atmosphere in MeCN-d3
at 298 K, were analyzed by 1H NMR. Quantitative analyses were made
1
on the basis of comparison of H NMR spectral integration between
products and their authentic samples.
Instrumentation. The EPR spectra were measured with a JEOL
X-band spectrometer (JES-RE1XE). The EPR spectra were recorded
under nonsaturating microwave power conditions. The magnitude of
modulation was chosen to optimize the resolution and the signal-to-
noise (S/N) ratio of the observed spectra. The g value was calibrated
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
1
by using a Mn2+ marker. H NMR spectra were recorded on a JEOL
to the literature procedures and distilled under Ar prior to use.28
A
A-300 spectrometer in MeCN-d3.
nonheme iron(II) complex, [(N4Py)FeII(MeCN)](ClO4)2, and its
corresponding iron(IV)−oxo complex, [(N4Py)FeIV(O)]2+, were
prepared by literature methods.19,29 Iodosylbenzene (PhIO) was
prepared by a literature method.30 Triflic acid was purchased from
Tokyo Chemical Industry. [FeII(Ph2phen)3](PF6)2 (Ph2phen = 4,7-
diphenyl-1,10-phenanthroline), [FeII(Clphen)3](PF6)2 (Clphen = 5-
chloro-1,10-phenanthroline), [RuII(4,4′-Me2bpy)3](PF6)2 (4,4′-
Me2bpy = 4,4′-dimethyl-2,2′-bipyridine), [(RuII(5,5′-Me2bpy)3](PF6)2
(5,5′-Me2bpy = 5,5′-dimethyl-2,2′-bipyridine), [RuII(bpy)3](PF6)2 and
[RuII(NO2phen)3](PF6)2 (NO2phen = 5-nitro-1,10-phenanthroline)
were prepared according to published procedures.31 [FeII(bpy)3]-
(PF6)2 was obtained by the addition of an aqueous solution containing
an excess amount of NaPF6 to an aqueous solution containing a
stoichiometric amount of the bpy ligand and FeSO4 to yield crystalline
solids.
Kinetic Studies. Kinetic measurements were performed on a
Hewlett-Packard 8453 photodiode-array spectrophotometer using a
quartz cuvette (path length = 10 mm) at 298 K. The C−H bond
cleavage of toluene derivatives and sulfoxidation of thioansiole
derivatives by [(N4Py)FeIV(O)]2+ were monitored by spectral changes
at 695 nm due to [(N4Py)FeIV(O)]2+ (2.5 × 10−4 M) with various
concentrations of toluene and thioanisole derivatives (2.5 × 10−3−1.0
× 10−1 M) in the absence and presence of acids, HOTf and Sc(OTf)3,
in MeCN at 298 K. The rates of the oxidation reactions of organic
substrates 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 HOTf and Sc(OTf)3 in MeCN.
The concentrations of toluene derivatives and thioanisole derivatives
were maintained at least more than 10-fold excess of [(N4Py)-
FeIV(O)]2+ to attain pseudo-first-order conditions.
RESULTS AND DISCUSSION
■
HOTf- and Sc(OTf)3-Promoted C−H Bond Cleavage of
Toluene Derivatives by [(N4Py)FeIV(O)]2+. Toluene is
known to be hardly oxidized by nonheme iron(IV)−oxo
complexes.19 However, oxidation of toluene by a nonheme
iron(IV)−oxo complex ([(N4Py)FeIV(O)]2+) in acetonitrile
(MeCN) is significantly enhanced by the presence of HOTf
(50 mM) as shown in Figure 1, to yield benzyl alcohol and
[(N4Py)FeIII]3+ (eq 1);
PhCH3 + 2[(N4Py)FeIV(O)]2+ + 2H+ + H2O
→ PhCH2OH + 2[(N4Py)FeIII(OH2)]3+
(1)
see Figures S1−S3 in Supporting Information (SI). The
[(N4Py)FeIV(O)]2+ complex acted as a one-electron oxidant,
when the yield of benzyl alcohol in the presence of HOTf was
50% based on [(N4Py)FeIV(O)]2+ (Figure S1 in SI). Other
toluene derivatives were also oxidized by [(N4Py)FeIV(O)]2+ in
the presence of HOTf to produce the alcohol products with the
maximum yield of 50%.
The rate of oxidation of toluene [(N4Py)FeIV(O)]2+ in the
absence and presence of HOTf in MeCN, monitored by
decrease in absorbance at 695 nm due to [(N4Py)FeIV(O)]2+
(Figure 1) obeyed first-order kinetics (Figure S4 in SI) with a
large excess of toluene and HOTf. The pseudo-first-order rate
constant (kf) increased linearly with increasing concentration of
toluene (Figure S5a in SI). Similarly the kf values were also
proportional to concentrations of toluene derivatives (Figures
S5b−S5f in SI). The observed second-order rate constants
(kobs) of oxidation of toluene derivatives by [(N4Py)FeIV(O)]2+
were determined from the slopes of plots of kf vs
concentrations of toluene derivatives in the absence and
ET from electron donors to [(N4Py)FeIV(O)]2+ in the presence of
Lewis acids, HOTf and Sc(OTf)3, was monitored by a Hewlett-
Packard 8453 photodiode-array spectrophotometer and a UNISOKU
RSP-601 stopped-flow spectrometer equipped with a MOS-type highly
sensitive photodiode array in MeCN at 298 K. These ET rates were
determined from the decay of [(N4Py)FeIV(O)]2+ (λmax = 695 nm),
respectively. All kinetic measurements were carried out under pseudo-
first-order conditions where the concentrations of electron donors
3619
dx.doi.org/10.1021/ic403124u | Inorg. Chem. 2014, 53, 3618−3628

Inorganic Chemistry
Article
dependence on [HOTf] at lower and higher concentrations
of HOTf, respectively, as given by eq 2 (Figure 2),
kobs = k0 + [HOTf](k1 + k2[HOTf])
(2)
(kobs − k0)/[HOTf] = k1 + k2[HOTf]
(3)
Figure 1. Visible spectral changes observed in the reactions of
[(N4Py)FeIV(O)]2+ (0.25 mM) with toluene (400 mM) in the (a)
absence and (b) presence of 50 mM of HOTf in MeCN at 298 K (left
panel). Right panels show the time course of absorbance monitored at
695 nm due to the decay of [(N4Py)FeIV(O)]2+.
Figure 2. Plots of kobs vs concentration of HOTf for oxidation of
toluene derivatives [(a) toluene, (b) 1,3,5-trimethylbenzene, (c)
1,2,4,5-tetramethylbenzene and (d) hexamethylbenzene] by [(N4Py)-
FeIV(O)]2+ in the presence of HOTf in MeCN at 298 K.
presence of HOTf (10 mM) in MeCN at 298 K as listed in
Table 1.
