Iron(iii) complexes of multidentate pyridinyl ligands: Synthesis, characterization and catalysis of the direct hydroxylation of benzene

Article abstract of DOI:10.1039/c4dt02032d

Three multidentate ligands, L₁-L₃, derived from bis(pyridin-2-ylmethyl)amine (L₁) were synthesized. Reaction of these ligands with FeCl₃·6H₂O in methanol led to the formation of the iron complexes Fe₁-Fe₃ (Fe₁: [FeL₁Cl₃]; Fe₂: [FeL₂Cl₃]; Fe₃: [FeL₃Cl₃]) in good yields. These complexes have been fully characterized. The structures of complexes Fe₁-Fe₃ have been determined using X-ray single crystal diffraction analysis. Electrochemical investigation revealed that complex Fe₃ partially converts to Fe₄ ([FeL₃Cl₂]PF₆) by the replacement of one of its three chlorides with its pendant triazolyl group in solution. Fe₄ was also synthesized by dechlorination using AgPF₆ as the Cl^- abstractor and its composition was further confirmed by both elemental analysis and X-ray single crystal diffraction analysis. All four complexes catalyze the direct hydroxylation of benzene to phenol with hydrogen peroxide as an oxidant in a mixed medium of water and acetonitrile. The reactivity of the complexes correlates well with their reduction potentials. The more negative the potential, the more reactive (high conversion rate) the catalysts. These complexes catalyze not only the oxidation of benzene, but also the further oxidation of the product, phenol. In the oxidation, a radical mechanism is certainly involved but an alternative pathway may also exist. This journal is

Full text of DOI:10.1039/c4dt02032d

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Graphical Abstract
Iron (Ⅲ) complexes of multidentate pyridinyl ligand: Synthesis,
characterization and catalysis on the direct hydroxylation of benzene
Beibei Xua, Wei Zhongb,c, Zhenhong Weia, Hailong Wangb, Jian Liua, Li Wua, Yonggang Fengb and
Xiaoming Liua,b
*
The correlation between the electrochemistry of multidentate pyridinyl iron complexes Fe1-Fe3
and their catalytic performance on the direct hydroxylation of benzene into phenol suggests that
increasing the electron-donating capability of the ligands coordinating to the metal centre is
beneficial to improving the catalysis.

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DOI: 10.1039/C4DT02032D
Cite this: DOI: 10.1039/c0xx00000x
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ARTICLE TYPE
Iron (Ⅲ) complexes of multidentate pyridinyl ligand: Synthesis,
characterization and catalysis on the direct hydroxylation of benzene
Beibei Xu,a Wei Zhong,b,c Zhenhong Wei,a Hailong Wang,b Jian Liu,a Li Wu,a Yonggang Fengb and
Xiaoming Liua,b
*
5
Received (in XXX, XXX) Xth XXXXXXXXX 20XX, Accepted Xth XXXXXXXXX 20XX
DOI: 10.1039/b000000x
Three multidentate ligands L1ꢀL3 derived from bis(pyridinꢀ2ꢀylmethyl)amine (L1) were synthesized.
Reaction of these ligands with FeCl3·6H2O in methanol led to the formation of iron complexes Fe1ꢀFe3
(Fe1: [FeL1Cl3]; Fe2: [FeL2Cl3]; Fe3: [FeL3Cl3]) in good yields, respectively. These complexes have been
10 fully characterized. The structures of complexes Fe1ꢀFe3 have been determined using Xꢀray single crystal
diffraction analysis. Electrochemical investigation revealed that complex Fe3 forms partially Fe4
([FeL3Cl2]PF6) by the replacement of one of its three chlorides with its pendant triazolyl group in
solution. Fe4 was also synthesized by dechlorination using AgPF6 as the Cl− abstractor and its
composition was further confirmed by both elemental analysis and Xꢀray single crystal diffraction
15 analysis. All the four complexes catalyze direct hydroxylation of benzene to phenol with hydrogen
peroxide as an oxidant in a mixed medium of water and acetonitrile. The reactivity of the complexes
correlates well to their reduction potentials. The more negative the potential, the more reactive (high
conversion rate) the catalysts. These complexes catalyze not only the oxidation of benzene, but also the
further oxidation of the product phenol. In the oxidation, radical mechanism is certainly involved but
20 alternative pathway may also exist.
cumene process. It is understandable that direct hydroxylation of
benzene, in the viewpoint of atomic economy, is the most
efficient way for the production of phenol. In this process, one of
the C–H bonds is activated and then hydroxylated. However, the
50 high bond energy (460 kJ mol−1) makes it impossible without the
help of a catalyst. Both heterogeneous and homogeneous
catalysts have been investigated to achieve this direct
hydroxylation. The heterogeneous catalysts such as TSꢀ1,7ꢀ9
1. Introduction
Iron is one of the most abundant metals on earth and has a
number of advantages over other transition metals as a catalyst,
for example, readily availability, low cost and environmentally
25 benign probably due to its good abundance. In nature, iron is an
indispensable element in many living organisms as the metal
centers of many enzymes. These enzymes play important
biological roles by catalyzing many essential reactions in living
organisms. For example, cytochrome P450 oxidizes selectively
30 the long aliphatic side chain of cholesterol in the biosynthesis of
female hormone progesterone.1 Methane monooxygenase, an
enzyme containing a diiron core, converts methane into methanol
as part of the metabolism of methanotrophs.2 Hydrogenases are
also ironꢀcontaining enzymes and catalyze both evolution and
35 oxidation of hydrogen at remarkable efficiency and rate.3 In
principle, ironꢀbased systems ought to have great potentials as
efficient, affordable and safe catalysts in industry.
