A Highly Reactive Oxoiron(IV) Complex Supported by a Bioinspired N3O Macrocyclic Ligand

Article abstract of DOI:10.1002/anie.201707872

The sluggish oxidants [Fe^IV(O)(TMC)(CH₃CN)]²⁺ (TMC=1,4,8,11-tetramethyl-1,4,8,11-tetraazacyclotetradecane) and [Fe^IV(O)(TMCN-d₁₂)(OTf)]⁺ (TMCN-d₁₂=1,4,7,11-tetra(methyl-d₃

Full text of DOI:10.1002/anie.201707872

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Accepted Article
Title: A Highly Reactive Oxoiron(IV) Complex Supported by a
Bioinspired N3O Macrocylic Ligand
Authors: Kallol Ray, Marcel Swart, Wonwoo Nam, Jason England,
Eckhard Bill, Erik R. Farquhar, Mi Yoo, Kumaran Elumalai,
Yong-Min Lee, Xenia Engelmann, and Inés Monte Pérez
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To be cited as: Angew. Chem. Int. Ed. 10.1002/anie.201707872
Angew. Chem. 10.1002/ange.201707872
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10.1002/anie.201707872
Angewandte Chemie International Edition
COMMUNICATION
A Highly Reactive Oxoiron(IV) Complex Supported by a
Bioinspired N3O Macrocyclic Ligand
Inés Monte Pérez,[a] Xenia Engelmann,[a] Yong-Min Lee,[b] Mi Yoo,[b] Kumaran Elumalai,[c] Erik R.
Farquhar,[d] Eckhard Bill,[e] Jason England,[c] Wonwoo Nam,*[b] Marcel Swart,*[f] and Kallol Ray*[a]
Abstract: The sluggish oxidants, [FeIV(O)(TMC)(CH3CN)]2+ (TMC
1,4,8,11-tetramethyl-1,4,8,11-tetraazacyclotetradecane) and
[FeIV(O)(d12-TMCN)(OTf)]+ (3d; d12-TMCN = 1,4,7,11-tetra-d3-
methyl-1,4,7,11-tetraazacyclotetradecane), are transformed into
a highly reactive oxidant, [FeIV(O)(TMCO)(OTf)]+ (1; TMCO =
corresponding triplet (S = 1) spin state. The biological FeIV=O
oxidants all contain at least two weak-field O-based ligands (either
carboxylate or water) cis to the oxo donor (Scheme 1A),[3] which
can play a significant role in the stabilization of the S = 2 state by
modulating the energy gap between the Fe d(xy) and d(x2-y2)
orbitals. A related species is also proposed to be the oxidant
formed in the reactions of N2O with coordinately unsaturated
iron(II) centers in carboxylate-based metal−organic frameworks
capable of catalyzing ethane hydroxylation.[4] While no direct
spectroscopic evidence is currently available, DFT studies
suggest that this intermediate is an S = 2 FeIV=O species
possessing an O-rich ligand environment.
=
4,8,12-trimethyl-1-oxa-4,8,12-triazacyclotetradecane),
upon
replacement of a -NMe donor in the TMC or TMCN ligand by an
O-atom. A rate enhancement of 5 – 6 orders of magnitude in both
H-atom and O-atom transfer reactions is observed upon oxygen
incorporation into the macrocyclic ligand and can be explained
based upon the higher electrophilicity of the iron center and the
higher availability of the more reactive S = 2 state in 1. This
rationalizes nature’s preference for using O-rich ligand
environments for the hydroxylation of strong C-H bonds in
enzymatic reactions.
H
igh-valent oxoiron(IV) (FeIV=O) species have been identified as
key reactive intermediates in the activation of dioxygen and the
oxygenation of organic substrates by mononuclear nonheme iron
enzymes.[1] The FeIV=O moiety in enzymes is always found in the
quintet (S = 2) spin state, which is predicted by DFT calculations[2]
to be more reactive in H-atom transfer (HAT) than the
[a]
Prof. Dr. K. Ray, Dr. I. Monte-Pérez, M. Sc. X. Engelmann
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, Dr. Y.-M. Lee, M. Yoo
Department of Chemistry and Nano Science
Ewha Womans University
[b]
Scheme 1. Structures of biological (A) and bioinspired (B) oxoiron(IV)
complexes.
