Crystal structure and guest uptake of a mesoporous metal-organic framework containing cages of 3.9 and 4.7 nm in diameter

Article abstract of DOI:10.1002/anie.200702324

(Figure Presented) Supersized pores: A new mesoporous metal-organic framework that is mainly composed of Tb³⁺ ions and tripodal carboxylate ligands has cages of 3.9 and 4.7 nm in diameter (see picture). The evacuated framework is robust and can

Full text of DOI:10.1002/anie.200702324

Communications
DOI: 10.1002/anie.200702324
Metal–Organic Frameworks
Crystal Structure and Guest Uptake of a Mesoporous Metal–Organic
Framework Containing Cages of 3.9 and 4.7nm in Diameter**
Young Kwan Park, Sang Beom Choi, Hyunuk Kim, Kimoon Kim, Byoung-Ho Won,
Kihang Choi, Jung-Sik Choi, Wha-Seung Ahn, Nayoun Won, Sungjee Kim, Dong Hyun Jung,
Seung-Hoon Choi, Ghyung-Hwa Kim, Sun-Shin Cha, Young Ho Jhon, Jin Kuk Yang, and
Jaheon Kim*
Porous crystalline solids with a regular array of well-defined
pores have proved to be very useful in gas storage, sensing,
separation, optics, nanoreactors, and catalysis.[1] Although
metal–organic frameworks (MOFs) have greatly expanded
the scope of porous materials, especially in terms of chir-
ality,[2] materials design,[3] and very high surface areas,[4] they
are largely restricted to the microporous regime. While
numerous inorganic porous solids such as silicates and
carbons exist with a wide range of pore sizes at the micro-
to mesoscales, few MOFs with mesopores have been
reported.[5] Therefore, it is a great challenge to synthesize
robust mesoporous MOFs, especially those with pore sizes
greater than 3 nm in diameter. Equally challenging is the task
of characterizing their structures at atomic resolution,
because the structural details of such frameworks are usually
sacrificed as the pore size increases.[6] Herein, we report the
crystal structure, permanent porosity, and luminescence
properties of a mesoporous MOF (1), the framework of
which is composed of fused 3.9- and 4.7-nm cages.
Mesoporous MOF 1 was prepared as truncated-octahe-
dral crystals by a solvothermal reaction between triazine-
1,3,5-tribenzoic acid (H3TATB) and Tb(NO3)3·5H2O in a
mixture of N,N-dimethylacetamide (DMA), methanol, and
water at 1058C for 2 days (Figure 1a; see the Supporting
Information). Single-crystal X-ray diffraction data were
collected up to 1.13-Š resolution in a face-centered cubic
unit cell at the beamline facilities designed for macromolec-
[*] Y. K. Park, S. B. Choi, Dr. Y. H. Jhon, Prof. J. K. Yang, Prof. J. Kim
Department of Chemistry, Soongsil University
Seoul 156-743 (Korea)
Fax: (+82)2-824-4383
E-mail: jaheon@ssu.ac.kr
H. Kim, Prof. K. Kim
National Creative Research Center for Smart Supramolecules and
Department of Chemistry, Pohang University of Science and
Technology, Pohang 790-784 (Korea)
B.-H. Won, Prof. K. Choi
Department of Chemistry, Korea University, Seoul 136-704 (Korea)
J.-S. Choi, Prof. W.-S. Ahn
Department of Chemical Engineering, Inha University
Incheon 420-751 (Korea)
N. Won, Prof. S. Kim
Department of Chemistry, Pohang University of Science and
Technology, Pohang 790-784 (Korea)
Dr. D. H. Jung, Dr. S.-H. Choi
Insilicotech Co. Ltd., Kolontripolis A-1101
Seongnam 463-805 (Korea)
G.-H. Kim, Dr. S.-S. Cha
Beamline Division, Pohang Accelerator Laboratory
Pohang 790–784 (Korea)
[**] This work was supported by the Hydrogen Energy R&D Center
(J.K.), the Carbon Dioxide Reduction and Sequestration R&D Center
(W-S.A.), the 21st Century Frontier R&D Program funded by the
Ministry of Science and Technology of Korea, the Creative Research
Initiatives and BK 21 Programs (K.K.), and a Korean Research
Foundation Grant funded by the Korean Government (MOEHRD;
KRF-2005-005J13102; S.K.). We thank Prof. Ji Man Kim (Department
of Chemistry, Sung Kyun Kwan University) for the SAXS measure-
ments. The X-ray diffraction experiments were performed at the
Pohang Accelerator Laboratory (beamlines 4A and 6B) supported by
MOST and Pohang University of Science and Technology.
Figure 1. a) Single crystals of 1 (left), and a sampled X-ray diffraction
pattern one such crystal (right). b) A three-dimensional electron-
density mapof a face of a truncated ST (left), and its corresponding
disordered (middle) and ordered (right) structural models. C gray,
H white, N blue, O red (space-filling model); outer (yellow) and inner
(blue) TATB ligands shown as stick models. The Tb3+ ions (light blue
spheres) sit on the triangular faces of a truncated ST. c) A cubic unit
cell viewed along the [101] direction. The large and small spheres
represent the inner spaces of the mesocages, and the sticks connect
the Tb3+ ions that are the vertices of the truncated STs.
