Guest-specific function of a flexible undulating channel in a 7,7,8,8-tetracyano-p-quinodimethane dimer-based porous coordination polymer

Article abstract of DOI:10.1021/ja073505z

We have designed and synthesized a flexible porous coordination polymer with highly selective capacity to separate benzene from cyclohexane. The undulating channel providing the suitable space for the benzene and the H-π interaction between the host framework and the guest molecules result in this high selectivity. Copyright

Full text of DOI:10.1021/ja073505z

Published on Web 08/17/2007
Guest-Specific Function of a Flexible Undulating Channel in a
7,7,8,8-Tetracyano-p-quinodimethane Dimer-Based Porous Coordination
Polymer
Satoru Shimomura,† Satoshi Horike,† Ryotaro Matsuda,‡ and Susumu Kitagawa*,†
Department of Synthetic Chemistry and Biological Chemistry, Kyoto UniVersity, Katsura, Nishikyo-ku,
Kyoto 615-8510, Japan
Received May 16, 2007; E-mail: kitagawa@sbchem.kyoto-u.ac.jp
In the field of porous coordination polymers (PCPs), separation
is one of the most significant topics. PCPs have regularly ordered
nanometer-size frameworks with readily modifiable pore surface
properties and structural flexibility, which give them potential
applications in separation,1,2 storage,3 and catalysis.4 The highly
selective separation of hydrocarbons having similar physical
properties, such as boiling point and molecular size, remains a
significant challenge that has much scientific and commercial value.
Because cyclohexane is usually produced by hydrogenation of
benzene, in the benzene/cyclohexane-miscible system used in the
petrochemical industry, separating their mixture is essential.5
However, the separation is difficult because these substances have
similar boiling points (benzene, 80.1 °C and cyclohexane, 80.7 °C),
molecular geometry, and Lennard-Jones collision diameters.6 To
face this chemical similarity, we need to design the high recognition
capability system, for instance, in a biological system, which
requires the specific binding sites such as cation-π, H-π, and
H-bonding-type interaction and the flexible structure to create
suitable space for the target molecules.7,8 7,7,8,8-Tetracyano-p-
quinodimethane (TCNQ) is a well-known molecule with a large π
surface and redox activity. A PCP whose pore surface is formed
by TCNQ will have a H-π-type interaction with benzene and
achieve selective separation of benzene from cyclohexane with a
suitable pore shape for benzene generated from the specific
flexibility of the PCP.
Figure 1. (a) Coordination environment of Zn(II) ion of 1⊃benzene. (b)
TCNQ dimer (green) connected to four 1D chains of Zn and bpy (gray).
(c) Benzene arranged in the cage of the undulating channel of 1⊃benzene.
The hydrogen atoms are omitted for clarity.
Green crystals of {[Zn(µ4-TCNQ-TCNQ)bpy]‚1.5benzene}n
(1⊃benzene) were synthesized by reacting Zn(NO3)2‚6H2O with
LiTCNQ and bpy (4,4′-bipyridine) in a MeOH/benzene mixture
(1:1). The Zn ions are octahedrally coordinated to the four cyanide
nitrogen atoms of TCNQ in the equatorial plane and the two
nitrogen atoms of the bpy at the axial sites (Figure 1a). The Zn
ions are linked by bpy to give a one-dimensional (1D) chain in the
direction of the a-axis. [TCNQ-TCNQ]2- acts as cross-linker
connecting the four 1D chains of Zn ions and bpy to form a 3D
open framework (Figure 1b). [TCNQ-TCNQ]2- is derived from a
σ dimerization of two TCNQ- anions. There are few reports about
coordination polymers containing the TCNQ dimer, despite the
scarcity, unique shape, and various coordination modes of the dimer
making the structures interesting.9 The dianionic ligand is a
nonplanar, staircase motif because of the sp3 C-C σ bond
connecting the two TCNQs, and the two electrons are separated
and delocalized in each planar part of the TCNQ. The channels
delimited by the ligands are of the undulating form, not a straight
but a unique form comprising an alternating arrangement of two
types of tubes of large and small diameter. The large diameter space
is adequate to accommodate a benzene molecule because of the
suitability of the cavity’s size and thickness (Figure 1c). The guest
benzene molecule is accommodated strongly in the cavity with the
size effect and the H-π interaction with the host framework. The
distance between the pore wall perpendicular to the benzene
molecule, which comprises the electron-rich π surface of the TCNQ
dimer and the center of the benzene molecule, is 5.05 Å. This is
consistent with the existence of the H-π interaction between the
host framework and the guest molecule (Figure 1c).8
Interestingly, although the benzene molecule can be removed
from and adsorbed to this compound, the aperture of the small tube
is not large enough for the benzene molecule to pass through it,
indicating that the framework is flexible. The X-ray powder
diffraction (XRPD) patterns also show the structural flexibility of
this compound, which is classified as a third-generation com-
pound.10 In the guest-removal process, the crystal is transformed
to a guest-free crystal phase 2 (Figure 2). Alternatively, exposing
2 to benzene vapor for 5 h produced the other crystal phase,
3⊃benzene, whose XRPD pattern quite similar to that of 1⊃ben-
zene. The flexibility may originate in the freedom in the coordina-
tion mode of TCNQ.
† Kyoto University.
The guests of 1⊃benzene were removed at 413 K for 10 h under
‡ Current address: Institute for Materials Chemistry and Engineering, Kyushu
University, Kasuga-koen 6-1, Kasuga-city, Fukuoka 816-8580, Japan.
low pressure, and the sorption isotherms of benzene and cyclo-
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J. AM. CHEM. SOC. 2007, 129, 10990-10991
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C O M M U N I C A T I O N S
flexibility to adapt to the target molecular shape and, with the
interaction site located in the right position of the pore surface,
makes PCPs an effective separation system.
Acknowledgment. This work was supported by Grant-In-Aid
for Science Research in a Priority Area “Chemistry of Coordination
Space” (Grant No. 464) and a CREST/JST program from the
Ministry of Education, Science, Sports and Culture, Japan. We thank
Dr. Kazuyuki Nakai for helpful discussions.
Supporting Information Available: Experimental methods includ-
ing synthesis, crystallographic data, NMR spectra, XRPD patterns, and
TGA for 1. This material is available free of charge via the Internet at
http://pubs.acs.org.
Figure 2. Sorption isotherm for each adsorbate in 2 at 298 K. (a) Benzene
adsorption (filled circles) and desorption (open circles). (b) Cyclohexane
adsorption (filled squares) and desorption (open squares).
hexane of 2 at 298 K were measured. The profiles differed
obviously between each isotherm. In the case of benzene, a flat
curve means no adsorption in the lower relative pressure region,
although an abrupt increase in adsorption and decrease in relative
pressure was observed in the process from point A to point B (see
the Supporting Information). This is similar to the gate-open type
adsorption behavior,2,11 which is related to the structural transforma-
tion of the host framework. The crystal initially has no gates large
enough to receive benzene molecules in its cavities until a certain
number of benzene molecules accumulate on the surface. Once the
threshold concentration, the so-called gate-opening pressure, is
achieved, the crystal begins to accommodate the guests, and the
sorption accelerates cooperatively, accompanied by the structural
transformation. The driving force to open the gates could be the
presence of a higher affinity interaction of the pore surface and
guest molecules. In contrast, no adsorption of cyclohexane occurs
even in the higher relative pressure region, indicating the gates
remain closed for cyclohexane.
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In conclusion, we have designed and synthesized a PCP with
highly selective capacity to separate benzene from cyclohexane.
Synergy of the undulating channel formed gives the PCP structural
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