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Highly porous metal–organic framework sustained
with 12-connected nanoscopic octahedra†
Weigang Lu,a Daqiang Yuan,*b Trevor A. Makal,a Zhangwen Wei,a Jian-Rong Lia and
Hong-Cai Zhou*a
Two dicopper(II)-paddlewheel-based metal–organic frameworks (PCN-81 and -82) have been synthesized
Received 17th October 2012,
Accepted 25th October 2012
by using tetratopic ligands featuring 90°-carbazole–dicarboxylate moieties. Both adopt 12-connected tfb
topology with nanoscopic octahedra as building units. The freeze-dried PCN-82 shows Brunauer–
Emmett–Teller (BET) and Langmuir surface areas as high as 4488 and 4859 m2 g−1, respectively. It also
DOI: 10.1039/c2dt32479b
exhibits high H2-adsorption capacity at low pressure (300 cm3 g−1 or 2.6 wt% at 77 K and 1 bar), which
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can be attributed to its high surface area, microporosity, and open metal sites.
Introduction
For years, metal–organic frameworks (MOFs) have received
growing attention not only for their structural aesthetics, but
also their potential applications in gas storage, gas/vapor separation, and shape/size-selective catalysis owing to their unprecedented large surface area and tunable pore size, geometry,
and functionality.1 Their efficacy, especially for applications in
gas storage, depends largely on surface area and microporosity.
Yaghi and coworkers demonstrated that significant enhancement of surface area could be achieved by ligand extension
within MOFs;2 indeed, this strategy also corroborates calculations on two series of isoreticular MOFs.3 However, the discrepancy between the calculated and the experimental surface
areas becomes larger as the ligand becomes more extended,
which can usually be traced back to interpenetration or partial
collapse of the framework upon guest-molecule removal. More
recently, freeze drying4 and CO2 supercritical activation5 were
developed to prevent the collapse of frameworks with mesoporous voids. Nevertheless, micropores can contribute more
surface area than mesopores on a volume basis, and surface
area is directly proportional to monolayered physisorption
capacity, especially for inert gas molecules, such as H2
and CH4.
a
Department of Chemistry, Texas A&M University, College Station, TX 77843, USA.
E-mail: zhou@mail.chem.tamu.edu; Fax: +1 979 845 4719; Tel: +1 979 845 4034
b
State Key Laboratory of Structure Chemistry, Fujian Institute of Research on the
Structure of Matter, Chinese Academy of Sciences, Fuzhou, 350002, P. R. China.
E-mail: ydq@fjirsm.ac.cn; Tel: +86 59183792525
† Electronic supplementary information (ESI) available: 1H-NMR spectra,
additional figures, sorption isotherms, TGA, IR spectra. CCDC 846792 and
846791. For ESI and crystallographic data in CIF or other electronic format see
DOI: 10.1039/c2dt32479b
1708 | Dalton Trans., 2013, 42, 1708–1714
To construct non-interpenetrated MOFs, one powerful strategy involves utilizing metal–organic polyhedra (MOPs) as
supermolecular building blocks (SBBs).6 One might view these
nano-scaled MOPs as super secondary building units (SBUs) in
the SBB-sustained framework, these MOPs are assembled from
the regular SBUs which are comprised of metal ions bridged
by organic ligands. Generally speaking, interpenetration tends
to occur when the length of organic linker is much larger compared to the size of SBU. When SBB is applied, the organic
linker has to be much longer in order to form interpenetrated
structures. Furthermore, MOPs usually have a higher degree of
connectivity than simple SBUs; the MOP as SBB approach provides a toolbox of building blocks for MOFs with rare or even
unprecedented connectivity.
To tailor-make MOFs for specific applications, it is extremely valuable if we can customize the resulting cavities or
channels (size, shape, and functionality) through predictable
approaches instead of serendipity.7 However, it is still quite a
challenge to absolutely control the network topology of the
constructed MOF, the outcome could be affected by many
factors, such as coordinating groups’ orientation, solvothermal
reaction conditions, etc. Nevertheless, a high degree of predictability can be integrated prior to synthesis with the SBB
approach. For example, a series of isoreticular (3,24)-connected
MOFs (PCN-6X, PCN stands for porous coordination network)8
having the same cuboctahedral cage as the SBB were synthesized under solvothermal conditions with dicopper(II)
paddlewheel and elongated ligands. Notably, PCN-6X exhibit
high surface areas and gas-adsorption capacities with no interpenetration observed, which was attributed to the incorporation of nanoscale cuboctahedral cages.
