Effect of Solvent Molecule in Pore for Flexible Porous Coordination

Article pubs.acs.org/IC

Effect of Solvent Molecule in Pore for Flexible Porous Coordination Polymer upon Gas Adsorption and Iodine Encapsulation Mürsel Arıcı,*,† Okan Zafer Yeşilel,† Murat Taş,‡ and Hakan Demiral§ †

Department of Chemistry, Faculty of Arts and Sciences, and §Department of Chemical Engineering, Faculty of Engineering and Architecture, Eskişehir Osmangazi University, 26480 Eskişehir, Turkey ‡ Department of Science Education, Education Faculty, Ondokuz Mayıs University, 55139 Samsun, Turkey S Supporting Information *

ABSTRACT: Four new Zn(II)-coordination polymers, namely, [Zn2(μ6-ao2btc)(μ-obix)2]n (1), [Zn2(μ4-ao2btc)(μ-obix)2]n (2), [Zn2(μ4-ao2btc)(μ-mbix)2]n (3), and {[Zn2(μ4-ao2btc)(μ-pbix)2]·2DMF·8H2O}n (4), where ao2btc = dioxygenated form of 3,3′,5,5′-azobenzenetetracarboxylate and obix, mbix, and pbix = 1,2-, 1,3-, and 1,4-bis(imidazol-1-ylmethyl)benzene, have been synthesized with azobenzenetetracarboxylic acid and isomeric bis(imidazole) ligands and characterized by elemental analyses, IR spectra, single-crystal X-ray diffraction, powder X-ray diffraction, and thermal analyses. X-ray results showed that 1, 2, and 4 had two-dimensional structures with 3,4L13 topology, while 3 was a three-dimensional coordination polymer with bbf topology. For 4, two types of activation strategies, solvent exchange + heating (which produced 4a) and direct heating (which produced 4b), were used to investigate the effect of a guest molecule in a flexible framework. Gas adsorption and iodine encapsulation properties of activated complexes were studied. The CO2 uptake capacities for 4a and 4b were 3.62% and 9.50%, respectively, and Langmuir surface areas calculated from CO2 isotherms were 167.4 and 350.7 m2/g, respectively. Moreover, 4b exhibited 19.65% and 15.27% iodine uptake in vapor phase and cyclohexane solution, respectively, which corresponded to 1.47 and 0.97 molecules of iodine/formula unit, respectively. Moreover, photoluminescence properties of the complexes were studied. have been used in many applications such as γ emitter (129I), cancer treatment (131I), and electrical conductivity.6,23,26 Despite their beneficial effects, iodine isotopes are dangerous for human health.22,27−29 In the synthesis of coordination polymers, rigid di-, tri-, or tetracarboxylate ligands have been widely utilized to obtain high-dimensional porous structures. In this study, 3,3′,5,5′azobenzenetetracarboxylate linker, which was used as an anionic ligand, was easily oxidized to generate an azoxy structure in the reaction medium.30,31 It can adopt diverse coordination modes to generate high-dimensional architectures.8,32 Recently, our group has synthesized mixed ligand coordination polymers using carboxylates and N-donor coligands.31,33 We used flexible bis(imidazole) derivatives as N-donor ligands because they display diverse conformations and connect to metal ions easily. This is because imidazole rings can freely rotate around the flexible -CH2- groups to connect to metal ions.34

1. INTRODUCTION During recent decades, there has been continued interest in the synthesis of porous coordination polymers, due to not only their intriguing topology but also their application fields such as gas storage/separation, catalysis, luminescence, sensor, drug delivery, and iodine encapsulation.1−13 Especially, flexible porous coordination polymers have attracted attention due to hysteretic gas adsorption behaviors.14−16 A lot of literature showed breathing or gate-opening behavior of flexible structures in the gas adsorption process with structural transformation.15,17−20 However, to the best of our knowledge, studies related to adsorption properties of flexible coordination polymers, using guest molecules in pores, are very rare.14 As known, porous coordination polymers usually contain guest (solvent) molecules in their pores. After activation, flexible structures can collapse or their pores may be closed or contracted.18 Hence, activation plays a key role in the gas adsorption process of a flexible structure. In this study, the influence of solvent in the pores for flexible structures upon gas adsorption was investigated. Moreover, iodine encapsulation has been widely studied with porous coordination polymers.6,7,9,21−25 Iodine and its isotopes © XXXX American Chemical Society

