Zn-MOFs Containing Flexible α,ω-Alkane (or Alkene)-Dicarboxylates

Article pubs.acs.org/crystal

Zn-MOFs Containing Flexible α,ω-Alkane (or Alkene)-Dicarboxylates and 1,2-Bis(4-pyridyl)ethane Ligands: CO2 Sorption and Photoluminescence In Hong Hwang,† Ha-Yeong Kim,§ Myoung Mi Lee,† Yu Jeong Na,† Jin Hoon Kim,† Hyun-Chul Kim,‡ Cheal Kim,*,† Seong Huh,‡ Youngmee Kim,*,§ and Sung-Jin Kim§ †

Department of Fine Chemistry, Seoul National University of Science and Technology, Seoul 139-743, Korea Department of Chemistry and Protein Research Center for Bio-Industry, Hankuk University of Foreign Studies, Yongin 449-791, Korea § Department of Chemistry and Nano Science and Institute of Nano-Bio Technology, Ewha Womans Univeristy, Seoul 120-750, Korea ‡

S Supporting Information *

ABSTRACT: The use of five flexible α,ω-alkane (or alkene)dicarboxylates (succinate (2), fumarate (3), adipate (5), and muconate (6)) produced three-dimensional (3-D) ZnII frameworks with 1,2-bis(4-pyridyl)ethane pillars, whereas glutarate gave a two-dimensional (2-D) ZnII compound (4). Structures 2 and 5 are 4-fold interpenetrated networks, and 3 and 6 are 5-fold interpenetrated networks. Both 5 and 6 displayed good CO2 sorption capabilities at 196 K, as evidenced by S-shape adsorption isotherms. Both compounds also exhibited selective CO2 sorption over N2 at low temperature, while 5 containing an adipate ligand showed higher CO2 uptake at 273 and 298 K than 6 with a more rigid muconate ligand. The isosteric heats of CO2 adsorption for 5 and 6 were 23.1 kJ mol−1 and 30.7 kJ mol−1, respectively. A photoluminescence study showed the emissions of 2, 4, and 5 to be blue-shifted relative to both the free acids and ligand, while 3 had ligand-based luminescence properties. The thermal stabilities of these complexes were also examined.

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and bipyridyl pillars.25 Two of the Cu-MOFs possessed very similar pore shapes with controllable pore dimensions and exhibited good selectivity for CO2 over N2 and H2. One MOF was shown to be an efficient, mild, and easily recyclable heterogeneous catalyst for the transesterification of esters.24 The Zn-MOFs containing malonates and bipyridyl pillars formed three-dimensional (3-D) frameworks and catalyzed the heterogeneous transesterification of phenyl acetate.25 As an extension of our previous work, we used various flexible α,ω-alkane (or alkene)-dicarboxylates with 1,2-bis(4pyridyl)ethane (bpa) pillars to prepare new functional MOFs with interesting structures and potential applications. Five flexible α,ω-alkane (or alkene)-dicarboxylates (Scheme 1) were employed in the formation of two-dimensional (2-D) or 3-D ZnII frameworks.

INTRODUCTION Metal−organic frameworks (MOFs) constructed with a variety of metal ions and polytopic bridging ligands have been used in a wide range of applications, including selective gas sorption,1−4 heterogeneous catalysis,5−11 separation,12−17 sensor,18 drug delivery,19,20 and biological imaging.21 Dicarboxylates have been commonly used in MOFs, giving structures of various dimensionalities with different coordination modes and pore sizes. Within the class of dicarboxylate ligands, rigid aromatic dicarboxylates2−8,13,15−17 have been the most popular choice for the synthesis of MOFs, and flexible cyclohexanedicarboxylates22,23 have also been used. One particular group of flexible dicarboxylates, α,ω-alkane-dicarboxylates, has been shown to be particularly suitable as ligands in MOFs of various topologies. Though less frequently employed in MOFs than aromatic dicarboxylates, a few examples of MOFs featuring α,ω-alkane dicarboxylates have been reported.24−31 A systematic investigation of MOFs containing these α,ω-alkane (or alkene)-dicarboxylate is warranted and important. Recently, we reported Cu-MOFs constructed of flexible α,ωalkane-dicarboxylate, glutarate, and bipyridyl ligands24 and ZnMOFs containing flexible α,ω-alkane-dicarboxylate, malonate, © XXXX American Chemical Society

