Design and characterization of a polarized coordination polymer of a zinc(II) biphenyldicarboxylate bearing a sulfone group

Article abstract of DOI:10.1246/cl.2010.28

A novel coordination polymer of zinc(II) biphenyldicarboxylate bearing a sulfone group, [Zn(sbdc)(H₂O)3]·(H2sbdc)· H ₂O (H₂sbdc: sulfone biphenyldicarboxylic acid), has been synthesized. It is composed of a one-dimensional chain structure of alternating Zn(II) cations and sbdc anions. This coordination polymer possesses noncoordinating uncharged acid H₂sbdc existing between the chains with hydrogen-bonding networks. The proton conductivity of [Zn(sbdc)(H ₂O)₃]· (H₂sbdc) · H₂O strongly depended on the relative humidity (RH), being 2.5 x 10^-7 S cm^-1 at 95% RH. The activation energy for proton conductivity was estimated to be 0.32 eV at 95% RH.

Full text of DOI:10.1246/cl.2010.28

doi:10.1246/cl.2010.28
28
Published on the web December 5, 2009
Design and Characterization of a Polarized Coordination Polymer of a Zinc(II)
Biphenyldicarboxylate Bearing a Sulfone Group
Katsuhiko Kanaizuka,1,2,³ Shoji Iwakiri,1 Teppei Yamada,*1 and Hiroshi Kitagawa*1,2,3
1Department of Chemistry, Faculty of Science, Kyushu University, 6-10-1 Hakozaki, Higashi-ku, Fukuoka 812-8581
2CREST, Japan Science and Technology Agency (JST), 5 Sanbancho, Chiyoda-ku, Tokyo 102-0075
3Department of Chemistry, Faculty of Science, Kyoto University, Sakyo-ku, Kyoto 606-8502
(Received September 30, 2009; CL-090885; E-mail: kitagawa@kuchem.kyoto-u.ac.jp, teppei@chem.kyushu-univ.jp)
A novel coordination polymer of zinc(II) biphenyldicarbox-
ylate bearing a sulfone group, [Zn(sbdc)(H2O)3]¢(H2sbdc)¢H2O
(H2sbdc: sulfone biphenyldicarboxylic acid), has been synthe-
sized. It is composed of a one-dimensional chain structure of
alternating Zn(II) cations and sbdc anions. This coordination
polymer possesses noncoordinating uncharged acid H2sbdc
existing between the chains with hydrogen-bonding networks.
The proton conductivity of [Zn(sbdc)(H2O)3]¢(H2sbdc)¢H2O
strongly depended on the relative humidity (RH), being
2.5 © 10¹7 S cm¹1 at 95% RH. The activation energy for proton
conductivity was estimated to be 0.32 eV at 95% RH.
Scheme 1. Schematic outline of the syntheses of H2sbdc ligand
and coordination polymer 1.
The design and synthesis of metal-organic frameworks
(MOFs) have been intensively studied over the past decade
because of their attractive applications in gas storage, molecular
adsorption, catalytic reactions, and molecular sensing.1-5 The
performance of the MOFs strongly depends on the size of the
cavity inside the frameworks. To date, many research groups
have focused on controlling the cavity size of the frameworks,
an effective way being to change the organic ligands and metal
ions.6-10 On the other hand, few examples have been reported for
the control of polarity of frameworks because of the difficulty of
synthesizing ligands bearing polarizing groups.11 By the design
and synthesis of a framework bearing polarizing groups, we can
control not only physical properties such as ionic conductivity
but also chemical properties such as catalytic activity.12-15
Biphenyldicarboxylic acid (H2bpdc) is well designed for
the construction of one-dimensional coordination polymers
and MOFs because H2bpdc has a simple linear structure. For
example, coordination polymers with oxalic acid and 2,5-
dihydroxybenzoquinone have been reported.16,17 We tried to
introduce electron-donating or electron-withdrawing moieties
onto the framework. The sulfone group is an excellent
substituent to introduce polarity to organic ligands because it
is expected to be a strong electron-withdrawing group. However,
the insertion reaction of sulfone to a MOF backbone has not
been reported. Here, we first report the syntheses of sulfone
biphenyldicarboxylic acid (H2sbdc) and the novel coordination
polymer [Zn(sbdc)(H2O)3]¢(H2sbdc)¢H2O (1) and discuss the
crystal structure and proton conductivity of 1.
Figure 1. Crystal structure of coordination polymer 1.
distilled water (30 mL) at 160 °C for 24 h in a Teflon-lined
autoclave by a hydrothermal method. After cooling to room
temperature, the mixture was washed with THF and water to
remove the unreacted ligand and metal ion. Colorless crystals
were obtained and used for the single-crystal X-ray diffraction
measurement, EA, thermogravimetry, and water uptake and
electrical conductivity measurements.
The H2sbdc ligand was synthesized as follows (Scheme 1).
Fuming sulfuric acid (5 mL) was added to H2bpdc (0.50 g,
2.1 mmol), and the mixture was reacted at 120 °C for 12 h in air.
After cooling to room temperature, the mixture was poured
onto ice (10 g). The yellow-white solid was filtered and dried
(90% yield). The H2sbdc was characterized by 1H NMR, IR, and
elemental analysis (EA).18 The MOF was made by mixing
H2sbdc (1 mmol) and zinc(II) sulfate hexahydrate (2 mmol) in
Figure 1 shows the crystal structure of 1, consisting of
one-dimensional chains of sbdc ligand and zinc ion.19 1 has
Chem. Lett. 2010, 39, 28-29
© 2010 The Chemical Society of Japan
www.csj.jp/journals/chem-lett/

