Expanding the group of Porous Interpenetrated Zr-Organic Frameworks (PIZOFs) with linkers of different lengths

Article abstract of DOI:10.1021/acs.inorgchem.6b01814

A Zr-based MOF of the PIZOF type, which consists of two independent and mutually interpenetrating UiO-type frameworks with [Zr6O4(OH)4(O2C)12] nodes, does not only form with a PEPEP dicarboxylic acid (P = phenylene, E = ethynylene). Also dicarboxylic acids with the shorter PPPP and PEPP spacers were found to give PIZOFs, denoted PPPP-PIZOF and PEPP-PIZOF, respectively. Reducing the spacer length even further to a PEEP segment caused a switchover to the formation of a UiO framework. The hysteresis in the Ar sorption curve of PEPP-PIZOF-1 and the slightly too large amount of combustion residue from PPPPPIZOF- 1 suggest structural defects. These hint at a mismatch between the requirement of the optimal linker length for PIZOF formation and the lengths of the PEPP and PPPP dicarboxylates. Nevertheless, these dicarboxylates prefer the formation of a PIZOF over the formation of a UiO structure. PEPEP-PIZOF-2, PPPP-PIZOF-1, and PEPP-PIZOF-1 are stable in air up to 325, 350, and 300 °C, respectively, and have BET surface areas of 2350, 2020, and 1650 m2 g-1, respectively. PEPEP-PIZOFs, even those with very hydrophilic oligo(ethylene glycol) side chains on the linkers, are very stable in water and also during drying from a water-wetted state. On the contrary, PEPP-PIZOF-1 and PPPP-PIZOF-1 that had been exposed to water required exchange of water for ethanol before drying to mostly preserve the framework. The results emphasize the importance of differentiating between framework damage caused through hydrolysis in water and through drying from a water-wetted state. The sensitivity of PEPP-PIZOF-1 and PPPP-PIZOF-1 against drying from a water-wetted state may be the consequence of defects. The drying stability of water-wetted PEPEP-PIZOFs lets us suggest that reversible bending of the linkers contributes to the stability of the PEPEP-PIZOFs.

Full text of DOI:10.1021/acs.inorgchem.6b01814

Article
pubs.acs.org/IC
Expanding the Group of Porous Interpenetrated Zr-Organic
Frameworks (PIZOFs) with Linkers of Different Lengths
Jann Lippke,† Birte Brosent,‡ Tobias von Zons,‡ Erika Virmani,§ Sebastian Lilienthal,† Thomas Preuße,‡
Miriam Hulsmann,‡ Andreas M. Schneider,† Stefan Wuttke,§ Peter Behrens,*,† and Adelheid Godt*,‡
̈
†Department of Inorganic Chemistry, Leibniz University Hannover, Callinstraße 9, 30167 Hannover, Germany
‡Faculty of Chemistry and Center for Molecular Materials (CM2), Bielefeld University, Universitatsstraße 25, 33615 Bielefeld,
̈
Germany
§Department of Chemistry and Center for NanoScience (CeNS), University of Munich (LMU), Butenandtstraße 11 (E), 81377
Munich, Germany
S
* Supporting Information
ABSTRACT: A Zr-based MOF of the PIZOF type, which
consists of two independent and mutually interpenetrating
UiO-type frameworks with [Zr6O4(OH)4(O2C)12] nodes,
does not only form with a PEPEP dicarboxylic acid (P =
phenylene, E = ethynylene). Also dicarboxylic acids with the
shorter PPPP and PEPP spacers were found to give PIZOFs,
denoted PPPP-PIZOF and PEPP-PIZOF, respectively. Re-
ducing the spacer length even further to a PEEP segment
caused a switchover to the formation of a UiO framework. The
hysteresis in the Ar sorption curve of PEPP-PIZOF-1 and the
slightly too large amount of combustion residue from PPPP-
PIZOF-1 suggest structural defects. These hint at a mismatch
between the requirement of the optimal linker length for
PIZOF formation and the lengths of the PEPP and PPPP dicarboxylates. Nevertheless, these dicarboxylates prefer the formation
of a PIZOF over the formation of a UiO structure. PEPEP-PIZOF-2, PPPP-PIZOF-1, and PEPP-PIZOF-1 are stable in air up to
325, 350, and 300 °C, respectively, and have BET surface areas of 2350, 2020, and 1650 m2 g−1, respectively. PEPEP-PIZOFs,
even those with very hydrophilic oligo(ethylene glycol) side chains on the linkers, are very stable in water and also during drying
from a water-wetted state. On the contrary, PEPP-PIZOF-1 and PPPP-PIZOF-1 that had been exposed to water required
exchange of water for ethanol before drying to mostly preserve the framework. The results emphasize the importance of
differentiating between framework damage caused through hydrolysis in water and through drying from a water-wetted state. The
sensitivity of PEPP-PIZOF-1 and PPPP-PIZOF-1 against drying from a water-wetted state may be the consequence of defects.
The drying stability of water-wetted PEPEP-PIZOFs lets us suggest that reversible bending of the linkers contributes to the
stability of the PEPEP-PIZOFs.
connected with 12 other IBUs via 12 dicarboxylate linkers
(Figure 1). For quite a while, the longest dicarboxylic acids with
which a MOF with the UiO-type structure had been reported
to form were substituted 1,1′:4′,1″-terphenyl-4,4″-dicarboxylic
acids (HO2C[P−P(R1,R2)−P]CO2H, where P stands for a
phenylene unit and R1 and R2 denote the substituents on the
central phenylene unit; abbreviated as PPP diacid; Figure
1b).5,37−41 Herein these MOFs will be denoted as PPP-UiO.
Only recently, during our own investigation, the even longer
dicarboxylic acid 4,4′-(buta-1,3-diyne-1,4-diyl)dibenzoic acid42
(HO2C[P−EE−P]CO2H, where E stands for an ethynylene
unit; abbreviated as PEEP diacid) was also reported to form a
UiO, here denoted as PEEP-UiO. A substituted
INTRODUCTION
■
Zirconium-based metal−organic frameworks (MOFs) have
become an intensely investigated family among MOFs.1−31
The main reason for this popularity is the high thermal and
outstanding chemical stability, especially against water, of many
members of this family.1,4,6,8,11,20,22,23,26−28,32 Stability in moist
air or in water as well as stability upon cycling between wet and
dry states are very important issues when it comes to
applications of MOFs.32−35 Most probably, the high stability
of Zr-based MOFs results from the high degree of
interconnection between the inorganic building units (IBUs;
also called secondary building units, SBUs) and the strong
bonds between the highly charged zirconium(IV) ions and the
carboxylate groups of the linker.1,36 The first Zr-based MOFs
were the UiOs,1 which feature [Zr6O4(OH)4(O2C)12] IBUs in
a cubic-close-packed arrangement (fcu topology). Each IBU is
Received: August 1, 2016
© XXXX American Chemical Society
A
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Figure 1. Structures of UiOs and PIZOFs (black, C; red, O; surrounded by coordination polyhedra, Zr). (a) [Zr6O4(OH)4(O2C)12] IBU. (b)
Possible linker backbones, without substituents, sorted by length; phenylene (P) and ethynylene (E) units are indicated. (c) Framework of UiO-66
with the fcu topology (P-UiO). (d) Single framework of a PEPEP-PIZOF with two different tetrahedral voids; voids containing blue balls are
occupied by IBUs of the second framework, and yellow balls are empty pores. (e) Two interpenetrated frameworks with the fcu topology. A pore
with a diameter of about 19 Å at the location of the octahedral void of one of the nets (yellow ball) has the same shape as the pores indicated by
yellow balls in (d). These pores form an interconnected three-dimensional pore system.
1,1′:4′,1″:4″,1‴-quaterphenyl-4,4‴-dicarboxylic acid43 (HO2C-
[P−P(R)−PP]CO2H; abbreviated as PPPP diacid) was
reported to form a UiO as well, a result which is in conflict
with our own findings (see below). Using the much longer
dicarboxylic acids HO2C[PE−P(R1,R2)−EP]CO2H (abbrevi-
ated as PEPEP diacid) resulted in the formation of porous
interpenetrated Zr-organic frameworks (PIZOFs; more pre-
cisely, PEPEP-PIZOFs).6 The PIZOFs consist of two
independent UiO-type frameworks that interpenetrate each
other (Figure 1e). Despite interpenetration, the PIZOFs are
highly porous and contain a continuous network of empty
pores with diameters of about 19 Å, which are well-accessible as
proven by postsynthetic modification.44 The PIZOF structure
forms from a large variety of PEPEP diacids differing widely in
the type of substituents R1 and R2 (Figures S1 and S2).6,42,45
With the structurally related PEAEP diacid (where A stands for
an anthracene unit), a similar interpenetrated structure
formed.46
We found nearly all of the PEPEP-PIZOFs tested to be
resistant against water (Figures S3 and S4) and against collapse
of the pores upon drying of the materials from a water-wetted
state. Drying from a water-wetted state has been reported to
cause collapse of UiO-67 (PP-UiO) and NU-1000.47 The water
and drying tolerance of the PEPEP-PIZOFs is contrary to the
reported limited stability of the structurally related UiO-67
(PP-UiO)5,18,48 and of PPP-UiOs37 and also contrary to our
experience that all PPP-UiOs deteriorate in the presence of
water and that many of them do so even in humid air.49 We
presumed that the pronounced stability of the PEPEP-PIZOFs
is a consequence of the interpenetration. Therefore, we were
curious whether the PIZOF structure is unique for the PEPEP-
linker and, if not, whether PIZOFs with linkers of other lengths
are as stable as the known PEPEP-PIZOFs. As a first step, we
checked the region of linker lengths between the PPP and the
PEPEP linkers using a PEEP diacid, a PEPP diacid, and a PPPP
diacid for Zr-MOF assembly. We found the crossover from
UiO-type MOFs to PIZOFs to occur between the PEEP diacid
and the PEPP diacid (Figure 1). Accordingly, the next longer
diacid, the PPPP diacid, also forms a PIZOF. Against our
expectation, the PEPP-PIZOF and the PPPP-PIZOF behaved
very differently from PEPEP-PIZOFs when tested for water
stability: they are highly sensitive to drying from a water-wetted
state. Nevertheless, they can be dried with only minor
destruction after exchange of water for ethanol. We assume
capillary forces to cause the framework collapse, as suggested by
Mondloch et al. for UiO-67 and NU-1000,47 and linker
bendability to enable PEPEP-PIZOFs to have an elastic
response to these forces.
