Donor Spacers - American Chemical Society

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Four 3D Cd(II)-Based Metal Organic Hybrids with Different N,N′Donor Spacers: Syntheses, Characterizations, and Selective Gas Adsorption Properties Biswajit Bhattacharya,† Rajdip Dey,† Pradip Pachfule,§ Rahul Banerjee,*,§ and Debajyoti Ghoshal*,† †

Department of Chemistry, Jadavpur University, Jadavpur, Kolkata, 700 032, India Physical/Materials Chemistry Division, CSIR-National Chemical Laboratory, Dr. Homi Bhabha Road, Pune-411008, India

§

S Supporting Information *

ABSTRACT: Four new metal organic frameworks with bivalent cadmium, disodium succinate (Na2suc), and four different N,N′-donor ligands, i.e., {[Cd(L1)(suc)]·(H2O)3}n (1), {[Cd(L2)(suc)]·(H2O)2}n (2), {[Cd(L3)(suc)]·(H2O)4}n (3), {[Cd3(L4)3(suc)2(H2O)2]·(NO3)2 (H2O)4}n (4) [L1 = 2,5-bis-(4-pyridyl)-3,4-diaza-2,4-hexadiene, L2 = trans 4,4′azobispyridine, L3 = 2,5-bis-(3-pyridyl)-3,4-diaza-2,4-hexadiene, L4 = 1, 2-bis(4-pyridyl) ethane and suc = succinate dianion] have been synthesized at room temperature and characterized by single-crystal X-ray diffraction and other physicochemical methods. Structure determination reveals that compounds 1 and 2 show honeycomb-like three-dimensional (3D) architecture with water-filled channels. The dehydrated frameworks of 1 and 2 exhibit hydrogen and carbon dioxide adsorption properties. In compound 3, change of linker (linear to bent) led to the blockage of such regular channels which also affected the porosity and adsorption properties of its dehydrated framework. In 4, the used spacer is linear but the resulting 3D framework contains blocked channels filled with nitrate (NO3−) anions and lattice water.

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made in the field of porous MOFs by using pyridyl N,N′-donor spacer ligands in order to synthesize porous frameworks with selective adsorption properties.14 The pyridyl based linker can pillar the metal-carboxylate motifs into higher dimensionality to alter the structural topology. Since the tailoring of the length and the chemical environment of pyridyl based linkers is synthetically feasible; thus a structural diversity can be generated by monitoring synthesis of such organic linkers. It has been observed that the longer pyridyl based organic linker forms MOFs with higher porosity compared to that synthesized with short linkers.15 Moreover, the positions of the pyridyl-N atom and introduction of functional groups in such pyridyl N,N′-donor linkers can also play a crucial role in altering the porosity of such MOFs. This type of predesigned functionalization could be very useful in binding guest molecules which makes the synthesized porous MOFs a selective gas adsorber.

INTRODUCTION The widespread applications of porous coordination polymers (PCPs) or metal−organic frameworks (MOFs) have made this field a rapidly developing one in the past few years.1 Diverse structural attributes with large surface area, tailored pore volume, and chemical environment make these materials promising for hydrogen storage,2 separation,3 sequestration of carbon dioxide or other harmful gases,4 heterogeneous catalysis,5 ion exchange,6 drug delivery,7 and as well as in sensing applications.8 Moreover, when transition metal ions are incorporated into the porous MOF structures, these materials show interesting ionic conductivity,9 optical,10 and magnetic properties11 as well, in addition to their porosity. In this context, different carboxylic acids having versatile coordination modes are being used as an essential tool to develop MOFs, which helps in their formation with the rich diversity in shape and connectivity.12 Among them, succinate dianion13 is quite useful in the creation of such structural diversity due to its moderately longer spacer length, flexibility, and the existence of several binding modes. Recently, a lot of progress has also been © XXXX American Chemical Society

Received: October 3, 2012 Revised: December 3, 2012

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dx.doi.org/10.1021/cg3014464 | Cryst. Growth Des. XXXX, XXX, XXX−XXX

