Metal-organic frameworks constructed from crown ether-based 1,4-benzenedicarboxylic acid derivatives

Article abstract of DOI:10.1039/c5dt04316f

A series of unprecedented crown ether- and thiacrown ether-derivatized benzene dicarboxylic acid (H₂bdc) ligands has been synthesized and incorporated into the prototypical isoreticular metal-organic framework (IRMOF) and UiO-66 materials. In the case of UiO-66, only MOFs comprised from a mixed-ligand composition, requiring both unsubstituted bdc and crown ether containing ligands, could be prepared. These are among the few ligand derivatives, and resulting MOFs, that incorporate a macrocyclic group directly on the bdc ligand, providing a new, modular platform for exploring new supramolecular and coordination chemistry within MOFs.

Full text of DOI:10.1039/c5dt04316f

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Page 1 of 16
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Metal−organic frameworks constructed from crown etherꢀbased
,4ꢀbenzenedicarboxylic acid derivatives
1
a
b
b
a*
TengꢀHao Chen, Andreas Schneemann, Roland A. Fischer, and Seth M. Cohen
a
Department of Chemistry and Biochemistry, University of California, San Diego, La Jolla,
California, 92093, USA
b
Lehrstuhl für Anorganische Chemie II, Organometallics & Materials Chemistry, Ruhrꢀ
Universität Bochum, Universitätsstrase 150, 44801 Bochum, Germany

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Abstract
A series of unprecedented crown etherꢀ and thiacrown etherꢀderivatized benzene dicarboxylic
acid (H bdc) ligands has been synthesized and incorporated into the prototype isoreticular
2
metal−organic framework (IRMOF) and UiOꢀ66 materials. In the case of UiOꢀ66, only MOFs
comprised from a mixedꢀligand composition, requiring both unsubstituted bdc and crown ether
containing ligands, could be prepared. These are among the few ligand derivatives, and resulting
MOFs, that incorporate a macrocyclic group directly on the bdc ligand, providing a new,
modular platform for exploring new supramolecular and coordination chemistry within MOFs.

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Introduction
Metal−organic frameworks (MOFs) are widely studied porous materials with controllable
1
,2,3
functionality, and have been proposed for many industrial and technological applications.
Many efforts have been devoted to frameworks with ultrahigh porosity providing largeꢀsized
4
,5
cavities or pore apertures for gas or guest molecule storage.
With the ability to construct
MOFs with large cavities, bulky organic groups such as macrocyclic molecules have been
6
introduced into these frameworks. Macrocyclic molecules are widely studied in supramolecular
chemistry as molecular cages, nanosized reaction vessels, switches and shuttles, liquid crystals,
7
8
catalysts, and sensors.
To date, various macrocyclic linkers based on crown ethers,
9
10
11
12
azamacrocycles, cyclodextrins, pseudorotaxanes, and rotaxanes have been incorporated
into MOFs. Crown etherꢀbased linkers normally consist of flexible macrocyclic polyethers,
8
which can form a complex with guest species through coordination or hydrogen bonds.
A
flexible azamacrocyclic ligand has been used to construct a MOF which displayed a selective
1
3
uptake of CO over N . Cyclodextrinꢀbased MOFs have been fabricated from sugarꢀbased,
2
2
14
macrocyclic building blocks and have shown strong and reversible binding of CO2.
Pseudorotaxane linkers enabled the resulting MOFs to exhibit specific stereoelectronic hostꢀ
guest interactions, essential for the creation of solidꢀstate molecular switches and molecular
1
5
machines based on mechanically interlocked molecules.
The difficult preparation of some macrocycles (high cost, low yield, multiple steps) has
been improved by the development of C−C crossꢀcoupling reactions and dynamic covalent
1
6,17
chemistry.
However, incorporating small crown ethers, such as 12ꢀcrownꢀ4, 15ꢀcrownꢀ5, and
18ꢀcrownꢀ6, into MOFs is still rare. Zhao and coworkers synthesized extended linear
1
8
dicarboxylic acid ligands with 12ꢀcrownꢀ4 or 15ꢀcrownꢀ5 anchored (Fig. 1, ligand 1 and 2).
These elongated linkers afforded interpenetrated structures, with frameworks that collapsed
under vacuum activation. Park and Suh enhanced isosteric heat (Q ) of H adsorption by the
st
2
1
9
entrapment of 15ꢀcrownꢀ5 and 16ꢀcrownꢀ6 ethers in a MOF. Later, the same group designed an
+
+
1
8ꢀcrownꢀ6ꢀbased tetradentate ligand (Fig. 1, ligand 3), which could bind K , NH , and methyl
4
8
viologen cations, for enhancing H storage in the MOF. In light of these prior studies, we
2
sought to develop facile synthetic methods to generate various crown etherꢀbased ligands for
readily constructing robust MOFs. Herein, we report the design and synthesis of bdc derivatives
containing 12ꢀcrownꢀ4, 15ꢀcrownꢀ5, and thiacrown ethers, and their resulting canonical IRMOFꢀ

