Porous interpenetrated zirconium-organic frameworks (PIZOFs): A chemically versatile family of metal-organic frameworks

Article abstract of DOI:10.1002/chem.201101015

We present the synthesis and characterization of porous interpenetrated zirconium-organic frameworks (PIZOFs), a new family of metal-organic frameworks obtained from ZrCl₄ and the rodlike dicarboxylic acids HO ₂C[PE-P(R¹,R²)-EP]CO₂H that consist of alternating phenylene (P) and ethynylene (E) units. The substituents R ¹,R² were broadly varied (alkyl, Oalkyl, oligo(ethylene glycol)), including postsynthetically addressable substituents (amino, alkyne, furan). The PIZOF structure is highly tolerant towards the variation of R ¹ and R². This together with the modular synthesis of the diacids offers a facile tuning of the chemical environment within the pores. The PIZOF structure was solved from single-crystal X-ray diffraction analysis. The PIZOFs are stable under ambient conditions. PIZOF-2, the PIZOF prepared from HO₂C[PE-P(OMe,OMe)-EP]CO₂H, served as a prototype to determine thermal stability and porosity. It is stable up to 325°C in air as determined by using thermogravimetry and powder X-ray diffraction. Argon sorption isotherms on PIZOF-2 revealed a Brunauer-Emmett-Teller (BET) surface area of 1250 m² g^-1 and a total pore volume of 0.68 cm³ g^-1. Copyright

Full text of DOI:10.1002/chem.201101015

DOI: 10.1002/chem.201101015
Porous Interpenetrated Zirconium–Organic Frameworks (PIZOFs):
A Chemically Versatile Family of Metal–Organic Frameworks
Andreas Schaate,[a] Pascal Roy,[b] Thomas Preuße,[b] Sven Jare Lohmeier,[a]
Adelheid Godt,*[b] and Peter Behrens*[a, c]
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FULL PAPER
Abstract: We present the synthesis and
characterization of porous interpene-
trated zirconium–organic frameworks
(PIZOFs), a new family of metal–or-
ganic frameworks obtained from ZrCl4
and the rodlike dicarboxylic acids
The PIZOF structure is highly tolerant
towards the variation of R1 and R2.
This together with the modular synthe-
sis of the diacids offers a facile tuning
of the chemical environment within the
pores. The PIZOF structure was solved
from single-crystal X-ray diffraction
analysis. The PIZOFs are stable under
ambient conditions. PIZOF-2, the
PIZOF prepared from HO2C
ACHTUNGTRENNUNG[PE-P-
T
prototype to determine thermal stabili-
ty and porosity. It is stable up to 3258C
in air as determined by using thermog-
ravimetry and powder X-ray diffrac-
tion. Argon sorption isotherms on
HO2C
G
N
that
consist of alternating phenylene (P)
and ethynylene (E) units. The substitu-
ents R1,R2 were broadly varied (alkyl,
PIZOF-2 revealed
a
Brunauer–
Emmett–Teller (BET) surface area of
1250 m2 gÀ1 and a total pore volume of
0.68 cm3 gÀ1.
Keywords: adsorption · coordina-
À
O alkyl, oligo(ethylene glycol)), in-
tion polymers
·
metal–organic
cluding postsynthetically addressable
substituents (amino, alkyne, furan).
frameworks · porosity · zirconium
Introduction
at atmospheric moisture.[10] Furthermore, Zr MOFs of the
UiO type are highly temperature resistant and exhibit large
surface areas.[9,10] Postsynthetic modifications of UiO-66-
NH2, a MOF isostructural to UiO-66 but with the 2-amino-
terephthalate as the linker,[11] prove good accessibility of the
pores and point out the potentials of Zr MOFs.
Metal–organic frameworks or porous coordination polymers
are porous materials that are constructed from metal-con-
taining secondary building units (SBUs), typically metal–
oxo clusters and organic linkers, most often dicarboxylates.
