Exploration of Structural Topologies in MetalâOrganic Frameworks

Article pubs.acs.org/crystal

Exploration of Structural Topologies in Metal−Organic Frameworks Based on 3‑(4-Carboxyphenyl)propionic Acid, Their Synthesis, Sorption, and Luminescent Property Studies Srinivasulu Parshamoni, Suresh Sanda, Himanshu Sekhar Jena, Kapil Tomar, and Sanjit Konar* Department of Chemistry, IISER Bhopal, Bhopal 462066, India S Supporting Information *

ABSTRACT: Four new compounds (two coordination polymers (CPs) and two metal organic frameworks (MOFs)), namely, [Zn(cpp)(H2O)]n (1), [Cu(cpp)(4-bpmh)]n·nH2O· nMeOH (2), [Cd2(cpp)2(3-bpmh)2]n4nH2O·2nMeOH (3), and [Cd(cpp)(bpy)(H2O)2]n·2nH2O (4), have been synthesized through the slow diffusion technique using cpp ligand and different neutral linkers (H2cpp = 3-(4-carboxyphenyl)propionic acid, 4-bpmh = N,N-bis-pyridin-4-ylmethylene-hydrazine, 3bpmh = N,N-bis-pyridin-3-ylmethylene-hydrazine, bpy = 4,4bipyridine). Single crystal X-ray analysis of compounds 1−4 reveals their structural diversities which might have been generated due to both rigidity (aryl carboxylate) and flexibility (aliphatic carboxylates) of cpp ligands as well their bridging modes and the orientation of nitrogen atoms of the neutral linkers. In addition, the dihedral angle in the aliphatic carboxylates of cpp ligand is also playing an important role in directing the final structural arrangement. Compound 1 exhibits a uninodal 6connected three-dimensional (3D) coordination polymer with point Schälfli symbol {33.59.63} and shows an uncommon lcy; 6/ 3/c1 topological type. Compound 2 reveals a 6-connected uninodal 3D framework with point Schälfli symbol {48.66.8} and shows a rare rob topology. Compound 3 formed a 4-connected uninodal two-dimensional framework with point Schälfli symbol {44.62} and displays a sql/Shubnikov tetragonal plane net topology, whereas 4 forms a one-dimensional CP which subsequently extended to 3D supramolecular networks through hydrogen bonding interactions. Gas adsorption studies reveal that compounds 1 and 2 show selective adsorption of CO2 over other gases (N2, CH4) at low temperature, whereas 3 and 4 show no uptake. Vapor sorption studies reveal that compounds 1, 2, and 4 show high uptake capacities for H2O over MeOH and EtOH. Solid state luminescence studies of compounds 1, 3, and 4 display significant red shifts compared to free ligands.

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INTRODUCTION The synthesis of metal organic frameworks (MOFs) or porous coordination polymers (PCPs) with interesting structure and functionality similar to the inorganic zeolites are an interesting task to scientists. These compounds show versatile structural features such as large surface area,1 porous structures,2 and potential applications in sensing,3 gas storage,4 separation,5 catalysis,6 luminescence,7 etc. Among all other applications proposed, the most important one is their capability for CO2 adsorption. In MOFs,8 some attempts have been made to regulate the selective capture of CO2 over other gases. Furthermore, the selectivity for CO2 adsorption can also be enhanced by introducing flexible carboxylates and neutral pyridyl-N,N′-donor spacers.9 Since altering of the length and functionalization of a spacer is synthetically feasible, structural diversity and variation of porosity can easily be attained by the use of flexible carboxylates and neutral linkers.10 Although various benzene polycarboxylates have been used for the synthesis of MOFs, the uses of flexible benzene polycarboxylates with long-spanning carboxyl groups are rare. In particular, the ligand 3-(4-carboxyphenyl)© XXXX American Chemical Society

