Effect of Ligand Fields on the Reactivity of O2-Activating Iron(II)-Benzilate Complexes of Neutral N5 Donor Ligands

Article abstract of DOI:10.1002/asia.202000142

Three new iron(II)-benzilate complexes [(N4Py)Fe^II(benzilate)]ClO₄ (1), [(N4Py^Me2)Fe^II(benzilate)]ClO₄ (2) and [(N4Py^Me4)Fe^II(benzilate)]ClO₄ (3) of neutral pentadentate nitrogen donor ligands have been isolated and characterized to study their dioxygen reactivity. Single-crystal X-ray structures reveal a mononuclear six-coordinate iron(II) center in each case, where benzilate binds to the iron center in monodentate mode via one carboxylate oxygen. Introduction of methyl groups in the 6-positions of the pyridine rings makes the N4Py^Me2 and N4Py^Me4 ligand fields weaker compared to that of the parent N4Py ligand. All the complexes (1–3) react with dioxygen to decarboxylate the coordinated benzilate to benzophenone quantitatively. The decarboxylation is faster for the complex of the more sterically hindered ligand and follows the order 3>2>1. The complexes display oxygen atom transfer reactivity to thioanisole and also exhibit hydrogen atom transfer reactions with substrates containing weak C?H bonds. Based on interception studies with external substrates, labelling experiments and Hammett analysis, a nucleophilic iron(II)-hydroperoxo species is proposed to form upon two-electron reductive activation of dioxygen by each iron(II)-benzilate complex. The nucleophilic oxidants are converted to the corresponding electrophilic iron(IV)-oxo oxidant upon treatment with a protic acid. The high-spin iron(II)-benzilate complex with the weakest ligand field results in the formation of a more reactive iron-oxygen oxidant.

Full text of DOI:10.1002/asia.202000142

CHEMISTRY
AN ASIAN JOURNAL
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Accepted Article
Title: Effect of Ligand Fields on the Reactivity of O2-Activating Iron(II)-
Benzilate Complexes of Neutral N5 Donor Ligands
Authors: Shrabanti Bhattacharya, Reena Singh, and Tapan Kanti
Paine
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Effect of Ligand Fields on the Reactivity of O
2
-Activating Iron(II)-
Benzilate Complexes of Neutral N5 Donor Ligands
a
,a
,a
Shrabanti Bhattacharya, Reena Singh* and Tapan Kanti Paine*
Dedicated to Professor Goutam Kumar Lahiri on his 60th Birthday
[
a]
Dr. S. Bhattacharya, Dr. R. Singh and Prof. T. K. Paine
School of Chemical Sciences, Indian Association for the Cultivation of Science, 2A&2B Raja S. C. Mullick Road, Jadavpur, Kolkata-700032, India.
E-mail: ictkp@iacs.res.in
Supporting information for this article is given via a link at the end of the document.
Abstract:
Three
new
(1), [(N4Py )Fe (benzilate)]ClO
and [(N4Py )Fe (benzilate)]ClO (3) of neutral pentadentate
iron(II)-benzilate
complexes
spin iron(IV)-oxoporphrin radical cation,[5] most of the nonheme
oxygenases employ high-valent iron-oxo species in high-spin
II
Me2
II
[
(N4Py)Fe (benzilate)]ClO
4
4
(2)
Me4
II
[6]
4
states. A high-spin iron(IV)-oxo intermediate has been trapped
nitrogen donor ligands have been isolated and characterized to
study their dioxygen reactivity. Single crystal X-ray structures reveal
a mononuclear six-coordinate iron(II) centre in each case, where
benzilate binds to the iron center in monodentate mode via one
carboxylate oxygen. Introduction of methyl groups in the 6-positions
and characterized in the catalytic cycle of taurine dioxygenase
(TauD).[7] Subsequently, similar intermediates have been
observed in other nonheme enzymes such as prolyl hydroxylase,
tyrosine hydroxylases and in halogenases.[
6,
8]
An iron(III)-
hydroperoxide intermediate has been crystallographically
characterized in the cis-dihydroxylation of aromatic compounds
catalyzed by Rieske dioxygenase.[9]
Me2
Me4
of the pyridine rings makes the N4Py
and N4Py ligands weaker
field compared to the parent N4Py ligand. All the complexes (1-3)
react with dioxygen to decarboxylate the coordinated benzilate to
benzophenone quantitatively. The decarboxylation takes lesser time
for the complex of more sterically hindered ligand and follows the
order 3>2>1. The complexes display oxygen atom transfer reactivity
to thioanisole and also exhibit hydrogen atom transfer reactions with
substrates containing weak C-H bonds. Based on interception
studies with external substrates, labelling experiment and Hammett
analysis, a nucleophilic iron(II)-hydroperoxo species is proposed to
form upon two-electron reductive activation of dioxygen by each
iron(II)-benzilate complex. The nucleophilic oxidants are converted
to the corresponding electrophilic iron(IV)-oxo oxidant upon
treatment with a protic acid. The high-spin iron(II)-benzilate complex
with the weakest ligand field results in the formation of a more
reactive iron-oxygen oxidant.
Functional modeling of enzymatic reactions provide useful
mechanistic information about the catalytic cycle. As in
enzymatic systems, synthetic high-spin iron(II) complexes are
more reactive toward dioxygen than the corresponding low-spin
complexes. A number of low-spin iron(II) complexes of nitrogen
rich polydentate ligands, which are otherwise unreactive, have
been shown to activate dioxygen in the presence of electron and
proton source by tuning the ligand field through use of
[
10]
appropriate solvent. Coordination of solvent molecules to iron
center regulates the iron(II)/iron(III) redox potential facilitating
2
the activation of O . Spin states of the iron centre have been
demonstrated to modulate the strength of Fe–O and O–O bonds,
and their cleavage in nonheme Fe-OOR/H species.[ The spin-
state effect is also manifested in the hydrogen atom transfer
11]
(
HAT) and oxygen atom transfer (OAT) reactivity of synthetic
iron(IV)-oxo complexes.[12] It has been demonstrated that
increasing the population in high-spin state increases the
reactivity.[13] This can be achieved by decreasing the energy gap
Introduction
between the low-spin and high-spin state through decrease in
ligand field strength. In synthetic complexes, polydentate
nitrogen ligands are extensively used and their ligand fields are
tunable by appropriate substitution on the ligand backbone.
Methyl substitution on one of the pyridine rings of tris(2-
Dioxygen activating mononuclear nonheme iron enzymes
perform a wide range of bio-transformation reactions.[1] Parallel
to their versatile reactivity, the active sites of these enzymes
exhibit a rich structural diversity.[2] In the dioxygen activation
process by these enzymes, additional reducing equivalents are
supplied either by cofactors or by substrates. In some cases,
electrons are supplied by electron transfer proteins.[1b, 3] The
cofactors/substrates bind to the active site iron center to
overcome the low one-electron redox potential of dioxygen.
