Soft 2D layer porous coordination polymers with 1,2-Di(4-pyridyl)ethane

Article abstract of DOI:10.1071/CH12467

Porous coordination polymer compounds consisting of Zn2+, 1,2-di(4-pyridyl)ethane, and dicarboxylates were synthesised and their crystal structures were determined. These are doubly interpenetrated 2D layer structures, and the flexibility of porous struct

Full text of DOI:10.1071/CH12467

RESEARCH FRONT
CSIRO PUBLISHING
Aust. J. Chem. 2013, 66, 464–469
Full Paper
http://dx.doi.org/10.1071/CH12467
Soft 2D Layer Porous Coordination Polymers
with 1,2-Di(4-pyridyl)ethane
A
B C
,
B
Keisuke Kishida, Satoshi Horike,
Kanokwan Kongpatpanich,
B D E F
, , ,
and Susumu Kitagawa
A
Research and Development Center, Showa Denko K. K., 2, Oaza, Nakanosu,
Oita 870-0189, Japan.
B
Department of Synthetic Chemistry and Biological Chemistry, Graduate School
of Engineering, Kyoto University, Katsura, Nishikyo-ku, Kyoto 615-8510, Japan.
C
Japan Science and Technology Agency, PRESTO, 4-1-8 Honcho, Kawaguchi,
Saitama 332-0012, Japan.
D
E
Institute for Integrated Cell-Material Sciences (WPI-iCeMS), Kyoto University,
Yoshida, Sakyo-ku, Kyoto 606-8501, Japan.
Japan Science and Technology Agency, ERATO, Kitagawa Integrated Pores Project,
Kyoto Research Park Bldg #3, Shimogyo-ku, Kyoto 600-8815, Japan.
Corresponding author. Email: kitagawa@icems.kyoto-u.ac.jp
F
Porous coordination polymer compounds consisting of Zn2þ, 1,2-di(4-pyridyl)ethane, and dicarboxylates were synthe-
sised and their crystal structures were determined. These are doubly interpenetrated 2D layer structures, and the flexibility
of porous structures is dependent on the substituent group of the dicarboxylate. From gas adsorption studies, distinct
adsorption isotherms were observed for CO2, CH4, C2H4, and C2H6 at 195 K and 273 K, respectively.
Manuscript received: 11 October 2012.
Manuscript accepted: 8 January 2013.
Published online: 29 January 2013.
Zn2þ, isophthalate, dpa, and one DMF molecule. The Zn2þ ion
is in a tetrahedral geometry, and coordinated by two nitrogen
atoms (N1 and N2) from two dpa molecules and two carbox-
ylates (O1 and O3). The Zn–N bond lengths are in the range of
Introduction
Synthesis and characterisation of porous coordination polymers
(PCPs) or metal-organic frameworks (MOFs) have been of
significant interest due to their potential applications for gas
storage, separation, magnetism, and catalysis.[1] To construct
open frameworks, a combination of a divalent metal cation,
neutral dipyridyl ligand, and an anionic dicarboxylate is a useful
approach.[2] Mixed ligand compounds have potential for organic
functionality in the pore, because one of the ligands enables
stable porosity, and the other provides a functional site. For
frameworks with dipyridyl and dicarboxylate, only a limited
number of examples have been reported.[3] To explore the
rational synthesis of PCP/MOFs, it is important to discover a
new series of mixed-ligand type compounds. Herein, we
demonstrate the syntheses of three two-dimensional (2D) layer
structures, {[Zn(5-R-isophthalate)(dpa)](DMF)}n (1’DMF:
R ¼ H, 2’DMF: R ¼ OMe, 3’DMF: R ¼ Me, dpa ¼ 1,2-di(4-
pyridyl)ethane) comprised of Zn2þ, 5-substituted isophthalate,
and dpa, by using DMF as a template solvent.
˚
˚
2.024–2.053 A, and the Zn–O bond lengths are 1.969–1.986 A,
respectively. All three compounds have a space group of
orthorhombic Pbca. The 2D layer is sql topology (Fig. 1a).[3]
Two 2D layers are interpenetrated into each other, and run along
the b axis. The interpenetrated layer structures are stacked with
the plane of isophthalate ligands of adjacent layers (Fig. 1b). 1D
2
˚
channels are created with a cross-section of ,3.0 ꢀ 4.5 A .
The channels are surrounded with aromatic rings of dpa and
dicarboxylate, and hydrophobic pores are formed. One DMF
molecule per unit cell is trapped as a guest molecule. Guest-
accessible void volumes of 1’DMF, 2’DMF, 3’DMF are
calculated to be 21.7, 23.6, and 23.0 % (probe molecule
[4]
˚
radius ¼ 1.4 A) using PLATON software.
1’DMF and 3’DMF are the isomers of previously reported
compounds [Zn(ip)(dpa)][5a,b] and [Zn(5-Meip)(dpa), where
ip ¼ isophthalate].[5c] The crystal structures of the reported
compounds are four-connected 3D (65.8)-dmp topology with
three-fold interpenetration or 2D sql topology with multiple
interpenetration. The 3D assembled structures are different from
1’DMF and 3’DMF. The reported compounds are obtained
using water or alcohols as solvent, whereas 1’DMF and
3’DMF are constructed with DMF. The difference of solvent
provides the different crystal structures. On the other hand, one
Results and Discussion
Single crystal X-ray structural analyses confirmed that
1’DMF, 2’DMF, and 3’DMF form structurally similar
motifs. These are two-fold interpenetrating 2D coordination
networks. Fig. 1 shows the crystal structure of 1’DMF. The
asymmetric unit of the three compounds consist of one each of
Journal compilation Ó CSIRO 2013
www.publish.csiro.au/journals/ajc

