Proton-promoted oxygen atom transfer vs proton-coupled electron transfer of a non-heme iron(IV)-oxo complex

Article abstract of DOI:10.1021/ja211641s

Sulfoxidation of thioanisoles by a non-heme iron(IV)-oxo complex, [(N4Py)Fe^IV(O)]²⁺ (N4Py = N,N-bis(2-pyridylmethyl)-N- bis(2-pyridyl)methylamine), was remarkably enhanced by perchloric acid (70% HClO₄). The observed second-order rate constant (k_obs) of sulfoxidation of thioaniosoles by [(N4Py)Fe^IV(O)]²⁺ increases linearly with increasing concentration of HClO₄ (70%) in acetonitrile (MeCN)at 298 K. In contrast to sulfoxidation of thioanisoles by [(N4Py)Fe^IV(O)]²⁺, the observed second-order rate constant (k_et) of electron transfer from one-electron reductants such as [Fe^II(Me₂bpy)₃]²⁺ (Me₂bpy = 4,4-dimehtyl-2,2′-bipyridine) to [(N4Py)Fe^IV(O)] ²⁺ increases with increasing concentration of HClO₄, exhibiting second-order dependence on HClO₄ concentration. This indicates that the proton-coupled electron transfer (PCET) involves two protons associated with electron transfer from [Fe^II(Me₂bpy) ₃]²⁺ to [(N4Py)Fe^IV(O)]²⁺ to yield [Fe^III(Me₂bpy)₃]³⁺ and [(N4Py)Fe^III(OH₂)]³⁺. The one-electron reduction potential (E_red) of [(N4Py)Fe^IV(O)]²⁺ in the presence of 10 mM HClO₄ (70%) in MeCN is determined to be 1.43 V vs SCE. A plot of E_red vs log[HClO₄] also indicates involvement of two protons in the PCET reduction of [(N4Py)Fe ^IV(O)]²⁺. The PCET driving force dependence of log k _et is fitted in light of the Marcus theory of outer-sphere electron transfer to afford the reorganization of PCET (λ = 2.74 eV). The comparison of the k_obs values of acid-promoted sulfoxidation of thioanisoles by [(N4Py)Fe^IV(O)]²⁺ with the k_et values of PCET from one-electron reductants to [(N4Py)Fe^IV(O)] ²⁺ at the same PCET driving force reveals that the acid-promoted sulfoxidation proceeds by one-step oxygen atom transfer from [(N4Py)Fe ^IV(O)]²⁺ to thioanisoles rather than outer-sphere PCET.

Full text of DOI:10.1021/ja211641s

Article
pubs.acs.org/JACS
Proton-Promoted Oxygen Atom Transfer vs Proton-Coupled Electron
Transfer of a Non-Heme Iron(IV)-Oxo Complex
Jiyun Park,† Yuma Morimoto,‡ Yong-Min Lee,† Wonwoo Nam,*,† and Shunichi Fukuzumi*,†,‡
†Department of Bioinspired Science, Ewha Womans University, Seoul 120-750, Korea
‡Department of Material and Life Science, Graduate School of Engineering, Osaka University, ALCA, Japan Science and Technology
Agency (JST), Suita, Osaka 565-0871, Japan
S
* Supporting Information
ABSTRACT: Sulfoxidation of thioanisoles by a non-heme
iron(IV)−oxo complex, [(N4Py)FeIV(O)]2+ (N4Py = N,N-
bis(2-pyridylmethyl)-N-bis(2-pyridyl)methylamine), was re-
markably enhanced by perchloric acid (70% HClO4). The
observed second-order rate constant (kobs) of sulfoxidation of
thioaniosoles by [(N4Py)FeIV(O)]2+ increases linearly with
increasing concentration of HClO4 (70%) in acetonitrile (MeCN)
at 298 K. In contrast to sulfoxidation of thioanisoles by [(N4Py)-
FeIV(O)]2+, the observed second-order rate constant (ket) of
electron transfer from one-electron reductants such as [FeII(Me2bpy)3]2+ (Me2bpy = 4,4-dimehtyl-2,2′-bipyridine) to
[(N4Py)FeIV(O)]2+ increases with increasing concentration of HClO4, exhibiting second-order dependence on HClO4
concentration. This indicates that the proton-coupled electron transfer (PCET) involves two protons associated with electron
transfer from [FeII(Me2bpy)3]2+ to [(N4Py)FeIV(O)]2+ to yield [FeIII(Me2bpy)3]3+ and [(N4Py)FeIII(OH2)]3+. The one-electron
reduction potential (Ered) of [(N4Py)FeIV(O)]2+ in the presence of 10 mM HClO4 (70%) in MeCN is determined to be 1.43 V
vs SCE. A plot of Ered vs log[HClO4] also indicates involvement of two protons in the PCET reduction of [(N4Py)FeIV(O)]2+.
