Homogeneous and heterogeneous styrene epoxidation catalyzed by copper(II) and nickel(II) Schiff base complexes

Article abstract of DOI:10.1007/s11243-014-9853-6

Both monomeric Schiff base complexes and 1D helical polymeric complexes of Cu(II) and Ni(II) were synthesized and characterized by physicochemical and spectroscopic methods. X-ray single-crystal studies were made on [K ₂(CuL)₂Ni(CN)<

Full text of DOI:10.1007/s11243-014-9853-6

Transition Met Chem (2014) 39:705–712
DOI 10.1007/s11243-014-9853-6
Homogeneous and heterogeneous styrene epoxidation catalyzed
by copper(II) and nickel(II) Schiff base complexes
•
•
Deng-Feng Liu Xing-Qiang Lu¨ Rong Lu
Received: 13 March 2014 / Accepted: 24 June 2014 / Published online: 4 July 2014
Ó Springer International Publishing Switzerland 2014
Abstract Both monomeric Schiff base complexes and 1D
helical polymeric complexes of Cu(II) and Ni(II) were syn-
thesized and characterized by physicochemical and spec-
troscopic methods. X-ray single-crystal studies were made
on [K2(CuL)2Ni(CN)4]nꢀ0.5nEt2O and [K2(NiL)2Ni(CN)4]-
nꢀ0.5nEt2O. The polymers were screened as heterogeneous
catalysts for styrene epoxidation. For comparison, the cata-
lytic properties of the homogeneous and heterogeneous
catalysts were also examined under identical reaction con-
ditions, and the influence of various solvents and oxidants
was studied. The polymeric catalysts showed better activities
in chloroform when using tert-butyl hydroperoxide as oxi-
dant, suggesting that heterogenization increased the activity
of the catalyst under this condition.
are among the most widely studied because of their
remarkable electronic tunability and potential use as cata-
lysts in epoxidation of styrene [2]. Cu(II) and Ni(II) Schiff
base complexes are among the most versatile catalysts
known for oxygenation reactions. Even though such com-
plexes have been well known for a long time, they have
scarcely been explored as homogeneous catalysts for styrene
epoxidation. Salen-type Cu(II) and Ni(II) complexes were
found to be less active for the epoxidation of styrene using
iodosylbenzene, sodium hypochlorite (NaClO), molecular
oxygen, hydrogen peroxide (H2O2), and tert-butyl hydro-
peroxide (TBHP) as oxygen sources under homogeneous
conditions [3–10]. Compared to their homogeneous coun-
terparts, heterogeneous systems have the inherent advanta-
ges of easy separation, better handling properties, as well as
higher stability; therefore, heterogenization of such com-
plexes has been receiving much attention [11–14]. Various
approaches for the immobilization of Salen-type Cu(II) and
Ni(II) complexes have been reported, e.g., grafting the cat-
alyst onto asolid inorganic support such as silica[15], MCM-
41 [16], MCM-48 [14] or SBA-15 [17, 18]; encapsulation
into the pores of zeolites [19, 20]; polymer supports [21];
clays [22, 23]; and activated carbon [24, 25]. However, there
are certain disadvantages with polymeric supports due to
their vulnerability to some chemicals and solvents. Encap-
sulation of metal complexes in porous materials typified by
zeolites leads to size restriction and possible catalyst leach-
ing problems.
Introduction
Over the last few decades, the catalytic oxidation of styrene
has been found to be a useful reaction for the synthesis of
styrene oxide, which is an important intermediate for a large
variety of fine chemicals such as perfumes, sweeteners,
plasticizers, epoxy paints, and drugs etc [1]. Transition metal
complexes (Cu, Fe, Ni, Co, V, and Mn) of Schiff base ligands
D.-F. Liu ꢀ X.-Q. Lu
¨ (&) ꢀ R. Lu
Shaanxi Key Laboratory of Degradable Medical Material,
Northwest University, Xi’an 710069, People’s Republic
of China
Herein, in the search to minimize the above disadvantages
of heterogeneous Cu(II) and Ni(II) catalysts on various solid
supports, a new type of 1D helical polymeric catalyst
formed, by self-assembly of the Salen-type precursor with
K2Ni(CN)4, has been designed and prepared. This material
has only moderate solubility in N,N0-dimethylformamide
and is insoluble in most other common solvents [26]. Hence,
e-mail: lvxing46@126.com
D.-F. Liu (&)
College of Chemistry and Chemical Engineering, Xi’an
University of Science and Technology, Xi’an 710054,
People’s Republic of China
e-mail: liudf78@xust.edu.cn
123

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Transition Met Chem (2014) 39:705–712
it can be used as a heterogeneous catalyst in most common
solvents. The catalytic performances of heterogeneous
Cu(II) and Ni(II) polymers were compared to those of their
homogeneous analogues, and the oxidation performances
using 30 % H2O2, 70 % TBHP, and 10 % NaClO as oxi-
dizing agents were also compared for the oxidation of sty-
rene. To the best of our knowledge, this is the first report of
the use of Cu(II) and Ni(II) polymer as heterogeneous cata-
lysts for the epoxidation of styrene.
