Synthesis, structure, luminescence and catalytic properties of cadmium(ii) coordination polymers with 9H-carbazole-2,7-dicarboxylic acid

Article abstract of DOI:10.1039/c3dt53109k

Two Cd(ii) metal-organic frameworks were synthesized from the NH-functionalized dicarboxylate ligand 9H-carbazole-2,7-dicarboxylic acid (2,7-H₂CDC). Compound 1, [Cd₄(CDC)₄(DMF) ₄]·4DMF·4H₂O, displays

Full text of DOI:10.1039/c3dt53109k

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Synthesis, structure, luminescence and catalytic
properties of cadmium(II) coordination polymers
with 9H-carbazole-2,7-dicarboxylic acid†
Cite this: Dalton Trans., 2014, 43,
3691
Xiu-Chun Yi, Meng-Xuan Huang, Yan Qi and En-Qing Gao*
Two Cd(II) metal–organic frameworks were synthesized from the NH-functionalized dicarboxylate ligand
9H-carbazole-2,7-dicarboxylic acid (2,7-H2CDC). Compound 1, [Cd4(CDC)4(DMF)4]·4DMF·4H2O, displays
2D square grid networks based on novel tetranuclear [Cd4(COO)8] secondary building units (SBUs) and
pairwise CDC2− linkers. Compound 2, (H3O)2[Cd3(2,7-CDC)4]·3DMF·4H2O, is also based on 4-connected
SBUs and pairwise CDC linkers, but the unusual trinuclear [Cd3(COO)8] SBUs lead to 2-fold interpene-
trated 3D diamond-type frameworks with guest accessible voids. Both compounds display strong blue
fluorescence in the solid state, and compound 2 shows high catalytic activity for Knoevenagel
condensation.
Received 4th November 2013,
Accepted 16th December 2013
DOI: 10.1039/c3dt53109k
www.rsc.org/dalton
1 Introduction
Metal–organic frameworks (MOFs) and/or porous coordination
polymers (PCPs), which can be readily self-assembled from
metal ions or clusters and organic ligands, have witnessed
rapid development over the last few decades. They attract both
scientific and technological interest for their diverse architec-
tures and their potential applications, such as gas adsorption/
separation,1 catalysis,2 and drug delivery.3 Besides the poro-
sity-related properties, MOFs are certainly very promising as
multifunctional luminescent materials.4
Scheme 1 (a) The ligand 2,7-H2CDC. (b) Knoevenagel condensation.
Among the variety of organic ligands thus far used for the
assembly of MOFs, di- and polycarboxylate compounds are the
most extensively explored because the versatile and strong
binding ability of the carboxylate group to metal ions can
afford exceptionally diverse, often robust, framework struc-
tures. We are interested in using 9H-carbazole-2,7-dicarboxylic
acid (2,7-H2CDC) as organic linkers to construct new MOFs.
The ligand was chosen for the following reasons. First, the
dicarboxylate ligand contains a unique, rigid backbone. In par-
ticular, the five-membered pyrrole ring between two benzoic
groups dictates a unique structure featuring a 150° angle
between the two coordinative carboxylic groups, so the ligand
may result in novel frameworks different from those with
common linear ligands such as 1,4-benzenedicarboxylic and
4,4′-biphenyl dicarboxylic acids. Second, the carbazole NH
groups could serve as potential reactive sites for post-synthetic
modification (PSM) of MOFs. NH2-containing MOFs have been
studied frequently as prototypical examples of PSM and for
MOF functionalization.5 Third, the secondary amine is a basic
site and could impart the resulting MOFs with potential cata-
lytic activity. Here we report two Cd(II) MOFs derived from 2,7-
H2CDC. The compounds display 2D square grid and 3D
diamond-type frameworks based on unusual tetranuclear
[Cd4(COO)8] and trinuclear [Cd3(COO)8] clusters, respectively.