The PCET driving force dependence of log ket is fitted in light of the Marcus theory of outer-sphere electron transfer to afford
the reorganization of PCET (λ = 2.74 eV). The comparison of the kobs values of acid-promoted sulfoxidation of thioanisoles by
[(N4Py)FeIV(O)]2+ with the ket values of PCET from one-electron reductants to [(N4Py)FeIV(O)]2+ at the same PCET driving
force reveals that the acid-promoted sulfoxidation proceeds by one-step oxygen atom transfer from [(N4Py)FeIV(O)]2+ to
thioanisoles rather than outer-sphere PCET.
of metal ions, such as Sc3+ and Ca2+, to a non-heme iron(IV)−
INTRODUCTION
■
oxo complex, [(TMC)FeIV(O)]2+ (TMC = 1,4,8,11-tetrameth-
Brønsted acid is well-known to enhance the electrophilicity of
yl-1,4,8,11-tetraazacyclotetradecane), and the crystal structure
electrophiles by interaction, making them possible to react with
of Sc3+-bound [(TMC)FeIV(O)]2+ was successfully determined
by X-ray crystallography.29 The binding of Sc3+ to [(TMC)-
FeIV(O)]2+ resulted in change in the number of electrons trans-
ferred from ferrocene (Fc) to [(TMC)FeIV(O)]2+ from one
electron in the absence of Sc3+ to two electrons in the presence
of Sc3+.29 Rates of electron transfer from a series of one-electron
reductants to an iron(IV)−oxo complex, [(N4Py)FeIV(O)]2+
(N4Py: N,N-bis(2-pyridylmethyl)-N-bis(2-pyridyl)methylamine),
were enhanced as much as 108-fold in the presence of metal
ions, such as Sc3+, Zn2+, Mg2+, and Ca2+, as compared with the
rates in the absence of metal ion.30 The rate enhancement by
metal ions exhibits a good correlation with the Lewis acidity of
metal ions.30 The one-electron reduction potential of [(N4Py)-
FeIV(O)]2+ has been demonstrated to be shifted to a positive
direction by as large as 0.84 V in the presence of Sc3+ ion.30−32
In contrast to the extensive studies on Brønsted acid-promoted
reactions of organic electrophiles containing CO bonds,1−9
nucleophiles, which would otherwise exhibit no reactivity.1−3
Metal ion salts, which act as Lewis acids, are also known to
enhance the electrophilicty of electrophiles by interacting with
Lewis acids to facilitate the reactions with nucleophiles.4−9 The
promoting effects of Brønsted acid and Lewis acids result from
the activation of the CX bond (X = O, NR, CR2), thereby
decreasing the LUMO energy and promoting the reactions with
nucleophiles.1−9 On the other hand, both Brønsted acid and
metal ions acting as Lewis acids promote electron-transfer reactions
from various electron donors to electron acceptors provided that
the one-electron reduced species of electron acceptors can bind
with Brønsted acid (protonation) and Lewis acids, respectively.
The former is classified as proton-coupled electron transfer
(PCET),10−14 whereas the latter is regarded as metal ion-
promoted electron transfer.15−24 It should be noted that the
term of PCET has also been used for the case when an electron
and a proton are transferred from the same molecule.25−28
Not only the CO bond but also the MO (M = metal)
bond can be activated by Brønsted acid and Lewis acids. In this
context, we have recently reported the first example of binding
Received: December 27, 2011
Published: February 16, 2012
© 2012 American Chemical Society
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intermediate generated) in the absence and presence of HClO4
(10 mM), respectively. In the cases of para-substituted thioanisoles,
methyl para-substituted phenyl sulfoxides were obtained quantitatively
by HPLC, as in the case of thioanisole.
there have been only a few reports on Brønsted aid-promoted
reactions of metal−oxo (MO) complexes.33 In particular, the
fundamental properties of high-valent metal-oxo complexes such
as the one-electron reduction potentials and the reorganization
energies in PCET have yet to be reported. In addition, the relation
between Brønsted acid-promoted reactions of high-valent metal-
oxo complexes with nucleophiles and the PCET with one-electron
reductants has yet to be clarified.
We report herein Brønsted acid-promoted oxygen atom
transfer from a non-heme iron(IV)-oxo complex, [(N4Py)-
FeIV(O)]2+, to thioanisoles34 in comparison with PCET from
one-electron reductants to [(N4Py)FeIV(O)]2+. The PCET rate
constants were evaluated in light of the Marcus theory of outer-
sphere electron transfer35,36 to determine the one-electron
reduction potential and the reorganization energy of PCET
reactions of [(N4Py)FeIV(O)]2+ in the presence of perchloric
acid (HClO4, 70 wt % in H2O) in acetonitrile (MeCN). The
detailed kinetic analyses on both acid-promoted oxygen atom
transfer from [(N4Py)FeIV(O)]2+ to thioanisoles and the PCET
reactions from one-electron reductants to [(N4Py)FeIV(O)]2+
provide valuable insight into the important mechanistic difference
between direct oxygen atom-transfer37 and outer-sphere PCET
pathways, both of which are enhanced by Brønsted acid.