This paper is organized as follows. After the syntheses of the
PEEP, PEPP, PPPP, and PEPEP diacids and the corresponding
Zr-based MOFs are described, the properties of the PEPP-
PIZOF and PPPP-PIZOF (structure, thermal stability, porosity,
and water compatibility) are presented and compared,
including PEPEP-PIZOF-2 as an archetype of the known
PEPEP-PIZOFs.
B
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μm). The size of the silica gel column is given as diameter × length.
Most often a slight pressure was applied during chromatography.
Thin-layer chromatography was performed on silica gel 60 F254-coated
aluminum foils. The spots were detected using a UV lamp (λ = 254
nm, λ = 366 nm). The compositions of solvent mixtures are given as
volume ratios.
EXPERIMENTAL SECTION
■
Material Characterization. Powder X-ray diffraction (PXRD)
measurements were carried out on a Stoe Stadi P diffractometer
operating with Ge(111)-monochromatized Cu Kα1 radiation (λ =
1.54060 Å) in transmission mode on samples placed between two foils.
Capillary measurements were performed on a Stoe Stadi P
diffractometer in Debye−Scherrer geometry. Indexing of PXRD
diagrams was performed with the software Stoe Win XPOW (version
1.08).
Thermogravimetric analysis (TGA) was performed using a Netzsch
STA 429 thermoanalyzer. Samples were filled into alumina crucibles
and heated under a flow of air at a heating rate of 5 °C/min up to 1000
°C.
Field-emission scanning electron microscopy (FE-SEM) images
were taken on a JEOL JSM-6700F microscope (acceleration voltage 2
kV) equipped with a semi in-lens detector. Samples were prepared by
suspending the material in ethanol and putting a drop of the
suspension onto a carbon block. After evaporation of the ethanol, the
sample was dried under reduced pressure at room temperature.
Argon sorption isotherms at 87 K were recorded on a
Quantachrome Autosorb-1 instrument. After Soxhlet extraction, the
samples were activated in vacuum at 80 °C directly before the
measurement. The pore volume was determined from the last point of
the isotherm before condensation between the particles occurs. The
“Micropore BET Assistant” of the ASiQwin software for Autosorb
instruments (version 2.0) from Quantachrome was used to determine
the Brunauer−Emmett−Teller (BET) range. The program determines
a BET range according to the theory of Rouquerol, which was shown
to be appropriate for different MOFs.50
NMR spectra were recorded at room temperature if not otherwise
mentioned and were calibrated using the solvent as an internal
standard: CDCl3, 7.25 ppm (1H) and 77.0 ppm (13C); DMSO-d6, 2.49
ppm (1H) and 39.50 ppm (13C). Signal assignments were supported
by COSY (all diesters), HMQC (all diesters), HMBC (PEPP diester),
and DEPT-135 (all compounds except PEEP diacid) experiments and
increment calculations. All of the mentioned compounds accompany-
ing a main product, such as a solvent or a side product, were identified
1
and quantified through H NMR spectroscopy. Signals assigned to
solvents are not listed with the NMR data.
Electrospray ionization (ESI) mass spectra were recorded using an
ion trap mass spectrometer equipped with a standard ESI source.
Samples were introduced by direct infusion with a syringe pump.
Nitrogen served both as the nebulizer gas and the dry gas.
Methyl 4-Ethynylbenzoate. We followed the reported synthesis6
but changed the amount of solvent and the procedure of isolation. To
a degassed solution of ethyl 4-iodobenzoate (25.1 g, 90.9 mmol) and
trimethylsilylethyne (14.0 mL, 101 mmol) in diethylamine (125 mL)
were added Pd(PPh3)2Cl2 (320 mg, 0.46 mmol) and CuI (173 mg,
0.91 mmol). The reaction mixture was stirred at room temperature for
22 h. The solvent was removed at room temperature and reduced
pressure. Diethyl ether and then water were added, and the aqueous
phase was extracted with diethyl ether. The combined organic phases
were washed with a saturated aqueous solution of NH4Cl. After the
solution had been dried with Na2SO4, the solvent was removed. Short-
path distillation at 120 °C (bath temperature) and 0.026 mbar gave a
colorless solid (22.5 g), which consisted mainly of ethyl 4-(2-
trimethylsilylethynyl)benzoate. This solid was dissolved in methanol
(120 mL), and K2CO3 (15.1 g, 109 mmol) was added. After the
suspension had been stirred for 10 min at 10−15 °C (cold water bath)
and 3.7 h at room temperature, water (100 mL) was added,
whereupon a precipitate formed. The precipitate was isolated through
filtration, washed with water, and dried at reduced pressure. Methyl 4-
ethynylbenzoate (12.3 g, 85% yield over two steps) was obtained as a
slightly beige-colored solid. For spectroscopic data, see ref 6.
Elemental analysis for C10H8O2 (160.172): calcd (%) C 74.99, H
5.03; found C 74.84, H 5.19.
Structure Simulations. Simulations were performed within the
modeling environment Materials Studio 7.0 of BIOVIA/Dassault
System
̀
es. We employed the universal force field (UFF)51 as
implemented in the Forcite module of Materials Studio to relax the
structural model (energy convergence tolerance 2 × 10−5 kcal·mol−1).
To avoid distortions of the structure of the IBUs, their crystallographic
coordinates as well as the lattice parameters were kept fixed during the
geometry optimization. Molecular dynamics simulations were carried
out employing the universal force field with 106 integration steps using
a time step of 1 fs at a temperature of 500 K.
Water Stability Tests. The powdery PIZOF (15 mg) was put into
a micro reaction vial, and ultrapure water (1.5 mL) was added. The
suspension was vortexed (digital vortex mixer from VWR operated at
775 rpm) for 24 h at room temperature. Afterward, the sample was
centrifuged, and the water was removed with a pipet. From each
PIZOF, two samples were tested. One was directly dried at reduced
pressure. The other one was suspended in ca. 1.5 mL of ethanol, and
the suspension was vortexed for 30 min, after which the ethanol was
removed at reduced pressure. The materials were analyzed with
PXRD.
Linker Syntheses. General. The synthesis of PEPEP diacid 4 has
been reported.6 1,4-Diiodo-2,5-dimethoxybenzene and ethyl 4-
ethynylbenzoate were prepared as reported.6 Pd(PPh3)2Cl2 was
synthesized according to the literature52 but using 2.1 times the
reported amount of methanol. All of the other starting compounds and
catalysts came from commercial sources and were used as received.
Tetrahydrofuran (THF) (HPLC grade) was distilled from sodium/
PEEP Dimethyl Diester. A solution of methyl 4-ethynylbenzoate
(798 mg, 4.98 mmol), Pd(PPh3)2Cl2 (72 mg, 0.10 mmol), and CuI
(19 mg, 0.10 mmol) in THF (15 mL) and piperidine (3 mL) was
stirred at room temperature for 22.5 h in air. During this time a
precipitate formed. Water (40 mL) was added, and the brown
precipitate was isolated by filtration. It was washed successively with
water (20 mL), 2 M aqueous HCl solution (20 mL), and water (10
mL) and then dried over P4O10 at 10−3 mbar. Column
chromatography of this solid (3 cm × 5 cm; CH2Cl2; the material
was loaded onto the silica gel as a solution in CH2Cl2) gave a colorless
solid mixed with a small amount of a red-brown solid. Crystallization
of this material in toluene (20 mL) gave PEEP dimethyl diester (499
mg, 63%) as a beige solid. Concentration of the mother lye to a
volume of approximately 5 mL gave additional PEEP dimethyl diester
(63 mg, 8%) as a beige solid. Decomposition: 177−178 °C. 1H NMR
(500 MHz, CDCl3) (Figure S10a): δ = 8.00 and 7.57 (AA′XX′ spin
system, 4H each, HAr ortho and meta to CO2Me, respectively), 3.91 (s,
3H, OCH3). 13C NMR (125 MHz, CHCl3) (Figure S10b): δ = 166.2
(CO2Me), 132.4 (CArH meta to CO2Me), 130.5 (CArCO2Me), 129.5
(CArH ortho to CO2Me), 126.1 (CAr para to CO2Me), 81.8 and 76.2
(CC), 52.3 (CH3). MS (EI, 70 eV): m/z (%) = 318.0 (100) [M]+•,
287.0 (84) [M − (OMe)]+, 259.0 (20) [M − (CO2Me)]+, 200.0 (21)
[M − 2(CO2Me)]+. Elemental analysis for C20H14O4 (318.33): calcd
(%) C 75.46, H 4.43; found C 75.55, H 4.44.
53
benzophenone and piperidine (p.a. grade) was distilled from CaH2
prior to their use in the coupling reactions. All other solvents that were
used to set up reactions were of p.a. grade. For column
chromatography and extractions, technical-grade solvents that had
been distilled at 40 °C and reduced pressure were used. Argon 4.6 was
passed through anhydrous calcium chloride prior to use.
If not mentioned otherwise, the reactions were carried out in an
argon atmosphere using the Schlenk technique. The temperatures
given refer to the bath temperatures. Degassed solutions or
suspensions were prepared through freeze−pump−thaw cycles.
Solvents were removed at 40 °C and reduced pressure. Residual
solvents were removed at room temperature and approximately 0.01
mbar. Column chromatography was performed on silica gel 60 (35−70
PEEP Diacid 1. This reaction was carried out in ambient
atmosphere. To a suspension of PEEP dimethyl diester (450 mg,
1.41 mmol) in THF (40 mL) was added a solution of KOH (7.91 g,
C
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141 mmol) in methanol (30 mL). The suspension had been stirred at
room temperature for 4 days, whereupon the color turned from yellow
to red-brown. The colorless precipitate was isolated through filtration
and then suspended in THF (5 mL). Trifluoroacetic acid was added
until a pH of 1 was reached. After the suspension had been stirred at
room temperature for 10 min, water (40 mL) was added. The
precipitate was isolated, washed with water (30 mL), and suspended in
CH2Cl2. The suspension was stirred for 1 h at room temperature. The
removed, and the residue was suspended in water (50 mL). The
suspension was filtered, and the isolated solid was washed with water
(6 × 20 mL). The isolated orange solid was suspended in methanol
(25 mL), ; tand the suspension was filtered. The isolated solid was
washed with methanol (6 × 20 mL) and was finally dried. Column
chromatography of this solid (3 cm × 20 cm, CH2Cl2; the material was
loaded onto the silica gel as a solution in CH2Cl2) gave PEPP dimethyl
diester (0.73 g, 59%; Rf = 0.36) as a light-brown solid. The alkyne
dimer, PEEP dimethyl diester, was eluted ahead of PEPP dimethyl
diester. 1H NMR (500 MHz, CDCl3) (Figure S13a): δ = 8.08 (half of
an AA′XX′ spin system, 2H, HAr meta to CC), 8.02 (half of an
AA′XX′ spin system, 2H, HAr meta to Ar), 7.63 and 7.62 (two halves of
two AA′XX′ spin systems, 2H each, HAr meta to CO2Me), 7.12 (s, 1H,
HAr ortho to CC), 6.89 (s, 1H, HAr meta to CC), 3.94 (s, 3H,
CO2CH3), 3.924 and 3.921 (2 s, 3H each, CO2CH3 and ArOCH3 meta
to CC), 3.79 (s, 3H, ArOCH3 ortho to CC). 13C NMR (125
MHz, CDCl3) (Figure S13b): δ = 166.9 (CO2CH3 para to CC),
166.5 (CO2CH3 para to Ar), 154.6 (CArOCH3 meta to CC), 150.2
(CArOCH3 ortho to CC), 142.5 (CArAr para to CO2CH3), 131.5
(CArH ortho to CC and meta to CO2CH3), 131.3 (CArAr ortho to
OCH3), 129.42 (CArH of terminal benzene ring), 129.40 (CArCO2CH3
or CArCC para to CO2CH3), 129.36 and 129.31 (CArH of terminal
benzene ring), 128.9 (CArCO2CH3 or CArCC para to CO2CH3),
128.0 (CArCC para to CO2CH3 or CArCO2CH3), 116.4 (CArH ortho
to OCH3 and CC), 113.7 (CArH ortho to OCH3 and meta to C
C), 111.9 (CArCC ortho to OCH3), 93.3 (ArCCArOMe), 88.6
(ArCCArOMe), 56.5 and 56.2 (ArOCH3), 52.2 and 52.1 (CO2CH3).
