Synthesis of bimagnetic ionic liquid and application for selective aerobic oxidation of aromatic alcohols under mild conditions

Article abstract of DOI:10.1039/c0cc04644b

The first bimagnetic ionic liquid based on Fe and TEMPO with cooperative functionalities not only exhibited strong paramagnetic behaviour at room temperature under an applied magnetic field of 5000 Oe but also proved to be an effective catalyst for select

Full text of DOI:10.1039/c0cc04644b

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COMMUNICATION
Synthesis of bimagnetic ionic liquid and application for selective aerobic
oxidation of aromatic alcohols under mild conditionsw
Cheng-Xia Miao,a Jin-Quan Wang,b Bing Yu,a Wei-Guo Cheng,b Jian Sun,b
a
a
b
´
Sebastien Chanfreau, Liang-Nian He* and Suo-Jiang Zhang*
Received 27th October 2010, Accepted 20th December 2010
DOI: 10.1039/c0cc04644b
The first bimagnetic ionic liquid based on Fe and TEMPO with
cooperative functionalities not only exhibited strong paramagnetic
behaviour at room temperature under an applied magnetic field
of 5000 Oe but also proved to be an effective catalyst for
selective aerobic oxidation of aromatic alcohols under mild
and clean conditions.
Scheme 1 Novel Fe/TEMPO-based bimagnetic IL with cooperative
Ionic liquids (ILs) are salts being composed of distinct cations
and anions that are capable of facilely tuning, and whereby
can be designed for task-specific applications through a smart
choice of the respective cation and/or anion. Nowadays, ILs
have been extensively investigated with a wide range of
interesting applications,1 because of their limitless attractive
properties, such as wide liquid temperature ranges, good
thermal stabilities, high ionic conductivity, wide electrochemical
potential windows, and high solvation interactions with both
polar and nonpolar compounds.
functionalities.
2,2,6,6-tetramethyl-1-piperidinyl-oxy-4-sulfate with potential
as conductive material, was also reported.7 Interestingly,
based on positive effects of ILs8 and rapid development of
iron catalysis,9 metal magnetic IL i.e. [C4min][FeCl4] combin-
ing attractive properties of both ionic liquid and FeCl3 was
found to have great application in catalysis.10 Despite those
successes with the magnetic ILs, the bimagnetic ILs with more
potential functionality were never found in the literature. In
this context, developing task-specific bimagnetic ILs would
be much promising.11 As a continuation of our interests
on ILs,12 we herein would like to report the first bimagnetic
imidazolium salt based on Fe and organic radical ion
(Scheme 1), which showed strong paramagnetic behaviour
(wg = 37.8 Â 10À6 emu gÀ1) and attractive catalytic properties.
The target bimagnetic IL was successfully synthesized from
commercially available 4-hydroxy-TEMPO, chloroacetic acid,
1-methylimidazole and FeCl3.3,13 The magnetic properties of
the ILs such as [Imin-TEMPO][FeCl4] and [Imin-TEMPO][Cl]
were examined by using the SQUID (superconducting quantum
interference device) method. The results are shown in Fig. 1A
and 1B and Fig. S3–S13 (see ESIw). The magnetizations for
[Imim-TEMPO][FeCl4] and [Imim-TEMPO][Cl] at 298 K were
measured in the magnetic field range of À50 000 to 50 000 Oe,
revealing an expected linear response to the applied magnetic
field (Fig. 1A and Fig. S13 (ESIw)). As a result, the gram and
molar magnetic susceptibilities of [Imim-TEMPO][FeCl4]
at 298 K were determined to be 37.8 Â 10À6 emu gÀ1 and
0.0187 emu molÀ1, respectively, being in well agreement with
the expected value of Fe(III).2 On the other hand, the effective
magnetic moment (meff) for [Imim-TEMPO][Cl] was found to
be 1.39 mB, being well consistent with the expected value for
TEMPO.7 Interestingly, the effective magnetic moment (meff)
for [Imim-TEMPO][FeCl4] was found to be 6.66 mB, suggesting
the cooperative contribution from both the S = 5/2 high-spin
On the other hand, the magnetic behaviour of ILs with
metal-containing anions has been less known until Hayashi and
Hamaguchi2 found paramagnetic behaviour of [C4mim][FeCl4]
with effective magnetic moment (meff = 5.8 mB) and high
conductivity (o2.0 Â 10À2 S cmÀ1). Subsequently, magnetic
ILs have been attracting more and more attention in view of
special properties and applications.3 A series of imidazole- or
tetraalkylphosphonium-based gadolinium(III), manganese(II),
iron(III) and cobalt(II) ILs have become available in Del Sesto’s
group, which show potential applications for magnetic and
electrochromic switching as well as magnetic transport.4 Further-
more, dysprosium(III)-based ILs exhibited strong luminescence
and response to magnetic fields (meff = 10.4 mB).5 Even the
FeCl4-based magnetic chiral ILs derived from amino acids and
their magnetic properties were also investigated.6 In addition,
non-metal organic magnetic IL based on a radical ion, such as
a State Key Laboratory of Elemento-Organic Chemistry,
Nankai University, Weijin Rd. 94, Tianjin, 300071, China.
