Synthesis, characterization, and catalytic activity for thioanisole oxidation of homogeneous and heterogeneous binuclear manganese(II) complexes with amino acid-based ligands

Article abstract of DOI:10.1007/s11243-013-9718-4

A binuclear Mn(II) complex [Mn₂(HL)₂(H ₂O)₄], HL = 2-[(2-Hydroxy-benzylidene)-amino]-3-(4- hydroxy-phenyl)-propionic acid, has been prepared and characterized by physico-chemical and spectroscopic methods. The r

Full text of DOI:10.1007/s11243-013-9718-4

Transition Met Chem (2013) 38:511–521
DOI 10.1007/s11243-013-9718-4
Synthesis, characterization, and catalytic activity for thioanisole
oxidation of homogeneous and heterogeneous binuclear
manganese(II) complexes with amino acid-based ligands
•
Massomeh Ghorbanloo Magdalena Jaworska
•
•
Piotr Paluch Guo-Dong Li Li-Jing Zhou
•
Received: 31 January 2013 / Accepted: 20 March 2013 / Published online: 4 April 2013
Ó Springer Science+Business Media Dordrecht 2013
Abstract
A
binuclear Mn(II) complex [Mn2(HL)2-
immobilized catalyst remains nearly unchanged after five
cycles, indicating that it is truly heterogeneous.
(H2O)4], HL = 2-[(2-Hydroxy-benzylidene)-amino]-3-(4-
hydroxy-phenyl)-propionic acid, has been prepared and
characterized by physico-chemical and spectroscopic
methods. The results suggest that the amino acid Shiff base
ligand is coordinated as a bivalent anion with a tridentate
NO2 donor set involving the phenolic and carboxylic acid
oxygens and azomethine nitrogen. A heterogenized binu-
clear manganese catalyst was synthesized by the covalent
anchoring of [Mn2(HL)2(H2O)4] onto a modified silica gel
surface, through the reactive (3-chloropropyl)-trimethox-
ysilane group. The surface properties of the functionalized
catalyst were analyzed by physico-chemical and spectro-
scopic methods. The silica-supported metal complex,
([Mn2L(HL)(H2O)4]/SiO2), catalyzes the oxidation of
thioanisole with H2O2, to give the sulfoxide and sulfone.
[Mn2L(HL) (H2O)4]/SiO2 shows lower catalytic activity
and turnover number compared to the homogeneous cata-
lyst [MnI2I–(HL)2(H2O)4]. However, the activity of the
Introduction
The growing applications of sulfoxides have encouraged
researchers to delve into new methods for their synthesis.
Organic sulfoxides are valuable synthetic intermediates that
are employed for the production of a range of chemically and
biologically active molecules such as anti-ulcer drugs (pro-
ton pump inhibitors), antifungal and anti-atherosclerotic
agents. The oxidation of sulfides by different methods pro-
vides a straightforward synthetic approach for preparing
numerous sulfoxides, and there are several reagents and
procedures available for this transformation. Hydrogen
peroxide is considered as an ideal ‘green’ oxidant, due to its
absence of toxic by-products [1]. Hence, new oxidative
processes based on the activation of H2O2 by robust, effi-
cient, and recyclable heterogeneous catalysts have been
developed, in view of the disadvantages of homogeneous
transition metal catalysts in terms of catalyst recovery and/or
reuse affecting the overall economics of the process [2].
Heterogeneous oxidation catalysts are of much current
interest because of advantages such as easy handling,
product separation, catalyst recovery, and reduced waste
[3]. Moreover, in heterogeneous catalysts, mixed inor-
ganic–organic materials are particularly attractive because
of the possibility to combine the functional variation of
organic chemistry with the thermal stability of an inorganic
substrate [4]. Fixation of such complexes by covalent
attachment to a functionalized support provides stable
heterogeneous catalysts [5]. A simple way to minimize the
leaching of the active catalyst in such materials is to anchor
polydentate ligands onto the support surface. Recent
Electronic supplementary material The online version of this
article (doi:10.1007/s11243-013-9718-4) contains supplementary
material, which is available to authorized users.
M. Ghorbanloo (&)
Department of Chemistry, Faculty of Science, University of
Zanjan, 45371-38791 Zanjan, Iran
e-mail: m_ghorbanloo@yahoo.com
URL: http://www.znu.ac.ir/members/ghorbanloo_massomeh
M. Jaworska ꢀ P. Paluch
Centre of Molecular and Macromolecular Studies, Polish
Academy of Sciences, Sienkiewicza 112, 90-363 Lodz, Poland
G.-D. Li ꢀ L.-J. Zhou
State Key Lab of Inorganic Synthesis and Preparative Chemistry,
College of Chemistry, Jilin University, 2699 Qianjin Street,
Changchun 130012, People’s Republic of China
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Transition Met Chem (2013) 38:511–521
reports have described the preparation of different hetero-
geneous forms of transition metal catalysts, based on
mesoporous silica [6] and USY zeolite [7], for the catalytic
oxidation of thioanisole; however, in spite of good yield
and chemoselectivity, a major problem is leaching of the
catalyst into solution [8].
