Microporous coordination polymer [Zn4(dmf)(ur)2(ndc)4] as a heterogeneous catalyst for the Knoevenagel reaction

Article abstract of DOI:10.1007/s11172-014-0748-7

The catalytic properties of the microporous metal-organic coordination polymer [Zn₄(dmf)(ur)₂(ndc)₄] (dmf is N,N'-dimethylformamide, ur is urotropine, and ndc^2- is 2,6-naphthalenedicarboxylate) for the Knoevenagel reaction were studied. The reaction between aromatic aldehydes (benzaldehyde, α-naphthaldehyde, 4-biphenylaldehyde, and 1-pyrenaldehyde) and malononitrile was studied. The coordination polymer was shown to be a heterogeneous catalyst that makes it possible to achieve 95% yield in the reaction between benzaldehyde and malononitrile. The selectivity of the catalyst depends on the size of aldehydes used in the reaction.

Full text of DOI:10.1007/s11172-014-0748-7

Russian Chemical Bulletin, International Edition, Vol. 63, No. 10, pp. 2363—2368, October, 2014
2363
Microporous coordination polymer [Zn4(dmf)(ur)2(ndc)4]
as a heterogeneous catalyst for the Knoevenagel reaction*
S. A. Sapchenko,a,b D. N. Dybtsev,a,b and V. P. Fedina,b
aA. V. Nikolaev Institute of Inorganic Chemistry, Siberian Branch of the Russian Academy of Sciences,
3 prosp. Akad. Lavrent´eva, 630090 Novosibirsk, Russian Federation
bNovosibirsk State University,
2 ul. Pirogova, 630090 Novosibirsk, Russian Federation.
Fax: +7 (383) 330 9489. Eꢀmail: cluster@niic.nsc.ru
The catalytic properties of the microporous metal—organic coordination polymer
[Zn4(dmf)(ur)2(ndc)4] (dmf is N,N´ꢀdimethylformamide, ur is urotropine, and ndc2– is
2,6ꢀnaphthalenedicarboxylate) for the Knoevenagel reaction were studied. The reaction beꢀ
tween aromatic aldehydes (benzaldehyde, ꢀnaphthaldehyde, 4ꢀbiphenylaldehyde, and 1ꢀpyrenꢀ
aldehyde) and malononitrile was studied. The coordination polymer was shown to be a heteroꢀ
geneous catalyst that makes it possible to achieve 95% yield in the reaction between benzaldeꢀ
hyde and malononitrile. The selectivity of the catalyst depends on the size of aldehydes used in
the reaction.
Key words: metal—organic coordination polymers, zinc, urotropine, heterogeneous catalysis.
The chemistry of porous coordination polymers has
been the subject of active investigation in recent two deꢀ
cades as evidenced by an increasing number of articles,
reviews, and special issues of scientific journals. In addiꢀ
tion to the sorption of gases and selective separation of
various mixtures, heterogeneous catalysis is one of the
promising areas for application of these materials.1,2
A high selectivity of sorption or catalytic process reported
for coordination polymers is determined in the first inꢀ
stance by the presence of various active sites on the interꢀ
nal surface with affinity to molecules of a certain type. As
a rule, these sites represent coordinatively unsaturated
metal cations with the Lewis type acidity.3 A considerable
number of works deals with the targeted preparation of
coordinatively unsaturated sites inside the porous strucꢀ
ture and the study of the catalytic activity of the considꢀ
ered coordination polymers.4—9 At the same time, many
chemical reactions and syntheses of important compounds
occur in the presence of alkalis or catalytic sites of the
basic type; however, examples for the syntheses of porous
coordination polymers containing active basic Lewis sites
(lone electron pairs) are rather few. Thus, the preparation
of new porous coordination polymers containing atoms
with lone electron pairs on the channel walls and the study
of their properties are important tasks.
We have recently synthesized the biporous framework
[Zn4(dmf)(ur)2(ndc)4] (dmf is N,N´ꢀdimethylformamide,
ur is urotropine, and ndc2– is 2,6ꢀnaphthalenedicarbꢀ
oxylate) with channels ~1 nm in diameter. Internal
surface of these frameworks contains tertiary nitrogen
atoms with the accessible lone electron pair (Fig. 1). The
compound exhibits interesting sorption and luminesꢀ
cence properties and unique stepꢀbyꢀstep substitution of
guest molecules.10,11 In this work, the catalytic activity
of the coordination polymer [Zn4(dmf)(ur)2(ndc)4] is
described.
Results and Discussion
The existence of lone electron pairs on the nitrogen
atoms of urotropine on the channel walls in the porous
compound [Zn4(dmf)(ur)2(ndc)4] suggests catalytic acꢀ
tivity of this compound. The condensation of malononiꢀ
trile and aldehydes (Knoevenagel reaction) was chosen as
a test reaction. It is known that the Knoevenagel reaction
is catalyzed by bases (Scheme 1).
The reaction was carried out in 1,4ꢀdioxane under arꢀ
gon using the Schlenk technique to avoid contact with air
moisture. Preliminary experiments showed that the presꢀ
ence of even insignificant amounts of water in the reaction
mixture destroys the catalyst and, therefore, anhydrous
calcium chloride was added to the reaction mixture. We
elucidated preliminarily that calcium chloride itself does
* Dedicated to Academician of the Russian Academy of Sciences
Yu. N. Bubnov on the occasion of his 80 birthday.
Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 10, pp. 2363—2368, October, 2014.
1066ꢀ5285/14/6310ꢀ2363 © 2014 Springer Science+Business Media, Inc.

