Synthesis of multicarboxylic acid appended imidazolium ionic liquids and their application in palladium-catalyzed selective oxidation of styrene

Article abstract of DOI:10.1039/b702573d

A series of multicarboxylic acid appended imidazolium ionic liquids (McaILs) with chloride [Cl]^- or bromide [Br]^- as anions have been synthesized and characterized. Deprotonation of these ionic acids gives the corresponding zwitterio

Full text of DOI:10.1039/b702573d

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Synthesis of multicarboxylic acid appended imidazolium ionic liquids and
their application in palladium-catalyzed selective oxidation of styrenew
Xuehui Li,*a Weiguo Geng,a Jixiang Zhou,a Wei Luo,a Furong Wang,a Lefu Wanga
and Shik Chi Tsang*b
Received (in Durham, UK) 19th February 2007, Accepted 13th July 2007
First published as an Advance Article on the web 9th August 2007
DOI: 10.1039/b702573d
A series of multicarboxylic acid appended imidazolium ionic liquids (McaILs) with chloride [Cl]À
or bromide [Br]À as anions have been synthesized and characterized. Deprotonation of these ionic
acids gives the corresponding zwitterions. Re-protonation of the zwitterions with strong Brønsted
acids gives a series of new ionic acid-adducts, many of which remained as room-temperature ionic
liquids. A new catalytic system, McaIL/PdCl2 for the selective catalytic oxidation of styrene to
acetophenone with hydrogen peroxide as an oxidant has been attempted. In the presence of
McaILs, it is found that the quantity of palladium chloride PdCl2 used can be greatly reduced
while the activity (TOF) and selectivity towards acetophenone are enhanced sharply. It is also
shown that the catalytic properties of this system could be finely tuned through the molecular
design of the McaILs. The best TOF value obtained so far is 146 hÀ1 with 100% conversion of
styrene at 93% selectivity to acetophenone. In addition, the catalytic activity has been maintained
for at least ten catalytic cycles.
an extraordinary stabilizing effect for zero-valent transition
Introduction
metals such as Pd(0) and Pt(0), and thus can increase their
Ionic liquids (ILs) incorporating functional groups on their
catalytic activities (formation of smaller metal nanoparticles of
cations and/or anions as new solvent media have started to
high surface area through the stabilization effect) and reduce
the extent of metal leaching into solution.9 Strong sulfonic
receive considerable attention, particularly in the areas of
homogeneous and heterogeneous catalysis.1 Functional
acid ILs are typical examples of functionalized ILs that can be
used as solvents and catalysts for etherification reactions.10
moieties attached in the cations or anions often show a
Imidazole11 and imidazolium salts12 with carboxylic groups
significant influence on certain processes, so that much effort
has been devoted to the design of the functionalized ILs
have known for many years, many of them being
in order to control processes where the IL is involved as
valuable starting materials for organic synthesis. Zwitterionic
solvent or catalyst.2 It is without doubt that the imidazolium
imidazolium salts with carboxylic group attached in the
2-position,13 giving weaker acidic properties are also well
based ILs are the focus of most investigations, due, in part, to
studied.14 Most recently, a series of Brønsted acidic ionic
the flexibility of varying the nature of the groups attached to
the N-atoms, as well as on the ring carbons, in order to modify
liquids with one or two carboxylic acid groups attached to
the imidazolium backbone have been reported.15 These ILs
the physical and chemical properties of the designed ILs.
Pyridinium,3 and ammonium based ILs have also been func-
show stronger acidic properties compared to the correspond-
tionalized,4 but to a much lesser extent. Upon incorporating a
ing non-ionic carboxylic acids, and can dissolve many group II
functional group into an ionic liquid structure specific proper-
metal carbonates and readily react with many transition
ties are imparted to the liquid. For example, ILs with groups
metals such as elemental zinc and cobalt, forming coordina-
containing lone pairs of electrons such as imidazole,5
tion polymers with interesting topologies.16 However, the
pyridine,6 pyrazole7 or even weaker nitrile groups8 can
potential applications of these acids ILs in organic processes
coordinate effectively to transition metals. They often show
have been seldom explored. By varying the cations and anions,
many of these ionic carboxylic acids can be obtained as
a School of Chemical and Energy Engineering, The Guangdong
Provincial Laboratory of Green Chemical Technology, South China
University of Technology, Guangzhou, 510640, P. R. China.
