Syntheses, crystal structures and knoevenagel condensation reactions of three coordination polymers assembled with Lewis basic ligand

Article abstract of DOI:10.1016/j.poly.2017.12.024

Three functional coordination polymers (CPs), [(CH₃)₂NH₂]₂[Cd₃L₂(H₂O)₂]·6H₂O (1), [Ba₄L₂(H₂O)₈]·H₂O (

Full text of DOI:10.1016/j.poly.2017.12.024

Accepted Manuscript
Syntheses, crystal structures and knoevenagel condensation reactions of three
coordination polymers assembled with Lewis basic ligand
Li Ma, Hua Wu, Jin Yang, Ying-Ying Liu, Jian-Fang Ma
PII:
S0277-5387(17)30807-0
https://doi.org/10.1016/j.poly.2017.12.024
POLY 12981
DOI:
Reference:
Polyhedron
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29 September 2017
21 December 2017
Please cite this article as: L. Ma, H. Wu, J. Yang, Y-Y. Liu, J-F. Ma, Syntheses, crystal structures and knoevenagel
condensation reactions of three coordination polymers assembled with Lewis basic ligand, Polyhedron (2017), doi:
https://doi.org/10.1016/j.poly.2017.12.024
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Syntheses, crystal structures and knoevenagel condensation reactions
of three coordination polymers assembled with Lewis basic ligand
Li Ma,a Hua Wu,b Jin Yang,a Ying-Ying Liu,a Jian-Fang Ma*,a
aKey Laboratory of Polyoxometalate Science, Department of Chemistry, Northeast
Normal University, Changchun 130024, P. R. China
bCollege of Science, Nanjing Agricultural University Nanjing 210095,P. R. China
* Correspondence authors
E-mail: majf247@nenu.edu.cn (J.-F. Ma)
Fax: +86-431-85098620 (J.-F. Ma)
ABSTRACT
Three functional coordination polymers (CPs), [(CH3)2NH2]2[Cd3L2(H2O)2]·6H2O (1),
[Ba4L2(H2O)8]·H2O
(2)
and
[Zn(H2L)(H2O)2]·H2O
were
(3)
(H4L
=
3,5-bis(3’,5’-dicarboxylphenyl)-1H-1,2,4-triazole),
synthesized
under
solvothermal conditions. Structures of 1-3 were confirmed by crystallography and
further physically characterized by elemental analysis, IR and TG. 1-3 exhibit
fascinating multi-dimensional framework structures. Knoevenagel condensation
reactions were systematically investigated by using 1-3 as heterogeneous catalysts
under solvent-free conditions. Among them, 1 as a recyclable catalyst shows highly
efficient catalytic performance in respect to 2 and 3, which may be attributed to the
open Lewis base sites and Lewis acid Cd(II) sites in the open channel of 1.
Keywords: Coordination polymers · Knoevenagel condensation · Heterogeneous
catalyst · Crystal structure
1

