Rational design and synthesis of porous polymer networks: Toward high surface area

Article abstract of DOI:10.1021/cm501922h

Head-on polymerization of tetrahedral monomers inherently imparts interconnected diamond cages to the resulting framework with each strut widely exposed. We have designed and synthesized a series of 3,3′,5,5′- tetraethynylbiphenyl monomers, in which the two phenyl rings are progressively locked into a nearly perpendicular position by adding substituents of different size at 2, 2′, 6, and 6′ positions, as evident from single crystal structures. Computational simulation suggests that these monomers, though not perfectly regular tetrahedra, could still be self-polymerized into three-dimensional frameworks with the same topology. Indeed, five porous polymer networks (PPNs) have been successfully synthesized with these newly designed monomers through Cu(II)-promoted Eglinton homocoupling reaction. Among them, PPN-13 shows exceptionally high Brunauer-Emmett-Teller (BET) surface area of 3420 m²/g. The total hydrogen uptake is 52 mg/g at 40 bar and 77 K, and the total methane uptake is 179 mg/g at 65 bar and 298 K.

Full text of DOI:10.1021/cm501922h

Article
pubs.acs.org/cm
Rational Design and Synthesis of Porous Polymer Networks: Toward
High Surface Area
Weigang Lu,† Zhangwen Wei,† Daqiang Yuan,*,‡ Jian Tian,† Stephen Fordham,† and Hong-Cai Zhou*,†
†Department of Chemistry, Texas A&M University, College Station, Texas 77842, United States
‡State Key Laboratory of Structure Chemistry, Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences,
Fuzhou, 350002, P. R. China
S
* Supporting Information
ABSTRACT: Head-on polymerization of tetrahedral monomers
inherently imparts interconnected diamond cages to the resulting
framework with each strut widely exposed. We have designed and
synthesized a series of 3,3′,5,5′-tetraethynylbiphenyl monomers, in
which the two phenyl rings are progressively locked into a nearly
perpendicular position by adding substituents of different size at 2,
2′, 6, and 6′ positions, as evident from single crystal structures.
Computational simulation suggests that these monomers, though not
perfectly regular tetrahedra, could still be self-polymerized into three-
dimensional frameworks with the same topology. Indeed, five porous
polymer networks (PPNs) have been successfully synthesized with these newly designed monomers through Cu(II)-promoted
Eglinton homocoupling reaction. Among them, PPN-13 shows exceptionally high Brunauer−Emmett−Teller (BET) surface area
of 3420 m2/g. The total hydrogen uptake is 52 mg/g at 40 bar and 77 K, and the total methane uptake is 179 mg/g at 65 bar and
298 K.
polymers (CMPs), in which transition-metal-catalyzed coupling
reactions were adopted to generate polymeric frameworks.1d,5
More recently, the surface area have reached new heights in
POPs via Yamamoto homocoupling reaction. By using
tetrahedral monomer tetrakis(4-bromophenyl)methane, PAF-
1 (PAF = porous aromatic framework) was synthesized with a
BET surface area of 5600 m2/g;6 by using tetrakis(4-
bromophenyl)silane instead, PPN-4 (PPN = porous polymer
network) was synthesized with over 6000 m2/g BET surface
area, which was translated into excellent gas storage capacities.7
Close examination reveals several key factors to achieve high
surface area in POPs.
Size of Monomer. Ideally, one strategy to maximize surface
area is to use long and slim organic strut,8 which, however,
likely lead to interpenetrated framework;9 if not, structural
collapse upon guest solvent removal.8b Although interpenetra-
tion has been shown to experimentally stabilize the structure, it
is not suggested to be a viable route for improving the surface
area. Investigation of the interpenetrated framework indicates
that it usually has much less surface accessible to gas molecules
than the corresponding noninterpenetrated one.10
INTRODUCTION
■
Recent decades have witnessed a rapid growth in the study of
porous polymers due to their potential applications in catalysis
and gas storage/separation.1 For instance, metal−organic
frameworks (MOFs) have greatly challenged our perception
of the surface area limit for solid-state materials; the current
record holder is NU-110E with a Brunauer−Emmett−Teller
(BET) surface area of 7140 m2/g.2 Nevertheless, the
susceptibility to exchange reactions of the coordination bonds
utilized to support the MOFs undermines their chemical
stability in most cases, deterring practical applications under
harsh conditions. On the other hand, porous organic polymers
(POPs), though amorphous, add new merits, such as low cost
and easy processing, to the adsorbents family. More
importantly, the robustness of the covalent bonds used to
construct the POPs renders most of them with exceptional
chemical stability. Thus, they can survive the vigorous post-
treatments either required to thoroughly empty the voids or to
introduce functionalities into the frameworks.
Compared to traditional porous materials, such as zeolites
and activated carbons, POPs have more potential of rational
design through control of the architecture and function. For
example, in the case of polymers with intrinsic microporosity
(PIMs), the voids were formed as a direct consequence of the
shape and rigidity of the component macromolecules.3 By using
reversible co-condensation reactions, covalent organic frame-
works (COFs) were synthesized as porous crystalline solids
featured with extended periodicity.4 High microporosity and
chemical resistance were observed in conjugated microporous
Geometry of Monomer. Most POPs are synthesized
through kinetic process. The dimensionality of the final product
is largely governed by the geometry of the monomer. Linear
monomers tend to form polymeric chains; monomers with
Received: May 26, 2014
Revised: July 10, 2014
Published: July 20, 2014
© 2014 American Chemical Society
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trigonal/square planar geometry form polymeric sheets. In
reality, however, they did generate cross-linked polymers, likely
due to the contingent bond rotation. Thus, the relationship
between monomer’s geometry and polymer’s dimensionality is
obscure in most cases. On the other hand, monomers with
tetrahedral/octahedral geometry inherently form three-dimen-
sional (3D) frameworks if head-on polymerization is applied.
