Article pubs.acs.org/cm
Tubular-Shape Evolution of Microporous Organic Networks Jiseul Chun,† Ji Hoon Park,† Jieun Kim,† Sang Moon Lee,‡ Hae Jin Kim,‡ and Seung Uk Son*,† †
Department of Chemistry and Department of Energy Science, Sungkyunkwan University, Suwon 440-746, Korea Korea Basic Science Institute, Daejeon 350-333, Korea
‡
S Supporting Information *
ABSTRACT: This work suggests a synthetic method for tubular-shape evolution of amorphous and microporous organic networks. The gradual addition of a dihalo building block to its mixture with tetrakis(4-ethynylphenyl)methane under conventional Sonogashira coupling conditions resulted in the formation of tubular materials. The resulting tubular materials were characterized by scanning and transmission (TEM) electron microscopies, Brunauer−Emmett−Teller and thermogravimetric analyses, and solid-phase 13C NMR spectroscopy. In comparison, when the mixture of a dihalo building block and tetrakis(4ethynylphenyl)methane was heated under conventional Sonogashira coupling conditions, granular materials were formed. We suggest that the tubular-shape evolution is attributed to the differences of steric situations in networking steps. In TEM, the oriented attachment of the tubular materials was observed. KEYWORDS: porous material, microporous organic network, tube, Sonogashira coupling, shape control, oriented attachment
1. INTRODUCTION During the past decade, amorphous and microporous organic networks (A-MON) have been extensively prepared via diverse organic reactions among building blocks.1 For example, the Cooper group reported the Sonogashira-coupling-based formation of A-MON using multialkynes and multihalides.2 Usually, related studies have focused on the inner porosity of materials and the resulting high surface area. In addition, by screening of the geometry of the building blocks, the pore sizes and surface areas of A-MONs could be controlled.2a Recently, several building blocks with tetrahedral geometry have been used as connectors for obtaining A-MONs with 3D inner network structures.3 However, studies on the outer-shape evolution of A-MONs having 3D inner networking are quite rare,4 possibly for the following two reasons. First, because the outer shape of A-MONs can be expected to be amorphous, control of the outer shape was not seriously considered. Second, an isotropic shape, such as in spherical materials, can be expected when tetrahedral building blocks were used to induce 3D inner network structure. Thus, the mechanism of the outer-shape evolution of A-MONs has not yet been seriously considered. The outer shape of A-MONs can be a key factor for their physical properties and ultimate applications. For example, the dispersion ability in solvents and the diffusion pathway of guest © 2012 American Chemical Society
molecules into porous networks can be dependent on the outer shape of A-MONs.4b In addition, our research group has recently prepared metal oxide incorporated A-MONs for application as anode materials of lithium-ion batteries.5 The metal oxides were successfully incorporated into microporous networks and showed enhanced stability in maintaining a discharge capacity, possibly because of the structural buffering action of the networks. In this case, the outer shape of the composites was important for their performance because the electrochemical reactions occurred on the surface of the materials. In continuous efforts for the synthesis of new functional AMON materials, we and other groups often encountered mixtures of the conventional spherical granules and intriguing tubular materials (as a minor product).6 We have tenaciously tried to elucidate the underlying mechanism of anisotropicshape evolution and ultimately to develop the synthetic process for purely tubular materials. In this work, we report a synthetic method for the tubular-shape evolution of amorphous MONs and suggestions for the underlying mechanism. Received: June 8, 2012 Revised: August 2, 2012 Published: August 21, 2012 3458
