Ligand-Based Phase Control in Porous Molecular Assemblies - ACS

Mar 26, 2018 - Functionalization of isophthalic acid ligands with linear alkoxide groups from ethoxy through pentoxy is shown to have a pronounced eff...
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Cite This: ACS Appl. Mater. Interfaces 2018, 10, 11420−11424

Ligand-Based Phase Control in Porous Molecular Assemblies Omar Barreda,‡ Gianluca Bannwart,‡ Glenn P. A. Yap,‡ and Eric D. Bloch*,‡,§ ‡

Department of Chemistry and Biochemistry, University of Delaware, Newark, Delaware 19716, United States Center for Neutron Science, Department of Chemical and Biomolecular Engineering, University of Delaware, Newark, Delaware 19716, United States

§

S Supporting Information *

ABSTRACT: Functionalization of isophthalic acid ligands with linear alkoxide groups from ethoxy through pentoxy is shown to have a pronounced effect on both the synthesis of porous paddlewheel-based molecular assemblies and their resulting surface areas and gas adsorption properties. Shorter chain length is compatible with either tetragonal or hexagonal two-dimensional materials, with the hexagonal phase favored with longer chain length. Precise tuning of reaction conditions affords discrete molecular species that are soluble in a variety of organic solvents. The isolated porous molecules display BET surface areas ranging from 125 m2/g to 545 m2/g. The pentoxide-based molecular assembly shows considerable promise for the separation of hydrocarbons with average isosteric heats of adsorption of −48 and −31 kJ/mol for ethylene and ethane, respectively. KEYWORDS: porous solids, gas adsorption, metal−organic materials, hydrocarbon separation, two-dimensional materials

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s a result of their solubility,1,2 amenability to postsynthetic modification and purification,3,4 and potential mobility in the solid state,5 porous molecular materials have received considerable recent attention.6−9 Although the surface areas displayed by these discrete molecular solids typically fall short of the record values exhibited by porous network solids such as aluminosilicates, 10 metal−organic frameworks (MOFs),11 and covalent organic frameworks (COFs),12 surface areas approaching 4000 m2/g have recently been achieved.13−15 All-organic porous molecules, or porous organic cages (POCs), have shown utility for a wide variety of applications ranging from gas storage and separation to catalysis and were utilized in the first porous liquid.16−18 However, these materials lack the metal sites that endow hybrid organic−inorganic materials with selective adsorption. Porous coordination complexes, often referred to as metal−organic polyhedra,19−22 nanoballs,23 nanocages,24 or supramolecular assemblies,25,26 lie at the interface of POCs and MOFs, because they are discrete molecular assemblies and contain metal and organic building units. Selective gas adsorption and BET surface areas in excess of 1100 m2/g have been previously reported for these materials.27−29 Although they are molecular by nature, the number of porous metal−organic compounds shown to display © 2018 American Chemical Society

appreciable solubility, which is one of the main advantages of porous molecular assemblies, are minimal.30−33 For the synthesis of porous, soluble molecular assemblies, we’ve targeted new structure types and modification of the isophthalic acid ligands in M24L24 carboxylate-based materials. The 5-position of isophthalic acid serves as an easy entry point for the latter strategy as a wide variety of functionalized ligands are commercially available. In our hands, most of these ligands afforded insoluble materials or molecules devoid of permanent porosity. The reaction of copper salts with 5-hydroxyisophthalic acid, however, affords Cu_OH-bdc,34 which is a material with excellent solubility in methanol. Furthermore, this ligand is easily functionalized via reaction with alkyl halides to afford 5alkoxy-isophthalic acid ligands. As is typical for the synthesis of new metal−organic materials, we initially surveyed the reaction of alkoxidefunctionalized ligands with copper(II) under a variety of conditions, varying solvent, concentration, reaction time, and copper source. Previous studies have suggested that the Received: February 2, 2018 Accepted: March 26, 2018 Published: March 26, 2018 11420