propionic acid) (H2cpp) is an interesting building block, mainly due to the possibility of different coordination sites, as well as its rigidity in one end and flexibility on the other.11 Moreover, the geometric disposition of both the aryl and aliphatic carboxylate arms easily adjusts their conformations to meet various coordination requirements of the metal ion by their ‘‘breathing’’ ability in the solid state. In addition, shorter aliphatic carboxylate along with rigid aryl carboxylates can also be conducive to avoid the formation of an interpenetrated network. Quite recently, very few MOFs have been reported using such versatile ligand with other neutral N-donor linkers.12 However for none of them were sorption properties studied. In this context herein, we have synthesized four compounds (two CPs and two MOFs), namely, [Zn(cpp)(H2O)]n (1), [Cu(cpp)(4-bpmh)]n·nH2O·nMeOH (2), [Cd2(cpp)2(3-bpmh)2]n· 4nH2O·2nMeOH (3), and [Cd(cpp)(bpy)(H2O)2]n·2nH2O (4), using H2cpp ligand and 4-bpmh, 3-bpmh, and bpy as Received: January 27, 2014 Revised: February 24, 2014

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dx.doi.org/10.1021/cg500149s | Cryst. Growth Des. XXXX, XXX, XXX−XXX

Crystal Growth & Design

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magnetic resonance (EPR) experiment was conducted using a BRUKER ESP-300 spectrometer operated at X-band frequency (9− 10 GHz) with 100 kHz frequency modulation. Single Crystal X-ray Diffraction. Single crystal X-ray data for compounds 1−4 were collected on a Bruker APEX II diffractometer equipped with a graphite monochromator and Mo Kα (λ = 0.71073 Å, 296 K) radiation. Data collections were performed using ϕ and ω scan. Non-hydrogen atoms located from the difference Fourier maps were refined anisotropically by full-matrix least-squares on F2, using SHELXS-97.14 All hydrogen atoms were included in the calculated positions and refined isotropically using a riding model. Determinations of the crystal system, orientation matrix, and cell dimensions were performed according to the established procedures. Lorentz polarization and multiscan absorption correction were applied. All calculations were carried out using SHELXL 97,15 PLATON 99,16 and WinGXsystemVer-1.64.17 Compounds 2 and 3 contain highly disordered solvent molecules, which have been removed by the SQUEEZE17 program. From the TG analysis, we have calculated that in compound 2, one noncoordinated methanol and one water molecules are present, whereas in 3 two noncoordinated methanol and four water molecules are present, and hence those are included in the molecular formula. Data collection and structure refinement parameters and crystallographic data in Table 1 and selected bond lengths and bond angles for compounds 1−4 are given in Table S1 in Supporting Information. Sorption Measurements. Gas adsorption measurements were performed by using BelSorp-max (BEL Japan) automatic volumetric adsorption instrument. All the gases used were ultrapure research grade (99.99%). Before every measurement, samples were pretreated for 12 h at 373 K under 10−2 kPa continuous vacuum using BelPrepvac II and purged with N2 on cooling. CO2 and CH4 isotherms were measured at 195 K (dry ice-MeOH cold bath). The adsorptions of different solvents such as MeOH, H2O, and EtOH at 298 K were measured in the vapor state by using BELSORP-aqua3 analyzer. Synthesis of [Zn(cpp)(H2O)]n (1). The sodium salt of H2cpp (0.2 mmol, 46 mg) and Zn(NO3)2·2.5H2O (0.2 mmol, 60 mg) were

neutral linkers (where 4-bpmh = N,N-bis-pyridin-4-ylmethylene-hydrazine, 3-bpmh = N,N-bis-pyridin-3-ylmethylene-hydrazine, bpy = 4,4′-bipyridine) (Scheme 1). The structural diversities, thermal stabilities, luminescence and sorption properties of all the compounds have been studied extensively. Scheme 1. Ligands Used in This Work