Instrumentation. UV−vis spectra were recorded on a UNISOKU
RSP-601 stopped-flow spectrometer equipped with a MOS-type highly
sensitive photodiode array or a Hewlett-Packard 8453 photodiode-
array spectrophotometer. 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 Instruments 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 = 9.65 GHz, microwave power = 1 mW, modulation amplitude =
10 G, gain = 5 × 103, time constant = 40.96 ms, conversion time =
81.00 ms. Electrospray ionization mass spectra (ESI MS) were collected
on a Thermo Finnigan (San Jose, CA, USA) LCQ Advantage MAX
quadrupole ion trap instrument, by infusing samples directly into the
source using a manual method. The spray voltage was set at 3.7 kV and
the capillary temperature at 100 °C. Product analysis for oxidation
reactions was performed on a DIONEX Summit Pump Series P580
equipped with a variable wavelength UV-200 detector (HPLC). Products
were separated on a Hypersil GOLD column (4.6 mm × 250 mm),
and product yields were determined with a UV Detector at 215 and
254 nm. 1H NMR spectra were measured with a Bruker model digital
AVANCE III 400 FT-NMR spectrometer. Cyclic voltammetry (CV)
measurements were performed on a BAS 630B electrochemical analyzer
in a deaerated MeCN solution containing 0.10 M TBA(PF6) as a
supporting electrolyte at 298 K. A conventional three-electrode cell
was used with a platinum working electrode, a platinum wire as a
counter electrode, and a Ag/AgNO3 reference electrode. The measured
potentials were recorded with respect to the Ag/AgNO3 (1.0 × 10−2 M).
The Eox and Ered values (vs Ag/AgNO3) are converted to those vs SCE
by adding 0.29 V.40 All electrochemical measurements were carried out
under an Ar atmosphere.
EXPERIMENTAL SECTION
■
Materials. All chemicals, which were the best available purity, were
purchased from Aldrich Chemical Co. and used without further puri-
fication 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.38 Iodosylbenzene (PhIO) was
prepared by a literature method.39 Non-heme iron(II) complex [(N4Py)-
FeII(MeCN)](ClO4)2 and its iron(IV)−oxo [(N4Py)FeIV(O)]2+ were
prepared by the literature methods.34 Perchloric acid (70 wt % in H2O)
was purchased from Wako Pure Chemical Ind. Ltd.
RESULTS AND DISCUSSION
■
Caution: Perchlorate salts are potentially explosive and should be
handled with care.
Brønsted Acid-Promoted Oxygen Atom Transfer from
[(N4Py)FeIV(O)]2+ to Thioanisoles. It has been well established
that oxygen atom transfer from a non-heme iron(IV)−oxo complex
([(N4Py)FeIV(O)]2+) to thioanisole occurs to yield the sulfoxide
and [(N4Py)FeII]2+ in MeCN (see Figure S1 in the Supporting
Information (SI) for ESI-MS).39 The UV−visible spectral
change observed in the reaction of oxygen atom transfer from
[(N4Py)FeIV(O)]2+ to thioanisole is shown in Figure 1a (left
panel), where the absorption band at 695 nm due to [(N4Py)-
FeIV(O)]2+ decreases, accompanied by increase in absorbance due
to [(N4Py)FeII]2+.
Kinetic Studies and Product Analyses. Kinetic measurements
were performed on a UNISOKU RSP-601 stopped-flow spectrometer
equipped with a MOS-type highly sensitive photodiode array or a
Hewlett-Packard 8453 photodiode-array spectrophotometer using a
10 mm quartz cuvette (10 mm path length) at 298 K. Sulfoxidation
reactions of thioanisole and its para-substituted derivatives (2.5 × 10−3
to 1.0 × 10−2 M) by [(N4Py)FeIV(O)]2+ (2.5 × 10−4 M) were carried
out and the rates were monitored by the decay of the absorption band
at 695 nm due to [(N4Py)FeIV(O)]2+ in the absence and presence of
HClO4 in MeCN at 298 K. The concentration of thioanisole deriva-
tives was maintained at least more than 10-fold excess of [(N4Py)-
FeIV(O)]2+ to attain pseudo-first-order conditions. 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 measure-
ments agreed within an experimental error of 5%. The pseudo-first-
order rate constants increased proportionally with an increase in
concentrations of substrates, from which second-order rate constants
were determined.
