Fine Control of the Redox Reactivity of a Nonheme Iron(III)–Peroxo Complex by Binding Redox-Inactive Metal Ions

Article abstract of DOI:10.1002/anie.201610828

Redox-inactive metal ions are one of the most important co-factors involved in dioxygen activation and formation reactions by metalloenzymes. In this study, we have shown that the logarithm of the rate constants of electron-transfer and C?H bond activation reactions by nonheme iron(III)–peroxo complexes binding redox-inactive metal ions, [(TMC)Fe^III(O₂)]⁺-Mⁿ⁺(Mⁿ⁺=Sc³⁺, Y³⁺, Lu³⁺, and La³⁺), increases linearly with the increase of the Lewis acidity of the redox-inactive metal ions (ΔE), which is determined from the g_zzvalues of EPR spectra of O₂^.?-Mⁿ⁺complexes. In contrast, the logarithm of the rate constants of the [(TMC)Fe^III(O₂)]⁺-Mⁿ⁺complexes in nucleophilic reactions with aldehydes decreases linearly as the ΔE value increases. Thus, the Lewis acidity of the redox-inactive metal ions bound to the mononuclear nonheme iron(III)–peroxo complex modulates the reactivity of the [(TMC)Fe^III(O₂)]⁺-Mⁿ⁺complexes in electron-transfer, electrophilic, and nucleophilic reactions.

Full text of DOI:10.1002/anie.201610828

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Chemie
International Edition: DOI: 10.1002/anie.201610828
German Edition: DOI: 10.1002/ange.201610828
Redox Reactivity
Fine Control of the Redox Reactivity of a Nonheme Iron(III)–Peroxo
Complex by Binding Redox-Inactive Metal Ions
Seong Hee Bae, Yong-Min Lee, Shunichi Fukuzumi,* and Wonwoo Nam*
(for example, Fe O2, Ni O2, and Cu O2);[10,11] metal–(su)per-
oxo complexes can be either electrophiles, which accept two
electrons, or nucleophiles, which donate two electrons,[12–16]
and binding of redox-inactive metal ions by the metal–
(su)peroxo complexes may affect the electrophilic and
nucleophilic reactivities of the (su)peroxo ligand in oxidation
reactions. However, effects of the redox-inactive metal ions
on the redox reactivity of the metal–(su)peroxo complexes in
electron-transfer and oxidation reactions have yet to be
scrutinized quantitatively.
À
À
À
Abstract: Redox-inactive metal ions are one of the most
important co-factors involved in dioxygen activation and
formation reactions by metalloenzymes. In this study, we
have shown that the logarithm of the rate constants of electron-
À
transfer and C H bond activation reactions by nonheme
iron(III)–peroxo complexes binding redox-inactive metal ions,
[(TMC)FeIII(O2)]+-Mn+ (Mn+ = Sc3+, Y3+, Lu3+, and La3+),
increases linearly with the increase of the Lewis acidity of the
redox-inactive metal ions (DE), which is determined from the
gzz values of EPR spectra of O2CÀ-Mn+ complexes. In contrast,
the logarithm of the rate constants of the [(TMC)FeIII(O2)]+-
Mn+ complexes in nucleophilic reactions with aldehydes
decreases linearly as the DE value increases. Thus, the Lewis
acidity of the redox-inactive metal ions bound to the mono-
nuclear nonheme iron(III)–peroxo complex modulates the
reactivity of the [(TMC)FeIII(O2)]+-Mn+ complexes in electron-
transfer, electrophilic, and nucleophilic reactions.