Modified Sharpless systems are among the most effec-
tive sulfoxidation systems. However, these systems gen-
erally give low turnover numbers, low chemoselectivity
and require expensive alkylhydroperoxides as oxidants [9,
10]. Thus, it is tempting to look for systems that would
combine the high chemoselectivity of Mn-based systems
with high turnover numbers [11].
Against this background, wedescribe here the anchoring of
a polydentate amino acid Schiff base manganese(II) complex
onto a silica substrate functionalized with (3-chloropropyl)-
trimethoxysilane. This type of Schiff base complex shows
considerable catalytic activity without leaching. These cata-
lysts are efficient and highly selective for oxidation of thio-
anisole with 30 % aqueous H2O2. Moreover, the manganese
complex containing an amino acid Schiff base ligand was
characterized by solid-state NMR and magnetic measure-
ments, both before and after immobilization on silica gel.
Fig. 1 FTIR spectra of a H3L, b [Mn2(HL)2(H2O)4], c CPTMS/SiO2,
d H2L/SiO2, e [Mn2L(HL)(H2O)4]/SiO2
band was observed at 1,612 cm-1, assigned to the azo-
methine t(C=N) linkage [15].
Comparison of the IR spectra of free H3L and its Mn(II)
complex, shown in Fig. 1, provides evidence for the
coordination mode of the ligand. The complex shows a
band at 1,648 cm-1 which can be assigned to t(C=N) of
the coordinated Schiff base ligand.
In the spectrum of the complex, an extremely broad
band at about 3,460 cm-1 is assigned to –OH, possibly
involved in intermolecular hydrogen bonding. The mas(COO)
is observed at 1,590 cm-1, while ms(COO) is identified as a
medium to strong bond at 1,389 cm-1. Hence, Dm (mas(COO) -
ms(COO)) is 191–225 cm-1, characteristic of monodentate
coordination of the carboxylate group [16]. A medium
band at 1,550 cm-1 is assigned to the (Ph–)C–C(=N) bond
[17] and is typical of a complex derived from salicylalde-
hyde [18]. The phenolic (C–O) stretching frequency was
observed at 1,247 cm-1 for free H3L, which was shifted to
1,280 cm-1 in the complex, indicating coordination
through phenolic oxygen [19]. Finally, strong to medium
bands at *400 and *530 cm-1 are attributed to t(M–N)
and t(M–O), respectively, for the complex [16].
The infrared spectra of the heterogeneous compounds
CPTMS/SiO2, H2L/SiO2, and [Mn2L(HL)(H2O)4]/SiO2 are
very similar and their profile is typical of the silica matrix.
Strong bonds at 460 cm-1 correspond to d(Si–O–Si). A
band at 960 cm-1 is due to t(Si–OH), while the band at
1,090 cm-1 is assigned to tas(Si–O–Si) [20].
The introduction of the CPTMS onto the SiO2 (Fig. 1c)
led to a number of new bands, such as C–H stretching
vibrations at 2,935 and 2,860 cm-1 [21]. The IR spectrum of
H2L/SiO2, Fig. 1d, shows bands at 2,965, 2,937, and
2,865 cm-1, associated with propyl groups, and other new
bands at 1,612, 1,515, and 3,215 cm-1. These suggest that
the Schiff base is successfully grafted onto the support. In
addition, the IR spectrum of ([Mn2L(HL)(H2O)4]/SiO2),
Results and discussion
Synthesis and immobilization
The Schiff base ligand H3L was synthesized by the con-
densation of salicylaldehyde with L-tyrosine in 1:1 molar
ratio in methanol [12]. Its manganese complex was pre-
pared with high yield from the reaction of H3L with an
equimolar amount of Mn(OAc)2ꢀ4H2O in dichloromethane.
The complex was obtained as a light green powder. The
elemental analysis corresponds to the general formula
[Mn2(HL)2(H2O)4], which was further confirmed by spec-
troscopic and magnetic measurements.
The synthesis of [Mn2(HL)2(H2O)4] is summarized in
Scheme 1. Both H3L and its complex were immobilized on
silica gel using (3-chloropropyl)-trimethoxysilane as a
linker, according to the reported method in [13]. Immobi-
lization of the Schiff base and its complex involves reac-
tion of their phenolic groups with (3-chloropropyl)-
trimethoxysilane (CPTMS) (Schemes 2, 3). Soxhlet
extractions of these compounds ensured that only the
covalently attached molecules remained on the support.