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2
ꢀ
1
(also known as MOFꢀ5 [ZnO (bdc) ] ; bdc = 1,4ꢀbenzenedicarboxylate) and UiOꢀ66
4 3 n
(
University of Oslo, [Zr (OH) O (bdc) ] ) structures. These studies provide a fundamental
6 4 4 6 n
understanding on the incorporation of crown etherꢀbased linkers into MOFs and shed light on the
design of robust crown ether MOFs for selective metal binding or molecular separations.
Fig. 1 Previous examples of small crown etherꢀappended ligands used for MOF synthesis.
Scheme 1 Synthetic scheme for crown etherꢀ and thiacrown etherꢀbased ligands and resulting
MOFs.

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Experimental
General. Starting materials and solvents were purchased and used without further
purification from commercial suppliers (SigmaꢀAldrich, Alfa Aesar, EMD, TCI, and others).
Chromatography was performed using a CombiFlash Rf 200 automated system from
1
13
TeledyneISCO (Lincoln, USA). H and C nuclear magnetic resonance (NMR) spectra were
collected on Varian Mercury spectrometers running at 400 and 500 MHz, respectively.
Chemical shifts are quoted in parts per million (ppm) referenced to the appropriate solvent peak
or 0 ppm for TMS. Electrospray ionization mass spectrometry (ESIꢀMS) was performed at the
Molecular Mass Spectrometry Facility (MMSF) in the Department of Chemistry & Biochemistry
at the University of California, San Diego. Elemental analysis was carried out at the NuMega
Resonance Labs, Inc.
General Synthesis for Compounds H L1−H L4. Anhydrous potassium carbonate (1.10
2
2
2
0
g, 7.90 mmol) and dimethylꢀ2,3ꢀdihydroxyterephthalate (1.50 g, 6.64 mmol) were dissolved in
00 mL of N,Nꢀdimethylformamide (DMF). Under vigorous stirring a dibromoalkylether
1
compound (6.64 mmol, Scheme 1) was added (see Supporting Information for details). The
mixture was heated at 85 °C overnight and was then filtered to remove K CO . DMF in the
2
3
mixture was removed in vacuo by rotary evaporation. The remainder was purified by silica gel
column chromatography (hexane/ethyl acetate gradient: 80/20 to 40/60) and the solvent from the
appropriate fractions was removed under vacuum by rotary evaporation. Water (100 mL) and
KOH (600 mg) were added to the resulting solid and the solution was heated to reflux overnight.
After cooling down to room temperature, the solution was acidified with 1M HCl in an ice bath.
The solution became turbid, but no significant precipitation was observed. The volume of water
was reduced to below 5 mL under vacuum and an offꢀwhite solid precipitated upon cooling in an
ice bath. The precipitate was filtered off, washed with water (100 mL), and dried overnight in
vacuo.
1
L1. Yield: 0.51 g (25%). H NMR (400 MHz, DMSOꢀd ): δ 7.43 (s, 2H, ArH), 4.14 (t, 4H,
6
1
3
CH , J = 6 Hz), 3.77 (t, 4H, CH , J = 6 Hz), 3.66 (s, 4H, CH ). C NMR (125 MHz, DMSOꢀd6):
2
2
2
−
1
67.1, 152.5, 131.1, 125.1, 74.5, 70.8, 70.0. ESIꢀMS(−): calculated m/z = 311.08 [M − H] ,
observed m/z = 311.07.
L2. Yield: 1.17 g (50%). H NMR (400 MHz, DMSOꢀd ): δ 7.38 (s, 2H, ArH), 4.10 (t, 4H,
1
6
1
3
CH , J = 6 Hz), 3.82 (t, 4H, CH , J = 6 Hz), 3.60 (s, 8H, CH ). C NMR (125 MHz, DMSOꢀd6):
2
2
2

