Three-dimensional porous coordination polymer functionalized with amide groups based on tridentate ligand: Selective sorption and catalysis

Article abstract of DOI:10.1021/ja067374y

To create a functionalized porous compound, amide group is used in porous framework to produce attractive interactions with guest molecules. To avoid hydrogen-bond formation between these amide groups our strategy was to build a three-dimensional (3D) coo

Full text of DOI:10.1021/ja067374y

Published on Web 02/09/2007
Three-Dimensional Porous Coordination Polymer
Functionalized with Amide Groups Based on Tridentate
Ligand: Selective Sorption and Catalysis
Shinpei Hasegawa, Satoshi Horike, Ryotaro Matsuda, Shuhei Furukawa,
Katsunori Mochizuki, Yoshinori Kinoshita, and Susumu Kitagawa*
Contribution from the Department of Synthetic Chemistry and Biological Chemistry, Graduate
School of Engineering, Kyoto UniVersity, Katsura, Nishikyo-ku, Kyoto, 615-8510, Japan
Received October 14, 2006; E-mail: kitagawa@sbchem.kyoto-u.ac.jp
Abstract: To create a functionalized porous compound, amide group is used in porous framework to produce
attractive interactions with guest molecules. To avoid hydrogen-bond formation between these amide groups
our strategy was to build a three-dimensional (3D) coordination network using a tridentate amide ligand as
the three-connector part. From Cd(NO3)2‚4H2O and a three-connector ligand with amide groups a 3D porous
coordination polymer (PCP) based on octahedral Cd(II) centers, {[Cd(4-btapa)2(NO3)2]‚6H2O‚2DMF}n (1a),
was obtained (4-btapa ) 1,3,5-benzene tricarboxylic acid tris[N-(4-pyridyl)amide]). The amide groups, which
act as guest interaction sites, occur on the surfaces of channels with dimensions of 4.7 × 7.3 Å2. X-ray
powder diffraction measurements showed that the desolvated compound (1b) selectively includes guests
with a concurrent flexible structural (amorphous-to-crystalline) transformation. The highly ordered amide
groups in the channels play an important role in the interaction with the guest molecules, which was
confirmed by thermogravimetric analysis, adsorption/desorption measurements, and X-ray crystallography.
We also performed a Knoevenagel condensation reaction catalyzed by 1a to demonstrate its selective
heterogeneous base catalytic properties, which depend on the sizes of the reactants. The solid catalyst 1a
maintains its crystalline framework after the reaction and is easily recycled.
Introduction
this synthetic approach to the PCP system has received little
attention. Two types of strategies are used to functionalize
In recent years, numerous studies of porous coordination
polymers (PCPs), also called metal-organic frameworks (MOFs),
have been reported1 because of their applications in gas
adsorption,2 molecular storage,3 and heterogeneous catalysis.4
PCPs have characteristic features that include (1) well-ordered
porous structures, (2) flexible and dynamic behaviors in response
to guest molecules, and (3) designable channel surface func-
tionalities. Although channel surface modification is essential
for creation of functionalized porous structures, application of
channel surfaces: immobilization of coordinatively unsaturated
(open) metal sites (OMS)5 and introduction of organic groups
to provide guest-accessible functional organic sites (FOS).4b,6
There is growing interest in the use of OMS for Lewis acid
catalysis and specific gas adsorption, but less attention has been
devoted to the study of FOS, despite their importance. The
paucity of information on FOS is because of the difficulty of
producing guest-accessible FOS on the pore surface: these
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M.; Yaghi, O. M. Science 2002, 295, 469-472. (b) Kitaura, R.; Kitagawa,
S.; Kubota, Y.; Kobayashi, T. C.; Kindo, K.; Mita, Y.; Matsuo, A.;
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Millange, F.; Loiseau, T.; Gue´gan, A. P. Chem. Commun. 2003, 2976-
2977. (d) Chu, H.; Dybtsev, D. N.; Kim, H.; Kim, K. Chem. Eur. J. 2005,
11, 3521-3529. (e) Rowsell, J. L. C.; Yaghi, O. M. Angew. Chem., Int.
Ed. 2005, 44, 4670-4679. (f) Matsuda, R.; Kitaura, R.; Kitagawa, S.;
Kubota, Y.; Belosludov, R. V.; Kobayashi, T. C.; Sakamoto, H.; Chiba,
T.; Takata, M.; Kawazoe, Y.; Mita, Y. Nature 2005, 436, 238-241. (g)
Dincaˇ, M.; Long, J. R. J. Am. Chem. Soc. 2005, 127, 9376-9377.
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Dalrymple, S. A.; Shimizu, G. K. H. Chem. Eur. J. 2002, 8, 3010-3015.
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2002, 1, 118-121. (d) Kim, H.; Suh, M. P. Inorg. Chem. 2005, 44, 810-
812.
(4) (a) Fujita, M.; Kwon, Y. J.; Washizu, S.; Ogura, K. J. Am. Chem. Soc.
1994, 116, 1151-1152. (b) Seo, J. S.; Whang, D.; Lee, H.; Jun, S. I.; Oh,
J.; Jeon, Y. J.; Kim, K. Nature 2000, 404, 982-986. (c) Wu, C. D.; Hu,
A.; Zhang, L.; Lin, W. J. Am. Chem. Soc. 2005, 127, 8940-8941. (d) Lor,
B. G.; Puebla, E. G.; Iglesias, M.; Monge, M. A.; Valero, C. R.; Snejko,
N. Chem. Mater. 2005, 17, 2568-2573. (e) Sato, T.; Mori, W.; Kato,
C. N.; Yanaoka, E.; Kuribayashi, T.; Ohtera, R.; Shiraishi, Y. J. Catal.
2005, 232, 186-198. (f) Dybtsev, D. N.; Nuzhdin, A. L.; Chun, H.;
Bryliakov, K. P.; Talsi, E. P.; Fedin, V. P.; Kim, K. Angew. Chem., Int.