From the architectural point of view, tetrahedral monomers
tend to form 3D framework with a diamond-topology, imposed
by the monomers themselves, featuring interconnected
diamond cages with each strut widely exposed to gas
molecules.11
Efficiency of Polymerization. Linear polymers usually can
reach high degrees of polymerization because the intermediates
are soluble in reaction solutions, which allow active sites to have
better chances to continue colliding and reacting. Unlike linear
polymers, in which all of the repeating units are surrounded and
therefore solvated by solvents, 3D polymers tend to precipitate
at much earlier stage possibly due to the less efficient
interaction between the highly cross-linked intermediates and
the solvents. Once precipitated, the polymeric propagation
would be virtually terminated because the chances of effective
collision between the active sites on the precipitates and on the
monomers in solution would be greatly reduced. To guarantee
efficient gas adsorption through accessible surface area,
however, degrees of polymerization in thousands or tens of
thousands are desired. Thus, it is essential to reach high degrees
of polymerization before precipitation. One approach to
achieve this is to apply highly efficient reactions, such as
Yamamoto homocoupling,6,12 Eglinton homocoupling,11 and
azide−alkyne “click chemistry”,13 in which the transition metals
are involved in the instantaneous activation of the monomers,
leading to the formation of 3D polymers in substantial degrees
before breaking apart from the solution.
In our previous study, PPN-1 was synthesized with
tetrakis(4-ethynylphenyl)methane (TEPM) through Eglinton
homocoupling of the terminal alkynes. The as-synthesized
PPN-1 was observed dramatic shrinkage upon guest solvent
removal (Supporting Information Figure S25), indicating that
the framework collapsed due to the magnitude of the formed
voids; a consequence of “nature abhors a vacuum”. To prevent
framework from collapsing in this case, an intuitive approach is
to shorten the arms of the monomer, thus, reduce the size of
the subsequently formed diamond-cage. Starting off from
TEPM, 3,3-diethynylpenta-1,4-diyne (DEPD) is the only
potential monomer (Supporting Information Scheme S1),
however, it is extremely air sensitive and synthetically
challenging.14
2′, 6, and 6′ positions, the two phenyl rings are locked into
perpendicular positions and four terminal ethynyl groups locate
at the corners of a tetrahedron. Computational simulation
suggests these monomers, though not perfectly regular
tetrahedra, could still be self-polymerized into the same
topological frameworks with diamondoid cages. More
importantly, the distance from the center of the monomer to
the terminal carbon of alkynes is reduced from ∼7.0 Å in
TEPM to ∼5.5 Å in 3,3′,5,5′-tetraethynylbiphenyls (Supporting
Information Scheme S1), which leads to rather significant
decreases in the simulated unit cell parameters (Supporting
Information Table S1). For example, the unit cell volume,
which is essentially a diamond cage, is reduced from 21 806 Å3
in PPN-1 to 9926 Å3 in PPN-13, greatly diminishing the risk of
framework collapsing upon guest-solvent removal, thus, high
porosity could be sustained. This hypothesis has been
experimentally corroborated; PPN-13, synthesized from
3,3′,5,5′-tetraethynyl-2,2′,4,4′,6,6′-hexamethylbiphenyl through
highly efficient Eglinton homocoupling, exhibits an exception-
ally high BET surface area of 3420 m2/g.
MATERIALS AND METHODS
■
Solvents, reagents, and chemicals were purchased from Sigma-Aldrich
and VWR International. Tetrahydrofuran (THF) was distilled from
sodium/benzophenone, and triethylamine (TEA) was distilled from
calcium hydride under nitrogen prior to use. Solid materials were
powdered. All reactions involving moisture sensitive reactants were
performed under a nitrogen atmosphere using oven-dried and/or
flame-dried glassware. All other solvents, reagents and chemicals were
used as purchased unless stated otherwise. 1,3,6,8-tetraethynylpyrene
was synthesized according to the literature procedure.15
Fourier transform infrared spectroscopy (FT-IR) data were
collected on a SHIMADZU IRAffinity-1 spectrophotometer; the
position of an absorption band was given in wave numbers ν in cm−1.
Elemental analyses (C and H) were obtained from Canadian
Microanalytical Service, Ltd. Melting points were measured on
Thomas-Hoover capillary melting point apparatus. Thermogravimetric
analyses (TGA) were performed under a nitrogen atmosphere on a
SHIMADZU TGA-50 thermogravimetric analyzer; with a heating rate
of 3 °C/min. Scanning electron microscopy (SEM) images were taken
on a JEOL JSM-6700F SEM. The samples were ground before
observation. 1H NMR spectra were recorded on a Mercury (300
MHz) spectrometer as solutions in CDCl3. Chemical shifts are
expressed in parts per million (ppm, δ) downfield from tetramethylsi-
lane (TMS) and referenced to CHCl3 (7.26 ppm) as internal standard.
All coupling constants (J) are absolute values and expressed in Hertz
(Hz). 19F NMR spectra were recorded on a mercury (282 MHz)
spectrometer as solutions in CDCl3. Chemical shifts are expressed in
parts per million (ppm, δ) without reference. 13C NMR spectra were
recorded on a mercury (75 MHz) spectrometer as solutions in CDCl3.
Chemical shifts are expressed in parts per million (ppm, δ) downfield
from tetramethylsilane (TMS) and referenced to CHCl3 (77.0 ppm)
as internal standard. The solid-state NMR spectra were recorded on a
Bruker AVANCE 400 spectrometer operating at 100.6 MHz for 13C.
The 13C CP/MAS (cross-polarization with magic angle spinning)
experiments were carried out at MAS rates of 13 and 10 kHz using
densely packed powders of the PPNs in 4 mm ZrO2 rotors.
Herein, we designed a series of tetraethynyl monomers with
biphenyl as backbone (Scheme 1); By adding substituents at 2,
Scheme 1. Illustration of Formation of a 3D Polymer with
Geometrically Constrained Biphenyl Monomer through
Head-on Homocoupling Reaction
Synthesis of 3,3′,5,5′-Tetraethynylbiphenyl. To a solution of
1,3,5-tribromobenzene (15.7 g, 50 mmol) in 200 mL of anhydrous
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diethyl ether at −78 °C under a nitrogen atmosphere, n-butyllithium
(2.5 mol/L hexane solution, 22.0 mL, 55 mmol) was added in 15 min.
The resulting mixture was stirred at room temperature for another 2 h.