H2L (16.4 mg, 0.05 mmol) in dry dichloromethane (3 ml).
The mixture was stirred vigorously at room temperature for
1 h. The resultant clear green solution was filtered, and the
filtrate was allowed to evaporate at room temperature. The
pale green microcrystalline product (CuL) was isolated by
filtration after several days [27]. Yield: 14.02 mg, 72 %.
Calc. for C18H18N2O4Cu (CuL): C 55.5, H 4.6, N 7.2 %;
found: C 55.4, H 4.7, N 7.1 %. FTIR (KBr, cm-1): 3,291
(b), 2,923 (w), 2,828 (w), 1,646 (s), 1,603 (w), 1,444 (vs),
1,240 (m), 1,215 (s), 728 (m), 697 (w), 636 (w), 582 (w),
531 (w), 463 (w).
Experimental
Materials and methods
Synthesis of [K2(CuL)2Ni(CN)4]nꢀ0.5nEt2O
The main reagents were commercially available and were
used without further purification. 1,2-diaminoethane (99 %,
Tianjin), o-vanillin (Aldrich), styrene (98 %, Tianjin), H2O2
(30 %, Aldrich), TBHP (70 %, Aldrich), NaClO (10 %,
Aldrich), Ni(CH3COO)2ꢀ4H2O, Cu(CH3COO)2ꢀH2O, K2-
Ni(CN)4, and all organic solvents were A.R. grade. Elemental
analyses were performed on a PerkinElmer 240C elemental
analyzer. Infrared spectra were recorded on a Nicolet Nagna-
IR 550 spectrophotometer in the region 4,000–400 cm-1
using KBr pellets. 1H and 13C NMR spectra were recorded on
a JEOL EX 400 MHz NMR spectrometer in DMSO-d6 at
room temperature, using Me4Si as internal reference.
To a stirred solution of CuL (19.5 mg, 0.05 mmol) in dry
N,N0-dimethylformamide (2 ml), a solution of K2Ni(CN)4
(12.1 mg, 0.05 mmol) in absolute DMF (2 ml) was added.
The mixture was then stirred vigorously at 40 °C for 1 h. The
clear purple solution was cooled to room temperature and
filtered. Diethyl ether was allowed to diffuse slowly into the
filtrate at room temperature, and purple microcrystals were
obtained in about 2 days. [K2(CuL)2Ni(CN)4]nꢀ0.5nEt2O
(1):Yield: 17.48 mg, 67 %. Calc. for C41H39Cu2K2N8O8.5-
Ni: C 47.1, H 3.7, N 10.7 %; found: C 47.1, H 3.8, N 10.7 %.
FTIR (KBr, cm-1):3,413 (b), 2,928 (m), 2,960 (m), 2,835
(m), 2,117 (m), 1,680 (w), 1,629 (vs), 1,601 (s), 1,544 (m),
1,472 (s), 1,443 (s), 1,399 (m), 1,308 (s), 1,242 (s), 1,221 (s),
1,169 (m), 1,080 (s), 985 (m), 964 (m), 851 (m), 783 (w), 736
(s), 648 (w), 605 (w), 538 (w), 450 (w).
Synthesis of H2L
The Salen-type Schiff base N,N’-bis(3-methoxy-salicylid-
ene)-ethylene-1,2-diamine (H2L) was synthesized by the
usual procedure [26] by condensation of o-vanillin (6.3 g,
40 mmol) and 1,2-diaminoethane (1.4 ml, 20 mmol) in
absolute EtOH under reflux for about 5 h. After cooling to
room temperature, the insoluble precipitate was filtered off
and recrystallized from absolute EtOH to give a yellow
polycrystalline solid. Yield: 4.98 g, 76 %. Calc. for
C18H20N2O4: C 65.8, H 6.1, N 8.5 %; found: C 65.8, H 6.2, N
8.5 %. FTIR (KBr, cm-1): 3,444 (b), 3,001 (w), 2,930 (w),
2,840 (w), 1,632 (s), 1,468 (s), 1,410 (m), 1,249 (s), 1,080
(m), 958 (m), 736 (m), 695 (w), 642 (w), 564 (w), 525 (w). 1H
NMR (400 MHz, DMSO, d): 13.5 (s, 2H, –OH), 8.6 (s, 2H,
–CH = N), 7.0–6.9 (m, 4H, –Ph), 6.8 (t, J = 10.0 Hz, 2H,
–Ph), 3.9 (s, 4H, –CH2), 3.7 (s, 6H, –OCH3). 13C NMR
(400 MHz, DMSO) (d, ppm): 160.1 (–CH = N), 150.5 (–C–O),
148.4 (–C–OCH3), 124.5 (–C–C = N), 123.8, 120.6, 119.5
(–CH), 56.3 (–OCH3), 60.8 (–CH2).