Received: August 15, 2015

A

DOI: 10.1021/acs.inorgchem.5b01869 Inorg. Chem. XXXX, XXX, XXX−XXX

Article

Inorganic Chemistry Table 1. Crystal Data and Structure Refinement Parameters for 1−4 empirical formula formula wt crystal system space group a, Å b, Å c, Å α, deg β, deg γ, deg V, Å3 Z Dc, g·cm−3 μ, mm−1 θ range, deg measd reflns indep reflns Rint S R1 wR2 Δρmax, e·Å−3 Δρmin, e·Å−3

1

2

3

4

C22H17N5O5Zn 496.78 monoclinic P21/c 9.9411(3) 15.7636(4) 14.3626(5) 90.00 111.548(2 90.00 2093.42(11) 4 1.576 1.22 2.3−33.4 29 211 4341 0.037 1.09 0.047 0.133 1.57 −0.96

C22H17N5O5Zn 496.78 triclinic P1̅ 8.5654(1) 10.1805(1) 12.9245(1) 76.03 88.046(1) 77.77 1068.69(2) 2 1.544 1.20 2.3−33.4 30 940 8241 0.023 1.08 0.073 0.236 5.32 −4.64

C22H17N5O5Zn 496.78 monoclinic P21/c 14.2505(6) 8.1972(3) 18.9816(7) 90.00 97.214(2) 90.00 2199.77(15) 4 1.500 1.16 2.7−33.2 21 951 4481 0.022 1.07 0.037 0.102 0.49 −0.84

C22H17N5O4Zn 480.77 orthorhombic Cmmm 14.639(2) 21.831(2) 9.791(1) 90.00 90.00 90.00 3129.1(6) 4 1.021 1.32 2.7−19.8 19 418 1232 1.02 0.103 0.303 0.47 −0.38

OLEX2 graphical user interface.37,38 For all compounds, anisotropic thermal parameters were refined for non-hydrogen atoms, and hydrogen atoms were calculated and refined with a riding model. Mercury program was used to draw crystal structures.39 Topological analyses were performed with ToposPro software.40 Sorption isotherms for CO2 and N2 were recorded with a Quantachrome Autosorb 1-C device at 273 and 77 K, respectively. 2.1. Synthesis of [Zn2(μ6-ao2btc)(μ-obix)2]n (1). A mixture of H4abtc (0.100 g, 0.279 mmol), Zn(NO3)2·6H2O (0.166 g, 0.558 mmol), and 1,2-bis(imidazol-1-ylmethyl)benzene (obix) ligand (0.067 g, 0.279 mmol) was stirred at 60 °C in the mixture of DMF/EtOH/ H2O (5:3:1 v/v/v) for 30 min. After 30 min, HNO3 (5 M, 0.17 mL) was added dropwise to the mixture until a clear solution was obtained. Then the clear solution was placed in a Pyrex tube and heated at 100 °C for 48 h. Orange crystals were obtained after 2 days. Yield: 0.103 g, 74% (based on H4abtc). Anal. Calcd for C22H17N5O5Zn: C, 52.86; H, 3.28; N, 14.15. Found: C, 53.19; H, 3.45; N, 14.10. IR (KBr, cm−1) 3126 (w), 2961 (w), 1624 (vs), 1575 (s), 1523 (m), 1483 (w), 1406 (s), 1363 (s), 1101 (m), 781 (m), 713 (m). 2.2. Synthesis of [Zn2(μ4-ao2btc)(μ-obix)2]n (2). The synthetic procedure for 2 is similar to that for 1. However, only the mixture of DMF/H2O (10:2 v/v) was used in the synthesis of 2. Yellow crystals of 2 were obtained after 1 day. Yield: 0.115 g, 83% (based on H4abtc). Anal. Calcd for C22H17N5O5Zn: C, 52.74; H, 3.80; N, 14.36. Found: C, 53.19; H, 3.45; N, 14.10. IR (KBr, cm−1) 3132 (m), 2956 (w), 1631 (vs), 1570 (s), 1527 (m), 1438 (m), 1341 (vs), 1097 (m), 790 (m), 733 (s). 2.3. Synthesis of [Zn2(μ4-ao2btc)(μ-mbix)2]n (3). The procedure for the synthesis of 3 was similar to that used for 1, except 1,3bis(imidazol-1-ylmethyl)benzene (mbix) (0.067 g, 0.279 mmol) was used instead of obix. After 2 days, yellow crystals were obtained. Yield: 0.030 g, 21% (based on H4abtc). Anal. Calcd for C22H17N5O5Zn: C, 52.85; H, 3.42; N, 13.55. Found: C, 53.19; H, 3.45; N, 14.10. IR (KBr, cm−1) 3117 (m), 2926 (w), 1630 (vs), 1575 (m), 1442 (m), 1384 (s), 1338 (s), 1087 (m), 779 (m), 729 (m). 2.4. Synthesis of {[Zn2(μ4-ao2btc)(μ-pbix)2]·2DMF·8H2O}n (4). A mixture of H4abtc (0.100 g, 0.279 mmol), ZnCl2 (0.076 g, 0.558 mmol), and 1,4-bis(imidazol-1-ylmethyl)benzene (pbix) ligand (0.067 g, 0.279 mmol) was stirred at 60 °C in the mixture of DMF/H2O (10:2 v/v) for 30 min, and then aqueous HNO3 (5 M, 0.22 mL)

