Highly reactive nonheme iron(III) iodosylarene complexes in alkane hydroxylation and sulfoxidation reactions

Article abstract of DOI:10.1002/anie.201402537

High-spin iron(III) iodosylarene complexes bearing an N-methylated cyclam ligand are synthesized and characterized using various spectroscopic methods. The nonheme high-spin iron(III) iodosylarene intermediates are highly reactive oxidants capable of activating strong C-H bonds of alkanes; the reactivity of the iron(III) iodosylarene intermediates is much greater than that of the corresponding iron(IV) oxo complex. The electrophilic character of the iron(III) iodosylarene complexes is demonstrated in sulfoxidation reactions.

Full text of DOI:10.1002/anie.201402537

Angewandte
Chemie
DOI: 10.1002/anie.201402537
Enzyme Models
Highly Reactive Nonheme Iron(III) Iodosylarene Complexes in
Alkane Hydroxylation and Sulfoxidation Reactions**
Seungwoo Hong, Bin Wang, Mi Sook Seo, Yong-Min Lee, Myoung Jin Kim, Hyung Rok Kim,
Takashi Ogura, Ricardo Garcia-Serres, Martin Clꢀmancey, Jean-Marc Latour,* and
Wonwoo Nam*
Abstract: High-spin iron(III) iodosylarene complexes bearing
an N-methylated cyclam ligand are synthesized and charac-
terized using various spectroscopic methods. The nonheme
high-spin iron(III) iodosylarene intermediates are highly
reactive oxidants capable of activating strong CꢀH bonds of
workers reported that iron(III)–OCl porphyrin and manga-
nese(IV) iodosylarene salen complexes are competent oxi-
[
5]
dants in OATreactions. McKenzie and Lennartson reported
a crystal structure of a nonheme iron(III) iodosylbenzene
[
6]
complex and used it in a sulfoxidation reaction. Lei and co-
workers reported spectroscopic evidence for a manganese
alkanes; the reactivity of the iron(III) iodosylarene intermedi-
ates is much greater than that of the corresponding iron(IV)
oxo complex. The electrophilic character of the iron(III)
iodosylarene complexes is demonstrated in sulfoxidation
reactions.
[7]
iodosylarene porphyrin adduct.
Over the past two decades, there has been continuing
interest in oxidation reactions in which multiple oxidants are
[
8,9]
involved.
In most cases, the multiple oxidant hypothesis
has been proposed based on product analysis using mecha-
nistic probes (e.g., radical-clock experiments and competitive
H
igh-valent iron oxo species, such as iron(IV) oxo porphy-
[
9]
rin p-cation radicals and nonheme iron(IV) oxo complexes,
have been invoked as key intermediates in a variety of
biological oxidation reactions, including alkane hydroxylation
epoxidation of cis- vs. trans-alkenes). In contrast to OAT
reactions, the involvement of metal–oxidant adducts has not
been proposed in the hydroxylation of alkanes with strong Cꢀ
[1]
and sulfoxidation. A large number of iron(IV) oxo com-
plexes have been synthesized and investigated in the oxida-
tion reactions as chemical models of heme and nonheme iron
H bonds. To our knowledge, no kinetic studies have been
n+
conducted on the proposed metal–oxidant species (M –OX)
in oxidation reactions. Herein, we report the synthesis and
characterization of high-spin iron(III) iodosylarene com-
plexes bearing an N-methylated cyclam ligand (proposed
structure given in Scheme 1). The coordination of iodosylar-
ene to the iron(III) center is evidenced spectroscopically and
[
2]
enzymes. It has been demonstrated that high-valent iron oxo
species are indeed strong oxidants capable of activating CꢀH
bonds of unactivated hydrocarbons.
Very recently, nonheme iron(III) superoxo and high-spin
iron(III) hydroperoxo complexes have been proposed or
demonstrated as active oxidants in CꢀH bond activation
reactions, although the intermediates are able to activate only
[
3,4]
weak CꢀH bonds in hydrocarbons.