Ed. 2006, 45, 916-920. (g) Uemura, T.; Kitaura, R.; Ohta, Y.; Nagaoka,
M.; Kitagawa, S. Angew. Chem., Int. Ed. 2006, 45, 4112-4116.
(5) (a) Noro, S.; Kitagawa, S.; Yamashita, M.; Wada, T. Chem. Commun. 2002,
222-223. (b) Kitaura, R.; Onoyama, G.; Sakamoto, H.; Matsuda, R.; Noro,
S.; Kitagawa, S. Angew. Chem., Int. Ed. 2004, 43, 2684-2687. (c) Chen,
B.; Fronczek, F. R.; Maverick, A. W. Inorg. Chem. 2004, 43, 8209-8211.
(d) Maggard, P. A.; Yan, B.; Luo, J. Angew. Chem., Int. Ed. 2005, 44,
2-5. (e) Chen, B.; Ockwig, N. W.; Millward, A. R.; Contreras, D. S.;
Yaghi, O. M. Angew. Chem., Int. Ed. 2005, 44, 4745-4749. (f) Cho,
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10.1021/ja067374y CCC: $37.00 © 2007 American Chemical Society
J. AM. CHEM. SOC. 2007, 129, 2607-2614
2607

A R T I C L E S
Hasegawa et al.
Scheme 1. 1,3,5-Benzene Tricarboxylic Acid
Tris[N-(4-pyridyl)amide] (4-btapa)
JNM-A500 FT NMR system, and chemical shifts are reported in δ
(ppm) values. Solid-state magic angle spinning/cross polarization
(CPMAS) 113Cd NMR experiment was carried out on a (300 MHz)
JEOL JNM-LA300WB spectrometer. IR spectra were recorded on a
Perkin-Elmer 2000 FT-IR spectrophotometer with samples prepared
with KBr pellets. Thermogravimetric analyses (TGA) were performed
using a Rigaku Thermo plus TG 8120 apparatus in the temperature
range between 298 and 723 K in a N2 atmosphere and at a heating rate
of 10 K min-1. X-ray powder diffraction (XRPD) data were collected
on a Rigaku RINT-2200HF (Ultima) diffractometer with Cu KR
radiation. The adsorption isotherm of nitrogen at 77 K and methanol
at 298 K were measured with BELSORP18 volumetric adsorption
equipment from Bel Japan, Inc.
Ligand (4-btapa). A solution of 4-aminopyridine (17.0 g, 180 mmol)
and distilled triethylamine (27 mL, 194 mmol) in distilled THF (200
mL) was added dropwise to a solution of 1,3,5-benzene tricarboxy-
trichloride (16.1 g, 60.0 mmol) in THF (60 mL) at 0 °C. Triethylamine
(9.0 mL, 65 mmol) was added, and the reaction mixture was stirred
for 7 h. The temperature was allowed to step up to room temperature.
The brown crude product of 4-btapa was collected by filtration and
washed with THF. The product was recrystallized from DMSO (700
mL) and H2O (1500 mL) by stirring for 1 h. The product was washed
with acetone (1000 mL) by stirring for 1 day. The yellowish white
powder was collected by filtration, washed with acetone, and dried
organic groups tend to coordinate metal ions via a self-assembly
process, resulting in frameworks in which FOS are completely
blocked.
The advantage of FOS is that the base-type catalyst is easy
to create. There are a variety of organic functional groups that
can serve as active base sites. In this report, we describe an
approach for the preparation of a PCP using base-type FOS and
demonstrate its base catalytic properties in a heterogeneous
reaction. These observations will contribute to the development
of new types of catalysts constructed from metal-organic
frameworks.
1
under vacuum for 35 h at room temperature (21.4 g, 81%). H NMR
(500 MHz, DMSO-d6): δ 10.95 (s, 3H, NH), 8.75 (s, 3H, -H2,4,6-ph),
8.52 (d, 6H, JH-H ) 5.0 Hz, H3,5-py), 7.82 (d, 6H, JH-H ) 5.0 Hz,
PCPs with amide groups are candidate compounds that have
FOS as guest interaction sites.7 The amide group is a fascinating
functional group because it possesses two types of hydrogen-
bonding sites: the -NH moiety acts as an electron acceptor
and the -CdO group acts as an electron donor.8 These
multifunctional moieties of amide groups tend to form hydrogen
bonds among themselves and interact negligibly with guest
molecules after construction of PCPs. If a PCP could be prepared
without forming amide-amide interactions, these groups would
constitute attractive interaction sites for selective sorption and/
or catalysis inside the channel. To retain amide groups as guest-
interaction FOS inside the network, we employed a three-
connector ligand containing amide groups. Three-connector
ligands are more useful for construction of 3D PCPs with
coordination bonds than bidentate or monodentate ligands.9 We
employed a three-connector ligand containing three pyridyl
groups as coordination sites and three amide groups as guest
interaction sites (1,3,5-benzene tricarboxylic acid tris[N-(4-
pyridyl)amide]10 (4-btapa, Scheme 1) and succeeded in produc-
ing a new base-type catalytic pore framework.
H2,6-py). FAB-MS (m/z): calcd for C24H18N6O3, 438.14; found, 438.
Anal. Calcd for C24H18N6O3 (438.5): C, 65.75; H, 4.14; N, 19.17.
Found: C, 64.84; H, 4.05; N, 18.81.