SBAꢀ1510, 11 and other molecular sieves,12ꢀ14 hydrotalcites,15 and
17
55 MWCNTs16,
loaded with metal ions have shown high
efficiency. But leaching of active species hinders their further
application at an industrial scale. Furthermore, their catalytic
performances could highly depend on their preparations.
Homogeneous catalysts, however, possess
a number of
60 advantages, for example, operational under mild conditions,
tunable in catalytic performance through ligand design.18ꢀ21 By
appropriately selecting a metal ion, catalytic performance could
be further improved.20, 21 But to date, only limited reports have
been devoted to developing homogeneous catalysts.18ꢀ24 The
65 advantages of the homogeneous systems need to be further
exploited.
As aforementioned, ironꢀbased catalytic systems have great
potentials as catalysts. Additionally, iron is one of the essential
metals in our own bodies. Therefore, it ought to possess the least
70 detrimental effect towards our environment as well as living
organisms. Indeed, among the reported homogeneous catalysts,
iron complexes showed promising efficiency in catalyzing the
Phenol is widely used in industry as an important chemicals for
the production of phenol resins, fibers, dyestuffs and medicines.4
40 Currently, phenol is mainly manufactured by a threeꢀstep cumene
5, 6
process.
Its major problems are low yield (~5% based on the
amount of benzene initially used), high energy consumption and
production of an equal amount of acetone as its byꢀproduct.
Therefore, it is extremely desirable to find alternative ways for its
45 production, which can ideally overcome the disadvantages of the
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24
direct hydroxylation of benzene.23,
In nature, it is not
atomic coordinates in neutral state are derived from
corresponding crystal structures. The initial geometries of the
reduced states were approximately established using the crystal
uncommon that iron is employed as a catalytic centre to activate
C–H bond. For example, cytochromes Pꢀ450, nonꢀheme iron
monoꢀ and dioxygenases perform hydroxylation of aromatic 60 structure parameters of their parent complexes. Vibrational
26
5
compounds via C–H bond activation.25,
Both the natural
frequencies were calculated based on the optimized geometries
and the absence of negative frequencies confirmed that the
structures were local minimum−energy structures.
systems and synthetic catalysts suggest that iron complexes could
be effective catalysts for the conversion of benzene to phenol if
the ligands are appropriately designed and both the electronic and
The free energies of solvation (∆G) of Fe3 and Fe4 in different
steric properties of the complexes are satisfactorily tuned. Under 65 states were evaluated by the selfꢀconsistent reaction field (SCRF)
10 these inspirations, we are prompted to develop ironꢀbased
homogeneous catalysts for benzene hydroxylation and explore
the correlations between the catalytic efficiency and both
electronic and structural features of the ironꢀbased catalysts.
approach based on the integral equation formalism of the
polarized continuum model (IEFPCM) level of theory using the
keyword SCFVAC. The choice of solvent was acetonitrile in
which the electrochemistry of the complexes was performed. To
Herein, we report three multidentate pyridinyl ligands L1ꢀL3 and 70 calculate the one−electron reduction potential, the free energy
15 their Fe(Ⅲ) complexes Fe1ꢀFe4. Complex Fe4 was derived from
Fe3 by the substitution of one of the bound chlorides by the
pendant triazolyl group. The catalysis of these complexes towards
the oxidation of benzene by H2O2 to phenol was investigated.
changes in the reduction processes were calculated by using
Born−Haber cycle.36 The calculated potentials (E1/2
) are
calibrated against Fc+ / Fc whose absolute oxidation potential is
−4.81 V calculated using the same approach as that for Fe3 and
Their catalysis and relevance to both electrochemistry and 75 Fe4. All calculations were carried out using Gaussian 03
20 structures were also discussed. Among the four complexes,
complex Fe2 exhibited the best yield in terms of the production of
phenol whereas Fe1 is the most active one which is of the most
negative reduction potential.
program.37
2.2 Synthesis
2.2.1 Preparation of bis(pyridin-2-ylmethyl)amine (L1). 2ꢀ
Picolinaldehyde (2.5 mL, 26 mmol) was added to a solution of 2ꢀ
80 pyridylmethylamine (2.7 mL, 26 mmol) in MeOH (15 mL),
which was stirred for 6 h at room temperature before sodium
borohydride (2.4 g, 63.8 mmol) was added to the mixture at iceꢀ
temperature. The reaction mixture was further stirred at room
temperature overnight. After removal of the solvents under
85 reduced pressure, 30 mL of distilled water was added, and then
extracted successively with dichloromethane (30 mL × 3). All the
extracts were combined before being dried with MgSO4.