Synthesis of NiL
To a stirred solution of H2L (16.40 mg, 0.05 mmol) in
absolute N,N’-dimethylformamide (3 ml), Ni(CH3COO)2-
4H2O (12.43 mg, 0.05 mmol) was added and the mixture
was stirred vigorously at room temperature for 1 h. The
resultant clear orange solution was filtered, and diethyl
ether was allowed to diffuse slowly into the filtrate at room
temperature. The purple microcrystalline product (NiL)
was isolated by filtration after 7 days [28]. Yield:
13.44 mg, 70 %. Calc.for C18H18N2O4Ni (NiL): C 56.2, H
4.7, N 7.3 %; found: C 56.1, H 4.8, N 7.4 %. FTIR (KBr,
cm-1): 3,291 (b), 2,923 (w), 2,828 (w), 1,646 (s), 1,603
(w), 1,444 (vs), 1,240 (m), 1,215 (s), 728 (m), 697 (w), 636
(w), 582 (w), 531 (w), 463 (w). 1H NMR (400 MHz,
DMSO, d): 8.5 (s, 2H, –CH = N), 7.1–7.0 (m, 4H, –Ph),
6.5 (t, J = 14.0 Hz, 2H, –Ph), 3.8 (s, 4H, –CH2), 3.7 (s, 6H,
–OCH3). 13C NMR (400 MHz, DMSO) (d, ppm): 162.0
(–CH = N), 150.1 (–C–O), 148.0 (–C–OCH3), 124.3
(–C–C = N), 123.2, 120.6, 118.6 (–CH), 56.1 (–OCH3),
46.4 (–CH2).
Synthesis of CuL
An absolute ethanol solution (3 ml) of Cu(OAc)2ꢀH2O
(10 mg, 0.05 mmol) was added to
a solution of
123

Transition Met Chem (2014) 39:705–712
707
Scheme 1 Synthetic routes of
ligand, Salen Cu(II), and Salen
Ni(II) complexes
Scheme 2 Structures of complexes CuL, NiL, 1, and 2
Synthesis of [K2(NiL)2Ni(CN)4]nꢀ0.5nEt2O
microcrystalline [K2(NiL)2Ni(CN)4]nꢀ0.5nEt2O (2) was
obtained in about 2 days. Yield: 16.55 mg, 64 %. Calc. for
C41H39Ni3K2N8O8.5: C 47.6, H 3.8, N 10.8 %; found: C
47.5, H 3.9, N 10.8 %. FTIR (KBr, cm-1): 3,443 (b), 3,129
(s), 2,932 (m), 2,844 (m), 2,359 (w), 2,117 (w), 1,621 (s),
1,554 (w), 1,476 (s), 1,395 (w), 1,322 (m), 1,247 (s), 1,132
(vs), 1,069 (vs), 955 (s), 858 (m), 738 (m), 621 (m), 546
(s) (Schemes 1, 2).
To a stirred solution of NiL (19.20 mg, 0.05 mmol) in
absolute N,N0-dimethylformamide (2 ml), a solution of
K2Ni(CN)4 (12.05 mg, 0.05 mmol) in absolute DMF (2 ml)
was added. The mixture was stirred vigorously at 80 °C for
1 h. The clear orange solution was then cooled to room
temperature and filtered. Diethyl ether was allowed to dif-
fuse slowly into the filtrate at room temperature, and red
123

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Transition Met Chem (2014) 39:705–712
Crystallography
Table 1 Summary of crystallographic data and structure refinements
for complexes 1 and 2
Single crystals of 1 and 2 of suitable dimensions were
mounted onto thin glass fibers. The structures of the crys-
tals were determined by X-ray crystallography. All the
intensity data were collected on a Bruker SMART CCD
Compound
1
2
Empirical
formula
C40H36Cu2K2N8O8.5Ni C40H39Ni3K2N8O9.5
Formula weight 1,028.76
1,038.12
0.71073
Tetragonal
I41cd
˚
diffractometer (Mo-Ka radiation and k = 0.71,073 A) in U
˚
Wavelength (A) 0.71073
and x scan modes. The observed data were weak, but the
best that could be obtained. Structures were solved by
Patterson methods followed by difference Fourier synthe-
ses, and then refined by full-matrix least-squares tech-
niques against F2 using SHELXTL-97 [29]. All other non-
hydrogen atoms were refined with anisotropic thermal
parameters. Absorption corrections were applied using
SADABS [30]. All hydrogen atoms were placed in calcu-
lated positions and refined isotropically using a riding
model. Crystallographic data and refinement parameters for
the complexes are presented in Table 1. Selected bond
distances and bond angles for 1 and 2 are given in Table 2.