Subsequent to dioxygen reduction at the iron center, different
iron-oxygen intermediates are generated depending upon the
structure and function of the enzymes.[1d] Along with the active
site geometry, the spin state of iron center plays important roles
in directing the reaction pathway. For example, a low-spin
iron(III)-hydroperoxo species has been observed in the catalytic
cycle of activated bleomycin (ABLM) enzyme.[4] While the active
species in heme oxygenases has been characterized as a low-
[14]
pyridylmethyl)amine (TPA) has been reported to change the spin
[14e]
state of the resulting iron(II) complexes.
The steric hindrance
of the 6-Me substituents is manifested in the longer metal-ligand
bonds and weaker ligand field. The effect of 6-Me substituents
on the pyridine rings of the pentadentate N4Py scaffold in
weakening the ligand field strength of the bis(6-methylpyridin-2-
yl)-N,N-bis((pyridin-2-yl)methyl)methanamine (N4PyMe2) ligand
in the equatorial plane has been reported recently.[15]
We have been investigating the reductive activation of dioxygen
by nonheme iron(II)-α-hydroxy acid complexes of ligands of
varying denticity and their oxidizing ability in O
2
-dependent
oxidation reactions. In that direction, we have shown that iron-
1
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coordinated benzilate acted as two-electron reductant and the
reduced oxygen species on the iron center exhibited versatile
reactivity.[16] The iron(II)-benzilate complexes of facial tridentate
ligands displayed better reactivity in substrate oxidation
compared to those of tetradentate ligands. Moreover, the
geometry and topology of tetradentate ligands have been shown
2
to play important roles in the oxygenation of substrates by O -
derived iron-oxygen oxidants.[16] With an objective to understand
the effect of methyl substitution on the pyridine rings of N4Py
backbone and the resulting ligand field strengths on the
oxidizing ability of the corresponding iron(II)-benzilate
complexes, we have explored the dioxygen reactivity of three
II
+
complexes,
[
(
[(N4Py)Fe (benzilate)]
(1),
(N4PyMe2)Fe (benzilate)] (2) and [(N4Py )Fe (benzilate)] (3)
N4PyMe4 = bis(6-methylpyridin-2-yl)-N,N-bis((6-methylpyridin-2-
II
+
Me4
II
+
yl)methyl)methanamine (Chart 1). With increasing number of
methyl substitution on the pyridine rings of N4Py, the ligand field
becomes weaker and the resulting iron-oxygen species
becomes a stronger oxidant. The characterization, dioxygen
reactivity of the iron(II)-benzilate complexes and the ligand field
Figure 1. Optical spectra of iron(II)-benzilate complexes (1-3) under nitrogen
atmosphere in acetonitrile (0.5 mM) at 298 K.
2
effect on O -dependent oxidation of substrates are presented in
this work.
The optical spectra of the complexes in acetonitrile display
bands in the region between 320 and 460 nm attributable to the
ligand-centric and metal to ligand charge-transfer transitions,
respectively (Figure 1). The band positions for 2 and 3 are blue
shifted with the maximum shift observed for complex 3. All the
complexes exhibit broad and paramagnetically shifted proton
resonances in between -25 and 55 ppm suggestive of the high-
spin state of the complexes in solution (Figure S1, Supporting
Information, SI). The room-temperature magnetic moment
B B
values between 5.1 µ and 5.2 µ further confirm the high-spin
nature of the complexes.
Chart 1. Ligands used in this work.
Results and Discussion
Synthesis and characterization
The N4Py ligand was synthesized using the procedure
reported in the literature.[17] The ligands, N4PyMe2 and N4PyMe4
,
were prepared by the reductive amination of bis(6-methylpyridin-
-yl)methanamine[18] with pyridine-2-carboxaldehyde and 6-
methylpyridine-2-carboxaldehyde, respectively, in
2
dichloromethane. Reactions of equimolar amounts of iron(II)
perchlorate, benzilic acid, triethylamine and the respective N5
ligand in methanol afforded bright yellow iron(II)-benzilate
Figure
(N4PyMe2)Fe (benzilate)] (2) and (b) [(N4Py )Fe (CH
3 2
CN)(H O)] (4) with
thermal ellipsoids at 30% probability.
2.
ORTEP diagrams of the cationic complexes (a)
complexes (1-3) in good yields (Experimental Section). The
II
+
Me4
II
2+
[
iron(II)-acetonitrile complex [Fe (N4PyMe4)(CH
II
CN)(H
O)]2+ (4)
3
2
was prepared by mixing the ligand and iron(II) perchlorate
hydrate in acetonitrile.
The complexes were further characterized by single crystal X-
ray diffraction. Complexes 1 and 2 crystallize in monoclinic
system with the P2(1)/c space group, whereas complex 3
crystallizes in the monoclinic P2(1)/n space group (Table S1). In
all the complexes, the coordination environment around the iron
center is distorted octahedral, and in each case, the iron center
is ligated by five nitrogen donors from the supporting ligand
(
Figures 2 and S2). The sixth coordination site is occupied by
one carboxylate oxygen of the monoanionic benzilate. In each
complex, the amine nitrogen (N3) and the carboxylate oxygen
(
O1) occupy the axial positions and the four pyridine nitrogens
2
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constitute the equatorial plane. The two methyl groups on the
pyridine donor of the ligand in complex 2 engender an
asymmetric ligand field in the equatorial plane thereby causing
more deviation in the axial N3-Fe-O1 angle from 180. To adjust
this deviation, the Fe1-N3 bond in complex 2 is slightly
elongated compared to that in 1 and 3. The average Fe-N bond
distances are observed in the range between 2.18 Å and 2.26 Å,
typical of high-spin iron(II) complexes (Table 1).[19] Upon
introduction of two and four methyl substituents on the pyridine
rings, the Fe-NPy bonds are elongated in the equatorial plane to
avoid steric interactions. The ligand field strength thus follows
and one water molecule giving rise to a distorted octahedral
coordination geometry. The steric bulk exerted by the 6-Me
substituents restrains coordination of one of the pyridyl nitrogen
atoms (N4). In the structure, the axial positions are occupied by
the amine nitrogen (N3) and the nitrogen atom (N6) of the
acetonitrile molecule with the N3-Fe1-N6 angle being 175.9(2)°
(Table 1). The three pyridine nitrogens (N1, N2, N5) from ligand
and the oxygen O1 from the water molecule constitute the
equatorial plane. The Fe-N and Fe-O bond distances are within
the range 2.12-2.29 Å and 2.05 Å, respectively, similar to those
observed in the high-spin iron complex 3 of the ligand (Table 1).
the order N4Py> N4PyMe2> N4PyMe4
.
