Reductive Activation of O2 by Non-Heme Iron(II) Benzilate Complexes of N4 Ligands: Effect of Ligand Topology on the Reactivity of O2-Derived Oxidant

Article abstract of DOI:10.1021/acs.inorgchem.6b02282

A series of iron(II) benzilate complexes (1-7) with general formula [(L)Fe^II(benzilate)]⁺ have been isolated and characterized to study the effect of supporting ligand (L) on the reactivity of metal-based oxidant generated in the reaction with dioxygen. Five tripodal N₄ ligands (tris(2-pyridylmethyl)amine (TPA in 1), tris(6-methyl-2-pyridylmethyl)amine (6-Me₃-TPA in 2), N¹,N¹-dimethyl-N²,N²-bis(2-pyridylmethyl)ethane-1,2-diamine (iso-BPMEN in 3), N¹,N¹-dimethyl-N²,N²-bis(6-methyl-2-pyridylmethyl)ethane-1,2-diamine (6-Me₂-iso-BPMEN in 4), and tris(2-benzimidazolylmethyl)amine (TBimA in 7)) along with two linear tetradentate amine ligands (N¹,N²-dimethyl-N¹,N²-bis(2-pyridylmethyl)ethane-1,2-diamine (BPMEN in 5) and N¹,N²-dimethyl-N¹,N²-bis(6-methyl-2-pyridylmethyl)ethane-1,2-diamine (6-Me₂-BPMEN in 6)) were employed in the study. Single-crystal X-ray structural studies reveal that each of the complex cations of 1-3 and 5 contains a mononuclear six-coordinate iron(II) center coordinated by a monoanionic benzilate, whereas complex 7 contains a mononuclear five-coordinate iron(II) center. Benzilate binds to the iron center in a monodentate fashion via one of the carboxylate oxygens in 1 and 7, but it coordinates in a bidentate chelating mode through carboxylate oxygen and neutral hydroxy oxygen in 2, 3, and 5. All of the iron(II) complexes react with dioxygen to exhibit quantitative decarboxylation of benzilic acid to benzophenone. In the decarboxylation pathway, dioxygen becomes reduced on the iron center and the resulting iron-oxygen oxidant shows versatile reactivity. The oxidants are nucleophilic in nature and oxidize sulfide to sulfoxide and sulfone. Furthermore, complexes 2 and 4-6 react with alkenes to produce cis-diols in moderate yields with the incorporation of both the oxygen atoms of dioxygen. The oxygen atoms of the nucleophilic oxidants do not exchange with water. On the basis of interception studies, nucleophilic iron(II) hydroperoxides are proposed to generate in situ in the reaction pathways. The difference in reactivity of the complexes toward external substrates could be attributed to the geometry of the O₂-derived iron-oxygen oxidant. DFT calculations suggest that, among all possible geometries and spin states, high-spin side-on iron(II) hydroperoxides are energetically favorable for the complexes of 6-Me₃-TPA, 6-Me₂-iso-BPMEN, BPMEN, and 6-Me₂-BPMEN ligands, while high spin end-on iron(II) hydroperoxides are favorable for the complexes of TPA, iso-BPMEN, and TBimA ligands.

Full text of DOI:10.1021/acs.inorgchem.6b02282

Article
pubs.acs.org/IC
Reductive Activation of O2 by Non-Heme Iron(II) Benzilate
Complexes of N4 Ligands: Effect of Ligand Topology on the
Reactivity of O2‑Derived Oxidant
Biswarup Chakraborty, Rahul Dev Jana, Reena Singh, Sayantan Paria, and Tapan Kanti Paine*
Department of Inorganic Chemistry, Indian Association for the Cultivation of Science, 2A & 2B Raja S. C. Mullick Road, Jadavpur,
Kolkata 700032, India
S
* Supporting Information
ABSTRACT: A series of iron(II) benzilate complexes (1−7) with
general formula [(L)FeII(benzilate)]+ have been isolated and
characterized to study the effect of supporting ligand (L) on the
reactivity of metal-based oxidant generated in the reaction with
dioxygen. Five tripodal N4 ligands (tris(2-pyridylmethyl)amine
(TPA in 1), tris(6-methyl-2-pyridylmethyl)amine (6-Me3-TPA in
2), N1,N1-dimethyl-N2,N2-bis(2-pyridylmethyl)ethane-1,2-diamine
(iso-BPMEN in 3), N1,N1-dimethyl-N2,N2-bis(6-methyl-2-
pyridylmethyl)ethane-1,2-diamine (6-Me2-iso-BPMEN in 4), and
tris(2-benzimidazolylmethyl)amine (TBimA in 7)) along with two
linear tetradentate amine ligands (N1,N2-dimethyl-N1,N2-bis(2-
pyridylmethyl)ethane-1,2-diamine (BPMEN in 5) and N1,N2-
dimethyl-N1,N2-bis(6-methyl-2-pyridylmethyl)ethane-1,2-diamine
(6-Me2-BPMEN in 6)) were employed in the study. Single-crystal X-ray structural studies reveal that each of the complex cations
of 1−3 and 5 contains a mononuclear six-coordinate iron(II) center coordinated by a monoanionic benzilate, whereas complex 7
contains a mononuclear five-coordinate iron(II) center. Benzilate binds to the iron center in a monodentate fashion via one of
the carboxylate oxygens in 1 and 7, but it coordinates in a bidentate chelating mode through carboxylate oxygen and neutral
hydroxy oxygen in 2, 3, and 5. All of the iron(II) complexes react with dioxygen to exhibit quantitative decarboxylation of
benzilic acid to benzophenone. In the decarboxylation pathway, dioxygen becomes reduced on the iron center and the resulting
iron−oxygen oxidant shows versatile reactivity. The oxidants are nucleophilic in nature and oxidize sulfide to sulfoxide and
sulfone. Furthermore, complexes 2 and 4−6 react with alkenes to produce cis-diols in moderate yields with the incorporation of
both the oxygen atoms of dioxygen. The oxygen atoms of the nucleophilic oxidants do not exchange with water. On the basis of
interception studies, nucleophilic iron(II) hydroperoxides are proposed to generate in situ in the reaction pathways. The
difference in reactivity of the complexes toward external substrates could be attributed to the geometry of the O2-derived iron−
oxygen oxidant. DFT calculations suggest that, among all possible geometries and spin states, high-spin side-on iron(II)
hydroperoxides are energetically favorable for the complexes of 6-Me3-TPA, 6-Me2-iso-BPMEN, BPMEN, and 6-Me2-BPMEN
ligands, while high spin end-on iron(II) hydroperoxides are favorable for the complexes of TPA, iso-BPMEN, and TBimA ligands.
substrates have been developed that use dioxygen directly in
the presence of reductants.1,16−26
INTRODUCTION
■
A large variety of biologically important oxidative trans-
formations such as aliphatic and aromatic C−C bond cleavage,
hydroxylation of alkanes and arenes, C−H bond halogenation,
epoxidation and cis-dihydroxylation of olefins, oxidation of
organosulfur compounds, etc. are catalyzed by non-heme iron
enzymes using dioxygen as the terminal oxidant.1−5 In the
catalytic cycles of these oxygenases, NADH or organic
cofactors/substrates facilitate the reduction of O2, leading to
the generation of iron−oxygen oxidants. The resulting metal-
based oxidants carry out different oxidation/oxygenation
reactions.1,6,7 Generation of different iron−oxygen intermedi-
ates through the reductive activation of dioxygen by synthetic
iron(II) complexes in the presence of electron and proton
donors has been documented.8−15 However, only a limited
number of iron complexes for oxyfunctionalization of organic
In bioinspired oxidation catalysis with metal-based catalysts
and dioxygen, selective electron transfer remains a major
challenge. With small-molecule systems, it is difficult to provide
electrons selectively for the reduction of dioxygen without
quenching the active oxidant. Uncontrolled electron transfers
from reductants to metal−oxygen species result in nonselective
oxidation of substrates. Therefore, most of the bioinspired
catalysts developed so far have used peroxides or other oxidants
to bypass the reduction process.27−42
Inspired by the reactions of the non-heme α-keto acid-
dependent enzymes,2,6 where α-keto acid cofactors bind to the
Received: September 20, 2016
© XXXX American Chemical Society
A
DOI: 10.1021/acs.inorgchem.6b02282
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
metal center prior to dioxygen activation and selectively
provide electrons to generate high-valent iron oxo species, we
have been investigating the dioxygen reactivity of ternary metal
complexes of polydentate supporting ligands and different
reductants as coligands.20−23 Recently, we have reported an
iron(II) benzilate complex of a facial tridentate N3 ligand,
[(TpPh2)FeII(benzilate)], which reacts with O2 to form
benzophenone in quantitative yield. The complex oxidizes
sulfides and activates aliphatic C−H bonds of various
substrates. In addition, the complex efficiently transforms
alkenes to the corresponding cis-diols with the incorporation of
both of the oxygen atoms of O2.20 In a subsequent study, the
reactivity of a series of iron(II) α-hydroxy acid complexes of the
same ligand toward various external substrates has been
investigated.21 On the basis of interception and mechanistic
studies, a nucleophilic iron−oxygen intermediate, generated
upon 2e reduction of O2, has been proposed as the active
oxidant.
Metal complexes of tetradentate nitrogen donor ligands that
provide two available cis sites for binding of oxidant have been
extensively used in peroxide-dependent catalytic oxidation of
substrates.29,30,38,42−48 The effect of ligand topology on the
catalytic oxidation of olefins and alkanes by biomimetic iron(II)
complexes has been reported.32,46,47,49,50 Therefore, iron
complexes of N4 ligands likely allow the formation of iron−
oxygen intermediates from O2 in the decarboxylation of α-
hydroxy acids. The tetradentate ligands shown in Chart 1 can
TPA = tris(2-pyridylmethyl)amine, 6-Me3-TPA = tris(6-
methyl-2-pyridylmethyl)amine, iso-BPMEN = N1,N1-dimethyl-
N2,N2-bis(2-pyridylmethyl)ethane-1,2-diamine, 6-Me2-iso-
BPMEN = N1,N1-dimethyl-N2 ,N2 -bis(6-methyl-2-
pyridylmethyl)ethane-1,2-diamine, BPMEN = N1,N2-dimethyl-
N1,N2-bis(2-pyridylmethyl)ethane-1,2-diamine, 6-Me2-BPMEN
= N1,N2-dimethyl-N1,N2-bis(6-methyl-2-pyridylmethyl)ethane-
1,2-diamine, and TBimA = tris(2-benzimidazolylmethyl)amine)
(Chart 1). While the iron(II) benzilate complexes of TPA, 6-
Me3-TPA, iso-BPMEN, and 6-Me2-iso-BPMEN have a dis-
torted-octahedral geometry,30,47 BPMEN and 6-Me2-BPMEN44
adopt a cis-α geometry in the corresponding six-coordinate
iron(II) benzilate complexes. The ligand TBimA, on the other
hand, stabilizes a five-coordinate complex with trigonal-
bipyramidal coordination geometry at the iron center. The
iron−oxygen oxidants derived from the reactions between
iron(II) complexes and O2 are intercepted by external
substrates. The effect of ligand topology on the reactivity of
O2-derived oxidants is discussed in this work.