DOI: 10.1021/acsami.8b02015 ACS Appl. Mater. Interfaces 2018, 10, 11420−11424

Letter

ACS Applied Materials & Interfaces

single-crystal quality tetragonal crystals appeared as poorly formed crystallites. Ultimately, after a total reaction time of 72 h, we exclusively observed single-crystal-quality hexagonal plates. This observation is in stark contrast to previous results wherein only tetragonal two-dimensional metal−organic frameworks were observed for the direct synthesis of various 5substituted isophthalic acid-based materials.36 Closer inspection of the structures depicted in Figure 1 reveals that in the hexagonal phase significantly more space is accessible for the alkoxide groups on the ligands. Incorporation of longer alkane groups should steer product formation toward the hexagonal phase. To investigate this, we prepared three additional alkoxide-functionalized ligands featuring propoxy, butoxy, and pentoxy groups. For OPr-bdc, similar timedependent product distribution is observed. The resulting structures adopt similar unit cells to those observed for both Cu_OEt-bdc materials. The OEt-bdc and OPr-bdc hexagonal sheets reported here are isostructural to those recently reported wherein they were synthesized via the hydrolytic decomposition of discrete molecular assemblies.39 An abrupt transition was observed upon moving to the butoxy-substituted ligand. Here, we isolated both phases as pure products by simply controlling reaction concentration, with the tetragonal phase favored at lower concentrations. For OPent-bdc, it was particularly difficult to isolate phase-pure tetragonal crystals. Under all conditions that were attempted we observed, at best, 10% tetragonal-phase formation. This is likely a result of the incompatibility of this ligand with the limited interligand space present in this structure. Although reaction time and reagent concentration can be used to alter product distribution, the formamide/ethanol solutions that we used were ultimately not compatible with the formation of discrete molecular assemblies, the ethoxide- and propoxide-functionalized versions of which were recently reported.39 For the synthesis of Cu_OEt-bdc, the reaction of copper nitrate with 1 equiv of OEt-bdc in a 60:40 DMF/EtOH solution at 92 °C for 12 h afforded crystalline material in high yield. Similar reaction conditions were utilized for the synthesis of Cu_OPent-bdc and Cu_OPr-bdc. For the former, solid crystallized after heating at 100 °C for 12 h. For the latter, an 80:20 DMA/EtOH mixture was utilized. The synthesis of Cu_OBu-bdc required dramatically different synthesis conditions, as we typically obtained phase-impure material contaminated with an unknown crystalline material. Ultimately, the room-temperature reaction of copper acetate with 1 equiv of ligand in pure DMF for 24 h afforded a crystalline solid. (See the Supporting Information for full experimental details.) All four molecular species reported here adopt the previously reported structure type wherein 24 ligands and 12 dicopper paddlewheel units assemble into a cuboctahedron (Figure 2).39 This is comprised of eight triangular faces and six square faces with the ligands on the edges and metal clusters at the vertices. No obvious structural correlations can be made between the ligand functional group and the space group, the details of which can be found in Table S14 in the Supporting Information. However, there is a clear trend in increasing the unit-cell volume from the shortest chain to the longest. Furthermore, the intermolecular distance, measured here as the shortest distance between an atom located in the center of adjacent molecules, increases from ethoxy to propoxy to butoxy from 23.6 Å to 29.1 Å as longer alkoxide groups on the ligands space the cages out at further distances. The distance in Cu_OPent-bdc decreases significantly as a result of the

formation of carboxylate-based cuboctahedral cages is incredibly rapid at room temperature in the presence of base.35 Ultimately the room-temperature reaction of 5-ethoxybenzenedicarboxylic acid (OEt-bdc) with copper acetate in 1:1 formamide/ethanol mixtures via layering of metal and ligand solutions afforded rectangular crystals at the solution/solution interface after 1 day (see the Supporting Information for full experimental procedures). Single-crystal structural determination indicated that the material adopts the two-dimensional structure previously reported.36 This solid is comprised of copper paddlewheel units coordinated to four different ligands (Figure 1). Ligands located trans to each other across the

Figure 1. Reaction scheme illustrating the synthesis of twodimensional layered materials and discrete, intrinsically porous molecular assemblies. Here, routes a and b in formamide/ethanol mixtures afford tetragonal and hexagonal phases at room temperature and elevated temperatures, respectively. Utilization of dialkylformamide/ethanol solvent mixtures affords discrete cages (route c).

