Trapping of a Highly Reactive Oxoiron(IV) Complex in the Catalytic Epoxidation of Olefins by Hydrogen Peroxide

Article abstract of DOI:10.1002/anie.201812758

The generation of a nonheme oxoiron(IV) intermediate, [(cyclam)Fe^IV(O)(CH₃CN)]²⁺ (2; cyclam=1,4,8,11-tetraazacyclotetradecane), is reported in the reactions of [(cyclam)Fe^II]²⁺ with aqueous hydrogen peroxide (H₂O₂) or a soluble iodosylbenzene (sPhIO) as a rare example of an oxoiron(IV) species that shows a preference for epoxidation over allylic oxidation in the oxidation of cyclohexene. Complex 2 is kinetically and catalytically competent to perform the epoxidation of olefins with high stereo- and regioselectivity. More importantly, 2 is likely to be the reactive intermediate involved in the catalytic epoxidation of olefins by [(cyclam)Fe^II]²⁺ and H₂O₂. In spite of the predominance of the oxoiron(IV) cores in biology, the present study is a rare example of high-yield isolation and spectroscopic characterization of a catalytically relevant oxoiron(IV) intermediate in chemical oxidation reactions.

Full text of DOI:10.1002/anie.201812758

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Accepted Article
Title: Trapping of a Highly Reactive Oxoiron(IV) Complex in the
Catalytic Epoxidation of Olefins by Hydrogen Peroxide
Authors: Xenia Engelmann, Deesha D. Malik, Teresa Corona, Katrin
Warm, Erik R. Farquhar, Marcel Swart, Wonwoo Nam, and
Kallol Ray
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To be cited as: Angew. Chem. Int. Ed. 10.1002/anie.201812758
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10.1002/anie.201812758
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Trapping of a Highly Reactive Oxoiron(IV) Complex in the Catalytic
Epoxidation of Olefins by Hydrogen Peroxide
Xenia Engelmann,[a] Deesha D. Malik,[b] Teresa Corona,[a] Katrin Warm,[a] Erik R. Farquhar,[c] Marcel
Swart,[d] Wonwoo Nam,*[b] Kallol Ray*[a]
Abstract: Oxoiron(IV) species are the active oxidants in catalytic
oxidation reactions by nonheme iron enzymes, but have been
rarely captured and/or identified as the actual oxidizing species in
catalytic epoxidation or hydroxylation reactions involving synthetic
nonheme iron complexes. We now report the generation of a
nonheme oxoiron(IV) intermediate, [(cyclam)FeIV(O)(CH3CN)]2+
(2; cyclam = 1,4,8,11-tetraazacyclotetradecane), in the reactions
of [(cyclam)FeII]2+ with aqueous hydrogen peroxide (H2O2) or a
soluble iodosylbenzene (sPhIO) as a rare example of an
oxoiron(IV) species that shows a preference for epoxidation over
allylic oxidation in the oxidation of cyclohexene. Complex 2 is
kinetically and catalytically competent to perform the epoxidation
of olefins with high stereo- and regioselectivity. More importantly,
2 is likely to be the reactive intermediate involved in the catalytic
epoxidation of olefins by [(cyclam)FeII]2+ and H2O2. In spite of the
predominance of the oxoiron(IV) cores in biology, the present
study represents a rare example of high-yield isolation and
our understanding of the mechanistic role of oxoiron(IV)
intermediates in nonheme iron enzymes and synthetic
accessibility of similar iron(IV) intermediates in model studies
have prompted investigators to view high-valent oxoiron(IV)
species as key intermediates responsible for oxygen-atom
transfer to organic substrates in catalytic oxidation reactions.