RESULTS AND DISCUSSION
■
Synthesis and Characterization. Reactions of equimolar
amounts of iron(II) perchlorate, benzilic acid, triethylamine,
and respective tetradentate ligand in methanol afforded light
yellow iron(II) benzilate complexes. Iron(II) complexes of TPA
and iso-BPMEN were isolated as tetraphenylborate salts by
addition of sodium tetraphenylborate to the reaction solution
(Scheme 1).
Chart 1. Tetradentate Ligands Used in This Study
The optical spectra of the complexes in acetonitrile display
absorption bands in the region between 370 and 390 nm
(Figure S1 in the Supporting Information). Room-temperature
magnetic moment values for the complexes are found in the
range of 4.8−5.2 μB, indicating the high-spin nature of the
complexes. A higher magnetic moment for 2 indicates a large
orbital contribution to the magnetic moment. All of the iron
complexes exhibit broad and paramagneticaly shifted reso-
1
nances of protons. The H NMR spectra of 1, 3, and 7 show
resonance signals between 0 and 140 ppm (Figure S2 in the
Supporting Information), whereas complexes 2, 4 and 6 exhibit
proton resonances in the region between −68 and 115 ppm
(Figure S3 in the Supporting Information). The number of
peaks observed in the 1H NMR spectra of 5 and 6 indicate that
the complexes have a C2-symmetric high-spin iron(II) center in
contrast to complexes 1−4 with respective ligand topologies in
solution.53 The 1H NMR spectrum of complex 7 bears a
resemblance to that of [(TBimA)FeII(CH3CN)]2+ and other
iron(II) complexes of the ligand, indicating a 3-fold symmetry
of the complex in solution (Figure S4 in the Supporting
Information).54
X-ray Crystal Structures. The structures of the complexes
(1−3, 5, and 7) in the solid state have been established from
single-crystal X-ray diffraction studies. Each of the mono-
cationic complexes, except complex 7, contains a six-coordinate
iron center surrounded by one N4 ligand and one monoanionic
benzilate (Figures 1 and 2). The average Fe−N distances are
found in the range of 2.056(2)−2.415(1) Å, which match well
with the reported high-spin iron(II) complexes of the
respective ligands (Table 1).53,55,56 The monoanionic benzilate
is coordinated to the metal center of 1 through one of the
carboxylate oxygens (O1) with an Fe1−O1 distance of
2.032(1) Å (Table 1). One methanol molecule binds at the
sixth site, giving rise to distorted-octahedral coordination
geometry at the metal center (Figure 1a). The pyridine
engender different geometries of iron complexes.51,52 Similarly,
the stabilities of iron−oxygen intermediates generated from
iron(II) benzilate complexes and their reactivities toward
external reagents are expected to vary depending upon the
topology/geometry engendered by tetradentate ligands. To test
this hypothesis, we have explored the dioxygen reactivity of
iron(II) α-hydroxy acid complexes of different N4 ligands.
Herein, we report the isolation and characterization of seven
ternary iron(II) benzilate complexes, [(TPA)FeII(benzilate)-
(MeOH)](BPh4) (1), [(6-Me3-TPA)FeII(benzilate)](ClO4)
(2), [(iso-BPMEN)FeII(benzilate)](BPh4) (3), [(6-Me2-iso-
BPMEN)FeI I (benzilate)](ClO4) (4), [(BPMEN)-
FeII(benzilate)](ClO4) (5), [(6-Me2-BPMEN)FeII(benzilate)]-
(ClO4) (6), and [(TBimA)FeII(benzilate)](ClO4) (7) (where
B
DOI: 10.1021/acs.inorgchem.6b02282
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
Scheme 1. Synthesis of Iron(II) Benzilate Complexes
Figure 2. ORTEP plots of the complex cations of (a) 3 and (b) 5 with
30% thermal ellipsoid parameters. The counterion and all H atoms
except that on O3 have been omitted for clarity.
Figure 1. ORTEP plots of the complex cations of (a) 1 and (b) 2 with
30% thermal ellipsoid parameters. The counterion and all H atoms
other than those on O4 and C57 in 1 and O3 in 2 are omitted for
clarity.
complex, [(6-Me3-TPA)FeII(mandelate)](BPh4) (Table 1).57
While the hydroxy oxygen O3 and pyridine nitrogen N4
occupy the axial positions with an O3−Fe1−N4 angle of
165.77(4)° in 2, the axial positions are occupied by the
carboxylate oxygen O1 and the amine nitrogen N2 with an
O1−Fe1−N2 angle of 174.45(11)° in 3. On the other hand, in
5, two pyridine nitrogens (N1 and N4) are positioned at the
axial sites with an N1−Fe1−N4 angle of 169.17(18)°. The
solid-state packing of 5 allows an intermolecular hydrogen
bond between the hydroxy hydrogen atom of O3 and the
noncoordinated carboxylate oxygen O2 of a neighboring
molecule.
nitrogen N4 and the oxygen atom O3 of methanol occupy the
axial positions with an N4−Fe1−O3 angle of 167.82(5)°. An
intermolecular hydrogen bond is observed between the
noncoordinated hydroxy group of benzilate and the coordi-
nated methanol molecule of another complex, forming a one-
dimensional hydrogen-bonded network structure.
In 2, 3, and 5, benzilate is coordinated to the iron center in a
bidentate chelating mode through the neutral hydroxy oxygen
(O3) and the anionic carboxylate (O1) with Fe1−O1 and
Fe1−O3 distances ranging between 2.013(9) and 2.074(5) Å
and between 2.084(5) and 2.214(9) Å, respectively (Figures 1b
and 2 and Table 1). The average Fe−N distance is longer in 2
and is closely comparable to a related iron(II) α-hydroxy acid
Complex 7 is a mononuclear five-coordinate iron complex,
where the tetradentate ligand wraps the metal center with three
imidazolyl nitrogens with an average Fe−Nimidazole distance of
C
DOI: 10.1021/acs.inorgchem.6b02282
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
Table 1. Selected Bond Lengths (Å) and Angles (deg) of 1−3, 5, and 7
bond distance/angle
1
2
3
5
7
Fe(1)−N(1)
Fe(1)−N(2)
Fe(1)−N(3)
Fe(1)−N(4)
Fe(1)−O(1)
Fe(1)−O(3)
2.205(1)
2.245(1)
2.195(1)
2.169(1)
2.032(1)
2.184(1)
2.205(1)
2.179(1)
2.240(1)
2.266(1)
2.013(1)
2.214(1)
2.134(4)
2.248(3)
2.149(4)
2.247(3)
2.043(2)
2.140(2)
2.191(5)
2.217(5)
2.233(5)
2.183(5)
2.074(4)
2.084(4)
2.415(1)
2.090(1)
2.085(1)
2.056(1)
2.019(1)
O(1)−Fe(1)−N(1)
O(1)−Fe(1)−N(2)
O(1)−Fe(1)−N(3)
O(1)−Fe(1)−N(4)
O(1)−Fe(1)−O(3)
N(1)−Fe(1)−N(2)
N(1)−Fe(1)−N(3)
N(1)−Fe(1)−N(4)
N(2)−Fe(1)−N(3)
N(2)−Fe(1)−N(4)
N(3)−Fe(1)−N(4)
O(3)−Fe(1)−N(1)
O(3)−Fe(1)−N(2)
O(3)−Fe(1)−N(3)
O(3)−Fe(1)−N(4)
90.34(5)
166.73(5)
118.69(5)
97.33(5)
90.17(4)
76.55(5)
150.90(5)
80.36(5)
74.50(5)
78.69(5)
96.72(5)
90.05(5)
91.84(4)
87.98(5)
167.82(5)
102.62(4)
165.28(4)
101.01(4)
114.30(4)
74.26(4)
77.40(5)
154.97(5)
95.89(4)
77.64(5)
80.17(4)
81.73(5)
93.03(4)
91.02(4)
85.49(4)
165.77(4)
107.18(12)
174.45(11)
99.92(13)
94.49(11)
75.05(9)
98.50(16)
101.29(17)
167.61(16)
91.65(16)
76.33(15)
76.81(18)
93.87(17)
169.17(18)
80.88(17)
97.58(17)
75.96(17)
89.80(16)
165.99(16)
104.46(16)
96.29(16)
175.38(5)
104.74(5)
102.15(5)
108.77(5)
76.28(15)
151.51(14)
95.56(14)
77.43(15)
80.75(12)
90.85(14)
89.10(11)
109.62(11)
89.52(12)
169.43(12)
74.74(5)
75.03(5)
75.75(5)
127.93(5)
102.71(5)
109.55(5)
2.077(1) Å. However, the iron−Namine distance becomes
elongated to 2.415(1) Å. Benzilate is coordinated to the
metal center in a monodentate mode through one anionic
carboxylate oxygen (O1) with an Fe1−O1 distance of 2.019(1)
Å (Figure 3 and Table 1). The geometry adapted by the metal
Figure 4. Coordination geometry at the metal center of iron(II)
benzilate complexes.
and analytical data leave no doubt about the composition of the
complexes. The optimized geometry of 4 obtained from DFT
calculations suggests a six-coordinate iron complex with
bidentate binding of monoanionic benzilate similar to that in
3 (Table S1 and Figure S5 in the Supporting Information).
Reactivity of the Iron(II) Benzilate Complexes. All of
the iron(II) α-hydroxy acid complexes (1−7) are air-sensitive
in solution. In reactions with O2, light yellow solutions of the
complexes in acetonitrile readily convert to light orange. In the
reaction between 1 and dioxygen, the intensity of the CT band
at 392 nm increases slowly with the formation of a broad band
at 326 nm (Figure 5). In the X-band EPR spectrum collected at
77 K, a gradual increment of the rhombic signal at g = 4.2 is
observed. In addition, three weak signals at g = 2.39, 2.23, 1.88
are observed (Figure S6 in the Supporting Information). The
signals in the EPR spectrum (Figure 5, inset) indicate the
formation of a mixture of high-spin iron(III) and low-spin
iron(III) species.59 The changes in optical and EPR spectra
clearly suggest the oxidation of 1. Reactions of other iron(II)
benzilate complexes with O2 exhibit similar spectral changes
(Figures S7−S12 in the Supporting Information). Complex 1
reacts over a period of 2 h, and complex 3 takes about 1 h. The
other complexes (2 and 4−7) require 2−6 h for complete
Figure 3. ORTEP plot of the complex cation of 7 with 30% thermal
ellipsoid parameters. The counterion and all H atoms other than those
on O3 and on selected nitrogen atoms have been omitted for clarity.
center is distorted trigonal bipyramidal (τ = 0.79),58 where the
axial positions are occupied by an amine nitrogen (N1) and
carboxylate oxygen (O1) with an N1−Fe1−O1 angle of
175.38(5)°.