Removal of the solvents under reduced pressure gave a crude
product which was further purified using flash chromatography
2. Experimental
25 2.1 General procedures and computations
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. Ligands
L1ꢀL327ꢀ29 and their Fe(Ⅲ) complexes Fe1ꢀFe330 were synthesized
30 following literature procedures with modifications. Elemental
analyses were performed on
a CHN elemental analyzer
(Elementar Vario MICRO). FTIR spectra were recorded on
Agilent 640 using a CaF2ꢀcell with a spacer of 0.1 mm. UVꢀvis
spectra were recorded in the range of 200ꢀ800 nm on a
35 spectrophotometer (VARIAN 50 Conc) in acetonitrile.
Crystallographic data of complexes Fe1ꢀFe4 were collected on a
Bruker SMART CCD diffractometer with graphiteꢀ
monochromated Mo–Kα radiation (λ = 0.71073 Å). The crystal
structures were solved using direct methods in SHELXS and
40 refined by fullꢀmatrix leastꢀsquares routines, based on F2, using
the SHELXL package.31 The crystallographic data of complexes
Fe1ꢀFe4 were deposited in the Cambridge Crystallographic Data
Center (CCDC 997390–997393).
90 on silica gel column (eluent: ethyl acetate
/ ethanol /
triethylamine = 5 : 1 : 0.1) to produce yellow oily liquid (3.32 g,
54%). 1H NMR (400 MHz, CDCl3) δ 8.47 (d, J = 4.7 Hz, 2H, py),
7.55 (t, J = 7.6 Hz, 2H, py), 7.28 (s, 2H, py), 7.14ꢀ6.98 (m, 2H,
py), 3.90 (s, 4H, CH2), 2.86 (s, 1H, NH).
95 2.2.2 Preparation of N,N-bis(pyridin-2-ylmethyl)prop-2-yn-1-
amine (L2). 3ꢀBromopropyne (0.42 mL, 5.3 mmol) was added to
the mixture of L1 (1.05 g, 5.3 mmol) and K2CO3 (0.73 g, 5.3
mmol) in THF (20 mL) in a round bottom flask (100 mL). The
reaction was refluxed for 24 h before removal of the solvents
100 under reduced pressure to give a crude product. The product was
further purified using flash chromatography on silica gel column
(eluent: ethyl acetate / ethanol / triethylamine = 15 : 1 : 0.1) to
produce a yellow oily liquid (0.73 g, 58%). IR (DCM, ν/cm−1):
Electrochemistry was performed in
45 electrode system in which a vitreous carbon disk (
a
gasꢀtighten threeꢀ
= 1 mm) was
Ф
used as a working electrode, a carbon strip as counter electrode,
and Ag / AgCl (inner reference solution : 0.45 mol L−1
[NBut4]BF4 + 0.05 mol L−1 [NBut4]Cl in dichloromethane)
against which the potential of ferrocenium / ferrocene couple is
50 0.55 V in 0.1 mol L−1 [NBut4]BF4 in acetonitrile as described else
where.32, 33 Ferrocene was added as an internal standard, and all
potentials are quoted against ferrocenium / ferrocene couple (Fc+ /
Fc).
1
3297, 2839, 2106 and 1591. H NMR (400 MHz, CDCl3) δ 8.58
105 (d, J = 4.7 Hz, 2H, py), 7.68 (td, J = 7.6, 1.6 Hz, 2H, py), 7.54 (d,
J = 7.8 Hz, 2H, py), 7.21 – 7.16 (m, 2H, py), 3.94 (s, 4H, CH2),
3.45 (d, J = 2.3 Hz, 2H, CH2), 2.32 (t, J = 2.3 Hz, 1H, CH).
2.2.3 Preparation of Azidomethylbenzene (1). A solution of
bromomethylbenzene (1.71 g, 10 mmol) and sodium azide (1.30
110 g, 20 mmol) in DMF (5 mL) was stirred at room temperature for
24 h. The reaction was diluted with water (5 mL) and extracted
with diethylether (4 × 10 mL). The organic fractions were
combined and washed successively with water (4 × 10 mL), dried
with anhydrous magnesium sulfate (MgSO4 ) and concentrated in
In theoretic calculations, Fe3 and Fe4 in neutral and reduced
55 states were fully optimized in gas phase without any symmetry
35
constraints at the (U)BP86 / TZVP level of theory.34,
The
2
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vacuo to produce azidomethylbenzene (1) as a yellow liquid (1.14
3.58, N%, 12.63.
g, 85%). The compound was used in next reaction without further
purification. IR (DCM, ν/cm−1): 3032, 2096, 1496 and 1455.
