acs%2Ecgd%2E5b00056

Synthesis and Characterization of Polyhedral-Based Metal−OrganicFrameworks Using a Flexible Bipyrazole Ligand: Topological Analysisand Sorption Property StudiesKapil Tomar, Richa Rajak,⊥ Suresh Sanda, and Sanjit Konar*

Department of Chemistry, IISER Bhopal, Bhopal 462066, India

*S Supporting Information

ABSTRACT: Six porous metal−organic frameworks (MOFs),{[Ni(BTC)0.66(BPz)2]·2MeOH·4H2O}n (1), {[Co(BTC)0.66-(BPz)2]·2MeOH·4H2O}n (2), {[Mn(BTC)0.66(BPz)2]·2MeOH·4H2O}n (3), {[Cd(BDC)(BPz)(H2O)]·2MeOH·DMF}n (4), {[Cd2(NH2-BDC)2(BPz)(H2O)]·MeOH·H2O·DMF}n (5), and {[Co(BDC)(BPz)(H2O)]}n (6) (whereH3BTC = 1,3,5-benzenetricarboxylic acid, H2BDC = 1,4-benzenedicarboxylic acid, NH2-H2BDC = 2-amino-1,4-benze-nedicarboxylic acid, and BPz = 3,3′,5,5′-tetramethyl-4,4′-bipyrazole), were obtained through a solvent diffusiontechnique and characterized. The networks exhibit a varietyof topologies: 1, 2, and 3 are isostructural and possessoctahedral and cuboctahedra type cages and exhibit 3,6-cbinodal net having loh1 topology, 4 is a two-dimensional MOF having one-dimensional open channels with a 4-c uninodal nethaving sql topology, 5 exhibits a three-dimensional (3D) porous MOF having a 3,3,4,8-c net with a new topology having thename, skr1, whereas 6 discloses a 3D nonporous network which exhibits a 4-c uninodal net having CdSO4 topology. Beingisostructural, gas sorption studies of 1−3 show nearly the same CO2 sorption at 195 K of ∼90 mL g−1, whereas 4 and 5 show amaximum uptake of 42 and 37 mL g−1 at 195 K. Vapor sorption studies of 1−3 reveal stepwise uptake of water with a finalamount reached to nearly 350 mL g−1, whereas 4 and 5 show maximum uptake of 110 and 90 mL g−1, respectively. Compared tothe free ligand BPz, photoluminescence studies of 4 and 5 show red shifts and emit in the blue-green region with λmax at 430 and472 nm for 4 and 5, respectively.

In the past few decades, the design and synthesis of porousmetal−organic frameworks (MOFs) have attracted extensive

interest owing to their variety of architectures and intriguingtopologies1−9 as well as potential applications.10−21 Especiallythe supramolecular assemblies of discrete cages and cagelikepolyhedral-based MOFs have become a topic of interest as theyare different from typical MOFs having open channel typepores, whereas polyhedral-based MOFs consist of individualcages carrying large voids which are connected through smallwindows.22−34 These features make polyhedral-based MOFsmore useful in storage of small molecules.35 The potential useof such compounds largely depends on their cavity shape andsize, which are largely controlled by the type of organic ligandused in the construction. These cage-based MOFs have beenprepared mostly by rigid linkers, whereas flexible or semirigidlinkers have been rarely used.36,37 It is a well-established factthat network topologies are greatly influenced by linkermolecules. Especially, in the case of flexible spacers, newtopologies can be discovered as the changeable conformationand flexibility may lead to richness and unpredictability of theformed MOF structures.38−43

In the category of semirigid ligand, 3,3′,5,5′-tetramethyl-4,4′-bipyrazole (BPz) is a well-known linker with which several

MOFs have been reported recently.44−56 The advantageassociated with BPz-based MOFs is that they possessesconsiderable flexibility due to the free rotation of C−C bondbetween the pyrazole rings. This results in various possibilitiesof coordination networks due to non-collinear orientation ofthe two N−metal bonds. Furthermore, BPz can act as a neutralbridging ligand to ligate two metal ions, or it can act as amonoanionic or dianionic linker to bridge more than two metalions. These features may result in a diverse range ofdimensionalities and topologies in BPz-based MOFs. Addition-ally, due to the robustness of the metal−azole bonds, the azole-based MOFs display good thermal stability and resistancetoward water. Although with BPz many porous MOFs arereported, this is the first time where polyhedral-based porousassemblies were obtained. Along with a substantial amount ofCO2 uptake, these MOFs showed unexpectedly high andstepwise water sorption (∼350 mL g−1). This high uptake ofwater is contradicting the fact that the cages are decorated withhydrophobic moieties (methyl groups of BPz).