29
A)
B)
References and Notes
³
1
2
3
4
5
6
7
Present address: Department of Applied Chemistry, Chuo Univer-
sity, 1-13-27 Kasuga, Bunkyo-ku, Tokyo 112-8551
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Figure 2. A) Plot of conductivity vs. relative humidity of
[Zn(sbdc)(H2O)3]¢(H2sbdc)¢H2O at 25 °C. B) Plot of conduc-
tivity vs. temperature of [Zn(sbdc)(H2O)3]¢(H2sbdc)¢H2O at
95% RH.
8
9
uncoordinating H2sbdc, and this H2sbdc is sandwiched by
chains. This unique structure is formed by the strong electron-
withdrawing nature of the sulfone group. Titration plots show
that the pKa of H2sbdc (6.59) is lower than that of H2bpdc
(8.25).20 The difference between the pKa values of the RCO2H
groups affects the coordination reaction between ligand and zinc
ion. Therefore, both coordinating and uncoordinating ligands
were included in the system of H2sbdc. It is also noted that this
crystal has two types of water molecules. Three water molecules
coordinate to the zinc ion, and the other is uncoordinating water
located between ligands constructing hydrogen-bonding net-
works into the MOF. Two carboxyl groups coordinate to the zinc
ion so that this zinc has five coordination bonds and adopts a
trigonal bipyramidal configuration.
H. Furukawa, J. Kim, N. W. Ockwig, M. O’Keeffe, O. M. Yaghi,
J. Am. Chem. Soc. 2008, 130, 11650.
10 G. Férey, Chem. Mater. 2001, 13, 3084.
11 Z. Wang, K. K. Tanabe, S. M. Cohen, Inorg. Chem. 2009, 48, 296.
12 M. Tadokoro, T. Inoue, S. Tamaki, K. Fujii, K. Isogai, H. Nakazawa,
S. Takeda, K. Isobe, N. Koga, A. Ichimura, K. Nakasuji, Angew.
Chem., Int. Ed. 2007, 46, 5938.
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2004, 104, 4637.
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131, 3144.
17 S. Morikawa, T. Yamada, H. Kitagawa, Chem. Lett. 2009, 38, 654.
18 1H NMR (DMSO): ¤H 8.35 (d, 2H), 8.38 (s, 2H), 8.43 (d, 2H), 13.8
(s, 2H). IR [KBr/cm¹1]: 3591, 3487, 2995, 2625, 2534, 1724, 1678,
1606, 1398, 1240, 1169, 1140, 1117, 1074, 908, 762, 717, 648, 588,
559, 532, 426. Elemental analysis calcd for C14H8O6S (304.3): C,
55.26; H, 2.65%. Found: C, 54.90, H, 2.72%.
From the presence of uncoordinating uncharged H2sbdc and
water molecules, it is expected that 1 would exhibit high proton
conduction. The proton conductivity of 1 was measured by ac
impedance.21 Figure 2A shows plots of the logarithm of con-
ductivity vs. relative humidity (RH). The conductivity strongly
depends on the humidity. The change in conductivity was
¹13
greater than six orders of magnitude, from 4.1 © 10
(30%
19 Crystal data were as follows: C28H22O16S2Zn, triclinic, space group
RH) to 2.5 © 10¹7 S cm (95% RH). As we noted above, one