PEPP Diacid 2. This reaction was carried out under ambient
atmosphere. To a suspension of PEPP dimethyl diester (725 mg, 1.68
mmol) in THF (65 mL) was added a solution of KOH (9.58 g, 171
mmol) in methanol (45 mL). Soon the reaction mixture turned turbid.
The suspension was stirred at room temperature for 40 h and then
concentrated (ca. 35 mL final volume). The suspension was cooled
with an ice bath, and trifluoroacetic acid (20 mL, 207 mmol) was
added dropwise. After the suspension had been stirred for 15 min at
room temperature, water (60 mL) was added to the ice-cooled
suspension. The precipitate was isolated and washed with water (3 ×
25 mL). The isolated material was suspended in water, and PEPP
diacid 2 (635 mg, 94%) was isolated by filtration as an off-white solid.
19F NMR spectroscopy proved the material to be free of trifluoroacetic
acid. Mp: 321−324 °C. 1H NMR (500 MHz, DMSO-d6) (Figure
S14a): δ = 13.07 (br s, 2H, CO2H), 7.99 and 7.97 (two halves of two
AA′XX′ spin systems, 2H each, HAr ortho to CO2H), 7.68 and 7.65
(two halves of two AA′XX′ spin systems, 2H each, HAr meta to
CO2H), 7.27 (s, 1H, HAr ortho to CC), 7.06 (s, 1H, HAr meta to
CC), 3.88 (s, 3H, OCH3 meta to CC), 3.76 (s, 3H, OCH3 ortho
to CC). 13C NMR (125 MHz, DMSO-d6) (Figure S14b): δ = 167.2
(CO2H para to CC), and 166.7 (CO2H para to Ar), 154.3
(CArOCH3 meta to CC), 149.7 (CArOCH3 ortho to CC), 141.8
(CArAr para to CO2H), 131.4 (CArH ortho to CC and meta to
CO2H), 130.8 and 130.4 (CArCO2H), 129.6, 129.5, and 129.1 (CArH
of terminal benzene rings), 126.9 (CArCC para to CO2H), 116.4
(CArH ortho to OCH3 and CC), 113.8 (CArH ortho to OCH3 and
meta to CC), 110.9 (CArCC ortho to OCH3), 92.9 (ArC
CArOMe), 89.0 (ArCCArOMe), 56.3 (OCH3). The signal of CArAr
ortho to OCH3 was not found. Elemental analysis (V2O5 was addded
as an oxidation agent) for C24H18O6 (402.11): calcd (%) C 71.64, H
4.51; found C 71.13, H 4.51.
1
colorless precipitate was isolated through filtration. Analysis with H
NMR spectroscopy revealed incomplete conversion. The colorless
solid was suspended in THF (30 mL), and a solution of KOH (5.00 g,
89 mmol) in methanol (ca. 25 mL) was added. The suspension was
stirred at room temperature for 2 days, and the precipitate was isolated
via filtration. The colorless precipitate was suspended in THF (5 mL),
and trifluoroacetic acid was added until a pH of 1 was reached. After
the suspension had been stirred at room temperature for 75 min, water
(40 mL) was added. The colorless precipitate was isolated via
filtration, washed with water (3 × 20 mL), and dried at reduced
pressure. The isolated solid was suspended in water, and the
suspension was stirred at room temperature for 8 h. The colorless
powder (295 mg) was isolated through filtration and dried at reduced
pressure. It was suspended in CH2Cl2 (15 mL), and the suspension
was stirred for 4 h. Filtering gave a colorless powdery solid (285 mg)
containing PEEP diacid 1 (about 72% isolated yield) and THF in a
molar ratio of about 10:1. 19F NMR spectroscopy proved the material
to be free of trifluoroacetic acid. Mp: >300 °C; darkening at 209 °C.
1H NMR (500 MHz, DMSO-d6) (Figure S11a): δ = 13.30 (br s, 2H,
CO2H), 7.97 and 7.74 (AA′XX′ spin system, 4H each, HAr ortho and
meta to CO2H, respectively). 13C NMR (125 MHz, DMSO-d6, 363 K)
(Figure S11c): δ = 165.8 (CO2H), 132.0 (CArH meta to CO2Me),
131.6 (CArCO2H), 129.0 (CArH ortho to CO2H), 124.0 (CAr para to
CO2H), 81.7 and 75.0 (CC).
Iodo-PEP Methyl Ester. Pd(PPh3)2Cl2 (57 mg, 0.08 mmol) and CuI
(33 mg, 0.17 mmol) were added to a degassed solution of 1,4-diiodo-
2,5-dimethoxybenzene (3.08 g, 7.88 mmol) and methyl 4-ethynylben-
zoate (892 mg, 5.57 mmol) in THF (36 mL) and piperidine (12 mL).
The yellow solution turned turbid shortly afterward. The suspension
was stirred at room temperature for 17 h. Water (30 mL) was added,
and the suspension was cooled with an ice bath. The precipitate was
isolated through filtration, washed with 2 N HCl and then with water,
and dried at reduced pressure. The material was dissolved in THF, and
silica gel was added to this solution, after which all of the volatiles were
removed (40 °C, reduced pressure). The resulting freely flowing
powder was transferred to the top of the silica gel column (4.5 cm ×
20 cm). Elution (CH2Cl2) gave the starting material 1,4-diiodo-2,5-
dimethoxybenzene (1.01 g, 32%; Rf = 0.70) as a red-brown solid, iodo-
PEP methyl ester (monocoupling product) (0.82 g, 25%; Rf = 0.47) as
a glittering orange solid, and PEPEP dimethyl diester54 (dicoupling
product) (0.62 g, 25%; Rf = 0.37) as a yellow-orange solid. The alkyne
dimer, PEEP dimethyl diester, was eluted together with some iodo-
PEP methyl ester (together 16 mg) immediately after the main part of
the iodo-PEP methyl ester had been eluted. Analytical data for iodo-
1
PEP methyl ester: H NMR (500 MHz, CDCl3) (Figure S12a): δ =
8.01 and 7.60 (AA′XX′ spin system, 2H each, HAr ortho and meta to
CO2Me, respectively), 7.32 (s, 1H, HAr ortho to I), 6.93 (s, 1H, HAr
meta to I), 3.92 (s, 3H, CO2CH3), 3.87 and 3.85 (2 s, 3H each,
ArOCH3). 13C NMR (125 MHz, CDCl3) (Figure S12b): δ = 166.5
(CO2CH3), 154.7 and 152.3 (CArOCH3 ortho and meta to CC,
respectively), 131.5 (CArH meta to CO2CH3), 129.6 (CArCO2), 129.5
(CArH ortho to CO2CH3), 127.8 (CAr para to CO2CH3), 122.4 (CArH
ortho to I), 115.0 (CArH meta to I), 112.4 (CArCC ortho to OCH3),
93.6 and 88.3 (CC), 87.5 (CArI), 57.0 and 56.7 (ArOCH3), 52.2
(CO2CH3).
PEPP Dimethyl Diester. Pd(PPh3)4 (108 mg, 0.09 mmol) was
added to a degassed suspension of a 73:10 mixture (1.30 g) of iodo-
PEP methyl ester (ca. 1.18 g, 2.79 mmol) and PEEP dimethyl
diester,55 4-methoxycarbonylbenzeneboronic acid (1.67 g, 9.28 mmol),
and Na2CO3 (1.15 g, 10.8 mmol) in toluene (20 mL), methanol (14
mL), and water (7 mL). The suspension was stirred at 100 °C for 87 h
and then cooled to room temperature. All of the volatiles were
PPPP Dimethyl Diester. Pd(PPh3)4 (139 mg, 0.12 mmol) was
added to a degassed suspension of 4,4′-diiodo-3,3′-dimethyl-1,1′-
biphenyl (2.51 g, 5.79 mmol), 4-methoxycarbonylbenzeneboronic acid
(4.18 g, 23.2 mmol), and Na2CO3 (3.71 g, 35.0 mmol) in a mixture of
toluene (75 mL), methanol (50 mL), and water (25 mL). The
suspension was stirred at 100 °C for 68 h. After standing at room
temperature for 3 days without stirring, the suspension was filtered.
The isolated slightly gray crystalline solid was washed with water and
dried. This solid (1.87 g) was extracted with methanol (70 mL) in a
Soxhlet device for 1 day. The extract was discarded. The solid residue
in the extraction thimble was extracted with CH2Cl2 (70 mL) in a
Soxhlet device for 6 h. From this second extract all of the volatiles were
D
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Scheme 1. Syntheses of Diacids 1−4 and the Corresponding Estimated Distances between the C Atoms of the Two Carboxylate
Groups (TFA = Trifluoroacetic Acid)
removed, leaving behind PPPP dimethyl diester (1.76 g, 68%) as a
zoate (5.05 g, 28.99 mmol) in THF (100 mL) and piperidine (20 mL).
