Copper-salen catalysts modified by ionic compounds for the oxidation of cyclohexene by oxygen

Article abstract of DOI:10.1016/j.molcata.2010.05.006

Copper-salen catalysts modified by ionic compounds were synthesized and used in the allylic oxidation of cyclohexene to 2-cyclohexen-1-ol and 2-cyclohexen-1-one with oxygen as the oxidant under mild conditions. Compared with their unmodified counterpart,

Full text of DOI:10.1016/j.molcata.2010.05.006

Journal of Molecular Catalysis A: Chemical 327 (2010) 25–31
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Journal of Molecular Catalysis A: Chemical
journal homepage: www.elsevier.com/locate/molcata
Copper-salen catalysts modified by ionic compounds for the oxidation of
cyclohexene by oxygen
Xiao Yun, Xingbang Hu, Zhiyuan Jin, Jinghui Hu, Chao Yan, Jia Yao, Haoran Li∗
Department of Chemistry, Zhejiang University, ZheDa Road, Hangzhou 310027, PR China
a r t i c l e i n f o
a b s t r a c t
Article history:
Copper-salen catalysts modified by ionic compounds were synthesized and used in the allylic oxidation of
cyclohexene to 2-cyclohexen-1-ol and 2-cyclohexen-1-one with oxygen as the oxidant under mild con-
ditions. Compared with their unmodified counterpart, the catalytic activities of these modified catalysts
are improved, thereby facilitating the development of highly efficient catalysts. The type of counteranion
could affect the reactivity of catalyst, which offers an opportunity to improve the catalysts via changing
counteranions. The cation–anion interaction can be adjusted by different solvents, which in turn influ-
encing the catalyst reactivity. Furthermore, these novel catalysts can be reused without sacrificing the
activity.
Received 28 December 2009
Received in revised form 10 May 2010
Accepted 14 May 2010
Available online 16 June 2010
Keywords:
Salen catalyst
Ionic compound
Oxygen oxidation
Cyclohexene
© 2010 Elsevier B.V. All rights reserved.
Cation–anion interaction
1. Introduction
bilization of catalysts. Jacobsen catalysts [20–23] and some other
catalysts [24–29] have been modified by ionic compounds. These
Metal-salen complexes have attracted the interest of chemists
for more than 80 years [1]. Developing high effective salen cat-
alysts has always been a challenging research. It has been long
recognized that the catalysts can be modified with various sub-
stituent groups on ligands [2–5]. However, these salen catalysts
suffer some problems in homogeneous medium, including diffi-
culty in separation from the reaction mixture and recycling. One
approach for overcoming these difficulties is to immobilize the
salen catalysts on some supports to create heterogeneous catalysts,
such as immobilization of salen catalysts using inorganic sup-
ports [6–9], mesoporous materials [10–13], and organic polymer
[14–16]. Unfortunately, comparing with the homogeneous salen
catalysts, the heterogeneous catalysts often suffer from various dis-
advantages, such as poor activity, low accessibility of substrates,
and leaching of the active species into the reaction medium. There-
fore, it is challenging to develop an efficient strategy for catalyst
recovery which can keep the reactivity of the catalyst or even
improve it.
catalysts have common advantages, facilitating product isolation
and offering an opportunity to reuse the catalysts. In addition,
some of them have been reported to obtain the advantage of
accelerating reactions [30,31]. Although there are extensive appli-
cations of catalysts supported by ionic compounds, the effect
of the counteranion on the reactivity of these supported cat-
alysts has been received little attention. As far as we know,
there are a few papers concerned on the catalysts supported
by ionic compounds with different anions. Dyson synthesized
[PdCl2(C3CNpy)2][X−] (X− = Cl−, PF6−, BF4−, PdCl42−, N(SO2CF3)2
)
−
with different anions. However, no significant differences on cat-
alytic activity with different anions were found in Suzuki coupling
reaction [32]. Designing a kind of ionic compound supported cat-
alyst, whose catalytic activity could be improved using various
cation–anion, would greatly simplify the process of modifying cat-
alyst.
Herein, we designed and synthesized copper-salen catalysts
modified by ionic compounds, M-[Salen-Py][X−]2 (Scheme 1).
The most appealing features of the catalysts were that the
cation of ionic salts connected with the metal center by
conjugated double bond and the anion interacted with the
cation through hydrogen bond, thus the cation–anion inter-
action could easily deliver to the metal center. The catalyst
reactivity could be improved by altering the type of counteran-
ion in ionic salts or changing the strength of the interaction
between cation and anion. The catalysts could expediently be
reused.
Ionic compounds have attracted attention in several scien-
tific disciplines owing to their intriguing physical and chemical
properties [17–19]. The properties of ionic salts can be tuned
by combining different cations and anions. More recently, ionic
compounds have been recognized as potential media for the immo-
∗
Corresponding author. Tel.: +86 571 8795 2424; fax: +86 571 8795 1895.
E-mail address: lihr@zju.edu.cn (H. Li).
