Introduction of cavities up to 4 nm into a hierarchically-assembled metal-organic framework using an angular, tetratopic ligand

Article abstract of DOI:10.1039/c0cc00779j

Cavities up to 4 nm have been introduced into a hierarchically-assembled metal-organic framework by adopting an angular, semi-flexible tetratopic ligand. The resulting MOF possesses permanent porosity and exhibits stepwise sorption isotherms for O₂ and N₂ gases.

Full text of DOI:10.1039/c0cc00779j

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Introduction of cavities up to 4 nm into a hierarchically-assembled
metal–organic framework using an angular, tetratopic ligandw
Wenjuan Zhuang,a Shengqian Ma,b Xi-Sen Wang,a Daqiang Yuan,a Jian-Rong Li,a
Dan Zhaoa and Hong-Cai Zhou*a
Received 5th April 2010, Accepted 28th May 2010
First published as an Advance Article on the web 23rd June 2010
DOI: 10.1039/c0cc00779j
Cavities up to 4 nm have been introduced into a hierarchically-
assembled metal–organic framework by adopting an angular,
semi-flexible tetratopic ligand. The resulting MOF possesses
permanent porosity and exhibits stepwise sorption isotherms for
O2 and N2 gases.
A solvothermal reaction between H4PMTB and Cu(NO3)2ꢀ
2.5H2O in dimethylacetamide (DMA) afforded blue cubic
crystals of PCN-21z (PCN represents porous coordination
network) with an empirical formula of Cu2(H2O)2(PMTB)ꢀxS
(S represents non-coordinated solvent molecules). Although
large crystals were harvested, attempts to collect diffraction
data on Bruker APEXII diffractometers were unsuccessful.
Single-crystal X-ray crystallographic study,y conducted
using synchrotron radiation at the Advanced Photon Source,
Argonne National Laboratory (ANL), revealed that PCN-21
crystallizes in the space group Im-3m with the unit cell
dimension of a E 67 A. In the structure of PCN-21, the
PMTB ligands connect with Cu2(COO)4(H2O)2 secondary
building units (SBUs, Fig. 1b) to form a complicated three-
dimensional framework containing giant cavities.
Porous metal–organic frameworks (MOFs)1 have experienced
rapid development in the last decade and have become an
exciting alternative to traditional porous materials such as
inorganic zeolites2 and activated carbon materials.3 MOFs
excel in surface areas4 and gas adsorption capacity.5 Recent
studies6,7 have revealed that porous MOFs containing cages
(or cavities if the interior is accessible by water molecules)
often demonstrate superior gas adsorption capacity. We have
been interested particularly in the construction of MOFs
containing nanoscopic cages for hydrogen and methane
storage applications.6 Our focus lies in the design of rigid,
multi-topic carboxylate ligands and the assembly of MOFs
containing nanoscopic cages or cavities using such ligands.6
Recent attempts to enlarge the sizes of the cavities via ligand
extension alone often led to framework collapse upon guest
removal.5e,6c,8 Most recently, we communicated the stabilization
of MOFs of high surface area through the introduction of
mesocavities with microwindows.6f To further expand the
synthetic toolbox for the introduction of even larger cavities
into porous MOFs, herein we propose a new strategy of using
an angular, tetratopic ligand in addition to ligand extension. It
is hoped that the angular ligands can wrap around the cavities
and sustain the integrity of the porous framework. In this
contribution, we report an angular, tetratopic ligand,
diphenylmethane-3,30,5,50-tetrakis(3,5-bisbenzoate) (PMTB;
Fig. 1a, with the synthesis details given in ESIw), and a stable,
hierarchically-assembled, porous MOF having cavities up to
4 nm constructed with this ligand. PMTB contains a methylene
group, which makes the ligand angular and ready to support
the cavities.6e,9 Furthermore, the angular ligand adds an extra
dimension to the planar, tetratopic ligands and will lead to
MOFs with hierachical complexity.10
Fig. 1 (a) The PMTB ligand as one of the secondary building units
(SBUs); (b) The Cu2(COO)4(H2O)2 paddlewheel SBU (C, black; O,
red; Cu, turquoise); (c) A tertiary building unit (TBU, green) formed
by 3 PMTB SBUs and 9 dicopper paddlewheel SBUs; (d) A quaternary
building unit (QBU, green) formed by 2 TBUs; (e) Structure of a QBU;
(f) Eight QBUs interlinking with each other through edge-sharing in a
unit cell, forming a quinary building unit (IBU) with a giant cavity
(blue) inside; (g) The giant cavity (blue, truncated octahedron) inside
an IBU; (h) The crystal structure and schematic representation of the
truncated octahedron, the yellow sphere represents the largest sphere
that can be fit inside the cavity; (i) The connectivity of the giant
cavities.