Dioxygen reactivity of iron(II)-benzilate complexes
Table 1. Selected bond length (Å) and angles (°) of complexes 1, 2, 3 and 4.
The iron(II)-benzilate complexes (1-3) react with dioxygen in
acetonitrile at ambient conditions to display oxidative
decarboxylation of benzilic acid to benzophenone. While
complex 1 does not show any appreciable change in the optical
spectra even after 24 h, the intensities of the CT bands (at 350
nm and 450 nm for complex 2, and at 370 nm for complex 3)
increase slowly generating broad featureless spectra (Figure 3).
Interestingly, the time required for complete decarboxylation
varies with the number of methyl substituents on the ligand
backbone. Complex 1 takes nearly 24 h for decarboxylation,
whereas complexes 2 and 3 quantitatively decarboxylate in 6 h
and 2 h, respectively. Thus, the rate of decarboxylation has clear
dependence on the ligand; sterically hindered the ligand faster is
the decarboxylation. The oxidized solutions of complexes 2 and
Bond distance/
bond angle
1
2
3
4
Fe1-O1benzilate
Fe1-O1H2O
Fe1-N6MeCN
Fe1-N5
1.972(8)
1.9970(19) 2.014(3)
-
-
-
-
-
-
2.053(5)
-
2.122(7)
2.138(11) 2.170(2)
2.251(4)
2.268(3)
2.251(4)
2.212(3)
2.293(3)
111.23(13)
110.17(12)
84.63(13)
98.13(12)
2.266(5)
Fe1-N4
2.190(9)
2.172(9)
2.232(9)
2.190(2)
2.226(2)
2.250(2)
-
Fe1-N2
2.293(5)
3
at 77 K are found to be EPR silent, whereas for complex 1, a
signal at g = 4.2 is observed in the X-band EPR spectrum (77 K)
after the reaction with O indicating the presence of a high spin
Fe1-N3
2.220(5)
2
Fe1-N1
2.198(11) 2.266(2)
2.209(6)
iron(III) species as the end product (Figure S3). Time-dependent
1
H NMR spectra of the complexes during the reaction with
O1-Fe1-N5
O1-Fe1-N4
N5-Fe1-N4
O1-Fe1-N2
N5-Fe1-N2
N4-Fe1-N2
O1-Fe1-N3
N5-Fe1-N3
N4-Fe1-N3
N2-Fe1-N3
O1-Fe1-N1
N5-Fe1-N1
N4-Fe1-N1
N2-Fe1-N1
N3-Fe1-N1
105.1(7)
94.0(4)
85.8(3)
100.8(4)
153.9(7)
88.5(3)
170.8(4)
77.2(6)
77.2(4)
76.7(4)
114.9(5)
88.9(4)
150.9(6)
83.8(4)
73.8(5)
94.30(8)
94.31(8)
87.75(9)
113.00(8)
152.62(9)
88.13(8)
167.95(8)
77.58(9)
76.66(9)
75.12(8)
114.44(8)
86.17(9)
150.96(9)
84.40(8)
74.31(8)
-
-
-
-
dioxygen show gradual changes with the disappearance of
some resonance signals yielding broad signals at the end of the
reactions (Figure S4). On the contrary, the NMR spectra of the
oxidized complexes from 2 and 3 bear resemblance to the
precursor iron(II) complexes. The 1H NMR spectrum of the
organic product derived from benzilate establishes quantitative
decarboxylation of benzilate to benzophenone in each case
150.61(14) 152.8(2)
(
Figure S5).
86.70(13)
168.70(12)
77.14(13)
77.51(13)
73.57(13)
98.60(11)
86.41(12)
151.18(14)
87.82(12)
84.63(13)
-
-
76.11(19)
-
76.83(19)
-
94.9(2)
-
94.9(2)
77.1(2)
A related iron(II) complex 4 was structurally characterized for
comparison with the iron(II)-benzilate complex 3. The iron center
in 4 is coordinated by four nitrogen donors from the N4PyMe4
ligand and the remaining sites are occupied by one acetonitrile
Figure 3. Optical spectral changes during the reaction of 3 (0.5 mM in
acetonitrile) with dioxygen at 295 K.
3
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Since no iron-oxygen species could be trapped even at low
temperature for direct characterization, indirect probes were
used to intercept the oxidant formed upon oxidative
decarboxylation of the iron(II)-benzilate complexes (Table 2).
When thioanisole (20 equiv.) is separately added to the
and Hammett analyses indicate that the oxidant from 2 is
relatively weaker nucleophile than that from 3.
2
acetonitrile solutions of the complexes 1-3 in the presence of O ,
thioanisole oxide is formed as the sole product (Figure S6 and
Table 2). When the equiv of thisoanisole is decreased (2 equiv.),
methyl phenyl sulfone is detected along with sulfoxide (Figure
S7 and Table 2). The yield of the product is however found to
decrease systematically from 3 to 1 (Table 2). Other substrates
such as 9,10-dihydroanthracene, fluorene, toluene and
cyclohexene are converted to their oxidized forms (Table 2).
Alkenes such as styrene, 1-octene, cyclooctene and
cyclohexane are, however, unreactive in the reaction conditions.
When p-bromobenzaldehyde (or benzaldehyde) is used as
intercepting agent, p-bromobenzoic acid (or benzoic acid) is
formed as the sole products (Table 2 and Figure S8).
Figure 4. Hammett plot of log krel versus σ
by complex 2 (blue square) and complex 3 (red circles). The krel value was
calculated by dividing the concentration of the product from p-XC SCH by
the concentration of the product from C SCH
p 6 4 3
for p-XC H SCH in the oxidation
Table 2. Oxidation of substrates with oxygen by the iron(II)-benzilate
complexes.
6
H
4
3
6
H
5
3
.
Substrates
equiv.)
Products
1
(%)
2
(%)
3
(%)
(
Based on the interception results, labelling experiment and
Hammett analysis, a nucleophilic iron(II)-hydroperoxo species is
proposed to form upon two-electron reductive activation of
dioxygen in the oxidative decarboxylation of benzilic acid to
benzophenone. Similar intermediates have been proposed in the
Thioanisole (20)
Thioanisole (2)
Thioanisole oxide
12
55
84
Thioanisole oxide +
Methyl phenyl sulfone
0+3
5+8
9+16
4
-Bromo-
benzaldehyde
20)
4-Bromobenzoic acid
Benzoic acid
5
12
20
decarboxylation of iron(II)-α-hydroxy acid complexes of
tridentate and tetradentate ligands.[16, 20b, 21]
(
Benzaldehyde
20)
3
10
15
Dioxygen reactivity of the iron(II)-benzilate complexes in the
presence of protic acid
(
9
,10-Dihydro-
Anthracene +
Aanthraquinone
18+3
36+2
50+5
The effect of protic acid on the reactivity of iron(II)-benzilate
complexes toward different organic substrates was investigated.