Crystal Growth & Design

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dynamic vacuum. Upon heating, complexes 1, 2, and 3 retain their crystalline nature and framework architecture which is supported by the PXRD measurement. Synthesis of {[Cd(L1)(suc)]·(H2O)3}n (1). An aqueous solution (4 mL) of disodium succinate (1 mmol, 0.162 g) was mixed with a methanolic solution (4 mL) of 2,5-bis-(4-pyridyl)-3,4-diaza-2,4hexadiene (L1) (1 mmol, 0.238 g) and stirred for 20 min to mix it well. Cd(NO3)2·4H2O (1 mmol, 0.308 g) was dissolved in 4 mL of water, and the solution was slowly layered to the above mixed ligand solution using 5 mL of buffer (1:1 of water and MeOH). The orange block crystals were obtained after two weeks. The crystals were separated and washed with a methanol−water (1:1) mixture and dried under air (Yield 78%). Anal. Calc. for C18H24CdN4O7: C, 41.51; H, 4.64; N, 10.76. Found: C, 41.44; H, 4.59; N, 10.65. IR spectra (in cm−1): ν(CN), 1605; ν(C−O), 1300−1220; ν(CH-Ar), 3100− 2900 and ν(CC), 1560−1410. Synthesis of {[Cd(L2)(suc)]·(H2O)2}n (2). This has been synthesized by the same procedure as that of 1 using trans-4,4′azobispyridine (L2) (1 mmol, 0.184 g) instead of L1. The reddish block crystals were obtained after five days (Yield 64%). Anal. Calc. for C14H16CdN4O6: C, 37.47; H, 3.59; N, 12.48. Found: C, 37.38; H, 3.54; N, 12.36. IR spectra (in cm−1): ν(NN), 1593; ν(C−O), 1300−1225; ν(CH-Ar), 3100−2900 and ν(CC), 1580−1410. Synthesis of {[Cd(L3)(suc)]·(H2O)4}n (3). Here also the same procedure was adopted as that of 1 using 2,5-bis-(3-pyridyl)-3,4-diaza2,4-hexadiene (L3) (1 mmol, 0.238 g) instead of L1. The yellow block crystals were obtained after one week (Yield 74%). Anal. Calc. for C18H26CdN4O8: C, 40.12; H, 4.86; N, 10.40. Found: C, 40.03; H, 4.83; N, 10.48. IR spectra (in cm−1): ν(CN), 1618; ν(C−O), 1300−1230; ν(CH-Ar), 3100−2900 and ν(CC), 1540−1425. Synthesis of {[Cd3(L4)3(suc)2(H2O)2]·(NO3)2(H2O)4}n (4). Using the same method as previous, this was synthesized using 1,2-bis(4pyridyl) ethane (L4) (1 mmol, 0.184 g). The colorless block crystals were obtained after three weeks (Yield 71%). Anal. Calc. for C44H56Cd3N8O20: C, 39.02; H, 4.16; N, 8.27. Found: C, 39.16; H, 4.04; N, 8.12. IR spectra (in cm−1): ν(C−O), 1300−1220; ν(CH-Ar), 3100−2900; ν(CC), 1610−1400; ν(NO2), 1295 and ν(NO), 1016. The bulk amounts of all four compounds have been synthesized in powder form by the direct mixing of the corresponding ligand mixture with aqueous solution of Cd(II) followed by overnight stirring. The purity of the compounds was checked and confirmed from similarity of XRPD patterns of the bulk phase with the simulated pattern from single crystal X-ray data. The compounds were also characterized by IR spectra and elemental analyses which was also found in accordance with the data obtained for the single crystals. Crystallographic Data Collection and Refinement. The single crystals of compounds 1−4 were mounted on the tips of glass fibers coated with Fomblin oil. X-ray single crystal data collection of all four crystals were collected at room temperature using a Bruker APEX II diffractometer, equipped with a normal focus, sealed tube X-ray source with graphite monochromated Mo−Kα radiation (λ = 0.71073 Å). The data were integrated using the SAINT17 program, and the absorption corrections were made with SADABS. All the structures were solved by SHELXS 9718 using the Patterson method and followed by successive Fourier and difference Fourier synthesis. Full matrix least-squares refinements were performed on F2 using SHELXL-9719 with anisotropic displacement parameters for all nonhydrogen atoms. During refinement of 1 and 3, two of the lattice water molecules are found disordered, and thus their occupancies were fixed at 0.5 before final refinement. All the hydrogen atoms except the hydrogen of disordered lattice water were fixed geometrically by HFIX command and placed in ideal positions in the case of all four structures. Calculations were carried out using SHELXL 97, SHELXS 97, PLATON v1.15,20 ORTEP-3v2,21 and WinGX system Ver-1.80.22 The coordinates, anisotropic displacement parameters, and torsion angles for all four compounds are submitted as Supporting Information in CIF format. Data collection and structure refinement parameters along with crystallographic data for all compounds are