The linear tetradentate ligand BPMEN adopts a cis-α
geometry in complex 5 (Figure 4). The presence of a fewer
number of proton resonances in the NMR spectra of 5 and 6
(Figures S2 and S3 in the Supporting Information) strongly
suggests that both the complexes retain their cis-α geometry in
solution as well.52 In contrast, complex 7 possesses a five-
coordinate trigonal-bipyramidal (tbp) geometry (Figure 4).
Although X-ray diffraction quality single crystals could not be
isolated for 4 and 6 after several attempts, the spectroscopic
D
DOI: 10.1021/acs.inorgchem.6b02282
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
quantitative formation of benzophenone in 4 h (Figure S17
in the Supporting Information).
The iron−oxygen oxidants, formed in situ upon two-electron
oxidative decarboxylation of iron(II) benzilate complexes, were
intercepted in the reaction with different substrates (Scheme
2). When 10 equiv of thioanisole is added separately to the
solutions of 1−7, thioanisole oxide is formed in each reaction.
In addition, methyl phenyl sulfone is observed in low yield
(<10%) for 2, 4, and 6 (Table 2 and Figure S18 in the
Table 2. Distribution of Sulfoxide and Sulfone Formed in the
Reaction between Iron(II) Benzilate Complexes and O2 with
Sulfides
iron(II) benzilate complex
amt of
Figure 5. Optical spectral changes with time during the reaction
between 1 (0.5 mM in CH3CN) and dioxygen at 298 K. Inset: X-band
EPR spectrum at 77 K of the oxidized solution.
PhSCH3
used
yield (%) of
PhSOCH3/
PhSO2CH3
(equiv)
1
2
3
4
5
6
7
1
2
PhSOCH3
PhSO2CH3
PhSOCH3
PhSO2CH3
PhSOCH3
PhSO2CH3
PhSOCH3
PhSO2CH3
PhSOCH3
PhSO2CH3
0
10
0
2
4
1
25
12
27
15
30
13
35
8
0
5
0
12
15
5
0
7
12
20
17
28
11
64
4
oxidation. Each of the oxidized solutions of 2−7 shows a
rhombic EPR (X-band at 77 K) signal at g = 4.2.
5
0
3
12
8
3
4
2
The reaction of 1 with dioxygen was also monitored by time-
dependent 1H NMR spectroscopy. A gradual change is
observed with time during the reaction in CD3CN. The
resonance signals of the pyridine rings of TPA observed at δ
137.7, 47, 46, and 20 ppm slowly disappear with a concomitant
appearance of three new peaks at 100, 16, and −4 ppm (Figure
S13 in the Supporting Information). The broad nature of the
final spectrum indicates the formation of iron(III) species at the
5
8
10
2
26
7
8
1
3
0
10
20
10
0
13
0
16
0
57
8
16
0
17
0
67
0
20
0
37
5
21
0
61
0
25
0
1
end of the reaction (Figure S13d). Moreover the H NMR
spectrum of the oxidized solution of 1 has features similar to
Supporting Information). The yield of thioanisole oxide is,
however, found to be <20% for 1, 3, 5, and 7, whereas 64%,
35%, and 57% of thioanisole oxide are found for 2, 4, and 6,
respectively (Table 2). The distribution of thioanisole oxide
and methyl phenyl sulfone varies with the amount of sulfide
added to the reaction mixture (Table 2). At a lower
concentration of sulfide, a higher amount of sulfone is formed
and the yield of sulfoxide decreases. With 1 equiv of thioanisole,
only sulfone is formed for complexes 1 and 5−7. On the other
hand, complexes 2−4 afford a mixture of sulfoxide and sulfone.
Of note, the iron(II) benzilate complex [(TpPh2)-
FeII(benzilate)] has been reported to yield only sulfone
(34%) in the presence of 1 equiv of thioanisole.21 Complexes
2, 4, and 5 oxidize dibenzothiophene to afford dibenzothio-
phene sulfoxide in 50%, 31% and 20% yields, respectively
(Table 3 and Figure S19 in the Supporting Information). In
contrast, the yield of dibenzothiophene sulfoxide is ≤6% with 1,
3, 6, and 7 (Scheme 2). Importantly, the oxidative
decarboxylation of benzilate is not affected in the presence of
sulfides.
those reported for an iron(III) oxo bridged dimer.53 Likewise,
1
the H NMR spectra of the oxidized solutions of 2 and 4 also
display broad signals due to the formation of iron(III) species
(Figures S14 and S15 in the Supporting Information).
Three proton resonances at 7.80, 7.68, and 7.56 ppm, which
are absent in the initial spectrum, are observed in the NMR
spectrum of the oxidized solution of 1 (Figure S13d in the
Supporting Information, inset). The peaks could be attributed
to the aromatic protons of noncoordinated benzophenone
1
formed in the decarboxylation of benzilate. The H NMR
spectrum of the organic product, isolated from the oxidized
solution after removal of the metal ion, exhibits resonances at δ
7.81 (d, 4H), 7.59 (t, 2H), and 7.48 ppm (t, 4H). The absence
of any peak of benzilic acid in the 1H NMR spectrum confirms
the quantitative conversion of benzilate to benzophenone. The
ESI-MS spectrum of the organic compound shows a molecular
ion peak at m/z 183.2 with an isotopic distribution calculated
for [C13H10O + H]+ (Figure S16 in the Supporting
Information). The reaction of 5 with dioxygen shows
Scheme 2. Reactions of the Iron(II) Benzilic Acid Complexes with Dioxygen and Sulfides
E
DOI: 10.1021/acs.inorgchem.6b02282
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
Table 3. Product(s) Derived from Substrates in the Reaction with Iron(II) Benzilate Complexes and Dioxygen
external substrate (amt (equiv))
product
1 (%)
2 (%)
50
3 (%)
4 (%)
31
5 (%)
20
6 (%)
7 (%)
dibenzothiophene (10)
dihydroanthracene (10)
fluorene (10)
dibenzothiophene oxide
anthracene
<5
22
28
65
34
<5
21
35
61
42
6
<5
38
20
72
68
26
28
24
34
fluorenone
25
33
34
36
benzaldehyde (5)
p-bromo-benzaldehyde (10)
styrene (100)
benzoic acid
82
41
74
84
p-bromobenzoic acid
1-phenylethane-1,2-diol, benzoic acid (b)
cis-cyclohexane-1,2-diol
cis-cyclooctane-1,2-diol
octane-1,2-diol
54
44
46
85
17, 40 (b)
35, 24 (b)
16, 20 (b)
42, 17 (b)
cyclohexene (100)
cyclooctene (100)
1-octene (100)
20
16
12
29
12
<5
<5
<10
12
21
cis-2-heptene
cis-2,3-heptanediol
24
Complex 2 is found to exhibit better reactivity for oxygen
atom transfer to thioanisole. Thus, labeling experiments for
thioanisole oxidation by complex 2 were performed with 18O2
to assess the fate of molecular oxygen into sulfide-derived
products. The GC-mass spectra of organic products display ion
peaks at m/z 140 and 156 corresponding to thioanisole oxide
and methyl phenyl sulfone that are shifted to m/z 142 and 160,
respectively (Figure S20 in the Supporting Information). The
results establish that the oxygen atoms incorporated into the
products are derived from dioxygen. Oxidation of thioanisole
by 2 with 16O2 in the presence of H218O results in sulfoxide
with no labeled oxygen atom incorporation from water. It is
important to mention here that the benzilate-derived product,
benzophenone, does not contain any labeled oxygen.
4 is a relatively weaker nucleophile in comparison to those from
complexes 2 and 6. Hammett analyses could not be performed
for other complexes due to the low yields of sulfoxide and
sulfone in their reactions with thioanisole. However, from the
product distribution, it is clear that the respective oxidants from
1, 3, 5, and 7 are less nucleophilic.
A nucleophilic iron−oxygen intermediate derived from the
reaction between [(TpPh2)FeII(benzilate)] complex and oxygen
has been reported to carry out cis-dihydroxylation of various
alkenes.21 To probe further the nature of iron−oxygen oxidants
derived from the iron(II) benzilate complexes of the
tetradentate ligands, the reactivity of the complexes toward
various alkenes such as styrene, cyclohexene, 1-octene, and
cyclooctene was investigated (Scheme 3 and Table 3).
The formation of sulfone at low sulfide concentration
indicates the involvement of a nucleophilic iron−oxygen
oxidant responsible for the oxygen atom transfer to
thioanisole.20,21 To test this hypothesis, Hammett analyses
were performed for sulfide oxidation by iron(II) complexes 2,
4, and 6. For Hammett analyses, competitive reactions were
carried out with 1/1 mixtures of thioanisole and different para-
substituted thioanisoles (p-XC6H4SCH3, where X = NO2, Br,
H, Me, OMe). A ρ value of +0.74 is obtained from the
Hammett plot of the relative rates (krel) versus σp for complex 2
(Figure 6), supporting the nucleophilic nature of the oxidant.
While a Hammett ρ value of +0.67 is obtained with complex 6,
complex 4 shows a ρ value of +0.34 (Figures S21 and S22 in
the Supporting Information). Thus, the oxidant generated from
Scheme 3. Products Obtained from the Reactions of Iron(II)
Benzilate Complexes 2 and 4−6 with Alkenes
Complexes 2 and 4−6 are capable of forming 1-phenyl-
ethane-1,2-diol with 17%, 35%, 16%, and 42% yields,
respectively, when reactions are carried out with 100 equiv of
styrene (Figure S23 in the Supporting Information). In
addition to 1-phenylethane-1,2-diol, benzoic acid is also formed
as an overoxidation product (Table 3).
When 100 equiv of cyclohexene is used, <20% cis-
cyclohexane-1,2-diol is observed only for 2, 5, and 6 (Scheme
3 and Figure S24 in the Supporting Information). Cyclooctene
and 1-octene remain unaffected in the reaction with 1, 3, and 4,
whereas cis-cyclooctane-1,2-diol and octane-1,2-diol (16% and
12%, respectively) are formed in the reaction with 2 (Figures
S25−S27 in the Supporting Information). Heptane-2,3-diol is
formed from cis-2-heptene with complexes 2 and 6 (Figure S28
in the Supporting Information). A labeling experiment in the
cis-dihydroxylation of 1-octene by 2 reveals that both of the
oxygen atoms are derived from aerial O2 (Figure 7). However,
no incorporation of oxygen from water (H218O) was found in
the cis-diol product in a mixed labeling experiment with 16O2
Figure 6. Hammett plot of log krel versus σp for p-XC6H4SCH3 for
complex 2. The krel value was calculated by dividing the concentration
of the product from p-XC6H4SCH3 by the concentration of the
product from PhSCH3.