Received: January 14, 2015Revised: March 16, 2015

Article

pubs.acs.org/crystal

© XXXX American Chemical Society A DOI: 10.1021/acs.cgd.5b00056Cryst. Growth Des. XXXX, XXX, XXX−XXX

Herein, we report six MOFs out of which three areisostructural polyhedral-based porous MOFs, namely, {[Ni-(BTC)0.66(BPz)2]·2MeOH·4H2O}n (1), {[Co(BTC)0.66-(BPz)2]·2MeOH·4H2O}n (2), and {[Mn(BTC)0.66(BPz)2]·2MeOH·4H2O}n (3). Also, we employed H2BDC/NH2-H2BDC and BPz ligands with Co2+ and Cd2+ ions to producethree new MOFs, namely, {[Cd(BDC)(BPz)(H2O)]·2MeOH·DMF}n (4), {[Cd2(NH2-BDC)2(BPz)(H2O)]·MeOH·H2O·DMF}n (5), and {[Co(BDC)(BPz)(H2O)]}n (6) (where,H3BTC = 1,3,5-benzenetricarboxylic acid, H2BDC = 1,4-benzenedicarboxylic acid, NH2-H2BDC = 2-amino-1,4-benze-nedicarboxylic acid) (Schemes 1 and 2).

■ EXPERIMENTAL SECTIONMaterials and General Procedure. All the reagents and solvents

were procured from S. D. Fine Chemicals, India. BPz was synthesizedaccording to a literature method.57 H3BTC, H2BDC, and NH2-H2BDC were obtained from the Sigma-Aldrich Chemical Co. and usedas received.The elemental analysis was carried out on Elementar Micro vario

Cube elemental analyzer. The IR spectrum of the compounds 1−6were recorded on a PerkinElmer FT-IR Spectrum BX using the KBrpellets in the region of 4000−400 cm−1. Thermogravimetric analysis(TGA) was carried out (PerkinElmer) in the temperature range of30−700 °C (heating rate 5 °C min−1). Powder X-ray diffraction(PXRD) data were collected on a PANalytical EMPYREANinstrument using Cu−Kα radiation.Sorption Studies. In order to evaluate the porous property of all

the complexes, the crystalline materials are ground to powder andactivated by drying under a vacuum at 100 °C for 10 h. Gas and

solvent vapor adsorption studies were performed using BELSORPMAX and BELSORP AQUA (BEL JAPAN) volumetric adsorptionanalyzer.

Crystal Data Collection and Structure Determination.Suitable single crystals of each of the complexes were mounted on aBruker SMART II diffractometer equipped with a graphitemonochromator and Mo−Kα (λ = 0.71073 Å, 140 K) radiation.Data collection was performed using a φ and ω scan. The structureswere solved using direct methods followed by full matrix least-squaresrefinements against F2 (all data HKLF 4 format) using SHELXTL.Subsequent difference Fourier synthesis and least-squares refinementrevealed the positions of the remaining non-hydrogen atoms.Determinations of the crystal system, orientation matrix, and celldimensions were performed according to the established procedures.Non-hydrogen atoms were refined with hydrogen atoms placedgeometrically and refined using the riding model. All calculations werecarried out using SHELXL 97,58 PLATON 99,59 and SHELXTL60

program packages. The solvent molecules in 1−5 are highlydisordered. Therefore, the SQUEEZE61 program was used, and anew .HKL file was generated. The structures were solved by using thenewly generated .HKL file. Structure refinement after modification ofthe data with the SQUEEZE led to better refinement and dataconvergence. Detail instruction from the .SQF file is included in thefinal cif file. The solvents molecules were calculated on the basis of acombined study of TGA, elemental analysis, and removed electroncounts which are included in the molecular formula. Data collectionand structure refinement parameters and crystallographic data for thesix complexes are given in Table 1.

Synthesis of {[Ni(BTC)0.66(BPz)2]·2MeOH·4H2O}n (1). Thesodium salt of H3BTC (0.2 mmol, 46 mg), Ni(NO3)2·6H2O (0.2mmol, 60 mg), and BPz (0.3 mmol, 57.6 mg) were dissolvedseparately in 12 mL of water and 15 mL of methanol solution,respectively. Two milliliters of the metal and BPz solution was slowlyand carefully layered above 2 mL of Na3+BTC3− solution in a narrowglass tube using 1 mL of buffer (1:1 H2O and MeOH) solution. Lightgreen, block-shaped single crystals were obtained from the junction ofthe layers after 1 week. The crystals were separated and washed withMeOH and air-dried (yield: 55%). Elemental analysis: Anal. Calcd ForC28H48N8O10Ni (%): C, 47.01; H, 6.76; N, 15.66; Found (%): C,47.45; H, 5.96; N, 16.04. FT-IR (KBr pellet, cm−1): 3444 (b), 1622(s), 1568 (s), 1400 (s), 1072 (m), 830 (w), 785 (w), 533 (w) (FigureS1a, Supporting Information).