The photoluminescence properties of the compounds were
studied, and compound 2 proved to have high catalytic activity
for Knoevenagel condensation (Scheme 1).
Shanghai Key Laboratory of Green Chemistry and Chemical Processes, Department of
Chemistry, East China Normal University, Shanghai 200062, P.R. China.
2 Experimental
E-mail: eqgao@chem.ecnu.edu.cn; Fax: +86-21-62233404; Tel: +86-21-62233404
†Electronic supplementary information (ESI) available: Powder XRD profiles.
CCDC 969455 (1) and 969456 (2). For ESI and crystallographic data in CIF or
other electronic format see DOI: 10.1039/c3dt53109k
2.1 Synthesis
All the solvents and reagents for synthesis were commercially
available and used as received. 9H-Carbazole-2,7-dicarboxylic
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acid (2,7-H2CDC) was prepared according to a procedure
reported elsewhere.6
Table 1 Crystal data and structure refinements for compounds 1 and 2
Compound
1
2
2.1.1 [Cd4(CDC)4(DMF)4]·4DMF·4H2O (1). A mixture of
Cd(NO3)2·6H2O (0.10 mmol, 35 mg) and 2,7-H2CDC
(0.05 mmol, 13 mg) was dissolved in DMF (3.0 mL) in a Teflon-
lined stainless steel vessel (23 ml), heated at 130 °C for 3 days,
and then cooled to room temperature. Yellow rhombus sheet
crystals of 1, contaminated with block-shaped crystals of 2,
were formed. 1 was sorted out manually with a yield of about
Formula
Mr
Crystal system
Temperature/K
C
80H92N12O28Cd4
C65H63N7O25Cd3
1679.42
Triclinic
2119.26
Triclinic
296(2)
296(2)
ˉ
P1
ˉ
Space group
P1
a/Å
b/Å
c/Å
α/°
11.699(3)
12.948(3)
15.679(4)
100.882(6)
91.372(7)
106.539(7)
2228.1(9)
1
1.579
1.025
22 661
7679
22.115(14)
22.212(14)
22.50(2)
107.732(11)
107.417(11)
101.832(8)
9497(13)
4
1.175
0.728
49 708
32 998
10%
based
on
Cd(NO3)2·6H2O.
Anal.
calcd
for
β/°
C80H92N12O28Cd4: C, 45.34; H, 4.38; N, 7.93. Found: C, 45.48;
H, 4.62; N, 8.01. % IR (KBr, cm−1): 1656w, 1579w, 1515s,
1494s, 1394vs, 1335w, 1217w, 1099w, 1001w, 823m, 774m,
671s, 520w.
2.1.2 (H3O)2[Cd3(2,7-CDC)4]·3DMF·4H2O (2). This com-
pound was initially obtained by the above method together
with 1. It can be synthesized as a single phase by the following
γ/°
V/Å3
Z
Dc/g cm−3
μ/mm−1
Reflns collected
Unique reflns
Rint
0.043
0.039
R1 [I > 2σ(I)]
wR2 (all data)
0.0553
0.1590
0.0474
0.1160
two methods. Method I:
A mixture of Cd(NO3)2·6H2O
(0.06 mmol, 19 mg), 1,3,5-benzenetricarboxylic acid
(0.02 mmol, 4.2 mg) and 2,7-H2CDC (0.03 mmol, 7.7 mg) was
dissolved in DMF (3.0 mL) in a Teflon-lined stainless steel
vessel (23 mL), heated at 130 °C for 3 days, and then cooled to
room temperature. A few yellow block crystals of 2 were
obtained for crystallographic measurements. Method II: A
mixture of Cd(NO3)2·6H2O (0.4 mmol, 137 mg) and 2,7-H2CDC
(0.8 mmol, 204 mg) was dissolved in DMF (6.0 mL) in a
glass vial (25 mL), heated at 110 °C for 2 days and at 130 °C
for 3 days, and then cooled to room temperature. Yellow micro-
crystals of 2 were obtained with a yield of about 50% based
on Cd(NO3)2·6H2O. Anal. calcd for C65H63N7O25Cd3: C, 46.48;
H, 3.78; N, 5.84. Found: C, 41.28; H, 3.81; N, 6.04. % IR (KBr,
cm−1): 3395w, 3065w, 2925w, 1611s, 1586m, 15 035m, 1382vs,
1327m, 1231m, 1103w, 890m, 816m, 772s, 677s, 551w.