In this work, four zinc(II) coordination polymers, namely, [Zn2(μ6-ao2btc)(μ-obix)2]n (1), [Zn2(μ4-ao2btc)(μ-obix)2]n (2), [Zn2(μ4-ao2btc)(μ-mbix)2]n (3), and {[Zn2(μ4-ao2btc)(μpbix)2]·2DMF·8H2O}n (4), were synthesized with 3,3′,5,5′azobenzenetetracarboxylic acid and isomeric flexible bis(imidazole) derivative ligands and characterized by elemental analyses, IR spectra, powder X-ray diffraction (PXRD), singlecrystal X-ray diffraction, and thermal analysis techniques [thermogravimetry (TG) and differential thermal analysis (DTA)]. For 4, two different activations were carried out to determine the effect of guest molecules in pores of flexible structures; their Brunauer−Emmett−Teller (BET) surface areas and CO2 and iodine uptake properties were also determined.

2. MATERIALS AND PHYSICAL MEASUREMENTS All chemicals were commercially available and were used without further purification. 3,3′,5,5′-Azobenzenetetracarboxylate (H4abtc)35 and bis(imidazole)36 ligands were synthesized according to literature procedures. PerkinElmer 2400C elemental analyzer was used for elemental analyses (C, H, and N). IR spectra were recorded on a Bruker Tensor 27 Fourier transform infrared (FT-IR) spectrometer in the range 400−4000 cm−1 by use of KBr pellets. UV−vis spectra were recorded on a T80 UV/vis spectrometer. Thermal analyses were performed on a PerkinElmer Diamond TG/DTA thermal analyzer in the static air atmosphere in the temperature range 30−700 °C with a heating rate of 10 °C/min. Photoluminescence spectra for solid samples were recorded on a PerkinElmer LS-55 spectrophotometer. PXRD patterns were obtained from 5 to 50° 2θ at a rate of 5°/min on a Rikagu Smartlab X-ray diffractometer operating at 40 kV and 30 mA with Cu Kα radiation (λ = 1.5406 nm). Energy-dispersive X-ray (EDX) analysis was carried out with Jeol JEM-1220 Electron Microscope. Single-crystal X-ray diffraction measurements for 1−4 were carried out on a Bruker APEX-II charge-coupled device (CCD) diffractometer with Mo Kα (0.710 73 Å) radiation at 293 K and an Agilent SuperNova diffractometer with a Cu Kα X-ray source at 293 K. Structures were solved by SHELXS and refined by the full-matrix leastsquares method on all data, by use of SHELXL in conjunction with the B