1381-1169/$ – see front matter © 2010 Elsevier B.V. All rights reserved.
doi:10.1016/j.molcata.2010.05.006

26
X. Yun et al. / Journal of Molecular Catalysis A: Chemical 327 (2010) 25–31
2.2.3. Synthesis of [Salen-Py][Br−]2 (3)
The above-obtained 2 (0.04 mol) dissolved in pyridine (100 ml)
was stirred under reflux for 3 h. On filtration, precipitate was col-
lected and recrystallized from acetonitrile. The obtained solid was
dried in vacuo to give a brown powder of 3. Yields: 64%. 1H NMR
(CDCl3, 500 MHz): ı ppm 11.36 (s, 2H), 9.13–9.14 (d, 4H, J = 5.7);
8.81–8.84 (t, 2H, J = 7.80); 8.31–8.34 (t, 4H, J = 7.05) 3.67–3.68 (t,
4H, J = 2.85), 1.78 (s, 6H), 1.72 (s, 6H). Compound 3: Calc. for
Scheme 1. Structure of Cu-[Salen-Py][X−
] (X = Br, NO3, BF4, CF3COO, PF6).
2
2. Experimental
C22H28Br2N4O2: C, 48.91; H, 5.22; N, 10.39. Found: C, 44.99; H, 5.41;
N, 9.49. FT-IR (KBr): 3449, 3041, 1626, 1611, 1556, 1469, 1431, 1365,
1336, 1288, 1211, 1128, 1069, 1024, 971, 878, 810, 784, 690, 574,
496 cm−1. Melting point: 162.6–164.8 ◦C.
2.1. Materials and methods
The cyclohexene and N-bromosuccinimide (NBS) were obtained
from Acros organics. Other reagents and solvents were of pure
analytical grade materials purchased from commercial sources
and used without further purification unless otherwise indi-
cated.
2.2.4. Synthesis of M-[Salen-Py][Br−]2 (4)
Cu-[Salen-Py][Br−]2: A solution of Cu(OAc)2·H2O (0.03 mol) in
ethanol (50 ml) was added dropwise to 50 ml ethanol solution con-
taining 0.03 mol 3, and then the mixture was stirred under reflux for
another 3 h. The resulting slurry was cooled to 5 ◦C for 2 h, filtered
and the precipitate recrystallized from acetonitrile, dried in vacuo,
Cu-[Salen-Py][Br−]2 was prepared. Yields: 64%. Cu-[Salen-Py][Br]2:
Calc. for C22H28Br2CuN4O2: C, 43.91; H, 4.35; N, 9.31. Found: C,
44.41; H, 5.51; N, 9.38. ESI-MS, m/z = 220.6 [(Cu-[Salen-Py][Br−]2-
2Br)/2]+. FT-IR (KBr): 3410, 3030, 1622, 1594, 1467, 1429, 1351,
1303, 1264, 1221, 1158, 1105, 1080, 1050, 1019, 961, 788, 732, 695,
The NMR spectra were detected by Bruker ARX 500 NMR spec-
trometer, and TMS as the internal standard. FT-IR spectra were
recorded on a Bruker APEX-III spectrometer using KBr pellets in
400–4000 cm−1 region. The ultraviolet–visible light (UV–vis) spec-
tra were recorded on a UV–vis SPECORD 200 spectrophotometer.
Electron spin resonance (ESR) signals were recorded at ambient
temperature (120 K) with a Bruker ESR A300 spectrometer. Cata-
lyst stability was recorded on Perkin Elmer Pyris 6 thermal-gravity
(TG) analysis equipment. The progress of reaction was monitored
and controlled by Shang Feng GC-112A gas chromatograph fitted
with a SE-30 column (50 m, 0.025 ␮m diameters) and a flame ion-
ization detector. The structure of products and by-products was
further identified using HP6890 GC/MS spectrometer by comparing
retention times and fragmentation patterns with authentic sam-
ples.
663, 634, 453 cm−1
.
2.2.5. Synthesis of Cu-[Salen-Py][X−]2
Cu-[Salen-Py][X−]2: AgX (X = NO3, BF4, CF3COO) or NH4PF6
(2 mmol) was added to the Cu-[Salen-Py][Br−]2 (1 mmol) in ace-
tonitrile (50 ml), and then the mixture was stirred away from light
at room temperature for 3 h. The resulted mixture was filtered
and the filtrate was evaporated under reduced pressure at 60 ◦C,
and the obtained solid was dried in vacuo. Cu-[Salen-Py][X−]2 was
prepared.
2.2. Preparation of the M-[Salen-Py][X−]2
Cu-[Salen-Py][NO3−]2: Yield: 91%. Atomic Absorption Spec-
The preparation of the M-[Salen-Py-IL][X−]2 is outlined in
Scheme 2.