a Department of Chemistry, Texas A&M University, College Station,
Texas 77843, USA. E-mail: zhou@mail.chem.tamu.edu;
Fax: +1-979-845 1595; Tel: +1-979-845 4034
b Chemical Sciences and Engineering Division, Argonne National
Laboratory, 9700 S. Cass Avenue, Argonne, IL 60439, USA
w Electronic supplementary information (ESI) available: Synthesis
and general characterizations (PXRD, IR and TGA), crystallographic
data and additional structure figures. CCDC 735398. For ESI and
crystallographic data in CIF or other electronic format see DOI:
10.1039/c0cc00779j
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For the convenience of discussion, five levels (primary,
secondary, tertiary, quaternary, and quinary) of building units
will be introduced to describe the hierarchically assembled
PCN-21. The primary building units are always atoms. The
secondary building units are the PMTB ligands and the
dicopper paddlewheel clusters. Building units of higher levels
of complexity were also classified as supramolecular building
blocks (SBBs) by Zaworotko and coworkers.1f,11 Every three
PMTBs are connected with each other through nine dicopper
paddlewheel SBUs to form a pseudo-hexagonal tertiary building
unit (TBU, Fig. 1c), and two such TBUs are linked by PMTB
bridges to form a pseudo-hexagonal prism with D3d symmetry,
which can be designated as a quaternary building unit (QBU,
Fig. 1d, e and S1w). Eight such QBUs occupy the eight vertices
of a cube and interlink with each other through edge-sharing
to give a quinary building unit (IBU) (Fig. 1f) enclosing a giant
cavity at the center of the IBU. The cavity can be viewed as a
truncated octahedron (Fig. 1g) with an internal diameter of
40 A (excluding van der Waals radii) (Fig. 1h). There are six
square windows (Fig. 2a) and eight trigonal windows (Fig. 2b)
with sizes of 17.2 (atom to atom distance along an edge unless
otherwise designated) and 15.9 A respectively on the truncated
octahedron cavity. Each IBU connects with eight adjacent
ones through face sharing to form a three-dimensional
network (Fig. 1i).
Fig. 3 Two types of cavities in PCN-21: (a) viewed down [0 0 1];
(b) viewed down [1 1 1].
cavities after the formation of a single layer at low pressure.
Another possible explanation is the co-existence of pores of
different sizes and distinct properties. Derived from the N2
adsorption data, PCN-21 has a BET surface area of 2718 m2 gꢂ1
and a total pore volume of 1.54 cm3 gꢂ1. At saturation, the
N2 adsorption capacity of PCN-21 is as high as 1000 cm3 gꢂ1
with an impressive Langmuir surface area of 4485 m2 gꢂ1
,
comparable to those of other high surface area MOFs.4c
Low-pressure H2 sorption isotherms at 77 K were collected
as well to evaluate its H2 adsorption performance. At 77 K and
760 Torr (Fig. 4b), PCN-21 can adsorb 1.6 wt% H2 gas,
consistent with previously reported results.5c We have also
tried to activate a sample using the critical point drying
method,4d but the resulting N2 sorption capacity is not as
high as that of the sample activated using the common
activation procedure (see Fig. S5 in ESIw).
Meanwhile, cavities with internal diameters of 24 A (purple
sphere in Fig. S2,w excluding van der Waals radii) between the
truncated octahedral cavities in the framework can also be
observed (Fig. 3 and S3w). In the framework, there exist square
and hexagonal open channels viewed down [1 0 0] and [1 1 1],
respectively (Fig. S4w). The cavities are filled with crystallo-
graphically disordered solvent molecules. Calculated using
PLATON,12 PCN-21 has a solvent accessible volume of
79.3% in the dehydrated structure (after removal of the
coordinated aqua ligands).