So far, cuboctahedral cages have been the most frequently
used SBBs owing to the chemical accessibility of 120°-angulardicarboxylate ligands. On the other hand, octahedral cages,
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mixture of THF/MeOH (100 mL, v/v = 1/1, THF = tetrahydrofuran, MeOH = methanol) and 50 mL aqueous solution of 2 M
KOH. The resultant mixture was stirred and refluxed until a
clear solution was obtained. After cooling to room temperature, organic solvents were removed, and the remaining solution was acidified with 1 N HCl to give a precipitate, which was
collected and washed with water. Recrystallization from
DMSO/MeOH (DMSO = dimethylsulfoxide) produced 0.61 g of
H4pbcd with a yield of 60% based on 1,4-diiodo-benzene.
1
H-NMR (300 MHz, DMSO-d6): 12.90 (4H, brs), 9.03 (4H, brs),
8.16 (4H, brd, J = 8.7 Hz), 8.04 (4H, s), 7.69 (4H, d, J = 8.7 Hz),
3.69 (6H, s). Elemental analysis (%): calcd for C34H20N2O8: C
69.86, H 3.45, N 4.79, O 21.90; found: C 69.83, H 3.40, N 4.78.
Fig. 1 Structural representations of a 6-connected (a) and 12-connected (b)
octahedral SBB.
another geometrically versatile building blocks which can be
assembled with 90°-angular-dicarboxylate ligands and square
planar building units (e.g. dicopper(II) paddlewheel), have
rarely been studied in the construction of three dimensional
MOFs, possibly because of the difficulty in ligand synthesis.
The octahedral cages can be linked to form frameworks
through two modes, 6-connected if extended from vertices
(Fig. 1a) and 12-connected from edges (Fig. 1b). In our previous work, we have successfully constructed a MOF with a
6-connected octahedral cage as SBB.9 Here, we extend the strategy to 12-connection through the rational design of ligands.
Experimental section
General information
Commercially available reagents were used as received. 1H
NMR data were collected on a Mercury 300 MHz NMR spectrometer. FT-IR data were recorded on an IRAffinity-1 instrument. TGA data were obtained on a TGA-50 (SHIMADZU)
thermogravimetric analyzer with a heating rate of 3 °C min−1
under a N2 atmosphere. The powder X-ray diffraction (PXRD)
patterns were recorded on a BRUKER D8-Focus Bragg–Brentano X-ray powder diffractometer equipped with a Cu sealed
tube (λ = 1.54178 Å) and graphite monochromator at room
temperature. Simulation of the PXRD pattern was carried out
by the single-crystal data and diffraction-crystal module of
the Mercury program available free of charge via internet at
http://www.iucr.org.
Synthesis of [9,9′-(1,4-phenylene)bis(9H-carbazole-3,6dicarboxylic acid)] (H4pbcd)
To a sealable flask was added dimethyl 9H-carbazole-3,6-dicarboxylate (1.0 g, 3.5 mmol), K3PO4 (2.0 g, 9.4 mmol), 1,4-diiodobenzene (0.50 g, 1.5 mmol), CuI (30 mg, 0.16 mmol), N,N′dimethylethylenediamine (0.1 mL), dioxane (20 mL), and a
magnetic stir bar under argon. The resulting mixture was
sealed and heated to 120 °C for 3 days. After cooling to room
temperature, water was added, the precipitate was collected,
washed with ethyl acetate and acetone, and suspended in a
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Synthesis of [9,9′-(2,5-dimethoxy-1,4-phenylene)bis(9H-carbazole-3,6-dicarboxylic acid)] (H4dpbcd)
H4dpbcd was synthesized by a procedure similar to that of
H4pbcd, with 1,4-dibromo-2,5-dimethoxybenzene (0.45 g,
1.5 mmol) in place of 1,4-diiodo-benzene, in a yield of 26%
based on 1,4-dibromo-2,5-dimethoxybenzene. 1H-NMR
(300 MHz, DMSO-d6): 12.81 (4H, brs), 8.99 (4H, d, J = 1.2 Hz),
8.14 (4H, dd, J = 8.7 and 1.2 Hz), 7.70 (2H, s), 7.52 (4H, d, J =
8.7 Hz), 3.69 (6H, s). Elemental analysis (%): calcd for
C36H24N2O10: C 67.08, H 3.75, N 4.35, O 24.82; found: C 67.03,
H 3.70, N 4.38.