The mixture was stirred at room temperature for 22 h. Water (100
mL) was added, and the precipitate was isolated by filtration, washed
three times with 2 N HCl, and dried at reduced pressure, giving a
yellow powder (6.51 g). A part of this powder (2.39 g) was transferred
into a Soxhlet extractor and was extracted with ethanol for 15.5 h. The
residue in the thimble was extracted with CH2Cl2 for 4 h. The solvent
of the second extract was removed, yielding PEPEP diethyl ester (2.04
g, 84%) as a yellow solid residue. For spectroscopic data see ref 6. Mp:
204−205 °C. Elemental analysis for C30H26O6 (422.48): calcd (%) C
74.67, H 5.43; found C 74.43, H 5.41.
1
colorless solid. H NMR (500 MHz, CDCl3) (Figure S15a): δ = 8.11
(half of an AA′XX′ spin system, 4H, HAr ortho to CO2CH3), 7.55 (d, 4J
3
4
= 1.6 Hz, 2H, HAr ortho to CH3), 7.52 (dd, J = 7.9 Hz, J = 1.6 Hz,
2H, HAr para to CH3), 7.45 (half of an AA′XX′ spin system, 4H, HAr
meta to CO2CH3), 7.32 (d, 3J = 7.8 Hz, 2H, HAr meta to CH3), 3.95 (s,
6H, CO2CH3), 2.35 (s, 6H, ArCH3). 13C NMR (125 MHz, CDCl3)
(Figure S15b): δ = 167.0 (CO2CH3), 146.3 (CAr para to CO2CH3),
140.2 and 140.0 (CArCH3 and CArAr meta to CH3), 135.7 (CArAr ortho
to CH3), 130.1 (CArH meta to CH3), 129.5 (CArH ortho to CO2CH3),
129.3 (CArH meta to CO2CH3), 129.2 (CArH ortho to CH3), 128.7
(CArCO2CH3), 124.6 (CArH para to CH3), 52.2 (CO2CH3), 20.6
(ArCH3).
MOF Syntheses. General. Benzoic acid (≥99.5%), N,N-
dimethylformamide (≥99.8%), ethanol (denatured, ≥99.8%), ZrCl4
(≥99.5%), ZrOCl2·8H2O (98%), and acetic acid (≥99.7%) were used
as received.
PPPP Diacid 3. This reaction was carried out in ambient
atmosphere. To a suspension of PPPP dimethyl diester (1.76 g, 3.91
mmol) in THF (170 mL) was added a solution of KOH (21.7 g, 387
mmol) in methanol (85 mL). After the slightly yellow suspension had
been stirred for 17 h at room temperature, it was concentrated (ca. 100
mL final volume). Trifluoroacetic acid (30 mL) was added dropwise
under ice bath cooling. Water (120 mL) was added, and the precipitate
was isolated through filtration. The isolated solid was suspended in
water, and the suspension was filtered. The obtained solid was dried
(P4O10, reduced pressure). To remove residual THF, the solid was
suspended in CH2Cl2 (50 mL), and the solvent was removed, leaving
behind PPPP diacid 3 (1.51 g, 91%) as a colorless solid. 19F NMR
spectroscopy proved the material to be free of trifluoroacetic acid. Mp:
General Procedure for Zr-MOF Synthesis. The MOFs were
obtained similarly to our previously published protocol for PEPEP-
PIZOFs.6 The Zr source, the modulator, and the linker diacid were
dissolved or suspended (some diacids dissolve only upon heating) in
DMF in glass bottles of 100 mL volume (VWR, Schott, borosilicate
3.3, ISO 4796). The tightly capped (PTFE coated seals) flasks were
put into an oven set to 120 °C for at least 24 h. We made the
observation that some of the Zr-MOF syntheses, especially those with
extensive crystallization on the glass walls of the flask, were not
finished after 24 h. In such cases, the bottle was removed from the
oven. The crystals were detached from the hot glass wall and
suspended using ultrasound (VWR ultrasonic cleaner: HF, 45 kHz, 60
W). Afterward, the reaction mixture was put back into the oven and
heated for another 24 h. If further crystallization on the glass walls
occurred, this procedure was repeated. A reaction was declared as
finished when no more crystallization on the glass walls took place and
product was present only at the bottom of the vessel. The overall
reaction time thus depended on the crystallization behavior of a
synthesis batch and amounted to 24, 48, 72, or 96 h.
1
>300 °C. H NMR (500 MHz, DMSO-d6) (Figure S16a): δ = 12.99
(s, 2H, CO2H), 8.02 (half of an AA′XX′ spin system, 4H, HAr ortho to
CO2CH3), 7.69 (s with slight indication of splitting into d, 2H, HAr
3
ortho to CH3), 7.63 (d with slight indication of splitting into dd, J =
7.9 Hz, 2H, HAr para to CH3), 7.53 (half of an AA′XX′ spin system,
3
4H, HAr meta to CO2CH3), 7.34 (d, J = 7.9 Hz, 2H, HAr meta to
CH3), 2.33 (s, 6H, ArCH3). 13C NMR (125 MHz, DMSO-d6) (Figure
S16b): δ = 167.2 (CO2H), 145.3 (CAr para to CO2H), 139.5 and
139.1 (CArAr meta to CH3 and CArCH3), 135.4 (CArAr ortho to CH3),
130.1 (CArH meta to CH3), 129.4 (CArCO2H), 129.28 and 129.25
(CArH ortho and meta to CO2H), 128.8 (CArH ortho to CH3), 124.3
(CArH para to CH3), 20.3 (CH3). MS (ESI, positive ions) m/z = 437.2
[M + Na]+. MS (ESI, negative ions): m/z = 209.8 [M]2−. Elemental
analysis for C28H22O4 (422.48): calcd (%) C 79.60, H 5.25; found C
79.54, H 5.27.
The precipitate was isolated from the mother liquor via
centrifugation and suspended in DMF followed by another
centrifugation. The suspension and centrifugation steps were repeated
using ethanol. The obtained powders were dried at reduced pressure
and then extracted with ethanol in a Soxhlet extractor for 24 h. The
products were finally dried at reduced pressure before further
characterization.
PEPEP Diethyl Diester. Pd(PPh3)2Cl2 (97 mg, 0.14 mmol) and CuI
(53 mg, 0.28 mmol) were added to a degassed solution of 1,4-diiodo-
2,5-dimethoxybenzene (5.38 g, 13.80 mmol) and ethyl 4-ethynylben-
PEEP-UiO. ZrOCl2·8H2O (55 mg, 0.17 mmol), benzoic acid (628
mg, 5.14 mmol), PEEP diacid 1 (HO2C[PEEP]CO2H; 50 mg, 0.17
E
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Figure 2. SEM images of (a) PEPP-PIZOF-1, (b) PPPP-PIZOF-1, and (c) PEPEP-PIZOF-2.
mmol), DMF (10 mL). PEEP-UiO was obtained as a slightly grayish
powder.
units in different numbers were used to fill the gap between the
PPP linker, the longest linker known to form a UiO-type MOF
at the time we started our investigation, and the PEPEP linker,
the linker of the parent PEPEP-PIZOFs. The dicarboxylic acids
prepared for this study, diacids 1−4, are shown in Scheme 1.
The central benzene rings were furnished with the substituents
methyl or methoxy in order to avoid problems with insufficient
solubility in DMF, the solvent of the MOF synthesis, as well as
to make synthetic precursors sufficiently soluble for purification
and conversion.
PEP(OMe,OMe)P-PIZOF (PEPP-PIZOF-1). ZrCl4 (40 mg, 0.17
mmol), benzoic acid (628 mg, 5.14 mmol), PEPP diacid 2
(HO2C[PE−P(OMe,OMe)−P]CO2H; 69 mg, 0.17 mmol), DMF
(10 mL). PEPP-PIZOF-1 was obtained as a slightly yellow powder.
PP(Me)P(Me)P-PIZOF (PPPP-PIZOF-1). ZrOCl2·8H2O (28 mg, 0.09
mmol), acetic acid (0.982 mL, 17.15 mmol), PPPP diacid 3
(HO2C[P−P(Me)−P(Me)−P]CO2H; 36 mg, 0.09 mmol), DMF (5
mL). PPPP-PIZOF-1 was obtained as a colorless powder.
PEP(OMe,OMe)EP-PIZOF (PEPEP-PIZOF-2). ZrCl4 (80 mg, 0.34
mmol), benzoic acid (1.256 g, 10.29 mmol), PEPEP diacid 4
(HO2C[PE−P(OMe,OMe)−EP]CO2H; 146 mg, 0.34 mmol), DMF
(20 mL). PEPEP-PIZOF-2 was obtained as a yellowish powder.
The backbones of diacids 1−4 were assembled through
transition-metal-catalyzed C−C cross-coupling (Scheme 1).56
C−C cross-coupling provided the diesters, which were
hydrolyzed under basic conditions to prepare diacids 1−4
(Scheme 1). Trifluoroacetic acid was used for protonation of
RESULTS AND DISCUSSION
■
the dicarboxylates to allow the detection of residual acid via 19
F
Synthesis of Diacids and Related Zr-MOFs. Linkers built
through the combination of phenylene (P) and ethynylene (E)
NMR spectroscopy. Residual acid acts as a modulator in the
F
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Figure 3. PXRD patterns (with 10-fold enhanced intensity on the right-hand side) of (a) PEPP-PIZOF-1, (b) PPPP-PIZOF-1, and (c) PEPEP-
PIZOF-2. Reflections that can be explained in space group Fd3m are indicated by black lines. Additional reflections appearing in the diffractogram of
̅
PEPP-PIZOF-1 are marked with red lines.
MOF synthesis and may influence the number and type of
defects.19
Soxhlet thimble with dichloromethane separated the PEPEP
diethyl diester from Pd black. The latter stayed in the extraction
thimble. Also, the isolation of the PPPP dimethyl diester
needed very little work: the precipitate of the reaction mixture
was isolated and then extracted in a Soxhlet extractor with
MeOH and finally with CH2Cl2.
