Bioinspired Olefin cis-Dihydroxylation and Aliphatic C-H Bond Hydroxylation with Dioxygen Catalyzed by a Nonheme Iron Complex

Article abstract of DOI:10.1021/acs.inorgchem.8b01353

A mononuclear iron(II)-α-hydroxy acid complex [(Tp^Ph,Me)Fe^II(benzilate)] (Tp^Ph,Me = hydrotris(3-phenyl-5-methylpyrazol-1-yl)borate) of a facial tridentate ligand has been isolated and characterized to explore its catalytic efficiency for aerial oxidation of organic substrates. In the reaction between the iron(II)-benzilate complex and O₂, the metal-coordinated benzilate is stoichiometrically converted to benzophenone with concomitant reduction of dioxygen on the iron center. Based on the results from interception experiments and labeling studies, different iron-oxygen oxidants are proposed to generate in situ in the reaction pathway depending upon the absence or presence of an external additive (such as protic acid or Lewis acid). The five-coordinate iron(II) complex catalytically cis-dihydroxylates olefins and oxygenates the C-H bonds of aliphatic substrates using O₂ as the terminal oxidant. The iron(II) complex exhibits better catalytic activity in the presence of a Lewis acid.

Full text of DOI:10.1021/acs.inorgchem.8b01353

Article
Cite This: Inorg. Chem. XXXX, XXX, XXX−XXX
pubs.acs.org/IC
Bioinspired Olefin cis-Dihydroxylation and Aliphatic C−H Bond
Hydroxylation with Dioxygen Catalyzed by a Nonheme Iron
Complex
†
†
Sayanti Chatterjee, Shrabanti Bhattacharya, and Tapan Kanti Paine*
Department of Inorganic Chemistry, Indian Association for the Cultivation of Science, 2A and 2B Raja S. C. Mullick Road, Jadavpur,
Kolkata 700032, India
*
S Supporting Information
ABSTRACT: A mononuclear iron(II)-α-hydroxy acid complex
Ph,Me
II
Ph,Me
[(Tp
)Fe (benzilate)] (Tp
= hydrotris(3-phenyl-5-methyl-
pyrazol-1-yl)borate) of a facial tridentate ligand has been isolated
and characterized to explore its catalytic efficiency for aerial
oxidation of organic substrates. In the reaction between the
iron(II)-benzilate complex and O , the metal-coordinated benzilate
2
is stoichiometrically converted to benzophenone with concomitant
reduction of dioxygen on the iron center. Based on the results from
interception experiments and labeling studies, different iron-oxygen
oxidants are proposed to generate in situ in the reaction pathway
depending upon the absence or presence of an external additive
(
such as protic acid or Lewis acid). The five-coordinate iron(II)
complex catalytically cis-dihydroxylates olefins and oxygenates the
C−H bonds of aliphatic substrates using O as the terminal oxidant. The iron(II) complex exhibits better catalytic activity in the
2
presence of a Lewis acid.
2
6,34,35
INTRODUCTION
of sacrificial reductants
catalytic systems prevail.
the crisis of selective and
■
Selective oxyfunctionalization of hydrocarbons is an important
step in the synthesis of useful materials in chemical industries.
Traditional oxidants, such as heavy-metal oxides/oxo-hydrox-
ides, chlorine, periodate, peroxides, and so on, are commonly
used for oxidation of alkanes and alkenes. In these reactions,
1
In our endeavor to develop bioinspired catalysts for
oxidations with O , we have been pursuing systematic studies
on the reductive activation of dioxygen by biomimetic iron(II)
complexes using α-hydroxy acids as sacrificial reductants.
2
36−40
2
The iron(II)−benzilate complexes have been reported to
oxidize substrates through in situ generation of iron−oxygen
oxidants (Scheme 1). Unfortunately, there is no direct
experimental evidence yet for such iron−oxygen oxidants.
We have proposed different iron−oxygen oxidants generated
under different experimental conditions based on indirect
evidence such as interception and mechanistic studies. Among
the reported complexes, the iron(II)−benzilate complex of the
the oxidant-derived products are often hazardous and the
reactions are not selective. Despite extensive research in this
3,4
field, development of sustainable methods for oxidation
reactions with high efficiency and selectivity still remains a
challenge. In biological systems, metalloenzymes are involved
in catalyzing these challenging oxidations under mild
5
−9
conditions using dioxygen as the terminal electron acceptor.
Ph2
Taking lessons from nature, bioinspired approaches toward
development of sustainable oxidation catalysts have attracted
considerable attention. While significant progress has been
made in bioinspired catalysis using peroxides or peracids for
the oxidation of hydrocarbon substrates by metal com-
Tp (hydrotris(3,5-diphenylpyrazolyl)borate)) ligand exhib-
its better reactivity in stoichiometric oxidation compared to
those supported by tetradentate ligands. However, the
Ph2
tendency of the ligand (Tp ) to undergo intramolecular
21,41
hydroxylation
under oxidizing conditions is expected to
make the iron complexes of this ligand less active for catalytic
oxidation. Recently, we demonstrated that the iron(II)
)Fe (BF)] (BF = monoanionic benzoylfor-
= hydrotris(3-phenyl-5-methylpyrazolyl)borate)
was capable of performing the catalytic aerobic oxidation of
alcohols and oxygen atom transfer reactions without detectable
1
0−18
plexes,
use of dioxygen in catalytic oxidations remains
less explored. The reductive activation of dioxygen by
bioinspired complexes and subsequent generation of metal−
oxygen oxidants for substrate oxidation requires electron and
Ph,Me
II
complex [(Tp
mate, Tp
Ph,Me
1
9−27
proton sources.
assisted activation of O by transition metal complexes offers
In biomimetic chemistry, cosubstrate-
2
5
,28−33
attractive alternatives for bioinspired catalysis.