Seoul 120–750, Korea
E-mail: wwnam@ewha.ac.kr
[c]
[d]
Prof. Dr. J. England, Dr. K. Elumalai
Division of Chemistry and Biological Chemistry, School of Physical
and Mathematical Sciences, Nanyang Technological University, 21
Nanyang Link, Singapore 637371
The reactivity of FeIV=O complexes with weak-field O-rich
ligand environments are, therefore, of particular interest.
Successful synthesis and characterization of such compounds
has been limited by their high reactivity and instability under
ambient conditions. For example, the short half-life of 20 s at 25
oC of the S = 2 [FeIV(O)(H2O)5]2+ complex[5] has inhibited a detailed
investigation of its spectroscopic and reactivity properties. On the
other hand, employment of strong-field pyridine and tertiary amine
donors[1a-b,6] or carbene donors[7] has led to the isolation and
characterization of several well-characterized S = 1 FeIV=O model
complexes with high stability and low reactivity. Efforts to stabilize
the more reactive S = 2 state by replacing one or more N-donors
with weak carboxylate ligands[8] have previously resulted in only
a small enhancement (5 – 10 folds) in the HAT abilities of the
complexes. In fact, no FeIV=O complexes supported by O-based
ligands that are able to oxidize substrates containing C-H bonds
Dr. E. R. Farquhar
Case Center for Synchrotron Biosciences
NSLS-II, Brookhaven National Laboratory
Upton, NY 11973, USA
[e]
[f]
Dr. E. Bill
Max-Plank-Institut für Chemische Energiekonversion,
Mülheim an der Ruhr (Germany)
Prof. Dr. M. Swart
ICREA, Pg. Lluís Companys 23, 08010 Barcelona, Spain
IQCC & Departament de Química
Universitat de Girona, Campus Montilivi, 17003, Girona, Spain
E-mail: marcel.swart@udg.edu
Supporting information for this article is given via a link at the end of
the document.
This article is protected by copyright. All rights reserved.

10.1002/anie.201707872
Angewandte Chemie International Edition
COMMUNICATION
with a bond dissociation energy (BDE) of > 80 kcal mol–1 are
presently known. This is surprising, considering nature’s
preference for employing FeIV=O complexes with O-rich ligand
environments to oxidize strong C-H bonds.
1), 848 nm (130 M–1 cm–1) and 992 nm (125 M–1 cm–1) (Figure 1,
right). The electrospray ionization mass spectrum (ESI-MS) of 1
(Figure S4) exhibited a signal at m/z = 464.1, which shifted by
two-mass units when tBuSO2C6H4I18O was used as an oxidant,
possessing a mass and isotope distribution pattern consistent
with a [FeIV(O)(TMCO)(OTf)]+ formulation. The presence of the
Fe=O unit in 1 was confirmed by EXAFS analysis that yields a
best-fit plot (Figure S5 and Table S3) with an O/N scatterer at
1.64 Å (assigned to the Fe=O unit) and a further shell of 5 O/N
scatterers at 2.10 Å (corresponding to the N/O donors of TMCO
and OTf- ligands). The Fe K-edge X-ray absorption spectrum of 1
(Figure 2A) reveals an edge energy of 7124.3 eV (versus
7121.5 eV for 2) and a broad pre-edge peak assigned to 1s→3d
transitions at 7114.2 eV with an area of 45 units (Table S4).
Furthermore, the 1H-, 2D- and 19F-NMR spectra of [FeIV(O)(d9-
TMCO)(OTf)]+ (1d) (Figure S6) indicate that a single Cs-
symmetric iron(IV) species is present, in which one of the OTf-
anions is coordinated to the metal center.