We report herein that the redox reactivity of a mononu-
clear nonheme iron(III)–peroxo complex bearing a macro-
cyclic N-tetramethylated cyclam ligand, [(TMC)FeIII(O2)]+ (1,
TMC = 1,4,8,11-tetramethyl-1,4,8,11-tetraazacyclotetrade-
cane),[17] in electron-transfer, electrophilic, and nucleophilic
reactions changes upon binding of redox-inactive metal ions
(Mn+) at the iron–peroxo moiety (Scheme 1). The change in
R
edox-inactive metal ions that function as Lewis acids are
known to play pivotal roles in a variety of oxidation reactions
by oxygen-containing metal intermediates, such as high-
valent metal–oxo species, in biological and biomimetic
systems.[1–3] In particular, a redox-inactive Ca2+ ion is an
essential component in the oxidation of water to evolve
dioxygen in the oxygen-evolving complex (OEC, a heteronu-
clear Mn4CaO5 cubane complex) of Photosystem II
(PS II),[4–6] although the role of the Ca2+ ion in the O O
bond formation reaction has yet to be clarified. In biomimetic
studies, it has been shown that reactivities of high-valent
metal–oxo complexes in electron-transfer, oxygen atom
À
Scheme 1. Molecular structure of [(TMC)FeIII(O2)]+-Mn+ (1-Mn+
;
Mn+ =Sc3+, Y3+, Lu3+, and La3+) and summary of the reactivities of 1-
Mn+ in electron-transfer, electrophilic, and nucleophilic reactions.
À
transfer, and C H bond activation reactions are enhanced
markedly upon binding of redox-inactive metal ions, resulting
from the positive shift of one-electron reduction potentials of
the metal–oxo complexes.[7–9] Redox-inactive metal ions are
also reported to bind to various metal–(su)peroxo complexes
the redox reactivity of the iron(III)-peroxo complex (1) by
binding of redox-inactive metal ions is then quantitatively
evaluated as a function of the Lewis acidity of metal ions
(DE), which was determined from the gzz values of EPR
spectra of O2CÀ-Mn+ complexes.[18] To the best of our knowl-
edge, the present study reports the first example showing the
effects of the Lewis acidity of redox-inactive metal ions bound
to a peroxo group in a mononuclear nonheme iron(III)–
peroxo complex on the reactivity of the iron–peroxo com-
plexes in electron-transfer and electrophilic and nucleophilic
reactions.
[*] S. H. Bae, Dr. Y.-M. Lee, Prof. Dr. S. Fukuzumi, Prof. Dr. W. Nam
Department of Chemistry and Nano Science
Ewha Womans University, Seoul 03760 (Korea)
E-mail: wwnam@ewha.ac.kr
Prof. Dr. S. Fukuzumi
Faculty of Science and Engineering
SENTAN (Japan) Science and Technology Agency (JST)
Meijo University, Nagoya, Aichi 468-8502 (Japan)
E-mail: fukuzumi@chem.eng.osaka-u.ac.jp
The iron(III)–peroxo complex, [(TMC)FeIII(O2)]+ (1),
was synthesized by reacting [FeII(TMC)(CF3SO3)2] with
5 equiv H2O2 in the presence of 2 equiv triethylamine in
acetonitrile (MeCN) at À408C and isolated as crystals for
further studies.[17] Redox-inactive metal ion-bound iron(III)-
Prof. Dr. W. Nam
State Key Laboratory for Oxo Synthesis and Selective Oxidation
Lanzhou Institute of Chemical Physics, Chinese Academy of Sciences
Lanzhou 730000 (China)
Supporting information for this article can be found under:
http://dx.doi.org/10.1002/anie.201610828.
peroxo complexes, [(TMC)FeIII(O2)]+-Mn+ (1-Mn+; Mn+
=
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Sc3+, Y3+, Lu3+, and La3+), were generated by adding metal
triflates (Mn+(CF3SO3)n) to the solution of 1 in MeCN at 08C
according to previously reported methods (see Figure 1 for
UV/Vis spectra of 1-Mn+).[19] We then investigated the
electron-transfer (ET) reactions of 1-Mn+ complexes with
ferrocene (Fc) derivatives. Upon addition of bromoferrocene
Scheme 2. Proposed mechanism of ET from BrFc to 1-Mn+
.