In spite of
a few reports on oxygen-dependent transformation of hydro-
carbon substrates by nonheme iron complexes in the presence
Received: May 17, 2018
©
XXXX American Chemical Society
A
DOI: 10.1021/acs.inorgchem.8b01353
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Inorganic Chemistry
Article
Scheme 1. Proposed Iron−Oxygen Oxidants Generated In Situ upon Reductive Activation of Dioxygen from
Ph2
II
36−38
[
(Tp )Fe (benzilate)] Complex under Different Reaction Conditions
4
2
intraligand hydroxylation. This motivated us to isolate the
Table 1. Crystallographic Data for
Ph,Me
II
Ph,Me
II
iron(II)−benzilate complex [(Tp
)Fe (benzilate)] (1) and
[(Tp
)Fe (benzilate)] (1)
explore its efficiency in the aerobic oxidation of substrates. In
the present investigation, we found that complex 1 not only
displayed selective oxygenation of substrates under stoichio-
metric condition, but also exhibited catalytic oxidation of
alkanes, alkenes and sulfides. The catalytic ability of 1 in the
1
empirical formula
formula weight
crystal system
space group
a, Å
b, Å
c, Å
α, deg
β, deg
C H BFeN O
3
44 39
6
766.47
triclinic
P-1
O -dependent oxidation reactions and its comparison with that
2
Ph2
II
11.878(4)
14.847(5)
15.937(6)
100.607(8)
100.605(8)
101.476(8)
2635.2(16)
2
of [(Tp )Fe (benzilate)] (2; Chart 1) are presented in this
manuscript.
Chart 1. Iron(II)−Benzilate Complexes of Facial N Ligands
3
γ, deg
volume, Å3
Z
Dcalcd, Mg/m3
0.967
0.322
μ Mo K , mm−
1
α
F(000)
800.96
1.34−28.44
12086
8208
0.0453
6826
504
1.153
0.0782
0.2132
θ range, deg
reflections collected
reflns unique
R(int)
data (I > 2σ(I))
parameters refined
goodness-of-fit on F2
R1 [I > 2σ(I)]
wR2
RESULTS AND DISCUSSION
■
Synthesis and Characterization. Complex 1 was isolated
Ph,Me
from the reaction of KTp
with iron(II) chloride and
sodium benzilate in a solvent mixture of dichloromethane and
methanol at room temperature under nitrogen atmosphere
1
(
Experimental Section). The H NMR spectrum of 1
displaying paramagnetically shifted proton resonances in the
range between +70 and −25 ppm supported the high spin
nature of the complex in solution (Figure S1, SI). The ESI-
mass spectrum of 1 in acetonitrile exhibits an ion peak at m/z
average Fe−N bond length of 2.105 Å is comparable to those
II
Ph,Me
44
of other Fe (Tp
) complexes, while the iron−oxygen
distances (r(Fe1−O1), 2.338(6) and r(Fe1−O2), 2.020(5) Å)
indicate asymmetric bidentate binding mode of carboxylate.
The geometry of the five-coordinate iron center and the
binding mode of benzilate is very similar to those of the
7
67.3 with the isotope distribution pattern calculated for
Ph,Me
+
[(Tp
)Fe(benzilate) + H ] along with other mass frag-
ments (Figure S2 and Experimental Section). The composition
of the complex was further confirmed by single crystal X-ray
diffraction studies (Table 1). The X-ray crystal structure of the
Ph2
36
reported complex of the Tp ligand. The hydroxy group of
benzilate remains noncoordinated in both the complexes. The
Fe−O2(carboxylate) and the Fe−N1 bonds in 1 are slightly
elongated compared to those in 2 (Table 2). Additionally, the
phenyl rings at 3-position of each pyrazole ring in 1 are slightly
tilted away from the metal center (Figure S3). These slight
structural variations likely prevent the intraligand hydroxyla-
tion.
neutral complex displays a five-coordinate iron center
Ph,Me
coordinated by the facial tridentate Tp
ligand and the
carboxylate oxygen atoms of benzilate monoanion (Figure 1).
In the ternary complex, the iron center adopts a distorted
square pyramidal coordination geometry. Two nitrogen donors
Ph,Me
(
N1 and N3) from the Tp
ligand, and two carboxylate
oxygen atoms (O1 and O2) from benzilate constitute the basal
Dioxygen Reactivity. The iron(II) complex (1) reacted
with pure dioxygen in benzene to convert the colorless solution
to light yellow (Figure S4). Time-dependent product analyses
revealed the quantitative conversion of iron-coordinated
43
plane of the distorted square pyramid (τ = 0.46) The apical
Ph,Me
position is occupied by N5 nitrogen from the Tp
ligand
with the Fe1−N5 distance of 2.076(6) Å (Table 2). The
B
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Inorganic Chemistry
Article
1
Figure 2. Time-dependent H NMR (500 MHz, CDCl , 295 K)
3
spectra monitoring the formation of benzophenone in the reaction
between complex 1 and O2.
Figure 1. ORTEP plot of [(TpPh,Me)Fe (benzilate)] (1). All the
hydrogen atoms other than those on B1 and O3 are omitted for
clarity.
II
mixture of methyl phenyl sulfoxide (93%) and methyl phenyl
sulfone (4%; Figure S6). The oxidized solution of 1 after the
reaction with thioanisole was found to be X-band EPR silent
and exhibited paramagnetically shifted proton resonances in
the NMR spectrum indicating the formation of an iron(II)
species (Figure S7). Thus, the in situ formed species was a
two-electron reduced iron−oxygen oxidant which after
oxidation of thioanisole generated an iron(II) complex.
