Article pubs.acs.org/IC
Sulfonic Group Functionalized Mixed Ligand Coordination Polymers: Synthesis, Characterization, Water Sorption, and Proton Conduction Studies Dilip Kumar Maity,† Kenichi Otake,‡ Saheli Ghosh,† Hiroshi Kitagawa,*,‡ and Debajyoti Ghoshal*,† †
Department of Chemistry, Jadavpur University, Jadavpur, Kolkata, 700 032, India Division of Chemistry, Graduate School of Science, Kyoto University, Kitashirakawa-Oiwakecho, Sakyo-ku, Kyoto 606-8502, Japan
‡
S Supporting Information *
ABSTRACT: Five sulfonic acid group functionalized mixed ligand coordination polymers (CPs), namely, {[Zn(bpeH)(5-sip)(H2O)]·(H2O)}n (1), {[Cu(pyz)(5-Hsip)(H2O)2]·(H2O)2}n (2), {[Cu(bpee)0.5(5sip)(H2O)2]·(H2O)4(bpeeH2)0.5}n (3), {[Cu(bpy)(5-Hsip)(H2O)]·(H2O)2}n (4), and {[Cu(bpy)2(5H2sip)2]·(H2O)6}n (5) [where sip3− = 5-sulfoisophthalate; bpe = 4,4′-bispyridylethane; pyz = pyrazine; bpee = 4,4′-bispyridylethylene; bpy = 4,4′-bipyridine], have been synthesized with varying different N,N′donor linkers using slow diffusion techniques at room temperature. The CPs possess guest water filled 1D channels and noncoordinating sulfonic acid or coordinated sulfonate groups, which are interconnected by means of extended intermolecular H-bonding interaction, which supports the humidity dependent proton conductivity of the samples. Under 95% relative humidity (% RH), the CPs exhibit the temperature dependent proton conductivity which is maximum up to in the range of ∼10−5−10−6 S cm−1 at 65 °C. In most of the cases, the framework shows activation energies with the value ranging from 0.35 to 0.54 eV, suggesting mostly the contribution of the Grotthuss mechanism of the proton conductivity.
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INTRODUCTION The present era of civilization truly hunted to find the alternative energy sources due to the limited availability of natural energy resources. Thus, in the past decades, several intelligent approaches have been explored to get the alternative energy, and that is why it took incredible attention of contemporary researchers. In this way, the applications of fuel cells1−3 become very significant, which can improve the production of clean energy2,4 in an economically viable way.3,5−7 The proton conduction measurement for metal− organic frameworks (MOFs)1−4 or chemically stable covalent organic frameworks (COFs)8−11 through the proton exchange membrane is the most important aspect for functioning of fuel cells as well as it is the vital parameter in judging the efficiency of the fuel cell. Hence, the synthesis of materials showing proton conductivity is one of the glaring topics in modern energy research related to fuel cell. Elevated proton conductivity and expanded working temperature range are the two main features of such materials.3 Although several inorganic and organic compounds7,12 are used nowadays, many of them have certain limitations including less thermal stability, nonordered structure, cost viability, low structural tunability, etc.2,13 Like other functionalities, proton conduction should also be realized in a better way if its pathway and its mechanism are properly revealed in terms of structure−property relationship.3 Therefore, a precise and careful assortment might be required to develop the low cost and highly effective proton conducting materials that can exhibit the above-mentioned structure property relationship for proton conduction in crystalline form at solid state and can operate at elevated © XXXX American Chemical Society
temperature. Therefore, the exploration for low cost materials involving easy synthesis has resulted in materials based on metal−organic frameworks (MOFs) or porous coordination polymers (PCPs),3,13 which can afford well designed functional pore surfaces14 for proton conducting pathways and also incorporate various guest molecules as a proton conducting media, such as free organic molecules (e.g., imidazole,15 triazoles16) or water molecules17 in the void of the frameworks to make the channel to be more hydrophilic in nature. To enhance the proton conductivity of PCPs, the use of substituted organic ligands18−20 with phosphonic, carboxylic, or hydroxyl groups is very logical as these groups can align themselves into the pore surfaces of the PCPs, making the channels more acidic. With this context, here we are presenting the highly acidic sulfonic acid (−SO3H) groups based mixed ligand MOFs where the functional groups are aligned along the pore channel. It is worth mentioning that MOFs containing uncoordinated protonated sulfonic acid (−SO3H) groups are very less explored14,21,22 because the sulfonate groups prefer to present as monoanionic form for binding with metal centers. The compounds sometime contain acidic uncoordinated −SO3H groups (in few cases, −SO3− group behaving as a proton acceptor) as well as coordinated waters, which are arranged along the pore surfaces. On the other hand, the compounds also contain lattice water molecules and/or protonated N,N′donor ligand within the pores of the framework in some cases, forming hydrophilic pore channels by means of mutual Received: November 4, 2016
A
DOI: 10.1021/acs.inorgchem.6b02674 Inorg. Chem. XXXX, XXX, XXX−XXX
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Inorganic Chemistry Table 1. Data Collection and Refinement Parameters for Single-Crystal Analysis for Complexes 1−5 formula formula weight 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 F(000) θ range/deg reflections collected unique reflections reflections I > 2σ(I) Rint goodness-of-fit (F2) R1 (I > 2σ(I))a wR2(I > 2σ(I))a Δρ min, max/e Å3 a
1
2
3
4
5
C20H20N2O9SZn 529.84 triclinic P1̅ 7.861(5) 9.523(5) 15.901(5) 79.092(5) 76.637(5) 66.689(5) 1057.2(9) 2 1.664 1.318 544 2.3−27.5 18252 4850 4262 0.024 1.06 0.0286 0.0769 −0.37, 0.28
C12H14N2O10SCu 441.85 monoclinic I2/m 11.728(5) 6.853(5) 20.166(5) 90 92.328(5) 90 1619.4(14) 4 1.812 1.537 900 2.0−27.5 12386 2020 1780 0.038 1.07 0.0372 0.1018 −0.55, 1.13
C20H26N2O13SCu 598.05 triclinic P1̅ 9.272(5) 10.787(5) 13.409(5) 106.404(5) 99.515(5) 100.001(5) 1233.9(10) 2 1.593 1.040 606 1.6−27.6 19796 5564 4773 0.020 1.04 0.0455 0.1318 −0.64, 2.01
C18H18N2O10SCu 517.97 monoclinic C2/c 24.024(5) 11.135(5) 16.298(5) 90 107.004(5) 90 4169(2) 8 1.644 1.208 2104 1.8−27.5 34595 4807 4361 0.023 1.06 0.0417 0.1412 −0.51, 1.34
C36H36N4O20S2Cu 972.38 monoclinic C2/c 17.8191(4) 11.0557(4) 22.3316(7) 90 112.858(3) 90 4053.9(2) 4 1.583 0.731 1980 2.0−27.6 27710 4698 3166 0.039 1.03 0.1073 0.2782 −2.17, 2.05
R1 = ∑||Fo| − |Fc||/∑|Fo|, wR2 = [∑(w(Fo2 − Fc2)2)/∑w(Fo2)2]1/2.