1. Introduction
Coordination polymers (CPs), as crystalline solid materials, are composed of
metal ions and organic ligands [1-3]. Thus far, an enormous amount of CPs materials
have been assembled by using metal ions as the nodes and various multi-functional
organic ligands as the linkers [4-10]. In recent years, various applications, such as ion
exchange, gas storage and separation, selective adsorption of organic molecules,
luminescence and catalysis, have been exploited in CPs materials [11-16]. Particularly,
CPs as the heterogeneous catalysts based on their structural elements such as metal
vertex, organic ligand and pore system have attracted considerable attention and
significant achievements have been gained [17-20].
In this regard, chemical transformation involving C-C bond formation reactions
promoted by base catalysis plays an important role in synthetic chemistry [21-24].
Particularly, C-C bond formation reactions represented by Knoevenagel condensation
are extensively applied in the syntheses of pharmaceutical products, fragrances and
chemical intermediates [25-27]. The conventional base catalysts are mainly focused
on organic nitrogen-containing compounds and alkali metal hydroxides [28,29].
Nevertheless, in contrast to heterogeneous catalysts, those conventional base catalysts
have some disadvantages in catalyst separation and recycling [30,31]. Very recently,
much effort has been devoted to design and synthesis of heterogeneous base catalysts
[32-39]. Nevertheless, the development of new heterogeneous catalytic materials with
efficient catalytic activity still remains challenging.
In this work, four new CPs, namely, [(CH3)2NH2]2[Cd3L2(H2O)2]·6H2O (1),
[Ba4L2(H2O)8]·H2O (2), and [Zn(H2L)(H2O)2]·H2O (3), have been synthesized
utilizing
functionalized
Lewis
basic
ligand
3,5-bis(3’,5’-dicarboxylphenyl)-1H-1,2,4-triazole (H4L). Remarkably, microporous
framework 1 possesses abundant Lewis base nitrogen sites and Lewis acid Cd(II) sites
in the skeleton and shows excellent heterogeneous catalytic performance for
Knoevenagel condensation reactions.
2

2. Experimental
2.1. Synthesis
2.1.1 Synthesis of [(CH3)2NH2]2[Cd3L2(H2O)2]·6H2O (1).
A mixture of Cd(NO3)2·4H2O (0.148 g, 0.48 mmol), H4L (0.095g, 0.24 mmol),
DMF (24 mL), and distilled water (6 mL) was placed in a Teflon reactor (100 mL)
and heated at 120 °C for 3 days. After cooling the mixture to room temperature,
colorless crystals of 1 were isolated in a 41% yield. Anal. calcd for C40H46Cd3N8O24
(Mr = 1360): C, 35.29; H, 3.38; N, 8.24. Found C, 35.00; H, 3.80; N, 7.98. IR data
(KBr, cm-1): 3394 (s), 1608 (s), 1552 (s), 1407 (s), 1356 (s), 1310 (m), 1190 (w), 1113
(w), 1024 (w), 920 (w), 771 (m), 749 (s), 592 (m), 458 (m).
2.1.2 Synthesis of [Ba4L2(H2O)8]·H2O (2)
The preparation of 2 was similar to that of 1 except that BaCl2·2H2O was used
instead of Cd(NO3)2·4H2O. Compound 2 was obtained as colorless crystals in a 51%
yield. Anal. calcd for C40H32Ba4N6O25 (Mr = 1498): C, 28.86; H, 2.15; N, 5.61. Found
C, 28.80; H, 2.05; N, 5.66. IR data (KBr, cm-1): 3209 (m), 1624 (m), 1550 (s), 1404
(s), 1370 (s), 1311 (m), 1189 (m), 1105 (m), 1026 (w), 920 (w), 869 (w), 818 (w), 790
(m), 754 (m), 723(m), 592 (w), 516 (w), 452(w).
2.1.3 Synthesis of [Zn(H2L)(H2O)2]·H2O (3)
A mixture of Zn(NO3)2·6H2O(0.143 g, 0.48 mmol), H4L (0.095g, 0.24 mmol),
ethanol (24 mL), and distilled water (24 mL) was placed in a Teflon reactor (100 mL)
and heated at 120 °C for 3 days. Compound 3 was obtained as colorless crystals in a
64% yield. Anal. calcd for C18H15ZnN3O11 (Mr = 514): C, 42.03; H, 2.92; N, 8.17.
Found C, 42.20; H, 2.90; N, 8.33. IR data (KBr, cm-1): 3648 (w), 3541 (w), 3191 (m),
2520 (w), 1665 (s), 1622 (s), 1660 (s), 1546 (m), 1512 (w), 1448 (m), 1417 (m), 1364
(s), 1322 (m), 1289 (s), 1197 (m), 1110 (m), 1020 (m), 919(m), 783 (m), 704 (m) 636
(w), 542 (w), 438(w).
2.2 Knoevenagel condensation reactions
In a typical procedure, aldehyde (1 mmol), malononitrile (2 mmol), and
as-synthesized catalyst (0.5 mol%) were stirred in a round-bottom flask at 60 °C for 1
3

