Schiff base structured acid-base cooperative dual sites in an ionic solid catalyst lead to efficient heterogeneous knoevenagel condensations

Article abstract of DOI:10.1002/chem.201201338

An acid-base bifunctional ionic solid catalyst [PySaIm]₃PW was synthesized by the anion exchange of the ionic-liquid (IL) precursor 1-(2-salicylaldimine)pyridinium bromide ([PySaIm]Br) with the Keggin-structured sodium phosphotungstate (Na

Full text of DOI:10.1002/chem.201201338

FULL PAPER
DOI: 10.1002/chem.201201338
Schiff Base Structured Acid–Base Cooperative Dual Sites in an Ionic Solid
Catalyst Lead to Efficient Heterogeneous Knoevenagel Condensations
Mingjue Zhang,[a] Pingping Zhao,[a] Yan Leng,[b] Guojian Chen,[a] Jun Wang,*[a] and
Jun Huang[a]
Abstract: An acid–base bifunctional
ionic solid catalyst [PySaIm]3PW was
synthesized by the anion exchange of
the ionic-liquid (IL) precursor 1-(2-sali-
cylaldimine)pyridinium bromide ([Py-
SaIm]Br) with the Keggin-structured
sodium phosphotungstate (Na3PW).
The catalyst was characterized by
FTIR, UV/Vis, XRD, SEM, Brunauer–
imine, and provides
a
controlled
95.8% and a selectivity of 100%; the
conversion is even much higher than
that (78.2%) with ethanol as a solvent.
The solid catalyst has a convenient re-
coverability with only a slight decrease
in conversion following subsequent re-
cyclings. Furthermore, the new catalyst
is highly applicable to many substrates
of aromatic aldehydes with activated
methylene compounds. On the basis of
the characterization and reaction re-
sults, a unique acid–base cooperative
mechanism within a Schiff base struc-
ture is proposed and discussed, which
thoroughly explains not only the highly
efficient catalytic performance of [Py-
SaIm]3PW, but also the lower activities
of various control catalysts.
nearby position for the acid–base dual
sites. The high melting and insoluble
properties of [PySaIm]3PW are relative
to the large volume and high valence
of PW anions, as well as the intermo-
lecular hydrogen-bonding networks
among inorganic anions and IL cations.
The ionic solid catalyst [PySaIm]3PW
leads to heterogeneous Knoevenagel
condensations. In solvent-free conden-
sation of benzaldehyde with ethyl cya-
noacetate, it exhibits a conversion of
Emmett–Teller (BET) theory, thermo-
1
G
N
ACHTUNGTRENNUNGcopy, ESI-MS, elemental analysis, and
melting points. Together with various
counterparts, [PySaIm]3PW was evalu-
ated in Knoevenagel condensation
under solvent and solvent-free condi-
tions. The Schiff base structure attach-
ed to the IL cation of [PySaIm]3PW in-
volves acidic salicyl hydroxyl and basic
Keywords: heterogeneous catalysis ·
heteropolyacids
· ionic liquids ·
Knoevenagel condensation · Schiff
bases
Introduction
media with the advantages of low volatility, high thermal
stability, adjustable solubility, and versatile structures.[7] In
particular, “task-specific” IL catalysts tethered with basic
and/or acidic functional groups have been reported for
Knoevenagel reactions.[3c,d,4] For example, Boronat et al.[5g]
prepared an acid–base IL catalyst, [diamine-A]BF4, with
Knoevenagel condensation of a carbonyl functionality with
an activated methylene compound is a classic C C bond-
À
forming reaction for producing important intermediates in
the fine-chemicals industry.[1,2] It is catalyzed by bases[3] or
acids.[4] A number of researchers have revealed the effec-
tiveness of acid–base bifunctional catalysts[5] on account of
the synchronic activation of carbonyl on acid sites and the
capture of the proton from methylene on basic sites.[6] On
the other hand, ionic liquids (ILs) are well-known reaction
À
À
every acid and base site being separated by a CH2 group.
It showed improved activity for the condensation of benzal-
dehyde with methylene compounds. However, that system
needed a large amount of IL (20 mol%), and the resultant
homogeneous reaction encountered difficulty in catalyst re-
cycling. To realize convenient catalyst recovery, efforts have
been made to prepare heterogeneous bifunctional catalysts
by supporting organic bases on acidic porous carriers;[5a,f]
nevertheless, this approach could not control the distance
between acid and base sites, which is a key influential factor
for an acid–base bifunctional catalyst to enhance the reactiv-
ity of a Knoevenagel condensation.[8] Thus, it is of interest
to design an efficient acid–base bifunctional catalyst with a
controllable acid–base distance for heterogeneous Knoeve-
nagel condensations.
[a] M. Zhang, P. Zhao, G. Chen, Prof. Dr. J. Wang, Prof. Dr. J. Huang
State Key Laboratory of Materials-Oriented
Chemical Engineering
College of Chemistry and Chemical Engineering
Nanjing University of Technology
Nanjing, Jiangsu, 210009 (P.R. China)
Fax : (+86)25-83172264
E-mail: junwang@njut.edu.cn
[b] Dr. Y. Leng
School of Chemical and Material Engineering
Jiangnan University, Wuxi, Jiangsu
214122 (P.R. China)
Heteropolyacids (HPAs) have been widely used as the
catalysts for organic transformations,[9] but the common
Keggin HPAs with a strong intrinsic acidity are rarely used
for Knoevenagel condensations. Significant advances in this
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.201201338.
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area are the use of a unusual dimeric disilicoicosatung-
state [H(g-SiW10O32)2A
(m-O)4]7À and its parent divacant g-
Keggin silicodecatungstate [g-SiW10O34A
(H2O)2]4À as homo-
CTHUNGTRENNUNG
CTHUNGTRENNUNG
geneous basic catalysts,[10] and the development of a heter-
ogeneous metal–organic framework (MOF) catalyst that
encapsulates a Cr3+-substituted lacunary HPA.[11] Another
strategy for modifying HPA catalysts is the combination
of HPA anions with organic cations through ionic linkag-
es,[12] which is also an interesting topic in IL chemistry.[13]
Recently, HPA–IL hybrids have been reported and ap-
plied in electrochemistry, phototropy, and materials sci-
ence.[14] Furthermore, the structures of single crystals for
some imidazolium salts with HPA anions have been deter-
mined.[15] In the field of catalysis, Luo et al.[16] modulated
À
the stereoselectivity of homogeneous asymmetric C C
bond-forming reactions over a bifunctional catalyst that
involved the basic sites of chiral amine ILs and the acidic
centers of Keggin phosphotungstic acid. Our group[17] de-
veloped a series of HPA–IL ionic catalysts by pairing
Keggin HPA anions with “task-specific” IL cations. Be-
cause of the high valence and large volume of HPA
anions, the resulting catalysts were insoluble solid-state
compounds, which mostly behaved as the heterogeneous
catalysts for organic syntheses.