65 °C under 95% RH, and their activation energy (Ea) calculated from Arrhenius plots belongs from 0.35 to 0.45, except 1, indicting the Grotthuss mechanism (Ea = 0.1−0.5 eV)20−24 supported proton conductivity. The slightly higher activation energy of 1 (i.e., Ea = 0.54 eV) indicates the Grotthuss-type proton transfer including increased contribution from the vehicle proton transfer (Ea = 0.5−0.6 eV).19,33
intermolecular H-bonding. Both of the above-mentioned phenomena are responsible individually or additively, to create an effective proton conducting medium13,23 in such compounds. It is clearly evident from the crystal structure analyses that the guest water molecules are strongly trapped by intermolecular H-bonding with −SO3−/−SO3H/−COO−/− COOH/coordinated water molecules or in between themselves, which must assist to show the proton conductivity according to the Grotthuss mechanisms.24−28 Here, the −COOH or −SO3H group of 5-sulfoisophthalic acid can easily transfer its acidic proton (H+) to the H2O molecule and forms its corresponding conjugate base and acid pairs (e.g., RO− and H3O+), respectively. Moreover, the intermediate hydronium ion (H3O+) can also stabilize the conjugate bases (e.g., −COO− or −SO3−) present in the system. Thus, the proton can easily transfer via the intermediate H3O+ ion, resulting in the formation of infinitely extended water bridges according to the Grotthuss-type proton-hopping mechanism.29,30 It is noticed that all the compounds exhibit increasing proton conductivity with the increase of relative humidity (% RH) up to 95%. Actually, under humid conditions, water molecules act as a conducting media through which proton transfers by forming extended H-bonding with the adjacent sulfonic acid/ sulfonate or carboxylic acid/carboxylate groups lining in the pore channel,31,32 which gives rise to efficient proton conduction. Herein, we report the synthesis of 5-sulfoisophthalate based five MOFs, namely, {[Zn(bpeH)(5-sip)(H2O)]·(H2O)}n (1), {[Cu(pyz)(5-Hsip)(H2O)2]·(H2O)2}n (2), {[Cu(bpee)0.5(5sip)(H2O)2]·(H2O)4(bpeeH2)0.5}n (3), {[Cu(bpy)(5-Hsip)(H2O)]·(H2O)2}n (4), and {[Cu(bpy)2(5-H2sip)2]·(H2O)6}n (5), which show both relative humidity (% RH) and temperature dependent proton conductivity linearly. They exhibit maximum conductivity from 9.9 × 10−8 to 3.7 × 10−5 at
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EXPERIMENTAL SECTION
Chemicals. The required materials, such as Cu(NO3)2·3H2O, Zn(NO3) 2·6H2O, pyrazine (pyz), 4,4′-bipyridine (bpy), 4,4′bispyridylethylene (bpee), 4,4′-bispyridylethane (bpe), and monosodium 5-sulfoisophthalic acid (5-NaH2SIP), for the synthesis of coordination polymers (CPs) were procured from Sigma-Aldrich Chemical Co., Inc. After that, 5-NaH2SIP was treated with Na2CO3 in aqueous medium to prepare the trisodium salt of monosodium 5sulfoisophthalic acid. Physical Measurements. The C, H, and N analyses were performed in a Heraeus CHNS analyzer taking the as-synthesized compounds of 1−5. The FT-IR spectra (4000−400 cm−1) were measured using a PerkinElmer made Spectrum BX-II IR spectrometer in KBr pellets of all the compounds. Thermogravimetric analysis (TGA) for all five compounds was done at the temperature range from 30 to 500 °C using a METTLER TOLEDO made TGA 850 thermal analyzer. During TGA analysis, a flow rate of nitrogen was maintained around 10 cm3/min and the furnace heating rate was set on 10 °C/ min. The bulk materials of all the compounds and the post impedance samples were characterized by powder X-ray diffraction (PXRD) study at room temperature using a Bruker D8 ADVANCE diffractometer. Sorption Measurements. The N2 adsorption isotherms (77 K) were measured for the dehydrated frameworks of 1−5 using a Quantachrome Autosorb-iQ adsorption instrument. High purity N2 gas (99.999% purity) was used for the adsorption measurements. The ambient pressure volumetric adsorptions for N2 were measured in the pressure range 0−1 bar at 77 K (maintained using liquid-nitrogen bath), taking the dehydrated samples of all the compounds. The assynthesized compounds of 1−5 (∼40 mg for each) were taken in the B
DOI: 10.1021/acs.inorgchem.6b02674 Inorg. Chem. XXXX, XXX, XXX−XXX