Received: June 28, 2013 Revised: October 8, 2013

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Scheme 1. Chemical Structures of α,ω-Alkane (or Alkene)dicarboxylates

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was layered onto the aqueous solution. Suitable crystals of 2 for X-ray analysis were obtained in three weeks. The yield of 2 was 30.4 mg (72%). The purity of a bulk sample of 2 was checked by PXRD (see Figure S1 in Supporting Information). IR (KBr): ν(cm−1) = 3374(brs), 1620(m), 1580(s), 1433(w), 1402(m), 1267(w), 1072(w), 1029(m), 833(m), 686(s). Anal. Calcd. for C16H22N2O7Zn (419.73), 2: C, 45.78; H, 5.29; N, 6.68%. Found: C, 45.43; H, 5.63; N, 6.61%. [Zn(μ-fumarate)(μ-bpa)]·H2O (3). 8.1 mg (0.07 mmol) of fumaric acid and 21.2 mg (0.07 mmol) of Zn(NO3)2·6H2O were dissolved in 4 mL of H2O. Four milliliters of an acetonitrile solution of 1,2-bis(4pyridyl)ethane (37.2 mg, 0.2 mmol) was layered onto the aqueous solution. Suitable crystals of 3 for X-ray analysis were obtained in two weeks. The yield of 3 was 12.1 mg (46%). The purity of a bulk sample of 3 was checked by PXRD (see Figure S2 in Supporting Information). IR (KBr): ν(cm−1) = 1614(w), 1579(m), 1431(w), 1368(s), 1209(w), 1029(w), 989(w), 835(m), 800(m), 695(s). Anal. Calcd. for C16H16N2O5Zn (381.68), 3: C, 50.35; H, 4.23; N, 7.34. Found: C, 50.48; H, 4.11; N, 7.62%. [{Zn(H2O)(μ-glutarate)}2(μ-bpa)] (4). 13.3 mg (0.1 mmol) of glutaric acid and 30.4 mg (0.1 mmol) of Zn(NO3)2·6H2O were dissolved in 4 mL of H2O. Four milliliters of an acetonitrile solution of 1,2-bis(4-pyridyl)ethane (37.2 mg, 0.2 mmol) was layered onto the aqueous solution. Suitable crystals of 4 for X-ray analysis were obtained in two weeks. The yield of 4 was 20.6 mg (67%). The purity of a bulk sample of 4 was checked by PXRD (see Figure S3 in Supporting Information). IR (KBr): ν(cm−1) = 1618(w), 1580(s), 1431(w), 1407(m), 1268(w), 1231(w), 1072(w), 1028(m), 834(m), 686(w). Anal. Calcd. for C22H28N2O10Zn2 (611.20), 4: C, 43.23; H, 4.63; N, 4.58. Found: C, 42.85; H, 4.87; N, 4.65%. [Zn(μ-adipate)(μ-bpa)]·H2O (5). 14.6 mg (0.1 mmol) of adipic acid and 30.4 mg (0.1 mmol) of Zn(NO3)2·6H2O were dissolved in 10 mL of H2O. Five milliliters of an ethanol solution of 1,2-bis(4pyridyl)ethane (37.2 mg, 0.2 mmol) was layered onto the aqueous solution. Suitable crystals of 5 for X-ray analysis were obtained in a

EXPERIMENTAL SECTION

Materials. Malonic acid, succinic acid, fumaric acid, glutaric acid, adipic acid, muconic acid, Zn(NO3)2·6H2O, NH4OH, 1,2-bis(4pyridyl)ethane, acetonitrile, ethanol, and dimethylformamide were purchased from Sigma-Aldrich and used as received. Instrumentation. Elemental analysis for carbon, nitrogen, and hydrogen was performed on a vario MACRO cube (Elementar Analysensysteme GmbH, Germany) in the Laboratory Center of Seoul National University of Science and Technology, Korea. IR spectra were measured on a BIO RAD FTS 135 spectrometer as KBr pellets. Thermogravimetric analyses (TGA) were performed on a Shimadzu TA50 integration thermal analyzer. Emission/excitation spectra were recorded on a Perkin−Elmer LS45 fluorescence spectrometer. Powder X-ray diffraction (PXRD) patterns were obtained by using a Rigaku MiniFlex diffractometer (30 kV, 15 mA, scan speed: 2° min−1, stepsize: 0.02°). The volumetric N2 adsorption−desorption analysis was performed using a Belsorp-miniII (BEL Japan) at 77 and 196 K (2propanol/dry ice bath). The as-prepared samples were dried at 393 K under high vacuum for 2 h. Low pressure CO2 adsorption−desorption measurements were performed using a Belsorp-miniII at 196 K, 273 K (ice bath), and 298 K (water bath). [Zn(μ-succinate)(μ-bpa)]·3H2O (2). Succinic acid (11.8 mg, 0.1 mmol), Zn(NO3)2·6H2O (30.4 mg, 0.1 mmol), and NH4OH (12.7 μL, 0.1 mmol) were dissolved in 4 mL of H2O. Four milliliters of an acetonitrile solution of 1,2-bis(4-pyridyl)ethane (37.2 mg, 0.2 mmol)