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Sapchenko et al.
Fig. 1. Structure of the [Zn4(dmf)(ur)2(ndc)4] framework based on the Xꢀray diffraction data. The unit cell is shown by lines.
Designations of atoms: Zn is blueꢀ, О is redꢀ, N is dark blueꢀ, and C is grayꢀcolored.
Note. Figure 1 is available in full color in the onꢀline version of the journal (http://www.springerlink.com).
Scheme 1
H is the vinyl proton, the signal intensity of H in the 1H NMR spectrum was used to determine the yield of the product.
not catalyze the reaction. In all experiments, the reaction
course was monitored by NMR spectroscopy taking aliꢀ
quots from the reaction mixture. The concentration of the
reaction products was determined by comparing integrals
of the singlet of the solvent and the singlet at  8.4 correꢀ
sponding to the vinyl proton of the condensation product.

Knoevenagel condensation
Russ.Chem.Bull., Int.Ed., Vol. 63, No. 10, October, 2014
2365
Table 1. Conditions and the product yield of the reaction СН2(CN)2 + R—CHO in the presence of
[Zn4(dmf)(ur)2(ndc)4]
Entry
Reaction conditionsa
Aldehyde : malononitrile Catalyst amount
Yield (%)
Aldehyde
/h
(mol.)
(mol.%)
1
PhCHO
PhCHO
PhCHO
PhCHO
PhCHO
1ꢀNaphCHO
4ꢀBpCHO
1ꢀPyrCHO
1 : 2
1 : 2
1 : 2
1 : 2
1 : 10
1 : 2
1 : 2
1 : 2
5
5
5
10
10
10
10
10
24
7
38
24
9
25
25
24
17
8
8
29
95
13
11
11
2ꢀ1
2ꢀ2b
3
4c
5
6
7
a All reactions, except that in entry 3, were carried out in argon in the presence of 50—100 mg of CaCl2 at
25 С. The structures of aldehydes are given in Fig. 2.
b Filtration in 7 h.
c Reaction temperature 40 С.
Y (%)
24
mained unchanged within 38 h after the beginning of
the reaction.
A twofold increase in the amount of the used catalyst
significantly accelerates the reaction: in the presence of
10 mol.% catalyst, the conversion attains 29% (see Table 1,
entry 3). A more considerable increase in the reaction rate
is induced by heating the reaction mixture to 40 C: conꢀ
version attains 95% within 9 h (see Table 1, entry 4).
In compound [Zn4(dmf)(ur)2(ndc)4] two nitrogen
atoms of each urotropine molecule participate in the forꢀ
mation of the coordination polymer, and the remaining
two nitrogen atoms are not coordinated. Based on the
assumption that all nonꢀcoordinated nitrogen atoms of
urotropine act as catalytically active sites, we determined
that the turnover frequency of the catalyst (N) for entries
1, 3, and 4 is 0.9, 0.6, and 2.4, respectively.
20
16
12
8
4
1
2, 3
4
10
20
30
40
t/h
Fig. 2. Kinetic curves for catalytic entries 1, 2 and 3, 4; Y is yield.
In entry 4, N > 1, indicating that the catalytic reaction
occurs between malononitrile and benzaldehyde. In enꢀ
tries 1 and 3, N is somewhat lower than unity, which
does not allow one to conclude that the catalytic reacꢀ
tion occurs. It appears that the coordination polymer
[Zn4(dmf)(ur)2(ndc)4] participates as a stoichiometric reꢀ
agent or the catalytic sites are rapidly blocked by the reacꢀ
tion products.
It is important to know whether the reaction occurs on
the external surface of crystals of the coordination polyꢀ
mer or inside the channels of [Zn4(dmf)(ur)2(ndc)4]. If
the reaction proceeds via the second mechanism, the coꢀ
ordination polymer [Zn4(dmf)(ur)2(ndc)4] should be sizeꢀ
selective, i.e., the reaction rate depends on the size of
substrate molecules. To check this assumption, we studied
the Knoevenagel reaction for aromatic aldehydes of variꢀ
ous sizes (Fig. 3).
The results of the catalytic experiments are presented in
Table 1 and Fig. 2.
When using the catalyst in an amount of 5 mol.% of
the reaction mixture at the reactant ratio 1 : 2 in the presꢀ
ence of calcium chloride at 25 C in argon, the yield of the
product is ~17% (see Table 1, entry 1). The choice of
the molar ratio of the reagents was based on the exꢀ
perimental data which indicated too low reaction rate in
the case of equimolar amounts of the reactants. Accordꢀ
ing to the Xꢀray diffraction data, the coordination polyꢀ
mer retained crystallinity during the process. The heteroꢀ
geneous character of catalytic transformations was conꢀ
firmed by the test to filtration (see Table 1, entry 2).
The reaction was carried out under the conditions similar
to conditions shown in entry 1; however, in 7 h the cataꢀ
lyst was removed from the reaction mixture. The reacꢀ
tion stopped completely. The yield of the product reꢀ
To be prove that all aldehydes exhibit comparable
activity in the Knoevenagel reaction, we carried out tests