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Inorganic Chemistry sample tube and dehydrated at 423 K, for 4 h under a vacuum of 1 × 10−1 Pa prior to the isotherm measurements. Helium gas (99.999% purity) was introduced at a certain pressure for its diffusion into the sample tube by controlling the valve. The gas adsorption volume was calculated from the difference of pressure (Pcal − Pe), where Pcal stands for the calculated pressure without any gas adsorption, whereas Pe denotes the observed pressure at equilibrium. Besides, the H2O adsorption isotherms were carried out at 298 K within the pressure range 0−24 Torr, in its vapor state by taking the dehydrated compounds of 1−5 using the same instrument. The samples of 1−5 (∼40 mg) were activated following a similar procedure, mentioned earlier. The water was heated to generate the vapor which was degassed fully by repetitive evacuation. Here, helium gas was used to measure the dead volume. The adsorbate was taken in the sample tube, and afterward, the pressure change was monitored. Like the gas adsorption, here also the degree of adsorption was calculated by the decrease in pressure at equilibrium. All operations for gas as well as water vapor were fully automatic and microprocessor controlled. Proton Conduction Measurement. Impedance measurements for compounds 1−5 were performed using the Solartron SI 1260 Impedance/Gain-Phase Analyzer and 1296 Dielectric Interface having the frequency ranging from 0.1 to 1 × 106 Hz. An Espec Corp. SH-221 incubator was used to control the temperature as well as the relative humidity (% RH). After that, the measurements were performed after setting each condition for approximately half a day. The proton conductivities of compounds 1−5 were evaluated using compacted pellet samples of diameter 2.5 mm and thickness of 2.24, 1.31, 2.16, 1.70, 1.89, and 1.40 mm for 1−5, respectively, that were prepared by pressing the powder samples at c.a. 400 MPa for 60 s. The measurements were performed using two gold wires (50 μm ϕ) and gold paste for the trivial two probe technique. The resistance values were determined from equivalent circuit fits of the first semicircle using Z-View software. Data Collection and Refinement for Single-Crystal Analysis. The X-ray single-crystal data for 1−5 were collected at room temperature using a Bruker made APEX II diffractometer. Prior to data collection, well separated single crystals of all the compounds were mounted on the tip of glass fibers using super glue. The graphite monochromated Mo−Kα radiation (λ = 0.71073 Å) was used as the sealed tube X-ray source in the said data collection. After the data collection, the data were integrated using the SAINT34 program and the absorption corrections were carried out with SADABS.35 The structures were solved by SHELXS-201536 using the Patterson method and followed by successive Fourier and difference Fourier synthesis. The full-matrix least-squares refinements were carried out on F2 for all non-hydrogen atoms using SHELXL-201537 with anisotropic displacement parameters. The hydrogen atoms were located geometrically in appropriate positions in all cases and fixed using the HFIX command. Besides, the lattice water molecules O3W, O4W, and O5W for 3, O3W for 4, and O1W for 5, respectively, being highly thermally disordered, no hydrogen atom has been attached with them. All the calculations were carried out using SHELXS-2015,36 SHELXL-2015,37 PLATON v1.15,38 and WinGX system Ver-1.80.39 The crystallographic data collection and structural refinement parameters for the compounds 1−5 are specified in Table 1. CCDC 1514613−1514617 contain the supplementary crystallographic data for this paper. Syntheses of the CPs. {[Zn(bpeH)(5-sip)(H2O)]·(H2O)}n (1). A 1 mmol portion of trisodium 5-sulfoisophthalate (sip3−) (0.312 g) was dissolved in 15 mL of water, and the resulting aqueous solution was mixed with the 15 mL methanolic solution of 1 mmol 4,4′bispyridylethane (bpe) (0.184 g). The mixture was allowed to stir through 1 h for making the homogeneous mixture of the two ligands. Zn(NO3)2·6H2O (1 mmol, 0.297 g) was dissolved in 15 mL of water in a separate beaker. After that, 4 mL of Zn(II) solution was slowly layered with the 8 mL of the above-mentioned mixed ligand solution in a crystal tube using 3 mL of buffer solution (1:1 of H2O and MeOH), where the buffer solution behaves as the junction between the two solutions. The tube was sealed and kept undisturbed at room temperature. The white-colored block-shaped crystals suitable for Xray diffraction analysis was obtained after 4 weeks. The crystals were
