Synthesis, characterization and catalytic property of tetradentate Schiff-base complexes for the epoxidation of styrene

Article abstract of DOI:10.1016/j.molcata.2006.01.055

A series of Schiff-base complexes has been synthesized by the condensation of 1,2-diaminocyclohexane with salicylaldehyde, 2-pyridinecarboxaldehyde, and 2-hydroxy-1-naphthaldehyde, followed by the metallation with manganese (1, 2, 3a), cobalt (3b), copper (3c) and iron (3d) salts. These Schiff-base ligands L₁-L₃ and complexes 1, 2, 3a-d were then characterized by IR, ¹H NMR, ¹³C NMR, UV-vis spectra, and DSC measurement. Schiff-base Mn complex (3a) resulting from N,N′-bis(2-hydroxy-1-naphthalidene)cyclohexanediamine (L₃) ligand was considerably active for the catalytic epoxidation of styrene under mild conditions, in which the highest yield of styrene oxide reached 91.2 mol%, notably higher than those achieved from simple salt catalysts Mn(Ac)₂·4H₂O and MnSO₄·H₂O. However, another two salen-Mn complexes 1 and 2 derived from ligands N,N′-bis(salicylidene)cyclohexanediamine (L₁) and N,N′-bis(2-pyridine carboxalidene)cyclohexanediamine (L₂) exhibited relatively poor activity under identical experimental conditions.

Full text of DOI:10.1016/j.molcata.2006.01.055

Journal of Molecular Catalysis A: Chemical 250 (2006) 62–69
Synthesis, characterization and catalytic property of tetradentate
Schiff-base complexes for the epoxidation of styrene
X.-H. Lua, Q.-H. Xiaa,∗, H.-J. Zhana,b, H.-X. Yuana, C.-P. Yea, K.-X. Sub, G. Xua
a Ministry-of-Education Key Laboratory for the Synthesis and Application of Organic Functional Molecules, Hubei University, Wuhan 430062, China
b Jingmen Technical College, Jingmen 448000, China
Received 29 June 2005; received in revised form 24 January 2006; accepted 26 January 2006
Available online 28 February 2006
Abstract
A series of Schiff-base complexes has been synthesized by the condensation of 1,2-diaminocyclohexane with salicylaldehyde, 2-
pyridinecarboxaldehyde, and 2-hydroxy-1-naphthaldehyde, followed by the metallation with manganese (1, 2, 3a), cobalt (3b), copper (3c) and
iron (3d) salts. These Schiff-base ligands L1–L3 and complexes 1, 2, 3a–d were then characterized by IR, 1H NMR, 13C NMR, UV–vis spectra,
and DSC measurement. Schiff-base Mn complex (3a) resulting from N,Nꢀ-bis(2-hydroxy-1-naphthalidene)cyclohexanediamine (L3) ligand was
considerably active for the catalytic epoxidation of styrene under mild conditions, in which the highest yield of styrene oxide reached 91.2 mol%,
notably higher than those achieved from simple salt catalysts Mn(Ac)2·4H2O and MnSO4·H2O. However, another two salen–Mn complexes 1 and
2 derived from ligands N,Nꢀ-bis(salicylidene)cyclohexanediamine (L1) and N,Nꢀ-bis(2-pyridine carboxalidene)cyclohexanediamine (L2) exhibited
relatively poor activity under identical experimental conditions.
© 2006 Elsevier B.V. All rights reserved.
Keywords: Tetradentate Schiff-base complexes; Ligand; Epoxidation; Hydrogen peroxide
1. Introduction
ultraviolet absorbent, flavoring agent, and so on, and is also
an important intermediate in organic synthesis, pharmacochem-
istry and perfumery. For example, catalytically-hydrogenated
product of styrene oxide is ␤-phenethyl alcohol, which is a
main component in the flower oil of attar, clove and neroli, and
extensively used for the preparation of food, tobacco, soap, cos-
metics essence etc. In recent years, the growth of domestic and
international demand for ␤-phenethyl alcohol and laevorotary
imidazole has resulted in the shortage of styrene oxide supply
on the market, which thus brings about vast foreground to the
research of styrene epoxidation.
