Synthesis, Structure, and Ligand-Centered Catalytic Properties of MII Coordination Polymers (M=ZnII, CdII, HgII) with Open Pyridyl N-Oxide Sites

Article abstract of DOI:10.1002/cplu.201600009

Four M^II (M=Zn^II, Cd^II, Hg^II) coordination polymers were designed and synthesized based on two pyridine N-oxide bridging ligands: 3,5-bis(4-pyridyl)pyridine N-oxide and 2,6-bis(3-pyridyl)pyridine N-oxide. The re

Full text of DOI:10.1002/cplu.201600009

DOI: 10.1002/cplu.201600009
Full Papers
Synthesis, Structure, and Ligand-Centered Catalytic
Properties of MII Coordination Polymers (M=ZnII, CdII, HgII)
with Open Pyridyl N-Oxide Sites
Jun-Yan Cheng, Feng-Wen Ding, Peng Wang, Chao-Wei Zhao, and Yu-Bin Dong*[a]
Four MII (M=ZnII, CdII, HgII) coordination polymers were de-
signed and synthesized based on two pyridine N-oxide bridg-
ing ligands: 3,5-bis(4-pyridyl)pyridine N-oxide and 2,6-bis(3-pyr-
idyl)pyridine N-oxide. The resulting polymers all feature a one-
dimensional chain motif and contain free pyridyl moieties.
More importantly, they exhibit interesting ligand-centered (pyr-
idyl N-oxide) catalytic behavior, and can be used as highly het-
erogeneous catalysts to promote the Knoevenagel condensa-
tion reaction under ambient conditions.
Introduction
Coordination polymers (CPs), as a new class of organic–inor-
ganic hybrid materials, have received intense attention owing
to their potential applications in gas adsorption and purifica-
tion,[1] luminescence,[2] drug delivery, and so on.[3] On the other
hand, CPs provide a practical platform to fabricate new types
of heterogeneous catalytic materials.[4] In principle, CPs can act
as solid catalysts, based on either catalytically active metal cen-
ters (metal-centered CPs catalysts)[5] or functional organic
groups attached to the ligands (ligand-centered CPs catalyst).[6]
For example, metal-centered CP catalysts, especially those with
open metal sites, can be considered as Lewis acid type catalyt-
ic species to promote reactions such as carbon–carbon and
carbon–heteroatom bond formation, CÀH bond activation, and
hydrogenation. On the other hand, ligand-centered CP cata-
lysts usually behave as Lewis base type catalysts owing to the
Lewis basicity of the organic functional groups in the ligand,
such as pyridine, amide, and amine moieties. Such CPs can be
used to facilitate ester polymerization, Knoevenagel condensa-
tion, the Michael reaction, and so on. Compared with metal-
centered CPs catalysts, the design and synthesis of the ligand-
centered CP counterparts are more challenging because the
embedded catalytically active organic groups might also coor-
dinate to the metal nodes during the self-assembly process as
a result of their Lewis base nature.
We have been studying CPs with different topological struc-
tural motifs composed of metal ions and functional organic
linkers.[7] In our previous study, bridging N-oxides were de-
signed and used as ligands to construct CPs with adsorption/
separation or fluorescence properties.[8] N-Oxides, as typical ho-
mogeneous catalysts, are widely used in the catalysis of nucle-
ophilic organic reactions in both metal-free or -mediated forms
owing to their pronounced nucleophilic character.[9] Despite
fruitful advancements in academia, similar to many homogene-
ous catalytic species, they cannot be more extensively applied
because of inherent difficulties in separation of the catalysts
from the reaction systems.
Compared with homogeneous catalytic systems, heteroge-
neous catalysts are more long-lived for recyclable use than
their homogeneous counterparts. To solve this problem, the
embedding of the catalytic active N-oxides species through co-
valent-bonding interactions into solid CPs can be performed.
Based on this approach, N-oxide species can become reusable
and eco-friendly catalysts for practical application. For exam-
ple, the pyridine N-oxide unit could be introduced into diver-
gent organic ligands that contain different types of coordinat-
ing donors. In addition, coordinating donors, other than N-
oxides, can bind relatively “soft” metal ions in CPs (Scheme 1).
The obtained CPs with free N-oxides are expected to be effec-
tive heterogeneous solid catalysts for promoting organic reac-
tions. To date, the catalytic activity of CPs with free N-oxides as
Lewis base sites has remained unexplored.