separated manually and washed with methanol−water (1:1) mixture and dried under air (Yield 62%). Anal. Calc. for C20H20N2O9SZn (%): C, 45.34; H, 3.80; N, 5.29. Found: C, 45.38; H, 3.83; N, 5.25. IR spectra (in cm−1): ν(H2O), 3479; ν(CH-Ar), 3064; ν(CC), 1620− 1433; ν(SO), 1108(s) and ν(C-O), 1203. {[Cu(pyz)(5-Hsip)(H2O)2].(H2O)2}n (2). Complex 2 was prepared similarly to that of 1 using pyrazine (pyz) (1 mmol, 0.80 g) instead of 4,4′-bispyridylethane (bpe) (1 mmol, 0.184 g) and Cu(NO3)2·3H2O (1 mmol, 0.241 g) instead of Zn(NO3)2·6H2O (1 mmol, 0.297 g). The needle-shaped blue-colored crystals appropriate for single-crystal X-ray diffraction studies were obtained after 4 weeks. The crystals were separated manually and washed with aqueous-methanolic (1:1) solution; after that, they were air-dried (Yield 65%). Anal. Calc. for C12H15N2O11SCu (%): C, 31.49; H, 3.29; N, 6.10. Found: C, 31.53; H, 3.27; N, 6.15. IR spectra (in cm−1): ν(H2O), 3402; ν(CH-Ar), 3103; ν(CC), 1552−1419; ν(SO), 1112(s) and ν(C-O), 1210. {[Cu(bpee)0.5(5-sip)(H2O)2]·(H2O)4(bpeeH2)0.5}n (3). Complex 3 was prepared similarly to that of 2 with the use of 4,4′-bispyridylethylene (bpee) (1 mmol, 0.182 g) in place of pyrazine (pyz) (1 mmol, 0.80 g). The block-shaped blue-colored crystals appropriate for single-crystal X-ray diffraction studies were obtained after 1 month. The crystals were separated manually and washed with aqueous methanolic (1:1) solution; after that, they were air-dried (Yield 65%). Anal. Calc. for C20H26N2O13SCu (%): C, 40.17; H, 4.38; N, 4.68. Found: C, 40.15; H, 4.73; N, 4.65. IR spectra (in cm−1): ν(H2O), 3446; ν(CH-Ar), 3056; ν(CC), 1603−1430; ν(SO), 1104(s) and ν(C-O), 1204. {[Cu(bpy)(5-Hsip)(H2O)]·(H2O)2}n (4) and {[Cu(bpy)2(5-H2sip)2]· (H2O)6}n (5). Complexes 4 and 5 were prepared similarly to that of 2 with the use of 4,4′-bipyridine (bpy) ligand (1 mmol, 0.156 g) instead of pyrazine (pyz) (1 mmol, 0.80 g). After 1 month, two types of single crystals, e.g., cyan-colored needle-shaped crystals of 4 (Yield 40%) and block-shaped violet-colored crystals of 5 (Yield 30%), suitable for X-ray diffraction, were obtained. The crystals were separated manually and washed with aqueous-methanolic (1:1) solution; after that they were air-dried. For 4, Anal. Calc. of C18H18N2O10SCu (%): C, 41.74; H, 3.50; N, 5.41. Found: C, 41.77; H, 3.54; N, 5.38. IR spectra (in cm−1): ν(H2O), 3453; ν(CH-Ar), 3101; ν(CC), 1609−1418; ν(SO), 1117(s) and ν(C-O), 1222. For 5, Anal. Calc. of C36H36N4O20S2Cu (%): C, 44.47; H, 3.73; N, 5.76. Found: C, 44.43; H, 3.77; N, 5.73. IR spectra (in cm−1): ν(H2O), 3454; ν(CH-Ar), 3074; ν(CC), 1606−1422; ν(SO), 1107(s) and ν(C-O), 1221. The purity of the complexes was investigated by measuring solid state PXRD at room temperature, which matches with the simulated PXRD patterns as well. The bulk purity of the samples was also proved by the elemental analysis and IR spectra as well, which also corroborated the data acquired from the single crystals.
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RESULTS AND DISCUSSION
Synthesis. The addition of trisodium salt of 5-sulfoisophthalate (sip3−) with different 4-pyridyl linkers [e.g., 4,4′bispyridylethane (bpe), pyrazine (pyz), 4,4′-bispyridylethylene (bpee), 4,4′-bipyridine (bpy)], and transition M(II) ions (e.g., Cu2+ and Zn2+) in MeOH/H2O medium using slow diffusion technique at room temperature yielded five new sulfonic group functionalized CPs of 1−5 (Scheme 1), namely, {[Zn(bpeH)(5-sip)(H2O)]·(H2O)}n (1), {[Cu(pyz)(5-Hsip)(H2O)2]· (H 2 O) 2 } n (2), {[Cu(bpee) 0.5 (5-sip)(H 2 O) 2 ]·(H 2 O) 4 (bpeeH2)0.5}n (3), {[Cu(bpy)(5-Hsip)(H2O)]·(H2O)2}n (4), and {[Cu(bpy)2(5-H2sip)2]·(H2O)6}n (5). Structural Descriptions of {[Zn(bpeH)(5-sip)(H2O)]·(H2O)}n (1). The single-crystal X-ray study confirms that compound 1 was crystallized in the triclinic P1̅ space group. The analysis of structure 1 reveals that the formation of a 1D chain with a pendent monoprotonated 4,4′-bispyridylethane (bpeH) ligand. In the asymmetric unit of 1, there are one Zn2+ ion, half coordinated monoprotonated 4,4′-bispyridylethane (bpeH) C
DOI: 10.1021/acs.inorgchem.6b02674 Inorg. Chem. XXXX, XXX, XXX−XXX
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Inorganic Chemistry
