Tuning the Moisture and Thermal Stability of Metal−Organic Frameworks through Incorporation of Pendant Hydrophobic Groups

An isostructural series of NbO-type porous metal−organic frameworks (MOFs) with different dialkoxy-substituents of formula Cu2(TPTC-OR) (TPTC-OR = 2′,5′-di{alkyl}oxy-[1,1′:4′,1′′-terphenyl]3,3′′,5,5′′-tetracarboxylate, R = Me, Et, Pr, Hex) has been synthesized and characterized. The moisture stability of the materials has been evaluated, and a new superhydrophobic porous MOF has been identified. The relationship between pendant side chain length and thermal stability has been analyzed by in situ synchrotron powder X-ray diffraction, showing decreased thermal stability as the side chain length is increased, contradictory to thermogravimetric decomposition studies. Additionally, the four materials exhibit moderate Brunauer−Emmett−Teller (BET) and Langmuir surface areas (1127−1396 m g−1 and 1414−1658 m g−1) and H2 capacity up to 1.9 wt % at 77 K and 1 bar. ■ INTRODUCTION Metal−organic frameworks (MOFs) are porous, crystalline materials that have been steadily growing in popularity due to their exciting chemistries and potential applications in gas storage, separations, catalysis, magnetism, luminescence, and drug delivery and storage, to name a few. The applicability of MOFs stems from their chemical tunability, high surface areas and porosities, and impressive thermal stability. MOFs for the storage of alternative fuels (hydrogen and natural gas) along with carbon capture from flue gas from coal-fired power plants have been at the forefront of the development of future materials for environmental conservation and remediation. However, since MOFs are constructed from coordination bonds between metal ions or clusters and organic linkers (typically, carboxylate or pyridyl moieties), the frameworks are often susceptible to decomposition resulting from ligand displacement by atmospheric water vapor. This is a significant concern in clean energy technologies, as water is certainly present in flue gas and is often an impurity in natural gas supplies. To this end, a number of researchers have investigated methods for enhancing the water stability of MOFs, including using high oxidation state metals (such as Fe, Al, and Zr), which form strong coordination bonds with linkers, incorporation of methyl groups near coordination sites, and postsynthetic grafting of hydrophobic groups onto linkers, among others. With the ready availability and inexpensiveness of copper(II) salts, along with the predictable nature of formation of dicopper(II) paddlewheel secondary building units (SBUs), we have turned our attention toward investigating methods for enhancing the water stability of MOFs formed from a combination of dicopper paddlewheel SBUs and carboxylate linkers. In particular, NbO-type MOFs are well-known as highly porous and robust frameworks with exceptional gas sorption properties and can predictably be assembled from solvothermal reactions of copper(II) salts and rectangular planar tetracarboxylic acids. A series of NbO-type MOFs (NOTT-10x) has been previously reported by Schröder and co-workers studying the role of pore size, ligand functionalization, and exposed metal sites in isoreticular copper(II) tetracarboxylate MOFs. This previous work highlighted the predictability and feasibility of designing and synthesizing NbO-type MOFs and their applicability in future gas sorption applications. From an inexpensive starting material, hydroquinone, four rectangular planar terphenyl tetracarboxylate organic linker precursors (H4TPTC-OR, Scheme 