Nonheme Fe(IV) Oxo Complexes of Two New Pentadentate Ligands and Their Hydrogen-Atom and Oxygen-Atom Transfer Reactions

Article abstract of DOI:10.1021/ic5029564

Two new pentadentate {N5} donor ligands based on the N4Py (N4Py = N,N-bis(2-pyridylmethyl)-N-bis(2-pyridyl)methylamine) framework have been synthesized, viz. [N-(1-methyl-2-benzimidazolyl)methyl-N-(2-pyridyl)methyl-N-(bis-2-pyridyl methyl)amine] (L¹) and [N-bis(1-methyl-2-benzimidazolyl)methyl-N-(bis-2-pyridylmethyl)amine] (L²), where one or two pyridyl arms of N4Py have been replaced by corresponding (N-methyl)benzimidazolyl-containing arms. The complexes [Fe^II(CH₃CN)(L)]²⁺ (L = L¹ (1); L² (2)) were synthesized, and reaction of these ferrous complexes with iodosylbenzene led to the formation of the ferryl complexes [Fe^IV(O)(L)]²⁺ (L = L¹ (3); L² (4)), which were characterized by UV-vis spectroscopy, high resolution mass spectrometry, and M?ssbauer spectroscopy. Complexes 3 and 4 are relatively stable with half-lives at room temperature of 40 h (L = L¹) and 2.5 h (L = L²). The redox potentials of 1 and 2, as well as the visible spectra of 3 and 4, indicate that the ligand field weakens as ligand pyridyl substituents are progressively substituted by (N-methyl)benzimidazolyl moieties. The reactivities of 3 and 4 in hydrogen-atom transfer (HAT) and oxygen-atom transfer (OAT) reactions show that both complexes exhibit enhanced reactivities when compared to the analogous N4Py complex ([Fe^IV(O)(N4Py)]²⁺), and that the normalized HAT rates increase by approximately 1 order of magnitude for each replacement of a pyridyl moiety; i.e., [Fe^IV(O)(L²)]²⁺ exhibits the highest rates. The second-order HAT rate constants can be directly related to the substrate C-H bond dissociation energies. Computational modeling of the HAT reactions indicates that the reaction proceeds via a high spin transition state.

Full text of DOI:10.1021/ic5029564

Article
pubs.acs.org/IC
Nonheme Fe(IV) Oxo Complexes of Two New Pentadentate Ligands
and Their Hydrogen-Atom and Oxygen-Atom Transfer Reactions
†
‡
§
†
†
⊥
Mainak Mitra, Hassan Nimir, Serhiy Demeshko, Satish S. Bhat, Sergey O. Malinkin, Matti Haukka,
¶
∥
§
#
∇
Julio Lloret-Fillol, George C. Lisensky, Franc Meyer, Albert A. Shteinman, Wesley R. Browne,
David A. Hrovat,○,◆ Michael G. Richmond, Miquel Costas, and Ebbe Nordlander*
◆
¶
,†
†
Chemical Physics, Department of Chemistry, Lund University, Box 124, SE-221 00, Lund, Sweden
‡
Department of Chemistry and Earth Sciences, College of Arts and Sciences, Qatar University, P.O. Box 2713, Doha, State of Qatar
§
Institute of Inorganic Chemistry, Georg-August-University Go
̈
ttingen, Tammanstrasse 4, D-37077 Go
̈
ttingen, Germany
∥
Department of Chemistry, Beloit College, 700 College Street, Beloit, Wisconsin 53511, United States
⊥
̈ ̈ ̈ ̈
Department of Chemistry, University of Jyvaskyla, P.O. Box-35, Jyvaskyla, FI-40014, Finland
#
Institute of Problems of Chemical Physics, Chernogolovka, Moscow District, 142432, Russian Federation
∇
Stratingh Institute for Chemistry, Faculty of Mathematics and Natural Sciences, University of Groningen, Nijenborgh 4, 9747AG
Groningen, The Netherlands
○
Center for Advanced Scientific Computing and Modeling, University of North Texas, Denton, Texas 76203, United States
◆
Department of Chemistry, University of North Texas, Denton, Texas 76203, United States
QBIS, Department of Chemistry, University de Girona, Campus Montilivi, E-17071 Girona, Spain
¶
*
S Supporting Information
ABSTRACT: Two new pentadentate {N5} donor ligands
based on the N4Py (N4Py = N,N-bis(2-pyridylmethyl)-N-
bis(2-pyridyl)methylamine) framework have been synthesized,
viz. [N-(1-methyl-2-benzimidazolyl)methyl-N-(2-pyridyl)-
1
methyl-N-(bis-2-pyridyl methyl)amine] (L ) and [N-bis(1-
methyl-2-benzimidazolyl)methyl-N-(bis-2-pyridylmethyl)-
2
amine] (L ), where one or two pyridyl arms of N4Py have
been replaced by corresponding (N-methyl)benzimidazolyl-
II
2+
1
2
containing arms. The complexes [Fe (CH CN)(L)] (L = L (1); L (2)) were synthesized, and reaction of these ferrous
3
IV
2+
1
2
complexes with iodosylbenzene led to the formation of the ferryl complexes [Fe (O)(L)] (L = L (3); L (4)), which were
ssbauer spectroscopy. Complexes 3 and 4 are
characterized by UV−vis spectroscopy, high resolution mass spectrometry, and Mo
̈
1
2
relatively stable with half-lives at room temperature of 40 h (L = L ) and 2.5 h (L = L ). The redox potentials of 1 and 2, as well
as the visible spectra of 3 and 4, indicate that the ligand field weakens as ligand pyridyl substituents are progressively substituted
by (N-methyl)benzimidazolyl moieties. The reactivities of 3 and 4 in hydrogen-atom transfer (HAT) and oxygen-atom transfer
(
(
OAT) reactions show that both complexes exhibit enhanced reactivities when compared to the analogous N4Py complex
IV
2+
[Fe (O)(N4Py)] ), and that the normalized HAT rates increase by approximately 1 order of magnitude for each replacement
IV
2
2+
of a pyridyl moiety; i.e., [Fe (O)(L )] exhibits the highest rates. The second-order HAT rate constants can be directly related
to the substrate C−H bond dissociation energies. Computational modeling of the HAT reactions indicates that the reaction
proceeds via a high spin transition state.
INTRODUCTION
bonds in a wide number of substrates, transforming them into
hydroxylated, unsaturated, or halogenated products.
The interesting chemistry that is exhibited by nonheme iron
■
4b,5−7,9
High valent Fe(IV) oxo species have been established as key
oxidizing intermediates in the activation of molecular oxygen in
many iron-containing enzymes. For example, nonheme high
spin (S = 2) Fe(IV) oxo intermediates have been identified as
active oxidizing species in the catalytic cycles of E. coli
enzymes has inspired extensive efforts to mimic their high
1
−3
2,10
valent intermediates and emulate their reactivities.
Over the
past decade, several nonheme Fe(IV) oxo (ferryl) complexes
supported by a wide range of pentadentate and tetradentate
4
2
,10,11
taurine:α-ketoglutaratedioxygenase (TauD), propyl-4-hydrox-
ligands have been prepared.
These Fe(IV) oxo complexes
5
6
7
ylase, halogenase CytC3, tyrosine hydroxylase, and the
8
aliphatic halogenase SyrB2 by means of various spectroscopic
Received: December 10, 2014
techniques. These reactive intermediates functionalize C−H
©
XXXX American Chemical Society
A
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Article
have been investigated for, inter alia, hydrogen-atom transfer
and oxygen-atom transfer reactions, and the reactivities of these
different nonheme Fe(IV) oxo complexes have been found to
hydrogen-atom transfer and oxygen-atom transfer processes
with their spectroscopic and electrochemical properties which
18
are controlled by the ligand scaffold. The oxo transfer process
to thioanisole by these five Fe(IV) oxo complexes has been
correlated to the redox potential of the Fe(IV)/Fe(III) couple
which was influenced by the ligands bound to the Fe-oxo unit;
however, such a direct correlation could not be concluded in
IV
2+
vary widely. For example, [Fe (O)(TMC)(NCMe)] (TMC
1,4,8,11-tetramethyl-1,4,8,11-tetraazacyclotetradecane) can
only oxidize substrates with C−H bond dissociation energies
=
12
IV
2+
(
BDE) of <80 kcal/mol, while [Fe (O)(N4Py)] (N4Py =
18
N,N-bis(2-pyridylmethyl)-N-bis(2-pyridyl)methylamine) and
the case of hydrogen abstraction processes. Due to the
IV
2+
IV
[
Fe (O)(Bn-tpen)] (Bn-tpen = N-benzyl-N,N′,N′-tris(2-
relative dearth of high spin Fe O complexes, the reactivity of
11
pyridylmethyl)-1,2-diaminoethane) (Figure 1) have been
this class of complexes is less well-explored. It has been
shown that such complexes can undergo hydrogen-atom
transfer (HAT) reactions, but in general with relatively low
reaction rates in comparison with the most reactive low spin
shown to oxidize strong C−H bonds, e.g., those in cyclohexane
1
3
(
C−H BDE 99.3 kcal/mol) at room temperature.
IV
19,20
IV
Fe O complexes.
Recently, one high spin Fe O
IV
2+
complex, [Fe (O)(TQA)(NCMe)]
(TQA = tris(2-
quinolylmethyl)amine), was found to be significantly more
IV
reactive toward HAT than other S = 2 Fe O complexes, and
21
was also found to effect alkene epoxidation.
IV
2+
Figure 1. Two nonheme Fe(IV) oxo complexes: [Fe (O)(N4Py)]
We wish to investigate whether the reactivity of ferryl
complexes can be tuned by modification of the steric and
electronic properties of ligands, and whether the reactivity of
such complexes can be improved while maintaining thermal
stability. For this purpose, we chose to modify/derivatize the
IV
2+
(left) and [Fe (O)(Bn-tpen)] (right).
The spin state on the iron center of the Fe(IV) oxo unit has
been proposed to play a crucial role in terms of reactivity. It has
been predicted theoretically that the high spin (S = 2) Fe(IV)
oxo complexes are more reactive than low spin (S = 1) Fe(IV)
2
2
IV
N4Py ligand framework. Among bioinspired Fe O
IV
2+
complexes, [Fe (O)(N4Py)] has been shown to exhibit the
unique combination of powerful oxidative reactivity toward
alkanes, enabling it to cleave strong CH bonds, while at the
same time possessing considerable thermal stability; indeed, the
stability has been proven sufficient to permit its structure to be
11,14,15
oxo complexes (vide infra).