Sharpless epoxidation of allylic alcohols was the first break-
through in this field [9,10], and this method led to the preparation
of a variety of different allylic epoxides, many of which have
been used for the synthesis of valuable target molecules with
very high selectivity [11]. Amongst various catalysts developed
so far for the selective epoxidation, salen–Mn(III) complexes
have been demonstrated to be efficient laboratory and industrial
homogeneous catalysts in the epoxidation of some unfunction-
alized alkenes using iodosylbenzene, sodium hypochlorite and
hydroperoxide as oxygen sources [12,13]. However, oxidation
reactions with environmentally-benign oxidants such as molecu-
The catalytic epoxidation of olefins has been a subject of
growing interest in the production of chemicals and fine chemi-
cals. Since epoxides are key starting materials for a wide variety
of products [1,2], much effort has been made to develop new
active and selective epoxidation catalysts for those processes
that require an elimination of by-products. Catalytic epoxida-
tion of carbon–carbon double bands is of pivotal importance in
organic chemistry, due to the wide-range applications of highly
regio-selective ring openings and other reactions of epoxides
in the synthesis of a variety of functionalized products [3–8].
It is well documented that epoxides can be easily transformed
into a large variety of compounds by means of regio-selective
ring-opening reactions.
Styrene is a major industrial chemical used extensively in
the production of plastics, resins and synthetic rubbers. Styrene
oxide can be used for producing epoxy resin diluting agent,
∗
Corresponding author. Tel.: +86 27 50865370; fax: +86 27 50865370.
E-mail address: xia1965@yahoo.com (Q.-H. Xia).
1381-1169/$ – see front matter © 2006 Elsevier B.V. All rights reserved.
doi:10.1016/j.molcata.2006.01.055

X.-H. Lu et al. / Journal of Molecular Catalysis A: Chemical 250 (2006) 62–69
63
lar oxygen and hydrogen peroxide have been intensively studied
during recent years [14–17]. Both oxidants are highly attrac-
tive since they are cheap and produce non-toxic waste prod-
ucts in contrast to many commonly employed oxidants, such
as MCPBA, PhIO, NaOCl, etc. The mild activity of hydro-
gen peroxide can be significantly enhanced by the addition of
various metal catalysts [18,19]. Oxidation of hydrocarbons is
of immense interest in the area of transition-metal complexes
mediated reactions under moderate reaction conditions. Two
main oxidation reactions viz. epoxidation and hydroxylation
have been reported using hydrogen peroxide as source of oxygen
atom and transition-metal complexes as catalysts.
It is generally recognized that manganese and iron complexes
are less environmentally damaging than other transition-metal
complexes, and that such complexes have received consider-
able attention as oxidation catalysts [20]. Manganese has been
involved in many biological processes as well, for instance as
the active site of several enzymes [21]. In order to mimic these
enzymes, many manganese complexes consisting of porphyrin,
phthalocyanin, triazamacrocycle and Schiff-base ligands have
been synthesized and studied in connection with oxidation state
[22–24], coordination number [25] and number of manganese
site present in these biological catalysts [26].
all the catalysts containing Mn metal showed excellent catalytic
activity for the epoxidation of styrene with dilute hydrogen per-
oxide (30%) under mild conditions.
2. Experimental
2.1. Materials
The main reagents used in the synthesis of Schiff-base
complexes were salicylaldehyde (98%, Tianjin), 2-pyridine-
carboxaldehyde (99%, Acros) and 2-hydroxy-1-naphthaldehyde
(>98%, Acros), 1,2-diaminocyclohexane (99%, Aldrich), in
which salicylaldehyde was redistilled three times prior to use,
while others were directly used as received. The freshly dis-
tilled solvents were absolute ethanol, chloroform, acetone and
dichloromethane. Other reagents included sodium bicarbon-
ate (0.2 M), N,Nꢀ-dimethylformamide (99.5%), styrene (>99%),
manganese(II) acetate tetrahydrate (99%, Mn(Ac)2·4H2O),
manganese(II) sulfate monohydrate (99%, MnSO4·H2O),
cobalt(II) acetate tetrahydrate (99.5%, Co(Ac)2·4H2O), cupric
acetate monohydrate (99%, Cu(Ac)2·H2O) and anhydrous ferric
trichloride (98%, FeCl3).
In the present study, three different Schiff-base lig-
ands N,Nꢀ-bis(salicylidene)cyclohexanediamine (L1), N,Nꢀ-
bis(2-pyridine carboxalidene)cyclohexanediamine (L2) and
N,Nꢀ-bis(2-hydroxy-1-naphthalidene)cyclohexanediamine (L3)
(Scheme 1) were synthesized. Subsequently, they were coordi-
nated with metals Mn, Co, Cu and Fe to achieve six Schiff-base
complex catalysts (1, 2, 3a–d) (Scheme 2). It was observed that
2.2. Synthesis of Schiff-base ligands
2.2.1. N,Nꢀ-bis(salicylidene)cyclohexanediamine (L1)
A solution of salicylaldehyde (0.877 mmol, 0.214 g) in 10 ml
of absolute ethanol was dropwise added over 1.5 h into the solu-
tion of 1,2-diaminocyclohexane (0.438 mmol, 0.100 g) in 10 ml
of warm absolute ethanol while stirring. Then the resulting mix-
Scheme 1. Synthetic routes of ligands and salen–metal complexes.