F
DOI: 10.1021/acs.inorgchem.6b02282
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
reaction. However, its methyl-substituted congener (6) shows
better reactivity. Thus, fine tuning of electronic and steric
factors eventually allows complex 2 (6-Me3-TPA ligand) to
show the best reactivity in oxygen atom transfer and olefin cis-
dihydroxylation.
The nucleophilic oxidants generated from the iron(II)
benzilate complexes (1−7) of N4 ligands are, however, less
reactive in comparison to that generated from the iron(II)
benzilate complex [(TpPh2)FeII(benzilate)]. A high-spin iron-
(III) hydroperoxo species supported by a hexadentate N4O2
donor ligand (H2bppa) has been reported to oxidize dimethyl
sulfoxide more efficiently than dimethyl sufide, supporting the
nucleophilic nature of the oxidant.60 A nucleophilic FeIII−OOH
species has been postulated as the active species in the cis-
dihydroxylation of alkenes with H2O2 catalyzed by biomimetic
complexes of N4 ligands.30,61,62 By analogy to the reactivity of
the oxidant from [(TpPh2)FeII(benzilate)],21,22 the reductive
activation of dioxygen on the iron(II) benzilate complexes of
tetradentate ligands is proposed to generate the corresponding
iron(II) hydroperoxide intermediate species that are nucleo-
philic in character. From the reactivity/interception studies
with the iron(II) benzilate complexes discussed in this work, it
is clear that the versatile reactivity exhibited by the complexes
toward an external substrate depends on the supporting ligand.
Density Functional Theory Calculations and Mecha-
nistic Proposal. The oxidative decarboxylation of mandelic
acid on a six-coordinate iron(II) complex of 6-Me3-TPA has
been proposed to initiate via opening of a pyridine arm.57 DFT
calculations on the oxidative aromatic ring fission of 2-
aminophenol on the (6-Me3-TPA)FeII center supported the
dioxygen binding mechanism following a pyridine arm
dissociation pathway. The calculated free energy barrier to
form a five-coordinate species from a six-coordinate iron(II) 2-
aminophenolate complex was found to be 7.98 kcal/mol.63 It
was thus assumed that, for the iron(II) benzilate complexes,
pyridine arm dissociation could also be a favorable pathway,
similar to the case for the reported biomimetic systems.
However, this could not be verified by any experimental
methods. Therefore, DFT calculations were conducted on the
complex [(6-Me3-TPA)FeII(benzilate)]+ to get a preliminary
understanding of the oxygen binding to the iron center. Two
different geometries of the five-coordinate species were
considered: (i) with one of the pyridyl arms of the ligand
open and (ii) with monodentate benzilate where the hydroxy
group remained noncoordinated. The calculated free energy
differences between six-coordinate and five-coordinate geo-
metries were found to be 8.99 and 10.43 kcal/mol, respectively
(Figures S32 and S33 in the Supporting Information). Thus,
dissociation of one of the pyridyl arms is proposed to take place
prior to dioxygen binding. However, the possibility of another
pathway cannot be ruled out completely.
Figure 7. GC-mass spectrum of octane-1,2-diol formed in the reaction
of 2 with 100 equiv of 1-octene in the presence of (a) 16O2 and (b)
18O2.
and H218O in the presence of 100 equiv of 1-octene. Thus, the
cis-dihydroxylation of alkenes without any incorporation of
oxygen atom from water in the diol product observed with 2
unambiguously suggests the formation of an iron−oxygen
oxidant that does not cleave its O−O bond prior to the cis-
dihydroxylation reaction.
When p-bromobenzaldehyde is used as an intercepting
reagent, only p-bromobenzoic acid is formed with all of the
complexes (Table 3 and Figure S29 in the Supporting
Information). Similarly, benzaldehyde forms benzoic acid as
the only product with all of the complexes (Table 3). A labeling
experiment with 18O2 suggests the incorporation of one labeled
oxygen atom into the benzoic acid (Figure S30 in the
Supporting Information). Each of the iron(II) benzilate
complexes (1−7) reacts with fluorene (20 equiv) to afford
fluorenone (Table 3 and Figure S31 in the Supporting
Information). 9,10-Dihydroanthracene forms anthracene in
the oxidation reactions (Scheme 4 and Table 3).
Scheme 4. Products Formed in the Reactions of Iron(II)
Benzilate Complexes 1−7 with Different Substrates
In comparison to iron(II) α-hydroxy acid (benzilate and
mandelate) complexes of tridentate ligands (TpPh2),20,21
complexes 1−7 react slowly with O2 and the reactions take
hours for completion (Table 4). The oxidants from 2, 4, and 6
react with thioanisole (10 equiv) to afford methyl phenyl
sulfoxide along with a small amount of methyl phenyl sulfone,
whereas those derived from 1, 3, 5, and 7 produce sulfone (5−
10%) only at low substrate concentration (<10 equiv). Among
all of the complexes, 2 and 6 show better reactivity in sulfide
oxidation. The oxidants generated from 1, 3, and 7 are able to
oxidize aliphatic substrates with weak C−H bonds but unable
to cis-dihydroxylate alkenes. In contrast, the oxidants from 2
and 4−6 are found to cis-dihydroxylate the CC bonds of
olefins (Table 4). Among those, the complexes of 6-Me3-TPA
and 6-Me2-BPMEN are found to cis-dihydroxylate alkenes in
relatively higher yields. The linear flexible tetradentate BPMEN
ligand gives rise to a mixture of isomers in solution,52 and its
iron(II) benzilate complex (5) showing cis-α geometry is not
efficient enough for an O2-dependent cis-dihydroxylation
The putative iron(II) hydroperoxide intermediate respon-
sible for substrate oxidation, however, could not be detected or
isolated for spectroscopic characterization. The geometries of
the intermediates [(L)FeII(OOH)]+ were optimized by DFT
calculations with the Cartesian coordinates of the atoms
obtained from the crystal structures of the respective iron(II)
benzilate complexes to understand the geometry and electronic
structure (Figure 8 and Tables S2−S4 and Figure S34 in the
Supporting Information). End-on iron(II) hydroperoxides were
found to be energetically more stable with TPA, iso-BPMEN,
and TBimA ligands (Figure 8a and Figure S34a,e), where in
each case the metal center adopts a distorted-trigonal-
G
DOI: 10.1021/acs.inorgchem.6b02282
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
Table 4. Comparison of Oxidizing Ability of Iron(II) Benzilate Complexes with Different Ligands and Geometry at the Metal
Center
bipyramidal geometry with an N4O coordination environment.
The axial positions are occupied by the amine nitrogen N2 and
the peroxo oxygen O1. The optimized geometries of [(6-Me3-
TPA)FeII(OOH)]+, [(6-Me2-iso-BPMEN)FeII(OOH)]+,
[(BPMEN)FeII(OOH)]+, and [(6-Me2-BPMEN)FeII(OOH)]+
reveal side-on iron(II) hydroperoxo species where mono-
anionic hydroperoxide is coordinated to the iron center
thorough both of the oxygen atoms with unsymmetrical
iron−oxygen binding (Figure 8b and Figure S34b−d and
Table S3). The iron center in each of the [(L)FeII(OOH)]+
Figure 8. Optimized geometries of [(L)FeII(OOH)]+ intermediate
species for 6-Me3-TPA, 6-Me2-iso-BPMEN, BPMEN, and 6-
((a) L = TPA and (b) L = 6-Me3-TPA) calculated by DFT.
Me2-BPMEN ligands has an N4O(OH) coordination environ-
Scheme 5. Iron−Oxygen Oxidants Formed in the Oxidative Decarboxylation of Iron(II) Benzilate Complexes
H
DOI: 10.1021/acs.inorgchem.6b02282
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
ment. The optimization of [(L)FeII(OOH)]+ was performed
with two possible geometries: i.e., the iron-coordinated oxygen
(O1) atom of the peroxide trans to the amine nitrogen (N2)
and cis to the amine nitrogen (N2) of the tetradentate ligand.
The ligands BPMEN and 6-Me2-BPMEN being centrosym-
metric, both geometrical isomers of [(BPMEN)FeII(OOH)]+
and [(6-Me2-BPMEN)FeII(OOH)]+ have identical energies.
DFT calculations could not locate any energy minima for the
end-on iron(II) hydroperoxide for [(BPMEN)FeII(OOH)]+,
whereas the Fe−O2 distance of 2.424 Å in [(6-Me2-
BPMEN)FeII(OOH)]+ predicts an intermediate state between
end-on and side-on geometry (Table S4). The shorter iron−
oxygen distances in the optimized geometries of end-on
iron(II) hydroperoxides suggest the presence of a strong
bonding interaction with the metal center. As an effect of ligand
topology, two different geometries of the iron(II) hydro-
peroxide are responsible for the diverse reactivity toward
external substrates. The side-on-bound iron(II) peroxides from
2 (6-Me3-TPA ligand), 4 (6-Me2-iso-BPMEN ligand), 5
(BPMEN ligand), and 6 (6-Me2-BPMEN ligand) are able to
cis-dihydroxylate the CC bonds of olefins, whereas the end-
on iron(II) hydroperoxides from 1 (TPA ligand), 3 (iso-
BPMEN ligand), and 7 (TBimA) cannot oxygenate alkenes.
Spin density calculations suggest that, in addition to the
electron density on the metal center, the hydroperoxide ligand
possesses more electron density than the supporting N4 ligand,
which is in agreement with the nucleophilic nature of the
iron(II) hydroperoxide oxidant (Table S2).
On the basis of the results discussed above, a mechanism
very similar to that proposed for the iron(II) benzilate
complexes of tridentate ligands is put forward (Scheme 5). In
the first step of the reaction, an iron(III) superoxide radical
intermediate (II) is formed via displacement of an iron−pyridyl
bond.57 On the other hand, an analogous iron(III) superoxide
radical intermediate (II′) could also be formed, considering a
monodentate benzilate anion. Subsequent to the hydrogen
atom abstraction from the hydroxy group of α-hydroxy acid by
species II or II′, iron(III) hydroperoxo oxyl radical
intermediates (III and III′) are generated which upon
decarboxylation generate iron(II) hydroperoxo intermediates
(IV and V). Depending upon the topology of the supporting
ligand, the iron(II) hydroperoxo intermediate may possess
either end-on (IV) or side-on (V) geometry. The nucleophilic
iron(II) hydroperoxide (IV or V) could oxidize sulfide to
sulfoxide and sulfone. In contrast, only side-on iron(II)
hydroperoxides (V) afford a cis-diol product in reactions with
olefins. In the absence of external substrates, the intermediate
decays via some unproductive pathways.
reaction support the nucleophilic nature of the O2-derived
metal-based oxidant. Formation of an iron(II) hydroperoxide
intermediate is proposed on the basis of interception and
labeling experiments. DFT calculations suggest two possible
orientations of the hydroperoxide group in [(L)FeII(OOH)]+:
end-on (L = TPA, iso-BPMEN, TBimA) and side-on (L = 6-
Me3-TPA, 6-Me2-iso-BPMEN, BPMEN, 6-Me2-BPMEN). The
side-on-bound iron hydroperoxides can carry out the cis-
dihydroxylation of alkenes. The effect of the supporting ligand
in directing the reactivity of iron(II) benzilate complexes has
been demonstrated. This study should help in designing
appropriate supporting ligands for the development of
bioinspired oxidation catalysts for O2-dependent transforma-
tion reactions.