Supporting information for this article is available on the WWW
under http://www.angewandte.org or from the author.
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ular crystallography at Pohang Accelerator Laboratory
(PAL). The crystal structure shows that a total of 1088 Tb3+
ions, or 272 Tb4 cluster units, are distributed in the unit cell
(Figure 1c; see the Supporting Information).[7] Each Tb4 unit
has a trigonal-planar geometry with an average interatomic
distance of 5.14 Š between the peripheral atoms and the
central atom. The separation between two peripheral Tb3+
ions is about 8.82 Š. In turn, four Tb4 moieties form a
tetrahedral arrangement, or a supertetrahedron (ST), with a
separation of about 16.27 Š between the central Tb3+ ions. In
the unit cell, 68 of these unique truncated STs are fused
together, producing 17 additional truncated STs and making a
three-dimensional extended network. The truncated ST can
be regarded as a structural subunit of 1 (Figure 1b). The
three-dimensional extended network built by the Tb3+ ions is
similar to those of MIL-100 and MIL-101, which exhibit the
MTN zeotype.[5b,c]
The metal ions and ligands in 1 are assembled in a very
unique way to produce the zeotype network. Owing to the
large separation between the Tb3+ ions (5.14 Š) compared to
the usual separation between carboxylate-bridged lanthanide
ions (ca. 4 Š),[8] the pair of TATB ligands on each face of the
truncated ST link nine Tb3+ ions in a very delicate way; the
outer TATB ligand joins three peripheral Tb3+ ions in a
bidentate fashion, while the inner TATB ligand binds three
central Tb3+ ions and another three peripheral Tb3+ ions in a
bi-monodentate fashion (Figure 1b and the Supporting
Information). This coordination mode makes the central
Tb3+ ions of the trigonal-planar Tb4 moieties and the four
internal TATB ligands almost unexposed to the pore. Two
TATB ligands stack with each other on each hexagonal face of
the truncated ST, such that the central triazine rings are
superimposed and the benzoate moieties are staggered.
Therefore, the truncated ST becomes thicker and heavier,
owing to the double coats. Crystallographic mirror planes
bisect all the independent truncated STs, causing the TATB
ligands to disorder over two sets of sites about the mirror
planes (Figure 1c and the Supporting Information).
Figure 2. a) The network of fused S and L mesocages formed by the
truncated STs. b) The doubly interpenetrating diamond-like net formed
by the L cages (blue spheres) and the centers (small red spheres) of
tetrahedra of S cages (yellow spheres). c) The S and L mesocages
drawn as space-filling models. In the S cage, the inner TATB ligands
are drawn in red, and outer ligand in blue; in the L cage, the atoms
are drawn in their atomic colors. C gray, H white, N blue, O red,
Tb light blue.
(Figure 2b). On the basis of the crystal-structure analysis,
the repeating unit was chosen to be {Tb16(TATB)16} with Z =
68, and by other chemical analyses, including elemental
analyses, the compound was formulated as [Tb16(TATB)16-
(DMA)24]·(DMA)91(H2O)108
.
Thermogravimetric analysis
(TGA) of the as-prepared crystals at ambient atmosphere
showed three weight-loss steps corresponding to the removal
of the free guests (47% at 1208C), the coordinating ligands
(9% at 3208C), and the TATB ligands with decomposition
(30% at 5208C; see the Supporting Information). It was
virtually impossible to clearly define the free guests inside the
pores with the current X-ray data, owing to the severe
disorder common to microporous MOFs.[4]
Above and below the trigonal-planar Tb4 moieties in each
truncated ST, a total of 12 TATB ligands are bound. The
TATB ligands link each truncated ST to four adjacent
truncated STs. Two truncated STs are linked in an eclipsed
fashion, despite having the possibility of adopting a gauche
geometry.[9] The fusion of the building blocks produces five-
and six-membered rings in the framework, which, considering
van der Waals radii, have free diameters of 13.0 and 17.0 Š,
respectively (Figure 2a and the Supporting Information).
These openings are the windows of two kinds of large cages,
the shapes of which are slightly distorted spheres. The smaller
cage S is surrounded by 20 truncated STs and has 12 pentag-
onal windows (Figure 2c). Its internal free diameter is 39.1 Š.