After cooled back to −78 °C, anhydrous copper(II) chloride (7.4 g, 55
mmol) was added in portions. The resulting mixture was stirred at
room temperature overnight. Then it was filtered; the filtrate was
evaporated to dryness. Flash chromatography with hexanes as eluent
and recrystallization with ethanol afforded 3,3′,5,5′-tetrabromobiphen-
8.067(8), b = 23.10(2), c = 4.340(4) Å, V = 808.9(14) Å3, Z = 2, Dc =
3.144 g/cm3, F000 = 676, T = 110(2) K, 7338 reflections collected,
1954 unique (Rint = 0.0427). Final GooF = 1.161, R1 = 0.0261, wR2 =
0.0706. Its cif file can be found in the Supporting Information.
To a degassed mixture of anhydrous THF (20 mL) and TEA (20
mL), 2,2′,4,4′,6,6′-hexafluoro-3,3′,5,5′-tetraiodobiphenyl (650 mg,
0.85 mmol), tetrakis(triphenylphosphine)palladium(II) (100 mg,
0.087 mmol, 10 mol %), copper(I) iodide (20 mg, 0.11 mmol), and
trimethylsilylacetylene (0.55 mL, 3.8 mmol) were added under a
nitrogen atmosphere. The resulting mixture was stirred at 90 °C
overnight. After it was cooled down to room temperature, it was
filtered; the filtrate was evaporated to dryness under reduced pressure.
The residue was then taken up in 10 mL of tetra-n-butylammonium
fluoride in THF (1 mol/L). After stirred for one hr, the mixture was
filtered; the filtrate was evaporated to dryness. Flash chromatography
with 10% dichloromethane/hexanes as eluent afforded 3,3′,5,5′-
tetraethynyl-2,2′,4,4′,6,6′-hexafluorobiphenyl as a white solid (170
mg, 0.47 mmol, 56% yield over two steps), Temperature for onset of
1
yl (4.5 g, 19.2%). H NMR (CDCl3, 300 MHz) 7.70 (t, J = 1.8 Hz,
2H), 7.59 (d, J = 1.8 Hz, 4H) ppm. These values are in good
agreement with the literature.16 A single crystal was obtained after
layering methanol on the top of its dichloromethane solution
overnight (see the Supporting Information). Crystal data: C12H6Br4,
FW = 469.81, colorless block, monoclinic, space group C2/c, a =
16.691(11), b = 7.314(5), c = 11.161(7) Å, V = 1271.1 (14) Å3, Z = 4,
Dc = 2.455 g/cm3, F000 = 872, T = 110(2) K, 6740 reflections
collected, 1426 unique (Rint = 0.0479). Final GooF = 1.075, R1 =
0.0289, wR2 = 0.0669. Its cif file can be found in the Supporting
Information.
1
decomposition: 250 °C. H NMR (300 MHz, CDCl3) δ: 3.56 (d, J =
0.6 Hz, 4H); 13C NMR (75 MHz, CDCl3) δ: 88.7 (d), 68.4 (s),
aromatic carbon signals are embedded in noise. 19F NMR (282 MHz,
CDCl3) δ: −98.65 (2F), −101.34 (4F). Anal. calcd for C20H4F6
(358.24): C, 67.05; H, 1.13; F, 31.82. Found: C, 67.24; H, 1.12.
Synthesis of 3,3′,5,5′-Tetraethynyl-2,2′,4,4′,6,6′-hexame-
thylbiphenyl. To a 500 mL three-necked flask containing 1.44 g of
To a degassed mixture of anhydrous THF (20 mL) and TEA (20
mL), 3,3′,5,5′-tetrabromobiphenyl (440 mg, 0.94 mmol), tetrakis-
(triphenylphosphine)palladium(II) (110 mg, 0.095 mmol, 10 mol %),
copper(I) iodide (20 mg, 0.11 mmol), and trimethylsilylacetylene
(0.60 mL, 4.2 mmol) were added under a nitrogen atmosphere. The
resulting mixture was stirred under at 60 °C overnight. After cooled to
room temperature, it was filtered; the filtrate was evaporated to
dryness under reduced pressure. The residue was then taken up in
dichloromethane/methanol/potassium carbonate (20 mL/20 mL/0.80
g). After stirred for 2 h, it was filtered; the filtrate was evaporated to
dryness. Flash chromatography with hexanes as eluent afforded
3,3′,5,5′-tetraethynylbiphenyl as a white solid (178 mg, 0.71 mmol,
1
76% yield over two steps), mp 187−189 °C. H NMR (300 MHz,
CDCl3) δ: 7.65 (d, J = 1.5 Hz, 2H), 7.61 (t, J = 1.5 Hz, 4H), 3.13 (s,
4H). Anal. calcd for C20H10 (250.29): C, 95.97; H, 4.03. Found: C,
95.91; H, 4.11.
magnesium pieces, degassed anhydrous THF (250 mL) and a pinch of
iodine were added under a nitrogen atmosphere. The resulting mixture
was heated to 80 °C, and then 2-bromomesitylene (7.0 mL) was
added dropwise. The reaction mixture was refluxed for another 3 h.
After it was cooled down to room temperature, it was transferred into
another 500 mL flask containing a mixture of anhydrous FeCl3 (0.22
g), 1,2-dibromoethane (2.4 mL), and anhydrous THF (3.0 mL) under
a nitrogen atmosphere. Stirring was continued for another 1 h, and
then the reaction was quenched by the addition of 1.0 mol/L aqueous
HCl solution (5.0 mL). Organic solvents were evaporated under
reduced pressure. The residue was extracted with dichloromethane.
The dichloromethane phase was dried over anhydrous MgSO4, and
filtered. Most of dichloromethane was removed under reduced
pressure, and then methanol was added. Bimesityl was collected as
Synthesis of 3,3′,5,5′-Tetraethynyl-2,2′,4,4′,6,6′-hexafluoro-
biphenyl. 2,2′,4,4′,6,6′-Hexafluorobiphenyl was synthesized accord-
ing to the literature procedure with a good yield.17 To confirm the
structure, single crystal was obtained by layering methanol on the top
of dichloromethane solution. Crystal data: C12H4F6, FW = 262.15,
colorless block, monoclinic, space group C2/c, a = 13.044(5), b =
6.284(3), c = 12.147(6) Å, V = 942.6(7) Å3, Z = 4, Dc = 1.847 g/cm3,
F000 = 520, T = 110(2) K, 5634 reflections collected, 1247 unique (Rint
= 0.0744). Final GooF = 1.097, R1 = 0.0336, wR2 = 0.0948. Its cif file
can be found in the Supporting Information.