Hammett analyses using 1:1 mixtures of thioanisole and
different para-substituted thioanisoles (p-XC H SCH , where
Table 2. Selected Bond Lengths (Å) and Bond Angles (deg)
Ph,Me
II
II
for [(Tp
[
)Fe (benzilate)] (1) and
Ph2
(Tp )Fe (benzilate)] (2)
II
[
(TpPh,Me
)
[(TpPh2)Fe (benzilate)]
II
39
Fe (benzilate)] (1)
(2)
6
4
3
Fe(1)−N(1)
Fe(1)−N(3)
Fe(1)−N(5)
Fe(1)−O(2)
Fe(1)−O(1)
C(1)−O(1)
C(1)−O(2)
C(1)−C(2)
2.147(7)
2.095(6)
2.076(6)
2.020(5)
2.338(6)
1.251(10)
1.275(9)
1.520(11)
1.430(10)
107.74(2)
131.22(2)
98.23(2)
134.94(2)
162.8(2)
104.3(2)
60.00(2)
91.57(2)
87.7(2)
2.075(3)
2.082(3)
2.120(3)
2.008(3)
2.346(3)
1.248(4)
1.273(5)
1.534(5)
1.421(5)
98.23(10)
134.82(11)
107.88(10)
131.04(11)
162.16(11)
103.85(12)
59.83(10)
91.87(11)
88.75(11)
88.16(11)
0.46
X = NO , Br, H, Me, OMe) resulted in a ρ value of +1.04
2
(
Figure S8) supporting the nucleophilic nature of the iron−
oxygen oxidant from 1. Nucleophilic iron−oxygen oxidants
from H O have been reported to cis-dihydroxylate
alkenes.
2
2
1
5,45
Reaction of 1 with cyclohexene yielded around
80% cis-cyclohexane-1,2-diol as the only product (Figure S9).
Cyclooctene afforded 86% cis-cyclooctane-1,2-diol (Figure
S10) and 1-octene was selectively converted to octane-1,2-
diol with about 92% yield (Figure S11). Electron-deficient
olefin, tert-butyl acrylate showed very high yield of the
corresponding diol (96%; Figure S12).
C(2)−O(3)
N(1)−Fe(1)−O(1)
N(1)−Fe(1)−O(2)
N(3)−Fe(1)−O(1)
N(3)−Fe(1)−O(2)
N(5)−Fe(1)−O(1)
N(5)−Fe(1)−O(2)
O(1)−Fe(1)−O(2)
N(1)−Fe(1)−N(3)
N(1)−Fe(1)−N(5)
N(3)−Fe(1)−N(5)
τ
37
As observed with complex 2, an electrophilic oxidant was
generated from 1 in the presence of Sc(OTf) (Figure S13). In
3
the reaction condition, thioanisole was selectively oxidized to
1
8
thioanisole oxide with the incorporation of one O atom from
1
8
H2 O into the sulfoxide product (Figure S14). The electro-
philic oxidant selectively oxidized alkenes to the corresponding
cis-diols, in which partial incorporation of oxygen atom from
water took place (Figure 3, Scheme 2). The product profile
and the results of labeling experiments clearly support that the
88.5(2)
0.46
3
+
oxidant from 1 and O with Sc can exchange its oxygen
2
benzilate to benzophenone in 45 min (Figure 2). The ESI-
mass spectrum of the oxidized solution showed an ion peak at
atoms with water.
Aliphatic C−H bonds were oxygenated with high selectivity
by the electrophilic oxidant (Scheme 2). Cyclohexane formed
Ph,Me
+
m/z 539.2 attributable to [(Tp
)Fe] ion. Unlike that
observed with 2, no peak corresponding to the iron complex of
the hydroxylated form of TpPh,Me was observed in the mass
spectrum of the oxidized solution of 1 (Figure S5, inset).
The two-electron oxidative decarboxylation of benzilate to
benzophenone suggests that dioxygen can undergo two-
electron reduction on the iron center. In the absence of any
observable intermediate in the reaction pathway, the in situ
formed iron−oxygen oxidant from 1 was intercepted using
external substrates as probes (Table 3, Experimental Section).
Complex 1 reacted with thioanisole (10 equiv) to afford a
cyclohexanol (60%) and cyclohexanone (5%) with high A/K
16
(12) ratio (Figure S15). Labeling experiments with O and
2
1
8
18
H2 O showed about 38% incorporation of O into cyclo-
hexanol (Figure S16). Adamantane and methylcyclohexane
were selectively transformed to 1-adamantanol (70% yield;
Figure S17) and 1-methylcyclohexanol (63% yield), respec-
tively (Figure S18). Except for the electron-deficient alkene
(tert-butyl acrylate), the yields of oxygenated products were
found to be higher compared to that found with complex 2
under similar experimental conditions (Table 3).
C
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Inorganic Chemistry
Article
a
Table 3. Reactivity of Iron(II)-Benzilate Complexes (1 and 2) of Monoanionic N Ligands toward Different Substrates
3
yield (%) of products with
yield (%) of products with
Ph2 II
b
Ph,Me
II
c
[(Tp
)Fe (benzilate)] (1)
[(Tp )Fe (benzilate)] (2)
without
additive
with
Sc(OTf)3
with
PyHClO4
without
additive
with
Sc(OTf)3
with
PyHClO4
substrate
product(s)
thioanisole (10 equiv)
PhSOCH3
93 ± 1
4 ± 1
96 ± 2
0
97 ± 1
0
84
7
90
0
90
0
PhSO CH
2
3
cyclooctene (100 equiv)
cis-cyclooctane-1,2-diol
cyclooctene oxide
octane-1,2-diol
86 ± 1
0
92 ± 2
0
80 ± 1
0
0
0
87 ± 3
0
93 ± 1
0
82 ± 2
0
0
0
0
80
0
85
0
60
0
0
80
0
85
0
60
0
0
0
70
0
67
0
35
22
5
58 ± 1
0
56 ± 2
0
31 ± 2
20 ± 2
5 ± 1
1
-octene (100 equiv)
1
,2-epoxy-octane
cyclohexene (100 equiv)
cis-cyclohexane-1,2-diol
2
2
-cyclohexen-1-one
-cyclohexen-1-ol
cyclohexene oxide
0
0
tert-butyl acrylate (100 equiv)
tert-butyl 2,3-
dihydroxypropanoate
96 ± 2
8 ± 1
96
10
cyclohexane (100 equiv)
cyclohexanol
cyclohexanone
1-methyl-cyclohexanol
0
0
60 ± 1
5 ± 1
63 ± 2
0
70 ± 4
0
58 ± 3
6 ± 1
60 ± 2
4 ± 1
65 ± 4
<2
0
0
12
3
20
4
50
5
45
13
70
0
45
8
1
-methyl-cyclohexane (100
equiv)
10 ± 1
0
20 ± 2
4 ± 1
44
15
47
18
0
2
-methyl-cyclohexanol
adamantane (50 equiv)
1-adamantanol
2
-adamantanol
2
-adamantanone
5 ± 1
0
<4
5
0
a
b
c
Reaction conditions: 0.02 mmol complex dissolved in dry benzene. Reaction time = 15 min. Reaction time = 45 min.