Characterization
The FTIR spectrum of H3L does not include bands at 3,245
and 1,745 cm-1, which would be expected for the t(NH2)
and t(C=O) of the starting materials [14]. Instead, a strong
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Transition Met Chem (2013) 38:511–521
513
Scheme 1 Ligand and complex synthesis procedure
Scheme 2 Immobilization of the ligand on a modified silica support
Scheme 3 Preparation of the supported complex [Mn2L(HL)(H2O)4]/SiO2
1
Fig. 1e, shows bands at 2,980, 2,933, and 2,904 cm-1
,
coupling. In this way, H MAS NMR spectra of satisfac-
tory resolution were been obtained. The three distinct
proton resonances of the –Si–CH2CH2CH2– moiety for
CPTMS/SiO2 are observed at d = 0.69, 1.35, and
2.98 ppm. The same signals are observed for H2L/SiO2 and
[Mn2L(HL)2(H2O)4]/SiO2 (Fig. 2b, c). For the 1H MAS
NMR spectra of the ligand and its metal complex immo-
bilized on silica, the resonances of the aromatic and imine
protons are also visible in the range of 4–12 ppm. Addi-
tionally, a 1H–1H NOESY experiment for the dipole–dipole
interactions between protons in H2L/SiO2 was performed
and the results are shown in Fig. 3.
associated with propyl groups, and other new bands at 1,472,
1,517, 1,597, and 1,648 cm-1 are shifted relative to the
ligand bands, indicating that the complex is grafted onto the
support. Hence, the FTIR characterization confirmed the
successful immobilization of the Mn(II) complex.
The liquid- and solid-state NMR spectra of H3L,
CPTMS/SiO2, H2L/SiO2, and [Mn2L(HL)2(H2O)4]/SiO2
confirm that the compounds are immobilized on the silica
1
gel surface. Figure 2 shows the H MAS SS NMR spectra
for samples of H3L, [Mn2L(HL)2(H2O)4], and CPTMS/
SiO2 recorded with a spinning rate of 55 kHz. The use of a
‘‘Very Fast’’ (VF) spinning rate makes it possible to
exceed the strength of the homonuclear proton dipolar
1H–1H NOESY NMR is suitable for the analysis of
long-range contacts between protons. The interactions
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Transition Met Chem (2013) 38:511–521
‘‘Dipolar Dephasing’’ CP-MAS NMR and standard liquid
13C NMR spectra for the pure Schiff base were recorded
(Fig. 4d, e, f). Comparison between the 13C CP-MAS NMR
spectra and 13C CP DD MAS NMR spectrum revealed the
presence of three quaternary carbon signals. The DD pulse
sequence is often used as a spectral editing technique. In
the simplest approach, the 1H decoupler is turned off for ca.
50 ls after CP. This is sufficient time for the 13C–1H
dipolar coupling to diphase the transverse magnetization of
1
any 13C isotope with a directly bonded H isotope. Con-
sequently, CH and CH2 are effectively suppressed, and
only quaternary carbon signals are observed. The com-
parison of the results of solid-state NMR and standard
liquid NMR (Fig. 4d, f) confirmed the structure of H3L and
its manganese complex and chemical bonding of these
species to the silica.
Additional and complementary data about the silica-
1
grafted catalyst were obtained from the H–29Si invHET-
COR NMR spectra performed under Very Fast MAS.
Pruski and coworkers have recently published a few
applications of inverse detected HETCOR NMR experi-
ments under Very Fast MAS in the structural studies of
inorganic–organic hybrid materials [22–25]. In our studies,
the 1H–29Si invHETCOR MAS NMR experiment was
especially useful for 29Si resonance detection and allowed
us to fully assign the spectra of the ligand and complex
grafted on silica. The results for [Mn2L(HL)(H2O)4]/SiO2
are presented in Fig. 5. Additionally, the 1H–29Si inv-
HETCOR MAS NMR spectra for CPTMS/SiO2 and H2L/
SiO2 are presented in the Supplementary Materials.