To a mixture of 2,2′,4,4′,6,6′-hexafluorobiphenyl (1.3 g, 5.0 mmol),
solid iodine (3.0 g, 11.8 mmol), and HIO4·2H2O (1.4 g, 6.1 mmol) in
a 100 mL round-bottom flask, CH3COOH/H2O/H2SO4 (60/12/1.8
mL) was added. The resulting mixture was refluxed for 3 days. After it
was cooled down to room temperature, it was diluted with 200 mL of
water. The resulting precipitate was filtered, and washed with water.
The pink solid was collected and dissolved in 100 mL of CHCl3, then
washed with aqueous Na2S2O3 solution to remove iodine residue
(color of solution changed from purple to colorless quickly). The
organic phase was dried over anhydrous MgSO4, filtered, and
evaporated under reduced pressure to ca. 20 mL. Then 80 mL of
methanol was added, and the precipitate was collect as a white solid
(3.2 g, 4.1 mmol, 84%). Single crystal was obtained after layering
methanol on the top of its dichloromethane solution overnight (see
the Supporting Information), and it was confirmed as 2,2′,4,4′,6,6′-
hexafluoro-3,3′,5,5′-tetraiodobiphenyl. Crystal data: C12 F6I4, FW =
765.72, colorless block, orthorhombic, space group P21212, a =
1
colorless solid. (4.0 g, 73.5% yield). H NMR (300 MHz, CDCl3) δ:
6.93 (s, 4H), 2.33 (s, 6H), 1.86 (s, 12H) ppm.
To a mixture of bimesityl (2.0 g, 8.4 mmol), solid iodine (3.5 g, 13.5
mmol), HIO4·2H2O (1.55 g, 6.7 mmol) in a 250 mL flask, add
CH3COOH/H2O/H2SO4 (120/24/3.6 mL). The resulting mixture
was stirred at 90 °C for 3 days. The reaction mixture was diluted with
250 mL of water. The precipitate was filtered, and washed thoroughly
with water. The pink solid was collected and dissolved in 100 mL of
CHCl3, then washed with saturated Na2S2O3 solution to remove
iodine residue. The organic phase was dried over anhydrous MgSO4,
filtered, and evaporated under reduced pressure to produce 3,3′,5,5′-
tetraiodo-2,2′,4,4′,6,6′-hexamethylbiphenyl as a white solid (4.5 g,
72.2% yield). The product was further purified by recrystallization in
hexanes/ethyl acetate (5/1) for characterization. 1H NMR (300 MHz,
CDCl3) δ: 2.05 (s, 12 H), 3.02 (s, 6 H). Single crystal was obtained
after layering methanol on the top of its dichloromethane solution
overnight (see the Supporting Information). Crystal data: C18H18I4,
FW = 741.92, colorless block, monoclinic, space group P21/n, a =
13.334(3), b = 20.948(4), c = 15.971(3) Å, V = 4062.5(14) Å3, Z = 8,
Dc = 2.426 g/cm3, F000 = 2704, T = 110(2) K, 48345 reflections
collected, 9693 unique (Rint = 0.0448). Final GooF = 1.046, R1 =
0.0247, wR2 = 0.0467. Its cif file can be found in the Supporting
Information.
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To a degassed mixture of anhydrous THF (50 mL) and TEA (50
mL), 3,3′,5,5′-tetraiodo-2,2′,4,4′,6,6′-hexamethylbiphenyl (2.0 g, 2.7
mmol), bis(triphenylphosphine)palladium(II) chloride (190 mg, 0.27
mmol), copper(I) iodide (50 mg, 0.28 mmol), and trimethylsilylace-
tylene (2.0 mL, 14.1 mmol) were added under a nitrogen atmosphere.
The resulting mixture was stirred at 90 °C for 3 days. After it was
cooled down to room temperature, it was filtered; the filtrate was
evaporated to dryness under reduced pressure. The residue was then
taken up in dichloromethane/methanol/potassium carbonate (50 mL/
50 mL/2.0 g) for 2 h. It was filtered; the filtrate was evaporated to
dryness. Flash chromatography with hexanes as eluent afforded
3,3′,5,5′-tetraethynyl-2,2′,4,4′,6,6′-hexamethylbiphenyl as a white
tetraethynyl-2,2′,4,4′,6,6′-hexamethylbiphenyl (100 mg, 0.30 mmol) in
pyridine (2.0 mL) was added in one portion. The resulting mixture
was stirred for 30 min. The precipitate was collected, washed lavishly
with methanol and water, and dried in vacuo to give PPN-13 as a
brown powder (95 mg, 95% yield). 13C CP/MAS (400 MHz) δ:
134.23 (3C), 118.08, 78.25 (2C), 13.66 (2C).
X-ray Single-Crystal Diffraction. Data were collected on a
Bruker AXS APEX-II CCD (charge-coupled device) diffractometer
with a fine-focus sealed-tube X-ray source (Mo−Kα). The structures
were resolved by the direct method and refined by full-matrix least-
squares fitting on F2 by the SHELX-97 software package.19 All non-
hydrogen atoms were refined with anisotropic thermal parameters. All
the hydrogen atoms were added at geometrically calculated positions
and refined as riding on their respective carbon atoms, with Uiso(H) =
1.2Ueq(C), a default treatment for hydrogen atom in SHELX.
Creation of PPN models. The theoretical noninterpenetrated
frameworks of PPNs were created by repeating the unit of the
monomer molecule. The unit cell of PPNs was subject to symmetry-
constrained geometry optimization runs based on molecular
mechanics simulations and their geometrical structures were optimized
using the Forcite Plus module with Universal force field in Material
Studio 6.0.20 The successive geometry optimization calculations were
performed until the difference between the two successive unit cell
dimensions was smaller than 0.001 Å. The porosity and pore volume
of these simulated PPN structures were calculated with Platon.21 All
the data are shown in Tables 1 and S1 (Supporting Information) for
comparison.