EXPERIMENTAL SECTION
■
Materials and Physical Measurements. All reagents and
solvents were purchased from commercial sources and were used
without further purification, unless otherwise noted. Solvents were
purified and dried prior to use. The preparation and handling of air-
sensitive materials were carried out under an inert atmosphere by
using standard Schlenk techniques or in a glovebox. Caution! Although
no problem was encountered during the synthesis of these complexes from
Fe(ClO4)2, perchlorate salts are potentially explosive and should be
handled with care.64 The ligands were synthesized according to the
procedure reported in the literature.56
Fourier transform infrared spectroscopy on KBr pellets was
performed on a Shimadzu FT-IR 8400S instrument. Elemental
analyses were performed on a PerkinElmer 2400 Series II CHN
analyzer. Electrospray ionization mass spectra were recorded with a
Waters QTOF Micro YA263 instrument. 1H NMR spectra were
measured at room temperature using a Bruker DPX-500 spectrometer.
Solution electronic spectra were measured on an Agilent 8453 diode
array spectrophotometer. Room-temperature magnetic data were
collected on a Gouy balance (Sherwood Scientific, Cambridge,
U.K.). Diamagnetic contributions were estimated for each compound
by using Pascal’s constants. X-band EPR measurements were
performed on a JEOL JES-FA 200 instrument. GC-MS measurements
were carried out with PerkinElmer Clarus 680 GC and SQ8T MS
instruments, using an Elite 5 MS (30 m × 0.25 mm × 0.25 μm)
column with a maximum temperature of 300 °C. Labeling experiments
were carried out with 18O2 gas (99 atom %) or H218O (98 atom %)
purchased from Icon Services Inc., USA.
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 5 mL of methanol. 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 3 h. While a
microcrystalline solid compound was obtained for complex 2 and 4−7
as perchlorate salts were isolated upon addition of diethyl ether to the
concentrated reaction solution, addition of 1 equiv of NaBPh4 (0.34 g,
1 mmol) resulted in the isolation of tetraphenylborate salts of
complexes 1 and 3.
CONCLUSIONS
■
We have synthesized and characterized a series of iron(II)
benzilate complexes using seven tetradentate nitrogen donor
ligands of varying topologies. Monodentate and bidentate
coordination modes of benzilate were observed in the X-ray
crystal structures of the mononuclear complexes 1−7. Although
a six-coordinate iron center with an N4O2 coordination
environment is found for complexes 1−6, the ligands wrap
around the metal center to display different geometries. While
the geometry around the metal center in 1−6 is distorted
octahedral, the supporting ligands adopt a cis-α geometry in 5
and 6. The complexes react with dioxygen under ambient
conditions, resulting in quantitative decarboxylation of metal-
coordinated benzilate. Hammett analyses in the sulfoxidation
[(TPA)FeII(benzilate)(MeOH)](BPh4) (1). Single crystals suitable for
X-ray diffraction were obtained by diffusion of diethyl ether into a
solution of the complex in THF/CH3OH. Yield: 0.66 g (71%). Anal.
Calcd for C57H53BFeN4O4 (924.71 g/mol): C, 74.04; H, 5.78; N, 6.06.
Found: C, 73.89; H, 5.82; N, 5.87. 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
CH3CN (λmax): 392 nm (ε = 1900 M−1 cm−1). Magnetic moment μeff
1
(298 K): 4.8 μB. H NMR in CD3CN (500 MHz; δ (ppm)): 137.7
(3H, Py-Hα), 55.3 (6H, CH2Py), 47.5 (3H, Py-Hβ), 46.4 (3H, Py-Hβ′),
20.4 (3H, Py-Hγ), 9.7 (4H, benzilate-o-H), 8.6 (4H, benzilate-m-H),
7.6 (2H, benzilate-p-H), 7.3 (8H, BPh4-Ar-o-H) 7.0 (8H, BPh4-Ar-m-
H), 6.8 (4H, BPh4-Ar-p-H).
I
DOI: 10.1021/acs.inorgchem.6b02282
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
[(6-Me3-TPA)FeII(benzilate)](ClO4) (2). Yield: 0.45 g (63%). Anal.
Calcd for C35H35ClFeN4O7 (714.97 g/mol): C, 58.80; H, 4.93; N,
7.84. Found: C, 58.46; H, 4.90; N, 7.77. IR (KBr, cm−1): 3080 (m),
2966 (m), 1622 (s), 1576 (m), 1456 (m), 1360 (m), 1090 (s), 800
(m), 700 (s), 623 (s). UV−vis in CH3CN (λmax): 367 nm (ε = 1500
M−1 cm−1). Magnetic moment μeff (298 K): 5.1 μB. 1H NMR in
CD3CN (500 MHz; δ (ppm)): 46.8 (3H, Py-Hβ and Py-Hβ′), 25.2
(3H, Py-Hγ), 12.6 (4H, benzilate-o-H), 8.5 (4H, benzilate-m-H), 7.3
(2H, benzilate-p-H), −49.7 (9H, CH3). Single crystals suitable for X-
ray diffraction of the tetraphenylborate salt of 2 were obtained by
vapor diffusion of diethyl ether into a solution of the complex in
CH2Cl2/CH3OH.
solution, and the mixture 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 sodium sulfate, and the solvent was
evaporated under reduced pressure. The products were analyzed and
quantified by 1H NMR spectroscopy with respect to the internal
standard. Quantification of benzophenone was done by comparing the
peak area of four aromatic ortho protons (δ 7.81 ppm) with the two
aromatic protons of 2,4-di-tert-butylphenol at δ 6.63 ppm in CDCl3.
Reactions of Iron(II) Benzilate Complexes with Substrates.
The iron(II) benzilate complex (0.02 mmol) was dissolved in 10 mL
of deoxygenated acetonitrile. To the resulting solution was added
external substrate. Pure oxygen gas was bubbled through the solution,
and the mixture was stirred at room temperature under an oxygen
atmosphere. The solution slowly changed from light yellow to light
orange. The solution was then concentrated and treated with 2 M HCl
(10 mL). The organic products were extracted with diethyl ether (3 ×
15 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.
[(iso-BPMEN)FeII(benzilate)](BPh4) (3). Single crystals suitable for
X-ray diffraction were obtained by vapor diffusion of diethyl ether into
a solution of the complex in a solvent mixture of THF and CH3OH.
Yield: 0.69 g (79%). Anal. Calcd for C54H53BFeN4O3 (872.68 g/mol):
C, 74.32; H, 6.12; N, 6.42. Found: C, 74.24; H, 6.36; N, 6.48. IR (KBr,
cm−1): 3410 (m), 3055 (m), 1620 (s), 1578 (m), 1479 (m), 1443 (m),
1373 (m), 1053 (m), 1028 (m), 758 (m), 733 (s), 704 (s). UV−vis in
CH3CN (λmax): 387 nm (ε = 1600 M−1 cm−1). Magnetic moment μeff
1
(298 K): 5.0 μB. H NMR in CD3CN (500 MHz; δ (ppm)): 137.9
1H NMR (500 MHz, CDCl3) of Organic Products. Thioanisole
oxide: δ 2.73 (s, 3H), 7.66 (m, 2H), 7.52 (m, 3H) ppm. Methyl phenyl
sulfone: δ 3.06 (s, 3H), 7.96 (d, 2H). 7.60 (m, 3H) ppm.
Dibenzothiophene sulfoxide: δ 8.00 (d, 3H), 7.81 (d, 2H), 7.61 (t,
2H), 7.51 (d, 2H) ppm. cis-Cyclohexane-1,2-diol: δ 3.77−3.82 (m,
2H) 3.58 (br s, 2H), 1.73−1.82 (m, 2H), 1.51−1.64 (m, 4H), 1.27−
1.35 ppm (m, 2H) ppm. 1-Phenylethane-1,2-diol: δ 7.37 (m, 4H), 7.30
(m, 1H), 4.85 (m, 1H), 3.80 (m, 1H), 3.70 (m, 1H) ppm. cis-Octane-
1,2-diol: δ 3.80−3.63 (m, 2H), 3.49 (m, 1H), 1.44−1.40 (m, 2H),
1.30−1.24 (m, 6H), 0.92−0.86 (m, 3H) ppm. cis-Cyclooctane-1,2-diol:
δ 3.94 (d, 2H), 1.93−1.86 (m, 2H), 1.70−1.63 (m, 4H), 1.55−1.49
(m, 6H) ppm. cis-Heptane-2,3-diol: δ 4.15 (br, 2H), 3.81−3.62 (m,
2H), 1.44−1.40 (m, 2H), 1.30−1.24 (m, 9H), 0.95 (m, 3H) ppm. The
quantification of organic products was carried out by comparing the
resonance signal with that of the ortho protons of benzophenone (δ
7.81 ppm).
Control Experiments. Control experiments were performed to
assess the role of the ligands in the oxidation reactions. Reactions of
iron(II) perchlorate with O2 in the presence of external substrates
were carried out by following the same procedure mentioned above
except that iron(II) perchlorate hexahydrate (0.01 mmol) was used
instead of an iron(II) benzilate complex. In the experiments, no
oxidized products derived from organic substrates were observed.
Another set of control experiments was performed with iron(II)
perchlorate hexahydrate and monoanionic benzilate (benzilic acid in
the presence of 1 equiv of triethylamine). In the reaction, a trace
amount (<2%) of benzophenone was observed but no oxidation
product from substrate was detected.
(2H, Py-Hα), 105.8 (4H, CH2Py), 91.0 (6H, N(CH3)2), 45.4 (2H, Py-
Hβ), 41.6 (2H, Py-Hβ′), 23.2 (4H, CH2CH2(NCH3)), 10.2 (2H, Py-
Hγ), 9.3 (4H, benzilate-o-H), 8.6 (4H, benzilate-m-H), 7.6 (2H,
benzilate-p-H), 7.3 (8H, BPh4-Ar-o-H), 7.0 (8H, BPh4-Ar-m-H), 6.8
(4H, BPh4-Ar-p-H).
[(6-Me2-iso-BPMEN)FeII(benzilate)](ClO4) (4). Yield: 0.46 g (67%).