Moreover, as well-known ligands, Schiff bases have long
attracted research interest for synthesis, structure, and reac-
tivity in coordination chemistry, catalysis, and materials sci-
ence.[18] In terms of ILs, Schiff bases have been attached to
imidazolium tags for producing novel Schiff base ILs for re-
covering metal catalysts[19] or coordinating metal ions.[20] Ac-
tually, a Schiff base molecule itself involves a Lewis basic
Scheme 1. Synthesis of the HPA-based Schiff base tethered ionic catalyst
[PySaIm]3PW and various counterparts.
À
which bears only base C=N; [MimSaIm]3PW with pyridini-
um replaced by imidazolium; and PySaIm(H)-PW, using the
strong acidic H3PW12O40 as the ion exchanger. Other control
samples [PyAM]3PW, [PyTA]3PW, and [PyHy]3PW were
prepared by pairing PW anions with the respective common-
ly seen Schiff base free mono-basic or mono-acidic IL cati-
ons (Scheme 2).
À
À
C=N and a weak Brønsted acidic OH group, with the
acid–base distance being controlled by the Schiff base struc-
ture. Therefore, it should be very interesting to employ a
Schiff base structure as an acid–base bifunctional catalytic
unit for Knoevenagel condensations; however, to the best of
our knowledge, there are no previous reports of Schiff base
molecules being used in this way.
In this study, we prepared a HPA-based ionic solid [Py-
SaIm]3PW by pairing Keggin phosphotungstic HPA anions
(PW) with pyridine-based Schiff base tethered IL cations
(PySaIm) (the top line of Scheme 1). This ionic catalyst is
task-specifically designed for Knoevenagel reactions, be-
cause this Schiff base is not used as a traditional ligand for
coordinating a metal, but as acid–base dual catalytic centers
Scheme 2. Synthesis of the control catalysts with Schiff base free mono-
basic or mono-acidic IL cations.
À
À
( C=N: base center; OH: acid center). To avoid protona-
Results and Discussion
À
tion of the C=N groups in IL cations by a highly acidic
HPA solution, neutral Na3PW12O40 was used as the ion ex-
changer instead of the customary H3PW12O40. Characteriza-
tions and catalytic performances for Knoevenagel reactions
were tested in detail over this HPA-based Schiff base teth-
ered ionic catalyst.
To examine the rationality and effectiveness of the design-
ing strategy for the target catalyst [PySaIm]3PW, various
counterpart hybrids were prepared for comparison
Characterizations for [PySaIm]3PW: [PySaIm]3PW was char-
acterized by FTIR, UV/Vis, XRD, SEM, Brunauer–
Emmett–Teller (BET) theory, thermogravimetric analysis
1
(TGA), H NMR spectroscopy, ESI-MS, elemental analysis,
and melting points (see the Experimental Section for de-
tails). The elemental analysis found (%) C 13.93, N 2.39,
and H 1.24, which are close to calculated (%) values of C
14.11, N 2.35, and H 1.26, which provides the formula for
[PySaIm]3PW composed of three Schiff base tethered IL
cations and one HPA anion. The TG curve (Figure S1 in the
À
(Scheme 1), including [PyHbIm]3PW with the acid OH and
base C=N sites separated by a phenyl ring; [PyBeIm]3PW,
À
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Knoevenagel Condensations
FULL PAPER
Supporting Information) showed a stable structure for [Py-
SaIm]3PW up to 2308C. The slight weight loss at the early
heating stage around 1308C owes itself to the release of
moisture and constitutional water, and the drastic weight
loss above 2308C is because of the decomposition of the or-
ganic moiety followed with the complete collapse of the in-
organic Keggin HPA structure to form P2O5 and WO3.[21]
The total weight loss of 19.42% in the range of 130–6208C
is close to the theoretical data of 19.75%, which again veri-
fies the chemical composition of [PySaIm]3PW shown in
Scheme 1. Although it has an IL-like structure, [Py-
SaIm]3PW has a high melting point of approximately 2208C
on account of the strong electrostatic interaction between
the large-volume organic cation and the inorganic HPA
anion with both large volume and high negative charge. [Py-
SaIm]3PW is also insoluble in common standard solvents, in-
cluding water, alcohols, acetone, acetonitrile, chlorinated hy-
drocarbons, THF, and DMF. These features make it possible
for [PySaIm]3PW to be tried as a heterogeneous solid cata-
lyst in liquid-phase organic transformations.
The SEM image in Figure 1(Top) illustrates spherical par-
ticles for [PySaIm]3PW with sizes of approximately 0.05–
0.2 mm. The observed submicrometer pores accumulated by
the particles account for its BET specific surface area of
8.1 m2 gÀ1. The XRD patterns of [PySaIm]3PW and pure
Na3PW12O40 are shown in Figure 1(Bottom). Na3PW12O40
presented a set of diffraction peaks for the typical crystal
structure of sodium HPA salt finely arranged by the Keggin
PW anions. For the organic HPA salt [PySaIm]3PW, howev-
er, those peaks disappeared, which indicates an amorphous
phase. It is suggested that the substitution of Na+ by the
large PySaIm+ IL cations has resulted in the loss of the
long-range crystal order of PW anions. The only weak and
broad Bragg reflection at approximately 8.68 implies micro-
sized gaps among cations and anions,[22] which makes no
contribution to the surface area tested by a nitrogen physi-
sorphtion.
Figure 1. Top) SEM image of [PySaIm]3PW. Bottom) XRD patterns of
a) Na3PW12O40 and b) [PySaIm]3PW.