1
solid (0.46 g, 51% yield over two steps), mp 157−159 °C. H NMR
(300 MHz, CDCl3) δ: 3.52 (s, 4H), 2.67 (s, 6H), 2.01 (s, 12H) ppm.
13C NMR (75 MHz, CDCl3) δ: 142.9, 139.1, 137.0, 120.7, 85.4, 81.3,
20.0, 18.7. Anal. calcd for C26H22 (334.45): C, 93.37; H, 6.63. Found:
C, 93.56; H, 6.72.
Synthesis of 3,3′,5,5′-Tetraethynyl-2,2′,6,6′-tetramethoxy-
4,4′-dimethylbiphenyl. 2,2′,6,6′-Tetramethoxy-4,4′-dimethylbi-
phenyl was synthesized as described in the literature.18
To a mixture of 2,2′,6,6′-tetramethoxy-4,4′-dimethylbiphenyl (2.0 g,
6.6 mmol), solid iodine (3.5 g, 13.5 mmol), and HIO4·2H2O (1.55 g,
6.7 mmol) in a 250 mL flask, CH3COOH/H2O/H2SO4 (120/24/3.6
mL) was added. The resulting mixture was stirred at 120 °C for 3 days.
After it was cooled down to room temperature, the reaction mixture
was diluted with 250 mL of water. The precipitate was filtered and
washed with water. The pink solid was dissolved in 100 mL of CHCl3
and washed with saturated Na2S2O3 solution to remove iodine residue.
The CHCl3 phase was evaporated to ca. 20 mL under reduced
pressure, and then, 100 mL of methanol was added. 3,3′,5,5′-
Tetraiodo-2,2′,6,6′-tetramethoxy-4,4′-dimethylbiphenyl was collected
Table 1. Surface Areas, Pore Volumes, and Porosities of
Synthesized PPNs
a
model space
group
Vp
porosity
(calc)
material
SLang/SBET/SCalc
(exp/calc)
PPN-10
PPN-11
PPN-12
PPN-13
PPN-14
Cmmm
P4322
P4322
1332/1128/2210
1551/1742/5677
1551/1742/4329
3966/3420/5703
1910/2160/5100
0.99/0.44
0.92/5.63
1.70/3.88
2.05/3.83
1.25/3.17
41.4%
90.6%
89.3%
84.8%
83.9%
I42d
̅
I41/amd
1
as a white solid (4.8 g, 90% yield). H NMR (300 MHz, CDCl3) δ:
a
3.58 (s, 12 H), 2.96 (s, 6 H). 13C NMR (75 MHz, CDCl3) δ: 158.5,
145.6, 121.0, 93.1, 60.8, 36.4. A single crystal was obtained after
layering methanol on the top of its dichloromethane solution
overnight (see the Supporting Information). Crystal data:
C18H18I4O4, FW = 805.92, colorless block, monoclinic, space group
Cc, a = 12.997(6), b = 13.035(6), c = 13.301(6) Å, V = 2252.7(19) Å3,
Z = 4, Dc = 2.376 g/cm3, F000 = 1480, T = 110(2) K, 12911 reflections
collected, 5348 unique (Rint = 0.0582). Final GooF = 1.057, R1 =
0.0399, wR2 = 0.0419. Its cif file can be found in the Supporting
Information.
SLang and SBET are experimental Langmuir and BET surface areas
(m2/g); SCalc is the calculated Connolly surface area (m2/g); Vp (exp/
calc) are the experimental pore volume (cm3/g) and the pore volume
(cm3/g) calculated from the simulated structure; porosity (calc) is the
calculated porosity from the simulated structure.
Low-Pressure Gas Sorption Measurements. Low pressure
(<800 Torr) gas sorption isotherms were measured using a
Micrometrics ASAP 2020 surface area and pore size analyzer. Pore
size distribution data were calculated from the nitrogen sorption
isotherms based on the density functional theory (DFT) model in the
Micromeritics ASAP 2020 software package (assuming slit pore
geometry). Prior to the measurements, the samples were degassed for
10 h at 80 °C. Ultrahigh purity grade N2, He, H2, and CH4 were used
for all measurements. Oil-free vacuum pumps and oil-free pressure
regulators were used for all measurements to prevent contamination of
the samples during the degassing process and isotherm measurement.
High-Pressure Gas Sorption Measurements. High pressure
excess adsorption of H2 and CH4 were measured using a HTP1-V
Sieverts-type volumetric hydrogen storage analyzer (Hiden) at 77 K
(liquid nitrogen bath) or 298 K (room temperature). About 200 mg of
sample was loaded into the sample holder. Prior to the measurements,
the samples were degassed at 80 °C overnight. The free volume was
determined by the expansion of low-pressure He (<5 bar) at room
temperature.
To a degassed mixture of anhydrous THF (50 mL) and TEA (50
mL), 3,3′,5,5′-tetraiodo-2,2′,6,6′-tetramethoxy-4,4′-dimethylbiphenyl
(2.0 g, 2.5 mmol), bis(triphenylphosphine)palladium(II) chloride
(190 mg, 0.27 mmol), copper(I) iodide (50 mg, 0.28 mmol), and
trimethylsilylacetylene (2.0 mL, 14.1 mmol) were added under a
nitrogen atmosphere. The resulting mixture was stirred at 90 °C for 3
days. After it was cooled down to room temperature, it was filtered;
the filtrate was evaporated to dryness under reduced pressure. The
residue was then taken up in dichloromethane/methanol/potassium
carbonate (50 mL/50 mL/2.0 g) for 2 h. It was filtered; the filtrate was
evaporated to dryness. Flash chromatography with gradient elution
(from hexanes to 10% ethyl acetate in hexanes) afforded 3,3′,5,5′-
tetraethynyl-2,2′,6,6′-tetramethoxy-4,4′-dimethylbiphenyl as a white
1
solid (0.75 g, 75% yield over two steps), mp 175−177 °C. H NMR
(300 MHz, CDCl3) δ: 3.76 (s, 12H), 3.50 (s, 4H), 2.64 (s, 6H) ppm.
