Ionic liquid-functionalized salen Mn(III) complexes as tunable separation catalysts for enantioselective epoxidation of styrene

Article abstract of DOI:10.1016/j.jcat.2008.02.015

A series of novel chiral salen Mn(III) complexes functionalized by ionic liquid (IL) of 1-propylamine-3-methylimidazolium tetrafluoroborate were synthesized through the reaction of amino group ({single bond}NH₂) of the IL with chloromethyl ({single bond}CH₂Cl) in the salen ligand at one side of the 5 position (complex 2) and at two sides of the 5, 5′ position (complex 3), as well as by direct axial coordination between {single bond}NH₂ of the IL and metal center of the salen Mn(III) complex (complex 4). All of the synthesized complexes were well characterized, and their performance in the enantioselective epoxidation of styrene was investigated systematically. Under optimum reaction conditions, a 99% styrene epoxide yield with 50% enantiometric excess (ee) could be obtained over the complex 2. Furthermore, the IL-functionalized chiral salen Mn(III) complexes of 2 and 3 could be conveniently separated from the reaction system by simple precipitation in hexane and subsequently used without significant loss of activity and enantioselectivity.

Full text of DOI:10.1016/j.jcat.2008.02.015

Journal of Catalysis 255 (2008) 287–295
www.elsevier.com/locate/jcat
Ionic liquid-functionalized salen Mn(III) complexes as tunable separation
catalysts for enantioselective epoxidation of styrene
Rong Tan a, Donghong Yin a,b,∗, Ningya Yu a,∗, Yong Jin a,b, Haihong Zhao a, Dulin Yin a
a
Institute of Fine Catalysis and Synthesis, Key Laboratory of Chemical Biology and Traditional Chinese Medicine Research (Ministry of Education),
Hunan Normal University, Changsha 410081, Hunan, China
b
Technology Center, China Tobacco Hunan Industrial Corporation, No. 426 Laodong Road, Changsha, Hunan, 410014, China
Received 1 November 2007; revised 29 January 2008; accepted 17 February 2008
Available online 19 March 2008
Abstract
A series of novel chiral salen Mn(III) complexes functionalized by ionic liquid (IL) of 1-propylamine-3-methylimidazolium tetrafluoroborate
were synthesized through the reaction of amino group (–NH ) of the IL with chloromethyl (–CH Cl) in the salen ligand at one side of the
2
2
ꢀ
5 position (complex 2) and at two sides of the 5, 5 position (complex 3), as well as by direct axial coordination between –NH of the IL and
2
metal center of the salen Mn(III) complex (complex 4). All of the synthesized complexes were well characterized, and their performance in the
enantioselective epoxidation of styrene was investigated systematically. Under optimum reaction conditions, a 99% styrene epoxide yield with
50% enantiometric excess (ee) could be obtained over the complex 2. Furthermore, the IL-functionalized chiral salen Mn(III) complexes of 2
and 3 could be conveniently separated from the reaction system by simple precipitation in hexane and subsequently used without significant loss
of activity and enantioselectivity.
© 2008 Elsevier Inc. All rights reserved.
Keywords: Chiral salen Mn(III) complex; Ionic liquid; Styrene; Enantioselective epoxidation
1. Introduction
branes [11]), thereby creating chiral heterogeneous catalyst that
can be readily recovered from reaction mixtures. Unfortunately,
despite their excellent performance in easy separation, the het-
erogenized catalysts often suffer from decreased catalytic effi-
ciency, likely due to the leaching of salen Mn(III) complexes
during reaction and/or the inaccessibility of the reagents to the
reactive centers [12]. Therefore, it is highly desirable to de-
velop a novel method to make the salen Mn(III)-type catalyst
effective and recoverable. More recently, we have synthesized
a polymeric chiral salen Mn(III) complex with chiral diamine
bridging [13]. The complex can be used as solvent-regulated
phase-transfer catalyst in the enantioselective epoxidation of
styrene and shows both facile separation and high catalytic ef-
ficiency, bearing comparison with the corresponding homoge-
neous chiral salen Mn(III) complex. Such a tunable separation
catalyst gives rise to an alternative approach to the heteroge-
nization of homogeneous catalysts.
Catalytic enantioselective epoxidation of alkenes presents
a powerful strategy for the synthesis of enantiometrically en-
riched epoxides, which are used as the important building
blocks in the synthesis of many pharmaceuticals and fine chem-
icals. Of several catalytic methods, the enantioselective epoxi-
dation of unfunctionalized alkenes catalyzed by homogeneous
chiral salen Mn(III) complexes developed by Zhang et al. is one
of the most relevant methods [1–3]. However, separation and re-
cycling of the homogeneous catalysts often prove difficult. Re-
cently, much effort has been aimed at overcoming this difficulty
by heterogenizing the homogeneous chiral salen Mn(III) com-
plexes (e.g., immobilization of salen Mn(III) complex on meso-
porous materials [4,5], activated carbon [6,7], zeolite Y [8],
poly-system [9], clay compounds [10], and inorganic mem-
Ionic liquids (ILs) with intriguing physical and chemical
properties have received considerable attention in several sci-
entific disciplines over the last several years [14]. They gen-
*
Corresponding authors. Fax: +86 731 8872531.