Table 1. Crystallographic Data for Compounds 1−6 empirical formula formula weight temperature (K) wavelength (Å) space group a (Å) b (Å) c (Å) α (°) β (°) γ (°) volume (Å3) Z density (calc) (Mg/m3) absorption coeff (mm−1) crystal size (mm3) reflections collected independent reflections data/restraints/parameters goodness-of-fit on F2 final R indices [I > 2σ(I)] R indices (all data) largest diff peak and hole (e·Å−3) a

1a

2

3

4

5

6

C11H13N2O5Zn 318.60 170(2) 0.71073 P21/n 7.2490(14) 23.930(5) 7.4190(15) 90.00 90.37(3) 90.00 1286.9(4) 4 1.644 1.926 0.25 × 0.20 × 0.04 6895 2477 [R(int) = 0.0239] 2477/2/179 1.082 R1 = 0.0321, wR2 = 0.0753 R1 = 0.0356, wR2 = 0.0764 0.593 and −0.916

C16H22N2O7Zn 419.73 170(2) 0.71073 P21/n 10.895(2) 13.052(3) 13.536(3) 90.00 95.74(3) 90.00 1915.2(7) 4 1.456 1.321 0.20 × 0.10 × 0.08 10581 3726 [R(int) = 0.0314] 3726/6/253 1.082 R1 = 0.0305, wR2 = 0.0805 R1 = 0.0381, wR2 = 0.0833 0.313 and −0.214

C16H14N2O5Zn 379.66 293(2) 0.71073 P1̅ 8.5220(17) 9.0910(18) 11.361(2) 91.44(3) 110.86(3) 98.38(3) 810.9(3) 2 1.555 1.543 0.10 × 0.10 × 0.02 4456 3096 [R(int) = 0.0961] 3096/0/212 0.808 R1 = 0.0796, wR2 = 0.1651 R1 = 0.1685, wR2 = 0.1860 0.996 and −0.704

C22H28N2O10Zn2 611.20 293(2) 0.71073 P21/c 11.025(2) 14.176(3) 16.191(3) 90.00 95.50(3) 90.00 2518.8(9) 4 1.612 1.962 0.20 × 0.10 × 0.10

C18H22N2O5Zn 411.75 293(2) 0.71073 Pnna 10.5081(9) 16.6714(14) 12.2960(10) 90.00 90.00 90.00 2154.1(3) 4 1.270 1.167 0.55 × 0.45 × 0.40 10481 2019 [R(int) = 0.0634] 2019/3/120 1.021 R1 = 0.0634, wR2 = 0.1784 R1 = 0.0790, wR2 = 0.1906 1.085 and −0.522

C36H40N4O12Zn2 851.46 293(2) 0.71073 P21/c 16.486(3) 14.244(3) 16.667(3) 90.00 99.74(3) 90.00 3857.4(13) 4 1.466 1.309 0.15 × 0.08 × 0.08

13953 4954 [R(int) = 0.0252] 4954/9/338 1.049 R1 = 0.0621, wR2 = 0.1836 R1 = 0.0824, wR2 = 0.2018 0.998 and −0.857

20718 7561 [R(int) = 0.0916] 7561/8/511 0.993 R1 = 0.0586, wR2 = 0.1314 R1 = 0.1585, wR2 = 0.1662 0.646 and −0.679

See ref 25. B

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Figure 1. (a) Crystal structure of 2 along the a-axis. Inset is the coordination environment around a ZnII ion. Symmetry operations: (i) 0.5 + x, 2.5 − y, 0.5 + z, (ii) 0.5 + x, 1.5 − y, −0.5 + z. (b) The 4-fold interpenetrated 3-D frameworks are shown in different colors. All hydrogen atoms and solvent molecules were omitted for clarity.