They find interest as materials in heterogeneous catalysis,[1]
gas storage,[2] separation,[3] electrodes,[4] and drug delivery.[5]
MOFs with short linkers are numerous but the synthesis of
MOFs with long linkers remains a challenge. Examples of
Here we report on an extension of the class of Zr-based
MOFs. The novel MOFs were constructed by using the very
(R1,R2)-EP]CO2H. These MOFs
long linkers HO2CACTHNUTRGENN[UG PE-PACHUTNGTRENNGUN
(R1,R2)-
very long linkers are the rodlike diacids HO2CACTHNUTRGNENG[U PE-PACHTUTGNRENNUGN
EP]CO2H (P stands for phenylene, E for ethynylene, and R1
and R2 denote the substituents at the central phenylene
unit; Figure 1a). With R1,R2 =Hex, OMe, and crown ether,
they have served for the preparation of coordination poly-
mers with Zn-based SBUs.[6] In general, Zn-based MOFs are
sensitive to moisture,[7] and often the framework collapses
upon guest removal.[2,8]
More stable MOFs are described for the highly charged
and highly oxygen-affine cation Zr4+. UiO-66 with the
linker terephthalate is stable in water,[9] and UiO-67 and
UiO-68-NH2 with biphenyl dicarboxylate and amino-substi-
tuted terphenyldicarboxylate as the linkers can be handled
[a] A. Schaate, Dr. S. J. Lohmeier, Prof. Dr. P. Behrens
Institute of Inorganic Chemistry
Leibniz University Hannover
Callinstrasse 9, 30167 Hannover (Germany)
Fax : (+49)511-762-3006
E-mail: peter.behrens@acb.uni-hannover.de
Figure 1. a) Structural formula of the dicarboxylic diacids HO2C
(R1,R2)-EP]CO2H; the substituents R1 and R2 are defined in Scheme 1.
b) Zr6O4(OH)4A(O2C)12 secondary building unit of the PIZOFs (black: C,
ACHTUNGTRENNUNG[PE-P-
[b] P. Roy, T. Preuße, Prof. Dr. A. Godt
Department of Chemistry
AHCTUNGTRENNUNG
CTHUNGTRENNUNG
Bielefeld University
red: O, gray: polyhedra around Zr). c) One of the two frameworks of
PIZOF-2 showing the arrangement of concave and convex tetrahedral
cavities. d) Topological representation of the interpenetrated structure of
the PIZOFs. e) Convex cavity. f) Concave cavity. g) Crystal structure of
the PIZOFs; the interpenetrating framework is depicted in blue. Due to
the fact that linkers from the second framework pass through the “octa-
hedral” voids of the first framework, these “octahedral” voids are effec-
tively reduced to “tetrahedral” voids. In c and e–g, the spheres represent
the largest spheres that fit into the cavities (diameters: yellowꢀ19 ꢂ,
blueꢀ14 ꢂ).
Universitꢁtsstrasse 25, 33615 Bielefeld (Germany)
Fax : (+49)521-1006-6417
E-mail: godt@uni-bielefeld.de
[c] Prof. Dr. P. Behrens
ZFM: Center for Solid-State Chemistry
and New Materials, Leibniz University Hannover
Callinstrasse 9, 30167 Hannover (Germany)
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.201101015.
Chem. Eur. J. 2011, 17, 9320 – 9325
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P. Behrens, A. Godt et al.
¯
constitute a novel series of isostructural porous interpene-
trated Zr–organic frameworks (PIZOFs; Figures 1 and 2)
with broadly variable substituents R1 and R2. The modulator
approach, introduced by Kitagawa and co-workers,[12] is an
essential prerequisite for the synthesis of the PIZOFs. Re-
cently, we have published that with such a modulator ap-
proach, the crystal size and the morphology of the Zr MOFs
UiO-66, UiO-67, and UiO-68-NH2 can be controlled.[10] Ad-
ditionally, the syntheses of these MOFs became highly re-
producible.
PIZOF-2 crystallizes in space group Fd3m with a=
39.81 ꢂ. The structure and connectivity of the SBUs are the
same as in UiO MOFs.[9] However, the PIZOFs show two
independent interpenetrating networks, whereas the UiO
MOFs are not interpenetrated. The SBUs consist of six Zr4+
ions with square-antiprismatic coordination by eight oxygen
atoms in the form of m3-O, m3-OH, and carboxylate groups.
Each SBU is connected to twelve other SBUs by dicarboxy-
lates (Figure 1b), thereby resulting in an expanded cubic
close packing of SBUs with four octahedral and eight tetra-
hedral voids (Figure 1c). Four of the latter are occupied by
the SBUs of a second, structurally identical framework that
interpenetrates the first one (Figure 1d). The arrangement
of the SBUs of the two frameworks resembles the positions
of Zn and S in zincblende. Since the linkers are bent,[13]
there are convex (Figure 1e) and concave tetrahedral cavi-
ties (Figure 1f) of substantially different free internal diame-
ters of 19 and 14 ꢂ (excluding van der Waals radii). The
convex tetrahedral voids are free of SBUs; the concave ones
contain the SBUs of the second framework (Figure 1g). The
twelve linkers of these SBUs penetrate the four trigonal
windows in groups of three, thereby extending into octahe-
dral cavities. Because each octahedral cavity shares its eight
trigonal faces with eight tetrahedral cavities, four of which
contain SBUs, each octahedral cavity is penetrated by six
linkers that form an additional SBU-free concave tetrahe-
dral cavity (Figure 1g). To the best of our knowledge, this is
the first description of an interpenetrated framework with a
twelvefold connected SBU. Interpenetration is possible due
to the length of the linkers, which form triangular windows
of sufficient size to let three other linkers pass.