(BrFc, Eox = 0.54 V vs. SCE) to the solution of 1-Mn+ (Mn+
=
Sc3+, Y3+, Lu3+, and La3+) in MeCN at 08C, the absorption
band of 1-Mn+ decreased with the concurrent appearance of
absorption bands at 820 nm owing to an iron(IV)–oxo
complex, [(TMC)FeIV(O)]2+,[20] and at 675 nm owing to the
bromoferrocenium ion (BrFc+) (Figure 2). Thus, ET from
ferrocene (Fc) derivatives to 1-Mn+ afforded the formation of
a putative [(TMC)FeII(O2)]-Mn+ species (Scheme 2, reac-
À
tion a), followed by a heterolytic O O bond cleavage of the
peroxide ligand in [(TMC)FeII(O2)]-Mn+ to form [(TMC)FeIV-
(O)]2+ (Scheme 2, reaction b), as reported previously in ET
reactions between 1-Mn+ and Fc.[19,21] The rate of ET was
determined by following the decrease in the absorption band
due to 1-Mn+. The second-order rate constant of ET from
BrFc to 1-Sc3+ was then determined to be 6.4(4) ꢀ 103 mÀ1 sÀ1 in
MeCN at 08C under second-order kinetic conditions owing to
a fast reaction between 1-Sc3+ and BrFc (Supporting Infor-
mation, Figure S1a). In the cases of other 1-Mn+ (Mn+ = Y3+,
Lu3+, and La3+) complexes, the rates obeyed first-order
kinetics under the pseudo first-order reaction conditions
([BrFc]/[1-Mn+
]
> 10). The first-order rate constants
increased linearly with the increase in the BrFc concentration,
and second-order rate constants (k2) were determined at 08C
(Supporting Information, Figure S1b and Table S1). The
logk2 values increased linearly with the Lewis acidity of the
metal ions (DE), which was determined from the gzz values of
EPR spectra of O2CÀ-Mn+ complexes, as shown in Figure 3.[18]
The DE is the energy splitting value of the pg levels due to the
binding of Mn+ to a superoxo species.[18] Thus, the DE value
has been regarded as the binding energy of Mn+ to O2CÀ,[18]
which may be comparable to the binding energy of Mn+ to the
peroxo moiety of [(TMC)FeIII(O2)]+. The stronger the Lewis
acidity of Mn+ is, the larger the splitting value is and the larger
the ket value becomes. The slope (18.4) of the linear plot with
Figure 1. UV/Vis spectra of 1 and 1-Mn+ (Mn+ =Sc3+, Y3+, Lu3+, and
La3+) in MeCN at 08C.
Figure 2. Stopped-flow absorption spectral changes observed in ET
from BrFc (0.50 mm) to 1-Sc3+ (0.50 mm, red line) in MeCN at 08C.
The reaction was completed within 10 s. Inset shows UV/Vis spectra of
1-Sc3+ (0.50 mm, red line) and the final reaction solution (black line);
the absorption bands at 675 and 820 nm correspond to BrFc+ and
[(TMC)FeIV(O)]2+, respectively.
Figure 3. Plots of logk2 values of ET from BrFc to 1-Mn+ (blue circles),
À
C H bond activation reaction of CHD (black circles), and aldehyde
deformylation reactions of 2-PPA (red circles) and CCA (green circles)
by 1-Mn+ in MeCN at 08C against the quantitative measure of the
Lewis acidity of metal ions (DE).
2
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blue line in Figure 3 indicates that about 100% of the change
the change in the spitting energy of the pg levels is reflected as
À
in the spitting energy of the pg levels is reflected as the change
in the transition state energy of ET from BrFc to 1-Mn+ at
08C, because F/(2.3RT) at 08C = 18.5. Virtually the same
slopes were also obtained in ET from Fc (Eox = 0.37 V vs.