dicarboxylate groups and bpa acted to bridge ZnII ions in the formation of 3-D frameworks (Figure 1a for 2, 2a for 3, 3a for 5, and 4a for 6). Each MOF contained solvated water molecules (n = 3 for 2, 1 for 3 and 5, and 2 for 6). Each carboxylate moiety bound ZnII in a monodentate fashion, with the entire dicarboxylate molecule bridging two ZnII ions. The 3-D frameworks of 2 and 5 were 4-fold interpenetrated and 5-fold interpenetrated for 3 and 6 (Figure 1b for 2, 2b for 3, 3b for 5, and 4b for 6). The solvent-free 3-D frameworks indicated void volumes of 19.5% (373.9 Å3/1915.2 Å3) for 2, 7.2% (58.6 Å3/ 810.9 Å3) for 3, 22.1% (477 Å3/2154 Å3) for 5, and 13.6% (523.8 Å/3857.4 Å3) for 6, based on PLATON analysis.36 The ZnII ions adopted distorted tetrahedral geometries, bound by two carboxylate oxygen atoms and two bpa nitrogen atoms (Figure 1a for 2, 2a for 3, 3a for 5, and 4a for 6). All four structures indicated a four-connected uninodal net with a Schläfli symbol of 66 assuming the ZnII ions act as nodes without any simplification based on by TOPOS analysis (version 4.0).37 The typical N−Zn−N and O−Zn−O bond angles, and the typical Zn−O and Zn−N bond distances, are listed in Table 2. [{Zn(H2O)(μ-O2CRCO2)}2(μ-bpa)] (O2CRCO2 = Glutarate (4)). The asymmetric unit contains two ZnII ions, two glutarates, a bpa ligand, and two water molecules. The glutarate and bpa ligands each bridge ZnII ions in the formation a 2-D layer (Figure 5a). The packing diagram of these layers is shown in Figure 5b. The carboxylates adopted both asymmetric chelating and monodentate coordination modes. The asymmetric chelating Zn−Oglutarate bond distances were 1.954(4)− 2.012(4) Å and 2.418(4)−2.469(4) Å (Table 2). Each ZnII ion was pentacoordinate, bound by two oxygen atoms of one glutarate, one oxygen atom of the other glutarate, one water oxygen atom, and one bpa nitrogen atom. The Oglutarate−Zn− Oglutarate bond angles ranged from 92.97(18) to 122.71(1)°, and the Oglutarate−Zn−Owater angle ranged from 83.19(14) to 123.8(2)° (Table 2). Employment of five flexible α,ω-alkane (or alkene)dicarboxylates (O2CRCO2 = malonate (1),25 succinate (2), fumarate (3), adipate (5), and muconate (6)) resulted in 3-D ZnII frameworks with bpa pillars, and glutarate produced a 2-D

week. The yield of 5 was 35.3 mg (86%). The purity of a bulk sample of 5 was checked by powder XRD (see Figure S4 in Supporting Information). IR (KBr): ν(cm−1) = 1584(s), 1401(m), 1269(w), 1072(w), 1028(w), 834(m), 687(w), 644(w). Anal. Calcd. for C18H20N2O4Zn (411.75), 5: C, 52.50; H, 4.91; N, 6.81. Found: C, 52.84; H, 5.32; N, 6.47%. [Zn(μ-muconate)(μ-bpa)]·2H2O (6). A mixture of muconic acid (28.4 mg, 0.2 mmol), Zn(NO3)2·6H2O (59.0 mg, 0.2 mmol), 1,2bis(4-pyridyl)ethane (73.7 mg, 0.4 mmol), and dimethylformamide (20 mL) was placed in a Teflon-lined high pressure bomb, which was sealed and heated at 85 °C for 72 h followed by cooling to room temperature. Suitable crystals of 6 for X-ray analysis were obtained. The yield of 6 was 38.0 mg (23%). The purity of a bulk sample of 6 was checked by PXRD (see Figure S5 in Supporting Information). IR (KBr): ν(cm−1) = 3399(brs), 1620(s), 1559(s), 1380(s), 1293(m), 1192(w), 1005(s), 866(m), 737(m), 704(w). Anal. Calcd. for C36H40N4O12Zn2 (851.46), 6: C, 50.78; H, 4.74; N, 6.58. Found: C, 50.66; H, 4.71; N, 6.45%. X-ray Crystallography. The X-ray diffraction data for all five compounds were collected on a Bruker SMART APEX diffractometer equipped with a monochromator in the Mo Kα (λ = 0.71073 Å) incident beam. Each crystal was mounted on a glass fiber. The CCD data were integrated and scaled using the Bruker-SAINT software package, and the structure was solved and refined using SHELXTL V6.12.32 All hydrogen atoms were placed in the calculated positions. The crystallographic data for compounds 1−6 are listed in Table 1. Structural information was deposited at the Cambridge Crystallographic Data Center (CCDC reference numbers are 890457 for 2, 890453 for 3, 890454 for 4, 890452 for 5, and 890455 for 6).