It is well known that the spectrum of SiO2 generally
exhibits three resonances at -110, -101, and -92 ppm,
corresponding to the Q4 [siloxane, (SiO)4Si], Q3 [single
silanol, (SiO)3Si(OH)], and Q2 [geminal sila-
nol,(SiO)2Si(OH)2] sites of the silica framework, respec-
tively [26]. In the case of [Mn2L(HL)(H2O)4]/SiO2, two
signals from the Si environment of Q4 (d = -102.10 ppm)
and Q3 (d = -100.70 ppm) are observed. In addition to
these two peaks, the sample displays two more peaks at
-68.50 ppm, assigned to T3 [C–Si(OSi)3], and at
-58.50 ppm, attributed to T2 [C–Si(OSi)2(OH)]. The
presence of T3 species confirms that the silica gel has been
modified by organic moieties [27]. The solid-state NMR
results thus provide direct evidence that the hybrid
CPTMS/SiO2 sample consists of a highly condensed
siloxane network with the organic group covalently bonded
to the silica. Moreover, the dipolar interactions between 1H
nuclei in the ligand and 29Si nuclei on the silica surface are
Fig. 2 1H MAS NMR spectra performed at 55 kHz spinning rate:
a CPTMS/SiO2, b H2L/SiO2, c [Mn2L(HL)(H2O)4]/SiO2
between the Schiff base and CPTMS/SiO2 protons (labeled
in Fig. 3) indicated that the ligand is chemically attached to
the silica matrix.
The 13C CP-MAS NMR spectra of CPTMS/SiO2, H2L/
SiO2, [Mn2L(HL)(H2O)4]/SiO2, and H3L recorded with a
spinning rate of 8 kHz at ambient temperature are shown in
Fig. 4.
The 13C CP-MAS NMR spectrum of CPTMS func-
tionalized silica gel is shown in Fig. 4a. For CPTMS/SiO2,
three resonances at 9.47, 26.01, and 46.06 ppm are
assigned to the –Si–CH2–, –CH2–, and –CH2–Cl groups,
respectively. Three similar signals are also observed for
[Mn2L(HL)(H2O)4]/SiO2 and H2L/SiO2 (Fig. 4b, c). In the
13C CP-MAS NMR spectra of the ligand and its complex
immobilized on silica (Fig. 4b, c), the resonances of the
aromatic and imine carbons are observed, in addition to the
expected aliphatic signals. In order to compare the results
obtained for the Schiff base grafted to silica, the 13C
1
visible. H–29Si correlations associated with Si–CH2, Si–
CH2–CH2–, and –CH2–Cl were present in the 1H–29Si
invHETCOR MAS NMR spectrum (Fig. 5).
This is further substantiated by the atomic absorption
(A.A). According to the A.A analysis, the loading of the
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Transition Met Chem (2013) 38:511–521
515
Fig. 3 1H–1H NOESY NMR
spectrum recorded at 55 kHz
spinning rate for H2L/SiO2
complex was 1.58 mmol CPTMS/g of silica gel,
0.18 mmol complex/g of silica gel, and 0.36 mmol Mn/g of
silica gel.
obtained from the Hamiltonian, where S1 = S2 = 5/2 and
x = J/kBT. The best fitting gave J = -1.70 cm-1 and
g = 2.07 with q = 2 % and TIP = 1.5 9 10-3 cm3
mol-1, indicating that the two Mn(II) atoms in the complex
are antiferromagnetically coupled.
Plots of vMT and v-M1 versus T (vM is the molar magnetic
susceptibility) of [Mn2L(HL)(H2O)4] are shown in Fig. 6.
The magnetic susceptibility measurement was performed
in the 1.8–300 K range under 0.1 T applied field on a
powdered sample. At room temperature, the vMT value is
9.24 cm3 K mol-1, which is slightly larger than the spin-
only value (8.75 cm3 K mol-1) expected for two uncou-
pled high-spin Mn(II) centers (S = 5/2 and g = 2.0). The
vMT value remains almost constant above 200 K, from
which it decreases, finally attaining a value of 0.67 cm3
K mol-1 at 1.8 K. This behavior, which is very similar to
that of reported dimanganese(II) complexes [28], is char-
acteristic of two high-spin (S = 5/2) Mn(II) centers expe-
riencing weak antiferromagnetic coupling.
Therefore, according to the magnetic properties, we
conclude that [Mn2(LH)2(H2O)4] is a binuclear Mn complex.
