Remarkable Acid Catalysis in Proton-Coupled Electron-Transfer Reactions of a Chromium(III)-Superoxo Complex

Article abstract of DOI:10.1021/jacs.8b02303

Much enhanced acid catalysis was observed in oxygen atom transfer (OAT) reactions by a mononuclear nonheme Cr(III)-superoxo complex, [(Cl)(TMC)Cr^III(O₂)]⁺ (1), in the presence of triflic acid. In the acid-catalyzed reactions, the reactivity of 1 in OAT of thioanisole was enhanced significantly, showing more than 10⁴-fold acceleration in rate. Electron transfer (ET) from electron donors to 1 also occurred only in the presence of HOTf. The enhanced reactivity of 1 by HOTf was explained by proton-coupled electron transfer from electron donors, such as ferrocene, to 1 in light of the Marcus theory of ET. The present study reports for the first time the dramatic proton effect on the chemical properties of metal-superoxo species.

Full text of DOI:10.1021/jacs.8b02303

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Communication
Remarkable Acid Catalysis in Proton-Coupled Electron-
Transfer Reactions of a Chromium(III)-Superoxo Complex
Tarali Devi, Yong-Min Lee, Wonwoo Nam, and Shunichi Fukuzumi
J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.8b02303 • Publication Date (Web): 27 Jun 2018
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Remarkable Acid Catalysis in Proton-Coupled Electron-Transfer Reactions of a
Chromium(III)-Superoxo Complex
†
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,†,‡
,†,§
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Tarali Devi, Yong-Min Lee, Wonwoo Nam,* and Shunichi Fukuzumi*
†
Department of Chemistry and Nano Science, Ewha Womans University, Seoul 03760, Korea
School of Chemistry and Chemical Engineering, Shaanxi Normal University, Xi’an 710119, P. R. China
Faculty of Science and Engineering, Meijo University, Nagoya, Aichi 468-8502, Japan
‡
§
Supporting Information Placeholder
ABSTRACT: Much enhanced acid catalysis was observed in oxygen
atom transfer (OAT) reactions by a mononuclear nonheme Cr(III)-
1 occurs only with HOTf and that the PCET reaction of 1 was much
accelerated by protons. The mechanism of the acid-catalyzed sulfox-
idation of thioanisoles by 1 is clarified by comparison with PCET
from electron donors to 1. To the best of our knowledge, the present
study reports the first example showing a much enhanced acid catal-
ysis on the reactivity of metal-superoxo species in OAT and ET re-
actions via PCET.
III
+
superoxo complex, [(Cl)(TMC)Cr (O )] (1), in the presence of
2
triflic acid. In the acid-catalyzed reactions, the reactivity of 1 in OAT
4
of thioanisole was enhanced significantly, showing more than 10 -
fold acceleration in rate. Electron transfer (ET) from electron do-
nors to 1 also occurred only in the presence of HOTf. The enhanced
reactivity of 1 by HOTf was explained by proton-coupled electron
transfer (PCET) from electron donors, such as ferrocene, to 1 in
light of the Marcus theory of ET. The present study reports for the
first time the dramatic proton effect on the chemical properties of
metal-superoxo species.
The Cr(III)-superoxo complex (1) was prepared and character-
4a
ized as reported previousy. 1 was reported to react with thioanisole
slowly in acetonitrile (MeCN) at 263 K with the second-order rate
–3
–1 –1 4b
constant (kox) of 5.2  10 M s . However, the sulfoxidation of
thioanisole by 1 has hardly occurred at a lower temperature (e.g.,
2
33 K) (Figure 1a). Interestingly, upon addition of one equiv of
HOTf to an MeCN solution containing 1 and thioanisole at 233 K,
the absorption bands at 550 and 675 nm due to 1 disappeared, ac-
Mononuclear nonheme metal-superoxo species have been in-
voked as key intermediates in various biological reactions, such as in
nonheme iron (e.g., isopenicillin N synthase, myo-inositol oxygen-
ase, and cysteine dioxygenase) and copper (e.g., dopamine β-
monooxygenase and peptidylglycine-α-amidating monooxygenase)
companied by the formation of
a Cr(IV)-oxo complex,
IV
+
4b
[(Cl)(TMC)Cr (O)] (2), with the absorption bands at 605 nm
and 960 nm (Figure 1b). The decay of 1 obeyed the first-order ki-
netics, and the pseudo-first-order rate constant increased linearly
with increasing thioanisole concentration to give the second-order
1
enzymes. Many metal-superoxo complexes have been successfully
–1 –1
synthesized and characterized structurally and/or spectroscopically
in biomimetic studies, and their reactivities have been explored in
rate constant (kox) of 3.5(3) M s at 233 K (Figure S1). Although
the oxidation of thioanisole by 1 occurred extremely slowly without
HOTf at 233 K (vide supra), the kox value was determined to be 3.6
2
-5
OAT and hydrogen atom transfer (HAT) reactions. In addition,
metal-superoxo complexes have been proposed as key intermediates
in superoxide reduction by biomimetic compounds of superoxide
–
4
–1 –1
× 10 M s at 233 K, using large concentrations of thioanisole
4
(Figure S2). The kox value of 1 toward thioanisole was 10 -fold larger
6
dismutases. However, chemical properties of the metal-superoxo
with HOTf at 233 K. Product analysis of the reaction solution re-
vealed formation of PhS(O)Me quantitatively (SI, Experimental
species have been less clearly understood and still remain elusive in
many aspects.