IV
2+
PhSMe + [Fe (O)(N4Py)]
II
2+
→ PhS(O)Me + [Fe (N4Py)]
(1)
Addition of HClO4 (70%, 10 mM) to an MeCN solution of
[(N4Py)FeIV(O)]2+ resulted in remarkable enhancement of the
reaction rate, as indicated by the comparison of the time profile
in the absence of HClO4 (right panel in Figure 1a) vs that in
the presence of HClO4 (right panel in Figure 1b). In the
presence of HClO4, the reaction rate was too fast to follow by a
conventional spectroscopic method; therefore, the rate was
followed by using a stopped-flow technique (see Experimental
Section). The rapid decrease in absorbance at 695 nm due to
[(N4Py)FeIV(O)]2+ in the presence of HClO4 was accom-
panied by an increase in absorbance due to [(N4Py)FeII]2+,
which was observed as the recovery of the bleaching as shown
in Figure 1b (right panel), where the spectrum after the
Typically, thioanisole (1.0 × 10−2 M) was added to an MeCN
solution (0.50 mL) containing [(N4Py)FeIV(O)]2+ (1.0 × 10−3 M) in
the absence and presence of HClO4 (1.0 × 10−2 M) in a vial. The
reaction was complete within 1 h under these conditions. Products
formed in the oxidation reactions of thioanisole by [(N4Py)-
FeIV(O)]2+, which were carried out in the absence and presence of
HClO4 under Ar atmosphere in MeCN at 298 K, were analyzed by
HPLC. Quantitative analyses were performed on the basis of
comparison of HPLC peak integration between products and their
authentic samples. In the case of thioanisole, methyl phenyl sulfoxide
was obtained as a sole product with 96% and 93% yield (based on the
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of HClO4 as shown in Figure 2a. The dependence of kobs of
other thioanisole derivatives on concentration of HClO4 also
exhibited linear relations (Figures 2b−d) as given by eq 2,
k
= k + k [HClO ]
0 1 4
(2)
obs
where k0 and k1 are the rate constant of oxygen atom transfer
from [(N4Py)FeIV(O)]2+ to para-X-substituted thioanisoles in
the absence and presence of HClO4, respectively. The observed
second-order rate constants (kobs) of sulfoxidation of
thioanisoles by [(N4Py)FeIV(O)]2+ in the absence and
presence of HClO4 (10 mM) are listed in Table 1.
Table 1. One-Electron Oxidation Potentials (Eox) of para-
X-Substituted Thioanisoles and Second-Order Rate Constants
of the Sulfoxidation by [(N4Py)FeIV(O)]2+ in the Absence
and Presence of HClO4 (10 mM) in MeCN at 298 K
kobs, M−1 s−1
Figure 1. Visible spectral changes observed in the reaction of
[(N4Py)FeIV(O)]2+ (0.50 mM) with 10 equiv of thioanisole (5.0 mM)
in the absence (a) and presence (b) of HClO4 (10.0 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+.
X
Eox (vs SCE, V)
without HClO4
with HClO4 (10 mM)
Me
H
1.24
1.34
1.37
1.41
1.61
1.3 0.1
(3.2 0.2) × 103
(1.5 0.1) × 103
(7.2 0.4) × 102
(8.4 0.4) × 102
(6.2 0.3) × 10
(8.7 0.4) × 10−1
(8.7 0.4) × 10−1
(1.5 0.1) × 10−1
(4.4 0.2) × 10−2
Cl
Br
reaction was subtracted from the observed spectra in the
stopped flow measurements. The yield of the sulfoxide in the
presence of HClO4 was quantitative, as is the case in the
absence of HClO4 (see Experimental Section).
CN
It should be noted that the kobs values of thioanisoles in the
presence of HClO4 (10 mM) are more than 103-fold larger than
the corresponding values in the absence of HClO4. The en-
hanced reactivity of [(N4Py)FeIV(O)]2+ by the presence of
HClO4 (70%) remarkably decreased by addition of H2O. The
kobs value of acid-promoted sulfoxidation of thioanisole by
[(N4Py)FeIV(O)]2+ decreased significantly with increasing
concentration of added water due to the decrease in the acidity
of HClO4 by H2O,12a,42 as shown in Figure 3. Because 70 wt %
HClO4 in H2O is employed in this work, the use of 100%
HClO4 may enhance the reactivity of [(N4Py)]FeIV(O)]2+
drastically. However, the possible explosion of 100% HClO4
Decay rates of the absorption band at 695 nm due to
[(N4Py)FeIV(O)]2+ in oxygen atom transfer from [(N4Py)-
FeIV(O)]2+ to large excess thioanisole in the absence and pre-
sence of large excess HClO4 obeyed pseudo-first-order kinetics
(Figure 1). The pseudo-first-order rate constant increased
linearly with increasing concentration of thioanisole (Figure S2
in the Supporting Information).41 The observed second-order
rate constant (kobs) obtained from the slope of a linear plot of
the pseudofirst-order rate constant vs concentration of thio-
anisole increased proportionally with increasing concentration
Figure 2. Plots of kobs vs HClO4 concentration in acid-promoted sulfoxidation of para-X-substituted thioanisoles (X = (a) Me, (b) H, (c) Br and
(d) CN) by [(N4Py)FeIV(O)]2+ in MeCN at 298 K.