Furthermore, an O2 sorption study of PCN-21 was also
carried out at 77 K. Similar to that of N2 at 77 K, the O2
adsorption of PCN-21 also exhibits a stepwise isotherm
(Fig. 5), which can be attributed to the same reasons discussed
above. Compared with N2 isotherms, a clear hysteresis at
p/po E 0.1 and p/po E 0.3 stands out in O2 isotherms, which
can be tentatively ascribed to different affinity of these two
gases to the MOF. The gas sorption studies also indicate that
guest-free PCN-21 can at least partially sustain its framework
integrity, which was further confirmed by X-ray powder
diffraction studies (Fig. S6w).
The remarkably complicated porous structure of PCN-21
with unusually large cavities (Fig. 3) prompted us to examine
its gas uptake behavior. A freshly prepared sample of PCN-21
was activated according to
a
literature procedure.6 In
previously reported work, MOFs with both micro- and
mesopores often exhibit reversible pseudo-type-I N2 isotherms
with small steps before the plateau appears.4b,6f,7a,f,11b,13 As
shown in Fig. 4a, PCN-21 exhibits a similar reversible,
stepwise nitrogen-sorption isotherm at 77 K. One of the likely
reasons is the occurrence of multilayer sorption in the large
Fig. 2 The square windows and trigonal windows around the
truncated octahedron cavity in PCN-21: (a) viewed along c-axis and
(b) viewed along the [1 1 1] direction.
Fig. 4 Gas sorption isotherms at 77 K: (a) N2 gas; (b) H2 gas (solid
symbols, adsorption; open symbols, desorption).
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38, 1294; (f) J. J. Perry IV, J. A. Perman and M. J. Zaworotko,
Chem. Soc. Rev., 2009, 38, 1400; (g) J.-R. Li, R. J. Kuppler and
H.-C. Zhou, Chem. Soc. Rev., 2009, 38, 1477.
2 (a) N. Texier-Mandoki, J. Dentzer, T. Piquero, S. Saadallah,
P. David and C. Vix-Guterl, Carbon, 2004, 42, 2744;
(b) Y. Gogotsi, R. K. Dash, G. Yushin, T. Yildirim, G. Laudisio
and J. E. Fischer, J. Am. Chem. Soc., 2005, 127, 16006.
3 (a) L. Zhou, Y. Zhou and Y. Sun, Int. J. Hydrogen Energy, 2004,
29, 475; (b) J. M. Blackman, J. W. Patrick and C. E. Snape,
Carbon, 2006, 44, 918.
4 (a) K. Koh, A. G. Wong-Foy and A. J. Matzger, J. Am. Chem.
Soc., 2009, 131, 4184; (b) G. Ferey, C. Mellot-Draznieks, C. Serre,
´
F. Millange, J. Dutour, S. Surble and I. Margiolaki, Science, 2005,
309, 2040; (c) H. K. Chae, D. Y. Siberio-Perez, J. Kim, Y. Go,
M. Eddaoudi, A. J. Matzger, M. O’Keeffe and O. M. Yaghi,
Nature, 2004, 427, 523; (d) A. P. Nelson, O. K. Farha,
K. L. Mulfort and J. T. Hupp, J. Am. Chem. Soc., 2009, 131, 458.
5 (a) A. G. Wong-Foy, A. J. Matzger and O. M. Yaghi, J. Am.
Chem. Soc., 2006, 128, 3494; (b) M. Dinca, A. Dailly, Y. Liu,
C. M. Brown, D. A. Neumann and J. R. Long, J. Am. Chem. Soc.,
2006, 128, 16876; (c) S.-i. Noro, S. Kitagawa, M. Kondo and
K. Seki, Angew. Chem., Int. Ed., 2000, 39, 2081; (d) H. Wang,
J. Getzschmann, I. Senkovska and S. Kaskel, Microporous
Mesoporous Mater., 2008, 116, 653; (e) X. Lin, I. Telepeni,
A. J. Blake, A. Dailly, C. M. Brown, J. M. Simmons, M. Zoppi,
G. S. Walker, K. M. Thomas, T. J. Mays, P. Hubberstey,
Fig. 5 O2 gas sorption isotherms at 77 K (P0 = 145 torr).