Catalytic activity studies
The catalytic performance of the complexes was checked
for thioanisole oxidation with 30 % aqueous H2O2 and the
results are listed in Table 1. In this reaction, hydrogen
peroxide should be used carefully, due to the possibility of
over-oxidation [29]. The effect of different amounts of
H2O2 was investigated, keeping a fixed amount of thio-
anisole (1.0 mmol) and catalyst [Mn2(LH)2(H2O)4] (1 mg)
in 3 mL of acetonitrile at 60 1 °C. As shown in Table 1
(entries 1–5), the conversion and selectivity drastically
increase with the molecular ratio of H2O2 to thioanisole,
until the ratio is equal to 3. The catalytic oxidation gave
95 % sulfoxide with 3 mmol of H2O2 in 120 min (Table 1,
entry 4). Higher ratios of H2O2 resulted in decreased yield,
due to the over-oxidation of the corresponding sulfone. The
excess H2O2 required in this catalytic system can be
The magnetic susceptibilities were fitted between 300
and 20 K (solid line in Fig. 6) by using Eq. 1:
~ ~
H ¼ ꢁ2JS1S2
~
2Ng2l2B e2x þ 5e6x þ 14e12x þ 30e20x þ 55e30x
kBT 1 þ 3e2x þ 5e6x þ 7e12x þ 9e20x þ 11e30x
Ng2l2BSðS þ 1Þ
vM
¼
ꢂ ð1 ꢁ qÞ þ
q þ TIP
ð1Þ
3kBT
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Transition Met Chem (2013) 38:511–521
Fig. 4 13C CP-MAS NMR
spectra of a CPTMS/SiO2,
b [Mn2L(HL)(H2O)4]/SiO2,
c H2L/SiO2, d H3L. 13C DD
CP-MAS NMR spectra of
e H3L and 13C NMR spectrum
in liquid DMSO-d6 of f H3L
attributed to its decomposition in the presence of the cat-
alyst. A variety of salen complexes [30] are able to catalyze
the oxidation of sulfides to sulfoxides with hydrogen
peroxide, but a drawback of these systems is often the use
of chlorohydrocarbon solvents or anhydrous H2O2 in eth-
anol as well as over-oxidation to sulfones.
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Transition Met Chem (2013) 38:511–521
517
Fig. 5 1H–29Si invHETCOR
MAS NMR spectrum performed
at 55 kHz spinning rate (Very
Fast NMR regime) of sample
[Mn2L(HL)(H2O)4]/SiO2
studies previously reported in the literature, solvents of high
hydrogen bonding capacity, such as methanol, favor the
formation of sulfoxide [31]. However, the best performance
of the catalyst was observed in acetonitrile, which is there-
fore the solvent of choice.
The oxidation is significantly influenced by the reaction
temperature (Table 3, entries 1–3). The optimum results
were obtained at 60 °C; higher temperatures gave lower
yields (Table 3, entry 3). It seems likely that higher tem-
peratures facilitate the decomposition of H2O2.
These results show that [Mn2(LH)2(H2O)4] is a good
catalyst for the oxidation of thioanisole to the sulfoxide
with hydrogen peroxide.
Comparing the homogeneous and heterogeneous sys-
tems reveals that the heterogeneous catalyst exhibits lower
activity than the homogeneous catalyst (Table 1, entry 9).
It seems likely that the use of a supported manganese
catalyst in the present case gives rise to additional prob-
lems related to the probable dismutation of H2O2 by the
inorganic support. This would explain why only a few
systems based on supported manganese catalysts and H2O2
have been reported for oxidation reactions [32]. Similar
difficulties were previously encountered for Mn(III) and
Mo(IV)—salen complexes immobilized on mesoporous
silica gel [3].
Fig. 6 vMT (box) and vM - 1 versus T (circle) plots of
[Mn2(HL)2(H2O)4].The solid line represents calculated data, as
described in the text
In control experiments without the catalyst, the yield of
sulfoxide is low (Table 1, entry 7). Furthermore, when the
catalyst was replaced with Mn(OAc)2ꢀ4H2O, extensive
peroxide decomposition was observed and no sulfoxide
was obtained (Table 1, entry 8).
Next, the catalytic oxidation was studied in various sol-
vents (Table 2). The catalytic activity decreases in the order
acetonitrile (dielectric constant e/e0 = 37.5) [ methanol
(32.7) [ chloroform (5.5) at 60 °C. Chloroform is not mis-
cible with aqueous H2O2 which could also be the reason for
low catalytic efficiency in this case. In agreement with
In order to investigate the possibility of recycling the
([Mn2L(HL)(H2O)4]/SiO2) catalyst, it was separated from
the reaction mixture by centrifugation after the reaction.
The supernatant was decanted, and the solid catalyst was
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Transition Met Chem (2013) 38:511–521
immobilized catalyst was observed; therefore, recycling is
possible in the case of this catalyst.
Table 1 Oxidation of thioanisole by the catalyst [Mn2(LH)2(H2O)4]
and ([Mn2L(HL)(H2O)4]/SiO2)/H2O2
The stability of the supported catalyst for oxidation of
thioanisole was also investigated. In these experiments, the
catalyst was separated from the reaction mixture after each
experiment by simple filtration, washed with methanol, and
dried before use in the subsequent run. After using catalyst
for five consecutive times, the oxidation yields were 60 %.
The filtrates were collected for determination of Mn
leaching. Analysis by atomic absorption method showed no
detectable (\0.02 ppm) manganese. Hence, the obtained
catalytic results are derived exclusively from the hetero-
geneous catalyst.