2.2.4 Preparation of 1-(1-benzyl-1H-1,2,3-triazol-4-yl)-N,N-
60 2.3 Catalytic assessment
A typical procedure is as follows: benzene (0.9 mL, 10 mmol),
acetonitrile (3.8 mL) and catalytic amount of the iron ( Ⅲ)
complex (0.2 mol%) were placed into a reaction vessel (10 mL)
equipped with cooling condenser and placed in an oilꢀbath. The
65 reaction was heated at appropriate temperature for period of a
time. When the reaction reached the specified temperature,
aqueous H2O2 (30 wt%, 1.2 mL, 12 mmol) was slowly and
carefully added in oneꢀgo. When the reaction was stopped, the
volume was calibrated to 10 mL with CH3CN. To the calibrated
70 reaction solution was added MgSO4 (3 g) to remove the water in
the reaction before being analyzed by gas chromatography (GC
6890) with a packed column of Restek capillary SEꢀ54 and using
toluene as an internal standard. The temperature of the GC
column was set at 60 ℃ for 1 min and then was programmed to
75 rise to 160 ℃ at the rate of 10 ℃ min−1. Under the employed
conditions, the retention time for benzene, toluene, benzoquinone
and phenol were 3.3 ± 0.2 min, 4.4 ± 0.2min, 6.6 ± 0.2 min, and
7.4 ± 0.2 min, respectively.
In establishing the calibration curves for quantitative analysis
80 of both benzene and phenol, 5 standard samples containing
quantitative benzene, phenol and toluene, respectively, were
prepared in appropriate ratios, heated and worked up in the same
manner as that for the above reaction. The samples were analyzed
by GC with the same GC parameters. Plotting the ratios of
85 area−readings of benzene and phenol over that of toluene,
respectively, against their quantities in the standard samples
produced two linear equations, yb = 0.0054 xb - 0.0183 ( R =
0.9986) and yp = 0.0022 xp - 0.0631 ( R = 0.9982) for benzene
5
bis(pyridin-2-ylmethyl)methanamine (L3).
A solution of
compound 1 (283 mg, 2.2 mmol) in THF (2 mL) was added to the
solution of ligand L2 (474 mg, 2 mmol) in THF (3 mL), followed
by the addition of a mixture of CuI (38 mg, 0.2 mmol) and Et3N
(2.5 mL, 2.2 mmol) in THF (2 mL). After being stirred for 18 h at
10 40 ℃, the yellow reaction solution turned to a black suspension.
Removal of the solvents under reduced pressure gave a crude
product which was further purified using flash chromatography
on silica gel column (eluent: ethyl acetate
/ ethanol /
triethylamine = 15 : 1 : 0.1 ) to produce a yellow oily liquid (303
15 mg, 41%). 1H NMR (400 MHz, CDCl3) δ 8.44 (d, J = 4.4 Hz, 2H,
py), 7.55 (tt, J = 14.1, 7.1 Hz, 2H, py), 7.51 – 7.39 (m, 3H, ph),
7.33 – 7.21 (m, 2H, py, 1H, CH), 7.22 – 7.13 (m, 2H, py), 7.06
(dd, J = 6.7, 5.5 Hz, 2H, ph), 5.45 (s, 2H, CH2), 3.79 (d, J = 8.2
Hz, 2H, CH2), 3.76 (s, 4H, CH2). 13C NMR (101 MHz, CDCl3) δ
20 159.09 , 148.96, 144.66, 136.42, 134.78, 128.99, 128.56, 127.87,
123.22, 122.95, 121.98, 77.44, 77.12, 76.80, 59.54, 53.96, 48.59.
2.2.5 Preparation of complexes [FeL1Cl3](Fe1), [FeL2Cl3](Fe2)
and [FeL3Cl3](Fe3). A solution of FeCl3·6H2O (542 mg, 2 mmol)
in methanol (2 mL) was mixed with a methanolic solution (3 mL)
25 of an equivalent of L1 (398 mg, 2 mmol). The reaction was stirred
for 3 h to produce a darkꢀyellow precipitate Fe1 (625 mg, 87%),
which was collected by filtration, washed successively with
methanol, dichloromethane and tetrahydrofuran, and dried in
vacuo. Crystals suitable for Xꢀray single crystal diffraction
30 analysis were obtained from its solution in a mixture DMF /
diethyl ether at room temperature. Elemental analysis for Fe1
(C12H13Cl3FeN3, FW= 361.46) : calc., C%, 39.87, H%, 3.63, N%,
11.63; found, C%, 40.06, H%, 3.60, N%, 11.22.
Fe2 (637 mg, 80%) and Fe3 (763 mg, 72%) were analogously
35 synthesized using the same procedure as described for the
preparation of Fe1. Crystals of complex Fe2 suitable for Xꢀray
analysis single crystal diffraction were obtained from its solution
in a mixture of acetonitrile / diethyl ether in few days at room
temperature. Elemental analysis for Fe2 (C15H15Cl3FeN3, FW =
40 399.50) : calc., C%, 45.10, H%, 3.78, N%, 10.52; found, C%,
45.01, H%, 3.81, N%, 10.30. Crystals of complex Fe3 suitable for
Xꢀray single crystal diffraction analysis were obtained from its
solution in a mixture of dichloromethane/diethyl ether in few
days at room temperature. Elemental analysis for Fe3
45 (C22H22Cl3FeN6, FW = 532.65): calc., C%, 49.61, H%, 4.16, N%,
15.78; found, C%, 50.09, H%, 4.37, N%, 15.79.
and phenol, respectively, where
y is designated as the
90 area−reading, x as the quantity of the substances, and b stands for
benzene and p for phenol. The two calibrating equations were
used to quantitative analysis of the two substances in all samples.