E-mail: cexhli@scut.edu.cn; Fax: 86 20 871 14707;
Tel: 86 20 8711 4707
room-temperature ionic liquids, and it would be interesting
to investigate their potential application as catalysts or
co-catalysts.
The Wacker oxidation reaction was chosen because it is an
important process widely used in industry17 and is also a
classical reaction for many catalytic studies.18 The traditional
process for Wacker reaction involves the use of organic solvent
and excess of H2O2.19 Recently, a technique for the use of a
much more convenient oxygen source, such as air and
b Surface and Catalysis Research Centre, School of Chemistry,
University of Reading, Whiteknights, Reading, UK RG6 6AD.
E-mail: s.c.e.tsang@reading.ac.uk; Fax: 0044 118 378 6632;
Tel: 0044 0118 378 6346
w Electronic supplementary information (ESI) available: DSC data.
See DOI: 10.1039/b702573d
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molecular oxygen was adopted.17b HCl is often used in the
system in order to re-oxidize the zero-valent palladium to
Pd(II). Since olefins are usually poorly soluble in water,
expensive phase-transfer agent has to be used. Oxidation
reactions of styrene with H2O2 or O2 using an ionic liquid or
mixture of ionic liquid and water as solvents have previously
been reported, but problems related to the solubility and the
recycling of the catalyst (PdCl2) apparently remained.20 The
low selectivity of this system often leads to the formation of a
mixture including acetophenone, benzaldehyde, benzoic acid
and other products which require further separation. Another
major drawback is that the ionic liquids and catalyst (PdCl2)
can be hardly reused due to the contamination of the side-
products that are formed during the oxidation process.
Moreover, the variety of products in the oxidation of styrene
with H2O2 could be oriented through careful design of the
catalyst system,21 which has a wider significance both in
theoretical design of catalyst and industrial production
process.
common polar solvents such as acetonitrile, acetone,
dichloromethane and chloroform, but insoluble in diethyl
ether or hexane. Heating 3a under reflux in an aqueous
solution of hydrochloric acid (37% w/w) gave ionic acid 3b
in quantitative yield. A slight excess of HCl was essential in
order to give full conversion of the esters to the acids. After the
reaction was complete, the side product and the solvent were
easily removed by heating the reaction mixture under mild
vacuum.
Similarly, 4b and 5b were obtained in high yield by reacting
4a and 5a with aqueous HBr or HCl. Unlike the
ester precursors, compounds 3b, 4b and 5b were found to be
well soluble in water, slightly soluble in DMSO and
totally insoluble in other solvents such as dichloromethane
and chloroform. The insolubility of these acids may be
due to the formation of extended polymeric networks in the
solid state arising from hydrogen bonds (for example
through COO–HÁ Á ÁF, COO–HÁ Á ÁO). Compared to the
1,3-dialkylimidazolium based ILs, there is increased
tendency of formation of hydrogen bonding in carboxylic
acid ILs.15,16
In this paper we describe the synthesis, characterization
and the reactivity of a novel series of imidazolium ionic liquids
incorporating multicarboxylic acid functionalities. In
addition, a new catalyst system, McaILs/PdCl2 employing
these McaILs and PdCl2 for selective oxidation of styrene
was investigated. The presence of the carboxylic groups not
only renders the catalysts to display higher conversion and
selectivity towards desirable products but also significantly
reduces the need for a high level of Pd content.
In the IR spectrum (data available in ESIw), 3b, 4b and 5b
displayed strong absorptions at 3350–3500 cmÀ1, representing
the O–HÁ Á ÁO/O–HÁ Á ÁCl(Br) hydrogen bonds. In addition,
strong–medium absorptions in the region 2500–2800 cmÀ1
were also observed, representing the C–HÁ Á ÁCl or C–HÁ Á ÁCl
interactions, typical of imidazolium halides.22 The 13C NMR
spectrum of 3b in D2O exhibited three sharp signals at 169, 170
and 171 ppm, indicative of the presence of three different
carboxylic groups. Similar results were also obtained from
4b and 5b.