Herein, we report four new MOFs, {[Cd(L1)(suc)]·(H2O)3}n (1), {[Cd(L2)(suc)]·(H2O)2}n (2), {[Cd(L3)(suc)]·(H 2 O) 4 } n (3), {[Cd 3 (L4) 3 (suc) 2 (H 2 O) 2 ]·(NO 3 ) 2 (H2O)4}n (4) with Cd(II), disodium succinate (Na2suc), and different N,N′-donor spacers. In the case of 1 and 2, the linkers L1 and L2 are slightly different in terms of length, but both the linkers have the pyridyl-N at the 4-position of the pyridine ring. This results in a formation of regular water filled channels within the framework structures in these two cases. The channel dimensions without lattice water molecules evidently exhibit a linear relation with the length of the linker as in 1 the channel is bigger than 2. Both the frameworks in the dehydrated state exhibit high surface area with moderately good hydrogen and carbon dioxide adsorption, whereas in the case of 3 the positional change of the pyridyl-N atom in L3 makes it bent, which leads to the partial blockage of regular channels in 3. Consequently, the pore dimension without lattice water molecules is reduced, and as a result almost no gas adsorption is observed in the case of dehydrated framework of 3. Compound 4 forms a completely different structure from the others, although the N, N′-donor linker L4 has the same length as that of L2, but differing in connecting environment of the pyridyl rings where the -NN- group of L2 linker has been replaced by the -(CH2)2- group. In 4 the channels are occupied by nitrate counteranion and lattice water molecules, and does not contain any significant solvent accessible void after the removal of lattice waters.

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EXPERIMENTAL SECTION

Materials. 2,5-Bis-(4-pyridyl)-3,4-diaza-2,4-hexadiene (L1), trans4,4′-azobispyridine (L2), and 2,5-bis-(3-pyridyl)-3,4-diaza-2,4-hexadiene (L3) were synthesized by the procedures reported earlier,16 and the starting material for these synthesis viz. 4-acetyl pyridine, 3-acetyl pyridine, 4-amino pyridine, and hydrazine hydrate were purchased from Aldrich Chemical Co. Inc. and used as received. High purity cadmium(II) nitrate tetrahydrate, succinic acid, and 1,2-bis (4-pyridyl) ethane (L4) were also purchased from Aldrich Chemical Co. Inc. and used as received. All other chemicals including solvents were of AR grade and used as received. Physical Measurements. Elemental analyses (carbon, hydrogen, and nitrogen) were performed using a Perkin−Elmer 240C elemental analyzer. Infrared spectra (4000−400 cm−1) were taken on KBr pellet, using a Perkin−Elmer Spectrum BX-II IR spectrometer. Thermal analysis (TGA) was carried out on a METTLER TOLEDO TGA 850 thermal analyzer under nitrogen atmosphere (flow rate: 50 cm3 min−1) at the temperature range of 30−400 °C with a heating rate of 2 °C/ min. X-ray powder diffraction (PXRD) patterns in different states of the sample were recorded on a Bruker D8 Discover instrument using Cu−Kα radiation. Measurement of Gas Adsorption. Low pressure volumetric gas adsorption measurements were performed at 77 K for N2 and H2, maintained by a liquid nitrogen bath, with pressures ranging from 0 to 1 bar on Quantachrome Quadrasorb automatic volumetric instrument, while CO2 adsorption measurements were done at 273 K in the same instrument at same pressure range. In all the adsorption measurements, ultra high-pure N2, H2, or CO2 was obtained by passing them through calcium aluminosilicate adsorbents to remove trace amounts of water and other impurities before introduction into the system. The microcrystals of each compound 1−3 were soaked in 1:1 dry dichloromethane and methanol mixture for 12 h. Fresh 1:1 dry dichloromethane and methanol mixture was subsequently added, and the crystals were allowed to stay for additional 48 h to remove free solvates (methanol and H2O) present in the framework. Then the sample was dried under a dynamic vacuum ( 2σ(I) Rint goodness-of-fit (F2) R1 (I > 2σ(I))a wR2 (I > 2σ(I))a Δρ max/min/e Å3 a

1

2

3

4

C18H24CdN4O7 520.81 monoclinic C2/c 29.200(2) 11.9144(9) 14.0146(10) 90 92.054(7) 90 4872.6(6) 8 1.403 0.937 2064 1.9−27.6 37730 5648 3822 0.062 1.08 0.0480 0.1516 −0.79, 0.88

C14H16CdN4O6 448.72 monoclinic C2/c 24.709(3) 11.5985(12) 14.2484(15) 90 103.464(7) 90 3971.2(8) 8 1.488 1.133 1760 1.7−27.4 30110 4482 4016 0.027 1.14 0.0321 0.1166 −0.47, 0.88

C18H26CdN4O8 538.83 monoclinic P21/c 12.6099(2) 12.1229(2) 14.2819(3) 90 98.410(1) 90 2159.78(7) 4 1.632 1.062 1064 1.6−27.6 35432 5002 4657 0.022 1.09 0.0245 0.0695 −0.60, 0.66

C44H56Cd3N8O20 1354.19 monoclinic C2/c 23.7901(9) 8.6694(3) 29.5375(9) 90 113.205(3) 90 5599.2(4) 4 1.597 1.206 2688 1.9−28.3 46939 6947 6557 0.023 1.30 0.0531 0.1456 −1.35, 2.65

R1 = Σ||Fo| − |Fc||/Σ|Fo|, wR2 = [Σ(w(Fo2 − Fc2)2)/ Σw(Fo 2)2]1/2.