PCN-81 synthesis
A mixture of H4pbcd (48 mg, 0.082 mmol) and Cu(NO3)2·2.5H2O (144 mg, 0.62 mmol) was added to a vial containing DMSO/DMA (6 mL/12 mL), and then 12 drops HBF4
(48% w/w aqueous solution) was added. The vial was sealed,
heated to 85 °C for a week, and the green block crystals of
PCN-81 were collected, washed with DMA, and dried in air to
produce 35 mg product with a yield of 56% based on H4pbcd.
Elemental analysis (%): calcd for C34H20Cu2N2O10: C, 54.92; H,
2.71; Cu, 17.09; N, 3.77; O, 21.52; found: C 52.07, H 3.07, N
4.14.‡
PCN-82 synthesis
PCN-82 was synthesized by a procedure similar to that of
PCN-81, with H4dpbcd (55 mg, 0.085 mmol) instead, in a yield
of 41% based on H4dpbcd. Elemental analysis (%): calcd for
‡ Crystal data for PCN-81: C34H20Cu2N2O10, M = 743.60, green block, 0.08 × 0.07
ˉ (No. 205), a = 39.4840(16), V = 61 555(4) Å3, Z
× 0.05 mm3, cubic, space group Pa3
= 24, Dc = 0.481 g cm−3, F000 = 9024, Bruker APEX II CCD area detector, synchrotron radiation, λ = 0.41328 Å, T = 173(2) K, 2θmax = 30.0°, 543 624 reflections collected, 21 027 unique (Rint = 0.0945). Final GooF = 1.236, R1 = 0.1125, wR2 =
0.2983, R indices based on 16 504 reflections with I > 2σ(I) (refinement on F2),
μ = 0.230 mm−1. CCDC 846792.
Crystal data for PCN-82: C36H24Cu2N2O12, M = 803.65, 0.08 × 0.07 × 0.06 mm3,
ˉm (No. 225), a = 40.026(6), V = 64 123(16) Å3, Z = 24, Dc =
cubic, space group Fm3
−3
0.499 g cm , F000 = 9792, Bruker APEX II CCD area detector, synchrotron radiation, λ = 0.40663 Å, T = 110(2) K, 2θmax = 26.1°, 75 012 reflections collected,
2305 unique (Rint = 0.2448). Final GooF = 1.229, R1 = 0.1254, wR2 = 0.3149,
R indices based on 1547 reflections with I > 2σ(I) (refinement on F2), μ =
0.223 mm−1. CCDC 846791.
Dalton Trans., 2013, 42, 1708–1714 | 1709
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C36H24Cu2N2O12: C, 53.80; H, 3.01; Cu, 15.81; N, 3.49; O,
23.89; found: C 51.29, H 3.56, N 4.07.‡
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Adsorption experiments
N2, H2, CH4, and CO2 physisorption isotherms were measured
up to 1 bar using a Micromeritics ASAP 2020 surface area and
pore size analyzer. A freeze-drying procedure4,10 was applied to
activate samples. After decanting all the mother liquor, assynthesized sample of PCN-82 was washed with DMA, then
washed with methanol three times over a 3 day period, and
then with dichloromethane three times over a 3 day period.
The resulting dark blue crystals were washed with benzene
three times and then soaked in benzene overnight before
loading into a BET sample cell. After decanting most of
benzene, the BET cell was then frozen at −5 °C. After three
freeze–thaw cycles, the sample cell was placed in an ice/H2O
bath and evacuated under a dynamic vacuum for 24 h. The ice/
H2O bath was removed and the sample was kept under
vacuum at room temperature for another 48 h, and then
heated under vacuum at 50 °C for 10 h. The resulting freezedried sample (74 mg) was used to perform gas-uptake
measurements. High-purity gases were used (N2: 99.999%, Ar:
99.999%). Pore size distribution data were calculated from the
N2 sorption isotherms based on DFT model in the Micromeritics ASAP 2020 software package.
Single crystal X-ray study
Single crystal X-ray structure determination of PCN-81 and -82
were performed at 173(2) K using the Advanced Photon Source
on beamline 15ID-B at Argonne National Laboratory. Structures were solved by direct methods and refined by full-matrix
least-squares on F2 using SHELXTL.11 Non-hydrogen atoms
were refined with anisotropic displacement parameters during
the final cycles. Organic hydrogen atoms were placed in calculated positions with isotropic displacement parameters set to
1.2 × Ueq of the attached atom. The solvent molecules of
PCN-81 and -82 were highly disordered, and attempts to locate
and refine the solvent peaks were unsuccessful. Contributions
to scattering due to these solvent molecules were removed
using the SQUEEZE routine of PLATON, structures were then
refined again using the data generated.12
Results and discussion
With the integration of two 90°-angular-dicarboxylate moieties
into the same ligand head to head, it is likely to form 12-connected topological network when assembled with dicopper(II)
paddlewheel cluster, as shown in Fig. 2. Every octahedron contains twelve carbazole moieties, each of which connects to
another octahedron through phenyl linkers.