[a] J.-Y. Cheng, F.-W. Ding, Dr. P. Wang, C.-W. Zhao, Prof. Dr. Y.-B. Dong
College of Chemistry, Chemical Engineering and Materials Science
Collaborative Innovation Center of Functionalized Probes
for Chemical Imaging in Universities of Shandong
Herein, we report two new, rigid, bent ligands, namely, 3,5-
bis(4-pyridyl)pyridine N-oxide (L1) and 2,6-bis(3-pyridyl)pyridine
Key Laboratory of Molecular and Nano Probes, Ministry of Education
Shandong Normal University, Jinan 250014 (P. R. China)
E-mail: yubindong@sdnu.edu.cn
Supporting information for this article can be found under http://
dx.doi.org/10.1002/cplu.201600009. It contains ORTEP figures, selected
bond lengths and angles for 1–5, and GC-MS and GC analysis data for
catalytic reactions based on 1–4.
This article is part of a Special Issue on “Coordination Polymers/MOFs”. A
link to the issue will appear here once it is compiled.
Scheme 1. Design of the N-oxide-containing organic ligand and correspond-
ing CPs with open N-oxide sites.
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N-oxide (L2), with both pyridyl and pyridyl N-oxide groups. As
shown in Scheme 2, ligands L1 and L2 were prepared by
Suzuki–Miyaura cross-coupling reactions between bromine-
substituted pyridine N-oxide and the corresponding pyridine
boronic acids in good yields. Ligands L1 and L2 have the fol-
lowing features: 1) both contain pyridyl and N-oxide donors
with different coordinating natures; and 2) they are generally
expected to link relatively soft metal ions, such as zinc(II) and
cadmium(II), to CPs with a free N-oxide moiety, as potential
Lewis base type catalysts for specific organic reactions. Further-
more, four CPs based on these ligands have been synthesized
(Scheme 2). They all display a 1D chain motif with an open pyr-
idine N-oxide site. Notably, compounds 1–4, with free pyridine
N-oxide moieties, are the first highly active heterogeneous CP
catalysts for Knoevenagel condensation.
Results and Discussion
Structural analysis
MII CPs based on L1
The crystallization of L1 with ZnCl2 and ZnI2 (metal-to-ligand
ratio 1:1) in a mixture of CH2Cl2/CH3OH at room temperature
afforded 1 and 2 as colorless crystals, respectively. Single-crys-
tal X-ray analysis revealed that 1 (Zn(L1)Cl2) and 2 (Zn(L1)I2)
were isostructural; thus only the structure of 1 is described in
detail herein (Table 1).
Compound 1 crystallizes in the monoclinic space group P21/
c. There is only one crystallographic zinc(II) center in 1. As
shown in Figure 1, the zinc(II) node lies in a slightly distorted
tetrahedral coordination sphere that consists of two terminal
Npyridyl donors on L1 and two ClÀ counterions. The correspond-
ing Cl2-Zn1-Cl1 and N2-Zn1-N3 angles are 128.2 and 109.538,
respectively. The ZnÀN bond lengths are 2.044(3) and
2.045(3) ꢁ, respectively, and they all are within the range of re-
ported ZnIIÀN bond lengths.[10] Notably, pyridyl-N-oxides
groups in 1 are free and uncoordinated to the zinc(II) centers.
As indicated in Figure 1, ligand L1 acts as a bidentate ligand to
bind zinc(II) cations through terminal pyridyl nitrogen atoms
into 1D helical chains that extend along the crystallographic
b axis. These one-handed 1D chains are further connected to
each other through interchain hydrogen-bonding interactions
into a chiral 2D wave-shaped layer. The hydrogen-bonding
system is composed of the oxygen atom of pyridyl N-oxide
and a hydrogen atom of pyridyl N-oxide on a neighboring
chain (Figure 1). The P and M layers alternatively arrange along
Scheme 2. Synthesis of L1, L2, and compounds 1–4. mCPBA=3-chloroper-
benzoic acid, UHP=urea hydrogen peroxide, TFAA=trifluoroacetic acid an-
hydride.
Table 1. Crystal data and structural refinement parameters for 1–5.