E-mail addresses: yindh@hunnu.edu.cn (D. Yin),
yuningya@yahoo.com.cn (N. Yu).
0021-9517/$ – see front matter © 2008 Elsevier Inc. All rights reserved.
doi:10.1016/j.jcat.2008.02.015

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R. Tan et al. / Journal of Catalysis 255 (2008) 287–295
erally are used as green solvents and/or catalysts in various
organic transformations [15]. Specifically, attempts have been
made to use ILs as reaction media for improving the ac-
tivity and separation of the catalysts in the enantioselective
epoxidation of olefins [16]; for example, Song and Roh [17]
investigated the performance of a homogeneous chiral salen
Mn(III) catalyst for enantioselective epoxidation of alkenes in
a [bmim][PF6]-containing reaction medium and found that the
use of [bmim]PF6 gave an elevated yield and ee value. Like-
wise, we found that some ILs are conducive to the epoxidation
of cyclohexene catalyzed by homogeneous salen Mn(III) com-
plexes [18]. However, disregarding the tunable dissolvability of
ILs determined by the structure of cation and the type of anion,
the bonding of ILs onto the structure of salen Mn(III) complex
to prepare tunable separation catalysts has not been reported to
date.
mass spectrometer (Applied Biosystems) with an electrospray
interface (ESI-MS/MS in MeOH). The operating conditions in
positive ionization mode were optimized using a +5500 V ion
spray voltage, 8 psi for curtain gas, 8 psi for drying gas heated
to 100 ◦C, and 7 psi for nebulizing gas. Elemental analyses
of C, H, and N were carried out on a Vario EL III elemen-
tal analyzer. The optical rotation of catalysts was measured in
dichloromethane on a WZZ-2A automatic polarimeter. The Mn
ion content was measured by the compleximetry method with
ethylenediamine tetraacetic acid (EDTA) as described previ-
ously [22]. The ee value for styrene epoxide and the product
analysis were determined using an Agilent Technologies 6890N
gas chromatograph equipped with 19091G-B213 chiral capil-
lary column (30 m × 0.32 mm × 0.25 µm) and a flame ioniza-
tion detector (FID).
Based on our previous work [13,15,18] and the concept of
“one-phase catalysis and two-phase separation” [19], in the
present study, an IL of 1-propylamine-3-methylimidazolium
tetrafluoroborate that is miscible with dichloromethane and im-
miscible with hexane was synthesized and successively bonded
onto a chiral salen Mn(III) complex. Different strategies have
been used to attach the IL onto different positions of the chi-
ral salen Mn(III) complex. All IL-functionalized chiral salen
Mn(III) complexes show particular properties of tunable sepa-
ration in the enantioselective epoxidation of styrene, and their
catalytic performance depends strongly on the location of the
attachment of chiral salen Mn(III) complex and the IL.
2.2. Preparation of ionic liquid
The IL of 1-propylamine-3-methylimidazolium tetrafluoro-
borate was synthesized as described previously [23] (see
Scheme 1). First, 100 mmol of 1-methylimidazole and 100
mmol of 3-bromopropylamine were dissolved in 50 mL of dry
ethanol under stirring. The resulting mixture was refluxed for
24 h under nitrogen protection. After removal of ethanol in
vacuum, the solid residue was dissolved in water. Then the
pH value of the solution was adjusted to 10 by the addition of
potassium hydroxide. The obtained solution was concentrated
under vacuum and then extracted with ethanol–tetrahydrofuran.
Subsequently, ion exchange with sodium tetrafluoroborate
(110 mmol) in ethanol/water was performed for 48 h at am-
bient temperature. After the solvent was removed, the result-
ing mixture was filtered, after which the filtrate was dried
under vacuum at 80 ◦C to give a pale-yellow viscous liquid
of 1-propylamine-3-methylimidazolium tetrafluoroborate (IL)
2. Experimental
2.1. Materials and methods
L(+)-tartaric acid, 1,2-diaminocyclohexane, 3-chloroper-
oxybenzoic acid (m-CPBA), 3-bromopropylamine, 4-phenyl-
pyridine N-oxide (4-PhPyNO), 4-methylmorpholine-N-oxide
(NMO), and 1-methylimidazole were purchased from Acros.
Pyridine N-oxide (PyNO) and sodium tetrafluoroborate were
obtained from Fluka. 2-tert-butyl phenol and 4-(3-phenylpro-
pyl) pyridine N-oxide (4-PPPyNO) were purchased from Alfa
Aesar and Aldrich, respectively. Other commercially available
chemicals were laboratory-grade reagents from local suppliers.
(R, R)-1,2-diammoniumcyclohexane mono-(+)-tartrate salt, 3-
tert-butyl-2-hydroxybenzaldehyde, and 3-tert-butyl-5-(chloro-
methyl)-2-hydroxybenzaldehyde were synthesized according to
the procedures described previously [3,20,21].
1
(13.85 g, yield of 65%). H NMR (CDCl3, 400 MHz): δ ppm
8.77 (s, 1 H), 7.50 (d, 1 H), 7.42 (d, 1 H), 4.31 (t, 2 H), 4.17
(t, 2 H), 2.70 (m, 2 H), 0.86 (t, 3 H). FT-IR (KBr): 3426, 3143,
2955, 2739, 2643, 2504, 2015, 1574, 1506, 1457, 1339, 1285,
1232, 1169, 1085, 1021, 831, 756, 621 cm−1
.