placed in calculated positions and refined using the riding
model. The crystals scatter very weakly even at low tempera-
ture. The best diffraction data set after several attempts are
still of limited quality. The structures have a large fraction of
voids containing a number of residual density peaks, which
may be attributed to disordered solvent molecules and could
not be satisfactorily modeled. The SQUEEZE routine in the
PLATON9 software package was applied to subtract the solvent
contribution. The numbers of the solvent molecules per
formula were estimated according to elemental analytic data
and thermogravimetric analyses. For 2, which has a 3D frame-
work with a Cd2+ to 2,7-CDC2− ratio of 3 : 4, since no appropri-
ate residues in the difference map can be assigned to protons
attached to carboxylate groups and all carboxylate oxygens are
strongly bonded to metal ions in the structure, it was assumed
that the framework is negatively charged and compensated by
H3O+ cations enclosed in voids. Selected crystal data for 1 and
2 are listed in Table 1.
2.2 Physical measurements
Elemental analyses were performed on an Elementar Vario
ELIII analyzer. The FT-IR spectra were recorded in the range of
500 − 4000 cm−1 using KBr pellets on a Nicolet NEXUS 670
spectrophotometer. Thermogravimetric analysis (TGA) was per-
formed on a Mettler TGALSDTA851e/5FL1100 instrument.
Fluorescence spectra were measured in a Hitachi F-4500 fluo-
rescence spectrometer. Powder X-ray diffraction (XRD)
measurements were performed on a Rigaku Ultima IV diffracto-
meter equipped with Cu Kα. Gas chromatography (GC) analyses
were performed on a LingHua GC 9890E instrument equipped
with a flame ionization detector (FID).
2.4 Catalytic studies
A given amount of the catalyst (4 mol%) was placed in a flask,
and after three cycles of vacuum pumping and nitrogen injec-
tion, the solution of active methylene substrates (1 mmol) in
DMF solvent (3 mL) was added and heated to a given tempera-
ture. After temperature equilibrium, benzaldehyde (0.5 mmol)
was added, and the reaction mixture was kept under a static
nitrogen atmosphere. The reaction was monitored by GC at
different time intervals.
2.3 Crystal data collection and refinement
Diffraction data for 1 and 2 were collected at 293 K on a Bruker
Apex II CCD area detector equipped with graphite-monochro-
mated Mo Kα radiation (λ = 0.71073 Å). Empirical absorption
corrections were applied using the SADABS program.7 The
structures were solved by direct methods and refined by the
3 Results and discussion
3.1 Synthesis
full-matrix least-squares method on F2, with all non-hydrogen Compounds 1 and 2 were initially obtained as a mixture in low
atoms refined with anisotropic thermal parameters.8 All the yield by reactions of Cd(NO3)2 and 2,7-H2CDC (molar ratio,
hydrogen atoms attached to carbon and nitrogen atoms were 2 : 1) at 130 °C in DMF. The efforts to synthesize 1 as a pure
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phase have been in vain. The reactions at the same or higher
temperature in DMF or DMF-ethanol yielded mixed phases,
sometimes including unknown phases. At lower temperature
the outcomes were usually clear solutions. Compound 2 can
be obtained as relatively large crystals (but in very low yield) by
the solvothermal reaction in the presence of 1,3,5-benzene-
tricarboxylic acid. The tricarboxylic acid is absent in the
final product but may serve as a H+ source. After some
attempts, compound 2 was obtained as a pure phase in
medium yield by performing the solvothermal reaction of
Cd(NO3)2 and 2,7-H2CDC (molar ratio, 1 : 2) at 110 °C for 2 days
and then at 130 °C for 3 days. The reaction was reproducible,
and no further optimization of the reaction conditions was
conducted.