given in Table 1. The selected bond lengths and bond angles are given in Tables 2−5.

the monoclinic space group C2/c, and the single crystal structure analysis reveals that both the compounds originate a three-dimensional (3D) coordination framework of Cd(II) connected by succinate (suc) and linear N, N′ linkers, L1 in the case of 1 and L2 in the case of 2. Here, in each case the geometry around each hepta-coordinated Cd(II) with the CdO5N2 chromophore can be best represented as distorted pentagonal bipyramidal (Figure 1a for 1 and Figure S1a for 2). The five oxygen atoms, O1, O2, O3a, O4a, and O4b (where a = 3/2 − x, 1/2 + y, 3/2 − z, b = x, −y, −1/2 + z for 1 and a = 1/ 2 − x, −1/2 + y, 1/2 − z, b = x, 1 − y, −1/2 + z for 2) of three different bridging succinate occupies the equatorial positions, whereas two 4-pyridyl nitrogen atoms N1 and N4c (where c = −1/2 + x, −1/2 + y, z for 1 and c = −1/2 + x, 1/2 + y, z for 2) from two different bridging linkers (L1 and L2) occupy the axial positions. In compounds 1 and 2, Cd−O bond length varies from 2.329(4) to 2.468(4) Å and 2.325(3) to 2.434(3) Ǻ , respectively, and the corresponding Cd−N bond lengths are in the range of 2.3336(4)−2.338(4) Å and 2.332(3)−2.340(3) Å for 1 and 2, respectively. The other selected bond lengths and bond angles are reported in Table 2 for 1 and Table 3 for 2. Here succinate dianion acts in a bridging bidented fashion and each succinate connects three Cd(II) centers to create the 2D sheet arrangement in the crystalographic bc plane (Figure 1b for 1 and Figure S1b for 2). In this 2D arrangement one end of succinate is chelated to the Cd(II) center, whereas on the other end of the succinate, chelation as well as oxo-linkage formation between two Cd(II) centers have been observed. The linear N, N′ linkers connect the 2D metal-carboxylate sheets to create a 3D arrangement with the 1D channel along the crystallographic c-axis, which is occupied by lattice guest water molecules. In the 3D framework linear N, N′ linkers are oriented in a crisscrossed manner to generate honeycomb-like arrangements when viewed along the c-axis (Figure 1c for 1 and Figure S1c

Table 2. Selected Bond Lengths (Å) and Bond Angles (°) for 1a Cd1−O1 Cd1−N1 Cd1−N4c Cd1−O4a O1−Cd1−O2 O1−Cd1−N1 O1−Cd1−O4b O1−Cd1−N4c O1−Cd1−O3a O1−Cd1−O4a O2−Cd1−N1 O2−Cd1−N4c O2−Cd1−O4a O4b−Cd1−N1 O4a−Cd1−N4c

2.368(4) 2.338(4) 2.336(4) 2.400(4) 54.08(15) 92.65(14) 142.47(14) 88.51(14) 92.28(15) 145.14(15) 85.92(15) 93.40(15) 160.60(14) 94.60(14) 90.37(14)

Cd1−O2 Cd1−O4b Cd1−O3a O2−Cd1−O4b O2−Cd1−O3a N1−Cd1−N4c O4a−Cd1−N1 O4b−Cd1−N4c O3a−Cd1−O4b O3a−Cd1−N4c O3a−Cd1−O4a O4b−Cd1−O4a O3a−Cd1−N1 Cd1−O4−Cd1d

2.429(4) 2.329(4) 2.468(4) 89.79(14) 146.15(1) 177.95(1) 89.65(14) 83.46(14) 124.00(1) 88.89(14) 52.86(14) 71.74(13) 92.76(14) 108.26(1)

Symmetry code: a = 3/2 − x, 1/2 + y, 3/2 − z; b = x, −y, −1/2 + z; c = −1/2 + x, −1/2 + y, z; d = 2x − 3/2, −2y + 1/2, −1 + 2z. a

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RESULTS AND DISCUSSION Synthesis. All the compounds 1−4 were synthesized by the slow diffusion technique in water−methanol medium by employing Cd(NO3)2·4H2O and appropriate ligand mixture as mentioned in Scheme 1. Structural Descriptions of {[Cd(L1)(suc)]·(H2O)3}n (1) and {[Cd(L2)(suc)]·(H2O)2}n (2). Compounds 1 and 2 are nearly isostructural differing only with respect to the linker (L1 and L2) and lattice water molecule. Both of them crystallize in C



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Scheme 1. Schematic Representation of the Synthesis of Four Metal Organic Hybrids with Four Different Linkers

Figure 1. (a) ORTEP drawing (40% probability ellipsoid) of 1 showing atom labeling scheme (L1 has been omitted for clarity); (b) 2D arrangement in 1 constructed by metal-succinate moiety (L1 has been omitted for clarity); (c) 3D honeycomb-like structure in 1 with water filled 1D channels.