However, the reactions exhibited by the model oxoiron(IV)
complexes are non-catalytic, with activities falling far short of the
activity of the biological catalysts.[1,2] Furthermore, synthetic
oxoiron(IV) compounds are known to prefer allylic oxidation over
epoxidation of cyclohexene[4] and are often not kinetically
competent to perform the rapid oxidation observed in iron-
catalyzed epoxidation and hydroxylation reactions.[1,2] These
results have previously allowed the exclusion of oxoiron(IV)
species as the actual oxidizing species in chemical catalysis. In
contrast, spectroscopically detected oxoiron(V) species in
catalytic systems based on synthetic iron complexes bearing
tetradentate N4-donor ligands with cis-binding sites, H2O2, and
carboxylic acid or water as an additive are demonstrated to be
highly efficient in the selective oxidations of C-H and C=C groups
of various organic substrates.[2f,2h,2i,2j,2k,5] Thus, although the
involvement of oxoiron(V) species in oxidation reactions is well-
established, the intermediacy of its one-electron reduced form
(i.e., oxoiron(IV) species) in catalytic oxidations by nonheme iron
spectroscopic characterization of
a
catalytically relevant
oxoiron(IV) intermediate in chemical oxidation reactions.
High-valent oxoiron species are nature’s tool for catalyzing a
startling array of chemical transformations, including alkane
hydroxylation, olefin epoxidation, cis-dihydroxylation, and
oxidative aromatic ring cleavage.[1-3] Oxoiron(IV) species have
been identified as the reactive intermediate responsible for
various oxidation reactions in several mono- and dinuclear non-
heme iron oxygenases.[3] Parallel studies with synthetic
oxoiron(IV) models of these enzymes have shown that they
exhibit intriguing oxidative reactivities, which in turn have provided
vital insight into the modelled enzymatic reactions.[1] Advances in
complexes remains elusive.
CH
3
CN
2+
2+
H
H
O
O
N
N
N
N
Fe
N
N
Fe
N
H
N
NCCH3
H
[(TMC)FeIV(O)(CH3
CN)]2+
2
[a]
Prof. Dr. K. Ray, M. Sc. X. Engelmann, Dr. T. Corona, M. Sc. K.
Warm
Scheme 1. Oxoiron(IV) complexes that are considered in this study
Department of Chemistry
Humboldt-Universität zu Berlin
Brook-Taylor-Straße 2, 12489 Berlin (Germany)
E-mail: kallol.ray@chemie.hu-berlin.de
Prof. Dr. W. Nam, M. Sc. D. D. Malik
Department of Chemistry and Nano Science
Ewha Womans University
The first clean epoxidation of olefins by H2O2 catalyzed by a
nonheme iron complex, [(cyclam)FeII]2+ (1; cyclam = 1,4,8,11-
tetraazacyclotetradecane), was reported by Valentine and co-
workers[5a] over two decades ago. In the study, olefins were
converted to the corresponding epoxides with high product yields
[b]
Seoul 120–750, Korea
E-mail: wwnam@ewha.ac.kr
and stereospecificity.
A hydroperoxoiron(III) species was
[c]
[d]
Dr. E. R. Farquhar
Case Center for Synchrotron Biosciences
NSLS-II, Brookhaven National Laboratory
Upton, NY 11973, USA
proposed as the plausible reactive intermediate without any
spectroscopic evidence supporting the catalytic ability of this
species for epoxidation.[3b,5] However, no further systematic
investigation on this highly intriguing catalytic olefin epoxidation
system has been explored. Herein, we report the trapping and
spectroscopic characterization of an oxoiron(IV) intermediate,
[(cyclam)FeIV(O)(CH3CN)]2+ (2; Scheme 1), that is kinetically and
catalytically competent to perform the epoxidation of olefins at
fast reaction rates and with high stereo- and regioselectivity. This
Prof. M. Swart
ICREA, Pg. Lluis Companys 23, 08010 Barcelona, Spain
IQCC, Universitat de Girona, Campus Montilivi, 17003,
Girona, Spain
Supporting information for this article is given via a link at the end of
the document.
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10.1002/anie.201812758
Angewandte Chemie International Edition
COMMUNICATION
is in contrast to the reactivity of the corresponding
demonstrating that the iron center remains in the intermediate-
spin configuration (S = 1).[9] The minor doublet (15%; δ = 0.26
mm/s and ΔEQ = 0.83 mm/s) presumably corresponds to an
iron(III) product formed in the reaction of 1-(OTf)2 with traces of
oxygen.