1
6
18
Figure 3. GC-mass spectrum of cis-cyclohexane-1,2-diol product formed in the reaction of 1 with cyclohexene and (a) O , (b) O , and (c)
O /H2 O in the presence of Sc (1 equiv).
2
2
2
16
18
3+
Scheme 2. Oxidation of Substrates by Complex 1 in the
Presence of Sc(OTf)3
Scheme 3. Oxidation of Substrates by Complex 1 with
Dioxygen in the Presence of Pyridinium Perchlorate (2
equiv)
The effect of protic acid on the reactivity of 1 toward
different substrates was also investigated (Scheme 3). The
reaction of 1 with thioanisole and pyridinium perchlorate (2
equiv) afforded thioanisole oxide. As in the case with Sc , the
oxidant here hydroxylated adamantane to form 1-adamantanol
(65%) and trace amounts of 2-adamantanol and 2-
adamantanone (<4% each). Cyclohexane was converted to
cyclohexanol (58%) and cyclohexanone (6%). Methylcyclo-
hexane was oxygenated to 1-methylcyclohexanol (60%) along
with a small amount of 2-methylcyclohexanol (4%). The
3
+
D
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Table 4. Catalytic Oxidation of Substrates by 1 and Its Comparison with 2
(TpPh,Me)Fe (benzilate)] (1)
II
[(TpPh2)Fe (benzilate)] (2)
II
[
h
i
TON (SOx
)
h
i
substrate
external additive
products (SOx
octane-1,2-diol
octane-1,2-diol
1,2-epoxyoctane
cis-cycloctane-1,2-diol
cis-cycloctane-1,2-diol
cyclooctene oxide
cis-cyclohexane-1,2-diol
cis-cyclohexane-1,2-diol
cyclohexene oxide
)
TON (SOx
)
TON (Ph CO)
TON (Ph CO)
2
2
b
1
-octene
17 ± 1
21 ± 1
6 ± 1
15 ± 0.5
19 ± 0.6
14 ± 0.7
14 ± 0.5
19 ± 1
4 ± 1
20 ± 0.5
25 ± 1
20 ± 1
20 ± 0.5
23 ± 1
18 ± 1
19 ± 1
21 ± 1
20 ± 0.5
1.3 ± 0.1
2 ± 0.2
nd
nd
nd
nd
nd
nd
nd
1.6 ± 0.1
2.3 ± 0.2
f
3
Sc(OTf)
PyNH+
g
nd
nd
nd
nd
nd
nd
nd
b
cyclooctene
f
3
Sc(OTf)
PyNH+
g
b
cyclohexene
f
3
Sc(OTf)
PyNH+
g
2
2
-cyclohexenol
-cyclohexenol
8 ± 1
6 ± 1
c
thioanisole
thioanisole oxide
thioanisole oxide
thioanisole oxide
1-adamantanol
5 ± 0.5
10 ± 1
8 ± 0.4
12 ± 1
1.2 ± 1
1.9 ± 1
8 ± 1
13 ± 0.5
16 ± 1
14 ± 0.5
18 ± 1
1 ± 0.1
3 ± 0.2
nd
2.4 ± 0.1
7 ± 0.2
nd
f
Sc(OTf)
3
PyNH+
g
d
f
adamantane
Sc(OTf)3
nd
nd
2
-adamantanol
2
-adamantanone
PyNH+
g
1-adamantanol
15 ± 0.5
nd
nd
2
2
-adamantanol
-adamantanone
1.1 ± 1
1.4 ± 1
10 ± 0.5
e
f
cyclohexane
Sc(OTf)3
cyclohexanol
16 ± 0.5
14 ± 1
15 ± 1
0.6 ± 0.1
0.05
nd
1.1 ± 0.1
cyclohexanone
cyclohexanol
cyclohexanone
1-methylcyclo-hexanol
0.9 ± 0.5
7 ± 0.5
0.5 ± 0.5
11 ± 2
4 ± 1
PyNH+
g
nd
nd
e
f
methyl-cyclohexane
Sc(OTf)3
nd
nd
2
2
-methylcyclo-hexanol
-methylcyclo-hexanone
1.3 ± 1
9 ± 2
PyNH+
g
1-methylcyclo-hexanol
15 ± 0.5
nd
2
2
-methylcyclo-hexanol
-methylcyclo-hexanone
5 ± 0.5
2 ± 1
j
1
9.0
.06
nd
nd
nd
nd
k
Sc(OTf)3
a
Reaction conditions: Dry benzene-acetonitrile (4:1) solvent mixture (4 h for 1 and 1 h for 2) with 0.02 mmol (0.0153 g) catalyst, and 30 equiv
b
(
1
0.150 g, 0.6 mmol) of sodium benzilate. Concentration of alkenes (1-octene, cyclooctene and cyclohexene): 100 equiv with respect to catalyst.