with the oxygen atoms of 5-sip3−, resulting in the formation of a supramolecular 2D structure (Figure 1c and Table S2). Structural Descriptions of {[Cu(pyz)(5-Hsip)(H 2 O) 2 ]· (H2O)2}n (2). The single-crystal X-ray study confirms that compound 2 was crystallized in the monoclinic C2/m space group. The analysis of structure 2 reveals the formation of a 1D chain with pendent disodium-5-sulfoisophthalate (5-Hsip2−) and bridging of N,N′-donor pyrazine ligand between adjacent Cu(II) centers. In the asymmetric unit of 2, there are one Cu2+ ion, one pyz ligand, one disodium-5-sulfoisophthalate (5Hsip2−) containing one −CO2H group, two coordinated and two lattice water molecules. Here, the hexacoordinated Cu(II) metal center displays a distorted octahedral geometry with a CuO4N2 coordination environment (Figure 2a). The basal plane is originated by four oxygen atoms (O1, O2, O1W, and O2W) from one monochelated 5-Hsip2− and two coordinated water molecules, respectively, whereas the axial positions are furnished by two symmetry related nitrogen atoms (i.e., N1 and N1a, where a = x, 1 − y, z) from two different pyrazine ligands. The Cu−O bond lengths are 1.948(4)−2.685(5) Å, and the Cu−N bond length is 2.046(3) Å (Table S3). The 1D chain (Figure 2b) of 2 contains pendent 5-Hsip2− with protonated −CO2H groups which are further stabilized by forming intermolecular H-bonding with the guest water molecules and forms a supramolecular 2D structure (Figure 2c and Table S4). Structural Descriptions of {[Cu(bpee)0.5(5-sip)(H2O)2]· (H2O)4(bpeeH2)0.5}n (3). The single-crystal X-ray analysis reveals that compound 3 was crystallized in the triclinic P1̅ space group. The analysis of structure 3 reveals the formation of a 1D ladder constructed by bridging 5-sulfoisophthalate (5-sip3−) and 4,4′-bispyridylethylene (bpee) ligands. In the asymmetric unit of 3, there are one Cu2+ ion, half coordinated 4,4′bispyridylethylene (bpee) ligand, one trisodium-5-sulfoisophthalate (5-sip3−), and two coordinated water molecules to create the CuO4N core around the pentacoordinated Cu(II) center. In addition to that, there are four water molecules and half biprotonated 4,4′-bispyridylethylene (bpeeH2) ligand
Scheme 1. Synthetic Outline of the Complexes 1−5
ligand, one trisodium-5-sulfoisophthalate (5-sip3−), and one coordinated water and one lattice water molecule. The tetracoordinated Zn(II) metal center exhibits a square planar geometry with a ZnO3N coordination environment (Figure 1a) originated by three oxygen atoms (O1, O7a, O1W, where a = x, 1 + y, z) from two different 5-sip3− along with one coordinated water molecule and one nitrogen pyridyl atom (N1) of bpeH. The Zn−O bond lengths are 1.927(2)−2.013(2) Å, where the Zn−N bond length is 2.0181(19) Å (Table S1). In 1, the 5sip3− bridges with two adjacent Zn(II) centers through the −COO− groups in a bis-monodentate fashion, resulting in the formation of a 1D chain (Figure 1b) along the crystallographic b-axis, where the 4,4′-bispyridylethane is pendent possibly due to the protonation of one pyridyl nitrogen atom. In the 1D chain, the pendent bpeH ligands and the lattice water molecules are stabilized by forming intermolecular H-bonding
Figure 1. (a) Atom labeling scheme showing the coordination environment around the tetracoordinated Zn(II) ion in 1; Zn (green), O (red), N (blue), and C (black). (b) 1D chain with pendent bpe ligand in 1. (c) Supramolecular 2D structure showing the extended intermolecular H-bonded network along the a-axis (magenta dotted lines represent the H-bonding interaction). D
DOI: 10.1021/acs.inorgchem.6b02674 Inorg. Chem. XXXX, XXX, XXX−XXX
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Figure 2. (a) Atom labeling scheme showing the coordination environment around the hexacoordinated Cu(II) ion in 2; Cu (green), O (red), N (blue), and C (black). (b) 1D chain with pendent 5-sulfoisophthalate along b-axis in 2. (c) View of supramolecular 2D structures through the formation of extensive intermolecular H-bonded network viewed along the c-axis (magenta dotted lines represent the H-bonding interaction).
Figure 3. (a) Atom labeling scheme showing the coordination environment around the pentacoordinated Cu(II) ion in 3; Cu (green), O (red), N (blue), and C (black). (b) 1D ladder constructed through the bridging 5-sulfoisophthalate and bpee ligand in 3. (c) Supramolecular 2D structure showing the extended intermolecular H-bonded network along the a-axis. (d) Simplified topological representation of 3-c uninodal net in 3 (magenta dotted lines represent the H-bonding interaction).