1
2
3
4
5
formula
Mr
crystal system
a [ꢁ]
b [ꢁ]
c [ꢁ]
C15H11Cl2N3OZn
385.56
C15H11I2N3OZn
568.44
C15H11I2N3OCd
615.48
C15H11I2N3OHg
703.66
monoclinic
9.930(2)
17.727(4)
9.872(2)
90
C60H44I12N12O4Cd6
3194.33
tetragonal
17.4268(16)
17.4268(16)
25.094(5)
90
monoclinic
7.3299(19)
20.053(5)
12.521(3)
90
monoclinic
7.2321(4)
18.4963(8)
13.2675(5)
90
monoclinic
9.7914(15)
17.734(3)
9.8466(15)
90
a [8]
b [8]
123.696(2)
90
1531.2(7)
P21/c
90.819(4)
90
1774.57(14)
P21/n
90.939(2)
90
1709.5(5)
C2/c
91.173(2)
90
1737.4(6)
C2/c
90
90
g [8]
V [ꢁ3]
7620.9(18)
I4(1)/a
4
2.784
6.557
space group
Z value
4
4
4
4
1calcd [gcmÀ3
]
1.672
1.956
293(2)
2.128
29.273
293(2)
2.391
4.893
293(2)
2.690
12.416
293(2)
m(MoKa) [mmÀ1
T [K]
]
293(2)
F (000)
776
1064
1136
1264
5776
GOF
1.005
1.104
1.128
1.027
1.224
R1/wR2[I>2s(I)]
0.0583/0.1155
0.0907/0.2577
0.0245/0.0605
0.0346/0.0834
0.0609/0.1464
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Figure 1. a) ORTEP drawing of the structure of 1. Ellipsoids are given at the 50% probability level. b) Hydrogen-bonding-driven 2D net composed of 1D
chains. c) Crystal packing of 1.
the crystallographic c axis and the interchain zinc(II) distance is
two pyridyl N-donors (dHgÀN bond lengths are 2.406(6) ꢁ) and
two IÀ counterions. Similarly to 1 and 2, the pyridine N-oxide
group in 4 is also uncoordinated.
about 6.2 ꢁ.
Figure 2b shows that L2 links HgI2 units through terminal
pyridyl N donors to create a one-handed 1D helix that extends
along the crystallographic c axis. The P and M helices are fur-
ther held together to provide double-stranded chains through
interchain H···p interactions (Figure 2c). In the solid state, these
double-stranded chains arrange along the crystallographic
a axis, and no detectable interactions are found among them.
MII CPs based on L2
The crystallization of L2 with CdI2 and HgI2 (metal-to-ligand
ratio 1:1) in mixtures of THF/CH3OH and CH2Cl2/CH3OH at
room temperature afforded 3 and 4 as colorless crystals, re-
spectively. Single-crystal XRD reveals that complexes
3
(Cd(L2)I2) and 4 (Hg(L2)I2) are isostructural; they all feature 1D
ripple-like chains (Table 1). Therefore, only the structure of 4 is
described in detail herein.
Catalytic properties
Compound 4 crystallizes in the monoclinic space group C2/
c (Figure 2a) and each mercury(II) node is located in a distorted
tetrahedral {HgN2I2} coordination sphere, which is composed of
The Knoevenagel reaction, which is the condensation of active
methylene compounds with aldehyde or ketone, is an impor-
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Figure 2. a) ORTEP drawing of the structure of 4. Ellipsoids are given at the 50% probability level. b) Single 1D helical chain. c) Crystal packing of 4.
tant CÀC coupling reaction and is widely used in the synthesis
of fine chemicals. This type of condensation is generally cata-
lyzed by Lewis bases or acids. As shown above, CPs 1–4 con-
tain an open pyridyl N-oxide group and MII nodes that could
be considered as bifunctional catalysts. Such coexisting acid–
base CPs would be more active than those of acid- or base-
centered catalysts. Based on their structural features, com-
pounds 1–4 are expected to show good catalytic behavior for
Knoevenagel condensation under milder conditions.
Consequently, the catalytic activity of compounds 1–4 as solid
heterogeneous catalysts in the Knoevenagel condensation re-
action of benzaldehydes with malononitrile was investigated.
Compound 1 was chosen to optimize the reaction condi-
tions. In a typical reaction, condensation is performed under
atmospheric pressure at room temperature. A mixture of ben-
zaldehyde, malononitrile (1:1.2, molar ratio), and 1 (4 mol%,
finely ground) was stirred at room temperature, and the reac-
tion was monitored by GC. The GC-MS spectra (see the Sup-
porting Information) indicated that the condensation reaction
was finished in 6 h to afford 2-benzylidenemalononitrile as the
expected product with excellent conversion (99.5%) and selec-
tivity (99.2%; Table 2, entry 1). Notably, the condensation reac-
tion described herein needed no additional organic solvents.