If no other procedure is known, the simplest (because of its
rather general applicability) way to isolate the diesters is
chromatography. However, chromatography is a rather labor-
intensive method involving large amounts of solvents and
adsorbents. Therefore, avoiding this technique is highly
desirable, even more so when taking into account the interest
in application of MOFs and thus the request for their
availability in multigram amounts. Thus, we worked on
improving our procedures6 for the synthesis of the important
terminal building block methyl 4-ethynylbenzoate and that of
the PEPEP diethyl diester. Methyl 4-ethynylbenzoate was
obtained through two synthetic steps performed at room
temperature involving only distillation and precipitation for
isolation in an overall yield of 85%.59 The isolation of PEPEP
diethyl diester was simplified by precipitation instead of
extraction to obtain the crude product, followed by Soxhlet
extraction instead of chromatography. Soxhlet extraction with
methanol removed PEEP dimethyl diester (butadiynes are
formed upon exposure of alkynes to air in the presence of Pd/
Cu salts), and subsequent extraction of the residue in the
The key to success in the synthesis of PEPEP-PIZOFs6 had
been the application of the modulation approach.5,15,19,61 The
same applies for our attempts to obtain Zr-based MOFs from
the PEEP, PEPP, and PPPP diacids. PEPEP-PIZOF-2 was
synthesized from diacid 4 and ZrCl4 with benzoic acid as the
modulator, a synthesis that we experienced to be very well
reproducible. On the contrary, the syntheses of the Zr-based
MOFs from diacids 1−3 turned out to be delicate: the quality
of the products varied from batch to batch, and/or the synthesis
required a change of the type of modulator and Zr source. The
synthesis of a Zr-based MOF with PEEP diacid 1, albeit of low
quality (Figure S5), was achieved with ZrOCl2·8H2O as the Zr
source and benzoic acid as the modulator. The synthesis of
PEPP-PIZOF-1 was attained with ZrCl4 as the Zr source and
benzoic acid as the modulator. To obtain a Zr-based MOF with
a PPPP-linker, we had to use ZrOCl2·8H2O as the Zr source
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and acetic acid as the modulator. When ZrCl4 and benzoic acid
were used instead, only materials of a very low degree of
crystallinity were obtained (Figure S6). All of the PIZOF
samples were Soxhlet-extracted with ethanol and then dried. In
our previous publication on the PEPEP-PIZOFs, we reported
that the materials were washed first with DMF and then with
ethanol and were finally dried at reduced pressure at room
temperature.6 As we found later, this procedure is insufficient to
remove the benzoic acid completely and yields materials that
contain a considerable amount of DMF. Instead, nearly guest-
free materials (see the TGA curves in Figure 6) were obtained
through Soxhlet extraction with ethanol.
Crystal Size and Shape. As scanning electron microscopy
(SEM) images reveal, all three of the PIZOFs show isolated and
intergrown octahedrally shaped crystals with sizes of about 1−3
μm (Figure 2). In the case of the PPPP-PIZOF, some crystals
were larger.
In the structure of PEPP-PIZOF-1, the asymmetric PEPP
linkers can be placed in two different orientations between two
IBUs. This could occur in a statistical fashion, leading to a
disordered structure. However, the appearance of additional
reflections in the PXRD pattern points to a reduction in
crystallographic symmetry, which can be taken as an indication
of an orientationally ordered arrangement of the linkers. Many
different patterns of linker orientation are possible. We chose
one that explains the experimental findings very well. For this
purpose, we constructed a structural model with the reduced
symmetry of the orthorhombic space group P212121. After
assembly, the structure was subjected to molecular dynamics
simulations. The resulting structural model of PEPP-PIZOF-1
is shown in Figure 4a. The corresponding simulated PXRD
pattern is depicted in Figure 5 together with the experimentally
obtained PXRD pattern. The two patterns show very good
agreement, especially in the region of small 2θ values (Figure
5b). The reflections that could not be indexed in space group
Structure Determination. In agreement with Marshall et
al.,42 our experiments with the PEEP diacid resulted in the
formation of a PEEP-UiO. However, our procedure provided
only material of a low degree of crystallinity, as manifested by
the broad reflections in the powder X-ray diffraction (PXRD)
pattern (Figure S5).
Fd3m in the experimentally obtained PXRD pattern also show
̅
up in the simulated PXRD pattern. We therefore infer that the
linkers are orientationally ordered. Differences between the
experimental and simulated patterns are attributed to some
randomness in linker orientation.
On the contrary, with PEPP diacid 2, PPPP diacid 3, and
PEPEP diacid 4, highly crystalline materials were obtained, as
reflected by the small widths of the reflections in the PXRD
patterns and baselines that stay nearly horizontal down to very
small diffraction angles (Figure 3). Whereas with PPPP diacid 3
and PEPEP diacid 4 the formation of materials with a very high
degree of crystallinity was easily reproduced, the quality of the
material obtained from PEPP diacid 2 varied slightly from batch
to batch (Figure S7). It was possible to index the PXRD
patterns of the two new materials in the cubic space group
In the case of PPPP-PIZOF-1, no ambiguity with respect to
the linker orientation exists, and therefore, we constructed a
cubic structural model in the space group Fd3m. The result is
̅
shown in Figure 4b. For the calculation of the simulated PXRD
pattern, the conformational isomerism was taken into account.
To identify low-energy conformations of the PPPP linker, we
performed molecular dynamic simulations, restricting the
terminal benzene rings and the planes of the carboxylate
groups to coplanarity. The two conformers that were found
have a dihedral angle of about 60° between the terminal
benzene ring and the directly bound benzene ring and differ in
the orientation ( 60° dihedral angle) of the two central
methyl-substituted benzene rings (Figure S8). To describe the
Fd3m, the same space group in which the PEPEP-PIZOFs
̅
crystallize.6 Additionally, the intensity patterns of the diffracto-
grams are similar to those found for the PEPEP-PIZOFs.
Therefore, we are confident that all three materials share the
same framework structure and that PIZOFs PEPP-PIZOF-1
and PPPP-PIZOF-1, which have shorter linkers than the parent
PEPEP-PIZOF, were obtained. The lattice parameters of the
three PIZOFs are listed in Table 1. The PXRD pattern of
disorder within the space group Fd3m, a superposition of the
̅
two conformers was used, so that the carbon atoms of the two
central phenylene units in each of the two orientations entered
the calculation with an occupancy factor of 0.5 and the carbon
atoms of the methyl groups with a factor of 0.25.
For the PPPP-PIZOF, we tried to gain more precise
structural information by carrying out Rietveld refinements of
the powder diffraction data. However, these attempts did not
give a significant improvement because the uncertain regions of
the structure concern the linkers, and these contain only atoms
that scatter very weakly in comparison with the Zr atoms.
Thermal Properties and Composition. Thermogravi-
metric analysis (TGA) was applied to determine the thermal
stability and the composition of the PIZOFs. The decom-
position of the frameworks in air starts at around 300 °C in the
case of PEPP-PIZOF-1 and around 350 °C in the case of PPPP-
PIZOF-1 (Figure 6). This thermal stability is comparable to
that of UiO-66 and PEPEP-PIZOF-2 (Figure 6c), which are
stable up to about 400 °C5 and 325 °C, respectively. As the
TGA curves show, the materials contain only a very small
amount of guest molecules after Soxhlet extraction with ethanol
and drying under reduced pressure at room temperature.
Assuming ZrO2 to be the sole residue at high temperatures, the
compositions of the materials were calculated from the TGA
data. These experimental results are listed together with the
data calculated for structurally perfect PIZOFs in Table 2. The
deviations between the experimental and calculated data fall
Table 1. Lattice Constants a and Unit Cell Volumes V of
PEPP-PIZOF-1, PPPP-PIZOF-1, and PEPEP-PIZOF-2
Based upon Indexing in Space Group Fd3m
̅
PIZOF
a/Å
V/Å3
PEPP-PIZOF-1
PPPP-PIZOF-1
PEPEP-PIZOF-2
36.4146(8)
38.8861(8)
39.8787(10)
48286.7(19)
58800.9(20)
63419.7(28)
PEPP-PIZOF-1 shows some additional weak reflections (Figure
3). As detailed below, we ascribe these reflections to a partial
orientational ordering of the PEPP linkers.
To derive structural models for PEPP-PIZOF-1 and PPPP-
PIZOF-1, we employed molecular modeling. In a first step, unit
cells with the lattice parameters obtained from the PXRD
diagrams (Table 1) were created. We put the IBUs at the
crystallographic positions known from the structure of PEPEP-
PIZOF-2,6 connected the IBUs with the substituted PPPP or
PEPP linkers (Figure 4), and simulated the PXRD patterns for
the obtained structural models.
H
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Figure 4. Structural models of (a) PEPP-PIZOF-1 and (b) PPPP-PIZOF-1 used to simulate the PXRD diagrams. For details, see the text. For the
sake of clarity, the disorder due to conformational isomerism that was taken into account in the simulation of PPPP-PIZOF-1 is not shown in the
structural model. Color code for the structural models: black, C; red, O; surrounded by coordination polyhedra, Zr.
within the range of accuracy of TGA measurements (1%) in the
cases of PEPP-PIZOF-1 and PEPEP-PIZOF-2, but the amount
of residue of the PPPP-PIZOF-1 sample is slightly too large.
The latter may be a hint that a small number of linkers is
missing or that a very small amount of amorphous material with
a different composition is present. Defects in PPPP-PIZOF-1
are not surprising considering the sensitivity of PPPP-PIZOF-1
formation to the reaction conditions.
do not. This hysteresis is taken as an indication of the presence
of mesopores, which are a sign of framework defects. Because
the composition derived from the TGA data fits very well with
the ideal composition, pieces of the two interpenetrating
networks and not only linkers must be missing. The PEPP
linker is the shortest among the linkers investigated here that
forms an interpenetrated structure. The next shorter one, the
PEEP linker, forms a non-interpenetrated structure. It is
possible that although the PEPP linker is long enough to allow
for the construction of two interpenetrated frameworks, it is
too short to form strainless frameworks, and therefore defects
occur. In line with this is the finding that repeating the synthesis
of PEPP-PIZOF-1 using the same procedure led to products
with slightly varying PXRD patterns (Figure S7).
Stability in Water and upon Drying. To determine the
water stability of the three PIZOFs, two samples of each PIZOF
were suspended in water for 24 h at room temperature. After
the water had been pipetted off, one of the two samples of each
PIZOF was subjected to reduced pressure directly. The other
sample was suspended in ethanol for 30 min. Afterward the
ethanol was removed at reduced pressure. Like acetone, ethanol
is expected to exert weaker capillary forces than water during
evaporation (acetone was used in the study of Mondloch et
al.47). When the water-wetted samples were dried, only PEPEP-
PIZOF-2 remained intact (Figure 8a−c), whereas PEPP-
PIZOF-1 and PPPP-PIZOF-1 collapsed. In contrast, when
the ethanol-wetted samples were dried, all of the materials gave
the same PXRD reflection patterns as the starting materials
(Figure 8d−f). These results support the proposal put forward
by Mondloch et al.47 that not hydrolysis but capillary forces can
be the predominant factor in the destruction of Zr-based
MOFs. The PXRD patterns of PEPP-PIZOF-1 and PPPP-
PIZOF-1 dried after water-for-ethanol exchange exhibit an
increased background in the region of small 2θ values
compared with those of the starting materials, indicating that
degradation had occurred, albeit only to a small extent. This
degradation can be the result of the action of capillary forces,
but it may also be the result of hydrolysis. We would like to add
that the water-for-ethanol exchange gave diverse results in
terms of framework preservation (Figure S9). The number of
experiments is insufficient to decide whether this is due to a
Physisorption. The Ar sorption isotherms of the three
PIZOFs are presented in Figure 7. All of the materials are
microporous. As expected, with increasing linker length the
characteristics of the isotherm shift to larger relative pressure,
and the BET surface area and pore volume both increase
(Table 3). A distinct difference between the behaviors of the
three PIZOFs is found in the regime of low gas pressures up to
a relative pressure of about 0.1: PEPP-PIZOF-1 adsorbs and
desorbs argon in one step, whereas for PPPP-PIZOF-1 and
PEPEP-PIZOF-2 the adsorption and desorption occur in two
steps, thus corresponding to the (newly defined) type I(b)
isotherm.62 Further details on the evaluation of the sorption
data are provided in section 7 of the Supporting Information.