Anal. Calcd for C32H37ClFeN4O7 (680.96 g/mol): C, 56.44; H, 5.48;
N, 8.23. Found: C, 54.97; H, 5.25; N, 8.47. IR (KBr, cm−1): 3424 (m),
2996 (m), 1626 (s), 1609 (s), 1464 (m), 1444 (m), 1356 (m), 1092
(s), 758 (m), 625 (m). UV−vis in CH3CN (λmax): 364 nm (ε = 1300
M−1 cm−1). Magnetic moment μeff (298 K): 5.2 μB. 1H NMR in
CD3CN (500 MHz; δ (ppm)): 69.2 (4H, −CH2(Py)), 53.4 (6H,
N(CH3)2), 28.8 (4H, Py-Hβ and Py-Hβ′), 12.8 (3H, Py-Hγ), 10.1 (4H,
benzilate-o-H), 9.5 (4H, benzilate-m-H), 7.3 (2H, benzilate-p-H),
−66.8 (6H, CH3).
[(BPMEN)FeII(benzilate)](ClO4) (5). Yield: 0.34 g (53%). Anal. Calcd
for C30H33ClFeN4O7 (652.9 g/mol): C, 55.19; H, 5.09; N, 8.58.
Found: C, 54.88; H, 5.30; N, 8.47. IR (KBr, cm−1): 3433 (m), 3061
(m), 2972 (m), 1626 (m), 1574 (m), 1445 (m), 1389 (m), 1095 (s),
762 (m), 704 (m), 623 (m). UV−vis in CH3CN (λmax): 357 nm (3200
M−1 cm−1). Magnetic moment μeff (298 K): 4.9 μB. 1H NMR in
CD3CN (500 MHz; δ (ppm)): 115.1 (2H, Py-Hα), 81.2 (4H, CH2Py),
49.4 (6H, NCH3), 42.3 (2H, Py-Hβ and Py-Hβ′), 10.0 (2H, Py-Hγ), 9.6
(4H, benzilate-o-H), 7.3 (4H, benzilate-m-H), 6.7 (2H, benzilate-p-H).
X-ray-quality single crystals of the complex as a tetraphenylborate salt
were grown from a solvent mixture of acetonitrile and diethyl ether.
[(6-Me2-BPMEN)FeII(benzilate)](ClO4) (6). Yield: 0.44 g (65%).
Anal. Calcd for C32H37ClFeN4O7 (680.9 g/mol): C, 56.44; H, 5.48; N,
8.23. Found: C, 55.88; H, 5.37; N, 8.27. IR (KBr, cm−1): 3429 (m),
3057 (m), 2976 (m), 2926 (m), 1607 (m), 1580 (m), 1462 (m), 1385
(m), 1094 (s), 766 (m), 704 (m), 623 (m). UV−vis in CH3CN (λma1x):
350 nm (781 M−1 cm−1). Magnetic moment μeff (298 K): 5.1 μB. H
NMR in CD3CN (500 MHz; δ (ppm)): 81.6 (4H, CH2Py), 62.5 (4H,
−CH2CH2(NMe)), 50.5 (4H, Py-Hβ), 48.2 (4H, Py-Hβ′), 13.6 (6H,
NCH3), 10.6 (2H, Py-Hγ), 9.5 (4H, benzilate-o-H), 8.9 (4H,
benzilate-m-H), 8.0 (2H, benzilate-p-H), −41.6 (6H, CH3).
Reactions of Iron(II) Benzilate Complexes with Substrates at
Dilute Concentration. An iron(II) benzilate complex (0.01 mmol)
dissolved in 50 mL of acetonitrile was allowed to react with
bromobenzaledehyde (10 equiv) under an oxygen atmosphere. No
change in product yield for substrate oxidation was observed, whereas
quantitative decarboxylation of benzilate took place.
Dioxygen Reactivity of Iron(II) Benzilate Complex in the
Presence of Chloride Ion. The iron(II) benzilate complex (2) (0.02
mmol) dissolved in 10 mL of deoxygenated acetonitrile was treated
with tetrabutylammonium chloride (10 equiv) followed by 10 min of
stirring under a nitrogen atmosphere. Pure dioxygen gas was then
bubbled through the solution, and the mixture was kept stirring in an
oxygen environment. The reaction was monitored by optical
[(TBimA)FeII(benzilate)](ClO4) (7). Yield: 0.50 g (64%). Anal. Calcd
for C38H32ClFeN7O7 (790.0 g/mol): C, 57.77; H, 4.08; N, 12.41.
Found: C, 57.19; H, 4.27; N, 12.19. IR (KBr, cm−1): 3273 (m), 1605
(m), 1452 (m), 1386 (m), 1146 (s), 1115 (s), 1088 (s), 746 (m), 704
(m), 629 (m). UV−vis in CH3CN (λmax): 355 nm (576 M−1 cm−1).
1
1
Magnetic moment μeff (298 K): 4.9 μB. H NMR in CD3CN (500
spectroscopy and by H NMR spectroscopy. A quantitative amount
MHz; δ (ppm)): 57.9 (6H, CH2Py), 42.3 (3H, NH), 26.2 (3H, ArH′),
13.9 (3H, ArH), 9.3 (4H, benzilate-o-H), 8.1 (4H, benzilate-m-H), 8.0
(2H, benzilate-p-H), 7.5 (3H, ArH).
of benzophenone was obtained after almost 13 h, which was much
longer than the time (4 h) taken for the reaction in the absence of
chloride anion.
Reaction of Iron(II) Benzilate Complexes with O2 and
Analysis of Organic Products. For a stoichiometric reaction, the
solid complex (0.1 mmol) was dissolved in 15 mL of dry acetonitrile
under a nitrogen atmosphere. Pure O2 was passed through the
X-ray Crystallographic Data Collection and Refinement of
the Structures. Diffraction data for 1−3, 5, and 7 were collected on a
Bruker Smart APEX II (Mo Kα radiation, λ = 0.71073 Å)
diffractometer. Details of the data collection and structure refinements
J
DOI: 10.1021/acs.inorgchem.6b02282
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
Table 5. Crystallographic Data for Complexes 1−3, 5, and 7
1
2
3
5
7
empirical formula
formula wt
cryst syst
space group
a (Å)
C57H53BFeN4O4
924.69
C59H55BFeN4O3
934.73
C54H43BFeN4O3
862.58
monoclinic
C2/c
C58H63BFeN4O4
946.78
monoclinic
P121/n1
17.096(5)
11.370(3)
25.264(7)
90
C38H32ClFeN7O7
790.00
monoclinic
P21/c
monoclinic
P21/n
monoclinic
P21/c
18.6143(14)
10.9186(9)
25.1226(19)
90
16.5701(13)
11.8991(10)
27.7929(18)
90.00
29.490(3)
11.5699(12)
31.273(3)
90
13.8121(6)
15.5546(7)
18.6260(8)
90
b (Å)
c (Å)
α (deg)
β (deg)
108.418(2)
90
119.643(3)
90.00
110.925(2)
90
93.956(9)
90
105.108(2)
90
γ (deg)
V (Å3)
4844.4(7)
4
4762.7(6)
4
9966.5(18)
8
4899(2)
4
3863.3(3)
4
Z
Dcalcd (Mg/m3)
μ(Mo Kα) (mm−1
F(000)
1.268
1.304
1.150
1.284
1.358
)
0.363
0.368
0.347
0.360
0.517
1944
1968
3600
2008.0
1.39−20.98
22838
1504
θ range (deg)
1.20−27.00
62365
1.58−25.00
44352
1.39−24.62
43914
2.01−27.85
135164
8738
no. of rflns collected
no. of unique rflns
R(int)
10459
8395
8309
5218
0.0325
0.0262
0.0553
6302
0.0950
3730
0.0230
7861
no. of data (I > 2σ(I))
no. of params refined
goodness of fit on F2
R1 (I > 2σ(I))
wR2
8987
7551
621
616
542
617
486
1.098
0.987
1.048
1.171
1.190
0.0375
0.0294
0.0715
0.1708
0.0545
0.1418
0.0391
0.1425
0.1315
0.0724
are provided in Table 5. Cell refinement, indexing, and scaling of the
data set were carried out using the APEX2 v2.1-0 software.65 The
structures were solved by direct methods and subsequent Fourier
analyses and refined by the full-matrix least-squares method based on
F2 with all observed reflections.66 In all the complexes the hydrogen
atoms were fixed except those on hydroxy oxygen O3. The routine
SQUEEZE was applied to intensity data of compound 1 in order to
take into account the disordered solvent molecules.67 Single crystals of
5, obtained in almost all combinations of the solvents, diffract at low
angles as the distinctive feature of the crystal.
DFT Calculations. Density functional theory (DFT) calculations
of the complexes were carried out using the Gaussian 09 program
package.68 The six-coordinate geometry of [(6-Me3-TPA)-
FeII(benzilate)]+ was optimized with the Cartesian coordinates
obtained from the crystal structure of complex 2, while the five-
coordinate geometry of [(6-Me3-TPA)FeII(benzilate)]+ with one
pyridine arm open or the hydroxy donor of benzilate noncoordinated
was drawn with the GaussView5 software package using the editing
tools therein.68 The structures of [(L)FeII(OOH)]+ were optimized
with the Cartesian coordinates obtained from the crystal structure of
the respective complexes (Tables S6−S13 in the Supporting
Information). The calculation was performed using the B3LYP
functional69,70 with LanL2DZ basis set71 for iron and 6-31G provided
for all other atoms.72 Geometries of [(L)FeII(OOH)]+ intermediates
with S = 2 spin states were optimized considering both side-on and
end-on binding modes of hydroperoxide. The geometries of the
corresponding low-spin states could not be optimized. Vibrational
frequency calculations with the optimized geometries were performed
to ensure the local minima with positive eigenvalues. The energies
reported herein for the optimized geometries included zero-point and
free energy corrections. Spin densities with the optimized geometries
were plotted with the GaussView5 software package.68
Spectral data (PDF)
Crystallographic data (CIF)
Crystallographic data (CIF)
Crystallographic data (CIF)
Crystallographic data (CIF)
Crystallographic data (CIF)
AUTHOR INFORMATION
Corresponding Author
*E-mail for T.K.P.: ictkp@iacs.res.in.
ORCID
■
Tapan Kanti Paine: 0000-0002-4234-1909
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
T.K.P. acknowledges the Science and Engineering Research
Board (SERB) of India for financial support (Project: EMR/
2014/000972). R.D.J. and R.S. thank the Council of Scientific
and Industrial Research (CSIR) of India for research
fellowships.
REFERENCES
■
(1) Costas, M.; Mehn, M. P.; Jensen, M. P.; Que, L., Jr. Oxygen
Activation at Mononuclear Nonheme Iron: Enzymes, Intermediates,
and Models. Chem. Rev. 2004, 104, 939−986.
(2) Hausinger, R. P. Fe(II)-α-Ketoglutarate-Dependent Hydroxylases
and Related Enzymes. Crit. Rev. Biochem. Mol. Biol. 2004, 39, 21−68.