Results and Discussion
The PIZOFs were obtained from ZrCl4 and diacids HO2C-
ACHTUNGTRENNUNG[PE-PACHTUNGTRENNUNG
(R1,R2)-EP]CO2H 1b–8b (Scheme 1) in DMF in the
presence of benzoic acid as the modulating agent.[10,12]
PIZOF-1, PIZOF-2, and PIZOF-8 were obtained as single
crystals with octahedral shape; PIZOF-3 to -7 were obtained
as powders. The framework structure was determined from
single crystals. PIZOF-2 serves as the example here.
Based on the single-crystal data, the powder X-ray dif-
fraction (PXRD) pattern of PIZOF-2 was simulated. The
excellent agreement of simulated and experimental PXRD
patterns (Figure S1 in the Supporting Information) together
with the close similarity of the PXRDs of PIZOF-1 to -8
(Figure 2) indicates a common framework structure even
though substituents R vary significantly; they range from
simple alkyl, alkoxy, and oligo(ethyleneglycol) chains of dif-
ferent lengths to the chemically addressable groups amino,
alkyne, and furan. Expectedly, all PIZOFs with their
common structure have similar cell dimensions (Table S1 in
the Supporting Information).
For further characterization, PIZOF-2 also serves as an
example. Thermogravimetry of PIZOF-2 in flowing air re-
veals two distinct steps (Figure 3a; data of other PIZOFs
can be found in Table S2). The residue is monoclinic ZrO2.
We assume that guests are released up to a temperature of
3008C. The mass loss above 3808C is ascribed to oxidation
of the linkers. PXRD patterns reveal that the framework is
stable up to 3258C in air and that only minor structural
changes occur up to this temperature (Figure 3b). PIZOF-2
decomposes upon heating to 3508C in air.
Scheme 1. Syntheses of the rodlike dicarboxylic acids HO2C
ACHTUNGTRNE[NUNG PE-P-
Despite interpenetration, the framework of PIZOF-2 ap-
pears to be accessible for guests. Only one half of the tetra-
hedral cavities of one framework is occupied with the SBUs
of the other, interpenetrating framework. The other half,
A
R
ACHTUNGTRENNUNG
À
CO2Alk 1a–8a. The diesters were obtained through Pd/Cu-catalyzed C
C cross coupling. The different substituents R1 and R2 are introduced
with the 1,4-dihalobenzene.
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Zirconium–Organic Frameworks
FULL PAPER
Figure 2. PXRD patterns of PIZOF-1 to PIZOF-8. The structure of the
central benzene ring of the corresponding dicarboxylic acid is depicted
on the right.
the convex ones (Figure 1e), is free of SBUs. As described
above, the octahedral voids of one framework obtain the
shape of tetrahedral voids by passage of linkers from the
other framework. All voids thus have a similar shape that is
described by the yellow spheres in Figure 1c,e,g and
Figure 4. As can be seen in Figure 4, these yellow spheres
overlap. Therefore, they form a continuous three-dimension-
al pore system with a topology related to the diamond struc-
ture (Figure 4). Each accessible void is surrounded by four
other voids in a tetrahedral manner (Figure 4b).
Argon sorption experiments proved the porosity of
PIZOF-2. The sorption isotherm (Figure 3c) shows a contin-
uous adsorption up to a pressure of around p/p0 =0.06 and a
steep increase at this pressure. The sharp increase corre-
sponds to the filling of the tetrahedral voids as proven by
fitting the curve using nonlinear density functional theory. It
corresponds to the presence of pores with a diameter of
around 20 ꢂ, which is in very good agreement with the
value of 19 ꢂ derived from the crystal structure. The adsorp-
tion at low pressure is assigned to the filling of smaller cavi-
ties between the SBUs and the linkers and between the link-
ers of the two networks. PIZOF-2 has a Brunauer–Emmett–
Teller (BET) surface area of 1250 m2 gÀ1 and a total pore
volume of 0.68 cm3 gÀ1.