Crystal system
Space group
Tetragonal
I41cd
˚
a (A)
24.843(2)
24.843(2)
28.398(5)
90
24.790(8)
24.790(8)
28.532(9)
90
˚
b (A)
˚
c (A)
a, (°)
b, (°)
c, (°)
90
90
90
90
3
˚
V (A )
17,527(4)
16
17,535(10)
16
Z
q (g/cm3)
1.547
1.573
T (K)
273(2)
293(2)
Crystal size
(mm3)
0.36 9 0.25 9 0.23
0.43 9 0.32 9 0.27
Epoxidation of styrene
Absorption
coefficient
1.637
1.529
Initial blank reactions were carried out without the catalyst,
as follows. Typically, a 100-ml two-necked round-bottom
flask equipped with a magnetic stirrer and a backflow
condenser was kept in a constant temperature (80 °C) in an
oil bath. Styrene (2 mmol) and the required solvent (5 ml)
were placed in the flask. The reaction was initiated by
adding 6 mmol of the required oxidant (30 % H2O2,
TBHP, or NaClO) at the required reaction temperature.
Aqueous NaOH solution (0.1 mol/l) was used to adjust the
pH of the reaction solution when only using 30 % H2O2 as
oxidant. Each reaction was sustained for 4 h before con-
ducting GC analysis.
(mm-1
)
F (000)
8,320
8,528
Theta range for
data collection
(°)
Limiting indices -26 B h B 31, -
28 B k B 31, -
35 B l B 35
1.64–26.62
1.64–26.68
-31 B h B 26, -
28 B k B 31, -
35 B l B 32
Reflections
collected/
unique
480,77/9,118
[R(int) = 0.1261]
47,497/8,920
[R(int) = 0.1462]
Completeness to 99.9
h = 26.62 (%)
99.2
All catalysts were dried overnight before they were used
in the epoxidation reactions. A mixture of catalyst
(0.01 mmol), styrene (2 mmol), and the required solvent
(5 ml) was stirred in a round-bottom flask supplied with a
backflow condenser, and then the required oxidant (6 mmol,
30 % H2O2, TBHP, or NaClO) was added dropwise within
1 h. Each reaction was sustained for 4 h at a constant tem-
perature (80 °C) prior to GC analysis.
Max. and min.
transmission
0.686 and 0.618
0.662 and 0.561
8,920/1/550
0.725
Data/restraints/
parameters
9,118/1/550
0.833
Goodness of fit
(GOF) on F2
R1,wR2
0.0591, 0.1542
0.0442, 0.0940
0.1501, 0.1135
[I [ 2r(I)]
R1,wR2 (all data) 0.1525, 0.1743
GC analysis of products
Quantitative GC analysis of the reaction products (using
the external standard method) was carried out with a
GC2060 instrument equipped with a FID detector and a
SE-30 capillary column (30 m 9 0.25 mm 9 0.25 lm).