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X.-H. Lu et al. / Journal of Molecular Catalysis A: Chemical 250 (2006) 62–69
Scheme 2. Structure of salen–metal complexes 1, 2 and 3a–d.
ture was refluxed at 78 ◦C for 5 h until the completion of reaction
(checked by TLC). Thereafter, the solvent was removed by a
rotary evaporator to receive a creamy compound, which was
further re-crystallized in chloroform to obtain pure yellowish
crystal ligand L1 with a yield of 92.0% (Table 1).
(150 MHz): δ, 24.640 (C-1 (cis)) ↔ 22.962 (C-1 (trans)), 33.562
(C-2 (cis)) ↔ 31.148 (C-2 (trans)), 73.094 (C-3 (cis)) ↔ 69.913
(C-3 (trans)), 117.227–117.482 (C-4 (cis)) ↔ 118.958 (C-4
(trans)), 119.046 (C-6), 119.119–119.294 (C-10), 131.927 (C-
7), 132.625 (C-5), 161.706 (C-8 (cis)) ↔ 161.432 (C-8 (trans)),
165.139–165.193 (C-9 (cis)) ↔ 164.605 (C-9 (trans)).
M.p.: 90.5 ◦C; Elemental analysis (C, H, N and O, wt.%)
calculated for C20H22N2O2 (L1): C, 74.51; H, 6.88; N, 8.69;
O, 9.93; Found: C, 75.09; H, 6.92; N, 8.36; O, 9.63; IR
2.2.2. N,Nꢀ-bis(2-pyridinecarboxalidene)-
cyclohexanediamine (L2)
1
(KBr) (cm−1): 3420, ν(OH); 1622, 1614 ν(C N)/ν(C C); H
NMR (CDCl3) (600 MHz): δ, 1.754–1.477 (m, 6H, H-1, H-2),
∼1.903 (m, 1H, H-2ꢀ (cis)) ↔ ∼1.971 (m, 1H, H-2ꢀ (trans)),
∼3.314 (d, 1H, H-3 (cis)) ↔ 3.607 (d, 1H, H-3 (trans)), 6.779
(d, 1H, H-4 (cis)) ↔ ∼6.795 (d, 1H, H-4 (trans)), ∼6.844
(d, 1H, H-6 (cis)) ↔ ∼6.879 (d, 1H, H-6 (trans)), ∼6.916 (t,
1H, H-5 (cis)) ↔ ∼7.140 (t, 1H, H-5 (trans)), ∼7.246 (t, 1H,
H-7 (cis)) ↔ ∼7.285 (t, 1H, H-7 (trans)), 8.254 (bs, 1H, H-
8 (cis)) ↔ 8.315 (bs, 1H, H-8 (trans)), 13.321 (s, 1H, –OH
A solution of 2-pyridinecarboxaldehyde (0.785 mmol,
0.168 g) in 10 ml of absolute ethanol was dropwise added over
1.5 h into the solution of 1,2-diaminocyclohexane (0.392 mmol,
0.089 g) dissolved in 10 ml of warm absolute ethanol while stir-
ring. Subsequently, the resultant mixture was treated for 6 h at
78 ◦Cuntilthecompletionofreaction(monitoredbyTLC). After
that, the solvent was evaporated off by a rotary evaporator to
yield a creamy compound, which was further purified by re-
crystallization in chloroform to obtain a pure yellowish crystal
ligand L2 with a yield of 73.1% (Table 1).