Synthesis of 2D MOFs with dpa Ligand
465
(a)
(b)
b
c
Fig. 1. Crystal structures of (a) 2D layer and (b) 3D assembly along the a axis of 1’DMF. Large and small grey spheres are Zn and C. Blue and pink spheres
are N and O. DMF molecules are omitted for clarity.
with that of the simulated patterns. The PXRD patterns of 1 and
2 are similar to those of 1’DMF and 2’DMF. This indicates
100
that the structures of these two compounds are robust, and they
do not show much structural rearrangement upon DMF release.
80
The PXRD patterns of 3’DMF and 3 are slightly different. This
suggests that 3 has structural flexibility. We confirmed that the
crystal structure of 3, after soaking with DMF, returned to
the same structure of 3’DMF by PXRD study; thus the
structural rearrangement is reversible.
60
We then measured gas adsorption isotherms of N2 (Fig. 4).
The N2 adsorption isotherms measured for 1–3 at 77 K reveal
40
low uptakes. Adsorption of CO2 and C1–C2 hydrocarbon gases
at 195K were conducted to evaluate their permanent porosity.
20
100
200
300
400
500
The study on adsorption properties of these gases is significant
for storage and separation.[7] All compounds show Type-I or
‘gate-opening’[8] type adsorption isotherms for these gases
which indicates their microporosity. A similar molecular-
sieving effect towards N2 has been observed in other PCP/MOFs
or zeolites,[9] and we consider this unusual behaviour is due to
the deficiency of the energy of N2 diffusion into the small
channels of 1–3. The Brunauer–Emmer–Teller (BET) surface
area of 2 and 3, calculated from CO2 adsorption, are 289 and
343 m2 gꢁ1. On the other hand, the CO2 adsorption isotherm of
1 shows a sudden jump at 80 kPa, and the total gas uptake
reaches 100 mL gꢁ1 at 100 kPa. This stepwise profile would be
due to the structural transformation. Desorption of CO2 in
1 occurs below 20 kPa, and we observe a large hysteresis in
the adsorption and desorption processes. We could consider the
pore structure of 1 has high affinity for CO2, and the condensa-
tion of CO2 into 1 promotes a structural transformation to
another phase. In the case of CH4 adsorption isotherms, 3 also
shows ‘gate-opening’ type behaviour as is the case with 1 for
CO2. There is no CH4 adsorption from 0 to 15 kPa, and it has a
plateau near the uptake of one CH4 molecule per formula unit at
30 kPa. In the desorption profile, we observe the hysteresis in the
range of pressure of 0 to 30 kPa. For C2 hydrocarbons, all
compounds show typical Type-I isotherms with a steep rise at
the low-pressure region.[10] The total gas uptakes of 1–3 for
Temperature [ЊC]
Fig. 2. TGA curves of 1’DMF (black), 2’DMF (blue) and 3’DMF
(pink).
of the sql structures with dipyridyl ligand and dicarboxylate is
synthesised from DMF; [Zn(5-MeOip)(dpe)] (dpe ¼ 1,2-di(4-
pyridyl)ethylene).[6] The compound has three-fold interpene-
tration of 2D layers, and no porosity forms. The structural
difference may be derived from the slightly smaller steric
hindrance of the dpe ligand than dpa, and also because of their
different rigidity.
Thermogravimetric analysis (TGA) of 1’DMF, 2’DMF,
and 3’DMF show weight loss at 2208C (Fig. 2). The weight
loss corresponds to the loss of DMF. The accommodated DMF
are removed by the evacuation procedure at 1508C.
The structural expansion and contraction behaviour of 2D
layer PCP/MOFs via gas adsorption have been reported. Fig. 3
shows the observed powder X-ray diffraction (PXRD) patterns
of 1’DMF, 2’DMF, and 3’DMF, and their degassed forms
1, 2, and 3 with simulated patterns of the single crystal
structures. PXRD patterns of 1’DMF, 2’DMF, and
3’DMF measured at room temperature are in good agreement