Figure 2 displays the FTIR spectra of the selected sam-
ples. For [PySaIm]3PW, the bands at 1631 cmÀ1 assigned to
À1
À
the stretching vibration of C=N in imine, 3444 cm to
À1
À
À
OH, and 1276 cm to C O in an aromatic ring, are re-
spectively similar to the observations at 1632, 3413, and
1277 cmÀ1 for the IL precursor [PySaIm]Br.[23,24] Further, for
À
the bending vibration of the C H bond in pyridine, [Py-
SaIm]3PW presented an almost identical band at 684 cmÀ1 to
that of [PySaIm]Br at 683 cmÀ1. These results demonstrate
that [PySaIm]3PW has the same pyridinium-based Schiff
base tethered IL cation as the precursor sample [PySaIm]Br.
On the other hand, the four bands at 1094, 953, 858, and
808 cmÀ1 featured for the Keggin structure of neat
À
Na3PW12O40 are attributed to P Oa (central oxygen), W=O
(terminal oxygen), W-Ob-W (corner-sharing oxygen), and
W-Oc-W (edge-sharing oxygen), respectively.[25] For [Py-
SaIm]3PW, the four Keggin bands were detected clearly,
thus indicating that the framework structure of the PW
anion is thoroughly preserved. However, the shifts of the
Keggin bands for [PySaIm]3PW indicate the distortion of
Figure 2. FTIR spectra of a) [PySaIm]3PW, b) [PySaIm]Br, and
c) Na3PW12O40.
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J. Wang et al.
À
the Keggin framework on account of the extension of the
conjugated p electrons from organic cations to inorganic
anions.[26]
icyl OH and imine groups in the Schiff base structure
would play dual roles of acidic and basic catalytic centers.
The previous result for the crystal structure of a pure
Schiff base revealed a fundamental chemical phenomenon
Knoevenagel condensation of benzaldehyde and ethyl cya-
noacetate over various catalysts with ethanol as the solvent:
It is known that the polarity of a solvent can significantly
affect the reaction rate of a Knoevenagel condensation, and
a polar protic solvent such as ethanol might help to activate
the carbonyl substrate.[30] Accordingly, we first carried out
the condensation of benzaldehyde and ethyl cyanoacetate in
the presence of ethanol solvent over various catalysts, with
results listed in Table 1. [PySaIm]3PW exhibited a GC con-
version of 93.8% and a selectivity for ethyl (E)-a-cyanocin-
namate of 100%, with an isolated yield of 86.5% (Table 1,
entry 1). On account of the insolubility of the ionic-solid cat-
alyst in this polar reaction mixture, the condensation process
was a liquid–solid heterogeneous catalysis system. In con-
trast, the IL [PySaIm]Br caused a homogeneous reaction
and demonstrated a perfect 100% selectivity with a conver-
À
called keto–enol tautomerism between the OH hydrogen
[23]
À
and C=N nitrogen atoms.
In Figure 2, the broad band
around 3413 cmÀ1 for [PyBeIm]Br also indicates the forma-
tion of the intermolecular hydrogen bonding within a Schiff
À
base structure between the proton of OH in salicyl and the
nitrogen in imine, as proposed in Scheme 3. However, for
Scheme 3. Resonance structures for the proposed Schiff base of the phe-
nolic form, intramolecular hydrogen bonding, and protonated imine in
pyridine-based IL cation [PySaIm]+.
Table 1. Comparison of the catalytic performances of [PySaIm]3PW with various control samples for the
Knoevenagel condensation of benzaldehyde and ethyl cyanoacetate with ethanol as the solvent.[a]
the HPA-paired analogue [Py-
SaIm]3PW, the intensity of the
corresponding
3444 cmÀ1 was weakened dras-
tically, which indicates that the
band
at
A
ACHTUNGTRENNUNG
presence of PW anions has
greatly hindered the formation
of the intramolecular hydrogen
bonding. Alternatively, the ex-
tended intermolecular hydro-
gen-bonding networks are gen-
erated between IL cations and
PW anions[27] in addition to the
strong electrostatic interaction.
This is a reason for the solid
nature of [PySaIm]3PW.
Entry
Catalyst
[PySaIm]3PW
[PySaIm]Br
PySaIm(H)–PW
[PyHbIm]3PW
[MimSaIm]3PW
[PyBeIm]3PW
[PyTA]3PW
[PyAM]3PW
[PyHy]3PW
without
Solubility in reaction
Conversion[b] [%]
Selectivity[c] [%]
1
2
3
4
5
6
7
8
9
G
insoluble
soluble
93.8
91.4
32.0
75.3
81.4
65.6
67.8
77.0
60.3
25.7
100 (86.5)
100
AHCTUNGTRENNUNG
insoluble
insoluble
insoluble
insoluble
insoluble
insoluble
insoluble
–
ACHTUNGTRNE(NUNG 26.8)
G
100
100
100
100
100
100
100
ACHTUNGTRENNUNG
ACHTUNGTRENNUNG
ACHTUNGTRENNUNG
ACHTUNGTRENNUNG
ACHTUNGTRENNUNG
10
[a] Reaction conditions: catalyst (0.1 g; 0.027 mmol for [PySaIm]3PW), benzaldehyde (8 mmol), ethyl cyano-
AHCTUNGTRENNGaUN cetate (8 mmol), ethanol (5 mL), 708C, 4 h. [b] GC conversion based on ethyl cyanoacetate. [c] GC selectivity
The intramolecular electronic
behavior in the organic moiety
of [PySaIm]3PW was further
measured by the UV/Vis spec-
trum (Figure S2 in the Support-
(isolated yield in parenthesis) for the target product ethyl (E)-a-cyanocinnamate, m.p.: 49–508C; 1H NMR
(300 MHz, CDCl3): d=1.40 (t, 3H), 4.39 (q, 2H), 7.48–7.59 (m, 3H), 8.00 (d, 2H), 8.25 ppm (s, 1H) (see Fig-
ure S3 in the Supporting Information).
ing Information). The adsorption bands in the range of 210–
300 nm are attributed to the p–p*-electron transitions of
benzene rings.[28] Though partly overlapped by the p–p*
bands, the shoulder peak at 315 nm assigned to the free phe-
nolic form near the imine group was observed clearly.[29] In
addition, the broad and weak band at 415 nm reflects the
ionic tautomer of the protonated imine (Scheme 3), which
results from the intramolecular proton transfer from the sal-
sion of 91.4% (Table 1, entry 2). One can see that the heter-
ogeneous catalyst [PySaIm]3PW is even more active than
the homogeneous [PySaIm]Br, with the added advantage of
much easier recovery from a reacted mixture.