While the dioxygen-derived oxidant from iron(II)-benzilate
complexes (1-3) are unreactive towards alkenes, the substrates
are converted to the corresponding epoxides by the oxidants
generated in the presence of pyridinium perchlorate (1 equiv.)
(Table 3). Cyclooctene is converted to cyclooctene oxide (Figure
S11) in 48% and 64% yields by 2 and 3, respectively. When 1-
octene is used, 1,2-epoxyoctane (Figure S12) is formed. The
oxidant also converts cis-2-heptene to 2-butyl-3-methyl oxirane
anthracene (5)
Fluorene (5)
Toluene (100)
Cyclohexene
Fluorenone
15
-
30
45
Benzaldehyde
6
15
2-Cyclohexen-1-ol
+ 2-Cyclohexen-1-one
-
12+5
21+10
(100)
The sulfoxidation reaction by 3 in the presence of H 1
8O does
2
not incorporate any labeled oxygen into the product. However,
the GC-mass spectra of the organic products formed in the
reaction with thioanisole and 18O
(
Figure S13) in 62% yield for 2 and 88% for 3. When styrene is
used, 2-phenyl oxirane is formed as the only product in 12% and
2% conversion by complex 2 and 3, respectively (Figure S14a).
The labeling experiment for styrene oxidation with 16O
and
18O by 3 reveals 10% incorporation of labeled oxygen atom
2
confirm the incorporation of
2
labeled oxygen into the oxygenated products (Figure S9). It is to
be noted that the decarboxylation product (benzophenone) does
not contain any labeled oxygen. Based on sulfone formation at
low sulfide concentrations, a nucleophilic iron-oxygen oxidant is
proposed as the active oxidant.[16, 20] The labeling experiment
2
H
2
into 2-phenyl oxirane (Figure S14b). Thus the oxidant formed in
the presence of protic acid can exchange its oxygen atom with
water. When cyclohexene is used as
a substrate, the
with 18O
and benzaldehyde suggests the incorporation of one
2
corresponding epoxide is formed in small amount. In the
reaction, allylic oxidation products are observed as major
product (Figure S15).
oxygen atom into benzoic acid (Figure S10).
For Hammett analysis, competitive oxidations were carried out
with 1:1 mixtures of thioanisole and different para-substituted
thioanisoles (p-XC
The ρ values of +0.63 and +0.81 for 2 and 3, respectively, are
obtained from the plots of the relative rates (krel) versus σ
Figure 4). Hammett analysis was not performed for 1 due to the
low yield of thioanisole oxide in that case. Product distribution
6 4 3 2
H SCH where X = NO , Cl, H, Me, OMe).
p
(
4
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indicate the generation of an electrophilic oxidant which could
carry out epoxidation of alkenes and the C-H bond oxidation of
Table 3. Oxidation of substrates by complexes 2 and 3 with dioxygen in the
presence of pyridinium perchlorate (1 equiv.).
Me2
cyclohexane (Table 3). The fact that the N4Py and N4Py
ligands can support the iron(IV)-oxo unit [ and the results from
mechanistic and interception studies together indicate that
iron(IV)-oxo species is generated in situ in the dioxygen-
dependent decarboxylation of the iron-benzilate complexes. As
15]
Subs-
trate
Products
2
3
[(TpPh,Me)
Fe
[(TpPh2)
Fe
II
II
(equiv)
(benzi-
late)] [
(benzi-
late)]
22]
[22]
Ph2
[23]
reported for the Tp
system,
an iron(IV)-oxo species is
Thio-
anisole
PhSOCH
3
53±2
48±2
85±1
64±1
97±1
90
proposed as the active oxidant from the respective iron(II)-
benzilate complexes that performs the epoxidation reactions in
(20)
the presence of
a protic acid. Importantly, the oxidative
Cyclo-
octene
Cyclooctene
oxide
58±1
70
decarboxylation is not affected in the presence of proton donor.
The reaction of complex 3 with thioanisole (20 equiv.) and
pyridinium perchlorate (1 equiv.) afforded thioanisole oxide as
the sole product (Figure S18). Hammett analyses from
competitive oxidations of equimolar amounts of para-substituted
thioanisoles and thioanisole by complex 3 in the presence of
protic acid and dioxygen reveal a negative ρ value of -0.90
(100)
Cyclo-
hexene
Cyclohexene
oxide
2-Cyclohexen-1-
ol
5
10
42
5
31±2
20 ±2
5±1
0
35
22
5
(100)
26
7
2-Cyclohexen-1-
(
±0.08) indicating that an electrophilic oxidant is responsible for
one
7-oxa-
2
9
-
sulfide oxidation (Figure 5).
bicyclo[4.1.0]hep
tan-2-one
1
-Octene
1,2-Epoxyoctane
2-Phenyloxirane
0
6±1
56±2
67
75
-
(100)
Styrene
100)
12±1
62±2
22±2
88±1
-
-
(
cis-2-
Heptene
2-Butyl-3-methyl
oxirane
(100)
Cyclo-
hexane
Cyclohexanol
Cyclohexanone
26±1
10±2
42±2
15±2
58±3
6±1
45
8
(100)
Adaman-
tane (5)
1-Adamantanol
2-Adamantanol
-
34±2
0
4±2
65±4
<2
<4
47
18
0
2-
Adamantanone
In the oxidation of cyclohexane by complexes 2 and 3 in
the presence of protic acid, cyclohexanol and cyclohexanone
are formed exhibiting A/K ratio of 2.6 and 2.8, respectively
Figure 5. Hammett plot for the oxidation of p-X-C
6
H
4
SMe by 3 with PyNHClO
4
(
Figure S16). Notably, no oxidized product is detected from
(1 equiv).
cyclohexane in the reaction with complex 1 alone. However,
complex 3 is found to oxidize adamantane with a C3/C2
normalized selectivity of 26 (Figure S17, Table 3). Recently,
similar C3/C2 selectivity has been reported in oxidation of
adamantane with m-CPBA or oxone by the iron(II) complex
The
iron(II)-benzilate
complexes
supported
by
tris(pyrazolyl)borate ligands have been reported to oxygenate
alkenes to cis-diols with the incorporation of both the oxygen
atoms of dioxygen. Further mechanistic studies suggested the
involvement of a nucleophilic iron-oxygen oxidant in the cis-
dihydroxylation reactions. However, the nucleophilic oxidant
changed its philicty in the presence of a Lewis acid[21] and protic
acid.[23] Lewis acid or protic acid cleaves the O-O bond of the
putative iron(II)-hydroperoxide heterolytically to generate an
electrophilic iron(IV)-oxo-hydroxo and iron(IV)-oxo-aquo species,
respectively. Furthermore, the electrophilic oxidants oxygenated
the strong C-H bonds of aliphatic substrates. In the absence of
any substrate, the oxidants intramolecularly hydroxylated one of
the phenyl rings of the TpPh2 ligand. The intraligand
II
Me2
[15]
[
Fe (N4Py )(CH
3
CN)](OTf)
2
complex.