3.2 Structure description
3.2.1 Compound 1. X-ray analysis on compound 1 revealed
a
two-dimensional (2D) layer based on
a
unique
[Cd4(COO)8(DMF)4] secondary building unit (SBU). The struc-
ture is depicted in Fig. 1. The asymmetric unit contains two
Cd(II) ions, two 2,7-CDC2− ligands and two coordinated DMF
molecules. The Cd2 ion is octahedrally coordinated by two
oxygen atoms from two μ-1,3-carboxylates (O4C and O8C), two
μ3-oxygens (O6 and O6E) from two carboxylate groups, a
μ2-oxygen (O1) from a chelate carboxylate and an oxygen atom
(O10) from DMF molecules with a little distortion. Cd1 adopts
a seven-coordinated distorted pentagonal bipyramidal geome-
try defined by seven oxygen atoms from two dissymmetric
chelate carboxylates (O1 and O2, O5 and O6), two μ-1,3-carboxy-
lates (O7A and O3B) and a DMF molecule (O9). The Cd1–O1
and Cd1–O6 distance (2.612(4)–2.700(4) Å) around Cd1 is sig-
nificantly longer than the other Cd–O distances (2.180(5)–
2.350(6) Å), reflecting the dissymmetric coordination of the
carboxylate groups. Four Cd atoms are bridged by a total of
eight carboxylates to give a butterfly-like cluster, for which the
Cd4 core takes a rhombus arrangement as shown in Fig. 1a.
The two body Cd ions (Cd2 and Cd2E) related by an inversion
center are bridged by two μ3-O (O6 and O6E), while the connec-
tions between the body and wing Cd ions (Cd1 and Cd2) are
either double oxygen bridges (μ2-O and μ3-O) or the triple
bridges consisting of two μ-1,3-carboxylates and a μ3-O. The
Cd⋯Cd distances in the tetranuclear SBU are in the range of
3.597(1)–3.964(1) Å.
In the structure, there are two independent CDC2− ligands
adopting different bridging modes (Scheme 2a and b): μ4,κ4
(a μ-1,3-carboxylate binds two Cd ions, while the other carboxy-
late chelates a Cd ion and binds another Cd using μ2-O) and
Fig. 1 (a) View of the [Cd4(COO)8(DMF)4] cluster in 1. Hydrogen atoms
are omitted for clarity. (b) The 2D layer with sql net. (c) The ABAB
μ5,κ4 (a carboxylate chelates a Cd ion and binds another two packing of the layer caused by the interlayer link-to-node hydrogen
bonds (yellow rods). Symmetry code: A = x − 1, y − 1, z; B = x, y, z − 1;
C = −x, −y + 2, −z + 2; D = −x + 1, −y + 3, −z + 1; E = −x, −y + 2, −z + 1.
Cd(II) ions through a μ3-O atom, with the other carboxylate
serving as
a μ-1,3 bridge between two Cd ions). Each
[Cd4(COO)8(DMF)4] SBU is furnished by four μ4,κ4 and four
μ5,κ4 CDC ligands, and neighboring SBUs are always linked by
two equivalent CDC ligands related by an inversion center.
Thus, although a single CDC is acentric, two CDC ligands
appear in pairs to set up centric bridges between SBUs. As a
consequence, the SBUs behave as 4-connected nodes and are
connected into a 2D network with the square lattice (sql) net
(Fig. 1b). The adjacent layers are associated into the 3D struc-
ture through strong N–H⋯O interactions. As shown in Fig. 1c,
the nitrogen atom (N2) of the carbazole moiety from one layer
donates its hydrogen atoms to a carboxylate oxygen atom
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Scheme 2 Coordination modes of the 2,7-CDC2− ligand in 1 and 2.