c
d e f g
00 equiv of thioanisole. 50 equiv adamantane. 100 equiv of cyclohexane/methylcyclohexane. 20 equiv of Sc(OTf) with respect to catalyst; 10
3
h i
Ox
equiv of pyridinium perchlorate was added. TON (S ) = mol of product formed/mol of catalyst used. TON (benzilate) = (mol of
benzophenone formed−mol of catalyst used)/mol of catalyst. reaction with complex 1 and sodium benzilate (30 equiv). reaction with complex 1
and sodium benzilate (30 equiv) in the presence of 20 equiv of Sc(OTf) . nd: not determined.
j
k
3
oxidant from 1 in the presence of protic acid oxidized alkenes
to the corresponding epoxides. With cyclooctene as substrate,
cyclooctene oxide was formed in 58% yield. The oxidant
converted 1-octene to 1,2-epoxyoctane in 56% yield (Figure
S19). When cyclohexene is used, the corresponding allylic
oxidation products were formed along with a small amount of
epoxide (Scheme 3 and Figure S20). Therefore, a different
electrophilic oxidant was generated from 1 in the presence of
protic acid which could carry out epoxidation of alkenes
instead of cis-dihydroxylation.
maximum catalytic activity of 1 was obtained with 20 equiv of
3
+
Sc (Figures S21 and S22) and 10 equiv of protic acid (Figure
S23). With increasing concentration of sodium benzilate, the
turnovers number (TON) of benzophenone increased and
reached a maximum value with 30 equiv of sodium benzilate
(
Figure 4). However, the TON of the oxidized product from 1-
Catalytic Activity. The high yields in substrate oxidations
without detectable intraligand hydroxylation under stoichio-
metric conditions encouraged us to explore the catalytic
activity of complex 1 in O -dependent oxidations using excess
2
benzilate. After several experimental trials with 1-octene, the
reaction condition was optimized (Experimental Section). The
catalytic reactions were then separately performed: (i) without
3
+
any external additive, (ii) with Sc , and (iii) with a proton
source in a solvent mixture of dry benzene and acetonitrile
(
Figure 4. Plot of (a) TON for benzophenone vs equiv of sodium
benzilate added and (b) TON of oxidized product(s) from 1-octene
vs equiv of sodium benzilate by complex 1 and O2.
4:1) for 4 h (Table 4). For catalytic reactions with additives,
E
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Scheme 4. Proposed Catalytic Cycle for the Oxygenation of Substrates with O by a Nonheme Iron(II)−Benzilate Complex
2
octene (diol/epoxide) was found to be additive dependent
Figure 4b). In the experimental conditions, highest catalytic
reactions took about an hour to attain maximum yield. The
activity of the complex, however, was found to be low. The
complex did not show catalytic turnover for benzophenone in
the absence of substrate. The oxidation of thioanisole or 1-
(
3+
activity was observed with Sc , affording 25 turnovers for the
conversion of benzilate to benzophenone and 21 turnovers for
octane-1,2-diol. Without any external additive, 20 turnovers for
benzophenone and 17 turnovers for octane-1,2-diol were
achieved (Table 4). With the protic acid, the TON of
benzophenone was maximized at 20 but the TON for 1,2-
epoxyoctane was found to be 6 (Table 4). Catalytic reactions
with 1-octene in the presence of Sc3 revealed that the TONs
of octane-1,2-diol increased with substrate concentration and
achieved a maximum TON at 100 equiv of substrate (Figure
S24). For cyclohexene, 14 turnovers of cis-diol were achieved
as quantified by acetylation of the oxidized product (Figure
S25). The TON for benzophenone not only depends on
additives but also on the nature of substrates; alkenes display
better reactivity compared to other substrates.
3
+
octene in the presence of Sc showed very low TON but the
complex underwent intraligand hydroxylation. The tendency of
the ligand to undergo intraligand hydroxylation was respon-
sible for low catalytic activity of 2. Among all other reported
3
6−40
iron(II)−benzilate complexes,
complex 1 is not only a
+
better system for stoichiometric oxidation, but also for selective
and catalytic oxidation of substrates (Tables 3, 4, and S1).
On the basis of the above experimental results and by
Ph2
analogy with the mechanistic proposal reported for the Tp
36−38
system,
a side-on bound nucleophilic iron(II)-hydro-
peroxide is proposed to form initially in the reductive
activation of dioxygen by complex 1 (Scheme 4). With a
Lewis acid, the iron(II)-hydroperoxide undergoes heterolytic
O−O bond cleavage to generate an electrophilic iron(IV)-oxo-
hydroxo oxidant where two oxygen atoms are disposed cis to
each other and can exchange with water. With protic acid, an
With aliphatic substrates, catalytic reactions were performed
separately with Sc3+ and protic acid (Table 4). Adamantane
was oxidized to 1-adamantanol with a TON of 12 in the
3
+
38
presence of Sc and a TON of 8 in the presence of protic acid.
In both the cases, 2-adamantanol and 2-adamantanone were
observed but with very low yields. Moreover, complex 1
catalyzed the hydroxylation of strong C−H bonds of
cyclohexane in the presence a Lewis acid or a protic acid (as
the case may be) with multiple TON. Methylcyclohexane was
catalytically oxidized to 1-methylcyclohexanol with a TON of
iron(IV)-oxo-aqua species is proposed as the active oxidant.