troscopy: Cu-[Salen-Py][NO3]2·5H2O: Calc. for C22H36CuN6O13
:
Cu, 9.69%. Found: Cu, 9.69%. ESI-MS, m/z = 503.2 (Cu-[Salen-
Py][NO3−]2-NO3)+. FT-IR (KBr): 3434, 3026, 2426, 1621, 1587, 1468,
1384, 1304, 1264, 1224, 1080, 1049, 1021, 961, 839, 794, 733, 700,
635, 460 cm−1
2.2.1. Synthesis of acetylacetone–ethylenediimine (1)
The acetylacetone–ethylenediimine (1) was synthesized
according to the modified procedure. Ethylenediimine (0.45 mol)
dissolved in acetyl acetone (100 ml) was heated to 105 ◦C under
stirring for 24 h, dewatering via water-separator. Upon fil-
tration, the precipitate was collected and recrystallized from
chloroform–pentane (1:1 (V/V)), dried in vacuo to get a pale-
yellow powder of 1. Yield: 59%. 1H NMR (CDCl3, 500 MHz): ı ppm
10.90 (s, 2H), 5.01 (s, 2H), 3.43–3.44 (t, 4H, J = 3.19), 2.01 (s, 6H),
1.92 (s, 6H). Compound 1: Calc. for C12H20N2O2: C, 64.26; H, 8.99;
N, 12.49. Found: C, 64.30; H, 8.98; N, 12.39. FT-IR (KBr): 3449,
2949, 1609, 1577, 1520, 1451, 1434, 1372, 1353, 1287, 1220, 1143,
1087, 1022, 979, 940, 851, 759, 739, 641, 521 cm−1. Melting point:
111.0–111.5 ◦C.
Cu-[Salen-Py][BF4−]2: Yield: 87%. Atomic Absorption Spec-
troscopy: Cu-[Salen-Py][BF4]2·5H2O: Calc. for C22H36CuF8N4O7:
Cu, 9.00%. Found: Cu, 7.53%. ESI-MS, m/z = 528.3 (Cu-[Salen-
Py][BF4−]2-BF4)+, 220.6 [(Cu-[Salen-Py][BF4−]2-2BF4)/2]+. FT-IR
(KBr): 3430, 1623, 1593, 1468, 1384, 1302, 1264, 1124, 1084, 1038,
785, 695, 533, 459 cm−1
Cu-[Salen-Py][PF6−]2: Yield: 77%. Atomic Absorption Spec-
troscopy: Cu-[Salen-Py][PF6]2·5H2O: Calc. for C22H36CuF12N4O7P2:
Cu, 8.27%. Found: Cu, 8.16%. ESI-MS, m/z = 586.4 (Cu-[Salen-
Py][PF6−]2-PF6)+, 220.7 [(Cu-[Salen-Py][PF6−]2-2PF6)/2]+. FT-IR
(KBr): 3430, 3060, 1686, 1625, 1595, 1471, 1430, 1355, 1299, 1266,
1223, 1203, 1127, 838, 791, 694, 558, 458 cm−1
Cu-[Salen-Py][CF3COO−]2: Yield: 64%. ESI-MS, m/z = 554.3
(Cu-[Salen-Py][CF3COO−]2-CF3COO)+,
221.0
[(Cu-[Salen-
Py][CF3COO−]2-2CF3COO)/2]+. FT-IR (KBr): 3448, 1685, 1628,
1474, 1432, 1287, 1202, 1139, 842, 798, 726, 459 cm−1
2.2.2. Synthesis of H2(3-Br-acacen) (2)
The above-obtained 1 (0.06 mol) dissolved in 50 ml acetonitrile,
followed by the addition of N-bromosuccinimide (0.12 mol) with
acetonitrile (50 ml) at 0 ◦C. The mixture was stirred for another
1 h at 0 ◦C. The resulted solid was filtered and recrystallized from
chloroform–ethanol (1:1 (V/V)), dried in vacuo, and 2 was pre-
pared. Yields: 47%. 1H NMR (DMSO, 500 MHz): ı ppm 11.62 (s, 2H),
3.49–3.51 (t, 4H, J = 2.98), 2.37 (s, 6H), 2.22 (s, 6H). Compound 2:
Calc. for C12H18BrN2O2: C, 37.72; H, 4.75; N, 7.33. Found: C, 37.59;
H, 3.58; N, 7.17. FT-IR (KBr): 3449, 2941, 1778, 1698, 1577, 1465,
1357, 1265, 1242, 1184, 1102, 1024, 986, 958, 880, 813, 757, 687,
635, 575 cm−1. Melting point: 140.9–141.7 ◦C.
2.3. Cyclohexene oxidation catalyzed by Cu-[Salen-Py][X−]2
The Cu-[Salen-Py][X−]2 were used as catalysts for the allylic oxi-
dation of cyclohexene. In a typical reaction, 40 mmol cyclohexene,
40 ml CH3CN, and 0.08 mmol Cu-[Salen-Py][X−]2 were added into
a 100 ml four-neck round bottom reactor, which was fitted with
an overhead stirrer and a reflux condenser. The reaction was per-
formed at 78 ◦C in a water bath with fast stirring. The oxygen was
flowing into the reactor at a constant flow rate (20 ml min−1). After

X. Yun et al. / Journal of Molecular Catalysis A: Chemical 327 (2010) 25–31
27
Scheme 2. Synthesis of M-[Salen-Py][X−
] (X = Br, NO3, BF4, CF3COO, PF6).
