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Mechanochemical Synthesis of Porous Molecular Assemblies Omar Barreda, Garrett A. Taggart, Casey A. Rowland, Glenn P. A. Yap, and Eric D. Bloch Chem. Mater., Just Accepted Manuscript • Publication Date (Web): 30 May 2018 Downloaded from http://pubs.acs.org on May 30, 2018
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Chemistry of Materials
Mechanochemical Synthesis of Porous Molecular Assemblies Omar Barreda, Garrett A. Taggart, Casey A. Rowland, Glenn P. A. Yap, Eric D. Bloch* Department of Chemistry & Biochemistry, University of Delaware, Newark, DE 19716, USA ABSTRACT: Solvent-free synthesis methods have been widely utilized in molecular chemistry and have shown promise for the synthesis of porous three-dimensional structures. Herein, we report the rapid mechanochemical synthesis of low-dimensional materials including functionalized 2-D frameworks and a novel highly-stable porous molecular coordination assembly. Solvent-free ball milling of copper salts and functionalized isophthalic acid ligands affords phase-pure material in as little as 18 minutes with reaction progress monitored by powder X-ray diffraction. Here discrimination between extended network solids and discrete porous cages is achievable via bulky ligand functionalization. Although the materials display moderate surface areas after rapid activation conditions, porosities were optimized by exchanging the materials with volatile solvents affording BET (Langmuir) surface areas of 55-522 (160-874) m2/g.
Alternative methods for the synthesis of novel porous materials, including metal-organic frameworks, covalent organic frameworks, and zeolitic imidazolate frameworks, are vital for the realization of the large-scale production of these solids. Their typical syntheses are not necessarily compatible with scale-up as they involve high boiling point, highly-polar organic solvents at elevated temperatures with reaction times on the order of days.1,2,3 A variety of green and/or solvent free synthetic methods have been reported for metal-organic frameworks and all-organic porous materials,4,5,6 including mechanochemistry,7,8,9,10,11,12,13,14,15 sonochemistry,16 and extrusion.17 These methods have been shown to afford highly-crystalline material with gas adsorption properties that rival those prepared via conventional techniques.18 Alternative synthesis methods are similarly important for porous molecular assemblies, including porous organic cages (POCs) and hybrid metal-organic coordination cages,19,20 also commonly referred to as nanocages,21 nanoballs,22 metal-organic polyhedra,23,24,25,26,27,28,29 and supramolecular assemblies.30,31,32 Organic cages have been prepared via mechanochemical methods where, for example, the condensation reaction of boronic acid groups proceeds in a ball mill.33 Similarly, mechanochemical syntheses of non-porous coordination cages have been reported.34 However, to the best of our knowledge, the solvent-free synthesis of porous molecular assemblies remains a challenge although the typical solvothermal routes to these materials are similar to those reported for three-dimensional solids. For the synthesis of porous molecular assemblies, we initially surveyed the reaction of copper(II) acetate with a variety of isophthalic acid-based ligands that have previously been shown to readily assemble porous cages under a wide range of synthetic conditions. Grinding Cu2(OAc)4 and 1.1 equivalents of isophthalic acid with a mortar and pestle released a strong odor of acetic acid, indicative of deprotonation of isophthalic acid via acetate. The resulting material displayed significantly different solubility and gas adsorption properties than the starting metal and ligand mixtures. However, it was poorly crystalline and contained significant amounts of starting reagents. Ultimately, solvent-free synthesis was carried out using a 65 mL steel vial with two 1 cm diameter steel ball bearings in a ball mill. Approximately 500 mg of copper acetate monohydrate and isophthalic acid were ground in a 1:1.1 molar ratio and the resulting
Figure 1. Powder X-ray diffraction patterns for mechanochemicallysynthesized Cu(OPent-bdc) sheets (top) and Cu24(tBu-amide-bdc)24 cage (bottom) milled at the indicated times as compared to solvothermally-synthesized material.