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EXPERIMENTAL SECTION

Materials. All the reagents and solvents for synthesis were purchased from commercial sources and used as supplied without further purification. Zn(NO3)2·6H2O, Cu(NO3)2·2.5H2O, Cd(NO3)2· 4H2O, 3-(4-carboxyphenyl)propionic acid (H2cpp), and 4,4-bipyridyl (bpy) were obtained from the Sigma-Aldrich Chemical Co. India. 4bpmh and 3-bpmh were synthesized by the literature procedure.13 Na2cpp was synthesized by the addition of Na2CO3 to H2cpp ligand in a 2:1 ratio in water and was allowed to evaporate at 80 °C until dryness. Elemental analysis was carried out on Elementar Micro vario cube elemental analyzer. Fourier transform infrared (FT-IR) spectra (4000−400 cm−1) was recorded on a KBr pellet with a Perkin-Elmer Spectrum BX spectrometer. Powder X-ray diffraction (PXRD) data were collected on a PANalytical EMPYREAN instrument using Cu Kα radiation. All the solid state fluorescence measurements were recorded on Horiba Jobin-Yvon Fluorolog3 instrument. An electron para-

Table 1. Crystallographic Data for Compounds 1−4

a

compound

1

2

3

4

CCDC number empirical formula chemical formula weight (g/mol) crystal shape color size (mm) crystal system space group a/Å b/Å c/Å α (deg) β (deg) γ (deg) cell volume V (Å3) cell formula units Z wavelength (Mo Kα) (Å) temperature (K) Mu (mm−1) density (g cm−3) theta range for data collection (°) goodness-of-fit R1 factor alla wR2b F(000)

976519 C10H10O5Zn 275.57 block colorless 0.24 × 0.21 × 0.17 orthorhombic P212121 6.3131(3) 8.0821(5) 20.8006(12) 90.00 90.00 90.00 1061.31(10) 4 0.71073 296(2) 2.316 1.725 5.66 to 28.31 0.991 0.0261 0.0957 560

976517 C23H24CuN4O6 516.98 block green 0.25 × 0.21 × 0.18 monoclinic I2/a 14.5663(6) 13.4855(5) 24.6581(14) 90.00 94.734(2) 90.00 4827.2(4) 8 0.71073 296(2) 0.937 1.282 2.30 to 35.31 1.152 0.0621 0.1422 1912

976518 C46H56Cd2N8O14 1169.81 block yellow 0.23 × 0.20 × 0.17 monoclinic P21/c 23.6532(9) 16.1209(6) 13.6472(5) 90.00 97.1590(10) 90.00 5163.3(3) 4 0.71073 296(2) 0.876 1.324 3.13 to 23.15 1.149 0.0924 0.2303 2064

976520 C20H24CdN2O8 532.83 block colorless 0.23 × 0.20 × 0.18 monoclinic P21/c 11.6251(9) 12.2299(11) 15.6845(13) 90.00 109.713(4) 90.00 2099.2(3) 4 0.71073 296(2) 1.091 1.689 2.82 to 28.44 1.128 0.0536 0.1572 1084

R1 = Σ∥Fo| − |Fc∥/Σ|Fo. bR2 = [Σ{w(Fo2 − Fc2)2}/Σ{w(Fo2)2}]1/2. B



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Scheme 2. Synthetic Scheme of Compounds 1−4