Therefore, because this reaction is easy to manipulate and
On the other hand, aromatic N-oxides are potential pollu-
tants of soils and aquatic environments because of their wide-
spread use as antibiotics, pesticides, and other industrial chem-
ical products.[11] Therefore, the development of corresponding
heterogeneous, eco-friendly, reusable, N-oxide-type catalysts
for Knoevenagel condensation is a topic of great interest.[12]
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Table 2. Catalytic data for the Knoevenagel condensation of malononi-
trile with different aromatic aldehyde substrates.
Entry
R
Solvent
t [h]
Conv. [%][c]
Selec. [%][c]
[a]
1
2
3
4
5
ÀH
–
6
2
2
6
6
99.5
99.5
99.5
99
99.2
100
100
99.6
97.1
ÀNO2
ÀBr
MeOH[b]
MeOH
ÀCH3
ÀtBu
–
–
97
[a] Solvent-free conditions. [b] MeOH was added to dissolve the sub-
strates 4-methylbenzaldehyde and 4-tert-butylbenzaldehyde. [c] As deter-
mined by GC-MS analysis.
avoids the use of toxic and volatile solvents, it could be con-
sidered as a clean, catalytic organic synthesis approach. Com-
pound 1 is stable in aqueous media over a wide pH range of
2–11 (see the Supporting Information), which makes it poten-
tially a green catalyst to promote other organic reactions in
aqueous solutions.
Figure 3. Above: Leaching test for catalytic condensation with compound 1.
Below: Results for the conversion and selectivity of the reaction between
benzaldehyde and malononitrile catalyzed by 1 in five catalytic cycles.
To explore the effect of substituents for this condensing re-
action, different types of benzaldehydes were used as sub-
strates under the reaction conditions. The results are summar-
yield of the condensation product was very low (ꢀ24%) after
6 h; this indicates that 1 is a highly active catalyst for the con-
densation of benzaldehyde and malononitrile under the reac-
ized in Table 2. When 4-nitro- and 4-bromobenzaldehyde, with tion conditions described herein. To gain an insight into the
ÀNO2 and ÀBr electron-withdrawing groups at the para posi-
tions are used, the conversions (99.5%, see the Supporting In-
formation) of the condensing products are equivalent to that
of benzaldehyde, but with the 100% selectivity. On the other
hand, when electron-donating groups, namely, methyl and
tert-butyl groups, were used as the substituents on benzalde-
hyde at the para position (4-methyl- and 4-tert-butylbenzalde-
hyde), slightly lower conversions (97–99%, see the Supporting
Information) and selectivities (97.1–99.6%, see the Supporting
Information) than those substrates containing electron-with-
drawing groups were observed. Such an observation suggests
that the electron-withdrawing substituents promote reactivity;
this might be related to an increase in the electrophilicity of
the substituted benzaldehydes. In addition, condensation reac-
tions of benzaldehydes containing an electron-withdrawing
group with malononitrile took only 2 h to reach 99.5% conver-
sion (Table 2, entries 2 and 3), which strongly supported the
above observation.
heterogeneous nature of 2, leaching tests were performed. As
indicated in Figure 3, no further reaction took place without
1 after initiation of the reaction at 4 h (Figure 3).
Compound 1 can be reused as a heterogeneous catalyst.
After each catalytic cycle, compound 1 could be easily recov-
ered by centrifugation and filtration, and directly reused in the
next run under the same reaction conditions. As indicated in
Figure 3, after five consecutive catalytic runs, the conversion
was still above 99% (see the Supporting Information), which
indicated almost no degradation in the catalytic activity of
1 during the recycling process. In addition, compound 1 was
demonstrated to be highly crystalline from analysis of the
powder XRD patterns, even after five catalytic runs (Figure 4),
which confirmed the reusability of 1 as a heterogeneous cata-
lyst.
In addition, the catalytic properties of 2–4 were also ex-
plored under the same reaction conditions. For example, the
treatment of benzaldehyde (1.0 mmol) with malononitrile
(1.2 mmol) in the presence of 2–4 (4 mol%) at room tempera-
ture afforded the condensation products with very high con-
versions (99.5%, as determined by GC; see the Supporting In-
formation), which were comparable to that of 1 (Table 3). Com-
pared with the ZnIIÀCPs 1 and 2, the Knoevenagel reactions
catalyzed by 3 and 4 with CdII centers required a longer reac-
tion time (8 h). Different catalytic performances of 1/2 and 3/4
might be due to different metal centers and pyridyl N-oxide
orientations resulting from the crystal-packing styles. The
stronger Lewis acid zinc(II) center and more exposed N-oxide
moieties in 1 and 2 would certainly facilitate this condensation
reaction.[13]
As shown above, compound 1 is nonporous. Thus, catalysis
based on 1 should occur on the external surface. To prove this
hypothesis, as-synthesized bulk crystals (average >100 mm) of
1 were used instead of a finely ground sample (average
<20 mm; see the Supporting Information). The yield for the
condensation of benzaldehyde and malononitrile was also up
to 99.5%, but took a much longer time (10.5 h). This observa-
tion strongly indicates that catalysis by 1 exhibits surface cata-
lytic behavior and is highly dependent on the catalyst particle
size.