The kobs values increased with increasing concentration of
HOTf, [HOTf], exhibiting first-order and second-order
Table 1. One-Electron Oxidation Potentials (Eox) of Toluene and Thioanisole Derivatives and Second-Order Rate Constants of
the C−H Bond Cleavage and Sulfoxidation by [(N4Py)FeIV(O)]2+ in the Presence of HOTf and Sc(OTf)3 (10 mM) in MeCN at
298 K
kobs, M−1 s−1
a
,
b
a,
b
e
no.
thioanisole and toluene derivative
Eox (vs SCE, V)
without acid
with Sc(OTf)3 (10 mM)
with HOTf (10 mM)
1
hexamethylbenzene
1,2,3,4,5-pentamethylbenzene
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
1.24
1.34
1.37
1.41
1.61
(4.8 0.2) × 10−2
(1.5 0.1) × 10−2
(7.8 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
1.3 0.1
(8.7 0.4) × 10−1
(4.0 0.2) × 10−1
(1.5 0.1) × 10−1
(4.4 0.2) × 10−2
(1.1 0.1) × 10−1
(2.9 0.1) × 10−2
(1.1 0.1) × 10−2
(8.2 0.4) × 10−3
(6.0 0.3) × 10−3
(5.6 0.3) × 10−3
(2.4 0.2) × 10−4
(4.2 0.2) × 102
(1.2 0.1) × 102
(3.9 0.2) × 10
(1.7 0.1) × 10−1
(5.0 0.2) × 10−2
(5.0 0.2) × 10−2
(6.5 0.2) × 10−4
(1.5 0.1) × 104
(3.2 0.2) × 103
(1.1 0.1) × 103
(1.0 0.1) × 103
(1.0 0.1) × 102
2
3
4
5
6
7
a
8
p-Me-thioanisole
(8.4 0.4) × 10
a
9
thioanisole
(1.9 0.1)× 10
a
10
11
12
p-Cl-thioanisole
4.2 0.2
a
p-Br-thioanisole
3.7 0.2
a
p-CN-thioanisole
(6.8 0.3) × 10−2
ket, M−1 s−1
c
no.
electron donor
Eox (vs SCE, V)
without acid
with Sc(OTf)3 (10 mM)
with HOTf (10 mM)
d
a
13
14
15
16
17
18
[FeII(Ph2phen)3]2+
[FeII(bpy)3]2+
[RuII(4,4′-Me2bpy)3]2+
[RuII(5,5′-Me2bpy)3]2+
[FeII(Clphen)3]2+
[RuII(bpy)3]2+
1.02
1.06
1.11
1.16
1.20
1.24
NR
(1.4 0.1) × 103
(7.9 0.4) × 103
(2.5 0.1) × 103
(5.0 0.2) × 102
(2.5 0.1) × 102
(2.5 0.1) × 102
(5.0 0.2) × 10
d
a
NR
(2.1 0.1) × 102
d
NR
(7.0 0.3) × 10
(4.0 0.2) × 10
d
NR
d
a
NR
9.4 0.4
d
a
NR
9.0 0.4
a
b
c
d
e
Taken from ref 23c and 32. Taken from ref 26. Taken from refs 23c 31, and 33. NR = no reaction. The rate constant of the natural decay of
[(N4Py)FeIV(O)]2+ in the presence of 10 mM of HOTf in MeCN at 298 K was determined to be 3.0 × 10−6 s−1, which is much smaller than the
observed pseudo-first-order rate constants of oxidation of substrates by [(N4Py)FeIV(O)]2+ in the presence of 10 mM of HOTf in MeCN at 298 K.
3620
dx.doi.org/10.1021/ic403124u | Inorg. Chem. 2014, 53, 3618−3628

Inorganic Chemistry
Article
where k0, k1 and k2 correspond to the rate constants of
oxidation of toluene derivatives by [(N4Py)FeIV(O)]2+, the
monoprotonated species ([(N4Py)FeIV(OH)]3+) and the
diprotonated species ([(N4Py)FeIV(OH2)]4+), which exhibit
zero-, first-, and second-order dependence on [HOTf],
respectively.26 Equation 2 is rewritten by eq 3, which exhibits
a linear correlation of (kobs − k0)/[HOTf] vs [HOTf] as shown
in Figure S6 in SI. This indicates that the protonation
equilibrium constants were too small to be detected as
indicated by no shift of the absorption maximum (695 nm)
due to [(N4Py)FeIV(O)]2+ in the presence of 50 mM HOTf
(Figure 1). Otherwise, the kobs values in Figure 2 would exhibit
a saturation behavior with increasing concentration of HOTf
due to the protonation equilibrium.
When HOTf was replaced by Sc(OTf)3, the kobs values of
oxidation of toluene derivatives by [(N4Py)FeIV(O)]2+ also
increased with increasing concentration of Sc(OTf)3 (Figure S7
in SI). The kobs values of oxidation of toluene derivatives by
[(N4Py)FeIV(O)]2+ in the presence of Sc(OTf)3 (10 mM) in
MeCN at 298 are also listed in Table 1. The kobs values of
oxidation of toluene derivatives (toluene and hexamethylben-
zenze) by [(N4Py)FeIV(O)]2+ in the presence of HOTf,
HClO4 (70%),26 and Sc(OTf)3 (50 mM) are compared in
Table S1 in SI, where kobs value increases in order: Sc(OTf)3 <
HClO4 < HOTf. In the case of hexamethylbenzene, kobs in the
presence of HOTf (50 mM) is 2.2 × 105 times larger than that
in the absence of HOTf.
HOTf- and Sc(OTf)3-Promoted Sulfoxidation of Thio-
anisole Derivatives by [(N4Py)FeIV(O)]2+. It is well-known
that oxidation of thioanisole derivatives by [(N4Py)FeIV(O)]2+
occurs to produce the corresponding sulfoxide products (eq
4).20a,23c,25
PhSCH3 + [(N4Py)FeIV(O)]2+
→ PhS(O)CH3 + [(N4Py)FeII]2+
(4)
The rate of sulfoxidation of thioanisole derivatives by
[(N4Py)FeIV(O)]2+ was remarkably enhanced in the presence
of Sc(OTf)3 (10 mM) and HClO4 (70%, 10 mM) as compared
with that in the absence of acids.23c,25 The mechanism of
sulfoxidation of thioanisole derivatives by [(N4Py)FeIV(O)]2+
is changed from a direct oxygen atom transfer (OAT) pathway
to a metal ion-coupled electron transfer (MCET) pathway in
the presence of Sc(OTf)3 and also to a PCET pathway in the
presence of HClO4.23c,25 The rate of sulfoxidation of
thioanisole derivatives by [(N4Py)FeIV(O)]2+ was further
enhanced by the presence of HOTf (10 mM) as compared
with that in the presence of HClO4 (70%, 10 mM).25 The kobs
values of oxidation of thioanisole derivatives by [(N4Py)-
FeIV(O)]2+ in the presence of HOTf (10 mM) were
determined from linear plots of kf vs concentrations of
thioanisole derivatives (Figure S8 in SI). The kobs values
increase with increasing concentration of HOTf in accordance
with eq 2 (Figure S9 in SI).23c The kobs values of oxidation of
thioanisole derivatives by [(N4Py)FeIV(O)]2+ in the presence
of HOTf (10 mM) in MeCN at 298 K are also listed in Table 1.
