A mononuclear non-heme high-spin iron(III)-hydroperoxo complex as an active oxidant in sulfoxidation reactions

Article abstract of DOI:10.1021/ja404152q

We report the first direct experimental evidence showing that a high-spin iron(III)-hydroperoxo complex bearing an N-methylated cyclam ligand can oxidize thioanisoles. DFT calculations showed that the reaction pathway involves heterolytic O-O bond cleavag

Full text of DOI:10.1021/ja404152q

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Communication
A Mononuclear Nonheme High–Spin Iron(III)–Hydroperoxo
Complex as an Active Oxidant in Sulfoxidation Reaction
Yun Mi Kim, Kyung-Bin Cho, Jaeheung Cho, Binju Wang, Chunsen Li, Sason Shaik, and Wonwoo Nam
J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/ja404152q • Publication Date (Web): 30 May 2013
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A Mononuclear Nonheme High–Spin Iron(III)–Hydroperoxo
Complex as an Active Oxidant in Sulfoxidation Reaction
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†
†,§
‡
‡
,‡
Yun Mi Kim, Kyung–Bin Cho, Jaeheung Cho, Binju Wang, Chunsen Li, Sason Shaik,* and Wonwoo
Nam*
,
†
†
Department of Bioinspired Science, Ewha Womans University, Seoul 120–750, Korea.
‡
Institute of Chemistry and the Lise Meitner–Minerva Center for Computational Quantum Chemistry, The Hebrew University of
Jerusalem, 91904 Jerusalem, Israel.
§
Department of Emerging Materials Science, DGIST, Daegu 711–873, Korea.
Supporting Information Placeholder
III
2+
ABSTRACT: We report the first direct experimental evidence
showing that a high–spin iron(III)–hydroperoxo complex bearing
an N–methylated cyclam ligand is capable of conducting the oxida-
tion of thioanisoles. DFT calculations show that the reaction path-
way involves a heterolytic O–O bond cleavage and that the choice
of heterolytic vs. homolytic pathway is spin–state selective and it
depends on the number of electrons in the dxz orbital of the
iron(III)–hydroperoxo species.
[(TMC)Fe (OOH)]
(1, TMC
=
1,4,8,11–tetramethyl–
1,4,8,11–tetraazacyclotetradecane), was synthesized and characte-
rized with various spectroscopic methods. Interestingly, this HS
Fe –OOH complex showed reactivities in nucleophilic and elec-
trophilic reactions, providing direct experimental evidence for
the involvement of a HS Fe –OOH complex in the oxidation of
organic substrates. More recently, reactivities of HS and low–spin
LS) nonheme Fe –OOH complexes were investigated experi-
1
0
III
1
0a
III
III
(
mentally and theoretically in hydrogen atom (H–atom) abstraction
Elucidation of the nature of reactive intermediates in the catalytic
cycles of dioxygen activation and oxygenation reactions by heme
and nonheme iron enzymes has been the subject of intense re-
search in bioinorganic and biological chemistry. High–valent iron–
III
reactions, showing that the HS Fe –OOH complex is an active
11
oxidant in the H–atom abstraction.
As mentioned above, the involvement of Cpd 0 in sulfoxidation by
P450 and its analogues has been debated over two decades. Simi-
6
IV
oxo species, such as Fe O porphyrin –cation radicals in cytoch-
larly, no direct experimental evidence has been obtained for the
IV
romes P450 (P450) and nonheme Fe O intermediates, have been
III
involvement of Fe –OOH species as active oxidants in oxygen
invoked as active oxidants that effect the oxygenation of organic
atom transfer (OAT) reactions by nonheme iron models. We now
1,2
substrates. In biomimetic studies, it has been demonstrated that
III
report that a HS Fe –OOH complex (1) is a competent oxidant in
IV
Fe O complexes are strong oxidants capable of oxygenating vari-
the oxidation of sulfides to give the corresponding sulfoxide prod-
ous organic substrates, such as in hydroxylation and sulfoxidation
III
ucts. In contrast, a LS Fe –OOH complex is shown to be a sluggish
3
reactions.