While both iron(II)-hydroperoxide and iron(IV)-oxo-hydroxo
can perform cis-dihydroxylations of alkenes, the iron(IV)-oxo
species generated with protic acid participates in olefin
epoxidation reaction instead of cis-dihydroxylation. In the
case of aliphatic C−H bond hydroxylation, only the electro-
philic high-valent iron-oxo species are involved. After oxidation
of substrates, the iron(II)−benzilate complex is regenerated
from sodium benzilate. The ESI-mass spectrum (positive ion
mode in benzene−acetonitrile) of the oxidized solution of 1
after the reaction with cyclohexene revealed an ion peak at m/z
3
+
1
1 with Sc and a TON of 9 with protic acid. Additionally,
complex 1 is active in catalytic oxo transfer reaction to convert
thioanisole to thioanisole oxide (Table 4). From the
experimental results, it is evident that Lewis acid enhances
the catalytic efficiency of 1 toward olefin cis-dihydroxylation,
aliphatic C−H bond oxygenation, and oxo transfer reaction. It
is important to mention here that these substrates were not
655.3 corresponding to the iron(II)-alkoxide (monodeproto-
Ph,Me
nated diol) complex of Tp
ligand (Figure S26). Although
the same complex could not be isolated, the addition of excess
sodium benzilate to the above solution resulted in catalytic
turnovers of cyclohexene. This result indicates that iron(II)-
coordinated monodeprotonated diols are labile enough to be
replaced by anionic carboxylate donor of benzilate, thereby
regenerating the catalyst.
catalytically oxidized under the reaction conditions of control
Ph,Me
experiments using a combination of the Tp
ligand and
iron(II) chloride/perchlorate (Experimental Section). These
results clearly demonstrated the role of benzilate in directing
the catalytic oxidation of substrates.
For comparison, the catalytic reactivity of [(TpPh2)-
Thus, complex 1 displays moderate activity as an oxidation
II
Fe (benzilate)] (2) was investigated (Table 4). Here, the
catalyst which makes use of O as the terminal oxidant to
2
F
DOI: 10.1021/acs.inorgchem.8b01353
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
catalyze selective oxygenation of a number of substrates. While
the TON for diols (dioxygenation) from alkenes are high
compared to monooxygenated products, there is a clear
mismatch for the TON of benzophenone and thioanisole, and
of benzophenone and monooxygenated products with protic
acid. It is important to mention here that neither benzilic acid
nor a combination of benzilic acid and scandium(III) triflate
exhibited catalytic oxidation. Additionally, the complex (1) in
the reaction with sodium benzilate (30 equiv) displayed only
stoichiometric conversion of benzilate in the absence of both
encountered during the synthesis of the complexes, perchlorate salts are
potentially explosive and should be handled with care. The ligand
Ph,Me
KTp
was prepared according to a procedure reported in the
46
literature. Complex 2 was prepared according to the reported
procedure.
39
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 (ESI) mass spectra were recorded
with a Waters QTOF Micro YA263 instrument. Solution electronic
spectra (single and time-dependent) were measured on an Agilent
3+
8
453 diode array spectrophotometer. All room temperature NMR
substrate and Sc . Interestingly, a combination of sodium
benzilate (30 equiv) and scandium(III) triflate (20 equiv) was
able to convert benzilate to benzophenone with a TON of 9
even in the absence of thioanisole (Table 4). In the presence of
thioanisole, the TON of benzophenone was increased to 16
along with 10 turnovers for thioanisole oxide. Therefore, the
electrophilic oxidants from 1 can oxidize noncoordinated
benzilate, causing the mismatch in benzilate and substrate
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
PerkinElmer Clarus 600 using Elite 5 MS (30 m × 0.25 mm ×
0
.25 μm) column with a maximum temperature of 300 °C. Labeling
18 18
experiments were carried out with O gas (99 atom %) or H
O
2
2
(
98 atom %) purchased from Icon Services Inc., U.S.A.
Ph,Me
II
Synthesis of [(Tp
)Fe (benzilate)] (1). Anhydrous iron(II)
3
+
TONs. For monooxygenation reactions with or without Sc , 1
equiv of hydroxide ion is expected to generate after each cycle.
The hydroxide ion either remains coordinated to the iron
chloride (0.06 g, 0.50 mmol) was added to a solution of the ligand
Ph,Me
KTp
(0.26 g, 0.50 mmol) in dichloromethane (5 mL). The
resulting mixture was stirred for 10 min to form a white suspension.
To the resulting solution was added a methanolic solution of sodium
benzilate (0.13 g, 0.50 mmol) and was stirred at room temperature
under nitrogen atmosphere for 2 h. The solution was then
concentrated under vacuum, washed with dichloromethane, and
filtered. The resulting filtrate was concentrated, washed with
acetonitrile, and filtered. The filtrate was further concentrated and
washed with hexane to isolate an off-white solid. The solid was further
purified by recrystallization from a solvent mixture of dichloro-
methane and methanol (1:1). Yield: 0.11 g (30%). Elem. anal. Calcd
center or in free form in solution. All attempts to isolate an
Ph,Me
iron(II)−hydroxide complex of the Tp
ligand failed, but
the addition of tetrabutylammonium hydroxide (3 equiv)
almost completely inhibited the catalytic conversion of
thioanisole by complex 1. Similarly, no turnover for substrate
oxidation was observed in the presence of pyridine (10 equiv).
The formation of pyridine in the reaction with pyridinium
perchlorate and the gradual accumulation of hydroxide in the
catalysis without any additive possibly cause low turnovers in
monooxygenation reactions. The hydroxide ion can be partially
quenched by Lewis acid showing relatively better activity in the
−
1
(
5
3
%) for C H BFeN O ·CH OH (799.52 g mol ): C, 67.60; H,
44 40 6 3 3
.55; N, 10.51. Found: C, 67.56; H, 5.38; N, 10.67. IR (KBr):
434(br), 3137−2962, 2551, 1728, 1618(br), 1546(m), 1485(m),
1448(m), 1413, 1344 (s) 1280, 1188(s), 1176(s), 1066(s) 975,
3+
catalytic reactions with Sc . The formation of catalytically
inactive complexes (hydroxide/pyridine-coordinated iron
complex, diiron(III) species) likely contribute to limiting the
turnovers (Scheme S1).
−1
7
61(vs), 696(vs) cm . ESI-MS (positive ion mode, benzene-
Ph,Me
+
CH CN) m/z (%): 789.3 (3%), {[(Tp
)Fe(benzilate)] + Na} ;
3
Ph,Me
+
7
67.3 (6%), {[(Tp
)Fe(benzilate)] + H} ; 697.3 (20%),
Ph,Me
+
Ph,Me
[
(Tp
)Fe + (3Ph,5MePzH)] ; 557.2 (10%), {[(Tp
)Fe] +
+
Ph,Me
+
Ph,Me
H O} ; 539.2 (90%), [(Tp
)Fe] ; 485.4 (20%), {(Tp
)-3Ph,5MePzH] + H} .