1) were designed and synthesized by grafting different dialkoxy substitutions on the central phenyl ring in order to modify the hydrophobic properties. By incorporating these hydrophobic groups prior to MOF formation, we can ensure full inclusion of functionalized linkers without any detrimental effects on the structure of the framework. We report here a series of four NbO-type Cu(II)based MOFs with different alkoxy substitutions that modify the moisture and thermal stability of the isostructural frameworks, as well as H2 capacity. Through in situ synchrotron powder X-ray diffraction (PXRD) measurements, we thoroughly investigated the stability of the crystalline materials. The hydrogen storage capacity is reported, and the role of pendant alkoxy groups on the linkers toward hydrogen uptake and, most importantly, thermal and moisture stability is evaluated. Received: June 19, 2013 Revised: September 12, 2013 Published: September 13, 2013 Article pubs.acs.org/crystal © 2013 American Chemical Society 4760 dx.doi.org/10.1021/cg4009224 | Cryst. Growth Des. 2013, 13, 4760−4768 ■ EXPERIMENTAL SECTION General Information. All initial reagents were purchased from commercial sources and used as received without further purification. Nuclear magnetic resonance (NMR) H data were collected on a 300 MHz Mercury 300 spectrometer. Thermogravimetric analyses (TGA) were performed under N2 flow on a Shimadzu TGA-50 thermogravimetric analyzer with a heating rate of 3 °C min−1. Synthesis of 1,4-Di-n-propoxybenzene (A, R = Pr). A, R = Pr, was synthesized by modification of a previously reported procedure. Hydroquinone (2.10 g, 19.1 mmol) and excess potassium carbonate were purged under a vacuum and backfilled with N2 three times in a dry 100 mL Schlenk flask. N,N-Dimethylformamide (DMF, 30 mL) was added via canula, and 1-bromopropane (4.1 mL, 45.2 mmol) was added via syringe. The reaction mixture was heated at 60 °C under N2 atmosphere overnight. After the mixture was cooled to room temperature, water was added and the light brown crystals were collected by vacuum filtration. Recrystallization from methanol yielded colorless crystals (26.5%). H NMR (300MHz CDCl3): δ 6.80 (d, 4H, J = 2.2 Hz), 3.85 (td, 4H, J = 6.6 Hz, 2.2 Hz), 1.78 (m, 4H), 1.01 (td, 6H, J = 7.4 Hz, 2.2 Hz). Synthesis of 1,4-Di-n-hexyloxybenzene (A, R = Hex). A, R = Hex, was synthesized similarly to A, R = Pr, except that 1-bromohexane (6.3 mL, 45.2 mmol) in place of 1-bromopropane. Recrystallization of the light brown crystals from ethanol yielded a flaky off-white precipitate (40.1%). H NMR (300 MHz CDCl3): δ 6.80 (s, 4H), 3.88 (t, 4H, J = 6.6 Hz), 1.73 (m, 4H), 1.50−1.26 (m, 12H), 0.88 (t, 6H, J = 7.0 Hz). Synthesis of 2,5-Diethoxy-1,4-di-iodo-benzene (B, R = Et). B, R = Et, was synthesized by modification of a previously reported procedure. To 20 mL of methanol at 0 °C were added dropwise 1.26 mL (3.91 g, 24.1 mmol) of iodine monochloride, followed by 1,4diethoxybenzene (A, R = Et, 1.0 g, 6.0 mmol). The reaction mixture was stirred and heated to reflux for 4 h, and the white precipitate was collected by vacuum filtration, washed with methanol, and dried in air to yield 0.681 g. Another 0.123 g was recovered upon cooling the filtrate in refrigerator (32.0% combined). H NMR (300 MHz CDCl3) 7.19 (s, 2H), 4.00 (q, 4H, J = 7.0 Hz) 1.44 (t, 6H, J = 7.0 Hz). Synthesis of 1,4-Di-iodo-2,5-di-n-propoxybenzene (B, R = Pr). B, R = Pr, was synthesized similarly to B, R = Et, except that 1,4-di-npropoxybenzene (A, R = Pr, 0.937 g, 4.82 mmol) was used instead of A, R = Et, in 15 mL of methanol