Efficient hydroxylation of aromatic compounds catalyzed by an iron(II) complex with H2O2

Article abstract of DOI:10.1002/aoc.3178

A mononuclear iron(II) complex, Et₄N[Fe(C₁₀H ₆NO₂)₃], coordinated by three 1-nitroso-2-naphtholate ligands in a fac-N₃O₃ geometry, was initiated to catalyze the direct hydroxylation of aromatic compounds to phenols in the presence of H₂O₂ under mild conditions. Various reaction parameters, including the catalyst dosage, temperature, mole ratio of H₂O₂ to benzene, reaction time and solvents which could affect the hydroxylation activity of the catalyst, were investigated systematically for benzene hydroxylation to obtain ideal benzene conversion and high phenol distribution. Under the optimum conditions, the benzene conversion was 10.2% and only phenol was detected. The catalyst was also found to be active towards hydroxylation of other aromatic compounds with high substrate conversions. The hydroxyl radical formed due to the reaction of the catalyst and H₂O₂ was determined to be the crucial active intermediate in the hydroxylation. A rational pathway for the formation of the hydroxyl radical was proposed and justified by the density functional theory calculations. Copyright

Full text of DOI:10.1002/aoc.3178

Full Paper
Received: 22 April 2014
Revised: 20 May 2014
Accepted: 20 May 2014
Published online in Wiley Online Library
(wileyonlinelibrary.com) DOI 10.1002/aoc.3178
Efficient hydroxylation of aromatic compounds
catalyzed by an iron(II) complex with H O
2
2
a
a
a
b
Xiao Wang , Tianyong Zhang , Bin Li *, Qiusheng Yang
a
and Shuang Jiang
A mononuclear iron(II) complex, Et N[Fe(C H NO ) ], coordinated by three 1-nitroso-2-naphtholate ligands in a fac-N O
3 3
4
10
6
2 3
geometry, was initiated to catalyze the direct hydroxylation of aromatic compounds to phenols in the presence of H
2 2
O under
mild conditions. Various reaction parameters, including the catalyst dosage, temperature, mole ratio of H O to benzene,
2
2
reaction time and solvents which could affect the hydroxylation activity of the catalyst, were investigated systematically for
benzene hydroxylation to obtain ideal benzene conversion and high phenol distribution. Under the optimum conditions,
the benzene conversion was 10.2% and only phenol was detected. The catalyst was also found to be active towards hydrox-
ylation of other aromatic compounds with high substrate conversions. The hydroxyl radical formed due to the reaction of the
catalyst and H
formation of the hydroxyl radical was proposed and justified by the density functional theory calculations. Copyright ©
014 John Wiley & Sons, Ltd.
2 2
O was determined to be the crucial active intermediate in the hydroxylation. A rational pathway for the
2
Additional supporting information may be found in the online version of this article at the publisher’s web-site.
Keywords: iron complex; hydroxylation; H O ; hydroxyl radical
2
2
acetonitrile and water, which are usually used as solvents in hydrox-
ylation reactions, and this may suppress its catalytic activity as a
homogeneous catalyst. Hence, in this study, the sodium ion in
PG8 was replaced by the tetraethylammonium ion, producing
soluble Et N[Fe(C H NO ) ] (PGN) to enhance catalytic activity.
Introduction
Phenols are important organic chemical raw materials that have
been produced by multistep reactions with high power consump-
tion for several decades. Recently, the one-step catalytic synthesis
4
10
6
2 3
[1–5]
of phenols
has attracted considerable interest in terms of atom
PGN was characterized as an iron complex coordinated by three
1-nitroso-2-naphtholate ligands through nitrogen and oxygen
atoms. In the presence of H O , PGN (Fig. 1) was used as an efficient
efficiency and environmental friendliness. The key technique for
the direct synthesis of phenols lies in the design and synthesis of
efficient catalysts. Although it is difficult to recycle and separate
compared with heterogeneous catalysts, homogeneous catalysts
usually have higher catalytic hydroxylation efficiency.