Scheme 1. Schematic Drawing of the Ligands

Scheme 2. Synthetic Scheme of 1−6

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DOI: 10.1021/acs.cgd.5b00056Cryst. Growth Des. XXXX, XXX, XXX−XXX

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Table 1. Crystal Data and Structure Refinement for 1−6

1 2 3 4 5 6

empirical formula C26H30NiN8O4 C26H30CoN8O4 C26H30MnN8O4 C18H20CdN4O5 C26H26Cd2N6O9 C18H20CoN4O5

formula weight 577.27 577.51 573.52 484.79 791.35 431.31crystal system trigonal trigonal trigonal monoclinic monoclinic monoclinicspace group R3c R3 c R3c P21/n P21/n P21/na (Å) 19.688(15) 19.791(6) 19.935(6) 10.340(5) 16.224(5) 11.218(9)b (Å) 19.688(15) 19.791(6) 19.935(6) 12.798(5) 12.979(4) 17.326(10)c (Å) 48.637(5) 48.771(15) 50.377(13) 19.845(5) 16.226(5) 11.235(8)α (deg) 90 90 90 90 90 90β (deg) 90 90 90 99.722 (5) 100.056 (10) 116.602 (5)γ (deg) 120 120 120 90 90 90V (Å3) 16327(2) 16544(9) 17339.0(9) 2588.4(17) 3364.1(17) 1952.7(2)Z 18 18 18 4 4 4temperature (K) 140(2) 140(2) 140(2) 140(2) 140(2) 140(2)θ range 2.00−28.21 2.47−27.92 2.28−27.67 2.34−27.47 2.24−27.12 2.37−27.38goodness-of-fit 1.155 0.999 1.168 0.989 0.996 0.827R1a 0.065 0.066 0.068 0.050 0.080 0.052

wR2b 0.132 0.173 0.152 0.126 0.194 0.105

F(000) 5436.0 5418.0 5382.0 976.0 1568.0 892.0aR1 = Σ∥Fo| − |Fc∥/Σ|Fo. bR2 = [Σ{w(Fo2 − Fc

2)2}/Σ{w(Fo2)2}]1/2.

Figure 1. (a−c) Coordination environments of Ni2+, Co2+, and Mn2+ in 1−3. (d) A view of the octahedral and cuboctahedral in 1. (e) 3D packingview of 1 along the c-axis. (f) N−H···O (N···O = 2.672(3) Å) type hydrogen bonding in 1. (g) Schematic diagram depicting the connectivity of twopolyhedral in 1−3. (h) Connolly surface in 1 showing the solvent accessible void volume. (i) Topology of 1−3, showing the 6,3 connected nodes.

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Synthesis of {[Co(BTC)0.66(BPz)2]·2MeOH·4H2O}n (2). The samediffusion technique of 1 was employed for the synthesis of compound2 using Co(NO3)2·6H2O (0.2 mmol, 58.2 mg), BPz (0.3 mmol, 57.6mg) and sodium salt of H3BTC (0.2 mmol, 46 mg). Purple-coloredcrystals were obtained after 1 week. The crystals were separated andwashed with MeOH and air-dried (yield: 59%). Elemental analysis:Anal. Calcd For C28H48N8O10Co (%): C, 46.99; H, 6.76; N, 15.66;Found (%): C, 46.32; H, 5.78; N, 15.14; FT-IR (KBr pellet, cm−1):3428 (b), 1627 (s), 1420 (s), 1386 (s),1308 (w), 1233 (m), 1191 (s),1116 (b), 1028 (m), 958 (m), 798 (s), 697 (s) (Figure S1b,Supporting Information).Synthesis of {[Mn(BTC)0.66(BPz)2]·2MeOH·4H2O}n (3). The

same diffusion technique of 1 was employed for the synthesis ofcompound 2 using MnCl2·4H2O (0.2 mmol, 39.58 mg), BPz (0.3mmol, 57.6 mg), and sodium salt of H3BTC (0.2 mmol, 46 mg).Colorless crystals were obtained after 12 days. The crystals wereseparated and washed with MeOH and air-dried (yield: 50%).Elemental analysis: Anal. Calcd For C28H48N8O10Mn (%): C, 47.26;H, 6.80; N, 15.75; Found (%): C, 47.05; H, 6.16; N, 15.04; FT-IR(KBr pellet, cm−1): 3428 (b), 1626 (s), 1420 (s), 1386 (s),1308 (w),1233 (m), 1191 (s), 1116 (b), 1028 (m), 958 (m), 798 (s), 697 (s)(Figure S1c, Supporting Information).Synthesis of {[Cd(BDC)(BPz)(H2O)]·2MeOH·DMF}n (4). The

same diffusion technique of 1 was employed for the synthesis ofcompound 4 using Cd(NO3)2·4H2O (0.2 mmol, 61.7 mg), BPz (0.3mmol, 57.6 mg), and sodium salt of H2BDC (0.2 mmol, 33.23 mg).Colorless crystals were obtained after 1 week. The crystals wereseparated and washed with MeOH and air-dried (yield: 62%).Elemental analysis: Anal. Cald. For C23H35CdN5O8. (%) C, 44.42; H,5.67; N, 11.26; Found (%): C, 43.85; H, 5.23; N, 11.07; FT-IR (KBrpellet, cm−1): 3428 (b), 1629 (s), 1420 (s), 1386 (s),1308 (w), 1233(m), 1191 (s), 1116 (b), 1028 (m), 958 (m), 798 (s), 697 (s) (FigureS1d, Supporting Information).Synthesis of {[Cd2(NH2-BDC)2(BPz)(H2O)]H2O·MeOH·DMF}n