Figure 3. a) Thermogravimetric measurement of PIZOF-2 in flowing air.
b) PXRD patterns of as-synthesized PIZOF-2 after being heated to 325
and 3508C in air for 1 h (from bottom to top). c) Argon-gas sorption iso-
therms for PIZOF-2 (heated to 3008C in air prior to sorption experi-
ment) at 87 K as linear scale plot; inset: logarithmic scale plot. Red sym-
bols: adsorption, blue symbols: desorption.
PIZOFs constitute a family of MOFs with a surprisingly
high tolerance in their formation towards changes of the
substituents at the linkers. This is probably caused by the
À
strong Zr O bond, the large driving force for the formation
of the SBUs, and the high connectivity of the SBUs. The
persistence of the framework structure together with a
linker synthesis (Scheme 1) that is modular and itself highly
compatible with a large variety of substituents R1,R2 allows
for deliberate choice of functional groups. The synthesis of
Figure 4. a) Topological representation of the PIZOF structure with
yellow spheres representing accessible pore spaces of convex tetrahedral
shape as depicted in Figure 1e. b) Connecting the centers of the yellow
spheres reveals the relation of the three-dimensional pore system to the
topology of cubic diamond.
(R1,R2)-EP]CO2H (1b–8b) makes
the linkers HO2CACHTUNGTRENNUNG[PE-PAHCTUNGTRENNGUN
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P. Behrens, A. Godt et al.
À
room temperature for 1–4 h, the suspension was centrifuged and the sol-
vent was decanted off. The obtained particles were washed with ethanol
(10 mL) in the same way as described for DMF. Finally, the solid was
dried under reduced pressure.
use of Pd/Cu-catalyzed C C cross coupling, a reaction that
very often gives high yields and is compatible with a variety
of functional groups. The coupling partners are 2,5-disubsti-
tuted 1,4-dihalobenzenes and alkyl 4-ethynylbenzoate. The
resulting diesters 1a–8a were saponified to obtain the di-
Characterization: Powder X-ray diffraction was carried out using a Stoe
Stadi P diffractometer operated with GeACHTUNGTRNEG(UN 111)-monochromatized CuKa1
A
radiation (l=1.54060 ꢂ) in transmission mode. TGA measurements
were performed using a Netzsch STA 429 thermoanalyzer. For this pur-
pose, the samples were filled into alumina crucibles and heated under a
flow of air at a heating rate of 58CminÀ1 up to 10008C. Argon sorption
isotherms were measured at 87 K using a Quantachrome Autosorb-1 in-
strument. Initially, the sample was heated to 3008C in air for 1 h to
remove all guests. The sample was further outgassed in vacuum at 1508C
for 24 h directly before the measurement to remove gases taken up from
the atmosphere during storage under ambient conditions. The pore-size
distribution was determined using the calculation model Ar at 87 K zeo-
lites/silica (cylinder pores, non-local DFT (adsorption branch model)) of
the AS1Win software for Autosorb-1 instruments (version 2.11) from
Quantachrome.
R1 and R2 was achieved through use of different 2,5-disub-
stituted 1,4-dihalobenzenes. The same route has been de-
scribed for the preparation of diacid HO2CACTHUNRTGNE[GNU PE-P(crown
ether)-EP]CO2H.[14] The ester as an intermediate makes pu-
rification—that is, the removal of metals, metal salts, catalyst
ligands, and 1,4-di[(4-alkyloxycarbonyl)phenyl]butadiyne,
the product that results from oxidative dimerization (Glaser
coupling) of alkyl 4-ethynylbenzoate—easy. This procedure
is much shorter than our previously reported synthesis that
starts from alkyl 4-iodobenzoate and 2,5-disubstituted 1-eth-
ynyl-4-(2-triisopropylsilylACHTUNGTRNEUNG
ethynyl)benzene.[15] Furthermore,
Single-crystal X-ray diffraction studies for PIZOF-1, PIZOF-2, and
PIZOF-8 were carried out using a Bruker KAPPA APEX II CCD dif-
fractometer equipped with a graphite crystal monochromator situated in
the incident beam for data collection at 173 K for PIZOF-1, at 123 K for
PIZOF-2, and at 103 K for PIZOF-8. The determination of unit-cell pa-
rameters and data collections were performed with CuKa radiation (l=
1.54178 ꢂ). The program SAINT was used for integration of the diffrac-
tion profiles. Adsorption corrections were applied using the SADABS
routine. The structures were solved by direct methods and refined by
full-matrix least-squares on F2 with anisotropic displacement using the
SHELXTL software package.[16] The hydrogen atoms were added theo-
retically, riding on the concerned atoms. Free solvent molecules were
highly disordered and attempts to locate and refine solvent molecules
were unsuccessful. The diffuse electron densities that resulted from these
solvent molecules were removed from the data set using the SQUEEZE
routine of PLATON[17] and refined further using the data generated.