2
2
catalyst for the direct hydroxylation of aromatic compounds, as iron
complexes containing nitrogen and phenolic oxygen donor atoms
have been widely used as efficient catalysts for hydroxylation of
[
26–28]
Iron-based catalyst has been an active research area for many
various substrates.
The hydroxylation proved to proceed
[6–9]
years,
as the geologically abundant and non-toxic iron ele-
with the participation of the hydroxyl radical, as supported by
density functional theory (DFT) calculations.
ment performs a number of chemically challenging oxidative
processes with high precision and reaction rates.
[
10–12]
Among
various iron-based catalysts, iron complexes have been accepted
[
13,14]
as effective homogeneous catalysts
for direct hydroxylation Experimental
of aromatics. In the hydroxylation of aromatic compounds in the
[15,16]
[17–19]
Materials and Methods
liquid phase, O
dants. The hydroxylation of aromatic compounds with H
very attractive from the viewpoint of industrial technology and
synthetic organic chemistry since H is cheap, environmentally
clean and easy to handle. The direct hydroxylation processes
oxidized by O generally need to employ severe operating condi-
tions because of the inactive triplet nature of O . Understanding
2
and H
2
O
2
are usually used as the oxi-
2 2
O
is
The chemicals were of analytical grade and used as received. FT-IR
spectra were recorded on a Thermo Nicolet 380 spectrophotometer
2 2
O
* Correspondence to: Bin Li, Collaborative Innovation Center of Chemical Science
and Engineering (Tianjin), Tianjin Key Laboratory of Applied Catalysis Science
and Technology, School of Chemical Engineering and Technology, Tianjin Uni-
versity, Tianjin 300072, People’s Republic of China. E-mail: libin@tju.edu.cn
2
2
the reaction mechanism between the catalysts and the oxidants
is still controversial but is of great significance for designing more
[20–23]
efficient catalysts.
C.I. Pigment Green 8 (Na[Fe(C10
plex widely used as a cheap organic pigment with excellent color
a Collaborative Innovation Center of Chemical Science and Engineering (Tianjin),
Tianjin Key Laboratory of Applied Catalysis Science and Technology, School of
Chemical Engineering and Technology, Tianjin University, Tianjin 300072,
People’s Republic of China
6 2 3
H NO ) ], PG8) is an iron(II) com-
[24]
performance,
which was recently adopted as an efficient
photocatalyst for selective oxidation of phenol in our group. In
the previous study, we found that PG8 was partly dissolved in
[25]
b School of Chemical Engineering and Technology, Hebei University of
Technology, Tianjin 300130, People’s Republic of China
Appl. Organometal. Chem. (2014)
Copyright © 2014 John Wiley & Sons, Ltd.

X. Wang et al.
1
1
(
512 (Ar–C–O), 1453 (Ar–N¼O), 1360 (¼C–H phenyl). H NMR
6
400 MHz, DMSO(d ) see Fig. S1, supporting information), δ
8.87 (d, J = 8.3 Hz, 3H, C9–H), 7.90 (d, J = 9.3 Hz, 3H, C4–H),
7.76 (d, J = 7.9 Hz, 3H, C6–H), 7.51 (dd, J = 7.4 Hz, 8.3 Hz, 3H,
C8–H), 7.31 (dd, J = 7.9 Hz, 7.4 Hz, 3H, C7–H), 7.12 (d, J = 9.3 Hz,
H, C3–H), 3.20 (q, J = 7.0 Hz, 8H, C11–H), 1.16 (t, J = 7.0 Hz, 12H,
3
Figure 1. Hydroxylation of aromatic compounds catalyzed by PGN (the
C¼C― double bonds stand for the naphthalene ring).
13
C12–H). C NMR (100 MHz, DMSO(d
51.4 (C1), 137.5 (C4), 130.6 (C5), 127.7 (C6), 125.3 (C10), 124.3
C8, C9), 123.2 (C7), 121.4 (C3), 51.4 (C11), 7.04 (C12).
6
) see Fig. S1), δ 183.6 (C2),
―
1
(
as a KBr pellet. UV–visible absorption spectra were taken on a
Thermo Scientific Evolution 300 UV–visible spectrophotometer.