466
K. Kishida et al.
(a)
(b)
1
2
2
DMF
1 DMF
1 DMF
simulation
2 DMF
simulation
10
20
30
10
20
30
2θ [deg.]
2θ [deg.]
(c)
3
3
DMF
DMF
3
simulation
10
20
30
2θ [deg.]
Fig. 3. Powder X-ray diffraction patterns of (a) 1’DMF, 1; (b) 2’DMF, 2; (c) 3’DMF, 3; and their simulated patterns from single crystal X-ray structures.
C1 and C2 hydrocarbons reach ,50–55 mL gꢁ1, which corre-
spond to the accommodation of one CH4, C2H4, or C2H6
molecule per Zn2þ ion in the structures. All compounds possess
a bottle-and-neck type channel structure, and they provide the
commensurate type adsorption of guest molecules.
195 K. In the case of 1, we observed Type-I isotherms for CO2,
C2H4, and C2H6, whereas a gradual curve for CH4 is observed
because of the lower interaction with the framework. The
pressure region studied (,900 kPa) is not enough to promote
the consequent structure transformation. In the case of 2, all
the gas isotherms are Type-I which is similar with 1, and the
isotherms of C2H4 and C2H6 have high steepness of uptake at
lower pressure regions. For 3, we also observed a stepwise
isotherm of CH4 and a second adsorption occurs at 150 kPa. The
step at 195 K is more abrupt, and this is because the gas diffusion
rate at the higher temperature (273 K) would be larger, which
provides the gradual uptake until 150 kPa.
Because the 3D assembled structures of these compounds are
the same, the observed different gas adsorption properties must
be caused by the different substituent groups on the isophtha-
lates in the structures. These compounds have higher affinity to
CO2 than the other gases at 195 K. 2 shows Type-I adsorption
isotherms for all gases, and it represents the robustness of the
porous structure. 1 and 3 have structural flexibility with gate-
opening type sorption behaviours. We have previously reported
that the structural flexibility depends on the substituent groups
of isophthalate ligands in the 2D interdigitated compounds.[11]
We also observe the similar effects of the substituent group on
the flexibility in the present compounds. The introduction of the
substituent groups into the porous framework may affect both
host-guest and host-host interaction, and it is hard to explain the
origin of the gate-opening type sorption behaviours.
Conclusion
In this work, we synthesised three compounds of 2D layer type
PCP/MOFs consisting of Zn2þ, dpa (dipyridylethane), and
5-substituted isophthalate ligands. The structure of the 2D layer
of these compounds is sql topology and each substituent group
provides different pore structures and structure flexibilities. We
measured gas adsorption profiles for CO2, CH4, C2H4, and C2H6
at 195 K and 273 K for these compounds. They show a prefer-
able adsorption of CO2 compared with other gases. For the
synthesis of these frameworks, the synthetic solvent (DMF) is
We also studied the gas adsorption isotherms for these
compounds at near room temperature and at pressure ranges
of 0 to 900 kPa (Fig. 5). The gas adsorption isotherms are
regarded as similar with those in the lower pressure region at