(O2H, H = −x + 1, −y + 2, −z + 1) from another layer with
N–H⋯O = 154.2°, H⋯O = 2.085(5) Å and N⋯O = 2.884(8) Å.
The interlayer link-to-node hydrogen bonds cause a ABAB
packing of the layers with the neighboring layers being
offset along the [110] direction. The packing leaves 1D void
ˉ
channels along the [110] direction, which are filled with
solvent molecules. PLATON analysis gave a total effective free
volume of 740.1 Å3 per cell (33.2% of the crystal volume). The
coordinated DMF molecules were taken as framework com-
ponents in the calculations.
Fig. 2 (a) Two crystallographically independent trinuclear [Cd3(COO)8]
clusters. (b) The alternating arrangement of the two independent 4-con-
nected SBUs. (c) A diamantane unit. (d) Two interpenetrating dia net-
works. Symmetry code: A = x − 1, y, z; B = x − 1, y − 1, z − 1; C = x + 1, y,
z + 1; D = x + 1, y + 1, z + 1; E = x − 1, y, z − 1; F = x + 1, y, z.
3.2.2 Compound 2. This compound exhibits negatively
charged 3D frameworks with 2-fold interpenetration. There are
six independent Cd ions in the asymmetric unit, which form
two crystallographically independent trinuclear [Cd3(COO)8]
clusters with minor differences in structural parameters. In
each cluster, three Cd(II) ions form an obtuse angle. The ion at
the corner (Cd2 or Cd5) is octahedrally coordinated by six
oxygen atoms from different carboxylate groups, while the
terminal metal ions (Cd1 and Cd3, or Cd4 and Cd6) are seven-
coordinated, each ligated by seven oxygens form three chelat-
ing carboxylates and a bridging carboxylate. The corner and
terminal Cd(II) ions are linked by a μ-1,3-carboxylate and two
μ2-oxygens (O28A and O32B, O8 and O19E, O3 and O22C, or
O12F and O16D) from two carboxylates chelating the terminal
metal ion, with the Cd⋯Cd distances in the range of 3.380(2)–
3.411(2) Å (Fig. 2a). The Cd–O distances around Cd2 and Cd5
are in the range of 2.179(5)–2.303(3) Å, while those around
terminal metal ions are in the range of 2.198(5)–2.567(5) Å, the
distances at the long limit being related to the dissymmetric
chelating coordination of some carboxylate groups.
4-connected (Fig. 2b). However, the connection topology in 2 is
very different from that in 1. In 1, owing to the centrosymmetry
of the tetranuclear cluster, the CDC linkers radiate from the
cluster in four coplanar directions, so a 2D sql network is
formed. Differently, the trinuclear cluster in 2 is asymmetric,
and the organic linkers radiate in four non-coplanar directions
to result in a 3D diamond (dia) framework (Fig. 2b), in which
the two crystallographically independent sets of clusters alter-
nate with each other. A diamantane unit is shown in Fig. 2c.
Note that sql and dia are respectively the default 2D and 3D
nets for 4-connected nodes. Interpenetration is a frequently
observed phenomenon for dia networks with long linkers. In
2, two networks related by inversion centers are interpene-
trated with each other (Fig. 2d). Despite the interpenetration,
the compound still has a large fraction of void volume (58.0%
estimated by PLATON).
In 2, there are eight independent CDC2− ligands, half in the
μ4,κ4 coordination mode (as found in 1) and half in the μ3,κ4
mode (the ligand uses a carboxylate to chelate a metal ion and
3.3 Stability
to bind another metal ion through a μ2-O, with the other car- Thermogravimetric analysis (TGA) was carried out under an air
boxylate binding only one metal ion in the chelating fashion, atmosphere from room temperature to 800 °C. The TGA plots
as shown in Scheme 2c). Despite the differences between 1 are given in Fig. 3. Compound 1 shows a weight loss of 17.3%
and 2 in the coordination mode and cluster structure, each upon heating up to 190 °C, which corresponds to the release
cluster in 2 is also surrounded by eight CDC ligands, which of all water and uncoordinated DMF molecules (calc., 17.2%).
