Distinct interpenetrated metal-organic frameworks constructed from crown ether-based strut analogue

Article abstract of DOI:10.1039/c2ce26401c

Rigid struts containing crown ether units (benzo-12-crown-4 and benzo-15-crown-5) coordinate with Zn₄O(CO₂)₆ clusters, affording two different interpenetrated metal-organic frameworks (MOFs), i.e., benzo-12-crown-4-based MOF presents a "close contact" interpenetration mode, while the benzo-15-crown-5-based one has a "body center" interpenetration mode.

Full text of DOI:10.1039/c2ce26401c

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Distinct interpenetrated metal–organic frameworks
constructed from crown ether-based strut analogue3
Cite this: CrystEngComm, 2013, 15, 841
Lei Liu,a Xiaojun Wang,a Quan Zhang,a Qiaowei Li*b and Yanli Zhao*ac
Received 30th August 2012,
Accepted 19th November 2012
DOI: 10.1039/c2ce26401c
www.rsc.org/crystengcomm
Rigid struts containing crown ether units (benzo-12-crown-4 and
benzo-15-crown-5) coordinate with Zn4O(CO2)6 clusters, afford-
ing two different interpenetrated metal–organic frameworks
led to a new type of porous domain—the active domain—wherein
an ordered distribution of guests within MOFs is maintained by
highly specific supramolecular recognition in addition to the size
and shape selectivity provided by the MOF itself.6 It is well known
that benzo-12-crown-4 and benzo-15-crown-5 have relatively small
cavities, which are suitable for forming stable complexes2n with Li+
(k = 1.05 6 103 M21 in dimethyl sulfoxide (DMSO)) and Na+ (k =
7.41 6 102 M21 in DMSO), respectively, for a wide range of
applications. In order to further explore the emerging research
area, simple rigid struts consisting of such crown ethers were to be
used for the preparation of MOFs.
(MOFs), i.e., benzo-12-crown-4-based MOF presents
a ‘‘close
contact’’ interpenetration mode, while the benzo-15-crown-5-
based one has a ‘‘body center’’ interpenetration mode.
Since the first report on cyclic polyethers by Charles Pedersen in
1967,1 these macrocyclic molecules and their analogs have
attracted increasing attentions on account of their abilities to
serve as host platforms for binding with organic and inorganic
guests, making them widely useful in various areas such as
supramolecular chemistry, biological science, materials science,
and nanoscience.2 As such, the design and synthesis of novel
crown ether derivatives with superior properties and proper
applications continue to be an important research area.
In the wake of the emergence and development of metal–
organic frameworks (MOFs),3 there is considerable interest in
incorporating crown ether derivatives into porous metal–organic
frameworks for potential applications. Some research groups have
fabricated4 such types of MOFs using struts consisting of the
crown ether units or crown ether based mechanically interlocked
molecules such as rotaxanes and catenanes. For example, Valente
et al. recently described the synthesis of a linear organic strut
possessing a bisparaphenylene[34]crown-10 recognition site and
its subsequent incorporation into MOFs.5 Such novel types of
struts still present the intrinsic properties of the crown ether units,
allowing the incorporation of specific features and functions into
MOFs. The introduction of the crown ether units into MOFs has
Herein, we report the design and synthesis of the rigid struts
(BC-4 and BC-5) containing benzo-12-crown-4 and benzo-15-
crown-5 units and their incorporation into MOFs (MOF–BC-4
and MOF–BC-5), respectively. The synthesis of the struts BC-4 and
BC-5 is outlined in Scheme 1. The intermediate 5 was prepared
from commercially available 1,2-dihydroxybenzene in five steps
(see the ESI ) according to a modified procedure.7 Cyclization of 5
3
with either 1,2-bis(2-chloroethoxy)ethane or bis[2-(2-chloroethox-
y)ethyl]ether afforded the macrocyclic intermediate 6a or 6b in a
moderate yield on account of the template-assisted ring closure
approach. Chemoselective Suzuki–Miyaura cross coupling of 6a or
6b with 4-(methoxycarbonyl)phenylboronic acid pinacol ester,
followed by saponification, led to the diacid BC-4 or BC-5.{
Direct evidence for the formation of the struts BC-4 and BC-5
was obtained in the single crystal forms by slow evaporation of BC-
4 and BC-5 solutions in N,N-dimethylformamide (DMF) and
DMSO, respectively. The crystal data and experimental and
refinement parameters for the two crystals are listed in Tables
S1 and S2 (see ESI ). 1H NMR, UV-Vis and mass spectra (MS) were
3
aDivision of Chemistry and Biological Chemistry, School of Physical and
Mathematical Sciences, Nanyang Technological University, 21 Nanyang Link,
Singapore 637371, Singapore. E-mail: zhaoyanli@ntu.edu.sg
bShanghai Key Laboratory of Molecular Catalysis and Innovative Material,
Department of Chemistry, Fudan University, 220 Handan Road, Shanghai, China
200433. E-mail: qwli@fudan.edu.cn
recorded (Fig. S10–S15 in the ESI ) for investigating the complexa-
3
tion between BC-4 and LiCl and between BC-5 and NaCl at
different ratios. The downfield shifts, approximately 0.49 for BC-4