Noticeably, relative to [PySaIm]3PW, all the PW-paired
IL-like counterparts presented lower catalytic activities for
the same condensation reaction (Table 1, entries 3–9). When
using H3PW12O40 instead of Na3PW12O40 as the starting ma-
terial to conduct the anion exchange with [PySaIm]Br, the
obtained hybrid PySaIm(H)–PW gave a conversion of
32.0%, which is at a similarly low level to the noncatalyst
system (Table 1, entries 3 and 10). The extremely low activi-
ty is associated with the partial elimination of the basicity of
imine groups in the Schiff base structure on account of the
protonation by the highly acidic H3PW12O40.[17c,d] Very inter-
À
icyl OH to the nitrogen atom of imine. The higher intensity
of the peak of the 315 nm band over that of 415 nm, togeth-
er with the lowered IR band at 3444 cmÀ1 (Figure 2,
curve a), allows one to infer that the phenolic form of the
Schiff base structure predominates in the pyridine-based IL
cation of [PySaIm]3PW. As a potential catalyst for Knoeve-
nagel condensations, [PySaIm]3PW with such unaltered sal-
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Knoevenagel Condensations
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estingly, [PyHbIm]3PW, although it has an analogous struc-
ture to [PySaIm]3PW, showed a remarkably lower conver-
sion of 75.3% (Table 1, entry 4). This result arises from the
much longer distance between acid and base sites separated
by a benzene ring in the organic cation unit, which agrees
with the earlier statement that the acid–base distance would
greatly influence Knoevenagel reactions.[8] Furthermore,
when the pyridine ring of [PySaIm]3PW was replaced by the
more commonly used imidazole ring in IL cations, the re-
sulting ionic solid [MimSaIm]3PW displayed a lower conver-
sion of 81.4% (Table 1, entry 5). This seems to be relative to
the stronger acidity of the C-2 protons in imidazolium com-
pounds.[3c,31] [PyBeIm]3PW is also an analogue sample of
[PySaIm]3PW, with the hydroxyl being removed from the
benzene ring, thus its much lower conversion of 65.6%
(Table 1, entry 6) owes itself to the loss of acidic sites. Other
like a heterogeneous catalyst and gave a very high conver-
sion of 95.8% and selectivity of 100%, coupled with an iso-
lated yield of 88.3% (Table 2, entry 1). The conversion is
much higher than the result of 78.2% (Table 2, entry 2)
under the ethanol solvent conditions with the same reaction
time of 2 h. In fact, a considerably high conversion of 90.3%
could be obtained without using any external solvents and
within a short time of 45 min (Table 2, entry 3). Moreover,
entry 4 of Table 2 even offered a perfect 100% conversion
with 100% selectivity if the amount of the substrate ethyl
cyanoacetate were reduced from 8 to 7 mmol under solvent-
free conditions. Consequently, one can see that [Py-
SaIm]3PW is an excellent heterogeneous catalyst under sol-
vent-free conditions. On the other hand, very high conver-
sion and selectivity could be observed over the parent IL
catalyst [PySaIm]Br, but it is a homogeneous catalyst
(Table 2, entry 5).
mono-base
and
mono-acid
catalysts
[PyAM]3PW,
[PyTA]3PW, and [PyHy]3PW, which were prepared by using
Schiff base free pyridine-based IL cations, also showed
much lower activities (Table 1, entries 7–9).
The above comparisons demonstrate that the high activity
of [PySaIm]3PW toward the heterogeneous Knoevenagel
condensation is closely related to its unique structure: the
high-valent PW anion endows the ionic catalyst with a solid
nature for the heterogeneity of the reaction, and the Schiff
base tethered pyridinium IL cation provides acid and base
dual active sites with suitable strength and spacing distance.
In contrast to [PySaIm]3PW, all the pyridine-based control
samples exhibited much lower activities. For the hydroxyl-
free catalysts [PyBeIm]3PW, [PyTA]3PW, and [PyAM]3PW
(Table 2, entries 6–8), the absence of ethanol solvent caused
sharp decreases in activities: their conversions were more or
less as low as that of the uncatalyzed reaction (Table 2,
entry 11). The mono-acid catalyst [PyHy]3PW, which bears
hydroxyl groups in its organic moiety, showed a conversion
of 35.6% (Table 2, entry 9); though far from a high value, it
is obviously higher than those of the three hydroxyl-free
counterparts. These results suggest a “solvent role” of the
hydroxyl-containing organic moiety in the absence of an ex-
ternal solvent. However, the hydroxyl-containing acid–base
bifunctional catalyst [PyHbIm]3PW (Table 2, entry 10) ex-
hibited a lower conversion of 50.3% than [PySaIm]3PW,
mostly due to the longer distance from the acid to base sites
separated by a benzene ring.
Solvent-free Knoevenagel condensation of benzaldehyde
and ethyl cyanoacetate over various catalysts: As has been
indicated in Table 1, [PySaIm]3PW is a good heterogeneous
catalyst for the Knoevenagel condensation in the presence
of ethanol solvent. However, there is already a considerable
amount of organic component in [PySaIm]3PW, and it is
thus reasonable to consider that the hydroxyl-containing or-
ganic moiety therein might function as a polar protic sol-
vent. Accordingly, we further attempted the solvent-free
Knoevenagel condensation of benzaldehyde and ethyl cya-
noacetate over [PySaIm]3PW, with results shown in Table 2.
As expected, without a solvent, [PySaIm]3PW still behaved
Proposed acid–base cooperative mechanism for the [Py-
SaIm]3PW-catalyzed Knoevenagel reaction: On the basis of
the above characterizations and reaction measurements, as
well as the previous results for the acid–base bifunctional
IL[5g] and supported catalysts,[5f] an acid–base cooperative
mechanism
for
the
[Py-
SaIm]3PW-catalyzed Knoevena-
gel condensation is proposed in
Scheme 4. The new reaction
route is very different from the
early imine-intermediated or
Table 2. Solvent-free Knoevenagel condensation of benzaldehyde with ethyl cyanoacetate over various cata-
lysts.[a]
Entry
Catalyst
[PySaIm]3PW
[PySaIm]3PW
[PySaIm]3PW
[PySaIm]3PW
[PySaIm]Br
[PyBeIm]3PW
[PyTA]3PW
[PyAM]3PW
[PyHy]3PW
[PyHbIm]3PW
without
Solubility in reaction
Conversion[b] [%]
Selectivity[c] [%]
1
N
insoluble
insoluble
insoluble
insoluble
soluble
insoluble
insoluble
insoluble
insoluble
insoluble
–
95.8
78.2
90.3
100
94.4
12.5
23.9
18.2
35.6
50.3
13.7
100 (88.3)
100
2[d]
3[e]
4[f]
5
ACHTUNGTRENNUNG
ion-pair
processes.[32]
In
R
100
100
100
100
100
100
100
100
A
Scheme 4, the weakly acidic
E
À
proton of OH in salicyl inter-
6
7
8
9
10
11
G
acts with the oxygen of a car-
bonyl group in benzaldehyde,
through which the C=O bond is
polarized with the increase in
the positive charge on the
carbon of the carbonyl group.
