Direct crystallographic observation of catalytic reactions inside the pores of a flexible coordination polymer

Article abstract of DOI:10.1002/chem.201200046

A new flexible porous coordination polymer (PCP), {[Gd₂(L) ₃(dmf)₄]·4 DMF·3 H₂O}_n (1), was synthesized under solvothermal condition by reacting [Gd(NO ₃)₃]·6 H₂O with the ligand 2,6,2',6'-tetranitro-biphenyl-4,4'-dicarboxylic acid (H₂L). Compound 1 had a 3D coordination polymeric structure with two types of 1D channels (A and B) that were occupied by DMF and water molecules. When crystals of 1 were separately exposed to vapors of various aromatic aldehydes, either the lattice or both the lattice and metal-bound solvent molecules were replaced by aldehyde molecules. The aldehyde molecules inside the pores spontaneously underwent cyanosilylation and Knoevenagel condensation reactions upon exposure to vapors of trimethylsilyl cyanide and malononitrile, respectively. These reactions took place at ambient temperature and pressure. Moreover, both the reactants and the products translocated from one cavity to another. The products that occupied the cavity were expunged upon exposure to the vapors of an aldehyde. Because crystallinity was maintained during these chemical transformations, direct crystallographic observation was possible. Herein, we showed that confinement of the reactants inside the void spaces of the PCP led to the products; we also assessed catalytic activities of this PCP in bulk quantities. Copyright

Full text of DOI:10.1002/chem.201200046

FULL PAPER
DOI: 10.1002/chem.201200046
Direct Crystallographic Observation of Catalytic Reactions inside the Pores
of a Flexible Coordination Polymer
Raj Kumar Das, Arshad Aijaz, Manish K. Sharma, Prem Lama, and
Parimal K. Bharadwaj*[a]
Abstract: A new flexible porous coor-
dination polymer (PCP), {[Gd2(L)3-
ACHTUNGTRENNUNG
bound solvent molecules were replaced
by aldehyde molecules. The aldehyde
molecules inside the pores spontane-
ously underwent cyanosilylation and
Knoevenagel condensation reactions
upon exposure to vapors of trimethyl-
silyl cyanide and malononitrile, respec-
tively. These reactions took place at
ambient temperature and pressure.
Moreover, both the reactants and the
products translocated from one cavity
to another. The products that occupied
the cavity were expunged upon expo-
sure to the vapors of an aldehyde. Be-
cause crystallinity was maintained
during these chemical transformations,
direct crystallographic observation was
possible. Herein, we showed that con-
finement of the reactants inside the
void spaces of the PCP led to the prod-
ucts; we also assessed catalytic activi-
ties of this PCP in bulk quantities.
ACHTUNGTRENNUNG
ligand 2,6,2’,6’-tetranitro-biphenyl-4,4’-
dicarboxylic acid (H2L). Compound 1
had a 3D coordination polymeric struc-
ture with two types of 1D channels (A
and B) that were occupied by DMF
and water molecules. When crystals of
1 were separately exposed to vapors of
various aromatic aldehydes, either the
lattice or both the lattice and metal-
Keywords: crystal engineering
·
polymers · pores · supramolecular
chemistry · X-ray diffraction
Introduction
zymes. It is now clear that molecules can behave quite dif-
ferently when they are isolated from the bulk solvent and
are restrained within a molecular reactor of defined shape,
volume, and chemical environment.[8]
Porous coordination polymers (PCPs) are a group of com-
pounds that feature pores of desired shape and size as mo-
lecular reactors, thereby allowing the encapsulation of spe-
cific molecules. PCPs have found several promising applica-
tions, such as in gas sorption,[1] sensing,[2] drug delivery,[3]
and heterogeneous catalysis.[4] Compared to discrete molec-
ular containers, the use of PCPs as heterogeneous catalysts
have several advantages, such as high thermal stability, easy
crystallization in high yields, insolubility in common organic
solvents, fine tuning of the pores by ligand design, etc. One
of the most-important discoveries in the area of PCPs is