with 1.0 mL (19.1 mmol) of ICl. Yield: 57.7% H NMR (300 MHz CDCl3) 7.11 (s, 2H), 3.83 (t, 4H, J = 6.4 Hz), 1.75 (m, 4H), 1.00 (t, 6H, J = 7.4 Hz). Synthesis of 2,5-Di-n-hexyloxy-1,4-di-iodo-benzene (B, R = Hex). B, R = Hex, was synthesized similarly to B, R = Et, except that 1,4-di-nhexyloxybenzene (A, R = Hex, 2.03 g, 7.29 mmol) was used instead of A, R = Et, in 20 mL of methanol with 1.65 mL (31.5 mmol) of ICl. Yield: 82.2% H NMR (300 MHz CDCl3) 7.17 (s, 2H), 3.92 (t, 4H, J = 6.4 Hz), 1.80 (qn, 4H, J = 6.3 Hz), 1.58−1.43 (m, 4H), 1.41−1.29 (m, 8H), 0.91 (t, 6H, J = 7.0 Hz). Synthesis of Diethyl 2′,5′-dimethoxy-[1,1′:4′,1′′-terphenyl]3,3′′,5,5′′-tetracarboxylate (C, R = Me). 1,4-Dibromo-2,5-dimethoxybenzene (0.51 g, 1.72 mmol), diethyl 5-(4,4,5,5-tetramethyl-1,3,2dioxaborolan-2-yl)-1,3-benzene-dicarboxylate (2.61 g, 7.5 mmol), NaHCO3 (1.23 g), CsF (3.0 g), and Pd(PPh3)4 (0.15 g) were mixed in a 250 mL Schlenk flask and then purged and refilled with N2 three times. 1,2-Dimethoxyethane (DME, 100mL) and distilled water (10mL) were mixed and purged with N2 for 30 min and then transferred under N2 to the reaction flask via canula. The reaction solution was heated at reflux for 7 days. Organic solvent was removed in vacuo and water was added. The aqueous phase was extracted with dichloromethane three times, and organic extracts were combined. The organic solution was washed with water, dried over MgSO4, filtered, and reduced in vacuo. The off-white solid was collected after washing with hot acetone and drying in air (43.8%). H NMR (300 MHz CDCl3): δ 8.67 (t, 2H, J = 1.6 Hz), 8.43 (d, 4H, J = 1.6 Hz), 7.00 (s, 2H), 4.43 (q, 8H, J = 7.1 Hz), 3.82 (s, 6H), 1.43 (t, 12H, J = 7.1 Hz). Synthesis of Diethyl 2′,5′-diethoxy-[1,1′:4′,1′′-terphenyl]3,3′′,5,5′′-tetracarboxylate (C, R = Et). B, R = Et, (0.804 g, 1.63 mmol), diethyl 5-(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)-1,3-benzene-dicarboxylate (2.26 g, 6.52 mmol), NaHCO3 (1.00 g), CsF (2.5 g), and Pd(PPh3)4 (0.1 g) were mixed in a 250 mL Schlenk flask and then purged and refilled with N2 three times. 1,2-Dimethoxyethane (DME, 100 mL) and distilled water (10 mL) were mixed and purged withN2 for 30min and then transferred under N2 to the reaction flask via canula. The reaction solution was heated at reflux for 5 days. Organic solvent was removed in vacuo and water was added. The aqueous phase was extracted with dichloromethane three times, and organic extracts were combined. The organic solution was washed with water, dried over MgSO4, filtered, and reduced in vacuo. The off-white solid was collected after recrystallization from acetone and drying in air (42.5%). H NMR (300 MHz CDCl3): δ 8.60 (t, 2H, J = 1.6 Hz), 8.43 (d, 2H, J = 1.6 Hz), 6.97 (s, 2H), 4.36 (q, 8H, J = 7.1 Hz), 3.98 (q, 4H, J = 6.9 Hz), 1.36 (t, 12H, J = 7.1 Hz), 1.27 (t, 6H, J = 7.0 Hz). Synthesis of Diethyl 2′,5′-di-n-propoxy-[1,1′:4′,1′′-terphenyl]3,3′′,5,5′′-tetracarboxylate (C, R = Pr). B, R = Pr, (1.24 g, 2.78 Scheme 1. Synthesis and Structures of Ligand Precursors H4TPTC-OR a (i) K2CO3, DMF, RBr; (ii) ICI, MeOH; (iii) diethyl 5-(4,4,5,5-tetramethyl-,3,2-dioxaborolan-2-yl)-1,3-benzene-dicarboxylate, NaHCO3, CsF, Pd(PPH3)4, 1,2-dimethoxyethane, H2O; (iv) 1. MeOH, THF, KOH 2. H3O . Crystal Growth & Design Article dx.doi.org/10.1021/cg4009224 | Cryst. Growth Des. 2013, 13, 4760−4768 4761 mmol), diethyl 5-(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)-1,3-benzene-dicarboxylate (2.60 g, 7.5 mmol), NaHCO3 (1.2 g), CsF (3.0 g), and Pd(PPh3)4 (0.15 g) were mixed in a 250 mL Schlenk flask and then purged and refilled with N2 three times. 