water molecule in the crystal is located as water of crystal-
lization; therefore, the uncoordinating ligands probably acted
as proton sources. The activation energy was estimated to be
0.32 eV from the slope shown in Figure 2B. The possible
mechanism of the conductivity is proton hopping through the
hydrogen-bonding network in the crystal. The distance between
the water molecules coordinating to the Zn metal center and
hydroxy group of the uncoordinating ligand is 2.743 ¡, indicat-
ing the existence of hydrogen bonding.22,23
¹1
ꢀ
P1(no. 2), a = 6.7486 ¡, b = 13.902(3) ¡, c = 14.842(5) ¡, ¡ =
89.995(6)°, ¢ = 88.158(4)°, £ = 84.556(4)°, V = 1385.5(5) ¡3,
Z = 2, R1 = 0.0710, wR2 = 0.2135, and GOF = 1.133. Elemental
analysis Calcd for C28H14O12S2Zn¢4H2O (744.0): C, 45.20; H,
2.98%. Found: C, 45.30, H, 2.99%. Details of the crystal structure of
the compound are available free of charge via Cambridge Crystallo-
graphic Data Centre under deposition number CCDC-749400.
20 The values of pKa were estimated in a mixture of H2O/DMSO
because the solubility of these ligands in water was poor. The values
in a mixture of water and organic solvent are higher than in water.
21 For the electrical conductivity study, the powdered sample was
compressed to approximately 0.5 mm in thickness and 2.5 mm in
diameter. Both sides of the pellet were attached to gold wires with
gold paste. The conductivity measurement was carried out using a
Solartron 1260 impedance/gain-phase analyzer by a quasi-four-
probe method in the frequency range 10 MHz to 1 Hz. Proton
conductivity was calculated from the diameter of the semicircle.
Samples were placed in a temperature-humidity-controlled chamber
(SH221, ESPEC Corp.) during the measurement.
In summary, we have carried out the synthesis and
characterization of a novel coordination polymer 1. The
structure of this compound is a one-dimensional chain, and
the proton conductivity strongly depends on the humidity.
This conductivity is thought to originate from H2sbdc as a
proton source and from hydrogen bonding as a proton pathway.
Because the change in proton conductivity of 1 is greater than
six orders over a range of humidity, this material is suitable for
highly sensitive humidity sensors.
22 R. Makiura, T. Yonemura, T. Yamada, M. Yamauchi, R. Ikeda, H.
Kitagawa, K. Kato, M. Takata, Nat. Mater. 2009, 8, 476.
This work was partly supported by the Grants-in-Aid for the
Global COE Program, “Science for Future Molecular Systems.”
23 H. Ōkawa, A. Shigematsu, M. Sadakiyo, T. Miyagawa, K. Yoneda,
M. Ohba, H. Kitagawa, J. Am. Chem. Soc. 2009, 131, 13516.
Chem. Lett. 2010, 39, 28-29
© 2010 The Chemical Society of Japan
www.csj.jp/journals/chem-lett/