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Figure 3. Plot of kobs vs additional H2O concentration in sulfoxida-
tion of thioanisoles by [(N4Py)FeIV(O)]2+ in the presence of HClO4
(10 mM) in MeCN at 298 K.
has precluded the examination of the enhancement of reactivity
of [(N4Py)FeIV(O)]2+ by HClO4 without water.
PCET from One-Electron Reductants to [(N4Py)-
FeIV(O)]2+. When [RuII(Clphen)3]2+ (Clphen: 5-chlorophena-
threne) was employed as an electron donor, no electron
transfer from [RuII(Clphen)3]2+ (Eox = 1.36 V vs SCE)30 to
[(N4Py)FeIV(O)]2+ (Ered = 0.51 V)36 occurred in MeCN, because
the free energy change of electron transfer is highly positive
(ΔGet = 0.85 eV), i.e., endergonic. In the presence of HClO4,
however, the electron transfer occurred efficiently as shown in
Figure 4, where the absorption band due to [RuII(Clphen)3]2+
Figure 5. EPR spectra of the products obtained in the reaction of
[(N4Py)FeIV(O)]2+ (1.0 mM) with [RuII(Clphen)3]2+ (2.0 mM) (a)
in the absence and (b) presence of HClO4 (10 mM). (c) EPR
spectrum of [RuIII(Clphen)3]3+ generated in the photocatalytic
oxidation of [RuII(Clphen)3]2+ (1.0 mM) with sodium persulfate
(50 mM) in the presence of HClO4 (50 mM) in MeCN at 298 K. The
EPR signal intensity observed at g = 2.004, which is attributed an
organic radical species, is negligible as compared with the intensity due
to [RuIII(Clphen)3]3+. (d) EPR spectrum of [(N4Py)FeIII(OH2)]3+
generated by PCET from ferrocene (1.0 mM) to [(N4Py)FeIV(O)]2+
(1.0 mM) in the presence of HClO4 (10 mM) in MeCN at 298 K. The
low spin [(N4Py)FeIII(OH2)]3+ observed at g⊥ = 2.46 and g||= 1.65 was
converted to a high spin species at gb = 4.3 and gc = 1.87 at prolonged
reaction time.
PCET from ferrocene to [(N4Py)FeIV(O)]2+ in the presence of
HClO4 in MeCN at 298 K, respectively. It was observed that
the low spin EPR signals due to [(N4Py)FeIII(OH2)]3+ were
converted to the high spin EPR signals (gb and gc), which can
be assigned to [(N4Py)FeIII]3+.
The concentration of [RuIII(Clphen)3]3+ produced in PCET
from [RuII(Clphen)3]2+ to [(N4Py)FeIV(O)]2+ (2.5 × 10−4 M)
in the presence of HClO4 (10 mM) increased with increasing
the initial concentration of [RuII(Clphen)3]2+ to reach the
initial concentration of [(N4Py)FeIV(O)]2+ as shown in Figure 6.
Figure 4. Visible spectral changes observed in PCET from
[RuII(Clphen)3]2+ (12.5 mM) to [(N4Py)FeIV(O)]2+ (0.25 mM) in
the presence of HClO4 (10 mM) in MeCN at 298 K. The inset shows
the time course monitored at 640 nm due to the formation of
[RuIII(Clphen)3]3+.
(λmax = 450 nm) and [(N4Py)FeIV(O)]2+ (λmax = 695 nm)
decreases to be changed to an absorption band due to
[RuIII(Clphen)3]3+ (λmax = 640 nm) with an isosbestic point at
548 nm.
Formation of [RuIII(Clphen)3]3+ and [(N4Py)FeIII(OH2)]3+
in the PCET reaction was confirmed by EPR measurements. In
the absence of HClO4, as shown in Figure 5a, no electron
transfer occurred because both reactants [RuII(Clphen)3]2+ and
[(N4Py)FeIV(O)]2+ are EPR silent. In the presence of HClO4,
PCET from [RuII(Clphen)3]2+ to [(N4Py)FeIV(O)]2+ occurred
to produce [RuIII(Clphen)3]3+ and [(N4Py)FeIII(OH2)]3+, as
indicated by EPR signals observed at g1 = 2.70, g2 = 2.46, and
g3 = 1.65 after the PCET reaction (Figure 5b). The observed
EPR signals agree with the superposition of the EPR signals of
[RuIII(Clphen)3]3+ (Figure 5c)43 and [(N4Py)FeIII(OH2)]3+
(Figure 5d), which were produced by the photosensitized
oxidation of [RuII(Clphen)3]2+ with sodium persulfate and by
Figure 6. Plot of concentration of [RuIII(Clphen)3]3+ produced in PCET
from [RuII(Clphen)3]2+ to [(N4Py)FeIV(O)]2+ (2.5 × 10−4 M) in the
presence of HClO4 (1.0 × 10−2 M) in deaerated MeCN at 298 K vs initial
concentration of [RuII(Clphen)3](PF6)2, [[RuII(Clphen)3](PF6)2]0.