TGA was performed using a TA SDT-Q600 apparatus and carried
Hydroxylation of Aromatic Compounds
ꢀ
1
out at dynamic temperature ramp at 10°C min from ambient
Hydroxylation of the aromatic compounds with H O2 (30% in
2
1
temperature to 800°C under nitrogen atmosphere. H NMR
water) was carried out in a 50 ml round-bottom flask equipped with
a reflux condenser and a magnetic stirrer. In a typical reaction, PGN
was dissolved in 20.0 ml acetonitrile. After the solution was heated
to the desired temperature, the aromatic substrates were added. A
certain amount of H O was added to initiate the hydroxylation. All
1
3
and C NMR spectra were recorded on a Bruker Avance III
00 M spectrometer using tetramethylsilane (TMS) as internal
4
standard in DMSO-d . Elemental analysis was carried out on a
6
Heraeus CHN-O-Rapid fully automatic elemental analyzer with
thermal conductivity detection (TMT CHN–O, Bestell–NR
2
2
the experiments were conducted under ambient conditions. After
stirring for several hours, a small amount of the mixture was
centrifuged and analyzed by gas chromatography.
2
215001). Electrochemical measurements were carried out in
ꢀ
4
ꢀ1
acetonitrile solution (10 mol L ) on a CHI660B (Shanghai
Chenhua Instruments) electrochemical workstation using a
three-electrode cell. The glassy carbon (0.071 cm ) working
2
electrode was polished with polishing α-alumina suspension
and rinsed with acetonitrile before use. The counter-electrode
Results and Discussion
was a platinum wire. All potentials were recorded with respect
Thermal Stability of PGN
+
to an Ag/Ag reference electrode (0.01 M AgNO
3
and 0.1 M n-
Thermal stability of PGN was checked by TG analysis and the
derivative thermogravimetry (DTG) curve is shown in Fig. 2. A
rapid weight loss originated from the decomposition of the
ligands from 280 to 300°C was observed. A slower weight loss
resulting from the volatilization of the early decomposition
products of PGN occurred from 600 to 800°C. We concluded from
the results of the TG and DTG analysis that PGN showed great
heat resistance.
4 4
Bu NClO in acetonitrile) and converted to values versus the
+
ferrocene/ferrocenium redox couple (Fc /Fc). X-ray diffraction
intensities were collected on a Rigaku MM-007 (rotating anode)
diffractometer equipped with a Saturn 70CCD. Data were col-
lected at 113 K using a confocal monochromator with Mo-K
α
radiation (λ = 0.71073 Å) in the ω–ϕ scanning mode. Data col-
lection, reduction and absorption correction were performed
[
29]
with the Crystalclear program.
The structure was solved by
direct methods and refined with the full-matrix least-squares
[
30]
technique using SHELXS-97 and SHELXL-97
programs, re-
Electrochemical Measurements of PGN
spectively. Hydrogen atoms were included in their calculated
positions. Some unresolved solvent present in the structure of
PGN was found to be highly disordered, so the SQUEEZE pro-
The cyclic voltammogram (CV) of PGN showed a reversible redox
2+
3+
couple and the oxidation event which assigned to Fe /Fe was
[
31]
+
cedure of the PLATON program
was used, suggesting 32
at a low potential of 0.017 V (vs. Fc /Fc) as illustrated in Fig. 3. The
electrons in one unit cell. The molecular structure diagram
low oxidation potential implied the strong electron donating
ability of the 1-niroso-2-naphtholate ligands to the iron center,
which also suggested that PGN could be oxidized easily to the
iron(III) species under appropriate conditions.
[
32]
(Fig. 4) was created with the SHELXTL program.
The quanti-
tative and qualitative analysis of the aromatics and phenols
were carried out with an Agilent 6890 N gas chromatograph
equipped with capillary column (30 m × 0.25 mm × 0.25 μm)
and flame ionization detector. Density functional theory calcu-
lations were carried out at B3LYP/6-31G + d level implemented
[
33]
with the Gaussian 09 program.
Preparation of PGN
[
25]
PG8 was synthesized according to the literature (yield 98.0%).