As part of our continuing efforts to uncover the structure-
reactivity relationship of non-heme oxometal complexes, we
report herein the synthesis and characterization of an S = 1
FeIV=O complex [FeIV(O)(TMCO)(OTf)]+ (1) (Scheme 1; TMCO =
4,8,12-trimethyl-1-oxa-4,8,12-triazacyclotetradecane),[9] bearing
a weak-field N3O macrocyclic ligand in place of the popular N4-
donor TMC-based ligands. 1 exhibits spectroscopic properties
distinct
from
[FeIV(O)(TMC)(CH3CN)]2+
(Scheme
1;
TMC=1,4,8,11-tetramethyl-1,4,8,11-tetraazacyclotetradecane)[10]
and [FeIV(O)(d12-TMCN)(OTf)]+ (3d) (Scheme 1; d12-
TMCN=1,4,7,11-tetra-d3-methyl-1,4,7,11-
tetraazacyclotetradecane) and, remarkably, 5 – 6 orders of
magnitude higher reactivity in HAT and O-atom transfer (OAT)
reactions.
Figure 2. A) Normalized Fe K-edge X-ray absorption spectra of 1 (solid line)
and 2 (dashed line) in a CH3CH2CN/CH2Cl2 solvent mixture (10% CH2Cl2 by
volume); B) The zero field Mössbauer spectrum of 1 recorded at 4.2 K. The solid
line is the simulation of the experimental spectrum (black dots) representing
>95% of the absorption. The major component (90%; black shaded area)
corresponds to 1; the minor component (10 %; gray shaded area) corresponds
to unreacted 2.
Figure 1. Left: X-ray crystal structure of 2. Right: UV-Vis absorption spectra of
1 (black) and 2 (gray) in CH2Cl2 at –90 oC. The inset shows the expansion of
the 500 – 1000 nm region.
Table 1. A comparison of the redox potentials, zero-field splitting and second-
order rate constants (k2) with cyclohexane (C6H12) and thioanisole (PhSMe) for 1,
3d and [FeIV(O)(TMC)(CH3CN)]2+
Combination of the tetradentate TMCO ligand (Scheme 1B;
see also Figure S1 for the ligand synthesis) with
Fe(OTf)2(CH3CN)2 yielded the expected iron(II) complex
[FeII(TMCO)(CH3CN)(OTf)](OTf) (2). The X-ray structure of 2
(Figure 1 left and Table S1) displays a 6-coordinate geometry
with axially bound triflate and CH3CN ligands. Notably, the
macrocyclic ring exhibits a configuration, where all the methyl-
groups point cis to CH3CN, with iron-ligand bond lengths
(Table S2) typical of a high-spin iron(II) ion. Consistent with
this spin-state assignment, the zero-field Mössbauer spectrum
of 2 (Figure S2) revealed a single doublet with an isomer-shift
(δ) of 1.17 mm/s and a large quadrupole splitting (ΔEQ = 3.32
Complex
D
[cm-1]
E(FeIV/III
vs SCE
)
k2[a](C6H12
[M–1 s–1
)
k2[a](PhSMe)
[M–1 s–1
]
]
1
35±3
-
0.90
0.37
0.39
1.0 x 10-2
2.5 x 102
4.0 x 10-3
1.3 x 10-3
3d
no reaction
no reaction
[FeIV(O)(TMC)(CH3CN)]2+ [1a]
26.95
±0.15
[a]
@ –40 oC
2
mm/s). In addition, the 1H-, 19F- (in CD2Cl2) and D-NMR (in
The zero field Mössbauer spectrum of 1 recorded at 4.2 K
exhibits a doublet representing ca. 90% of the iron with ΔEQ
CH2Cl2) spectra of [FeII(d9-TMCO)(CH3CN)(OTf)](OTf) (2d; d9-
=
TMCO
=
4,8,12-tri-d3-methyl-1-oxa-4,8,12-
1.58 mm s−1 and δ = 0.21 mm s−1 (Figure 2B); the remaining 10%
of the signal corresponds to unreacted 2. Applied magnetic fields,
B, elicit magnetic hyperfine interactions (Figure S7) with
parameters typical of S = 1 FeIV O complexes. Within resolution,
the zero-field splitting (ZFS) tensor, described by the parameters
D and E, was found to be axial (D = 35 (± 3) cm–1; E/D = 0.0
(+0.05). Notably, the D value of 1 is significantly higher than that
reported for [FeIV(O)(TMC)(CH3CN)]2+ and all other oxoiron(IV)
triazacyclotetradecane) display paramagnetically shifted peaks
(Figure S3), indicative of coordination of both triflate anions and
retention of the pseudo-Cs symmetric solid-state structure in
solution.