{[Cd(4-btapa)2(NO3)2]‚6H2O‚2DMF}n (1a). A solution of Cd-
(NO3)2‚4H2O (9.3 mg, 0.030 mmol) in MeCN (1.5 mL) was carefully
layered on a solution of in 4-btapa (20 mg, 0.045 mmol) in DMF (1.5
mL) with MeCN/DMF (0.75 mL/0.75 mL) placed between the two
layers. Colorless single crystals began to form in a few days. One of
these crystals was used for X-ray crystallographic analysis. The bulk
product was obtained by the following procedure. A solution of Cd-
(NO3)2‚4H2O (0.93 g, 3.0 mmol) in DMF (50 mL) was added to a
solution of 4-btapa (2.0 g, 4.5 mmol) in DMF (50 mL). The reaction
mixture was stirred for 3.5 h. The crystalline yellowish white powder
1a was obtained by filtration, washed with DMF, and dried under
vacuum for 1 day at room temperature (2.86 g, 93% yield based on
4-btapa). Anal. Calcd for {[Cd(4-btapa)2(NO3)2]‚6H2O‚2DMF}n (1a)
C54H62CdN16O20 (1367.58): C, 47.42; H, 4.57; N, 16.39. Found for
single crystal: C, 48.23; H, 4.66; N, 16.75. Found for bulk product:
C, 48.29; H, 4.30; N, 16.52. For X-ray crystallographic analysis some
guest molecules that show positional disorder could not be fixed in
the structure model like the early compounds.5b,11 The formula {[Cd-
1
Experimental Section
(4-btapa)2(NO3)2]‚6H2O‚2DMF}n (1a) was assigned by EA, TGA, H
NMR, IR, as well as single-crystal X-ray diffraction studies.12 Further
details of experimental studies are given in the Supporting Information.
{[Cd(4-btapa)2]‚2NO3}n (1b). The amorphous yellowish white
powder 1b was obtained by drying 1a under vacuum for 7.5 h at
140 °C. Removal of all solvent molecules was checked by TGA. The
amorphous state of 1b was checked by XRPD. Anal. Calcd for [Cd-
General Information. 4-Aminopyridine and 1,3,5-benzene tricar-
boxylic acid trichloride were obtained from Tokyo Kasei Industrial.
Cd(NO3)2‚4H2O was obtained from Wako without purification. El-
emental analysis (EA) was carried out with a Thermo Finnigan EA1112.
1H NMR spectra were measured in DMSO-d6 with a (500 MHz) JEOL
(7) (a) Uemura, K.; Kitagawa, S.; Kondo, M.; Fukui, K.; Kitaura, R.; Chang,
H. C.; Mizutani, T. Chem. Eur. J. 2002, 8, 3586-3600. (b) Uemura, K.;
Kitagawa, S.; Fukui, K.; Saito, K. J. Am. Chem. Soc. 2004, 126, 3817-
3828. (c) Tzeng, B. C.; Chen, B. S.; Yeh, H. T.; Lee, G. H.; Peng, S. M.
New J. Chem. 2006, 30, 1087-1092.
(11) (a) Hoskins, B. F.; Robson, R. J. Am. Chem. Soc. 1989, 111, 5962-5964.
(b) Ohmori, O.; Fujita, M. Chem. Commun. 2004, 1586-1587. (c) Luo,
T. T.; Tsai, H. L.; Yang, S. L.; Liu, Y. H.; Yadav, R. D.; Su, C. C.; Ueng,
C. H.; Lin, L. G.; Lu, K. L. Angew. Chem., Int. Ed. 2005, 44, 6063-6067.
(12) Some guest molecules that show positional disorder could not be fixed in
the structure model. During the final stages of refinement, several Q peaks
were found, which probably correspond to highly disordered molecules.
However, each guest molecule was detected by EA, TGA, IR, and 1H NMR.
Crystal data for 1a: C48H36CdN12O12, Mr ) 1085.30, cubic, Ia-3 (#206),
(8) (a) Lee, C. M.; Kumler, W. D. J. Am. Chem. Soc. 1962, 84, 571-578. (b)
Bent, H. A. Chem. ReV. 1968, 68, 587-648. (c) Huyskens, P. L. J. Am.
Chem. Soc. 1977, 99, 2578-2582.
(9) (a) Batten, S. R.; Murray, K. S. Coord. Chem. ReV. 2003, 246, 103-130.
(b) Chae, H. K.; Siberio-pe´rez, D. Y.; Kim, J.; Go, Y.; Eddaoudi, M.;
Matzger, A. J.; O’Keeffe, M.; Yaghi, O. M. Nature 2004, 427, 523-527.
(c) Ohmori, O.; Kawano, M.; Fujita, M. J. Am. Chem. Soc. 2004, 126,
16292-16293.
(10) Kumar, D. K.; Jose, D. A.; Dastidar, P.; Das, A. Chem. Mater. 2004, 16,
2332-2335.
a ) 24.756(4) Å, V ) 15171.6(43) Å3, Z ) 8, F(000) ) 4423.68, Fcalcd
)
0.952 gcm-3, µ ) 0.342 mm-1, GOF ) 0.904. A total of 84 868 reflections
were collected in the range 2θ ) 6.2-55.0°, of which 2896 were unique.
Final R1 and wR2 are 0.080 and 0.216, respectively, for 112 parameters
and 1350 reflections [I > 3σ(I)].
9
2608 J. AM. CHEM. SOC. VOL. 129, NO. 9, 2007

Selective Sorption and Catalysis
A R T I C L E S
Figure 2. Solid-state 113Cd CPMAS NMR spectrum of 1a at a MAS
spinning rate of 4382 Hz.