dissolved separately in 12 mL of water and 15 mL of methanol solution respectively. Two milliliters of the metal solution was slowly and carefully layered above 2 mL of ligand solution in a narrow glass tube using 1 mL of buffer (1:1 H2O and MeOH) solution. Colorless block-shaped single crystals were obtained from the junction of the layers after two weeks. The crystals were separated and washed with MeOH and air-dried (yield: 55%). Elemental analysis: Anal. Calcd. for C10H10O5Zn (%): C, 43.58; H, 3.65; Found (%): C, 43.4; H, 3.1; FTIR (KBr pellet, cm−1): 3444 (b), 1622 (s), 1568 (s), 1400 (s), 1072 (m), 830 (w), 785 (w), 533 (w). Synthesis of [Cu(cpp)(4-bpmh)]n·nH 2O·nMeOH (2). An aqueous solution (12 mL) of Na2cpp (0.2 mmol, 46 mg) was mixed with a methanolic solution (12 mL) of 4-bpmh and stirred for 1 h to mix well. Cu(NO3)2·2.5H2O (0.2 mmol, 48.2 mg) was dissolved in 15 mL of water in a separate beaker. Two milliliters of the above mixed ligand solution was slowly and carefully layered above 2 mL of metal solution in a narrow glass tube using 2 mL of buffer (1:1 H2O and MeOH) solution. Green-colored block-shaped single crystals were obtained from the junction of the layers after two weeks. The crystals were separated and washed with MeOH and air-dried (yield: 70%). Elemental analysis: Anal. Calcd. for C23H24CuN4O6 (%). C, 53.53; N, 10.85; H, 4.6; Found (%): C, 53.6; N, 11. 05; H, 4.2; FT-IR (KBr pellet, cm−1): 3409 (b), 1612 (s), 1555 (s), 1425 (s), 1371 (s), 1313 (b), 1277 (s), 1233 (s), 1092 (m), 952 (s), 828 (s). EPR (solid state, 298 K): gav = 2.110, A = 73 G. Synthesis of [Cd2(cpp)2(3-bpmh)2]n·4nH2O·2nMeOH (3). The same diffusion technique of 2 was employed for the synthesis of compound 3 using the Cd(NO3)2·4H2O, and 3-bpmh in place of Cu(NO3)2·2.5H2O and 4-bpmh in same 1:1:1 molar ratio using methanol as a solvent. Yellow-colored crystals were separated after 4 days. The crystals were separated and washed with MeOH and airdried (yield: 85%). Elemental analysis: Anal. Cald. for C46H56Cd2N8O14. (%). C, 47.22; N, 9.5; H, 4.8; Found (%): C, 47.8; N, 9.3; H, 4.13; FT-IR (KBr pellet, cm−1): 3428 (b), 1630 (s), 1420 (s), 1386 (s),1308 (w), 1233 (m), 1191 (s), 1116 (b),1028 (m),958 (m),798 (s), 697 (s). Synthesis of [Cd(cpp)(bpy)(H2O)2]n·2nH2O (4). The same diffusion technique of compound 3 was employed for the synthesis of compound 4 using bpy in place of 3-bpmh in same 1:1:1 molar ratio

using ethanol as a solvent. Colorless block-shaped crystals were separated after two weeks. The crystals were separated and washed with EtOH and air-dried (yield: 30%) Elemental analysis: Anal. Cald. for C20H24CdN2O8 (%). C, 45.08; N, 5.25; H, 4.54; Found (%): C, 44.5; N, 5.9; H, 4.03; FT-IR (KBr pellet, cm−1): 3436 (b), 1594 (s), 1485 (s), 1361 (s), 1221 (s), 1063 (b), 821 (w), 718 (w). The bulk amount of the compounds 1−4 were prepared in powder form by the direct mixing of the ligand mixture with a corresponding solution of metal followed by overnight stirring. To confirm the phase purity of the bulk materials, powder X-ray diffraction (PXRD) experiments were carried out on compounds 1−4. All major peaks in experimental PXRD of compounds 1−4 match well with simulated PXRD, which indicates equitable crystalline phase purity (Figures S1− S4 in Supporting Information).

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RESULTS AND DISCUSSION Synthetic Aspects. Usage of long flexible ligands over rigid ones for the construction of MOFs facilitates the degrees of conformational isomerism and affects control of the porosity in the resulting MOFs.18 However, the use of conformationally flexible ligands may offer possibilities to generate novel dynamic “soft” materials that are well-suited for use as switches, catalysts, or selective adsorption−desorption agents due to their mechanical motion of local bonds and shift in angles or change during guest removal or accommodation.19 In this aspect, herein the flexible H2cpp ligand has been chosen to explore its effect on the final structural arrangement, sorption properties, and luminescent properties of the synthesized coordination polymers as well as MOFs. Compound 1 was synthesized using only cpp ligands and Zn(NO3)2·6H2O. Compounds 2−4 were synthesized using cpp ligands along with different neutral linkers with varying lengths such as 4-bpmh (2), 3-bpmh (3), and bpy (4) respectively using the corresponding metal nitrate at neutral pH conditions (Scheme 2). All compounds were synthesized through the slow diffusion technique using ethanol (or) methanol/water (1:1) mixture at room temperature. Structural illustration of compounds 1−4 reveals that they C