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Such a titration curve indicates that PCET from [RuII(Clphen)3]2+
(Eox = 1.36 V vs SCE)30 to [(N4Py)FeIV(O)]2+ in the presence of
HClO4 (10 mM) is in equilibrium as shown in eq 3. The PCET
equilibrium constant (Ket) was determined to be 14.0 by a
fitting curve based on the PCET equilibrium (eq 3); see the
Supporting Informationfor determination of Ket.
respectively) as shown in the solid line (Figure 7). Under the
conditions such that Kred2[HClO4] ≫ 1, eq 5 is rewritten by
eq 6, when the slope of a linear correlation between Ered and
log[HClO4] corresponds to 2(2.3RT/F) = 118 mV at 298 K, as
observed in Figure 7.
E
= E° + 2(2.3RT/F) log(K
red
K
[HClO ])
red
red red2
4
(6)
IV
2+
III
II
2+
+
[(N4Py)Fe (O)] + [Ru (Clphen) ] + 2H
3
(3)
K
The protonation of two protons to [(N4Py)FeIII(O)]+ is also
supported by the kinetic measurements (vide infra).21 The rate
of PCET from [FeII(Me2bpy)3]2+ to [(N4Py)FeIV(O)]2+ was
determined by monitoring a decrease in absorbance at 695 nm
due to [(N4Py)FeIV(O)]2+. The rate obeyed pseudo-first-order
kinetics in the presence of large excess [FeII(Me2bpy)3]2+
and HClO4 (Figure S5 in the Supporting Information). The
pseudo-first-order rate constant increased linearly with
increasing concentration of [FeII(Me2bpy)3]2+ (Figure S6 in
the Supporting Information). From the slope, the second-order
rate constant (ket) of PCET was determined. The ket values of
PCET from various one-electron reductants to [(N4Py)-
FeIV(O)]2+ were determined from the slopes of k1 vs concentra-
tions of reductants. The ket values increased with increasing
concentration of HClO4 and showed second-order dependence
at the high concentration of HClO4, as shown in Figure 8.
et
3+
III
3+
XoooY [(N4Py)Fe (OH )] + [Ru (Clphen) ]
2
3
The reduction potential (Ered) of [(N4Py)FeIV(O)]2+ in the
presence of HClO4 (10 mM) in MeCN at 298 K was then
determined from the Ket value and Eox value of [RuII(Clphen)3]2+
(Eox = 1.36 V vs SCE)30 using the Nernst equation (eq 4, where R
E
= E + (RT/F) ln K
ox
(4)
red
et
is the gas constant, T is absolute temperature, and F is the
Faraday constant) to be 1.43 V vs SCE. The Ket values in the
presence of various concentrations of HClO4 were determined
by global fitting of plots of concentrations of [RuIII(Clphen)3]3+
vs initial concentrations of [RuII(Clphen)3]2+ (Figure S4 in the
Supporting Information). The one-electron reduction poten-
tials of [(N4Py)FeIV(O)]2+ (Ered) in the presence of various
concentrations of HClO4 were also determined from the Ket
values and the Eox value of [RuII(Clphen)3]2+ using eq 4.
The dependence of Ered of [(N4Py)FeIV(O)]2+ on log([HClO4])
is shown in Figure 7, where the slope of the apparent linear
Figure 8. Plots of ket vs concentrations 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.
Figure 7. Dependence of Ered of [(N4Py)FeIV(O)]2+ on log([HClO4])
in deaerated MeCN at 298 K. The blue line is fitted by eq 6.
correlation is close to 120 mV/log([HClO4]). This slope
indicates that two protons are involved in the PCET reduction
of [(N4Py)FeIV(O)]2+ according to the Nernst equation for the
dependence of Ered on [HClO4] (eq 5), where Kred1 and Kred2
This result shows sharp contrast to the case of proton-promoted
oxygen atom transfer from [(N4Py)FeIV(O)]2+ to thioanisoles in
Figure 2, where the kobs values increased linearly with increasing
concentration of HClO4.