In summary, a porous MOF, with cavities up to 4 nm and
surface area as high as 4485 m2 gꢂ1, was constructed using an
angular, tetratopic ligand. It is evident that the addition to the
geometric complexity of a bridging ligand leads to a hierarchically
assembled MOF. It has also been discovered that the bent
feature of an angular ligand helps to wrap around a cavity and
stabilizes the resulting framework. PCN-21 exhibits a rare
stepwise, reversible N2 sorption isotherm and excellent gas
adsorption capacity. Further examination of its high-pressure
gas adsorption properties is currently underway.
N. R. Champness and M. Schroder, J. Am. Chem. Soc., 2009,
¨
131, 2159; (f) K. Li, D. H. Olson, J. Y. Lee, W. Bi, K. Wu, T. Yuen
and Q. Xu, and J. Li, Adv. Funct. Mater., 2008, 18, 2205.
6 (a) S. Ma and H.-C. Zhou, J. Am. Chem. Soc., 2006, 128, 11734;
(b) D. Sun, S. Ma, Y. Ke, D. J. Collins and H.-C. Zhou, J. Am.
Chem. Soc., 2006, 128, 3896; (c) S. Ma, D. Sun, M. Ambrogio,
J. A. Fillinger, S. Parkin and H.-C. Zhou, J. Am. Chem. Soc., 2007,
129, 1858; (d) S. Ma, D. Sun, J. M. Simmons, C. D. Collier,
D. Yuan and H.-C. Zhou, J. Am. Chem. Soc., 2008, 130, 1012;
(e) X.-S. Wang, S. Ma, P. M. Forster, D. Yuan, J. Eckert,
This work was supported by the U.S. Department of Energy
(DE-FC36-07GO17033) and the U.S. National Science
Foundation (CHE-0449634). S. Ma acknowledges the
Director’s Postdoctoral Fellowship from Argonne National
Laboratory. The microcrystal diffraction of PCN-21 was
carried out at the Advanced Photon Source on beamline
15ID-B of ChemMatCARS Sector 15, which is principally
supported by the National Science Foundation/Department of
Energy under grant number CHE-0535644. Use of the Ad-
vanced Photon Source was supported by the U.S. Department
of Energy, Office of Science, Office of Basic Energy Sciences,
under Contract No. DE-AC02-06CH11357. We thank Dr
Yu-Sheng Chen for assistance in single crystal X-ray experiments,
Dr Theodore F. Baumann for assistance in supercritical CO2
activation, and Dr Qian-Rong Fang for discussion.
J. J. Lopez, B. J. Murphy, J. B. Parise and H.-C. Zhou, Angew.
´
Chem., Int. Ed., 2008, 47, 7263; (f) D. Zhao, D. Yuan, D. Sun and
H.-C. Zhou, J. Am. Chem. Soc., 2009, 131, 9186.
7 (a) Y. Yan, X. Lin, S. Yang, A. J. Blake, A. Dailly,
N. R. Champness, P. Hubberstey and M. Schroder, Chem.
¨
Commun., 2009, 1025; (b) Y.-B. Zhang, W.-X. Zhang,
F.-Y. Feng, J.-P. Zhang and X.-M. Chen, Angew. Chem., Int.
Ed., 2009, 48, 5287; (c) J. Zhang, T. Wu, S. Chen, P. Feng and
X. Bu, Angew. Chem., Int. Ed., 2009, 48, 3486; (d) B. Chen,
N. W. Ockwig, A. R. Millward, D. S. Contreras and
O. M. Yaghi, Angew. Chem., Int. Ed., 2005, 44, 4745; (e) T. Wu,
J. Zhang, C. Zhou, L. Wang, X. Bu and P. Feng, J. Am. Chem.
Soc., 2009, 131, 6111; (f) H. Chun, J. Am. Chem. Soc., 2008, 130,
800.