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Sapchenko et al.
Y (%)
25
a
b
PhCHO (8.5×6.0 Å)
20
15
10
5
1ꢀNaphCHO (8.5×8.5 Å)
1ꢀBpCHO (13.0×7.0 Å)
1ꢀPyrCHO (10.5×7.0 Å)
1
3
4
2
5
10
15
20
25 t/h
Fig. 3. (a) Conversion of aldehydes PhCHO (1), 4ꢀBpCHO (2), 1ꢀNaphCHO (3), and 1ꢀPyrCHO (4) vs reaction time; (b) molecular
models of aldehydes and their characteristic van der Waals dimensions.
with PhCHO, 1ꢀNaphCHO, 4ꢀBpCHO, and 1ꢀPyrCHO
under homogeneous conditions using urotropine as a catꢀ
alyst. In all cases, the reaction rates were equally high, the
yield was close to the quantitative one, and no dependence
on the molecular size of aldehyde involved in the reaction
was observed.
Completely different results were obtained for these
aldehydes under heterogeneous conditions using
[Zn4(dmf)(ur)2(ndc)4] as a catalyst (Table 2).
All reactions were carried out under the same condiꢀ
tions: 10 mol.% [Zn4(dmf)(ur)2(ndc)4] were used, the corꢀ
responding aldehydes and malononitrile were taken in the
molar ratio 1 : 2, and the reaction occurred in dioxane in
the presence of CaCl2 in argon at 25 C. As can be seen
from Table 2, the formation of the product with the highest
yield is observed in the reaction with benzaldehyde. At the
same time, the yields in the reactions with other three
larger aldehydes are comparable and significantly lower
than that in the case of PhCHO. It is most likely that the
dimensions of benzaldehyde molecule allow the compound
to diffuse easily to the channel of the coordination polyꢀ
mer [Zn4(dmf)(ur)2(ndc)4] and to interact with the basic
sites of nitrogen, whereas the diffusion of 1ꢀNaphCHO,
4ꢀBpCHO, and 1ꢀPyrCHO is more difficult and the
catalytic reaction occurs on the external surface of
[Zn4(dmf)(ur)2(ndc)4] crystals.
Note that the crystals of [Zn4(dmf)(ur)2(ndc)4] isolatꢀ
ed from the reaction mixtures with 1ꢀNaphCHO and
1ꢀPyrCHO are slightly colored, most likely, due to the
partial adsorption of the aldehydes on the crystal surface
of the coordination polymer. The color does not disappear
upon washing the crystals with dioxane, which indicates
fairly strong interaction of the aldehydes with the surface.
After the reaction mixture is kept in air for 30 min only,
the color of the crystals significantly changes (from light
yellow to bright orange), and the color corresponds to the
color of the product of the Knoevenagel reaction. Since
the formation of the product was confirmed by NMR specꢀ
troscopy, it is evident that the reaction continues to occur
involving the solid phase. This interaction can involve alꢀ
dehydes adsorbed on the crystal surface and malononitrile
molecules, some of which exist in the polymer channels
and some molecules are located as the adsorbed species on
the polymer surface. Thus, the coordination polymer
[Zn4(dmf)(ur)2(ndc)4] can catalyze the reaction not only
in solutions but in the solid state as well. At present the
reactions in the solid state evoke considerable interest of
researchers, because this reactions can be important for
the development of new ecologically pure processes.
Thus, the catalytic activity of the porous coordination
polymer [Zn4(dmf)(ur)2(ndc)4] was demonstrated. The
catalytic activity is due to the presence of nitrogen atoms in
the nanosized channels which play the role of basis sites.
The coordination polymer is a heterogeneous catalyst that
makes it possible to attain 95% yield in the Knoevenagel
reaction between malononitrile and benzaldehyde. The
catalyst also demonstrates selectivity to the substrate size.
Unfortunately, the contribution of the catalytic sites localꢀ
ized on the crystal surface significantly decreases the selecꢀ
tivity, but it can be hoped that a further study of the catalytic
properties of the [Zn4(dmf)(ur)2(ndc)4] framework provides
examples for reactions with a more pronounced selectivity.
Table 2. Yield of the product of the catalytic reacꢀ
tion vs size of the aldehyde
Aldehyde
/h
Yield (%)
PhCHO
24
25
25
24
24
13
11
11
1ꢀNaphCHO
4ꢀBpCHO
1ꢀPyrCHO