1
(cis)) ↔ 13.469 (s, 1H, –OH(trans)); 13C{ H} NMR (CDCl3)
M.p.: 129.1 ◦C; Elemental analysis (C, H and N, wt.%) cal-
culated for C18H20N4 (L2): C, 73.94; H, 6.89; N, 19.16; Found:
C, 74.19; H, 7.04; N, 18.67; IR (KBr) (cm−1): 1645 ν(C N);
1H NMR (CDCl3) (600 MHz): δ, 1.528–1.515 (m, 6H, H-1, H-
2), 1.846 (m, 1H, H-2ꢀ (cis)) ↔ ∼1.882 (m, 1H, H-2ꢀ (trans)),
3.544–3.529 (d, 2H, H-3), 7.230–7.210 (d, 2H, H-5), 7.265 (d,
2H, H-6), 7.656–7.631 (d, 2H, H-7), 7.889–7.875 (d, 2H, H-
4), 8.311 (bs, 1H, H-8 (cis)) ↔ ∼8.552 (bs, 1H, H-8 (trans));
Table 1
Isolated yield of Schiff-base ligands and complexes
Sample
Schiff-base ligand yield (wt.%)
Complex yield (%)
1
2
3a
3b
3c
3d
92.0
73.1
97.8
65.0
95.0
97.0
80.0
93.6
92.0
13C{ H} NMR (CDCl3) (150 MHz): δ, 24.801 (C-1), 33.173
1
(C-2), 73.995 (C-3), 121.781 (C-7), 124.890 (C-5), 136.863 (C-
6), 149.672 (C-4), 155.096 (C-9), 161.897 (C-8).
Samples 3a–d used the same ligand L3.

X.-H. Lu et al. / Journal of Molecular Catalysis A: Chemical 250 (2006) 62–69
65
2.2.3. N,Nꢀ-bis(2-hydroxy-1-naphthalidene)-
cyclohexanediamine (L3)
ν(C C); 1255, 1311 ν(CO); 570 ν(Mn O), 426 ν(Mn N).
UV–vis (toluene) λmax (nm): 261, 275, 283, 298.
Catalyst 2: {[bis(2-pyridinecarboxlidene)-1,2-cyclohexa-
nediaminato]dioxo-manganese}. IR (KBr) (cm−1): 1645
ν(C N); 480 ν(Mn N). UV–vis (toluene) λmax (nm): 259, 266,
282, 326.
A solution of 2-hydroxy-1-naphthaldehyde (0.570 mmol,
0.198 g) in 10 ml of absolute ethanol was added dropwise
over 1.5 h into a stirred solution of 1,2-diaminocyclohexane
(0.285 mmol, 0.065 g) in 10 ml of warm absolute ethanol. Then,
the resulting cloudy mixture was refluxed at 78 ◦C for 6 h until
the completion of reaction (checked by TLC). The precipitate
was recovered by filtration. The filtrate was further concentrated
to yield a yellowish solid powder. Thereafter, the recovered pre-
cipitate and solid powder were combined to undergo a further
purification by re-crystallization in chloroform. The pure prod-
uct was a yellow crystal with a yield of 97.8%, designated as L3
(Table 1).
Catalysts 3a–d: {[bis(2-hydroxynaphthylidene)-1, 2-cyclo-
hexanediaminato]dioxo-M (M = Mn, Co, Cu, Fe)} (designated
as 3a, 3b, 3c and 3d). IR (KBr) (cm−1): 3a—1622, ν(C N);
1247, 1311 ν(C O); 567 ν(Mn O), 420 ν(Mn N); 3b—1618,
ν(C N); 1247, 1311 ν(C−O); 570 ν(Co O), 451 ν(Co N);
3c—1618, ν(C N); 1247, 1311 ν(C O); 555 ν(Cu O), 408
νCu N; 3d—1616, ν(C N); 1247, 1311 ν(C O); 553 ν(Fe O),
410 ν(Fe N). UV–vis (toluene) λmax (nm): 3a—260, 263,
275, 281, 284, 299; 3b—263, 276, 280, 304, 322, 365,
381, 424; 3c—264, 282, 305, 319, 384,402; 3d—276, 282,
306, 378.
