A dinuclear iron(II) complex bearing multidentate pyridinyl ligand: Synthesis, characterization and its catalysis on the hydroxylation of aromatic compounds

Article abstract of DOI:10.1016/j.ica.2016.02.005

A dinuclear iron(II) complex Fe₂L₂(μ₂-Cl)₂Cl₂ (L = N,N-bis(pyridin-2-ylmethyl)prop-2-yn-1-amine) was prepared and fully characterized by UV–Vis spectroscopy, elemental analysis, electrochemical analysis and X-ray single crystal diffraction analysis. The catalytic activity of the complex was assessed for the hydroxylation of aromatic compounds by using aqueous H₂O₂ as an oxidant in acetonitrile. The catalytic system was applicable in a wide range of substrates including aromatic compounds with both electron-donating and electron-withdrawing substituents and showed moderate to good catalytic activity and selectivity in the oxidation reactions. Particularly, in the case of benzene the selectivity of phenol achieve to 74% with the reaction conversion of 24.8%.

Full text of DOI:10.1016/j.ica.2016.02.005

Inorganica Chimica Acta 444 (2016) 159–165
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A dinuclear iron(II) complex bearing multidentate pyridinyl ligand:
Synthesis, characterization and its catalysis on the hydroxylation
of aromatic compounds
a
b,
⇑
b
c
b
b,
⇑
Erxing Gu , Wei Zhong , Hongxia Ma , Beibei Xu , Hailong Wang , Xiaoming Liu
a
School of Metallurgy and Chemical Engineering, Jiangxi University of Science and Technology, Ganzhou 341000, China
College of Biological, Chemical Sciences and Engineering, Jiaxing University, Jiaxing 314001, China
School of Chemistry, Nanchang University, Nanchang 330031, China
b
c
a r t i c l e i n f o
a b s t r a c t
Article history:
2 2 2 2
A dinuclear iron(II) complex Fe L (l2-Cl) Cl (L = N,N-bis(pyridin-2-ylmethyl)prop-2-yn-1-amine) was
prepared and fully characterized by UV–Vis spectroscopy, elemental analysis, electrochemical analysis
and X-ray single crystal diffraction analysis. The catalytic activity of the complex was assessed for the
2 2
hydroxylation of aromatic compounds by using aqueous H O as an oxidant in acetonitrile. The catalytic
system was applicable in a wide range of substrates including aromatic compounds with both electron-
donating and electron-withdrawing substituents and showed moderate to good catalytic activity and
selectivity in the oxidation reactions. Particularly, in the case of benzene the selectivity of phenol achieve
to 74% with the reaction conversion of 24.8%.
Received 2 December 2015
Received in revised form 31 January 2016
Accepted 2 February 2016
Available online 6 February 2016
Keywords:
Diiron complex
Hydroxylation
Aromatic compounds
Catalysis
Ó 2016 Elsevier B.V. All rights reserved.
Phenols
1
. Introduction
A variety of iron complexes have been widely used as homoge-
specific functionalities. Thus, the search for new catalytic systems
based on iron complexes with nitrogen ligands has been the sub-
ject of extensive studies, with the final aim to mimic the catalytic
properties of natural enzymes.
nous catalysts for the preparation of important organic molecules,
including commodity chemicals, pharmaceuticals and polymers
Inspired by some base metals complexes such as iron [17–20]
and copper [21–24] (either mono- or multi-nuclear) exhibiting
functionalities of catalysis on hydroxylation of aromatic
compounds, we have been interested in exploring the catalytic
chemistry of iron and copper complexes bearing multidentate
ligand containing N, O atoms by tuning their electronic and struc-
tural properties. Recently, we have reported such catalytic systems
basing on iron [25] and copper complexes [26,27] for the hydrox-
ylation of benzene. The results showed that the catalytic efficiency
of the complexes correlates well with their reduction potentials,
that is, the more negative the reduction potential of these
complexes, the higher the benzene conversion. Even though the
conversion of benzene achieves over 40%, the selectivity of phenol
is less than 50%. Thus, how to avoid the over-oxidation of phenol to
improve the selectivity/yield becomes a huge challenge in the
homogeneous catalytic hydroxylation of benzene.