h. The yields were analyzed by GC and 1H NMR spectra.
3. Results and discussion
3.1. Description of the structures
3.1.1 Crystal structure of [(CH3)2NH2]2[Cd3L2(H2O)2]·6H2O (1)
Single-crystal X-ray diffraction revealed that 1 crystallizes in the space group
C2/c. The asymmetric unit of 1 consists of three Cd(II) ions, two L4- anions, two
[(CH3)2NH2]+ cations, two coordinated water molecules and six free water molecules.
The [(CH3)2NH2]+ from the decomposed DMF as a positive charge balanced the
framework [40-42]. As illustrated in Fig. 1a, Cd1 and Cd3 exhibit analogous
pentagonal bipyramid coordination geometries. Both Cd1 and Cd3 are encircled by
four L4- anions and one water molecule, while Cd2 is coordinated by five carboxylate
oxygen atoms of distinct L4- anions. Three Cd(II) ions, Cd2, Cd3 and
symmetry-related Cd1#2, are connected by carboxylate oxygen atoms of nine L4-
anions to generate a trinuclear secondary building unit (SBU) with the Cd1···Cd2
separation of 4.153Å and Cd2···Cd3 distance of 4.345Å. The Cd–O–Cd bond angles
range from 114.7° to 115.8°, and the Cd–O bond lengths vary from 2.575 Å to 2.191
Å (Fig. 1b, Table S1). As shown in Fig. 1c, one L4- anion links six Cd(II) ions and
another L4- links seven Cd(II) ions through carboxylate groups of four isophthalate
moieties, yielding a 3D microporous framework (Fig. 1d). The free solvent-accessible
volume calculated from PLATON is 39.2% of the unit cell [43].
4

(a)
(b)
(c)
5

(d)
Fig. 1. (a) Coordination environments of the Cd(II) ions in 1. (b) View of the
trinuclear Cd(II) unit connected by the carboxylate groups from nine L4- anions. (c)
Coordination modes of 1. (d) View of the 3D microporous framework of 1.
3.1.2 Crystal structure of [Ba4L2(H2O)8]·H2O (2)
2 crystallizes in the space group P-1. The asymmetric unit of 2 is composed of
four Ba(II) ions, two L4-, eight coordinated water molecules and one free water
molecule. As illustrated in Fig. 2a, both Ba1 and Ba2 are seven-coordinated. Ba1 is
encircled by four carboxylate oxygen atoms and three water molecules, while Ba2 is
surrounded by six carboxylate oxygen atoms and one water molecule. Ba3 is
eight-coordinated by six carboxylate oxygen atoms of four different L4- anions and
three coordinated water molecules. Ba4 is nine-coordinated by eight carboxylate
oxygen atoms. The Ba–O bond lengths range from 3.275 Å to 2.659 Å, and the
O–Ba–O bond angles vary from 149.0° to 43.2° (Table S2). As shown in Fig. 2b, the
L4- anion bridges nine Ba(II) ions in two different ways to yield a 3D framework (Fig.
2c).
6

(a)
(b)
7

(c)
Fig. 2. (a) Coordination environments of the Ba(II) in 2. (b) Coordination modes of 2.
(c) View of the 3D framework of 2.
3.1.3 Crystal structure of [Zn(H2L)(H2O)2]·H2O (3)
3 crystalline in the space group C2/c. In 3, each Zn(II) ion is in an octahedral
coordination style, surrounded by four carboxylate oxygen atoms from two H2L2−
anions and two water oxygen atoms (Fig. 3a). The Zn–O bond lengths vary from
1.998 Å to 2.125 Å, and the O–Zn–O bond angles range from 85.9° to 140.7° (Table
S3). As shown in Fig. 3b, each H2L2− anion bridges three Zn(II) cations with four
isophthalate arms to yield a layer. Remarkably, there exists π-π stackings among the
benzene rings of the H2L2− anions between neighboring layers (inter-plane and
inter-center lengths of 3.33 and 3.52 Å, respectively), leading to a 3D supramolecular
architecture (Figs. 3c and 3d).
8