for 2). In 1, the dimension of the channel is about 4.4 × 5.3 Å2 and that in 2 is about 4.2 × 3.1 Å2, which is around 40% less (in terms of area) than that of 1. The effective solvent accessible void calculated using PLATON20 are 32.3% (for 1) and 31.2% (for 2) relative to the total crystal volume. Although the pores have very similar morphology in 1 and 2, the organization of guest water molecules in the channels are a little bit different in these two (Figure 1c for 1 and Figure S1c for 2). Structural Description of {[Cd(L3)(suc)]·(H2O)4}n (3). Compound 3 crystallizes in the monoclinic space group P21/c, and the X-ray structure determination reveals that the 3D coordination framework of Cd(II) connected by the succinate and a bent bis (3-pyridyl) N,N′ linker (L3). The coordination structure of 3 is very similar to that of 1 and 2. Here also in

each asymmetric unit, there is one hepta coordinated Cd(II) along with one bridging succinate ligand, one bridging L3 linker and four lattice water molecules. The hepta coordinated Cd(II) centers with CdO5N2 chromophore show distorted pentagonal bipyramidal geometry satisfied by five oxygen atoms (O1, O2, O3a, O4a, and O4b where a = 1 − x, −1/2 + y, 3/2 − z, b = x, 1/2 − y, 1/2 + z) of three different succinate, which creates the basal plane and two 3-pyridyl nitrogen atoms of two different L3 (N1 and N4c where c = 1 + x, y, z) occupy the epical position (Figure S2a). Cd1−O and Cd1−N bond lengths around Cd1 are 2.3328(14)−2.4824(16) Å and 2.3372(18)− 2.3852(18) Å, respectively (Table 4). Like 1 and 2, one end of succinate is chelated to the one Cd(II) center, whereas in other end there is a simultaneous chelation as well as oxo-linkage D



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Table 3. Selected Bond Lengths (Å) and Bond Angles (°) for 2a Cd1−O1 Cd1−N1 Cd1−N4c Cd1−O4a O1−Cd1−O2 O1−Cd1−N1 O1−Cd1−O4b O1−Cd1−N4c O1−Cd1−O3a O1−Cd1−O4a O2−Cd1−N1 O2−Cd1−N4c O2−Cd1−O4a O3a−Cd1−O4b O3a−Cd1−O4a

2.357(3) 2.340(3) 2.332(3) 2.425(3) 54.33(9) 96.11(10) 143.35(10) 85.65(10) 91.18(8) 143.95(9) 84.68(10) 93.11(9) 161.28(10) 124.13(10) 52.77(9)

Cd1−O2 Cd1−O4b Cd1−O3a O2−Cd1−O4b O2−Cd1−O3a O4b−Cd−N4c O4b−Cd1−O4a O4a−Cd1−N4c O4b−Cd1−N1 N1−Cd1−N4c O3a−Cd1−N1 O4a−Cd1−N1 O3a−Cd1−N4c Cd1−O4−Cd1d

2.434(3) 2.325(3) 2.427(2) 90.82(10) 145.04(9) 85.39(10) 71.99(11) 92.84(10) 90.81(11) 175.58(1) 94.77(10) 88.12(11) 89.24(9) 108.03(1)

Symmetry code: a = 1/2 − x, −1/2 + y, 1/2 − z; b = x, 1 − y, −1/2 + z; c = −1/2 + x, 1/2 + y, z; d = 2x − 1/2, 1/2 − 2y, 2z. a

Table 4. Selected Bond Lengths (Å) and Bond Angles (°) for 3a Cd1−O1 Cd1−N1 Cd1−O3a Cd1−O4b O1−Cd1−O2 O1−Cd1−N1 O1−Cd1−N4c O1−Cd1−O3a O1−Cd1−O4a O1−Cd1−O4b O2−Cd1−N1 O3a−Cd1−N4c O4b−Cd1−N4c O4a−Cd1−O4b O3a−Cd1−O4a

2.3644(16) 2.3372(18) 2.4824(16) 2.3328(14) 54.27(5) 88.72(6) 89.38(6) 90.56(5) 143.75(5) 144.58(6) 93.75(6) 87.52(6) 93.02(6) 71.66(5) 53.19(5)

Cd1−O2 Cd1−N4c Cd1−O4a O2−Cd1−N4c O2−Cd1−O4a O3a−Cd1−N1 O4b−Cd1−N1 O2−Cd1−Oa O2−Cd1−O4b N1−Cd1−N4c O4a−Cd1−N1 O3a−Cd1−O4b O4a−Cd1−N4c Cd1−O4−Cd1d

2.4368(16) 2.3852(18) 2.3741(16)

Figure 2. 3D structure of 3 with water filled 1D channels.