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RESULTS AND DISCUSSION Five Zn-MOFs containing flexible α,ω-alkane (or alkene)dicarboxylates and bpa ligands were synthesized. The Fouriertransform infrared (FT-IR) spectra of 2−6 were fully consistent with their formulations. Very strong, slightly broadened bands at ∼1600 cm−1 and ∼1400 cm−1 indicated the asymmetric and symmetric CO stretching modes of the bound malonate moieties.33−35 The absence of any bands in the area of ∼1710 cm−1 indicated full deprotonation of all carboxylate groups in 2−6,33−35 consistent with X-ray analysis. Structure Description. [Zn(μ-O2CRCO2)(μ-bpa)]·nH2O (O2CRCO2 = Succinate (2), Fumarate (3), Adipate (5), and Muconate (6)). Both the α,ω-alkane (or α,ω-alkene)C

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ZnII structure (4) (Table 3). Structures 2, 3, 5, and 6 are formulated as [Zn(μ-O2CRCO2)(μ-bpa)] with water solvates. Both dicarboxylate and bpa ligands bridge ZnII ions in the formation of the 3-D frameworks in 2, 3, 5, and 6. These 3-D frameworks are 4- or 5-fold interpenetrated, uninodal, fourconnected diamond networks with a Schläfli symbol of 66 by TOPOS analysis. For instance, 2 and 5 each display a 4-fold interpenetrated 3-D network with Zt = 2 and Zn = 2 (class IIIa), and 3 and 6 are 5-fold-interpenetrated 3-D networks with Zt = 5 and Zn = 1 (class Ia). Flexible α,ω-alkane-dicarboxylates without double bonds produced 4-fold interpenetrated frameworks, and α,ω-alkene-dicarboxylates with double bonds gave 5-fold interpenetrated frameworks. Apparently, the interpenetration of each MOF is greatly dependent on subtle variations in the bridging ligand. The ZnII ions in 2, 3, 5, and 6 assume a tetrahedral geometry comprised of two carboxylate oxygen atoms and two bpa nitrogens. 1 and 4 are formulated as [{Zn(H2O)(O2CRCO2)}2(μ-bpa)] containing coordinated water ligands. 1 has one-dimensional (1-D) channels, with a structure indicative of a seven-connected uninodal net with a Schläfli symbol of 36·46·59.25 The ZnII ion in 1 took an octahedral geometry constructed by four carboxylate oxygen atoms, one oxygen atom from water, and one bpa nitrogen atom. In 4, both glutarate and bpa ligands bridge ZnII ions to form 2-D layers. The ZnII ion in 4 is pentacoordinate, bound by two oxygen atoms from a chelating glutarate, one oxygen atom from a monodentate glutarate, one bpa nitrogen atom, and one oxygen atom from water. The coordinated water ligand in both 1 and 4 provided structures different than the diamond-like structures of 2, 3, 5, and 6. The coordination modes of carboxylates are monodentate for 2, 3, 5, and 6; both chelating and monodentate for 4; and bridging for 1. Thus, coordination geometry and coordination mode also play important roles in structure construction. Despite the small void volumes of 5 and 6 due to interpenetration, both structures showed good CO2 sorption at 196 K, which will be described next section. CO2 Sorption. Because porous MOFs are promising materials for selective sorption of CO2 over N2, H2, and CH4 due to the high quadrupolar moment of CO2 and strong interactions between CO2 molecules and frameworks,2,38−40 the gas sorption capabilities of 5 and 6 were investigated volumetrically using several gases at varying temperatures as depicted in Figure 6. Neither solvent-free 5 nor 6 adsorbed significant amounts of N2 at 77 and 196 K as shown in Figure 6a,b. The evacuated 5 sorbed 13.8 cm3 g−1 and ∼0 cm3 g−1 at 77 and 196 K, respectively, while the respective uptake values of N2 for the evacuated 6 were 35.0 cm3 g−1 (77 K) and 2.3 cm3 g−1 (196 K). Interestingly, however both compounds exhibited meaningful sorption of CO2 as depicted in Figure 6c,d. Among the aforementioned coordination polymers, only 5 and 6 exhibited significant CO2 adsorption. The evacuated sample of 5 adsorbed 79.9 cm3 g−1 (3.56 mmol g−1, 15.7 wt %), 41.1 cm3 g−1 (1.84 mmol g−1, 8.1 wt %), and 35.2 cm3 g−1 (1.57 mmol g−1, 6.9 wt %) of CO2 at 196, 273, and 298 K, respectively. The evacuated sample of 6 adsorbed 81.5 cm3 g−1 (3.60 mmol g−1, 16.0 wt %), 16.9 cm3 g−1 (0.75 mmol g−1, 3.3 wt %), and 10.69 cm3 g−1 (0.48 mmol g−1, 2.1 wt %) of CO2 at 196, 273, and 298 K, respectively. A comparison of uptake capacities at 196 K indicated little difference between 5 and 6. At higher adsorption temperatures (273 and 298 K), 5 exhibited much greater uptake than 6. Structurally speaking, 6 contains a more rigid dicarboxylate ligand than 5, and the framework of 6 is 5-fold interpenetrated, whereas 5 is 4-fold interpenetrated. These