2
completion of the reaction, the solvent was removed by evaporated
under reduced pressure at 60 ◦C and the products were collected
by evaporated under reduced pressure at 100 ◦C. Subsequently, the
catalyst was used without further purification. The progress of reac-
tion was monitored by GC, and the selectivity of 2-cyclohexen-1-ol
and 2-cyclohexen-1-one were determined by gas chromatography
internal standard method (with benzyl ethanoate as the inter-
nal standard). The products were analyzed by GC–MS. Besides, a
serious of comparative experiments with different catalysts and
different conditions were carried out.
high as the TOF value for Cu-[Salen-Py][Br−]2 (20.8) (Entry 6). This
may be due to the fact that the cation of the ionic salts connected
with the metal center by conjugated double bonds, thus the vari-
ation via altering the type of counteranion could deliver to the
metal center of catalysts. We observed that the catalytic activi-
ties of catalysts with different counteranions followed the order
in a trend expected of the nucleophilicity of the counteranion in
CH3CN: PF6− > BF4− > CF3COO− > NO3− > Br− (Entry 2–6) [33–39]. It
is worth noting that though the counteranion could improve the
catalytic activity of Cu-[Salen-Py][X−]2, in comparison to unsup-
ported catalyst, its selectivity did not lose (selectivity of –ol and
–one >90%).
Different counteranions in ionic compounds can change the
electric structure of the active center of the catalysts, in turn
adjusting the properties of the catalysts via altering the type of
counteranion. It is worth noting that altering the counteranion only
needs an ion change process with high yield.
3. Results and discussion
3.1. The effect of the counteranion on the reactivity of the
catalysts
Many studies [33–36] found that the catalytic activity of metal-
salen complexes depended on the Lewis acidity of the catalyst and
the nucleophilicity of the axial ligand. As far as we know, there
is little attention on the effect of the counteranion in ionic com-
pound. Due to the unconjugated bond structure between cation and
metal center in common metal-salen catalysts modified by ionic
compound, the variation of anion almost has no influence on the
reactivity of the catalyst. Herein, we proposed a novel metal-salen
catalyst modified by ionic compounds (Cu-[Salen-Py][X−]2), whose
metal center connected with the cation by conjugate bonds.
The supported catalysts Cu-[Salen-Py][X−]2 were evaluated in
the allylic oxidation of cyclohexene (Scheme 3). As shown in
Table 1, we could find that the reactivity of unsupported catalyst
Cu-[Salen] was low (Entry 1), in comparison to Cu-[Salen-Py][X−]2.
This indicated that ionic salt had been successfully functional-
ized on the Cu-salen. Based on UV–vis spectra, the charge transfer
transition of salen ligand changed from 239 to 268 nm, when
ionic salts were functionalized on the salen ligand (see supporting
information). It was interesting to find that the type of counteran-
ion could affect the c−atalytic activity (Entry 2–6). The TOF value
An extensive comparison between Cu-[Salen-Py][X−]2 and the
reported ones was carried out then. As shown in Table 2, the
homogeneous catalysts showed high activity and selectivity for the
oxidation of cyclohexene. However, most of them are difficult to
separate from the products. Some of them have been immobilized
on the support, however, the heterogeneous catalysts suffered from
low activity and selectivity. A new method for immobilizing salen
complex onto ionic salt was introduced in this study, and the sup-
−
ported catalysts Cu-[Salen-Py][PF6
] displayed satisfied activity
2
and selectivity for the oxidation of cyclohexene (Entry 1). The reac-
tion was carried out continuously and was monitored by GC. After
16 h, the conversion of cyclohexene was up to 100% and the major
products were 2-cyclohexen-1-ol and 2-cyclohexen-1-one (Fig. 1).
3.2. Changing the reactivity by solvent-sensitive cation–anion
interaction
The ionic compound can be contacted or separated ion pairs
in different solvents. It prefers to be contacted ion pairs in low
polarity solvent whereas separated in high polarity solvent [45–48].
These findings inspired us to use different solvents to alter the
cation–anion interaction, hence improving the catalyst reactivity.
for Cu-[Salen-Py][PF6
] (45.5) (Entry 2) is more than twice as
2
Table
Py][PF6
3 summarizes catalytic performances of Cu-[Salen-
−
]
2 and Cu-[Salen-Br] in various solvents. Interestingly, the
−
catalytic activity of Cu-[Salen-Py][PF6
] changes in different sol-
2
vents (Entry 1, 3, 5, 7, 9), whereas the polarity of solvent has
no significant influence on the catalytic activity of Cu-[Salen-Br]
(Entry 2, 4, 6, 8, 10). The order of the activity of Cu-[Salen-
Scheme 3. The oxidation of cyclohexene.

28
X. Yun et al. / Journal of Molecular Catalysis A: Chemical 327 (2010) 25–31
Table 1
The results of the oxidation of cyclohexene with Cu-[Salen-Py][X−] and Cu-[Salen] in CH3CN.a.
b
Entry
X
Time (h)
Conversion (%)
Selectivity (%)
–ol
TOF (h−1
)
–one
1
2
3
4
5
6
Cu-[Salen]
Br−
24
24
16
14
13
22
33.6
100
100
100
100
100
54.9
34.8
32.5
31.9
31.3
31.1
41.3
58.6
60.1
61.7
62.6
63.2
7.0
20.8
31.3
35.7
38.5
45.5
−
NO3
CF3COO−
−
BF4−c
PF6
a
The oxidation of cyclohexene was performed with cyclohexene (40 mmol), catalyst (2‰ molar percentage) in 40 ml CH3CN at 78 ◦C.
Turnover Frequency (TOF) is calculated by expression of (moles of cyclohexene consumed)/[(mole of catalyst) × time (h)].