Meanwhile, as a Lewis basic
site, the lone-electron-pair-bear-
G
R
G
G
100
[a] Reaction conditions: catalyst (0.1 g; 0.027 mmol for [PySaIm]3PW), benzaldehyde (8 mmol), ethyl cyano-
acetate (8 mmol), 708C, 2 h. [b] GC conversion based on ethyl cyanoacetate. [c] Selectivity (isolated yield in
ACHTUNGTRENNUNG
parentheses) for ethyl (E)-a-cyanocinnamate. [d] Ethanol as solvent, 2 h, other conditions (see Table 1).
[e] Reaction time: 45 min. [f] Using 7 mmol ethyl cyanoacetate.
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1655 cmÀ1 was detected, which strongly indicates the occur-
rence of polarized C=O groups that arise from the activa-
[5a]
À
tion of carbonyls by the acidic proton of salicyl OH.
However, the recovered [PySaIm]3PW catalyst from a react-
ed mixture (Scheme 4, curve c) did not show that polarized
C=O band, thereby confirming the catalytic role of the salic-
yl hydroxyl as a weak acid. It is known that the effect of a
commonly used polar protic solvent such as ethanol for a
Knoevenagel reaction is the aided activation of the carbonyl
substrate.[30] Now that the hydroxyl in the Schiff base struc-
ture of the organic moiety of [PySaIm]3PW has acted as the
catalytic acid center for activating the carbonyl substrate, as
indicated in Scheme 4, the excellent catalytic performance
of [PySaIm]3PW in the absence of an external polar solvent
is understandable (Table 2).
The acid–base cooperative mechanism within a Schiff
base structure in Scheme 4 satisfactorily accounts for the de-
creased activities of various control catalysts relative to [Py-
SaIm]3PW (Tables 1 and 2). The protonated imine in Py-
SaIm(H)–PW greatly eliminates the basicity of the Schiff
base, and thus hinders the acid–base cooperative function,
thereby resulting in drastically lower conversion. For [PyH-
bIm]3PW, the weak hydroxyl acid and the imine base are
concurrent in one molecule, which seems similar to [Py-
SaIm]3PW, but the longer distance from acid to base sepa-
rated by a benzene ring would slow the reaction rate for
Scheme 4. The acid–base cooperative mechanism proposed for the heter-
ogeneous Knoevenagel condensation catalyzed by the Schiff base teth-
ered [PySaIm]3PW ionic hybrid.
ing nitrogen of an imine group attacks the electron-deficient
methylene hydrogen, thus forming a methylene carbanion.
These catalytic activation processes take place within the
tethered Schiff base structure of [PySaIm]3PW. It is the
Schiff base structure that provides a proximate position for
the acid and base sites with a suitably short distance, there-
by allowing a prompt further reaction of the methylene
carbACHTUNGTRENNUNGanion with the positive-charged carbon of carbonyl into
À
the C C-bonded intermediate that links the two reactants.
Finally, the product ethyl cyanocinnamate is obtained by an
immediate elimination of a water molecule from the inter-
mediate, from which the ionic catalyst [PySaIm]3PW is re-
leased for the next catalysis cycle.
The catalytic roles of the basic nitrogen sites in amino or
imino functional groups for Knoevenagel condensations
have been revealed by previous reports,[3,5,6] which supports
the idea of the catalytic function of the basic imine in the
Schiff base unit of [PySaIm]3PW for activating the methyl-
ene substrate in Scheme 4. In addition, the participation of
À
generating the C C-bonded intermediate. On the other
hand, the fact that both the mono-base (e.g., [PyBeIm]3PW,
[PyAM]3PW, or [PyTA]3PW) and the mono-acid (e.g.,
[PyHy]3PW) catalysts showed sharp decreases in activity
strongly implies the highly efficient performance of the
acid–base cooperative mechanism for [PySaIm]3PW.
À
the salicyl OH as a catalytic acid site in the condensation
reaction is evidenced by the FTIR spectra in Figure 3. Rela-
tive to the fresh [PySaIm]3PW (Scheme 4, curve a), the
It is also interesting to see that the imidazolium-substitut-
ed control sample [MimSaIm]3PW, which contains exactly
the same Schiff base structure as the pyridinium-based [Py-
benzACHTUNGTRENNUNGaldehyde-treated catalyst (Scheme 4, curve b) showed a
band at 1688 cmÀ1 that is assigned to the physically adsorbed
benzaldehyde. More importantly, a small new band at
SaIm]3PW, presented a much lower activity (Table 1).
Taking into account of the stronger acidity of C-2 protons in
imidazolium,[3c,31] a resonance structure that originated from
the intramolecular protonation of the organic moiety of
[MimSaIm]3PW is proposed in Scheme 5. The protonation
Scheme 5. Proposed resonance structures for the imidazolium-based IL
cation of [MimSaIm]3PW.
of the basic imine by the C-2 proton of an imidazolium
might suppress the full extent of the acid–base cooperative
procedure within the Schiff base structure, which is responsi-
ble for the low activity of [MimSaIm]3PW.
According to Scheme 4, one might consider that the IL
precursor [PySaIm]Br should display the highest activity be-
cause it has an identical pyridine-based Schiff base tethered
organic cation to that of [PySaIm]3PW, together with the
Figure 3. FTIR spectra of a) fresh [PySaIm]3PW, b) benzaldehyde-treated
[PySaIm]3PW, and c) recovered [PySaIm]3PW.