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167.2, 152.3, 131.0, 124.9, 74.2, 70.8, 70.0, 69.9. ESIꢀMS(−): calculated m/z = 355.10 [M −
−
H] , observed m/z = 355.02.
1
L3. Yield: 0.44 g (20%). H NMR (400 MHz, DMSOꢀd ): δ 7.46 (s, 2H, ArH), 4.28 (t, 4H,
6
1
3
CH , J = 5.5 Hz), 3.04 (t, 4H, CH , J = 5.5 Hz), 2.97 (s, 4H, CH ).
C NMR (125 MHz,
2
2
2
DMSOꢀd ): 167.0, 152.3, 130.2, 125.4, 77.5, 33.0, 32.2. ESIꢀMS(−): calculated m/z = 343.03
6
−
[
M − H] , observed m/z = 342.95.
1
L4. Yield: 0.45 g (17%). H NMR (400 MHz, DMSOꢀd ): δ 7.44 (s, 2H, ArH), 4.13 (t, 4H,
6
CH , J = 8 Hz), 2.95 (t, 4H, CH , J = 8 Hz), 2.83 (t, 4H, CH , J = 8 Hz), 2.80 (s, 4H, CH , J = 8
2
2
2
2
1
3
Hz). C NMR (125 MHz, DMSOꢀd ): 167.0, 151.6, 131.1, 125.3, 74.3, 32.0, 30.7, 30.6. ESIꢀ
6
−
MS(−): calculated m/z = 403.03 [M − H] , observed m/z = 402.97.
MOF Synthesis.
[ZnO (L2) ] ), IRMOFꢀ1ꢀL3
Zn(NO ) ·6H O (75 mg, 0.24 mmol) and a crown ether bdc ligand (L1, L2, L3, L4, 0.08 mmol)
Syntheses of IRMOFꢀ1ꢀL1 ([ZnO (L1) ] ), IRMOFꢀ1ꢀL2
4 3 n
(
([ZnO (L3) ] ),
and
IRMOFꢀ1ꢀL4
([ZnO (L4) ] ).
4 3 n
4
3 n
4
3 n
3
2
2
were dissolved in 8 mL of DMF in a 20 mL scintillation vial. The vial was placed in a sand bath
in a preheated isothermal oven at 100 °C for 24 h. The mother liquor was decanted, and the
resulting colorless crystals were washed with DMF (3×10 mL), rinsed with dichloromethane
(
CH Cl , 3×10 mL), and soaked for 3 d in CH Cl , which was replaced with fresh solvent every
2 2 2 2
2
4 h. The crystals were stored in CH Cl until needed. Yield: IRMOFꢀ1ꢀL1 22 mg (67%),
2 2
IRMOFꢀ1ꢀL2 22 mg (62%), IRMOFꢀ1ꢀL3 25 mg (75%), and IRMOFꢀ1ꢀL4 28 mg (77%).
Syntheses of UiOꢀ66ꢀL10.33 and UiOꢀ66ꢀL20.25. ZrCl (250 mg, 1.10 mmol), H bdc (90
4
2
mg, 0.55 mmol), and a crown ether bdc ligand (L1, L2, 0.55 mmol) were dissolved in 40 mL of
DMF and 9.5 mL of acetic acid in a 100 mL scintillation bottle. The vial was placed in a sand
bath in a preheated isothermal oven at 120 °C for 24 h. After cooling, the solids were isolated
via centrifugation at 6,000 rpm for 15 min using a fixed angle rotor, and washed thoroughly with
fresh DMF (3×20 mL). The solid was left to soak in CH Cl for 3 d, and the liquid was replaced
2
2
with fresh CH Cl (20 mL) every 24 h. After 3 d of soaking, the solid was isolated via
2
2
centrifugation (6,000 rpm for 15 min using a fixed angle roto), and dried under vacuum. Yield:
UiOꢀ66ꢀL10.33 256 mg (71%) and UiOꢀ66ꢀL20.25 240 mg (68%).
1
Analysis by H NMR. For IRMOFs, ~5 mg of dried solid was digested with sonication
in 500 ꢁL of DMSOꢀd and 5 ꢁL of 35% DCl in D O. For UiOꢀ66 MOFs, ~5 mg of dried solid
6
2
was digested with sonication in 500 ꢁL of DMSOꢀd and 5 ꢁL of 40% HF.
6