dx.doi.org/10.1021/cm301786g | Chem. Mater. 2012, 24, 3458−3463
Chemistry of Materials
Article
Then, 3,5-bis(4-bromophenyl)pyridine (70 mg, 0.18 mmol) in toluene (10 mL) was slowly added to the reaction mixture for 1 h using a syringe pump. After the reaction mixture was stirred for 24 h at 90 °C, the resulting precipitates were retrieved by centrifugation, washed with methanol, methylene dichloride, and acetone, and dried under vacuum. Preparation of Granular A-MONs. 3,5-Bis(4-bromophenyl)pyridine (93 mg, 0.24 mmol) and tetrakis(4-ethynylphenyl)methane (50 mg, 0.12 mmol) were dissolved in a mixture of toluene (12 mL) and triethylamine (6 mL) in a flame-dried 50 mL two-necked Schlenk flask. Copper iodide (2.3 mg, 0.012 mmol) and bis(triphenylphosphine)palladium dichloride (8.4 mg, 0.012 mmol) were added, and the reaction mixture was heated at 90 °C for 24 h under nitrogen. The isolated precipitates were washed with methanol, methylene dichloride, and acetone, and dried under vacuum. For the preparation of granular A-MONs in Figure S1 in the SI, 3,5-bis(4bromophenyl)-N-methylpyridinium iodide (0.24 mmol) was used instead of 3,5-bis(4-bromophenyl)pyridine. Preparation of the Methyl Adduct of Tubular A-MONs. In a flame-dried 50 mL Schlenk flask, tubular A-MONs (60 mg) were suspended in acetonitrile (8 mL). After the addition of methyl iodide (1.0 mL, 1.6 mmol), the reaction mixture was heated at 80 °C for 48 h. The resulting precipitates were retrieved by centrifugation, washed with acetone, and dried under vacuum.
2. EXPERIMENTAL SECTION Transmission electron microscopy (TEM) and high-resolution TEM images were obtained using a JEOL 2100F unit operated at 200 kV. Samples for TEM were prepared on a copper grid by drop-casting a methylene chloride solution of the materials. The scanning electron microscopy (SEM) images were taken by a field-emission scanning electron microscope (JSM6700F). Powder X-ray diffraction (PXRD) patterns were obtained using a Rigaku MAX-2200 and filtered Cu Kα radiation. Solid-phase 13C NMR spectra were recorded on a Varian 600 MHz NOVA600 spectrometer at KBSI (Daegu). Adsorption− desorption isotherm curves for N2 (77 K) were recorded by using BELSORP II-mini volumetric adsorption equipment. Thermogravimetric analysis (TGA) curves were obtained by Seiko Exstar 7300. Elemental analysis was performed on a CE EA1110 elemental analysis instrument. Preparation of Building Block A, Its Methyl Salt, and B. The building block A, 3,5-bis(4-bromophenyl)pyridine was prepared by the method in the literature.8 (4-Bromophenyl)acetonitrile (10 g, 0.051 mol) and toluene (35 mL) were added to a flame-dried 250 mL twonecked Schlenk flask. A 1.5 M toluene solution (45 mL, 0.068 mol) of diisobutylaluminum hydride in a flame-dried dropping funnel was slowly added to the Schlenk flask at −5 °C. At this temperature, the reaction mixture was stirred for an additional 1.5 h. Ethanol (20 mL) was added to the reaction mixture at −5 °C. Toluene (35 mL) was added at 0 °C, and then 1 M sulfuric acid (120 mL) was slowly added. At room temperature, the reaction mixture was extracted with brine three times and the resultant toluene solution was separated. In a 250 mL one-necked Schlenk flask, the toluene solution above and morpholine (20 mL) were mixed, and the reaction mixture was stirred overnight. Then, after the solvent was evaporated, the solid was recrystallized using cyclohexane and retrieved by centrifugation. After washing with cyclohexane, the resulting solid, 2-(N-morpholino)-4′bromostyrene, was dried under vacuum. For the preparation of hexahydra-1,3,5-tri-tert-butyl-1,3,5-triazine, a 37% formaldehyde solution (45 mL) was added dropwise to tert-butylamine (32 mL) in a flame-dried 100 mL one-necked Schlenk flask at 5 °C. The reaction mixture was stirred at room temperature for 2 h. The organic phase was separated using a separatory funnel, and the remaining water was removed using KOH. For the preparation of 3,5-bis(4-bromophenyl)pyridine (building block A), 2-(N-morpholino)-4′-bromostyrene (3.2 g, 12 mmol), hexahydra-1,3,5-tri-tert-butyl-1,3,5-triazine (1.6 g, 6.3 mmol), and p-xylene (43 mL) were added to a flame-dried 100 mL two-necked Schlenk flask. The reaction mixture was refluxed for 72 h. Then, after cooling to room temperature, the solid was recrystallized using hot ethanol. The resulting solid, 3,5-bis(4-bromophenyl)pyridine, was dried under vacuum. 