with Li+ and 0.46 ppm for BC-5 with Na+, of H resonances
corresponding to the protons in crown ether rings are indicative of
the inclusion complex formation. In addition, absorption and MS
studies clearly demonstrate the complexation of BC-4 with Li+ and
cSchool of Materials Science and Engineering, Nanyang Technological University, 50
Nanyang Avenue, Singapore 639798, Singapore
Electronic supplementary information (ESI) available: Synthesis and characteriza-
3
BC-5 with Na+. As shown in Fig. S12 and S13 (ESI ), the absorption
3
tion data, and CCDC 883383–883386. For ESI and crystallographic data in CIF or
other electronic format see DOI: 10.1039/c2ce26401c
intensities of BC-4 and BC-5 in DMF decrease upon the addition of
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Fig. 1 Crystal superstructures of (a) MOF–BC-4 and (b) MOF–BC-5. Hydrogen
atoms and solvent molecules are omitted for the sake of clarity. Crown ether
units with polyether chains and hydroquinone rings are colored dark blue/
brown, and the metal joints are colored pale blue. The space filling
representations for the crystal superstructures of (c) MOF–BC-4 and (d) MOF–BC-
5. Atoms in the metal joints and the rigid backbones of the struts were located
by single X-ray diffraction, and the crown ether units in both structures were
modeled by Materials Studio.
Scheme 1 Synthetic route leading to the production of dicarboxylic acid-
terminated struts BC-4 and BC-5, and their X-ray single crystal structures. C, grey;
O, red. Hydrogen atoms are omitted for clarity.
space group for MOF–BC-5, both in pcu topology.10 The crown
ethers were found to be highly disordered, thus cannot be located
precisely. Nonetheless, the positions of all the atoms in the
inorganic SBUs and the rigid backbone of the struts are
unambiguous.
Li+ and Na+ in the range of 250–330 nm, leading to high binding
constants of 1.67 6 104 and 9.94 6 105
M
21, respectively.
Moreover, the absorption bands of the crown ether units around
280 nm are slightly shifted after the coordination with the metal
cations. These spectral changes indicate the complex formation of
BC-4 with Li+ and BC-5 with Na+, according to previous reports.8
The ESI/MS signals at 471.23 and 531.17 (m/z) corresponding to
[BC-4?Li]+ and [BC-5?Na]+ were detected for the samples of BC-4
with 1.0 equiv. of Li+ and BC-5 with 1.0 equiv. of Na+, respectively,
further supporting the complex formation.
The side length of the cube defined by eight SBUs and twelve
struts is 21.5 Å for both MOF–BC-4 and MOF–BC-5. It is not
surprising that the slender nature of the struts brings interpene-
tration to the frameworks. Similar to IRMOF-15, both SBU and BC-
5 in MOF–BC-5 were disordered over two sites with an equal
probability. The SBUs from the second framework (Fig. 2) occupy
the body center of the first framework cube in MOF–BC-5. In
MOF–BC-4, however, the SBUs in the second framework are away
from the body center of the first framework by 6.5 Å in the c
direction (Fig. 2). Analysis by TOPOS11 indicates that the
interpenetration of MOF–BC-4 belongs to class IIa, while the
interpenetration of MOF–BC-5 belongs to class Ia (the two
interpenetrated networks are generated by translation). A closer
examination of MOF–BC-4 reveals that the closest distance
between two struts from two frameworks is 3.8 Å, which suggests
a p–p stacking interaction between two central phenyl rings of the
two struts along the c direction perpendicular to each other
(Fig. 2). Two types of the strut conformations were observed in
MOF–BC-4
[Zn4O(C26H22O8)3]
and
MOF–BC-5
[Zn4O(C28H26O9)3] were successfully prepared using a solvothermal
approach. Either BC-4 or BC-5 in DMF in the presence of
Zn(NO3)2?6H2O was heated up in a programmable oven with a
temperature increase step of 10 uC h21 to 70 uC, and the reaction
mixture was then kept at this temperature for 48 h. Colorless
crystals of MOF–BC-4 and MOF–BC-5 were analyzed by X-ray
crystallography at low temperature, revealing the superstructures
(Fig. 1) isoreticular with IRMOF-15, a MOF prepared from the
triphenyl rigid strut without the crown ether unit.9 Rigid struts
emanate from each secondary building unit (SBU)—Zn4O(CO2)6
cluster—in an octahedral array to form a framework with the P42/
¯
ncm space group for MOF–BC-4 and a framework with the Im3m
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adsorption isotherms measured at 77 K. Both activated MOF–BC-4
and MOF–BC-5 present relatively low metal ion sorption capacity
towards Li+ (0.08 wt%) and Na+ (0.06 wt%), respectively, when
immersing the MOF crystals into the corresponding metal ion
solution (1 mM) in DMF/H2O (v/v = 100 : 1) overnight at room
temperature followed by the ICP-MS (inductively coupled plasma
mass spectrometry) analysis. Powder X-ray diffraction (PXRD)
measurements showed that bulky crystalline samples of as-
synthesized MOF–BC-4 and MOF–BC-5 have good phase purities
(Fig. S19 in the ESI ). PXRD of the samples after activation shows
3
lower quality of patterns with broader peak width, even though the
main diffraction peaks remain (Fig. S20 in the ESI ).