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Thermal Analysis. ~5−10 mg of dried MOF material was used for thermogravimetric
analysis (TGA) measurements. Samples were analyzed under a stream of dinitrogen (80 mL/min)
e
using a Mettler Toledo TGA/DSC 1 STAR System running from 30 to 800 °C with a ramping
rate of 5 °C/min.
PXRD Analysis. ~15−30 mg of MOF was filtered and airꢀdried (∼5 min) before powder
Xꢀray diffraction (PXRD) analysis. PXRD data were collected at ambient temperature on a
Bruker D8 Advance diffractometer using a LynxEye detector at 40 kV and 40 mA for Cu Kα (λ
=
1.5418 Å), with a scan speed of 1 s/step, a step size of 0.02 in 2θ, and a 2θ range of 5−40°.
Gas Sorption Analysis. All gases used were of 99.999% purity. ~50−100 mg of MOF
(
soaking in CH Cl ) was transferred to a preꢀweighed sample tube and evacuated on a vacuum
2 2
line for 10 min at room temperature. The sample was then degassed at 150 °C on a
Micromeritics ASAP 2020 Adsorption analyzer for a minimum of 10 h. The sample tube was reꢀ
weighed to obtain a consistent mass of the degassed sample. BrunauerꢀEmmettꢀTeller (BET)
2
surface area measurements (m /g) were collected on three independent samples at 77 K with N2
on the same instrument using the volumetric technique. The Q of H adsorption was estimated
st
2
from the H sorption data measured at 77 K and 87 K. The data were fit to virialꢀtype equations
2
8
according to literature.
Scanning Electron Microscopy. MOFs were transferred to conductive carbon tape on a
sample holder disk, and coated by Irꢀsputtering for 7 sec. A Philips XL ESEM instrument was
used for acquiring images using a 10 kV energy source under vacuum at a working distance of
10 mm.
Results and Discussion
The focus of this study was to synthesize prototypical frameworks, such as IRMOFꢀ1 and
UiOꢀ66, using crown etherꢀappended bdc ligands. Combining dimethylꢀ2,3ꢀ
2
0
dihydroxyterephthalate with various dibromoalkyl ether compounds (Scheme 1, see Supporting
Information for details), we were able to synthesize compounds L1−L4 via the Williamson ether
2
1
synthesis in yields ranging from 17−50%. To the best of our knowledge, L3 and L4 are the
first examples of sulfurꢀcontaining crown ether ligands to be employed for MOF synthesis.
Using conventional solvothermal conditions, the desired IRMOF analogues (IRMOFꢀ1ꢀ
L1−L4) were obtained as blockꢀshape crystals. These crystals exhibited PXRD patterns as