Tetraethylammonium chloride (166 mg, 1.0 mmol) was added to
a N,N-dimethylformamide solution (40.0 ml) of PG8 (595 mg,
1
.0 mmol) and the mixture was stirred for 2 h at room tempera-
ture. After the addition of diethyl ether (100 ml) to the solution,
PGN precipitated out and was collected as a dark-green solid
(630 mg, 89.7%). Anal. Calcd for PGN (C38
4 6
H38FeN O ): C, 64.96;
H, 5.41; N, 7.98. Found: C, 64.73; H, 5.54; N, 7.75%. UV–visible λmax
3
ꢀ1
ꢀ1
(
2
acetonitrile) (nm): 345, 430, 690 (ε (dm mol cm ) 37 200,
–1
4 800, 32 400). FT-IR (KBr) υmax (cm ): 1593 (C¼C phenyl),
Figure 2. The TG and DTG analysis of PGN.
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Copyright © 2014 John Wiley & Sons, Ltd.
Appl. Organometal. Chem. (2014)

Iron complex catalyzed hydroxylation
Figure 3. Cyclic voltammogram of PGN in de-aerated acetonitrile
ꢀ
1
4 4
containing 0.1 M n-Bu NClO at 25°C, scan rate 0.05 V s .
Figure 4. X-ray crystal structure of PGN (ellipsoids at 50% probability
level, the hydrogen atoms are omitted for clarity). Selected bonds lengths
(
1
Å): Fe1―N1 1.887(2), Fe1―O1 1.9652(16), Fe1―N2 1.854(2), Fe1―O3
.9635(16), Fe1―O5 1.9636(19), Fe1―N3 1.858(2).
X-Ray Crystal Structure Determination
In the literature, structures of metal complexes coordinated by
the 1-nitroso-2-naphtholate ligand feature coordination geome-
[
34,35]
tries based on six oxygen atoms
and three oxygen atoms.
or three nitrogen atoms
one triangular face, establishing PGN to be the fac-isomer; the
three coordinated oxygen atoms occupied the trans positions
to nitrogen on the opposite triangular face.
[
36,37]
In order to obtain the exact struc-
ture of PGN, an X-ray crystal structure determination was carried
out. Single crystals suitable for X-ray crystallographic analysis
were obtained by slow diffusion of n-hexane into the dichloro-
methane solution of PGN. Details of the crystallographic data
are summarized in Table 1. CCDC975043 contains the supple-
mentary crystallographic data for this paper. The molecular
structure of PGN (Fig. 4) showed a slightly distorted octahedral
iron center coordinated by the 1-nitroso-2-naphtholate ligands
through nitrogen and oxygen atoms, each forming a five-
membered chelate ring. All the three nitrogen atoms occupied
Hydroxylation of Benzene to Phenol
Hydroxylation of benzene was performed in acetonitrile with
H O as the oxidant. A blank experiment (Table 2, entry 1) was
2
2
conducted in which no phenol or dihydroxybenzenes were
detected. We also observed that the soluble PGN exhibited
higher benzene conversion and phenol distribution than PG8,
which was partly dissolved under the same conditions (Table 2,
entries 2 and 3). Thus PGN was used as a homogeneous catalyst
in this study. Some parameters such as the catalyst dosage,
temperature, mole ratio of H O to benzene, reaction time and
2 2
solvents were investigated systematically to obtain ideal benzene
conversion and high phenol distribution.
Table 1. Crystallographic data
Empirical formula
38 4 6
C H38FeN O
Formula weight
Space group
Crystal size (mm)
Crystal system
a (Å)
702.57
C2/c
0.10 × 0.20 × 0.20 mm
Monoclinic
16.827(3)
14.023(3)
28.417(6)
91.40(3)
a
Table 2. Blank experiment and effects of different catalysts
c
Entry
Catalyst
Benzene b
conversion
Product distributions (%)
b (Å)
d
e
c (Å)
Phenol
CAT
HQ
(
%)
β (º)
V (Å )
3
f
6704(2)
1
2
3
None
PG8
0
N.D.
N.D.
21.4
19.8
N.D.