A control reaction was performed with benzaldehyde in the
absence of 1, at room temperature without solvent, and the
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Figure 4. Simulated powder XRD pattern of 1 and those recorded after each
catalytic cycle.
Table 3. Knoevenagel condensation of benzaldehyde and malononitrile
by 2–4.[a]
Entry
CP catalyst
t [h]
Conv. [%][b]
Selec. [%][b]
1
2
3
2
3
4
6
8
8
99.5
99.5
99.5
>99
>99
>99
[a] Reaction conditions: benzaldehyde (1 mmol), malononitrile (1.2 mmol),
catalyst (4% with respect to the amount of benzaldehyde), solvent-free
conditions. [b] Yield determined by GC analysis.
Figure 5. Above: ORTEP drawing of the structure of 5. Ellipsoids are given at
the 50% probability level. The N-oxide coordinates to the CdII center with
CdÀO bond lengths of 2.319(8) and 2.510(8) ꢁ. Below: b) Twofold interpene-
trating network in 5.
To confirm the catalytic function of the open pyridyl N-
oxide, we tried to prepare the CP without a free pyridyl N-
oxide moiety. Through the combination of L1 and CdI2 in
oxide was the dominating active catalytic site for Knoevenagel
condensation.
CH2Cl2/MeOH, compound
5 (Cd6(L1)4I12) was generated.
Single-crystal analysis indicated that 5 crystallized in the tetrag-
onal space group I4(1)/a (Table 1). Different from 1 and 2,
ligand L1 in 5 acts as a tridentate linker instead of a bidentate
one to bind cadmium(II) nodes into a twofold interpenetrating
Based on the above observation, the condensation between
benzaldehyde and malononitrile might follow the reaction
pathway shown in Scheme 3.
To date, a series of CPs as catalysts for the Knoevenagel con-
densation of benzaldehyde and malononitrile have been re-
ported (Table 4). The conversions in our system based on 1–4
are usually higher than those reported for other zinc(II) and
cadmium(II) CPs. In addition, most of reactions are performed
under solvent-assistant conditions and require a longer time or
higher temperature. Therefore, CPs 1–4 are good solid cata-
lysts with the advantages of being easy to prepare, energy
saving, and resulting in clean synthesis. More importantly, the
ligand-centered CP catalysts described herein might provide
a useful approach to access new types of CP catalysts.
3D network with
a
two-nodal net (Schlꢂfli symbol
{4^2;6}2{4^4;6^2;8^2;10^7}; see the Supporting Information),
in which all pyridyl N-oxides on L1 coordinate to cadmium(II)
centers and no open N-oxide sites remain (Figure 5). Indeed,
when the condensation reaction of benzaldehyde and malono-
nitrile was performed in the presence of 5 under the same re-
action conditions, the reaction afforded much lower conver-
sion (only ꢀ51% based on GC analysis), even after 12 h. We
believe that this is only a metal-centered (Lewis acid) catalytic
process.
In addition, a control reaction for the condensation of ben-
zaldehyde with malononitrile was performed in the presence
of 4,4’-bispyridyl-N,N’-dioxide as a catalyst under the same re-
action conditions, the yield for the expected condensing prod-
uct was 99.5% (5 h); this yield was the same as those obtained
for reactions catalyzed by 1–4 (see the Supporting Informa-
tion). This result further demonstrated that the open pyridyl N-
Conclusion
We successfully synthesized four MII CPs (M=Zn, Cd, Hg) based
on pyridyl N-oxide bridging ligands. All four CPs featured a 1D
chain motif and contained open pyridyl N-oxide sites. More im-
portantly, these CPs could be used as highly active heteroge-
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Experimental Section
Materials and methods
HgI2, ZnCl2, CdI2, and ZnI2 (Acros) were used as obtained without
further purification. IR samples were prepared as KBr pellets, and
spectra were obtained in the n˜ =400–4000 cmÀ1 range by using
a PerkinElmer 1600 FTIR spectrometer. Elemental analyses were
performed on a PerkinElmer model 2400 analyzer. 1H NMR data
were collected by using an AM-300 spectrometer. Chemical shifts
are reported in d relative to tetramethylsilane (TMS). Powder XRD
patterns were obtained on a D8 ADVANCE powder X-ray diffrac-
tometer with CuKa radiation (l=1.5405 ꢁ). GC-MS analysis data
were recorded on a J&K S011525-300 gas chromatograph (Agilent
6890GC-5973MS). Separation data were obtained by using an Agi-
lent 1260 Infinity HPLC system equipped with an Agilent C18 re-
verse-phase column (150ꢃ4.6 mm, 5 mm).