adjacent Cu(II) centers by its −COO− and −SO3− groups through bis-monodentate fashion, forming a 1D chain (Figure 3b). Such two parallel metal-carboxylate chains are joined by the N,N′-donor 4,4′-bispyridylethylene (bpee) ligand, resulting in the formation of a 1D ladder along the crystallographic a-axis (Figure 3b). In the 1D ladder, the pendent −CO2− groups are stabilized by forming intermolecular H-bonding with the guest water molecules and protonated lattice 4,4′-bispyridylethylene (bpeeH2) ligand, resulting in the formation of a supramolecular
present in the lattice. The basal plane of this distorted trigonal bipyramidal geometry consists of two oxygen atoms (O1 and O3a, where a = −1 + x, y, z) of two different 5-sip3− and one pyridyl nitrogen atom (N1) of the 4,4′-bispyridylethylene (bpee) ligand, respectively, whereas the axial positions are taken by two oxygen atoms (O1W and O2W) of two coordinated water molecules. The Cu−O bond lengths are in between 1.926(2) and 2.846(3) Å, where the Cu−N bond length is 1.981(3) Å (Table S5). Here, each 5-sip3− bridges with two E
DOI: 10.1021/acs.inorgchem.6b02674 Inorg. Chem. XXXX, XXX, XXX−XXX
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Figure 4. (a) Atom labeling scheme showing the coordination environment around the pentacoordinated Cu(II) ion in 4; Cu (green), O (red), N (blue), and C (black). (b) 1D ladder with pendent 5-sulfoisophthalate along the b-axis in 4. (c) View of supramolecular 2D structure through the formation of extended intermolecular H-bonded network along the a-axis. (d) Simplified topological representation of 3-c uninodal net in 4 (magenta dotted lines represents the H-bonding interaction).
Figure 5. (a) Atom labeling scheme showing the coordination environment around the hexacoordinated Cu(II) ion in 5; Cu (green), O (red), N (blue), and C (black). (b) Diagram of a 2D sheet with pendent 5-sulfoisophthalate in 5. (c) View of supramolecular 3D structure through the formation of extended intermolecular H-bonded network along the a-axis. (d) Simplified topological representation of 4-c uninodal net in 5 (magenta dotted lines represent the H-bonding interaction).
2D structure (Figure 3c and Table S6). The TOPOS40,41 analysis of structure 3 reveals a 3-c uninodal net with corresponding Schläfli symbol for the net {42.6} (Figure 3d). Structural Descriptions of {[Cu(bpy)(5-Hsip)(H2O)]·(H2O)2}n (4). The single-crystal X-ray analysis confirms that compound 4 was crystallized in the monoclinic C2/c space group. The analysis of structure 4 indicates the formation of a 1D ladder
with pendent disodium-5-sulfoisophthalate (5-Hsip2−). In the asymmetric unit of 4, there are one Cu2+ ion, one bpy ligand, one disodium-5-sulfoisophthalate (5-Hsip2−) with one −CO2H group, and three water molecules including two lattice water molecule. The pentacoordinated Cu(II) center displays a distorted trigonal bipyramidal geometry with a CuO3N2 environment (Figure 4a), where the basal plane is originated F
DOI: 10.1021/acs.inorgchem.6b02674 Inorg. Chem. XXXX, XXX, XXX−XXX
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Inorganic Chemistry
°C maintaining the heat rate at 10 °C/min, which is depicted in Figure S11. Compound 1 exhibits the weight loss in two steps up to 6.83% at 201 °C (calcd. 6.8%) for the loss of two water molecules (Figure S11). The dehydrated framework is stable up to 288 °C without showing any weight loss; after that, it sharply decreases and collapses into an unidentified product. For compound 2, two lattice water molecules have been released before starting the measurement. The TGA curve of 2 (Figure S11) exhibits weight loss up to 7.9% at 101 °C (calcd. 7.95%) which corroborates the loss of two coordinated water molecules, and up to 225 °C, there is no further weight loss, indicating the robustness of the dehydrated framework. After that, it sharply decreases, and finally, the framework decomposes into an unidentified product. In the case of 3, one disordered lattice water molecule has been released before starting the measurement. Compound 3 looses 15.1% weight experimentally at 142 °C (calcd. 15.53%), signifying the loss of three guest water molecules. The dehydrated framework is stable up to 296 °C without showing any further loss of weight, and after that temperature, it sharply decreases and decomposes into an unidentified product. Compound 4 exhibits a weight loss of 10.65% at 147 °C (calcd. 10.43%) for loss of one coordinated and two guest water molecules, and the deaquated framework is stable up to 312 °C without showing any weight loss. After that, it sharply decreases, and finally, the framework decomposes into an unidentified product. Finally, compound 5 looses 10.84% weight at 156 °C (calcd. 11.12%) in two step processes, signifying the loss of non-hydrogen bonded and hydrogen bonded six guest water molecules, respectively. The dehydrated framework is stable from 156 to 250 °C with no weight loss. After that, it sharply decreases, and finally, the framework decomposes into an unidentified product. Sorption Study. To realize the correlation between the relative humidity (% RH) and the adsorbed amount of water molecules, we have measured the water vapor sorption isotherms at 298 K for all the compounds 1−5 (Figures 6 and S12−S16). Hence, the compounds 1−5 are preheated at 150 °C for 4 h under a 1 × 10−1 Pa vacuum. The water vapor adsorption for all the compounds except 4 exhibit three-step total water uptake profiles and reaches up to 122, 227, 225, 152, and 171 cc/g at P/Po = ∼0.9. The three-step uptake of water occurs with a small uptake at low humidity, followed by a two-