The adsorption behavior of PEPEP-PIZOF-2 was elucidated
with simulations of the Ar sorption mechanism at 87 K.63 It was
found that there are differently attractive sorption sites inside
the large tetrahedral pores. These pores are accessible in spite
of the interpenetration. The first Ar atoms are adsorbed at sites
close to the IBUs. Then other sorption sites at the linkers
become occupied. Finally, in a process that resembles capillary
condensation, the interior of the pores becomes filled. These
processes correspond to the two steps in the adsorption
isotherms of PPPP-PIZOF-1 and PEPEP-PIZOF-2. The one-
step profile of the adsorption curve of PEPP-PIZOF-1 reveals
that the energy difference between the different sorption sites is
too small to distinguish the individual adsorption steps. This is
probably a consequence of the shortness of the linker and the
fact that argon condensation takes place at lower relative
pressures than in the case of the other PIZOFs because of the
smaller pores of PEPP-PIZOF-1.
Another difference between the sorption isotherms of PEPP-
PIZOF-1 and the other two PIZOFs is that the former shows
hysteresis at relative pressures beyond 0.45 whereas the others
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Figure 5. (a, b) Experimental and simulated PXRD patterns of PEPP-
PIZOF-1, with (b) showing a zoom into the region of small 2θ with
the focus on the reflections that cannot be indexed in space group
Figure 6. Results of thermogravimetric analysis of (a) PEPP-PIZOF-1,
(b) PPPP-PIZOF-1, and (c) PEPEP-PIZOF-2.
Fd3m. These reflections are indicated with red lines and were also
̅
found in the simulated PXRD diagram of a model with the symmetry
of the orthorhombic space group P212121. (c) Experimental and
simulated PXRD patterns of PPPP-PIZOF-1.
frameworks, because of the strongly hydrophilic oligo(ethylene
glycol) substituents. Nevertheless, these PIZOFs stay intact
during drying from the water-wetted state (Figure S3). We take
this as a hint that an important factor for withstanding the
destruction is the bendability of the linker. Linkers with
alternating phenylene and ethynylene units are easily bent,67
and indeed the linkers of PEPEP-PIZOF-2 are bent toward the
IBUs of the interpenetrating network.6 The bendability may
allow the frameworks of PEPEP-PIZOF to respond elastically
to deformation caused by capillary forces. The longer the linker,
the larger the deformation amplitude can be before bonds get
broken.
difference in PIZOF quality or is caused by unnoted differences
in how the experiment was conducted. Nevertheless, the data
presented in Figure 8 clearly prove the effect of the water-for-
ethanol exchange.
We assume that framework defects not only offer sites
reactive toward hydrolysis in water but also play a role in
framework collapse during drying from a water-wetted state.
Defects in terms of missing linkers are known for the
structurally related UiO-66 and UiO-67.19,64−66 Water
molecules will take the unoccupied coordination sites at
defective IBUs and will form hydrogen bonds to the carboxyl
groups of linkers that are missing an IBU to which they can
coordinate. These water molecules will act as a strong link
between the framework and the water phase in the pore and
will therefore allow capillary forces to induce framework
deformation and eventually framework collapse during drying.
It has to be assumed that strong interactions between the
pore water and the framework also exist for PEPEP-PIZOF-7
and PEPEP-PIZOF-10 (Figure S1), even in case of defectless
CONCLUSION
■
The formation of PIZOFs is not unique to the PEPEP diacids.
PIZOFs form as well from the shorter PEPP and PPPP diacids.
Upon further shortening the linker to a PEEP diacid and
therefore decreasing the distance between the C atoms of the
two carboxyl groups from 17 to 15 Å, crossover from the
twofold-interpenetrated PIZOF to a non-interpenetrated Zr-
MOF of the UiO type occurs. Among the PIZOFs, PEPEP-
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Table 2. Mass Losses of PEPP-PIZOF-1, PPPP-PIZOF-1, and PEPEP-PIZOF-2 Determined Experimentally by
Thermogravimetric Analysis and Calculated for Structurally Perfect PIZOFs
PEPP-PIZOF-1
w/o guests
PPPP-PIZOF-1
w/o guests
PEPEP-PIZOF-2
w/o guests
exptl
calcd
exptl
calcd
exptl
calcd
guests/%
linkers/%
residue/%
4.2
73.7
22.1
−
76.9
−
76.0
5.3
71.0
23.7
−
75.0
−
76.9
3.8
74.5
21.7
−
77.4
−
77.1
23.1
24.0
25.0
23.1
22.6
22.9
Figure 7. (a−c) Ar adsorption (solid symbols) and desorption (open symbols) isotherms of (a) PEPP-PIZOF-1, (b) PPPP-PIZOF-1, and (c)
PEPEP-PIZOF-2. (d) Ar adsorption isotherms of the three PIZOFs in the region of low relative pressures.
PIZOF-1 and PEPEP-PIZOF-2 occurs in two steps that are
correlated with two energetically distinct adsorption sites. On
the contrary, the argon sorption isotherm of PEPP-PIZOF-1
reveals only one step, probably because of the short linkers and
small pores. Another special feature of the argon sorption
isotherm of PEPP-PIZOF-1 is the hysteresis in the range of
relative pressures above 0.45.
Table 3. BET Surface Areas and Pore Volumes for the Three
PIZOFs
material
SBET/m2·g−1
Vpore/cm3·g−1
PEPP-PIZOF-1
PPPP-PIZOF-1
PEPEP-PIZOF-2
1650
2020
2350
0.73
0.83
0.97
A striking difference between PEPEP-PIZOF-2 and the other
two PIZOFs is the resistance of PEPEP-PIZOF-2 against
framework collapse during drying from the water-wetted state.
PXRD patterns contain no hint of decomposition of PEPEP-
PIZOF-2. On the contrary, PEPP-PIZOF-1 and PPPP-PIZOF-
1 were destroyed when dried from the water-wetted state.
Degradation could be largely avoided through suspension of the
water-wetted materials in ethanol before drying at reduced
pressure. Degradation may be caused by framework defects.
The TGA data of PPPP-PIZOF-1, the sorption hysteresis of
PEPP-PIZOF-1, and the difficulties with obtaining highly
crystalline materials are taken as indications of the presence of
defects caused by a mismatch between the lengths of the PEPP
and PPPP linkers and the requirement for the linker length for
the formation of a perfect PIZOF. The defects are considered
as strong binding sites for water molecules and the bound water
molecules as a point of action for capillary forces. To explain
the distinct robustness of PEPEP-PIZOFs, including those with
PIZOFs are obtained most easily: PEPEP-PIZOF-2 forms
reproducibly in high quality, whereas the quality of PEPP-
PIZOF-1 varies from batch to batch and PPPP-PIZOF-1
requires another modulator and another Zr source than used in
the PEPEP-PIZOF synthesis. The synthesis procedures
described for the preparation of the PIZOFs are possibly also
of interest for the preparation of other Zr-based MOFs.
Removal of the modulator and DMF from the PIZOFs requires
prolonged extraction with ethanol, a requirement that was met
with extraction in a Soxhlet device, as proven with TGA in air,
which revealed a content of guest molecules of only 4−5 mass
%. TGA in air showed decomposition of the three PIZOFs to
start in the temperature range of 300 to 350 °C. The
compositions of PEPP-PIZOF-1 and PEPEP-PIZOF-2 deter-
mined by TGA fit very well to those calculated for the
structurally perfect PIZOFs, but the sample of PPPP-PIZOF-1
contained a small excess of Zr. Argon sorption on PPPP-
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Figure 8. PXRD diagrams of (a, d) PEPP-PIZOF-1, (b, e) PPPP-PIZOF-1, and (c, f) PEPEP-PIZOF-2 before (black curves) and after (red curves)
exposure to water and drying. The PXRD patterns shown in red in the left-hand column (a−c) were obtained from samples dried from the water-
wetted state. The PXRD patterns shown in red in the right-hand column (d−f) stem from samples that were exposed to water and dried after water
had been exchanged for ethanol. In (c) and (f), the diffraction patterns nearly coincide, and the red curves are hardly visible.
highly hydrophilic substituents on the linkers, we suggest an
elastic response to the action of capillary forces through
bending of the long linkers.
1; NMR spectra of linkers; and further details of
physisorption investigations (PDF)
Our results emphasize the need to distinguish between
hydrolysis and drying effects when investigating water stability.
Summarizing for the class of Zr-based MOFs with UiO-type
networks, UiO-66 and some derivatives thereof, Zr-fum-MOF,
PEPEP-PIZOF-1 to -8, and PEPEP-PIZOF-10 to -12 can be
dried from the water-wetted state, whereas UiO-67, PEPP-
PIZOF-1, and PPPP-PIZOF-1 require exchange of water for
ethanol before drying.
AUTHOR INFORMATION
■
Corresponding Authors
*E-mail: peter.behrens@acb.uni-hannover.de.
*E-mail: godt@uni-bielefeld.de.
Author Contributions
The manuscript was written through contributions of all
authors. All authors have given approval to the final version of
the manuscript.
ASSOCIATED CONTENT
■
Notes
S
* Supporting Information
The authors declare no competing financial interest.
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.inorg-
chem.6b01814.
ACKNOWLEDGMENTS
■
Overview of all synthesized PEPEP-PIZOFs; results of
water stability tests; PXRD results for PEEP-UiO; further
information about the synthesis, structure simulation,
and water stability of PEPP--PIZOF-1 and PPPP-PIZOF-
We are thankful for the financial support by the Deutsche
Forschungsgemeinschaft (DFG) within SPP 1362 (GO 555/5-
2, BE1664/17-2). We thank Dr. Pascal Roy and Dr. Andreas
Schaate for performing preliminary experiments.