(3) Kovaleva, E. G.; Lipscomb, J. D. Versatility of biological non-
heme Fe(II) centers in oxygen activation reactions. Nat. Chem. Biol.
2008, 4, 186−193.
(4) Bruijnincx, P. C. A.; van Koten, G.; Gebbink, R. J. M. K.
Mononuclear non-heme iron enzymes with the 2-His-1-carboxylate
facial triad: recent developments in enzymology and modeling studies.
Chem. Soc. Rev. 2008, 37, 2716−2744.
ASSOCIATED CONTENT
■
S
* Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.inorg-
chem.6b02282.
K
DOI: 10.1021/acs.inorgchem.6b02282
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
(5) Barry, S. M.; Challis, G. L. Mechanism and Catalytic Diversity of
Rieske Non-Heme Iron-Dependent Oxygenases. ACS Catal. 2013, 3,
2362−2370.
(22) Chatterjee, S.; Paine, T. K. Olefin cis-Dihydroxylation and
Aliphatic C-H Bond Oxygenation by a Dioxygen-Derived Electrophilic
Iron-Oxygen Oxidant. Angew. Chem., Int. Ed. 2015, 54, 9338−9342.
(23) Chatterjee, S.; Paine, T. K. Hydroxylation versus Halogenation
of Aliphatic C-H Bonds by a Dioxygen-Derived Iron−Oxygen
Oxidant: Functional Mimicking of Iron Halogenases. Angew. Chem.,
Int. Ed. 2016, 55, 7717−7722.
(6) Krebs, C.; Fujimori, D. G.; Walsh, C. T.; Bollinger, J. M., Jr. Non-
Heme Fe(IV)−Oxo Intermediates. Acc. Chem. Res. 2007, 40, 484−492.
(7) Ray, K.; Pfaff, F. F.; Wang, B.; Nam, W. Status of Reactive Non-
heme Metal-Oxygen Intermediates in Chemical and Enzymatic
Reactions. J. Am. Chem. Soc. 2014, 136, 13942−13958.
(24) Sahu, S.; Goldberg, D. P. Activation of Dioxygen by Iron and
Manganese Complexes: A Heme and Nonheme Perspective. J. Am.
Chem. Soc. 2016, 138, 11410−11428.
(8) Thibon, A.; England, J.; Martinho, M.; Young, V. G., Jr.; Frisch, J.
R.; Guillot, R.; Girerd, J.-J.; Munck, E.; Que, L., Jr.; Banse, F. Proton-
̈
(25) Kim, S. O.; Sastri, C. V.; Seo, M. S.; Kim, J.; Nam, W. Dioxygen
Activation and Catalytic Aerobic Oxidation by a Mononuclear
Nonheme Iron(II) Complex. J. Am. Chem. Soc. 2005, 127, 4178−4179.
(26) Funabiki, T. Functional model oxygenations by nonheme iron
complexes. In Advances in Catalytic Activation of Dioxygen by Metal
Complexes; Simandi, L. I., Ed.; Kluwer Academic: Dordrecht. The
́
Netherlands, 2003; pp 157−226.
(27) Kim, C.; Chen, K.; Kim, J.; Que, L., Jr. Stereospecific Alkane
Hydroxylation with H2O2 Catalyzed by an Iron(II)-Tris(2-
pyridylmethyl)amine Complex. J. Am. Chem. Soc. 1997, 119, 5964−
5965.
(28) Costas, M.; Chen, K.; Que, L., Jr. Biomimetic Nonheme Iron
Catalysts for Alkane Hydroxylation. Coord. Chem. Rev. 2000, 200−202,
517−544.
(29) Chen, K.; Que, L., Jr. Stereospecific Alkane Hydroxylation by
Nonheme Iron Catalysts: Mechanistic Evidence for an FeV = O Active
Species. J. Am. Chem. Soc. 2001, 123, 6327−6337.
(30) Chen, K.; Costas, M.; Kim, J.; Tipton, A. K.; Que, L., Jr. Olefin
cis-Dihydroxylation versus Epoxidation by Nonheme Iron Catalysts:
Two Faces of an FeIII-OOH Coin. J. Am. Chem. Soc. 2002, 124, 3026−
3035.
and Reductant-Assisted Dioxygen Activation by a Nonheme Iron(II)
Complex to Form an Oxoiron(IV) Intermediate. Angew. Chem., Int. Ed.
2008, 47, 7064−7067.
(9) Hong, S.; Lee, Y.-M.; Shin, W.; Fukuzumi, S.; Nam, W. Dioxygen
Activation by Mononuclear Nonheme Iron(II) Complexes Generates
Iron−Oxygen Intermediates in the Presence of an NADH Analogue
and Proton. J. Am. Chem. Soc. 2009, 131, 13910−13911.
(10) Martinho, M.; Blain, G.; Banse, F. Activation of dioxygen by a
mononuclear non-heme iron complex: characterization of a Fe(III)
(OOH) intermediate. Dalton Trans. 2010, 39, 1630−1634.
(11) Lee, Y.-M.; Hong, S.; Morimoto, Y.; Shin, W.; Fukuzumi, S.;
Nam, W. Dioxygen Activation by a Non-Heme Iron(II) Complex:
Formation of an Iron(IV)−Oxo Complex via C−H Activation by a
Putative Iron(III)−Superoxo Species. J. Am. Chem. Soc. 2010, 132,
10668−10670.
(12) Badiei, Y. M.; Siegler, M. A.; Goldberg, D. P. O2 Activation by
Bis(imino)pyridine Iron(II)-Thiolate Complexes. J. Am. Chem. Soc.
2011, 133, 1274−1277.
(13) Li, F.; Van Heuvelen, K. M.; Meier, K. K.; Munck, E.; Que, L.,
̈
Jr. Sc3+-Triggered Oxoiron(IV) Formation from O2 and its Non-Heme
Iron(II) Precursor via a Sc3+−Peroxo−Fe3+ Intermediate. J. Am. Chem.
Soc. 2013, 135, 10198−10201.
(31) Oldenburg, P. D.; Shteinman, A. A.; Que, L., Jr. Iron-Catalyzed
Olefin cis-Dihydroxylation Using a Bio-Inspired N, N, O-Ligand. J. Am.
Chem. Soc. 2005, 127, 15672−15673.
(14) Nishida, Y.; Lee, Y.-M.; Nam, W.; Fukuzumi, S. Autocatalytic
Formation of an Iron(IV)−Oxo Complex via Scandium Ion-Promoted
Radical Chain Autoxidation of an Iron(II) Complex with Dioxygen
and Tetraphenylborate. J. Am. Chem. Soc. 2014, 136, 8042−8049.
́
(32) Mas-Balleste, R.; Costas, M.; van den Berg, T.; Que, L., Jr.
Ligand Topology Effects on Olefin Oxidations by Bio-Inspired
[FeII(N2Py2)] Catalysts. Chem. - Eur. J. 2006, 12, 7489−7500.
(33) Bruijnincx, P. C. A.; Buurmans, I. L. C.; Gosiewska, S.;
Moelands, M. A. H.; Lutz, M.; Spek, A. L.; van Koten, G.; Gebbink, R.
J. M. K. Iron(II) Complexes with Bio-Inspired N,N,O Ligands as
Oxidation Catalysts: Olefin Epoxidation and cis-Dihydroxylation.
Chem. - Eur. J. 2008, 14, 1228−1237.
́ ́ ̀ ́ ́
(15) Segaud, N.; Anxolabehere-Mallart, E.; Senechal-David, K.;
Acosta-Rueda, L.; Robert, M.; Banse, F. Electrochemical study of a
nonheme Fe(II) complex in the presence of dioxygen. Insights into the
reductive activation of O2 at Fe(II) centers. Chem. Sci. 2015, 6, 639−
647.
(16) Sawyer, D. T.; Liu, X.; Redman, C.; Chong, B. Iron(II)/
reductant(DH2)-induced activation of dioxygen for the hydroxylation
and ketonization of hydrocarbons; mimics for the cytochrome P-450
hydroxylase/reductase system. Bioorg. Med. Chem. 1994, 2, 1385−
1395.
(17) Jaafar, H.; Vileno, B.; Thibon, A.; Mandon, D. Tuning the
conversion of cyclohexane into cyclohexanol/one by molecular
dioxygen, protons and reducing agents at a single non-porphyrinic
iron centre and chemical versatility of the tris(2-pyridylmethyl)amine
TPAFe(II)Cl2 complex in mild oxidation chemistry. Dalton Trans.
2011, 40, 92−106.
(34) Sun, C.-L.; Li, B.-J.; Shi, Z.-J. Direct C-H Transformation via
Iron Catalysis. Chem. Rev. 2011, 111, 1293−1314.
(35) Que, L., Jr.; Tolman, W. B. Biologically inspired oxidation
catalysis. Nature 2008, 455, 333−340.
(36) Christmann, M. Selective Oxidation of Aliphatic C-H Bonds in
the Synthesis of Complex Molecules. Angew. Chem., Int. Ed. 2008, 47,
2740−2742.
(37) Bryliakov, K. P.; Talsi, E. P. Active sites and mechanisms of
bioinspired oxidation with H2O2, catalyzed by non-heme Fe and
related Mn complexes. Coord. Chem. Rev. 2014, 276, 73−96.
(38) Chen, M. S.; White, M. C. A Predictably Selective Aliphatic C−
H Oxidation Reaction for Complex Molecule Synthesis. Science 2007,
318, 783−787.
(18) Mandon, D.; Jaafar, H.; Thibon, A. Exploring the Oxygen
sensitivity of FeCl2 complexes with tris(2-pyridylmethyl)amine-type
ligands: O2 coordination and a quest for superoxide. New J. Chem.
2011, 35, 1986−2000.
́
(39) Company, A.; Gomez, L.; Guell, M.; Ribas, X.; Luis, J. M.; Que,
̈
L., Jr.; Costas, M. Alkane Hydroxylation by a Nonheme Iron Catalyst
that Challenges the Heme Paradigm for Oxygenase Action. J. Am.
Chem. Soc. 2007, 129, 15766−15767.
(19) Schroder, K.; Join, B.; Amali, A. J.; Junge, K.; Ribas, X.; Costas,
̈
M.; Beller, M. A Biomimetic Iron Catalyst for the Epoxidation of
Olefins with Molecular Oxygen at Room Temperature. Angew. Chem.,
Int. Ed. 2011, 50, 1425−1429.
(40) Chen, M. S.; White, M. C. Combined Effects on Selectivity in
Fe-Catalyzed Methylene Oxidation. Science 2010, 327, 566−571.
(41) Gelalcha, F. G. Biomimetic Iron-Catalyzed Asymmetric
Epoxidations: Fundamental Concepts, Challenges and Opportunities.
Adv. Synth. Catal. 2014, 356, 261−299.
(42) Oloo, W. N.; Que, L., Jr. Bioinspired Nonheme Iron Catalysts
for C-H and CC Bond Oxidation: Insights into the Nature of the
Metal-Based Oxidants. Acc. Chem. Res. 2015, 48, 2612−2621.