Knoevenagel condensation
Russ.Chem.Bull., Int.Ed., Vol. 63, No. 10, October, 2014
2367
mixture (50 L) was dissolved in DMSOꢀd6 (200 L). The idenꢀ
tification and quantitative analysis of the products were perꢀ
formed using NMR spectroscopy (Fig. 4).
Experimental
Synthesis of [Zn4(dmf)(ur)2(ndc)4]. Colorless crystals of the
coordination polymer [Zn4(dmf)(ur)2(ndc)4] were obtained by
the earlier published procedure.11
Catalytic experiments. Catalytic experiments were carried
out in a temperatureꢀcontrolled preꢀevacuated twoꢀnecked glass
flask in argon. Prior to use in catalytic experiments, compound
[Zn4(dmf)(ur)2(ndc)4] was heated for 2 h in a dynamic vacuum
at 45 C to remove sorbed water. An aliquot of the reaction
Reaction between benzaldehyde and malononitrile. Test.
Malononitrile (0.132 g, 2.00 mmol) and benzaldehyde (0.106 g,
1.00 mmol) were dissolved in dioxane (2.5 mL). Anhydrous
CaCl2 (101 mg) was added to the reaction mixture to reꢀ
move water traces, after which 10 mol.% urotropine (0.014 g,
0.1 mmol) was added. The reaction was carried out at
25 C. Samples were taken every 30 min. To show the abꢀ
sence of catalytic activity of CaCl2 in this reaction, an exꢀ
a
1.0000
4.3605
10
9
8
7
6
5
4

10.0100
b
1.0000
10.0
9.5
9.0
8.5
8.0
7.6

c
10.0185
1.0000
0.4177
10.0
9.5
9.0
8.5
8.0
7.6

Fig. 4. NMR spectra of the reaction mixture without the catalyst (a, b) and 24 h after the addition of the catalyst (c) in entry 1.