Inverse Kinetic Isotope Effect in Proton-Coupled
Electron Transfer Reduction of [(N4Py)FeIV(O)]2+. When
HOTf was replaced by the deuterated acid (DOTf), the ket
value of PCET from an electron donor ([RuII(bpy)3]2+; bpy =
2,2′-bipyridine) to [(N4Py)FeIV(O)]2+ with DOTf in MeCN at
298 K was larger than that with the same concentration of
HOTf as shown in Figure 4. Both the ket values with DOTf and
HOTf increased with increasing concentration of DOTf and
HOTf, respectively, in accordance with eq 2. The KIE value
When toluene was replaced by the deuterated compound
(toluene-d8), a large deuterium kinetic isotope effect (KIE) was
observed in the absence of HOTf (KIE = 31) as shown in
Figure 3. Such a large KIE value suggests that the hydrogen
(kobs(HOTf)/kobs(DOTf)) was determined to be 0.25
0.05 at
HOTf (DOTf) = 2.5 mM, increasing to be a constant value of
0.40 0.04 at the higher concentrations of HOTf (DOTf) as
Figure 3. Plot of KIE vs concentration of HOTf in the oxidation of
toluene and toluene-d8 with [(N4Py)FeIV(O)]2+ (0.25 mM) in the
presence of HOTf in MeCN at 298 K. Red-colored numbers show the
KIE values obtained experimentally.
atom transfer occurs via tunneling in the rate-determining step
of oxidation of toluene by [(N4Py)FeIV(O)]2+.34−37 In the
presence of HOTf, the KIE value decreased with increasing
concentration of HOTf to reach KIE = 1 at concentrations of
HOTf larger than 50 mM (Figure 3). Such a drastic change in
KIE from 31 to 1 indicates that the rate-determining step of
oxidation of toluene by [(N4Py)FeIV(O)]2+ is changed from
hydrogen atom transfer (HAT) in the absence of HOTf to
proton-coupled electron transfer (PCET) in the presence of
HOTf (> 50 mM).
Figure 4. Plots of ket vs [HOTf] (black circles) and [DOTf] (red
circles) for PCET from [RuII(bpy)3]2+ to [(N4Py)FeIV(O)]2+ in the
presence of HOTf and DOTf in MeCN at 298 K, respectively.
3621
dx.doi.org/10.1021/ic403124u | Inorg. Chem. 2014, 53, 3618−3628

Inorganic Chemistry
Article
shown in Figure S10a in SI. Such an inverse KIE indicates that
binding of protons to the oxo group to produce the O−H (or
O−D) bond is involved in the rate-determining step of PCET
from [RuII(bpy)3]2+ to [(N4Py)FeIV(O)]2+, because an inverse
KIE results from a larger zero-point energy difference between
the O−H and O−D bonds in the transition state relative to the
ground state.38 Thus, the O−H bonds in the reduced species
([(N4Py)FeIII(OH)]2+ and [(N4Py)FeIII(OH2)]3+) are much
stronger than those in [(N4Py)FeIV(OH)]3+ and [(N4Py)-
FeIV(OH2)]4+, respectively. The larger the difference is in the
O−H bond strength between the reduced and oxidized species,
the larger may be the inverse KIE. Thus, the larger inverse KIE
value at the lower concentration of HOTf suggests that the
difference in O−H bond strength between [(N4Py)-
FeIII(OH)]2+ and [(N4Py)FeIV(OH)]3+ (monoprotonated
species) may be larger than that between [(N4Py)-
FeIII(OH2)]3+ and [(N4Py)FeIV(OH2)]4+ (diprotonated).
A similar inverse KIE was observed for the kobs values of
oxidation of toluene by [(N4Py)FeIV(O)]2+ in the presence of
large concentrations of DOTf (>50 mM) in comparison with
those in the presence of HOTf as shown in Figure 5. The KIE
FeIV(O)]2+ increased with increasing concentration of HOTf in
accordance with eq 2 as shown in Figure 6 (red line). The
Figure 6. Plots of ket vs [HOTf] (red circles) and [Sc(OTf)3] (black
circles) for PCET and MCET from Fc to [FeIV(O)(N4Py)]2+ in the
presence of HOTf and Sc(OTf)3 in MeCN at 298 K, respectively.
Inset shows plot of (ket − k0)/[HOTf] vs [HOTf].
linear plot of (ket − k0)/[HOTf] vs [HOTf] (eq 3) is shown in
the inset of Figure 6. The k1 and k2 values in eq 3 were
determined from the intercept and the slope, respectively.
Similarly, we determined the k1 and k2 values of MCET from
Fc to [(N4Py)FeIV(O)]2+ in the presence of Sc(OTf)3 in
MeCN at 298 K. The dependence of ket on [Sc(OTf)3] is
shown in Figure 6 (black line).23a The ket values with HOTf are
always larger than those with Sc(OTf)3 at the same
concentrations of acids. The k1 and k2 values with various
metal triflates were determined previously from the intercepts
and slopes of plots of (ket − k0)/[Mn+(OTf)n] vs [Mn+(OTf)n],
respectively.23a
Figure 7 shows plots of log k1 and log k2 vs a quantitative
measure of Lewis acidity (Δhν) for both PCET and MCET
from Fc to [(N4Py)FeIV(O)]2+ in the presence of HOTf and
Mn+(OTf)n, respectively. The Δhν values were obtained from
the red shifts of the fluorescence emission energies of 10-
methylacridone (Δhν) due to binding of metal triflates to the
carbonyl oxygen from that in the absence of acids.41 According
to the shift of fluorescence emission energy (Δhν), the relative
Lewis acidity of the proton was determined to be 0.26 eV,
which is slightly higher than that of Sc(OTf)3 (0.25 eV) as
shown in Figure S11 in SI.41 Good linear correlations were
obtained for plots of both log k1 and log k2 of PCET and
MCET from Fc to [(N4Py)FeIV(O)]2+ vs Δhν (Figure 7). The
stronger the acidity of Lewis acids is, the stronger becomes the
binding of acids to [(N4Py)FeIV(O)]2+ as well as to the excited
state of AcrCO, resulting in the acceleration of PCET and
MCET from Fc to [(N4Py)FeIV(O)]2+.
Figure 5. Plots of kobs vs [HOTf] (black circles) and [DOTf] (red
circles) for oxidation of toluene by [(N4Py)FeIV(O)]2+ in the presence
of HOTf and DOTf in MeCN at 298 K, respectively.