L
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Modulation as a Tool To Increase the Catalytic Activity of Metal−
Organic Frameworks: The Unique Case of UiO-66(Zr). J. Am. Chem.
Soc. 2013, 135, 11465−11468.
REFERENCES
■
(1) Cavka, J. H.; Jakobsen, S.; Olsbye, U.; Guillou, N.; Lamberti, C.;
Bordiga, S.; Lillerud, K. P. A New Zirconium Inorganic Building Brick
Forming Metal Organic Frameworks with Exceptional Stability. J. Am.
Chem. Soc. 2008, 130, 13850−13851.
́
(20) Furukawa, H.; Gandara, F.; Zhang, Y.-B.; Jiang, J.; Queen, W. L.;
Hudson, M. R.; Yaghi, O. M. Water Adsorption in Porous Metal−
Organic Frameworks and Related Materials. J. Am. Chem. Soc. 2014,
136, 4369−4381.
(2) Garibay, S. J.; Cohen, S. M. Isoreticular Synthesis and
Modification of Frameworks with the UiO-66 Topology. Chem.
Commun. 2010, 46, 7700−7702.
(21) Devic, T.; Serre, C. High Valence 3p and Transition Metal
Based MOFs. Chem. Soc. Rev. 2014, 43, 6097−6115.
́
(3) Guillerm, V.; Gross, S.; Serre, C.; Devic, T.; Bauer, M.; Ferey, G.
(22) Zahn, G.; Schulze, H. A.; Lippke, J.; Konig, S.; Sazama, U.;
̈
A Zirconium Methacrylate Oxocluster as Precursor for the low-
Temperature Synthesis of Porous Zirconium(IV) Dicarboxylates.
Chem. Commun. 2010, 46, 767−769.
Froba, M.; Behrens, P. A Water-Born Zr-Based Porous Coordination
̈
Polymer: Modulated Synthesis of Zr-Fumarate MOF. Microporous
Mesoporous Mater. 2015, 203, 186−194.
(4) Kandiah, M.; Nilsen, M. H.; Usseglio, S.; Jakobsen, S.; Olsbye, U.;
Tilset, M.; Larabi, C.; Quadrelli, E. A.; Bonino, F.; Lillerud, K. P.
Synthesis and Stability of Tagged UiO-66 Zr-MOFs. Chem. Mater.
2010, 22, 6632−6640.
(5) Schaate, A.; Roy, P.; Godt, A.; Lippke, J.; Waltz, F.; Wiebcke, M.;
Behrens, P. Modulated Synthesis of Zr-Based Metal−Organic
Frameworks: From Nano to Single Crystals. Chem. - Eur. J. 2011,
17, 6643−6651.
(6) Schaate, A.; Roy, P.; Preuße, T.; Lohmeier, S. J.; Godt, A.;
Behrens, P. Porous Interpenetrated Zirconium−Organic Frameworks
(PIZOFs): A Chemically Versatile Family of Metal−Organic Frame-
works. Chem. - Eur. J. 2011, 17, 9320−9325.
(7) Kim, M.; Cohen, S. M. Discovery, Development, and
Functionalization of Zr(IV)-Based Metal−Organic Frameworks.
CrystEngComm 2012, 14, 4096−4104.
(8) Guillerm, V.; Ragon, F.; Dan-Hardi, M.; Devic, T.;
Vishnuvarthan, M.; Campo, B.; Vimont, A.; Clet, G.; Yang, Q.;
(23) SK, M.; Bhowal, S.; Biswas, S. Synthesis, Characterization,
Stability, and Gas Adsorption Characteristics of a Highly Stable
Zirconium Mesaconate Framework Material. Eur. J. Inorg. Chem. 2015,
2015, 3317−3322.
(24) Lv, X.-L.; Tong, M.; Huang, H.; Wang, B.; Gan, L.; Yang, Q.;
Zhong, C.; Li, J.-R. A High Surface Area Zr(IV)-Based Metal−Organic
Framework Showing Stepwise Gas Adsorption and Selective Dye
Uptake. J. Solid State Chem. 2015, 223, 104−108.
(25) Marshall, R. J.; Griffin, S. L.; Wilson, C.; Forgan, R. S. Single-
Crystal to Single-Crystal Mechanical Contraction of Metal−Organic
Frameworks through Stereoselective Postsynthetic Bromination. J. Am.
Chem. Soc. 2015, 137, 9527−9530.
(26) Wang, S.; Wang, J.; Cheng, W.; Yang, X.; Zhang, Z.; Xu, Y.; Liu,
H.; Wu, Y.; Fang, M. A Zr Metal−Organic Framework Based on
Tetrakis-(4-carboxyphenyl) Silane and Factors Affecting the Hydro-
thermal Stability of Zr-MOFs. Dalt. Trans 2015, 44, 8049−8061.
(27) Kalidindi, S. B.; Nayak, S.; Briggs, M. E.; Jansat, S.; Katsoulidis,
́
Maurin, G.; Ferey, G.; Vittadini, A.; Gross, S.; Serre, C. A Series of
̀
A. P.; Miller, G. J.; Warren, J. E.; Antypov, D.; Cora, F.; Slater, B.;
Isoreticular, Highly Stable, Porous Zirconium Oxide Based Metal−
Organic Frameworks. Angew. Chem., Int. Ed. 2012, 51, 9267−9271.
(9) Wißmann, G.; Schaate, A.; Lilienthal, S.; Bremer, I.; Schneider, A.
M.; Behrens, P. Modulated Synthesis of Zr-Fumarate MOF.
Microporous Mesoporous Mater. 2012, 152, 64−70.
Prestly, M. R.; Martí-Gastaldo, C.; Rosseinsky, M. J. Chemical and
Structural Stability of Zirconium-Based Metal−Organic Frameworks
with Large Three-Dimensional Pores by Linker Engineering. Angew.
Chem., Int. Ed. 2015, 54, 221−226.
́ ́
(28) Deria, P.; Gomez-Gualdron, D. A.; Bury, W.; Schaef, H. T.;
(10) Schaate, A.; Duhnen, S.; Platz, G.; Lilienthal, S.; Schneider, A.
̈
Wang, T. C.; Thallapally, P. K.; Sarjeant, A. A.; Snurr, R. Q.; Hupp, J.
T.; Farha, O. K. Ultraporous, Water Stable, and Breathing Zirconium-
Based Metal−Organic Frameworks with ftw Topology. J. Am. Chem.
Soc. 2015, 137, 13183−13190.
M.; Behrens, P. A Novel Zr-Based Porous Coordination Polymer
Containing Azobenzenedicarboxylate as a Linker. Eur. J. Inorg. Chem.
2012, 2012, 790−796.
(11) Yang, Q.; Guillerm, V.; Ragon, F.; Wiersum, A. D.; Llewellyn, P.
L.; Zhong, C.; Devic, T.; Serre, C.; Maurin, G. CH4 Storage and CO2
Capture in Highly Porous Zirconium Oxide Based Metal−Organic
Frameworks. Chem. Commun. 2012, 48, 9831−9833.
(29) Reinsch, H.; Bueken, B.; Vermoortele, F.; Stassen, I.; Lieb, A.;
Lillerud, K.-P.; De Vos, D. Green Synthesis of Zirconium-MOFs.
CrystEngComm 2015, 17, 4070−4074.
(30) Reinsch, H.; Stassen, I.; Bueken, B.; Lieb, A.; Ameloot, R.; De
Vos, D. First Examples of Aliphatic Zirconium MOFs and the
Influence of Inorganic Anions on their Crystal Structures.
CrystEngComm 2015, 17, 331−337.
(31) Bueken, B.; Vermoortele, F.; Cliffe, M. J.; Wharmby, M. T.;
Foucher, D.; Wieme, J.; Vanduyfhuys, L.; Martineau, C.; Stock, N.;
Taulelle, F.; Van Speybroeck, V.; Goodwin, A. L.; De Vos, D. A
Breathing Zirconium Metal−Organic Framework with Reversible Loss
of Crystallinity by Correlated Nanodomain Formation. Chem. - Eur. J.
2016, 22, 3264−3267.
(32) Qadir, N. ul; Said, S. A. M.; Bahaidarah, H. M. Structural
Stability of Metal Organic Frameworks in Aqueous Media −
Controlling Factors and Methods to Improve Hydrostability and
Hydrothermal Cyclic Stability. Microporous Mesoporous Mater. 2015,
201, 61−90.
(33) Burtch, N. C.; Jasuja, H.; Walton, K. S. Water Stability and
Adsorption in Metal−Organic Frameworks. Chem. Rev. 2014, 114,
10575−10612.
(34) Canivet, J.; Fateeva, A.; Guo, Y.; Coasne, B.; Farrusseng, D.
Water Adsorption in MOFs: Fundamentals and Applications. Chem.
Soc. Rev. 2014, 43, 5594−5617.
(12) Bon, V.; Senkovskyy, V.; Senkovska, I.; Kaskel, S. Zr(IV) and
Hf(IV) Based Metal−Organic Frameworks with reo-Topology. Chem.
Commun. 2012, 48, 8407−8409.
́
(13) Morris, W.; Volosskiy, B.; Demir, S.; Gandara, F.; McGrier, P.
L.; Furukawa, H.; Cascio, D.; Stoddart, J. F.; Yaghi, O. M. Synthesis,
Structure, and Metalation of Two New Highly Porous Zirconium
Metal−Organic Frameworks. Inorg. Chem. 2012, 51, 6443−6445.
(14) Jasuja, H.; Zang, J.; Sholl, D. S.; Walton, K. S. Rational Tuning
of Water Vapor and CO2 Adsorption in Highly Stable Zr-Based
MOFs. J. Phys. Chem. C 2012, 116, 23526−23532.
́
(15) Katz, M. J.; Brown, Z. J.; Colon, Y. J.; Siu, P. W.; Scheidt, K. A.;
Snurr, R. Q.; Hupp, J. T.; Farha, O. K. A Facile Synthesis of UiO-66,
UiO-67 and their Derivatives. Chem. Commun. 2013, 49, 9449−9451.
(16) Bon, V.; Senkovska, I.; Weiss, M. S.; Kaskel, S. Tailoring of
Network Dimensionality and Porosity Adjustment in Zr- and Hf-Based
MOFs. CrystEngComm 2013, 15, 9572−9577.
(17) Bon, V.; Senkovska, I.; Baburin, I. A.; Kaskel, S. Zr- and Hf-
Based Metal−Organic Frameworks: Tracking Down the Poly-
morphism. Cryst. Growth Des. 2013, 13, 1231−1237.