[(TMC)FeIV(O)(CH3CN)]2+ (TMC
= 1,4,8,11-tetraaza-1,4,8,11-
tetramethylcyclotetradecane) complex (scheme 1),[6] where all the
amino functions of the cyclam ligand are methylated, which is
reported to be a sluggish oxidant in oxygen-atom transfer and C-
H bond oxidation reactions. In fact, 2 represents a rare example
of an oxoiron(IV) species that shows a preference for epoxidation
over allylic oxidation and is likely to be the reactive intermediate
involved in the nonheme iron cyclam-catalyzed epoxidation of
olefins by H2O2.
Treatment of 1-(OTf)2 with a slow addition of 50 equiv H2O2
over a period of 1 min in the presence of excess cyclohexene in
acetonitrile (CH3CN) at 25 °C under an inert atmosphere resulted
in the formation of cyclohexene oxide (40% based on H2O2) with
a turnover number of 20 (Table 1), consistent with the previous
report.[5a] Notably, only small amounts of allylic oxidation products
(2-cyclohexen-1-ol and 2-cyclohexen-1-one) were formed (Table
1), suggesting that typical radical reactions are not involved.
Similarly, no dihydroxylation products were observed, which may
exclude oxoiron(V) species as the actual oxidizing species in this
reaction; the oxoiron(V) species have been frequently proposed
as intermediates giving dihydroxylation products in the oxidation
of olefins by nonheme iron catalysts and H2O2.[2f,2h,2i,2j,2k,5] When
cis-stilbene was used as the substrate, the major product was cis-
stilbene oxide (33% based on H2O2),[5a] indicating that the
epoxidation reaction is stereoselective. Replacement of H2O2 with
iodosylbenzene (PhIO) as an oxidant also gave high yields of
epoxide products (Table 1). To gain insight into the nature of the
oxidant, we monitored the reaction of 1-(OTf)2 with 30 equiv of
Combination of the tetradentate cyclam ligand with
–
Fe(OTf)2(CH3CN)2 (OTf = CF3SO3 ) yielded the iron(II) cyclam
complex, [(cyclam)FeII(OTf)2] (1-(OTf)2).[5a] The X-ray structure of
1-(OTf)2 displays a 6-coordinate geometry with an average Fe-N
distance of 2.148(5) Å and two cis-positions occupied by triflate
ligands (Scheme 2; Tables S1 – S2). The macrocyclic ring
exhibits a cis-V configuration,[7] where two out of the four -NH
hydrogens are oriented cis to the two cis triflates.[8]
TfO
OTf
R
R
H
H
2+
N
Fe
O
N
H
O
H
N
H
N
Fe
N
R
R
N
N
H
N
H
1
-(OTf)
2
H
CH3CN
2+
CH3CN
H2O2 or
a
soluble iodosylbenzene derivative (2‐(tert‐
R
R
CH3CN
butylsulfonyl)iodosylbenzene; sPhIO)[10] and 10 equiv of
cyclohexene at −40 °C in CH3CN in a cryo-stopped-flow
experiment using a diode-array UV-vis detector (Figure S2). The
growth and decay of a metastable species 2 with visible
absorption features at 737 nm was observed in both cases. These
may point to the plausible involvement of a common reaction
intermediate 2 in the 1-(OTf)2-mediated epoxidation reactions by
PhIO or H2O2.
H
O
CH
CH
2+
3
H
CN
3
3
CN
O
2+
H
N H
N
H
H
O
H
NCCH
H
H2O2
H
N
H
N
Fe
N
Fe
N
Fe
N
H2O
N
N
H
N
N
1
-CH3CN
N
H
H
H
2
Scheme 2. Role of hydrogen bonding interactions of the cyclam –NH hydrogens
in the formation of 2 and its subsequent reaction with olefins. The inset shows
the molecular structure of 1-(OTf)2 (Fe, orange; C, grey; H, white; N, blue; O,
red; S, yellow); crystal data and selected bond distances and angles are
provided in Tables S1 and S2.