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FULL PAPER
most mass-transfer-favored homogeneous nature for the re-
action. Nevertheless, [PySaIm]Br was slightly less active
than the heterogeneous catalyst [PySaIm]3PW (Tables 1 and
2). The stronger intramolecular interaction between the hy-
droxyl of salicyl and the nitrogen of imine has been revealed
in the Schiff base structure of the IL cation of [PySaIm]Br.
Consequently, the formation of a significant amount of in-
tramolecular hydrogen bonding and/or protonated imine
(Figure 2 and Scheme 3) lowers the efficiency of the acid–
base cooperative mechanism. Another reason is ascribed to
the widely accepted concept of “pseudoliquid behavior” for
a HPA catalyst,[33] that is, the ionic compound [PySaIm]3PW
behaves as a bulk-type catalyst that allows polar substrates
to penetrate its amorphous secondary structure, in which the
substrates seem to be dissolved in the organic moiety for
starting the acid–base cooperative catalysis. As a result, the
bulk-type catalysis of [PySaIm]3PW does not relate to the
low specific BET surface area of 8.1 m2 gÀ1.
Figure 4. Catalytic reusability of [PySaIm]3PW for the Knoevenagel con-
densation of benzaldehyde and ethyl cyanoacetate: a) without adding
fresh catalyst, b) with the addition of fresh catalyst to make the constant
catalyst amount of 0.1 g.
Based on the conversion of 90.3% in entry 3 of Table 2,
[PySaIm]3PW gives a turnover number (TON) of 118.9 mol
base siteÀ1 hÀ1 in the condensation of benzaldehyde with
ethyl cyanoacetate. Thus, the present bulk-type heterogene-
ous catalyst is much more active than the previously report-
ed surface-type acid–base bifunctional catalyst, dihydroimi-
dazole-functionalized, mesoporous molecular sieve SBA-15
(TON: 18.7 molbase siteÀ1 hÀ1), even with a very high BET
surface area of 610 m2 gÀ1.[5f] The main reason for the lower
activity of this supported acid–base bifunctional catalyst is
the impossibility to control the distance between the loaded
organic basic sites and the acid sites on the surface of the
silica support.
the catalyst recovery operation and the leaching into the re-
action solution. The similar decrease in activities for reused
catalysts was also observed in previous reports on other
HPA-IL ionic catalysts for esterifications.[17a,b]
To ascertain whether the loss of catalyst is the only reason
for the reduced activities over recovered catalysts, another
five run set of tests was investigated by using a constant cat-
alyst amount of 0.1 g by mixing a small amount of fresh cat-
alyst into the recovered one at recycling intervals (Figure 4,
curve b). It showed higher conversions than that of Figure 4,
curve a. However, the conversion still decreased, albeit very
slowly. This result indicates that the reduced conversion is
due not only to the loss of catalyst, but also the partial deac-
tivation nature of the recovered catalyst itself. The FTIR
spectrum for the recovered catalyst (Figure 3, curve c) illus-
trates generally similar bands to those of the fresh one,
thereby implying the structural durability of the catalyst in
this reaction. However, the extra bands detected at 1724
and 1608 cmÀ1 are assigned to the C=O bond of ethyl cya-
noacetate and the C=C of ethyl cyanocinnamate, respective-
ly, which indicates that the recovered ionic solid involves
residues from the reaction mixture that must have partially
contaminated the acid/base sites, thereby resulting in a
slight deactivation of the recovered catalyst. Additionally,
the detection of the irremovable functional groups of sub-
strate molecules in the recovered catalyst might be evidence
of the supposed bulk-type catalysis due to the “pseudoliquid
behavior” of [PySaIm]3PW.
Therefore, the high efficiency of the acid–base coopera-
tive mechanism for the [PySaIm]3PW-catalyzed Knoevena-
gel reaction thoroughly reflects the rational design strategy
for the present catalyst, [PySaIm]3PW.
Reusability of [PySaIm]3PW catalyst for Knoevenagel con-
densation of benzaldehyde and ethyl cyanoacetate: Since
[PySaIm]3PW-catalyzed Knoevenagel condensation is
a
liquid–solid biphasic heterogeneous reaction, the catalyst
[PySaIm]3PW could be recovered from a reacted mixture by
filtration, washing, and vacuum drying. Figure 4 plots two
parallels of five-run catalyst recycling results for solvent-free
Knoevenagel condensation of benzaldehyde with ethyl cya-
noacetate. By using only recovered catalyst without adding
any fresh one at intervals of two runs (Figure 4, curve a), a
perfect selectivity of 100% could always be obtained (not
shown); but the conversion decreased gradually. The reused
catalyst in the second run showed a conversion of 80.4%,
which is clearly lower than the fresh one (95.8%). After the
second run, the rate of decrease of the conversions slowed,
and by the fifth run the recycled catalyst yielded a conver-
sion of 74.8%. The decrease in conversion is associated with
the catalyst recovery yield of 86.7% for the second run and
83.2% for the fifth run, which are calculated on the basis of
the fresh catalyst amount of 0.1 g for the first run. The cata-
lyst reduction quantity includes the inevitable loss during
Solvent-free Knoevenagel condensations with various sub-
strates over the [PySaIm]3PW catalyst: To examine the
scope of the [PySaIm]3PW catalyst for Knoevenagel conden-
sations, we tested the reactivity of various substrates of car-
bonyl or methylene compounds under solvent-free condi-
tions. The results are summarized in Table 3. When reacting
with ethyl cyanoacetate, the aldehyde derivative of furalde-
hyde showed a higher conversion of 97.0% at a much short-
er time of 0.2 h than benzaldehyde did (95.8%, 2 h)
(Table 3, entries 1 and 2). Similarly, the strong electron-with-
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Table 3. Solvent-free Knoevenagel condensations with various substrates catalyzed by [PySaIm]3PW.[a]
Entry
Substrate
t [h]
Conversion[b]
[%]
Selectivity[c]
[%]
R1
R2
X
Y
1
2
Ph
2-fural
H
H
H
H
H
H
H
CN
CN
CN
CN
CN
CN
CN
CN
CN
CO2Et
CO2Et
CO2Et
CO2Et
CO2Et
CO2Et
CO2Et
CO2Et
CO2Et
CN
2
95.8
97.0
93.4
95.6
92.7
77.6
68.4
62.3
61.7
100
100
100
100
100
100
100
100
100
100
100
100
100
0.2
0.5
0.4
6
6
8
12
12
0.1
20
20
3[d]
4
4-NO2 Ph
À
À
2-Cl Ph
5[d]
6
4-OH Ph
À
nC3H7
nC7H15
À
7
8
9
10
11
12
À
cC5H10
CH3
Ph
Ph
CH3
H
H
CN
COCH3
CO2Et
CO2Et
CO2Et
67.4
44.8
Ph
H
[a] Reaction conditions: catalyst 0.1 g (0.027 mmol), aldehyde or ketone (8 mmol), methylene compound (8 mmol), 708C. [b] Entries 1–9: GC conversion
based on ethyl cyanoacetate; entries 10–12: GC conversion based on benzaldehyde. [c] Selectivity for the respective main products. [d] With ethanol as
the solvent (5 mL) for dissolving the solid substrates of aldehydes.