value decreases with increasing concentration of HOTf (Figure
S10b in SI) to reach a constant value of 0.42 0.04, which
agrees with the KIE value of PCET from [RuII(bpy)3]2+ to
[(N4Py)FeIV(O)]2+ (0.40 0.04) in Figure S10a in SI. The
same inverse KIE of the oxidation of toluene as that of PCET
together with the absence of KIE between toluene-d8 and
toluene (Figure 3) strongly indicates that the oxidation of
toluene by [(N4Py)FeIV(O)]2+ in the presence of large
concentrations of HOTf occurs via PCET as the rate-
determining step.39,40
Comparison of Proton-Coupled Electron Transfer vs
Metal Ion-Coupled Electron Transfer. Rates of electron
transfer from electron donors to [(N4Py)FeIV(O)]2+ are
accelerated by binding of protons and metal ions to
[(N4Py)FeIV(O)]2+ via PCET and MCET, respectively.23b In
order to compare the acceleration effect of HOTf with that of
metal triflates (Mn+(OTf)n), we determined that rate constant
of PCET from ferrocene (Fc) to [(N4Py)FeIV(O)]2+ in the
presence of HOTf in MeCN at 298 K. The observed second-
order rate constant (ket) of PCET from Fc to [(N4Py)-
One Electron Reduction Potentials of [(N4Py)-
FeIV(O)]2+ in the Presence of HOTf. In order to examine
the driving force dependence of PCET from electron donors to
[(N4Py)FeIV(O)]2+ in the presence of HOTf in MeCN, the
one-electron reduction potentials of [(N4Py)FeIV(O)]2+ in the
presence of various concentrations of HOTf were determined
by the redox titration (vide infra). When [RuII(NO2phen)3]2+
(NO2phen = 5-nitrophenathrene) was employed as an electron
donor, no electron transfer from [RuII(NO2phen)3]2+ (Eox
=
3622
dx.doi.org/10.1021/ic403124u | Inorg. Chem. 2014, 53, 3618−3628

Inorganic Chemistry
Article
[(N4Py)FeIV(O)]2+ + [RuII(NO2phen)3]2+ + 2H+
ket
⇄ [(N4Py)FeIII(OH2)]3+ + [RuIII(NO2phen)3]3+
(5)
Figure 7. Plots of log k1 (red circles) and log k2 (blue squares) vs Δhν
for PCET and MCET from Fc to [(N4Py)FeIV(O)]2+ in the presence
of HOTf and metal triflates in MeCN at 298 K, respectively. The Δhν
values were determined from the fluorescence emission energies of
AcrCO in the presence of HOTf and metal triflates relative to the
energy in their absence.
Figure 9. Plot of concentration of [RuIII(NO2phen)3]3+ produced in
PCET from [RuII(NO2phen)3]2+ to [(N4Py)FeIV(O)]2+ (0.25 mM) in
the presence of HOTf (10 mM) in deaerated MeCN at 298 K vs initial
concentration of [RuII(NO2phen)3](PF6)2, [RuII(NO2phen)3]-
(PF6)2]0.
1.45 V vs SCE, Figure S12 in SI) to [(N4Py)FeIV(O)]2+ (Ered
=
0.51 V vs SCE)42 occurred in the absence of acids in MeCN,
because the free energy change of electron transfer is highly
positive (ΔGet = 0.94 eV), i.e., endergonic. However, the
electron transfer occurred efficiently in the presence of HOTf
(10 mM) as shown in Figure 8, where the absorption band at
The PCET equilibrium constant Ket was determined by
fitting the plot in Figure 9 to be Ket = 24. The Ered value of
[(N4Py)FeIV(O)]2+ in the presence of HOTf (10 mM) MeCN
at 298 K was determined from the Ket value (24) and the Eox
value of [RuII(NO2phen)3]2+ (Eox = 1.45 V vs SCE) using an
Nernst equation (eq 6, where R is the gas constant, T is
absolute temperature, and F is the Faraday constant) to be 1.55
V vs SCE,
Ered = Eox + (RT/F) ln Ket
(6)
which is much higher than that in the presence of Sc(OTf)3 (10
mM; 1.19 V vs SCE) reported previously.23b,43 The Ered values
in the presence of various concentrations of HOTf and
Sc(OTf)3 were also determined from the Ket values and the Eox
value of [RuII(NO2phen)3]2+ using eq 6.
The dependence of Ered of [(N4Py)FeIV(O)]2+ on log-
[HOTf] and log[Sc(OTf)3] is shown in Figure 10, where the
slopes are determined to be 113 6 mV for HOTf and 118
8 mV for Sc(OTf)3. The Nernst equation for the dependence
of Ered on log[Acid] (Acid = HOTf and Sc(OTf)3) is given by
eq 7, where Kred1 and Kred2 are the equilibrium constants for the
first and second binding of acids to [(N4Py)FeIII(O)]+,
respectively. Under the conditions such that Kred2[Acid] ≫ 1,
eq 7 is rewritten by eq 8,
Figure 8. Visible spectral changes observed in PCET from
[RuII(NO2phen)3]2+ (2.5 mM) to [(N4Py)FeIV(O)]2+ (0.25 mM) in
the presence of HOTf (10 mM) in MeCN at 298 K.
Ered = E0 + (2.3RT/F) log(Kred1[Acid]
red
695 nm due to [(N4Py)FeIV(O)]2+ (λmax = 695 nm)
disappeared, accompanied by the appearance of a new
absorption band at 650 nm due to [RuIII(NO2phen)3]3+ (λmax
= 650 nm) with an isosbestic point at 730 nm. The
concentration of [RuIII(NO2phen)3]3+ produced in PCET
from [RuII(NO2phen)3]2+ to [(N4Py)FeIV(O)]2+ in the
presence of HOTf (10 mM) is plotted against the initial
concentration of [RuII(NO2phen)3]2+ as shown in Figure 9,
which indicates that there is a PCET equilibrium as given in eq
5.
+ Kred1
Ered = E°red + 2(2.3RT/F) log(Kred1
K
red2[Acid]2)
(7)
(8)
K
red2[Acid])
where the slope of the plot of Ered vs log[Acid] is 2(2.3RT/F) =
118 mV at 298 K, which agrees well with the experimental
values in Figure 10. Such agreement indicates that PCET and
MCET reduction of [(N4Py)FeIV(O)]2+ involve binding of two
acid molecules of HOTf and Sc(OTf)3 to [(N4Py)FeIII(O)]+,
respectively. The second-order rate constants (ket) of electron
3623
dx.doi.org/10.1021/ic403124u | Inorg. Chem. 2014, 53, 3618−3628

Inorganic Chemistry
Article
to [(N4Py)FeIV(O)]2+ and the reorganization energies of the
electron donors are virtually the same as reported for
photoinduced electron-transfer reactions of aromatic com-
pounds.32 However, the log ket values of PCET from
coordinatively saturated metal complexes (nos. 13−18 in
Table 1) to [(N4Py)FeIV(O)]2+ are significantly smaller than
log kobs values of oxidation of toluene and thioanisole
derivatives (red line in Figure 11).
Similarly a unified linear correlation is observed for plots of
log kobs of oxidation of toluene and thioanisole derivatives by
[(N4Py)FeIV(O)]2+ in the presence of Sc(OTf)3 (10 mM) vs
the Eox values of toluene and thioanisole derivatives when the
Eox values are lower than 1.6 V vs SCE (black line in Figure 11).
Such a unified correlation indicates that oxidation of toluene
and thioanisole derivatives by [(N4Py)FeIV(O)]2+ in the
presence of Sc(OTf)3 (10 mM) proceeds via MCET from
toluene and thioanisole derivatives to [(N4Py)FeIV(O)]2+. In
the case of 1,2,4-trimethylbenzene (No. 4), 1,4-dimethylben-
zene (No. 5), 1,3,5-trimethylbenzene (No. 6) and toluene (No.
7), which have higher Eox values than 1.6 V vs SCE, however,
the log kobs values are significantly larger than the linear
correlation (black line in Figure 11). The log ket values of
MCET from coordinatively saturated metal complexes (Nos.
13−18 in Table 1) to [(N4Py)FeIV(O)]2+ are also significantly
smaller than the log kobs values of oxidation of toluene and
thioanisole derivatives (black line in Figure 11).