Intermediate 2 could be generated in high yields in the
absence of any substrates that allowed its characterization by
various spectroscopic methods. First, the reaction of 1-(OTf)2 with
1.5 equiv of sPhIO in anhydrous CH3CN at −40 °C led to the
immediate formation of a brownish intermediate 2 (t1/2 ~200 s at
−20ꢀ°C) with λmax (εmax) centered at 737 nm (230 M–1 cm–1) (Figure
1A). The electrospray ionization mass spectrum (ESI-MS) of 2
exhibited a signal at m/z = 421.08, which shifted by two-mass
units when sPhI18O was used as an oxidant, possessing a mass
Table 1: Comparison of the oxidation products of cyclohexene under
catalytic (0.02 mmol 1-(OTf)2; 1 mmol H2O2 or PhIO in CH3CN at 25 oC) and
stoichiometric conditions (1 mmol 2 in CH3CN at –20 oC).
OH
O
[ox]
+
+
O
and isotope distribution pattern consistent
[(cyclam)FeIV(O)(OTf)]+ formulation (Figure S3). However, based
on NMR and X-ray absorption spectroscopy (XAS),
with
a
a
c
b
a
Yield2
[%]
[(cyclam)FeIV(O)(CH3CN)]2+ assignment has been concluded for
2 (Figure 1B and Figure S4C). For example, 19F NMR spectrum
of [(cyclam‐d4)FeIV(O)(CH3CN)]2+ (2-d4; cyclam‐d4 = cyclam
ligand containing four –ND groups; Figure S4C) in CD3CN shows
a singlet at –78.84 ppm, which is indicative of the presence of free
triflate anions.[1k] Furthermore, 1H-NMR spectrum of 2-d4 (Figure
S4C) reveals 13 signals integrating to 23 protons, thereby
suggesting that the cis-V cyclam configuration, observed in the X-
ray structure of 1-(OTf)2, is also present in 2 with two out of four –
NH hydrogens being cis to the oxo ligand, which in turn is cis to a
bound CH3CN ligand (Scheme 1).[8] Extended X-ray absorption
fine structure (EXAFS) analysis of 2 confirmed the presence of
the Fe=O unit that yields a best fit comprised of an O/N scatterer
at 1.66 Å, which is assigned to an Fe=O unit, and a further shell
Oxidant [ox]
Products / product yield (mmol)1
Cyclohexene
1/H2O2
a
b
0.01
-
c
0.004
-
41
38
75
0.40
0.38
1/PhIO
2
0.60
0.10
0.05
1The formation of diol products was not observed; 2 Yield based on H2O2/PhIO
under catalytic conditions and based on 2 under stoichiometric conditions.
The zero-field Mössbauer spectrum (Figure S1) of 1-(OTf)2 at 35
K revealed a major doublet [85%] with an isomer-shift (δ) of 0.57
mm/s and a small quadrupole splitting (ΔEQ) of 0.41 mm/s,
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10.1002/anie.201812758
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of 5 O/N scatterers at 2.01 Å, which correspond to the N donors
of the cyclam and CH3CN ligands (Figure 1B; Table S3). The Fe
K-edge XAS of 2 reveals a pre-edge energy of 7114.2 eV with an
area of 25 units and an edge energy of 7125.0 eV, which are
typical of previously reported S = 1 oxoiron(IV) complexes.[1,2]
Mössbauer spectrum of 2 recorded at 35 K exhibits a doublet
representing approximately 85% of the total iron with δ = 0.10
very few exceptions,[11] afford one-electron oxidized Fe(III)
products.[2b] Energy profile for the reaction of 1-(OTf)2 and H2O2,
as obtained by density functional theory (DFT) calculations
(Figure S8) reveal that the transition state leading to the formation
of 2 is preceded by an intermediate, where one of the cyclam–NH
groups hydrogen-bonds (H-bonds) to one of the HOOH oxygens
and the other HOOH oxygen is rotated away from Fe. Notably, in
the absence of such H-bonding interaction, for example for the
TMC ligand, where the nitrogen atoms are methylated,
stoichiometric generation of the oxoiron(IV) complex using H2O2
as an oxidant is only possible in the presence of an externally
added base.[11c]
To understand the potential role of the oxoiron(IV)
intermediate 2 in 1-(OTf)2-catalyzed olefin epoxidation reactions
by H2O2 or PhIO, the reactivity of the independently generated 2
with olefins was studied experimentally (Table S6). Figure S9
shows that addition of cyclohexene to the solution of 2 in CH3CN
at –20ꢀ°C resulted in the decay of the characteristic band at 737
nm. The first‐order rate constant, determined by pseudo‐first‐
order fitting of the kinetic data for the decay of 2, increased linearly
with increasing cyclohexene concentration, thereby giving a
second‐order rate constant of 1.5 x 10–2ꢀ M−1 s−1 at −20 °C.