SCE) and dibromoferrocene (Br2Fc; Eox = 0.71 V vs. SCE) to
1-Mn+ complexes (Supporting Information, Figure S2 and
Table S1).
The reactivity of iron(III)–peroxo complexes binding
different redox-inactive metal ions, 1-Mn+, was investigated
in hydrogen atom transfer (HAT) reactions. While 1 was not
the change in the transition state energy of the C H bond
activation of CHD by 1-Mn+ at 08C. This percent is smaller
than the case of electron-transfer from BrFc to 1-Mn+ (ca.
100%, see above) because of a partial charge transfer in the
electrophilic reaction instead of a full electron transfer. The k2
values of C H bond activation by 1-Sc with alkylaromatic
3+
À
À
compounds bearing weak C H bonds, such as xanthene
(75.5 kcalmolÀ1) and 9,10-dihydroanthracene (DAH, 77 kcal
molÀ1), were determined as well (Supporting Information,
Figure S5). The logk2 values increase linearly with decreasing
À
À
able to activate the C H bonds of 1,4-cyclohexadiene (CHD),
the C H bond dissociation energies (BDEs) of the substrates.
1-Mn+ showed a reactivity with CHD. Upon addition of CHD
to a solution of 1-Sc3+ in MeCN at 08C, the UV/Vis
absorption band of 1-Sc3+ disappeared with a pseudo first-
order decay profile (Figure 4a), and product analysis of the
reaction solution revealed the formation of benzene (92 Æ
6%) as a product. Furthermore, [(TMC)FeIII(OH)]+ was
formed as a decay product of 1-Sc3+ in the reaction solution
(Supporting Information, Figure S3 for EPR and CSI-MS
spectra). We have shown recently that a high-spin nonheme
iron(III)–hydroperoxo complex, [(TMC)FeIII(O2H)]2+, is
The effect of the Lewis acidity of metal ions (DE) on the
reactivity of 1-Mn+ was also investigated in aldehyde defor-
mylation reactions. The nucleophilic character of 1-Mn+ was
demonstrated by the reactions with 2-phenylpropionaldehyde
(2-PPA) in MeCN at 08C. Addition of 2-PPA to a solution of
1-Sc3+ caused the decay of the 1-Sc3+ intermediate with the
concomitant formation of the corresponding iron(IV)–oxo
complex, [(TMC)FeIV(O)]2+, as detected by UV/Vis and CSI-
MS (Figure 4c; Supporting Information, Figure S6); we have
shown the formation of [(TMC)FeIV(O)]2+ in the reaction of
[(TMC)FeIII(O2H)]2+ and aldehydes.[17a] The pseudo first-
order rate constant increased proportionally with the increase
of the concentration of 2-PPA, giving k2 value of 2.1 ꢀ
10À2 mÀ1 sÀ1 (Figure 4d). The product analysis of the reaction
solutions of 1-Sc3+ with 2-PPA revealed the formation of
acetophenone (90 Æ 5% based on the intermediate). The
second-order rate constants (k2) of the nucleophilic reactions
of other 1-Mn+ (Mn+ = Y3+, Lu3+, and La3+) with 2-PPA were
also determined (Supporting Information, Figure S7 and
Table S1).
À
capable of activating C H bonds of hydrocarbons with
[17]
À
weak C H bonds.
The first-order rate constant was
proportional to the substrate concentration, from which
a second-order rate constant (k2) was determined (Figure 4b).