paddlewheel are pointed in alternating directions throughout the structure, which results in a two-dimensional framework featuring square channels based on stacked sheets. A parallel reaction isolated after 72 h was found to contain hexagonal crystals. This material crystallizes in P3̅m1 with a = b = 18.725(4) and c = 6.999(2) and is similarly comprised of copper paddlewheel units. Viewed down the a-axis, these two structures appear nearly identical. However, this framework adopts a Kagome lattice featuring a trihexagonal tiling of pores.37,38 In this structure, the ethoxide groups point into the hexagonal channel. In order to determine if there was a transition from Cu_OEt-bdc_tet to Cu_OEt-bdc_hex over the course of the reaction or if two different phases formed in separate reaction vials as a result of minute changes in concentration, solvent volume, etc. a single vial was monitored for three days. Indeed, after 24 h at room temperature, we observed tetragonal crystals in the reaction vessel (phase purity was later confirmed via powder X-ray diffraction; see Figure S2 in the Supporting Information). After an additional 24 h, the 11421

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displays similar surface area as the starting compound (see Figure S14 in the Supporting Information). Although the surface areas displayed by the four materials reported here are modest compared to both metal−organic frameworks and many porous molecular cages, the alkoxide functionalization on the surface of the molecules is intriguing for selective gas adsorption. These types of functional groups have been shown to impart selective adsorption properties in previously reported metal−organic frameworks.40 To assess the ability of Cu_OR-bdc porous cages to selectively adsorb ethane or ethylene, isotherms were collected at 298, 308, and 318 K. For both gases, Cu_OEt-bdc displays the highest saturation capacity at room temperature, approaching 2 mmol/g, which is almost double that of the other materials (see Figure 3).

Figure 2. Space-filling representation of Cu_OPr-bdc illustrating porosity intrinsic to individual cages and extrinsic porosity between cages.

favorable alkane−arene interaction in this structure (Figure S17 in the Supporting Information). Here, six of the pentoxy groups on a plane in the structure interact with six adjacent cages, essentially forming a two-dimensional array of cuboctahedra. There are large solvent-accessible voids in the crystal structures of all four materials. Accordingly, thermogravimetric analysis (TGA) of as-synthesized samples reveal mass losses ranging from 15 wt % for Cu_OPent-bdc to 40 wt % for Cu_OEt-bdc (Figure S5 in the Supporting Information). To facilitate activation of these materials, solvent exchanges were attempted with DMF, DMA, methanol, and ethanol. However, their high solubility in hot amide-based solvents and solvent mixtures prevented the use of these for exchanges. Ultimately, room-temperature DMF and diethyl ether exchanges were utilized. With the exception of the previously reported chromium(II)-based material, Cr_tBu-bdc,28,29 these types of molecular assemblies have typically shown limited thermal stability in terms of their optimal activation temperatures. Accordingly, we initiated degas surveys for all four compounds at room temperature. Although Cu_OEt-bdc and Cu_OPr-bdc displayed Langmuir surface areas of >300 m2/g upon activation at room temperature, the butoxy- and pentoxy-functionalized materials had significantly diminished surface areas. Therefore, we screened activation conditions up to 200 °C in 25 °C increments. Both Cu_OEt-bdc and Cu_OPr-bdc achieved optimal surface areas after activation at 125 °C. The surface areas of Cu_OBu-bdc and Cu_OPent-bdc were optimized at significantly higher temperatures of 150 and 175 °C, respectively. All four materials displayed decreased surface areas upon heating to 200 °C. Fits to N2 adsorption data at 77 K result in BET (Langmuir) surface areas ranging from 125 to 545 m2/g (280−727 m2/g) with increasing alkoxide functionalization resulting in lower surface areas (Table S14). The potential benefits of soluble porous materials are particularly illustrated by Cu_OPent-bdc. During activation condition screening we obtained a nonporous phase at high degas temperatures. By dissolving this material in chloroform and allowing the solvent to slowly evaporate over the course of 2 days, crystalline material isostructural to the solvothermally prepared material was isolated. This recrystallized material

Figure 3. Ethane (upper left) and ethylene (upper right) adsorption isotherms at 298 K for Cu-OEt-bdc (red), Cu_OPr-bdc (blue), Cu_OBu-bdc (orange), and Cu_OPent-bdc (green). Isosteric heats of adsorption for ethane (lower left) and ethylene (lower right) in these materials are also shown.