(5). The same diffusion technique of 1 was employed for the synthesisof compound 5 using Cd(NO3)2·4H2O (0.2 mmol, 61.7 mg), BPz (0.3mmol, 57.6 mg), and sodium salt of NH2-H2BDC (0.2 mmol, 36.23mg). Colorless crystals were obtained after 1 week. The crystals wereseparated and washed with MeOH and air-dried (yield: 65%)Elemental analysis: Anal. Calcd For C30H39Cd2N7O12(%): C, 39.40;H, 4.30; N, 10.72. Found (%): C, 39.05; H, 4.47; N, 11.07. FT-IR(KBr pellet, cm−1): 3413.04 (b), 3222.82 (b), 2929.34 (w), 1595.66(b), 1415.95 (b), 1258.4 (w), 1187.77 (w) (Figure S1e, SupportingInformation).Synthesis of {[Co(BDC)(BPz)(H2O)]}n (6). The same diffusion

technique of 1 was employed for the synthesis of compound 6 usingCo(NO3)2·6H2O (0.2 mmol, 58.2 mg), BPz (0.3 mmol, 57.6 mg), andsodium salt of H2BDC (0.2 mmol, 33.23 mg). Colorless crystals wereobtained after 1 week. The crystals were separated and washed withMeOH and air-dried (yield: 60%). Elemental analysis: Anal. Calcd ForC18H20CoN4O5 (%): C, 50.12; H, 4.67; N, 12.99. Found (%): C,49.58; H, 4.36; N, 12.35. FT-IR (KBr pellet, cm−1): 3423.04 (b),3226.82 (b), 2927.34 (w), 1581.66 (b), 1418.95 (b), 1261.4 (w),1188.77 (w) (Figure S1f, Supporting Information).The bulk amount of the compounds 1−6 were prepared in powder

form by the direct mixing of the ligands mixture with a correspondingsolution of metal salt followed by overnight stirring. All major peaks inexperimental PXRD of compounds 1−6 match well with simulatedPXRD, which indicates equitable crystalline phase purity (Figure S2,Supporting Information).

■ RESULTS AND DISCUSSIONStructural Description of Complexes 1, 2, and 3. Since

all three complexes are isostructural, the detailed structuraldescription of only complex 1 is discussed here. Complex 1crystallizes in trigonal system and R3 c space group. Theasymmetric unit contains one-half occupied Ni(II) ion, one BPzligand, and one-third of BTC3− molecule as well as somedisordered solvent molecules. As shown in Figure 1a, the Ni(II)

ion is coordinated by four nitrogen atoms of four BPzmolecules and two oxygen atoms from two BTC3− molecules inan NiN4O2 environment giving it a distorted octahedralcoordination geometry. The equatorial plane is formed byfour nitrogens, while the two oxygen atoms occupy the axialpositions with measured bond distances of Ni−N in the rangeof 2.092(3)−2.098(2) Å, while the Ni−O distance is 2.102(5)Å. The O−Ni−O angle is 173.77 (5)°. The interpyrazoledihedral angle for the BPz ligand is 63.30(4)° with exo-bidentate coordination mode. The BTC3− anion is in a (κ1-κ0)-(κ1-κ0)-(κ1-κ0)-μ3 coordination mode (Scheme 3). This type of

monodentate coordination mode of BTC3− ligand is rare inMOFs and is observed only in a few compounds reported byMukherjee and Rosseinsky groups.62−64 The free oxygen of the-COOH group is engaged in hydrogen bonding with the -NHmoiety of the pyrazole ring (N2−H2···O2, N2···O2, 2.672(3)Å) (Figure 1f).More insight into the structure revealed that [Ni-(BTC)0.66]n

is arranged into a 2D neutral (6,3) connected sheets withhexagonal rings along the c-axis (Figure S3a, SupportingInformation). The second honeycomb sheet was stacked onfirst with an offset and was pillared by BPz molecules, whichultimately resulted in a pillar-layered porous 3D MOF (FigureS3b, Supporting Information). From a topological point ofview, the overall framework is made up of two types ofpolyhedral cages, cuboctahedral and octahedral. The cubocta-hedral cage is constructed from 12 Ni(II) ions, 12 BPz, andeight BTC3− molecules. The cage consists of eight triangularand six square faces and can enclose a sphere of diameter 6 Å.The octahedral cage involves six Ni(II) ions, six BPz, and twoBTC3− molecules and can enclose a sphere of diameter 3 Å(Figure 1d). In the crystal, one cuboctahedral cage is joined tosix others by sharing a square window and to eight otheroctahedral cages by sharing a triangular window to form a 3Dporous architecture (Figure 1e).The formed voids in the two polyhedral cages are

interconnected through relatively narrow passages as thetriangular and square faces are effectively blocked by methylgroups of the BPz molecules. This feature also prevents theinterpenetration of the framework and allows a substantialamount of porosity. The total solvent-accessible volume for 1was estimated to be 36.9% (6029.2 Å3 out of 16326.8 Å3) by