1,2-Dimethoxyethane (DME, 100mL) and distilled water (10mL) were mixed and purged with N2 for 30 min and then transferred under N2 to the reaction flask via canula. The reaction solution was heated at reflux for 5 days. Organic solvent was removed in vacuo and water was added. The aqueous phase was extracted with dichloromethane three times and organic extracts were combined. The organic solution was washed with water, dried over MgSO4, filtered, and reduced in vacuo. The white solid was collected after washing with acetone and drying in air (60.7%). H NMR (300 MHzCDCl3): δ 8.60 (t, 2H, J = 1.6Hz), 8.42 (d, 4H, J = 1.6 Hz), 6.96 (s, 2H), 4.36 (q, 8H, J = 7.1 Hz), 3.87 (t, 4H, J = 6.3 Hz), 1.65 (m, 4H), 1.36 (t, 12H, J = 7.1 Hz), 0.90 (t, 6H, J = 7.4 Hz). Synthesis of Diethyl 2′,5′-di-n-hexyloxy-[1,1′:4′,1′′-terphenyl]3,3′′,5,5′′-tetracarboxylate (C, R = Hex). B, R = Hex, (1.50 g, 2.82 mmol), diethyl 5-(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)-1,3-benzene-dicarboxylate (2.60 g, 7.5 mmol), NaHCO3 (1.2 g), CsF (3.0 g), and Pd(PPh3)4 (0.15 g) were mixed in a 250 mL Schlenk flask and then purged and refilled with N2 three times. 1,2-Dimethoxyethane (DME, 100mL) and distilled water (10mL) were mixed and purged with N2 for 30 min and then transferred under N2 to the reaction flask via canula. The reaction solution was heated at reflux for 5 days. Organic solvent was removed in vacuo and water was added. The aqueous phase was extracted with dichloromethane three times and organic extracts were combined. The organic solution was washed with water, dried over MgSO4, filtered, and reduced in vacuo. The white solid was collected after recrystallization from acetone and drying in air (44.0%). H NMR (300 MHz CDCl3): δ 8.66 (t, 2H, J = 1.6 Hz), 8.47 (d, 4H, J = 1.6 Hz), 7.02 (s, 2H), 4.42 (q, 8H, J = 7.1 Hz), 3.95 (t, 4H, J = 6.4 Hz), 1.68 (qn, 4H, J = 6.3 Hz), 1.43 (t, 12H, J = 7.1), 1.39−1.19 (m, 12H) 0.83 (t, 6H, J = 6.8 Hz). Synthesis of 2′,5′-Dimethoxy-[1,1′:4′,1′′-terphenyl]-3,3′′,5,5′′-tetracarboxylic acid (H4TPTC-OMe).To a suspension ofC, R =Me, (0.435 g, 0.753 mmol) in 50 mL of THF/methanol (1/1 by volume) was added KOH (3 g in 25 mL of water). The reaction mixture was heated at reflux until no more solid was present. The organic solvent was removed in vacuo, and the peach-colored solution was acidified with 20% HCl (in water). The white precipitate was collected by vacuum filtration, washed with water, and dried in air (quant. yield). H NMR (300 MHz DMSOd6): δ 13.35 (br, 4H), 8.46 (t, 2H, J = 1.6 Hz), 8.32 (d, 4H, J = 1.6 Hz), 7.18 (s, 2H), 3.81 (s, 6H). Synthesis of 2′,5′-Diethoxy-[1,1′:4′,1′′-terphenyl]-3,3′′,5,5′′-tetracarboxylic acid (H4TPTC-OEt). To a suspension of C, R = Et, (0.401 g, 0.690 mmol) in 50 mL of THF/methanol (1/1 by volume) was added KOH (3 g in 25 mL water). The reaction mixture was heated at reflux overnight. The organic solvent was removed in vacuo, and the peachcolored solution was acidified with 20% HCl (in water). The white precipitate was collected by vacuum filtration, washed with water, and dried in air (60.0%). H NMR (300 MHz DMSO-d6): δ 13.30 (br, 4H), 8.45 (t, 2H, J = 1.6 Hz), 8.39 (d, 4H, J = 1.6 Hz), 7.20 (s, 2H), 4.09 (q, 4H, J = 7.1 Hz), 1.24 (t, 6H, J = 7.0 Hz). Synthesis of 2′,5′-Di-n-propoxy-[1,1′:4′,1′′-terphenyl]-3,3′′,5,5′′tetracarboxylic acid (H4TPTC-O Pr). To a suspension of C, R = Pr, (1.07 g, 1.69 mmol) in 50 mL of THF/methanol (1/1 by volume) was added KOH (3 g in 25 mL of water). The reaction mixture was heated at reflux overnight. The organic solvent was