1H NMR spectrum of the product matched well with the reported one.8 1H NMR (300 MHz, CDCl3): δ 7.79 (d, J = 6.6 Hz, 4H), 7.70 (s, 1H), 7.54 (m, 3H), 7.37 (d, J = 6.6 Hz, 4H). For the preparation of methyl salts of building block A, 3,5bis(4-bromophenyl)pyridine (0.50 g, 1.3 mmol) was suspended in acetonitrile (30 mL) using a flame-dried 50 mL Schlenk flask. After the addition of methyl iodide (0.24 mL, 3.9 mmol), the reaction mixture was refluxed overnight. After cooling to room temperature, the precipitates were separated by filtration, washed with acetone, and dried under vacuum. Characterization data of the methyl salt of building block A are given. 1H NMR (300 MHz, DMSO-d6): δ 9.41 (s, 2H), 9.14 (s, 1H), 7.98 (d, J = 8.7 Hz, 4H), 7.85 (d, J = 8.7 Hz, 4H), 4.45 (s, 3H). 13C NMR (75 MHz, DMSO-d6): δ 142.2, 139.3, 138.0, 132.4, 134.1, 129.7, 124.1, 48.3. HRMS (FAB). Calcd for C18H14NBr2 ([M − I]+): m/z 401.9493. Found: m/z 401.9497. The building block B, tetrakis(4-ethynylphenyl)methane, was prepared by the method in the literature.3b Preparation of Tubular A-MONs. In a flame-dried 50 mL twoneck Schlenk flask, 3,5-bis(4-bromophenyl)pyridine (23 mg, 0.060 mmol) and tetrakis(4-ethynylphenyl)methane (50 mg, 0.12 mmol) were dissolved in a mixture of toluene (2 mL) and triethylamine (6 mL). After the addition of bis(triphenylphosphine)palladium dichloride (8.4 mg, 0.012 mmol) and copper iodide (2.3 mg, 0.012 mmol), the reaction mixture was heated at 90 °C under nitrogen.
3. RESULTS AND DISCUSSION Basically, A-MONs in this work were prepared by Sonogashira coupling between tetrahedral multialkyne building blocks and dihalo building blocks (Scheme 1). In previous observations in Scheme 1. Synthetic Methods for A-MONs in This Study
our group, when imidazolium or viologen salt containing dihalo building blocks was used, we observed the exclusive formation of tubular materials.4b,7 In comparison, when neutral dihalo building blocks such as 1,3- or 1,4-bis(4-iodophenyl)benzene were used, spherical materials were obtained.5,7 Considering this, at first glance, it was speculated that a certain undisclosed assembly related to the ionic character of salt-containing dihalo building blocks may exist in a relatively less polar solvent for induction of the tubular shape. To get more straightforward information by a systematic approach, we prepared 3,5-bis(4-bromophenyl)pyridine (A).8 By transformation of building block A to its methylpyridinium salt form, a change in the outer shape of the materials from spheres to tubular materials would be expected. However, in both cases (A and its pyridinium derivative), we obtained irregular granules (Figure S1 in the SI). In the continuous search for synthetic conditions for purely tubular materials, finally, we found that maintaining a subequivalent amount of dihalo building blocks against tetrahedral building blocks in networking is a key reaction condition for successful tubularshape evolution (vide infra). The resulting optimized synthetic procedure for anisotropic-shape evolution is shown in Scheme 1. 3459
dx.doi.org/10.1021/cm301786g | Chem. Mater. 2012, 24, 3458−3463
Chemistry of Materials
Article
First, the conventional method was applied for the preparation of A-MONs using building block A and tetrakis(4-ethynylphenyl)methane (B). After 2 equiv of A and 1 equiv of B were completely dissolved in a mixture of toluene and triethylamine, the reaction mixture was heated at 90 °C for 24 h using 5 mol % (PPh3)2PdCl2 and 5 mol % CuI catalysts. During the reaction, dark-yellow precipitates gradually formed. SEM of the obtained A-MONs showed irregular granules (Figure 1e).