To calibrate any uncertainty resulting from batches, a synthesized
sample was always analyzed along with reaction samples in each
95 batch. Reaction yield / selectivity and conversion rate are
calculated as follows: phenol (mmol)
(mmol)×100% and benzene−reacted (mmol) / benzene initially
used (mmol) ×100%, respectively.
/ benzene−reacted
3. Results and discussion
100 3.1 Synthesis and characterization
2.2.6 Preparation of complex [FeL3Cl2]PF6 (Fe4). A solution of
AgPF6 (72 mg, 0.28 mmol) in acetonitrile (4 mL) was added to a
solution of complex Fe3 (151 mg, 0.28 mmol) in acetonitrile (2
50 mL). The solution was stirred for 6 h. An inorganic solid (AgCl)
was filtered off and the filtrate was concentrated in vacuo to give
a yellow solid (133 mg, 74%), which was washed, in portions,
with a small amount of cold petroleum ether and diethyl ether,
respectively. Crystals suitable for Xꢀray single crystal diffraction
55 analysis were obtained from its solution in a mixture of
dichloromethane/methanol in few days at room temperature.
Elemental analysis for Fe4 (C22H22Cl2F6FeN6P, FW = 642.18):
calc., C%, 41.15, H%, 3.45, N%, 13.09; found, C%, 41.09, H%,
Scheme 1 Synthesis of ligands L1ꢀL3. Reaction conditions: ⅰ,
MeOH , room temperature; ⅱ, NaBH4; ⅲ, K2CO3, THF, room
temperature; ⅳ, CuI, Et3N, THF, 40 ℃.
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The three ligands L1ꢀL3 were prepared following the literature
procedures,27ꢀ29 Scheme 1. L1 was prepared by first forming a
Schiff base and then reduction by NaBH4. Since the reduction
involving hydrogen gas evolution, the addition of NaBH4 was
carried out in portions with care and the reaction was kept at iceꢀ
temperature. Reaction of the secondary amine (L1) with 3ꢀ
bromopropyne led to ligand L2. Ligand L3 was prepared by
reacting ligand L2 with azidomethylbenzene via “click reaction”.
Complexes Fe1ꢀFe3 were synthesized from the reaction of
ligands (L1-L3) with FeCl3·6H2O, respectively, in methanol. All
the three complexes adopt octahedral coordination geometry as
revealed by their crystal structures (Fig. 1). The structural
similarity is also reflected in their UVꢀvis spectrum (Fig, S1).
5
10
15 When the secondary amine of ligand L1 is converted into tertiary
amine (L2 and L3), the solubility of their corresponding iron (Ⅲ)
complexes differs drastically. Fe1 is soluble in DMF but hardly
soluble in other common solvents. But Fe2 and Fe3 have much
improved solubility, which are soluble in most organic solvents,
20 for example, acetonitrile, ethanol and dichloromethane. The
improvement ought to be attributed to the further introducing of
organic moieties into the ligands. Complex Fe4 was prepared
from complex Fe3 in good yield by using one equivalent of
AgPF6, Scheme 2.
c
d
Fig. 1 Crystal structures of complexes Fe1 (a), Fe2 (b), Fe3 (c) and
Fe4 (d).