Other metal–oxidant
n+
adducts (M –OX; for example, X = IAr, OH, OR, halides)
have also been synthesized and characterized spectroscopi-
cally and/or structurally, and their reactivities have been
investigated in oxygen-atom transfer (OAT) reactions, such as
epoxidation and sulfoxidation. For example, Fujii and co-
Scheme 1. Proposed structure of high-spin iron(III) iodosylarene com-
plexes bearing an N-methylated cyclam ligand and reactions investi-
1
8
gated in this work. Filled red circle= O.
[
+]
[+]
[
*] Dr. S. Hong, Dr. B. Wang, Dr. M. S. Seo, Dr. Y.-M. Lee, M. J. Kim,
Dr. R. Garcia-Serres, Dr. M. Clꢀmancey, Prof. Dr. J.-M. Latour
CEA, DSV, iRTSV, LCBM, PMB
Prof. Dr. W. Nam
Department of Chemistry and Nano Science
Ewha Womans University
Seoul 120–750 (Korea)
38054 Grenoble (France)
E-mail: jean-marc.latour@cea.fr
+
[
] These authors contributed equally to this work.
E-mail: wwnam@ewha.ac.kr
[
**] We gratefully acknowledge research support of this work by the NRF
Prof. Dr. H. R. Kim
Research Center for Green Catalysis, Korea Research Institute of
Chemical Technology, Daejeon 305-600 (Korea)
of Korea through CRI (NRF-2012R1A3A2048842 to W.N.), GRL
(NRF-2010-00353 to W.N.), and Basic Research Program (2010-
0002558 to M.S.S.). J.M.L. acknowledges the support of Labex
Prof. Dr. T. Ogura
Picobiology Institute
Graduate School of Life Science, University of Hyogo
Koto 3-2-1, Kamigori-cho, Ako-gun, Hyogo 678-1297 (Japan)
ARCANE (ANR-11-LABX-0003-01). T.O. acknowledges support by
Grant-in-Aid for Scientific Research on Innovative Areas (No.
25109540) from MEXT (Japan).
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201402537.
Dr. R. Garcia-Serres
University of Grenoble Alpes, LCBM, 38054 Grenoble (France)
Dr. M. Clꢀmancey
CNRS UMR 5249, LCBM, 38054 Grenoble (France)
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1
8
by conducting experiments with O-labeled water. The
iron(III) iodosylarene intermediates are highly reactive
oxidants capable of activating strong CꢀH bonds (e.g.
The UV/Vis absorption spectra of 2 and 3 exhibit an
absorption band at l = 660 nm (Figure 1a for 2; Figure S3 for
3). The CSI-TOF MS of 2 exhibits a prominent ion peak at
m/z 766.1 (Figure 1b; Figure S4 for full mass spectrum of 2),
cyclohexane; Scheme 1). The electrophilic character of the
iron(III) iodosylarene adducts is also demonstrated in sulfox-
idation reactions (Scheme 1).
The reactions of [Fe (13-TMC)(CF SO ) ] (1, 13-TMC =
,4,7,10-tetramethyl-1,4,7,10-tetraazacyclotridecane)
whose mass and isotope distribution patterns correspond to
+
[Fe(OIPh)(13-TMC)(CF CH O)(CF SO )]
(calculated
3
2
3
3
II
18
18
m/z 766.1). When 2 was prepared with O-labeled PhI O,
a mass shift from m/z 766.1 to 768.1 was observed (Figure 1b,
inset), indicating that 2 contains an oxygen atom. The CSI-
TOF MS of 3 was also taken and analyzed similarly (Fig-
ure S5). The rRaman spectra of 2 and 3, obtained upon l =
441.6 nm excitation in CF CH OH at ꢀ408C, show isotopi-
3
3 2
[10]
1
with
iodosylarenes, such as iodosylbenzene (PhIO) or pentafluor-
oiodosylbenzene (F PhIO), in a solvent mixture of acetone
5
and CF CH OH (3:1) at ꢀ408C resulted in the formation of
3
2