their flexible and dynamic properties that are characteristic
of the cooperative action of a metal–ligand ensemble.[5,6]
Flexible coordination polymers can reversibly change the
shape and size of their pores in response to the penetration
of guest molecules.[7] Moreover, the dynamic guest accom-
modation of these soft structures would enable the uniform
arrangement and orientation of guest molecules. This flexi-
ble nature would be essential for producing molecular rec-
ognition and efficient conversion, such as that found in en-
Cyanosilylation and Knoevenagel condensation reactions
À
are important C C bond-forming reactions that are cata-
lyzed by Lewis acids and Lewis acids/bases, respectively.[9,10]
In PCPs, vacant coordination site(s) on the metal and/or
functional organic sites for possible interactions with reac-
tants have been reported to catalyze these reactions.[11,12]
Herein, we report the synthesis of a flexible porous GdIII-
based
coordination
polymer,
{[Gd2(L)3-
AHCTUNGTREN(GNUN dmf)4]·4DMF·3H2O}n (1), that showed reversible guest-ex-
change in the cavities as well as in the metal-coordination
sites. Compound 1 was an excellent heterogeneous catalyst
for the cyanosilylation and Knoevenagel condensation reac-
tions. When the reactants were put inside the pores of this
PCP, they spontaneously underwent both of these reactions
at room temperature and atmospheric pressure owing to iso-
lation from the surroundings and being in close proximity to
one another. This reaction was directly observed by X-ray
crystallography because the host crystallinity was main-
tained throughout. To the best of our knowledge, this is the
first example of the single-crystal to single-crystal (SC-SC)
observation of cyanosilylation and Knoevenagel condensa-
tion reactions inside the pores of a flexible coordination
polymer.
[a] R. K. Das, A. Aijaz, M. K. Sharma, P. Lama,
Prof. Dr. P. K. Bharadwaj
Department of Chemistry
Indian Institute of Technology
Kanpur 208016 (India)
E-mail: pkb@iitk.ac.in
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.201200046.
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Results and Discussion
twisted (dihedral angle: 77.082 (10)8) owing to the steric
hindrance of the NO2 groups.
The reaction of [Gd
thermal conditions formed block-shaped yellow crystals of
{[Gd2(L)3A(dmf)4]·4DMF·3H2O}n (1) in 55% yield. The
(NO3)3·6H2O] with H2L under solvo-
The 3D structure showed two types of 1D channels (A
and B) running along the crystallographic a axis (Figure 3).
The dimensions of the channels were 3.33ꢂ8.65 ꢁ2 for chan-
CHTUNGTRENNUNG
ligand was chosen to afford restricted rotation about the
À
C C bond that joined the two dinitrobenzoic acid moieties
so as to impart a number of topological variations. More-
over, the presence of nitro groups hindered the formation of
interpenetrated structures. The crystalline product, once
formed, was insoluble in most organic solvents and water,
thereby making it possible to study the substitution reac-
tions in a number of solvents. Compound 1 crystallized in
the monoclinic space group P21/c with a 3D coordination
polymeric structure. The core structure consisted of a Gd2
dimer (Figure 1) that was constructed from two bridging car-
Figure 3. Embedded DMF molecules in channels A and B in the crystal
structure of compound 1; hydrogen atoms and water molecules are omit-
ted for clarity.
nel A and 2.40ꢂ8.24 ꢁ2 for channel B (channel sizes were
measured by considering the van der Waals radii of the con-
stituting atoms); these channels provided a void space of
51.7% of the total crystal volume, as estimated by
PLATON.[14] The overall 3D structure (Figure 2) was stabi-
lized by an array of H-bonding interactions that involved
lattice molecules as well as metal-bound solvent molecules.
The larger size of channel A versus channel B resulted from
the different relative orientations of the phenyl rings. Chan-
nel A was occupied by six water molecules and six DMF
molecules whilst channel B contained two DMF molecules.