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Scheme 3. Various Bridging Modes of H2cpp (Harris Notation21) and Neutral Linkers Observed in Compounds 1−4

Scheme 4. Orientation of Aliphatic Carboxyl Groups in the Flexible cpp Linker

coordinated water molecules. In addition, in compound 1, a single band is observed at 1399 cm−1, which corresponds to the νsym(OCO) stretching vibrations, whereas in compounds 2−4 it splits into two bands (1371, 1425 cm−1 for 2), (1386, 1420 cm−1 for 3), and (1404, 1430 cm−1 for 4). The splitting of symmetric stretching bands in compounds 2−4 suggests that the carboxylate groups of cpp ligands exhibits both μ-1,2 and μ1,1 bridging modes, whereas in compound 1 they exhibit only μ-1,2 bridging mode. 20 The noted difference (Δν = ν(COO)asym − ν(COO)sym) in the bridging modes of carboxylates groups of cpp ligands is further supported by the structural analysis (supra). X-band EPR spectrum of 2 in the polycrystalline state recorded at room temperature shows a broad signal due to overlapping of some lines (Figure S6 in Supporting Information). Structural Description of [Zn(cpp)(H2O)]n (1). Compound 1 crystallizes in the orthorhombic crystal system with non-centrosymmetric P212121 space group. The asymmetric unit of 1 contains one Zn(II) ion, one cpp ligand, and one coordinated water molecule (Figure S7 in Supporting Information). Each Zn(II) ion adapts distorted tetrahedral geometry with the contribution from three different oxygen atoms from three different cpp ligands, and the fourth coordination site is occupied by a water molecule (Figure S8 in Supporting Information). The Zn···O bond distances are in the range of 1.943−1.972 Å, and the bond angles around the Zn(II) centers are in the range of 99.48(9)−121.93(9)°. In 1, the coordination geometry around the Zn(II) center can be best described as distorted tetrahedral geometry (t4 = 0.90),22 where cpp ligands are in the basal plane and the coordinated

exhibit different structural topologies which might be due to the different bridging modes of aryl and aliphatic carboxylates of cpp ligands and different orientation of neutral linkers (Scheme 3). Furthermore, it has been examined that in compounds 1−4 the dihedral angle in the aliphatic carboxylates of cpp ligand is different (Scheme 4), which might have affected the final structural arrangements as well as sorption properties. The IR data for compounds 1−4 are given in Table S2 in Supporting Information. FT-IR analyses (Figure 1 and Figure S5 in Supporting Information) show that in all compounds 1− 4, a broad band in the region of 3390−3600 cm−1 was observed. The band indicates the presence of free or

Figure 1. FT-IR spectra of compounds 1−4, highlighting the different bridging modes of cpp ligands. D



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Figure 2. (a) Illustration of the 3D zigzag arrangement of compound 1 along the ac plane. Color code: oxygen (red), carbon (light gray), and zinc (olive). (b) Representation of SBU found in 1. (c) A view of 3D coordination polymer showing the arrangement of SBUs down a axis. (d) View of the {33.59.63} lcy; 6/3/c1 topology featuring a 6-connected uninodal net.