1‰ molar percentage catalyst was used.
b
c
Table 2
Allylic oxidation of cyclohexene with different kinds of catalysts.a.
c
Entry
Catalysts
Reaction conditions
Conversion (%)
Selectivityb (%)
TOF (h−1
)
Ref.
−
1
2
3
4
5
6
Cu-[Salen-Py][PF6
Sal-Phe-Mn
]
2
22 h (1000:1)d
12 h (3350:1)d
12 h (5000:1)d
8 h (8000:1)d
7 h (80:1)d
100
94.3
75.1
83.7
100
84.6
91.9
45.5
208.8
342.9
359.3
10.8
–
74.8
82.3
53.9
94.5
98.1
[40]
[41]
[42]
[43]
[44]
Mn(III) complexes
{[Ni([16]aneN5)]2R}(ClO4)4
Cobalt resinate
Fe(III)/SiO2
10 h (196:1)d
19.2
a
The homogeneous and heterogeneous catalysts used previously and in this study.
The selectivity of 2-cyclohexen-1-ol and 2-cyclohexen-2-one.
b
c
Turnover Frequency (TOF) is calculated by expression of (moles of cyclohexene consumed)/[(mole of catalyst) × time (h)].
d
The ratio of substrate to catalyst.
−
Py][PF6
]
2 in different solvents was CHCl3 > CH3OH > CH3CN > DMF
high polarity solvent, the cation and anion were separated by sol-
vent [45–48], thus the anion had little influence on the reactivity
of catalyst.
(Entry 1, 3, 5, 9), which was contrary to the polarity of solvents. It
is mainly because the Cu-[Salen-Py]+· · ·[PF6−] interaction in Cu-
−
[Salen-Py][PF6
]
2
changes in different solvents. High polar solvent
ESR spectroscopy can give the information about the electronic
environment of transition metal, due to the multiplet structure
originating from dn configuration of the transition metal. The peaks
of Cu-[Salen-Py][X−]2 were quiet different from that of Cu-[Salen-
Br] both in solid (Fig. 2) and DMSO (Fig. 3). It indicated that ionic
compounds significantly affected the electronic environment of Cu.
There is a remark shift on the peaks of Cu-[Salen-Py][X−]2 with dif-
ferent counteranions in solid (Fig. 2) but no significant change in
DMSO (Fig. 3). As we know, the anion should directly interact with
the cation in solid. The ESR results suggested that the direct inter-
action can effectively deliver the influence of anion to the metal
center. Direct interaction between the anion and cation has been
proved to be existent in low polarity solvent [45–48]. Hence, we
can expect a direct influence of anion in low polarity solvent, such
as CHCl3. However, when DMSO was used as solvent, because the
anion cannot interact with the cation directly, the influence of dif-
ferent anions was not remark. These ESR results could well explain
the above reaction results listed in Table 4, why the difference of the
catalytic activity of Cu-[Salen-Py][X−]2 with various counteranions
could separate [PF6−] from Cu-[Salen-Py]+, thus weakening the Cu-
[Salen-Py]+· · ·[PF6−] −interaction. As a result, the catalytic activity
of Cu-[Salen-Py][PF6
] is lower in high polarity solvent. The Cu-
2
[Salen-Br] remains neutral molecule in different solvents, hence the
polarity of solvent has little influence on the catalytic activity of Cu-
[Salen-Br]. The reactivity of Cu-[Salen-Py][PF6
−
] was not satisfied
2
in CH3COOH (Entry 7), due to the fact that CH3COOH is apt to be
protonated. The proton acted on [PF6−], which could weaken the
Cu-[Salen-Py]+· · ·[PF6−] interaction, thus Cu-[Salen-Py][PF6
]2 had
−
lower activity in CH3COOH.
The most interesting results were obtained when Cu-[Salen-
Py][X−]2 were used in different solvents (Table 4). It was found that
the reactivity of catalysts with different counteranions were differ-
ent in CHCl3 (Entry 1–3) whereas the difference of these catalysts
was not so obvious in DMSO (Entry 7–9). The reason is that Cu-
[Salen-Py][X−]2 might be contacted ion pairs in low polarity solvent
[45–48] and the direct interaction between anion and cation made
the reactivity of the catalyst different from each other. However, in
Table 3
The results of the oxidation of cyclohexene in different solvents over the complex Cu-[Salen-Py][PF6−] and Cu-[Salen-Br].a.
b
Entry
Catalyst
Solvent
Conversion (%)
Selectivity (%)
–ol
TOF (h−1
)
–one
−
−
−
−
−
1
2
3
4
5
6
7
8
9
Cu-[Salen-Py][PF6
Cu-[Salen-Br]
Cu-[Salen-Py][PF6
Cu-[Salen-Br]
Cu-[Salen-Py][PF6
Cu-[Salen-Br]
Cu-[Salen-Py][PF6
Cu-[Salen-Br]
]
]
]
]
]
DMF
45.7
29.2
46.1
28.6
48.3
29.0
25.9
22.1
57.4
29.7
38.9
47.6
23.3
25.7
35.1
37.3
34.8
36.2
31.7
32.3
58.2
39.8
62.2
61.1
56.3
53.8
60.8
54.1
60.8
45.3
19.0
12.2
19.2
11.9
20.1
12.1
10.8
9.2
CH3CN
CH3OH
CH3COOH
CHCl3
Cu-[Salen-Py][PF6
Cu-[Salen-Br]
23.9
12.4
10
a
The oxidation of cyclohexene was performed with cyclohexene (40 mmol), catalyst (1‰ molar percentage) in 40 ml solvent at 58 ◦C, 24 h.