powder was analyzed via powder X-ray diffraction (PXRD). Reaction time was optimized by monitoring the presence of H2bdc and copper acetate, with neither present as crystalline phases after 150 minutes of reaction time. The as-synthesized material displayed a relatively low Langmuir surface area of 110 m2/g compared to the reported value of 248 m2/g (BET) for a solvothermally-synthesized cage.35 However, rapid room-temperature ethanol washing afforded a material with a BET (Langmuir) surface area of 167 (194) m2/g
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and a solvated PXRD pattern in excellent agreement with the powder pattern predicted from a solvothermally-synthesized material (Figure S1). We are particularly interested in the mechanochemical synthesis of functionalized isophthalic acid-based materials as these have been shown to display high solubility in a variety of organic solvents in addition to selective gas adsorption.36,37,38 Alkoxide-functionalized isophthalic acid-based metal-organic materials are particularly interesting given their compatibility with a variety of different structure types, including tetragonal and hexagonal two-dimensional metalorganic frameworks and discrete porous cages.39 In order to avoid the formation of two-dimensional materials, we targeted pentoxidefunctionalized ligand as longer functional groups have been shown to disfavor extended structures.38 Here the mechanochemical reaction of 5–OPent-H2bdc with copper acetate was monitored via PXRD (Figure 1). After six minutes, starting material peak intensities decreased with concomitant rise of new peaks. At 18 minutes of milling, the PXRD pattern shows complete absence of copper acetate and 5OPent-H2bdc peaks with the appearance of new peaks at 5, 10, and 12 degrees. Comparison of this powder pattern, however, with predicted patterns of known materials reveals the material adopts the hexagonal two-dimensional phase. However, the mechanochemicallysynthesized material displays a Langmuir surface area after ethanol/ether washing and activation of 285 m2/g whereas the solvothermal material was nonporous.36 In order to avoid the formation of two-dimensional phases, bulkier ligand substituents are necessary. Although previously reported methods for alkoxide-functionalization are broadly applicable for linear functional groups,40 incorporation of branched functional groups via this method is difficult given the relative lack of commercially available bulky alkyl halide reagents. Additionally, secondary and tertiary alkyl halides display decreased reactivity with 5hydroxyisophthalic acid as compared to primary alkyl halides. For the facile synthesis of isophthalic analogs with bulky functional groups, we ultimately pursued amide-functionalization. Here the reaction of trimethylacetyl chloride with 5-aminoisophthalic acid in THF affords the amide-functionalized ligand in quantitative yield. A particular benefit of this strategy is the synthesis does not require carboxylic acid protection/deprotection as in the synthesis of 5alkoxide-functionalized ligands. The mechanochemical reaction of this ligand with copper acetate affords a highly crystalline material (Figure 1). The reaction was optimized by monitoring the disappearance of metal and ligand diffraction peaks via PXRD, which suggested a 90-minute reaction time. Rapid activation of assynthesized material of an unknown structure, afforded a solid with a Langmuir surface area of 115 m2/g. To determine the structure of the material that was synthesized via milling, we employed more common solvothermal synthesis techniques. For this, the reaction of copper acetate with H2tBuamide-bdc in dimethylformamide afforded diffraction quality single crystals after combining metal and ligand solution at room temperature followed by slow evaporation of solvent over the course of seven days. The material, Cu24(tBu-amide-bdc)24, adopts the expected cuboctahedral structure consisting of 12 dicopper paddlewheel units and 24 ligands (Figure 2). This material crystallizes in I4/m with a = b = 32.566(2) Å and c = 46.870(2) Å. The PXRD pattern of this material after methanol exchanges is in excellent agreement with the powder pattern observed for this material obtained via solvent-free synthesis. An important feature of this structure is depicted in Figure 2. The cage is highly ordered in three
Figure 2. (Top) Crystal structure of Cu24(tBu-amide-bdc)24 prepared via solvothermal synthesis. Green, red, gray, and blue spheres represent copper, oxygen, carbon, and nitrogen, respectively. Hydrogen atoms have been omitted. The large blue sphere in the center of the cage is for clarity. (Bottom) Packing of the cage illustrating large gasaccessible voids and intermolecular interactions between ligand functional groups.