again appear in pairs to serve as double linkers between The further weight drop above 190 °C could be due to the loss
clusters. Thus the trinuclear [Cd3(COO)8] SBUs are also of coordinated DMF molecules followed immediately by
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suggests that the intraligand energy levels are altered by the
perturbation of metal coordination.10 The remarkable
enhancement of the intensity for 1 and 2 compared with that
of the free ligand may be ascribed to the increased rigidity of
the ligand upon metal coordination, which effectively reduces
the loss of energy.11
3.5 Catalytic properties
The Knoevenagel condensation of a carbonyl group with the
methylene group activated by two electron-withdrawing groups
is an important C–C bond coupling reaction widely used in the
synthesis of fine chemicals and pharmaceuticals. The conden-
sation can be catalyzed by bases or Lewis acids, homogeneous
or heterogeneous. Considering the demand for heterogeneous
catalysts and the special features of MOFs, the development of
environmentally friendly MOF catalysts for the Knoevenagel
condensation is a topic of great interest.12 The amino group is
well known as a catalyst site in Knoevenagel condensation.13 It
has been demonstrated that NH2-functionalized MOFs, such
as IRMOF-3, can catalyze the reaction in a heterogeneous way
with higher activity than aromatic amines such as aniline.13b
Our MOFs with CDC2− should also be a potential basic catalyst
for Knoevenagel condensation due to the incorporation of NH
groups. We tested the catalytic properties of compound 2 for
the reactions between benzaldehyde and different methylene
substrates in DMF (Table 2). DMF was chosen as a solvent
because 2 is stable in it and also because this polar solvent
has proven to be good for Knoevenagel condensation.13a,b,e
The catalyst was used as synthesized without pre-activation.
In the presence of the catalyst (4 mol% with respect to the
amount of benzaldehyde), the reaction of benzaldehyde and
ethyl cyanoacetate (1 : 2 molar ratio) at 40 °C occurred
smoothly and gave ethyl (E)-α-cyanocinnamate as the only
product with a yield of 84% after 3 h, and complete conversion
was achieved within 8 h (Fig. 5). For comparison, the blank
experiment in the absence of any catalyst led to only 24% con-
version after 8 h under the same conditions. The conversion in
the presence of the catalyst can reach almost completion
Fig. 3 TGA plots of 1 and 2.
decomposition of the dicarboxylate ligands. For 2, the weight
loss of 18.2% before 190 °C indicates the loss of water and
DMF molecules (calc., 18.5%).
The structural data indicate that the frameworks of 2
possess large void volume filled with solvent molecules.
However, according to powder X-ray diffraction measurements,
the frameworks collapsed into an amorphous phase upon eva-
cuation by heating. The N2 adsorption of the desolvated phase
revealed only surface adsorption. The compound retains its
phase integrity upon being immersed in DMF for days (see
ESI† for the powder XRD profiles of the samples before and
after the DMF treatment), but changed into unknown phases
upon guest exchange with other solvents such as acetone,
methanol and dichloromethane.
3.4 Fluorescence properties
The solid-state photoluminescence spectra of compounds 1
and 2 were measured, and are compared in Fig. 4 with the free
2,7-H2CDC ligand. Upon excitation at 322 nm, compound 1
shows a blue fluorescent emission band around 462 nm and
compound 2 exhibits a broad emission band at 439 nm, while
the free 2,7-H2CDC ligand gives weaker emission around
495 nm. The blue-shift of the emission for the two complexes
Table 2 Catalytic data for the Knoevenagel condensation of benz-
aldehyde with different methylene substrates
Methylene
substrates
Catalyst
(4 mol%)
Temp.
(°C)
Time
(h)
Yield
(%)
Entry
1
2
2
—
40
40
3
3
84
15
3
4
5
6
7
2
2
80
80
80
80
r.t.