To rule out the
involvement of a free radical mechanism, substrate oxidations
were carried out in the presence of radical scavengers such as
tert-butanol
cyclohexanol/cyclohexanone ratio does not change in
cyclohexane oxidation but the yield of 7-oxa-
and
1,4-benzoquinone.
Although
the
bicyclo[4.1.0]heptan-2-one formed from cyclohexene is
completely reduced. This result suggests the generation of over-
oxidized product from cyclohexene via radical pathway is
inhibited by radical scavenger. A kinetic isotope effect (KIE)
value of 3.8 in the competitive oxidation of a mixture of
cyclohexane and cyclohexane-d12 by complex 3 also supports
the involvement of a metal-based oxidation. These results
hydroxylation was inhibited by replacing the 5-phenyl groups on
the pyrazole rings of the ligand by methyl groups.[22] The iron(II)-
5
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Chemistry - An Asian Journal
10.1002/asia.202000142
FULL PAPER
benzilate complex of the TpPh,Me ligand displayed enhanced
reactivity under stoichiomteric conditions. Furthermore, the
complex exhibited catalytic oxygenations and the catalytic
activity was improved upon addition of a Lewis acid. The iron(II)-
benzilate complexes of tetradentate nitrogen ligands also
displayed diverse reactivity toward alkenes depending upon
ligand topology. It was proposed that the side-on-bound iron(II)-
hydroperoxides were able to cis-dihydroxylate the C=C bonds of
olefins, whereas the end-on iron(II)-hydroperoxides was unable
to oxygenate the alkenes.[16]
The iron(II)-benzilate complexes discussed in this work display
moderate reactivity in oxygen atom transfer (OAT) and in
hydrogen atom transfer (HAT) reactions on substrates with weak
C-H bonds. The iron(II)-benzilate complexes of ligands with 6-
methyl substitution (2 and 3) show better reactivity compared to
that of 1. Of note, complexes 2 and 3 exhibit higher reactivity in
OAT transfer reaction in comparison to that of the iron(II)
complexes of tetradentate systems.[16] In fact, the OAT ability of
the oxidants from 2 and 3 closely match to that of the complexes
of tridentate ligand (Table 4). The increased reactivity of the
complexes over the reported iron(II) complexes of tetradentate
ligands can be attributed to the significant steric constraint of the
ligands that prevent dimerization and other unwanted
bimolecular side reactions.
On the basis of the results discussed above, a mechanism
similar to that proposed for other reported iron(II)-benzilate
complexes is put forward.[16] From the crystal structure of 3, it is
clear that the average iron-Npyridine distance is longer than those
in other complexes. Although complex 4 is not reactive toward
dioxygen, its crystal structure (Figure 2b) provides information
about a possible pathway of the reaction between iron(II)-
2
benzilate complex 3 and O . One of the hemilabile pyridine arms
may get de-coordinated to avoid steric congestion, which in turn
facilitates dioxygen activation at the iron(II) center in 3. The de-
coordination step is most facile in 3 among the three complexes
and least in complex 1. Upon binding of dioxygen and
subsequent reduction by the co-ligand benzilate, an end-on
bound nucleophilic iron(II)-hydroperoxo (I) intermediate is
formed. Transfer of one of the oxygen atoms to external
substrates results in iron(II)-hydroxide species as the final
species. The UV-vis spectra of the oxidized solutions after the
reaction with dioxygen show featureless broad spectra.
II
O)]2+ is
Furthermore, an ion peak corresponding to [LFe -OH(H
2
observed at m/z 486 for the oxidized complex of 2 in the ESI-
mass spectrometry (Figure S19). No such species is detected
for the oxidized solution of complex 1.
Table 4. Comparison of the reactivity of iron(II)-benzilate complexes
II
+
[
(L)Fe (benzilate)] of different ligands.
L
Reaction
time
Yields (%) of the products from substrates
PhSOCH
+
3
Octane-
1,2-diol
from
1-
octene
cis-
Anthra-
cene
from
Cyclo-
octane-
1,2-diol
from
cyclo-
octene
PhSO
2
CH
3
from thio-
anisole
9,10-
Dihydro
anthra-
cene
TpPh2 [22]
15 min
84 + 7
85
80
-
TpPh,Me [22]
TPA[16]
45 min
2 h
93+ 4
17
92
-
86
-
-
22
26
6
-Me3-
4 h
67
12
16
[
16]
TPA
Scheme 1. Mechanistic proposal for the formation of iron-oxygen
oxidants in the oxidative decarboxylation of α-hydroxy acids in the
presence of a protic acid.
iso-
BPMEN
1.5 h
2 h
20
-
-
21
28
[
16]
6-Me
2
-iso-
[
37 + 5
16]
BPMEN
-
-
In the presence of protic acid, the de-coordinated pyridine ring
likely gets protonated (Scheme 1). The protonated pyridine upon
interaction with the hydroperoxide moiety may facilitate the
heterolytic O-O bond cleavage with the formation of iron(IV)-oxo
species. The high-valent iron-oxo species can exchange its
oxygen atom with water and performs epoxidation of alkenes
and C-H bond hydroxylation of aliphatic substrates including
those of cyclohexane.
BPMEN[16]
4.5 h
5.5 h
21
61
<5
21
<5
12
24
34
6
-Me
2
-
[
16]
BPMEN
TBimA[16]
N4Py
2.2 h
20 h
6 h
25
12
55
84
-
-
-
-
-
-
-
-
38
18
36
50
N4PyMe2
N4PyMe4
2 h
6
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Chemistry - An Asian Journal
10.1002/asia.202000142
FULL PAPER
Conclusion
dichloromethane (3 x 50 mL). The combined organic extracts were dried
over Na
.45 g (48%). 1H NMR (500 MHz, DMSO-d
m, 4H), 7.05 (dd, 4H), 6.99 (dd, 2H), 6.87 (dd, 2H), 5.18 (s, 1H), 4.71 (s,
2
SO
4
and evaporated to dryness to yield a pale yellow oil. Yield:
0
6
, 298 K): δ(ppm) 7.40-7.50
Three new iron(II)-benzilate complexes supported by
pentadentate N4Py-derived ligands were isolated and
structurally characterized. The ability of the complexes in
(
4
H), 2.48 (s, 6H), 2.45 (s, 6H). ESI-MS (+ve ion mode, acetonitrile): m/z
+
+
424.1 (100%, [M+H] ), 446.1 (40%, [M+Na] ).