(a)
(b)
(c)
9

(d)
Fig. 3. (a) Coordination environment of the Zn(II) ion in 3. (b) Coordination modes of
3. (c) View of the layer of 3. (d) View of the 3D supramolecular architecture by π-π
stackings.
3.2 Knoevenagel condensation reaction
Catalytic activities of 1-3 were investigated in Knoevenagel condensation
reaction of benzaldehyde (BA) with malononitrile [44]. Typically, the catalytic
reactions were carried out at 60 °C with 0.5 mol% of catalyst loading (with respect to
the amount of benzaldedyde). The yields of benzylidene malononitrile (BM) were
confirmed by 1H NMR studies and gas chromatography (GC) (Fig. S1 and S2) [37].
When 2 and 3 were used as catalysts for Knoevenagel condensation reactions, only
conversion yields of 5% and 7.2% were achieved, respectively, after 1 hour.
Meanwhile, a lower conversion yield of 11.2% was acquired with the H4L as
heterogeneous catalyst. In contrast to 2 and 3, 1 exhibits much higher catalytic activity
as the heterogeneous catalyst with high selectivity more than 99% and turnover
frequency (TOF) of 200 h−1 (Table 1). Notably, no catalysis was observed in the
absence of catalysts 1-3 and H4L, demonstrating that 1-3 and H4L indeed exhibit the
catalytic activities towards the Knoevenagel condensation reaction. Furthermore,
PXRD demonstrates that the framework of 1-3 retained after treatments (Fig. S3). In
contrast 2 and 3, the large pore of 1 facilitates the mass transfer of guest molecules,
leading to excellent catalytic performance for Knoevenagel condensation reaction
[19,38].
10

Table 1. Knoevenagel condensation reactions of malononitrile with benzaldehyde
catalyzed by 1-3 and H4L.
Entry catalyst
Yields (%) TOF (h-1)
1
2
3
4
5
blank
Trace
>99
11.5
5
0
286
23
[(CH3)2NH2]2[Cd3L2(H2O)2]·6H2O (1)
H4L
10
[Ba4L2(H2O)8]·H2O (2)
[Zn(H2L)(H2O)2]·H2O (3)
7.2
14.4
Reaction conditions: benzaldehyde (1 mmol), malononitrile (2 mmol), catalyst (0.5
mol% of the substrate), 1 hour, 60 °C.
To check the heterogeneity of 1 in the Knoevenagel condensation reaction, a
filtration experiment was tested. After half an hour of the catalytic reaction, the
catalyst was removed. Further, the reaction mixture was stirred for additional 30
minutes, but no additional BM products were achieved (Fig. 4). The results
demonstrate that the catalytic reaction is indeed heterogeneous and no catalysts were
dissolved into the filtrate. Moreover, recycled Knoevenagel reaction was performed
with catalyst 1 under the same condition. The experiment result shows that catalyst 1
still exhibits good catalytic activity after three runs, demonstrating the excellent
reusability of 1 in the Knoevenagel condensation reaction. Meanwhile, the PXRD
patterns confirmed that the framework was retained after three runs (Fig. S3a).
11