E-mail: heln@nankai.edu.cn; Fax: +86 22 2350 4216
b Key Laboratory of Green Process and Engineering,
Institute of Process Engineering, Chinese Academy of Sciences,
P.O. Box 353, Beijing, 100190, China.
E-mail: sjzhang@home.ipe.ac.cn; Fax: +86 10 8262 7080
w Electronic supplementary information (ESI) available: General
experimental part, characterization of magnetic imidazolium salts and
its intermediates, supporting tables and figures, 1H NMR and 13C NMR
data for products identification. See DOI: 10.1039/c0cc04644b
c
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Encouraged by those results, we further examined the utility
and generality of this approach for transformation of various
alcohols including the benzyl alcohols and heterocyclic alcohols
to carbonyl compounds by performing the reaction under the
given conditions (Table 2). Notably, the activities of the
substituted benzyl alcohols were promoted by addition of
water (entries 2 vs. 3, 4 vs. 5, Table 2). Fortunately, all
primary benzylic alcohols were converted into the corres-
ponding aldehydes in high yield and excellent selectivity
(entries 1, 3, 5–9) and the secondary benzylic alcohols also
gave high to moderate yields by prolonging the reaction time
(entries 10 and 11). It is noteworthy that the heterocyclic
alcohols are compatible in this bimagnetic imidazolium
IL-catalyzed oxidation reaction (entries 12–14), which are
generally problematic with other transition metal-mediated
aerobic oxidation protocols; and the reactivity follows the
order 2-furfuryl alcohol > 2-thiophenemethanol > 2-pyridinyl-
methanol. Unfortunately, allylic and aliphatic alcohols were
inert to this catalytic system (entries 1–5, Table S2, ESIw).
Furthermore, one feature of this bimagnetic IL-catalyzed
aerobic oxidation of alcohols would be facile separation of the
catalyst. In each run, [Imim-TEMPO][FeCl4] was readily
recovered by ether extraction, then was subjected to a sub-
sequent run of the reaction by charging a fresh substrate and
NaNO2. The results show that the catalyst can be reused for at
least five times with retention of high activity and selectivity.14
Although the mechanism for the present catalytic oxidation
is not yet clear, valuable insight and discussion would be
herein presented. The control experiments reveal that TEMPO
and [FeCl4]À species/NaNO2 are indispensable parts of this
efficacious catalytic system (entries 1–6, Table 1). Additionally,
NaNO2 as the electron-transfer-medium is required for
completing the catalytic cycle.15 In other words, NaNO2 could
release NO and NO2, and the oxidation of NO into NO2
undergoes easily with O2. On the other hand, the aliphatic
alcohol was inert in this study (Table S2, ESIw), suggesting
that the reaction could not undergo an ‘oxoammonium’
mechanism16 or a FeCl3/TEMPO/NaNO2 process.17 ILs were
reported to stabilize the anionic oxygen radical intermediate
and thus accelerate electron transfer reactions.18 Indeed, the
EPR (electron paramagnetic resonance) experiment could
show the existence of the radical species in situ generated in
the reaction (Fig. S1, ESIw).19 Moreover, the iron valence
could be changeable during the reaction being detected by the
ESI-MS technique (electrospray ionization mass spectrometry,
Fig. S2, ESIw). In short, the oxidation would go through an
electron transfer process mediated by [Imim-TEMPO][FeCl4]
and NaNO2.
Fig.
1
A: Field dependence of the mass susceptibility wg for
[Imim-TEMPO][FeCl4] at 298 K; B: Temperature dependence of the
mass susceptibility wg at a field of 500 Oe for [Imim-TEMPO][FeCl4]:
electronic state of iron(III) (5.92 mB)2 and the S = 1/2 radical
spin of TEMPO (1.73 mB).7 In comparison with the bimagnetic
ILs [Imim-TEMPO][FeCl4], [Imim-TEMPO][Cl] gave much
smaller values of the gram and molar magnetic susceptibilities
at 298 K, i.e. 24.5 Â 10À7 emu gÀ1 and 0.000816 emu molÀ1
,
respectively.