2.3. Preparation of IL-functionalized chiral salen ligand 1
(IL-CL1) and IL-functionalized chiral salen ligand 2 (IL-CL2)
The preparation of IL-CL1 and IL-CL2 is outlined in
Scheme 2.
FT-IR spectra were obtained as potassium bromide pellets
with a resolution of 4 cm−1 and 32 scans in the range of
400–4000 cm−1 using an AVATAR 370 Thermo Nicolet spec-
trophotometer. The ultraviolet–visible light (UV–vis) spectra
were recorded on a UV–vis Agilent 8453 spectrophotometer.
The solution of samples in dichloromethane (ca. 1.0 mM) was
poured into a 1-cm quartz cell for UV–vis adsorption with
dichloromethane as the reference. 1H NMR spectra of the
samples were recorded with a Varian-400 spectrometer using
tetramethylsilane (TMS) as an internal reference. Mass spec-
trometry analyses were performed using an API 3000 tandem
2.3.1. Synthesis of chiral half-unit of ligand 1 (HL1) and
chiral half-unit of ligand 2 (HL2)
In this procedure, 11.2 mmol of (R, R)-1,2-diammonium-
cyclohexane mono-(+)-tartrate salt and 22.5 mmol of potas-
sium carbonate were dissolved in 15 mL of distilled water,
followed by the addition of 6 mL of dry ethanol under vig-
orous stirring. The resulting cloudy mixture was refluxed for
another 2 h. The liberated diamine was extracted with chlo-
roform (4 × 5 mL). Then 20 mL of chloroform containing
11.2 mmol of 3,5-di-tert-butyl salicylaldehyde (for synthe-
sis of the compound HL1) or 3-tert-butyl-5-(chloro-methyl)-

R. Tan et al. / Journal of Catalysis 255 (2008) 287–295
289
Scheme 1. Synthesis of the ionic liquid (IL) containing primary amine.
Scheme 2. Synthesis of chiral salen ligand CL1 and chiral salen ligand CL2.
2-hydroxybenzaldehyde (for synthesis of the compound HL2)
was added dropwise to the solution of diamine at 0 ◦C un-
der stirring for 48 h. On filtration, precipitate was collected
and recrystallized from ethanol to give a pale-yellow powder
of HL1 (3.52 g, 95%) or HL2 (3.23 g, 89%). HL1: Calc. for
C21H34N2O: C, 76.31; H, 10.37; N, 8.48. Found: C, 76.43; H,
10.32; N, 8.41%. 1H NMR (CDCl3, 400 MHz): δ ppm 13.74 (s,
1 H), 8.30 (s, 1 H), 7.21 (s, 1 H), 7.00 (s, 1 H), 3.34 (s, 1 H),
1.95–1.46 (m, 10 H), 1.40 (s, 9 H), 1.24 (s, 9 H). FT-IR (KBr):
3440, 2953, 2864, 1630, 1593, 1469, 1439, 1391, 1361, 1325,
1270, 1252, 1240, 1202, 1174, 1135, 1085, 1063, 1037, 981,
939, 879, 862, 828, 803, 772, 731, 711, 644 cm−1. HL2: Calc.

290
R. Tan et al. / Journal of Catalysis 255 (2008) 287–295
Scheme 3. Synthesis of the IL-functionalized chiral salen Mn(III) complexes 2, 3, and 4.
for C18H27ClN2O: C, 66.96; H, 8.43; N, 8.68. Found: C, 66.89;
H, 8.52; N, 8.57%. 1H NMR (CDCl3, 400 MHz): δ ppm 13.78
(s, 1 H), 8.29 (s, 1 H), 7.28 (s, 1 H), 7.11 (s, 1 H), 4.58 (s, 2 H),
3.32 (s, 1 H), 2.05–1.44 (m, 10 H), 1.42 (s, 9 H). FT-IR (KBr):
3449, 2956, 2858, 1630, 1582, 1547, 1480, 1468, 1442, 1386,
1351, 1340, 1325, 1291, 1270, 1240, 1224, 1200, 1173, 1153,
tion. After cooling to 5 ◦C overnight, the complex thus obtained
was collected by removal of toluene, washed completely with
hexane for several times, and dried in vacuum to obtain a deep-
yellow solid of IL-CL1 (2.26 g, 91%) or IL-CL2 (3.89 g, 82%).
IL-CL1: Calc. for C40H63BF4N5O2: C, 65.56; H, 8.67; N, 9.56.
1
Found: C, 65.50; H, 8.74; N, 9.73%. H NMR (CDCl3, 400
1081, 1054, 1043, 942, 859, 823, 798, 717, 646 cm−1
.