Acids play important roles in PCET reactions in biological redox
reactions, such as the four-electron reduction of dioxygen in respira-
7
tion and the four-electron oxidation of water in Photosystem II. The
reactivity of high-valent metal-oxo complexes in OAT, HAT, and ET
reactions is also markedly influenced by addition of external pro-
8
,9
tons. However, such a proton effect has never been explored in
metal-superoxo species. As our ongoing efforts in elucidating the
chemical properties of metal-superoxo species and the proton effect
on the reactivity of metal-oxygen intermediates, we have investi-
gated the proton effect in oxidation reactions by metal-superoxo spe-
cies. In this communication, we report a remarkable acid catalysis in
the sulfoxidation of thioanisoles by a mononuclear nonheme
Figure 1. UV-visible spectral changes observed in the sulfoxidation of thioan-
isole (10 mM) by 1 (1.0 mM) in the (a) absence and (b) presence of HOTf
(1.0 mM) in MeCN at 233 K. The insets show the time profile at 550 nm to
monitor the decay of 1.
III
+
Cr(III)-superoxo complex, [(Cl)(TMC)Cr (O
1,4,8,11-tetramethyl-1,4,8,11-tetraazacyclotetradecane),
CF SO
2
)] (1, TMC =
4
with
3
3
H (HOTf). We also report that ET from electron donors to
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Journal of the American Chemical Society
Page 2 of 5
+
Section). In addition, the oxygen source in PhS(O)Me and 2 was
H ions in 3, agreeing with 2  (2.3RT/F) at 233 K = 93
1
8
1
2
3
4
5
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1
1
1
1
1
1
1
1
1
1
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confirmed to be 1 by performing O–labeled experiments with
mV/log([HOTf]) (eq 5). The Ered value of 1 is positively shifted
from –0.52 V vs SCE without HOTf to 1.12 V vs SCE with HOTf
(2.5 mM), because the ET reduction of 1 occurs at the ligand center
III 18
+
18
[(Cl)(TMC)Cr ( O )] , in which O was found in the
2
1
8
IV 18
+
PhS( O)Me and [(Cl)(TMC)Cr ( O)] products (Figures S3
and S4) (eq 1). It should be noted that HOTf acts as an efficient cat-
1
1
to produce 3 in which two protons are bound.
The diprotonation in 3 in the presence of HOTf is also supported
by the kinetic measurements (vide infra). The PCET rate constants
II
2+
from [Fe (bpy)
crease in absorbance at 520 nm due to [Fe (bpy)
3
] to 1 were determined by monitoring the de-
II
2+
3
] (Table S4 and
Figure S10). The second-order rate constants (ket) increased with
2
0
1
2
3
4
5
6
7
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2
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increasing [HOTf], being proportional to [HOTf] (Table S5 and
alyst without being consumed in eq 1. It should be also noted that
the absorption spectrum of 1 remained the same upon addition of
HOTf (Figure S5), indicating that no protonation on the superoxo
Figure S11), as observed in the sulfoxidation of para-MeO-thioan-
isole by 1 with HOTf (eq 2; Figure 2).
10
moiety in 1 occurs by HOTf.
Rate constants of ET from ferrocene derivatives to 1 were also de-
termined (Table S4 and Figures S12 and S13). From the spectral ti-
tration experiments, the ET stoichiometry was established as given
by eq 3, where ET from Fc to 1 occurs with the 1:1 stoichiometry
and two protons are required for the PCET reaction (Figure S14).
As shown in Figure 2 (see also Table S1), the kox value increased
2
with increasing [HOTf], being proportional to [HOTf] (eq 2).
2
k
ox = k
2
[HOTf]
(2)
III
The kox values of para-substituted thioanisoles (X = MeO, Me, H,Cl,
and Br) were determined as listed in Table S2 (see also Figures S6
and S7).
The formation of Cr -species was confirmed with cold-spray ioniza-
tion mass spectrometer and EPR (Figure S15). The yield of 3 was
determined to be 86(5)% by the iodometric titration (Figure S16).