When single crystals of compound 1 were exposed to ben-
zaldehyde vapor at ambient temperature for 5 days,
Figure 1. Coordination environment around the Gd2 dimer in compound
1.
boxylates, two chelating carboxylates, and two (syn, anti)
carACHTUNGTRENNUNG
boxylates from six different L2À units. These dimeric Gd2
units were further connected to other similar units through
L2À linkers to generate an infinite 3D structure (Figure 2).
{[Gd2(L)3ACHTNUTRGNEUNG(dmf)4]·2PhCHO·4H2O}n (2) was formed without
a loss in crystallinity and preservation of the coordination
environment of the Gd2 dimeric unit and of the overall
framework structure (Figure 4), although the space group
changed to C2/c (see the Supporting Information, Table S1).
During this process, all of the lattice guests molecules in
Figure 2. 3D packing in the crystal structure of compound 1.
Each metal ion was also coordinated to one DMF molecule,
thus giving a total coordination number of nine. The coordi-
nation geometry could be described as distorted tri-capped
trigonal prismatic (see the Supporting Information, Fig-
À
ure S1). The Gd O distances were in the range 2.350(4)–
2.712(4) ꢁ,[13] within normal statistical errors. Two phenyl
rings of the ligand were not planar; rather, they were highly
Figure 4. Coordination environment around the Gd2 dimer in compound
2.
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Catalytic Reactions in the Pores of a Coordination Polymer
FULL PAPER
further hydrolyzed into the cyanohydrin by acidic work-up.
From the crystallographic investigation of compound 2, we
found that, once benzaldehyde had entered into the cavity,
it remained there through H-bonding interactions between
its aromatic hydrogen atoms and the framework. To better
understand the possible mechanism for substrate activation,
IR spectroscopy was performed. The IR spectrum of com-
pound 2 showed a C=O stretching frequency of benzalde-
hyde at 1674 cmÀ1, which was red-shifted by about 26 cmÀ1
with respect to neat benzaldehyde (see the Supporting Infor-
mation, Figures S5 and S6). This result suggested that non-
bonding interactions between benzaldehyde and the frame-
work led to activation of the encapsulated benzaldehyde
molecules.
Figure 5. Single-crystal to single-crystal guest-exchange process in com-
pound 1; the crystal structure of compound 1 shows embedded DMF
molecules in channels A and B. The framework is shown as a ball and
stick model; guest molecules are shown as CPK models; hydrogen atoms
and water molecules are omitted for clarity.
Next, we investigated the Knoevenagel condensation re-
actions in a similar manner. The exposure of a single crystal
of compound 2 to malononitrile vapor for 2 h afforded
channel A were replaced by four benzaldehyde molecules
(Figure 5; also see the Supporting Information, Figure S2),
whilst all of the DMF molecules in channel B were replaced
by eight water molecules. When a single crystal of com-
pound 2 was exposed to trimethylsilyl cyanide vapor for 2 h
{[Gd2(L)3ACTHNGUTRENNU(G dmf)4]·2(2-benzylidinemalononitrile)·3(malononi-
trile)·5H2O}n (4) without a loss in crystallinity. As above,
the coordination environment around the Gd center re-
mained intact (see the Supporting Information, Figure S7).
Out of the four benzaldehyde molecules, two translocated
from channel A to channel B and all of them reacted with
malononitrile to yield the Knoevenagel condensation prod-
at ambient temperature, {[Gd2(L)3ACHTUNRTGNEUNG(dmf)4]·(2-hydroxylphenyl
acetonitrile)·3H2O}n (3) was formed without a loss in crys-
tallinity. The coordination ge-
ometry around each metal
center remained intact (see the
Supporting Information, Fig-
ure S3), whilst four trimethyl-
silyl cyanide molecules reacted
with the four benzaldehyde
molecules in channel A to form
cyanohydrin
trimethylsilyl
ether. As all four cyanohydrin
trimethylsilyl ether molecules
could not be accommodated in
channel
A because of their
bulkier size, so two of them
came out from this channel.