MHz): δ ppm 14.25 (s, 1 H), 13.68 (s, 1 H), 8.93 (s, 1 H), 8.45
(s, 1 H), 8.29 (s, 1 H), 7.58 (d, 1 H), 7.47 (d, 1 H), 7.30 (d, 1 H),
7.26 (d, 1 H), 6.99 (d, 1 H), 6.89 (d, 1 H), 4.56 (s, 2 H), 4.32 (m,
2 H), 4.17 (t, 2 H), 3.60–3.34 (m, 2 H), 2.70 (m, 2 H), 2.4 (s,
1 H), 1.88–1.49 (m, 8 H), 1.40 (s, 9 H), 1.28 (s, 18 H), 0.88 (t,
3 H). FT-IR (KBr): 3443, 3144, 2956, 2864, 2013, 1630, 1547,
1530, 1468, 1439, 1390, 1363, 1313, 1269, 1252, 1240, 1204,
1173, 1135, 1084, 1037, 982, 939, 879, 828, 772, 731, 712, 642,
621 cm−1. IL-CL2: Calc. for C44H72B2F8N8O2: C, 57.52; H,
7.90; N, 12.20. Found: C, 57.41; H, 7.97; N, 12.32%. 1H NMR
(CDCl3, 400 MHz): δ ppm 13.68 (s, 2 H), 8.91 (s, 2 H), 8.42
(s, 2 H), 7.60 (d, 2 H), 7.49 (d, 2 H), 7.31 (d, 2 H), 7.23 (d,
2 H), 4.58 (s, 4 H), 4.29 (m, 4 H), 4.15 (t, 4 H), 3.55–3.30 (m,
2 H), 2.70 (m, 4 H), 2.4 (s, 2 H), 1.89–1.44 (m, 8 H), 1.32 (s, 18
H), 0.85 (t, 6 H). FT-IR (KBr): 3448, 3146, 3089, 2953, 2861,
2013, 1630, 1547, 1530, 1471, 1438, 1390, 1363, 1339, 1313,
1272, 1246, 1204, 1174, 1089, 1031, 874, 830, 772, 712, 642,
2.3.2. Synthesis of chiral salen ligand 1 (CL1) and chiral
salen ligand 2 (CL2)
In this procedure, 8 mmol of the previously obtained HL1
(for the synthesis of CL1) or HL2 (for the synthesis of CL2)
was dissolved in 20 mL of dry ethanol. The solution was added
dropwise to 20 mL of dry ethanol containing 8 mmol of 3-
tert-butyl-5-(chloro-methyl)-2-hydroxybenzaldehyde at ambi-
ent temperature. The resulting mixture was gradually heated to
60 ◦C and stirred for 8 h. The slurry thus obtained was cooled
in ice-water bath over 3 h. Then the solid obtained was fil-
tered and recrystallized from ethanol to give pale-yellow pow-
der of CL1 (3.55 g, 85%) or CL2 (3.38 g, 82%). CL1: Calc. for
C33H47ClN2O2: C, 73.51; H, 8.79; N, 5.20. Found: C, 73.58;
H, 8.75; N, 5.13%. 1H NMR (CDCl3, 400 MHz): δ ppm 14.29
(s, 1 H), 13.67 (s, 1 H), 8.44 (s, 1 H), 8.31 (s, 1 H), 7.30 (d,
1 H), 7.26 (d, 1 H), 6.99 (d, 1 H), 6.89 (d, 1 H), 4.43 (s, 2 H),
3.55–3.32 (m, 2 H), 1.97–1.46 (m, 8 H), 1.40 (s, 9 H), 1.23 (s,
18 H). FT-IR (KBr): 3446, 2954, 2865, 1630, 1591, 1479, 1468,
1439, 1391, 1362, 1271, 1253, 1241, 1202, 1174, 1135, 1085,
1037, 982, 939, 879, 828, 803, 772, 731, 711, 644 cm−1. CL2:
Calc. for C30H40Cl2N2O2: C, 67.79; H, 7.58; N, 5.27. Found:
621 cm−1
.
2.4. Preparation of IL-functionalized chiral salen Mn(III)
complexes 2, 3, and 4
The preparation of complexes 2, 3, and 4 is outlined in
Scheme 3. Under vigorous stirring, 15 mL of dry ethanol con-
taining 8 mmol of manganese acetate was added dropwise to
15 mL of dry ethanol solution containing 4 mmol of chiral salen
ligand IL-CL1 (for the synthesis of complex 2) or IL-CL2 (for
the synthesis of complex 3) at 50 ◦C under nitrogen protection.
The mixture was refluxed for 5 h, then cooled to ambient tem-
perature. Then 10 mL of ethanol containing 24 mmol of lithium
chloride was added to the mixture under stirring for 3 h. Af-
ter bubbling with a gentle stream of air for another 2 h, the
mixture was exposed to air overnight. The resulting slurry was
cooled to 5 ◦C for 2 h, filtered, and washed with 50 mL of wa-
ter. The solid thus obtained was dried under vacuum at 40 ◦C to
1
C, 67.71; H, 7.69; N, 5.32%. H NMR (CDCl3, 400 MHz):
δ ppm 13.68 (s, 2 H), 8.42 (s, 2 H), 7.31 (d, 2 H), 7.26 (d, 2 H),
4.58 (s, 4 H), 3.55–3.30 (m, 2 H), 1.89–1.44 (m, 8 H), 1.42 (s,
18 H). FT-IR (KBr): 3449, 2952, 2866, 1630, 1594, 1545, 1466,
1439, 1390, 1361, 1308, 1266, 1229, 1200, 1167, 1142, 1091,
1030, 974, 930, 871, 778, 753, 669 cm−1
.