The difference in log ket and log kobs values between PCET
and MCET is expected to result from the difference in the Ered
values of [(N4Py)FeIV(O)]2+ in the presence of HOTf (10
mM) and Sc(OTf)3 (10 mM). Driving forces of the PCET and
MCET reactions (−ΔGet in eV) are obtained from the one-
electron oxidation potentials (Eox) of electron donors
(coordinatively saturated metal complexes, toluene and
thioanisole derivatives) and the one-electron reduction
potentials (Ered) of [(N4Py)FeIV(O)]2+ in the presence of
acids as given by eq 9, where e is the elementary charge.
Figure 10. Dependence of Ered of [(N4Py)FeIV(O)]2+ on log[HOTf]
(red circles) and log[Sc(OTf)3] (black circles) in deaerated MeCN at
298. Black and red lines are fitted by eq 8.
transfer from electron donors to [(N4Py)FeIV(O)]2+ in the
presence of HOTf (10 mM) and Sc(OTf)3 (10 mM) were
determined as listed in Table 1 (Figures S13 and S14 in SI).
Unified Driving Force Dependence of Rate Constants.
The dependence of log kobs of oxidation of toluene and
thioanisole derivatives by [(N4Py)FeIV(O)]2+ in the presence
of HOTf (10 mM) and Sc(OTf)3 (10 mM) on one-electron
oxidation potentials (Eox) of electron donors including toluene
and thioanisole derivatives is shown in Figure 11 (red line). A
−ΔGet = e(Ered − Eox)
(9)
The driving force dependence of log ket of electron transfer
from coordinatively saturated metal complexes to [(N4Py)-
FeIV(O)]2+ in the absence and presence of 10 mM of HOTf
(black line in Figure 12) is well fitted by the Marcus equation of
outer-sphere electron transfer (eq 10),44
ket = Z exp[−(λ/4)(1 + ΔGet/λ)2/kBT]
(10)
where Z is the frequency factor, which is kBTK/h (kB is the
Boltzmann constant, T is absolute temperature, K is the
formation constant of the precursor complex and h is the
Planck constant), using the same value of reorganization energy
of electron transfer (λ = 2.74 eV).
The Z value of outer-sphere electron-transfer reactions is
normally taken as 1.0 × 1011 M−1 s−1.45−48 This indicates that
the K value of outer-sphere electron-transfer reactions is as
small as 0.020 M−1, because there is little interaction in the
precursor complex for outer-sphere electron-transfer reactions
of coordinatively saturated metal complexes.
Figure 11. Plot of log kobs vs oxidation potentials (Eox) of toluene and
thioanisole derivatives [(1) hexamethylbenzene, (2) 1,2,3,4,5-pentam-
ethylbenzene, (3) 1,2,4,5-tetramethylbenzene, (4) 1,2,4-trimethylben-
zene, (5) 1,4-dimethylbenzene, (6) 1,3,5-trimethylbenzene, (7)
toluene, (8) p-Me-thioanisole, (9) thioanisole, (10) p-Cl-thioanisole,
(11) p-Br-thioanisole, and (12) p-CN-thioanisole] in the C−H bond
cleavage of toluene derivatives and sulfoxidation of thioanisole
derivatives by [(N4Py)FeIV(O)]2+ in the presence of 10 mM of
HOTf (red circles) and Sc(OTf)3 (black circles) in MeCN at 298 K.
unified correlation is observed for plots of log kobs of oxidation
of toluene and thioanisole derivatives by [(N4Py)FeIV(O)]2+ in
the presence of HOTf (10 mM) vs the Eox values of toluene
and thioanisole derivatives. Such a unified correlation suggests
that oxidation of both toluene and thioanisole derivatives by
[(N4Py)FeIV(O)]2+ in the presence of HOTf (10 mM)
proceeds via PCET from toluene and thioanisole derivatives
The log kobs values of oxidation of toluene and thioanisole
derivatives by [(N4Py)FeIV(O)]2+ in the presence of HOTf and
Sc(OTf)3 (10 mM) as well as the log ket values of MCET from
coordinatively saturated metal complexes to [(N4Py)-
FeIV(O)]2+ in the presence of Sc(OTf)3 (10 mM) are larger
than those expected by eq 10 with λ = 2.74 eV. The log kobs
values of oxidation of toluene and thioanisole derivatives by
3624
dx.doi.org/10.1021/ic403124u | Inorg. Chem. 2014, 53, 3618−3628

Inorganic Chemistry
Article
thioanisole derivatives is expected as compared with that with
coordinatively saturated metal complexes. This was confirmed
by examining the dependence of the pseudo-first-order rate
constants (kf) on concentrations of toluene and thioanisole
derivatives in the large concentration range (vide infra). The kf
values of oxidation of p-CN-thioanisole by [(N4Py)FeIV(O)]2+
in the presence of HOTf (10 mM) and Sc(OTf)3 (10 mM) are
shown respectively in a and b of Figure 13. In both cases, the kf
values increase with increasing concentration of p-CN-
thioanisole to approach constant values. Such a saturation
behavior indicates formation of the precursor complex prior to
electron transfer when kf is given by eq 11, where kET is the
first-order rate constant of electron transfer in the precursor
complex, K is the formation constant of the precursor complex,
and [S] is concentration of a substrate. Equation 11 is rewritten
by eq 12, which predicts a linear correlation between kf−1 and
[S]−1.
Figure 12. Plots of log kobs for oxidation of toluene and thioanisole
derivatives [(1) hexamethylbenzene, (2) 1,2,3,4,5-pentamethylben-
zene, (3) 1,2,4,5-tetramethylbenzene, (4) 1,2,4-trimethylbenzene, (5)
1,4-dimethylbenzene, (6) 1,3,5-trimethylbenzene, (7) toluene, (8) p-
Me-thioanisole, (9) thioanisole, (10) p-Cl-thioanisole, (11) p-Br-
thioanisole, and (12) p-CN-thioanisole] by [(N4Py)FeIV(O)]2+ in the
absence (black) and presence of 10 mM of HOTF (red) and Sc(OTf)3
(blue) in MeCN at 298 K vs the driving force of electron transfer
[−ΔG = e(Ered − Eox)] from toluene derivatives (squares) and
thioanisole derivatives (triangles) to [(N4Py)FeIV(O)]2+ in the
presence of HOTf (red) and Sc(OTf)3 (blue). The red and blue
circles show the driving force dependence of the rate constants (log
ket) of electron transfer from electron donors [(13)
[FeII(Ph2phen)3]2+, (14) [FeII(bpy)3]2+, (15) [RuII(4,4′-Me2phen)3]2+,
(16) [RuII(5,5′-Me2phen)3]2+, (17) [FeII(Clphen)3]2+, and (18)
[RuII(bpy)3]2+] to [(N4Py)FeIV(O)]2+ in the presence of HOTf (10
mM) and Sc(OTf)3 (10 mM) in MeCN at 298 K. The black line is
drawn using eq 10 with λ = 2.74 eV.25,42 The black circles show the
driving force dependence of the rate constants (log ket) of electron
transfer from electron donors [(19) decamethylferrocene, (20)
octamethylferrocene, (21) 1.1′-dimethylferrocene, (22) n-amylferro-
cene, and (23) ferrocene] to [(N4Py)FeIV(O)]2+ in the absence of
acids in MeCN at 298 K.42
kf = kETK[S]/(1 + K[S])
(11)
−1
kf −1 = (kETK[S])−1 + kET
(12)
From the intercepts and slopes of linear plots of kf−1 and [S]−1
(Figure S15 in SI), the K values were determined as listed in
Table 2. The K values of oxidation of 1,2,4,5-tetramethylben-
Table 2. Formation Constants of Precursor Complexes in
Oxidation of Toluene and Thioanisole Derivatives by
[(N4Py)FeIV(O)]2+ in the Presence of Acids (10 mM),
HOTf and Sc(OTf)3, in MeCN at 298 K
formation constant (K, M−1
)
toluene and
thioanisole derivative
10 mM of Sc(OTf)3
10 mM of HOTf
1,2,4,5-tetramethylbenzene
1,2,4-trimethylbenzene
p-Cl-thioanisole
3.0 0.2
2.5 0.1
3.1 0.2
2.7 0.2
(1.2 0.4) × 102
(1.0 0.1) × 102
(9.6 0.5) × 10
(4.0 0.5) × 10
p-CN-thioanisole
zene and 1,2,4-trimethylbenzene by [(N4Py)FeIV(O)]2+ in the
presence of HOTf (10 mM) and Sc(OTf)3 (10 mM) were also
determined as listed in Table 2 (Figures S16−S18 in SI).