Scheme 3. Various Bridging Modes of Ligands in 1−6

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summing voxels more than 1.2 Å away from the frameworkusing PLATON software.Topologically, each Ni(II) ion has been linked by six ligands;

therefore it can be simplified into a 6-connected node. The BPzligand can be omitted due to its linear geometry, and oneBTC3− molecule is connected to three Ni(II) molecules;therefore it can be considered as a three connected node. Thewhole network can thus be viewed as a 3,6-c binodal net(Figure 1i) with stoichiometry (3-c)2(6-c)3. It has a Pointsymbol of {43}2{4

6.66.83}3 which is assigned to topological type,loh1 (personal.ttd).Structural Description of Complex 4. The asymmetric

unit has one each of Cd(II), BPz, BDC2− and one coordinatedwater molecule along with disordered solvent molecules. Asshown in Figure 2a, the Cd(II) ion is coordinated from fouroxygen atoms of BDC2−, two nitrogen atoms of BPz, and anoxygen atom of coordinated water in a CdO5N2 environmentgiving it a distorted pentagonal bipyramidal geometry. Thenitrogen atoms occupy the axial positions, while the oxygenatoms form the equatorial plane. The Cd−O bond distances fallin the range 2.299(4)−2.567(3) Å, while the Cd−N distancesare measured as 2.337(3) and 2.363(2) Å. The BDC2− anion isin a chelating (κ1)-((κ1)-μ2 coordination mode. Similar tocomplexes 1−3, here also, the -COOH group is engaged inhydrogen bonding with the -NH moiety of the pyrazole ring.The [Cd-BDC]n moiety forms infinite parallel chains which

are connected to each other by BPz molecules to form a 2Dopen gridlike undulated sheet structure which is shown inFigure 2b. A similar type of 2D grid was also observed in theZn-BDC-BPz system reported by He et al.,65 but in 3D packingthe open pores vanished due to 2-fold interpenetration. TwoCd···Cd distances in the grid is measured as 10.340 (4) and11.302 (3) Å. The dihedral angle between the pyrazole rings inBPz is found to be 75.77 (4)°, which is quite higher thancomplexes 1−3, having an exo-bidentate co-ordination mode.The 2D sheets stack upon each other in such a way that a

continuous 1D channel formation occurs along the b-axis withrough dimensions of 8.88 × 6.78 Å (Figure 2c). In the 3Dpacking, the sheets were held together by O−H···O (O···O =2.738(8) Å) type hydrogen bonds between coordinated watermolecules and carboxylate groups of BDC2− molecules in theadjacent sheets (Figure 2d). The channels are filled withdisordered solvent molecules which were removed by theSQUEEZE command of PLATON. The total solvent accessiblevoid volume as estimated by PLATON is found to be 31.5%(815.0 Å3 out of 2588.4 Å3).The topological analysis reveals that the Cd(II) center acts as

a four-connected node, while the BDC2− and BPz ligands wereconsidered as simple linkers; therefore one 2D layer can beenvisioned as a 4-c uninodal net (Figure 2f). It has a Pointsymbol of {44.62}, which is assigned to topology type sql/Shubnikov tetragonal plane net (topos & RCSR.ttd).

Structural Description of Complex 5. The asymmetricunit of 5 consists of two crystallographically independentCd(II) ions, two NH2−BDC2− molecules, one bipyrazole, andone coordinated water molecule along with disordered solventmolecules in the lattice. The Cd1 and Cd2 acquire distortedpentagonal bipyramidal geometry (Figure 3a). The Cd1 iscoordinated to four oxygen atoms of two BDC2−, one oxygenatom of water molecule, one nitrogen atom of the -NH2 groupof BDC2−, and one nitrogen atom from BPz ligand. The Cd2 iscoordinated to five oxygen atoms of three BDC2−, one nitrogenatom of -NH2, and one nitrogen atom of BPz ligand. In boththe metal centers nitrogen atoms are located at the axialpositions, while the oxygen atoms form the equatorial plane.The middle of the two Cd2···Cd2 centers is present on theinversion center with a Cd2···Cd2 distance of 3.881 Å. TheCd−O bond distances for Cd1 are in the range 2.290 (3)−2.557 (6) Å, while for Cd2 the range is 2.316 (3)−2.561 (4) Å.The Cd−N distances for Cd1 and Cd2 are 2.316 (3) Å, 2.385(5) Å and 2.316 (4) Å, 2.459 (6) Å, respectively. Both theBDC2− molecules bind to the metal in a chelating mode

Figure 2. (a) Coordination environment of Cd2+ in 4. (b) A square grid type 2D sheet in 4. (c) Stacking of 2D layers to form porous network in 4.(d) O−H···O (O···O = 2.738(8) Å) type hydrogen bonding between 2D layers. (e) Connolly surface in 4 showing the pore shape. (f) 4 connectedsql topology in 4.