Nitrogen was used as carrier gas. The temperatures of the
injector and detector were both 240 °C, and the tempera-
ture of the column oven was programmed to increase from
20 to 130 °C at a rate of 10 °C/min. The performance of
the catalysts was evaluated quantitatively as the conversion
of styrene (Conv %), the yield of styrene oxide (Yield %),
and the selectivity to styrene oxide (Sel %), such that
Conv % ¼ ð½styreneꢁI ꢂ ½styrene]FÞ=½styreneꢁI ꢃ 100
where [styrene]I = the concentration of styrene at initial
reaction time and [styrene]F = the concentration of styrene
at final reaction time
123

Transition Met Chem (2014) 39:705–712
709
˚
Table 2 Selected bond lengths (A) and angles (deg) with esds for
complexes 1 and 2
1
2
Cu(1)–O(2)
1.906(6)
1.889(6)
1.947(8)
1.950(9)
1.880(6)
1.875(6)
1.893(8)
1.895(7)
3.162(7)
2.888(7)
2.767(6)
2.902(7)
3.313(7)
2.918(7)
3.062(7)
2.738(10)
93.2(3)
Ni(1)–O(2)
1.857(4)
1.860(5)
1.873(6)
1.845(7)
1.848(4)
1.847(4)
1.827(6)
1.825(6)
3.106(6)
2.906(5)
2.792(5)
2.891(5)
3.230(5)
2.930(5)
3.007(5)
2.805(8)
94.4(3)
Cu(1)–O(3)
Ni(1)–O(3)
Cu(1)–N(1)
Ni(1)–N(1)
Cu(1)–N(2)
Ni(1)–N(2)
Cu(2)–O(6)
Ni(2)–O(6)
Cu(2)–O(7)
Ni(2)–O(7)
Cu(2)–N(3)
Ni(2)–N(3)
Cu(2)–N(4)
Ni(2)–N(4)
K(1)–O(1)
K(1)–O(1)
K(1)–O(2)
K(1)–O(2)
K(1)–O(3)
K(1)–O(3)
K(1)–O(4)
K(1)–O(4)
K(1)–O(5)
K(1)–O(5)
K(1)–O(6)
K(1)–O(6)
K(1)–O(7)
K(1)–O(7)
K(1)–N(6)
K(1)–N(6)
N(1)–Cu(1)–O(2)
N(2)–Cu(1)–O(3)
N(1)–Cu(1)–O(3)
N(2)–Cu(1)–O(2)
N(3)–Cu(2)–O(6)
N(4)–Cu(2)–O(7)
N(3)–Cu(2)–O(7)
N(4)–Cu(2)–O(6)
N(1)–Ni(1)–O(2)
N(2)–Ni(1)–O(3)
N(1)–Ni(1)–O(3)
N(2)–Ni(1)–O(2)
N(3)–Ni(2)–O(6)
N(4)–Ni(2)–O(7)
N(3)–Ni(2)–O(7)
N(4)–Ni(2)–O(6)
92.8(3)
95.1(3)
178.9(4)
178.2(4)
94.3(3)
178.1(3)
178.8(3)
95.5(2)
94.7(3)
95.7(2)
Fig. 1 Perspective drawing of the asymmetrical unit of 1, hydrogen
atoms, and solvates is omitted for clarity
179.6(3)
177.0(4)
178.8(3)
178.2(3)
with the usual distorted tetragonal anti-prism geometry,
provided by four atoms from one Schiff base ligand, three
inner O atoms from the neighboring Schiff base, and one N
atom from the Ni(CN)2- anions. Depending on the nature of
the oxygen atoms, the K–O bond lengths vary from 2.767(6)
Yield % ¼ ðNY=NEÞ ꢃ 100
where NY = the moles of styrene oxide obtained and
NE = the theoretically expected moles of styrene oxide
˚
to 3.313(7) A.
Sel % ¼ NY=ðNI ꢂ NFÞ ꢃ 100
The coordination bonds from the K atom to the mono-
mer CuL lead to the formation of a 1D polymeric chain. It
is interesting that the helix of the Cu–MeO Salen units,
along with other two homochiral helical arrangements of K
and Ni atoms, can generate a ‘‘fake’’ example of a triple-
stranded helix in the 1D helical polymeric chain along the
crystallographic fourfold screw axis. The structure of
complex 2 is similar to that of complex 1. The X-ray
crystal structure analysis of complex 2 unambiguously
revealed the asymmetric unit of 2 and the presence of 1D
helical polymeric chain, as depicted in Fig. 2.
where NI = the moles of styrene at the initial reaction time
and NF = the moles of styrene at final reaction time.
