Superhydrophobic perfluorinated metal-organic frameworks

Article abstract of DOI:10.1039/c3cc41564c

Three perfluorinated Cu-based metal-organic frameworks (MOFs) were prepared starting from extensively fluorinated biphenyl-based ligands accessed via C-H functionalization. These new materials are highly hydrophobic: with water contact angles of up to 151

Full text of DOI:10.1039/c3cc41564c

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Superhydrophobic perfluorinated metal–organic
frameworks†
Cite this: Chem. Commun., 2013,
49, 6846
Teng-Hao Chen, Ilya Popov,* Oussama Zenasni, Olafs Daugulis and
Received 1st March 2013,
Accepted 7th June 2013
ˇ
´
Ognjen S. Miljanic*
DOI: 10.1039/c3cc41564c
www.rsc.org/chemcomm
Three perfluorinated Cu-based metal–organic frameworks (MOFs)
were prepared starting from extensively fluorinated biphenyl-based
ligands accessed via C–H functionalization. These new materials are
highly hydrophobic: with water contact angles of up to 151 Æ 18,
they are among the most water-repellent MOFs ever reported.
Owing to their high thermal and chemical stability and unique
physical properties, fluorinated polymers1 are desirable materials in
industrial and household applications. The recent explosion of
interest in modularly synthesized metal–organic frameworks
(MOFs)2 quickly prompted efforts to amalgamate these two classes
of materials through the preparation of extensively fluorinated
MOFs. Such ‘‘Teflon-coated MOFs’’ promise to show superhydro-
phobic behavior and increased robustness towards moisture3 and
Scheme 1 Synthesis of ligands 3 and 4. See ESI† for experimental details.
their fluorophilic character could facilitate the sequestration and The Cu-catalyzed deprotonative arylation of C–H bonds combines
analysis of fluorinated pollutants, including Freons. Efforts to intro- low cost of catalysts and high generality with respect to the coupling
duce heavily fluorinated linkers into MOFs were limited in their partners.6 This approach was successfully used to prepare linear
scope by the relatively small pool of readily available fluorinated perfluorinated carboxylic acid 3 and tetrazole 4 (Scheme 1). After
organic linkers.4 Most significantly, fluorinated analogs of large extensive optimization of the reaction parameters, 2,3,5,6-tetrafluoro-
aromatic oligocarboxylates—which currently represent a mainstay benzonitrile (1) was found to be an efficient cross-coupling partner
of MOF chemistry—are essentially unknown, with the notable in the Cu-catalyzed reaction with 2,3,5,6-tetrafluoro-4-iodobenzo-
exception of the commercially available perfluoroterephthalic acid.5 nitrile, affording the key dinitrile intermediate 2. Hydrolysis of 2
In this communication, we present a general strategy for the pre- was achieved under strongly acidic conditions: heating compound 2
paration of such large, perfluorinated and rigid aromatic carboxylic with an excess of triflic acid in trifluoroacetic acid solvent afforded
acids and tetrazoles, and proceed to demonstrate that these novel diacid 3 in excellent yield. On the other hand, the reaction between
ligands can be reticulated into MOFs under solvothermal conditions. dinitrile 2 and NaN3 in the presence of ZnCl2 provided bistetrazole 4
These new materials—which we propose to name MOFFs, high- in high yield.7 The developed cross-coupling should be readily
lighting their fluorinated character—have highly hydrophobic struc- extendable to the synthesis of diversely substituted benzenes with
tures with H2O contact angles as high as 151 Æ 11.
polyfluoroarene substituents.8
Our investigations were initially aimed towards developing
With linkers 3 and 4 in hand, we proceeded to develop synthetic
a method for the synthesis of fluorinated polyaryl ligands. conditions for their incorporation into single-crystalline MOFs.