The much enhancement of the reactivity of 1 was also observed in
ET from electron donors to 1 with HOTf. No ET from
The deuterium kinetic isotope effect (KIE) was also examined by
comparing the k value of ET from 1,1′-dibromoferrocene to 1 with
et
II
2+
[
Fe (bpy)
3
] (Eox = 1.06 V vs SCE) to 1 occurred without HOTf, as
HOTf (2.5 mM) and that with DOTf (2.5 mM) (Figure S17). The
KIE value observed for the PCET reaction was 0.85. Such an inverse
KIE in the PCET reaction with HOTf vs DOTf was also reported for
PCET from toluene to an iron(IV)-oxo complex with HOTf vs
DOTf, showing sharp contrast to the large KIE (31) in concerted
1
1
expected from Ered of 1 (–0.52 V vs SCE). In the presence of HOTf,
II
2+
however, PCET from [Fe (bpy)
3
]
to 1 occurred to produce
III
3+
III
2+
[
Fe (bpy)
3
]
and [(Cl)(TMC)Cr (H
2
O )] (3), which is in
2
1
2
equilibrium (eq 3; Figure S8), as indicated by the redox titrations
PCET from toluene and toluene-d to an iron(IV)-oxo complex
8
1
3
without acid.
The rate constants (ket) of ET from the one-electron donors to 1
are also evaluated in light of the Marcus theory of adiabatic ET as
given by eq 6,
II
2+
in Figure 3, where [[Fe (bpy)
[HOTf] (Figure S9). The ET equilibrium constants (Ket) in eq 3
were determined by global fitting of plots in Figure 3, where Ket
3
] ] decreased with increasing
2
=
k
et = Zexp[–(λ/4)(1 + ∆G/λ) /k
B
T ]
(6)
0
+ 2
1
1
–1 –1
K
et [H ] . The Ered values of 1 at various concentrations of HOTf
where Z is the frequency factor that is taken as 10
M s , (=
(Figure 3, inset) were also determined from the Ket values (Table S3)
k
B
TK/h; k is the Boltzmann constant, T is the absolute temperature,
B
II
2+
8
and the Eox value of [Fe (bpy)
3
] using the Nernst equation (eq 4).
K is the formation constant of the precursor complex, and h is the
Ered = Eox + (2.3RT/F)logKet
(4)
Planck constant), Get is the ET free energy change, and λ is the ET
1
4
+
reorganization energy. The driving force dependence of ket is
shown in Figure 4, where log ket values of ET from electron donors
to 1 with HOTf (2.5 mM) at 233 K are plotted against the driving
force (–∆Get) of PCET. The –∆Get values were determined from Eox
of electron donors and Ered of 1 with HOTf, as given by eq 7,
The dependence of Ered of 1 on [H ] is given by the Nernst equa-
8
tion (eq 5; see SI for the derivation of eq 5).
0
+ 2
E
red = E red + (2.3RT/F)log(1 + Kred[H ] )
(5)
where Kred is the equilibrium constant of the diprotonation in 3. A
plot of Ered vs log([HOTf]) is shown in Figure 3 (inset), exhibiting a
linear correlation with a slope of 93 mV/log([HOTf]), which indi-
cates that the ET reduction of 1 is accompanied by binding of two
Figure 3. Spectroscopic redox titrations at 520 nm for the decrease in
II
2+
[[Fe (bpy)
MeCN solution of [Fe (bpy)
cles, 2.5 mM; red circles, 3.0 mM) at 233 K. Inset shows the dependence of
Ered of 1 on log([HOTf]).
3
] ] as a function of the initial concentration of 1 added to an
II
2+
3
] and HOTf (blue circles, 1.5 mM; black cir-
Figure 2. Dependence of kox on [HOTf] for the sulfoxidation of para-MeO-
thioanisole (0.50 mM) by 1 (0.50 mM) with HOTf (0–3.0 mM) at 233 K.
2
Inset shows plot of kox vs [HOTf] .
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Scheme 1. Proposed Mechanism of the Acid-Catalyzed Sulfoxida-
–∆Get = e(Ered – Eox)
1
2
3
4
5
6
7
8
9
tion of p-Methoxythioanisole by 1
where e is the elementary charge. The –∆Get dependence of ket of
PCET from ferrocene derivatives to 1 with HOTf (2.5 mM) is well
fitted by using the Marcus equation (eq 6) with the best fit λ value of
II
2+
2
.32 eV, whereas the ket values of PCET from [Fe L
3
] to 1 are fitted
using the somewhat smaller λ value of 1.93 eV due to the smaller re-
II
2+
organization energy of the electron-self exchange between [Fe L]
III
3+
+ 15
and [Fe L] , as compared with that between Fc and Fc .