A significant aspect of the
observed reactivities and spin states of ferryl complexes is that
they can be modulated by the various ligand environments
supporting the Fe(IV) oxo unit. A high spin (S = 2) Fe(IV) oxo
unit may be achieved by adopting a trigonal bipyramidal (TBP)
geometry using tetradentate ligands with sufficient steric
constraints, as observed in the case of [Fe (O)(TMG tren)]
TMG tren = tris(tetramethylguanidino)tren) and [Fe (O)-
2
3
characterized by X-ray crystallography. A number of
modifications of the N4Py ligand framework have been
24,25
IV
2+
made,
but they do not include the replacement of the
3
16
IV
(
(
pyridyl moieities of this ligand with other nitrogen donor
moieties. We were therefore interested in attempting to modify
the stability and reactivity of N4Py by tuning the ligand
coordination environment, and we chose to introduce (N-
methyl)benzimidazolyl moieties as these are relatively close
analogues of histidine imidazolyls. Although there are a few
examples of benzimidazolyl donors exerting profound influence
3
−
17
H buea)] (H buea = tris(tert-butylureaylethylene)aminato)
3
3
leading to degeneracy of the second highest occupied molecular
2
2
orbitals, which have significant dxy and dx −y character. A
detailed investigation was recently made on five low spin
Fe(IV) oxo complexes comprising pentadentate pyridine- and
amine-based ligands to examine the correlation of the rates of
1
2
Scheme 1. Schematic Synthetic Routes for Ligands L and L
B
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Article
on the reactivities of bioinspired nonheme iron catalysts in
Scheme 2. Synthesis of the Fe(II) Complexes
1
5,26,27
II
1
oxidation,
there is only one previous study involving the
[Fe (CH CN)(L )](ClO ) (1·(ClO ) ) and
3 4 2 4 2
II 2
coordination of such donor moieties to an isolated/detected/
[Fe (CH CN)(L )](ClO ) (2·(ClO ) )
3
4 2
4 2
IV
15
identified Fe O complex. The five-coordinate complex
II
[
Fe (Me NTB)(NCMe)](OTf) (Me NTB = tris((N-methyl-
3
2
3
benzimidazol-2-yl)methyl)amine), the (N-methyl)-
benzimidazolyl equivalent of the tripodal tris(2-pyridylmethyl)-
amine (TPA) ligand, reacts with m-CPBA at −40 °C to form
IV
2+
the very reactive (S = 1) [Fe (O)(Me NTB)] complex,
3
which is proposed to be more reactive than Cyt P450 model
IV
+• 2+
compound I ([Fe (O)(TDCPP) ] , TDCPP = meso-tetrakis-
(
15
2,6-dichlorophenyl)porphinatodianion). However, this ferryl
species with a solvent acting as a sixth ligand and with a
plausible pseudo-octahedral coordination geometry around the
iron is unstable and decays fast even at −40 °C (t = 2 min).
1
2
The UV−vis spectra of the ligands L and L in acetonitrile
solution show high intensity bands in the UV region
(wavelength <300 nm) (Figure S3, Supporting Information).
Upon coordination to the Fe(II) ion, two new charge transfer
1
/2
1
2
The two new pentadentate ligands L and L that have been
prepared are thus based on the N4Py ligand framework, with
(
(
CT) bands appear in the visible region. For complex 1·
1
2
one (L ) or two (L ) of the (2-pyridyl)methyl arms of N4Py
being exchanged by one (or two) (N-methylbenzimidazolyl)-
methyl arm(s). The (N-methyl)benzimidazole moiety contains
not only greater steric bulk but is generally considered to be a
−1
ClO ) , the CT bands appear at λ = 387 nm (ε ≈ 5200 M
4
2
max
−1
−1
−1
cm ) and 466 nm (ε ≈ 4300 M cm ), while for complex 2·
−1
(
ClO ) , the CT bands appear at λ = 404 nm (ε ≈ 3200 M
4
2
max
−1 −1 −1
28
cm ) and 477 nm (ε ≈ 2100 M cm ) (Figure 2). These CT
better σ-donor than the pyridine moiety. It has recently been
shown that such a simple change of a ligand donor moiety can
affect the catalytic activities of Fe(II) complexes in alkane and
27
alkene oxidation reactions, and it was envisioned that the
IV
chemistry of high valent Fe(IV) oxo species, e.g., [Fe (O)-
2+
(
N4Py)] , may also be sensitive to such a change. Here we
report two new Fe(II) complexes based on the two ligands, the
generation and characterization of their corresponding Fe(IV)
oxo complexes, and the reactivities of these ferryl species
toward hydrogen-atom transfer and oxygen-atom transfer
processes.
RESULTS AND DISCUSSION
■
Synthesis and Characterization of Ligands and
1
Figure 2. UV−vis spectra of complexes 1·(ClO ) (red) and 2·
Complexes. Ligand L was synthesized by reaction of N-
4 2
2
9
(
ClO ) (black) (0.25 mM) in acetonitrile recorded at room
[
1
di(2-pyridyl)methyl]-N-(2-pyridylmethyl)methylamine with
equiv of 2-chloromethyl-1-methylbenzimidazole in refluxing
dry acetonitrile, in the presence of K CO and tetrabutylam-
4 2
temperature (298 K).
2
3
2
monium bromide (Scheme 1). Ligand L was synthesized by
bands can be assigned as metal to ligand charge transfer
(MLCT) bands arising from electron transfer from low spin
Fe(II) t2g orbitals to the π* orbitals of the ligand. The weaker
absorbance/smaller observed extinction coefficients for com-
plex 2 relative to 1 may be explained by the presence of a
mixture of high spin and low spin forms of the complex in
solution (vide infra).
3
0
reaction of bis(2-pyridyl)methylamine with 2 equiv of 2-
chloromethyl-1-methylbenzimidazole in the presence of
aqueous NaOH solution (Scheme 1).
II
1
The corresponding Fe(II) complexes, [Fe (CH CN)(L )]-
3
II
2
(
(
1
ClO4)2 (1·(ClO ) ) and [Fe (CH CN)(L )](ClO ) (2·
4 2
3
4 2
1
2
ClO ) ), were prepared by reaction of 1 equiv of L /L with
4
2
The natural abundance 57Fe Mo
ssbauer spectra of the solid
equiv of hydrated Fe(ClO ) in a minimum amount of dry
̈
4
2
samples of complexes 1·(ClO ) and 2·(ClO ) measured at 80
acetonitrile solvent at room temperature (Scheme 2). Both
4 2
4 2
K confirm the presence of low spin Fe(II) ions in both
complexes 1·(ClO ) and 2·(ClO ) were isolated as air-stable
4
2
4 2
complexes in the solid state (Figure 3). The isomeric shift
solids.
The ESI mass spectrum of 1·(ClO ) in acetonitrile shows
4
2
peaks at m/z = 238 and m/z = 258.5 corresponding to the
II
1
2+
II
1
formulations [Fe (L )]
(calcd 238) and [Fe (L )-
2+
(
CH CN)] (calcd 258.5), respectively, as well as peaks at
3
m/z = 575.1 and m/z = 616.1 corresponding to the
II
1
+
II
1
formulations [Fe (L )(ClO )] (calcd 575.1) and [Fe (L )-
4
+
(
CH CN)(ClO )] (calcd 616.1), respectively (Figure S1,
3 4
Supporting Information). Similarly, the ESI-MS of complex 2·
(
ClO ) in acetonitrile shows prominent peaks at m/z = 264.6
4
2
and m/z = 628.1 corresponding to the formulations
II
2
2+
II
2
+
[
6
Fe (L )] (calcd 264.6) and [Fe (L )(ClO )] (calcd
Figure 3. Zero-field Mo
̈
ssbauer spectra of complexes 1·(ClO ) (left)
4 2
4
28.1), respectively (Figure S2, Supporting Information).
and 2·(ClO ) (right) collected at 80 K.
4
2
C
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values (δ) and quadrupole splitting values (ΔE ) are listed in
from high spin to low spin in acetonitrile solution as the
Q
Table 1. Single-point DFT energy calculations on the S = 0, S =
solution is cooled, the electrochemistry of 2·(ClO ) was also
4
2
measured as a function of temperature. A small shift of 0.07 V
to a lower potential was observed as the sample was cooled
from 21 to −40 °C (Figure S13, Supporting Information).
Crystal and Molecular Structures of 1·(ClO ) and 2·
Table 1. Mo
̈
ssbauer Parameters for Complexes 1·(ClO4)2
2
and 2·(ClO )
4
4
2
isomeric shift value (δ) in quadrupole splitting value (ΔE )
Q
mm s−
1
−1
(ClO ) . Single crystals suitable for X-ray diffraction were grown
4 2
complex
in mm s
for both 1·(ClO ) and 2·(ClO ) , and their crystal structures
4
2
4 2
1
·(ClO4)2
·(ClO4)2
0.41
0.44
0.19
0.21
were determined in order to confirm the proposed molecular
structures. The structures of the two cationic complexes are
shown in Figure 4; selected bond distances are collated in Table
2
1
, and S = 2 spin states of 1·(ClO ) and 2·(ClO ) using the
4 2 4 2
structures obtained from the X-ray diffraction data confirm the
S = 0 spin state as the ground state of each complex.
Magnetic moment measurements performed on solid
samples of 1·(ClO ) and 2·(ClO ) show that both complexes
4
2
4 2
remain low spin in the solid state within the temperature range
1
0
−300 K (Figures S4−5, Supporting Information). The H
NMR spectrum of 1·(ClO ) in CD CN indicates the presence
of a low spin Fe(II) center in solution at room temperature
Figure S6, Supporting Information). However, the H NMR
4
2
3
1
(
spectrum of complex 2·(ClO ) in CD CN measured at 298 K
4
2
3
shows a presence of the high spin form of 2·(ClO ) as a minor
4
2
component in solution, in equilibrium with the diamagnetic low
spin form (Figure S7, Supporting Information). The variable
temperature NMR measurement (VT-NMR) in the temper-
ature range 298−243 K on 2·(ClO ) in CD CN reveals that
Figure 4. ORTEP plots of the molecular structures of the cations of 1·
(ClO4)2 (left) and 2·(ClO4)2 (right), showing the atom labeling
scheme. Thermal ellipsoids are plotted at 30% probability ellipsoids;
hydrogen atoms have been omitted for clarity.
4
2
3
the high spin Fe(II) center becomes low spin with a decrease in
temperature (Figure S8, Supporting Information). The differ-
ent behaviors of 2·(ClO ) in the solid state and in solution can
4
2
explain the observed low intensity in the absorbance value of 2·
ClO ) relative to 1·(ClO ) (Figure 2) and implies that the
Table 2. Selected Bond Distances (Å) for Complexes 1·
(
(ClO ) and 2·(ClO )
4 2 4 2
4
2
4 2
successive introduction of (N-methyl)benzimidazolyl moieties
into the N4Py ligand framework gradually weakens the ligand-
field strength. Thus, there is an increase of the absorbance value
of the UV−vis spectrum of complex 2·(ClO ) in acetonitrile
solution when the temperature is lowered from room
temperature (298 K) to 243 K (Figure S9, Supporting
Information). This may be rationalized by an increased
population of the low spin state, with the spectral properties
of this state predominating over the transitions connected with
the high spin state.
1
·(ClO4)2
2·(ClO4)2
Fe(1)−N(6)
Fe(1)−N(1)
Fe(1)−N(4)
Fe(1)−N(3)
Fe(1)−N(2)
Fe(1)−N(5)
Fe(1)−N(7)
Fe(1)−N(4)
Fe(1)−N(5)
Fe(1)−N(6)
Fe(1)−N(1)
Fe(1)−N(3)
1.909(6)
1.948(6)
1.957(6)
1.960(6)
1.977(6)
1.980(6)
1.901(3)
1.964(3)
1.979(2)
1.983(3)
1.983(3)
2.028(3)
4
2
2, and relevant crystallographic data are summarized in Table
Electrochemistry of Complexes 1·(ClO4)2 and 2·
S1 (Supporting Information). The crystal structures are similar
II
(
ClO ) . The cyclic voltammograms of complexes 1·(ClO )
to that of the “parent” complex [Fe (N4Py)(CH CN)]-
4
2
4 2
3
31
1
2
and 2·(ClO ) were measured in acetonitrile at 298 K using a
(ClO ) and show that the pentadentate ligands L and L
4
2
4 2
glassy carbon electrode as the working electrode and SCE as
the reference electrode. All potentials are reported versus the
ferrocene/ferrocenium potential (0.4 V vs NHE) as an internal
standard. Complex 1·(ClO ) showed a reversible oxidation
coordinate as envisaged, with the sixth coordination site at the
iron ion being occupied by a (solvent) acetonitrile molecule.