Product analysis of the reaction mixture revealed the formation of
cyclohexene oxide (60% based on 2) as a major product (Table
1). Some allylic oxidation products, such as 2-cyclohexen-1-ol
(10%) and 2-cyclohexen-1-one (5%), were also obtained as minor
products, which is in agreement with the observed preference for
epoxidation over allylic oxidations under catalytic reaction
conditions (vide supra).[5a]
mm/s and ΔEQ = 1.09 mm/s (Figure 1D), which is close to the
Mössbauer parameters of the related [(TMC)FeIV(O)(CH3CN)]2+
complex (δ = 0.17 mm/s and ΔEQ = 1.24 mm/s)[10] and compares
well to integer electronic spin systems with S = 1 Fe(IV) (δ range
0.01 to 0.14 mm/s).[1,2] The remaining 15% of the signal with δ =
0.53 mm/s and ΔEQ = 0.61 mm/s corresponds to unreacted
[(cyclam)FeII(CH3CN)2]2+ complex (1-(CH3CN)2) (Table S4 and
Figure S5), which exists as a minor species in a CH3CN solution
of 1-(OTf)2.
B)
A)
C)
D)
Reactivities of 2 with other substrates were also investigated
(Table S6); a preference for epoxidation reaction was observed in
each case. In particular, in the reaction with cis-stilbene, cis-
stilbene oxide was the sole product, further confirming the
possible involvement of 2 as a reactive intermediate in the
stereoselective epoxidation reactions by H2O2 catalysed by 1-
(OTf)2. In addition to epoxidation reactions, complex 2 was also
capable of oxidizing a variety of substrates containing weak C–H
bonds, such as xanthene, dihydroanthracene, cyclohexadiene,
fluorene and indene (Figure S10). The experimentally determined
second-order rate constants, k2, were then adjusted for reaction
stoichiometry to yield k2’ based on the number of equivalent target
C–H bonds of substrates and correlated with the BDEs of the
substrates,[12] which gave a linear fit (Figure S10). Furthermore, a
kinetic isotope effect (KIE) value of 6.7 was obtained when
xanthene and xanthene-d2 were used as substrates (Figure S11).
These results are consistent with the rate-determining step of the
reaction being C–H bond cleavage, as it is expected in hydrogen
atom transfer (HAT) reactions. Product analysis of the reaction
mixture for xanthene reaction revealed the formation of xanthone
(64% yield) as a major product. Notably, the self-decay of 2 in the
presence of excess H2O2 is 2 – 3 orders of magnitude faster than
its reaction with substrates containing C-H or C=C bonds (Table
S6; Figure S7A); correspondingly, a very slow addition of H2O2 is
a prerequisite for achieving maximum yields of the substrate
oxidation products.
Figure 1. A): UV-vis absorption spectra of 1-(OTf)2 (black) and 2 (red) in CH3CN
at –40 °C. B): Fourier-transformed Fe K-edge EXAFS spectrum of
2
(experimental data, dotted line; simulation, solid line). The inset shows the
corresponding EXAFS data in wave-vector scale before the Fourier
transformation. C): Normalized Fe K-edge X-ray absorption spectra of 2 (black
solid line) and 1-(OTf)2 (dashed line). D): Mössbauer spectrum of 2 recorded in
CH3CN at 35 K. The solid line is the simulation of the experimental spectrum
(black dots). The major component (85%; orange shaded area; δ/ΔEQ
0.10/1.09 mm/s) corresponds to 2; the minor component (15%; purple shaded
area; δ/ΔEQ 0.53/0.61 mm/s) corresponds to unreacted
[(cyclam)FeII(CH3CN)2](OTf)2.