Results and discussion
Synthesis of multicarboxylic acid imidazolium halide precursors
Synthesis of multicarboxylic acid imidazolium zwitterions and
McaILs
The synthetic route for novel multicarboxylic acid imidazo-
lium halide precursors (McaILs) is illustrated in Scheme 1.
The precursors 1 and 2 were synthesized following a literature
method by Michael addition of acrylate to imidazole.11
Quaternization of 1 and 2 with chloroacetic acid ethyl
ester or ethyl bromide gave the corresponding imidazolium
salts 3a, 4a and 5a, respectively, with the carboxylic acid
ester groups being attached. The ester compounds were
all very hygroscopic. They were found to be soluble in
Reaction of the ionic acids 3b, 4b and 5b with the organic
base triethylamine in 1 : 1 molar ratio gave corresponding
zwitterions 3c, 4c and 5c (Scheme 2). Similar to the ionic
acids 3b, 4b and 5b, these zwitterions were found to be very
soluble in water, but only slightly soluble in DMSO and
totally insoluble in other common organic solvents. This
enabled the facile separation of the zwitterions from the
Scheme 1 Synthetic route of acidic imidazolium salts 3a/3b–5a/5b: Reagents and conditions: (i) ClCH2COOEt (for 3a and 5a), BrEt (for 4a), 60 1C,
24 h; (ii) 37% HCl (3b and 5b) or 48% HBr (for 4b) aqueous solution, 100 1C, 2 h.
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Scheme 2 Synthetic route to acidic imidazolium salts 3c–5c and 3d–3g, 4d–4g and 5d–5g: d, A = BF4, e: A = PF6, f: A = CF3CO2, g: A =
CF3SO3.
reaction mixture, since the zwitterions were insoluble in
this solvent whereas the formed ammonium salts were well
solubilised. Formation of zwitterions using other bases such as
tributylamine or pyridine was also studied. 3c, 4c and 5c
were characterized using standard spectroscopic methods. In
D2O solution, the protons of the carboxylic groups can not be
observed, and even the 2-H of the imidazolium ring undergoes
rapid H–D exchange. It is interesting to note that for all the
zwitterions 3c, 4c and 5c, use of excess base did not lead to
further deprotonation of the other carboxylic protons.
Synthetic route to acidic imidazolium salts 3c–5c and 3d–3g,
4d–4g and 5d–5g: d, A = BF4, e: A = PF6, f: A = CF3CO2,
g: A = CF3SO3.
The carboxylic group COOÀ can act as a proton acceptor,
and thus may be protonated using stronger acids. This
differÀs significantly from other zwitterions, for example
–SO3 groups, which can not be eaÀsily protonated. Imidazo-
lium based zwitterions with SO3 attached to the side-
chain react with aqueous HCl gave a mixture of the zwitterion
and HCl. Upon washing the obtained mixture with
cold acetone the HCl can be removed, giving the zwitterion
back. Reaction of the zwitterions 3c, 4c and 5c with strong
Brønsted acids HBF4, HPF6, CF3COOH and CF3SO3H gave
a series of new ionic acids 3d–3g, 4d–4g and 5d–5g. Since all
the precursors were acids, the products 3d–3g, 4d–4g and
5d–5g can be considered as acid–acid adducts. All these new
acids were viscous liquids at room temperature and become
waxes on cooling at a temperature of À20 1C; only glass
transitions were observed and no obvious melting point
was detected (data available in ESIw). These acids were found
to be stable and showed no decomposition even up to 250 1C
with decomposition only at a temperature above 280 1C
(data available in ESIw).
Application of McaILs in Wacker oxidation process
The ionic and acidic nature of 3b, 3d–3g, 4b, 4d–4g and 5b,
5d–5g made these compounds interesting candidates as cata-
lysts. The oxidation process of styrene is always complicated
and often leads to the formation of mixtures including
acetophenone, benzaldehyde, benzoic acid and other products
(Scheme 3).