[
1–5]. The enthusiasm in the research of iron-based catalysts has
been due to not only that iron is inexpensive, earth-abundant
and non-toxic, but also that in many enzymes, iron-based enzymes
catalyze some important reactions under physiological conditions,
2
such as the activation of O and CAH bonds [6–9]. Largely due to
the biological inspiration, various dinuclear iron complexes were
synthesized to mimic such diiron enzyme and their catalytic
properties were well studied. [FeFe]-hydrogenase [10–13] and sol-
uble methane monooxygenase [14] are such an example. The for-
mer catalyzes both evolution and oxidation of hydrogen at
remarkable efficiency and rate and the latter catalyzes the hydrox-
ylation of methane into methanol [15,16]. In general, the coordina-
tion chemistry of the iron centers in those iron enzymes is well
satisfied by the ligands (usually amino acid residues of proteins)
of N, O, S ligating atoms, which alongside with the proximal
environment offered by protein domains render the enzymes
In this piece of work, we report the fully characterization of a
2 2 2 2 2
dinuclear iron(II) complex Fe L (l -Cl) Cl (L = N,N-bis(pyridin-2-
ylmethyl)prop-2-yn-1-amine) containing multidentate pyridinyl
ligand and its catalysis on the hydroxylation of a variety of
aromatic compounds including benzene under mild conditions
⇑
Corresponding authors. Tel./fax: +86 (0)573 83643937.
E-mail addresses: weizhong@mail.zjxu.edu.cn (W. Zhong), xiaoming.liu@mail.
zjxu.edu.cn (X. Liu).
http://dx.doi.org/10.1016/j.ica.2016.02.005
0
020-1693/Ó 2016 Elsevier B.V. All rights reserved.

1
60
E. Gu et al. / Inorganica Chimica Acta 444 (2016) 159–165
(
at 70 °C in acetonitrile solution) with hydrogen peroxide.
reaction was heated and when the temperature reached 70 °C,
aqueous H (30 wt%, 1.2 mL, 12 mmol) was slowly and carefully
added in one-go. After heated at 70 °C for 2 h, the reaction was
Although the influence was not significant, our results did show
that the electronic property of the substituent group(s) on an aro-
matic compound exerted influence on both conversion and
selectivity.
2 2
O
stopped and the volume of the mixture was calibrated to 10 mL
3 4
with CH CN. To the calibrated reaction solution was added MgSO
(
3.0 g) to remove the water in the reaction before being analyzed
by gas chromatography (GC 7820A) with a packed column of
Restek capillary SE-54 and using toluene as an internal standard.
2
. Experimental
The temperature of the GC column was set at 60 °C for 1 min and
2.1. General procedures
À1
then was programed to rise to 160 °C at the rate of 10 °C min
.
Under the employed conditions, the retention time for benzene,
toluene, benzoquinone and phenol were 3.3 ± 0.2 min,
Unless otherwise stated, all chemicals and solvents were of AR
grade and purchased from local suppliers unless otherwise stated.
The organic solvents used in this work were appropriately dried
when necessary. Elemental analyses were performed on a CHN ele-
mental analyzer and used Elementar Vario MICRO. UV–Vis spectra
were recorded in the range of 200–800 nm on a VARIAN 50 Conc in
acetonitrile. Electrochemistry was performed in a gas-tighten
three-electrode system in which
U = 1 mm) was used as a working electrode, a carbon strip as
counter electrode, and Ag/AgCl (inner reference solution:
a
vitreous carbon disk
(
À1
n
À1
n
0
.45 mol L
[N Bu
]BF
4 4
+ 0.05 mol L
4
[N Bu ]Cl in dichloro-
methane) against which the potential of ferrocenium/ferrocene
À1
n
4 4
couple is 0.55 V in 0.5 mol L [N Bu ]BF in dichloromethane as
described elsewhere [28,29]. Ferrocene was added as an internal
standard and all potentials are quoted against ferrocenium/
+
ferrocene couple (Fc /Fc). Crystallographic data of complex
Fe
tometer with graphite-monochromated Mo
k = 0.71073 Å). The crystal structures were solved using direct
2
L
2
(l
2
-Cl)
2
Cl
2
were collected on a Bruker SMART CCD diffrac-
K
a
radiation
(
methods in SHELXS and refined by full-matrix least-squares routines,
2
based on F , using the SHELXL package [30].