2
carrying out O -dependent oxidations particularly oxygen atom
transfer (OAT) and hydrogen atoms transfer (HAT) was
investigated. The complexes activate dioxygen and display
quantitative decarboxylation of benzilic acid to benzophenone.
Enhanced reactivity of the complexes in sulfide oxidation is
observed compared to the related complexes of tetradentate
ligands. Moreover, the complexes exhibit HAT reactions on
substrates with weak C-H bonds. Based on the interception and
General Procedure for the Syntheses of Iron(II) Benzilate
Complexes. Iron(II) perchlorate hydrate (0.36 g, 1 mmol) was added to a
solution of the ligand (1 mmol) in methanol(5 mL). To the resulting
solution was dropwise added an equimolar mixture of benzilic acid (0.23
g, 1 mmol) and triethylamine (240 μL, 1 mmol). The solution was then
stirred at room temperature for 5 h to afford the microcrystalline yellow
solid compounds 1-3.
labelling
intermediate is proposed as the active oxidant. The presence of
protic acid changes the philicity of the iron-oxygen
experiments,
a
putative
iron(II)-hydroperoxo
II
[
(N4Py)Fe (benzilate)](ClO
4
)
2
(1). Single crystals suitable for X-ray
diffraction were obtained by diffusion of hexane into a solution of the
complex in dichloromethane. Yield: 0.69 g (83%). Anal. Calcd for
a
intermediate from a nucleophilic iron(II)-hydroperoxo to an
electrophilic iron(IV)-oxo species. Effect of substitution on ligand
backbone in directing the reactivity of iron(II)-benzilate
complexes is demonstrated. The reactivity trend of the iron(II)-
benzilate complexes discussed in this work implicate the role of
ligand field in performing dioxygen-dependent oxidation
reactions and that weak-field ligands are more appropriate to
C
5
38
H
34Cl
3
FeN
5 7
O (834.90 g/mol): C, 54.67; H, 4.10; N, 8.39. Found: C,
4.61; H, 4.05; N, 8.31. IR (KBr, cm−1): 3053 (m), 3038 (m), 3001 (m),
1638 (s), 1605 (s), 1576 (m), 1481 (s), 1443 (s), 1346 (m), 1157 (m),
1016 (m), 760 (m), 737 (s), 706 (s). UV−vis in CH CN (λmax): 374 nm (ε =
340 M−1 cm−1), 450 nm (ε = 918 M−1 cm−1). Magnetic moment μeff (298
K): 5.1 μ (500 MHz; δ (ppm)): 90.3 (Py-H ), 54.3
. 1H NMR in CDCl
CH Py), 48.3 (Py-H ), 46.5 (Py-Hβ′), 32.5 (Py-H ), 25.5 (NCH-Py-H
NCH-Py-Hβ′), 22.8 (NCH-Py-H ), 9.4 (benzilate-o-H), 8.2 (benzilate-m-H),
.3 (benzilate-p-H), 7.7 (NCH-Py-H ), 7.2 (N-CH).
3
1
B
3
α
(
2
β
γ
β
,
γ
promote
2
O -based reactivity using two-electron sacrificial
7
α
reductants. Further studies to trap the proposed iron-oxygen
intermediates from iron(II)-benzilate complexes of polydentate
ligands are presently being pursued in our laboratory.
(N4PyMe2)Fe (benzilate)](ClO
diffraction were obtained by diffusion of diethyl ether into a solution of the
complex in acetonitrile. Yield: 0.61 (78%). Anal. Calcd for
36ClFeN (778.03 g/mol): C, 60.21; H, 4.66; N, 9.00. Found: C,
II
[
4 2
) (2). Single crystals suitable for X-ray
g
C
39
H
5 7
O
5
1
1
1
9.31; H, 4.58; N, 8.89. IR (KBr, cm−1): 3053 (m), 3038 (m), 3001 (m),
638 (s), 1605 (s), 1576 (m), 1481 (s), 1443 (s), 1346 (m), 1157 (m),
Experimental Section
016 (m), 760 (m), 737 (s), 706 (s). UV−vis in CH
580 M−1 cm−1), 433 nm (ε = 1260 M−1 cm−1). Magnetic moment μeff (298
. 1H NMR in CDCl
(500 MHz; δ (ppm)): 87.8 (Py-H ), 51.0
Py), 49.4 (Py-H ), 46.1 (Py-Hβ′), 38.5 (Py-H ), 28.2 (Me-Py-H , Me-
Py-Hβ′), 27.7 (Me-Py-H ), 9.4 (benzilate-o-H), 8.3 (benzilate-m-H), 7.4
benzilate-p-H), 7.3 (N-CH), -15.2 (Me-Py-CH ).
3
CN (λmax): 356 nm (ε =
Materials and Methods. All chemicals and reagents were obtained from
commercial sources and were used without further purification unless
otherwise noted. Solvents were distilled and dried before use.
Preparation and handling of air-sensitive materials were carried out
under an inert atmosphere in a glove box. Fourier transform infrared
spectroscopy on KBr pellets was performed on a Shimadzu FT-IR 8400S
instrument. Elemental analyses were performed on a Perkin Elmer 2400
series II CHN analyzer. Electro-spray ionization (ESI) mass spectra were
K): 5.2 μ
B
3
α
(CH
2
β
γ
β
γ
(
3
(N4PyMe4)Fe (benzilate)](ClO
II
[
4 2
) (3). Single crystals suitable for X-ray
diffraction were obtained by diffusion of diethyl ether into a solution of the
complex in dichloromethane. Yield: 0.70 g (86%). Anal. Calcd for
C H40ClFeN O (806.08 g/mol): C, 61.09; H, 5.00; N, 8.69. Found: C,
41 5 7
recorded with
a Waters QTOF Micro YA263 instrument. Solution
electronic spectra (single and time-dependent) were measured on an
Agilent 8453 diode array spectrophotometer. All room temperature NMR
spectra were collected on a Bruker Avance 500 MHz spectrometer. X-
band EPR measurements were performed on a JEOL JES-FA 200
instrument. GC-MS measurements were carried out with a Perkin Elmer
Clarus 600 using Elite 5 MS (30m x 0.25mm x 0.25μm) column with a
maximum temperature of 300C. Labeling experiments were carried out
5
1
1
1
9.91; H, 4.88; N, 8.81. IR (KBr, cm−1): 3053 (m), 3038 (m), 3001 (m),
638 (s), 1605 (s), 1576 (m), 1481 (s), 1443 (s), 1346 (m), 1157 (m),
016 (m), 760 (m), 737 (s), 706 (s). UV−vis in CH
320 M−1 cm−1). Magnetic moment μeff (298 K): 5.2 μ
Py), 46.6 (Py-H ), 44.7 (Py-Hβ′), 22.2 (Py-
, NCH-Py-Hβ′), 11.0 (NCH-Py-H ), 10.0(benzilate-o-
-Py-
3
CN (λmax): 380 nm (ε =
. 1H NMR in CDCl
B
3
(500 MHz; δ (ppm)): 54.9 (CH
2
β
18
18
H
γ
), 16.8 (NCH-Py-H
β
γ
with
Services Inc., USA.