Article

Inorganic Chemistry

Figure 1. (a) Molecular structure of 1 showing the atom numbering scheme. (i) −x + 1, −y + 1, −z + 1; (iii) −x, −y + 1, −z + 1; (v) x + 1, y, z. (b) 2D structure in 1 (obix ligand was omitted for clarity). (c) 3D supramolecular structure. (d) Schematic representation of 3,4-connected binodal net with -ABA- stacking. solution was dropped into the mixture to produce a clear solution. Then the clear solution was placed in a Pyrex tube and heated at 100 °C. Orange crystals were obtained after 6 h. Yield: 0.124 g, 89% (based on H4abtc). The abtc and pbix ligands are highly disordered in 4. The formula of 4 can be given as {[Zn2(μ4-ao2btc)(μ-pbix)2]·2DMF· 8H2O}n, which was obtained from elemental and thermal analyses and EDX spectrum. Anal. Calcd for C25H32N6O10Zn: C, 46.78; H, 5.02; N, 13.09. Found: C, 46.20; H, 5.07; N, 13.59. EDX results: Zn, 9.56; C, 45.70; N, 14.23; O, 25.39. IR (KBr, cm−1) 3440 (s), 3130 (m), 2929 (w), 1666 (vs), 1625 (vs), 1571 (s), 1525 (m), 1489 (m), 1384 (s), 1356 (s), 1097 (s), 727 (m).

H4abtc. Asymmetric vibrations are observed between at 1631 and 1624 cm−1 for 1−4. Symmetric stretching vibrations of carboxylate groups are also observed at 1363−1338 cm−1. For 4, the sharp strong band at 1666 cm−1 is due to ν(CO) stretching vibration of DMF solvent molecule. Crystal data and refinement details of complexes are given in Table 1. Selected bond distances and angles and hydrogenbond geometries are listed in Tables S1−S4. Crystallographic data for the structural analysis have been deposited with the Cambridge Crystallographic Data Centre, CCDC nos. 1037888−1037890 and 1413764 for 1−4, respectively. Copies of this information may be obtained free of charge from the Director, CCDC, 12 Union Road, Cambridge CB2 1EZ, U.K. (fax + 44-1223-336033; e-mail [email protected]; www:http://www.ccdc.cam.ac.uk). 3.1.1. Structure of [Zn2(μ6-ao2btc)(μ-obix)2]n (1). The crystal structure of 1 with atom numbering scheme is shown in Figure 1a. X-ray analysis demonstrates that 1 crystallizes in the monoclinic system with space group P21/c. The asymmetric unit of 1 consists of a Zn(II) ion, one obix ligand, and half an ao2btc ligand (ao2btc = dioxygenated form of 3,3′,5,5′azobenzenetetracarboxylate). As seen in Figure 1a, the Zn(II) ion is five-coordinated, with a distorted trigonal bipyramidal geometry (τ = 0.667),41 by three O atoms from three different ao2btc ligands and two N atoms from two different obix ligands. Each ao2btc ligand acts as a hexadentate ligand: 3,3′-carboxyl groups display a monodentate mode and 5,5′-carboxyl groups display a bis(monodentate) bridging mode to connect to six

3. RESULTS AND DISCUSSION 3.1. Synthesis and Characterization. All four coordination polymers were synthesized from three isomers of bis(imidazol-1-ylmethyl)benzene under similar conditions. In the syntheses of 1 and 2, although the same components were used, two different structures were obtained depending on the solvents. Synthesized complexes were characterized by IR spectra, elemental analysis, and single-crystal X-ray diffraction. Elemental analysis results agreed with single-crystal X-ray results. In the IR spectra of complexes, the bands observed between 3132 and 3117 cm−1 are attributed to aromatic ν(C− H) stretching. Weak aliphatic ν(C−H) stretching vibrations for 1−4 are observed in the range 2961−2926 cm−1. The strong asymmetric and symmetric stretching vibrations corresponding to carboxylate groups of H4abtc observed at 1710 and 1278 cm−1, respectively, disappeared after conversion to 1−4. This indicates the complete deprotonation of carboxylate groups of C


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Inorganic Chemistry

Figure 2. (a) Molecular structure of 2 showing the atom numbering scheme. (i) x, y − 1, z; (ii) −x + 2, −y, −z. (b) 2D structure in 2 (obix ligand was omitted for clarity). (c) 3D supramolecular structure. (d) Schematic representation of 3,4-connected 3,4L13 net with -AAA- stacking.