À
À
drawing groups ( NO2 and Cl) attached to aromatic alde-
hydes were also very reactive substrates (Table 3, entries 3
and 4). In contrast, the electron-donating hydroxyl-tethered
aromatic aldehyde (Table 3, entry 5) required a much longer
time to reach a high conversion. On the other hand, the ali-
phatic aldehydes (Table 3, entries 6 and 7) and ketones
(Table 3, entries 8 and 9) exhibited lower conversions of 62–
78% at long reaction times of around 6–12 h.
tance. Also, the large volume and high valence of HPA
anions result in the solid-state nature and insoluble proper-
ties of the ionic compound [PySaIm]3PW. In solvent-free
condensation of benzaldehyde with ethyl cyanoacetate, [Py-
SaIm]3PW exhibits heterogeneity, high conversion (95.8%),
and perfect selectivity (100%), with the yield even being
much higher than in the presence of the solvent ethanol.
The solid catalyst [PySaIm]3PW can be conveniently recov-
ered after a reaction, and a five-run recycling test shows
only slight decreases in conversion for reused catalysts. Fur-
thermore, the new catalyst is widely applicable to many sub-
strates of aromatic aldehydes with activated methylene com-
pounds. A unique acid–base cooperative mechanism within
the Schiff base structure is proposed to account for the good
catalytic performance of [PySaIm]3PW, as well as to explain
the lower activities of various control catalysts. The catalysis
strategy of this study might be a significant step towards the
design of heterogeneous ionic-solid catalysts for more tradi-
tional liquid-phase organic syntheses.
With benzaldehyde as the constant carbonyl substrate,
entry 10 of Table 3 shows that the highly activated methyl-
ene compound malononitrile offered an excellent and com-
plete conversion of 100% to the target alkene product
within a short time of 0.1 h. Nevertheless, when ethyl ace-
toacetate (Table 3, entry 11) and diethyl malonate (Table 3,
entry 12) were used as the methylene substrates, only low
conversions were obtained, even after a very long time of
20 h.
Consequently, the heterogeneous catalyst [PySaIm]3PW is
widely applicable to Knoevenagel condensations of aromatic
aldehydes with the methylene compounds activated by elec-
tron-withdrawing functional groups. It is suggested that com-
paratively inert substrates such as aliphatic aldehydes, ke-
tones, diethyl malonate, and so on demand stronger acid–
base cooperative catalysts, which requires further study.
Experimental Section
Materials and methods: All chemicals were of analytical grade and used
as received. FTIR spectra for catalyst samples in KBr discs were record-
ed using a Nicolet 360 FTIR instrument in the 4000–400 cmÀ1 region. The
solid UV/Vis spectrum was measured using a PE Lambda 950 spectrome-
ter with BaSO4 as an internal standard. XRD patterns were collected
using a Bruker D8 Advance powder X-ray diffractometer with an Ni-fil-
tered CuKa radiation source at 40 kV and 20 mA, from 5 to 508 at a scan
rate of 28CminÀ1. Before measurements, the samples were dried at
1008C for 2 h. SEM images were recorded using a Hitachi S-4800 field-
emission scanning electron microscope. BET surface areas were meas-
Conclusion
We have developed a new acid–base bifunctional catalyst,
[PySaIm]3PW, for Knoevenagel condensations. The synthesis
strategy for this ionic solid catalyst requires pairing pyri-
dine-based Schiff base tethered IL cations with Keggin-
structured HPA anions. The novelty of this synthetic strat-
egy for [PySaIm]3PW is the involvement of the catalytically
active sites of both acid (the proton of salicyl hydroxyl) and
base (the nitrogen of imine) within a Schiff base structure,
which adds the advantage of a controllable acid–base dis-
ured at the temperature of liquid nitrogen using
a Micromeritics
ASAP2010 analyzer. The samples were degassed at 1508C to a vacuum
of 10À3 Torr before analysis. TG analysis was carried out using an
STA409 instrument in dry air at a heating rate of 108CminÀ1 1H NMR
.
spectra were measured using a Bruker DPX 300 spectrometer at ambient
temperature in CDCl3 or [D6]DMSO using TMS as the internal refer-
ence. The CHN elemental analysis was obtained using an elemental ana-
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Knoevenagel Condensations
FULL PAPER
lyzer (FlashEA 1112). Melting points were determined using an X4 digi-
tal microscopic melting-point apparatus with an upper limit of 2508C.
Electrospray ionization mass spectrometry (ESI-MS) was recorded using
a Finnigam Mat APISSQ 710 mass spectrometer.
the procedure, the solvent-free condensation reaction was also carried
out at a shorter reaction time of 2 h without adding the ethanol solvent.
After reaction, the reaction mixture was centrifuged to remove the solid
catalyst, and the liquid was analyzed using a gas chromatograph (GC SP-
6890) equipped with an FID detector and a capillary column (SE-54;
30 mꢁ0.32 mmꢁ0.25 mm). n-Dodecane was used as the internal standard
to calculate the reaction conversion. Isolated yield was obtained by using
column chromatography. A five-run catalyst recycling was carried out for
testing the reusability of the catalyst. The catalyst was recovered from a
reacted mixture by filtration, washing with diethyl ether three times, and
vacuum drying. Another five-run catalyst recycling was carried out by
adding a small amount of fresh catalyst to the recovered catalyst to make
a constant total catalyst amount of 0.1 g. Also, the reactivity of various
substrates of carbonyl or methylene compounds for Knoevenagel con-
densations was tested over the catalyst [PySaIm]3PW. In addition to [Py-
SaIm]3PW, catalytic behaviors of the prepared control catalysts were
measured under the typical reaction conditions for comparison.