Fig. 1. Crystal structure of complex Fe L (l2-Cl) Cl .
2
2
2
2
2 2 2 2
2.2. Synthesis of iron(II) complex Fe L (l-Cl) Cl
Table 1
The ligand L (474 mg, 2 mmol) was dissolved in degassed
methanol (10 mL) and then the degassed methanolic solution of
FeCl O (397 mg, 2 mmol) was added into the above solution.
Crystallographic details and refinement data for complex Fe
2
L
2
(l
2 2 2
-Cl) Cl .
Empirical formula
C
30
H
30Cl
4 6 2
N Fe
2
Á4H
2
F
w
728.10
monoclinic
P2(1)/c
The mixture was stirred for 3 h under argon atmosphere at room
temperature to afford yellow solid. The solution was removed
under reduced pressure to collect the yellow solid (550 mg, 76%),
which was washed with water (3 Â 30 mL) and diethyl ether
Crystal system
Space group
Unit cell dimensions
a (Å)
b (Å)
c (Å)
10.0015(7)
17.0150(12)
10.0015(7)
90
110.01
90
1599.25(19)
2
1.512
(
3 Â 30 mL) successively, and then dried in vacuo. Dark yellow
crystals suitable for X-ray analysis were obtained from its solution
in a mixture of diethyl ether–methanol in a few days at room tem-
a
(°)
b (°)
(°)
c
perature. Elemental analysis calculated for C30
H
30
N
6
2 4
Fe Cl : Calc. C,
3
V (Å )
4
9.49; H, 4.15; N, 11.54; found: C, 49.46; H, 4.10; N, 11.18%.
Z
D
calc (Mg m3)
2
.3. Catalytic assessment
F(000)
744
Reflections collected (Rint
Reflections independent
Goodness-of-fit on F2
)
14765 (0.0223)
3936
1.024
0.0235, 0.0643
0.0261, 0.0662
A typical procedure is as follows: Benzene (0.9 mL, 10 mmol),
acetonitrile (3.8 mL) and catalytic amount of the iron(II) complex
2‰) were placed into the reaction vessel (10 mL), which is
R , wR
1
2
[I > 2
r(I)]
(
R1, wR2 (all data)
equipped with cooling condenser and placed in an oil-bath. The
N
N
Cl
Cl
Cl
N
MeOH
r.t. 3 h
N
Fe
+
FeCl2 4H O
Fe
N
2
N
N
Cl
N
L
N
Fe L (­ -Cl) Cl
2
2
2
2
2
Scheme 1. Synthesis of dinuclear iron(II) complex Fe
2
L
2
(l
2
-Cl)
2
Cl
2
.

E. Gu et al. / Inorganica Chimica Acta 444 (2016) 159–165
161
Table 2
76% (Scheme 1). The complex is easy to purify by successively
washing with water and diethyl ether for several times. The iden-
2 2 2 2 2
Selected bond distances (Å) and angles (°) for complex Fe L (l -Cl) Cl .
Fe(1)AN(1)
Fe(1)AN(2)
Fe(1)AN(3)
Fe(1). . .Fe(1A)
N(1)AFe(1)AN(2)
N(2)AFe(1)AN(3)
N(1)AFe(1)AN(3)
N(1)AFe(1)ACl(1)
N(2)AFe(1)ACl(1)
N(3)AFe(1)ACl(1)
N(1)AFe(1)ACl(2)
N(2)AFe(1)ACl(2)
2.3126(11)
2.2083(11)
2.2156(11)
3.803(4)
74.80(4)
99.64(4)
72.55(4)
159.81(3)
93.78(3)
93.56(3)
97.92(3)
89.29(3)
Fe(1)ACl(1)
Fe(1)ACl(2)
Fe(1)ACl(2A)
2.3667(4)
2.4797(4)
2.5445(4)
tity of complex Fe
sis and X-ray crystal structure determination.