25.7
25.0
Z
8
17.2
25.7
52.9
55.2
ꢀ
3
ρ (g cm
μ (mm
F(000)
)
1.392
PGN
ꢀ
1
)
0.504
a
6
0°C; benzene, 1.0 ml; catalyst, 0.14 mmol; acetonitrile, 20.0 ml;
reaction time, 3 h; n(H ):n(benzene) = 3.
2944
2 2
O
θ
max(°)
27.9
b
Moles of benzene converted based on the products/initial
moles of benzene.
No. of measured reflections
31753
No. of unique reflections and Rint
No. of observed reflections [I ≥ 2σ(I)]
7856 and 0.061
6064
c
Moles of phenol (CAT or HQ) produced/moles of all the
products detected by GC.
2
Goodness-of-fit on F
1.10
d
CAT, catechol.
R, wR2 [I ≥ 2σ(I)]
R, wR2 (all data)
0.058, wR2 = 0.132
0.078, wR2 = 0.144
e
HQ, hydroquinone.
N.D., not detected.
f
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X. Wang et al.
Effects of PGN Dosage
The influence of catalyst dosage on benzene conversion and
product distribution are illustrated in Table 3. The benzene con-
version increased first, which is initially attributed to more PGN
being exposed to react with H
diates to oxidize benzene; conversion then decreased as a result
of accelerated non-productive decomposition of H with incre-
2 2
O , forming more active interme-
2 2
O
mental PGN dosage. Accompanied by the increased benzene
conversion, catechol and hydroquinone were detected due to
the further oxidation of phenol, which could be concluded from
the observed lower phenol distribution when 0.1 g or more
2 2
PGN was used. Under the condition that more H O decomposed
on the surface of the catalyst when more than 0.1 g PGN was
added, we also found that the phenol distribution slightly
increased owing to the reduced amount of H O which could
2
2
be used to oxidize phenol to dihydrobenzenes. Although the
phenol distribution was 100% when 0.05 g PGN was added,
benzene conversion was low. The highest benzene conversion
was obtained when 0.1 g PGN was added. From the economic
point of view, 0.1 g catalyst was the appropriate dosage since
the phenol distribution was comparable with that obtained when
more PGN was used.
Figure 5. Effects of temperature on hydroxylation of benzene.
Benzene, 1.0 ml; PGN, 0.1 g; acetonitrile, 20.0 ml; reaction time, 3 h,
n(H O ):n(benzene) = 3.
2
2
Effects of Temperature
The effects of temperature on the hydroxylation of benzene are
shown in Fig. 5. With an increase in temperature, benzene
conversion increased first and then decreased. Although the
phenol distribution was 100% at 30 and 40°C, only a small
amount of benzene was converted because a longer induction
period under low temperature prevented the formation of
phenols. Higher temperature sped up the reaction, which re-
sulted in higher benzene conversion. Decreased phenol distribu-
tion was also observed as a result of the accelerated further
oxidation of phenol under higher temperature. The relative high
benzene conversion (16.6%) and phenol distribution (76.0%)
were obtained at 50°C.
2 2
Figure 6. Effects of mole ratio of H O to benzene on hydroxylation of
benzene. 50°C; benzene, 1.0 ml; PGN, 0.1 g; acetonitrile, 20.0 ml; reaction
time, 3 h.
Effects of Mole Ratio of H
2 2
O to Benzene
As illustrated in Fig. 6, benzene conversion and product distri-
bution were greatly influenced by the mole ratio of H to ben-
zene. The benzene conversion increased with added increments
of H , which was lower than 10% under a lower mole ratio of
to benzene. Dihydroxybenzenes were not detected until
the mole ratio of H to benzene reached 2. The phenol distri-
2 2
O
gradually because of the further oxidation of phenol with excess
H O . When the mole ratio of H O to benzene was 2, higher phe-
nol distribution (92.3%) and ideal benzene conversion (12.9%)
was observed.
2
2
2 2
2 2
O
H O
2 2
2 2
O
bution decreased and dihydroxybenzene distribution increased
Effects of Reaction Time
a
Table 4 showed that the higher benzene conversion and
lower phenol distribution were achieved over long reaction
Table 3. Effects of PGN dosage on hydroxylation of benzene
Entry PGN (g)
Benzene
Product distribution (%)
times. When the mole ratio of H O2 to benzene was 2, only
2
conversion
phenol was detected in the first 2 h, which was then oxidized
to dihydroxybenzenes after a longer time. As a result of fur-
ther oxidation, a decreased phenol distribution was obtained.