) +
2
+
Ph,Me
+
SUMMARY AND CONCLUSIONS
2H} ; 327.2 (100%), {[(Tp
■
Reaction of 1 with Dioxygen. The iron(II)−benzilate complex
0.02 mmol) was dissolved in 10 mL of dioxygen-saturated benzene.
In conclusion, the catalytic efficiency of a nonheme iron(II)-α-
hydroxy acid complex of a hydrotris(3-phenyl-5-
methylpyrazolyl)borate ligand toward selective oxygenation
(
The solution was allowed to stir at room temperature for 45 min.
After the reaction, the solution was concentrated and the residue was
treated with 5 M HCl solution (10 mL). The organic products were
extracted with diethyl ether (3 × 15 mL), and the organic layer was
dried over anhydrous sodium sulfate. After removal of the solvent, the
of organic substrates using O as the terminal oxidant has been
2
explored. The complex catalyzes selective oxidation of organic
substrates with multiple turnovers. Based on interception and
labeling experiments, involvement of iron−oxygen oxidants
such as iron(II)-hydroperoxide (with no external additive),
iron(IV)-oxo-hydroxo (with a Lewis acid) and iron(IV)-oxo-
aqua (with a protic acid) has been proposed in the oxidation
reactions. Lewis acid enhances the catalytic efficiency of the
iron(II)-benzilate toward oxygenation reactions. Thus, a “proof
of concept” of using sacrificial reductant for sustainable
1
colorless residue was analyzed by GC-MS and H NMR spectroscopy.
For dioxygen reactivity in the presence of Lewis acid or protic acid,
the required amount of acid was dissolved in acetonitrile and added to
the reaction solution immediately after purging dioxygen to the
iron(II)−benzilate complex.
Reactions of 1 with Substrates. The iron(II)−benzilate complex
0.02 mmol) was dissolved in benzene (10 mL) under a nitrogen
(
atmosphere. To the solution was added the substrate, and dry
dioxygen was purged through the solution for 2 min. For the reactions
in the presence of additive, the dioxygen-saturated solution was
treated with a Lewis acid/protic acid dissolved in acetonitrile followed
by the addition of external substrate. The reaction solution was
allowed to stir at room temperature for 40−45 min. After oxidation,
the solvent was removed under reduced pressure and the iron
complex was decomposed with 10 mL of 5 M HCl solution. Organic
products were then extracted either with diethyl ether or with
chloroform (3 × 15 mL), and the organic layer was dried over
anhydrous sodium sulfate. After removal of the solvent, organic
catalytic oxidation with O by a bioinspired iron complex has
2
been established. The results presented here would provide
useful information in developing oxidatively robust and
efficient catalytic systems for O -dependent oxidations of
2
organic substrates.
EXPERIMENTAL SECTION
■
Materials and Methods. All chemicals and reagents were
obtained from commercial sources and were used as such 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 glovebox. Although no problem was
1
products were analyzed by GC-MS and H NMR spectroscopy. For
1
reactions with alkenes, the products were analyzed by H NMR or
injected to GC-MS after esterification with acetic anhydride following
G
DOI: 10.1021/acs.inorgchem.8b01353
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
4
7
1
the literature procedure. Quantification of organic products by
NMR was done by comparing the peak area of four aromatic ortho
protons (7.81 ppm) of benzoquinone. For reactions with alkenes in
the presence of protic acid, the solution after reaction with dioxygen
was passed through 15 cm silica (60−120 mesh size) column using
dichloromethane as eluent. The combined organic phase was then
analyzed by GC-mass spectrometry. 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
H NMR (500 MHz, CDCl ) Data of Organic Products.
3
Benzophenone: δ 7.80 (d, 4H), 7.59 (t, 2H), 7.50 (t, 4H).
Thioanisole oxide: δ 7.66 (m, 2H), 7.52 (m, 3H), 2.73 (s, 3H).
Methyl phenyl sulfone: δ 7.96 (d, 2H), 7.60 (m, 3H), 3.06 (s, 3H).
cis-Cyclohexane-1,2-diol: δ 3.77−3.82 (m, 2H), 1.73−1.82 (m, 2H),
1.51−1.64 (m, 4H), 1.27−1.35 (m, 2H). 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),
0.920 (m, 3H). cis-Cyclooctane-1,2-diol: δ 3.94 (d, 2H), 1.93−1.86
(
2
m, 2H), 1.70−1.63 (m, 4H), 1.55−1.49 (m, 6H). 1-Adamantanol: δ
1
8
.14 (s, 3H), 1.72−1.71 (m, 6H), 1.62−1.61 (m, 6H). 2-
authentic compounds. For mixed labeling experiment, H2 O (12 μL)
water was added to the solution of the complex prior to exposure to
dioxygen. All the products were quantified by H NMR spectroscopy
except for cyclohexanol, cyclohexanone, 1-methylcyclohexanol, 2-
methylcyclohexanol, 2-methylcyclohexanone,2-cyclohexenone, 2-cy-
clohexenol, and the epoxides derived from alkenes (cyclohexene
oxide, 1,2-epoxyoctane, and cyclooctene oxide), which were
quantified by GC-MS.