3
3
3
3
3
99a
87b
39
2,7-H2CDC
Cd(NO3)2
2
45
100a
8
9
2
2
r.t.
80
3
10
97b
<5
Fig. 4 The emission spectra of and compound 1 (green), compound 2
(pink) and free 2,7-H2CDC ligand (black) in the solid state at room
temperature.
a With fresh catalyst. b With catalyst separated from the first run.
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4 Conclusions
We have successfully synthesized and characterized two Cd(II)
compounds with a novel NH-functionalized dicarboxylate
ligand (2,7-H2CDC). Compound 1 displays 2D square grid
frameworks based on tetranuclear [Cd4(COO)8] clusters and
pairwise linkers. Compound
2 is based on trinuclear
[Cd3(COO)8] clusters and pairwise CDC2− linkers and exhibits
doubly interpenetrated 3D diamond-type frameworks. Both
compounds show strong blue fluorescence in the solid state.
Compound 2 can be used as an efficient catalyst, with no need
of pre-activation, for Knoevenagel condensation of benz-
aldehyde with malononitrile and ethyl cyanoacetate. Efforts to
obtain new MOFs from the ligand are underway in our labora-
tory, which would allow postsynthetic modification via the
amine group of the carbazole backbone.
Fig. 5 The Knoevenagel condensation between benzaldehyde
(1 mmol) and ethyl cyanoacetate (2 mmol) in DMF (6 ml) at 40 °C in the
presence and absence of 2, and after filtering off the catalyst.
within 3 h when the temperature is raised to 80 °C (Table 2,
entry 3). The results clearly confirm the catalytic activity of 2
for the Knoevenagel reaction.
Acknowledgements
This work was supported by the National Science Foundation
of China (NSFC no. 91022017 and 21173083) and the Research
Fund for the Doctoral Program of Higher Education of China.
With a solid catalyst for liquid-phase reactions, an issue of
great concern is whether the catalytic process is heterogeneous
or homogeneous. To clarify this point for 2, in a catalytic
experiment, the catalyst was removed by hot filtration after
reacting for 1 h, and the filtrate was kept under the same con-
ditions and monitored by GC at different intervals. As shown
in Fig. 5, after filtration, the conversion of benzaldehyde is
much slower than that for the reaction without filtration, and
the increasing rate of the conversion is similar to that for the
blank test. These results demonstrate that the catalysis is
heterogeneous in nature and that the slow reaction after fil-
tration should be due to thermal activation rather than cataly-
tic species leaching into the liquid phase. Furthermore, the
reaction was performed in the presence of the free H2CDC
ligand or Cd(NO3)·6H2O (Table 2, entries 5 and 6). The conver-
sions for these homogeneous systems are much lower than
that for 2, although the amount of the homogeneous catalysts
has been controlled to correspond to the amount of the ligand
or Cd(II) in 2. The increased catalytic activity for amino groups
incorporated with a framework structure has been observed for
IRMOF-3 compared with aniline.13b
The catalytic properties of 2 for the Knoevenagel conden-
sation of benzaldehyde with malononitrile or ethyl aceto-
acetate were also investigated. As shown in Table 2,
malononitrile shows much higher reactivity, the conversion
after 2 h reaching 100% even at room temperature, while ethyl
acetoacetate leads to very low conversion at 80 °C. The
decrease in reactivity in the order malononitrile > ethyl cyano-
acetate > ethyl acetoacetate is well consistent with the increase
in pKa of these activated methylene substrates.14
Notes and references
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2009, 38, 1477; (b) L. J. Murray, M. Dinca and J. R. Long,
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The recyclability of 2 as a catalyst was tested for the reac-
tions of benzaldehyde with malononitrile and ethyl cyano-
acetate. As can be seen (Table 2, entries 4 and 8), the activity of
the catalyst (filtered out and washed with DMF after the first
run) remains at high levels.
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