The K value of the precursor complexes of [(N4Py)-
FeIV(O)]2+ with 1,2,4,5-tetramethylbenzene in the presence of
10 mM HOTf (K = 120 M−1) is much larger than that of
[(N4Py)FeIV(O)]2+ with the same substrate in the presence of
10 mM Sc(OTf)3 (K = 3.0 M−1). The same trend is observed
[(N4Py)FeIV(O)]2+ in the presence of HOTf and Sc(OTf)3
larger than the log ket values of electron transfer from
coordinatively saturated metal complexes to [(N4Py)-
FeIV(O)]2+ in the presence of HOTf may result from the
difference in the K values of precursor complexes, because the
stronger interaction of [(N4Py)FeIV(OH2)]4+ with toluene and
Figure 13. Plots of pseudo-first-order rate constants (kf) of oxidation of p-CN-thioanisole by [(N4Py)FeIV(O)]2+ (0.25 mM) in the presence of (a)
HOTf (10 mM) and (b) Sc(OTf)3 (10 mM) in MeCN at 298 K vs concentration of p-CN-thioanisole.
3625
dx.doi.org/10.1021/ic403124u | Inorg. Chem. 2014, 53, 3618−3628

Inorganic Chemistry
Article
for other substrates as listed in Table 2. The significantly
smaller K values with Sc(OTf)3 (10 mM) than those with
HOTf (10 mM) may be ascribed to the large steric effect of
two molecules of Sc(OTf)3 bound to [(N4Py)FeIV(O)]2+ as
compared with that of HOTf. In each case, the K values are
significantly larger than those employed for outer-sphere
electron-transfer reactions of coordinatively saturated metal
complexes (K = ∼0.020 M−1).
Although K values may be changed depending on the Eox
values of toluene and thioanisole derivatives, the kET values
were evaluated by using averaged K values with HOTf [(9 2)
× 10 M−1] and Sc(OTf)3 (2.8 0.2 M−1) in Table 2. Figure 14
unified as a blue line using the same λ value of 2.27 eV. When
the driving forces of MCET (Sc(OTf)3) and PCET (HOTf)
are more negative than −0.5 eV, the kET values become larger
than those predicted by the Marcus lines (Figure 14). The λ
value of the MCET (2.27 eV) smaller than that of the PCET
(2.74 eV) may be ascribed to the smaller change in the Fe−O
distance associated with the MCET than that with the PCET at
the same concentration of acids probably due to the steric effect
of two molecules of Sc(OTf)3 bound to the oxo group.
The unified correlations of driving force dependence of log
kET of PCET and MCET from all kinds of electron donors to
[(N4Py)FeIV(O)]2+ in Figure 14 indicate that oxidation of
toluene and thioanisole derivatives by [(N4Py)FeIV(O)]2+ in
the presence of HOTf and Sc(OTf)3 proceeds via the rate-
determining electron transfer from toluene and thioanisole
derivatives to [(N4Py)FeIV(O)]2+ which binds with two
molecules of Sc(OTf)3 and HOTf, respectively, following
formation of the precursor complexes as shown in Scheme 1,
provided that the driving force of the electron transfer is larger
than −0.5 eV. In the case of oxidation of toluene derivatives by
[(N4Py)FeIV(O)]2+ in the presence of HOTf and Sc(OTf)3,
the rate-determining PCET and MCET may be followed by
rapid proton transfer and the oxygen rebound to produce the
corresponding benzyl alcohol derivatives and [(N4Py)FeII]2+,
which is oxidized by [(N4Py)FeIV(O)]2+ to [(N4Py)FeIII]3+ in
the presence of HOTf and Sc(OTf)3. In the case of oxidation of
thioanisole derivatives by [(N4Py)FeIV(O)]2+ in the presence
of HOTf and Sc(OTf)3, the PCET and MCET may be
followed by rapid O•− transfer to produce the corresponding
sufoxide derivatives and [(N4Py)FeII]2+, which is also oxidized
by [(N4Py)FeIV(O)]2+ to [(N4Py)FeIII]3+ in the presence of
HOTf and Sc(OTf)3. However, the detailed mechanism after
the rate-determining PCET and MCET has yet to be clarified.
In the absence of HOTf and Sc(OTf)3, the kox values of
oxidation of toluene and thioanisole derivatives by [(N4Py)-
FeIV(O)]2+ are much larger than those predicted from PCET or
MCET reactions (black circles in Figure 14). Thus, C−H bond
cleavage of toluene derivatives and sulfoxidation of thioanisole
derivatives proceed via direct hydrogen atom transfer from
toluene derivatives to [(N4Py)FeIV(O)]2+ and oxygen atom
transfer from thioanisole derivatives to [(N4Py)FeIV(O)]2+,
respectively, rather than an electron-transfer pathway.