Turnover Frequency (TOF) is calculated by expression of (moles of cyclohexene consumed)/[(moles of catalyst) × time (h)].
b

X. Yun et al. / Journal of Molecular Catalysis A: Chemical 327 (2010) 25–31
29
Table 4
The results of the oxidation of cyclohexene over Cu-[Salen-Py][X−] with different counteranions in different solvents.a.
b
Entry
Anion
Solvent
Conversion (%)
Selectivity (%)
–ol
TOF (h−1
)
–one
1
2
3
4
5
6
7
8
9
Br−
CHCl3
36.8
47.2
57.4
31.7
40.3
46.1
56.4
58.4
70.9
33.1
31.2
31.7
28.7
23.9
23.3
43.7
40.0
41.9
57.7
60.2
60.8
59.8
61.3
62.2
51.4
55.2
54.3
15.4
19.7
23.9
13.2
16.8
19.2
11.8
12.2
14.8
−
BF4−
PF6
Br−
CH3CN
DMSOc
−
BF4−
PF6
Br−
−
BF4−
PF6
a
The oxidation of cyclohexene was performed with cyclohexene (40 mmol), catalyst (1‰ molar percentage) in 40 ml solvent at 58 ◦C, 24 h.
Turnover Frequency (TOF) is calculated by expression of (moles of cyclohexene consumed)/[(mole of catalyst) × time (h)].
2‰ molar percentage catalyst was used.
b
c
Fig. 1. The conversion and selectivity vs. reaction time plot for oxidation of cyclo-
Fig. 3. ESR spectra of Cu-[Salen-Py][X−
]
2
and Cu-[Salen-Br] in DMSO at 120 K.
−
hexene catalyzed by Cu-[Salen-Py][PF6
] in CH3CN.
2
as axial ligands, the reactivity of Cu-salen was not improved. The
catalytic activity among the axial ligands followed the order pyri-
dine > 4-methylprridine > N-methylimidazole (Entry 2, 4, 6), which
was contrary to the pKb value of the axial ligand. It was interesting
in low polar solvent (CHCl3) was higher than in high polar solvent
(DMSO).
3.3. The effect of axial ligand on Cu-salen
−
to find that the catalytic activity of Cu-salen improved if PF6 was
joined as axial ligand, especially for N-methylimidazole (Entry 7).
It is worth noting that the catalytic activity of Cu-salen modified by
pyridine hexafluorophosphate remarkably improved, in compari-
son to pyridine hexafluorophosphate as axial ligand (Entry 8). This
indicated that ionic salt (pyridine hexafluorophosphate) had been
successfully functionalized on the Cu-salen, and the ionic salt could
affect the reactivity of the catalyst.
The effects on the activation of the Cu-salen with different axial
ligands were evaluated (Table 5). When nitrogen bases were used
3.4. Recycling of the catalyst
One of the features of catalysts modified by ionic compound was
that they have almost no vapour pressure and nice thermal stability
[17–19]. Thermal analysis had been used to monitor the decompo-
sition profiles of Cu-[Salen-Py][X−]2 (See supporting information).
We could find that the decomposition temperatures of Cu-[Salen-
Py][X−]2 are all higher than 200 ◦C. Therefore, we could use this
thermal stability to recover the catalyst. When the oxidation was
completed, the products and solvent were removed by evaporated
under reduced pressure, and then the catalyst was reused for subse-
−
quent reaction. We took Cu-[Salen-Py][PF6
]
2 as catalyst in CH3CN
−
for recycling. As shown in Table 6, the Cu-[Salen-Py][PF6
] could
2
be reused at least four times with only a slight loss of catalytic
activity and selectivity.
Fig. 2. ESR spectra of Cu-[Salen-Py][X−
] and Cu-[Salen-Br] in solid at 120 K.
2

30
X. Yun et al. / Journal of Molecular Catalysis A: Chemical 327 (2010) 25–31
Table 5
The results of the oxidation of cyclohexene over Cu-[Salen] with axial ligand.a.
b
Entry
Catalyst
Axial ligand
Time (h)
Conversion (%)
Selectivity (%)
TOF (h−1
)
–ol
–one
1
2
3
4
5
6
7
8
Cu-[Salen]
Cu-[Salen]
Cu-[Salen]
Cu-[Salen]
Cu-[Salen]
Cu-[Salen]
Cu-[Salen]
Cu-[Salen-Py][PF6
None
Pyridine
24
24
24
24
24
24
24
22
33.6
47.7
56.9
28.8
32.5
18.3
83.2
100
54.9
38.3
37.5
44.3
46.2
43.9
19.0
31.1
41.3
52.3
54.9
33.1
43.1
35.1
52.3
63.2
7.0
9.9
11.9
6.0
6.8
3.8
Pyridine hexafluorophosphate
4-Methylpyridine
4-Methylpyridine hexafluorophosphate
N-Methylimidazole
N-Methylimidazole hexafluorophosphate
None
17.3
45.5
−
c
]
a
The oxidation of cyclohexene was performed with cyclohexene (40 mmol), catalyst (2‰ molar percentage), axial ligand (2‰ molar percentage) in 40 ml CH3CN at 78 ◦C.