where (i) = 2 − x, 1 − y, −z and (ii) = 2 − x, −y, −z] (Figure 2b). In 2, the obix ligand displays a trans conformation, in contrast to obix in 1. As seen in Figure 2c, there are 1D tubular channels (size 3.418 × 9.102 Å2) with obix ligands serving as walls in 2D networks. The 2D layered structures are stacked in an -AAA- fashion. According to PLATON analysis, the complex has voids of 14.3 Å3, which represents 1.3% per unit cell volume.42 Complex 2 has 3,4-connected 3,4L13 topological structure like complex 1 (Figure 2d). 3.1.3. Structure of Zn2(μ4-ao2btc)(μ-mbix)2]n (3). The crystal structure of 3 with atom numbering scheme is shown in Figure 3a. Complex 3 crystallizes in the monoclinic system with space group P21/c. A 3D coordination polymer was obtained when mbix ligand replaced the obix used in syntheses of 1 and 2 (Figure 3b). The asymmetric unit of 3 contains one Zn(II) ion, one mbix ligand, and half an ao2btc ligand. The Zn(II) ion is four-coordinated, with a distorted tetrahedral geometry, by two O atoms from two different ao2abtc ligands and two N atoms from two different mbix ligands. Zn(II) ions are bridged by mbix ligands to form helical 1D chains (Figure 3d). Moreover, as seen in Figure 3b, 3,3′- and 5,5′-carboxylate oxygen atoms of ao2abtc ligand are coordinated to four Zn(II) ions to form undulated 2D networks. The mbix ligands link the neighboring chains into 2D structures to form 3D coordination polymers with 4,4-connected bbf topology (Figure 3b,c). According to PLATON analysis, the total potential solvent volume is 56.3 Å3, which indicates 2.6% of the total cell volume (2199.8 Å3).42

zinc centers. Two Zn(II) ions are bridged by 5,5′-carboxyl groups of ao2btc ligand with Zn···Zn distance of 3.9072(5) Å to form bimetallic subunits. These subunits are stabilized by the coordination of obix ligand displaying cis conformation. Bimetallic subunits linked by oxygen atoms of 3,3′-carboxyl groups of ao2btc generate two-dimensional (2D) structures (Figure 1b). In the 2D structure, both 16-member and 26member rings occur. The 2D structures stack in an -ABAfashion to form a three-dimensional (3D) supramolecular structure (Figure 1c,d). Moreover, topological analysis demonstrated that 1 exhibited a 3,4-connected binodal net with point symbol {4.62}2{42.62.82} (Figure 1d). 3.1.2. Structure of [Zn2(μ4-ao2btc)(μ-obix)2]n (2). Although 2 contained the same components as 1, it was obtained in a different solvent medium. For 2, the mixture DMF/H2O as solvent was used instead of DMF/H2O/EtOH. Thus, in 2, ao2btc ligand successively coordinated to Zn(II) ions with a different coordination mode. As seen in Figure 2a, there is one Zn(II) ion, one obix ligand, and half an ao2btc ligand in the asymmetric unit of 2. Each Zn(II) ion adopts a distorted [ZnO2N2] tetrahedral geometry by coordinating to two O atoms from two different ao2btc ligands and two N atoms from two different obix ligands. Zn(II) ions are bridged by 3,3′- and 5,5′-carboxylate oxygen atoms of ao2btc ligand as monodentate to form a onedimensional (1D) structure, which is further coordinated to obix ligand in trans conformations to generate 2D networks [Zn1···Zn1i = 6.2540(3) versus Zn1···Zn1ii = 7.5434(5) Å, D


Article

Inorganic Chemistry

Figure 3. (a) Molecular structure of 3 showing the atom numbering scheme. (i) x, −y + 1/2, z + 1/2; (ii) −x + 2, y + 1/2, −z + 3/2. (b) 3D framework. (c) Topologic representation of 3D 4,4-connected net. (d) 1D chain formed by obix ligands and Zn(II) ions.

Figure 4. (a) Coordination environment of Zn(II) ion in 4. (b) View of 2D tubular structure. (c) Space-filling mode of 3D supramolecular structure in 4. (d) Topological view of -ABA- stacking of 2D layers in 4.