Table 5. Selected Bond Lengths (Å) and Bond Angles (°) for 4a

85.94(6) 161.41(5) 91.51(6) 89.10(6) 144.22(5) 90.64(5) 177.86(6) 91.63(6) 124.84(5) 89.32(6) 108.35(6)

Cd1−O1W Cd1−N2c Cd1−O1Wb Cd2−O1 Cd2−O3 Cd2−N1 Cd2−O4a O1W−Cd1−O2 O1W−Cd1−N2c O1W−Cd1−N2d O1W−Cd1−O1Wb O1W−Cd1−O2b O2−Cd1−N2c O2−Cd1−N2d O1Wb−Cd1−O2 O2−Cd1−O2b N2c−Cd1−N2d O1Wb−Cd1−N2c O2b−Cd1−N2c O1Wb−Cd1−N2d O2b−Cd1−N2d O1Wb−Cd1-O2b O1−Cd2−O2 O1−Cd2−O3 O1−Cd2−N1 O2−Cd2−N1

a Symmetry code: a = 1 − x, −1/2 + y, 3/2 − z; b = x, 1/2 − y, 1/2 + z; c = 1 + x, y, z; d = 1/2 − x, y, 1 − z.

formation between two Cd(II) centers to create the 2D sheet in the crystallographic bc plane (Figure S2b). The 3D structure is entirely different from 1 and 2. In the previous two structures the 2D metal-carboxylate sheets are joined in a staggered way, but due to the angular nature of the N, N′ linker (L3), here in 3 it connects the 2D metal-carboxylate sheets in parallel fashion to create a 3D arrangement with the 1D channel along the crystallographic a-axis, which are occupied by lattice guest water molecules (Figure 2). In the 3D framework arrangement the L3 linkers are not criss-crossed like 1 and 2, but they arranged in a pillared layer fashion probably due to the increased steric strain of the bent linker (Figure S2c). Here the dimension of the channel is about 3.8 × 4.0 Å2, and upon removal of the water molecules the framework shows 21.6% void space with respect to the total crystal volume as suggested by the PLATON20 crystallographic software. Structural Description of {[Cd 3(L4)3(suc)2(H2O)2]·(NO 3 ) 2 (H 2 O) 4 } n (4). Compound 4 crystallizes in the monoclinic C2/c space group, and the single-crystal X-ray structure analysis revealed that the formation of the 3D structure constituted by both the hexa-coordinated and heptcoordinated Cd(II) center (Cd1 and Cd2 respectively) along

2.320(5) 2.293(4) 2.320(5) 2.309(4) 2.388(4) 2.317(4) 2.319(5) 98.03(16) 90.81(17) 89.19(17) 180.00 81.97(16) 90.11(16) 89.89(16) 81.97(16) 180.00 180.00 89.19(17) 89.89(16) 90.81(17) 90.11(16) 98.03(16) 53.40(12) 140.55(14) 87.14(15) 91.47(15)

Cd1−O2 Cd1−N2d Cd1−O2b Cd2−O2 Cd2−O4 Cd2−N3 O3−Cd2−N3 O3−Cd2−O4a O4−Cd2−N1 O4−Cd2−N3 O4−Cd2−O4a N1−Cd2−N3 O4a−Cd2-N1 O4a−Cd2-N3 Cd1−O2−Cd2 Cd2−O4−Cd2a O1−Cd2−O4 O1−Cd2−N3 O1−Cd2−O4a O2−Cd2−O3 O2−Cd2−N3 O2−Cd2−O4a O3−Cd2−O4 O2−Cd2−O4 O3−Cd2−N1

2.332(4) 2.293(4) 2.332(4) 2.530(4) 2.481(5) 2.322(4) 93.60(17) 124.86(15) 90.49(16) 92.54(16) 72.53(15) 170.71(17) 86.48(16) 86.06(17) 137.38(16) 107.47(17) 167.03(14) 87.95(16) 94.59(15) 87.15(13) 91.92(15) 147.99(14) 52.37(15) 139.47(13) 95.21(15)

Symmetry code: a = 2 − x, 1 − y, 1 − z; b = 3/2 − x, 3/2 − y, 1 − z; c = x, 1 - y, 1/2 + z; d = 3/2 - x, 1/2 + y, 1/2 − z. a

with the bridging succinate and a linear N, N′-donor linker L4. In each asymmetric unit, two hepta-coordinated and one hexacoordinated Cd(II) are present. The hexa-coordinated Cd1 with CdO4N2 chromophore shows nearly octahedral geometry where the equatorial sites are occupied by two O atoms (O2, E



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Figure 3. (a) ORTEP drawing (40% probability ellipsoid) of 4 showing the atom labeling scheme (L4 has been omitted for clarity); (b) 2D arrangement in 4 constructed by metal-succinate moiety (L4 has been omitted for clarity; hexa-coordinated Cd1 represented in magenta color and the hepta-coordinated Cd2 in green); (c) 3D rectangular grid-like structure with water and nitrate counteranion in 1D channels along the crystallographic c-axis of 4.