Crystals of complex Fe -Cl) Cl suitable for X-ray single
2 2 2 2 2
L (l -Cl) Cl was confirmed by its microanaly-
2
L
2
(l
2
2
2
crystal diffraction analysis were grown from a methanol solution
into which the vapor of diethyl ether was diffused. Its structure
is shown in Fig. 1 and its crystallographic data and selected bond-
ing parameters are tabulated in Tables 1 and 2, respectively. The
structure consists of a dimeric unit generated through a crystallo-
graphically imposed inversion center. Each iron center is bridged
by two chloride ligands and bound terminally by a chloride and a
ethyne-substituted bis(pyridin-2-ylmethyl)amine ligand to con-
N(3)AFe(1)ACl(2)
164.46(3)
92.42(3)
163.22(3)
86.46(3)
101.470(14)
81.625(12)
98.539(14)
N(1)AFe(1)ACl(2A)
N(2)AFe(1)ACl(2A)
N(3)AFe(1)ACl(2A)
Cl(1)AFe(1)ACl(2A)
Cl(2)AFe(1)ACl(2A)
Cl(1)AFe(1)ACl(2)
3 3
struct a pseudo-octahedral geometry with ‘‘N Cl ” donor atoms.
4
.4 ± 0.2 min, 6.6 ± 0.2 min, and 7.4 ± 0.2 min, respectively. Other
aromatic compounds were analyzed by GC (7820A) and GC–MS
7890-5975C). The temperature of the GC column/GC–MS column
As expected, the FeANamine distance (2.3126(11) Å) is more than
0.1 Å longer than FeANpyridine (2.2083(11), 2.2156(11) Å), which
is similar to reported analogous iron complexes [32–36]. The two
FeACl(bridging) bond distances (Fe(1)ACl(2) = 2.4797(4) Å, Fe
(1)ACl(2A) = 2.5445(4) Å) are over 0.1 Å longer than the FeACl
(
was set at 60 °C for 1 min and then was programed to rise to 250 °C
at the rate of 15 °C min
À1
.
(
terminal) bond distances (Fe(1)ACl(1) = 2.3667(4) Å), but both of
them still falls within the range observed in structurally related
complexes [32–37]. The Fe. . .Fe distance is 3.803(4) Å, which falls
at the top end of the range found for related complexes [32–37].
The UV spectra of ligand L and complex Fe L (l2-Cl) Cl2 were
3
. Results and discussion
3.1. Synthesis and characterization of complex Fe L (l -Cl) Cl
2 2 2 2 2
2
2
2
recorded in acetonitrile. It exhibits a very strong absorption in
⁄
Ligand L was prepared using modified literature procedure [31]
and has reported in our previous work [25]. Treatment of ligand L
and FeCl O in methanolic solution for 3 h leads to the forma-
Á4H
tion of dinuclear iron(II) complex Fe -Cl) Cl in a yield of
the UV, corresponding mainly to pyridines
p
–p
transitions, and
a less intense one near visible region related to metal-to-ligand
charge transfer (MLCT) that are responsible for the yellow color
of the solution (Fig. 2).