13C NMR (75 MHz, CDCl3) δ: 161.2, 146.9, 119.9, 112.4, 85.4, 78.5,
61.1, 19.5. Anal. calcd for C26H22O4 (398.45): C, 78.37; H, 5.57; O,
16.06. Found: C, 78.01; H, 5.62.
General Procedure for the Synthesis of PPNs (Take PPN-13
as an Example). To a clear solution of Cu(OAc)2·H2O (1.2 g, 1.9
mmol) in pyridine (20 mL) at 100 °C, a solution of 3,3′,5,5′-
RESULTS AND DISCUSSION
■
Dihedral Angle. A close-to-90° dihedral angle is key for the
biphenyl monomer to adopt a nearly perfect tetrahedral
geometry, which is essential to impose wide-open and
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interconnected diamond cages to efficiently expose each strut
to the gas molecules in the resulting polymeric framework. The
dihedral angle of biphenyl, however, is largely dependent on the
substituents at 2, 2′, 6, and 6′ positions. To study the
correlation between the size of the substituents and dihedral
angles, we designed and synthesized a series of 3,3′,5,5′-
tetraethynylbiphenyl monomers 1−5 (Scheme 2). High-quality
The structures of PPN-13 and PPN-14 have been
characterized on a molecular level by solid-state CP/MAS
13C NMR. The signal assignments for the spectra displayed in
Figure 1 were made based on compounds with similar structure
a
Scheme 2. Monomers and Their Corresponding Precursors
Figure 1. Solid-state CP/MAS 13C NMR spectra of PPN-13 (top) and
PPN-14 (bottom); 4 mm, bypass, 10 kHz, field = −2000, p15 = 2000.
(inset) Corresponding monomers’ chemical shifts in their 13C NMR
spectra in CDCl3.
a
In parentheses are the dihedral angles from single crystal structures. *
Average value for the two molecules in one asymmetric unit.
elements reported before,6,7,11,23 as well as a comparison with
the solution NMR data of their respective monomers in CDCl3.
Overall, the solid-state 13C NMR signals shifted upfield
compared to their corresponding monomers’ solution 13C
NMR in CDCl3. For example, two methyl carbons, C1 and C2,
resonance at 19.98 and 18.66 ppm respectively in the spectrum
of PPN-13 monomer’s solution NMR, while they appear
overlapped at 13.36 ppm in the solid-state NMR spectrum of
PPN-13 (Figure 1 top), which could be attributed to the
relatively low resolution of solid-state NMR. For the same
reason, aromatic carbons (C6, C7, and C8) are recorded as one
broad signal in the solid-state NMR spectrum of PPN-13
(134.23 ppm). Another aromatic carbon C5, however, is well
resolved at a relative upfield shift (118.08 ppm) due to the
direct shield effect from the alkyne group. Interestingly, two
distinctive alkyne-carbon signals, one at 85.42 ppm for the
terminal carbon C3 and the other at 81.27 ppm for C4, are
observed in monomer’s solution NMR; these two carbons’
signals emerge as one broad peak (78.25 ppm) in the solid-state
NMR spectrum of PPN-13, which could be due partly to
relatively low resolution of solid-state NMR, and partly to C4 in
PPN-13 polymer not being a terminal carbon any more, and
therefore its signal shifted upfield.
The same assumption can be applied to PPN-14, all the
carbon signals in the solid-state spectrum shifted upfield about
4−6 ppm compared to their corresponding carbon signals in
monomer’s solution NMR spectrum (Figure 1 bottom). Two
distinctive alkyne carbon signals in monomer’s solution NMR,
78.46 and 85.36 ppm, end up as one broad peak (78.68 ppm)
in the spectrum of PPN-14’s solid-state NMR. The character-
istic resonance at 55.82 ppm in the spectrum of PPN-14
corresponds to the methoxyl carbons at 2, 2′, 6, and 6′
positions, which proves that this functional group stays intact
during both the coupling reaction and following activation
single crystals were successfully obtained for their precursors
(either iodide or bromide; heavy atoms facilitate the single
crystal growth and structure solution) by using a liquid/liquid
diffusion method. As we can see in Scheme 2, the dihedral
angles increase from 37.3° to 57.1° when hydrogens were
replaced by fluorines at 2, 2′, 6, and 6′ positions, to 83.5° when
replaced by methyl groups, and to almost 90° when replaced by
methoxyl groups.
Although the dihedral angle from single crystal structure does
not necessarily reflect the real conformation of the monomer in
the reaction solution, or the real configuration in the polymeric
framework, it is correlated to the torsional barrier in principle;
the closer-to-90° dihedral angle, the higher torsional barrier.
Thus, monomers with a perpendicular position would be
dominating in the reaction solution for the framework
construction and the probability of deformation of the resulting
diamond-cage could be minimized.
Chemical Composition and Physical Properties of the
PPNs. PPN-10, PPN-11, PPN-12, PPN-13, and PPN-14
(Scheme 2) were synthesized from their corresponding
monomers through Eglinton homocoupling reaction. They
are brown powders that are insoluble in any common solvents.
Unlike PPN-1, which undergoes dramatic shrinkage upon
drying and ends up being rock hard solid (Supporting
Information Figure S25), these new PPNs remain fluffy
powders with a limited shrinkage. SEM images reveal that
PPN-13 is comprised of solid spheres with submicrometer
dimensions, which is typical for highly cross-linked polymers
(Supporting Information Figure S26).22 TGA analyses indicate
that these PPNs have lower thermal stability (<300 °C) than
PAF-1 (520 °C),6 which is probably due to the intrinsically
weak polyyne unit (Supporting Information Figure S20).
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procedure. Overall, the spectra of PPN-13 and PPN-14 suggest
the homogeneity of the materials and the efficiency of the
Eglinton coupling.
and all of the PPNs show the CC stretch of the disubstituted
alkyne at ∼2200 cm−1 (highlighted in gray, Figure 2 right)
instead of the CC stretch of the terminal alkyne at ∼2120
cm−1.
Solid-sate CP/MAS 13C NMR of PPN-11 (Supporting
Information Figure S16) is not as informative probably because
of its much disordered structure. In the case of nonsubstituted
biphenyl, the equilibrium dihedral angle is 44.4° (37.3°
measured herein) and the torsional barrier is ca. 1.4 kcal/mol
at 0 °C.24 The value is much lower than the energy barrier (ca.