Scheme 3. The proposed mechanism for the Knoevenagel condensation of
benzaldehyde and malononitrile in 1.
Ligand synthesis
Synthesis of 3,5-dibromopyridine N-oxide: 3,5-Dibro-
mopyridine (3.2 g, 13.5 mmol) was dissolved in THF
Table 4. Comparison of the catalytic activity of typical CPs in the Knoevenagel reac-
(50 mL), mCPBA (4.6 g, 26.5 mmol) was added, and the
mixture was heated at reflux for 48 h. A saturated aque-
ous solution of Na2CO3 (25–30 mL) was added until the
resulting mixture became basic. The aqueous layer was
extracted with CH2Cl2 (2ꢃ50 mL) and the combined or-
ganic layers were dried over Na2SO4. The crude product
was purified by column chromatography on silica gel
with CH2Cl2 as the eluent to afford 3,5-dibromopyridine
tion of benzaldehyde and malononitrile.
Entry CP catalyst
Conditions
Conv. [%] Ref.
1
2
3
Zn2dobdc
Zn2(tpt)2(2-atp)I2
[Ni4(m6-MTB)2(m2-
toluene/708C/24 h 77
[14]
[15]
[16]
ethanol/608C/2 h
p-xylene/1308C/
6 h
CH3CN/608C/24 h
benzene/RT/12 h
CH3CN/RT/24 h
THF/408C/1.5 h
toluene/RT/3 h
solvent-free/RT/4 h 96
DMF/RT/3 h
toluene/RT/6 h
xylene/1308C/6 h
xylene/1308C/1 h quant
xylene/1308C/3 h quant
99
78
H2O)4(H2O)4]·10DMF·11 H2O
4
[Tb(BTATB)(DMF)(H2O)2] ·DMF·2H2O
99
98
quant
98
[17]
[18]
[19]
[20]
[21]
[22]
[23]
[24]
[25]
[25]
[25]
this
study
5
[Cd(4-btapa)2(NO3)2]·6H2O·2MDF
N-oxide as
a white crystalline solid (1.5 g, 59.3%).
1H NMR (300 MHz, DMSO, 258C, TMS): d=8.60 (s, 2H;
ÀC5H3NBr2), 7.97 ppm (s, 1H; -C5H3NBr2).
6
Pb(cpna)2·2MDF·6H2O
7
[ZnL(H2O)2]·nDMF
8
ZIF-8
100
9
TMU-5
Synthesis of 2,6-dibromopyridine N-oxide: 2,6-Dibro-
mopyridine (4.7 g, 20 mmol) was dissolved in CH2Cl2
(50 mL) and the UHP adduct (4.0 g, 42 mmol) was
added. The mixture was cooled to 08C and then TFAA
was slowly added. A saturated aqueous solution of
Na2S2O3 (25–30 mL) and a solution of Na2CO3 were
added until the resulting mixture became basic. The
aqueous layer was extracted with CH2Cl2 (2ꢃ50 mL) and
the combined organic layers were dried over Na2SO4.
The crude product was purified by column chromatogra-
phy on silica gel with CH2Cl2 as the eluent to afford 2,6-
dibromopyridine N-oxide as a white crystalline solid
10
11
12
13
14
15
(H3O)2[Cd3(2,7-CDC)4]·3DMF·4H2O
ZIF-9
TS-1
Cu-BTC
Fe-BTC
1–4
quant
99
96
solvent-free/RT/6–
8 h
99.5
dobdc=2,5-dioxidoterephthalate; tpt=2,4,6-tris(4-pyridyl)-1,3,5-triazine; 2-atp=2-
aminoterephthalate; MTB=methanetetrabenzoate; BTATB=4,4’4“-benzene-1,3,5-triyl-
tris(azanediyl)tribenzoate; 4-btapa=1,3,5-benzene tricarboxylic acid tris[N-(4-pyridyl)a-
mide]; cpna=N-(4-carboxyphenyl)isonicotinamide 1-oxide; L=5-acetamidoisophthalic
acid; 2,7-CDC=9H-carbazole-2,7-dicarboxylic acid; BTC=benzene-1,3,5-tricarboxylate;
quant=quantitative.