The short Fe−N bond lengths (1.9−2.0 Å) observed for both
complexes are in agreement with the presence of low spin
Fe(II) centers in 1·(ClO ) and 2·(ClO ) , as observed for
4
2
wave at E1/2 = 0.467 V, and complex 2·(ClO ) showed a
4 2
4 2
4 2
II
31
quasireversible oxidation wave at E1/2 = 0.398 V (Figure S12,
[Fe (N4Py)(CH CN)](ClO )
and some previously re-
3
4 2
III
24,32,33
Supporting Information). Peak separations for both these Fe /
ported related Fe(II) complexes.
A comparison between
II
Fe couples are similar to that of ferrocene. A small impurity of
different Fe−N bond distances in 1·(ClO ) , 2·(ClO ) , and
4
2
4 2
3
1
II
the aquo complex was also observed at 0.1 V for 1·(ClO )
[Fe (N4Py)(CH CN)](ClO ) is shown in Table 3. It is not
4
2
3 4 2
II
and at 0.2 V for 2·(ClO ) . The [Fe (CH CN)(N4Py)]-
possible to discern clear structural trends, as most distances are
equivalent within experimental error, but the following
observations may be made: For each replacement of a pyridyl
moiety by an (N-methyl)benzimidazolyl moiety (going from
the parent complex to 1·(ClO ) to 2·(ClO ) ), the specific
4
2
3
(
ClO ) complex has a relatively high redox potential value
4 2
25,31
(
1.01 V vs SCE or 0.85 V vs ferrocene).
Successive
replacement of the pyridyl moieties in N4Py by (N-methyl)-
benzimidazolyl moieties led to an incremental lowering of the
oxidation potential in agreement with the benzimidazolyl
groups being better σ-donors, thus increasing the electron
density on the metal relative to the N4Py complex. Because
NMR spectroscopy shows that complex 2·(ClO ) changes
4
2
4 2
Fe−N bond that is subject to the change is lengthened.
Replacement of a pyridyl moiety by a second (N-methyl)-
benzimidazolyl moiety in 2·(ClO ) results in a lengthening of
4
2
both equatorial Fe−NPy (Py = pyridyl) bond distances with
4
2
D
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Table 3. Comparison of Different Fe−N Bond Distances (Å)
between 1·(ClO ) , 2·(ClO ) , and
Complex 4 has a characteristic absorbance band with a
−1
−1
maximum at λmax = 725 nm with ε ≈ 380 M cm (Figure
4
2
4 2
II
31
[
Fe (N4Py)(CH CN)](ClO )
5). These near-IR bands are due to ligand-field (d−d)
3 4 2
3
4,35
transitions centered on the Fe(IV) ion.
The change of
II
entry
1·(ClO4)2 2·(ClO4)2 [Fe (N4Py)(CH CN)](ClO )
4 2
IV
3
λmax of the near IR spectral Fe O signature for the
Fe−Npy (av)
1.953
1.974
1.972
IV
2+
complexes [Fe (O)(N4Py)] (four equatorial pyridyl moi-
eties; λmax = 695 nm), 3 (three equatorial pyridyls, one
equatorial (N-methyl)benzimidazolyl moiety; λmax = 708 nm),
and 4 (two equatorial pyridyls, two equatorial (N-methyl)-
benzimidazolyl moieties; λmax = 725 nm) may be attributed to
the change in the ligand environment and suggests that the
Fe−NBzIm (av)
1.977
1.981
Fe−NMeCN
1.909(6)
1.980(6)
1.901(3)
2.028(2)
1.915(3)
1.961(3)
Fe−Namine
respect to 1·(ClO ) . This elongation may be rationalized by
4
2
the angular requirements imposed by the five-membered (N-
methyl)imidazolyl ring versus the six-membered pyridyl ring,
and the consequent structures of the five-membered Fe−
(
equatorial) ligand field becomes progressively weaker as (N-
methyl)benzimidazolyl substituents are introduced in the ligand
scaffold.
that of the related, but considerably less stable, complex
13,18
These absorption maxima may be compared with
N
−C−C−N
chelation rings formed upon coordination
amine
donor
IV
n
n
of the ligands. Thus, the Nδ−C−Nε angles for the
benzimidazolyl substituents in 1·(ClO ) and 2·(ClO ) vary
[Fe (O)( Bu-P2DA)] for which λ = 770 nm ( BuP2DA =
max
4
2
4 2
N-(1′,1′-bis(2-pyridyl)pentyl)iminodiacetate, i.e., the same
ligand framework as L , L , and N4Py, but with the two (N-
methyl)benzimidazolyl moieties of L replaced by two acetate
groups). This spectroscopic feature indicates that the BuP2-
DA ligand exerts a considerably weaker field than L and L , yet
the ground state for [Fe (O)( Bu-P2DA)] is S = 1. Solomon,
Que, and co-workers have deconvoluted the near-IR absorption
of [Fe (O)(N4Py)] using MCD spectroscopy and computa-
tional modeling. This relatively broad absorption involves
1 2
from 112.2(3)° to 113.2(6)° with the C−C−N
angles in
donor
2
the corresponding chelate rings varying from 120.1(3)° to
36
n
1
21.0(3)°. For the corresponding pyridyl moiety in 1·(ClO ) ,
4
2
1
2
the angles are “reversed”: within the pyridyl ring, C23−C22−
N6 = 119.7(11)°, and within the chelation ring, C21−C22−N6
IV
n
=
114.6(8)°. In other words, the benzimidazolyl units form a
IV
2+
more obtuse C−C−Ndonor angle within the chelation ring than
the pyridyl unit, and shortening of the Ndonor−Fe distance for
the benzimidazolyl entities would likely lead to larger strain
several bands, including a d to d
to the strength of the Fe−O bond and dxz/yz-pyridine π overlap,
and a d to d
transition which is related
xy
xz/yz
within the coordination ring. The average Fe−N bond
Py
distance in 1·(ClO ) is approximately 1.95 Å while the average
2
2
4
2
transition that is directly related to the
xy
x −y
34b
Fe−N bond distance in 2·(ClO ) is approximately 1.97 Å
Py
4 2
equatorial field strength of the ligand.
(
Table 3). The Fe−N
bond distance, trans to the
amine
Complexes 3 and 4 were also characterized by high
resolution mass spectrometry (HRMS). The HRMS of an
acetonitrile solution containing complex 3 showed major peaks
at m/z = 246.0681, corresponding to the formulation
[
5
acetonitrile ligand, is lengthened by approximately 0.05 Å,
going from 1·(ClO ) to 2·(ClO ) .
4
2
4 2
Generation and Characterization of High Valent
Fe(IV) Oxo Complexes. The two Fe(II) complexes contain
a relatively labile coordinated solvent (acetonitrile) molecule,
and they are easily converted into the corresponding Fe(IV)
oxo species upon reaction with the strong O atom donor agent
iodosylbenzene (PhIO). Complex 1·(ClO4)2 reacted with
excess (10 equiv) solid PhIO in acetonitrile at room
IV
1
2+
Fe (O)(L )] (m/z calcd 246.0675), and at m/z =
IV
1
91.0847, corresponding to the formulation [Fe (O)(L )-
1+
(
ClO )] (m/z calcd 591.0841) (Figures S16−18, Supporting
4
Information). Similarly, the HRMS of complex 4 (with triflate
II
2
counteranion, derived from [Fe (L )(CH CN)](CF SO ) , cf.,
Experimental Section for synthesis) in acetonitrile showed
major peaks at m/z = 272.5837, corresponding to the
formulation [Fe (O)(L )] (m/z calcd 272.5808) and at m/
3
3
3 2
IV
1
2+
temperature to form a pale green complex, [Fe (O)(L )]
3) (Scheme 3).
(
IV
2
2+
IV
2
z = 694.1153, corresponding to the formulation [Fe (O)(L )-
1+
Scheme 3. Generation of the Fe(IV) Oxo Complexes 3 and 4
from the Precursor Fe(II) Complexes 1 and 2, Respectively
(
CF SO )] (m/z calcd 694.1142) (Figures S19−21, Support-
3
3
ing Information).
Mossbauer spectroscopy was employed to confirm the
oxidation state of the iron ions in complexes 3 and 4. The
zero-field Mossbauer spectrum of an acetonitrile solution
containing a partly (20%) of Fe enriched sample of 3 showed
̈
̈
57
−1
an isomeric shift value, δ = −0.03 mm s , and quadrupole
−1
splitting value, ΔE = 1.1 mm s (Figure 6), indicating the
Q
presence of an Fe(IV) species. Approximately 86% of the Fe
sample accounts for the presence of 3, with the remaining
absorption corresponding to an Fe(III) byproduct. In the case
IV
n
Formation of 3 was monitored using UV−vis spectropho-
tometry, which indicated that complex 1 can be converted into
complex 3 within 20 min of stirring at room temperature
of [Fe (O)( Bu-P2DA)] (vide supra), a corresponding Fe(III)
impurity has been suggested to be an (L)Fe −O−Fe (L)
species, and the Mossbauer parameters of the present
III
III
36
̈
−1
−1
(
Figure S14, Supporting Information). Complex 3 has a
impurity (δ = 0.45 mm s ; ΔE = 1.50 mm s ) are similar
n −1
Q
characteristic absorbance band in the near IR region, with a
to those of the dinuclear Bu-P2DA complex (δ = 0.46 mm s ;
−1
−1
−1
maximum at λmax = 708 nm and ε ≈ 400 M cm , a common
feature for low spin (S = 1) Fe(IV) oxo species (Figure 5).
Similarly, complex 2·(ClO ) also formed a pale green species,
ΔE = 1.65 mm s ). We therefore tentatively assign the ferric
impurity as [{(L )Fe} (μ-O)] , generated by coupling of the
Q
1
4+
2
IV
analogous Fe O species with its Fe(II) precursor, although,
4
2
IV
2
2+
n
[
Fe (O)(L )] (4), upon reaction with excess (10 equiv)
unlike the dinuclear Bu-P2DA complex, the existence of such
PhIO in acetonitrile at low temperature (−10 °C) (Scheme 3).
an Fe(III)OFe(III) dimer could not be confirmed by mass
E
DOI: 10.1021/ic5029564
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Inorganic Chemistry
Article
Figure 5. Absorbance spectra of complexes 3 (blue) and 4 (red) [0.5 mM] in acetonitrile (left) recorded at room temperature; expanded plot at long
wavelengths, with corresponding molar extinction coefficient values (ε) for complexes 3 (blue) and 4 (red) (right).
Figure 6. (Left) Zero-field Mo
subspectrum corresponds to ca. 14% of an Fe(III) impurity, tentatively assigned as the oxo-bridged dinuclear complex [{(L )Fe} (μ-O)] (see text).
̈
ssbauer spectrum of 57Fe enriched 3 (light gray) in acetonitrile solution (5 mM) measured at 80 K. The minor black
1
4+
2
(
Right) Zero-field Mo
̈
ssbauer spectrum of 57Fe enriched 4 (light gray) in acetonitrile solution (2 mM) measured at 80 K. The minor black
2
4+
subspectrum corresponds to ca. 20% of an Fe(III) impurity, tentatively assigned as [{(L )Fe} (μ-O)] .