Figure 2. (a) N2 adsorption isotherm curves by BET analysis, (b) TGA curves, (c) PXRD patterns, and solid-state 13C NMR spectra of tubular (red) and granular (black) A-MONs.
Actually, the synthetic method for the tubular A-MON was devised from an insightful interpretation of tubular materials, which were obtained using salt-containing dihalo building blocks even by the conventional synthetic method in Scheme 1.4b,7 We speculated that the salt-containing dihalo building blocks would have relatively poor solubility and hence might gradually join in networking with B, which can be related to anisotropic-shape evolution. In the case of building block A in this study, it has good solubility in the reaction medium used, and thus the anisotropic-shape evolution of A-MONs using A is quite challenging. However, we could induce a similar reaction situation with the cases of salt-containing dihalo building blocks via the gradual addition of the soluble A using a syringe pump to the reaction mixture containing B. Considering this, we screened the amount of A in the syringe and in the reaction mixture in the Schlenk flask. Figure 3 summarizes the results (see the Experimental Section for the detailed procedure). First, 2 equiv of A was slowly added to 1 equiv of B in a Schlenk flask for 1 h under conventional Sonogashira coupling conditions, and then the reaction mixture was stirred for 24 h. However, black materials with irregular shape were obtained possibly by the self-coupling of B. Second, when 1.75 or 1.5 equiv of A was slowly added for 1 h to a mixture of 1 equiv of B and 0.25 or 0.5 equiv of A, respectively, tubular materials were obtained exclusively. Third, when the amount of A in the mixture with 1 equiv of B in the Schlenk flask was increased from 1 to 1.5 and 2 equiv, conventional granules started to form. Through these experiments, the key points could be summarized as follows. To obtain tubular A-MONs with convincing quality, a partial amount of A is necessary in a mixture with B in a Schlenk flask to suppress the self-coupling of B. Although more specific amounts of A in a mixture with 1 equiv of B could be screened for further optimization, tubular A-MONs with sufficiently good quality were obtained by the gradual addition of 1.5 equiv of A to a mixture solution of 0.5 equiv of A and 1 equiv of B.
Figure 1. SEM image of a tubular A-MON (a), pore-size distributions (based on a DFT method) of tubular (b) and granular (c) A-MONs, TEM image of a tubular A-MON (d), and SEM image of a granular AMON (e).
Second, in a comparison with the conventional method above, when 1.5 equiv of A was slowly added to the mixture of 0.5 equiv of A and 1 equiv of B at 90 °C for 1 h by a syringe pump and the reaction mixture was heated for 24 h, the tubes were exclusively obtained. Parts a and e of Figure 1 show typical SEM images of the tubular and granular A-MONs, respectively. The TEM images in Figures 1d and S2 and S4 in the SI showed the hollow inner space of the tubes. Brunauer−Emmett−Teller (BET) analysis showed the microporous character of A-MONs (Figures 1b,c and 2a). Interestingly, the tubular A-MON showed a significantly higher surface area value (580 m2/g) than the granular A-MON (506 m2/g). In addition, while pore-size distribution patterns obtained by the density functional theory (DFT) method were similar, tubular A-MONs showed sharper pore-size distribution in a size range of