After that, two guest water
molecules translocated from
channel B to channel A and hy-
drolyzed two lattice cyanohy-
drin trimethylsilyl ether mole-
cules to form cyanohydrin (Fig-
ure 6a; also see the Supporting
Information, Figure S4). As
a result, only six water mole-
cules remained in channel B. It
is interesting to note that, when
these two reactants were al-
lowed to react in bulk quanti-
ties in the presence of a catalytic
amount
of
compound
1 (10 wt.%), we observed the
formation of cyanohydrin tri-
Figure 6. Single-crystal to single-crystal observation of the cyanosilylation (a) and Knoevenagel condensation
reactions (b). The framework is shown as a ball and stick model; guest molecules are shown as CPK models;
methylsilyl ether, which was hydrogen atoms and water and malononitrile molecules are omitted for clarity.
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P. K. Bharadwaj et al.
uct (Figure 6b; also see the Supporting Information, Fig-
ure S8). Channel A also contained ten water molecules and
channel B contains six unreacted malononitrile molecules.
This result suggested that eight malononitrile molecules en-
tered into channel B and two entered into channel A. The
pore-size of the initially empty channel B expanded owing
to rotation of the phenyl rings on the ligand to accommo-
date the malononitrile molecules. The increase in the
number of water molecules was a by-product of the conden-
sation reaction. Moreover, all eight of the water molecules
from channel B moved to channel A. Therefore, the mole-
cules translocated from one channel to the other as and
when required. When crystals of compound 3 were exposed
to benzaldehyde vapor for 5 days at room temperature,
Figure 7. Plot of time versus conversion for the cyanosilylation of various
aromatic aldehydes by compound 1.
{[Gd2(L)3ACHTUNGTRENNUNG(dmf)4]·2PhCHO·6H2O}n (2’) was formed, which
was structurally similar to that of compound 2. The only dif-
ference was that channel B contained a few more water mol-
ecules. However, when compound 4 was exposed to benzal-
dehyde vapor for 5 days, the crystallinity of the product
became extensively deteriorated and no structural data was
obtained.
To probe the catalytic activity of compound 1 in the cya-
nosilylation reactions in bulk, an aromatic aldehyde and tri-
methylsilyl cyanide were added in a 1:1 molar ratio to a sus-
pension of powdered compound 1 (10 wt.%) in dry CH2Cl2
under a nitrogen atmosphere. In the case of benzaldehyde
and 2-pyridinecarboxaldehyde (2-PyCHO), the reactions
proceeded to completion within 1 hour (Table 1 and
Table 2).
Figure 8. Plot of time versus conversion for the Knoevenagel condensa-
tion reactions of various aromatic aldehydes by compound 1.
Table 1. Cyanosilylation reactions catalyzed by compound 1.
Entry
R
t
Yield
[%]
[h]
1
2
3
4
5
6
Ph
2-Py
3-Py
1
1
6
6
12
12
92
84
45
35
6
4-Py
1-naphthyl
9-anthryl
0
Figure 9. Crystal structures of a) compound 5 and b) compound 6, which
shows the aromatic guest molecules inside the channels. Compound 7
was structurally similar to compound 6. The framework is shown as a ball
and stick model; guest molecules are shown as CPK models; hydrogen
atoms and water molecules are omitted for clarity.
In each case, the conversion was monitored by GCMS
(Figure 7 and Figure 8). The rate and the yield of the indi-
vidual reactions depended upon the size as well as the
nature of the reactants.
À
For both C C bond-forming reactions, 2-pyridinealdehyde
behaved like benzaldehyde, whereas 3-pyridinealdehyde and
4-pyridinealdehyde showed lower conversion at much
slower rates. The separate exposure of a single crystal of
compound 1 to the vapor of each aldehyde afforded
ing Information, Figure S9) and twelve water molecules oc-
cupied channel B.