blue intermediates. These intermediates, denoted 2 (from the
3
2
ꢀ1
reaction with PhIO) and 3 (from the reaction with F PhIO),
cally sensitive bands at 783 and 772 cm , respectively (Fig-
5
respectively, have maximum UV/Vis absorption bands at l =
ure 1c for 2; Figure S6 for 3). The peaks were shifted to 751
and 740 cm when the intermediates 2 and 3 were prepared
ꢀ
1
6
60 nm (Supporting Information, Figure S1). The intermedi-
1
8
18
ates were highly unstable and decayed to an iron(IV) oxo
with PhI O and F PhI O, respectively. The observed shift
5
18
IV
2+
16
ꢀ1
complex, [Fe (O)(13-TMC)] (4), with a half-life (t ) of
from the O to O samples of D = 32 cm is in good
agreement with the calculated value of 39 cm expected for
a diatomic O–I oscillator. The rRaman spectrum of the
decayed product, [Fe (O)(13-TMC)] (4), shows one iso-
1
/2
ꢀ1
approximately 20 s (Figures S1 and S2). Interestingly, when
the same reactions were carried out in the presence of
[11]
IV
2+
1
.2 equivalents of HClO , the stability of the intermediates
4
ꢀ1 [10]
increased markedly (t1/2 ꢁ 300 s; Figure 1a for 2; also see
Figure S3a for 3). Thus, we were able to characterize
intermediates 2 and 3 with various spectroscopic methods,
including coldspray ionization time-of-flight mass spectrom-
etry (CSI-TOF MS), UV/Vis absorption, electron paramag-
netic resonance (EPR), Mçssbauer, and resonance Raman
topically sensitive band at 833 cm .
The X-band EPR spectra of 2 and 3, recorded in a frozen
acetone/CF CH OH (3:1) solution at 5 K, show signals that
3
2
III
[12]
are characteristic of S = 5/2 Fe (Figure S7). The Mçss-
bauer spectrum of 2, recorded at 4.2 K (Figure S8), in the
absence of a strong applied magnetic field displays a doublet
ꢀ1
(
rRaman) spectroscopies.
typical of iron(IV) oxo species (d = 0.12 mms , DE =
Q
ꢀ
1 [2e,f,10b]
1
.98 mms )
and
two doublets corre-
sponding to Fe(III)
centers
(d =
ꢀ
1
0
.43 mms ,
DE =
Q
ꢀ
1
ꢀ
1.02 mms and d =
.36 mms ,
ꢀ
1
0
DE =
Q
ꢀ
1
ꢀ
0.60 mms ).
spectrum also displays
diffuse absorption
The
spreading over a large
velocity domain. To
characterize this ab-
sorption, spectra were
recorded in variable
applied
magnetic
fields (Figure 1d). A
spin Hamiltonian fit
allowed simulation of
this absorption as an
III
S = 5/2 Fe
plex
com-
[
10b]
accounting for
approximately 30%
Figure 1. a) UV/Vis absorption spectral changes of 2 formed from the reaction of [Fe(13-TMC)(CF SO ) ] (1.0 mm)
3
3 2
of total iron, with d =
with PhIO (3.0 equiv) in the presence of HClO (1.2 equiv) in acetone/CF CH OH (3:1) at ꢀ408C. Compound 2,
4
3
2
ꢀ
1
0
.37 mms and DE =
Q
with a maximum absorption band at l=660 nm, was generated within 20 s. Inset: change in absorption intensity,
monitoring at l=660 nm, with respect to time, for the natural decay of 2. b) CSI-TOF MS spectrum of 2 in the
presence of HClO (1.2 equiv) in acetone/CF CH OH (3:1) at ꢀ508C. Black plots: CSI-TOF MS of 2 prepared using
ꢀ
1
ꢀ0.59 mms
(Table
4
3
2
S1
for
complete
1
6
18
III 16
3+
PhI O. Red plot: CSI-TOF MS of 2 prepared using PhI O. c) rRaman spectra of [Fe ( OIPh)(13-TMC)] (4.0 mm,
parameters), that was
assigned to 2. The Fe-
III 18
3+
black line) and [Fe ( OIPh)(13-TMC)] (4.0 mm, red line) upon l=441.6 nm excitation in CF CH OH at ꢀ408C.