Interestingly, there is not a straightforward relationship between one bar uptakes and surface areas. Although the latter decrease with increasing alkoxide length, the former follows the trend Et > Pent > Prop > But. Ultimately, the combination of surface area and ligand functionalization dictate the saturation capacity. Isosteric heats of ethane and ethylene adsorption were calculated using the Clausius−Clapeyron equation and are plotted in Figure 3. All four materials display similar ethane binding enthalpies, ranging from ∼−30 to −35 kJ/mol up to a loading of 0.5 mmol/g. The ethoxide-, propoxide-, and butoxide-functionalized cages are all expected to show limited ethylene/ethane selectivity as their ethylene adsorption enthalpies similarly fall in the range from −33 kJ/mol to −40 kJ/mol. Cu_OPent-bdc, however, shows a significantly higher ethylene adsorption enthalpy, averaging −48 kJ/mol from 0 to 0.5 mmol/g, as compared to −31 kJ/mol for ethane. To our knowledge, this represents the first case of olefin/paraffin selectivity displayed by a porous molecular assembly. The foregoing results demonstrate the use of isophthalic acid functionalization to control phase formation in low-dimensional metal−organic materials. The limited ligand bulk in the ethoxide- and propoxide-functionalized materials allows for the isolation of tetragonal sheets, hexagonal sheets, and discrete 11422

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(5) Holst, J. R.; Trewin, A.; Cooper, A. I. Porous Organic Molecules. Nat. Chem. 2010, 2, 915−920. (6) McKeown, N. B. Nanoporous Molecular Crystals. J. Mater. Chem. 2010, 20, 10588−10597. (7) Tian, J.; Thallapally, P. K.; McGrail, B. P. Porous Organic Molecular Materials. CrystEngComm 2012, 14, 1909−1919. (8) Giri, N.; Davidson, C. E.; Melaugh, G.; Del Popolo, M. G.; Jones, J. T. A.; Hasell, T.; Cooper, A. I.; Horton, P. N.; Hursthouse, M. B.; James, S. J. Alkylated Organic Cages: From Porous Crystals to Neat Liquids. Chem. Sci. 2012, 3, 2153−2157. (9) Mastalerz, M. Modular Synthesis of Shape-Persistent Organic Cage Compounds: Molecular Precursors for a New Class of Permanent Porous Materials. Synlett 2013, 24, 781−786. (10) Wang, H.; Wang, Z.; Huang, L.; Mitra, A.; Holmberg, B.; Yan, Y. High-Surface-Area Zeolitic Silica with Mesoporosity. J. Mater. Chem. 2001, 11, 2307−2310. (11) Farha, O. K.; Eryazici, I.; Jeong, N. C.; Hauser, B. G.; Wilmer, C. E.; Sarjeant, A. A.; Snurr, R. Q.; Nguyen, S. T.; Yazaydin, A. O.; Hupp, J. T. Metal-Organic Framework Materials with Ultrahigh Surface Areas: Is the Sky the Limit? J. Am. Chem. Soc. 2012, 134, 15016− 15021. (12) Uribe-Romo, F. J.; Hunt, J. R.; Furukawa, H.; Klock, C.; O’Keeffe, M.; Yaghi, O. M. A Crystalline Imine-Linked 3-D Porous Covalent Organic Framework. J. Am. Chem. Soc. 2009, 131, 4570− 4571. (13) Cooper, A. I. Molecular Organic Crystals: From Barely Porous to Really Porous. Angew. Chem., Int. Ed. 2012, 51, 7892−7894. (14) Mastalerz, M.; Oppel, I. M. Rational Construction of an Extrinsic Porous Molecular Crystal with an Extraordinary High Specific Surface Area. Angew. Chem., Int. Ed. 2012, 51, 5252−5255. (15) Zhang, G.; Presly, O.; White, F.; Oppel, I. M.; Mastalerz, M. A. A Permanent Mesoporous Organic Cage with an Exceptionally High Surface Area. Angew. Chem., Int. Ed. 2014, 53, 1516−1520. (16) Kewley, A.; Stephenson, A.; Chen, L.; Briggs, M. E.; Hasell, T.; Cooper, A. I. Porous Organic Cages for Gas Chromatography Separations. Chem. Mater. 2015, 27, 3207−3210. (17) Hasell, T.; Miklitz, M.; Stephenson, A.; Little, M. A.; Chong, S. Y.; Clowes, R.; Chen, L.; Holden, D.; Tribello, G. A.; Jelfs, K. E.; Cooper, A. I. Porous Organic Cages for Sulfer Hexafluoride Separation. J. Am. Chem. Soc. 2016, 138, 1653−1659. (18) Giri, N.; Del Popolo, M. G.; Melaugh, G.; Greenaway, R. L.; Ratzke, K.; Koschine, T.; Pison, L.; Costa Gomes, M. F.; Cooper, A. I.; James, S. L. Liquids with Permanent Porosity. Nature 2015, 527, 216− 220. (19) Eddaoudi, M.; Kim, J.; Wachter, J. B.; Chae, H. K.; O’Keeffe, M.; Yaghi, O. M. Porous Metal-Organic Polyhedra: 25Å Cuboctahedron Constructed from 12 Cu2(CO2)4 Paddle-Wheel Building Blocks. J. Am. Chem. Soc. 2001, 123, 4368−4369. (20) Ke, Y.; Collins, D. J.; Zhou, H.-C. Synthesis and Structure of Cuboctahedral and Anticuboctahedral Cages Containing 12 Quadruply Bonded Dimolybdenum Units. Inorg. Chem. 2005, 44, 4154− 4156. (21) Furukawa, S.; Horike, N.; Kondo, M.; Hijikata, Y.; CarneSanchez, A.; Larpent, P.; Louvain, N.; Diring, S.; Sato, H.; Matsuda, R.; Kawano, R.; Kitagawa, S. Rhodium-Organic Cuboctahedra as Porous Solids with Strong Binding Sites. Inorg. Chem. 2016, 55, 10843− 10846. (22) Teo, J. M.; Coghlan, C. J.; Evans, J. D.; Tsivion, E.; HeadGordon, M.; Sumby, C. J.; Doonan, C. J. Hetero-Bimetallic MetalOrganic Polyhedra. Chem. Commun. 2016, 52, 276−279. (23) Moulton, B.; Lu, J.; Mondal, A.; Zaworotko, M. J. Nanoballs: Nanoscale Faceted Polyhedra with Large Windows and Cavities. Chem. Commun. 2001, 863−864. (24) Niu, Z.; Fang, S.; Liu, X.; Ma, J.-G.; Ma, S.; Cheng, P. Coordination-Driven Polymerization of Supramolecular Nanocages. J. Am. Chem. Soc. 2015, 137, 14873−14876. (25) Chatterjee, B.; Noveron, J. C.; Resendiz, M. J. E.; Liu, J.; Yamamoto, T.; Parker, D.; Cinke, M.; Nguyen, C. V.; Arif, A. M.; Stang, P. J. Self-Assembly of Flexible Supramolecular Metallacyclic