The reaction of
2
with 1.5 equiv of 2-(tert-
butylsulfonyl)iodosylbenzene (tBuSO2C6H4IO)[11] in anhydrous
CH2Cl2 at −90 °C led to the immediate formation of a pale green
intermediate 1 (t1/2 = 1.5 h; −50ꢀ°C) with absorption maxima λmax
(εmax) centered at 390 nm (>3000 M–1 cm–1), 585 nm (200 M–1 cm–
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10.1002/anie.201707872
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COMMUNICATION
complexes (Table 1 and Table S4); this may point to a more
accessible S = 2 excited state in 1.[1]
From the 1H- and 2D-NMR spectra of [FeII(d12-
TMCN)(CH3CN)]2+, it was discovered that the chemically
equivalent methyl groups (and ethylene and propylene units) of
TMCN are magnetically inequivalent (Figure S11). This indicates
that these two methyl substituents point in different directions and
results in the complex being C1 symmetric. The zero-field
Mössbauer spectrum of [FeII(TMCN)(CH3CN)]2+ (Figure S12)
revealed a single doublet with δ = 1.20 mm s–1 and ΔEQ = 2.90
mm s–1 typical of a high-spin iron(II) ion.
The oxidative reactivity of 1 was examined in the oxidation of
hydrocarbons with C–H BDEs of 77 – 99.3 kcal mol–1 (Table S5
and Figure S8). The experimentally determined (see SI for details)
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; the k2’ values were correlated with the
BDEs of the substrates (Figure 3 left and Table S5). This
observation is consistent with the rate-determining step of the
reaction being C–H bond cleavage, as is expected in HAT
reactions. Furthermore, a deuterium kinetic isotope effect (KIE) of
24.8 was obtained when ethylbenzene-d10 was used as a
substrate (Figure 3 right). Such a large KIE value implies a
tunneling mechanism in H-atom abstraction, as is frequently
proposed in C–H bond activation reactions of FeIV=O species.[1,6]
Product analysis of the reaction mixture for cyclohexane reaction
revealed the formation of cyclohexanol (35% yield),
cyclohexanone (5%) and high-spin iron(III) (>90% yield;
Figure S9A) products. Thus, 1 performs HAT in an oxygen non-
rebound mechanism,[12] as proposed earlier for other nonheme
FeIV=O complexes.
t
The reaction of [FeII(TMCN)(CH3CN)]2+ with BuSO2C6H4IO in
CH3CN at –40 oC revealed the formation of
a transient
intermediate 3, whose fast decay prevented its isolation. The
instability of 3 can be presumably attributed to its self-decay via
intramolecular attack of a ligand methyl C−H bond (cis to the oxo
group) by the Fe=O unit (see DFT calculation below). With an
assumption that a large KIE would be involved for such a self-
decay mechanism, the perdeuterated d12-TMCN ligand was
employed in an effort to stabilize the corresponding complex
[FeIV(O)(d12-TMCN)(OTf)]+ (3d). Indeed, the reaction of [FeII(d12-
TMCN)(CH3CN)]2+ with tBuSO2C6H4IO in CH3CN at 0 oC led to a
o
clean conversion to 3d (t1/2 = ~8 min at 0 C), as confirmed by
NMR (Figure S13), UV-Vis (Figure S14; λmax = 820 nm;  = 260
M–1 cm–1) and ESI-MS (Figure S15). The oxidative reactivity of 3d,
when tested in intermolecular HAT and OAT reactions, shows it
to be a sluggish oxidant similar to [FeIV(O)(TMC)(CH3CN)]2+
(Tables 1 and S4).