Table 1. Selected Bond Distances (Å) and Angles (deg) for
{[Cd(4-btapa)2(NO3)2]‚6H2O‚2DMF}n (1a)a
Cd (1)-N1
2.372 (7)
2.372 (8)
2.372 (8)
90.8 (3)
Cd1-N11
2.372 (8)
2.372 (8)
2.372 (7)
90.8 (3)
89.2 (3)
90.8 (3)
89.2 (3)
89.2 (3)
89.2 (3)
90.8 (3)
Cd1-N12
Cd1-N13
Cd1-N14
Cd1-N15
N11-Cd1-N1
N13-Cd1-N1
N15-Cd1-N1
N13-Cd1-N11
N15-Cd1-N11
N14-Cd1-N12
N14-Cd1-N13
N15-Cd1-N14
N12-Cd1-N1
N14-Cd1-N1
N12-Cd1-N11
N44-Cd1-N41
N13-Cd1-N12
N15-Cd1-N12
N15-Cd1-N13
Figure 1. ORTEP drawing of 1a with ellipsoids probability 30% (asterisk
indicates atoms that are symmetrically related). Cd(II) centers are octahe-
drally coordinated to N atoms of different six 4-btapa. H2O molecules and
hydrogen atoms are omitted for clarity.
89.2 (3)
180.0 (4)
180.0 (4)
90.8 (3)
90.8 (3)
(4-btapa)2(NO3)2] (1b) (1113.30): C, 51.78; H, 3.26; N, 17.61. Found
for bulk product: C, 49.52; H, 3.57; N, 16.66.
a Symmetry transformation used to generate equivalent atoms: 1 )
(y, z, x); 2 ) (z, x, y); 3 ) (-y, -z, -x); 4 ) (-z, -x, -y); 5 ) (-x, -y,
-z); 6 ) (-z + 1/2, -x, y + 1/2).
{[Cd(4-btapa)2(NO3)2]‚6MeOH‚yH2O}n (1c). The methanol-
exchanged colorless compound 1c was obtained by exposing 1b on
the methanol vapor for 30 h at room temperature. The amount of
1
adsorbed methanol molecules was checked by TGA and H NMR.
Results and Discussion
X-ray Structure Determination. The colorless single crystal of 1a
was mounted on glass fibers with epoxy resin. X-ray data collection
for the single crystal was carried out on a Rigaku Mercury diffracto-
meter with graphite-monochromated Mo KR radiation (λ ) 0.71070
Å) and a CCD two-dimensional detector at 238 K in a cold nitrogen
stream. The X-ray condition for 1a was 50 kV × 40 mA. The structure
was solved by direct method using the SHELXS-97 program and refined
by the full-matrix least-squares method on F2 with appropriate software
implemented in the crystals program package. The final cycles of the
full-matrix least-squares refinements were based on the observed
reflections [I > 3σ(I)]. All calculations were performed using the crystal
structure program package. All non-hydrogen atoms except for those
of disordered solvent molecules in 1a were refined anisotropically.
Hydrogen atoms were added at their geometrically ideal positions and
refined isotropically. Further refinement was unsuccessful because
counteranions and some solvent molecules were highly disordered, and
the high symmetric position in 1a would make occupancies of these
atoms decrease.
Structural Description. The coordination environment around
Cd(II) of {[Cd(4-btapa)2(NO3)2]‚6H2O‚2DMF}n (1a) is shown
in Figure 1. The Cd(II) center is octahedrally coordinated to
six pyridyl nitrogen atoms from different bridging (4-btapa)
connectors. To the best of our knowledge, 1a is the first
compound in which Cd(II) is coordinated to six pyridyl groups
as the ideal octahedral coordination environment. All the Cd-N
bond lengths and angles are similar to each other (Table 1).
The solid-state cross-polarization magic-angle spinning (CP-
MAS) 113Cd nuclear magnetic resonance (NMR) spectrum for
1a also supports the ideal octahedral coordination environment
of Cd(II). The spectrum has one resonance centered at 183.6
ppm (Figure 2). It is well known that each aromatic nitrogen
donor ligand in the CdN4 plane contributes approximately +50
ppm.13 Therefore, in this compound the chemical shift should
have a value of ca. +200 ppm directed normal to the 4N plane.
The observed value of 183.6 ppm is in agreement with this
value. Furthermore, one can safely state that the one single
resonance peak indicates a highly symmetrical coordination
environment around the Cd(II) center.
Selective Accommodation and Activation of Reactants. For each
reagent (malononitrile, ethyl cyanoacetate, and cyano-acetic acid tert-
butyl ester), the accommodation procedure was carried out. The as-
synthesized 1a (0.10 g) and a reagent (2 mmol) in dehydrated benzene
(10 mL) were stirred for 6 h. The precipitates were filtrated and dried
in the open air for 1.5 h and then identified by IR (KBr pellet) and 1H
NMR (DMSO-d6) spectroscopies.
3-D Porous Framework Functionalized with Amide Groups.
The framework of 1a was constructed from the Cd(II) center
as a six-connected node using 4-btapa as a three-connected
linker. This arrangement produces a large six-membered ring
composed of three octahedral Cd(II) moieties linked together
by three 4-btapa units. The six-membered ring is distorted
because of the rotation of 4-btapa between the benzene ring
and the amide group (Figure 3a). The framework expands in
Knoevengael Condensation Reaction Catalyzed by 1a. A solution
of benzaldehyde (0.21 mL, 2.1 mmol) and malononitrile (or ethyl
cyanoacetate or cyano-acetic acid tert-butyl ester) (0.132 g, 2.0 mmol)
in benzene (10 mL) was stirred for 5 min. Then 1a (0.10 g, 0.08 mmol,
4 mol %) was added. The suspension was stirred at room temperature
1
for 12 h. The progress of the reaction was monitored by H NMR
(DMSO-d6). The catalyst was removed by filtration, washed with
benzene, and recovered.