The results in Figure 8 are well analyzed by eq 7, where ket1
is the rate constant of electrontransfer from a one-electron
E
= E° + (2.3RT/F) log(K
red
[HClO ]
red1
red
4
× (1 + K
[HClO ]))
red2
(5)
4
2
k
= k [HClO ] + k [HClO ]
(7)
et
et1
4
et2
4
are the equilibrium constants for the first protonation and the
second protonation of the one-electron reduced complex,
[(N4Py)FeIII(O)]+.44 The plot of Ered of [(N4Py)FeIV(O)]2+ vs
log([HClO4]) in Figure 7 is well fitted by eq 5 with the best fit
values of Kred1 and Kred2 (1.2 × 1013 and 3.0 × 106 M−1,
reductant to the monoprotonated complex ([(N4Py)FeIV-
(OH)]3+) and ket2 is the rate constant of electron transfer to
the diprotonated complex ([(N4Py)FeIV(OH2)]4+). Equa-
tion 7 is written by eq 8, which predicts a linear relation
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between ket/([HClO4]) and [HClO4]. Linear plots of ket/
([HClO4]) and [HClO4] were confirmed as shown in Figure 9,
the presence of 10 mM HClO4 is determined to be 2.74 eV,
which is the same as the λ value of electron transfer from one-
electron reductants to [(N4Py)FeIV(O)]2+ (2.74 eV) as shown
in Figure 10.36 The same value of the reorganization energy
Figure 10. Plots of log kobs for sulfoxidation of para-X-substituted
thioanisoles (X = (1) Me, (2) H, (3) Cl, (4) Br, and (5) CN) by
[(N4Py)FeIV(O)]2+ in the absence and presence of HClO4 (70%,
10 mM) MeCN at 298 K vs the driving force of electron transfer [−ΔG =
e(Ered − Eox)] from thioanisoles to [(N4Py)FeIV(O)]2+ in the absence
(black closed circles) and the presence of HClO4 (10 mM) (red open
circles). The blue open circles show the driving force dependence of
the rate constants (log ket) of PCET from one-electron reductants ((6)
[FeII(Me2bpy)3](PF6)2, (7) [RuII(Me2bpy)3](PF6)2, (8)
[FeII(Clphen)3](PF6)2, and (9) [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 the Supporting Information). The green open
triangles show the driving force dependence of the rate constants
(log ket) of electron transfer from one-electron reductants ((10)
decamethylferrocene, (11) octamethylferrocene, (12) 1.1′-dimethyl-
ferrocene, (13) n-amylferrocene, and (14) ferrocene) to [(N4Py)-
FeIV(O)]2+ in the absence of HClO4 in MeCN at 298 K.34.
Figure 9. Plots of ket/[HClO4] vs [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 in MeCN at 298 K.
indicating the involvement of two protons in the PCET
reactions.
k /[HClO ] = k + k [HClO ]
(8)
et
4
et1
et2
4
Direct Oxygen Atom Transfer vs Electron Transfer in
Proton-Promoted Sulfoxidation of Thioanisoles by
[(N4Py)FeIV(O)]2+. The first-order dependence of kobs of the
acid-promoted oxygen atom transfer from [(N4Py)FeIV(O)]2+
to thioanisoles on [HClO4] in Figure 2 suggests that the rate-
determining step of the acid promoted oxygen atom transfer
cannot be PCET from thioanisoles to [(N4Py)FeIV(O)]2+,
because the PCET rate constants exhibit a second-order
dependence on [HClO4] (Figure 8). The mechanistic differ-
ence between proton-promoted oxygen atom transfer and
PCET can be clarified by comparing the dependence of kobs vs
ket on the PCET deriving force (vide infra).
The PCET deriving force (−ΔGet) was obtained from the
difference between the one-electron oxidation potentials of thio-
anisoles (Eox vs SCE)45 and the one-electron reduction potentials
of [(N4Py)FeIV(O)]2+ (Ered vs SCE) in the absence and presence
of HClO4 at 298 K.36 It should be noted that the Eox values of
thioanisoles are not changed in the presence of HClO4, whereas
the Ered value of [(N4Py)FeIV(O)]2+ is significantly shifted to the
positive direction from 0.51 V vs SCE in the absence of HClO4 to
1.43 V vs SCE in the presence of 10 mM of HClO4 (vide supra).36
The driving force dependence of rate constants of PCET
from one-electron reductants to [(N4Py)FeIV(O)]2+ in the
presence of 10 mM HClO4 is well fitted in light of the Marcus
theory of adiabatic outer-sphere electron transfer (eq 9), where
(2.74 eV) obtained from electron transfer from one-electron
reductants to [(N4Py)FeIV(O)]2+ ([(N4Py)FeIV(OH2)]4+) in
the presence and absence of HClO4 indicates that the large
reorganization energy results from the one-electron reduction
of the FeIV center to the FeIII center. In fact, a large reorganiza-
tion energy (2.34 eV) was reported for the electron-transfer
oxidation of an organoiron(III) porphyrin to the corresponding
organoiron(IV) porphyrin.46
It is interesting to note that two Sc3+ ions are involved in
Sc3+-promoted electron transfer from one-electron reductants
to [(N4Py)FeIV(O)]2+ as the case of the PCET reactions of
[(N4Py)FeIV(O)]2+, in which two protons are also involved.