2.3.3. Synthesis of IL-CL1 and IL-CL2
First, 15 mL of dry toluene containing 5 mmol of the CL1
(for the synthesis of IL-CL1) or the chiral salen ligand CL2
(for the synthesis of IL-CL2) was added to equiv. of IL. The
resulting mixture was refluxed for 48 h under nitrogen protec-

R. Tan et al. / Journal of Catalysis 255 (2008) 287–295
291
Scheme 4. Molecular structure of chiral salen Mn(III) complexes 1, 2, 3, and 4.
give a brown powder of IL-functionalized chiral salen Mn(III)
complex 2 (3.02 g, 87%) or 3 (3.65 g, 89%). Complex 2: FT-
IR (KBr): 3408, 3147, 2953, 2864, 2013, 1613, 1558, 1538,
1458, 1437, 1390, 1361, 1339, 1310, 1270, 1251, 1200, 1171,
1085, 1027, 877, 836, 772, 750, 645, 621, 568 cm−1; UV–vis
(CH2Cl2): 508, 434, 326 nm; Calc. for C40H61BClF4MnN5O2:
C, 58.51; H, 7.49; N, 8.53. Found: C, 58.43; H, 7.61; N, 8.46%;
621, 569 cm−1; UV–vis (CH2Cl2): 520, 447, 327 nm; Calc. for
C43H69BClF4MnN5O2: C, 59.69; H, 8.04; N, 8.09. Found: C,
59.62; H, 8.11; N, 8.13%; Mn ion content: 1.21 mmol/g (the-
oretical value: 1.23 mmol/g); ESI-MS, m/z = 866 (M+H)+,
33
599 (M-IL-Cl)+; [α]D = +544 (C = 0.04, CH2Cl2).
2.5. Enantioselective epoxidation of styrene
Mn ion content: 1.19 mmol/g (theoretical value: 1.25 mmol/g);
33
ESI-MS, m/z = 822 (M+H)+, 543 (M-CH2-IL-Cl)+; [α]D
=
The enantioselective epoxidation of styrene was typically
performed according to the following procedure. First, 0.5
mmol of styrene and a desirable amount of axial base were
added to 1 mL of dichloromethane containing 0.02 mmol of
catalyst under stirring. The mixture was precooled to the spec-
ified temperature, after which 1 mmol of m-CPBA was added
in five equal portions over 5-min periods. The reaction products
were analyzed by Agilent Technologies 6890N gas chromatog-
raphy (FID, 19091G-B213 chiral capillary column [30 m ×
0.32 mm × 0.25 µm]) using nitrogen as a carrier gas with a flow
rate of 30 mL/min. The injector temperature, detector temper-
ature, and oven temperature were 250, 250, and 100 ◦C, respec-
tively. The retention times of styrene, R-configuration styrene
epoxide, and S-configuration styrene epoxide were 3.37, 10.1,
and 10.5 min, respectively. The authentic sample of R- or S-
configuration styrene epoxide was used as the standard product
to determine the yields through comparison of peak height and
area. After completion of the reaction, the catalyst was precip-
itated from the reaction system by the addition of hexane and
subsequently used without further purification.
+562 (C = 0.04, CH2Cl2). Complex 3: FT-IR (KBr): 3408,
3143, 3087, 2953, 2861, 2013, 1613, 1586, 1547, 1538, 1461,
1442, 1390, 1358, 1308, 1198, 1169, 1087, 1044, 930, 871,
832, 751, 626, 621, 568 cm−1; UV–vis (CH2Cl2): 508, 433,
325 nm; Calc. for C44H70B2ClF8MnN8O2: C, 52.48; H, 7.01;
N, 11.13. Found: C, 52.54; H, 7.05; N, 11.06%; Mn ion
content: 1.02 mmol/g (theoretical value: 1.04 mmol/g); ESI-
33
MS, m/z = 1008 (M+H)+, 487 [M-2(CH2+IL)-Cl]+; [α]D
=
+545 (C = 0.04, CH2Cl2).
The IL-functionalized chiral salen Mn(III) complex 4 was
prepared as follows. First, 1 mmol of neat salen Mn(III) com-
plex 1 synthesized as described previously [2] and 1 mmol of
IL were dissolved in 15 mL of dry ethanol at ambient tem-
perature, after which the mixture was stirred vigorously for
another 48 h. After the removal of ethanol, the resulting solid
was washed with hexane several times and dried under vac-
uum to get the complex 4 (0.7 g, 86%). FT-IR (KBr): 3408,
2956, 2865, 1613, 1575, 1538, 1462, 1433, 1390, 1361, 1340,
1311, 1271, 1252, 1172, 1087, 1028, 928, 837, 780, 749, 659,

292
R. Tan et al. / Journal of Catalysis 255 (2008) 287–295
Fig. 2. UV–vis spectra of the IL-functionalized chiral complexes 2 (a), 3 (b),
and 4 (c), as well as the neat chiral salen Mn(III) complex 1 (d).
Fig. 1. FT-IR spectra of the IL-functionalized chiral salen Mn(III) complexes 2
(a), 3 (b), and 4 (c), the neat chiral salen Mn(III) complex 1 (d), and the ionic
liquid A (e).