23 to 33 kcal/mol)25 required to prohibit spontaneous
conformational isomerization of the substituted biphenyls at
room temperature, not to mention the polymerization reactions
applied here took place at raised temperatures. Therefore,
frameworks constructed from nonsubstituted biphenyls could
easily collapse upon solvent removal because of the “free”
rotation around the C−C bond connecting the phenyl rings.
On the other hand, frameworks constructed from 2,2′,6,6′-
substituted biphenyls would more likely sustain under the same
conditions if the substituents are large enough to prevent “free”
rotation, as in the cases of PPN-13 and -14.
Porosity of the PPNs. Framework models were built for
these PPNs based on the default diamond-topology without
taking interpenetration into consideration (Scheme 1). The
Connolly surface areas,26 pore volumes, and porosities were
calculated using these models (Tables 1 and S1 (Supporting
Information). Based on the calculation, PPN-1, PPN-13, and
PPN-13′ (built from 3,3-diethynylpenta-1,4-diyne, see Scheme
S1 and Table S1 in the Supporting Information for details)
show comparable surface areas (over 5000 m2/g). The
somewhat smaller surface area of PPN-1 compared to PPN-
13 reveals the benefit of longer strut for higher surface area is
not necessarily valid, especially for the extremely elongated
strut. On the other hand, the volumetric surface area of PPN-13
is almost doubled compared to that of PPN-1 based on the
calculated densities; low density usually comes as a liability of
using the long strut.
Solid-sate CP/MAS 13C NMR of PPN-12 (Supporting
Information Figure S17) is also not as informative due partly
to its disordered structure, as its dihedral angle (57.1°) suggests
a low torsional barrier, and partly to the carbon signals being
deeply split by two different sets of fluorine groups on the
benzene ring. In the 1H NMR spectrum of PPN-12’s monomer,
the proton on the alkyne carbon gives a signal at 3.56 ppm, split
by fluorine(s) with a coupling constant of 0.6 Hz (Supporting
Information Figure S3). In its 13C NMR spectrum, the alkyne
carbon directly attached to benzene ring gives a signal at 88.67
ppm, split by adjacent fluorine(s) into multiple peaks; and the
other alkyne carbon gives a signal at 68.35 ppm with no
The porosities of these PPNs were experimentally studied via
nitrogen sorption at 77 K (Figure 3). PPN-12 shows type I
obvious splitting (Supporting Information Figure S4). In its 19
F
NMR spectrum, two sets of fluorine signals were observed with
a ratio of 1:2 (Supporting Information Figure S5).
FT-IR spectra (Figure 2) also suggest the completion and
efficiency of the Eglinton homocoupling reaction. All of the
PPNs do not show the terminal alkyne C−H vibrations of the
monomers at ∼3300 cm−1 (highlighted in gray, Figure 2 left),
Figure 3. Nitrogen sorption isotherms for PPN-10 (red), PPN-11
(black), PPN-12 (magenta), PPN-13 (blue), and PPN-14 (olive) at 77
K; adsorption (filled circle)/desorption (open circle).
nitrogen sorption isotherms based on the IUPAC classification,
indicating extensive microporosity within the frameworks,27
while other four PPNs display a type I isotherm with type IV
character at higher relative pressures, which were confirmed by
their respective pore size distributions calculated by using DFT
models (Supporting Information Figure S27). It is worth
pointing out that the small amount of mesoporosity could be
caused by the voids between submicrometer agglomerates in
these PPNs, which is rather common in porous organic
polymers.28 The surface area data obtained from nitrogen
sorption isotherms reveal that PPN-13 has the highest surface
area (SBET: 3420 m2/g), followed by PPN-14 (SBET: 2160 m2/
g). BET surface area calculations were carried out by using
ASAP 2020 software. Isotherm points chosen to calculate the
BET surface area were subject to the three consistency criteria
detailed by Walton and Snurr.29 First, the pressure range
selected has values of V(Po − P) increasing with P/Po
(Supporting Information Figure S28). Second, the points
used to calculate the BET surface area are linear with an upward
slope (Supporting Information Figure S29). Third, the line has
a positive y-intercept (Supporting Information Figure S29).
Figure 2. FT-IR spectra of monomers (left) and PPNs (right); PPN-
10 and its monomer (red), PPN-11 and its monomer (black), PPN-12
and its monomer (magenta), PPN-13 and its monomer (blue), and
PPN-14 and its monomer (olive).
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The experimental surface areas for PPN-13 and PPN-14 exhibit
the same trend compared to the calculated values (Table 1),
but the disparity between experimental values is much larger
than that of calculated ones. This could be attributed partly to
the larger methoxyl group posing as an obstruction to the
polymerization reaction and partly to its the negative electronic
effect; the first step of Eglinton homocoupling is deprotonation
of the terminal alkyne by pyridine, which is relatively less
efficient in the PPN-14’s case because of methoxyl group’s
electron donating effect. All cavities in PPN-13 are larger than 1
nm, while the other PPNs have considerable amount of cavities
with size less than 1 nm. In the cases of PPN-11 and -12, the
relative high ratio of small cavities is presumably due to the
framework “cave-in” from the “free” rotation of biphenyl.
Applying supramolecular building block (SBB) strategy,
MOFs have been demonstrated to achieve noninterpenetration
with extended ligands.2,8b,30 Conversely, PPNs tend to
interpenetrate because they do not have bulky metal clusters
as building blocks; monomers usually are connected through
single σ bonds. By reducing the size of the monomer, as in the
cases of PPNs reported here, one would expect non-
interpenetration. Although we cannot completely rule out the
possibility of interpenetration, the comparatively short strut
makes it less likely to undergo extensive interpenetration;
therefore, large voids and high surface areas can be expected.
Many POPs with high surface areas have been synthesized
recently.4c,6,7,31 Until now, the highest surface area in POPs
typical for organic polymers lacking special adsorption sites.