(13) Griffith, E. A. H.; Li, H. Y.; Amma, E. L. Inorg. Chim. Acta 1988, 148,
203-208.
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A R T I C L E S
Hasegawa et al.
Figure 3. Crystal structure of {[Cd(4-btapa)2(NO3)2]‚6H2O‚2DMF}n (1a).
(a) The distorted 6-membered ring composed of three octahedral Cd(II)
linked together by three 4-btapa units. (b) The framework expanding three
directions to form a 3-D framework. Orange and sky blue represent Cd(II)
and the framework. H atoms and guests are omitted for clarity.
Figure 6. View of the ordered amide groups on the channel surface. H2O
molecules are interacted by hydrogen bonds with the NH group of the amide
groups. Light brown, red, blue, gray, and purple represent channel surface,
Cd(II), O, N, C, and H, respectively.
Figure 4. (a-c) Two-fold interpenetrating 3-D crystal structure of 1a to
form three-dimensionally running channels of dimensions 4.7 × 7.3 Å.2
The perspective view is down from the b axis. Guests and H atoms are
omitted for clarity.
Figure 7. Thermogravimetric analyses of (a) 1a and (b) 1c over the
temperature range from 25 to 450 °C at a heating rate of 10 °C min-1
under N2 atmosphere. Amount of guests in figures is based on one 4-btapa
ligand of each compound.
to-N(amide) distances are about 24.8 Å in the network and 4.0
Å between the two adjacent networks. The distance of N(amide)-
to-O(amide) is about 9.7 Å. Consequently, the amide groups,
which are highly ordered on the surfaces of the channels, could
interact with guest molecules. The X-ray crystallographic results
clearly show that H2O molecules interact weakly via hydro-
gen bonds with the NH moieties of all the amide groups
(O[water]‚‚‚N[amide] ) ca. 3.1 Å) (Figure 6). The amide groups
inside the pores would act as important FOS in the interaction
between the host and guest molecules.
Figure 5. Crystal structure of 1a to form another type of zigzag channels
with dimensions of 3.3 × 3.6 Å.2 Guests and H atoms are omitted for clarity.
Thermogravimetric Analyses of 1a and 1c. As seen in the
crystal structure analysis and 1H NMR results, 1a includes H2O
and DMF molecules as guests. The desorption process was
monitored by TGA. The observed weight loss of the guest
molecules is in agreement with that calculated for the corre-
sponding crystal structure. The TGA data for 1a are indicative
of two stages of weight loss for the guest molecules: six H2O
molecules and two DMF molecules per formula unit (Figure
7a). The resultant species, [Cd(4-btapa)2(NO3)2]n (1b), is stable
up to 250 °C and then gradually decomposes with the loss of
4-btapa. The TGA curve showed that the weight loss below
150 °C of the methanol-exchanged compound 1c corresponds
to three methanol molecules per 4-btapa ligands (Figure 7b).
The amount of methanol adsorbed is in good agreement with
the number of amide groups in the host framework.
three directions, with a Cd(II)‚‚‚Cd(II) separation of 17.5 Å, to
form a 3D network (Figure 3b). The two frameworks mutually
interpenetrate with a nearest-neighbor Cd(II)‚‚‚Cd(II) distance
of 12.4 Å between Cd(II) atoms in the adjacent frameworks,
producing three-dimensionally running channels (so-called 3D
pores) with dimensions of 4.7 × 7.3 Å,2 which are available
for guest accommodation and exchange (Figure 4). The channel
is not straight but zigzags and occupied by nitrate anions and
H2O and DMF molecules. Two nitrate anions, six H2O
molecules, and two DMF molecules per formula unit have been
confirmed by elemental analysis (EA), thermogravimetric
analysis (TGA), infrared spectroscopy (IR), and 1H NMR,14 in
addition to the crystallographic results. 1a also has another type
of zigzag channel, with dimensions of 3.3 × 3.6 Å,2 running in
three directions (Figure 5). In 1a, 43.8% of the void space is
accessible to the solvent molecules.15,16 The nearest N(amide)-
Recoverable Collapse on Desolvation and Resolvation. 1a,
1b, and 1c are insoluble in water or common organic solvents
and only soluble in dimethyl sulfoxide. Removal of the guest
molecules causes a significant structural change in the frame-
work of 1a. As-synthesized crystalline 1a changes to the
amorphous state 1b after heating under reduced pressure. The
(14) More detailed measurement results are available in the Supporting
Information.
(15) The void space was calculated by PLATON.16 The value was taking the
-
volume of NO3 (77.0 Å3) into account. The size is calculated by
-
considering van der Waals radii for NO3 molecules. The value is in the
following reference. Israelachvili, J. N. Intermolecular and surface forces,
(16) Spek, A. L. PLATON, a multipurpose crystallographic tool; Utrecht
2nd ed.; Academic Press: New York, 1992.
University: Utrecht, The Netherlands, 2001.
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Selective Sorption and Catalysis
A R T I C L E S
Figure 8. XRPD patterns of (a) 1a simulation based on the single-crystal
structure, (b) the as-synthesized 1a, (c) the desolvated compound 1b, which
is obtained from 1a under vacuum for 7.5 h at 140 °C, and (d) the methanol-
exchanged compound 1c obtained by exposing 1b to methanol vapor for
30 h.