IL-functionalized salen Mn(III) complexes 2 and 3 is main-
tained, provided that the IL is bonded onto the salen ligand. But
complex 4, in which the IL was attached via direct axial coor-
dination of the metal center with IL, exhibited a red shift in the
wavelength from 433 to 447 nm and from 508 to 520 nm due
to strong coordination between the metal center and the amino
group of the IL. These findings suggest that the electronic prop-
erties of the active site for chiral salen Mn(III) complex 4 should
change slightly. Therefore, the different attachment locations of
IL with salen Mn(III) complex should have crucial effects on
their catalytic performance in the epoxidation of alkene.
3. Results and discussion
3.1. Characterization of samples
The synthesized IL-functionalized chiral salen Mn(III) com-
plexes 2, 3, and 4 were characterized by FT-IR (see Fig. 1).
All of the IL-functionalized chiral salen Mn(III) complexes dis-
played major characteristic bands around 1613, 1538, 1463,
1432, 1390, 1361, 1252, 1175, 836, and 568 cm−1 (Figs. 1a,
1b, and 1c), which are similar to the neat chiral salen Mn(III)
complex 1 (Fig. 1d). The results indicated the identity of the
IL-functionalized chiral salen Mn(III) complexes 2, 3, and 4
with the neat chiral salen Mn(III) complex 1. Furthermore,
the IL-functionalized samples exhibited an additional feature
at 3408 and 621 cm−1 assigned to the stretching vibration of
N–H groups [24] and characteristic peaks of imidazole frag-
ment [25], respectively. These observations suggest that the
IL has been successfully bonded with the chiral salen Mn(III)
complex. 1H NMR further characterized the IL-CL1. Compared
with CL1, some new peaks at 2.4 ppm originated from the sec-
ondary amine (–NH) proton, and also at 8.93, 7.58, 7.47, 4.32,
4.17, 2.70, and 0.88 ppm originated from the protons of imida-
3.2. Catalytic performances
Table 1 summarizes the catalytic performance of the synthe-
sized complexes 2, 3, and 4 and the neat chiral salen Mn(III)
complex 1 in the enantioselective epoxidation of styrene. In
comparison with the neat chiral salen Mn(III) complex 1, com-
plexes 2 and 3 showed comparable catalytic activity (TOF)
and enantioselectivity (ee). This is because complexes 2 and 3,
which can dissolve in dichloromethane, act as homogeneous
catalysis in the reaction system presented here, thus leading
to excellent catalytic activity. Furthermore, Table 1 shows that
different attachment positions of chiral salen Mn(III) complex
with the amino group in the IL had crucial effects on the cat-
alytic performance of the corresponding IL-functionalized chi-
ral salen Mn(III) complex. Clearly, complex 3 demonstrated
similar catalytic activity and ee value as the neat complex 1, due
to the fact that the IL was bonded onto two sides of 5, 5ꢀ position
of complex 3, which imparts the same C2-symmetric structure
as in complex 1. In complex 2, with the IL bonded onto one side
of the 5 position, the non-C2-symmetric structure gave rise to an
elevated ee value (40%) under the same reaction conditions. It
is believed that the electronic and steric features of the non-C2-
symmetric chiral salen Mn(III) complex 2 improve the enan-
tioselectivity collectively [27,28]; however, when complex 4
was used as catalyst, an ee value of only 21% with a 75% yield
1
zole were observed in the H NMR spectrum of IL-CL1. It is
proposed that the secondary amine group in the IL-CL1 resulted
from the reaction between chloromethyl group (–CH2Cl) at the
5-position of the salen ligand and the amino group (–NH2) of
the IL.
The UV–vis electronic spectra of the neat chiral salen
Mn(III) complex 1 and the IL-functionalized complexes 2, 3,
and 4 are shown in Fig. 2. It can be seen that complexes 2 and 3
demonstrate broad peaks at near 433 nm and at around 508 nm,
which were identical with those of complex 1. The peak at
433 nm was assigned to charge-transfer transitions between
the metal and ligand, and the peak at 508 nm was assigned
to the d–d transitions in the salen Mn(III) complex [26], re-
spectively. The observation implies that the active site of the

R. Tan et al. / Journal of Catalysis 255 (2008) 287–295
293
Table 1
The results of the asymmetry epoxidation of styrene over different chiral salen
Mn(III) complexes
b
c
d
−3 −1
)
Catalyst
Run times
Yield (%)
ee (%)
TOF × 10 (s
No
Complex 1
Complex 2
32
92
99
98
98
98
98
98
98
98
75
55
38
0 (R)
–
a
a
Fresh
Fresh
2nd
3rd
5th
10th
Fresh
2nd
10th
Fresh
2nd
35 (R)
40 (R)
38 (R)
38 (R)
38 (R)
38 (R)
35 (R)
35 (R)
34 (R)
21 (R)
8 (R)
3.19
3.44
3.41
3.41
3.41
3.41
3.42
3.42
3.42
2.60
1.91
1.32
a
a
Complex 3
Complex 4
3rd
0
a
Catalyst (4% of styrene), styrene (0.5 mmol), PyNO (1 mmol), m-CPBA
Fig. 3. UV–vis spectra of the fresh complex 2 (A-a), the complex 2 after the
first reaction (A-b), and the complex 2 after the 10th reaction (A-c); the fresh
complex 3 (B-a), the complex 3 after the first reaction (B-b), and the complex 3
after the 10th reaction (B-c); the fresh complex 4 (C-a), the complex 4 after the
third reaction (C-b).