In our continuing efforts to explore MOFs with 90°-angulardicarboxylate subunits,13 we report here the design and synthesis of two dicopper(II) paddlewheel-based MOFs (PCN-81
and -82). Both have pre-designed octahedra in a 12-connected
topology, as predicted. Compared to a recently reported
1710 | Dalton Trans., 2013, 42, 1708–1714
Fig. 2 (a) Ligands pbcd4− (R: H) and dpbcd4− (R: OCH3); (b) octahedral cage,
six dicopper(II) paddle-wheel clusters as vertices, twelve carbazole moieties as
edges; (c) phenyl linker between octahedral cages; (d) illustration of each octahedron connected to twelve other octahedra by phenyl linkers in an edgedirected manner, a different colour was used to highlight the connectivity.
12-connected metal–organometallic phosphate framework,14
the octahedron in our case does not just serve as a building
unit, it is a microporous cage that contributes to surface area
and microporosity of the resulting frameworks. PCN-81 and
-82 are also rare examples of MOFs assembled with octahedra
as SBB. Our group previously reported the stepwise assembly
of a three-dimensional MOF, in which the same octahedra were
connected in a vertex-to-vertex manner with 4,4′-bipyridine
coordinating to terminal copper sites.9 In the PCN-81 and -82
cases, the connection of octahedra were covalent and edgedirected (Fig. 2d), which possibly leads to increased stability of
the resultant frameworks.
The acids of tetratopic ligands, H4pbcd and H4dpbcd, were
synthesized by a Cu(I)-catalyzed hetero-coupling reaction15
between dimethyl 9H-carbazole-3,6-dicarboxylate and 1,4dibromobenzene or 1,4-dibromo-2,5-dimethoxybenzene followed by hydrolysis with 2 M KOH in MeOH/THF/H2O (1/1/1)
mixture.
Solvothermal reaction of ligand precursors and Cu(NO3)2·2.5H2O in DMA and DMSO mixture in the presence of
HBF4 afforded cubic green crystals of PCN-81 and PCN-82 with
a formula of [Cu2(ligand)]·2X (X is coordinating solvent).
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Dalton Transactions
ˉ space group with a =
PCN-81 crystallizes in cubic Pa3
39.4840(16) Å. While, surprisingly, PCN-82 crystallizes in the
ˉm space group with a = 40.0256(59) Å.
higher symmetry Fm3
Closer inspection of the ligands in the crystal structures
reveals the two carbazole dicarboxylate moieties on the same
bpcd4− ligand in PCN-81 have a dihedral angle of ∼18°. The
considerable free rotation of C–N bond facilitates the out-ofplane bending which may be the most thermally stable position. On the other hand, the two carbazole moieties on the
same dbpcd4− ligand in the crystal structure of PCN-82 are
nearly in the same plane. The two pendant methoxy groups
restrict the C–N bond rotation and, as such, force the two carbazole moieties to adopt the in-plane orientation, which possibly leads to lower energy of the resulting framework and
higher symmetry. Our group has previously reported the construction of MOFs with a similar tetracarboxylate ligand, N,N,
N′,N′-tetrakis(4-carboxyphenyl)-1,4-phenylenediamine (tcppda4
−
); in this case the network formation is rather unpredictable,
possibly because of the ligand’s conformational flexibility.16
Another benefit of the two pendant methoxy groups is their
stabilizing effect on the framework. With PCN-81, in which the
phenyl linker has no substituent, there is very little nitrogen
sorption observed, implying a collapse of the framework. The
same conclusion is also drawn from the PXRD pattern, which
reveals that PCN-81 lost its crystallinity completely upon activation. In view of Aristotle’s observation that “nature abhors a
vacuum”, the change of the R group from hydrogen to
methoxy or other larger groups on the central phenyl ring of
the linker could possibly lead to a robust framework by serving
to fill the void space. Indeed, the framework of PCN-82
remains intact when the freeze-drying procedure was applied
to activate the sample.