In both compounds 6 and 7, four corresponding pyridine
aldehyde molecules occupied channel A and two occupied
channel B. However, unlike in compound 5, in compounds 6
and 7, half of the metal-bound DMF molecules were re-
placed by pyridine aldehyde molecules, with the pyridine ni-
trogen atom coordinating to the metal ion (Figure 10 and
Figure 11). In both compounds 6 and 7, the channels were
{[Gd2(L)
(3-PyCHO)2]·3(3-PyCHO)·3H2O}n (6), and {[Gd2(L)
(4-PyCHO)2]·6(4-PyCHO)·DMF}n (7) without any loss in
3ACHTUNGTRENNUNG(dmf)4]·2(2-PyCHO) ·6H2O}n (5), {[Gd2(L) CHTUNGTNER(NUNG dmf)2-
3A
G
CHTUNGTRENNUNG
ACHTUNGTRENNUNG
crystallinity. In compound 5, four 2-pyridinealdehyde mole-
cules occupied channel A (Figure 9a; also see the Support-
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Catalytic Reactions in the Pores of a Coordination Polymer
FULL PAPER
and the framework remained stable up to about 3208C in
each case (see the Supporting Information, Figures S20–
S26).
To test the heterogeneous nature of the catalyst in both
the cyanosilylation and Knoevenagel condensation reactions,
we performed control experiments. Removal of the catalyst
from the reaction mixtures by filtration resulted in no fur-
ther conversion, even after a long time (see the Supporting
Information, Figures S27 and S28), which suggested that
there was no dissolution of the catalyst during the reaction.
Figure 10. Coordination environment around the Gd2 dimer in compound
6.
Also, [GdACTHNUGRTNEUNG(NO3)3], as well as ligand L, were unable to cata-
lyze the reaction, even at higher loading and over a long
period of time. The powder XRD pattern of the recovered
catalyst revealed that it had maintained its crystallinity
during the course of the reactions (Figure 12). We also
Figure 11. Coordination environment around the Gd2 dimer in compound
7.
Table 2. Knoevenagel condensation reactions catalyzed by compound 1.
Entry
R
t
Yield
[%]
AHCTUNGTERG[NNUN min]
1
2
3
4
5
6
Ph
2-Py
3-Py
20
30
180
180
300
300
96
86
40
30
5
4-Py
1-naphthyl
9-anthryl
Figure 12. Powder XRD patterns of compound 1 before and after the cy-
anosilylation and Knoevenagel condensation reactions.
0
crowded (Figure 9b; also see the Supporting Information,
Figure S10 and Figure S11) for another guest to enter with
tested the recyclability of the catalyst by using the recovered
catalyst in up to 3 cycles of the cyanosilylation and Knoeve-
nagel condensation reactions of benzaldehyde. In each case,
the catalytic activity remained unchanged (see the Support-
ing Information, Figures S29 and S30).
À
difficulty, which explained the incomplete and sluggish C C
bond-formation reactions (Table 1 and Table 2).
The transparency and morphology of the crystals were re-
tained during all of the single-crystal to single-crystal trans-
formations (see the Supporting Information, Figure S12),
which ruled out the initial dissolution of crystals in succes-
sive vapors of the reagents followed by crystallization of re-
nucleation at the surface and growth of the new phases.
During the course of all single-crystal to single-crystal trans-
formations, the nitro groups moved with respect to one an-
other, which led to changes in the space and shape of the
voids. These changes were induced by the introduction of
guest molecules inside the cavity.
The phase purity of all of the compounds was confirmed
by powder XRD (see the Supporting Information, Figur-
es S13–S19). TGA showed a gradual weight loss, owing to
the expulsion of the lattice guest molecules as well as metal-
bound DMF molecules in the temperature range 45–2508C
Conclusion
It is well-known that both cyanosilylation and Knoevenagel
condensation reactions are Lewis acid and/or Lewis base
catalyzed. Herein, we have shown, for the first time, that
both of these reactions could take place in the confined
space that was provided by the pores of the flexible coordi-
nation polymer. All of the catalytic reactions were moni-
tored in a single-crystal to single-crystal manner. This study
may have applications in observing many other chemically
and biologically important reactions inside the pores of crys-
talline hosts and should facilitate investigations of their
mechanisms.