=
=
Replacement of sPhIO by H2O2 also leads to the formation of
2 (Figure S6; note that the characteristic near-IR feature in the
absorption spectrum of 2 is red-shifted to 760 nm, presumably
because of the binding of triflate anion in a non-coordinating
solvent like acetone), albeit with a less stability and a lower yield
(70%), presumably because of a subsequent reaction of nascent
2 with excess oxidant. Correspondingly, 2 could be stabilized only
at –80 oC in acetone when H2O2 was used as an oxidant; at higher
temperature, 2 decayed spontaneously to a bis(hydroxo)diiron(III)
species 3 (Table S5 and Figure S7). Titration experiments
confirmed a 1:1 stoichiometry between 1-(OTf)2 and H2O2 (Figure
S6B). The high yield formation of 2 in the reaction of 1-(OTf)2 with
H2O2 is notable in itself, since reactions of Fe(II) with H2O2, with
Notably, the reactivity of 2 is strikingly different from that of the
previously reported [(TMC)FeIV(O)(CH3CN)]2+ complex,[6] where
all the amino functions of the cyclam ligand are methylated. For
example, 2 performs HAT at a rate two orders of magnitude faster
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10.1002/anie.201812758
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than [(TMC)FeIV(O)(CH3CN)]2+ (Table S6). The greater reactivity
of 2 is also reflected in the inability of [(TMC)FeIV(O)(CH3CN)]2+ to
perform epoxidation of olefins under catalytic reaction
conditions.[6] The higher reactivity of 2 may arise from kinetic
and/or thermodynamic factors.[13] DFT calculations (see SI for
details)[14] on the two complexes show that the Fe=O core in 2 in
the experimentally determined cis-V configuration is sterically less
hindered than in [(TMC)FeIV(O)(CH3CN)]2+ (Figure S12), which
would make 2 a kinetically better oxidant. Note that for both 2 and
[(TMC)FeIV(O)(CH3CN)]2+, a triplet S = 1 state has been
calculated as the ground state with the excited S = 2 state being
separated by 7.9 kcal/mol and 4.7 kcal/mol, respectively (Table
S7). Thus, a two-state reactivity model[15] cannot account for the
higher reactivity of 2 as the more reactive S = 2 excited state is
placed higher in energy in 2 than in [(TMC)FeIV(O)(CH3CN)]2+.
Interestingly, a KIE value of 1.4 has been determined in the
epoxidation of cyclohexene, when 2-d4 containing four –ND
groups) was employed in the reaction (Figure S13). The presence
of KIE may corroborate the involvement of the cyclam–NH groups
in assuring preferential epoxidation over allylic oxidation reactions
in 2. Notably, [(TMC)FeIV(O)(CH3CN)]2+ and other oxoiron(IV)
complexes, which lack any –NH hydrogens, undergo preferential
allylic oxidation reactions.[1,2] We presume that because of the H-
bonding interaction with the cyclam –NH hydrogens, the oxidizing
FeIV=O core lies in closer proximity to the C=C unit (than the –
CH2- unit) of the C=C-CH2- substrates, thereby leading to stereo-
and regioselective epoxidation reactions mediated by 2. However,
a more detailed mechanistic investigation is needed to fully
understand the preference of the olefin epoxidation over the C-H
bond activation in the oxidation of cyclohexene by 2.
the NRF of Korea through CRI (NRF-2012R1A3A2048842 to WN)
and GRL (NRF-2010-00353 to WN) for financial support. TC
gratefully acknowledges the Alexander von Humboldt Foundation
for a postdoctoral fellowship. XAS experiments were performed at
SSRL beamline 4-1 (SLAC National Accelerator Laboratory,
USA) benefited from support of the US DOE Office of Science
(DE‐AC02‐76SF00515) and the US NIH (P30‐EB‐009998 and
P41‐GM‐103393).