Figure 4. PXRD patterns in different states. (a) Simulated from X-ray single crystal data; (b) bulk as-synthesized compound; (c) at 150 °C of 1, 2 and 3 respectively .

from 2.309(4)−2.530(4)Ǻ , whereas the axial positions are occupied by two 4-pyridyl nitrogen atoms (N1 and N3) of two different L4 linkers with Cd2−N bond distances 2.317(4) and 2.322(4) Å (Figure 3a). Each hepta-coordinated Cd2 center is attached with three different bridging succinates by means of chelation and oxo-bridging, whereas each hexa-coordinated Cd1 center is oxo-bridged with two different succinates which are chelated with Cd2. Thus, two geometrically different Cd(II) centers along with succinate creates a 2D arrangement in the crystallographic ab plane (Figure 3b). It is interesting to note that the close integrity of the Cd2, created by the succinate ligand, produces the unit cell with double number of Cd2 with

O2b where b = 3/2 − x, 3/2 − y, 1 − z) from two different oxobridged succinate and two O atoms of coordinated water (O1w, O1wb) [bond lengths are in the range of 2.320(5)−2.332(4) Å]. Two 4-pyridyl nitrogen atoms, N2c and N2d (where c = x, 1 − y, 1/2 + z, d = 3/2 − x, 1/2 + y, 1/2 − z), of two different L4 linkers occupies the axial position of the octahedron [Cd1−N length 2.293 Å ] (Figure 3a). However, the hepta-coordinated Cd2 shows distorted pentagonal bipyramidal geometry with CdO5N2 chromophore. In the case of Cd2 the equatorial positions are occupied by five oxygen atoms (O1, O2, O3, O4 and O4a, where a = 2 − x, 1 − y, 1 − z) from three different bridging succinate moiety with Cd2−O bond distances varying F



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respect to Cd1. The flexible linear N, N′-donor linker (L4) connects the 2D metal-carboxylate in a pillared fashion to create a rectangular grid-like 3D network with lattice guest water molecules and nitrate counteranion occupied 1D channel along the crystallographic b-axis (Figure 3c). Unlike the previous three structures, this one does not show any significant void after the removal of the guest water molecules due to the presence of nitrate counteranion in the channels. Moreover, the flexible nature of the L4 and the coordinated water also has a crucial role in squeezing the channels. Thermogravimetric Analysis and the Framework Stability. Thermogravimetric analysis (TGA) of all four compounds shows that after the loss of lattice water molecules the frameworks are quite stable up to a moderately high temperature. Compound 1 shows a weight loss of ∼10.5% around 40−130 °C, which corresponds to the loss of three lattice water molecules (calc. wt % 10.3; Figure S3), and the dehydrated form is stable up to 240 °C. The PXRD pattern (Figure 4) after complete removal of the water molecules shows a pattern similar to that of as-synthesized 1, which clearly suggests the rigid nature of the framework. Similarly, a weight loss of ∼8.1% is observed in case of 2, which was completed at ∼125 °C and the dehydrated framework is stable up to 190 °C. The weight loss corresponds to the loss of two guest water molecules (calc. wt % 8; Figure S4). Here also the PXRD patterns (Figure 4) of dehydrated and as-synthesized compound shows an identical nature, indicating the robustness of the framework. Compound 3 shows a weight loss of ∼13.5% around 40−105 °C corresponding to the loss of four guest solvent water molecules (calc. wt % 13.37; Figure S5), and the dehydrated from is stable up to 240 °C. The PXRD pattern of dehydrated framework of 3 also shows a similar nature to that of the as-synthesized compound (Figure 4). For compound 4, a weight loss of ∼8.6% is observed at around 40−130 °C indicating the loss of four lattice waters and two coordinated water molecules (calc. wt % 7.9; Figure S6) in a single step and the dehydrated framework remains stable up to 260 °C. Gas Adsorption Measurements. Dehydrated frameworks of 1, 2, and 3 contain solvent accessible void, and pore sizes comparable to the kinetic diameter of N2 (3.65 Å) and CO2 (3.40 Å). Thus to evaluate the porosity and the adsorption capability of these three compounds, we have carried out the N2, H2, and CO2 adsorption measurements of the three compounds. Dehydrated framework of all three compounds does not show any N2 adsorption, possibly due to the very close pore size comparable to the kinetic diameter of N2. Moreover, there is only a one-dimensional channel system in all three frameworks (Figures S7−S9), and there are no additional channels along the crystallographic a and b axes in the case of the dehydrated framework of 1 and 2 (Figures S10 and S11) and the b and c axes in that of 3 (Figure S12). Therefore, at a low measuring temperature (77 K) the window of the channels is blocked by the entering N2 molecule, which prevent further diffusion of N2 molecules. Another possible reason for this may be due to the quadruple interactions of N2 with the electrostatic field gradients near the pore surface, which subsequently block other molecules from entering the pores.23 The H2 and CO2 adsorption properties of 1 and 2 are shown in Figure 5. All these compounds show reversible type-I H2 and CO2 adsorption isotherms at 77 and 273 K, respectively. In addition, the presence of adsorption−desorption hysteresis indicates that H2 and CO2 both are reversibly adsorbed by compounds 1 and 2 reported here. At 1 bar pressure and 77 K