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P. K. Bharadwaj et al.
calcd (%) for C66H74N20O47Gd2: C 35.80, H 3.36, N 12.65; found: C 35.71,
H 3.52, N 12.58.
Experimental Section
Synthesis of {[Gd2(L)3ACTHUNRGTNEUNG(dmf)4]·2(benzaldehyde)·4H2O}n (2): Single crys-
Materials: 4-Chloro-3,5-dinitrobenzoic acid (97%), 2-pyridinecarboxalde-
hyde (99%), 3-pyridinecarboxaldehyde (98%), 4-pyridinecarboxaldehyde
(97%), trimethylsilyl cyanide (95%), malononitrile (99%), benzaldehyde
tals of compound 1, upon exposure to benzaldehyde vapor for 5 days, af-
forded compound 2 without a loss in crystallinity. IR (KBr): n˜ =3422
(br), 3083 (m), 2933 (m), 2875 (w), 1674 (s), 1629 (s), 1546 (s), 1460 (m),
1386 (s), 1342 (s), 1253 (w), 1203 (w), 1103 (m), 1062 cmÀ1 (w); elemental
analysis calcd (%) for C66H62N18O48Gd2: C 36.20, H 2.85, N 11.51; found:
C 36.15, H 2.93, N 11.45.
(ꢀ99%), and [Gd
ACHTUNGTRENNUNG(NO3)3]·6H2O (99.99%) were purchased from Sigma–
Aldrich and used as received. All of the solvents and copper powder
(electrolytic grade) were purchased from S.D. Fine Chemicals, India, and
purified prior to use following standard procedures.
Physical measurements: IR spectra (KBr disk, 400–4000 cmÀ1) were re-
corded on a PerkinElmer Model 1320 spectrometer. Powder X-ray dif-
fraction (Cu Ka radiation, scan rate 38minÀ1, 293 K) was performed on
a Bruker D8 Advance Series 2 powder X-ray diffractometer. Thermogra-
vimetric analysis (TGA; heating rate 58CminÀ1 under normal atmos-
Synthesis of {[Gd2(L)3ACHTUNTRGNE(UNG dmf)4]·(2-hydroxyphenylacetonitrile)·3H2O}n (3):
Single crystals of compound 2, upon exposure to trimethylsilyl cyanide
vapor for 2 h, afforded compound 3 without a loss in crystallinity. IR
(KBr): n˜ =3345 (br), 3091 (m), 2934 (m), 2213 (m) 1651 (s), 1631 (w),
1538 (s), 1497 (w), 1385 (s), 1342 (s), 1252 (w), 1187 (w), 1107 (m),
1060 cmÀ1 (w); elemental analysis calcd (%) for C62H53N17O44Gd2:
C 36.24, H 2.60, N 11.59; found: C 36.34, H 2.65, N 11.53.
phere) was performed on
a
Mettler Toledo Star System. 1H NMR
(500 MHz) and 13C NMR (125 MHz) spectra were recorded on a JEOL
JNM-LA500 FT instrument in [D6]DMSO. Elemental analysis was car-
ried out on a CE-440 elemental analyzer (Exeter Analytical Inc.).
Synthesis of {[Gd2(L)3ACHTNUTRGENNG(U dmf)4]·2(2-benzylidene-malononitrile)·3(malono-
nitrile)·5H2O}n (4): Single crystals of compound 2, upon exposure to ma-
lononitrile vapor for 2 h, afforded compound 4 in a SC-SC transforma-
tion. IR (KBr): n˜ =3555 (br), 3089 (m), 2965 (s), 2932 (s), 2273 (s), 2205
(w), 1666 (m), 1627 (s), 1547 (s), 1463 (m), 1393 (s), 1343 (m), 1253 (w),
1188 (w), 1106 (m), 1062 cmÀ1 (w); elemental analysis calcd (%) for
C83H68N26O45Gd2: C 40.52, H 2.79, N 14.80; found: C 40.46, H 2.85,
N 14.74.