Ligand 3 was combined with Cu(NO3)2Á212H2O in a 1:18:1 mixture
of N,N-dimethylformamide (DMF), MeOH and H2O. After 4 d of
heating at 40 1C, greenish-blue plate-shaped crystals of MOFF-1 were
isolated. Their structural analysis using single crystal X-ray diffrac-
tion9 revealed an infinite two-dimensional network (Fig. 1), in which
pairs of Cu atoms form paddlewheel-shaped Cu2(COO)4 clusters that
are capped with one MeOH molecule at each Cu. This structure
permits formulating the obtained material as Cu2+(3-2H+)(MeOH).
Department of Chemistry, University of Houston, 136 Fleming Building, Houston,
Texas 77204-5003, USA. E-mail: ilja.popovs@gmail.com, miljanic@uh.edu;
Fax: +1-713-743-2709; Tel: +1-832-842-8827
† Electronic supplementary information (ESI) available: Synthetic procedures and
characterization data for all new compounds and MOFs, and crystallographic
information files (CIFs) for MOFFs 1–3. CCDC 902654, 902655 and 902657. For
ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/
c3cc41564c
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Fig. 1 X-ray crystal structure of MOFF-1, Cu2+(3-2H+)(MeOH). (A) Secondary
building unit; (B) representative segment of the two-dimensional layer structure;
(C) side-on view of interlayer orientation.
Fig. 3 X-ray crystal structure of MOFF-3, Cu2+(4–2H+)(H2O). (A) Secondary
building unit; (B) view along the one-dimensional channels in the structure;
(C) view of the structure perpendicular to the orientation of the channels.
Two-dimensional sheets of MOFF-1 organize into a three-
dimensional crystal through parallel offset stacks, in which the
Cu2(COO)4 cluster of one layer fits into the void space of the
adjacent layers. The two-dimensional grid structure of this MOF is
similar to that of its non-fluorinated analog MOF-118,10 but the two
frameworks differ in their three-dimensional organization: while
MOF-118 presents a rare interpenetrated square grid network,
MOFF-1 is composed of parallel stacked two-dimensional layers.
In the presence of a bifunctional pillaring ligand 1,4-diazabi-
cyclo[2.2.2]octane (DABCO), diacid 3 was converted into a pillared
three-dimensional Cu-based framework MOFF-2. Specifically, heat-
ing a solution of ligand 3, DABCO and Cu(NO3)2Á212H2O in a 3:18:1
mixture of DMF, MeOH and H2O at 60 1C for 2 d resulted in
greenish single crystals. Their analysis using X-ray diffraction
connectors. The structure is two-fold interpenetrated, in contrast
to its non-fluorinated constitutional analog DMOF-1-bpdc.11
Finally, linker 4 was used to demonstrate that perfluorinated
tetrazolate-based linkers can also be coordinated into MOFs.
A solution of 4 and CuCl2Á2H2O in DMF was heated at 70 1C
for 4 d. Blue rod-shaped single crystals that resulted were
analyzed using X-ray diffraction, revealing (Fig. 3) a porous
three-dimensional structure of MOFF-3. In this structure, octa-
hedral Cu atoms are coordinated to peripheral tetrazolate
nitrogens in four separate molecules of 4, and an H2O molecule
acts as a bridge between each pair of adjacent Cu centers. This
structure is analogous to Long’s previously reported example of
Cu-bistetrazolate MOFs,12 and it also changes significantly
upon heating as the coordinated H2O molecules are removed.
Thermal stabilities of MOFFs 1–3 were evaluated using thermo-
gravimetric analysis (TGA, see ESI† for details). MOFF-1 shows a
relatively featureless TGA trace, suggesting that the initial slow loss
of coordinated and encapsulated solvent overlaps with the more
rapid weight loss caused by framework decomposition occurring
at around 220 1C.13 MOFF-2 does not crystallize with included
solvent and thus shows no weight loss until it starts to rapidly
decompose at around 270 1C. For MOFF-3, loss of solvent
(14.5% weight) occurs under 80 1C; the desolvated framework
then remains stable until B230 1C, when final decomposition
begins. Decomposition temperatures for these MOFs are compar-
able to those for similar non-fluorinated networks, suggesting
that the cleavage of the strong C–F bond (110–120 kcal molÀ1) is
unlikely during framework decomposition.