No. Sulfoxide/sulfone H2O2:Substrate
yield (%)a
Sulfoxide
selectivity(%)b
TONc
molar ratio
1
2
3
4
5
6
7
8
9
26.4/3.6
40.5/4.5
55.8/4.2
95/0
1:1
88
90
187
281
375
593
619
–
1.5:1
2:1
93
3:1
100
96
95/4
4:1
0
0:1
0
14/25
0
3:1d
3:1e
3:1
36
–
0
100f
–
60/0
375
These recycling results are promising when compared to
the literature. Heterogeneous Mn(II) catalysts, with ace-
tylacetone-based Schiff base ligands, have been reported
for alkene epoxidation with H2O2/CH3COONH4 [33].
However, they are not stable and could not be recycled.
Mn(III)—salen complex immobilized into pillared clays—
was applied in the heterogeneous epoxidation of styrene
with PhIO [34]. Again this system was not very stable,
leaching and deactivation took place.
Reaction conditions: catalyst ([Mn2(LH)2(H2O)4]) 1 mg (0.0016 mmol
catalyst), thioanisole 1 mmol, CH3CN 3 ml, temperature 60 1 °C,
and time 120 min
a
Conversions are based on the starting substrate
b
Sulfoxide selectivity: %sulfoxide/(sulfoxide ? sulfone)
c
TON = moles of converted sulfide/moles of catalyst
d
Without catalyst
e
Mn(OAC)2ꢀ4H2O is used as catalyst
Reaction conditions: catalyst (([Mn2L(HL)(H2O)4]/SiO2)) 9 mg
g
(0.0016 mmol catalyst), substrate 1 mmol, CH3CN 3 ml, temperature
60 1 °C, and time 120 min
Conclusion
Table 2 Solvent effect on the oxidation of thioanisole with
[Mn2(HL)2(H2O)4]/H2O2
In summary, the present study has described the synthesis
of a Schiff base ligand, H3L, and its MnI2I complex and
immobilization of these two compounds onto a modified
silica gel surface. A range of characterization techniques
confirm that the metal complex is attached to the silica gel
support.
Entry
Solvent
Conv. (%)a
Sulfoxide selectivity (%)b
1
2
3
CHCl3
20
90
95
90
99
CH3OH
CH3CN
100
[Mn2(HL)2(H2O)4] and [Mn2(HL)L(H2O)4]/SiO2 pro-
vide homogeneous and heterogeneous catalysts for oxida-
tion of thioanisole by hydrogen peroxide in acetonitrile.
The supported catalyst [Mn2L(HL)(H2O)4]/SiO2 gave low
activity and turnover number compared to its homogeneous
counterpart. However, the supported catalyst was easily
separated from the reaction mixture via filtration and could
be recycled for several catalytic runs, without significant
loss of activity. This system could be useful for many other
base-catalyzed oxidations.
Reaction conditions: catalyst ([Mn2(LH)2(H2O)4]) 1.0 mg (0.0016
mmol), MeSPh 1.0 mmol, solvent 3 ml, H2O2 3 mmol, time 2 h,
temperature 60 1 °C
a
Conversions are based on the starting substrate
b
Sulfoxide selectivity: %sulfoxide/(sulfoxide ? sulfone)
Table 3 Temperature effect on the oxidation of thioanisole with
[Mn2(HL)2(H2O)4]/H2O2
Entry
Temp(°C)
Conv. (%)a
Sulfoxide selectivity(%)b
1
2
3
25
60
80
11
95
80
60
100
95
Experimental
Reaction conditions: catalyst ([Mn2(LH)2(H2O)4]) 1.0 mg (0.0016
mmol), MeSPh 1.0 mmol, solvent 3 ml, H2O2 3 mmol, time 2 h
All solvents, L-tyrosine, salicylaldehyde (Sal), Mn(OAc)2ꢀ
4H2O, and MeSPh were purchased from Merck and used as
received. Silica gel (0.063–0.200 mm) was purchased from
Merck and activated at 550 °C for 6 h before use. Aqueous
30 % hydrogen peroxide (12.5 mol/L) was used and its
exact concentration was determined before use by titration
with standard KMnO4. Elemental analyses were
a
Conversions are based on the starting substrate
b
Sulfoxide selectivity: %sulfoxide/(sulfoxide ? sulfone)
washed twice by adding acetonitrile and centrifugation.