35
Details of the crystallographic data of complexes Fe1ꢀFe4 are
summarized in Table 1. Selective bonding parameters for
complexes Fe1ꢀFe4 are tabulated in Table 2. The structural views
of complexes Fe1-Fe4 are shown in Fig. 1. As shown in Fig. 1, the
40 complexes Fe1-Fe3 are mononuclear and adopt an octahedral
geometry with the same donorꢀset, “N3ꢀCl3”, and the complex Fe4
with “N4ꢀCl2”. Due to the πꢀaccepting nature of the pyridinyl
ring, the distances of Fe(1)ꢀN(2) and Fe(1)ꢀN(3) bonds assigned
to FeꢀN(Py) in complexes Fe1-Fe4 are slightly shorter than the
45 Fe(1)ꢀN(1).
25
3.3 Electrochemistry
In the direct hydroxylation of benzene catalyzed by a transition
metal complex, the step of binding of the oxidant H2O2 to the
metal center is essential.38 Therefore, the electron density on the
50 metal center ought to have influence on its catalysis. And the
electron density is certainly correlated to the capability of its
accepting electron. Thus, it is worthy of examining the
electrochemistry of these complexes. In acetonitrile, all the
complexes exhibited a quasiꢀreversible reduction process between
55 -0.5 and -0.3 V, Fig. 2. But further looking into the
electrochemistry of complex Fe3 reveals that its reduction is
composed of at least two processes. To find out what happened,
its electrochemistry was examined at various scanning rates and
temperatures, Figs. 3 and 4. The results shown in Figs. 3 and 4
60 indicate that both increasing the scanning rate and lowering the
temperature could suppress the first reduction, which suggests a
chemical reaction is involved. As revealed by its crystal structure
(Fig. 1), Fe3 possesses a free triazolyl group which is both a σꢀ
donor and πꢀacceptor. If one of the three chlorides bound in Fe3
65 could dissociate, the pendant triazolyl group would readily move
in to replace the dissociated chloride. Since chloride is a πꢀdonor,
its substitution by the triazolyl group will certainly shift
positively the reduction potential of the product. The
irreversibility of the second reduction and the relation of the
70 reduction and the oxidation potentials allow us proposing the
mechanism of electron transfer and the coupled chemical
reaction, Scheme 3. To confirm the proposed mechanism, one
chloride was abstracted from Fe3 using one molar equivalent of
AgPF6 to give a yellow solid. Micro analysis results indicate that
Scheme 2 Synthesis of iron complexes Fe1ꢀFe4.
3.2 Structural analysis
30
a
b
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its composition is in agreement with that defined by [FeL3Cl2]PF6
(Fe4) whose absolute structure was also determined
crystallographically (Fig. 1). By adding the complex into the
solution of Fe3, it was found out that the first reduction was
enhanced as suggested by the pulse differential voltammograms,
Fig. 5. To further rationalize the mechanism depicted in Scheme
3, DFT calculations were performed. The results show, indeed,
that dissociating a chloride from Fe3 followed by the binding of
the pendant triazolyl group to form Fe4 is thermodynamically
5
10 favored. But the tendency of “kicking” out a chloride from its
−
reduced form Fe3 is much stronger as suggested by the large
Gibbs free energy of the process. This is sensible since upon
reduction, the electron density on the metal centre increases
further and cleaving a chloride will offꢀset this increase in
15 electron density. The theoretical results alongside with the
experimental observations validate firmly the mechanism shown
in Scheme 3.
30 Fig. 4 Cyclic voltammograms of complex Fe3 (2.91 mmol L−1) in
acetonitrile at various temperatures: room temperature, 10, 0, -
10, -20, -30 and -40 ℃.
Fig. 5 Differential pulse voltammograms of complex Fe3 (2.86
35 mmol L−1) and Fe3 (2.86 mmol L−1) + Fe4 (1.29 mmol L−1) in
acetonitrile (v = 0.1 V s−1, 298 K).
20 Fig. 2 Cyclic voltammograms of complexes Fe1 (2.94 mmol L−1),
Fe2 (2.93 mmol L−1) and Fe3 (2.91 mmol L−1) in acetonitrile (v =
0.1 V s −1, 298 K).
Scheme 3 Mechanism of electron transfer and the coupled
40 chemical reaction. Please note that both reduction peak potentials
were quoted for consistence.
3.4 Catalytic hydroxylation of benzene to phenol
The catalytic activity of the complexes was assessed using
45 benzene oxidation reaction by H2O2 in acetonitrile. Reaction
conditions for the catalysis were optimized using Fe2 as a
catalyst. Full details of the optimization are given in supporting
information. Throughout the work, reaction temperature and time
are set at 70 ℃ and 2h, respectively, and the quantities of other
50 components are as follows, the catalyst (equivalent of iron: 0.02
mmol), benzene (0.9 mL, 10 mmol), H2O2 (1.2 mL, 12 mmol)
25 Fig. 3 Cyclic voltammograms of complex Fe3 (2.91 mmol L−1) in
acetonitrile at various scanning rates: 0.1, 0.5, 1.0, 2.0, 3.0, 4.0,
5.0 and 6.0 V s−1.
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and solvent acetonitrile (3.8 mL).
Selecting an appropriate quantity of the oxidant is a dilemma
electrochemistry of the catalysts, particularly the reduction
processes, is of importance. It is known that the higher the
as decreasing the oxidant would improve the selectivity but the 45 election density, the more negative the reduction potentials of the
conversion rate would drop as shown in Table 3. The root cause
of this is that the product phenol undergoes further catalytic
oxidation. Varying the amount of the catalyst does not
significantly alter the conversion rate of benzene, Fig. 6. But for
catalysts. As shown in Table 3, the conversion rate of benzene
correlates well with the reduction potentials of complexes Fe1ꢀ
Fe4. With a slight variation in ligating donor set in complex Fe4,
these complexes adopt essentially a similar geometry and
5
the reaction yield / selectivity, 0.2%ꢀ0.3% of the examined 50 coordinating environment, thus the electron density of the metal
catalyst gave relatively good results. Thus, throughout the
10 investigation, 0.2% was used for all the complexes.
centre is the key factor determining the catalytic behaviors of
these complexes. The correlation suggests that increasing the
electronꢀdonating capability of the ligands coordinating to the
metal centre is beneficial to improving the catalysis. For
55 comparison, the catalytic performance of FeCl3 was also
examined as shown in Table 3. It is inferior to all the complexes
in both conversion rate and yield / selectivity. Although its
reduction potential in acetonitrile is more negative than that of
Fe4, its catalysis is poorer. This is probably due to the
60 polynuclear nature and / or nonꢀoctahedral geometry,40 which are
different from the mononuclear complexes which are of
octahedral coordination.