2+
+
1
revealed (Fig. 2) the expected constitution Cu (3-2H )(DABCO)
2
and the pillared arrangement of layers mediated by DABCO
Nitrogen sorption (see ESI† for details) within the pores of
MOFF-1 and MOFF-2 was characterized by typical type I isotherms—
with the corresponding BET surface areas of 580 and 444 m2 gÀ1
,
respectively. In contrast, MOFF-3 showed the highest and very
hysteretic uptake of N2, perhaps indicative of its breathing behavior;
this issue will be further explored in the forthcoming full paper.
To evaluate the hydrophobic/philic characteristics of MOFFs
1–3, we performed contact angle measurements with H2O
(Table 1). Samples of MOFF-1 are wettable by H2O if air-dried,
+
Fig. 2 X-ray crystal structure of MOFF-2, Cu2+(3-2H )(DABCO) . The disorder in
1
2
the DABCO ligand is apparent. (A) Secondary building unit; (B) view along the
one-dimensional channels in the structure; (C) side-on view of the interpene-
trated framework, where two independent nets are shown in different colors.
c
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Table 1 Water contact angle measurements for MOFFs 1–3a
ˇ
Foundation (to O. D.). O. S. M. is a Cottrell Scholar of the
Research Corporation for Science Advancement.
Framework
Contact angle
MOFF-1
B01 b
MOFF-2
MOFF-3
151 Æ 11 c
134 Æ 11 c
108 Æ 21 c
135 Æ 21 d
Notes and references
a
b
c
Average of three measurements. Air-dried. Dried in a vacuum oven
1 (a) Modern Fluoropolymers: High Performance Polymers for
Diverse Applications, ed. J. Scheirs, Wiley, Hoboken, NJ, 1997;
(b) Fluoropolymers, ed. L. A. Wall, Wiley, New York, NY, 1972.
2 (a) Metal–Organic Frameworks: Applications from Catalysis to Gas
Storage, ed. D. Farrusseng, Wiley-VCH, Weinheim, 2011; (b) Metal–
Organic Frameworks: Design and Applications, ed. L. R. MacGillivray,
Wiley, Hoboken, NJ, 2010.
3 (a) C. Yang, U. Kaipa, Q. Z. Mather, X. Wang, V. Nesterov, A. F.
Venero and M. A. Omary, J. Am. Chem. Soc., 2011, 133, 18094–18097;
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d
(120 1C, 24 h). Dried with supercritical CO2, see ESI for details.
which is probably caused by the coordinated hydrophilic mole-
cules of MeOH. Upon oven-drying, these solvent molecules are
removed and the residual framework becomes water-repellent
(H2O contact angle of 108 Æ 21), as does oven-dried MOFF-3
(H2O contact angle of 134 Æ 11). The most hydrophobic
material among these new fluorinated MOFs is MOFF-2, with
a H2O contact angle of 151 Æ 11. As MOFF-2 crystallizes without
included solvent molecules, its structure and hydrophobicity
are unaffected upon drying. Further evidence for the super-
hydrophobic behavior of the prepared MOFFs came from the
water vapor adsorption studies (see ESI† for details). These
revealed that MOFFs 1–3 adsorb negligible amounts of water,
even at 90% relative humidity (o2 kg mÀ3)—which is compar-
able to the very low water adsorption of Omary’s perfluorinated
FMOF-1.3 Since other large perfluorinated ligands are expected
to be hydrophobic, this direct-synthesis route to highly hydro-
phobic MOFs appears to be broadly applicable and comple-
mentary to Cohen’s postsynthetic functionalization approach14
to superhydrophobic MOFs.