The larger cage L, defined by 28 truncated STs and having
12 pentagonal and 4 hexagonal windows, has an internal free
diameter of 47.1 Š (Figure 2c). The hexagonal windows are
tetrahedrally located in an L cage, and through these windows
all the L cages are fused together to form a diamond-like
network. The S cages are fused to the L cages by sharing
pentagonal windows and also form a diamond-like net, if the
centers of four S cages joined together are considered
A nitrogen adsorption measurement on the evacuated as-
prepared crystals at 77 K and up to 760 Torr showed a nearly
reversible isotherm (Figure 3a). This sorption behavior is
different from that of other similar mesoporous MOFs, such
as MIL-100 and MIL-101, in terms of the number of steps and
their distinctness.[5b,c] If it is assumed that the steps at 200 and
300 Torr are due to the S and L pores, respectively, the
isotherm more or less resembles the isotherm found for
calcined MCM-41, for which the pore-filling mechanism is
still not absolutely clear.[10] The Brunauer–Emmett–Teller
(BET) surface area was calculated as 1419 m2 g, and the
Langmuir surface area calculated with the amount of
adsorbed nitrogen and the molecular area of N2 (16.2 Š2)
was estimated as 2887 m2 g.
When the samples were activated at 1608C, the surface-
area values increased to 1783 m2 g (BET) and 3855 m2 g
(Langmuir), with retention of the original sorption behavior
(Figure 3a). This change in the surface area was also
confirmed by CO2 adsorption measurements at ambient
temperature (Figure 3b). The as-prepared sample could
store 14 mmolgÀ1 (ca. 43 bar), which increased to
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Figure 4. a) Microscope images of a crystal of 1 taken in transmission
mode (left) and in fluorescence mode (middle), and its luminescence
spectrum (right). b) As in (a), for a bulk sample. c) As in (a), for
ferrocene-containing crystals. In the fluorescence image, half of the
background color was subtracted to show the crystals. d) As in (a), for
the crystals used in (c) after evacuation at 508C for 1 day.
Figure 3. a) Nitrogen sorption isotherms at 77 K for samples of 1
activated at 808C and 1608C, respectively. b) High-pressure carbon
dioxide adsorption isotherms at ambient temperature for the same
samples used in (a).
actually stronger than that from the ferrocene molecules
alone, though the mechanism of the energy transfer requires
further investigation (see the Supporting Information).
It is interesting to note that the included ferrocene
molecules could be released under vacuum at an elevated
temperature, and that afterwards the green emission from the
crystal of 1 was recovered completely. Moreover, the spec-
trum exactly matched that of the evacuated host crystals, and
the ferrocene emission disappeared (Figure 4d).
This work demonstrates that mesoporous MOFs with
pores of at least 4.7 nm in diameter can be synthesized and
that their crystal structures can be directly determined at
atomic resolution by X-ray crystallography, without assuming
the building-block structures as in previous cases.[5b,c] More-
over, the experimental observations on 1 also support that
host frameworks built with coordination linkages can be
robust enough to tolerate a mesoscale void space. These
results are believed to be valuable for the exploration of
mesoporous MOFs, into which a variety of functionality can
be introduced as for their microporous relatives.
18 mmolgÀ1 (ca. 45 bar) for the activated sample. These
values are located between the uptakes of MOF-177 and of
zeolite 13X.[11] The unsaturated uptake values of 1 indicate
that most of the pore space is not filled with CO2 molecules.
The as-prepared crystals of 1 emit strong green light at
488, 541, 584, and 620 nm (Figure 4a), which is very similar to
that emitted by other Tb3+-containing MOFs with very
different framework structures.[8a,b] The emission spectrum
of a single crystal was identical to that of the bulk (Figure 4b).
As the host framework has both a high thermal stability and
sufficient rigidity under vacuum, ferrocene guest molecules
could be included by a sublimation procedure at 1008C.[12]
The crystal color changed to dark brown, and 1H NMR
spectra and elemental analyses suggested that at least
65 ferrocene molecules per formula unit, which is equivalent
to 4420 ferrocene molecules per unit cell, were included in the
pores of 1 (see the Supporting Information). For the
ferrocene-containing crystals, no strong green emission was
observed; instead, a weak and broad emission from the
included ferrocene molecules appeared (Figure 4c). As the
ferrocene molecule absorbs light at wavelengths that overlap
with the emission region of the framework of 1,[13] it is likely
that an efficient energy transfer from the host framework to
the ferrocene molecules occurs. The crystal of 1 appears to act
as an antenna harvesting photons for the included ferrocene
molecules, because the emission from the host–guest system is
Received: May 26, 2007
Published online: August 7, 2007
Keywords: adsorption · luminescence · mesoporous materials ·
.
metal–organic frameworks · terbium
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[7] Crystal data for 1: C384H192N48O96Tb16, Mr = 9556.58, cubic, space
3
¯
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group F43m (No. 216), a = 123.901(1) Š, V= 1902061(27) Š ,
Z = 68, dcalcd = 0.567 gcmÀ3 for the framework only, T= 90(2) K,
crystal size 0.35 ” 0.35 ” 0.35 mm3, l = 0.89999 Š, 2q = 46.848,
1358 parameters, R1 = 0.2074 (I > 2s(I), 29225 reflections),
wR2 = 0.4932 (all data, 30902 reflections), GOF = 1.101. See
details in the Supporting Information. CCDC-648613 contains
the supplementary crystallographic data for this paper. These
data can be obtained free of charge from The Cambridge
Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_
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