The effects of reaction time on the hydroxylation of benzene
to phenol implies that the dihydroxybenzenes were formed
only when phenol had accumulated to a certain extent. In
the first 3 h, 12.9% benzene was consumed at high reaction
rate, which then slowed with reduced reactant concentrations
as time elapsed.
Phenol
CAT
HQ
(%)
1
2
3
4
0.05
0.1
7.5
100
55.2
N.D.
19.8
18.2
17.6
N.D.
25.0
24.4
22.7
25.7
21.2
17.6
0.2
57.4
59.7
0.3
a
6
0°C; benzene, 1.0 ml; acetonitrile, 20.0 mL; reaction time, 3 h; n
2 2
(H O
):n(benzene) = 3.
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Appl. Organometal. Chem. (2014)

Iron complex catalyzed hydroxylation
a
[42,43]
Table 4. Effects of reaction time on hydroxylation of benzene
As reported in the literature,
formed in benzene hydroxylation, [Phenol]
linear with [Ethanol]. [Phenol] /[Phenol] was plotted as a function
if the hydroxyl radical is
0
/[Phenol] should be
Entry Time (h)
Benzene
Products distributions (%)
0
conversion
Phenol
CAT
HQ
of [Ethanol], which was almost linear as shown in Fig. 7; we thus
concluded that the hydroxyl radical was formed and acted as the
key active species for direct hydroxylation of benzene to phenol.
In the PGN-catalyzed hydroxylation of benzene, under the
optimum conditions (1.0 ml benzene, 0.1 g PGN, 50°C, 20.0 ml
acetonitrile, mole ratio of H O to benzene = 2), benzene conver-
(%)
1
2
3
4
5
1
2
3
4
5
6.8
100
100
92.3
72.3
57.2
N.D.
N.D.
2.4
N.D.
N.D.
5.3
10.2
12.9
13.3
14.9
10.1
19.6
17.6
23.2
2
2
sion was 10.2% and phenol distribution was 100%. Benzene
conversion was higher in the hydroxylation catalyzed by similar
a
50°C; benzene, 1.0 mL; PGN, 0.1 g; acetonitrile, 20.0 mL; n(H
2 2
O ):n
[
14]
(benzene) = 2.
iron polyazadentate complexes
in the presence of H O due
2 2
to the participation of the highly selective iron-oxo species. In
our study, benzene conversion was at a relative low level in
which the hydroxyl radical was proved to be the active species.
In the catalytic hydroxylation of benzene to phenol with the
non-selective and highly active hydroxyl radicals as the active
species, the higher conversion of the substrates was usually
accompanied by lower selectivity as the hydroxyl radical would
react with the hydroxylated product to produce further oxidation
products such as dihydroxybenzenes. Thus the benzene
conversion should be carefully controlled to obtain high phenol
selectivity.
Effects of Solvents
The hydroxylation of benzene to phenol was carried out in different
solvents and the results are shown in Table 5. The results showed
that the activity of the catalyst was greatly influenced by the sol-
vents. It appears that acetonitrile was the only effective solvent
among the studied solvents, although its influence on hydroxyl-
ation has not been unambiguously elucidated in this study. In the
catalytic hydroxylation system involving an iron-based catalyst
and H O , the hydroxyl radical is likely to be formed and act as
2
2
[
38–40]
the active intermediate, as reported in the literature.
bining the fact that acetone and ethanol are usually used as hy-
droxyl radical scavengers
Com-
Hydroxylation of Aromatic Compounds
[
41]
and the results that no phenol was
Furthermore, PGN was also able to catalyze the hydroxylation of a
series of aromatic compounds to corresponding phenols, which
revealed good substrate adaptability. As shown in Table 6,
substrates with activating ortho-/para-directing substituents like
toluene, phenol and fluorobenzene were converted to both
ortho- and para-phenols. The nature of the phenols of the
hydroxylation of different substrates by PGN suggested that the
reaction involved a non-selective species, which was consistent
with the conclusion that the hydroxyl radical was formed in the
hydroxylation from the previous experiments in this study.