The initial screening of the ionic carboxylic acids, 3b, 4b and
5b and 3d–3g/4d–4g/5d–5g in the absence of palladium
chloride suggested that all these McaILs show little catalytic
activities on their own: oxidation of styrene took place slowly
(entry 1, Table 1) and the selectivity of acetophenone was very
poor with the main product being benzaldehyde. Without the
presence of the McaILs, styrene could be catalytically oxidized
to acetophenone by palladium chloride with relatively higher
selectivity although such oxidation took place very slowly with
a TOF value of only 21 hÀ1 after 8 h reaction time. (entry 2,
Table 1).
When a combination of McaILs and palladium chloride was
used, all the McaIL/PdCl2 systems showed better catalytic
properties with the yield and selectivity improved significantly.
After investigating all the possible combinations we found that
the reaction temperature was the most important factor for the
rate of the oxidation, independent of the nature of the cation
and anions. Another important factor was the particular
structure of the McaIL used. In general, the ILs with ClÀ or
BrÀ as anions, namely the ILs 3b, 4b and 5b gave the highest
conversions of the styrene and highest selectivities towards
acetophenone. This is probably due to the favorable inter-
actions of PdCl2 with ILs 3b, 4b and 5b. Addition of PdCl2
to the acidic ILs 3b gave immediately an orange solution.
Apparently, the PdCl2 reacted with the coordinating anion
2À
ClÀ or BrÀ to form PdCl42À/PdCl2Br2 anions, as evidenced
The IR and NMR spectroscopic data of the acid–acid
adducts 3d–3g, 4d–4g and 5d–5g showed a close similarity
to the acids 3b, 4b and 5b, and the anions had little
effect on the chemical shifts in the 1H and 13C NMR
spectra. In the ESI-MS spectra, all the ILs showed strong
absorptions of the cation, anions and their aggregates
(e.g. [(Cation)3(Anion)2]+, [(Cation)4(Anion)3]+ in the posi-
tive mode). Similar results have been observed in other ionic
compounds.23
by the presence of a peak at 124 from the negative mode
Scheme 3 Oxidation of styrene.
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a
Table 1 Selective oxidation of styrene catalyzed by McaILs/PdCl2
Entry
IL
IL/Pd (mol/mol)
TOFd/hÀ1
C (%)
S (%)
1
2
3
4
5
6
a
3bb
—
3b
3d
4b
5b
—
0
1.1
2.0
1.4
2.3
—
21
146
125
116
92
8
25
100
100
84
7
86
93
91
92
92
c
80
Reaction conditions: reaction was carried out in a 250 mL round-
bottomed flask fitted with a condenser; styrene (250 mmol, 26.03 g),
30% H2O2 (287.5 mmol, 32.59 g), PdCl2 (0.25 mmol, 0.0446 g),
b
c
temperature = 55 1C. Without PdCl2, 8 h. Without McaIL,
8 h. TOF: turnover frequency: number of moles of styrene converted
d
per mole of palladium catalyst per hour. C (%): conversion of styrene;
S (%): selectivity of acetophenone.
Fig. 1 Effect of temperature on the oxidation of styrene with H2O2 in
the presence of 3b/PdCl2: selectivity of acetophenone (B), selectivity of
benzaldehyde (n), conversion (K) and TOF value (&). Reaction
conditions: reactions were carried out in a 250 mL round-bottomed
flask fitted with a condenser until the conversion of styrene maintained
invarient; styrene (250 mmol, 26.03 g), 30% H2O2 (287.5 mmol,
32.59 g), PdCl2 (0.25 mmol, 0.0446 g), 3b : PdCl2 = 1.1 : 1.0; TOF:
turnover frequency, number of moles of styrene converted per mole of
palladium catalyst per hour; C (%): conversion of styrene; S (%):
selectivity of acetophenone.
ESI-MS (for 3b and 5b) and 168 (for 4b) representing
the presence of [PdCl4]2À , [PdCl2Br2]2À
1
2
(
m/z due to the
À2 charge).