Structural Description of [Cu(cpp)(4-bpmh)]n·nH2O· nMeOH (2). Compound 2 crystallizes in the monoclinic system, with I2/a space group. The asymmetric unit of 2 contains one Cu(II) ion, one cpp ligand, and one 4-bpmh linker (Figure S12 in Supporting Information). In 2, each Cu(II) center is hexa-coordinated with the contribution from the four carboxylate oxygen atoms from three different cpp ligands which are occupied in the equatorial position and two nitrogen atoms from two different 4-bpmh ligands in the axial position and results a distorted octahedral geometry (Figure S13 in Supporting Information). The Cu−O bond lengths are in the range of 1.962(3)−2.212(3) Å, whereas the Cu−N bond lengths are in the range of 2.020(3)−2.022(3) Å. The bond angles around Cu(II) center are in the range of 85.1(1)− 175.1(1)°. In 2, each cpp ligand coordinates to three Cu(II) ions through both of its aryl and aliphatic carboxylates oxygen atoms through μ-1,2 (2.11 in Harris notation) and chelating bridging modes (1.11 in Harris notation) respectively, Scheme 3b. In 2, the carbon atoms in the aliphatic carboxylate (C− CH2−CH2−C) of cpp ligand adopted a nearly cis conformation having a dihedral angle of 61.71° (C1−C2−C3−C4) between them (Scheme 4), whereas in 1 they are trans. The noted change in orientation of aliphatic carboxylates might be due to the introduction of neutral linker and change in metal ion. Since each cpp ligands coordinates three Cu(II) centers, it can be called a three-connected linker. It was found that compound 2 contains a repeated [Cu2(cpp)4(4-bpmh)4] unit and can be considered as SBUs (Figure 3a). The distance between two Cu(II) centers in the SBU is 4.081(6) Å. Each SBU is further connected to four by cpp ligands and remaining through the 4-bpmh ligand. The Cu(II)···Cu(II) separation between the SBUs linked by cpp ligands is 10.110(1) Å and linked by 4-bpmh ligands is 15.352(1) Å. In 2, the M(II)···M(II) separation through 4bpmh ligands are in the range of our recently reported

water molecule is in the apical site away from the vertical plane. It was observed that Zn(II) center is sited at a distance of 0.419 Å from the mean plane containing cpp ligands. The coordinated water molecule is involved in a bifurcated Hbonding interaction with the free aryl carboxylate oxygen atom of cpp ligand O1w···O2; 2.646(3) Å (O1w−H1wB···O2 = 160.6(2)°, −1 + x, y, z) and 2.656(3) Å (O1w−H1wA···O2 = 110.3(2)°, −1/2 + x, 3/2 − y, −z) and extended as a column of repeating R23(10) units as shown in (Figure S9 in Supporting Information). In 1, the carbon atoms in aliphatic carboxylate (C−CH2−CH2−C) of cpp ligand adopted a nearly anti conformation having a dihedral angle of 170.75° (C1−C2− C3−C4) between them (Scheme 4). Each cpp ligand coordinates three Zn(II) ions having both of its aryl and aliphatic carboxylates through monodentate (1.10 in Harris notation21) and μ-1,2 (2.11 in Harris notation) bridging mode respectively (Scheme 3a). In 1, cpp ligands coordinate to three Zn(II) centers and hence can be called a three-connected linker. The aryl carboxylate oxygen atoms of cpp ligand connects the Zn(II) center only in the c axis to form a zigzag chain (Figure S10 in Supporting Information), which is further extended along a and down the b directions by aryl carboxylate oxygen atoms of cpp ligands to form a three-dimensional (3D) zigzag type arrangement (Figure 2a). The distance between two Zn(II) centers in the noted 1D chain is 13.097 Å. In overall, compound 1 exhibits a single repeating unit [Zn(cpp)3(H2O)], which can be termed as secondary building units (SBUs) (Figure 2b). Further, each SBU is connected to six nearby SBUs through cpp ligands and hence leads to a uninodal 6-connected 3D coordination polymer (Figure 2c, Figure S11 in Supporting Information) with point Schalfli symbol {33.59.63}. This network is identified by the uncommon topological type lcy; 6/3/c1 in the Reticular Chemistry Structure Resource (RCSR) database23 (Figure 2d). PLATON analysis16 reveals no solvent-accessible free volume, which substantiates its nonporous nature. E



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Figure 3. (a) A view of SBU found in compound 2. Color code: oxygen (red), nitrogen (blue), carbon (light gray), and copper (cyan). (b) Illustration of 2D brick wall arrangement found in 2 by connection of SBUs by cpp ligands in the ab plane (4-bpmh linkers are omitted for clarity). (c) Illustration of 3D honeycomb-like structure having 1D channels running along the a axis found in 2. (d) View of the {48.66.8} rob topology featuring a 6-connected uninodal net.