removed in vacuo, and the peach-colored solution was acidified with 20% HCl (in water). The white precipitate was collected by vacuum filtration, washed with water, and dried in air (quant. yield). H NMR (300 MHz DMSO-d6): δ 13.26 (br, 4H), 8.45 (t, 2H, J = 1.6 Hz), 8.39 (d, 4H, J = 1.6 Hz), 7.19 (s, 2H), 4.00 (t, 4H, J = 6.2 Hz), 1.64 (m, 4H, J = 6.7Hz), 0.90 (t, 6H, J = 7.4Hz). Synthesis of 2′,5′-Di-n-hexyloxy-[1,1′:4′,1′′-terphenyl]-3,3′′,5,5′′tetracarboxylic acid (H4TPTC-O Hex). To a suspension of C, R = Hex, (0.896 g, 1.25 mmol) in 50 mL of THF/methanol (1/1 by volume) was added KOH (3 g in 25 mL of water). The reaction mixture was heated at reflux overnight. The organic solvent was removed in vacuo, and the peach-colored solution was acidified with 20% HCl (in water). The white precipitate was collected by vacuum filtration, washed with water, and dried in air (quant. yield). H NMR (300 MHz DMSO-d6): δ 13.30 (br, 4H), 8.45 (t, 2H, J = 1.6 Hz), 8.35 (d, 4H, J = 1.6 Hz), 7.17 (s, 2H), 4.00 (t, 4H), 1.57 (m, 4H), 1.45−1.05 (m, 12H), 0.76 (t, 6H). Synthesis of Cu2(TPTC-OR)·xS (S = unidentified guest species). Cu2(TPTC-OMe)·xS. To a 10 dram glass vial was added 0.050 g (0.107 mmol) of H4TPTC-OMe and 0.150 g (0.645 mmol) of Cu(NO3)2· 2.5H2O dissolved in 15 mL of N,N-dimethylacetamide (DMA) with 40 drops of HBF4 (48% w/w aqueous solution) and 40 drops of water. The reaction was kept at 85 °C in an oven for 4 days. The resultant blue-teal crystalline powder of Cu2(TPTC-OMe)·xS was then decanted, washed with fresh DMA, and collected. Yield: 45.3% (activated sample) based on H4TPTC-OMe. Cu2(TPTC-OEt)·xS. To a 10 dram glass vial was added 0.050 g (0.101 mmol) of H4TPTC-OEt and 0.150 g (0.645 mmol) of Cu(NO3)2· 2.5H2O dissolved in 15 mL of N,N-dimethylacetamide (DMA) with 20 drops of HBF4 (48% w/w aqueous solution). The reaction was kept at 85 °C in an oven for 4 days. The resultant blue-teal crystalline powder of Cu2(TPTC-OEt)·xS was then decanted, washed with fresh DMA, and collected. Yield: 39.4% (activated sample) based on H4TPTC-OEt. Cu2(TPTC-O Pr)·xS. To a 10 dram glass vial was added 0.050 g (0.096 mmol) of H4TPTC-O Pr and 0.150 g (0.645 mmol) of Cu(NO3)2· 2.5H2O dissolved in 15 mL of N,N-dimethylacetamide (DMA) with 20 drops of HBF4 (48% w/w aqueous solution). The reaction was kept at 85 °C in an oven for 4 days. The resultant blue-teal crystalline powder of Cu2(TPTC-O Pr)·xS was then decanted, washed with fresh DMA, and collected. Yield: 66.5% (activated sample) based on H4TPTC-O Pr. Cu2(TPTC-O Hex)·xS. To a 10 dram glass vial was added 0.050 g (0.082 mmol) of H4TPTC-O Hex and 0.150 g (0.645 mmol) of Cu(NO3)2·2.5H2O dissolved in 15 mL of N,N-dimethylacetamide (DMA) with 20 drops of HBF4 (48% w/w aqueous solution). The reaction was kept at 85 °C in an oven for 4 days. The resultant blue-teal crystalline powder of Cu2(TPTC-O Hex)·xS was then decanted, washed with fresh DMA, and collected. Yield: 63.2% (activated sample) based on H4TPTC-O Hex. X-ray Crystallography. Powder X-ray diffraction (PXRD) patterns were collected on a Bruker D8-Focus Bragg−Brentano X-ray powder diffractometer equipped with a Cu sealed tube (λ = 1.54178 Å) and graphite monochromator at a scan rate of 1 s deg−1, solid state detector, and a routine power of 1600 W (40 kV, 40 mA). The samples were dispersed on Si single crystal zero diffraction plate for analysis. Simulation of the PXRD pattern was carried out by the single-crystal data and diffraction-crystal module of the Mercury program available free-of-charge via the Internet at http://www.ccdc.cam.ac.uk/products/