oxidant in the sulfoxidation reaction. The mechanisms of the sul-
IV
In addition to the Fe O intermediates, iron(III)–hydroperoxo
III
foxidation by 1 and a LS Fe –OOH complex are investigated by
III
species (Fe –OOH) have been proposed as a “second electrophilic
12
density functional theory (DFT) calculations, proposing a hetero-
4,5
oxidant” in oxygenation reactions, particularly in the sulfoxida-
lytic O–O bond breaking transition state in the OAT reaction by 1
6
III
tion of thioether substrates by P450 and its analogues. The Fe –
OOH species have also been proposed as active oxidants in non-
heme iron systems, including bleomycin, Rieske dioxygenases, and
(Figure 1).
Complex 1, which was prepared by adding 10 equiv of HClO
4
to a
III
+
solution of [(TMC)Fe (O
2
)] in acetone/CF
3
CH
2
OH (3:1)
7
synthetic iron catalysts. However, experimental evidence against
o
–3 –1
at -20 C, decayed with the rate of kobs = 3.3 x 10 s (Figure 2a,
III
the Fe –OOH species as a second electrophilic oxidant appeared
8
recently in iron model reactions. In addition, computational stu-
III
dies by one of us have shown that Fe –OOH species are sluggish
oxidants in oxygenation reactions, including in sulfoxidation by
III
Fe –OOH porphyrin species known as Compound 0 (Cpd 0) in
9
P450. Thus, there has been an intriguing, continuing controversy
III
on the involvement of Fe –OOH species as a second electrophilic
oxidant in the reactions of heme and nonheme iron enzymes and
4–9
their model compounds.
Figure 1. DFT–optimized transition state of the sulfoxidation of thioanisole
by 1. Hydrogens, except for O–H, are omitted for clarity; Fe, purple; N,
blue; O, red; S, yellow; C, black; H, white. Fe–O, O–O, and O–S distances
are shown in Å .
III
Recently, a high–spin (HS) Fe –OOH complex bearing a macro-
cyclic N–methylated cyclam as
a
supporting ligand,
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when the thioanisole oxidation was performed with O–labeled 1
(
III 18 18
2+
i.e., [(TMC)Fe ( O OH)] ) (SI, Figure S5). In addition, by
analyzing the reaction solution with electron paramagnetic reson-
ance (EPR) spectroscopy and electrospray ionization mass spec-
trometer (ESI MS), we found that Fe species was formed as a
major decomposed product of 1 (SI, Figures S6 and S7).
II
1
4
III
For comparison, the reactivity of a LS Fe –OOH complex,
(N4Py)Fe (OOH)] (3, N4Py = N,N–bis(2–pyridylmethyl)–
III
2+
[
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1
5
N–bis(2–pyridyl)methylamine), was investigated in the sulfoxi-
dation reaction. Addition of para–X–C SCH (e.g., 4–
methoxythioanisole) to a reaction solution of 3 in CH
did not affect the disappearance rate of 3 much (SI, Figure S8),
6
H
4
3
o
3
OH at 25 C
1
6
III
suggesting that the LS Fe –OOH complex is a sluggish oxidant for
the oxidation of thioanisoles.
The reactivity of 1 was also compared with that of the corres-
ponding Fe O complex 2. Interestingly, 1 was more reactive than 2
by a factor of 10 in the oxidation of 4–methoxythioanisole (e.g., 1.8
x 10 and 1.7 x 10 M s for 1 and 2, respectively) (SI, Figure
S9). However, by considering the proton effect on the reactivity of
nonheme Fe O complex, 2 was more reactive than 1 in the pres-
ence of HClO (10 equiv to 2) (SI, Figure S10). It is worth noting
that, different from the Fe O complexes, there is no significant
proton effect on the reactivity of the Fe –OOH complex in the
IV
–1
–2
–1 –1
IV
17
4
Figure 2. (a) UV–vis spectral changes showing the conversion of
III
+
2+
IV
[
(TMC)Fe (O
line) upon addition of 10 equiv HClO
methoxythioanisole (100 mM) in acetone/CF
2
)] (1 mM, blue line) to [Fe(TMC)(OOH)] (1) (red
and the reaction of 1 with 4–
CH OH (3:1) at –20 °C.