X-ray powder diffraction (XRPD) patterns for 1a and 1b are
shown in Figure 8. This crystalline-to-amorphous transformation
is accompanied by loss of the H2O and DMF guest molecules
in the channels, and the structural behavior has also been
observed in other compounds.17 However, when amorphous 1b
is immersed in methanol or exposed to methanol vapor, it
changes immediately to the original crystalline phase (1c), which
was detected by XRPD (Figure 8). We can rule out the
possibility of recrystallization because of the low solubility and
the results of experiments on vapor exposure. The amorphous-
to-crystalline transformation is also induced by H2O or DMF.18
These types of flexible frameworks provide pore structures that
are suitable for given guest molecules and much more useful
for molecular recognition or selective guest inclusion than are
robust porous structures.19
Figure 9. XRPD patterns for desolvated compound 1b immersed in a few
drops of (a) methanol, (b) ethanol, (c) n-propanol, (d) n-butanol, (e)
n-pentanol, and (f) n-hexanol solution.
Selective Binding of Alcohols Inspired by Hydrogen
Bonding. The desolvated host 1b can easily include certain
alcohols via hydrogen bonding with the amide groups. 1b was
immersed in a few drops of various alcohols, and the XRPD
was measured; several of these experiments are shown in Figure
9. Short-chain alcohols (methanol, ethanol, n-propanol, and
n-butanol) exhibited guest inclusion with structural transforma-
tion. Conversely, long-chain alcohols (n-pentanol and n-hexanol)
caused no change in the XRPD patterns, indicating that 1b did
not undergo a structural change with guest adsorption. This
selective inclusion system reflects the size of the guest molecule.
1b also exhibits unique selectivity responsive to functional
groups of guests. Figure 10 shows the XRPD patterns of 1b
that was immersed in n-butanol, n-pentane, or n-pentene. Unlike
its response to n-pentane and n-pentene, which have no OH
groups, 1b immersed in n-butanol exhibited guest inclusion with
structural transformation. Considering the fact that n-butanol,
n-pentane, and n-pentene are similar in size and shape, this
selective inclusion system reflects the presence/absence, in the
guest, of OH groups that would be available for hydrogen
bonding. On the other hand, in IR spectra for 1a, 1b, and 1a
Figure 10. XRPD patterns for desolvated compound 1b immersed in a
few drops of (a) n-butanol, (b) n-pentane, and (c) n-pentene solution.
with other guests, the NO symmetric stretching vibration band
-
of NO3 can be seen at 1385 cm-1 (Figure S1), supporting no
coordination or strong interaction of nitrate anions with the
amide moieties in the framework.20 In the case of the chloride
derivative of 1b, the Cl- ions clearly interact with the amide
moieties inside the channels, resulting in no porous properties
such as sorption or catalysis.7c Therefore, weaker participation
-
of anions such as NO3 in this study might be important for
successful formation of functional PCP system with the amide
groups.
Gas Sorption Properties with Dynamic Structural Trans-
formation. The mechanism of guest binding, with structural
transformation, should be investigated in detail. The adsorption
and desorption behaviors of some guests were examined. 1b
showed no N2 adsorption at 77 K, as shown in Figure 11. This
result indicates that 1b does not retain channels available for
N2 (kinetic diameter ) 3.64 Å).21 Because 1b undergoes an
amorphous-to-crystalline transformation when exposed to metha-
nol vapor, methanol adsorption and desorption experiments were
carried out on 1b. The adsorption and desorption isotherms for
(17) (a) Maspoch, D.; Ruiz-Molina, D.; Wurst, K.; Domingo, N.; Cavallini, M.;
Biscarini, F.; Tejada, J.; Rovira, C.; Veciana, J. Nat. Mater. 2003, 2, 190-
195. (b) Carlucci, L.; Ciani, G.; Moret, M.; Proserpio, D. M.; Rizzato, S.
Angew. Chem., Int. Ed., 2000, 39, 1506-1510.
(18) The XRPD patterns are shown in Figures S3 and S4.
(19) (a) Lee, E. Y.; Jang, S. Y.; Suh, M. P. J. Am. Chem. Soc. 2005, 127, 6374-
6381. (b) Maji, T. K.; Mostafa, G.; Matsuda, R.; Kitagawa, S. J. Am. Chem.
Soc. 2005, 127, 17152-17153. (c) Kitagawa, S.; Uemura, K. Chem. Soc.
ReV. 2005, 34, 109-119. (d) Uemura, K.; Matsuda, R.; Kitagawa, S.
J. Solid State Chem. 2005, 178, 2420-2429.
(20) (a) Newman, S. P.; Jones, W. J. Solid State Chem. 1999, 148, 26-40. (b)
Xu, Z. P.; Zeng, H. C. J. Phys. Chem. B 2001, 105, 1743-1749.
(21) Pan, L.; Adams, K. M.; Hernandez, H. E.; Wang, X.; Zheng, C.; Hattori,
Y.; Kaneko, K. J. Am. Chem. Soc. 2003, 125, 3062-3067.
9
J. AM. CHEM. SOC. VOL. 129, NO. 9, 2007 2611

A R T I C L E S
Hasegawa et al.
Figure 11. Gas adsorption/desorption isotherms for the uptake of N2,
MeOH. P0 is the saturated vapor pressure, 102.3 kPa, of N2 (77 K), and
16.94 kPa, of methanol (298 K).
methanol at 298 K are shown in Figure 11. The adsorption
isotherm showed gradual uptake to P/P0 ) 0.9. Conversely, the
desorption isotherm did not trace the adsorption profile and
decreased slightly until a sudden drop at P/P0 ) 0.1, with a
large-range hysteresis loop. This characteristic adsorption profile
shows the conversion of amorphous 1b to the crystal 1c. The
interaction between the host framework and methanol is strong
enough to transform and maintain the channel structure, so that
the large hysteresis profile appears. The amount of adsorbed
methanol is 3.2 molecules per 4-btapa ligand at P/P0 ) 0.9,
which is close to that calculated from the TGA measurement
for 1c. This result also supports the proposition that the adsorbed
methanol is bound to the amide groups of 1c in a 1:1 ratio.