The used catalyst was then recycled for further reactions,
up to five times. No significant loss in the activity of the
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Transition Met Chem (2013) 38:511–521
519
determined on a CHN Perkin–Elmer 2400 analyzer. FTIR
spectra were recorded on a Perkin–Elmer 597 spectrome-
ter. Manganese contents were determined by a Varian
spectrometer AAS-110 spectrophotometer, using a flame
approach, after alkali melting of a known amount of the
complex in a platinum crucible. The magnetic suscepti-
bility of the complex was measured with a Quantum
Design MPMS XL7 magnetometer in the 1.8–300 K range,
C5, C50), 116.30 (CHAr,C12), 117.76 (CHAr, C10), 117.99
(CAr, C8), 129.93 (2CHAr, C4, C40), 130.23 (CAr, C3),
131.60 (CHAr, C9), 132.18 (CHAr, C11), 155.27 (CAr, C6),
163.31 (CH=N, C7), 164.50 (CAr, C13), 171.24 (COOH,
C14) ppm. 13C CP-MAS NMR, 100.63 MHz, d = 37.20
(CH2, C2), 56.53 (CH, C1), 116.90 (2CHAr, C5, C50),
118.40 (2CHAr, C10, C12), 124.10 (2CAr, C8, C3), 131.2
(4CHAr, C4, C40, C9, C11), 156.00 (2CAr, C6, C13, also
CH=N, C7), 176.10 (COOH, C14) ppm.
1
under 0.1 T applied field. H NMR spectra were recorded
on a 250 MHz Bruker spectrometer and a 500 MHz Bruker
Avance III spectrometer. The solid-state cross-polarization
magic angle spinning (CP-MAS) NMR were obtained on a
400 MHz Bruker Avance III spectrometer, equipped with a
MAS probe head using 4 mm ZrO2 rotors, at a frequency
of 100.61 MHz for 13C. A sample of 13C-labeled tyrosine
was used to set the Hartmann–Hahn condition for 13C. A
sample of Q8M8 was used to set the Hartmann–Hahn
condition for 29Si. The acquisition was made with a
SPINAL decoupling sequence [35]. The 1H–29Si HETCOR
(with indirect detection of 29Si) experiments were per-
formed using the pulse sequence described by Mao et al.
[24]. The following parameters were used: 55 kHz spin
rate, proton 90° pulse length of 2.5 ls, first contact time of
9.5 ms, second contact time of 9.5 ms or 200 ls, and
proton p pulse (5 ls) in the middle of the evolution period
Synthesis of the complex
A mixture of L-tyrosine (0.36 g, 2.0 mmol) and NaOH
(0.08 g, 2.0 mmol) in CH2Cl2 (10 mL) was added to a
solution of salicylaldehyde (0.244 g, 2.0 mmol) in CH2Cl2
(10 mL) and refluxed, where upon the colorless solution
rapidly turned yellow. The mixture was refluxed for 2 h,
then Mn(OAc)2ꢀ4H2O (2 mmol, 0.498 g) was added and
the mixture was stirred under reflux for 5 h. The light green
precipitate was collected and washed with methanol and
dried in air. Yield: 0.5 g (67 %). Anal. calc. for
C32H34Mn2N2O10: C, 51.3; H, 4.5; N, 3.7; Mn 14.6. Found:
C, 52.9; H, 4.0; N, 3.6; Mn, 15.3 %. IR (KBr, cm-1): 3,424
(br), 3,077 (w), 3,035(w), 2,921 (m), 2,854 (w), 1,648 (vs),
1,589 (vs), 1,549 (w), 1,515 (m), 1,471 (s), 1,444 (s), 1,385
(s), 1,280 (m), 1,187 (m), 823 (m), 757 (s), 526(m), 449 (m).
1
(instead of CW H decoupling as mentioned by Ishii and
Tycko) [36]. The reaction products of the oxidation were
analyzed by an HP Agilent 6890 gas chromatograph,
equipped with a HP-5 capillary column (phenyl methyl
siloxane 30 m 9 320 lm 9 0.25 lm) with flame-ioniza-
tion detector.
Grafting of the linker
Anchoring of (3-chloropropyl)-trimethoxysilane on the
surface of silica gel was carried out according to the
reported method [13]. The light cream powder, CPTMS/
SiO2, was dried at room temperature. Linker load-
ing = 1.16 mmol/g silica gel, IR (KBr, cm-1): 3,429 (br,
w), 2,968 (w), 2,935 (w), 2,860 (w), 1,086 (vbr, vs), 821
(vs), 471 (s). 1H MAS NMR, 600 MHz, d = 0.26 (protons
of silica gel), 0.69 (Si–CH2–), 1.35 (Si–CH2–CH2–), 2.98
(–CH2–Cl), 5.57 (–OH) ppm. 13C CP-MAS NMR,
100.63 MHz, d = 9.47 (Si–CH2), 26.01 (Si–CH2–CH2–),
46.06 (–CH2–Cl) ppm. 29Si CP-MAS (as determined in
1H–29Si HETCOR NMR experiment), 116.22 MHz, d =
-110.10 (Q4 [siloxane, (SiO)4Si]), -102.10 (Q3 [single
silanol,(SiO)3Si(OH)]), -57.40 (T3 [C–Si(OSi)3]), -50.50
(T2 [C–Si(OSi)2(OH)]) ppm.