O gas (99 atom %) or H O (98 atom %) purchased from Icon
2
2
H), 8.2 (benzilate-m-H), 7.3 (benzilate-p-H), -7.2 (N-CH), 3.1 (CH
2
CH ), -22.0 (NCH-Py-CH ).
3
3
Ligand Synthesis. The N4Py and N4PyMe2 ligands were synthesized
[Fe (N4PyMe4)(CH
0.36 g, 1 mmol) and ligand (0.42 g, 1 mmol) were mixed together in
acetonitrile (5 mL). The resulting solution was stirred at room
temperature under N atmosphere for 5 h. Single crystals suitable for X-
II
according to the procedure reported in the literature.53,41 Bis(6-
3
2 4 2
CN)(H O)](ClO ) (4). Iron(II) perchlorate hydrate
(
methylpyridin-2-yl)methanone, bis(6-methylpyridin-2-yl)methanoneoxime,
and bis(6-methylpyridin-2-yl)methanaminewere prepared according to
the procedures reported in literature.[18] Although no problem was
encountered during the synthesis of these complexes, perchlorate salts
are potentially explosive and should be handled with care![24]
2
ray diffraction were obtained by diffusion of diethyl ether into the
acetonitrile solution of the complex. Yield: 0.64 g (86%). Anal. Calcd for
C
29
H
34Cl
2
FeN
7.01; H, 4.63; N, 11.31. ESI-MS (+ve ion mode in CH
100%) [Fe(N4PyMe4)]2+.IR (KBr, cm−1): 3053 (m), 3038 (m), 3001 (m),
O
6 9
(737.37 g/mol): C, 47.24; H, 4.65; N, 11.40. Found: C,
4
3
CN): m/z 239.6
Synthesis of N4PyMe4
yl)methanamine (0.500 g, 2.35 mmol) and sodium triacetoxyborohydride
3.44 g, 16.2 mmol) in dichloromethane (50 mL) was added 6-methyl-
.
To
a mixture of bis(6-methylpyridin-2-
(
1
638 (s), 1605 (s), 1576 (m), 1481 (s), 1443 (s), 1346 (m), 1157 (m),
016 (m), 760 (m), 737 (s), 706 (s). UV−vis in CH CN (λmax): 376 nm (sh).
. 1H NMR (500 MHz in CD
CN):
(
1
3
pyridine-2-carboxaldehyde (0.594 g, 4.9 mmol). The mixture was allowed
to stir for 48 h at ambient temperature. Subsequently, a saturated
aqueous sodium hydrogen carbonate solution was added followed by
stirring for an additional 1 h. The organic product was then extracted with
Magnetic moment μeff (298 K): 5.2 μ
δ(ppm) 82.4 (CH Py-Fe-coordinated), 61.1 (H
7.3 (Hβ′-CH -Py-Fe-coordinated), 53.0 (H
γ
B
3
2
β
-CH
2
-Py-Fe-coordinated),
-Py-Fe-coordinated),
5
2
-CH
2
7
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Chemistry - An Asian Journal
10.1002/asia.202000142
FULL PAPER
4
(
7.8 (CH
NCH-Py-H
coordinated), -6.3 (N-CH), -27.2 (CH
2
-Py), 25.6 (CH
NCH-Py-Hβ′), 8.4 (NCH-Py-H
-CH -Py), -40.8 (NCH-Py-CH
2
-Py-H
β
, CH
2
-Py-Hβ′), 18.4 (CH
2
-Py-H
γ
), 13.8
-Py-Fe-
).
TKP gratefully acknowledges the Council of Scientific and
Industrial Research (CSIR), India for the financial support
β
,
γ
), -5.0 (CH
3
-CH
2
3
2
3
(
Project No. 01(2926)/18/EMR-II). SB thanks CSIR for research
fellowships. RS acknowledges SERB (Project:
YSS/2015/001741) for the Young Scientist Project.
2
Reaction of Iron(II)-Benzilate Complexes with O and Analysis of
Organic Products. For a stoichiometric reaction, solid complex (0.02
mmol) was dissolved in dry acetonitrile (15 mL) under nitrogen
atmosphere. Pure O
2
was passed through the solution, and the mixture
Keywords: N-ligand • iron • benzilate • dioxygen • oxidation
was stirred at room temperature, during which the initial light yellow
solution slowly turned light orange. The solution was concentrated and
the residue was treated with 2 M HCl (10 mL). The organic products were
then extracted with diethyl ether (3 × 20 mL) and dried over anhydrous
sodium sulfate. The organic layer was filtered and evaporated to dryness.
The products were analyzed by 1H NMR and GC-MS without further
purification. Quantification of the products were done by using 1,3,5-
trimethoxy benzene and naphthalene as internal standards for 1H NMR
and GC-MS, respectively.
[
1]
2]
(a) R. P. Hausinger, Crit. Rev. Biochem. Mol. Biol. 2004, 39, 21-68; (b)
M. Costas, M. P. Mehn, M. P. Jensen, L. Que, Jr., Chem. Rev. 2004,
104, 939-986; (c) T. D. H. Bugg, S. Ramaswamy, Curr. Opin. Chem.
Biol. 2008, 12, 134-140; (d) P. C. A. Bruijnincx, G. van Koten, R. J. M.
K. Gebbink, Chem. Soc. Rev. 2008, 37, 2716-2744; (e) E. G. Kovaleva,
J. D. Lipscomb, Nat. Chem. Biol. 2008, 4, 186-193; (f) K. Ray, F. F.
Pfaff, B. Wang, W. Nam, J. Am. Chem. Soc. 2014, 136, 13942-13958.