III
4
sulfoxidation reaction (SI, Figure S11).
3
2
Insets show the time courses of the absorbance change of 1 at 526 nm for
DFT Calculations. A deeper understanding of the reaction itself
can be obtained by use of theoretical calculations. As a starting
point to model 1, we have used the published X–ray structure of
] , wherein the TMC methyl substituents are cis
to Fe–OO. Figure 3 shows the energetics for the sulfoxidation
the natural decay (● ) and the reaction of 4–methoxythioanisole (●). (b)
+
Hammett plot of log k
plot of log k
2
against σ
p
of para–X–Ph–SCH
(right panel).
3
(left panel) and
2
against Eox of para–X–Ph–SCH
3
III
+
[(TMC)Fe (O)
2
1
0a
IV
2+
inset), resulting in the formation of [(TMC)Fe (O)] (2) (Sup-
*
+
18
1
0a
reaction of thioanisole by 1 at B3LYP/LACVP //LACVP level
including CPCM solvation (trifluoroethanol) correction with
porting Information (SI), Figure S1). Interestingly, upon addi-
1
9
tion of 4–methoxythioanisole to the solution of 1, this intermediate
2
0
10,11
–2
–1
o
Gaussian 09. In agreement with experiments,
the S = 5/2
disappeared at a faster rate (kobs = 2.1 x 10 s at –20 C) under the
condition of pseudo–first–order kinetics (Figure 2a). The rate
constant increased linearly with the increase of the substrate con-
centration (SI, Figure S2 and Table S1). The reaction rate was
dependent on reaction temperature, from which a linear Eyring
plot was obtained between 233 K and 263 K to give the activation
high–spin state of 1 was found to be the ground state, with an or-
1
1
1
1
1
bital configuration of dxy , dxz , dyz , dx2-y2 , and dz2 on Fe. We define
here our coordinate system as the x–axis along the O–OH bond
and the z–axis along the Fe–O bond, and we disregard the orbital
mixings with the peroxide moiety for simplicity (see also SI, DFT
Section text). The energetically lowest reaction proceeds through a
concerted pathway, where the O–O bond break occurs concomi-
tantly with the bond formation to the substrate S (Figure 1). Mulli-
ken spin density distribution (SI, Table S5) shows that the leaving
OH group has only 0.06 in spin at the transition state (TS), indicat-
ing that the bond break is heterolytic. Further orbital analyses cha-
‡
–1
‡
–1
–1
parameters of H = 39 kJ mol and S = 119 J mol K (SI,
Figure S3). When 1 was reacted with para–substituted thioanisoles,
para–X–C
6
H
4
SCH
3
(X = OCH
3
, CH , F, and H), to investigate the
3
electronic effect of substrates on the sulfoxidation reaction, a ρ
value of –1.1 was obtained in the Hammett plot of the second–
+
order rate constants vs. σ
p
(Figure 2b, left panel; SI, Figure S4 and
III
+
+
racterize the constituent groups at TS as [Fe O] and OH (vide
infra). This reaction is followed by a barrier–less second step where
a proton transfer (PT) occurs involving a return of the OH proton
Table S1). This result indicates the electrophilic character of the
hydroperoxo group of 1 in the sulfoxidation of sulfides, as frequent-
ly observed in sulfoxidation reactions by high–valent metal–oxo
III
2+
1
3
to form [(TMC)Fe OH] . The energy barrier for this reaction
complexes. In addition, we observed a good linear correlation
-1
was found to be 14.5 kcal mol (Figure 3); hence, the reaction is
feasible also from a theoretical point of view in agreement with
with the slope of –2.5 when the rates were plotted against oxidation
potentials (Eox) of para–X–C
6
H
4
SCH
3
(Figure 2b, right panel; SI,
2
1
1
experiments. The intermediate spin–state S = 3/2 (dxy , dxz , dyz , dx2-
Table S1), suggesting that the oxidation of sulfides by 1 occurs via a
1
0
1
3
y2 , dz2 ) was found to follow the same reaction pathway but with a
direct oxygen atom transfer (OAT) mechanism.