E


Article

Inorganic Chemistry

Figure 5. TG curves and PXRD patterns of as-synthesized 4, 4a, and 4b.

3.1.4. Structure of {[Zn2(μ4-ao2btc)(μ-pbix)2]·2DMF·8H2O}n (4). The crystal structure of 4 with atom numbering scheme is shown in Figure 4a. Complex 4 crystallizes in the orthorhombic system with space group Cmmm. Zn(II) ions are bridged by 3,3′- and 5,5′-carboxylate oxygen atoms of abtc ligand in a monodentate fashion to form 1D structures, which are further coordinated to pbix ligand to generate 2D metal−organic nanotubular structures (Figure 4b). Two pbix ligands bridge two Zn(II) ions to form a Zn2(pbix)2 26-membered ring with dimensions of 10.585 × 8.432 Å2 along the c-axis. As known, metal−organic nanotubular structures are important owing to their applications in gas and solvent adsorption, ion exchange, etc., and their topological structures.43,44 The 3D supramolecular structure occurred from weak interactions (Figure 4c). The SQUEEZE routine in PLATON was applied to remove the diffraction contribution from highly disordered solvent molecules. PLATON analysis showed an accessible void volume of 3129.1 Å3, representing 36% void per unit cell volume. Complex 4 has 3,4-connected 3,4L13 topology with point symbol {4.62}(2{42.62.82} (Figure 4d). 3.2. Thermal Analysis and X-ray Powder Diffraction Results. Thermal stabilities and behaviors of complexes 1−4 were studied by thermal analysis in the temperature range 30− 700 °C in a static air atmosphere (Figure S1). As depicted in Figure S1, 1−3 are stable up to 279, 239, and 347 °C, respectively. Upon further heating, frameworks are decomposed with exothermic picks. The final residual products of 1− 3 are possible ZnO (found 15.69%, calcd 16.30% for 1; found 15.87%, calcd 16.30% for 2; found 17.70%, calcd 16.30% for 3). For 4, the weight loss of 24.2% from 30 to 250 °C corresponds to release of water and DMF molecules (calcd 22.60%). Upon further heating, the final product is possible ZnO (found 13.10%, calcd 12.56%). Moreover, TG results for 4 are consistent with EDX results. Powder X-ray diffraction (PXRD) measurements of 1−4 were carried out, confirming the phase purity of the bulk materials (Figure S2). PXRD patterns of the complexes are well-matched with simulated patterns from their single-crystal structures, indicating the phase purity of the complexes. 3.3. Effect of Guest Molecule in Pores for Gas Sorption Studies of Flexible Structures. Gas and iodine adsorption behaviors of 1−4 were studied. However, 1−3 did not show any gas and iodine adsorption. The porous structure of 4, with the one-dimensional channels, encouraged us to perform gas sorption measure-

ments. As mentioned, 4 contains water and DMF solvent molecules in the pores. Before the gas sorption measurements, 4 was activated in two different ways, solvent exchange + heating and direct heating, to investigate pore properties. For the first activation method, 4 was immersed in MeOH for 1 week (MeOH was changed every day) at room temperature to replace guest molecules, and then it was heated at 105 °C under vacuum for 1 day for full activation, producing 4a (Figure S3). For the other activation method, 4 was heated at 105 °C to remove only water molecules in pores, producing 4b. After activation, TG curves, elemental analyses, and PXRD patterns of 4a and 4b were performed; the results are given in Figure 5 and Table 2. TG and elemental analysis results for 4a Table 2. Elemental Analysis Results of Activated Complexesa

a

complex

C, %

H, %

N, %

4a 4b

52.74 (53.19) 51.94 (52.69)

3.29 (3.45) 4.26 (4.24)

14.41 (14.10) 14.38 (14.75)

Theoretical results are given in parentheses.