M.p.: 198.4 ◦C; Elemental analysis (C, H, N and O, wt.%)
calculated for C28H26N2O2 (L3): C, 79.59; H, 6.20; N, 6.63;
O, 7.57; Found: C, 80.07; H, 6.25; N, 6.15; O, 7.53; IR (KBr)
1
(cm−1): 3420, ν(OH); 1622, ν(C N); 1247, 1311 ν(CO); H
2.4. Structural characterization of ligands and complexes
NMR (CDCl3) (600 MHz): δ, 1.786–1.525 (m, 6H, H-1, H-2),
∼2.026 (m, 1H, H-2ꢀ (cis)) ↔ ∼2.217 (m, 1H, H-2ꢀ (trans)),
3.445 (d, 1H, H-3 (cis)) ↔ 3.860 (d, 1H, H-3 (trans)), 6.856
(d, 1H, H-7 (cis)) ↔ ∼6.912 (d, 1H, H-7 (trans)), ∼7.132
(t, 1H, H-4 (cis)) ↔ ∼7.198 (t, 1H, H-4 (trans)), ∼7.237 (t,
1H, H-8 (cis)) ↔ ∼7.293 (t, 1H, H-8 (trans)), ∼7.353 (d,
1H, H-9 (cis)) ↔ ∼7.461 (d, 1H, H-9 (trans)), ∼7.468 (d,
1H, H-6 (cis)) ↔ ∼7.640 (d, 1H, H-6 (trans)), ∼7.725 (d,
1H, H-5 (cis)) ↔ ∼7.822 (d, 1H, H-5 (trans)), 8.759 (bs, 1H,
H-10 (cis)) ↔ 8.879 (bs, 1H, H-10 (trans)), 14.659 (s, 1H,
The differential scanning calorimetric analysis of N,Nꢀ-bis-
(salicylidene)cyclohexanediamine (L1), N,Nꢀ-bis(2-pyridine-
carboxalidene)cyclohexanediamine (L2) and N,Nꢀ-bis(2-hydr-
oxy-1-naphthalidene)cyclohexanediamine (L3) was performed
by a Shimadzu DSC-60 instrument (differential scanning
calorimeter) in the range of room temperature −400 ◦C. Ele-
mental analyses (C, H, N and O) of the Schiff-base ligands
were conducted on an Elementar VarioEL-III instrument. IR
spectra (KBr pellets) were recorded on a Shimadzu IR Prestige-
21 Fourier Transform Infrared Spectrometer. UV–vis spectra of
samples in toluene were determined with a Shimadzu UV-2550
spectrometer. 1H NMR spectra (600 MHz) and 13C NMR spec-
tra (150 MHz) of samples dissolved in CDCl3 were measured
on a Varian Inova-600 (600 MHz) NMR instrument using TMS
((CH3)4Si) as an internal standard of chemical shifts (ppm).
–OH (cis)) ↔ 14.793 (s, 1H, –OH(trans)); 13C{ H} NMR
1
(CDCl3) (150 MHz): δ, 24.676 (C-1), 33.192 (C-2), 69.587
(C-3 (cis)) ↔ 65.464 (C-3 (trans)), 107.561 (C-14), 118.857
(C-4), 123.320–123.288 (C-7 (cis)) ↔ 123.778 (C-7 (trans)),
127.102 (C-9), 127.011 (C-8), 128.273 (C-6), 129.541 (C-12
(cis)) ↔ 129.328 (C-12 (trans)), 133.844 (C-5 (cis)) ↔ 133.659
(C-5 (trans)), 137.071 (C-13 (cis)) ↔ 136.939 (C-13 (trans)),
59.677 (C-11), 172.605 (C-10).
2.5. Epoxidation of styrene
2.3. Preparation of catalysts 1, 2 and 3a–d
2.5.1. Blank reactions without any catalyst
An appropriate amount of Schiff-base ligands L1
(0.253 mmol, 0.081 g) and L2 (0.253 mmol, 0.074 g) was
separately dissolved in absolute ethanol in nitrogen atmosphere
under vigorous stirring. Then was added Mn(Ac)2·4H2O
(0.253 mmol, 0.062 g), and the resulting mixture was refluxed
for 7 h in nitrogen atmosphere until the complete reaction of
ligand compound (checked by TLC). Finally, the precipitate
complexes (1 and 2) were recovered by filtration, and washed
several times with absolute ethanol and dichloromethane,
dried in vacuum at 65 ◦C for 5 h, and again purified by
re-crystallization in acetone. Syntheses of the catalysts 3a–d
with Schiff-base ligand L3 and various metallic salts were
similar to the procedure performed in the preparation of
complexes 1 and 2. The yields of various complexes are listed in
Table 1.
Reactions were run in a round-bottom flask at 0 ◦C by mixing
styrene (0.384 mmol), 1.53 ml of 0.2 M NaHCO3 solution (pH
8) and 2.0 ml of solvent, and then adding dropwise 1.68 mmol
30% H2O2 under stirring. The solvent was chosen from vari-
ous solvents DMF, CH3CN, CH2Cl2 and ButOH. Each reaction
was sustained for 3, 10, 17, 20, and 30 h before conducting GC
analysis.
2.5.2. Epoxidation with metal salts catalysts
The epoxidation of styrene with H2O2 catalyzed by metal
salts was carried out according to the procedure: 0.634 mmol
of MnSO4·H2O catalyst (or 0.896 mmol of Mn(Ac)2·4H2O),
1.53 ml of 0.2 M NaHCO3 solution (pH 8), 2.0 ml DMF and
0.384 mmol of styrene were mixed in a round-bottom flask
at 0 ◦C while stirring; subsequently, 1.68 mmol of aqueous
30%H2O2 was added within 1.5 h by a dropping funnel. Each
reaction was sustained for 3, 4.5, 6, 9, and 24 h prior to GC
determination.