It is believed that iron (Ⅱ) catalysts form Fe (Ⅱ)ꢀOOH species
and involves radical mechanism when H2O2 is used as an
65 oxidant.41 When PhIO is used as an oxidant, high valent ironꢀoxo
species Fe (Ⅳ=O) is involved and alternative mechanism is
adopted. To shed some light on possible mechanism in the
oxidation of benzene catalyzed by Fe1ꢀFe4, the simple radical
quencher (ethanol) was used in the catalysis, Table 3. Indeed, the
70 presence of ethanol decreases significantly the conversion rate
and yield / selectivity. But further addition of ethanol by another
1 mL changed hardly the catalytic performance. This observation
may suggest that another catalytic mechanism parallel to the
radical one may also exists, for example, involving high valent
75 ironꢀoxo species.
Fig. 6 Variations of the conversion rate and yield / selectivity
with the quantity of the catalyst (Fe2) used. (nc: mole of Fe2, nb:
15 mole of benzene). Reaction conditions: H2O2 (1.2 mL, 12 mmol),
benzene (0.9 mL, 10 mmol), and acetonitrile (3.8 mL), 70 ℃, 2h.
Under the optimized conditions, the catalytic activities of the
three complexes were examined. For comparison, complex Fe4
20 generated from Fe3 by abstracting one of the three chlorides was
also investigated. All the results are tabulated in Table 3. It is
found that Fe2 shows the best catalytic performance. Although all
the systems do not show excellent performance in both selectivity
and reaction yield, complex Fe2 exhibits the best reaction yield
25 and selectivity. It must be pointed out that no significant variation
in performance was observed for all the complexes. This is
probably due to that all the complexes (Fe1ꢀFe3) are highly
similar in both structure and coordinating atmosphere. As shown
in Table 3, high conversion rate does not lead to high reaction
30 yield. The poor selectivity of these catalytic systems indicates
that overꢀoxidation of phenol exists severely. This suggests that
the reported complexes catalyses not only the oxidation of
benzene, but also the further oxidation of the product phenol. In
fact, GCꢀMS results showed the formation of benzoquinone and
35 catechol, which then could be further degraded by iron species in
the catalytic process.39 This may explain that all the iron
complexes do not exhibit good selectivity for the production of
phenol.
4. Conclusions
In summary, we have reported the synthesis, characterization and
catalysis on the oxidation of benzene into phenol of four Fe (Ⅲ)
complexes derived from bis(pyridinꢀ2ꢀylmethyl)amine (L1) and
80 its derivatives (L2 and L3). Structural analysis indicates that all
the complexes adopt an octahedral geometry. These complexes
catalyze the oxidation of benzene to phenol at moderate
temperature. They catalyze also the further oxidation of phenol
using H2O2 as an oxidant which leads to the poor selectivity of
85 the oxidation. The catalysis involves certainly radical mechanism
without ruling out alternative mechanism. The catalytic activity
of the four complexes correlates positively to the electron density
of the metal centre. This correlation indicates that in the design of
iron (Ⅲ)−based catalysts, increasing the electronꢀdonating
90 capability of a ligand to coordinate to Fe(Ⅲ) is certainly
beneficial. Although the conversion rate are comparable to the
reported ones,23, 42, 43 the selectivity is poor which is caused by
over oxidation of the product. Therefore, how to avoid over
oxidation becomes one of the major issues in designing Fe (Ⅲ)–
95 based catalysts for the direct oxidation of benzene to phenol.
As mentioned earlier, in the catalysis the binding of the oxidant
40 H2O2 with the metal centre of the catalyst is one of the essential
steps. Thus the electron density of metal centre would correlate to
the catalysis. To probe the electron density, examining the
Acknowledgments
6
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Page 8 of 9
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DOI: 10.1039/C4DT02032D
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We thank the National Natural Science Foundation of China
(Grant Nos. 21171073, 21301071), the Natural Science
Foundation of Zhejiang Province (LQ13B010002) and the
Government of Zhejiang Province (Qianjiang professorship for
XL) for supporting this work.
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Table 1 Crystallographic details for complexes Fe1ꢀFe4.