From these results, we conclude that the amide groups on the
channel surfaces function effectively and give rise to an
attractive interaction with the methanol guest molecules via
hydrogen bonding. The structural transformation is inspired by
the interaction between the guest molecules and the host
framework.
Selective Inclusion and Activation of Reactants in Selective
Catalytic Reactions. Amide groups have two sites that are
involved in their function in different ways. In 1a, the -NH
moiety acts as a hydrogen donor for short-chain alcohols in the
channels, whereas the -CdO moiety acts as an electron donor.
Recently, a study reported a PCP with base carbonyl oxygen
atoms as the catalytic interaction sites on the channel wall.4g
We also expected the -CdO moiety in the channels of 1a to
provide guest interaction sites as base sites. To confirm the
selective accommodation and activation of reactants by 1a,
selective adsorption experiments were performed to identify the
substrates of 1a. The substrates chosen for the reaction were
malononitrile (molecular size, 6.9 × 4.5 Å2), ethyl cyanoacetate
(10.3 × 4.5 Å2), and cyano-acetic acid tert-butyl ester (10.3 ×
5.8 Å2).22 As-synthesized 1a was immersed in benzene contain-
ing each reagent and stirred. The powder was filtered off and
dried in air, and then its 1H NMR and IR spectra were acquired.
The 1H NMR spectra showed that 1a adsorbs 2.9, 0.7, and 0.6
molecular stoichiometric amounts of malononitrile, ethyl cy-
anoacetate, and cyano-acetic acid tert-butyl ester per 4-btapa
ligand of 1a, respectively (Figure 12). The inclusion amount of
malononitrile for 1a was 4-5 times larger than those of the
other reagents, implying that the smallest molecule, malononi-
trile, was most easily introduced in the channels of 1a. From
Figure 12. 1H NMR (DMSO-d6) spectra of 1a that adsorbed each guest
molecule: (a) malononitrile, (b) ethyl cyanoacetate, and (c) cyano-acetic
acid tert-butyl ester. The peaks marked with an asterisk indicate the 4-btapa.
Figure 13. IR spectra in the region of CtN stretching vibration bands at
room temperature of (a) malononitrile (reactant), (b) 1a containing
malononitrile, (c) 1a containing ethyl cyanoacetate, and (d) 1a containing
cyano-acetic acid tert-butyl ester. The bands marked with an asterisk indicate
the CtN stretching vibration bands due to each substrate that is not
activated.
the IR spectra, only malononitrile interacted with the host 1a.
In general, when an active methylene compound interacts with
part of the R-H atom, the IR band attributed to the CtN
as
stretching vibration shifts to a lower wavenumber and υs
υ
CN-
CN
as
splitting appears.23 The υs
υ
CN-
splitting at 2180 and 2200
CN
cm-1 was observed for malononitrile in 1a, whereas the other
reagents showed only the original peak (Figure 13). Malono-
nitrile in the channels of 1a may interact with the -CdO moiety
of the amide groups via hydrogen bonding. Unfortunately, the
shift in the peak for the -CdO stretching vibration was not
large because the amide groups of the original 1a had already
interacted with the guest molecules, which maintain the channels
of 1a. Therefore, it was difficult to observe the shift in the peak
of the -CdO stretching band. It is worth noting that the IR
spectrum of 1a containing malononitrile shows two types of
guests: one interacting with the host 1a and the other not
1
interacting. These H NMR and IR results indicate that the
smallest molecule, malononitrile, is selectively adsorbed and
activated in the channels of 1a.
(23) Binev, I. G.; Binev, Y. I.; Stamboliyska, B. A.; Juchnovski, I. N. J. Mol.
Struct. 1997, 435, 235-245.
(22) These molecular sizes are calculated based on the CPK model.
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Selective Sorption and Catalysis
A R T I C L E S
Table 2. Knoevenagel Condensation Reaction of Benzlaldehyde
with Substrates, Catalyzed by 1a
Figure 14. (a) Conversion (%) vs time (h) for Knoevenagel condensation
reaction of benzaldehyde with malnonitrile in benzene catalyzed as-
synthesized, 1a (blue square), desolvated, 1b (green circle), and 4-btapa
ligand (black triangle). (b) Reaction of solvent-free condition (red circle)
is much faster than that in benzene solution (blue square). Each conversion
1
was detected by H NMR based on starting material.
catalytic activity of 1a. The catalyst 1a shows good recyclability.
1a is easily isolated from the reaction suspension by filtration
alone and can be reused without loss of activity. The XRPD
patterns of the 1a before and after the reactions were the same,
indicating the high stability of 1a. Even when 1a changes its
form to the amorphous state 1b, it is easily recovered in the
crystalline state by immersing it in DMF or short-chain alcohols,
which cause the amorphous-to-crystalline structural transforma-
tion of the host. Therefore, this compound containing effective
FOS, amide groups, can be considered to exhibit a sufficient
catalytic activity with base property. Under solvent-free condi-
tions, remarkable improvements were observed in the conver-
sion, via Knovenagel condensation of benzaldehyde with
malononitrile, catalyzed by 1a (Figure 14b). The reaction of
malononitrile (2 mmol) with liquid benzaldehyde (10 mL) was
performed without benzene, and 1a achieved a conversion to
the adduct of 100% after a 2.5 h reaction time.
a All reaction were performed with 2 mmol substrates in 10 mL of
benzene using 0.1 g (0.08 mmol, 4 mol %) of the as-synthesized 1a for 12
1
h. b Determined by H NMR, based on starting materials.