Turnover Frequency (TOF) is calculated by expression of (moles of cyclohexene consumed)/[(mole of catalyst) × time (h)].
1‰ molar percentage catalyst was used.
b
c
Table 6
The recycling studies of Cu-[Salen-Py][PF6−] in the oxidation of cyclohexene.a.
b
Entry
Run times
Conversion (%)
Selectivity (%)
–ol
TOF (h−1
)
–one
1
2
3
4
5
Fresh
2nd
3rd
4th
5th
100
100
100
95.6
88.0
31.1
36.7
31.6
31.8
34.8
63.2
54.8
59.7
59.1
52.6
45.5
45.5
45.5
43.5
40.0
a
The oxidation of cyclohexene was performed with cyclohexene (40 mmol), catalyst (1‰ molar percentage) in 40 ml CH3CN at 78 ◦C for 22 h.
Turnover Frequency (TOF) is calculated by expression of (moles of cyclohexene consumed)/[(mole of catalyst) × time (h)].
b
4. Conclusions
[5] L.P. Guo, C.M. Wang, W.J. Zhao, H.R. Li, W.L. Sun, Z.Q. Shen, Dalton Trans. 27
(2009) 5406–5410.
[6] R.I. Kureshy, N.H. Khan, S.H.R. Abdi, I. Ahmael, S. Singh, R.V. Jasra, J. Catal. 221
(2004) 234–240.
[7] R.I. Kureshy, I. Ahmael, N.H. Khan, S.H.R. Abdi, S. Singh, K. Pathak, R.V. Jasra, J.
Catal. 238 (2006) 134–141.
[8] C. Baleiza, H. Garcia, Chem. Rev. 106 (2006) 3987–4043.
[9] T. Balser, E.N. Jacobsen, Adv. Synth. Catal. 350 (2008) 967–971.
[10] M. Cardoso, A.R. Silva, B. Castro, C. Freire, Appl. Catal. A. Gen. 285 (2005)
110–118.
[11] H.Q. Yang, L. Zhang, W.G. Su, Q.H. Yang, C. Li, J. Catal. 248 (2007) 204–212.
[12] K. Yu, Z.C. Gu, R.N. Ji, L.L. Lou, F. Ding, C. Zhang, S.X. Liu, J. Catal. 252 (2007)
312–320.
[13] H.D. Zhang, Y.M. Wang, L. Zhang, G. Gerritsen, H.C.L. Abbenhuis, R.A.V. Santen,
C. Li, J. Catal. 256 (2008) 226–236.
[14] L. Canali, E. Cowan, H. Deleuze, C.L. Gibson, D.C. Sherrington, J. Chem. Soc.,
Perkin Trans. 1 (2000) 2055–2066.
[15] K. Smith, C.H. Liu, Chem. Commun. 8 (2002) 886–887.
[16] B.W. Gong, X.K. Fu, J.X. Chen, Y.D. Li, X.C. Zou, X.B. Tu, P.P. Ding, L.P. Ma, J. Catal.
262 (2009) 9–17.
The salen complexes modified by ionic compound M-[Salen-
Py][X−] were synthesized, and this kind of catalysts provided high
activity and selectivity for the allylic oxidation of cyclohexene.
The types of counteranion in ionic compounds could affect the
catalytic activ−ity of the catalysts, especially, the highest nucle-
ophilicity PF6 gave the best catalytic activity. The polarity of
solvent could also change the cation–anion interaction, and this
can be used to improve the reactivity of catalysts. The differences
of the catalytic activity of Cu-[Salen-Py][X−]2 with various coun-
teranions in low polarity solvent were remarkable, however, the
counteranion had little influence in high polarity solvent. Thus, if it
is desired to exclude the influence of anion, the solvent with high
dielectric constant is recommended. If it is anticipant that the reac-
tion is influenced mainly by the cooperation between the cation
and anion, the solvent with low dielectric constant is preferential.
Moreover, the catalyst could be used at least four times without
significant loss of catalytic activity and selectivity. These observa-
tions suggested that the easily reconstructed complexes presented
here are promising catalysts for allylic oxidation of cyclohexene.
[17] T. Welton, Chem. Rev. 99 (1999) 2071–2084.
[18] J. Dupont, R.F.D. Souza, P.A.Z. Suarez, Chem. Rev. 102 (2002) 3667–3692.
[19] V. Parvulescu, C. Hardacre, Chem. Rev. 107 (2007) 2615–2665.
[20] R. Tan, D.H. Yin, N.Y. Yu, H.H. Zhao, D.L. Yin, J. Catal. 263 (2009) 284–291.
[21] R. Tan, D.H. Yin, N.Y. Yu, Y. Jin, H.H. Zhao, D.L. Yin, J. Catal. 255 (2008) 287–
295.