Synthesis of the ligand
A mixture of L-tyrosine (0.36 g, 2.0 mmol) and NaOH
(0.08 g, 2.0 mmol) in toluene (10 mL) was added to a
solution of salicylaldehyde (0.244 g, 2.0 mmol) in toluene
(10 mL) and refluxed for 2 h. The resulting yellow solid
was filtered off, washed repeatedly with methanol, and
dried in air. Yield 0.50 g (88 %). IR (KBr, cm-1): 3,208
(w) (O–H), 3,026 (w), 2,963 (w), 2,930 (w), 1,612 (vs)
(C=N), 1,609 (s), 1,514 (m), 1,455 (m), 1,364 (m), 1,332
(s), 1,245 (s), 1,100 (m), 842 (s), 740 (m), 650 (s), 576 (s),
1
530 (s), 493 (w), 434 (w). H NMR, 500 MHz, DMSO-d6,
d = 2.77 (1H, dd, J = 9.0 Hz, J = 13.5 Hz, CH2, H-2),
3.13 (1H, dd, J = 4.0 Hz, J = 15.0 Hz, CH, H-1), 3.74
(1H, dd, J = 4.0 Hz, J = 9.5 Hz, CH2, H-2), 6.56 (2H, d,
J = 8.5 Hz, CHAr), 6.63 (1H, d, J = 6.5 Hz, CHAr), 6.69
(1H, d, J = 8.0 Hz, CHAr), 6.90 (2H, d, J = 8.5 Hz,
CHAr), 7.16–7.20 (2H, m, CHAr), 8.04 (1H, s, CH = N,
H-7), 9.14 (1H, br s, –COOH), 14.29 (1H, br s, OH) ppm.
13C NMR, 125.77 MHz, DMSO-d6, d = 36.96 (CH2, C2,
overlapped by DMSO), 74.44 (CH, C1), 114.84 (2CHAr,
Immobilization of the ligand
In a typical synthesis, a mixture of CPTMS/SiO2 (0.5 g)
and LH3 (0.1 g) was refluxed in toluene for 24 h, followed
by Soxhlet extraction with CH2Cl2 for 4 h in order to
remove adsorbed H3L. The yellow material, H2L/SiO2, was
dried at room temperature. H3L loading = 0.30 mmol/g
123

520
Transition Met Chem (2013) 38:511–521
silica gel. IR (KBr, cm-1): 3,477 (br, w), 2,935 (w), 2,860
(w), 1,612 (s), 1,084 (vbr, vs), 811 (vs), 653 (m), 577(w),
Xiamen University of P. R. C. for technical assistance in magnetic
measurement experiments.
1
474 (s); H MAS NMR, 600 MHz, d = 0.26 (protons of
silica gel), 1.36 (Si-CH2), 2.38 (CH2–O), 3.81 (CHAr), 5.97
(CHAr), 8.18 (–CH=N), 9.46 (–OH) ppm. 13C CP-MAS
NMR, 100.63 MHz, d = 10.55 (Si–CH2), 26.49 (Si–CH2–
CH2–), 37.06 (CH2, C2), 47.59 (–CH2–Cl), 56.95 (CH,
C1), 115.51 (2CHAr, C5, C50), 118.60 (2CHAr, C10, C12),
123.96 (2CAr, C8, C3), 130.99 (4CHAr, C4, C40, C9, C11),
155.93 (2CAr, C6, C13, also CH=N, C7), 175.78 (COOH,
C14) ppm. 29Si CP-MAS (as determined in 1H–29Si
HETCOR NMR experiment), 116.22 MHz, d = -100.70
(Q4 [siloxane, (SiO)4Si]), -102.10 (Q3 [single sila-
nol,(SiO)3Si(OH)]), -60.50 (T3 [C–Si(OSi)3]), -59.90 (T2
[C–Si(OSi)2(OH)]) ppm.
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SiO2 (0.5 g) and [Mn2(LH)2(H2O)4] (0.1 g) were refluxed in
CH2Cl2 for 24 h, followed by Soxhlet extraction with
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Oxidation reactions were carried out under air at
60 1 °C, with acetonitrile as a solvent and aqueous 30 %
H2O2 (12.5 M) as oxidant. In a typical experiment, a
mixture of 1.0 mg as the homogeneous catalyst, 3.0 mL
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glass flask. After the mixture was heated to 60 °C, H2O2
was added. At appropriate intervals, aliquots were removed
and analyzed immediately by GC. The oxidation products
were identified by comparing their retention times with
those of authentic samples. Yields are based on the added
substrate and were determined by a calibration curve.
´
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Acknowledgments Authors are thankful to University of Zanjan for
financial support of this study. We also thank Dr. Jun Tao from
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