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expected for an IRMOF lattice (Fig. 2). Noticeably, the peak at 2θ = 9.7° splits in the PXRD
pattern of IRMOFꢀ1ꢀL2. This phenomenon was also observed and described in studies of
2
2
IRMOFꢀ1 when the pores were occupied by organic and inorganic species. The crown ether
group of L2 and trapped solvent molecules might contribute to the peak splitting in the PXRD of
IRMOFꢀ1ꢀL2. The sizes of the crystals of IRMOFꢀ1ꢀL1 and ꢀL2 were large enough
3
(
>0.05×0.05×0.05 mm ) for single crystal Xꢀray diffraction analysis. However, under polarized
light, the crystals showed polycrystalline features (Fig. S19, S20) and suitable single crystal Xꢀ
ray diffraction data could not be obtained. The tiny crystals of IRMOFꢀ1ꢀL3 and ꢀL4 were also
polycrystalline as confirmed by SEM (Fig. S23). However, the PXRD data (Figure 2)
unambiguously show that these materials possess the canonical IRMOF architecture.
For the synthesis of MOFs with the UiOꢀ66 architecture, starting with 1:1 molar ratio of
ZrCl and L1 or L2 under solvothermal conditions, amorphous white powders were obtained
4
(
Fig. S24, S25). Keeping the 1:1 molar ratio of ZrCl to ligand, and adjusting the ratio between
4
bdc and L1 or L2, allowed the formation of crystalline compounds when the ratio of bdc:L1 (or
L2) was 1:1 (Fig. 1). SEM images revealed the octahedral morphology, typical of the UiOꢀ66
2
3
framework (Fig. S26).
After digestion of the resulting MOFs in HF/D O solution, the
2
framework constructed from bdc and L1 was found to contain 33% of L1 (Fig. S27), while the
product from bdc and L2 was found to contain 25% of L2 (Fig. S28). Thus, we denoted these
frameworks as UiOꢀ66ꢀL10.33 and UiOꢀ66ꢀL20.25. For L3 and L4, despite numerous attempts,
under a variety of solvothermal conditions, no crystalline product was obtained with ZrCl4,
2
4
perhaps due to coordination of the thiacrown ether to Zr(IV). Adding more equivalents of L3
or L4 produced white precipitates, but the products were amorphous as confirmed by PXRD
(data not shown). Postsynthetic exchange (PSE) of L3 or L4 into asꢀsynthesized UiOꢀ66 was
2
5
also unsuccessful in our hands (data not shown).

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Fig. 2 PXRD of IRMOFꢀ1ꢀL1−L4 (left), and UiOꢀ66ꢀL10.33 and –L20.25 (right) after airꢀdrying
from DMF.
TGA analysis showed similar results for all of the crown etherꢀbased MOFs. Although
the solids were activated at 150 °C in vacuum for 10 h before the measurements, the TGA data
(Figure 3) indicated obvious weight losses below 100 °C suggesting that certain amounts of
water from the atmosphere were adsorbed by the hydrophilic crown ether groups during the
sample preparation. These materials began to decompose between 220 and 260 °C. We attribute
the low decomposition temperature to the degradation of crown ether units. The elemental
analysis reveals that the activated materials (Table S1) contain 24−38 wt% carbon, which is
4−15 wt% less than the calculated values according to the theoretical formulas. Presumably, this
is due to a certain amount of metals trapped either in the pores of the frameworks or in the
cavities of crown ethers or thiacrown ethers. Especially for IRMOFꢀ1ꢀL3 and ꢀL4, the complex
1
H NMR spectra of the crystals digested in DCl/D O (Fig. S29, S30) showed multiple broad
2
2
6
peaks indicating a strong coordination between Zn(II) cations and thiacrown ethers. In case of
UiOꢀ66ꢀL10.33 and UiOꢀ66ꢀL20.25, it is wellꢀknown that Zrꢀbased MOFs commonly display
2
7
defects where some organic linkers in the structure may be missing. This may also account for
the large discrepancy between the experimental and calculated values of elemental analysis,
especially when bulky crown ether ligands are incorporated.