Synthesis of 1-(2-salicylaldimine)pyridinium phosphotungstate [Py-
SaIm]3PW
(Scheme 1):
1-(2-Aminoethyl)pyridinium
bromide
([PyAM]Br) was synthesized according to the literature.[34] Pyridine
(7.91 g, 100 mmol) and 2-bromoethylamine hydrobromide (20.49 g,
100 mmol) were dissolved in ethanol (50 mL) with stirring at 758C for
24 h under a nitrogen atmosphere. Upon completion, the solvent was re-
moved by filtration, and the residue was washed twice with ethanol to
afford [PyAM]Br·HBr as a white solid. KOH was added to the aqueous
solution of the above solid for neutralization, followed by the evapora-
tion under vacuum. Ethanol (25 mL) was then added into the resulting
mixture with the appearance of the precipitated salt. After filtration, the
filtrate was evaporated to give the product [PyAM]Br as dark brown oil
(yield: 85%). Afterwards, a mixture of [PyAM]Br (2.03 g, 10 mmol) and
salicylaldehyde (1.22 g, 10 mmol) was stirred at room temperature for
12 h without a solvent, followed by washing with diethyl ether (3ꢁ
30 mL) and vacuum evaporation,[20] which gave the IL product [Py-
SaIm]Br (1-(2-salicylaldimine)pyridinium bromide) as dark burgundy oil
(yield: 79%). 1H NMR (300 MHz, [D6]DMSO, TMS): d=4.21 (t, 2H),
5.06 (t, 2H), 6.88 (q, 2H), 7.34 (m, 1H), 7.41 (d, 1H), 8.21 (t, 2H), 8.55
(s, 1H), 8.65 (t, 1H), 9.20 (d, 2H), 12.4 ppm (s, 1H) (see Figure S4 in the
Supporting Information). Finally, [PySaIm]Br (1.84 g, 6.0 mmol) was
added to the aqueous solution of Na3PW12O40 in ethanol (5.76 g,
2.0 mmol) with further stirring at room temperature for 24 h. Bright
yellow precipitate formed. It was filtered and washed with water three
times, followed by drying in a vacuum to give the final product [Py-
SaIm]3PW as a solid (yield: 95%). MS (ESI)+: m/z: 227.1 [PySaIm]+
(Figure S5A in the Supporting Information). Negative-ion ESI-MS
Acknowledgements
The authors thank greatly the Key Program of the National Natural Sci-
ence Foundation of China (no. 21136005).
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3À
showed a fragmentation pattern identical to that of PW12O40 (Fig-
ure S5B in the Supporting Information),[35a] with the following assign-
ments for the clusters: m/z: 122.1 (H4PW2O13/5), 198.8 (H3W4O16/5),
226.4 (H4P2W3O18/4), 254.1 (PW5O20/5), 446.7 (H4PW9O34/5), 485.8
(PW10O35/5), 578.6 (H2PW7O26/3), 655.8 (H2PW8O29/3). Meanwhile, the
patterns identical to those of possible cation/anion aggregates were ob-
served (Figure S5B in the Supporting Information)[35b] with the following
assignments:
m/z:
684.4
[(C14H15N2O)4P2W7O30/4],
705.6
[(C14H15N2O)3W12O40/5], 781.7 [(C14H15N2O)3PW10O36/4].
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Synthesis of various control catalysts: [PyHbIm]3PW and [PyBeIm]3PW
were prepared according to Scheme 1, with the same method as descri-
bed above for [PySaIm]3PW, except for the use of 4-hydroxybenzalde-
hyde and benzaldehyde as the respective starting raw materials instead of
salicylaldehyde. PySaIm(H)–PW was prepared by using H3PW12O40 to
make the anion exchange instead of Na3PW12O40. The chemical composi-
tion of PySaIm(H)–PW is shown in Scheme 1, which was determined by
elemental analysis (found (%): C 11.77, N 2.14, H 1.13). [MimSaIm]3PW
was obtained according to the bottom line of Scheme 1, with imidazole
replacing pyridine to prepare the Schiff based tethered IL cation.
[PyAM]3PW, [PyTA]3PW, and [PyHy]3PW were prepared according to
Scheme 2. For [PyAM]3PW, [PyAM]Br (1.22 g, 6.0 mmol) was added to
the aqueous solution of Na3PW12O40 in ethanol (5.76 g, 2.0 mmol), and
then the mixture was stirred at room temperature for 24 h. Next, the
formed dark yellow precipitate was filtered and washed with water three
times, followed by drying in
a vacuum to give the solid product
[PyAM]3PW (yield: 94%). [PyTA]3PW and [PyHy]3PW were prepared
by following the same procedure, but with 2-piperidinoethyl chloride hy-
drochloride and ethylene bromohydrin as the starting materials, respec-
tively.
Procedures for Knoevenagel condensations: A typical procedure for
Knoevenagel condensation of benzaldehyde with ethyl cyanoacetate is as
follows. Ethanol (solvent, 5 mL), benzaldehyde (0.849 g, 8 mmol), and
ethyl cyanoacetate (0.905 g, 8 mmol) were added to a round-bottomed
flask reactor (25 mL) equipped with a condenser under a nitrogen atmos-
phere. After the heating temperature was adjusted to 708C, the catalyst
[PySaIm]3PW (0.1 g, 0.027 mmol) was added into the reactor, and then
the reaction slurry was stirred for 4 h under reflux conditions. Following
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Received: April 19, 2012
Published online: && &&, 0000
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10
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www.chemeurj.org
ꢀ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Eur. J. 0000, 00, 0 – 0
ÝÝ
These are not the final page numbers!

Knoevenagel Condensations
FULL PAPER
Schiff Bases
M. Zhang, P. Zhao, Y. Leng, G. Chen,
J. Wang,* J. Huang .......... &&&& — &&&&
Schiff Base Structured Acid–Base
Cooperative Dual Sites in an Ionic
Solid Catalyst Lead to Efficient Heter-
ogeneous Knoevenagel Condensations
Cooperative catalyst: The ionic solid
prepared by pairing pyridine-based
Schiff base tethered cations with phos-
photungstate anions is proved to be a
recoverable efficient heterogeneous
catalyst for Knoevenagel condensa-
tions by means of the unique bulk-type
acid–base cooperative mechanism (see
scheme).
Chem. Eur. J. 2012, 00, 0 – 0
ꢀ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
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ÞÞ
These are not the final page numbers!