Results and discussion
Crystal structures
Complex 1 consists of 1D polymeric [K2(CuL)2Ni(CN)4]n
chains and 0.5nEt2O solvate molecule. The asymmetric unit
of complex 1, Fig. 1, shows the maintenance of the monomer
CuL units, in which the Cu atom lies in a four-coordinated
environment and adopts a slightly distorted square-planar
geometry, with two sets of unequal Cu–N(imino) [1.893(8)–
Catalytic styrene epoxidation
In the catalytic epoxidation of styrene, the choice of oxygen
donor and solvent are both of crucial importance. In order to
investigate the effect of solvent, experiments with chloro-
form, acetonitrile, and dichloromethane were carried out
˚
˚
1.950(9) A] and Cu–O(phenolic) [1.875(6)–1.906(6) A]
bond lengths and N–Cu–O bond angles [92.8(3)–94.7(3)°
and 177.0(4)–179.6(3)°]. The K atoms are eight-coordinated
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Transition Met Chem (2014) 39:705–712
Table 3 Product selectivity and percent conversion of styrene in
solvent when 30 % H2O2 as oxidant
Catalysts
Solvents Conversion
(%)
Product selectivity (%)
Styrene
oxide
Benzaldehyde
K2Ni(CN)4 CH3CN 48.3
14.0
16.2
13.8
22.3
17.4
15.7
16.5
16.2
26.6
21.3
13.5
14.9
27.1
12.0
30.3
24.1
57.3
26.2
67.5
49.9
16.2
16.8
17.0
26.7
22.9
14.8
17.2
18.3
32.6
30.7
CuL
NiL
1
CH3CN 66.5
CH3CN 57.4
CH3CN 87.5
CH3CN 62.5
2
K2Ni(CN)4 CH2Cl2 22.6
CuL
NiL
1
CH2Cl2 33.8
CH2Cl2 33.6
CH2Cl2 41.6
CH2Cl2 36.4
2
K2Ni(CN)4 CHCl3
14.6
34.5
28.4
41.1
36.2
CuL
NiL
1
CHCl3
CHCl3
CHCl3
CHCl3
2
Reaction conditions: the molar ratio of H2O2/styrene = 3:1, solvent
5 ml, catalyst 0.01 mmol, pH = 8.0, temperature 80 °C, duration 4 h
blank reactions exhibited extremely low yields of styrene
oxide and benzaldehyde. Table 2 shows the product
selectivity and conversion of styrene with the homoge-
neous and heterogeneous catalysts using TBHP as oxidant.
In the presence of the homogeneous catalyst CuL, styrene
oxide was the major product, along with moderate yields of
benzaldehyde, 90.2 % conversion of styrene, and 48.9 %
selectivity to styrene oxide was obtained in chloroform. For
the homogeneous catalyst NiL, 86.2 % conversion of sty-
rene and 67.9 % selectivity to styrene oxide were obtained
in chloroform. Oxidation of styrene with TBHP in the
presence of heterogeneous catalyst 1 gave styrene oxide as
major product along with moderate yields of benzaldehyde.
93.4 % conversion of styrene and 56.8 % selectivity to
styrene oxide were obtained in chloroform. For the heter-
ogeneous catalyst 2 in chloroform, the conversion of sty-
rene was 94.7 % with maximum selectivity to styrene
oxide (81.6 %). Lower yields of styrene oxide and benz-
aldehyde were produced when using dichloromethane or
acetonitrile as the solvent. Hence, the heterogeneous cat-
alysts 1 and 2 showed higher conversion to styrene and
better selectivity to styrene oxide in chloroform when using
TBHP as oxidant, and the performance of the heteroge-
nized Ni(II) catalyst was better than that of the heteroge-
neous Cu(II) catalyst in chloroform (Table 4).
Fig. 2 Perspective drawing of the asymmetrical unit of 2, hydrogen
atoms, and solvates is omitted for clarity
using 30 % aqueous hydrogen peroxide as oxidant. Aque-
ous 0.1 mol/l sodium hydroxide was added to maintain a pH
of 8.0. Blank reactions with each of the three solvents
exhibited extremely low yields of styrene oxide and benz-
aldehyde from styrene. As shown in Table 1, the selectivity
of the Cu(II) catalysts for the production of benzaldehyde
from styrene is higher than that for styrene oxide in aceto-
nitrile solvent, the heterogeneous catalyst 1 showed 87.5 %
conversion of styrene and 67.5 % selectivity for benzalde-
hyde, while homogeneous catalyst CuL showed 66.5 %
conversion of styrene and 57.3 % selectivity for benzalde-
hyde, while using dichloromethane or chloroform as the
solvent produced low yields of both styrene oxide and
benzaldehyde. In contrast, both the homogeneous catalyst
NiL and heterogeneous catalyst 2 produced only low yields
of styrene oxide and benzaldehyde in each solvent. The
catalytic performances of the Cu(II) and Ni(II) species were
better than those of the homogeneous catalysts. Moreover,
the Cu(II) catalysts showed better activity in comparison
with Ni(II) catalysts using H2O2 as oxidant. However, H2O2
as primary oxidant caused poor selectivity for styrene oxide
and decomposed at higher temperature, similar to the
observations made by Sharma et al. [18, 21] (Table 3).