2
spectrometric measurements on 3 (or 4, vide infra). Similarly to
, the Mossbauer spectrum of 4 measured under the same
Table 4. Comparison of Wavelengths and Half-Lives of
Different Low Spin (S = 1) Fe(IV) Oxo Complexes Bearing
Pentadentate Ligands
3
̈
−1
conditions showed an isomeric shift value, δ = −0.02 mm s ,
−1
and quadrupole splitting value, ΔE = 1.34 mm s , indicating
Q
λmax, nm (ε in
the presence of an Fe(IV) species, with ∼80% of the Fe sample
accounting for the presence of 4 (Figure 6). The isomeric shift
values for 3 and 4 are very similar to those obtained for
−1
−1
complex
M
cm
)
t1/2 at RT
ref
this
IV
1
2+
[
Fe (O)(L )] (3)
708 (400)
40 h
work
this
work
13
IV
2+
−1
−1 13
IV
2
2+
[
Fe (O)(N4Py)] (δ = −0.04 mm s ; ΔE = 0.93 mm s )
[Fe (O)(L )] (4)
725 (380)
2.5 h
Q
IV
2+
−1
and [Fe (O)(Bn-tpen)] (δ = 0.01 mm s ; ΔE = 0.87 mm
Q
IV
2+
−
1
13
[Fe (O)(N4Py)]
696 (400)
770 (220)
739 (400)
834 (260)
810 (270)
60 h
n.a.
6 h
7 h
5 d
s ), suggesting that both 3 and 4 are low spin (S = 1)
nonheme Fe(IV) oxo complexes, and DFT calculations (vide
infra) support the low spin state as the favored (ground) state.
Again, a ferric impurity/byproduct assigned as the oxo-bridged
IV
n
[
[
[
[
Fe (O)( Bu-P2DA)]
36
13
37
38
IV
2+
Fe (O)(Bn-tpen)]
IV
2+
Fe (O)(TMC-py)]
IV
Fe (O)(Me cyclam-CH C(O)
2
4+
−1
3
2
dimer [{(L )Fe} (μ-O)] (δ = 0.49 mm s ; ΔE = 1.36 mm
2+
2
Q
NMe
IV
2
)]
−
1
1
2+
2+
s ) was detected (Figure 6).
[Fe (O)(BP )]
730 (400)
730 (380)
736 (310)
740 (340)
n.a.
n.a.
n.a.
n.a.
39
39
40
18
IV
2
The half-lives (t1/2’s) for complexes 3 and 4 were determined
at room temperature. The t1/2 for complex 3 is 40 h, while for
complex 4 it is 2.5 h, demonstrating that complex 4 is thermally
much less stable compared to complex 3. In Table 4, the half-
lives and the characteristic wavelengths of these two new
Fe(IV) oxo complexes are compared to some previously
reported low spin Fe(IV) oxo complexes bearing pentadentate
ligands; it may be noted that the thermal stability of complex 3
is very similar to that of the parent N4Py complex.
Hydrogen-Atom Transfer (HAT) Reactions. The Fe(IV)
oxo complexes 3 and 4 react with the C−H bonds of a number
of substrates at room temperature. A series of alkane substrates
having different C−H bond dissociation energies (BDE) (range
from 81 to 99.3 kcal/mol) were investigated, and the relative
reactivities between complexes 3 and 4 were evaluated.
[Fe (O)(BP )]
IV
2+
[Fe (O)(MePy TACN)]
[Fe (O)(Me2TACNPy2)]2+
2
IV
Addition of 20 equiv of triphenylmethane (C−H BDE = 81
kcal/mol) to 3 resulted in rapid decay of 3 to its Fe(II)
precursor species as identified by UV−vis spectroscopy (Figure
S22, Supporting Information) and formation of triphenylme-
thanol with ∼89% yield. On the other hand, complex 3 reacted
with the comparatively less reactive substrate cyclohexane (C−
H BDE = 99.3 kcal/mol) at a slower rate to form cyclohexanol
(A) and cyclohexanone (K) (yield of A ∼8%, yield of K ∼10%,
total yield ∼18%). Similarly, complex 4 reacted with triphenyl-
methane with a faster rate producing triphenylmethanol with
∼90% yield (Figure 7, left), while its reactivity slowed for
F
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Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
Figure 7. (Left) Kinetic decay of complex 4 (≈0.5 mM in acetonitrile) upon addition of 30 equiv of triphenylmethane at room temperature. The
2
4+
decay product is most likely a mixture of complex 2 and the byproduct [{(L )Fe} (μ-O)] , meaning that the 477 nm band of 2 grows in as a
2
shoulder in the spectrum. (Right) Determination of the kinetic isotope effect (KIE) for separate reactions with toluene and toluene-d with complex
8
3
at room temperature.
Table 5. C−H Bond Dissociation Energies of Different Alkane Substrates and the Second-Order Rate Constants (k ) of 3, 4,
2
IV
2+
a
and [Fe (O)(N4Py)] for HAT Reactivities
k × 10 for 3 (M s−1
3
−1
)
k × 10 for 4 (M s−1
3
−1
)
k × 10 for [Fe (O)(N4Py)] (M−1 s−1)14
3
IV
2+
substrate
BDE (kcal/mol)
2
2
2
triphenylmethane
cumene
81
31 ± 1
8 ± 0.3
270 ± 10
60 ± 1
48 ± 1
12 ± 5
26 ± 1
37.0
2.0
84.5
87
ethylbenzene
toluene
7.6 ± 0.2
1.3 ± 0.4
1.5 ± 0.1
0.32 ± 0.03
0.3 ± 0.02
4.0
90
0.15
cyclooctane
95.3
96.5
99.3
2,3-dimethyl butane
2.5 ± 0.3
2.9 ± 0.1
0.12
0.05
cyclohexane
a
Adapted from ref 14.
substrates with higher C−H BDE. The details of product
analyses are provided in the Supporting Information. The decay
of the Fe−oxo absorbance band of 3 monitored at 708 nm (and
of 4 monitored at 725 nm) followed pseudo-first-order
conditions in the presence of excess substrate (50−400
equiv) for a range of substrates. The second-order rate
constants (k ) for the different substrates were obtained from
2
the slope of plots of the observed pseudo-first-order rate
constant, k , versus molar concentration, taken at three
obs
different substrate concentrations. The k values thus obtained
2
are listed in Table 5.
A plot of logarithmic values of second-order rate constants
Figure 8. Plot of log k ′ versus C−H bond dissociation energies of
2
IV
2+
(
log k ′) (k ′ is the second-order rate constant divided by the
different alkane substrates for 3 and 4 and [Fe (O)(N4Py)]
2
2
number of equivalent C−H bonds in the substrate) for
absorbance decay versus bond dissociation energies for various
hydrocarbons shows a linear correlation (Figure 8).
(adapted from ref 14) with 5% error range. The second-order rates
were measured at 25 °C in all cases.
41
A primary kinetic isotope effect (KIE) value of ∼14 was
obtained by determining the second-order rate constants
corresponding to separate reactions of toluene and its d8-
isotopomer with 3 (Figure 7, right), using the same
methodology as that described above. The corresponding KIE
TSR, vide infra). The nonclassical values observed for 3 and 4
suggest that these complexes follow similar reaction paths as
IV
2+
[Fe (O)(N4Py)] (cf., Discussion).
The reactivity of complex 4 with substrates like 9,10-
dihydroanthracene (9,10-DHA) and 1,4-cyclohexadiene (1,4-
CHD) was also investigated in order to examine any possible
influence of the bulky (N-methyl)benzimidazolyl moieties on
the access to the Fe(IV) oxo center by the substrate. Both
substrates have similar C−H bond dissociation energies (C−H
BDE = 77 kcal/mol for 9,10-DHA and 78 kcal/mol for 1,4-
CHD), but 9,10-DHA is a sterically bulkier substrate relative to
value for 4 was ∼11. The linear correlation between log k ′ and
2
C−H BDE of substrates, as well as the observed large KIE
value, constitutes strong evidence of the reactions taking place
via hydrogen-atom transfer (HAT). The above-mentioned KIE
values are larger than “classical” KIE values (which are expected
to be less than or equal to 7) but lower than that observed for
IV
2+
41
the reaction of [Fe (O)(N4Py)] (KIE ∼20). Klinker et al.
1,4-CHD. The second-order rate constants obtained for
IV
−1 −1
have argued that the relatively high KIE value for [Fe (O)-
complex 4 measured at 243 K were (1.4 ± 0.2) M
s
with
+
−1 −1
(
N4Py)]2 may be explained by a tunneling-like HAT
9,10-DHA and (1.2 ± 0.3) M
s
with 1,4-CHD. Similar
values of k for 4 indicated that access to the Fe(IV) oxo center
mechanism involving a crossover from a triplet (S = 1) ground
state to a quintet (S = 2) transition state (two-state reactivity,
2
by the substrates were equally feasible and not influenced by
G
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Inorganic Chemistry
Article
2
the nearby (N-methyl)benzimidazolyl arms. It should be
mentioned that 4 acted as a one-electron oxidant in these
cases, thereby requiring 2 equiv of complex 4 to form benzene
10− M−1 s−1 was obtained in the oxidation of thioanisole by
complex 3. Similarly, 4 was reacted with thioanisole under the
same conditions, and a faster rate with respect to complex 3
and methyl phenyl sulfoxide was obtained with a yield of ∼88%.
(
identified by GC) from 1 equiv of 1,4-CHD (Scheme 4).
The second-order rate constant (k ) for complex 4 in oxo
transfer reaction was found to be (3.1 ± 0.11) × 10
Table 6 shows a comparison in the reaction rates of OAT
processes between complex 3, 4, and previously reported
Fe(IV) oxo complexes. In parallel with the observed trend for
2
−1
−1 −1
Scheme 4. Reaction of an Fe(IV) Oxo Complex with 1,4-
Cyclohexadiene
M
s .
HAT, the k values suggest that complex 4 is more reactive (by
2
1
order of magnitude) than complex 3 in the OAT reaction
(
i.e., thioanisole oxidation).
While the steric hindrance in complex 4 has not been probed
by an extensive range of substrates, the results above indicate
that there is relatively little steric discrimination of substrates by
DISCUSSION
■
The enhanced HAT and OAT reactivities of 3 and 4 when
IV
2+
4
(and implicitly by 3).
The HAT plot clearly indicates that 4 consistently reacted
compared with [Fe (O)(N4Py)] may be considered using
33,42
two popular approaches:
Shaik’s exchange-enhanced-
15
with faster rates of reactions with different hydrocarbon
substrates with respect to 3. The slopes observed for the
reactivity (EER) model and/or the Bell−Evans−Polanyi
4
3,44
principle (BEP) of linear free energy relations (LFER),
which has been fruitfully developed by Mayer for HAT
IV
2+ 13
HAT plots of [Fe (O)(N4Py)] , complex 3, and complex 4
are not parallel (Figure 8), but the approximate colinearity of
the plots/slopes for the different complexes indicates that the
effect in reaction rates upon introducing the second (N-
methyl)benzimidazolyl arm is roughly the same irrespective of
the substrate. Considering that there does not seem to be a
significant steric discrimination of substrates for complex 4
45
reactions. These approaches are not mutually exclusive but
rather complementary, as the EER approach applies to the
kinetics and the BEP to the thermodynamics of the process.
According to the EER, or two-state reactivity (TSR)
1
4b,46
IV
model,
the HAT reactivity of triplet Fe O (S = 1)
complexes may be explained by a more reactive quintet (S = 2)
transition state that is populated via spin crossover during
movement over the energy surface along the reaction
coordinate. The EER/TSR model shows that an increase of
the number of unpaired and spin-identical electrons on the iron
center will be favorable to the reactivity of the high spin state in
comparison with that of the low spin state. The model states
that a greater number of unpaired electrons will lead to a more
favorable transition state (hence increased reactivity), and
therefore, the proximity of the quintet excited state to the
(vide supra), the observed HAT reactivities suggest that the
influence of the replacement of a pyridyl side arm by a (N-
methyl)benzimidazolyl side arm is primarily electronic. Since
the reaction of an Fe(IV) oxo species with a substrate R−H
involves the rate-determining transfer of a hydrogen-atom (one
electron coupled with a proton), it may be concluded that the
effective hydrogen-atom affinity of the ferryl unit is increased by
the successive introduction of (N-methyl)benzimidazolyl side
arms in this family of complexes; it may be noted that the
addition of one extra (N-methyl)benzimidazolyl moiety
increases the rate constant by 1 order of magnitude (Figure 8).