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Fig. 3 TGA traces for IRMOFꢀ1ꢀL1−L4 and UiOꢀ66ꢀL10.33−L20.25
.
2
IRMOFꢀ1ꢀL1 and ꢀL2 exhibited BET surface areas of 1204 and 899 m /g, respectively,
after activation at 150 °C under vacuum (Fig. 4). These BET surface areas are significantly
lower than that of IRMOFꢀ1, but consistent with the porous MOF architecture. The reduced
surface areas are likely attributable to the bulky crown ether groups. The frameworks of
IRMOFꢀ1ꢀL3 and ꢀL4 collapsed after activation at both room temperature and 150 °C (Fig. S31),
which hindered further surface area analysis. The BET surface areas of UiOꢀ66ꢀL10.33 and UiOꢀ
2
6
6ꢀL20.25 were 999 and 795 m /g, respectively (Fig. 4, right). The trend of decreasing surface
areas due to incorporation of bulkier ligands observed in the UiOꢀ66 series is consistent with that
1
9
of IRMOFꢀ1ꢀL1 and ꢀL2, as well as many other reports on similar systems.

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Fig. 4 N sorption isotherms of IRMOFꢀ1ꢀL1 and IRMOFꢀ1ꢀL2 (left) at 77 K. N sorption
2
2
isotherms of UiOꢀ66ꢀL10.33 and UiOꢀ66ꢀL20.25 (right) at 77 K. Samples were activated at 150 °C
under vacuum. Filled symbols are the adsorption curves; open symbols are the desorption curves.
The four MOFs that displayed appreciable surface areas were also examined for H2
adsorption. The zeroꢀcoverage isosteric heat (Q ) of the H adsorption was estimated from the
st
2
8
H adsorption isotherms measured at 77 K and 87 K under 1 atm using virial equation (Fig. 5).
2
The total amounts of H uptake in these MOFs are less than those of their parent MOFs, IRMOFꢀ
2
1
(Fig. 5a) and UiOꢀ66 (Fig. 5b), again likely due to the incorporation of bulky crown ethers.
IRMOFꢀ1ꢀL1 and ꢀL2 have approximately oneꢀthird the BET surface area of IRMOFꢀ1, but their
H uptakes at 77 K are only 16 and 33% less than that of IRMOFꢀ1. Interestingly, the calculated
2
Q values of H adsorption of these four materials are slightly higher than those of the parent
st
2
MOFs (Fig. 5c, Table S2, S3). For IRMOFꢀ1ꢀL2 the Q value (6.52 kJ/mol) is 26% higher than
st
that of IRMOFꢀ1 (5.16 kJ/mol), and this observation is comparable to other crown etherꢀ
1
9
derivatized MOFs.
2
8
Because crown ethers are able to selectively bind alkali metal ions, which are predicted
2
9
to help enhance H storage in MOFs, we further tried to metalate IRMOFꢀ1ꢀL1, IRMOFꢀ1ꢀL2,
2
UiOꢀ66ꢀL10.33, and UiOꢀ66ꢀL20.25
.
L1ꢀ and L2ꢀbased MOFs were exposed to 0.1 M
LiNO /DMF and NaNO /DMF solution, respectively, at room temperature for 1 d and then
3
3
washed with DMF. After solvent exchange with CH Cl for 3 d, the materials were activated at
2
2
1
50 °C under vacuum. The PXRD patterns (Fig. S32, S33) and N 77 K adsorption isotherms
2
(Fig. S34) confirmed the maintenance of crystallinity and porosity after activation. As controls,