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through −COOH groups, while the -NH2 group alsocoordinates to another Cd(II) ion giving a (κ2-κ1-μ2)-(κ

1)-(κ1)-μ4 coordination mode. Both the NH2-H2BDC and BPzligands were also used by Lin et al.66 with Zn (II) ions wherethey obtained a 3D porous MOF with six connected topology.But coordination from the -NH2 group was not observed intheir case. The dihedral angle between the two pyrazole rings is58.81°, which is the lowest among complexes 1−6 with exo-bidentate coordination mode. The BDC2− molecule connectsto the Cd(II) ion in such a way that it forms a neutral doublelayer type 2D structure along the b-axis, which is shown inFigure S4, Supporting Information. These layers are furtherconnected by BPz molecules to form a 3D porous MOF(Figure 3b) with irregular-shaped pores along the b-axis that arefilled with disordered solvent molecules. The methyl groups ofBPz blocked the channels in such a way that it formed cageswhich are connected to each other through a small opening andthat is revealed from the analysis of Connolly surface as shownin Figure 3d. The disordered solvent molecules were removedby SQUEEZE command. Also, the total solvent accessiblevolume is found to be 24.4% (822.0 Å3 out of 3364.1.4 Å3).Better insight into the complicated 3D architecture can be

achieved by topological analysis. Th cluster method inTOPOS67 has to be applied as there are carboxylate bridgeddinuclear Cd(II) ions. The dinuclear Cd(II) center has beenassigned an eight-connected node, the other mononuclearCd(II) center is considered as a four connected node, while thetwo NH2-BDC

2− ligands were considered as three connectednodes. Finally, the analysis revealed the complex to be a 3,3,4,8-c net with stoichiometry (3-c)2(3-c)2(4-c)2(8-c) having a 4-nodal net (Figure 3c). It has a Point symbol of {42.64}2{4

2.6}4-{44.610.88.106}, which is assigned to a new topological type,having the name skr1 (topos & RCSR.ttd).Structural Description of Complex 6. The asymmetric

unit of 6 contains one molecule each of Co(II) ion, BPz, andBDC2− along with one coordinated water molecule. The Co(II)ion is coordinated from three oxygen atoms of two BDC2−, twonitrogen atoms of pyrazole, and one oxygen atom ofcoordinated water molecule in an CoO4N2 environment givingit a distorted octahedral geometry (Figure 4a). The Co(II)−O

distances fall in the range 2.031(4)−2.195(3) Å, while theCo(II)−N distances are found to be 2.137(2) and 2.151(4) Å.The oxygen atoms from BDC2− form the equatorial plane,while the nitrogen atoms from BPz occupy the axial positionswith a N−Co−N angle of 177.79 (5)°, which is very close tothe ideal angle of 180°. The dihedral angle between thepyrazole rings is measured to be 80.40°, which is the highestvalue among complexes 1−6 having endo-bidentate coordina-tion mode. Out of two types of crystallographicallyindependent BDC2− molecules, one has monodentate, whilethe other has a chelating mode of coordination with (κ1)-((κ1)-μ2 coordination mode. These BDC2− molecules form a 1Dinfinite chain with Co(II) ion and are further connected by BPzmolecules to form a 3D network (Figure 4c). Although thenumber and type of ligands coordinated in 6 is similar to 4, theresulting geometries are different. In 4, both the -COOHgroups coordinated in chelating mode, while in 6 one of the-COOH is showing monodentate mode (due to the highcoordination number of Cd2+ than Co2+). Along with this, onemore difference is in the dihedral angles of the BPz ligand inboth complexes (75.77° in 4 and 80.40° in 6). These twodifferences gave entirely different structures with a 2D networkin 4 (sql topology) and 3D network in 6 (CdSO4 topology).Topological analysis shows that Co(II) can be considered as

a 4-connected node (four ligands are connected to Co(II)),whereas BDC2− and BPz are considered only as simple linkers;therefore the whole network is classified as a 4-c uninodal netwith a Point symbol of {65.8} (Figure 4d). The assignedtopology is cds CdSO4; 4/6/t4; sqc5 (topos & RCSR.ttd).