Keywords: Bioinorganic/Biomimetic Chemistry • Nonheme
Oxoiron Intermediate • Stereoselective Epoxidation •
Regioselectivity • Catalysis
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Catalysis systems based on iron(II) complexes of tetradentate
N4-donor ligands and H2O2 have attracted particular attention in
the past decade due to the high efficiency and selectivity in the
preparative oxidations of C═C groups of various organic
molecules.[2] High-valent oxoiron(V) species have been proposed
and in few cases isolated as the reactive intermediate in these
reactions.[2f,2h,2i,2j,2k,5] In this work, we have demonstrated an
oxoiron(IV) complex, 2, as a catalytically relevant intermediate in
the [(cyclam)FeII]2+/H2O2 catalytic system for the stereo- and
regioselective epoxidation of olefins. Notably, 2 is generated as a
common reactive intermediate in the reactions of [(cyclam)FeII]2+
with H2O2 and sPhIO, which may explain the comparable catalytic
efficiency observed for both the oxidants. In spite of the
predominance of the oxoiron(IV) cores in biology, the present
study, to the best of our knowledge, represents a rare example
where an oxoiron(IV) species has been isolated as a kinetically
and catalytically competent reactive intermediate in chemical
oxidation reactions. Thus, this work provides a new fundamental
framework for understanding the nature of the iron-based species
responsible for performing olefin epoxidation reactions in a
synthetic biomimetic system that may have enzymatic relevance.
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Acknowledgements
We thank the Deutsche Forschungsgemeinschaft (UniCat; EXC
314-2 and Heisenberg Professorship to KR), MINECO
(CTQ2017-87392-P to MS), FEDER (UNGI10-4E-801 to MS) and
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Que, Jr., A. Company, M. Costas, J. Am. Chem. Soc. 2015, 137, 15833-
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[14] The DFT calculated Mössbauer parameters of the cis-V configuration of
2 (δ = 0.15 mm/s and ∆EQ = 0.89 mm/s) are in excellent agreement with
the experiment (δ = 0.10 mm/s and ∆EQ = 1.10 mm/s). The calculated
values for the corresponding trans-isomer are δ =0.10, and ΔEQ=1.914
mm/s.
[8]
In solution, CH3CN replaces the bound triflate ligands of 1-(OTf)2 with a
concomitant change to the trans-I configuration (e.g., all four –NH
hydrogens cis to each other) to form 1-(CH3CN), as evident from 1H and
II
19F NMR spectra of 1-(OTf)2 and [(cyclam-d4)Fe (OTf)2] (1-(OTf)2-d4) (in
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CD3CN) (see Scheme
2 and Figures S4A and S4B). The cis-V
configuration of 2 is presumably stabilized by additional H-bonding
interactions of the –NH hydrogens with OTf-anion (see Table S8).
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[9]
[10] PhIO is insoluble in CH3CN and hence for kinetic measurements the
soluble form of iodosylbenzene (sPhIO) is used. See (D. Macikenas, E.
Skrzypczak-Jankun, J. D. Protasiewicz, J. Am. Chem. Soc. 1999, 121,
7164-7165).
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Entry for the Table of Contents
COMMUNICATION
An oxoiron(IV) complex 2 is
trapped and characterized as a
kinetically and catalytically
competent intermediate in the
[(cyclam)FeII]2+/H2O2 catalytic
system for the epoxidation of
olefins. Complex 2 represents a
rare example of an oxoiron(IV)
species that shows a preference
for epoxidation over allylic
oxidation reactions.
Xenia Engelmann, Deesha D.
Malik, Teresa Corona, Katrin
Warm, Erik R. Farquhar,
Marcel Swart, Wonwoo Nam,*
Kallol Ray*
Page No. – Page No.
Trapping of a Highly
Reactive Oxoiron(IV)
Complex in the Catalytic
Epoxidation of Olefins by
Hydrogen Peroxide
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