molecules, depending on synthesis conditions. The four discrete porous cages display BET surface areas ranging from 125 m2/g to 545 m2/g. In addition to affecting the phase of the material that is formed, the functional groups also impart selective adsorption properties in the materials as Cu_OPentbdc displays high selectivity for the adsorption of ethylene over ethane. Based on the results presented here, we expect discrete, porous metal−organic assemblies to be the favored phase in cases where functionalization in the 5-position is longer than a butoxide group. Future work along these lines will target the synthesis of selective materials with increased capacity, compared to the solids presented here.

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.8b02015. Detailed experimental details, adsorption data and fitting, NMR and infrared spectra, thermogravimetric analysis plots, and crystal structure information (PDF) Crystallographic data for C24H24Cu2O12 (CIF) Crystallographic data for C192H72Cu24O142 (CIF) Crystallographic data for C20H16Cu2O12 (CIF) Crystallographic data for C268H248Cu24N10O144 (CIF) Crystallographic data for C24H10Cu2O12 (CIF) Crystallographic data for C258H192Cu24N12O144 (CIF) Crystallographic data for C22H20Cu2O12 (CIF) Crystallographic data for C18.50H7Cu2O12 (CIF) Crystallographic data for C320H350Cu24N18O150 (CIF) Crystallographic data for C23.67H22Cu2O12 (CIF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Eric D. Bloch: 0000-0003-4507-6247 Author Contributions

The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We are grateful to the University of Delaware for generous start-up funds. REFERENCES

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