3
2
The peaks marked with an asterisk are ascribed to solvent. d) Mçssbauer spectrum of 2 recorded at 4.2 K in a 7 T
(III) centers giving rise
parallel applied magnetic field (vertical bars). The solid black line is the theoretical spectrum generated by adding
IV
to the doublets were
confirmed to have an
individual spin-Hamiltonian simulations (colored lines). Components are assigned to 2 (orange), an Fe product
III
converted from 2 (blue), and an Fe dinuclear complex (pink).
2
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Chemie
S = 0 ground state and were assigned to a m-oxo dimer. Based
on the spectroscopic characterization of 2 and 3, it can be
concluded that high-spin iron(III) iodosylarene complexes,
III
3+
III
assigned as [Fe (OIPh)(13-TMC)]
for
2 and [Fe -
3
+
(
OIPhF )(13-TMC)] for 3, were produced in the reactions
5
of 1 with iodosylarenes.
The coordination of iodosylarene in 2 and 3 was further
1
8
confirmed by carrying out an isotopically O-labeled water
experiment. This experiment was based on the previous
report that metal–iodosylarene adducts exchange the oxygen
n+
18
[5c,13]
atom (e.g. M -O-IAr) with labeled water, H2 O.
Com-
1
6
pounds 2 and 3 were firstly prepared with PhI O and
F PhI O, respectively, and then a small amount of H
1
6
18
O
5
2
was added to these reaction solutions. Although the UV/Vis
spectra of 2 and 3 remained intact, the CSI-TOF MS of the
resulting solutions showed that approximately 70% of the
1
8
oxygen atoms in 2 and 3 exchanged with H2 O within 30 s
Figure S9 and S10). This result provides the first direct
(
evidence that iron(III) iodosylarene complexes exchange
1
8
[14]
their oxygen atom with H2 O at a fast rate [see Eq. (1)].
III 16
3þ
18
½
ð13-TMCÞFe - O-IArꢂ þ H2 O Ð
III 18
ð1Þ
3þ
16
½
ð13-TMCÞFe - O-IArꢂ þ H2
O
The reactivities of the iron(III)–iodosylarene adducts, 2
and 3, were investigated in the CꢀH bond activation of
hydrocarbons at ꢀ408C. Upon addition of cumene to
a solution of 2, the intermediate was converted into a new
species (5) which gives rise to an isosbestic point at l =
5
37 nm (Figure 2a). First-order rate constants, determined
by the pseudo-first-order fitting of the kinetic data for the
decay of 2 (Figure 2a, inset), increased linearly with increas-
ing cumene concentration, giving rise to a second-order rate
Figure 2. a) UV/Vis absorption spectral changes observed in the
reaction of 2 (1.0 mm) with cumene (100 equiv) in the presence of
HClO (1.2 equiv) in acetone/CF CH
OH (3:1) at ꢀ408C. Inset:
change in absorption intensity of 5, monitoring at l=660 nm, with
4
3
2
ꢀ1
ꢀ1 ꢀ1
constant of 2.1 ꢀ 10 m
s
at ꢀ408C. A kinetic isotope effect
respect to time. b) Plots of pseudo-first-order rate constants (kobs
)
(
(
1
KIE) of 9.1(5) was obtained for the oxidation of cumene by 2
against the concentration of cumene (black circles) or
Figure 2b). In the case of 3, a second-order rate constant of
[
D ]cumene (red circles) to determine second-order rate constants
12
ꢀ
1
ꢀ1 ꢀ1
.4 ꢀ 10 m
s
with a KIE of 13(2) was determined for the
(
k ) in the oxidation reaction of cumene or [D ]cumene by 2. Reaction
2
1
2
oxidation of cumene at ꢀ408C (Figure S11). This result
carried out in the presence of HClO (1.2 equiv) in acetone/CF CH OH
4
3
2
suggests that the reactivity of 2 is slightly greater than that of
(3:1) at ꢀ408C. c) Plot of logk
2
’ against the CꢀH BDE of substrates
for the reactions of 2. Second-order rate constants, k , were deter-
3. We also determined second-order rate constants for the
2
mined and then adjusted for reaction stoichiometry to yield k ’ based
2
oxidation of other substrates by 2 (Table S2 and Figure S12),
showing a linear correlation between the reaction rates and
the CꢀH bond dissociation energy (BDE) of substrates
on the number of target CꢀH bonds within substrates (see data in
Table S2 and Figure S12).