by three oxygen atoms (O1, O2a, and O1W, where a = 1 − x, y, 3/2 − z) from two different 5-Hsip2− and one coordinated water molecule, respectively, where the axial positions are satisfied by two pyridyl nitrogen atoms (N1 and N2b, where b = x, 1 + y, z) of two different 4,4′-bipyridine ligands. The Cu−O bond lengths are 1.949(2)−2.075(2) Å, and Cu−N bond lengths vary from 2.023(2) to 2.030(2) Å (Table S7). Here, 4,4′-bipyridine ligand bridges with two adjacent Cu(II) centers to form a 1D chain. Such two parallel metal−ligand chains are joined by the 5-Hsip2− coligand through its one −COO− group in a bidentate fashion, resulting in the formation of a 1D ladder along the crystallographic b-axis (Figure 4b). In the 1D ladder, the pendent −CO2H groups and the lattice water molecules are stabilized by forming intermolecular H-bonding with the adjacent oxygen atoms of 5-Hsip2−, resulting in the formation of a supramolecular 2D structure (Figure 4c and Table S8). The TOPOS40,41 analysis of structure 4 reveals a 3-c uninodal net with corresponding Schläfli symbol for the net {42.6} (Figure 4d). Structural Descriptions of {[Cu(bpy)2(5-H2sip)2]·(H2O)6}n (5). The single-crystal X-ray analysis confirms that compound 5 was crystallized in the monoclinic C2/c space group, forming a 2D sheet with pendent monosodium 5-sulfoisophthalate (5H2sip−). In the asymmetric unit of 5, there are one Cu2+ ion, one bpy ligand, one monosodium 5-sulfoisophthalate (5H2sip−) containing two protonated −CO2H groups, and six lattice water molecules. The hexacoordinated Cu(II) metal center displays a distorted octahedral geometry with a CuO2N4 coordination environment (Figure 5a), where the basal plane is generated by four nitrogen atoms (N1, N2a, N3, and N3b; a = x, −1 + y, z and b = 1 − x, y, 3/2 − z) from four different 4,4′bipyridine ligand, and the axial positions are occupied by two symmetry related oxygen atoms (O6 and O6b) of two different sulfonate (−SO3−) groups of monosodium 5-sulfoisophthalate (5-H2sip−). The axial Cu−O bond lengths are 2.631(6) Å, and Cu−N bond lengths vary from 1.979(9) to 2.042(4) Å (Table S9). Compound 5 exhibits a 2D sheet containing pendent 5H2sip− by the bridging of the 4,4′-bipyridine (bpy) ligand (Figure 5b). The 2D sheet contains the 1D guest water filled channel along the a-axis, which is further stabilized by the formation of intermolecular H-bonding with the oxygen atoms/ −CO2H group of 5-H2sip−, which results in the formation of a supramolecular 3D structure (Figure 5c and Table S10). The TOPOS40,41 analysis of structure 5 suggests a 4-c uninodal net with corresponding Schläfli symbol for the net {44.62} (Figure 5d). Powder X-ray Diffraction (PXRD) Study. At room temperature, solid state PXRD measurements were performed to verify the bulk purity of the as-synthesized powder samples of 1−5 (Figures S6−S10). For all cases, the as-synthesized bulk materials confirm the phase purity with identical emergence of peak positions with their corresponding simulated patterns. The PXRD of compounds 1−5 after proton conductivity study (Figures S6−S10) has been investigated at room temperature, where all the PXRD patterns suggest the retention of the framework structure after proton conductivity measurement inferred from the identity of significant peak positions with the as-synthesized patterns. It is worth to mention that Au paste was used for the conductivity measurements, and thus, the Au peaks at 2θ ∼ 38° were observed for after-measurement PXRD patterns of 1−5. Thermogravimetric Analysis. Thermogravimetric analysis (TGA) of compounds 1−5 has been performed from 30 to 500
Figure 6. Water vapor adsorption profiles for compounds 1−5 measured at 298 K. G
DOI: 10.1021/acs.inorgchem.6b02674 Inorg. Chem. XXXX, XXX, XXX−XXX
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Inorganic Chemistry step pore-filling uptake beginning at ∼9−20% relative humidity (% RH). For compounds 1−5, the pore-filling steps began at a low relative humidity (% RH), e.g., 20, 9, 12, 9, and 14%, respectively, which suggest more hydrophilic character of most of the samples.42 The large hysteresis in between the adsorption−desorption cycles for all cases (Figures S12−S16) suggests the presence of hydrophilic channels in the structure.43 It is quite obvious that the proton conductivity gradually increases with the gradual increment of relative humidity (% RH) and maximum at 95% RH (Figure 8), which is due to the formation of H-bonded networks to a greater extent at higher relative humidity (% RH). The nitrogen gas adsorption isotherm has also been measured for the compounds, and that exhibits a negligible amount of surface adsorption for all the compounds 1−5 (Figure S17). These negligible amounts of N2 uptake compared to water vapor uptake by the dehydrated frameworks of 1−5 imply that the dehydrated frameworks do not have sufficient spaces between the layer to diffuse the larger sized N2 gas (kinetic diameter: 3.64 Å) compared to smaller sized water molecules (kinetic diameter: 2.64 Å) which are strongly adsorbed through hydrogen bonding interactions. Proton Conductivity Study. To assess the proton conductivity, alternating current (AC) impedance spectroscopy at 15−65 °C was carried out using pelletized powder samples of 1−5. The Nyquist plots of the impedance spectra are shown in Figures 7 and S18−S21. The conductivities with varying both
Figure 8. Plots of log(σ) vs RH at 25 °C for compounds 1−5. The filled symbols correspond to the adsorption processes.