It has been proved that the first step of the reaction be-
tween the aromatic compounds and hydroxyl radical involves
the electrophilic addition of the hydroxyl radical to the aro-
detected in acetone or ethanol, we speculate that the hydroxyl-
ation of benzene to phenol catalyzed by PGN proceeded through
a hydroxyl radical mechanism.
Effects of Hydroxyl Radical Scavenger
In order to verify our postulate that the hydroxyl radical was formed
in the hydroxylation of benzene to phenol in the presence of H O ,
2 2
the effects of addition of hydroxyl radical scavenger on the produc-
tion of phenol were investigated by adding ethanol to the reaction
system. Since the hydroxyl radical produced in the hydroxylation
will react with both benzene and ethanol, the relationship between
the concentration of phenol and that of ethanol can be expressed
by the following equation:
[
44,45]
matic compound to form cyclohexadienyl radical.
Under
the same conditions, the hydroxyl radical was prone to attack
the electron-rich benzene ring. Consequently, phenol and
toluene, which are more prone to react with the hydroxyl
½
Phenolꢁ0 ≈
k2½Ethanolꢁ
k1½Benzeneꢁ
1 þ
½
Phenolꢁ
where [Phenol] and [Phenol] represent phenol produced with-
0
out and with added ethanol; [Ethanol] and [Benzene] are initial
concentration of ethanol and benzene; and k and k are rate
1
2
constants of the reactions between benzene or ethanol and
the hydroxyl radical.
a
Table 5. Effects of solvents on hydroxylation of benzene
Entry
Solvent
Benzene
Product distribution (%)
conversion
Phenol
CAT
HQ
(%)
1
2
3
5
Acetonitrile
Acetone
10.2
—
100
N.D.
N.D.
N.D.
N.D.
N.D.
N.D.
N.D.
N.D.
N.D.
N.D.
N.D.
Ethanol
—
1,4-Dioxane
—
a
50°C; benzene, 1.0 ml; PGN, 0.1 g; solvents, 20.0 ml; reaction time,
2
h; n(H ):n(benzene) = 2.
2 2
O
Figure 7. Effects of ethanol concentration on formation of phenol.
Appl. Organometal. Chem. (2014)
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X. Wang et al.
a
Table 6. Hydroxylation of different substrates catalyzed by PGN
Entry
Substrate
Conversion (%)
Products distributions (%)
p-Cresol 55.8
1
2
3
4
5
Toluene
16.5
18.1
11.3
5.4
o-Cresol 44.2
Phenol
Catechol 45.9
Hydroquinone 54.1
Naphthalene
Fluobenzene
Benzene
1-Naphthol 100
o-Fluorophenol 42.6
p-Fluorophenol 57.4
10.2
Phenol 100
a
5
0°C; aromatic compounds, 11.3 mmol; PGN, 0.1 g; acetonitrile, 20.0 ml; reaction time, 2 h; n(H O ):n(aromatics) = 2.
2 2
radical, compared with naphthalene, fluorobenzene and benzene,
exhibited higher conversions.
low spin iron(II) center of PGN and the oxygen atom of the
2+
3+
naphtholate ligand. The low oxidation potential of the Fe /Fe
+
event (0.017 V vs. Fc /Fc) illustrated in Fig. 3 also indicated the easy
cleavage of the Fe–O bond via protonation due to the strong
electron-donating ability of the 1-nitroso-2-naphtholate
Proposed Mechanism and DFT Calculations
[
46]
According to the experiments above and the conclusion that
the hydroxyl radical was the active species in the direct
hydroxylation of the aromatic compounds catalyzed by PGN
with H O , we proposed a rational catalytic cycle with the par-
ligands. The penta-coordinated neutral complex IM2
was
formed by the configuration inversion with an energy barrier
ꢀ
1
of 17.75 kJ mol , which provided an available open site for
the coordination of H to the iron(II) center. The homolysis
2
2
2 2
O
ticipation of PGN involving production of the hydroxyl radical,
as illustrated in Fig. 8.