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Sapchenko et al.
periment without urotropine was carried out under similar
conditions.
2.00 mmol) were dissolved in dioxane (2.5 mL). Anhydrous CaCl2
(101 mg) was added to the reaction mixture to remove water
traces, and 10 mol.% urotropine (0.014 g, 0.1 mmol) was then
added. Samples were taken every 30 min.
Study of the catalytic activity of [Zn4(dmf)(ur)2(ndc)4].
1ꢀPyrenaldehyde (0.230 g, 1.00 mmol) and malononitrile (0.132 g,
2.00 mmol) were dissolved in dioxane (2.5 mL). Anhydrous CaCl2
(101 mg) was added to the reaction mixture to remove water
traces, and 10 mol.% [Zn4(dmf)(ur)2(ndc)4] (148 mg, 0.1 mmol)
was then added. Samples were taken 9.5 and 25 h after the reacꢀ
tion onset.
Study of the catalytic activity of [Zn4(dmf)(ur)2(ndc)4]. (1)
Malononitrile (0.132 g, 2.00 mmol) and benzaldehyde (0.106 g,
1.00 mmol) were dissolved in dioxane (2.5 mL). Anhydrous CaCl2
(0.110 g) was added to the reaction mixture to remove water
traces, and 5 mol.% [Zn4(dmf)(ur)2(ndc)4] (74 mg, 0.05 mmol)
was then added. The reaction was carried out at 25 C. Samples
were taken 9, 19, and 42 h after the reaction onset. (2) Test to
filtration. The reaction was carried out under similar conditions.
Seven hours after the reaction onset, the catalyst was separated
from the reaction mixture by centrifugation. Samples were taken
7, 17, and 38 h after the reaction onset. (3) Optimization of the
yield. The experiment was carried out under similar condiꢀ
tions, but CaCl2 (0.040 g) and 10 mol.% [Zn4(dmf)(ur)2(ndc)4]
(148 mg, 0.1 mmol) were used. Samples were taken 0.5, 6.25, 19,
and 24 h after the reaction onset. (4) Malononitrile (0.660 g,
10.00 mmol) and benzaldehyde (0.106 g, 1.00 mmol) were disꢀ
solved in dioxane (2.5 mL). Anhydrous CaCl2 (0.062 g) was
added to the reaction mixture to remove water traces, and
10 mol.% [Zn4(dmf)(ur)2(ndc)4] (148 mg, 0.1 mmol) was then
added. The reaction was carried out at 40 C.
Reaction between ꢀnaphthaldehyde and malononitrile. Test.
ꢀNaphthaldehyde (0.156 g, 1.00 mmol) and malononitrile
(0.132 g, 2 mmol) were dissolved in dioxane (2.5 mL). Anhydrous
CaCl2 (101 mg) was added to the reaction mixture to remove
water traces, and 10 mol.% urotropine (0.014 g, 0.1 mmol) was
then added. Samples were taken at an interval of 30 min.
Study of the catalytic activity of [Zn4(dmf)(ur)2(ndc)4].
ꢀNaphthaldehyde and benzaldehyde were dissolved in dioxane
(2.5 mL). Anhydrous CaCl2 (101 mg) was added to the reacꢀ
tion mixture to remove water traces, and 10 mol.%
[Zn4(dmf)(ur)2(ndc)4] (148 mg, 0.1 mmol) was then added.
Samples were taken 8 and 25 h after the reaction onset.
Reaction between 4ꢀbiphenylaldehyde and malononitrile. Test.
4ꢀBiphenylaldehyde (0.182 g, 1.00 mmol) and malononitrile
(0.132 g, 2.00 mmol) were dissolved in dioxane (2.5 mL). Anꢀ
hydrous CaCl2 (101 mg) was added to the reaction mixture
to remove water traces, and 10 mol.% urotropine (0.014 g,
0.1 mmol) was then added. Samples were taken every 30 min.
Study of the catalytic activity of [Zn4(dmf)(ur)2(ndc)4].
4ꢀBiphenylaldehyde (0.182 g, 1.00 mmol) and malononitrile
(0.132 g, 2.00 mmol) were dissolved in dioxane (2.5 mL). Anꢀ
hydrous CaCl2 (101 mg) was added to the reaction mixture to
remove water traces, and 10 mol.% [Zn4(dmf)(ur)2(ndc)4]
(148 mg, 0.1 mmol) was then added. Samples were taken 8, 21,
and 25 h after the reaction onset.
The authors are grateful to I. Yu. Skobelev for valuable
advices and discussion of the results.
This work was financially supported by the Russian
Foundation for Basic Research (Project No. 14ꢀ03ꢀ
00141 A).
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Received June 2, 2014;
in revised form September 10, 2014