(a) E. L. Hegg, L. Que, Jr., Eur. J. Biochem. 1997, 250, 625-629; (b) G.
D. Straganz, B. Nidetzky, ChemBioChem 2006, 7, 1536-1548; (c) C.
Loenarz, C. J. Schofield, Nat. Chem. Biol. 2008, 4, 152-156.
Reactions of Iron(II) Complexes with Substrates. The iron(II) benzilate
complex (0.02 mmol) were dissolved in dioxygen-saturated organic
solvent under a nitrogen atmosphere. Substarte was added to the
solution and was allowed to stir at room temperature for 2 h. After
oxidation, the solvent was removed under reduced pressure and the iron
complex was decomposed by addition of 3 M HCl solution (10 mL). The
organic products were extracted by either diethyl ether or chloroform (3 ×
[
[
[
3]
4]
S. Martinez, R. P. Hausinger, J. Biol. Chem. 2015, 290, 20702-20711.
L. V. Liu, C. B. Bell, III, S. D. Wong, S. A. Wilson, Y. Kwak, M. S. Chow,
J. Zhao, K. O. Hodgson, B. Hedman, E. I. Solomon, Proc. Natl. Acad.
Sci. U. S. A. 2010, 107, 22419-22424, S22419/22411-S22419/22413.
J. Rittle, M. T. Green, Science 2010, 330, 933-937.
15 mL), and the organic layer was dried over anhydrous sodium sulfate.
[
[
5]
6]
After removal of the solvent, organic products were analyzed by GC-MS
D. Galonić Fujimori, E. W. Barr, M. L. Matthews, G. M. Koch, J. R.
Yonce, C. T. Walsh, J. M. Bollinger, Jr., C. Krebs, P. J. Riggs-Gelasco,
J. Am. Chem. Soc. 2007, 129, 13408-13409.
1
and H NMR spectroscopy. For reactions with alkenes, the products were
analyzed by GC-MS spectroscopy. Quantification of the organic products
by NMR was done by comparing the peak area of four aromatic ortho
protons (7.81 ppm) of benzophenone. For GC analyses, naphthalene
was used as an internal standard and the products were identified by
comparison of their GC retention times and GC-MS with those of
authentic compounds. 1H NMR Data: Benzophenone: δ 7.81 (d, 4H),
[
[
7]
8]
9]
(a) J. C. Price, E. W. Barr, B. Tirupati, J. M. Bollinger, Jr., C. Krebs,
Biochemistry 2003, 42, 7497-7508; (b) D. A. Proshlyakov, T. F.
Henshaw, G. R. Monterosso, M. J. Ryle, R. P. Hausinger, J. Am. Chem.
Soc. 2004, 126, 1022-1023.
(a) M. L. Matthews, C. M. Krest, E. W. Barr, F. H. Vaillancourt, C. T.
Walsh, M. T. Green, C. Krebs, J. M. Bollinger, Jr., Biochemistry 2009,
7
2
.59 (t, 2H), 7.48 (t, 4H), 4- bromobenzoic acid: δ 7.95 (d, 2H), 7.65 (d,
H). Methyl phenyl sulfone: δ 7.94 (d, 2H), 7.61 (m, 3H), 3.10 (s, 3H).
48, 4331-4343; (b) J. M. Bollinger, Jr., W.-c. Chang, M. L. Matthews, R.
J. Martinie, A. K. Boal, C. Krebs, RSC Metallobiol. 2015, 3, 95-122.
A. Karlsson, J. V. Parales, R. E. Parales, D. T. Gibson, H. Eklund, S.
Ramaswamy, Science 2003, 299, 1039-1042.
Control Experiments. For control experiments, iron(II) perchlorate was
allowed to react with O in the presence of external substrate. The
[
[
2
reaction was carried out following the same procedure mentioned above
except that iron(II) perchlorate hexahydrate (0.01 mmol) was used
instead of iron(II) benzilate complex. No oxidized product derived from
organic substrates was observed in any of the experiments. In a different
set of control experiments, iron(II) perchlorate hexahydrate and
monoanionic benzilate (benzilic acid in the presence of 1 equiv. of
10] (a) S. Hong, Y.-M. Lee, W. Shin, S. Fukuzumi, W. Nam, J. Am. Chem.
Soc. 2009, 131, 13910-13911; (b) A. Thibon, J. England, M. Martinho,
V. G. Young, Jr., J. R. Frisch, R. Guillot, J.-J. Girerd, E. Münck, L. Que,
Jr., F. Banse, Angew. Chem. Int. Ed. 2008, 47, 7064-7067.
[
11] (a) F. Namuswe, T. Hayashi, Y. Jiang, G. D. Kasper, A. A. N. Sarjeant,
P. Moënne-Loccoz, D. P. Goldberg, J. Am. Chem. Soc. 2010, 132, 157-
triethylamine) was used that resulted
a trace amount (<5%) of
167; (b) A. Franke, C. Fertinger, R. van Eldik, Chemistry – A European
benzophenone. However, substrate oxidation was not observed in the
above reaction.
Journal 2012, 18, 6935-6949; (c) F. Namuswe, G. D. Kasper, A. A. N.
Sarjeant, T. Hayashi, C. M. Krest, M. T. Green, P. Moënne-Loccoz, D.
P. Goldberg, J. Am. Cherm. Soc. 2008, 130, 14189-14200; (d) N.
Lehnert, R. Y. N. Ho, L. Que, Jr., E. I. Solomon, J. Am. Chem. Soc.
2001, 123, 8271-8290.
X-ray Crystallographic Data Collection, Refinement and Solution of
the Structure. X-ray single-crystal data for the complexes were collected
using Mo Kα (λ = 0.7107 Å) radiation on a SMART-APEX diffractometer
equipped with CCD area detector. Details of the data collection and
structure refinement are provided in (Table S1). Data collection, data
reduction, structure solution and refinement were carried out using the
software package of APEX II.[25] The structure was solved by intrinsic
methods and subsequent Fourier analyses and refined by the full-matrix
least-squares method basedon F2 with all observed reflections.[26] The
non-hydrogen atoms were treated anisotropically. The disordered carbon
atoms were treated isotropically. CCDC 1960560, 1960559, 1960558,
and 1960561 contain the supplementary crystallographic data for 1, 2, 3
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Entry for the Table of Contents
Playing with Ligand Field: The oxidizing ability of iron(II)-benzilate complexes of pentadentate nitrogen donor ligands is greatly
influenced by electronic and structural tuning of the supporting ligand. With decreasing ligand fields, the reactivity of dioxygen-derived
iron-oxygen oxidants increases in oxygen-and hydrogen-atom transfer reactions towards organic substrates.
1
0
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