8 (a) L. Ma, J. Y. Lee, J. Li and W. Lin, Inorg. Chem., 2008, 47, 3599;
(b) S. Wu, L. Ma, L.-S. Long, L.-S. Zheng and W. Lin, Inorg.
Chem., 2009, 48, 2436.
9 (a) L. Ma and W. Lin, J. Am. Chem. Soc., 2008, 130, 13834;
(b) L. Ma and W. Lin, Angew. Chem., Int. Ed., 2009, 48, 3637.
´
10 A. C. Sudik, A. P. Cote, A. G. Wong-Foy, M. O’Keeffe and
O. M. Yaghi, Angew. Chem., Int. Ed., 2006, 45, 2528.
11 (a) A. J. Cairns, J. A. Perman, L. Wojtas, V. C. Kravtsov,
M. H. Alkordi, M. Eddaoudi and M. J. Zaworotko, J. Am. Chem.
Soc., 2008, 130, 1560; (b) F. Nouar, J. F. Eubank, T. Bousquet,
L. Wojtas, M. J. Zaworotko and M. Eddaoudi, J. Am. Chem. Soc.,
2008, 130, 1833.
Notes and references
z Preparation of PCN-21, Cu2(H2O)2(PMTB)ꢀxS.
A mixture of
H4PMTB (0.005 g, 7.7 ꢃ 10ꢂ6 mol), Cu(NO3)2ꢀ2.5H2O (0.015 g,
6.5 ꢃ 10ꢂ5 mol), and 4 drops of HBF4 (48% w/w aq. solution) in
1.5 mL of dimethylacetamide (DMA) was sealed in a Pyrex tube under
vacuum and heated to 85 1C, kept at that temperature for 72 h, and
cooled to room temperature. The resulting blue cubic crystals were
washed with DMA and collected, the yield: 60% based on H4PMTB.
y X-ray crystal data for PCN-21: C41H28Cu2O10, Mr = 807.71, Cubic,
Space group: Im-3m, a = 67.283(10) A, V = 304590(78) A3, synchrotron
12 A. L. Spek, J. Appl. Crystallogr., 2003, 36, 7.
13 (a) A. Sonnauer, F. Hoffmann, M. Froba, L. Kienle, V. Duppel,
¨
M. Thommes, C. Serre, G. Ferey and N. Stock, Angew. Chem., Int.
´
radiation, l = 0.41328 A, Z = 96, T = 173 K, Dc = 0.423 g cmꢂ3
,
R1 (I > 2s(I)) = 0.0487, wR2 = 0.1244, GOF = 1.006. CCDC
735398.
Ed., 2009, 48, 3791; (b) Y. K. Park, S. B. Choi, H. Kim, K. Kim,
B.-H. Won, K. Choi, J.-S. Choi, W.-S. Ahn, N. Won, S. Kim,
D. H. Jung, S.-H. Choi, G.-H. Kim, S.-S. Cha, Y. H. Jhon,
J. K. Yang and J. Kim, Angew. Chem., Int. Ed., 2007, 46, 8230;
(c) Q.-R. Fang, G.-S. Zhu, Z. Jin, Y.-Y. Ji, J.-W. Ye, M. Xue,
H. Yang, Y. Wang and S.-L. Qiu, Angew. Chem., Int. Ed., 2007, 46,
6638; (d) K. Koh, A. G. Wong-Foy and A. J. Matzger, Angew.
Chem., Int. Ed., 2008, 47, 677.
1 (a) O. M. Yaghi, M. O’Keeffe, N. W. Ockwig, H. K. Chae,
M. Eddaoudi and J. Kim, Nature, 2003, 423, 705; (b) G. Ferey,
´
Chem. Soc. Rev., 2008, 37, 191; (c) S. Kitagawa, R. Kitaura and
S.-i. Noro, Angew. Chem., Int. Ed., 2004, 43, 2334; (d) M. P. Suh,
Y. E. Cheon and E. Y. Lee, Coord. Chem. Rev., 2008, 252, 1007;
(e) L. J. Murray, M. Dinca and J. R. Long, Chem. Soc. Rev., 2009,
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Chem. Commun., 2010, 46, 5223–5225 | 5225