Next, the epoxidation of styrene with 70 % TBHP as
oxidant was investigated in the same three solvents. Again,
Next, the same experiments were carried out using
NaClO as the primary oxidant. As before, blank reactions
with all three solvents gave very low yields of styrene
oxide and benzaldehyde. As seen from Table 3, both
123

Transition Met Chem (2014) 39:705–712
711
NaClO as oxidant. However, the conversion was 58.9 %
with 37.7 % styrene oxide selectivity and 65.4 % with
51.0 % styrene oxide selectivity when using NiL and 2,
respectively, in acetonitrile. The performance of the Ni(II)
catalyst was better than that of the Cu(II) catalyst when
using NaClO as oxidant. Furthermore, the heterogeneous
Ni(II) catalyst was better than the homogeneous Ni(II)
catalyst (Table 5).
Table 4 Product selectivity and percent conversion of styrene in
solvent when 70 % TBHP as oxidant
Catalysts
Solvents Conversion
(%)
Product selectivity (%)
Styrene
oxide
Benzaldehyde
K2Ni(CN)4 CH3CN 59.6
21.5
35.7
33.7
41.6
50.1
26.6
27.0
26.5
37.0
48.2
36.1
48.9
67.9
56.8
81.6
17.2
29.5
23.1
29.8
18.8
29.5
27.3
14.1
19.2
15.3
18.9
16.3
14.6
14.2
13.1
CuL
NiL
1
CH3CN 77.9
CH3CN 84.1
CH3CN 94.8
CH3CN 93.3
2
Conclusions
K2Ni(CN)4 CH2Cl2 45.5
CuL
NiL
1
CH2Cl2 81.9
CH2Cl2 55.4
CH2Cl2 83.7
CH2Cl2 84.6
Copper(II) and Ni(II) Schiff base complexes were synthe-
sized and examined as homogeneous and heterogeneous
catalysts for the epoxidation of styrene. The solvent, pri-
mary oxidant, and choice of catalyst were all important
variables for the epoxidation reaction. The Cu(II) catalysts
gave better results than the Ni(II) catalysts using H2O2 as
oxidant, with the heterogeneous catalyst giving the best
results. In contrast, the Ni(II) catalysts were better when
using TBHP as oxidant, although once again the hetero-
geneous catalyst 2 exhibited the highest catalytic activities
in chloroform. For NaClO as primary oxidant, the best
results were obtained with heterogeneous Ni(II) catalyst 2
in acetonitrile. In conclusion, the results of this study offer
new insights into heterogeneous epoxidation catalysts.
2
K2Ni(CN)4 CHCl3
37.0
90.2
86.2
93.4
94.7
CuL
NiL
1
CHCl3
CHCl3
CHCl3
CHCl3
2
Reaction conditions: the molar ratio of TBHP/styrene = 3:1, solvent
5 ml, catalyst 0.01 mmol, temperature 80 °C, duration 4 h
Table 5 Product selectivity and percent conversion of styrene in
solvent when 10 % NaClO as oxidant
Catalysts
Solvents Conversion
(%)
Product selectivity (%)
Supplementary material
Styrene
oxide
Benzaldehyde
The crystallographic data for 1 and 2 have been deposited
at the Cambridge Crystallographic Data Center, CCDC
reference number for 1 is 957,476 and CCDC reference
number for 2 is 957,477. The data can be obtained free of
charge via http://www.ccdc.cam.ac.uk/conts/retrieving.
html or from the Cambridge CB21EZ, UK.
K2Ni(CN)4 CH3CN 35.6
21.8
30.1
37.7
37.5
51.0
17.1
27.3
16.5
11.6
38.6
21.7
23.7
27.8
28.7
37.3
18.3
18.8
19.2
19.7
12.2
17.8
17.5
16.4
13.7
26.9
23.9
24.4
21.8
22.2
36.9
CuL
NiL
1
CH3CN 42.8
CH3CN 58.9
CH3CN 51.1
CH3CN 65.4
2
K2Ni(CN)4 CH2Cl2 30.1
CuL
NiL
1
CH2Cl2 35.1
CH2Cl2 42.1
CH2Cl2 39.6
CH2Cl2 46.2
Acknowledgments This work was supported by the National Nat-
ural Science Foundation (21373160, 21173165), the Program for New
Century Excellent Talents in University from the Ministry of Edu-
cation of China (NCET-10-0936), the research fund for the Doctoral
Program (20116101110003) of Higher Education of China, the Sci-
entific Research Program Foundation of Shaanxi Provincial Education
Department (2013JK0701, 2012JK0577), and the Scientific Research
Culture Foundation of Xi’an University of Science and Technology
(201117).