X-ray structural studies: Single-crystal X-ray data were collected at
100 K on a Bruker SMART APEX CCD diffractometer by using graph-
ite-monochromated Mo Ka radiation (l=0.71073 ꢁ). The linear absorp-
tion coefficients, scattering factors for the atoms, and the anomalous dis-
persion corrections were taken from the international tables for X-ray
crystallography. Data integration and reduction were processed with the
SAINT[15] software. An empirical absorption correction was applied to
the collected reflections with SADABS[16] by using XPREP.[17] The struc-
ture was solved by direct methods using SHELXTL[18] and was refined
on F2 by the full-matrix least-squares technique using the SHELXL-97[19]
program package. The non-hydrogen atoms were refined anisotropically
(except as noted). The H atoms of the coordinated water molecule of
compound 1 were located by difference Fourier synthesis. The H atoms
of the uncoordinated water molecules in compound 1 and all of the
water molecules of the regenerated crystals were not found on a differ-
ence Fourier map. All other H atoms were placed at calculated positions
by using idealized geometries (riding model) and were assigned fixed iso-
tropic displacement parameters. Several DFIX commands were used to
fix the distances of solvent molecules. The crystal and refinement data
are collected in the Supporting Information, Table S1.
Synthesis of {[Gd2(L)3ACTHUNRGTNEUNG(dmf)4]·2(2-pyridinecarboxaldehyde)·6H2O}n (5):
Single crystals of compound 1, upon exposure to 2-pyridinecarboxalde-
hyde vapor for 5 days, afforded compound 5 without a loss in crystallini-
ty. IR (KBr): n˜ =3418 (br), 3087 (m), 2929 (m), 1714 (s), 1660 (m) 1621
(s), 1540 (s), 1453 (m), 1388 (m), 1342 (s), 1295 (w), 1171 (w), 1093 (m),
1011 cmÀ1 (w); elemental analysis calcd (%) for C68H60N16O46Gd2:
C 37.96, H 2.81, N 10.41; found: C 37.89, H 2.89, N 10.36.
Synthesis of {[Gd2(L)3ACTHNURTGNEUNG(dmf)2(3-pyridinecarboxaldehyde)2]·3(3-pyridine-
carboxaldehyde)·3H2O}n (6): Single crystals of compound 1, upon expo-
sure to 3-pyridinecarboxaldehyde vapor for 5 days, afforded compound 6
without a loss in crystallinity. IR (KBr): n˜ =3070 (m), 1707 (s), 1663 (m)
1627 (s), 1593 (s), 1541 (s), 1459 (m) 1387 (s), 1341 (s), 1244 (w), 1212
(w), 1191 (w), 1106 (m), 1028 cmÀ1 (w); elemental analysis calcd (%) for
C78H57N19O46Gd2: C 40.54, H 2.49, N 11.52; found: C 40.57, H 2.41,
N 11.48.
CCDC-843413 (1), CCDC-843414 (2), CCDC-843415 (3), CCDC-843416
(4), CCDC-843417 (5), CCDC-843418 (6), CCDC-843419 (7), and
CCDC-861225 (2’) contain the supplementary crystallographic data for
this paper. These data can be obtained free of charge from The Cam-
bridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_re-
quest/cif.