dimensions via interaction of tert-butyl groups on a given molecule with groups on two adjacent cages. Every tert-butyl group on the cage displays this interaction, essentially forming a stable network where a cage interacts with 14 neighboring cages. This feature is also displayed by Cr24(tBu-bdc)24 and is likely the reason the material displays excellent thermal stability upon solvent evacuation.41,42 To accurately compare surface areas of materials prepared via both routes, methanol exchanges were performed on both prior to sample activation. After room temperature evacuation, the ball milled sample displays a considerably higher surface area than the solvothermal material with Langmuir surface areas of 750 and 154 m2/g, respectively. This trend continues at higher activation temperatures. Materials prepared via both routes display increasing surface areas up to an activation temperature of 125 °C (Figure 3). Cu24(tBu-amide-bdc)24-mech obtains an optimized BET (Langmuir) surface area of 493 (874) m2/g at this temperature before decreasing rapidly at higher temperatures. The surface area of Cu24(tBu-amide-bdc)24-solvo, however, increases up to an activation temperature of 225 °C where it displays a BET (Langmuir) surface area of 522 (697) m2/g. The lower thermal stability of the
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Chemistry of Materials
Detailed experimental procedures, powder X-ray diffraction data, infrared spectra, gas adsorption data, thermogravimetric analysis (PDF). Crystal structure of Cu24(tBu-amidebdc)24 (CIF).
AUTHOR INFORMATION Corresponding Author *
[email protected] Funding Sources This publication was made possible by the Delaware COBRE program, supported by a grant from the National Institute of General Medical Sciences – NIGMS (5 P30 GM110758-02) from the National Institutes of Health. Figure 3. Surface areas as a function of activation temperature for solvothermally- and mechanochemically-synthesized Cu24(tBu-amidebdc)24.
mechanochemically-synthesized material is likely a result of the decreased crystallinity of the material and the presence of a significant number of defects in the crystal structure. Although the mechanochemical synthesis of Cu(OPent-bdc) and Cu24(tBu-amide-bdc)24 illustrate the potential utility of ballmilling for the solvent-free synthesis of low-dimensional porous metalorganic materials, a particular drawback of this method is illustrated by the mechanochemical reaction of copper acetate with 5-NH2bdc. The methanol-washed material displays a BET (Langmuir) surface area of 360 (520) m2/g upon activation at 100 °C. The resulting product of this reaction is moderately crystalline, as determined by PXRD. However, the powder pattern does not match that of any reported material based on Cu2+ and this particular ligand. Our attempts to produce an isostructural material via solvothermal methods were ultimately unsuccessful. The lack of solubility of the mechanochemically-synthesized material is not indicative of a particular phase as two-dimensional metal-organic frameworks and most discrete porous cages based on paddlewheel building blocks are insoluble. We predict this synthesis method will be particularly useful for soluble porous cages where product can be extracted from unreacted metal and ligand, rather than utilizing methanol washes to remove these from insoluble product. The work presented here illustrates the potential utility of mechanochemical methods for the synthesis of porous molecular coordination assemblies. Utilization of a ball mill affords phase-pure material in high yields in as little as 15 minutes. Although for the materials presented here moderate surface areas were obtained for direct activation of as-synthesized material, surface areas were optimized by washing the materials with the appropriate volatile solvents. Here, surface areas comparable to those observed via typical solvothermal methods were attained. We envision this approach being widely applicable to a variety of porous molecular species. Future work in this area will focus on the synthesis of functionalized materials amenable to extraction into non-coordinating, volatile organic solvents.
ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website.
ACKNOWLEDGMENT This work was supported by the University of Delaware, whom we thank for generous start-up funds. We thank Professor Svilen Bobev for access to the ball mill used in this work. We also thank Professor Raul Lobo for the powder X-ray diffractometer used here.
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