(
Figure 2c). Under identical reaction conditions, 4 did not
react with cumene, indicating that the reactivity of the
iron(III)–iodosylarene adducts is much greater than that of
the corresponding iron(IV) oxo species. On the basis of the
large KIE value and the good correlation between reaction
rates and BDEs of substrates, we conclude that the CꢀH bond
2-phenylpropan-2-ol product (Figure S13). The high ratio of
alcohol to ketone products obtained and the incorporation of
O into the products demonstrates that the products are
derived from the metal-mediated oxidation reaction, not from
an autooxidation reaction. We also analyzed 5, a decomposi-
tion product of 2 in the oxidation of cumene, using ESI-MS
and EPR and assigned 5 as an iron(III) species (Experimental
Section and Figures S14 and S15).
1
8
activation of substrates by the iron(III)–iodosylarene adducts
is the rate-determining step.
Analysis of the products obtained from the reaction
mixtures revealed the formation of alcohols as a major
product in the oxidation of alkanes by 2 and 3 (Table S3). For
example, the oxidation of cumene by 2 under an inert
atmosphere yielded 2-phenylpropan-2-ol (20%), a-methyl-
styrene (21%), and acetophenone (3%). When the cumene
The reactivities of 2 and 3 were also investigated in
sulfoxidation reactions. Upon addition of 10 equivalents of
thioanisole, 2 and 3 disappeared within 0.2 s with a first-order
decay profile (Figure 3a for 2; Figure S16 for 3). The pseudo-
first-order rate constants increased proportionally with the
concentration of thioanisole to afford second-order rate
1
8
oxidation was performed with approximately 70% of O-
1
8
18
3
3
ꢀ1 ꢀ1
labeled 2 (2- O), approximately 67% of O was found in the
constants of k = 3.5 ꢀ 10 and 2.7 ꢀ 10 m
s
at ꢀ608C for 2
2
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F PhI. In iron porphyrin systems, an equilibrium exists
5
between an iron(IV) oxo porphyrin p-cation radical complex
and an iron(III) iodosylarene porphyrin complex in the
presence of iodoarene and it is the iron(IV) center which is
[
17]
the reactive species in olefin epoxidation reactions. If there
is an equilibrium between an iron(III)–iodosylarene adduct
and an iron(V) oxo species, which is formed by OꢀI bond
cleavage of the iron(III)–iodosylarene adduct, the presence of
an excess amount of iodoarene should decrease the reaction
rate (see Scheme 2). Compound 3 was employed in the
Scheme 2. See text for details.
cumene hydroxylation and thioanisole oxidation reactions in
the presence of F PhI (50 equiv). It was found that the
5
reaction rates were the same as those obtained in the
Figure 3. a) UV/Vis absorption spectral changes observed in the
reaction of 2 (0.50 mm) with thioanisole (10 equiv) in the presence of
HClO (1.2 equiv) in acetone/CF CH OH (3:1) at ꢀ608C. Inset:
reactions carried out in the absence of F PhI (Figure S20).
5
These results demonstrate unambiguously that the iron(III)–
iodosylarene adduct (Scheme 2, pathway a), not the iron(V)
oxo complex (Scheme 2, pathway b), is the reactive species
for the hydroxylation and sulfoxidation reactions.
4
3
2
change in absorption intensity, monitoring at l=660 nm, with respect
to time. b) Plot of logk against the s values (left) and E (right) of
2
p
ox
para-substituted thioanisoles for the reactions of 2.