the second alternative, the coupling of 2,5-disubstituted 1,4-
diethynylbenzene with alkyl 4-iodobenzoate,[14] appears
much less attractive to us for several reasons: 1) more con-
secutive steps, therefore less efficient synthesis, 2) a larger
number of consecutive steps with the moiety that is varied
to tune the interior of the PIZOFs, and 3) variation of the
substituents of 1,4-diethynylbenzene results in different
Glaser coupling products during coupling with alkyl 4-iodo-
benzoate. Therefore, individual protocols for their removal
would have to be devised and separation may not always be
possible.[15] Additionally, 1,4-diethynylbenzenes tend to be
instable, whereas alkyl 4-ethynylbenzoate can be stored for
months.
Crystal data for PIZOF-1: C72H24O16Zr3, Mr =1418.57; space group
Fd3m, cubic; a=39.845(12) ꢂ; V=63257(34) ꢂ3; T=173 K; Z=16;
¯
m
N
flections (Rint =0.1601); R1 (I>2s(I))=0.0485, wR2=0.1078; GOF=
0.960.
Conclusion
Crystal data for PIZOF-2: C78H24O22Zr3, Mr =1586.63; space group
(R1,R2)-
In conclusion, the use of diacids HO2CACTHNUTRGENNUG[PE-PACHTUNGTREUNGNN
Fd3m, cubic; a=39.8144(11) ꢂ; V=63113(3) ꢂ3; T=123 K; Z=16;
¯
EP]CO2H 1b–8b as linkers led to PIZOF-1 to PIZOF-8, a
new family of Zr MOFs with the characteristics of very long
linkers, stability towards atmospheric moisture, high heat re-
sistance, large voids, and a broad variability of substituents
at the linkers, including functional groups ready for postsyn-
thetic modification. Thus, PIZOFs with different chemical
functionalities and environments within the pores can be ob-
tained in a straightforward manner.
m
N
flections (Rint =0.0769); R1 (I>2s(I))=0.0488, wR2=0.1056; GOF=
1.084.
¯
Crystal data for PIZOF-8: C72H24O22Zr3, M=1514.57; space group Fd3m,
cubic; a=39.8015(8) ꢂ; V=63052(2) ꢂ3; T=103 K; Z=16; m
ACHTUNGTRENNUNG(CuKa)=
1.880 mmÀ1; 38000 reflections measured, 2613 independent reflections
(Rint =0.0854); R1 (I>2s(I))=0.0936, wR2=0.2468; GOF=1.046.
CCDC-803459 (PIZOF-1), 803460 (PIZOF-2), and 803461 (PIZOF-8)
contain the supplementary crystallographic data for this paper. These
data can be obtained free of charge from The Cambridge Crystallograph-
ic Data Centre via www.ccdc.cam.ac.uk/data_request/cif.
Experimental Section
Synthesis of linkers 1b–8b: For a detailed description of the syntheses,
please refer to the Supporting Information.
Acknowledgements
This work was supported as a part of the priority program 1362 (Porous
Metal–Organic Frameworks) by the Deutsche Forschungsgemeinschaft.
We thank Ms. Hꢃlsmann, Ms. Senger, Mr. Heinrich, and Ms. Brosent for
their contributions to the linker syntheses and Mr. Lippke for his contri-
bution to the syntheses of the PIZOFs.
Synthesis of PIZOF-n: The syntheses of the PIZOFs were performed in
100 mL Teflon-capped glass flasks. For a typical synthesis, ZrCl4 (0.080 g,
0.343 mmol) and benzoic acid (1.256 g, 10.287 mmol) were dissolved in
DMF (20 mL) by using ultrasound to give a clear solution. One of the di-
acids HO2CACHTUNGTRENNUNG[PE-PACHTUNGTRENNUNG
(R1,R2)-EP]CO2H 1b–8b (0.343 mmol) was added and
dissolved by the application of ultrasound. The tightly capped flask was
kept in an oven at 1208C under static conditions for 24 h. The suspension
was cooled to room temperature and the precipitate was isolated by cen-
trifugation. The solid was suspended in DMF (10 mL). After standing at
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Zirconium–Organic Frameworks
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Received: April 3, 2011
Published online: July 27, 2011
Chem. Eur. J. 2011, 17, 9320 – 9325
ꢀ 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
9325