Analytical data for all the complexes are as follows:
Catalyst 1: {[bis(salicylidene)-1,2-cyclohexanediaminato]-
dioxomanganese}. IR (KBr) (cm−1): 1622 ν(C N), 1614

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X.-H. Lu et al. / Journal of Molecular Catalysis A: Chemical 250 (2006) 62–69
2.5.3. Epoxidation with catalysts 1, 2, and 3a–d
A mixture of 0.02 mmol catalyst, 1.53 ml of 0.2 M NaHCO3
solution (pH 8), 2.0 ml DMF and 0.384 mmol of styrene was
stirred in a round-bottom flask at 0 ◦C for 10 min while stirring
before 1.68 mmol of aqueous 30% H2O2 was dripped within
1.5 h. Similarly, each reaction was sustained for 3.5, 6, 9, 20 and
30 h prior to GC determination.
2.5.4. GC analysis of products
The quantitative GC analysis of the epoxidized products
was carried out by a GC-900A equipped with an FID detec-
tor and a SUPELCO BETA-DEXTM 120 capillary column
(30 m × 0.25 mm × 0.25 ␮m). Hydrogenwasusedascarriergas.
GC error for the determination was within 3%.
3. Results and discussion
As shown in Section 2, the elemental contents (C, H, N and
O) of all the ligands synthesized are relatively close to those
calculated based on molecular formulae proposed, which indi-
cates the correctness of molecular compositions proposed. IR
spectra of synthesized ligands (L1 and L3) and complexes (1
and 3a) are presented in Figs. 1 and 3. The strong band emerg-
ing at 3440 cm−1 in the IR spectra of Schiff-base ligands L3
can be assigned to the vibration of ν(O H). Upon complex-
ation, the broad ν(O H) band will disappear, indicating the
occurrence of coordination of naphtholic oxygen to metal. In
the spectra of ligands L1 and L3, the complexation leads to the
splitting of the characteristic imido (C N) bands in the region
of 1610–1640 cm−1 and the occurrence of more or less shift-
ing of these bands, as has been reported in the literature [27].
Fig. 2. IR spectra of 2-pyridinecarboxaldehyde and Schiff-base ligand L2.
The pattern of νC N absorbance suggests non-equivalence of
two imido groups and possible deviation of C N group from the
aromatic ring plane. Upon coordination with metals, the absorp-
tion bands near 567 and 420 cm−1 are observed from the spectra
of complexes 1 and 3a, ascribable to Mn O and Mn N bands
[28,29].
Fig. 2 shows IR spectra of 2-pyridinecarboxaldehyde and
Schiff-base ligand L2. Prior to coordination, the ligand L2 shows
two characteristic C O bands at about 1400 and 1700 cm−1
,
and two C−H bands of the aldehyde group near 2700 and
2840 cm−1; however, upon condensation with diamine these
peaks disappear totally. Instead, a characteristic C N band
emerges at 1640 cm−1 for ligand L2 and its complexes, dis-
tinctly different from the cases of ligands L1 and L3. Note
that this peak cannot be observed in the IR spectra of 2-
pyridinecarboxaldehyde.
1H NMR spectra of synthesized L1–L3 were measured in
CDCl3. For Schiff-base ligand L1, twin H NMR peaks at
13.321 (cis) and 13.469 (trans) ppm are assigned to phenolic
OH protons. The H NMR peak at 9.906 ppm resulting from
CH O proton of salicylaldehyde, disappears completely for
ligand L1, due to the condensation between salicylaldehyde
and 1,2-diaminocyclohexane. The peaks at 6.779–7.285 ppm
are attributed to aromatic protons ( C H), and those at
1.477–1.754 ppm, 1.903 (cis) and 1.971 ppm (trans) to the pro-
tons of CH2 in the ring of cyclohexane. Two proton peaks at
8.254 (cis) and 8.315 (trans) ppm can be assigned to CH N
protons, consistent with the observations made by Belokon et
al. [30], who had assigned H NMR peaks of 2H at 8.2–8.4 ppm
in the salen compounds to CH N protons. In a similar man-
ner to Schiff-base ligand L1, the H NMR peak at 10.102 ppm
Fig. 1. IR spectra of Schiff-base ligand L1, complex 1 and Mn(OAc)2·4H2O
(peak ␤: 569 cm−1 Mn O; peak ␥: 426 cm−1 Mn N).

