Rational Design and Synthesis of Porous Polymer Networks: Toward

Jul 20, 2014 - ABSTRACT: Head-on polymerization of tetrahedral monomers inherently imparts interconnected diamond cages to the resulting framework ...
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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 threedimensional 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.

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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 posttreatments 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 frameworks (COFs) were synthesized as porous crystalline solids featured with extended periodicity.4 High microporosity and chemical resistance were observed in conjugated microporous © 2014 American Chemical Society

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, PAF1 (PAF = porous aromatic framework) was synthesized with a BET surface area of 5600 m2/g;6 by using tetrakis(4bromophenyl)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 interpenetration 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 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 4589

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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 exceptionally high BET surface area of 3420 m2/g.

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-dimensional (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 Herein, we designed a series of tetraethynyl monomers with biphenyl as backbone (Scheme 1); By adding substituents at 2,

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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 tetramethylsilane (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. 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

Scheme 1. Illustration of Formation of a 3D Polymer with Geometrically Constrained Biphenyl Monomer through Head-on Homocoupling Reaction

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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′-tetrabromobiphenyl (4.5 g, 19.2%). 1H 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. 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, 76% yield over two steps), mp 187−189 °C. 1H 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. Synthesis of 3,3′,5,5′-Tetraethynyl-2,2′,4,4′,6,6′-hexafluorobiphenyl. 2,2′,4,4′,6,6′-Hexafluorobiphenyl was synthesized accord-

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 decomposition: 250 °C. 1H 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′-hexamethylbiphenyl. To a 500 mL three-necked flask containing 1.44 g of

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 colorless solid. (4.0 g, 73.5% yield). 1H 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.

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 = 4591

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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 leastsquares fitting on F2 by the SHELX-97 software package.19 All nonhydrogen 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 symmetryconstrained 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.

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 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 hexanes as eluent afforded 3,3′,5,5′-tetraethynyl-2,2′,4,4′,6,6′-hexamethylbiphenyl as a white solid (0.46 g, 51% yield over two steps), mp 157−159 °C. 1H NMR (300 MHz, CDCl3) δ: 3.52 (s, 4H), 2.67 (s, 6H), 2.01 (s, 12H) ppm. 13 C 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′-tetramethoxy4,4′-dimethylbiphenyl. 2,2′,6,6′-Tetramethoxy-4,4′-dimethylbiphenyl 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 as a white solid (4.8 g, 90% yield). 1H NMR (300 MHz, CDCl3) δ: 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. 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 solid (0.75 g, 75% yield over two steps), mp 175−177 °C. 1H NMR (300 MHz, CDCl3) δ: 3.76 (s, 12H), 3.50 (s, 4H), 2.64 (s, 6H) ppm. 13 C 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′-

Table 1. Surface Areas, Pore Volumes, and Porosities of Synthesized PPNsa material

model space group

SLang/SBET/SCalc

Vp (exp/calc)

porosity (calc)

PPN-10 PPN-11 PPN-12 PPN-13 PPN-14

Cmmm P4322 P4322 I4̅2d I41/amd

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%

a 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 (