Base Catalytic Properties of 1a. Although several
studies about PCPs with acid properties have been report-
ed,4a,c-e,5e-f,24 studies of PCPs with base properties have been
few.4b,4g,25 To characterize the base-type properties of 1a, a
Knoevenagel condensation reaction catalyzed by 1a was per-
formed. The Knoevenagel reaction is well known, not only as
a weak base-catalyzed model reaction but also as a reaction
that generates a C-C bond.26 However, to our knowledge, the
reaction has never been reported in the context of PCP.
Knoevenagel condensation reactions of benzaldehyde with each
of the active methylene compounds (malononitrile, ethyl cy-
anoacetate, and cyano-acetic acid tert-butyl ester) were catalyzed
by 1a. As a result, the malononitrile was a good substrate,
producing 98% conversion of the adduct, whereas the other
substrates reacted negligibly (Table 2). This guest-selective
reaction suggests that the reaction occurs in the channels and
not on the surface of 1a.
Only as-synthesized 1a promoted the reaction with a good
yield compared with desolvated 1b or the 4-btapa ligand (Figure
14a). Desolvated 1b has no channels, and the 4-btapa ligand
has no free amide groups because it forms an intermolecular
hydrogen bond among themselves.10 We also checked that no
reaction occurred with Cd(NO3)2‚4H2O. These results also
support that the reaction occurs within the channels function-
alized with the amide group of 1a. The heterogeneity and
recyclability of 1a in the Knoevenagel condensation of benzal-
dehyde and malononitrile were examined. After the reaction
mixture was stirred for 3 h in the presence of 1a, 1a was
removed by filtration. After removal of 1a from the reaction
system, the reaction did not proceed further, demonstrating the
Conclusion
This work describes the construction of a 3D PCP containing
guest-accessible amide groups and characterizes the selective
inclusion of guest molecules with the structural transformation
of the host. Moreover, we demonstrated a PCP that acts as a
heterogeneous base catalyst based on a FOS located on its
channel surfaces.
First, we successfully synthesized a 3D PCP functionalized
with amide groups, {[Cd(4-btapa)2(NO3)2]‚6H2O‚2DMF}n (1a),
from the reaction between Cd(NO3)2‚4H2O and a three-
connector-type amide ligand (4-btapa). The amide groups of
1a are ordered uniformly on the channel surfaces. 1a undergoes
reversible dynamic structural transformation via an amorphous
state (1b) in response to the removal and rebinding of guest
molecules. This is inspired by hydrogen bonding between the
guest molecules and amide groups. We observed selective guest
inclusion via the hydrogen bond, which is based on not only
the size and shape of the incoming guest but also its affinity
for the amide group, as demonstrated by X-ray crystallographic
analysis and adsorption measurements.
(24) Vimont, A.; Goupil, J. M.; Lavalley, J. C.; Daturi, M.; Surble´, S.; Serre,
C.; Millange, F.; Fe´rey, G.; Audebrand, N. J. Am. Chem. Soc. 2006, 128,
3218-3227.
(25) Shin, D. M.; Lee, I. S.; Chung, Y. K. Cryst. Growth Des. 2006, 6, 1059-
1061.
(26) (a) The number of published articles containing the keyword “Knoevenagel
condensation” is shown in Figure S5. (b) Rao, P. S.; Venkataratnam,
R. V. Tetrahedron Lett. 1991, 132, 5821-5822. (c) Reddy, T. I.; Varma,
R. S. Tetrahedron Lett. 1997, 38, 1721-1724. (d) Rodriguez, I.; Iborra,
S.; Corma, A.; Rey, F.; Jorda´, J. L. Chem. Commun. 1999, 593-594. (e)
Harjani, J. R.; Nara, S. J.; Salunkhe, M. M. Tetrahedron Lett. 2002, 43,
1127-1130. (f) Yadav, J. S.; Reddy, B. V. S.; Basak, A. K.; Visali, B.;
Narsaiah, A. V.; Nagaiah, K. Eur. J. Org. Chem. 2004, 546-551. (g)
Tamami, B.; Fadavi, A. Catal. Commun. 2005, 6, 747-751.
Second, 1a selectively accommodated and activated guests
in its channels because of the active amide groups. As a result,
the Knoevenagel condensation reaction, which is a well-known
base-catalyzed model reaction, was selectively promoted in good
yield. This selectivity depends on the relationship between the
size of the reactants and the pore window of the host. 1a displays
a unique recoverable framework; even when 1a changes to 1b
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J. AM. CHEM. SOC. VOL. 129, NO. 9, 2007 2613

A R T I C L E S
Hasegawa et al.
after complete guest removal, the porous crystalline structure
can be easily recovered by guest adsorption and demonstrated
the same catalytic activity as the initial one.
Acknowledgment. This work was supported by a Grant-In-
Aid for Science Research in a Priority Area “Chemistry of
Coordination Space” (#434) and a CREST program from the
Ministry of Education, Culture, Sports, and Science and
Technology, Government of Japan.
This research is particularly relevant in the context of porous
solid-state chemistry in the generation of new materials with
unusual properties. This result suggested that the combination
of metal ions and designed organic ligand can provide PCP with
FOS showing base catalytic performance. PCPs functionalized
with FOS herald the next advance in porous materials that can
be used as a fascinating class of adsorbents and heterogeneous
catalysts.
Supporting Information Available: General experimental
details and characterization data. This material is available free
of charge via the Internet at http://pubs.acs.org.
JA067374Y
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2614 J. AM. CHEM. SOC. VOL. 129, NO. 9, 2007