under 95% RH, respectively. Though 1 and 2 both exhibit a 1D chain, the conductivity value of 2 is 10 times greater than that of 1, indicating the presence of one additional protonated −COOH group lining in the hydrophilic pore channel. In the cases of 3 and 4, though both show a 1D ladder structure, the difference in proton conductivity behavior may arise due to the different number of protons associated with 5-sulfoisophthalate. The 5-sulfoisophthalate exists as a dinegative form in 4 containing one protonated −COOH group in the pore wall where that is trinegative in the case of 3. The conductivity value of 4 is 100 times greater than that of 3 possibly due to the existence of free proton in 4 as mentioned above. Compound 5 having a 2D sheet structure exhibits a similar proton conductivity value as that of 4, due to the presence of protonated −COOH as we found in the case of 4. The leastsquares fit of the Arrhenius plots for the heating cycles produced the activation energies (Ea) with the value of 0.54, 0.35, 0.40, 0.43, and 0.45 eV for compounds 1−5, respectively (Figure 9), indicating the Grotthuss mechanism24−28 supported proton conductivity for all cases. The slightly higher activation
Figure 7. Nyquist plots for compound 1 at 95% RH and various temperatures.
the temperature (Figures 7 and S18−S21) and the relative humidity (% RH) (Figure 8) were measured after setting each condition for approximately half a day. The Nyquist plots for all the compounds (Figures 7 and S18−S21) show the temperature dependent conductivity at 95% RH. The conductivity value for each samples increases with elevation of temperature and attains a maximum up to 2.5 × 10−6, 3.5 × 10−5, 9.9 × 10−8, 5.8 × 10−6, and 1.4 × 10−6 S cm−1 at 65 °C under 95% RH for compounds 1−5, respectively. In comparison to their conductivity value at 65 °C under 95% RH, compounds 1−5 exhibit much lower conductivity values up to 1.1 × 10−7, 4.5 × 10−6, 1.1 × 10−8, 5.4 × 10−7, and 1.2 × 10−7 S cm−1 at 15 °C
Figure 9. Plots of log(σT) vs 1000T−1/K at 95% RH for compounds 1−5. The solid lines represent the best fit of the data. H
DOI: 10.1021/acs.inorgchem.6b02674 Inorg. Chem. XXXX, XXX, XXX−XXX
Inorganic Chemistry
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energy of 1 (i.e., Ea = 0.54 > 0.5 eV) would be due to the increased contribution from the vehicle proton transfer (Ea = 0.5−0.6 eV).19,33 Here in all the cases, the notable humidity dependence of proton conductivity clearly indicates that the water molecules exist in interlayer spaces are essential for showing the proton conductivity. The pathway of proton conduction in compounds 1−5 involves both lattice and coordinated water molecules, along with oxygen atoms from the sulfonate/hydrogen sulfonate and/or carboxylate/carboxylic acid groups of 5-sulfoisophthalate that are interconnected through hydrogen bonding. In some cases, the lattice/ coordinated water molecules are also H-bonded in intermolecular fashion with the lattice/coordinated protonated N,N′donor linkers which may help to generate the proton conductivity. The hydrogen bond distances between the water molecules in the channel for compounds 1−5 are summarized in Tables S2, S4, S6, S8, and S10, where the water molecules form strong hydrogen bonds with the O···O distances within the range 2.57−2.931 Å, constructing the extended intermolecular hydrogen bonded network in the 1D channel. Such an infinitely extended H-bonded network of water molecules is one of the key factors for efficient proton conductivity through proton transfers between the molecules, which is known as the Grotthuss mechanism.24,28 It is quite obvious that the activation energy (Ea) values could be an outcome of the heterogeneity of the functional groups having different pKa’s present in interlayer spaces.25,31 It is quite obvious that the hydrophilic and/or acidic25 inner surfaces of PCPs are anticipated to have an effect on the mobility as well as extent of proton carriers for showing the significant proton conductivity.
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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/acs.inorgchem.6b02674. The IR, PXRD, TGA, and additional application based figures (Figures S1−S21) and tables related to the crystal structures (Tables S1−S10) of compounds 1−5 (PDF) X-ray crystallographic data of compounds 1−5 (CIF)
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AUTHOR INFORMATION
Corresponding Authors
*E-mail:
[email protected] (H.K.). *E-mail:
[email protected]. Fax: +9133 2414 6223 (D.G). ORCID
Hiroshi Kitagawa: 0000-0001-6955-3015 Debajyoti Ghoshal: 0000-0001-8820-8209 Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS We acknowledge the financial assistance by SERB, India (Grant No. SB/S1/IC-06/2014). D.K.M. acknowledges UGC for his research fellowship.
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REFERENCES
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CONCLUSION
Herein, we report the synthesis of 5-sulfoisophthalate based five MOFs with sulfonic acid/sulfonated group functionalized pore walls, in which the compounds possess 1D channels of guest water molecules. Those guest waters are found to be responsible to create moderately high proton conductivity for the compounds 1−5 through the formation of an extended intermolecular hydrogen bonded network with other oxygen rich carboxylic or sulfonic acid groups of 5-sulfoisophthalate following the Grotthuss mechanism. Moreover, the proton conductivity of all the compounds show temperature as well as humidity (% RH) dependence. With increasing value of relative humidity (% RH), the proton conductivity of the compounds has significantly increased over 4−5 orders of magnitude, suggesting the essential role of water molecules in proton conductivity, through the transfer of protons by means of the extended intermolecular H-bonded network. All the compounds except 1 exhibit the activation energy value less than 0.5 eV, supporting the Grotthuss-type proton transfer, whereas the slightly higher activation energy of 1 indicates the Grotthusstype proton transfer including increased contribution from the vehicle proton transfer. In summary, this work opens a new route for the design and synthesis of mixed ligand CPs with sulfonated proton conducting frameworks with an endeavor of searching materials having a unique proton conductive nature based on the functionality of the components in the pore channels of CPs. I
DOI: 10.1021/acs.inorgchem.6b02674 Inorg. Chem. XXXX, XXX, XXX−XXX
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