PXRD and TGA Studies. To examine the thermal stabilityand phase purity of the synthesized frameworks, TGA andPXRD were carried out. The similarity of the PXRD data withthe simulated patterns based on single crystal data confirms thephase purity of the as-synthesized samples (Figure S2,Supporting Information). The PXRD patterns of the desolvatedcompounds 1−6 show that they retain their crystallinity afterpretreatment at 100 °C for 12 h. TGA analysis shows that afterthe loss of lattice solvent molecules all the frameworks are quietstable up to a moderately high temperature (Figure S5,Supporting Information). Further, details of the TGA are in theSupporting Information.

Figure 3. (a) Coordination environment of Cd2+ in 5. (b) 3D porousframework along the b-axis (methyl groups of BPz have been omittedfor clarity). (c) 3,3,4,8-c net in 5 with a new topological type, skr1. (d)Connolly surface in 5 showing the closed pores.

Figure 4. (a) Coordination environment of Co2+ in 6. (b) A portion ofthe 2D sheet struture in 6. (c) 3D packing diagram of 6. (d) Figuredepicting the 4-connected CdSO4 topology of 6.

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Sorption Studies. The CO2 sorption studies of complexes1−3 reveal that the isotherms are quiet similar owing to theirsimilar isostructural features and showed complete reversibilityand no or little hysteresis with a typical type I adsorption curve.As shown in Figure 5a−c, the adsorption curves showedselective sorption of CO2 gas at 195 K as compared to CH4 (at195 K) and N2 (at 77 K). The CO2 sorption increases abruptlyat low pressures and reaches a maximum uptake of 91, 81, and87 mL g−1 at 1 atm. In comparison, sorption of CH4 and N2 at195 and 77 K is very less, which amounts to only a maximumuptake of 10 mL g−1 of CH4 and 11 mL g−1 of N2 atatmospheric pressure. This can be due to the comparably largekinetic diameter of N2 (3.6 Å) and CH4 (3.8 Å) than CO2 (3.3Å); furthermore the low kinetic energy of the N2 molecules at77 K does not allow entering of the small windows of 1−3.Moreover, at atmospheric pressure, the CO2 uptakes of 1−3 at273 K reach 26.8, 27.5, and 27 mL g−1, respectively (Figure S6,Supporting Information). For complexes 4 and 5, the CO2

uptake at 195 K reaches a value of 42.3 and 37.2 mL g−1 at 1atm (Figure 5d,e). The open hysteresis loop of CO2 in 5 maybe attributed to the phenomenon of capillary condensation.68

The other possible reason could be the incomplete equilibriumduring the adsorption stage at low temperature and lowpressure region.69 This happens because at low temperaturediffusion and mobility of molecules is slower. The Brunauer−Emmett−Teller (BET) surface areas were estimated from theCO2 isotherms at 298 K to be 405−400 m2/g for 1−3 and 147and 132 m2/g for 4 and 5, respectively. The surface areas ofcomplexes 1−3 are comparable to some of the reported porousMOFs.70,71

We have also calculated the isosteric heat of adsorption (Qst)for CO2 using adsorption data collected at 298 and 273 K(Figure S7, Supporting Information). At the onset ofadsorption, Qst for compounds 1−3 is in the range of 74.2−67.6 kJ/mol, which is comparable to the Qst value in MIL-100(63 kJ/mol),72 and reflecting a strong and selective interactionof CO2 with the framework. From the crystal structures of 1−3it can be seen that all the carboxylate groups of BTC3− adopt amonodentate binding mode; that is, one O atom remains freein the polyhedral cages. In addition, one more interacting siteavailable for CO2 interaction inside the cage is the -NH moietyof the BPz linker. The high Qst values of 1−3 are mainly due to

Figure 5. (a) Gas adsorption isotherms of compounds (a) 1, (b) 2, (c) 3, (d) 4, (e) 5 at 195 K (CO2, CH4) and 77 K (N2), (f) Selectivity of CO2over N2 in 1−5 at 273 K, and (g) selectivity of CO2 over CH4 in 1−5 at 273 K.

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these polar sites inside the polyhedral cages. Furthermore, theQst for compound 5 (42 kJ/mol) is comparably higher than 4(37.6 kJ/mol), which can be attributed to the presence ofadditional -NH2 groups in 5, which provides additionalinteracting sites for CO2 molecules.The adsorption selectivities of CO2 with respect to N2 and