However, it is larger than those of PPN-4 (4.1 kJ/mol)7 and
PAF-1 (4.6 kJ/mol)6 probably because of its relatively small
pore size, which allow stronger overall interactions of the guest
gas molecuels due to the additional interaction with the
opposite wall.34 In addition, the polyyne motifs may help
increase the gas affinity as well.35 The heat of adsorption of
PPN-10 at zero loading is surprisingly large (8.4 kJ/mol), which
could be attributed to the uneven distribution of electron
density in the pyrene system, but it drops quickly over the
whole gas loading range, indicating only limited amount of
hydrogen molecules could interact with the pyrene system. The
heat of adsorption of PPN-12 at zero loading is aslo large (8.1
kJ/mol), and it remains almost constant over the whole gas
loading range, which could possibly be attributed the high
density of fluorine; fluorine is believed to have strong affinity
toward gas molecules because of its high electronegativity. The
hydrogen affinity difference is more reflected in the hydrogen
sorption isotherms at a lower pressure range (less than 0.5 bar)
(Supporting Information Figure S21), where the hydrogen
adsorption amounts in PPN-10 and PPN-12 rise more steeply.
In the higher pressure range, where the surface area and pore
volume become dominant, PPN-13 has the highest hydrogen-
uptake.
High-pressure gas sorption was performed on PPN-13 using
volumetric measurement method. At 77 K, the total hydrogen-
uptake of PPN-13 was 52 mg/g (4.9 wt %) at 40 bar (Figure 4).
came from Yamamoto homocoupling, such as PPN-4 (SBET
:
6461 m2/g)7 and PAF-1 (SBET: 5600 m2/g).6 Not only does
Yamamoto homocoupling require stoichiometric amount of
Ni(COD)2, but also strict anaerobic reaction condition for
efficient polymerization in order to achieve high surface area. In
Eglinton homocoupling, on the other hand, low-cost Cu(OAc)2
was used, and the reaction was carried out in open air. POPs
synthesized from reversible organic condensation reactions also
achieved very high surface area, such as COF-102 (SBET: 3472
m2/g)4c and COF-104 (SBET: 4210 m2/g).4c More importantly,
high crystallinity observed by PXRD patterns demonstrates a
balance can be struck between two competing factors,
thermodynamics and kinetics. These COF materials exhibit
high thermal stabilities (400−500 °C) because they are entirely
constructed from strong covalent bonds. However, the use of
relatively susceptible B−O bond could possibly compromise
their chemical stabilities. The nitrogen-rich carbonaceous
polymer materials (CTF-1) were synthesized under ionother-
mal conditions at over 400 °C via the formation of
polyaryltriazine networks, which present surface areas and
porosities up to 3300 m2/g and 2.4 cm3/g, respectively.31g
Hydrogen Storage. Hydrogen storage based on phys-
isorption using adsorbents is an immensely important topic in
the clean energy area.32 The U.S. Department of Energy
(DOE) reset the gravimetric and volumetric storage targets for
on-board hydrogen storage for year 2017 (0.055 kg H2/kg
system, 0.040 kg H2/L system) and ultimate (0.075 kg H2/kg
system, 0.070 kg H2/L system).33 The high porosity of these
synthesized PPNs makes them good candidates for this
purpose.
Figure 4. Hydrogen adsorption isotherms of PPN-13 at 77 K. Excess
adsorption is the amount of gas molecules stored on surface
interacting with the adsorbent. Total adsorption is excess adsorption
plus the amount of gas molecules compressed within the pores.
Although the value is lower than those of PPN-4 (total uptake:
158 mg/g, 13.6 wt %, 80 bar)7 and PAF-1 (total uptake: 116
mg/g, 10.4 wt %, 48 bar),6 it can be anticipated that the amount
of hydrogen trapped in PPN-13 will be much increased at
higher pressure.
Methane Storage. The need for alternative fuels is greater
now than ever before due to the rapid rising levels of
atmospheric carbon dioxide produced from burning of fossil
fuels. With considerable sources available and low pollution
factor, methane is a natural choice as petroleum replacement in
the clean energy applications.36 However, efficient storage
methods are still lacking to implement the application of
methane in the automotive industry. Advanced porous
materials, MOFs and POPs, have received considerable
attention in sorptive storage applications owing to their
In the low pressure region, the hydrogen-adsorption capacity
is largely controlled by the hydrogen affinity toward the
framework, which can be quantified by isosteric heat of
adsorption. Variable-temperature measurements reveal an
isosteric heat of adsorption of 5.8 kJ/mol for PPN-13 at zero
loading (Supporting Information Figure S22); this value is
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exceptionally high surface areas and chemically tunable
structures.37
Among them, PPN-13 exhibits an exceptionally high BET
surface area of 3420 m2/g.
Isosteric heats of adsorption of PPNs for methane, calculated
with three sets of isotherm data at different temperatures (273,
286, and 295 K), are around 15−20 kJ/mol (Supporting
Information Figure S23), which is deemed to be in the
appropriate range for methane storage at close to ambient
temperatures. The optimal value of heat of adsorption for
methane was calculated to be 18.8 kJ/mol.38
Therefore, we tested methane storage capacity for PPN-13.
As with hydrogen storage, the gravimetric methane-uptake is
directly proportional to the surface area. The excess methane-
uptake of PPN-13 is 81 mg/g at 298 K and 65 bar, while the
total methane-uptake is 179 mg/g based on the free volume
obtained from nitrogen isotherms at 77 K (Figure 5), the value
is lower than that of PPN-4 (389 mg/g at 55 bar)7 but is still
among the highest reported for organic porous materials.
ASSOCIATED CONTENT
■
S
* Supporting Information
1H NMR, 13C NMR, 19F NMR, solid-state NMR, gas sorption
isotherm, TGA, SEM, and crystallographic data (CIF). This
material is available free of charge via the Internet at http://
pubs.acs.org.
AUTHOR INFORMATION
■
Corresponding Authors
*E-mail: ydq@fjirsm.ac.cn (D.Y.).
*E-mail: zhou@chem.tamu.edu (H.-C.Z.).
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This work has been financially funded by Office of Naval
Research (N00014-13-1-0753). We acknowledge Dr. Vladimir
Bakhmoutov for his help with solid-state NMR and the
Supercomputing Facility at Texas A&M for Molecular
Simulation and other support. D.Y. thanks the National Science
Foundation of China (No. 21271172) and the “One Hundred
Talent Project” from the Chinese Academy of Sciences.
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