2
K2Ni(CN)4 CHCl3
24.0
50.2
64.0
61.7
68.3
CuL
NiL
1
CHCl3
CHCl3
CHCl3
CHCl3
2
References
Reaction conditions: the molar ratio of NaClO/styrene = 3:1, solvent
5 ml, catalyst 0.01 mmol, temperature 80 °C, duration 4 h
1. Zhan WC, Guo YL, Wang YQ, Guo Y, Liu XH, Wang YS, Zhang
ZG, Lu GZ (2009) J Phys Chem C 113:7181
2. Gupta KC, Sutar AK, Lin CC (2009) Coord Chem Rev 253:1926
3. Zolezzi S, Spodine E, Decinti A (2003) Polyhedron 22:1653
4. Iglesias AL, Aguirre G, Somanathan R, Hake MP (2004) Poly-
hedron 23:3051
heterogeneous catalysts and both homogeneous catalysts
showed poor conversion to styrene and selectivity to sty-
rene oxide in dichloromethane and chloroform when using
123

712
Transition Met Chem (2014) 39:705–712
5. Rayati S, Zakavi S, Koliaei M, Wojtczak A, Kozakiewicz A
19. Qi B, Lu XH, Zhou D, Xia QH, Tang ZR, Fang SY, Pang T,
Dong YL (2010) J Mol Catal A: Chem 322:73
20. Ghost R, Shen XF, Villegas JC, Ding YS, Malinger K, Suib SL
(2006) J Phys Chem B 110:7592
(2010) Inorg Chem Commun 13:203
6. Naskar S, Figge HM, Sheldrick WS, Chattopadhyay SK (2011)
Polyhedron 30:529
7. Monfaerd HH, Bikas R, Mayer P (2010) Inorg Chim Acta
363:2574
8. Li ZK, Li Y, Lei L, Che CM, Zhou XG (2005) Inorg Chem
Commun 8:307
9. Samsel EG, Srinivasan K, Kochi JK (1985) J Am Chem Soc
107:7606
10. DiazRequejo MM, Belderrain TR, Perez PJ (2000) Chem Com-
mun 19:1853
11. Lu XH, Xia QH, Zhan HJ, Yuan HX, Ye CP, Su KX, Xu G
(2006) J Mol Catal A: Chem 250:62
12. Jiang J, Ma K, Zheng YF, Cai SL, Li R, Ma JT (2009) Appl Clay
Sci 45:117
13. Yang Y, Zhang Y, Hao SJ, Guan JQ, Ding H, Shang FP, Qiu PP,
Kan QB (2010) Appl Catal A Gen 381:274
14. Gomez S, Garces LJ, Villegas J, Ghosh R, Giraldo O, Suib SL
(2005) J Catal 233:60
21. Sharma S, Sinha S, Chand S (2012) Ind Eng Chem Res 51:8806
22. Sen R, Bhunia S, Mal D, Koner S, Miyashita Y, Okamoto KI
(2009) Langmuir 25:13667
23. Das P, Biernacka IK, Silva AR, Carvalho AP, Pires J, Freire C
(2006) J Mol Catal A: Chem 248:135
24. Cardoso M, Silva AR, Castro BD, Freire C (2005) Appl Catal A
Gen 285:110
25. Silva AR, Figueiredo JL, Freire C, Castro BD (2005) Catal Today
102–103:154
26. Chai WL, Jin WJ, Lu¨ XQ, Bi WY, Song JR, Wong WK, Bao F
(2008) Inorg Chem Commun 11:699
27. Chen LL, Ding LQ, Chu Z, Long Y, Lu¨ XQ, Song JR, Fan DD,
Jin WJ (2011) Appl Organomet Chem 25:310
28. Jin WJ, Ding LQ, Chu Z, Chen LL, Lu¨ XQ, Zheng XY, Song JR,
Fan DD (2011) J Mol Catal A: Chem 337:25
29. Sheldrick GM (1997) SHELXL-97. A program for crystal
¨ ¨
structure refinement. University of Gottingen, Gottingen
¨
30. Sheldrick GM (1996) SADABS. University of Gottingen,
¨
Gottingen
15. Hu JL, Li KX, Li W, Ma FY, Guo YH (2009) Appl Catal A Gen
364:211
16. Jana S, Dutta B, Bera R, Koner S (2007) Langmuir 23:2492
17. Yang Y, Zhang Y, Hao SJ, Kan QB (2011) Chem Eng J 171:1356
18. Yang Y, Guan JQ, Qiu PP, Kan QB (2010) Appl Surf Sci
256:3346
123