X.-H. Lu et al. / Journal of Molecular Catalysis A: Chemical 250 (2006) 62–69
67
Fig. 3. IR spectra of Schiff-base ligand L3, complex 3a and Mn(OAc)2·4H2O
(peak ␤: 567 cm−1 Mn O; peak ␥: 420 cm−1 Mn N).
Fig. 4. UV–vis spectra of salen complexes coordinated with different metals
(3a–d).
resulting from CH O proton of 2-pyridinecarboxaldehyde, dis-
appears totally for ligand L2. The peaks at 7.210–7.889 ppm
are attributed to C H of the pyridyl group, and those at
1.528–1.515 ppm, 1.846 (cis) ppm and 1.882 (trans) ppm to the
protons of CH2 in the ring of cyclohexane. The bands at 8.311
(bs, 2H, H-8 (cis)) and 8.552 (bs, 2H, H-8 (trans)) ppm in the H
NMR spectra are due to CH N protons.
for complex 3b, six bands at 264, 282, 305, 319, 384, 402 nm
for complex 3c, and four bands at 276, 282, 306, 378 nm for
complex 3d.
The effect of various solvents such as DMF, CH3CN, CH2Cl2
and ButOH on the epoxidation of styrene with 30% aqueous
hydrogen peroxide was investigated. When no catalyst was
added, the blank reaction with solvent DMF exhibited extremely
low reactivity towards the yield of styrene oxide from styrene
substrate; within 10 h almost no conversion occurred. When the
reaction time was prolonged to 17 h, the yield of styrene oxide
increased to 42.3 mol%. Along with increasing time to 20 h, the
yield of styrene oxide was rapidly elevated to 70 mol%. How-
ever, in the same reaction time span (20 h) using CH3CN as the
solvent only 0.55 mol% of styrene oxide was yielded, and in the
presence of CH2Cl2 and ButOH there was no epoxide detectable
from the reaction mixture. The results propose that DMF was
a preferred solvent for the epoxidation of styrene under our
experimental conditions. Thus, in unspecified cases DMF was
chosen as the solvent to test the catalytic activity of various
catalysts.
Fig. 5 shows the relationship between the yield of styrene
oxide and the reaction time, where the reaction temperature was
fixed at 0 ◦C. It is evident that for blank reaction, the yield of
70 mol% was achieved within 20 h, and further increasing the
reaction time to 30 h merely exhibited an extremely small incre-
ment to 70.2 mol%. Very notably, both catalysts Mn(Ac)2·4H2O
and MnSO4·H2O showed relatively high activity for this reac-
tion with almost the same conversion tendency, considerably
similar to the results reported by Burgess et al. [32–34]. At 3 h,
the yield of 28.3–32.2 mol% was achieved; thereafter, it lin-
For Schiff-base ligand L3, twin H NMR peaks at 14.659
(cis) ppm and 14.793 (trans) ppm can be attributed to naph-
tholic OH protons. The disappearance of the H NMR peak
at 10.797 ppm resulting from CH O proton of 2-hydroxy-1-
naphthaldehyde, and the emergence of two new proton peaks
at 8.759 (cis) ppm and 8.879 (trans) ppm assigned to CH N
protons proves the condensation reaction between 2-hydroxy-
1-naphthaldehyde and 1,2-diaminocyclohexane. The peaks at
6.856–7.822 ppm are attributable to aromatic protons ( C H),
andthoseat1.525–1.786 ppm, 2.026(cis)ppm, and2.217(trans)
ppmtotheprotonsofCH2 intheringofcyclohexane(Scheme1).
UV–vis spectra of salen–Mn complexes (1, 2 and 3a) show
absorbance bands at 261, 275, 283, 298 nm for complex 1, at
259, 266, 282, 326 nm for complex 2, and at 260, 263, 275,
281, 284, 299 nm for complex 3a, respectively, slightly different
from those at 259, 263, 282, 305, 313 nm of the corresponding
metal salt Mn(Ac)2·4H2O [31], which shows the complexation
between ligands and metal Mn ions. Fig. 4 compares UV–vis
spectra of salen complexes (3a–d) coordinated with different
metals Mn, Co, Cu, and Fe. Evidently, these complexes coor-
dinated with various metal ions possess considerably different
ultraviolet absorbance characteristics. For example, there are six
recognizable bands at 260, 263, 275, 281, 284, 299 nm for com-
plex 3a, eight bands at 263, 276, 280, 304, 322, 365, 381, 424 nm

68
X.-H. Lu et al. / Journal of Molecular Catalysis A: Chemical 250 (2006) 62–69
Table 2
Catalytic activity of different metal centers
Metal
Epoxide yield (mol%)
3.5 h
6 h
9 h
20 h
30 h
Mn
Co
Cu
Fe
69.9
34.8
–
85.6
45.4
–
88.9
46.0
–
89.9
46.2
–
91.2
46.2
–
–
–
–
–
–
–: Not detectable. Epoxide yield (mol%) was determined by GC.