To further examine the deduced pathway for production of the
of the O–O bond of the H–O–O–H cluster attached to the iron
center in IM3 then caused the production of a hydroxyl radical
and IM4, followed by oxidation of the ferrous iron to the ferric
≠
ꢀ1
hydroxyl radical, DFT calculations were conducted. The sum of
in IM4 (ΔG = ꢀ15.08 kJ mol ), which also could be concluded
+
2+ 3+
the free energies of PGN (the anion), H
3
O and H
2
O
2
were taken
from the low oxidation potential of the Fe /Fe couple. Here,
the theoretical calculations in conjunction with the electro-
chemistry results demonstrated a reasonable pathway of
oxidizing PGN to IM4. On the basis of the above analysis, we
draw a conclusion that the proposed production pathway of
the hydroxyl radical was thermodynamically rational.
as zero energy. The optimized structure of PGN (the anion)
matched well the geometric data determined by X-ray diffrac-
tion, which supported our choice for the function and basis set
and thus lent confidence to the correctness of the computation.
Optimized structures of PGN (the anion) as well as other interme-
diates are shown in Figs S2–S8, and selected bond lengths and
angles are listed in Tables S1–S7.
The main results of the DFT study are also illustrated by the
free energy profile (shown in parentheses in Fig. 8). Since PGN
was a hexa-coordinated complex with an iron(II) center, there
The aromatics were attacked readily by the highly active hy-
droxyl radical to form the cyclohexadienyl radical, which could
then react with IM4 to produce phenol. Simultaneously, IM5 with
an iron(II) center was formed from IM4 via accepting one elec-
tron from the cyclohexadienyl radical inferred from the litera-
[
47,48]
was no open site for the attack of H
2
O
2
molecule to the iron(II)
ture.
This needs further verification because the reaction
center. The first step should be the protonation of PGN, which
caused Fe–O bond cleavage (ΔG < < 0 kJ mol ) between the
process and the calculations including the active radicals were
complicated. The nucleophilic intermediate IM5 was protonated
easily to produce IM6, in which the water molecule could be
released to form IM2 with an energy barrier of 10.36 kJ mol
to fulfill the catalytic cycle.
≠
ꢀ1
ꢀ
1
Conclusions
In this study, PGN was explored for catalytic potential. The mono-
nuclear iron complex PGN was coordinated by the 1-nitroso-2-
naphtholate ligands, which define a fac-N O donor set. Under
3
3
the conditions that 1.0 ml benzene, 0.1 g PGN, 50°C, 20.0 ml
acetonitrile as the solvent, the mole ratio of H O to benzene
2
2
was 2, benzene conversion was 10.2% and phenol distribution
was 100%. Under the same conditions, PGN was also able to
catalyze the hydroxylation of other aromatic compounds to
the corresponding phenols with high substrate conversion.
The hydroxyl radical mechanism was proved to dominate the
hydroxylation. A rational pathway for the formation of the
hydroxyl radical was proposed in which PGN was oxidized to
an iron(III) species accompanied by the release of a hydroxyl
radical justified by the theoretical results. Given the reactivity
and broad substrate scope demonstrated, we anticipate that
this study reveals a new way for the direct hydroxylation of
aromatic compounds. Based on the activity of PGN, we deduce
Figure 8. Proposed catalytic cycle (the naphthalene ring is omitted as
ꢀ
1
the ―C¼C― double bond). The Gibbs free energy profiles (kJ mol ) of
the intermediates are shown in parentheses.
wileyonlinelibrary.com/journal/aoc
Copyright © 2014 John Wiley & Sons, Ltd.
Appl. Organometal. Chem. (2014)

Iron complex catalyzed hydroxylation
[
23] D. T. Sawyer, C. Kang, A. Llobet, C. Redman, J. Am. Chem. Soc. 1993,
15, 5817.
that the 1-niroso-2-naphtholate ligand, which offers both nitrogen
and oxygen atoms as the coordination atoms, shows promise for
the design of efficient iron complexes for the hydroxylation of
aromatic compounds.
1
[
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1
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