O-Atom Transfer (OAT) Reactions: Oxidation of
Sulfides. The oxo transfer reactivities of 3 and 4 were also
IV
triplet (S = 1) ground state in the Fe O complex and
correspondingly the easiness of triplet/quintet spin crossover is
significant for its reactivity. Oxygen-atom transfer reactions are
two-electron processes and hence should not exhibit EER.
46
investigated, using thioanisole (PhSCH ) as a substrate.
Complex 3 reacted with thioanisole at 243 K and transferred
the oxygen to form methyl phenyl sulfoxide in high yield
The TSR model is difficult to verify experimentally, and the
concept that a high spin (transition) state is inherently more
reactive than a low spin (transition) state has been challenged.
Computational modeling by Que, Solomon, and co-workers has
3
(∼84%) (Scheme 5). During the course of the reaction,
IV
complex 3 was converted into its Fe(II) precursor (complex 1)
as identified by UV−vis spectrophotometry (Figure 9). The
reaction showed pseudo-first-order behavior under conditions
of excess of substrate (5−20 equiv with regard to complex 3),
shown that the reaction barriers for HAT involving [Fe (O)-
2+
IV
2+
(TMG tren)] (S = 2) and [Fe (O)(N4Py)] (S = 1) are
3
composed of electronic components that are approximately
similar, while the difference in reactivity of the two complexes is
47
and the observed rate constant (k ) was linearly dependent on
due to different steric contributions to the reaction barrier.
obs
substrate concentration (Figure 9). From this linear plotting, a
In order to elucidate the HAT mechanism(s) for reaction of
alkanes with 3 and 4, and establish the spin states of the active
second-order rate constant (k ) with a value of (3.3 ± 0.09) ×
2
Scheme 5. Oxygen-Atom Transfer (OAT) to Thioanisole by Complex 3 and 4 at 243 K
H
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Inorganic Chemistry
Article
Figure 9. (Left) Conversion of complex 3 into its Fe(II) precursor in the presence of thioanisole (5 equiv) at 243 K. (Right) Determination of the
second-order rate constant for the oxygen-atom transfer reaction from the kobs vs concentration plot.
Table 6. Comparison of Rates of OAT Process (Thioanisole
Oxidation) between Complex 3, Complex 4, and Some
Other Relevant Low Spin Fe(IV) Oxo Complexes
k (in
temp of
measurement
2
−
1
−1
complex
M
s
)
ref
IV
1
2+
−2
[
[
[
[
[
[
[
Fe (O)(L )] (3)
3.3 × 10
3.1 × 10
1.4 × 10
2.4 × 10
3.3 × 10
1.4 × 10
2.1 × 10
243 K
243 K
263 K
233 K
263 K
233 K
233 K
this work
IV
2
2+
−1
−2
−4
−1
−2
4
Fe (O)(L )] (4)
this work
IV
2+
Fe (O)(N4Py)]
18
15
18
15
15
IV
2+
Fe (O)(N4Py)]
3
3
3
IV
2+
2+
Figure 11. B3LYP-optimized transition-state structures TS AB CD
Fe (O)(Bn-tpen)]
5
5
5
(
left) and TS AB CD (right).
IV
Fe (O)(Bn-tpen)]
IV
2+
Fe (O)(Me NTB)]
3
III
1
2+
[
Fe (OH)(L )] (C) and methyl radical D. Collapse of
species on the potential energy surface, DFT calculations were
carried out on the triplet and quintet states of 3 ( A and A),
using methane (B) as the substrate. Figure 10 shows the lowest
3
3
5
CD completes the formal hydroxylation and gives the MeOH-
coordinated complex [Fe (MeOH)(L )] ( E). The overall
process is exergonic by 7.5 kcal/mol.
II
1
2+
3
Population of the energetically favored quintet surface
3
5
proceeds through a A → A spin crossover. The quintet oxo
5
species ( A) undergoes HAT reaction through a linear, σ-
5
5
5
directed transition structure, TS AB CD, which lies 12.6 kcal/
mol lower in energy than the triplet transition structure
3
3
3
(
TS AB CD). This energetic stabilization more than compen-
sates for the ground-state exchange destabilization experienced
5
5
by A. The collapse of the geminate radical pair, CD, affords
the MeOH-coordinated complex [Fe (MeOH)(L )] ( E)
with a net release of 25.8 kcal/mol.
II
1
2+
5
51
The experimental observation of an increase in the rate of
HAT for each successive substitution of a pyridyl unit in
IV
2+
[
Fe (O)(N4Py)] by (N-methyl)benzimidazolyl moiety is
also supported by DFT calculations. The calculated barrier for
HAT of methane by 3 is 1.9 kcal/mol lower in energy than that
IV
2+
⧧
for [Fe (O)(N4Py)] (ΔG = 23.2 kcal/mol). The same
⧧
3
5
barrier calculated for 4 (ΔG = 20.5 kcal/mol) is an additional
Figure 10. B3LYP potential energy surface for reaction of A/ A and
methane (B) to give methanol-substituted complexes 3E and 5E.
0.8 kcal/mol lower in energy. We are convinced that the
computed trend is correct, but it should be noted that these
differences in activation energies, which are based on gas phase
calculations, may not be expected to be quantitatively translated
to observed reaction rates.
3
Energy values are in ΔG in kcal/mol relative to A + B.
energy pathways that have been computed for methane
activation, while Figure 11 shows the pertinent transition-
state structures responsible for H-abstraction. The results are in
concert with a two-state reactivity model, as popularized by
Shaik, involving the triplet (S = 1) and quintet (S = 2) reaction
The BEP/LFER principle establishes the linear correlation of
⧧
the reaction barrier height (ΔΔG ) with the thermodynamic
driving force of reaction determined by the free energy value
(ΔΔG°). Such linear free energy relationships have been
established for a wide range of heme and nonheme ferryl
1
4b,48−50
surfaces.
The side-on approach of methane to the oxo moiety in A
3
52
45
IV
complexes. According to Mayer, the reactivities of Fe O
3
3
3
leads to TS AB CD through a π-directed manifold. This
complexes in HAT reactions may be related to their redox
3
transition structure lies 33.9 kcal/mol above the triplet oxo A
potentials and the pK of the FeOH products according to
a
and affords the corresponding geminate radical pair
the Bordwell−Polanyi equation.
I
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Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
o
DOH = 23.06E + 1.37pK + C
increases in the corresponding values for the rate constants by 1
order of magnitude for each nitrogen donor replacement.
Despite the increased steric bulk of the (N-methyl)-
benzimidazolyl groups relative to the pyridyl moieties, no
influence of this replacement on the access to the Fe(IV) oxo
center by the substrate molecules could be detected.
a
The rates of HAT reactions effected by ferryl species should
IV
thus be mainly correlated to the redox potentials of Fe O
complexes, with potential deviations related to contributions by
the second term of this equation (pK ) or the nonadiabaticity
of the reaction (EER model). For equatorial donors an increase
of the donor strength of the ligand will lower the redox
a
As expected, the introduction of equatorial (N-methyl)-
benzimidazolyl donors in the N4Py framework reduces the
IV
2
2
potential of the Fe O moiety and increase the pK of the
energy difference between the iron d and d
orbitals, but
a
xy
x −y
III
2
2
Fe OH product formed upon hydrogen-atom transfer. It
the resulting d /d
gap is not sufficiently small to make the
xy x −y
has been found that ligand donor moieties in cis-position to the
oxo unit (equatorial donors) exert less influence on the
high spin S = 2 state the ground state of 3 and 4. However,
even though DFT calculations find the low spin S = 1 state to
be the ground state for both 3 and 4, computational modeling
of the HAT/hydroxylation of methane by 3 and 4 (vide supra)
supports spin crossover, since the high spin S = 2 transition
states for 3 and 4 are preferred over the low spin S = 1
transition state by, respectively, 12.6 and 15.1 kcal/mol.
IV
reactivity of Fe O (S = 1) octahedral complexes as
4
8
compared to those ligated trans (axial) as “trans-ligands can
interact with both σ- and π-orbitals involved in the FeO
53
bond but cis-ligands cannot”. However, in the present case,
the effect of replacing equatorial pyridyl donors with (N-
methyl)benzimidazolyl units is quite significant. The (number
and positions of) equatorial donors in octahedral complexes
can modulate the spin state and reactivity of the oxo unit by
EXPERIMENTAL SECTION
■
Materials. The reagents and solvents were purchased from Sigma-
Aldrich and Fisher chemicals. All solvents were of at least 99.5% purity
and used as received. Reagents were of at least 99% purity and used
without any further purification. N-[Di(2-pyridinyl)methy]-N-(2-
altering the energies, and thus the occupancies, of the iron d
xy
2
2
and d
orbitals. The relative λ of the low energy d−d
bands in the series [Fe (O)(N4Py)] , 3, and 4 indicate that
the replacement of equatorial pyridyl moieties by (N-methyl)-
benzimidazolyl units leads to a weaker ligand field but is still
sufficiently strong to support a d /d
complexes that leads to spin pairing to form the S = 1 ground
states for 3 and 4. Still, the weaker ligand field provided by the
N-methyl)benzimidazolyl units should decrease the triplet/
x −y
max
IV
2+
2
9
30
pyridinylmethyl)methylamine, bis(2-pyridyl)methylamine, and 2-
5
4
chloromethyl-1-methylbenzimidazole were prepared according to
literature procedures.
IV
2
2
gap in the Fe O
xy x −y
Physical Methods. UV−vis spectra and all kinetic experiments
were performed on a 8453 UV−vis Agilent Technologies equipped
with a diode-array detector and a Unisoku which permits monitoring
of the temperature of the experiments from −90 to 100 °C. All UV−
vis spectra were measured in 1 cm quartz cell. NMR spectra were
collected on Varian Inova 500 MHz spectrometer in CDCl3 and
(
IV
2+
quintet gap relative to that of [Fe (O)(N4Py)] . This is in
fact what our DFT calculations show with ΔG = 4.0 kcal/
TQ
IV
2+
mol for [Fe (O)(N4Py)] , ΔG = 2.6 kcal/mol for 3, and
CD CN solvents and referenced to the residual signal of the solvent.
3
TQ
Elemental analysis was performed using an ElementarVario EL III
instrument. The mass spectrometry (ESI) was performed with a
Bruker HCT ultra mass spectrometer. The high resolution mass
spectrum (HRMS) was performed using a Bruker FTICR APEX IV
instrument. The electrochemical analyses were performed on a model
CHI760B Electrochemical Workstation (CH Instrument). Tetrabuty-
lammonium hexafluorophosphate [(t-Bu N)HPF ] was used as a
ΔG = 1.7 kcal/mol for 4. The decrease of field strength may
TQ
thus, in principle, facilitate the spin crossover required for two-
state reactivity (vide supra) and can thus explain the increase in
HAT rates that are observed upon substitution of pyridyl
moieties by (N-methyl)benzimidazolyl moieties in the ligand
framework.