2
2
2
L (l
2
2
2
2
The cyclic voltammogram (CV) of complex Fe
2 2 2 2 2
L (l -Cl) Cl
showed a broad quasi-reversible redox couple and the oxidation
2
+
3+
event which assigned to Fe /Fe was at a low potential of
+
À0.129 V (E1/2) (versus Fc /Fc) as illustrated in Fig. 3. This broad
redox couple can be explained by the structure of complex
2 2 2 2 2
Fe L (l -Cl) Cl composing two iron(II) metal center. Although
those two iron(II) metal center possess exactly the same coordina-
tion sphere as revealed by its solid state structure, the CVs of them
may not exhibit the same redox events. This assumption was also
supported by the differential pulse voltammograms shown in
Iron(II) catalyst (2‰)
OH
R
+ H O2 (aq)
2
R
Acetonitrile, 70 oC, 2 h
R = H, electron-donating groups, electron-withdrawing groups
Scheme 2. Hydroxylation of aromatic compounds catalyzed by complex
Fig. 2. UV–Vis spectra of ligand L and complex Fe
2
L
2 2
(l
-Cl)
2
Cl
2
in MeCN.
2 2 2 2 2
Fe L (l -Cl) Cl .
À1
Fig. 3. Left: Cyclic voltammogram of complex Fe
acetonitrile (v = 0.1 V s , 298 K).
2 2
L (
2
l -Cl)
2
Cl
2
in acetonitrile (v = 0.1 V s , 298 K); right: differential pulse voltammogram of complex Fe
2
L (l
2
2
-Cl)
Cl
2 2
in
À1

Table 3
Products distribution and yields (area ratio %) for the hydroxylation of aromatic compounds with H
a
2 2
O .
Conversionb (%)
Other products
R
R
R
HO
R
HO
OH
R = H
24.8 ± 0.5
74 ± 4
–
–
O
OH
OH
OH
O
HO
(
1.0 ± 0.5)
(20 ± 2)
(
5 ± 2)
R = Cl
R = Br
23.4 ± 0.2
21.1 ± 0.3
35 ± 2
27 ± 2
–
51 ± 4
HO
Cl
O
O
O
OH
10 ± 1)
Cl
4 ± 1)
(
(
47 ± 2
–
HO
Br
O
O
O
OH
Br
(21 ± 2)
(5 ± 1)
R = CN
R = NO
20.0 ± 0.3
19.8 ± 0.7
25.7 ± 0.2
44 ± 3
12 ± 1
21 ± 2
24 ± 1
61 ± 1
20 ± 2
32 ± 1
27 ± 1
–
–
–
2
R = Mec
O
OH
O
O
CHO
COOH
O
O
(25 ± 1)
(8 ± 1)
(15 ± 1)
(8 ± 1)
(3 ± 1)
R = OMe
R = Ph
27.8 ± 0.9
28.0 ± 0.4
45 ± 1
25 ± 2
–
24 ± 1
25 ± 1
HO
OMe
OH
HO
(22 ± 2)
(9 ± 1)
23 ± 1
OH
OH
OH
O
O
OH
OH
O
OH
(17 ± 1)
O
(3 ± 1)
(
1.0 ± 0.2)
(6 ± 1)
t
R = Bu
18.0 ± 0.9
–
16 ± 1
49 ± 3
O
tBu
tBu
tBu
HO
O
OH
17 ± 1)
O
4 ± 1)
(
O
(
(14 ± 1)
a
b
c
Calculated by GC–MS.
Calculated by single addition method with toluene as internal standard substance, GC.
Benzene as internal standard substance.

E. Gu et al. / Inorganica Chimica Acta 444 (2016) 159–165
163
Fig. 3, which displayed two successive oxidation peaks at À0.237 V
and À0.024 V, respectively.
conditions by varying reaction temperature, reaction time, catalyst
dosage and the amount of H . Full details of the optimization of
the reaction conditions are reported in our recent work [25]. By
employing the optimized reaction conditions, benzene conversion
achieves 24.8% with a selectivity of 74% by using complex
2 2
O
3.2. Catalytic hydroxylation of aromatic compounds
Fe
2
L
2
(
l
2
-Cl)
Inspired by higher selectivity than other catalytic systems we
reported previously [25,26], we tried to extend the hydroxylation
2 2
Cl as a catalyst.
We began our evaluation of this iron-based catalyst by hydrox-
ylation of benzene using hydrogen peroxide as an oxidant in ace-
tonitrile and the hydroxylation was carried out under different
Table 4
a
Variation in product distribution at various reaction time.