Figure 5. H2 and CO2 uptake properties of 1 and 2. (a) H2 adsorption isotherms below 1.0 bar pressure for 1 (green) and 2 (blue) at 77 K. (b) CO2 adsorption isotherms below 1.0 bar pressure for 1 (green) and 2 (blue) at 273 K temperature. Filled and open circles represent adsorption and desorption respectively.

temperature, 1 and 2 adsorb 0.49 and 0.38 wt % of H2, respectively (Figure 5a). The difference between the H2 uptakes of 1 and 2 is well anticipated as comparatively larger pores (4.4 × 5.3 Å2) of 1 with higher solvent accessible void (32.3%) compared to the pore size (4.2 × 3.1 Å2) and solvent accessible (31.2%) void of compound 2. Although H 2 adsorption isotherms are collected at 77 K, the probable reason for H2 uptake at same temperature is less kinetic diameter of H2 (2.89 Å) compared to N2 (3.65 Å). The CO2 adsorption isotherms collected for compounds 1 and 2 at 273 K show reversible uptake of 5.86 and 4.47 wt % of CO2 respectively, as the pressure approaches to 1 atm (Figure 5b). The CO2 uptake shown by compounds 1 and 2 can be well justified as mentioned earlier due to difference in pore size and solvent accessible void. A possible reason for the selective CO2 adsorption over N2 at 273 K is at this high temperature the quadruple interactions overcome by the thermal energy in the case of CO2 adsorption, which does not block the pore openings even exists in only one direction.23 Although the CO2 uptake shown by 1 and 2 is not so high, this type of reversible uptake shown at 273 K and atmospheric pressure can be comparable with commercially available activated carbons like BPL carbon24 and well-known ZIFs. This type of gas adsorption phenomena is also found in reported similar frameworks of Cd(II).25 The H2 and CO2 adsorption capacity shown by 1 and 2 is comparable with known reported cadmium based MOFs, whereas some of other Cd(II) functionalized MOFs or MOFs with open metal sites shows higher H2 and CO2 uptake than those of 1 and 2 (Table S1 in Supporting Information). In spite of the existence of comparable solvent accessible void and pore size dimension, compound 3 does not show any significant uptake of CO2 (Figure S13) and a very less H2 uptake (Figure S14). This is probably due to the squeezing of the pore raised by the use of bent linker L3. The bent organization of the sheets prevent the incoming gas molecule and devoid it to show any measurable uptake. G



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CONCLUSION We have successfully synthesized four new 3D MOFs of Cd(II) by using succinate dianion and different N,N′-donor spacers. Here the size and nature of the solvent filled pore are nicely controlled by the structure and length of different linkers. By using the straight L1 and L2 linker in 1 and 2, 3D frameworks containing water-filled channels are observed, whereas in the case of 3, the change of the linker from linear to bent (L3) leads to the partial blockage of water filled channels which also affected the porosity and adsorption properties of its dehydrated framework. In 4, although the used spacer is linear, it forms a 3D framework, very much different from the others, with blocked channels filled with nitrate counteranion and lattice water. This has been observed due to the flexible nature of the -(CH2)2- group of L4 spacer in the case of 4. The dehydrated frameworks of 1 and 2 exhibit significant hydrogen and carbon dioxide adsorption properties, but 3 shows almost negligible adsorption and in 4 there is no void at all. Thus this work is a nice example of tuning of the structure of the metal−organic architecture, which is reflected in their functionality too. Here the structure of the pore has nicely been modified simply by the modification of ligands, which is the key point for the design of functional porous material. In summary, this work represents the design of different pore morphologies by the suitable design of building blocks.

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ASSOCIATED CONTENT

S Supporting Information *

X-ray crystallographic data in CIF format for the structures reported in this paper along with different figures related to the crystal structure, thermogravimetric analysis, and adsorption property of compound 3 are reported. These are available in free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*(D.G.) E-mail: [email protected]; fax: +9133 2414 6223. (R.B.) E-mail: [email protected]; fax: +91 20 25902636. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Authors gratefully acknowledge the financial assistance given by the DST and UGC, Government of India (grant to D.G.). The fellows also acknowledge UGC (B.B.) and CSIR (R.D.) for their senior research fellowship. The X-ray diffractometer facility under the DST-FIST program of Department of Chemistry (JU) is also gratefully acknowledged.

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