4
6
IV
It has previously been found that the activities of Fe O
supporting electrolyte, and the measurements were carried out using 3
mm diameter Teflon-shrouded glassy carbon working electrode, a Pt
wire auxiliary electrode, and an SCE reference electrode. Product
analyses were performed on an Agilent technologies 7820A with 16
sample automatic liquid sampler and flame ionization detector. The
complexes with tetramethylcyclam (TMC) ligands in oxygen-
atom transfer reactions correlate directly with the electro-
IV
12
philicity of the Fe O moiety. One may thus expect
complexes 3 and 4 to be less electrophilic and poorer OAT
IV
2+
products were identified by their GC retention times. Mo
spectra were recorded with a 57Co source in a Rh matrix using an
alternating constant acceleration Wissel Mossbauer spectrometer
̈
ssbauer
reagents than [Fe (O)(N4Py)] ; however, our results show
that the trend is the opposite. The redox properties of 3 and 4
will need to be established in order to establish a quantitative
̈
operated in the transmission mode and equipped with a Janis
closed-cycle helium cryostat. Isomer shifts are given relative to iron
metal at ambient temperature. Simulation of the experimental data was
performed with the Mfit program (E. Bill, Max-Planck Institute for
IV
correlation to the OAT reactivities of the Fe O complexes.
SUMMARY AND CONCLUSIONS
■
=
̈
Chemical Energy Conversion, Mulheim/Ruhr, Germany).
The octahedral ferryl complexes 3 (t = 40 h, RT) and 4 (t1/2
2.5 h, RT) with pentadentate N -donor ligands (cf., Table 5)
that have been prepared by us are significantly more stable than
the previously studied [Fe (O)(Me NTB)] complex, and
1
/2
1
Synthesis of Ligand L [N-(1-Methyl-2-benzimidazolyl)methyl-N-
(2-pyridyl)methyl-N-(bis-2-pyridylmethyl)amine]. A two-necked
round-bottom flask was charged with N-[di(2-pyridinyl)methy]-N-
(2-pyridinylmethyl)methylamine (1.00 g, 3.6 mmol), 2-chloromethyl-
1-methylbenzimidazole (0.651 g, 3.6 mmol), K CO (2.5 g, 18 mmol),
5
IV
2+
3
have thus permitted a detailed study of ferryl complexes with
2
3
(
3
N-methyl)benzimidazolyl donor moieties. More importantly,
and 4 demonstrated excellent hydrogen-atom transfer (HAT)
and oxygen-atom transfer (OAT) activities, surpassing the
parent [Fe (O)(N4Py)] complex by approximately 1−2
orders of magnitude (Tables 5 and 6, Figure 8). For example, in
the oxidation of cyclohexane (which has the strongest C−H
bonds among the substrates used in the present study) the
successive replacement of pyridyl moieties of the N4Py ligand
by one and two (N-methyl)benzimidazolyl moieties led to
and tetrabutylammonium bromide (0.23 g, 0.7 mmol). A total of 50
mL of dry CH CN was added under a vigorous flow of nitrogen. The
3
mixture was refluxed for 24 h under nitrogen, and then the reaction
IV
2+
mixture was filtered through a Celite pad. A small amount of CH Cl
2 2
was added to wash the Celite pad. The resulting filtrate was
evaporated, and the residue was dissolved in a 1 (M) NaOH solution
and extracted with CH Cl . The organic portion was washed with
2
2
brine solution and dried over Na SO . Evaporation of the organic
2
4
1
solvent gave the crude ligand L as a red oil. This crude product was
purified by passing through a silica column using a mixture of
J
DOI: 10.1021/ic5029564
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
CH Cl :CH OH:Et N (10:2:1) as eluent. Yield: 1.01 g (67%). ESI-
bath and stored overnight. The precipitate was collected by filtration,
washed with ethyl acetate, dried under vacuum, and obtained as red
2
2
3
3
+
1
MS: 421.2 [M + H] . H NMR (500 MHz, CDCl ) δ (ppm): 8.6−
3
8
.55 [m, 3H, Py-H], 7.76 [d, 2H, J = 8 Hz, BzIm-H], 7.69−7.6 [m, 3H,
solid. Yield: 101 mg (78%). ESI-MS (in CH CN): m/z 264.6
3
II
2
2+
II
2
+
Py-H], 7.5 [m, 3H, Py-H], 7.2−7.15 [m, 5H, BzIm-H, Py-H], 5.33 [s,
[Fe (L )] (z = 2) calcd 264.6, 678.1 [Fe (L )(CF SO )] (z = 1)
3
3
1
3
1
1
5
H, NCH], 4.18 [s, 2H, NCH Py], 3.98 [s, 2H, NCH BzIm], 3.75 [s,
calcd 678.1. Anal. Calcd (%): C 45.63, H 3.48, N 12.9. Found (%): C
2
2
H, NCH3]. 13C NMR (125 MHz, CDCl ) δ (ppm): 159.58 (s),
46.05, H 3.27, N 12.53. H NMR (500 MHz, CD CN, 298 K) δ
1
3
3
59.21 (s), 152.0 (s), 149.38 (s), 148.98 (s), 136.52 (s), 136.1 (s),
24.65 (s), 123.72 (s), 122.39 (d), 121.885 (d), 119.580 (s), 72.14 (s),
7.63 (s), 49.36 (s), 30.25 (s).
(ppm): 18.55, 16.80, 12.40, 8.44, 5.62, 5.04, 3.65, 3.34.
Crystal Structure Determinations. The crystal of 1·(ClO ) and
4
2
2·(ClO ) were immersed in cryo-oil, mounted in a Nylon loop, and
4
2
2
Synthesis of Ligand L [N-Bis(1-methyl-2-benzimidazolyl)methyl-
measured at a temperature of 100 K. The X-ray diffraction data were
N-(bis-2-pyridylmethyl)amine]. A total of 0.403 g of 2-chloromethyl-
collected on a Bruker Kappa Apex II and Bruker Kappa Apex II Duo
55
1-methylbenzimidazole (2.23 mmol) was dissolved in 2 mL of a 5 M
diffractometers using Mo Kα radiation (λ = 0.710 73 Å). The APEX2
NaOH solution. After about 10 min of stirring, bis(2-pyridyl)-
methylamine (0.206 g, 1.115 mmol) in 2 mL of 5 M NaOH was
added. The stirring was continued for 3 days at room temperature.
After stirring was finished, the sticky solid that was formed was
program package was used for cell refinements and data reductions.
The structure was solved by charge flipping technique (SUPER-
5
6
57
FLIP) or direct methods using the SIR2011 program with the
5
8
Olex2 graphical user interface. A semiempirical numerical absorption
5
9
collected, and HPF was added dropwise to precipitate a brick-red
correction based on equivalent reflections (SADABS) was applied to
6
60
solid. The resultant solid was dissolved in hot water. After
recrystallization from hot water, 5 M NaOH was added to basify the
reaction mixture (pH > 12). The product was extracted with
dichloromethane and dried over Na SO , and the dichloromethane
all data. Structural refinements were carried out using SHELXL-97.
The crystal of 1·(ClO ) was diffracting only weakly, and therefore,
4
2
atoms N2, C16, C17, C18, C19, and C20 were restrained to have the
2
4
same U components within the standard uncertainty of 0.02. In 2·
ij
2
extract was evaporated to give the desired ligand L as a pale brownish
solid. Yield: 0.749 g (71%). ESI-MS: 474.2 [M + H] . H NMR (500
(ClO4)2 one molecule of the acetonitrile of crystallization was
disordered over two sites with equal occupancies. Hydrogen atoms
were positioned geometrically and were also constrained to ride on
their parent atoms, with C−H = 0.95−0.1.00 Å, and U = 1.2−1.5U
+
1
MHz, CDCl ) δ (ppm): 8.635 [d, 2H, J = 8 Hz, Py-H], 7.714−7.645
3
[
m, 6H, Py-H], 7.62 [dt, 2H, BzIm-H], 7.19 [m, 4H, BzIm-H, Py-H],
iso
eq
5
.402 [s, 1H, NCH], 4.285 [s, 4H, NCH BzIm], 3.619 [s, 6H, NCH ].
(parent atom). The crystallographic details are summarized in Table
S1, Supporting Information.
2
3
1
3
C NMR (125 MHz, CDCl ) δ (ppm): 158.8 (s), 151.9 (s), 149.15
3
(
(
s), 136.47 (s), 136.05 (s), 124.97 (s), 122.8 (s), 122.39 (d), 119.41
s), 109.1 (s), 48.31 (s), 29.8 (s).
Hydrogen-Atom Transfer (HAT) Reactions. Fe(IV) oxo
solutions (in the concentration range 0.5−1.0 mM) in CH CN were
3
II
1
Synthesis of [Fe (CH CN)(L )](ClO ) (1·(ClO ) ). A total of 100 mg
prepared by using excess solid PhIO to optimize yield; after filtration
of unreacted PhIO, the solutions represent a reaction system without
any methanol or per-acid contaminants. On deaeration of the solutions
and a temperature equilibration at 25 °C in the UV−vis cuvette,
substrates were added to the stirred solutions. The concentrations of
substrates used ranged from 25 to 800 mM and were adjusted to
achieve convenient times for the reduction of Fe(IV) oxo species. The
time course decay of the Fe(IV) oxo was then monitored at 25 °C by
the UV−vis spectrophotometer. Time courses were subjected to
pseudo-first-order fit, and second-order rate constants were evaluated
from the concentration dependence data.
3
4 2
4 2
1
(
0.23 mmol) of ligand L was taken in a vial and dissolved in a
minimum amount of CH CN. To this solution was added 58.6 mg
(
3
0.23 mmol) of Fe(ClO4)2 in CH CN under stirring at room
3
temperature under air. A red precipitate appeared within 5 min of
stirring. After stirring for about 30 min, the reaction mixture was
placed into an ethyl acetate bath and stored overnight. The precipitate
was collected by filtration, washed with ethyl acetate, dried under
vacuum, and obtained as a red solid. Yield: 147 mg (82%). ESI-MS (in
II
1
2+
II
1
CH CN): m/z 238 [Fe (L )] (z = 2) calcd 238, 258.5 [Fe (L )-
3
2+
II
1
+
(
CH CN)] (z = 2) calcd 258.5, 575.1 [Fe (L )(ClO )] (z = 1)
3
4
II
1
+
calcd 575.1, 616.1 [Fe (L )(CH CN)(ClO )] (z = 1) calcd 616.1.
To isolate the organic products, the solutions after the end of the
reaction were passed through a silica column, using ethyl acetate as the
eluent, in order to remove the metal complex. The ethyl acetate
solutions were then analyzed by GC using a known strength of
biphenyl solution as the quantification standard. All the data obtained
from these studies were collected in Supporting Information Table S2.
Oxygen-Atom Transfer (OAT) Reactions. The Fe(IV) oxo
solutions were prepared as described before. The solutions were
placed in cuvette, and the temperature of the UV−vis instrument was
monitored to −30 °C. Then, appropriate amounts of thioanisole
substrate were added to the Fe(IV) oxo solution, and the subsequent
decay was monitored. Time courses were subjected to pseudo-first-
order fit and second-order rate constants were evaluated from the
concentration dependence data.
3
4
Anal. Calcd (%): C 47.58, H 3.99, N 14.8. Found (%): C 46.97, H
3
1
.98, N 14.93. H NMR (500 MHz, CD CN, 298 K) δ (ppm): 9.42 [s,
3
1
H, Py-H], 9.165 [d, 2H, J = 25 Hz, Py-H], 8.015 [d, 1H, J = 5 Hz,
BzIm-H], 7.94−7.88 [m, 2H, BzIm-H], 7.61 [t, 1H, J = 15 Hz, Py-H],
7
1
7
.53−7.38 [m, 6H, Py-H], 7.27 [d, 1H, J = 10 Hz, BzIm-H], 6.77 [s,
H, NCH], 4.795 [d, 2H, J = 15 Hz, NCH Py], 4.57 [dd, 2H, J = 27.5,
2
.5 Hz, NCH BzIm], 3.61 [s, 3H, NCH ].