Fig. 4. Conversion of the benzaldehyde in the Knoevenagel condensation reactions
catalyzed by 1 (pink) and by the filtrate of 1 after 30 min of the catalytic reaction
(black).
In the following catalytic reactions, a series of benzaldehyde derivatives with
different functional groups were selected as substrates. As shown in Table 2, the
Knoevenagel condensation reactions of malononitrile with 4-cyanobenzaldehyde
(entry 3) and 4-nitrobenzaldehyde (entry 4) were completely converted to products
after 1 hour at 60 °C. The high catalytic performance can be explained by the electron
withdrawing substituents, which facilitate nucleophilic attack in the Knoevenagel
condensation [27,45]. The synergistic effect by the inductive effect and the
conjugation effect make the electron-withdrawing property of the
halogenobenzaldehyde in the order of -Cl > -Br > -F. Thus, the conversion of the
halogenobenzaldehyde decreases in the same order (entries 2 and 6). In contrast, the
electron-rich benzaldehyde derivatives, such as 4-methylbenzaldehyde,
4-ethylbenzaldehyde and 4-methoxybenzaldehyde, gave the products with much
lower yields (entries 5, 7 and 8).
Table 2. Knoevenagel condensation reaction of malononitrile with benzaldehyde
derivatives catalyzed by 1.
12

Entry Aldehydes
1
Products
Yields (%) TOF (h-1)
>99
>99
>99
>99
>99
39
200
200
200
200
200
78
2
3
4
5
6
7
8
85
170
86
43
Reaction conditions: benzaldehyde derivatives (1 mmol), malononitrile (2 mmol),
catalyst (0.5 mol% of the substrate), 1 hour, 60 °C.
4. Conclusion
We have synthesized three functional coordination polymers with exposed Lewis
basic sites. The Knoevenagel condensation reactions were investigated using 1-3 as
heterogeneous catalysts under solvent-free conditions. Remarkably, porous 1 shows
much higher conversion and selectivity in Knoevenagel condensation reaction
because of its exposed Lewis basic sites and Lewis acid Cd(II) sites in the open
13

channels. Moreover, catalyst 1 can be easily separated and reused three times without
obvious loss of the catalytic activity. The high catalytic activity under mild reaction
conditions makes 1 a promising material for Knoevenagel condensation reaction as
heterogeneous catalyst.
Appendix A. Supplementary data
CCDC 1572038, 1572039 and 1572041 contains the supplementary crystallographic
data for 1-3. These data can be obtained free of charge via
http://www.ccdc.cam.ac.uk/conts/retrieving.html,
or
from
the
Cambridge
Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: (+44)
1223-336-033; or e-mail: deposit@ccdc.cam.ac.uk.
Acknowledgements
This work was supported by the National Natural Science Foundation of China (Grant
No. 21471029).
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16

Table 3. Crystallographic data and structure refinements.
compound
formula
Mr
1
2
3
C40H46Cd3N8O24 C36H32Ba4N6O25 C18H15ZnN3O11
1360
1498
514
crystal system monoclinic
triclinic
P-1
monoclinic
C2/c
space group
a (Å)
b (Å)
c (Å)
α(°)
C2/c
33.2297(12)
20.0890(9)
15.9769(5)
90
10.122(5)
14.450(5)
16.551(5)
107.990(5)
98.667(5)
104.517(5)
2159.4(15)
2
13.526(5)
17.183(5)
16.667(5)
90.000(5)
103.208(5)
90.000(5)
3771(2)
8
β(°)
100.360(3)
90
γ(°)
V (Å3)
10491.5(7)
8
Z
Dcalc (g cm-3) 1.722
2.304
1.813
F(0 0 0)
Rint
5424
1428
2096
0.0342
0.0559
0.0360
1.0370
0.0499
0.1341
GOF on F2
0.9340
1.0260
R1a [I>2σ(I)] 0.0420
0.0688
b
wR2 (all data) 0.0987
0.1323
2
2 2
aR1 = Σ||Fo| - |Fc||/Σ|Fo|. bwR2 = {Σ[w(Fo -Fc2)2]/Σw(Fo ) ]} 1/2
.
17

Table of Contents
Three coordination polymers (CPs) have been constructed with Lewis basic ligand,
where the microporous framework of 1 possesses abundant Lewis base nitrogen sites
and Lewis acid Cd(II) sites in the skeleton and shows excellent heterogeneous
catalytic performance for Knoevenagel condensation reaction under the solvent-free
conditions.
18