In summary, we have utilized C–H functionalization to
access novel perfluorinated aromatic linkers, which were in
turn reticulated into highly hydrophobic, extensively fluori-
nated metal–organic frameworks (MOFFs). The preparative
route to ligands presented here is simple and general, and
other extensively fluorinated ligands (and the derived MOFs)
could be generated through straightforward adaptation of our
protocol. As the extended aromatic ligands shown here open up
pathways to highly porous fluorinated MOFs, it should be
possible to explore and capitalize upon unique adsorption
and binding properties anticipated for these materials. Finally,
these new fluorinated precursors have B300 times higher
acidities than their non-fluorinated counterparts,15 and can be
coordinated into MOFs at temperatures as low as 40 1C—which
could be of interest in the effort to produce high-resolution
patterned MOF arrays on surfaces.16
4 For recent examples of MOFs incorporating –CF3 functionalities,
see: (a) P. Pachfule, C. Dey, T. Panda, K. Vanka and R. Banerjee,
Cryst. Growth Des., 2010, 10, 1351–1363; (b) P. Pachfule, C. Dey,
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Cheetham, Dalton Trans., 2009, 1131–1135. For fluoropyridine
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Y. J. Oh, M. J. Seo, Y. H. Jhon, T.-B. Lee, D. Kim, S. H. Choi and
J. Kim, Catal. Today, 2007, 120, 324–329; ( f ) B. Chen, Y. Yang,
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Chem., 2006, 45, 8882–8886; (g) H. Chun, D. N. Dybtsev, H. Kim and
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7 Z.-H. Zhao, H. Jin, Y.-X. Zhang, Z. Shen, D.-C. Zou and X.-H. Fan,
Macromolecules, 2011, 44, 1405–1413.
8 Synthesis of other perfluorinated MOF ligands will be described in a
forthcoming full paper.
9 CCDC 902654, 902655 and 902657†.
We thank Drs Antonio DiPasquale (UC Berkeley) and James
D. Korp (University of Houston, UH) for the collection and 10 H. Furukawa, J. Kim, N. W. Ockwig, M. O’Keeffe and O. M. Yaghi,
J. Am. Chem. Soc., 2008, 130, 11650–11661.
11 (a) P. V. Dau, M. Kim, S. J. Garibay, F. H. L. Munch, C. H. Moore and
refinement of crystal structure data, Dr Casey Wade and Prof.
˘
Mircea Dinca (MIT) for assistance with N2 and H2O sorption
S. M. Cohen, Inorg. Chem., 2012, 51, 5671–5676; (b) K. Seki, Chem.
Commun., 2001, 1496–1497.
experiments, Prof. Allan J. Jacobson (UH) for assistance with
powder X-ray diffraction and TGA, and Prof. T. Randall Lee
(UH) for providing access to contact angle measurement equip-
ment. We acknowledge the financial support from the Univer-
˘
12 M. Dinca, A. F. Yu and J. R. Long, J. Am. Chem. Soc., 2006, 128, 8904–8913.
13 Calculations of MOFFs’ percent weight loss only roughly correlate
with the amount of solvent observed in the crystal structure. The
electron densities associated with the disordered solvent (present
within MOFF pores) were subtracted using the SQUEEZE routine
and their weight contributions to the crystals were not quantified.
14 (a) J. G. Nguyen and S. M. Cohen, J. Am. Chem. Soc., 2010, 132, 4560–
4561. See also: (b) C. R. Wade, T. Corrales-Sanchez, T. Narayan and
ˇ
sity of Houston (to O. S. M.), the Norman Hackerman Advanced
Research Program (to O. D.), the National Science Foundation
ˇ
(award CHE-1151292 to O. S. M.), the donors of the American
˘
M. Dinca, Energy Environ. Sci., 2013, 6, DOI: 10.1039/c3cc40876k.
Chemical Society Petroleum Research Fund (award 50390-DNI10
15 D. Jiang, A. D. Burrows and K. J. Edler, CrystEngComm, 2011, 12, 6916–6919.
16 For a review, see: D. Zacher, R. Schmid, C. Woll and R. A. Fischer,
ˇ
to O. S. M.), the Welch Foundation (awards E-1571 to O. D. and
¨
ˇ
E-1768 to O. S. M.) and the Camille and Henry Dreyfus
Angew. Chem., Int. Ed., 2011, 50, 176–199.
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6848 Chem. Commun., 2013, 49, 6846--6848
This journal is The Royal Society of Chemistry 2013