CH4 were calculated by the ideal adsorbed solution theory(IAST) method.73 For this purpose, the sorption isotherms ofCO2, CH4, and N2 gases at 273 K were taken (Figures S6 andS8, Supporting Information). The predicted adsorptionselectivities for equimolar CO2 and N2/CH4 mixtures in 1−5are shown in Figure 5f,g. The selectivity of CO2 over N2 for 1−3 has values in the range of 62−66 at 273 K. The selectivitydecreases with an increase in the bulk pressure. These valuesare comparable to those of well-known porous MOFs.74 Theselectivity of CO2 could be attributed to the small aperture ofthe windows which may discriminate between CO2 (smallkinetic diameter = 3.3 Å) and N2 (kinetic diameter = 3.6 Å) orCH4 (kinetic diameter = 3.8 Å). Additionally, the presence ofpolar groups (-COOH and -NH) may be responsible for astronger CO2−framework interaction due to a higher quadru-pole moment of CO2. For complexes 4 and 5 selectivity valuesare 29 and 34, respectively. A higher value for 5 than 4 can beattributed to the extra -NH2 groups in the framework whichincreases the interaction of CO2 molecules with the framework.The selectivity values of CO2 over CH4 at 273 K for 1−5 are∼15 for complexes 1−3, 5 for 4, and 7 for 5. Although thesevalues are less than CO2/N2 selectivity, they are stillcomparable with some of the reported microporousMOFs.75−77

The solvent vapor sorption study of complexes 1−3 shows astepwise adsorption of water as a function of pressure. Asshown in Figure 6a, compound 1 exhibits a sinusoidal typeadsorption curve with a steep rise in adsorption after a pressurethreshold of 0.4 P/P0 with a saturation uptake of 350 mL g−1,which is greater than the well-known Zn-BTC MOF (208 mL

g−1).78 Complexes 2 and 3 also show similar types of sinusoidaltype adsorption curves (Figure 6b,c) with large hysteresisresulting from different type of interactions at different pressurepoints which indicates the presence of two energeticallydifferent processes. Similar type of ‘S’ shaped isotherm wasalso observed in water sorption of MIL-101.79 The notablehysteresis in the low pressure region indicates that the largepolyhedral cages retain some of the water molecules due to thesmall window aperture. For 2 and 3, the water isotherms areinterrupted. This can be attributed to the accumulated watermolecules inside the cage which cannot escape easily during thedesorption process.80 Canivet et al. suggested that theinterruption can also arise from the weak flexibility of theframework (due to the rotation of the two pyrazole rings in BPzligand).81 As compared to high water sorption in 1−3,methanol and ethanol uptake is very less and amounts tonearly or equal to 100 mL g−1 of methanol and 50 mL g−1 ofethanol at 1 atm. The small size of water molecules ascompared to methanol or ethanol could be the reason for highuptake of water as there are no open metal or hydrophilic sitein 1−3. These studies reveal their possible application as adesiccant material. Complexes 4 and 5 also show comparablymore water sorption than ethanol or methanol with final uptakeof 90 and 110 mL g−1 respectively (Figure S9a,b, SupportingInformation).

Solid State Emission Studies. The emission spectra of 4and 5 were examined in the solid state at room temperature(Figure 7). The free ligand BPz displays emission maxima at422 nm. The complex 4 shows λmax at 430 nm, and 5 at 472nm, under the excitation at 326 nm. As Zn(II) or Cd(II) isdifficult to oxidize or reduce due to the d10 configuration,emission of 4 and 5 cannot be ascribed to LMCT or MLCT.Thus, intraligand and ligand to ligand charge transitions areresponsible for emission properties of 4 and 5.82 The red shiftof 4 and 5 relative to BPz are 8 and 50 nm, respectively. Thesered shifts may be attributed to ligand coordination to the metal

Figure 6. Vapor adsorption isotherms of compounds (a) 1, (b) 2, and (c) 3.

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ion, which increases the rigidity of the ligand and reduces theloss of energy by radiation less.

■ CONCLUSIONUsing polycarboxylates and bipyrazole ligand and differenttransition metal salts, we have successfully prepared six MOFsout of which five shows permanent porosity. Complexes 1−3exhibit isostructural features having octahedral and cuboctahe-dral type cages with loh1 topology. Compound 4 exhibits a 1Dopen channel, while compound 5 possesses a 3D network withclosed cages and exhibits a new topology having the name skr1.Compound 6 has a nonporous 3D MOF with CdSO4 topology.Complexes 1−5 exhibit selective CO2 sorption over N2 andCH4. Complexes 1−3 also exhibit high water sorption with finalamount of nearly 350 mL g−1 at room temperature.Compounds 4 and 5 exhibit solid state emission propertieswith λmax at 430 and 472 nm.

■ ASSOCIATED CONTENT*S Supporting InformationMore gas/vapor sorption isotherms, IR spectra, PXRD, TGAplots, isosteric heat plots, table for bond distances and angles,and extra images for compounds 1−6. This material is availablefree of charge via the Internet at http://pubs.acs.org.

■ AUTHOR INFORMATIONCorresponding Author*E-mail: [email protected] authors declare no competing financial interest.⊥(R.R.) Undergraduate researcher.

■ ACKNOWLEDGMENTSR.R. thanks IISER Bhopal for the INSPIRE fellowship. K.T.thanks CSIR, New Delhi, India, for Research Associateship.S.K. thanks CSIR and IISER Bhopal for generous financial andinfrastructural support.

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Figure 7. Solid-state emission spectra of 4 and 5 at room temperature.

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