2
3
II
2
Synthesis of [Fe (CH CN)(L )](ClO ) (2·(ClO ) ). The procedure for
3
4 2
4 2
the synthesis of complex 2 is identical to that for 1. A total of 109 mg
2
(
0.23 mmol) of ligand L was taken in a vial and dissolved in a
minimum amount of acetonitrile. To this solution, 58.6 mg (0.23
mmol) of Fe(ClO ) in acetonitrile was added under stirring at room
4
2
temperature under air. A red precipitate appeared within 5 min of
stirring. After about 30 min of stirring, the reaction mixture was placed
into an ethyl acetate bath and stored overnight. The precipitate was
collected by filtration, washed with ethyl acetate, dried under vacuum,
and obtained as red solid. Yield: 136 mg (77%). ESI-MS (in CH CN):
m/z 264.6 [Fe (L )] (z = 2) calcd 264.6, 628.1 [Fe (L )(ClO )] (z
The products were quantified following the procedure already
described. The chirality of the sulfoxide product was not determined.
Computational Details and Modeling. All DFT calculations
61
were carried out with the Gaussian 09 package of programs using the
3
II
2
2+
II
2
+
B3LYP hybrid functional. This functional is composed of Becke’s
4
6
2
=
(
1) calcd 628.1. Anal. Calcd (%): C 49.14, H 4.18, N 16.01. Found
three-parameter hybrid exchange functional (B3) and the correlation
1
63
%): C 48.99, H 4.17, N 16.77. H NMR (500 MHz, CD CN, 298 K)
functional of Lee, Yang, and Parr (LYP). The iron atom was
3
δ (ppm): 18.7 [br, 2H, Py-H], 16.74 [br, 1H, NCH], 12.47 [s, 4H,
described with the Stuttgart−Dresden effective core potential and
6
4
65
NCH BzIm], 8.48 [s, 6H, NCH ], 5.62 [s, 2H, Py-H], 5.02 [s, 2H, Py-
SDD basis set, and the 6-31G(d′) basis set was employed for all
2
3
H], 3.6 [br, 2H, BzIm-H], 3.3 [s, 2H, BzIm].
remaining atoms.
II
2
Synthesis of [Fe (CH CN)(L )](CF SO ) (2·(CF SO ) ). A total of 71
All reported geometries were fully optimized, and analytical second
derivatives were evaluated at each stationary point to determine
whether the geometry was an energy minimum (no negative
eigenvalues) or a transition structure (one negative eigenvalue).
Unscaled vibrational frequencies were used to make zero-point and
thermal corrections to the electronic energies. The resulting potential
3
3
3 2
3
3 2
2
mg (0.15 mmol) of ligand L was taken in a vial and dissolved in a
minimum amount of acetonitrile. To this solution, 65.4 mg (0.15
mmol) of [Fe(CH CN) (CF SO ) ] in acetonitrile was added under
3
2
3
3 2
stirring at room temperature under nitrogen atmosphere. After about
0 min of stirring, the reaction mixture was placed into an ethyl acetate
3
K
DOI: 10.1021/ic5029564
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
energies and enthalpies are reported in kcal/mol relative to the
specified standard. Standard state corrections were applied to all
species to convert concentrations from 1 atm to 1 M according to the
(7) Eser, B. E.; Barr, E. W.; Frantom, P. A.; Saleh, L.; Bollinger, J. M.,
Jr.; Krebs, C.; Fitzpatrick, P. F. J. Am. Chem. Soc. 2007, 129, 11334−
11335.
(8) Matthews, M. L.; Krest, C. M.; Barr, E. W.; Vaillancourt, F. H.;
Walsh, C. T.; Green, M. T.; Krebs, C.; Bollinger, J. M., Jr. Biochemistry
6
6
treatise of Cramer. Internal reaction coordinate (IRC) calculations
3
3
3
5
5
5
were performed on TS AB CD and TS AB CD in order to establish
the reactant and product species associated with these transition-state
structures. The geometry-optimized structures have been drawn with
2
(
009, 48, 4331−4343.
9) (a) Krebs, C.; Fujimori, D. G.; Walsh, C. T.; Bollinger, J. M., Jr.
6
7
the JIMP2 molecular visualization and manipulation program.
Acc. Chem. Res. 2007, 40, 484−492. (b) Proshlyakov, D. A.; Henshaw,
T. F.; Monterosso, G. R.; Ryle, M. J.; Hausinger, R. P. J. Am. Chem.
Soc. 2004, 126, 1022−1023.
ASSOCIATED CONTENT
■
(
10) (a) Nam, W. Acc. Chem. Res. 2007, 40, 522−531. (b) Que, L., Jr.
*
S
Supporting Information
Acc. Chem. Res. 2007, 40, 493−500. (c) Lyakin, Y. O.; Shteinman, A. A.
Kinet. Catal. 2012, 53, 694−713.
(11) Hohenberger, J.; Ray, K.; Meyer, K. Nat. Commun. 2012, 3,
720−733.
1
ESI-MS, H NMR, and FT-IR spectra of complexes 1·(ClO )
4
2
and 2·(ClO ) ; the UV−vis spectra of the ligands; the cyclic
4
2
voltametric diagrams of complexes 1·(ClO ) and 2·(ClO ) ;
4
2
4 2
the kinetic absorbance spectra of formation of complexes 3 and
; HRMS of complexes 3 and 4; detailed product analyses;
figures of the optimized ground-state and transition-state
(12) Sastri, C. V.; Lee, J.; Oh, K.; Lee, Y. J.; Lee, J.; Jackson, T. A.;
Ray, K.; Hirao, H.; Shin, W.; Halfen, J. A.; Kim, J.; Que, L., Jr.; Shaik,
S.; Nam, W. Proc. Natl. Acad. Sci. U. S. A. 2007, 104, 19181−19186.
4
(
13) Kaizer, J.; Klinker, E. J.; Oh, N. Y.; Rohde, J. U.; Song, W. J.;
structures for the toluene and methane oxidation cycles
3
Stubna, A.; Kim, J.; Munck, E.; Nam, W.; Que, L., Jr. J. Am. Chem. Soc.
004, 126, 472−473.
14) (a) Hirao, H.; Que, L., Jr.; Nam, W.; Shaik, S. Chem. - Eur. J.
008, 14, 1740−1756. (b) Hirao, H.; Kumar, D.; Que, L., Jr.; Shaik, S.
involving E; along with tables of atomic coordinates and
2
(
2
electronic energies for all optimized structures. Crystallographic
details in CIF format. The Supporting Information is available
free of charge on the ACS Publications website at DOI:
J. Am. Chem. Soc. 2006, 128, 8590−8606. (c) Cho, K. B.; Shaik, S.;
Nam, W. Chem. Commun. 2010, 46, 4511−4513. (d) Kumar, D.;
Hirao, H.; Que, L., Jr.; Shaik, S. J. Am. Chem. Soc. 2005, 127, 8026−
8027. (e) Hirao, H.; Kumar, D.; Thiel, W.; Shaik, S. J. Am. Chem. Soc.
10.1021/ic5029564.
AUTHOR INFORMATION
■
*
2
005, 127, 13007−13018. (f) Bernasconi, L.; Louwerse, M. J.;
Baerends, E. J. Eur. J. Inorg. Chem. 2006, 2007, 3023−3033.
g) Janardanan, D.; Wang, Y.; Schyman, P.; Que, L., Jr.; Shaik, S.
Corresponding Author
E-mail: Ebbe.Nordlander@chemphys.lu.se.
(
Notes
Angew. Chem., Int. Ed. 2010, 49, 3342−3345. (h) Chen, H.; Lai, W.;
Shaik, S. J. Phys. Chem. Lett. 2010, 1, 1533−1540. (i) Geng, C.; Ye, S.;
Neese, F. Angew. Chem., Int. Ed. 2010, 49, 5717−5720. (j) Ye, S.;
Neese, F. Curr. Opin. Chem. Biol. 2009, 13, 89−9. (k) Godfrey, E.;
Porro, C. S.; de Visser, S. P. J. Phys. Chem. A 2008, 112, 2464−2468.
(l) de Visser, S. P.; Oh, K.; Han, A. R.; Nam, W. Inorg. Chem. 2007, 46,
4632−4641. (m) de Visser, S. P.; Latifi, R.; Tahsini, L.; Nam, W.
Chem. - Asian J. 2011, 6, 493−504.
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This research has been carried out within the framework of the
International Research Training Group Metal Sites in
Biomolecules: Structures, Regulation and Mechanisms (www.
biometals.eu) and has also been supported by COST Action
CM1003. M.M. thanks the European Union for an Erasmus
Mundus fellowship. M.G.R. thanks the Robert A. Welch
Foundation (Grant B-1093) and the Wenner−Gren Founda-
tion for financial support and acknowledges computational
resources through UNT’s High Performance Computing
Services funded by NSF (CHE-0741936).
(
15) Seo, M. S.; Kim, N. H.; Cho, K. B.; So, J. E.; Park, S. K.;
Clemancey, M.; Garcia-Serres, R.; Latour, J. M.; Shaik, S.; Nam, W.
Chem. Sci. 2011, 2, 1039−1045.
(16) England, J.; Martinho, M.; Farquhar, E. R.; Frisch, J. R.;
Bominaar, E. L.; Munck, E.; Que, L., Jr. Angew. Chem., Int. Ed. 2009,
8, 3622−3626.
17) Macbeth, C. E.; Golombek, A. P.; Young, V. G., Jr.; Yang, C.;
4
(
Kuczera, K.; Hendrich, M. P.; Borovik, A. S. Science 2000, 289, 938−
9
41.
REFERENCES
(18) Wang, D.; Ray, K.; Collins, M. J.; Farquhar, E. R.; Frisch, J. R.;
Gomez, L.; Jackson, T. A.; Kerscher, M.; Waleska, A.; Comba, P.;
■
(
1) (a) Costas, M.; Mehn, M. P.; Jensen, M. P.; Que, L., Jr. Chem.
Costas, M.; Que, L., Jr. Chem. Sci. 2013, 4, 282−291.
Rev. 2004, 104, 939−986. (b) Abu-Omar, M. M.; Loaiza, A.; Hontzeas,
N. Chem. Rev. 2005, 105, 2227−2252. (c) Shan, X. P.; Que, L., Jr. J.
Inorg. Biochem. 2006, 100, 421−433. (d) Kryatov, S. V.; Rybak-
Akimova, E. V.; Schindler, S. Chem. Rev. 2005, 105, 2175−2226.
(19) Bigi, J. P.; Harman, W. H.; Lassalle-Kaiser, B.; Robles, D. M.;
Stich, T. A.; Yano, J.; Britt, R. D.; Chang, C. J. J. Am. Chem. Soc. 2012,
34, 1536−1542.
20) England, J.; Guo, Y.; Van Heuvelen, K. M.; Cranswick, M. A.;
Rohde, G. T.; Bominaar, E. L.; Munck, E.; Que, L., Jr. J. Am. Chem. Soc.
011, 133, 11880−11883.
21) Biswas, A. N.; Puri, M.; Meier, K. K.; Oloo, W. N.; Rohde, G. T.;
Bominaar, E. L.; Munck, E.; Que, L., Jr. J. Am. Chem. Soc. 2015, 137,
428−2431.
22) Lubben, M.; Meetsma, A.; Wilkinson, E. C.; Feringa, B. L.; Que,
L., Jr. Angew. Chem., Int. Ed. Engl. 1995, 34, 1512−1514.
23) Klinker, E. J.; Kaizer, J.; Brennessel, W. W.; Woodrum, N. L.;
1
(
(
e) Bruijnincx, P. C. A.; van Koten, G.; Klein Gebbink, R. J. M. Chem.