Catalytic Reactions. For catalytic experiments, the iron(II)
complex (0.02 mmol) was dissolved in 10 mL of benzene-acetonitrile
solvent mixture (4:1) under nitrogen atmosphere. To the solution,
sodium benzilate (30 equiv, 0.15 g) was added followed by addition of
excess substrate (100 equiv/50 equiv for adamantane). Then
dioxygen was purged through the solution for 2 min. The reaction
solution was allowed to stir at room temperature for 4 h. For catalysis
in the presence of Lewis acid, 20 equiv of scandium(III) triflate was
added in four equal portions at an interval of 10 min. Similarly for
catalytic experiments in the presence of protic acid, 10 equiv of
pyridinium perchlorate was added in four equal portions at an interval
of 10 min. After the reaction, the solvent was removed under reduced
pressure and the iron complex was decomposed by addition of 5 M
HCl solution (10 mL). The organic products were extracted either by
diethyl ether or by chloroform (3 × 15 mL), and the organic layer was
dried over anhydrous sodium sulfate. After removal of the solvent,
Adamantanol: δ 4.5 (s, 1H), 2.06 (s, 2H), 1.72−1.71 (m, 6H),
1.62−1.61 (m, 6H). (1R, 2R)-Cyclohexane-1,2-diyl diacetate: δ 5.14
1
(
(
d, 2H), 2.24 (s, 6H), 2.04 (m, 2H), 1.81−1.78 (m, 2H), 1.71−1.76
m, 2H), 1.33−1.23 (m, 2H). 3-tert-Butoxy-3-oxopropane-1,2-diyl
diacetate: δ 4.15 (m, 2H), 3.65 (m, 1H), 2.72 (s, 3H), 2.58 (s, 3H),
1
.44 (s, 9H).
X-ray Crystallographic Data Collection and Refinement of
the Structures. X-ray single-crystal data were collected at 120 K
using MoKα (λ = 0.7107 Å) radiation on a SMARTAPEX
diffractometer equipped with CCD area detector. Data collection,
data reduction, structure solution and refinement were carried out
48
using the software package of APEX II. The structure was solved by
direct method and subsequent Fourier analyses and refined by the
full-matrix least-squares method on F with all observed reflections.
The nonhydrogen atoms were treated anisotropically for complex 1.
Routine SQUEEZE was applied to data intensities of compound 1 in
2
49
50
order to take into account the disordered solvent molecules. The
hydrogen atoms on oxygen (O1) and that on N(7) could not be
assigned. CCDC 1508424 (for 1) contains the supplementary
crystallographic data for this paper. The data can be obtained free
of charge from the Cambridge Crystallographic Data Centre via www.
ccdc.cam.ac.uk/data_request/cif.
1
ASSOCIATED CONTENT
organic products were analyzed by GC-MS and H NMR spectros-
■
copy. For catalytic experiments, quantification of the products were
done by adding 3,5-di-tert-butyl benzoquinone as a standard (to the
organic layer after acidic work up). All experiments were performed in
triplicate and the average values are reported.
Control Experiments. For control experiments, iron(II) per-
chlorate hydrate (0.007 g, 0.02 mmol) was added to a solution of the
*
S
Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.inorg-
chem.8b01353.
Spectroscopic data of oxygenated compounds (PDF).
Ph,Me
ligand KTp
(
(0.01 g, 0.02 mmol) in 10 mL benzene-acetonitrile
4:1) solvent mixture. To the resulting solution, substrate (100 equiv
Accession Codes
or 50 equiv for adamantane) was added and dioxygen was purged to
the solution for 2 min. The solution was allowed to stir for 4 h. After
the reaction, the solvent was removed under reduced pressure and the
iron compound was decomposed by addition of 5 M HCl solution
CCDC 1508424 contains the supplementary crystallographic
data for this paper. These data can be obtained free of charge
via www.ccdc.cam.ac.uk/data_request/cif, or by emailing
data_request@ccdc.cam.ac.uk, or by contacting The Cam-
bridge Crystallographic Data Centre, 12 Union Road,
Cambridge CB2 1EZ, UK; fax: +44 1223 336033.
(
1
10 mL). The organic products were extracted by diethyl ether (3 ×
5 mL), and the organic layer was dried over anhydrous sodium
sulfate. After removal of the solvent, organic products were analyzed
1
by GC-MS and H NMR spectroscopy.
AUTHOR INFORMATION
For reactions with alkenes in the presence of protic acid, the
■
solution after reaction with O was passed through 15 cm silica (60−
Corresponding Author
E-mail: ictkp@iacs.res.in. Fax: +91-33-2473-2805. Tel.: +91-
3-2473-4971.
2
120 mesh size) column using dichloromethane as eluent. The
*
combined organic phase was then analyzed by GC-mass spectrometry.
All the substrates (thioanisole, 1-octene, cyclooctene and cyclo-
hexene, cyclohexane, methylcyclohexane and adamantane) were
tested for control experiments used in catalytic experiments. For
most of the substrates, no oxidized product was observed under the
reaction conditions of control experiments. Trace amounts of 2-
adamantanone (5%), cyclohexanone (<2%), and 2-methyl cyclo-
hexanone (3%) were detected from adamantane, cyclohexane, and
methylcyclohexane, respectively. Similar results were observed in
3
ORCID
Tapan Kanti Paine: 0000-0002-4234-1909
Author Contributions
†
S.C. and S.B. contributed equally to this work.
Notes
The authors declare no competing financial interest.
control experiments using the in situ generated iron(II)−chloro
Ph,Me
complex from the Tp
ligand and FeCl .
2
ACKNOWLEDGMENTS
■
Control experiments were also performed with cyclohexanol and
1
8
T.K.P. gratefully acknowledges the Science and Engineering
Research Board (SERB), India, for the financial support
(Project: EMR/2014/000972). S.C. thanks CSIR and S.B.
thanks DST (INSPIRE) for research fellowships.
Sc(OTf)3 or pyridinium perchlorate using H2 O in benzene−
acetonitrile solvent mixture in the presence and absence of iron(II)
salt. In both the cases, no exchange of water with alcohols was
observed.
H
DOI: 10.1021/acs.inorgchem.8b01353
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
Derived Oxidant of a Biomimetic Nonheme Iron Complex. Angew.
Chem., Int. Ed. 2009, 48, 1780−1783.
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