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their parent MOFs (IRMOFꢀ1 and UiOꢀ66) were also treated under the same metalation
conditions. The PXRD data indicated that IRMOFꢀ1 lost its crystallinity after the metalation
treatment, while UiOꢀ66 remained crystalline (Fig. S32, S33). These results reveal that the
MOFs composed of compact crown ether ligands are stable in the alkaliꢀmetalꢀcontaining
solutions and remain intact even after conventional activation. The four activated crown ether
MOFs after metalation were examined for H adsorption. However, the Q of H adsorption of
2
st
2
the resulting materials did not display any significant improvement. Other metalation conditions
were explored (e.g. different metal salts, solvents, temperatures, see Table S2, S3) but none
resulted in materials with enhanced H sorption. Furthermore, the inductively coupled plasmaꢀ
2
optical emission spectrometry (ICPꢀOES) analysis of these MOFs indicated no significant uptake
of lithium or sodium cations in the frameworks. These results suggest that the crown ether
MOFs were not efficiently metalated under the reaction conditions explored here. The metalꢀ
binding ability of the free ligands was examined by dissolving L1 and L2 in DMSOꢀd and
6
titrating in excess equivalents of alkali metals; however, no chemical shift was observed in the
1
H NMR spectra (Fig. S35 and S36). In contrast, compounds 1 and 2 (Figure 1) showed
18
significant changes in chemical shift when treated with alkali metalꢀcontaining solutions. It
may be that the electronꢀwithdrawing carboxylic acid groups adjacent to the crown ether in L1
and L2 substantially decrease the metalꢀbinding ability of these crown ether ligands.
Fig. 5 H sorption isotherms at 77 K (square) and 87 K (triangle); a) Black: IRMOFꢀ1, blue:
2
IRMOFꢀ1ꢀL1, and green: IRMOFꢀ1ꢀL2; b) black: UiOꢀ66, blue: UiOꢀ66ꢀL10.33, and green: UiOꢀ
6
6ꢀL20.25. Filled shape: adsorption; open shape: desorption. c) Isosteric heat (Q ) of H2
st
adsorption; black: IRMOFꢀ1, red: IRMOFꢀ1ꢀL1, blue: IRMOFꢀ1ꢀL2, green: UiOꢀ66, magenta:
UiOꢀ66ꢀL10.33, and orange: UiOꢀ66ꢀL20.25
.

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Conclusion
In this study, we have synthesized a variety of novel crown etherꢀappended bdc ligands
that can be incorporated into prototypical MOFs. L1−L4 are able to combine with Zn(II) to
form IRMOFꢀ1 architectures. The bulkiness of crown ether groups and the relatively small pores
of UiOꢀ66 framework permitted the synthesis of UiOꢀ66 analogues only from combinations of
bdc and crown etherꢀsubstituted bdc (L1 and L2) ligands. Ligands L3 and L4, containing soft
sulfur heteroatoms, appear to hinder the formation of UiOꢀ66 architecture. Unlike most of the
crown etherꢀcontaining MOFs that have been reported, the MOFs constructed from L1 and L2
show high stability toward conventional activation at high temperatures under vacuum and
metalation treatment. Although the crown ether MOFs appeared unable to bind alkaliꢀmetals
which could enhance hydrogen storage, these crown ether ligands may be of interest for hostꢀ
guest chemistry within MOFs. This work provides a route to incorporate the fundamental units
of supramolecular chemistry into MOFs. Combining macrocyclic groups with modular
architectures of MOFs could potentially lead to interesting applications in guest molecule capture
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and separations.
ASSOCIATED CONTENT
Supporting Information
Synthesis and characterization of starting materials and MOFs. This material is available free of
charge via the internet at http://pubs.acs.org.
AUTHOR INFORMATION
Corresponding authors
*Eꢀmail: scohen@ucsd.edu. Telephone: (858) 822ꢀ5596
Funding
This work was supported by a grant from the Department of Energy, Office of Basic Energy
Sciences, Division of Materials Science and Engineering under Award No. DEꢀFG02ꢀ
08ER46519. A.S. gratefully acknowledges the German Academic Exchange Service (DAAD)
for a 6ꢀmonth visiting graduate fellowship to conduct research at UC San Diego. A.S.
furthermore acknowledges the research cluster SusChemSys for a doctoral fellowship. The
project “Sustainable Chemical Synthesis (SusChemSys)” is coꢀfinanced by the European

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Regional Development Fund (ERDF) and the state of North RhineꢀWestphalia, Germany, under
the Operational Programme “Regional Competitiveness and Employment” 2007–2013.
Notes
The authors declare no competing financial interests.
ACKNOWLEDGEMENTS
We thank the Molecular Mass Spectrometry Facility (Dr. Yongxuan Su, UC San Diego) for
assistance with the MS experiments, and Prof. Paterno Castillo and Dr. Christopher MacIsaac
(Scripps Institution of Oceanography, UC San Diego) for assistance with the ICPꢀOES
experiments.
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