Fe1
Fe2
Fe3
C22H22Cl3FeN6·0.5CH2Cl2
575.12
Fe4
Empirical formula
Fw
Crystal system
space group
C12H13Cl3FeN3
C15H15Cl3FeN3
C22H22Cl2F6FeN6P
361.45
Orthorhombic
Pna2(1)
399.50
Monoclinic
P2(1)/c
642.18
Monoclinic
P2(1)/n
Orthorhombic
Fdd2
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DOI: 10.1039/C4DT02032D
a/Å
b/Å
c/Å
α (°)
β (°)
15.6262 (18)
8.4836 (10)
22.772 (3)
90
13.725 (2)
14.961 (2)
8.2443 (12)
90
92.129(2)
90
1691.7(4)
4
1.569
30.029 (4)
33.845 (5)
9.7572 (14)
90
90
90
9917 (2)
16
1.541
8.9480(8)
8.8509(8)
32.824(3)
90
96.0490(10)
90
2585.1 (4)
4
1.650
90
90
γ (°)
V, Å3
3018.8 (6)
8
1.591
Z
Dcalc, Mg/m3
F(000)
1464
812
4704
1300
Reflections collected
Reflections independent(Rint)
Goodnessꢀofꢀfit on F2
R1, wR2[I > 2σ(I)]
R1, wR2 (all data)
17373
5879, 0.0179
1.048
0.0212, 0.0553
0.0234, 0.0563
15797
4245, 0.0166
1.039
0.0233, 0.0624
0.0271, 0.0650
19600
4132, 0.0337
1.089
0.0275, 0.0712
0.0285, 0.0776
23978
6400, 0.0437
1.023
0.0544, 0.1251
0.0940, 0.1437
Table 2 Contrast of bond lengths (Å) and angles (°) for complexes Fe1ꢀFe4.
lengths (Å)
Fe(1)ꢀN(1)
Fe(1)ꢀN(2)
Fe(1)ꢀN(3)
Fe(1)ꢀN(4)
Fe(1)ꢀCl(1)
Fe(1)ꢀCl(2)
Fe(1)ꢀCl(3)
angles (°)
Fe1
Fe2
Fe3
Fe4
2.210(2)
2.197(2)
2.187 (2)
2.304(1)
2.186(1)
2.202(1)
2.257(2)
2.191(3)
2.167(2)
2.261(3)
2.158(3)
2.121(3)
2.143(3)
2.244(1)
2.262(1)
2.279(7)
2.289(7)
2.318(7)
Fe1
2.261(4)
2.307 (5)
2.314 (5)
Fe2
2.295(8)
2.299 (8)
2.286 (9)
Fe3
Fe4
N(3)ꢀFe(1)ꢀN(2)
N(3)ꢀFe(1)ꢀN(1)
N(2)ꢀFe(1)ꢀN(1)
N(3)ꢀFe(1)ꢀCl(2)
N(2)ꢀFe(1)ꢀCl(2)
N(1)ꢀFe(1)ꢀCl(2)
N(3)ꢀFe(1)ꢀCl(1)
N(2)ꢀFe(1)ꢀCl(1)
N(1)ꢀFe(1)ꢀCl(1)
N(3)ꢀFe(1)ꢀCl(3)
N(2)ꢀFe(1)ꢀCl(3)
N(1)ꢀFe(1)ꢀCl(3)
Cl(2)ꢀFe(1)ꢀCl(3)
Cl(1)ꢀFe(1)ꢀCl(3)
Cl(2)ꢀFe(1)ꢀCl(1)
N(4)ꢀFe(1)ꢀCl(1)
N(4)ꢀFe(1)ꢀCl(2)
N(4)ꢀFe(1)ꢀN(1)
N(4)ꢀFe(1)ꢀN(2)
N(3)ꢀFe(1)ꢀN(4)
78.77(7)
76.39(8)
76.31(7)
92.30(6)
164.46(5)
89.30(6)
93.53(6)
94.35(5)
167.32(6)
162.54(6)
87.16(5)
90.36(6)
98.95(3)
97.82(3)
98.91(3)
87.64(4)
73.27(4)
76.03(4)
90.28(3)
166.68(3)
90.78(3)
92.57(3)
94.09(3)
162.83(3)
166.18(3)
84.08(3)
93.95(3)
95.23(2)
99.02(2)
99.14(2)
81.02(9)
78.44(8)
75.27(9)
166.08(7)
88.04(6)
90.51(6)
92.99(7)
94.72(6)
167.63(7)
90.83 (7)
166.87(6)
93.09 (7)
98.28(3)
95.98(4)
96.48(3)
151.63(1)
76.39(1)
75.29(1)
92.43(8)
90.77(8)
93.12(8)
103.36(8)
103.76(8)
166.43(8)
100.44(4)
89.76(7)
169.26(8)
76.67(1)
83.49(1)
88.42(1)
Table 3 Oxidation of benzene with H2O2 using various catalysts under the optimized reaction conditions.
cEp
ꢀ0.433
ꢀ0.363
ꢀ0.330
ꢀ0.210
ꢀ0.235
/
Conversion
42.0%
37.7%
35.2%
31.5%
26.7%
17.1%
16.9%
Selectivity
26.8%
52.9%
34.7%
41.2%
34.4%
35.5%
24.2%
Yield
11.2%
19.9%
12.2%
13.0%
9.2%
TON
56
TOF/ h−1
28
50
30
32
23
15
10
Fe1
Fe2
100
61
Fe3
65
Fe4
FeCl3
aFe2
bFe2
46
6.0%
30
/
4.1%
20
5
a 1 mL of ethanol was added while the total volume of the reaction mixture was kept unchanged. b 2 mL of ethanol was added.
8
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