(18) DeCoste, J. B.; Peterson, G. W.; Jasuja, H.; Glover, T. G.;
Huang, Y.; Walton, K. S. Stability and Degradation Mechanisms of
Metal−Organic Frameworks Containing the Zr6O4(OH)4 Secondary
Building Unit. J. Mater. Chem. A 2013, 1, 5642−5650.
(35) Howarth, A. J.; Liu, Y.; Li, P.; Li, Z.; Wang, T. C.; Hupp, J. T.;
Farha, O. K. Chemical, Thermal and Mechanical Stabilities of Metal−
Organic Frameworks. Nat. Rev. Mater. 2016, 1, 15018.
(36) Low, J. J.; Benin, A. I.; Jakubczak, P.; Abrahamian, J. F.; Faheem,
S. A.; Willis, R. R. Virtual High Throughput Screening Confirmed
(19) Vermoortele, F.; Bueken, B.; Le Bars, G.; Van de Voorde, B.;
Vandichel, M.; Houthoofd, K.; Vimont, A.; Daturi, M.; Waroquier, M.;
Van Speybroeck, V.; Kirschhock, C.; De Vos, D. E. Synthesis
M
DOI: 10.1021/acs.inorgchem.6b01814
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
Experimentally: Porous Coordination Polymer Hydration. J. Am.
Chem. Soc. 2009, 131, 15834−15842.
dimethyl diester through column chromatography is difficult because
of their similar chromatographic behavior. PEEP dimethyl diester does
not hamper the Suzuki coupling and was removed during isolation of
PEPP dimethyl diester.
(56) The reported procedures for the synthesis and isolation of PPPP
diacid 357 and PEEP diester 1(Me)42,58 differ from ours.
(57) Hylland, K.; Øien-Ødegaard, S.; Lillerud, K.; Tilset, M. Efficient,
Scalable Syntheses of Linker Molecules for Metal-Organic Frame-
works. Synlett 2015, 26, 1480−1485.
(58) Fasina, T. M.; Collings, J. C.; Burke, J. M.; Batsanov, A. S.;
Ward, R. M.; Albesa-Jove, D.; Porres, L.; Beeby, A.; Howard, J. A. K.;
́ ̀
(37) Jiang, H.-L.; Feng, D.; Liu, T.-F.; Li, J.-R.; Zhou, H.-C. Pore
Surface Engineering with Controlled Loadings of Functional Groups
via Click Chemistry in Highly Stable Metal−Organic Frameworks. J.
Am. Chem. Soc. 2012, 134, 14690−14693.
(38) Gui, B.; Hu, G.; Zhou, T.; Wang, C. Pore Surface Engineering in
a Zirconium Metal−Organic Framework via Thiol-ene Reaction. J.
Solid State Chem. 2015, 223, 79−83.
(39) Gui, B.; Meng, X.; Chen, Y.; Tian, J.; Liu, G.; Shen, C.; Zeller,
M.; Yuan, D.; Wang, C. Reversible Tuning Hydroquinone/Quinone
Reaction in Metal−Organic Framework: Immobilized Molecular
Switches in Solid State. Chem. Mater. 2015, 27, 6426−6431.
(40) Wang, C.; Volotskova, O.; Lu, K.; Ahmad, M.; Sun, C.; Xing, L.;
Lin, W. Synergistic Assembly of Heavy Metal Clusters and
Luminescent Organic Bridging Ligands in Metal−Organic Frame-
works for Highly Efficient X-ray Scintillation. J. Am. Chem. Soc. 2014,
136, 6171−6174.
(41) Kutzscher, C.; Nickerl, G.; Senkovska, I.; Bon, V.; Kaskel, S.
Proline Functionalized UiO-67 and UiO-68 Type Metal−Organic
Frameworks Showing Reversed Diastereoselectivity in Aldol Addition
Reactions. Chem. Mater. 2016, 28, 2573−2580.
Scott, A. J.; Clegg, W.; Watt, S. W.; Viney, C.; Marder, T. B. Synthesis,
Optical Properties, Crystal Structures and Phase Behaviour of
Symmetric, Conjugated Ethynylarene-Based Rigid Rods with Terminal
Carboxylate Groups. J. Mater. Chem. 2005, 15, 690−697.
(59) Several other procedures have been described.42,58,60
́
(60) Zhao, Y.-L.; Liu, L.; Zhang, W.; Sue, C.-H.; Li, Q.; Miljanic, O.
̌
S.; Yaghi, O. M.; Stoddart, J. F. Rigid-Strut-Containing Crown Ethers
and [2]Catenanes for Incorporation into Metal−Organic Frameworks.
Chem. - Eur. J. 2009, 15, 13356−13380.
(61) Zahn, G.; Zerner, P.; Lippke, J.; Kempf, F. L.; Lilienthal, S.;
Schroder, C. A.; Schneider, A. M.; Behrens, P. Insight into the
̈
Mechanism of Modulated Syntheses: in situ Synchrotron Diffraction
Studies on the Formation of Zr-Fumarate MOF. CrystEngComm 2014,
16, 9198−9207.
(42) Marshall, R. J.; Griffin, S. L.; Wilson, C.; Forgan, R. S.
Stereoselective Halogenation of Integral Unsaturated C-C Bonds in
Chemically and Mechanically Robust Zr and Hf MOFs. Chem. - Eur. J.
2016, 22, 4870−4877.
(62) Thommes, M.; Kaneko, K.; Neimark, A. V.; Olivier, J. P.;
Rodriguez-Reinoso, F.; Rouquerol, J.; Sing, K. S. W. Physisorption of
gases, with special reference to the evaluation of surface area and pore
size distribution (IUPAC Technical Report). Pure Appl. Chem. 2015,
87, 1051−1069.
(43) Manna, K.; Zhang, T.; Greene, F. X.; Lin, W. Bipyridine- and
Phenanthroline-Based Metal−Organic Frameworks for Highly Effi-
cient and Tandem Catalytic Organic Transformations via Directed C−
H Activation. J. Am. Chem. Soc. 2015, 137, 2665−2673.
(44) Roy, P.; Schaate, A.; Behrens, P.; Godt, A. Post-Synthetic
Modification of Zr-Metal−Organic Frameworks through Cyclo-
addition Reactions. Chem. - Eur. J. 2012, 18, 6979−6985.
(45) Doan, T. L. H.; Nguyen, H. L.; Pham, H. Q.; Pham-Tran, N.-N.;
Le, T. N.; Cordova, K. E. Tailoring the Optical Absorption of Water-
Stable ZrIV- and HfIV-Based Metal−Organic Framework Photo-
catalysts. Chem. - Asian J. 2015, 10, 2660−2668.
(46) Chen, D.; Xing, H.; Wang, C.; Su, Z. Highly Efficient Visible-
Light-Driven CO2 Reduction to Formate by a New Anthracene-based
Zirconium MOF via Dual Catalytic Routes. J. Mater. Chem. A 2016, 4,
2657−2662.
(47) Mondloch, J. E.; Katz, M. J.; Planas, N.; Semrouni, D.; Gagliardi,
L.; Hupp, J. T.; Farha, O. K. Are Zr6-Based MOFs Water Stable?
Linker Hydrolysis vs. Capillary-Force-Driven Channel Collapse. Chem.
Commun. 2014, 50, 8944−8946.
(63) Lilienthal, S. Metall-organische Gerustverbindungen: Simulation
̈
von Strukturen und Adsorptionseigenschaften mittels Monte Carlo-
und Molekuldynamik-Methoden. Doctoral Thesis, Leibniz University
̈
Hannover, Hannover, Germany, 2015.
(64) Shearer, G. C.; Chavan, S.; Ethiraj, J.; Vitillo, J. G.; Svelle, S.;
Olsbye, U.; Lamberti, C.; Bordiga, S.; Lillerud, K. P. Tuned to
Perfection: Ironing Out the Defects in Metal−Organic Framework
UiO-66. Chem. Mater. 2014, 26, 4068−4071.
́
(65) Gutov, O. V.; Hevia, M. G.; Escudero-Adan, E. C.; Shafir, A.
Metal−Organic Framework (MOF) Defects under Control: Insights
into the Missing Linker Sites and Their Implication in the Reactivity of
Zirconium-Based Frameworks. Inorg. Chem. 2015, 54, 8396−8400.
́
(66) Trickett, C. A.; Gagnon, K. J.; Lee, S.; Gandara, F.; Burgi, H.-B.;
̈
Yaghi, O. M. Definitive Molecular Level Characterization of Defects in
UiO-66 Crystals. Angew. Chem., Int. Ed. 2015, 54, 11162−11167.
(67) Jeschke, G.; Sajid, M.; Schulte, M.; Ramezanian, N.; Volkov, A.;
Zimmermann, H.; Godt, A. Flexibility of Shape-Persistent Molecular
Building Blocks Composed of p-Phenylene and Ethynylene Units. J.
Am. Chem. Soc. 2010, 132, 10107−10117.
(48) Shearer, G. C.; Forselv, S.; Chavan, S.; Bordiga, S.; Mathisen, K.;
Bjorgen, M.; Svelle, S.; Lillerud, K. P. In Situ Infrared Spectroscopic
and Gravimetric Characterisation of the Solvent Removal and
Dehydroxylation of the Metal Organic Frameworks UiO-66 and
UiO-67. Top. Catal. 2013, 56, 770−782.
(49) Virmani, E.; Wuttke, S.; von Zons, T.; Godt, A. Unpublished
results.
(50) Bae, Y.-S.; Yazaydın, A. O.; Snurr, R. Q. Evaluation of the BET
Method for Determining Surface Areas of MOFs and Zeolites that
Contain Ultra-Micropores. Langmuir 2010, 26, 5475−5483.
́
(51) Rappe, A. K.; Casewit, C. J.; Colwell, K. S.; Goddard, W. A., III;
Skiff, W. M. UFF, a Full Periodic Table Force Field for Molecular
Mechanics and Molecular Dynamics Simulations. J. Am. Chem. Soc.
1992, 114, 10024−10035.
(52) Heck, R. F. Palladium Reagents in Organic Synthesis; Academic
Press: New York, 1985; p 18.
(53) Recent experiments suggest that the alkynyl−aryl cross-coupling
works well with piperidine that has not been dried.
(54) The corresponding ethyl ester has been described.6 The NMR
data of the methyl ester are identical except for the signal of the methyl
group instead of that of an ethyl group.
(55) The starting material came from a synthesis in which, because of
insufficient degassing, an unusually large amount of PEEP dimethyl
diester formed. Separation of iodo-PEP methyl ester and PEEP
N
DOI: 10.1021/acs.inorgchem.6b01814
Inorg. Chem. XXXX, XXX, XXX−XXX