130.3(3) Oglut−Zn−Oglut 83.19(14)−123.8(2) 105.04(7)

104.3(2), 106.1(2)

98.86(16) 103.5(3) 102.21(7)

96.26(17), 96.41(18)

Article

See ref 25.

Oglut−Zn-Owater 92.97(18)−122.71(1) a

Zn−N (Å) N−Zn− N (°) O−Zn− O (°)

Zn−Ofuma 2.002(6), 2.036(6)

Zn−Oglut 1.954(4)−2.012(4) (asymm 2.418(4), 2.469(4)) Zn−Owater 1.973(4) 2.048(4), 2.056(4)

2.060(3) 2.017(7), 2.060(7) 2.0369(17)−2.0653(17) Zn−Owater 2.1113(18) 2.117(6)

2.037(4)−2.056(5)

Zn−Omuco 1.918(4)−1.967(4) Zn−Oadip 1.905(3) Zn−Osucc 1.9585(15)−1.9882(15) Zn−Omalo 2.1037(18)−2.1329(18) Zn−O (Å)

Table 2. Selected Bond Distances and Angles for Compounds 1−6

3

4

5 2 1a

6

Crystal Growth & Design

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Figure 2. (a) Crystal structure of 3. Inset is the coordination environment around a ZnII ion. (b) The 5-fold interpenetrated 3-D frameworks are shown in different colors. All hydrogen atoms and solvent molecules were omitted for clarity.

Figure 3. (a) Crystal structure of 5 along the a-axis. Inset is the coordination environment around a ZnII ion. Symmetry operations: (i) x, 0.5 − y, 0.5 − z. (b) The 4-fold interpenetrated 3-D frameworks are shown in different colors. All hydrogen atoms and solvent molecules were omitted for clarity.

compounds38,39 or electrostatic attractions between CO2 molecules.40 The former is related to the structural transition of the framework from an activated form to another form during CO2 uptake. The latter is the case when CO2 molecules, preadsorbed in a low CO2 dosage regime, induce easier adsorption of incoming CO2 molecules at higher CO2 pressure as a result of attractive interactions. The result, in both cases, is the sudden increase of CO2 adsorption volume resulting in an S-shape isotherm. Interestingly, both 5 and 6 exhibited hysteretic behaviors in adsorption and desorption, especially at 196 K. In order to quantitatively estimate the zero-coverage heats of CO2 adsorption for 5 and 6, isosteric heats of adsorption (Qst) using Clausius−Clapeyron equation were obtained based on the adsorption data acquired at 196 and 273 K (Figure 7).24 5