Figure 14. Plots of log kET for C−H bond cleavage of toluene
derivatives and sulfoxidation of thioanisole derivatives [(1) hexame-
thylbenzene, (2) 1,2,3,4,5-pentamethylbenzene, (3) 1,2,4,5-tetrame-
thylbenzene, (4) 1,2,4-trimethylbenzene, (5) 1,4-dimethylbenzene, (6)
1,3,5-trimethylbenzene, (7) toluene, (8) p-Me-thioanisole, (9)
thioanisole, (10) p-Cl-thioanisole, (11) p-Br-thioanisole and (12) p-
CN-thioanisole] by [(N4Py)FeIV(O)]2+ in the absence (black) and
presence of acids (10 mM), HOTf (red) and Sc(OTf)3 (blue), in
MeCN at 298 K vs the driving force of electron transfer [−ΔG = e(Ered
− Eox)] from toluene derivatives (squares) and thioanisole derivatives
(triangles) to [(N4Py)FeIV(O)]2+ in the presence of HOTf (red) and
Sc(OTf)3 (blue). The red and blue circles show the driving force
dependence of the rate constants (log ket) of electron transfer from
electron donors [(13) [FeII(Ph2phen)3]2+, (14) [FeII(bpy)3]2+, (15)
[RuII(4,4′-Me2phen)3]2+, (16) [RuII(5,5′-Me2phen)3]2+, (17)
[FeII(Clphen)3]2+ and (18) [RuII(bpy)3]2+] to [(N4Py)FeIV(O)]2+ in
the presence of acids (10 mM), HOTf and Sc(OTf)3, in MeCN at 298
K, respectively. The black circles show the driving force dependence of
the rate constants (log ket) of electron transfer from electron donors
[(19) decamethylferrocene, (20) octamethylferrocene, (21) 1.1′-
dimethylferrocene, (22) n-amylferrocene and (23) ferrocene] to
[(N4Py)FeIV(O)]2+ in the absence of acids in MeCN at 298 K.42
CONCLUSION
■
Oxidation of toluene and thioanisole derivatives by [(N4Py)-
FeIV(O)]2+ in deaerated MeCN at 298 K was remarkably
accelerated by the presence of HOTf and Sc(OTf)3 to yield a
stoichiometric amount of the corresponding benzyl alcohol and
sulfoxide derivatives, respectively. No KIE was observed when
toluene was replaced by toluene-d8 for the oxidation by
[(N4Py)FeIV(O)]2+ in the presence of HOTf (>50 mM),
suggesting that the rate-determining step is PCET from toluene
to [(N4Py)FeIV(O)]2+. An inverse KIE was observed for PCET
from both [RuII(bpy)3]2+ and toluene to [(N4Py)FeIV(O)]2+
when HOTf was replaced by DOTf, suggesting that the O−H
bond of the protonated FeIV(O) complex at the transition state
of PCET is significantly stronger than that of the ground state.
The unified correlations of driving force (−ΔGet) dependence
of log kET of oxidation of toluene and thioanisole derivatives by
[(N4Py)FeIV(O)]2+ as well as electron transfer from
coordinatively saturated metal complexes to [(N4Py)-
FeIV(O)]2+ in the presence of HOTf and Sc(OTf)3 (Figure
14) indicate that oxidation of toluene and thioanisole
shows unified plots of log kET of PCET and MCET from
electron donors to [(N4Py)FeIV(O)]2+ in the presence of
HOTf (10 mM) and Sc(OTf)3 (10 mM). The driving force
dependence of log kET (or log kobs) of PCET from all kinds of
electron donors (coordinatively saturated metal complexes,
toluene and thioanisole derivatives) to [(N4Py)FeIV(O)]2+ in
the presence of HOTf (10 mM) in MeCN at 298 K is unified
as a red line together with that of log kET of electron transfer
from coordinatively saturated metal complexes to [(N4Py)-
FeIV(O)]2+ in the absence of HOTf using the same λ value of
2.74 eV.42 The driving force dependence of log kET of MCET
from all kinds of electron donors to [(N4Py)FeIV(O)]2+ in the
presence of Sc(OTf)3 (10 mM) in MeCN at 298 K is also
3626
dx.doi.org/10.1021/ic403124u | Inorg. Chem. 2014, 53, 3618−3628

Inorganic Chemistry
Article
Scheme 1. Proposed Unified Mechanism of Oxidation of Toluene and Thioanisole Derivatives by [(N4Py)FeIV(O)]2+ in the
Presence of HOTf and Sc(OTf)3
derivatives by [(N4Py)FeIV(O)]2+ in the presence of HOTf and
Sc(OTf)3 proceeds via the rate-determining PCET and MCET,
respectively. Remarkable enhancement of PCET and MCET
reactivity of [(N4Py)FeIV(O)]2+ results from binding of two
molecules of HOTf and Sc(OTf)3 to the oxo group of the
FeIV(O) complex, which causes large positive shifts of the Ered
values in the presence of HOTf and Sc(OTf)3, respectively.
Formation of strong precursor complexes of [(N4Py)-
FeIV(O)]2+, which binds with two molecules of HOTf and
Sc(OTf)3, with organic substrates (toluene and thioanisole
derivatives) as compared with coordinatively saturated metal
complexes also results in enhancement of the PCET and
MCET reactivity. A boundary between electron transfer vs
concerted pathways is determined by the driving force of
electron transfer (−ΔGet). PCET and MCET pathways are
dominant when −ΔGet > −0.5 eV, whereas concerted pathways
become dominant when −ΔGet < −0.5 eV. The unified view of
enhancement of oxidative C−H bond cleavage of toluene
derivatives and sulfoxidation of thioanisole derivatives by a
nonheme iron(IV)−oxo complex via PCET and MCET
demonstrated in this study provides generalized understanding
of a variety of PCET and MCET pathways for oxidation of
substrates by metal−oxygen species.
edge support from JSPS by a Grant-in-Aid for JSPS fellowship
for young scientists.
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.
(d) Grinkova, Y. V.; Denisov, I. G.; McLean, M. A.; Sligar, S. G. Biol.
Biophys. Res. Commun. 2013, 430, 1223.
(2) (a) Sharma, V. K. Oxidation of Amino Acids, Peptides, and Proteins;
John Wiley & Sons Ltd: Chichester, England, 2012. (b) Sono, M.;
Roach, M. P.; Coulter, E. D.; Dawson, J. H. Chem. Rev. 1996, 96, 2841.
(c) Watanabe, Y. J. Biol. Inorg. Chem. 2001, 6, 846. (d) 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,
̈
1037.
(4) (a) Usharani, D.; Lacy, D. C.; Borovik, A. S.; Shaik, S. J. Am.
Chem. Soc. 2013, 135, 17090. (b) de Visser, S. P.; Rohde, J.-U.; Lee, Y.-
M.; Cho, J.; Nam, W. Coord. Chem. Rev. 2013, 257, 381. (c) McDonald,
A. R.; Que, L., Jr. Coord. Chem. Rev. 2013, 257, 414.
ASSOCIATED CONTENT
(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) Bakac, A. Coord. Chem. Rev. 2006, 250, 2046. (b) Arias, J.;
Newlands, C. R.; Abu-Omar, M. M. Inorg. Chem. 2001, 40, 2185.
■
S
* Supporting Information
Table S1 and Figures S1−S18. This material is available free of
charge via the Internet at http://pubs.acs.org.
AUTHOR INFORMATION
■
Corresponding Authors
*E-mail: fukuzumi@chem.eng.osaka-u.ac.jp (S.F.)
*E-mail: wwnam@ewha.ac.kr (W.N.)
(c) Sarauli, D.; Meier, R.; Liu, G.-F.; Ivanovic-
Eldik, R. Inorg. Chem. 2005, 44, 7624.
́ ́
Burmazovic, I.; van
(7) Hayashi, H.; Fujinami, S.; Nagatomo, S.; Ogo, S.; Suzuki, M.;
Uehara, A.; Watanabe, Y.; Kitagawa, T. Inorg. Chim. Acta 2000, 300−
302, 587.
Notes
The authors declare no competing financial interest.
(8) Das, D.; Sarkar, B.; Kumar Mondal, T.; Mobin, S. M.; Fiedler, J.;
Kaim, W.; Kumar Lahiri, G. Inorg. Chem. 2011, 50, 7090.