In summary, we have reported the synthesis, character-
ization, and reactivity of nonheme iron(III) iodosylarene
complexes. The intermediates are highly reactive in both
alkane hydroxylation and sulfoxidation reactions. Based on
the results of the mechanistic studies and product analysis, we
were able to conclude that the iron(III)–iodosylarene adducts
are the reactive species responsible for the oxidation reac-
tions. These results provide an example of metal–oxidant
adducts which can transfer their oxygen atom to organic
and 3, respectively (Figure S17). The reactivity of 2 is slightly
greater than that of 3, as observed in the alkane hydroxylation
reactions. To investigate the electronic effect of para-sub-
stituents on the oxidation of thioanisoles by iron(III)–
iodosylarene adducts, 2 was treated with para-substituted
thioanisoles, para-X-PhSCH (X = Me, H, F, Cl, and CN). A
3
Hammett plot of the second-order rate constants versus s of
p
[8]
substrates gave a 1 value of ꢀ1.9 (Figure 3b, left; Table S4).
Such a negative 1 value illustrates the electrophilic character
of the iron(III)–iodosylarene adducts in OAT reactions, as
frequently observed in the sulfoxidation of thioanisoles by
high-valent metal oxo, metal superoxo, and metal hydro-
substrates prior to the conversion into metal oxo species.
Received: February 18, 2014
Published online: && &&, &&&&
[15,16]
Keywords: alkane hydroxylation · bioinorganic chemistry ·
peroxo complexes of heme and nonheme ligands.
In
.
enzyme models · nonheme iron complexes · sulfoxidation
addition, we observed a good linear correlation when the
rates were plotted against oxidation potentials (E ) of
ox
thioanisoles (Figure 3b, right). The negative slope of ꢀ3.7
indicates that the oxidation of sulfides by iron(III)–iodosylar-
ene adducts proceeds by a direct oxygen-atom transfer
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933.
1
8
2
2
, on the basis of an O-labeling experiment performed with
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8
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Figure S18). In contrast to the alkane hydroxylation reaction,
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2
an iron(II) complex (Figure S19).
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[
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Angew. Chem. Int. Ed. 2014, 53, 1 – 6
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Angewandte
Chemie
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[
1
6
18
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IV
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[
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[5b]
7
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[
[
[
[13] W. Nam, J. S. Valentine, J. Am. Chem. Soc. 1993, 115, 1772.
[14] We cannot exclude the possibility that the oxygen exchange
1
8
between iron(III) iodosylarene complexes and H2 O may occur
after the iodosylarene in the iron(III) iodosylarene complexes
1
8
18
dissociates first and exchanges with H2 O. Then, the O-labeled
1
8
iodosylarenes are rebound to an iron(III) species to give the O-
labeled iron(III) iodosylarene complexes.
654; c) M. Newcomb, P. F. Hollenberg, M. J. Coon, Arch.
Biochem. Biophys. 2003, 409, 72.
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[
6641; c) J. P. Collman, A. S. Chien, T. A. Eberspacher, J. I.
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B. S. Mandimutsira, R. Todd, B. Ramdhanie, J. P. Fox, D. P.
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2004, 43, 5228; f) W. J. Song, M. S. Seo, S. D. George, T. Ohta, R.
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Angew. Chem. Int. Ed. 2014, 53, 1 – 6
ꢀ 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.angewandte.org
5
These are not the final page numbers!

.
Angewandte
Communications
Communications
Enzyme Models
S. Hong, B. Wang, M. S. Seo, Y.-M. Lee,
M. J. Kim, H. R. Kim, T. Ogura,
R. Garcia-Serres, M. Clꢁmancey,
J.-M. Latour,* W. Nam*
&&&& — &&&&
An iron boost: High-spin iron(III) iodo-
sylarene complexes bearing an N-methy-
lated cyclam ligand are prepared. The
nonheme high-spin iron(III) iodosylarene
intermediates are highly reactive oxidants
capable of activating strong CꢀH bonds
of alkanes. The electrophilic character of
the iron(III) iodosylarene complexes is
demonstrated in sulfoxidation reactions.
Highly Reactive Nonheme Iron(III)
Iodosylarene Complexes in Alkane
Hydroxylation and Sulfoxidation
Reactions
6
www.angewandte.org
ꢀ 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2014, 53, 1 – 6
These are not the final page numbers!