1
(3.3 g, 58.9%). H NMR (300 MHz, [D6]DMSO, 258C, TMS):
d=7.95 (d, 2H; -C5H3NOBr), 7.12 ppm (t, 1H; -C5H3NOBr).
Synthesis of L1: A solution of 3,5-dibromopyridine N-
oxide (1.73 g, 10.0 mmol), pyridine-4-boronic acid (2.95 g,
24.0 mmol), K2CO3 (17.25 g, 125.0 mmol), and [Pd(PPh3)4] (1.16 g,
1.0 mmol) in EtOH/H2O was heated at reflux for 40 h under N2
(Scheme 1). After removal of solvent under vacuum, the residue
was purified by column chromatography to afford L1 (72%). IR
(KBr pellet): n˜ =3048 (m), 2469 (w), 1603 (s), 1552 (m), 1502 (w),
1462 (w), 1406 (s), 1348 (w), 1217 (m), 1174 (m), 1069 (w), 1013
(m), 943 (w), 821 (s), 679 (w), 625 (w), 557 cmÀ1 (w); 1H NMR
(300 MHz, [D6]DMSO, 258C, TMS): d=8.84 (s, 2H; -C5H3NO-), 8.73–
8.72 (d, 4H; -C6H4N-), 8.18 (s, 1H; -C6H3NO-), 7.95–7.94 ppm (d, 4H;
-C6H4N-); elemental analysis calcd (%) for C15H11N3O: C 72.58, H
4.44, N 16.87; found: C 72.01, H 4.85, N 16.24.
neous catalysts to promote the Knoevenagel reaction under
mild conditions. The catalytic functions of free pyridyl N-oxides
were demonstrated. We believe that this new ligand-design
approach will be very useful for the synthesis of more ligand-
centered CP catalysts. Compared with metal-centered CP coun-
terparts, the ligand-centered CP catalysts are beneficial supple-
ments to the CP catalyst family. More heterogeneous CP cata-
lysts of this type are currently under investigation in our labo-
ratory.
Synthesis of L2: A solution of 2,6-dibromopyridine N-oxide (1.73 g,
10.0 mmol), pyridine-3-boronic acid (2.95 g, 24.0 mmol), K2CO3
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(17.25 g, 125.0 mmol), and [Pd(PPh3)4] (1.16 g, 1.0 mmol) in EtOH/
H2O was heated at reflux for 40 h under N2 (Scheme 1). After re-
moval of solvent under vacuum, the residue was purified by
column chromatography to afford L2 (75%). IR (KBr pellet): n˜ =
3046 (s), 1603 (s), 1552 (m), 1503 (w), 1463 (w), 1408 (s), 1343 (w),
1225 (m), 1174 (m), 1069 (w), 1011 (m), 943 (w), 822 (s), 679 (w),
625 (w), 557 cmÀ1 (w); 1H NMR (300 MHz, [D6]DMSO, 258C, TMS):
d=8.84 (s, 2H; -C5H3NO-), 8.73–8.72 (d, 4H; -C6H4N-), 8.18 (s, 1H;
-C6H3NO-), 7.95–7.94 ppm (d, 4H; -C6H4N-); elemental analysis calcd
(%) for C15H11N3O: C 72.58, H 4.44, N 16.87; found: C 72.11, H 4.96,
N 16.45.
Typical catalytic experiment
A mixture of benzaldehyde (1.0 mmol, 0.1 mL), and malononitrile
(1.2 mmol, 0.079 g) was stirred at ambient temperature for 5 min.,
and then the CP catalyst (4%) was added. The mixture was stirred
for 6–8 h at room temperature (monitored by GC) and the conver-
sions and products were determined by GC-MS. The catalyst was
recovered by filtration, washed with methanol, and then directly
reused in the next run under the same reaction conditions.