◦
(1 mmol), 1 mL CH Cl , 2 h, 0 C.
2
2
b
c
d
Yield of the isolated epoxide.
Determined by GC.
Turnover frequency (TOF) is calculated by the expression of [product]/
−1
[catalyst] × time (s ).
of the styrene epoxide was obtained. A possible explanation for
this finding might be that axial ligation has a critical influence
on the catalytically active species [29]; an overly strong coor-
dination between the metal center of the chiral salen Mn(III)
complex and the amino group of the IL results in occupation of
most of the coordination sites, eventually leading to decreased
formation of the active oxygen transfer species (Mn(V)–oxo) in
the enantioselective epoxidation of styrene [30].
We also found that the synthesized IL-functionalized chi-
ral salen Mn(III) complexes 2, 3, and 4 were immiscible with
hexane and thus could be readily separated from the reaction
system by the addition of hexane for the subsequent catalytic
runs. Table 1 characterizes the recovery and reusability of these
complexes. Obviously, complexes 2 and 3 could be reused at
least 10 times with no significant loss of activity and enantiose-
lectivity, whereas the recycling of complex 4 led to an apparent
drop in both activity and enantioselectivity. This finding may
indicate that the strong coordination of amino group (–NH2) in
the IL with the metal center of the chiral salen Mn(III) complex
led to an increase in electron density of the metal center, mak-
ing complex 4 less resistant to oxidative degradation [31,32].
In addition, the undesirable axial coordination may have led to
a distortion of the coordination sphere of the manganese in the
triplet state, decreasing the stability of the complex [29].
Fig. 4. FT-IR spectra of the fresh complex 2 (a) and the complex 2 after the
ꢀ
10th reaction (a ); the fresh complex 3 (b) and the complex 3 after the 10th
ꢀ
reaction (b ).
catalytic system, complex 2 was selected as the typical cata-
lyst to investigate the effects of various solvents, axial bases,
and reaction temperatures on the enantioselective epoxidation
of styrene.
The reuse properties of the three new complexes were further
confirmed by UV–vis (Fig. 3) and FT-IR spectra (Fig. 4). No
significant changes were seen in the UV–vis and FT-IR spectra
of complexes 2 and 3 even after 10 reuses; however, complex 4
lost the characteristic absorbance peaks in UV–vis spectra com-
pletely after 3 reuses (see Fig. 3), implying that complex 4 is
destroyed during the oxidative reaction. Nevertheless, the novel
complexes 2 and 3 showed high efficiency in the enantiose-
lective epoxidation of styrene. More importantly, they had the
property of tunable separation, facilitating their recycling and
reuse. In the goal to gain a better understanding of this unique
3.3. Effect of solvent
Table 2 summarizes the results of a comparative study of
the enantioselective epoxidation of styrene over the catalyst of
complex 2 in various solvents. As expected, the catalytic ac-
tivity and enantioselectivity were related to the solvent. If the
IL-functionalized chiral salen Mn(III) complex 2 could be dis-
solved completely in the reaction system, then the yield of
epoxide and the ee value should be high. It was found that
only a 33% yield with a 12% ee value was obtained when

294
R. Tan et al. / Journal of Catalysis 255 (2008) 287–295
Table 2
Table 4
The results of the enantioselective epoxidation of styrene in different solvent
over the complex 2
The results of the enantioselective epoxidation of styrene at different tempera-
ture over the complex 2
a
a
b
c
d
−3 −1
◦
b
c
d
−3 −1
)
Solvent
Yield (%)
ee (%)
TOF × 10 (s
)
Temperature ( C)
Yield (%)
ee (%)
TOF × 10 (s
n-Hexane
33
99
98
96
87
12 (R)
40 (R)
35 (R)
35 (R)
27 (R)
1.15
3.44
3.41
3.34
3.02
−78
−40
−10
0
20
30
94
99
99
99
93
89
35 (R)
50 (R)
47 (R)
44 (R)
37 (R)
34 (R)
3.27
3.44
3.44
3.44
3.22
3.08
Dichloromethane
Ethyl acetate
Acetone
Acetonitrile
a
Catalyst (4% in 1 mL solvent), styrene (0.5 mmol), PyNO (1 mmol),
◦
a
m-CPBA (1 mmol), 2 h, 0 C.
Catalyst (4% in 1 mL CH Cl ), styrene (0.5 mmol), NMO (2 mmol),
2 2
m-CPBA (1 mmol), 2 h.
b
Same as in Table 1.
Same as in Table 1.
Same as in Table 1.
c
b
Same as in Table 1.
Same as in Table 1.