The observed second-order rate constants of Sc3+-promoted
oxygen atom transfer from [(N4Py)FeIV(O)]2+ to thioanisoles
fit well with the driving force dependence of ket for Sc3+-
promoted electron-transfer reactions using the Marcus equation
(eq 9). Such an agreement with the Marcus equation indicates
that the Sc3+-promoted oxygen atom transfer from [(N4Py)-
FeIV(O)]2+ to thioanisoles in the presence of Sc3+ proceeds via
Sc3+ ion-coupled electron transfer from thioanisoles to
[(N4Py)FeIV(O)]2+, which is the rate-determining step, followed
by rapid transfer of the oxygen atom from [(N4Py)FeIII(O)]+ to
the radical cation (ArSR•+).31
2
k
= Z exp[−(λ/4)(1 + ΔG /λ) /k T]
(9)
et
et
B
Z is the collision frequency, taken as 1 × 1011 M−1 s−1, λ is the
reorganization energy of electron transfer, kB is the Boltzmann
constant, and T is the absolute temperature.35 The best fit λ value
of PCET from one-electron reductants to [(N4Py)FeIV(O)]2+ in
In the absence of HClO4, the driving force of electron transfer
(−ΔGet) is largely negative. This indicates that electron transfer
from thioanisoles to [(N4Py)FeIV(O)]2+ is highly endergonic and
thereby the electron transfer is thermodynamically infeasible.
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The kobs values in the absence of HClO4 are more than 10
orders magnitude larger than the expected ket values of outer-
sphere electron transfer from thioanisoles to [(N4Py)FeIV(O)]2+
without HClO4 (extrapolated blue line in Figure 10). In such
a case, direct oxygen atom transfer (Scheme 1a) occurs
us to conclude that acid-promoted sulfoxidation of thioanisoles
by [(N4Py)FeIV(O)]2+ proceeds via direct oxygen atom transfer
from the monoprotonated complex ([(N4Py)FeIV(OH)]3+) to
thioanisoles rather than outer-sphere PCET from thioanisoles
to [(N4Py)FeIV(OH)]3+. Such an activation of FeIVO species
by the protonation will expand the scope of acid-promoted
reactions of high-valent metal−oxo complexes as the case of
organic compounds containing a CO bond.
Scheme 1
ASSOCIATED CONTENT
* Supporting Information
■
S
Table of oxidation potentials, reductants, and second-order rate
constants; ESI-MS spectrum; 1H NMR spectrum; concen-
tration vs concentraion plots; visible spectral changes during
reactions; and plots of the pseudo-first-order rate constants vs
concentrations. This material is available free of charge via the
Internet at http://pubs.acs.org.
rather than an outer-sphere electron-transfer pathway
(Scheme 1b).31
In the presence of HClO4 (10 mM), the kobs values are still
two to four orders magnitude larger than the expected ket values
of outer-sphere PCET from thioanisoles to [(N4Py)FeIV(O)]2+
in the presence of HClO4 (10 mM) (blue line in Figure 10).
Thus, the proton-promoted sulfoxidation of thioanisoles also
occurs via direct oxygen atom transfer from the monoproto-
nated complex ([(N4Py)FeIV(OH)]3+) to thioanisoles rather
than outer-sphere PCET from thioanisoles to [(N4Py)-
FeIV(O)]2+. The difference in the reaction pathways between
proton-promoted direct oxygen atom transfer and Sc3+ ion-
coupled electron transfer may result from the steric effect.
Direct oxygen atom transfer from the monoprotonated
complex ([(N4Py)FeIV(OH)]3+) to thioanisoles may be made
possible by the small size of the proton, whereas the direct
oxygen atom transfer from Sc3+-bound complex [(N4Py-
FeIV(O)−Sc(OTf)3)2+] to thioanisoles may be prohibited due
to the steric effect of bulky Sc(OTf)3 bound to the FeIV(O)
complex. In the case of the Sc(OTf)3-promoted reaction, only
outer-sphere electron transfer can occur because outer-sphere
electron transfer requires little interaction between electron
donor and acceptor molecules, thereby being insensitive to
steric effects. In both the Sc3+ ion-coupled electron transfer and
PCET from one-electron reductants to [(N4Py)FeIV(O)]2+,
two Sc3+ ions and two protons are involved in the electron-
transfer reactions, exhibiting the second-order dependence of
the rate constant on concentrations of Sc(OTf)3 and HClO4,
respectively. In the case of proton-promoted direct oxygen
transfer, only the monoprotonated complex ([(N4Py)-
FeIV(OH)]3+) is involved and the diprotonated complex
([(N4Py)FeIV(OH2)]4+) is less reactive due to the steric effect
of two protons, whereas, in the PCET reaction, the
diprotonated complex ([(N4Py)FeIV(OH2)]4+) is the most
reactive species because of the much higher reduction potential
as indicated by eq 5. The proton-promoted direct oxygen transfer
from thioanisoles to [(N4Py)FeIV(OH)]3+ may be regarded as
inner-sphere electron transfer,47−49 in which electron transfer is
accompanied by the oxygen−ligand transfer.50
AUTHOR INFORMATION
Corresponding Author
fukuzumi@chem.eng.osaka-u.ac.jp; wwnam@ewha.ac.kr
■
Notes
The authors declare no competing financial interest.
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.), and 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.).
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