Single-crystal structure determination
Suitable single crystals of 1–5 were selected and mounted in air
onto thin glass fibers. X-ray intensity data were measured at
298(2) K on a Bruker SMART APEX CCD-based diffractometer (MoKa
radiation, l=0.71073 ꢁ). The raw frame data for 1–5 were integrat-
ed into SHELX-format reflection files and corrected for Lorentz and
polarization effects by using SAINT.[26] Corrections for incident and
diffracted beam adsorption effects were applied by using
SADABS.[10] None of the crystals showed evidence of crystal decay
during data collection. All structures were solved by a combination
of direct methods and difference Fourier syntheses and structural
analysis refined against F2 by the full-matrix least-squares tech-
nique. Crystal data, data collection parameters, and refinement sta-
tistics for 1–5 are listed in Table 1. Relevant interatomic bond
lengths and angles for 1–5 are given in Tables S1–S5 in the Sup-
porting Information. CCDC 1443513–1443517 contain the supple-
mentary crystallographic data for this paper. These data can be ob-
tained free of charge from The Cambridge Crystallographic Data
Centre.
Synthesis of MII CPs
Synthesis of Zn(L1)Cl2 (1): A solution of ZnCl2 (2.7 mg, 0.02 mmol)
in CH3OH (5 mL) was carefully layered on a solution of L1 (5 mg,
0.02 mmol) in CH2Cl2 (5 mL). The solution was left for 1 week at
room temperature, and colorless crystals of 1 were obtained (53%
(based on L1)). IR (KBr pellet): n˜ =3099 (w), 1616 (s), 1552 (m),
1514 (w), 1402 (m), 1342 (m), 1199 (s), 1075 (m), 1028 (m), 949 (w),
823 (s), 691 (w), 643 (m), 604 (w), 499 cmÀ1 (m); elemental analysis
calcd (%) for C15H11ZnCl2N3O: C 46.70, H 2.85, N 10.90; found: C
47.14, H 2.48, N 10.55.
Synthesis of Zn(L1)I2 (2): A solution of ZnI2 (7 mg, 0.02 mmol) in
CH3OH (5 mL) was carefully layered on a solution of L1 (5 mg,
0.02 mmol) in CH2Cl2 (5 mL). The solution was left for 1 week at
room temperature, and colorless crystals of 2 were obtained (50%
(based on L1)). IR (KBr pellet): n˜ =3069 (s), 1602 (s), 1550 (s), 1503
(w), 1401 (s), 1344 (s), 1068 (m), 1010 (m), 946 (w), 819 (s), 674(m),
626 (m), 552 cmÀ1 (w); elemental analysis calcd (%) for
C15H11HgI2N3O: C 25.60, H 1.56, N 5.97; found: C 25.88, H 1.48, N
6.21.
Acknowledgements
We are grateful for financial support from the NSFC (grant
nos. 21475078 and 21271120), the 973 Program (grant
nos. 2012CB821705 and 2013CB933800), and the Taishan Schol-
ar’s Construction Project.
Synthesis of Cd(L2)I2 (3): A solution of CdI2 (7.33 mg, 0.02 mmol)
in CH3OH (5 mL) was carefully layered on a solution of L2 (5 mg,
0.02 mmol) in THF (5 mL). The solution was left for 1 week at room
temperature, and yellow crystals of 3 were obtained (47% (based
on L2)). IR (KBr pellet): n˜ =3070 (s), 1601 (s), 1551 (m), 1501 (w),
1465 (w), 1412 (s), 1344 (w), 1324 (w), 1174 (m), 1014 (m), 942 (m),
824 (m), 782 (m), 692 (m), 628 cmÀ1 (m); elemental analysis calcd
(%) for C15H11CdI2N3O: C 29.25, H 1.79, N 6.82; found: C 29.67, H
1.94, N 6.51.
Keywords: coordination polymers · heterogeneous catalysis ·
Knoevenagel condensation
metals
· pyridyl N-oxides · transition
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1171 (s), 1069 (w), 1010 (m), 938 (m), 826 (s), 679 (w), 628 (w),
556 cmÀ1 (w); elemental analysis calcd (%) for C60H44Cd6I4N12O4: C
33.05, H 2.02, N 7.71; found: C 33.27, H 1.88, N 7.46.
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Manuscript received: January 8, 2016
Revised: March 3, 2016
Accepted Article published: March 11, 2016
Final Article published: && &&, 0000
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J.-Y. Cheng, F.-W. Ding, P. Wang,
C.-W. Zhao, Y.-B. Dong*
&& – &&
Synthesis, Structure, and Ligand-
Centered Catalytic Properties of MII
Coordination Polymers (M=ZnII, CdII,
HgII) with Open Pyridyl N-Oxide Sites
Metal infiltration: A series of one-di-
mensional MII coordination polymers
(CPs) with open pyridyl N-oxide sites are
reported. These CPs can be used as
highly active heterogeneous catalysts
for Knoevenagel condensation under
ambient conditions (see scheme).
&
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ꢀ 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
ÝÝ These are not the final page numbers!