Synthesis of 2D MOFs with dpa Ligand
467
100
80
60
40
20
0
(a)
(b) 100
80
60
40
20
0
0
20
40
60
80
100
0
20
40
60
80
100
Pressure [kPa]
Pressure [kPa]
(c) 100
80
60
40
20
0
0
20
40
60
80
100
Pressure [kPa]
Fig. 4. Gas adsorption and desorption isotherms of (a) 1, (b) 2, and (c) 3 at 77 K (purple: N2) and 195 K (red: CO2, black: CH4, blue: C2H4, green: C2H6). Solid
symbols are adsorption and open symbols are desorption, respectively.
key for construction of these new types of 2D layer structures.
We have demonstrated a promising approach to construct
PCP/MOFs by solvent optimisation.
Synthesis of {[Zn(5-MeOip)(dpa)](DMF)}n (2.DMF)
Zn(NO3)2ꢂ6H2O in DMF (0.1 M, 0.3 mL), 5-methoxyisophthalic
acid in DMF (0.1 M, 0.3 mmol), 1,2-di(4-pyridyl)ethane in
DMF (0.1 M, 0.3 mL) and DMF (0.2 mL) were reacted in a
1.5 mL vial for 72 h at 808C. Colourless single crystals
were obtained and one of these was used for X-ray crystallo-
graphic analysis. The bulk sample was obtained by the
following procedure. Zn(NO3)2ꢂ6H2O (1.49 g, 5.00 mmol),
5-methoxyisophthalic acid (0.932 g, 4.75 mmol), and 1,2-di(4-
pyridyl)ethane (0.924 mg, 5.02 mmol) were reacted with
100 mL of DMF in a 300 mL round bottomed flask and stirred
for 16 h at 1208C under N2 atmosphere. A light brown micro-
crystalline precipitate was filtered, washed with DMF and
methanol several times and dried under vacuum at room
temperature (1.19 g, 52 %).
Experimental
All chemicals and solvents used in the syntheses were of
reagent grade and used without further purification. Zn(NO3)2ꢂ
6H2O and N,N0-dimethylformamide (DMF) were purchased
from Wako Pure Chemical Industries Inc. and isophthalic
acid (H2-ip), 5-methoxyisophthalic acid (H2-5-MeOip), 5-
methylisophthalic acid (H2-5-Meip), and 1,2-di(4-pyridyl)ethane
(dpa) were from Tokyo Chemical Industry Co.
Synthesis of {[Zn(ip)(dpa)](DMF)}n (1.DMF)
Zn(NO3)2ꢂ6H2O in DMF (0.1 M, 0.2 mL), isophthalic acid in
DMF (0.1 M, 0.2 mmol), 1,2-di(4-pyridyl)ethane in DMF
(0.1 M, 0.2 mL) and H2O (0.5 mL) were reacted in a 1.5 mL vial
for 72 h at 808C. Colourless single crystals were obtained and
one of these was used for X-ray crystallographic analysis. The
bulk sample was obtained by the following procedure. Zn
(NO3)2ꢂ6H2O (0.600 g, 2.02 mmol), isophthalic acid (0.339 g,
2.04 mmol), and 1,2-di(4-pyridyl)ethane (0.393 mg, 2.13 mmol)
were reacted with 60 mL of DMF and 50 mL of water in a
300 mL round bottomed flask and stirred for 16 h at 808C under
N2 atmosphere. The white microcrystalline precipitate was fil-
tered, washed with DMF and methanol several times and dried
under vacuum at room temperature (0.240 g, 26 %).
Synthesis of {[Zn(5-Meip)(dpa)](DMF)}n (3.DMF)
Zn(NO3)2ꢂ6H2O in DMF (0.1 M, 0.4 mL), 5-methylisophthalic
acid in DMF (0.1 M, 0.4 mmol), 1,2-di(4-pyridyl)ethane in
DMF (0.1 M, 0.2 mL), and DMF (0.1 mL) were reacted in a
1.5 mL vial for 72 h at 808C. Colourless single crystals were
obtained and one of these was used for X-ray crystallographic
analysis. The bulk sample was obtained by the following
procedure. Zn(NO3)2ꢂ6H2O (1.50 g, 5.04 mmol), 5-
methylisophthalic acid (0.901 g, 5.00 mmol) and 1,2-di(4-
pyridyl)ethane (0.928 mg, 5.04 mmol) were reacted with
100 mL of DMF in a 300 mL round bottomed flask and stirred