The electrochemistry of 1, [FeIV(O)(TMC)(CH3CN)]2+, and 3d
was then investigated as a means to gain insight into the
observed reactivity and provide a more quantitative basis for the
influence of the TMCO, TMC and d12-TMCN ligands, respectively,
on the FeIV=O center (Figure S16). Cyclic voltammetry (CV) of
[FeIV(O)(TMC)(CH3CN)]2+ has previously revealed an FeIV/III
potential of +0.39 V vs. SCE in CH3CN.[16] A comparable FeIV/III
potential of +0.37 V has been determined for 3d in CH3CN. In
contrast, a large positive shift of the FeIV/III potential to +0.90 V was
observed for 1 in trifluoroethanol (CF3CH2OH), which justifies the
Figure 3. Left: Plot of log k2’ vs C-H BDE of substrates for 1. Right: Plot of the
pseudo-first-order rate constants, kobs (s–1), against substrate concentrations to
determine second-order rate constants, k2, and C–H kinetic isotope effect (KIE)
value for the reaction of 1 with ethylbenzene.
much higher HAT and OAT capabilities of
[FeIV(O)(TMC)(CH3CN)]2+ and 3d.
1 relative to
Comparison of the HAT reactivities of 1 and other oxoiron(IV)
complexes (Tables 1 and S4) shows that the rate of cyclohexane
oxidation by 1 at –40 oC (0.01 M–1 s–1) is only an order of
magnitude lower than that of the two most reactive complexes
reported to date; namely, the S = 2 [FeIV(O)(TQA)(CH3CN)]2+
(0.37 M–1 s–1)[13] and S = 1 [FeIV(O)(Me3NTB)(CH3CN)]2+ (0.25 M–
DFT calculations (Figure S17 and Table S6) for
[FeIV(O)(TMCO)(CH3CN)]2+
(1-CH3CN) and
[FeIV(O)(TMC)(CH3CN)]2+ provided further insight into their
differing reactivity. For both complexes, a ground-state (GS) with
a triplet (S = 1) spin quantum number and a low-lying quintet (S =
2) spin state were predicted. In [FeIV(O)(TMC)(CH3CN)]2+, the S =
2 state is 5 kcal mol–1 higher in energy than the S = 1 GS, but for
1-CH3CN, this spin-state splitting is reduced to only 1 kcal mol–1.
Free energies, which has been shown[17] to better rationalize the
different reactivities of the oxoiron(IV) complexes than electronic
energies, have also been determined for 1-CH3CN and
[FeIV(O)(TMC)(CH3CN)]2+ in the temperature range 10 – 370 K
(Tables S7, S8). While the S = 1 state is preferred for
[FeIV(O)(TMC)(CH3CN)]2+ at all temperatures, the S = 2 state is
the predicted GS at the experimental temperature of 223 K for
reactivity studies. The reaction profile for HAT from cyclohexane
by 1-CH3CN and [FeIV(O)(TMC)(CH3CN)]2+ in the S = 1 and S = 2
spin states is displayed in Figure S18. The obtained barriers
(Table S6) are consistent with the experimental findings, with a
1
s–1)[14] complexes. In particular, the closely related S = 1
[FeIV(O)(TMC)(CH3CN)]2+ complex does not react at all with
cyclohexane even at 25 °C. Furthermore, comparison of the OAT
abilities of 1 (Table 1, Figure S10) and [FeIV(O)(TMC)(CH3CN)]2+
towards thioanisole (PhSMe) shows that the former reacts five-
orders of magnitude faster than the latter at –40 oC.[15]
The TMCO ligand differs from TMC not only in being a N3O
rather than a N4 macrocycle, but also in having the two propylene
(and also the two ethylene) spacers adjacent, rather than
alternating with respect to one another (Scheme 1B).