The plot of log kox for the acid-catalyzed sulfoxidation of thioan-
isole derivatives by 1 with HOTf (2.5 mM) at 233 K vs –Get is also
fitted by using eq 6 with the significantly smaller value of 1.30 eV
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
(
red line in Figure 4). The much larger kox values than those expected
from outer-sphere ET may result from the much larger K values of
the precursor complexes, because the stronger interaction of 1 is ex-
II
2+
pected with organic substrates as compared with that of [Fe L
3
]
14
(
vide infra).
The dependence of k of the oxidation of thioanisole (S) by 1 with
f
In conclusion, a remarkable acid catalysis was observed in the sul-
foxidation of thioanisole by a Cr(III)-superoxo complex (1) with
HOTf, which proceeds through the rate-determining PCET from
thioanisoles to 1. The driving force dependence of ket of PCET from
electron donors to 1 with HOTf was well evaluated in light of the
Marcus theory of ET, providing valuable insights into the acid catal-
ysis in the oxidation of substrates by metal-superoxo species through
PCET.
HOTf (1.0 and 2.5 mM) on [S] at 233 K exhibits a saturation behav-
ior (Figure S18), as expressed by eq 8, resulting from the formation
of the precursor complex (K) prior to ET (kET). The kET and K values
–
are obtained from the intercept and the slope of the linear plot of k
vs [S] (eq 8).
f
1
–1
–1
–1
–1
–1
k
f
=(kETK) [S] + kET
(8)
–
1
The average K value in Figure S18 is determined to be 76(4) M ,
–1
which is much larger than the value (0.021 M at 233 K) normally
used for outer-sphere ET. The difference in the log K values between
ASSOCIATED CONTENT
II
2+
thioanisole and [Fe L ] is 3.5, which agrees with the observed dif-
3
Supporting Information.
ference (3.5) between the log kox value of thioanisole (no. 11, red line)
and the log ket value (blue line) in Figure 4. Such an agreement indi-
cates that the acid-catalyzed sulfoxidation of thioanisole by 1 pro-
ceeds via rate-determining PCET from thioanisole to 1 through the
Experimental details, Tables S1 – S6, and Figures S1 – S18. This ma-
terial is available free of charge via the Internet at
http://pubs.acs.org.
III
+
+
16
precursor complex with [(Cl)(TMC)Cr (O
2
)] -(H ) , followed
2
AUTHOR INFORMATION
by the O–O bond cleavage by thioanisole radical cation to produce
IV
•–
the sulfoxide and the Cr (O) complex via the O transfer and re-
leasing two protons (Scheme 1). The ET reactivity of 1 binding two
protons is much enhanced in the rate-determining step. In addition,
no protons are consumed or produced in the overall reaction.
Corresponding Author
fukuzumi@chem.eng.osaka-u.ac.jp
wwnam@ewha.ac.kr
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENT
This work was supported by the NRF of Korea through CRI (NRF-
2
(
012R1A3A2048842 to W.N) and Basic Science Research Program
2017R1D1A1B03029982 to Y.M.L. and 2017R1D1A1B03032615
to S.F.) and the Grants-in-Aid (no. 16H02268 to S.F.) from MEXT.
(
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Figure 4. The –∆Get dependence of log ket or log kox of PCET from fer-
rocene derivatives [(1) 1,1′-dimethylferrocene, (2) ferrocene, (3) bro-
II
2+
(2) (a) Itoh, S. Curr. Opin. Chem. Biol. 2006, 10, 115. (b) Yao, S.; Driess, M.
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II
2+
II
2+
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[
Fe (Me
2
bpy)
3
2+
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phen)
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(9) (a) Pegis, M. L.; Wise, C. F.; Martin, D. J.; Mayer, J. M. Chem. Rev. 2018,
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(10) The protonation of [(Cl)(TMC)Cr (O)] was not observed, either.
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0
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0
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0
(
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(12) The spectroscopic titrations in Figure 3 fit with eq 3, where H
2
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III
III
to the Cr complex. If H O is bound to the Cr complex, the addition of
2
2
(5) Pestovsky, O.; Bakac, A. Dalton Trans. 2005, 3, 556.
2 2
H O would result in no change in the equilibrium (eq 3) as confirmed in
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III
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(b) Warren, J. J.; Tronic, T. A.; Mayer, J. M. Chem. Rev. 2010, 110, 6961.
(16) If one-step OAT from thioanisoles to [(Cl)(TMC)Cr (O )] -(H ) is the
2
2
(
c) Weinberg, D. R.; Gagliardi, C. J.; Hull, J. F.; Murphy, C. F.; Kent, C. A.;
rate-determining step, the kox values would be much larger than those ex-
pected from the Marcus line in Figure 4, because any interaction required
for one-step OAT would be much larger than that for ET.
Westlake, B. C.; Paul, A.; Ess, D. H.; McCafferty, D. G.; Meyer, T. J. Chem.
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