Synthesis of {[Gd2(L)3ACTHNURTGNEUNG(dmf)2(4-pyridinecarboxaldehyde)2]·6(4-pyridine-
carboxaldehyde)·DMF}n (7): Single crystals of compound 1, upon expo-
sure to 4-pyridinecarboxaldehyde vapor for 5 days, afforded compound 7
without a loss in crystallinity. IR (KBr): n˜ =3395 (br), 3087 (m), 2931
(m), 1715 (s), 1678 (w) 1638 (w), 1539 (s), 1458 (m), 1391 (s), 1341 (s),
1215 (w), 1190 (w), 1106 (m), 1059 cmÀ1 (w); elemental analysis calcd
(%) for C99H81N23O47Gd2: C 44.48, H 3.05, N 12.57; found: C 44.52,
H 3.13, N 12.50.
Synthesis of H2L: The ligand H2L (Scheme 1) was synthesized according
to a literature procedure.[20]
Synthetic procedure for the cyanosilylation reactions: A solution of aro-
matic aldehyde (1.5 mmol) and trimethylsilyl cyanide (3.0 mmol) in
CH2Cl2 (10 mL) was stirred for 5 min under an argon atmosphere and
then catalyst 1 (10 wt.%) was added whilst maintaining the temperature
at 58C. The suspension was stirred under ice-cold conditions for 30 min;
the progress of the reaction was monitored by TLC. Once the reaction
was complete, the catalyst was removed by filtration, washed with
CH2Cl2, and dried in air for reuse. The organic layer was washed with
10% HCl solution. On evaporation of the organic layer under reduced
pressure, a crude product was obtained that was purified by column chro-
matography on silica gel (EtOAc/n-hexane). All of the products were
characterized by standard methods and the data were correlated with lit-
erature data.
Scheme 1. Ligand H2L.
Synthesis of {[Gd2(L)
(NO3)3]·6H2O (0.5 mmol) and H2L (0.5 mmol) in DMF (10 mL) and
EtOH (5 mL) was heated at 1008C under autogenous pressure in
a Teflon-lined steel bomb for 2 days, followed by slow cooling (58ChÀ1
3ACHTUNGTRENNUNG(dmf)4]·4DMF·3H2O}n (1): A mixture of [Gd-
Synthetic procedure for the Knoevenagel condensation reactions: A solu-
tion of aromatic aldehyde (1.5 mmol) and malononitrile (1.5 mmol) in
benzene (10 mL) was stirred for 5 min under an argon atmosphere. Com-
pound 1 (10 wt.%) was added to the solution and the suspension was
stirred at RT. The reaction was monitored by TLC. On completion of the
reaction, the catalyst was removed by filtration, washed with CH2Cl2, and
dried in air for reuse. The filtrate was evaporated to dryness under re-
ACHTUNGTRENNUNG
)
to RT. Yellowish block-shaped crystals were collected, washed with
DMF, and preserved in dry DMF. Yield: 64%; IR (KBr): n˜ =3397(br),
3085 (m), 2933 (m), 2880 (w), 1675 (s), 1631 (s), 1543 (s), 1461 (m), 1387
(s), 1341 (s), 1253 (w), 1187(w), 1093 (m), 1062 (w); elemental analysis
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Catalytic Reactions in the Pores of a Coordination Polymer
FULL PAPER
duced pressure and the residue was crystallized from EtOAc/n-hexane
(1:10) to afford the pure product. The properties of each product were
matched with literature data.
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Received: January 5, 2011
Published online: && &&, 2012
Chem. Eur. J. 2012, 00, 0 – 0
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P. K. Bharadwaj et al.
Coordination Polymers
R. K. Das, A. Aijaz, M. K. Sharma,
P. Lama, P. K. Bharadwaj* . &&&& — &&&&
Direct Crystallographic Observation of
Catalytic Reactions inside the Pores of
a Flexible Coordination Polymer
Rich man, pore man: Molecules inside
confined environments can behave
very differently to their bulk state
owing to different reactivities. Both
Knoevenagel condensation and cyano-
silylation reactions were catalyzed by
the confined space that was provided
by a single coordination polymer.
These processes were monitored in
a single-crystal to single-crystal fashion
(see figure).
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These are not the final page numbers!