Twelve dbpcd4− ligands in the structure of PCN-82 form
octahedral SBB with eight 4-connected dicopper(II) paddlewheel SBUs. The octahedral SBB serves as a 12-connected
node to generate a 12-connected network. There are three
types of microporous cages with different sizes. S-cage
(∼11.0 Å internal pore diameter) is an octahedron formed by
six metal SBUs and twelve 90°-angular carbazole-3,6-dicarboxylate moieties from one end of the dpbcd4− ligands (Fig. 3a);
the same cage was previously reported as a discreet molecule
by our group.9 M-cage (∼12.9 Å internal pore diameter) is
formed by twelve dicopper(II) paddlewheel SBUs and six
dpbcd4− ligands (Fig. 3b). L-cage (∼18.1 Å internal pore diameter) forms in a similar fashion to S-cage with 90°-angulardicarboxylates coming from carbazole moieties on opposite
ends of the ligand (Fig. 3c). The overall structure consists of
these three types of cages packed in a 1 : 1 : 1 ratio (Fig. 3d).
From a topological viewpoint, PCN-82 can be described as a
12-connected network with fcu topology when octahedral SBBs
are taken as nodes. Alternatively, the structure may be viewed
with the metal SBUs as 4-connected nodes and the dpbcd4−
ligands as 3,3-connected nodes. As a result, PCN-82 adopts the
very rare 3,4-c 2-nodal net (Fig. 3d) with tfb-type topology.
The calculated free volume in fully desolvated PCN-82 is
77.8% by PLATON (1.8 Å probe radius),12 and the pore volume
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Fig. 3 (a) Small size cage (S-cage); (b) middle size cage (M-cage); (c) large size
cage (M-cage); (d) packing of cages and schematic representations of the
tfb-type topology of PCN-82. (e) Solvent accessible surface area in PCN-82.
is 1.64 cm3 g−1. To confirm the porosity, PCN-82 was freezedried and further degassed under a dynamic vacuum at 80 °C
for 10 h after solvent exchange with methanol, dichloromethane, and then benzene. A color change from green (in
methanol) to deep blue (in dichloromethane and benzene) to
purple (activated, Fig. 4a inset) was observed, typical for dicopper(II)-paddlewheel frameworks in which open copper sites
are generated.27 The N2 sorption for freeze-dried PCN-82 at
77 K exhibited a reversible Type-I isotherm as shown in
Fig. 4a, a characteristic of microporous materials.28 PCN-82
exhibits high N2 uptake (ca. 1100 cm3 g−1). By applying the
BET model (up to P/Po ≈ 0.05), the apparent surface area is
estimated to be ∼4488 m2 g−1 (calculated ∼4307 m2 g−1) and
Langmuir surface area ∼4859 m2 g−1, which is similar to that
of MOF-177 (BET ∼4500 m2 g−1).2 The data is among the
highest reported to date for porous MOFs.
It is interesting to point out that MOFs with BET surface
area over 5000 m2 g−1 all have a considerable amount of mesoporous nature, which are evident from their cryogenic N2 sorption isotherms and the calculated pore size distributions.
Microporosity of PCN-82 was confirmed by the three distinct
sorption regions within PCN-82 (Fig. 4b), corresponding to the
three different-sized cavities in the crystal structure discussed
above, and this is consistent with pore size distribution analysis by DFT methods using N2 gas at 77 K (Fig. 4b inset). To the
best of our knowledge, PCN-82 and MOF-177 are the two
microporous MOFs with the highest BET surface area
(Table 1).