Substrate
Benzene
0.5 h
1 h
1.5 h
2.0 h
OH
OH
OH
OH
(
10.3%)
(12.0%)
(15.3%)
(19.8%)
O
O
O
O
O
O
(
0.8%)
(1.4%)
(2.1%)
OH
OH
(
0.6%)
OH
HO
(
0.7%)
Toluene
CHO
CHO
CHO
CHO
(
11.6%)
(13.1%)
(12.9%)
(14.6%)
O
O
O
O
O
O
O
O
(
2.9%)
(2.3%)
(3.6%)
(3.9%)
OH
OH
OH
OH
(
(
11.9%)
2.2%)
(10.5%)
(2.4%)
(13.0%)
(4.1%)
(13.3%)
OH
OH
OH
OH
(5.0%)
(1.3%)
OH
OH
(
0.9%)
COOH
OH
(
2.9%)
Chlorobenzene
Cl
Cl
Cl
Cl
OH
OH
OH
(
13.9%)
(14.6%)
(15.1%)
(15.3%)
OH
OH
OH
OH
Cl
2.6%)
Cl
(4.0%)
Cl
(4.7%)
Cl
(4.7%)
(
HO
Cl
OH
(
2.0%)
O
O
O
Cl
(
0.8%)
a
The yield of products (area ratio %) are calculated by GC–MS.

1
64
E. Gu et al. / Inorganica Chimica Acta 444 (2016) 159–165
reaction to substituted aromatic compounds under the reaction
conditions we optimized (Scheme 2) [25]. The hydroxylation of
aromatic compounds with both electron-donating substituents
and electron-withdrawing substituents afforded the corresponding
phenols in moderate yield (10–20%) with the distribution of iso-
mers of ortho-(o), meta-(m) and para-(p), although most of them
produced other by-products with a certain percentage (Table 3).
In general, aromatic compounds with electron-donating sub-
stituents (Me, OMe, Ph) could be hydroxylated in higher reaction
conversion than those with electron-withdrawing substituents
Table 5
The influence of radical scavengers on the hydroxylation of benzene performances.
Entry
Radical scavenger
Conversion (%)
Yield (%)
Selectivity (%)
1a
–
–
–
24.8 ± 0.5
0
0
24 ± 1
18.7 ± 0.9
14.9 ± 0.5
11.3 ± 0.7
9.6 ± 0.7
0
18.4 ± 0.6
0
0
18 ± 1
14.3 ± 0.7
11.6 ± 0.7
8.9 ± 0.3
8 ± 1
74 ± 4
0
0
76 ± 2
77 ± 3
77.6 ± 0.9
79 ± 1
79 ± 1
0
b
2
c
3
4
5
6
7
8
1 mmol EtOH
6 mmol EtOH
12 mmol EtOH
24 mmol EtOH
48 mmol EtOH
1 mmol TEMPO
1 mmol TEMPO
6 mmol TEMPO
12 mmol TEMPO
24 mmol TEMPO
b
9
0
0
(
2
NO , CN, Br, Cl). The only exception is tert-butylbenzene, which
1
1
1
0d
1
2
0
0
exhibited the lowest conversion probably due to the steric effect
of the tert-butyl group. On the contrary, aromatic compounds with
electron-withdrawing substituents (74–100%) showed higher
hydroxylation selectivity than those with electron-donating sub-
stituents (41–73%), although the distribution of o-, m- and p-posi-
tion phenols do not have distinct regularity. The m-hydroxylated
products obtained higher than other isomers suggest that the
hydroxylation occurred preferably at the meta-position for the aro-
matics with electron-withdrawing groups such as nitrobenzene
and bromobenzene. But no formation of m-hydroxylated product
was found when chlorobenzene used as the substrate and
cyanobenzene favor to form o-hydroxylated product. The predom-
inance of the p-substituted phenol and no o-substituted phenol
formation in tert-butylbenzene oxidation indicate that the oxidiz-
ing species is sterically demanding, which is consistent with the
reported analogous systems [38,39]. For the aromatic compounds
with electron-donating substituents, some by-products such as
di-hydroxylated derivatives, quinones and oxidative ring-opening
adducts were detected. In the case of toluene, the oxidation took
place at the methyl group, giving benzaldehyde, benzoic alcohol
and benzoic acid in a yield of 6.4%, 3.9% and 2%, respectively. Inter-
estingly, phenol was obtained in 6.1% yield in the case of anisole,
which indicates demethylation happened in the oxidation process.