-
1
considerably higher transition barrier (25.5 kcal mol ) and is there-
Product analysis of the reaction solution revealed that methyl
phenyl sulfoxide was produced as a sole product with a high yield
fore ruled out. Interestingly, computationally the low–spin S = 1/2
(dxy , dxz , dyz , dx2-y2 , dz2 ) state reaction follows a stepwise pathway
with a distinct intermediate. The spin density distribution on the
leaving OH group corresponds to half a radical at the intermediate
2
2
1
0
0
(
~80% based on the amount of 1 used), and the source of oxygen in
the sulfoxide product was found to be the hydroperoxo ligand of 1
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Figure 3. Calculated energy profile for the sulfoxidation reaction of thioani-
sole by 1. While the S = 1/2 state proceeds through a stepwise, homolytic
mechanism, the S = 3/2 and 5/2 states occur through a concerted hetero-
lytic mechanism.
Figure 4. The most important orbitals involved in a sulfoxidation reaction
by an Fe –OOH species. Blue arrows represent electrons present only in
the high–spin configuration, and red only in the low–spin configuration
III
stage with another half radical on the substrate S (SI, Table S5).
Together with orbital analyses, we interpret these data as showing a
IV
2+
(black is present in both). The fate of the two electrons in the  orbital is
x
homolytic O–O bond cleavage, forming an [Fe O] compound
with a loosely associated HO–S moiety sharing an unpaired elec-
tron. In the second step, the O–S bond is consolidated and a pro-
ton–coupled electron–transfer (PCET) occurs to form
different depending on a homolytic (low–spin) or heterolytic (high–spin)
bond break. This is governed by the number of electrons in the dxz orbital of
*
the iron, which will form xz and  xz orbitals with the p
x
orbital of the prox-
imal O (see text).
III
2+
[
(TMC)Fe OH] .
somewhat simplified view (see SI, DFT Section text), it is found
sufficiently useful to describe the underlying principles governing
the reactions in all the calculations we present in this study, includ-
ing different spin states and ligands (vide infra).
Even though the rate–limiting barrier for the low–spin state is too
-1
high to matter (31.7 kcal mol ), it still poses a fundamental ques-
tion about the origins of this spin–selective reactivity. Therefore, an
in–depth analysis of the orbitals involved in the reaction was done
in order to discern what factors govern the choice of heterolytic vs.
homolytic O–O bond cleavages. Figure 4 describes the sulfoxida-
tion reaction seen from an orbital mixing point of view. We found
that during the O–O bond–breaking reaction, the most important
In order to assess the generality for the above orbital interaction
description, we also calculated the sulfoxidation reaction catalyzed
IV
by 3 (see SI, Tables S3, S6, S9 and Figure S12). In the Fe O spe-
cies with N4Py, the ground state of 3 is in a low–spin S = 1/2 state,
1
1,15
again in concord with experiments.