The catalytic application of the task-specific bimagentic
imidazolium salts [Imim-TEMPO][FeCl4] for aerobic oxida-
tion of alcohols was further investigated. An initial experiment
was carried out by using benzyl alcohol as the substrate in the
presence of 5 mol% of the bimagnetic imidazolium salts under
organic solvent-free conditions. The results are summarized in
Table 1. The reaction did not occur in the absence of any
component of the catalytic system (entries 1–6, Table 1). Further-
more, FeCl3/NaNO2 shows no activity without any TEMPO
and the IL (entry 6). In other words, [Imim-TEMPO][FeCl4],
NaNO2 and O2 are prerequisite to performing those reactions
smoothly. In particular, [Imim-TEMPO][Cl] and [Imim][FeCl4]
were found to be inactive, indicating that combination of both
[FeCl4] and TEMPO parts probably plays a crucial role in
promoting the aerobic oxidation (entries 4 and 5). After rough
optimization of the reaction conditions, quantitative yields
with excellent selectivity could be achieved at mild conditions
(30 1C, 0.2 MPa O2, 1.5 h) (entries 7–10, Table 1; entries 12 and
13, Table S1 (ESIw)). It is worth mentioning that the reaction
also performed well even while using air as an oxidant, and was
totally immune to moisture (entries 11 and 12).
Table 1 Oxidation of benzylic alcohol catalyzed by the designed
bimagnetic imidazolium salta
Entry
PO2/Mpa
t/h
Conv.b/%
Yieldb/%
1
0
1
1
1
1
1
1
0.6
0.2
0.1
0.1
0.2
1
1
1
1
1
1
2
1
1
0.4
1
0.5
0
0
100
69
1
0.3
1
0.5
0
2c
3d
4e
5f
6g
7
0
99
68
99
99
63
99
8
9
10
11h
12i
In conclusion, a type of novel bimagnetic imidazolium salt
([Imim-TEMPO][FeCl4]) was developed and its paramagnetic
properties were confirmed by the use of SQUID measure-
ments. Notably, [Imim-TEMPO][FeCl4] with cooperative
functionalities proved to be a highly effective catalyst for
selective oxidation of a variety of aromatic alcohols under
mild conditions without utilization of any organic solvent.
Thus, the bimagnetic imidazolium salt able to be designed for
specific applications through the choice of the respective
magnetic core appears to be very appealing in view of creating
novel chemistry as well as developing extremely valuable
1.5
24
48
100
100
65
1.5
100
a
Reaction conditions: benzyl alcohol (1.93 mmol), [Imim-TEMPO][FeCl4]
b
(5 mmol%), NaNO2 (5 mmol%), 30 1C. Yields determined
by GC. Without [Imim-TEMPO][FeCl4]. Without NaNO2.
c
d
e
[Imim-TEMPO][FeCl4] was replaced by [Imim-TEMPO][Cl].
g
f
[Imim][FeCl4] was used instead of [Imim-TEMPO][FeCl4]. FeCl3
(5 mmol%), NaNO2 (5 mmol%) in the absence of any TEMPO and
h
the IL. Air as oxygen resource. 0.3 mL H2O was added.
i
c
2698 Chem. Commun., 2011, 47, 2697–2699
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Table 2 Oxidation of various alcoholsa
Entry
Alcohol
Product
T/1C
t/h
1.5
10
3
10
3
14
3
2
2
24
24
Conv.b (%)
Yieldb (%)
1c
2c
3
PhCH2OH
PhCHO
30
30
30
30
30
30
60
100
100
30
100
7
80
3
88
90
91
100
82
100
67
99
6
80(78)
2
87(83)
90
91
100(94)
81(80)
>99
64
4-MeO–C6H4CH2OH
4-MeO–C6H4CH2OH
3-MeO–C6H4CH2OH
3-MeO–C6H4CH2OH
2-MeO–C6H4CH2OH
4-Me–C6H4CH2OH
4-NO2–C6H4CH2OH
2-NO2–C6H4CH2OH
PhCH(OH)CH3
4-MeO–C6H4CHO
4-MeO–C6H4CHO
3-MeO–C6H4CHO
3-MeO–C6H4CHO
2-MeO–C6H4CHO
4-Me–C6H4CHO
4-NO2–C6H4CHO
2-NO2–C6H4CHO
PhC(O)CH3
4c
5
6
7
8
9
10
11
PhCH(OH)Ph
PhC(O)Ph
100
12
13
30
30
4
100
100
98
98
12
14
30
24
73
71
a
b
Reaction conditions: alcohol (1.93 mmol), [Imim-TEMPO][FeCl4] (5 mol%), NaNO2 (5 mol%), H2O (0.3 mL), O2 (0.2 MPa). Determined by
c
GC, values in parentheses refer to isolated yields. In the absence of H2O.
materials with great potential applications. Further studies on
mechanistic investigations and the use of a magnet to recover
magnetic ILs are in progress and will be reported in due
course.
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14 Catalyst recycling. Conversion and yield of acetophenone: run 1,
100% and 99.6%; run 2, 99.7% and 98.4%; run 3, 94.2% and
93.9%; run 4 (when NaNO2 was not replenished), 0% and 0%; run
5, 92.1% and 91.3%.
Support by the grant from the National Natural Sciences
Foundation of China (No. 20672054, 20872073), National
Natural Science Funds for Distinguished Young Scholar
(20625618) and National Basic Research Program of China
(973 program, 2009CB219901) is gratefully acknowledged.
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c
This journal is The Royal Society of Chemistry 2011
Chem. Commun., 2011, 47, 2697–2699 2699