structural differences clearly have a greater effect on CO2 sorption capacity at higher temperatures than at lower temperatures. For the practical CO2 capture application, the separation of CO2 out of an industrial flue gas is usually operated under ambient conditions. Thus, the CO2 uptake capacity of 5 at 298 K was compared with other standard high surface MOFs including Co-MOF-74 (127.0 cm3 g−1), Cr-MIL101 (21.4 cm3 g−1 at 319 K), HKUST-1 (93.9 cm3 g−1), MOF5 (20.4 cm3 g−1), and MOF-177 (33.2 cm3 g−1).2 Therefore, 5 is a superior CO2 sorbent to MOF-5 and MOF-177, but it is an inferior sorbent to Co-MOF-74, Cr-MIL-101, and HKUST-1. Both compounds exhibited an S-shape CO2 adsorption isotherm at 196 K. Despite the small amount of CO2 adsorption, 6 also displayed an S-shape isotherm at 273 K. This S-shape may result from structural changes in the E

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Figure 4. (a) Crystal structure of 6 along the a-axis. Inset is the coordination environment around a ZnII ion. (b) The 5-fold interpenetrated 3-D frameworks are shown in different colors. All hydrogen atoms and solvent molecules were omitted for clarity.

Figure 5. (a) Crystal structure of 4 along the a-axis. Inset is the coordination environment around a ZnII ion. Symmetry operations: (i) 1 − x, 0.5 + y, 2.5 − z. (b) packing diagram of 2-D layers are shown in different colors. All hydrogen atoms were omitted for clarity.

and 6 exhibited 23.1 kJ mol−1 and 30.7 kJ mol−1, respectively. Although the Qst for 6 is larger than that of 5, both values fall in the usual range of Qst for MOFs.2 Interestingly, the Qst values for 5 gradually increase upon increase of CO2 sorption volume, while those for 6 initially decrease, then increase slightly, and finally decrease. Despite the same level of CO2 uptake capacities for 5 and 6 at 196 K, the different behaviors of Qst could be attributed to the differences in ligand flexibility at higher temperature. This implies that different types of pillar ligands induce distinct CO2 sorption affinity.

Structural Transformation of 5. The choice of the flexible adipate ligand for 5 strongly induced selectivity of CO2 over N2 at low temperature and pressure. Therefore, the possible transformations of structure of 5 during gas sorption were investigated. The PXRD pattern of 5 significantly changed during the gas-adsorption activation process at 120 °C (Figure 8), indicating a structural transformation in the framework caused by dehydration. To study the effect of dehydration on the framework, variable temperature PXRD patterns were collected (Figure 9). 5 was heated for 10 h at each temperature to remove solvated water. Above 100 °C, the structure of 5 was F

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Table 3. α,ω-Alkane (or Alkene)-dicarboxylates and the Related Structures of Zn-MOFs

a

Figure 8. PXRD patterns of 5 before and after CO2 gas sorption analysis. Simulated pattern from X-ray structure (a), as-prepared sample (b), and sample after gas sorption analysis (c).

See ref 25.

Figure 9. Variable temperature PXRD patterns of 5. As-prepared sample (a), samples dried at 50 °C (b), 60 °C (c), 70 °C (d), 80 °C (e), 100 °C (f), and 110 °C (g).

transformed into a new structure with a triclinic unit cell (a = 11.85(2), b = 9.010(6), c = 14.50(1) Å, α = 96.50(8), β = 106.16(9), γ = 96.37(7)°, V = 1460.7 Å3), as determined by DICVOL91.41 These variable temperature PXRD patterns indicate a flexible framework, as predicted by CO2 sorption experiments. These results indicate that the flexible η1coordinated adipate ligand and the flexibility of the tetrahedral coordination geometry of ZnII ions may contribute to the structural transformation observed upon dehydration. Photoluminescence. Recently, luminescent compounds, especially those with d10 metals, have been the focus of intense interest as they have potential in a variety of applications, including chemical sensors, photochemistry, and electroluminescence displays.42−44 Therefore, the emission spectra of ZnII complexes 2−6, together with their corresponding acids and bpa, were measured in the solid state at room temperature (Figure 10). Measurements of emission maxima and excitations for compounds 2−6, acids and bpa ligand are listed in Table 4. Compared with the free acids and ligand, the emission bands of complexes 2, 4, and 5 are blue-shifted, indicating that the emission peaks of 2, 4, and 5 might be due to a mixture characteristics of intraligand and ligand-to-ligand charge transitions (LLCT).45−48 According to the literature, the ZnII ion is difficult to oxidize or reduce due to its d10 configuration. Therefore, the emissions of 2, 4, and 5 are neither metal-toligand charge transfer (MLCT) nor ligand-to-metal charge transfer (LMCT) in nature. Complex 4 showed a particularly