Using the ionic acid 3b and PdCl2, the optimal temperature
was found at 55 1C (Fig. 1), under which conditions
the conversion of styrene was virtually 100% within 7 h of
reaction time and the measured TOF value (146 hÀ1) was
six times higher than that of PdCl2 catalyst. Furthermore, the
selectivity of acetophenone was as high as 93%. For
all McaIL/PdCl2 catalyst systems, with the increase of
molar ratio of McaIL to PdCl2, the selectivity of acetophenone
increased, however, it was found that the conversions
of styrene and the TOF values peaked at a certain molar
ratio, which was different in different McaIL systems. For
example, using the same cation the optimized molar ratio for
3b to PdCl2 was 1.1 (entry 3), 2.0 for 3d (entry 4) while the
optimized molar ratio for 3b, 5b with ClÀ anion and 4b with
BrÀ to PdCl2 was 1.1 (entry 3), 2.3 (entry 6) and 1.4 (entry 5),
respectively.
acid, which is always the by-production of the selective
oxidation of styrene, cannot be detected upon any McaIL/
PdCl2 samples, implying that McaILs/PdCl2 could inhibit the
generation of deep oxidation products of styrene. This could
be rationalised in that the deep oxidation products are sup-
pressed as the transition process of the intermediate transient
state is prevented to form carboxylic groups by these carboxyl
groups present in the McaIL/PdCl2 systems. Further study
on the detailed catalytic mechanism of McaILs/PdCl2 is on-
going in our laboratory and the results will be communicated
elsewhere.
Other acids with BF4À, PF6À, CF3COOÀ and CF3SO3
To illustrate the advantage of the McaIL/PdCl2 catalytic
systems we compared our McaILs/PdCl2 with C4mimBF4/
PdCl2, C4mimPF6/PdCl2, oxalic acid/PdCl2, succinic acid/
PdCl2 and acetic acid/PdCl2 systems. For the C4mimBF4/
PdCl2 and C4mimPF6/PdCl2 systems we achieved similar
results as those reported in the literature with the selectivity
of acetophenone being slightly lower.26 In the presence of
palladium chloride, these neutral carboxylic acid–PdCl2
systems could catalyze the oxidation of styrene but they all
gave consistently lower conversions (28–46%) with lower
selectivity towards acetophenone (73–84%).
À
anions gave slightly lower conversions than 3b/PdCl2,
4b/PdCl2 and 5b/PdCl2. In most cases, the conversion can
reach as high as 90%, but the selectivity to the side-product
benzaldehyde was significantly increased (2–23%) at the ex-
pense of acetophenone. Anion effects have been observed in
other catalytic systems where the anions of the ionic liquids
played a major role in the catalytic performance.24 As in this
case, the reduced activity of 3d–3g, 4d–4g and 5d–5g/PdCl2
systems could be attributed to the interaction(s) between
palladium chloride and the McaILs. In contrast to the ILs
3b, 4b and 5b, ILs 3d–3g, 4d–4g and 5d–5g did not form a
homogeneous solution upon addition of PdCl2. The suspen-
sions obtained instead can be partially dissolved upon heating
the mixture to 55 1C. The solids, isolated from the suspension
through centrifugation, all showed an absorption at 1650 cmÀ1
in the IR spectra, a remarkable shift compared to the absorp-
tion value of the corresponding free acid (1720 cmÀ1), which
strongly indicates a coordination of Pd ion with carboxylic
groups as (COO–Pd).25 Though the coordination mode the
carboxylic groups to the palladium could be complicated, we
believe, due to the presence of the multi-carboxylic groups,
formation of polymeric structures is possible. Phenylformic
A further major advantage of using McaILs was reflected in
their facile separation and recycling with the palladium
catalyst. After distillation of the reaction mixture,27 the
impurities formed during the oxidation process can all be
removed by washing the McaIL–palladium containing
mixture with cold acetone. Due to the low solubility of the
McaILs, most of the McaILs/Pd can be recovered. In our
previous paper, the susceptibility of dialkylimidazolium ionic
liquids to oxidative degradation under oxidizing conditions
was reported,28 but it should be mentioned here that the
oxidation of McaILs could not be observed in the present
work. Thus, the obtained McaIL/PdCl2 mixtures can be
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