Soc. Rev. 2008, 37, 2716−2744. (f) Meunier, B.; de Visser, S. P.; Shaik,
2
(
S. Chem. Rev. 2004, 104, 3947−3980.
(
4
(
1
2) McDonald, A. R.; Que, L., Jr. Coord. Chem. Rev. 2013, 257, 414−
28.
3) (a) Kovaleva, E. G.; Lipscomb, J. D. Nat. Chem. Biol. 2008, 4,
86−193. (b) Groves, J. T. J. Inorg. Biochem. 2006, 100, 434−447.
4) (a) Bollinger, J. M., Jr.; Price, J. C.; Hoffart, L. M.; Barr, E. W.;
2
(
(
(
Krebs, C. Eur. J. Inorg. Chem. 2005, 2005, 4245−4254. (b) Price, J. C.;
Barr, E. W.; Tirupati, B.; Bollinger, J. M., Jr.; Krebs, C. Biochemistry
Cramer, C. J.; Que, L., Jr. Angew. Chem., Int. Ed. 2005, 44, 3690−3694.
(24) (a) McQuilken, A. C.; Jiang, Y.; Siegler, M. A.; Goldberg, D. P. J.
Am. Chem. Soc. 2012, 134, 8758−8761. (b) Sahu, S.; Widger, L. R.;
Quesne, M. G.; de Visser, S. P.; Matsumura, H.; Moenne-Loccoz, P.;
Siegler, M. A.; Goldberg, D. P. J. Am. Chem. Soc. 2013, 135, 10590−
10593. (c) Widger, L. R.; Davies, C. G.; Yang, T.; Siegler, M. A.;
2
(
003, 42, 7497−7508.
5) Hoffart, L. M.; Barr, E. W.; Guyer, R. B.; Bollinger, J. M., Jr.;
Krebs, C. Proc. Natl. Acad. Sci. U. S. A. 2006, 103, 14738−14743.
6) Galonic, D. P.; Barr, E. W.; Walsh, C. T.; Bollinger, J. M., Jr.;
Krebs, C. Nat. Chem. Biol. 2007, 3, 113−116.
(
L
DOI: 10.1021/ic5029564
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
Troeppner, O.; Jameson, G. N. L.; Ivanovic-Burmazovic, I.; Goldberg,
by Shaik et al. on related systems. (b) Usharani, D.; Janardanan, D.; Li,
C.; Shaik, S. Acc. Chem. Res. 2013, 46, 471−482.
D. P. J. Am. Chem. Soc. 2014, 136, 2699−2702.
(
25) Draksharapu, A.; Li, Q.; Logtenberg, H.; van den Berg, T. A.;
(52) (a) Shaik, S.; Lai, W.; Chen, H.; Wang, Y. Acc. Chem. Res. 2010,
43, 1154−1165. (b) Ye, S.; Geng, C. Y.; Shaik, S.; Neese, F. Phys.
Chem. Chem. Phys. 2013, 15, 8017−8030.
Meetsma, A.; Killen, J. S.; Feringa, B. L.; Hage, R.; Roelfes, G.;
Browne, W. R. Inorg. Chem. 2012, 51, 900−913.
(
26) Wang, B.; Wang, S.; Xia, C.; Sun, W. Chem. - Eur. J. 2012, 18,
(53) Zhou, Y.; Shan, X.; Mas-Balleste, R.; Bukowski, M. R.; Stubna,
A.; Chakrabarti, M.; Slominski, L.; Halfen, J. A.; Munck, E.; Que, L., Jr.
Angew. Chem., Int. Ed. 2008, 47, 1896−1899.
(54) Amrutkar, S. V.; Bhagat, U. D.; Pargharmol, P.; Kotgire, S. S.;
Ranawat, M. S. Int. J. Pharm. Pharm. Sci. 2010, 2, 84−92.
(55) APEX2Software Suite for Crystallographic Programs; Bruker
AXS, Inc.: Madison, WI, 2009.
7
(
332−7335.
27) Mitra, M.; Lloret-Fillol, J.; Haukka, M.; Costas, M.; Nordlander,
E. Chem. Commun. 2014, 50, 1408−1410.
28) (a) Goodgame, M.; Cotton, F. A. J. Am. Chem. Soc. 1992, 84,
(
1
2
543−1548. (b) Johnson, C. R.; Shepherd, R. E. Inorg. Chem. 1983,
2, 3506−3513. (c) Goodgame, D. M. L.; Goodgame, M.; Weeks, M.
J. J. Chem. Soc. 1964, 5194−5199.
(
(56) Sheldrick, G. M. Acta Crystallogr., Sect. A: Found. Crystallogr.
2008, A64, 112−122.
29) Roelfes, G.; Branum, M. E.; Wang, L.; Que, L., Jr.; Feringa, B. L.
J. Am. Chem. Soc. 2000, 122, 11517−11518.
(57) Farrugia, L. J. J. Appl. Crystallogr. 1999, 32, 837−837.
(58) Spek, A. L. J. Appl. Crystallogr. 2003, 36, 7−13.
(59) Sheldrick, G. M. SADABS, Bruker AXS Scaling and Absorption
Correction; Bruker AXS, Inc.: Madison, WI, 2008.
(
30) Niemers, E.; Hiltmann, R. Synthesis 1976, 1976, 593−595.
(
31) Roelfes, G.; Lubben, M.; Chen, K.; Ho, R. Y. N.; Meetsma, A.;
Genseberger, S.; Hermant, R. M.; Hage, R.; Mandal, S. K.; Young, V.
G., Jr.; Zang, Y.; Kooijman, H.; Spek, A. L.; Que, L., Jr.; Feringa, B. L.
Inorg. Chem. 1999, 38, 1929−1936.
(
60) Sheldrick, G. M. SHELXTL, Bruker Analytical X-ray Systems;
Bruker AXS, Inc.: Madison, WI, 2005.
61) Frisch, M. J.; et al. Gaussian 09, Revision E.01; Gaussian, Inc.:
Wallingford, CT, 2009.
(
(
32) Patra, A. K.; Olmstead, M. M.; Mascharak, P. K. Inorg. Chem.
2
(
002, 41, 5403−5409.
33) Wong, E.; Jeck, J.; Grau, M.; White, A. J. P.; Britovsek, G. J. P.
Catal. Sci. Technol. 2013, 3, 1116−1122.
34) (a) Decker, A.; Rohde, J.-U.; Que, L., Jr.; Solomon, E. J. Am.
(
(
62) Becke, A. D. J. Chem. Phys. 1993, 98, 5648−5652.
63) Lee, C.; Yang, W.; Parr, R. G. Phys. Rev. B: Condens. Matter
Mater. Phys. 1988, 37, 785−789.
(
(64) (a) Dolg, M.; Wedig, U.; Stoll, H.; Preuss, H. J. Chem. Phys.
Chem. Soc. 2004, 126, 5378−5379. (b) Decker, A.; Rohde, J.-U.;
Klinker, E. J.; Wong, S. D.; Que, L., Jr.; Solomon, E. I. J. Am. Chem.
Soc. 2007, 129, 15983−15996.
1
1
(
987, 86, 866. (b) Walch, S. P.; Bauschlicher, C. W. J. Chem. Phys.
983, 78, 4597−4605.
65) (a) Petersson, G. A.; Bennett, A.; Tensfeldt, T. G.; Al-Laham,
(
35) Assuming an idealized C4 symmetry, the band may be
v
M. A.; Shirley, W. A.; Mantzaris, J. J. Chem. Phys. 1988, 89, 2193−
218. (b) Petersson, G. A.; Al-Laham, M. A. J. Chem. Phys. 1991, 94,
6081−6090.
66) Cramer, C. J. Essentials of Computational Chemistry, 2nd ed.;
Wiley: Chichester, U.K., 2004.
67) JIMP2, version 0.091, a free program for the visualization and
manipulation of molecules: (a) Hall, M. B.; Fenske, R. F. Inorg. Chem.
attributed to a d−d transition(s) where the excited state is localized in
2
the x-y plane and is related to the equatorial ligand field. This has been
IV
2+
shown for the complex [Fe (O)(TMC)(NCMe)] , which has a
closely related optical spectrum (see ref 34b).
(
(
36) McDonald, A. R.; Guo, Y.; Vu, V. V.; Bominaar, E. L.; Munck,
E.; Que, L., Jr. Chem. Sci. 2012, 3, 1680−1693.
37) Thibon, A.; England, J.; Martinho, M.; Young, V. G.; Frisch, J.
(
(
1
972, 11, 768−775. (b) Manson, J.; Webster, C. E.; Hall, M. B. Texas
R.; Guillot, R.; Girerd, J.-J.; Munck, E.; Que, L., Jr.; Banse, F. Angew.
Chem., Int. Ed. 2008, 47, 7064−7067.
A&M University, College Station, TX, 2006; http://www.chem.tamu.
edu/jimp2/index.html.
(
38) England, J.; Bigelow, J. O.; Van Heuvelen, K. M.; Farquhar, E.
R.; Martinho, M.; Meier, K. K.; Frisch, J. R.; Munck, E.; Que, L., Jr.
Chem. Sci. 2014, 5, 1204−1215.
(
39) Bukowski, M. R.; Comba, P.; Lienke, A.; Limberg, C.; Laorden,
C. L. D.; Mas-Balleste, R.; Murz, M.; Que, L., Jr. Angew. Chem., Int. Ed.
006, 45, 3446−3449.
40) Company, A.; Sabenya, G.; Gonzalez-Bejar, M.; Gomez, L.;
2
(
Clemancey, M.; Blondin, G.; Jasniewski, A. J.; Puri, M.; Browne, W. R.;
Latour, J. M.; Que, L., Jr.; Costas, M.; Perez-Prieto, J.; Lloret-Fillol, J. J.
Am. Chem. Soc. 2014, 136, 4624−4633.
(
41) Klinker, E. J.; Shaik, S.; Hirao, H.; Que, L., Jr. Angew. Chem., Int.
Ed. 2009, 48, 1291−1295.
(
(
(
(
42) Borovik, A. S. Chem. Soc. Rev. 2011, 40, 1870−1874.
43) Bell, R. P. Proc. R. Soc. London, Ser. A 1936, 154, 414−429.
44) Evans, M. G.; Polanyi, M. Trans. Faraday Soc. 1938, 34, 11−24.
45) (a) Mayer, J. M. Acc. Chem. Res. 1998, 31, 441−450. (b) Warren,
J. J.; Tronic, T. A.; Mayer, J. M. Chem. Rev. 2010, 110, 6961−7001.
(
c) Mayer, J. M. Acc. Chem. Res. 2011, 44, 36−46.
(46) Shaik, S.; Chen, H.; Janardanan, D. Nat. Chem. 2011, 3, 19−27.
(47) Wong, S. D.; Bell, C. B., III; Liu, L. V.; Kwak, Y.; England, J.;
Alp, E. E.; Zhao, J.; Que, L., Jr.; Solomon, E. I. Angew. Chem., Int. Ed.
011, 50, 3215−3218.
48) Janardanan, D.; Usharani, D.; Chen, H.; Shaik, S. J. Phys. Chem.
Lett. 2011, 2, 2610−2617.
49) Usharani, D.; Janardanan, D.; Shaik, S. J. Am. Chem. Soc. 2011,
33, 176−179.
50) The DFT-optimized structures for all species on the triplet π
and quintet σ surfaces have been deposited as Supporting Information.
51) (a) We have also probed the activation of methane via a quintet
π manifold and find this H-abstraction to lie ca. 7 kcal/mol higher in
2
(
(
1
(
(
3
3
3
energy than TS AB CD in excellent agreement with the recent review
M
DOI: 10.1021/ic5029564
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