(9) (a) Groves, J. T.; Shalyaev, K.; Lee, J. In The Porphyrin
Handbook; Kadish, K. M., Smith, K. M., Guilard, R., Eds.; Academic
Press: Elsevier Science (USA), 2000, Vol. 4, pp 17−40. (b) Watanabe,
Y. In The Porphyrin Handbook; Kadish, K. M., Smith, K. M., Guilard,
R., Eds.; Academic Press: Elsevier Science (USA), 2000, Vol. 4, pp
97−117. (c) Groves, J. T. J. Inorg. Biochem. 2006, 100, 434.
ACKNOWLEDGMENTS
■
The research at OU was supported by an ALCA funding from
Japan Science and Technology Agency (JST), Japan (to S.F.).
The research at EWU was supported by NRF/MEST of Korea
through CRI (NRF-2012R1A3A2048842 to W.N.) and GRL
(NRF-2010-00353 to W.N.). J.P. and Y.M. gratefully acknowl-
3627
dx.doi.org/10.1021/ic403124u | Inorg. Chem. 2014, 53, 3618−3628

Inorganic Chemistry
Article
(10) (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.
(11) (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.
(12) (a) Shaik, S.; Chen, H.; Janardanan, D. Nature Chem. 2011, 3,
19. (b) Chen, H.; Lai, W.; Shaik, S. J. Phys. Chem. Lett. 2010, 1, 1533.
(13) (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.
(28) Armarego, W. L. F.; Chai, C. L. L. Purification of Laboratory
Chemicals, 6th ed.; Pergamon Press: Oxford, 2009.
(29) Lubben, M.; Meetsma, A.; Wilkinson, E. C.; Feringa, B.; Que, L.,
Jr. Angew. Chem., Int. Ed. 1995, 34, 1512.
(30) Organic Syntheses; Saltzman, H.; Sharefkin, J. G., Eds.; Wiley:
New York, 1973; Vol. V, p 658.
(31) (a) Lin, C. T.; Bottcher, W.; Chou, M.; Creuts, C.; Sutin, N. J.
Am. Chem. Soc. 1976, 98, 6536. (b) Fussa-Rydel, O.; Zhang, H. T.;
Hupp, J. T.; Leidnert, C. R. Inorg. Chem. 1989, 28, 1533. (c) Leidner,
C. R.; Murray, R. W. J. Am. Chem. Soc. 1984, 106, 1606.
(32) Fukuzumi, S.; Ohkubo, K.; Suenobu, T.; Kato, K.; Fujitsuka, M.;
Ito, O. J. Am. Chem. Soc. 2001, 123, 8459.
(33) Skarda, V.; Cook, M. J.; Lewis, A. P.; McAuliffe, G. S. G.;
Thomson, A. J. J. Chem. Soc., Perkin Trans. 1984, 1309.
(34) Kwart, H. Acc. Chem. Res. 1982, 15, 401.
(35) (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.
(36) (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, Bioenerg. 2006,
1757, 981. (d) McCusker, K. P.; Kilnman, J. P. J. Am. Chem. Soc. 2010,
132, 5114.
(37) Fukuzumi, S.; Kobayashi, T.; Suenobu, T. J. Am. Chem. Soc.
2010, 132, 1496.
(38) (a) Parkin, G. Acc. Chem. Res. 2009, 42, 315. (b) Kotani, H.;
Hanazaki, R.; Ohkubo, K.; Yamada, Y.; Fukuzumi, S. Chem.−Eur. J.
2011, 17, 2777.
(39) (a) Wolfsberg, M. Acc. Chem. Res. 1972, 5, 225. (b) Parkin, G.
Acc. Chem. Res. 2009, 42, 315.
(40) Tanner, M. J.; Brookhart, M.; DeSimone, J. M. J. Am. Chem. Soc.
1997, 119, 7617.
(14) 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.
(15) Tang, H.; Guan, J.; Zhang, L.; Liu, H.; Huang, X. Phys. Chem.
Chem. Phys. 2012, 14, 12863.
(16) Kumar, D.; Sastry, G. N.; de Visser, S. P. J. Phys. Chem. B 2012,
116, 718.
(17) (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.
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.
(18) Chiavarino, B.; Cipollini, R.; Crestoni, M. E.; Fornarini, S.;
Fornarini, S.; Lapi, A. J. Am. Chem. Soc. 2008, 130, 3208.
(19) Kaizer, J.; Klinker, E. J.; Oh, N. Y.; Rohde, J.-U.; Song, W. J.;
(41) (a) Fukuzumi, S.; Ohkubo, K. J. Am. Chem. Soc. 2002, 124,
10270. (b) Fukuzumi, S.; Ohkubo, K. Chem.−Eur. J. 2000, 6, 4532.
(42) Lee, Y.-M.; Kotani, H.; Suenobu, T.; Nam, W.; Fukuzumi, S. J.
Am. Chem. Soc. 2008, 130, 434.
Stubna, A.; Kim, J.; Munck, E.; Nam, W.; Que, L., Jr. J. Am. Chem. Soc.
̈
2004, 126, 472.
(20) (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.
(21) (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,
(43) The one-electron reduction potentials of [(N4Py)FeIV(O)]2+
(Ered) in the presence of acids may also have linear correlation with the
quantitative measure of the Lewis acidity of acids, although the Ered
values in the presence of other metal triflates have not been
determined.23b
(44) (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.
(45) (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.
́
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.
(46) (a) Fukuzumi, S.; Honda, T.; Kojima, T. Coord. Chem. Rev.
2012, 256, 2488. (b) Fukuzumi, S.; Karlin, K. D. Coord. Chem. Rev.
2013, 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.
(47) (a) Takai, A.; Gros, C. P.; Barbe, J.-M.; Guilard, R.; Fukuzumi,
S. Chem.−Eur. J. 2009, 15, 110. (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.
(22) (a) Yiu, S.-M.; Man, W.-L.; Lau, T.-C. J. Am. Chem. Soc. 2008,
130, 10821. (b) Lam, W. W. Y.; Yiu, S.-M.; Lee, J. M. N.; Yau, S. K. Y.;
Kwong, H.-K.; Lau, T.-C.; Liu, D.; Lin, Z. J. Am. Chem. Soc. 2006, 128,
2851. (c) Yiu, S.-M.; Wu, Z.-B.; Mak, C.-K.; Lau, T.-C. J. Am. Chem.
Soc. 2004, 126, 14921.
(23) (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.
(48) (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.
(24) Morimoto, Y.; Park, J.; Suenobu, T.; Lee, Y.-M.; Nam, W.;
Fukuzumi, S. Inorg. Chem. 2012, 51, 10025.
(25) Park, J.; Morimoto, Y.; Lee, Y.-M.; Nam, W.; Fukuzumi, S. J.
Am. Chem. Soc. 2012, 134, 3903.
(26) Park, J.; Lee, Y.-M.; Nam, W.; Fukuzumi, S. J. Am. Chem. Soc.
2013, 135, 5052.
(27) (a) Fukuzumi, S. Prog. Inorg. Chem. 2009, 56, 49. (b) Fukuzumi,
S.; Ohkubo, K. Coord. Chem. Rev. 2010, 254, 372. (c) Fukuzumi, S.;
Ohkubo, K.; Morimoto, Y. Phys. Chem. Chem. Phys. 2012, 14, 8472.
3628
dx.doi.org/10.1021/ic403124u | Inorg. Chem. 2014, 53, 3618−3628