Structural Description of [Cd2(cpp) 2(3-bpmh)2] n· 4nH2O·2nMeOH (3). Compound 3 crystallizes in the monoclinic system with P21/c space group. The asymmetric unit of 3 contains two Cd(II) ions, two cpp ligands, and two 3bpmh linkers, respectively (Figure S17 in Supporting Information). Each Cd(II) ion acquires pentagonal bipyramidal geometry (Figure S18 in Supporting Information) and is ligated by the five oxygen atoms from three different cpp ligands sited in the equatorial position and two nitrogen atoms from two different 3-bpmh ligands present in the axial position. The Cd− O bond lengths are measured in the range of 2.372(7)− 2.589(7) Å, and the Cd−N bond lengths are lying in the range of 2.331(6)−2.343(7) Å. The bond angles around the Cd(II) center are found in the range of 84.5(2)−177.9(2)°. In 3, two cpp ligands coordinate to the Cd(II) center in different bridging modes. One cpp ligand coordinates to four Cd(II) ions by only a μ-1,1′,2 bridging mode (3.21 in Harris notation), whereas the other cpp ligands link two Cd(II) centers via only chelating mode (1.11 in Harris notation) (Scheme 3c,d). Comparing the bridging modes of both aryl and aliphatic carboxylates of cpp ligands in compound 1−3, it is found that in 1 and 2 they exhibit different bridging modes, whereas in 3 they exhibit the same bridging modes. To be precise, in 1 and 2,

compounds.24 It was found that SBUs that are connected by only carboxylate oxygen atoms of cpp ligands arranged to form 2D brick wall arrangement along the ab-plane (Figure 3b), whereas those linked by 4-bpmh linkers are extended only in a 1D chain. In the noted 2D arrangement, the cpp ligands are arranged in a helical fashion (Figure S14 in Supporting Information) having a Cu(II)···Cu(II) separation of 12.332(1) Å, and the phenyl rings of the cpp ligands are slipped over each another at a distance of 4.614 Å. Furthermore, the adjacent 2D layers are pillared by nitrogen atoms of 4-bpmh linkers along the c-direction to generate a porous 3D network with one-dimensional (1D) channels running along the a-axis (Figure 3c). The alignment of bpmh linkers in the 3D framework are criss-crossed and canted, which results in a honeycomb-like arrangement along the a-axis (Figure S15 in Supporting Information). In overall, compound 2 leads to 6-connected uninodal 3D network with point Schalfli symbol {48.66.8} (Figure S16 in Supporting Information). This network is identified by rob topological type (Figure 3d) in the RCSR database. PLATON analysis revealed that compound 2 exhibits a large pore-accessible void volume of 1065.9 Å3 out of 4827.2 Å3 that represents 22.1% per unit cell volume. F



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Figure 4. (a) A view of SBU found in compound 3. Color code; oxygen (red), carbon (light gray), nitrogen (blue), and cadmium (magenta). (b) Structural illustration showing a 2D sheetlike structure along the b-direction, (c) Packing diagram showing 3D networks through offset π···π stacking interactions between 2D layers. (d) View of the {44.62} sql/Shubnikov tetragonal plane net topological type featuring a 4-connected uninodal net.