Figure 6. N2 adsorption−desorption isotherms measured at 77 and 196 K for 5 (a) and 6 (b). CO2 adsorption−desorption isotherms measured at 196 K, 273 K, and 298 K for 5 (c) and 6 (d). Solid and open symbols indicate adsorption and desorption isotherms, respectively.

Figure 7. CO2 uptake dependence of isosteric heats of CO2 adsorption for 5 and 6.

G

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CONCLUSION Five flexible α,ω-alkane-(or alkene)-dicarboxylates (malonate (1),25 succinate (2), fumarate (3), adipate (5), and muconate (6)) produced 3-D ZnII frameworks with bpa pillars, and glutarate gave a 2-D ZnII structure (4). All of the compounds were structurally characterized. Structures 2, 3, 5, and 6 are 4or 5-fold interpenetrated networks, and 1 has 1-D channels. Only 5 and 6 display good CO2 sorption abilities at 196 K, with each displaying an S-shape isotherm. These characteristic Sshape isotherms are attributable to the flexible nature of the frameworks upon desolvation. Depending on the types of carboxylate bridging ligands, the CO2 uptake values at higher temperature showed significant differences. Both compounds exhibit selective CO2 adsorption over N2 at low temperature. A photoluminescence study showed that the emissions of 2, 4, and 5 were observed at 351−366 nm (λex = 322 nm) for 2, 358 nm (λex = 328 nm) for 4, and 370 nm (λex = 300 nm) for 5. The strong emission observed for 4 indicates that it may be a good candidate for research on luminescent materials.

Figure 10. Solid-state photoluminescence spectra of complexes 2−6, free acids, and bpa at room temperature.

Table 4. Emission and Excitation Data for Complexes 2−6, Acids, and Free bpa at Room Temperature compound

excitation (λ/nm)

emission (λmax/nm)

2 3 4 5 6 succinic acid fumaric acid glutaric acid adipic acid muconic acid free bpa

322 300 328 300 300 322 300 299 300 288 287

351−366 447−463 328 370 447−463 447−463 447−463 447−463 447−463 447−463 447−463

Article

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ASSOCIATED CONTENT

S Supporting Information *

TGA profiles and PXRD patterns for 2−6. This information is available free of charge via the Internet at http://pubs.acs.org/.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (C.K.). *E-mail: [email protected] (Y.K.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Financial support from Converging Research Center Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education, Science, and Technology (2012001725, 2011K000675, and 2012008875), the Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education, Science, and Technology (20110008018), and RPGrant 2013 of Ewha Womans University is gratefully acknowledged.

intense emission compared with those of complexes 2 and 5 at room temperature. The intense fluorescence may be attributed to ligand chelation to the metal center, which effectively increases the rigidity of the coordination polymer and then reduces the dissipation of energy through radiationless decay.49−52 These results suggest that 4 may be a good candidate for exploration as a hybrid inorganic−organic photoactive material. Thermogravimetric Analysis. To study the thermal stabilities of these new MOFs, thermogravimetric analyses (TGA) of 2−6 were performed. For 2 (Figure S6, Supporting Information), the 10.75% weight loss in the range of 25−89 °C may be attributed to the removal of three solvated water molecules (calcd. 12.88%). The TGA curve of 3 (Figure S7, Supporting Information) showed an observed weight loss of 4.63% in the temperature range of 25−333 °C corresponding to the removal of one solvated water molecule (calcd. 4.72%). The TGA curve for 4 (Figure S8, Supporting Information) displayed a weight loss of 5.88% from 73 to 130 °C, which is attributed to the loss of two water molecules (calcd. 5.90%). TGA profile of 5 (Figure S9, Supporting Information) indicated a loss of one water molecule between 30 and 92 °C, as the observed weight loss (4.38%) was identical to the calculated value (calcd. 4.38%). 6 (Figure S10, Supporting Information) exhibited 8.46% weight loss within the temperature range of 32−260 °C, which may be attributed to the removal of four water molecules (calcd. 8.52%).

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REFERENCES

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