Same as in Table 1.
d
c
d
Table 3
The results of the enantioselective epoxidation of styrene catalyzed by the com-
plex 2 in the presence of different axial base
alyst. Rather low activity (52% yield) and only racemic styrene
epoxides were obtained in the absence of an axial base even at
low reaction temperatures, suggesting that the axial base plays
a very important role in improving catalytic activity and enan-
tioselectivity. As expected, significant increases in the yield and
enantioselectivity of the epoxide could be obtained by adding
the axial base PyNO, 4-PhPyNO, 4-PPPyNO, or NMO, demon-
strating that the addition of axial base is essential to the enan-
tioselective epoxidation of styrene presented here. The axial
base of NMO gave the highest enantioselectivity (44% ee) for
the large steric hindrance on the N atom. In addition, Pietikii-
inen observed that in CH2Cl2, a salt is generated between NMO
and m-CPBA that oxidizes the salen Mn(III) catalyst even at
low temperature to form the catalytically active species Mn(V)–
oxo complexes [35], thereby giving a high yield of the styrene
epoxide (99%).
b
c
d
−3 −1
)
Axial base
–
PyNO
Yield (%)
ee (%)
TOF × 10 (s
52
99
99
98
99
0
1.80
3.44
3.44
3.41
3.44
a
40 (R)
39 (R)
39 (R)
44 (R)
a
4-PPPyNO
a
4-PhPyNO
e
NMO
a
Catalyst (4% in 1 mL CH Cl ), styrene (0.5 mmol), axial base (1 mmol),
2
2
◦
m-CPBA (1 mmol), 2 h, 0 C.
b
Same as in Table 1.
Same as in Table 1.
Same as in Table 1.
c
d
e
Catalyst (4% in 1 mL CH Cl ), styrene (0.5 mmol), NMO (2 mmol),
2
2
◦
m-CPBA (1 mmol), 2 h, 0 C.
the reaction was performed in hexane, due to the immiscibil-
ity of the complex 2 with hexane. For dichloromethane, ethyl
acetate, and acetone, a yield of >96% and ee of 35% could be
obtained, because complex 2 can be miscible with these sol-
vents. The enantioselectivity was lower in ethyl acetate and
acetone than in dichloromethane, possibly indicating that the
solvent containing oxygen or nitrogen atoms with a lone elec-
tron pair can induce coordination with the metal center of the
chiral salen Mn(III) complex, suppressing formation of the ac-
tive oxygen transfer species (Mn(V)–oxo) in the epoxidation
of styrene [30]. In particular, a yield of only 87% with an ee of
27% was obtained in acetonitrile, due to the strong coordination
of nitrogen atoms with manganese ions, even though complex 2
was miscible with acetonitrile. Therefore, dichloromethane is
considered a suitable solvent for the catalytic enantioselective
epoxidation presented here, giving the highest epoxide yield
(99%) and ee value (40%).
3.5. Effect of reaction temperature
Table 4 summarizes the catalytic performance of the com-
plex 2 at different reaction temperatures. The tables shows that
the yield and enantioselectivity to the styrene epoxide with an
R-configuration increased with decreasing reaction temperature
from 30 to −40 ◦C. It is known that the enantiofacial selectivity
in the initial C–O bond-forming step will be enhanced and that
the trans-pathway in the second C–O bond-forming step will be
suppressed when the epoxidation reaction is performed at low
temperature [36]. Moreover, the second C–O bond-forming step
provides an enantiometric leakage pathway for terminal olefins
[37,38]. Therefore, the ee value can be increased with decreas-
ing reaction temperature. Table 4 also shows that the yield of
styrene epoxide decreased with an increase of reaction tem-
perature from 0 to 30 ◦C; this is because the decomposition of
the oxidant of m-CPBA increased, decreasing the catalytic effi-
ciency of complex 2. But further attempts to decrease reaction
temperature from −40 to −78 ◦C resulted in decreases in epox-
ide yield and ee value. The solubility of the IL-functionalized
chiral salen Mn(III) complex 2 likely was decreased at −78 ◦C
in the reaction system; indeed, some granules of catalyst could
be observed after the reaction. After optimizing the reaction
conditions, a 99% epoxide yield with an ee of 50% could be
3.4. Effect of axial bases
Axial bases, which are only weakly bound to the man-
ganese center, have remarkable effects on both the activity and
enantioselectivity of the enantioselective epoxidation by acti-
vating and stabilizing the catalyst [33–35]. Table 3 summarizes
the comparative results of the enantioselective epoxidation of
styrene in the presence of various axial bases of PyNO, 4-
PhPyNO, 4-PPPyNO, and NMO using complex 2 as the cat-

R. Tan et al. / Journal of Catalysis 255 (2008) 287–295
295
obtained when the reaction was performed at −40 ◦C using
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In this work, novel IL-functionalized chiral salen Mn(III)
complexes were synthesized and used as catalysts in the enan-
tioselective epoxidation of styrene. FT-IR, UV–vis spectra, el-
emental analyses, and mass spectra analyses indicated that
the IL of 1-propylamine-3-methylimidazolium tetrafluorobo-
rate can be successfully bonded onto the chiral salen Mn(III)
complexes. The attachment position of chiral salen Mn(III)
complex with IL has significant effects on catalytic activity
and enantioselectivity. The chiral salen Mn(III)complex 2 pre-
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with the salen ligand at one side of the 5 position exhib-
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tion. Moreover, the synthesized IL-functionalized chiral salen
Mn(III) complexes could be conveniently separated from the re-
action system through simple hexane extraction. Among these,
the complexes of 2 and 3 are stable and can be readily re-
covered for reuse. These observations suggested that the tun-
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catalysts for the enantioselective epoxidation of nonfunctional
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This project was supported by the Natural Science Foun-
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