It is noteworthy that for most of the substrates, a derivative of
maleic anhydride was observed. This can certainly be attributed
to the well established fact that the deep oxidation of benzene
skeleton produced this anhydride. This argument was further val-
idated by monitoring the reaction progress, Table 4. As shown in
the table, along the reaction coordinate, the reaction products
became complicated and for the chlorobenzene, the chloro-
substituted maleic anhydride was observed in the products at
the reaction time of 2 h.
16.4 ± 0.9
12 ± 1
9.9 ± 0.4
12.7 ± 0.7
9 ± 1
8.0 ± 0.9
77 ± 2
79 ± 2
81 ± 2
13
a
Reaction conditions: complex Fe
2 2 2 2 2
L (l -Cl) Cl (0.01 mmol), benzene (0.9 mL,
10 mmol), H (1.2 mL, 12 mmol), acetonitrile as solvent (3.8 mL), temp. = 70 °C,
2 2
O
reaction time = 2 h.
b
2 2
Under the standard conditions without H O .
Under the standard conditions without complex Fe
c
2
L
2 2
(l
-Cl)
2
Cl
2
.
d
Under the standard conditions without complex Fe
tion for 2 h.
L
2 2
(l
2
-Cl)
2
Cl
2
and irradia-
same radical mechanism. However, as we pointed out previously
25], the participation of a high valent iron-oxo species cannot be
[
ruled out since the oxidation occurred still even when excess rad-
ical scavenger was present.
4
. Conclusions
In summary, a dinuclear iron(II)-based catalyst was prepared
and fully characterized. The complex was employed as a catalyst
on the hydroxylation of substituted aromatics with H as an oxi-
dant. Aromatic compounds with electron-donating substituents
showed higher reaction conversion than those with electron-with-
drawing substituents while the selectivity of hydroxylation was on
the opposite. The results indicated that the hydroxylating products
are not only influenced by the electronic effect, but also by the
steric one.
2 2
O
Acknowledgements
The authors would like to thank the National Natural Science
Foundation of China (Grant nos. 21301071, 21171073,
2
1371079), the Natural Science Foundation of Zhejiang Province
3
.3. Mechanistic considerations
(
(
LQ13B010002) and the Government of Zhejiang Province
Qianjiang Professorship for X.L.) for supporting this work.
In order to obtain some insights into the general type of mech-
anism involved in the studied oxidations, we selected benzene as
an example to perform the reaction in the presence of a radical
scavenger as well as some control experiments. The results are tab-
ulated in Table 5.
Appendix A. Supplementary material
CCDC 1429939 contains the supplementary crystallographic
data for the complex. These data can be obtained free of charge
from The Cambridge Crystallographic Data Centre via www.
ccdc.cam.ac.uk/data_request/cif.
Supplementary data associated with this article can be found, in
the online version, at http://dx.doi.org/10.1016/j.ica.2016.02.005.
2 2
Generally, in the oxidation of benzene by H O using iron com-
plex as catalyst, the hydroxyl radicals are considered as active spe-
cies for the hydroxylation [40,41]. When a radical scavenger, such
as ethanol and TEMPO (2,2,6,6-tetramethylpiperidine-1-oxyl), was
2 2 2 2 2
added into the oxidation of benzene by complex Fe L (l -Cl) Cl ,
the phenol yield decreases steadily while the phenol selectivity
increases slightly (Table 5, entries 4À8 and 11À13). These observa-
tions are in agreement with what we reported previously for Fe
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