A
comparison of the reactivity of with that of the
1
[FeIV(O)(TMCN)(CH3CN)]2+ (3; TMCN = 1,4,7,11-tetramethyl-
1,4,7,11-tetraazacyclotetradecane) is, therefore, warranted for a
proper analysis of the effect of N- vs O-donors in an otherwise
identical ligand environment.
large reduction of the
barrier upon going from
[FeIV(O)(TMC)(CH3CN)]2+ to 1-CH3CN in both the S = 1 and S = 2
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10.1002/anie.201707872
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reactive S = 2 spin state for 1 may rationalize its enhanced
oxidative reactivity relative to [FeIV(O)(TMC)(CH3CN)]2+.
Calculations were also performed for the complex
[FeIV(O)(TMCN)(CH3CN)]2+, for which one of the N-methyl groups
was predicted to be syn to the Fe=O unit (consistent with
experiment) in its most stable configuration (Figure S17C). The
barrier for self-decay of [FeIV(O)(TMCN)(CH3CN)]2+ by an
intramolecular HAT from the syn methyl group is 13 – 14 kcal mol-
[2]
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1
lower than the oxidation barrier for cyclohexane (Figure S18),
explaining the inherent instability of [FeIV(O)(TMCN)(CH3CN)]2+.
In summary, we have generated a highly reactive oxoiron(IV)
complex 1 by weakening the ligand field about the FeIV=O unit via
introduction of an O-atom into the N4 macrocycle of the TMC or
TMCN ligands. The FeIV=O center goes from being a sluggish
oxidant (within an N4 macrocycle) to being an extremely reactive
species (in an N3O environment) that can hydroxylate alkanes
with C-H bonds as strong as 99.3 kcal mol–1. Furthermore, a
dramatic enhancement of the rate of OAT to PhSMe by five-
orders of magnitude was observed upon swapping to the N3O
coordination sphere. Complex 1 represents the only example of a
highly reactive FeIV=O complex supported by an O-containing
ligand system, and explains nature’s preference for using O-rich
ligand environment for the hydroxylation of strong C-H bonds.
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Acknowledgements
Financial supports from the DFG (Cluster of Excellence “Unifying
Concepts in Catalysis”; EXC 314-2, KR), MINECO (CTQ2014-
59212-P
and
CTQ2015-70851-ERC,
MS),
GenCat
M. T. S. Amorim, S. Chaves, R. Delgado, J. J. R. Fraústo da Silva, Dalton
Trans. 1991, 3065.
(2014SGR1202, MS), FEDER (UNGI10-4E-801, MS), and COST
action CM1305 “ECOSTBio” are gratefully acknowledged. K.R.
also thanks the Heisenberg-Program of DFG for financial support.
J.E. thanks NTU for funding. W.N. acknowledges financial
support from NRF of Korea through CRI (NRF-
2012R1A3A2048842), GRL (NRF-2010-00353) and Basic
Science Research Program (2017R1D1A1B03029982 to Y.M.L.).
XAS experiments were conducted at SSRL beamline 7-3 (SLAC
National Accelerator Laboratory, USA), with support from the
DOE Office of Science (DE-AC02-76SF00515) and NIH (P30-EB-
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Keywords: Oxoiron(IV) Complex • Oxygen-Based Ligands • Spin
State Effect • Hydrogen Atom Transfer • Oxygen Atom Transfer .
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10.1002/anie.201707872
Angewandte Chemie International Edition
COMMUNICATION
Entry for the Table of Contents
COMMUNICATION
The drastically enhanced
reactivity of an oxoiron(IV)
center in an N3O
environment relative to an
N4 environment explains
nature’s preference for
using oxygen rich ligand
environment for the
Inés Monte Pérez, Xenia
Engelmann,Yong-Min Lee, Mi
Yoo, Erik Farquhar, Eckhard
Bill, Kumaran Elumalai, Jason
England, Wonwoo Nam*,
Marcel Swart,* and Kallol Ray*
Page No. – Page No.
hydroxylation of strong C-H
bonds.
A Highly Reactive
Oxoiron(IV) Complex
Supported by a Bioinspired
N3O Macrocyclic Ligand
This article is protected by copyright. All rights reserved.