confirmed the removal of all solvent molecules, and TG and elemental analysis results of 4b showed that it contained only one DMF molecule in the pore. The PXRD pattern of 4a showed very poor crystallinity. This may be attributed to close or collapse of framework after activation.45 The PXRD pattern of 4b shows that the framework is robust after activation, because most of the peaks remain unchanged. N2 adsorption isotherms of 4a and 4b were recorded at 77 K and 1.0 bar (Figure S4). Total pore volumes and BET surface areas are 0.05 and 0.07 cm3/g and 25.63 and 78.25 m2/g for 4a and 4b, respectively. According to N2 BET isotherms, the complexes seem to be nonporous. This situation may be due to closing or contraction of pores after activation and pores not enough big for N2 adsorption (kinetic diameter of 3.64 Å).26 N2 adsorption results for 4a and 4b show the flexible structure of the framework. According to the obtained results, N 2 adsorption of 4b is surprisingly higher than that of 4a, although there are solvents in the pores of 4b. This observation may reflect the closing or contraction of pores after full activation. Hence, lower adsorption was observed due to the flexible structure of the complex. For 4b, containing only DMF molecules in the pores, DMF molecule acted as a column in the pore, so that pores did not close and higher adsorption was observed. F


Article

Inorganic Chemistry To further understand the flexibility of the structure and the importance of guest solvent molecules in pores upon adsorption, CO2 adsorption isotherms were recorded volumetrically at 273 K and 1.0 bar for 4a and 4b (Figure 6). The

uptake, respectively, which corresponded to 1.47 molecules of iodine per formula unit of 4b (Figure 8, Table 3).

Figure 8. TG curves of 4b, 4b@I2 (in vapor), and 4b@I2 (in solvent).

Figure 6. CO2 adsorption−desorption isotherms for 4a and 4b at 273 K.

Table 3. Elemental Analysis Results after Iodine Uptake in Vapor and Solventa

CO2 adsorption measurements of 4a and 4b exhibit typical type I isotherms at 273 K, characteristic of microporous structures. The CO2 uptake capacities for 4a and 4b are 18.43 cm3/g (3.62%) and 48.40 cm3/g (9.50%), respectively. Langmuir surface areas calculated from CO2 isotherms are 167.4 and 350.7 m2/g for 4a and 4b, respectively. Complex 4b, containing guest molecules in the pores, shows higher CO2 sorption than 4a. These results confirm the importance of solvent molecules in pores for the flexible structure. Moreover, CO2 adsorption value of 4b is better than some metal−organic frameworks (MOFs), like MOF-5 (6.2%), MOF-602 (5.0%), and SNU-15 (7.0%) at 1 bar and 273 K.26,46−49 3.4. Iodine Uptake and Release. The I2 uptake and release experiments were performed for activated derivatives of 4 both in vapor phase and in solvent. For this aim, a 100 mg sample of each complex (4a and 4b) was exposed to I2 vapor at 75 °C for 3 days in a closed container to study volatile I2 uptake in vapor phase. The iodine uptake was monitored by the naked eye via color change. After 3 days, the complexes were washed with cyclohexane to remove I2 residing on crystal surfaces. The color change of 4b indicated the presence of I2 (Figure 7),

complex 4b@I2 (in vapor) 4b@I2 (in solvent) a

molecules of iodine/ formula unit of 4b

C, %

H, %

N, %

1.475

31.63 (31.80)

2.61 (2.56)

8.86 (8.90)

0.97

36.16 (36.79)

3.07 (2.96)

10.13 (10.30)

Theoretical results are given in parentheses.

To investigate sorption behavior in solution, 100 mg portions of single crystals of 4a and 4b were immersed in 2 mL (0.1 M) of I2 cyclohexane solution. Complex 4a did not show any color change, indicating no I2 adsorption in solution again. The dark brown solutions of I2 fade slowly to very pale red for 4b after adsorption of 4b for 51 h (Figure S5). I2 capsulated sample (4b@I2 in solvent) was washed with cyclohexane to remove I2 residing on crystal surfaces. After iodine uptake in solvent, the amount of iodine uptake was calculated by thermal and elemental analyses. These results showed 15.27 and 15.08 wt % iodine uptake, respectively, which corresponded to 0.97 molecule of iodine per formula unit of 4b (Figure 8, Table 3). These overall results indicated that 4b took up the maximum amount of I2 in vapor phase. IR spectra and PXRD patterns of 4b and 4b@I2 were recorded to determine the interaction between 4b and I2 (Figure 9). IR spectra of 4b and 4b@I2 are similar, indicating host−guest interaction. In PXRD patterns, some peaks of 4b@ I2 were broadened and intensity of the peaks was changed. This situation can be attributed to loss of crystallinity of 4b after iodine encapsulation. I2 release experiments were carried out for 4b. Delivery of I2 from 4b@I2 (in both vapor and solution) immersed in EtOH was followed by naked eye (Figure 10) and by UV−vis spectra at room temperature (Figure S6). UV−vis spectrum for iodine in ethanol solution showed that λmax at 205, 290, and 355 nm increased depending on increasing I2 content. Intensity of the adsorption band at 205 nm is proportional to concentration of