The high surface area of PCN-82 prompted us to study its
gas-adsorption capacity, especially that for hydrogen,
methane, and carbon dioxide. MOF-based hydrogen storage
has attracted remarkable attention recently because of its fast
kinetics and favorable thermodynamics in hydrogen adsorption and release.29 The hydrogen-adsorption capacity of
PCN-82 is shown in Fig. 4c. In the low pressure region, the
Dalton Trans., 2013, 42, 1708–1714 | 1711
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Table 1
Porosity data for PCN-82 and selected MOFs
Materials
17
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MOF-5
NOTT-11218
PCN-8013
PCN-668b
Be(BTB)19
MIL-10120
Bio-MOF-10021
PCN-82this work
MOF-1772
NU-11122
PCN-688a
UMCM-223
NU-10024
MOF-21025
ABETa
ACalca,b
Vpa
Micro/Meso
3800
3800
3850
4000
4030
4100
4300
4488
4500
5000
5109
5200
6143
6240
3527
4286
3584
3935
3600
3076
3039
4307
5340
4915
4811
4475
5894
6497
1.55
1.62
1.47
1.63
1.48
2.00
4.30
1.70
1.89
2.38
2.13
2.32
2.82
3.60
Micro
Micro
Micro
Micro
Micro
Micro/Meso
Micro/Meso
Micro
Micro
Micro/Meso
Micro/Meso
Micro/Meso
Micro/Meso
Micro/Meso
a
Acronyms: ABET and ACalc are the BET and calculated surface areas
(m2 g−1); Vp is the measured pore volume (cm3 g−1). b The accessible
surface area is calculated from a simple Monte Carlo integration
technique where the probe molecule is “rolled” over the framework
surface.26
Fig. 5
Fig. 4 (a) N2 sorption isotherms for PCN-82 at 77 K, adsorption (●)/desorption
(○); (b) amount of nitrogen adsorbed in PCN-82 at 77 K vs. log(P/Po); inset: pore
size distribution, incremental surface area (cm3 g−1) vs. pore width (Å); (c)
H2-adsorption for PCN-82 at low pressure, adsorption (●)/desorption (○), inset:
H2 heat of adsorption (kJ mol−1) vs. uptake (mg g−1).
hydrogen-adsorption capacity is largely controlled by the
hydrogen affinity towards the framework, which can be quantified by isosteric heat of adsorption. Variable-temperature
measurements reveal an isosteric heat of adsorption of 6.6 kJ
mol−1 for PCN-82 at zero loading (Fig. 4c inset). This value
compares favourably with MOFs lacking special sorption sites;
their heats of adsorption are typically in the 4–5 kJ mol−1
range.29b–d The high performance of PCN-82 can be attributed
to the availability of open metal sites, microporous nature, and
high surface area. As a result, PCN-82 can take up as high as
300 cm3 g−1 (2.6 wt%) of H2 at 77 K and 1 bar. The value is
among the highest of reported porous materials at low pressure.29b–d And it is lower than PCN-12, which can take up
3.05 wt% H2 under the same conditions while having a much
lower BET surface area (1943 m2 g−1), possibly because
1712 | Dalton Trans., 2013, 42, 1708–1714
PXRD patterns of PCN-82.
PCN-12 has higher density and more accessible alignments of
open metal sites.30
The nitrogen uptake of activated PCN-82 was measured
eight times at 77 K consecutively with no obvious loss of porosity (Fig. S12†). Thermogravimetric analysis revealed PCN-82 to
be thermally stable up to 250 °C. PXRD was also utilized to
confirm the framework’s stability upon removal of guestsolvent molecules (Fig. 5); the activated PCN-82 sample was
handled under air-free conditions due to its high sensitivity to
moisture. PXRD patterns revealed good agreement between the
simulated, as-synthesized, and desolvated materials, which
indicates that the framework retains its crystallinity upon
guest-solvent removal.
Conclusions
In this report, two metal–organic frameworks (PCN-81 and -82)
have been designed and synthesized with tetratopic ligands
featuring 90° carbazole-3,6-dicarboxylate moieties. From
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PCN-81 to -82, the framework is stabilized by replacing two
hydrogens with methoxy groups on the central phenyl ring of
the linker. The freeze-dried PCN-82 retains its porosity and
high crystallinity with multi-cycle gas-adsorption measurements and PXRD patterns. It also shows a very high H2-adsorption capacity at low pressure (2.6 wt% at 77 K and 1 bar),
possibly because of its high surface area, microporosity, and
open metal sites. This work is a successful demonstration of
supramolecular building block strategy with octahedral cages
to assemble 12-connected topological networks. Further
studies will focus on exploring other metal clusters for waterstable MOFs and incorporating functionalities onto the phenyl
linker for gas application and catalysis.
Acknowledgements
This work was supported by the U.S. Department of
Energy (DOE DE-SC0001015, DE-FC36-07GO17033, and DEAR0000073), the National Science Foundation (NSF
CBET-0930079 and CHE-0911207), and the Welch Foundation
(A-1725). The microcrystal diffraction of PCN-81 and -82 were
carried out with the assistance of Yu-Sheng Chen at the
Advanced Photon Source on beamline 15ID-B at ChemMatCARS Sector 15, which is principally supported by the NSF/
DOE under grant number CHE-0535644. Use of the Advanced
Photon Source was supported by the U.S. DOE, Office of
Science, Office of Basic Energy Sciences, under Contract No.
DE-AC02-06CH11357.
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