3
Thermal gravimetric analysis was performed (Fig. S23 and S24
in the ESI ) in order to evaluate the thermal stabilities of MOF–BC-
3
4 and MOF–BC-5. The percentage weight-loss patterns of both as-
synthesized MOF–BC-4 and MOF–BC-5 reveal nearly identical
decomposition temperatures at approximately 350 uC. The
stepwise weight-loss patterns between 350–550 uC for MOF–BC-4
and MOF–BC-5 are attributed to the subsequent decomposition of
the organic struts, since BC-4 and BC-5 show similar decomposi-
tion profiles.
In summary, we have successfully incorporated the struts
containing benzo-12-crown-4 and benzo-15-crown-5 units into the
three-dimensional frameworks, MOF–BC-4 and MOF–BC-5,
respectively, through the coordination with Zn(NO3)2?6H2O. The
single crystal superstructures reveal that both MOF–BC-4 and
MOF–BC-5 adopt doubly interpenetrated frameworks, but with
significantly different interpenetration modes. Their supramole-
cular recognitions towards alkali metal ions were further
investigated. The expected binding ability in the solid–solution
interface was hindered on account of framework collapse after
vacuum activation, together with increased rigidity of the crown
ethers after their incorporations into the interpenetrated MOFs
with restrained pore environments. The present work sheds light
on the introduction of macrocyclic molecules into three-dimen-
sional frameworks for the construction of novel functional
materials.
Fig. 2 Representation of different interpenetration modes for (a) MOF–BC-4
and (b) MOF–BC-5. (c) The p–p stacking interaction between two central phenyl
rings of the two struts (in blue and pink) in MOF–BC-4. Struts with coplanar
carboxylates are colored green, and struts with perpendicular carboxylates are
colored grey.
MOF–BC-4. For each SBU, there are two struts with coplanar
carboxylates at the ends along the c axis, and the other four struts
connect neighboring SBUs with carboxylates perpendicular to each
other (in which crown ether units could be fully located by single
Acknowledgements
This work was supported by the Singapore National Research
Foundation Fellowship (NRF2009NRF-RF001-015), Singapore
National Research Foundation CREATE program – Singapore
Peking University Research Centre for a Sustainable Low-
Carbon Future, and National Natural Science Foundation of
China (21101030). We thank Dr. Yongxin Li and Dr. Rakesh
Ganguly at Nanyang Technological University, and Prof.
Linhong Weng at Fudan University for their assistance in
X-ray crystallography.
crystal X-ray diffraction, see Fig. S17 in the ESI ). The resulting
3
reduction in symmetry, along with the p–p stacking interaction,
makes the doubly interpenetrated ‘‘close-contact’’ fashion favor-
able for MOF–BC-4. Compared to MOF–BC-5, the smaller crown
ether ring size in MOF–BC-4 enables the crown ether units to be
less bulky and flexible, thus the struts are easier to accommodate
in the confined geometry and conformation.
The calculated densities of MOF–BC-4 and MOF–BC-5 are 0.60
and 0.56 g cm23, respectively, and they are in the same range with
other highly porous MOFs, such as MOF-5 (0.59 g cm23).12 In
comparison, the densities of BC-4 and BC-5 crystals obtained are
1.32 and 1.35 g cm23 (including crystallized solvent molecules).
After the activation of the MOFs through solvent exchange method
from DMF to EtOH (or to CHCl3) followed by vacuum drying at
room temperature, low surface areas (103.8 m2 g21 for MOF–BC-4
and 80.1 m2 g21 for MOF–BC-5) were determined from the N2
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