Figure 7. Color change of 4b after exposure to I2 vapor at 75 °C.

while 4a did not show any adsorption for molecular I2 (color change for 4a was not observed). Probably, the pores of desolvated sample (4a) were closed after activation due to the flexible framework and the pores were not sufficiently large for I2 adsorption. After iodine uptake in vapor phase, the amount of iodine uptake was calculated by thermal and elemental analyses. These results showed 19.65 and 19.82 wt % iodine G


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Inorganic Chemistry

Figure 9. IR spectra and PXRD patterns of 4b and 4b@I2.

properties of activated structures were studied to determine the effect of solvent molecules in pores. The results showed that the pores of fully activated 4a could be closed or contracted and lower adsorption was observed, but 4b, containing DMF solvent molecules in the pores, displayed higher CO2 and iodine adsorption. The pores of 4b were not closed due to the presence of DMF molecules in the pores. Hence, guest solvent molecules in pores of flexible frameworks have important effects for gas sorption or iodine encapsulation studies. Moreover, 4b can release iodine in ethanol solution.

Figure 10. Photographs of iodine release in ethanol solution from 4b@I2.

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ASSOCIATED CONTENT

S Supporting Information *

I2. As shown in Figure S6, delivery of I2 increases with time in complex 4b@I2. UV−vis spectra demonstrate that the I2 adsorption process is reversible and there is a host−guest interaction between 4b and I2 molecules. 3.5. Photoluminescent Properties. Solid-state photoluminescence spectra of the complexes and free ligand H4abtc (conjugated π system) were recorded under the same conditions at room temperature (Figure S7). Free ligand H4abtc exhibits emissions at 410, 420, 463, 487, and 532 nm upon excitation at 344 nm. These emissions are due to π* → n or π* → π transitions of H4abtc. Emission spectra of the complexes and H4abtc are similar. Complexes 1−4 showed emissions at 409, 421, 462, 487, and 531 nm (1); at 404, 421, 464, 487, and 531 nm (2); at 407, 423, 463, 487, and 531 nm (3); and at 407, 421, 463, 487, and 531 nm (4) upon excitation at 344 nm. Emissions of the complexes are neither ligand-tometal charge transfer nor metal-to-ligand charge transfer. Zn(II) ions are difficult to reduce or oxidize due to their d10 configurations. Hence, the emissions of 1−4 can be due to intraligand transitions of H4abtc.50 When the emission intensities of 1 and 2, which have the same components, are compared, the emission intensity of 1 is higher than that of 2. The difference between emission intensities may stem from the conformations of obix ligand in the structures.

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.5b01869. Additional text, seven figures showing TG and PXRD curves and UV−vis spectra, and four tables listing bond distances and angles of 1−4 (PDF) Crystallographic data for 1 (CIF) Crystallographic data for 2 (CIF) Crystallographic data for 3 (CIF) Crystallographic data for 4 (CIF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Telephone: +90-2222-393750. Fax: +90-2222-393578. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work has been supported by The Scientific and ̇ AK) Technological Research Council of Turkey (TUBIT (Project number 113Z313).

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4. CONCLUSIONS In this study, four 2D and 3D Zn(II)-coordination polymers with azobenzenetetracarboxylate and flexible isomeric bis(imidazol-1-ylmethyl)benzene ligands were synthesized and structurally characterized. CO2 and I2 absorption properties were also studied. Promising results were obtained only for 4. Complex 4, which had a flexible porous structure, was activated in two ways: solvent exchange + heating (leading to 4a) and direct heating (leading to 4b). CO2 and iodine adsorption

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