The second-order rate constants (k2) of C H bond activation
À
of 1,4-cyclohexadiene by other 1-Mn+ (Mn+ = Y3+, Lu3+, and
La3+) complexes were also determined (Supporting Informa-
tion, Figure S4 and Table S1). The logk2 values increase
linearly with increasing the Lewis acidity of metal ions (DE;
Figure 3, black line), and the slope (8.9) indicates that 48% of
In contrast to the cases of electrophilic reactions of 1-Mn+
,
the logk2 values decrease linearly with increasing the DE
values (Figure 3, red and green lines). The slope (À5.7) of the
linear plot with the red line in Figure 3 indicates that 31% of
the change in the spitting energy of the pg levels is reflected as
the change in the transition state energy of the nucleophilic
reaction of 2-PPA with 1-Mn+ at 08C. It should be noted that
the sign of the slopes of the nucleophilic reactions is negative,
which is different from the positive slopes determined in
electron-transfer and electrophilic reactions (Figure 3), since
the direction of the charge transfer in the transition state is
opposite. The slope (À5.2) of the linear plot with the green
line in Figure 3 for the nucleophilic reaction of CCA with 1-
Mn+ is slightly less negative as compared to the nucleophilic
reaction of 2-PPA with 1-Mn+, indicating that the degree of
the charge transfer is less in this reaction. The nucleophilic
character of 1-Sc3+ was also demonstrated by the reactions
with para-substituted benzaldehydes bearing a series of
electron-donating and -withdrawing substituents at the para-
position of the phenyl group (para-Y-C6H4CHO; Y= OMe,
Me, H, and F); a positive 1 value of 2.0 was obtained in the
Hammett plot (Figure 5; Supporting Information, Table S2).
In conclusion, we have shown that redox-inactive metal
ion-bound iron(III)–peroxo complexes, [(TMC)FeIII(O2)]+-
Mn+ (1-Mn+; Mn+ = Sc3+, Y3+, Lu3+, and La3+), act as both
electrophiles and nucleophiles as well as electron acceptors.
The electron-transfer and electrophilic reactivities of the 1-
Figure 4. a) UV/Vis spectral changes observed in the electrophilic
oxidation of CHD (200 mm) by 1-Sc3+ (0.50 mm) in MeCN at 08C.
Inset: time course monitored at 530 nm due to 1-Sc3+. b) Plot of the
pseudo first-order rate constant (kobs) versus the concentration of CHD
to determine the second-order rate constant. c) UV/Vis spectral
changes observed in the nucleophilic oxidation of 2-PPA (200 mm) by
1-Sc3+ (0.50 mm) in MeCN at 08C. Inset: time course monitored at
530 nm due to 1-Sc3+. d) Plot of the pseudo first-order rate constant
(kobs) versus the concentration of 2-PPA to determine the second-order
rate constant.
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Acknowledgements
The authors acknowledge financial support from the NRF of
Korea through CRI (NRF-2012R1A3A2048842 to W.N.),
GRL (NRF-2010-00353 to W.N.), and from a Grant-in-Aid
(no. 16H02268 to S.F.) from the Ministry of Education,
Culture, Sports, Science and Technology (MEXT) and
a SENTAN project (to S.F.) from the Japan Science and
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Keywords: iron–peroxo intermediates · Lewis acidity ·
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Manuscript received: November 6, 2016
Final Article published: && &&, &&&&
Angew. Chem. Int. Ed. 2016, 55, 1 – 6
ꢀ 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.angewandte.org
5
These are not the final page numbers!

Angewandte
Communications
Chemie
Communications
Redox Reactivity
Inactive but influential: The Lewis acidity
of the redox-inactive metal ions bound to
a mononuclear nonheme iron(III)–peroxo
S. H. Bae, Y.-M. Lee, S. Fukuzumi,*
W. Nam*
&&&& — &&&&
complex, [(TMC)FeIII(O2)]+-Mn+ (1-Mn+
;
Mn+ =Sc3+, Y3+, Lu3+, and La3+), deter-
mines the reactivities of 1-Mn+ in elec-
tron-transfer, electrophilic, and nucleo-
philic reactions.
Fine Control of the Redox Reactivity of
a Nonheme Iron(III)–Peroxo Complex by
Binding Redox-Inactive Metal Ions
6
www.angewandte.org
ꢀ 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2016, 55, 1 – 6
These are not the final page numbers!