[22] L.-L. Lou, K. Yu, F. Ding, X.J. Peng, M.M. Dong, C. Zhang, S.X. Liu, J. Catal. 249
(2007) 102–110.
[23] E.K. Noh, S.J. Na, S. Sujith, S.W. Kim, B.Y. Lee, J. Am. Chem. Soc. 129 (2007)
8082–8083.
[24] T.J. Geldbach, P.J. Dyson, J. Am. Chem. Soc. 126 (2004) 8114–8115.
[25] S.D. Miao, Z.M. Liu, B.X. Han, J. Huang, Z.Y. Sun, J.L. Zhang, T. Jiang, Angew. Chem.
Int. Ed. 45 (2006) 266–269.
Acknowledgement
This work was supported by the National Natural Science Foun-
dation of China (20773109, 20990221, 20803062 and J0830413)
[26] G. Sievers, O. Jimenez, T.E. Muller, S. Steuemage, J.A. Lercher, J. Am. Chem. Soc.
128 (2006) 13990–13991.
[27] T. Konno, K. Haneishi, M. Hirotsu, T. Yamaguchi, T. Ito, T. Yoshimura, J. Am.
Chem. Soc. 125 (2003) 9244–9245.
[28] M. Haumann, K. Dentler, J. Joni, A. Riisager, P. Wasserscheid, Adv. Synth. Catal.
349 (2007) 425–431.
[29] X.B. Hu, J.Y. Mao, Y. Sun, H. Chen, H.R. Li, Catal. Commun. 10 (2009) 1908–1912.
[30] W. Miao, T.H. Chan, Acc. Chem. Res. 39 (2006) 897–908.
[31] W. Miao, T.H. Chan, Adv. Synth. Catal. 348 (2006) 1711–1718.
[32] D.B. Zhao, Z.F. Fei, T.J. Geldbach, R. Scopelliti, P.J. Dyson, J. Am. Chem. Soc. 126
(2004) 15876–15882.
Appendix A. Supplementary data
Supplementary data associated with this article can be found, in
the online version, at doi:10.1016/j.molcata.2010.05.006.
References
[33] C.X. Miao, J.Q. Wang, Y. Wu, Y. Du, L.N. He, ChemSusChem 1 (2008) 236–241.
[34] C.T. Cohen, T. Chu, G.W. Coates, J. Am. Chem. Soc. 127 (2005) 10869–10878.
[35] L.P.C. Nielsen, C.P. Stevenson, D.G. Jacobsen, J. Am. Chem. Soc. 126 (2004)
1360–1362.
[36] K. Qiao, F. Ono, Q. Bao, D. Tomida, C. Yokoyama, J. Mol. Catal. A Chem. 303 (2009)
30–34.
[1] S. Yamada, Coord. Chem. Rev. 1 (1966) 415–437.
[2] W. Zhang, J.L. Loebach, S.R. Wilson, E.N. Jacobsen, J. Am. Chem. Soc. 112 (1990)
2801–2803.
[3] J.Y. Mao, N. Li, H.R. Li, X.B. Hu, J. Mol. Catal. A Chem. 258 (2006) 178–184.
[4] D.L. Xiong, Z.H. Fu, S. Zhong, X.F. Jiang, D.L. Yin, Catal. Lett. 113 (2007) 155–159.

X. Yun et al. / Journal of Molecular Catalysis A: Chemical 327 (2010) 25–31
31
[37] W.H. Guan, C.M. Wang, X. Yun, X.B. Hu, Y. Wang, H.R. Li, Catal. Commun. 9
(2008) 1979–1981.
[38] A.C. Pinto, A.A.M. Lapis, B.V.D. Silva, R.S. Bastos, J. Dupont, B.A.D. Neto, Tetrahe-
dron Lett. 49 (2008) 5639–5641.
[39] C.M. Wang, W.H. Guan, P.H. Xie, X. Yun, H.R. Li, X.B. Hu, Y. Wang, Catal. Commun.
10 (2009) 725–727.
[40] R.M. Wang, C.J. Hao, Y.P. Wang, S.B. Li, J. Mol. Catal. A Chem. 147 (1999) 173–178.
[41] J.H. Tong, Y. Zhang, Z. Li, C.G. Xia, J. Mol. Catal. A Chem. 249 (2006) 47–52.
[42] M. Salavati-Niasari, J. Mol. Catal. A Chem. 272 (2007) 207–212.
[43] C.X. Yin, Z.H. Yang, B. Li, F.M. Zhang, J.Q. Wang, Catal. Lett. 131 (2009) 440–443.
[44] J.Y. Mao, X.B. Hu, H.R. Li, Y. Sun, C.M. Wang, Z.R. Chen, Green Chem. 10 (2008)
827–831.
[45] J. Dupont, P.A.Z. Suarez, R.F.D. Souza, R.A. Burrow, J.P. Kintzinger, Chem. Eur. J.
6 (2000) 2377–2381.
[46] J. Dupont, J. Braz. Chem. Soc. 15 (2004) 341–350.
[47] C. Hardacre, J.D. Holbrey, M. Nieuwenhuyzen, T.G.A. Youngs, Acc. Chem. Res.
40 (2007) 1146–1155.
[48] W. Jiang, Y. Wang, G.A. Voth, J. Phys. Chem. B 111 (2007) 4812–4818.