(43) White, M. C.; Doyle, A. G.; Jacobsen, E. N. A Synthetically
Useful, Self-Assembling MMO Mimic System for Catalytic Alkene
(20) Paria, S.; Que, L., Jr.; Paine, T. K. Oxidative Decarboxylation of
Benzilic Acid by a Biomimetic Iron(II) Complex: Evidence for an
Iron(IV) − Oxo−Hydroxo Oxidant from O2. Angew. Chem., Int. Ed.
2011, 50, 11129−11132.
(21) Paria, S.; Chatterjee, S.; Paine, T. K. Reactivity of an Iron−
Oxygen Oxidant Generated upon Oxidative Decarboxylation of
Biomimetic Iron(II) α-Hydroxy Acid Complexes. Inorg. Chem. 2014,
53, 2810−2821.
L
DOI: 10.1021/acs.inorgchem.6b02282
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
Epoxidation with Aqueous H2O2. J. Am. Chem. Soc. 2001, 123, 7194−
7195.
(44) Costas, M.; Tipton, A. K.; Chen, K.; Jo, D.-H.; Que, L., Jr.
Modeling Rieske Dioxygenases. The First Example of Iron-Catalyzed
Asymmetric cis-Dihydroxylation of Olefins. J. Am. Chem. Soc. 2001,
123, 6722−6723.
(45) Suzuki, K.; Oldenburg, P. D.; Que, L., Jr. Iron-Catalyzed
Asymmetric Olefin cis-Dihydroxylation with 97% Enantiomeric Excess.
Angew. Chem., Int. Ed. 2008, 47, 1887−1889.
(46) Costas, M.; Que, L., Jr. Ligand Topology Tuning of Iron-
Catalyzed Hydrocarbon Oxidations. Angew. Chem., Int. Ed. 2002, 41,
2179−2181.
(47) England, J.; Britovsek, G. J. P.; Rabadia, N.; White, A. J. P.
Ligand Topology Variations and the Importance of Ligand Field
Strength in Non-Heme Iron Catalyzed Oxidations of Alkanes. Inorg.
Chem. 2007, 46, 3752−3767.
(48) Prat, I.; Company, A.; Postils, V.; Ribas, X.; Que, L., Jr.; Luis, J.
M.; Costas, M. The Mechanism of Stereospecific C-H oxidation by
Fe(Pytacn) Complexes: Bioinspired Non-Heme Iron Catalysts
Containing cis-Labile Exchangeable Sites. Chem. - Eur. J. 2013, 19,
6724−6738.
(61) Fujita, M.; Costas, M.; Que, L., Jr. Iron Catalyzed Olefin cis-
Dihydroxylation by H2O2: Electrophilic versus Nucleophilic Mecha-
nisms. J. Am. Chem. Soc. 2003, 125, 9912−9913.
(62) Zang, C.; Liu, Y.; Xu, Z.-J.; Tse, C.-W.; Guan, X.; Wei, J.;
Huang, J.-S.; Che, C.-M. Highly Enantioselective Iron-Catalyzed cis-
Dihydroxylation of Alkenes with Hydrogen Peroxide Oxidant via an
FeIII-OOH Reactive Intermediate. Angew. Chem., Int. Ed. 2016, 55,
10253−10257.
(63) Chakraborty, B.; Bhunya, S.; Paul, A.; Paine, T. K. Reactivity of
Biomimetic Iron(II)-2-aminophenolate Complexes toward Dioxygen:
Mechanistic Investigations on the Oxidative C−C Bond Cleavage of
Substituted 2-Aminophenols. Inorg. Chem. 2014, 53, 4899−4912.
(64) Wolsey, W. C. Perchlorate salts, their uses and alternatives. J.
Chem. Educ. 1973, 50, A335−A337.
(65) APEX 2, v2.1-0; Bruker AXS, Madison, WI, 2006.
(66) Sheldrick, G. M. SHELXL-97, Program for crystal structure
refinement; University of Gottingen, Gottingen, Germany, 1997.
̈
̈
(67) van der Sluis, P.; Spek, A. L. BYPASS: an effective method for
the refinement of crystal structures containing disordered solvent
regions. Acta Crystallogr., Sect. A: Found. Crystallogr. 1990, 46, 194−
201.
(68) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.;
Robb, M. A.; Cheeseman, J. R.; Scalmani, G.; Barone, V.; Mennucci,
B.; Petersson, G. A.; Nakatsuji, H.; Caricato, M.; Li, X.; Hratchian, H.
P.; Izmaylov, A. F.; Bloino, J.; Zheng, G.; Sonnenberg, J. L.; Hada, M.;
Ehara, M.; Toyota, K.; Fukuda, R.; Hasegawa, J.; Ishida, M.; Nakajima,
T.; Honda, Y.; Kitao, O.; Nakai, H.; Vreven, T.; Montgomery, J. A., Jr.;
Peralta, J. E., Jr.; Ogliaro, F.; Bearpark, M.; Heyd, J. J.; Brothers, E.;
Kudin, K. N.; Staroverov, V. N.; Kobayashi, R.; Normand, J.;
Raghavachari, K.; Rendell, A.; Burant, J. C.; Iyengar, S. S.; Tomasi,
J.; Cossi, M.; Rega, N.; Millam, J. M.; Klene, M.; Knox, J. E.; Cross, J.
B.; Bakken, V.; Adamo, C.; Jaramillo, J.; Gomperts, R.; Stratmann, R.
E.; Yazyev, O.; Austin, A. J.; Cammi, R.; Pomelli, C.; Ochterski, J. W.;
Martin, R. L.; Morokuma, K.; Zakrzewski, V. G.; Voth, G. A.; Salvador,
(49) Britovsek, G. J. P.; England, J.; White, A. J. P. Iron(II),
manganese(II) and cobalt(II) complexes containing tetradentate
biphenyl-bridged ligands and their application in alkane oxidation
catalysis. Dalton Trans. 2006, 1399−1408.
(50) Hong, S.; Lee, Y.-M.; Cho, K.-B.; Sundaravel, K.; Cho, J.; Kim,
M. J.; Shin, W.; Nam, W. Ligand Topology Effect on the Reactivity of
a Mononuclear Nonheme Iron(IV)-Oxo Complex in Oxygenation
Reactions. J. Am. Chem. Soc. 2011, 133, 11876−11879.
(51) Gavrilova, A. L.; Bosnich, B. Principles of Mononucleating and
Binucleating Ligand Design. Chem. Rev. 2004, 104, 349−384.
(52) Ng, C.; Sabat, M.; Fraser, C. L. Metal Complexes with Cis α
Topology from Stereoselective Quadridentate Ligands with Amine,
Pyridine, and Quinoline Donor Groups. Inorg. Chem. 1999, 38, 5545−
5556.
̈
P.; Dannenberg, J. J.; Dapprich, S.; Daniels, A. D.; Farkas, O.;
Foresman, J. B.; Ortiz, J. V.; Cioslowski, J.; Fox, D. J. Gaussian 09,
Revision A.1; Gaussian, Inc., Wallingford, CT, 2009.
(53) Chiou, Y.-M.; Que, L., Jr. Models for α-Keto Acid-Dependant
Non-Heme Iron Enzymes: Structures and Reactivities of [FeII(L)-
(O2CCOPh)](ClO4) Complexes. J. Am. Chem. Soc. 1995, 117, 3999−
4013.
(69) Lee, C.; Yang, W.; Parr, R. G. Development of the Colle-Salvetti
correlation-energy formula into a functional of the electron density.
Phys. Rev. B: Condens. Matter Mater. Phys. 1988, 37, 785−789.
(70) Becke, A. D. Density-functional thermochemistry. III. The role
of exact exchange. J. Chem. Phys. 1993, 98, 5648−5652.
(71) Dunning, T. H., Jr.; Hay, P. J. In Modern Theoretical Chemistry;
Schaefer, H. H, III, Ed.; Plenum: New York, 1976; Vol. 3, pp 1−28.
(72) Ditchfield, R.; Hehre, W. J.; Pople, J. A. Self-Consistent
Molecular Orbital Methods. IX. An Extended Gaussian-Type Basis for
Molecular-Orbital Studies of Organic Molecules. J. Chem. Phys. 1971,
54, 724−728.
(54) Tsou, C.-C.; Yang, W.-L.; Liaw, W.-F. Nitrite Activation to
Nitric Oxide via One-fold Protonation of Iron(II)-O,O-nitrito
Complex: Relevance to the Nitrite Reductase Activity of Deoxy-
hemoglobin and Deoxyhemerythrin. J. Am. Chem. Soc. 2013, 135,
18758−18761.
(55) Men
Reactivity of
́
age, S.; Zang, Y.; Hendrich, M. P.; Que, L., Jr. Structure and
Bis(μ-acetato-O,O′)diiron(II) Complex,
a
[Fe2(O2CCH3)2(TPA)2](BPh4)2. A Model for the Diferrous Core of
Ribonucleotide Reductase. J. Am. Chem. Soc. 1992, 114, 7786−7792.
(56) Britovsek, G. J. P.; England, J.; White, A. J. P. Non-heme
Iron(II) Complexes Containing Tripodal Tetradentate Nitrogen
Ligands and Their Application in Alkane Oxidation Catalysis. Inorg.
Chem. 2005, 44, 8125−8134.
(57) Paine, T. K.; Paria, S.; Que, L., Jr. Oxidative decarboxylation of
α-hydroxy acids by a functional model of the nonheme iron oxygenase,
CloR. Chem. Commun. 2010, 46, 1830−1832.
(58) Addison, A. W.; Rao, T. N.; Reedijk, J.; van Rijn, J.; Verschoor,
G. C. Synthesis, structure, and spectroscopic properties of copper(II)
compounds containing nitrogen−sulphur donor ligands; the crystal
and molecular structure of aqua[1,7-bis(N-methylbenzimidazol-2′-yl)-
2,6-dithiaheptane]copper(II) perchlorate. J. Chem. Soc., Dalton Trans.
1984, 1349−1356.
(59) Guisado-Barrios, G.; Zhang, Y.; Harkins, A. M.; Richens, D. T.
Low temperature reaction of [Fe(TPA)(CH3CN)2]2+ with excess 3-
chloroperoxybenzoic acid in semi-frozen acetonitrile; EPR detection of
an acylperoxo iron(III) adduct. Inorg. Chem. Commun. 2012, 20, 81−
85.
(60) Wada, A.; Ogo, S.; Nagatomo, S.; Kitagawa, T.; Watanabe, Y.;
Jitsukawa, K.; Masuda, H. Reactivity of Hydroperoxide Bound to a
Mononuclear Non-Heme Iron Site. Inorg. Chem. 2002, 41, 616−618.
M
DOI: 10.1021/acs.inorgchem.6b02282
Inorg. Chem. XXXX, XXX, XXX−XXX