The yield of epoxide on salen–Co complex catalyst 3b is about
half of that on salen–Mn catalyst 3a. In 3.5 h, the catalyst 3b
yielded 34.8 mol% of styrene oxide, while at this time the yield
of styrene oxide reached 70 mol% on the catalyst 3a. Similarly,
conversion equilibrium could be reached at about 9 h for both
catalysts; however, the highest yield on the catalyst 3b was only
46.2 mol%, but that on the catalyst 3a was 91.2 mol%. Regard-
less of the reaction time, Fe-complex and Cu-complex did not
show any catalytic activity for the epoxidation of styrene with
H2O2 under identical conditions. These results could be due to
several factors, such as the nature of metals, the ability of coor-
dination with reactants, the solubility of complexes in solvent,
etc.
Fig. 5. The relationship between epoxide yield and reaction time with or with-
out catalysts (ꢀ: blank reaction; ꢁ: Mn(OAc)2·4H2O; ꢂ: MnSO4·H2O; ×:
salen–Mn complex 3a).
4. Conclusions
early increased to 73.0–78.2 mol% within only 1 h. When the
reaction time was prolonged to 6 h, the yield of styrene oxide
continuously increased to 81.2–82.5 mol%. Further increasing
the reaction time to 9 h slightly increased the yield of styrene
oxide to 83.2–83.9 mol%. Then, when the reaction time was
prolonged to 24 h, the yield of epoxide slowly reached about
85.3 mol%. As expected, the salen–Mn complex 3a exhibited an
excellent activity for the conversion of styrene-to-styrene oxide.
Within 3.5 h, the yield of styrene oxide was reached 70.0 mol%.
When the reaction time was increased to 6 h, the yield linearly
increased to 85.6 mol%. The epoxidation of styrene with H2O2
catalyzed by complex 3a approached conversion equilibrium
at about 9 h to achieve 89.9 mol% of the yield. After that, even
whenthereactiontimewasprolongedto30 h, theyieldofstyrene
oxide was merely elevated to 91.2 mol%.
It was observed that the efficiency of catalysts was strongly
dependent on the structure of Schiff-base ligands. When N,Nꢀ-
bis(salicylidene)cyclohexanediamine ligand L1 and N,Nꢀ-bis(2-
pyridinecarboxalidene)cyclohexanediamine ligand L2 were
coordinated with manganese, the resulting complex catalysts
1 and 2 reached the epoxide yield of 71.7 and 53.9 mol%
within 6 h, notably lower than 85.6 mol% of the catalyst 3a and
even much lower than 81.2–82.5 mol% of inorganic precursors
Mn(Ac)2·4H2O and MnSO4·H2O. This could be due to the poor
coordination stability of complex catalysts 1 and 2, which easily
underwent decomposition in the reaction environment, similar
to the observations made by Ambroziak et al. [35].
Several Schiff-base complexes have been synthesized by the
condensation of 1,2-diaminocyclohexane with salicylaldehyde,
2-pyridinecarboxaldehyde, and 2-hydroxy-1-naphthaldehyde,
followed by the metallation with manganese (1, 2, 3a), cobalt
(3b), copper (3c) and iron (3d) salts. The structure of ligands
L1–L3 and complexes 1, 2, 3a–d were confirmed by means
of DSC, FTIR, elemental analysis, H NMR, 13C NMR and
1
UV–vis spectroscopy. The selective epoxidation of styrene with
dilute hydrogen peroxide (30%) was performed in the presence
of Schiff-base complex catalysts (1, 2, 3a–d). Under our experi-
mental conditions, Schiff-base Mn complex (3a) resulting from
N,Nꢀ-bis(2-hydroxy-1-naphthalidene)cyclohexanediamine lig-
and (L3) showed effectiveness on the catalytic epoxida-
tion of styrene, leading to the highest yield of styrene
oxide of 91.2 mol%. However, the catalytic efficiency of
other salen–Mn complexes 1 and 2 derived from ligands
N,Nꢀ-bis(salicylidene)cyclohexanediamine (L1) and N,Nꢀ-bis(2-
pyridinecarboxalidene)cyclohexanediamine (L2) was poor,
even notably lower than that of simple inorganic salt catalysts
Mn(Ac)2·4H2O and MnSO4·H2O.
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