468
K. Kishida et al.
(a) 60
(b) 60
40
20
0
40
20
0
0
200
400
600
800
0
200
400
600
800
Pressure [kPa]
Pressure [kPa]
(c) 60
40
20
0
0
200
400
600
800
Pressure [kPa]
Fig. 5. Gas adsorption and desorption isotherms of (a) 1, (b) 2, and (c) 3 at 273 K (red: CO2, black: CH4, blue: C2H4, green: C2H6). Solid symbols are
adsorption and opened symbols are desorption, respectively.
for 16 h at 1208C under N2 atmosphere. A light brown micro-
crystalline precipitate was filtered, washed with DMF and
methanol several times and dried under vacuum at room
temperature (1.32 g, 56 %).
Table 1. Summary of the crystal data for compounds
Compound 2’DMF 3’DMF
1’DMFA
Molecular formula C20H16N2O4Zn C24H25N3O6Zn C24H25N3O5Zn
Molecular weight
Temperature [K]
Space group
˚
a [A]
˚
b [A]
413.74
223
516.84
100
500.84
100
Single Crystal X-Ray Diffraction
The colourless single crystals of 1’DMF, 2’DMF, and
3’DMF were mounted on glass fibres with epoxy resin. X-ray
data collection for the single crystals was carried out on a Bruker
AXS SMART APEX II Ultra diffractometer with graphite
Pbca
Pbca
Pbca
10.1459(11)
18.125(2)
23.509(3)
4323.2(9)
8
10.2480(12)
18.957(2)
23.615(3)
4673.7(10)
8
10.1606(13)
19.747(3)
23.294(3)
4587.8(9)
8
˚
c [A]
˚
3
V [A ]
˚
monochromated MoKa radiation (l ¼ 0.71073 A) and a CCD
2D detector at 223K (for 1’DMF) or 100 K (for 2’DMF and
3’DMF) in a cold nitrogen stream. The conditions for X-ray of
1’DMF were 50 kV ꢀ 100 mA. Absorption corrections were
applied by using the multi-scan program SADABS and the
structures were solved with direct methods and refined with a
full-matrix least-squares technique with the SHELXTL program
package. All non-hydrogen atoms were refined anisotropically
and hydrogen atoms were included in calculated positions and
refined using a riding model. The crystallographic data for
compound 1’DMF, 2’DMF, and 3’DMF are listed in
Table 1. CCDC deposit numbers of 1’DMF, 2’DMF,
3’DMF are 905315, 905316, and 905317, respectively.
Z
Dc [g cmꢁ3
]
1.271
1.469
1.436
m [mmꢁ1
GOF
]
1.160
1.096
1.111
1.761
1.009
1.078
R1 [I .2s(I)]
wR2 [all data]
0.0563
0.2150
0.0256
0.0761
0.0286
0.0773
AWe used the SQUEEZE program for 1’DMF because the accommodated
DMF molecule is heavily disordered. The amount of DMF was confirmed by
TGA analysis.
of 10 K minꢁ1. Powder X-ray diffraction (PXRD) data were
collected on a Rigaku Multiflex diffractometer with CuKa
radiation. Adsorption isotherms at 273 K were measured with
BEL-HP volumetric adsorption equipment from BEL Japan,
Inc. Adsorption isotherms at 195 K were measured with
BEL-mini instrument.
Other Physical Measurements
Thermogravimetric analyses (TGA) were performed using a
Shimadzu DTG-60A apparatus in the temperature range
between 298 and 723 K in a N2 atmosphere and at a heating rate

Synthesis of 2D MOFs with dpa Ligand
469
Acknowledgements
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Development Organization (NEDO) and the Japan Science and Technology
Agency PRESTO program, Grants-in-Aid for Scientific Research, Japan
Society for the Promotion of Science (JSPS), and the Japan Science and
Technology Agency ERATO program. iCeMS is supported by World
Premier International Research Initiative (WPI), MEXT, Japan.
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