The rate–limiting barriers
orbitals on the hydroperoxide ligand are the doubly occupied
-1
here are 20.3, 29.3, and 19.6 kcal mol for S = 1/2, 3/2, and 5/2,
respectively. The high and low–spin barriers are on the verge of
what is considered to be possible barriers for the reactions to pro-
ceed, hence predicting a slow or no reaction using this ligand. This
is therefore compatible with our experimental result showing no
reactivity of this compound under the reaction conditions. Still, the
calculations show the same homolytic/heterolytic path selection
depending on the dxz occupation, as described above. Interestingly,
we find for the high–energy S = 3/2 state an example of a homolytic,
concerted reaction, indicating that the homolytic and heterolytic
bond breakings are not always equivalent to stepwise and concerted
mechanisms, respectively. The spin selection rule is kept for a third
tested ligand (TPA, tris(2–pyridylmethyl)amine) (see SI, Tables
S4, S7 and S10).
*
bonding 
x
orbital and the unoccupied anti–bonding 
x
orbital
(Figure 4, top center). During the O–O bond breaking, these two
orbitals split into their constituent p orbitals localized on each of
x
the oxygen. The leaving OH group will then form a bond with the
substrate S (Figure 4, right), where the orbital mixings result in a
*
bonding SO orbital and an anti–bonding  SO orbital. On the other
hand, the p
x
orbital of the proximal O will mix with the dxz orbital of
*
the iron to form the bonding xz and anti–bonding  xz orbitals
(
Figure 4, left). If dxz is doubly occupied, as it is in the low–spin case,
its two electrons plus the one coming from the peroxide  orbital
will create a three–electron xz/ xz system, whereas the remaining
electron from peroxide  will follow the leaving OH group to po-
x
*
x
pulate the *SO orbital (i.e., homolytic O–O bond break). This
electron will then follow the positively charged proton back to the
The above results can be put in further general context. In a recent
IV
2+
[
(TMC)Fe O] moiety in the second step during PCET. On the
11
theoretical work using 1 and 3 for C–H activation reactions with
other hand, if dxz is originally singly occupied, as in the case of in-
xanthene, the former one (in a ground state of S = 5/2) was found
to break the O–O bond heterolytically, with the substrate showing
a spin of 0.33 at TS. This is in striking similarity to our sulfoxidation
reaction described here. For 3 (with a ground state of S = 1/2), the
O–O bond break occurred homolytically. While no electrons were
donated from the substrate in this case, it is anyway not required for
a homolytic O–O bond break, and it is still in accord with our
pathway selection rules.
termediate– and high–spin states, the electron deficiency in the
*

xz/ xz system will favor a heterolytic O–O bond break, where both
electrons originating from the peroxide  orbital will stay with the
x
proximal oxygen. The final step of the reaction is therefore a pure
PT. Hence, we find that the number of electrons in the dxz orbital
governs the choice of heterolytic vs. homolytic O–O bond cleavage
in sulfoxidation reactions. Although the above description is a
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In conclusion, we have provided the first direct experimental evi-
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dence demonstrating that HS Fe –OOH species can perform sul-
III
foxidation reactions, whereas LS Fe –OOH species are less reac-
tive in OAT reactions. Furthermore, we have suggested a homolyt-
ic/heterolytic pathway selection rule based on the dxz orbital occu-
(7) (a) Liu, L. V.; Bell, C. B.; Wong, S. D.; Wilson, S. A.; Kwak, Y.; Chow,
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III
pancy in the sulfoxidation reactions by nonheme Fe –OOH spe-
cies. If the dxz orbital is doubly occupied, the reaction will occur
homolytically, and heterolytically otherwise. This rule applies to all
the nonheme cases we have investigated here regardless of ligands
and spin states.
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ASSOCIATED CONTENT
Eur. J. 2012, 18, 6935.
Supporting Information. Experimental and DFT details, Tables S1 –
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AUTHOR INFORMATION
Corresponding Author
(
*
E–mail: wwnam@ewha.ac.kr, sason@yfaat.ch.huji.ac.il
ACKNOWLEDGMENT
W.N. acknowledges research support of this work by the NRF of Korea
through CRI and GRL (2010–00353). S.S. acknowledges support by
the Israeli Science Foundation. J.C. acknowledges the R & D program
of the MEST of Korea (13–BD–0403).
(
(
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2
2
O
2
(
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