linker extended only in the 1D chain. This difference might be due to the different angular orientation of coordinating sites (both of them are N) of the bpmh linkers (Scheme 4). The 2D sheets in 3 are linked by the offset π···π stacking interaction (3.33(1)−3.37(1) Å) between the 3-bmph linker to form supramolecular 3D network (Figure 4c). In overall, compound 3 leads to 4-connected uninodal network with point Schalfli symbol {44.62}. This network is identified by sql/Shubnikov tetragonal plane net topological type (Figure 4d, Figure S20 in Supporting Information) in the RCSR database. PLATON analysis revealed that compound 3 exhibits a large poreaccessible void volume of 1353.4 Å3 out of 5163.3 Å3 that represents 26.2% per unit cell volume. Structural Description of [Cd(cpp)(bpy)(H 2O) 2 ] n· 2nH2O (4). Compound 4 crystallizes in the monoclinic crystal system with P21/c space group. The asymmetric unit of 4 contains one Cd(II) ion, one cpp ligand, one bpy linker, two coordinated water molecules, and two lattice water molecules (Figure S21 in Supporting Information). Each Cd(II) center coordinates to three oxygen atoms from two different cpp ligands and two water molecules in the equatorial position and two bpy linkers in the axial site, which results a pentagonal bipyramidal geometry (Figure S22 in Supporting Information). In 4, the Cd−O bond lengths are measured in the range of 2.328−2.487 Å, and the Cd−N bond lengths are 2.276 Å and 2.305 Å respectively. The bond angles around the Cd(II) center are lying in the range of 75.4(1)−179.3(2)°. In 4, each cpp ligand coordinates to two Cd(II) centers by only an aliphatic carboxylate group via μ-1,1′,2 bridging mode (3.21 in Harris notation) leaving the aryl carboxylate uncoordinated (Scheme

both the aryl and aliphatic carboxylates coordinate to M(II) centers in a different way, whereas in 3 they link the Cd(II) center in a similar fashion. The observes difference in bridging modes of cpp ligands from compounds 1−3 might be due to the collective effect of metal ions and the linkers. Similar to compound 1, in 3, the aliphatic carboxylate (C−CH2−CH2− C) of both the cpp ligands adopted nearly anti conformation having a dihedral angle of 175.52° (C1−C2−C3−C4) and 176.03° (C15−C16−C17−C18) respectively. It was found that compound 3 contains a repeated [Cd2(cpp)4(3-bpmh)4] unit and can be considered as SBU (Figure 4a). The distance between two Cd(II) centers in the SBU is 3.825(1) Å. The shorter M···M distance found in compound 3 than in 2 might be due to the increase in size of the metal ions (Cd(II) > Cu(II)) and μ-1,1′,2 bridging mode of cpp ligands. Further, each SBUs are connected to two SBUs by only cpp ligands and two other SBUs by both cpp ligands and 3-bpmh linkers. Packing analysis of 3 reveals that each SBU is extended by both 3-bpmh linker and μ-1,1′,2 bridged cpp ligands in c direction to form a 1D chain (Figure S19 in Supporting Information). In the noted 1D chain, the cpp ligands are sandwiched between the 3-bpmh linkers, and also the linkers are associated with parallel displaced π···π stacking interactions (3.770 Å and 3.732 Å) through their pyridine rings and stabilizes the cpp ligands in between. Further the 1D chain is linked through chelated cpp ligands in the a direction to form a two-dimensional (2D) sheetlike structure along the ac plane (Figure 4b). Comparing the effect of linkers in compounds 2 and 3, it was found that in 2 the 4bpmh linker connects the 2D layers, whereas in 3 the 3-bpmh G



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Figure 5. (a) A view of SBU found in compound 4. (b) Illustration of 1D chain showing the arrangement of cpp ligands and bpy linkers. (c) Packing diagram showing 3D supramolecular networks through hydrogen bonding interactions. (d) A view of a rarely occurring distorted hydrogen bonded cube between coordinated water molecules, free carboxylate group of cpp ligands, and lattice guest water molecules. Color code: same as in Figure 4.

3d). The free carboxylate oxygen atoms are involved in hydrogen bonding interaction with the lattice water molecules. The hydrogen bond parameters are listed in Table S3 in Supporting Information. Comparing the bridging modes of both aryl and aliphatic carboxylates of cpp ligands in compounds 1−4, diversity has been noticed which might be due to the effect of metal ions. Notably, although compounds 3 and 4 contains same Cd(II) ion, the cpp ligand exhibits a different bridging mode in both cases. In compound 4, it may be the lattice water molecules that enforce the aryl carboxylate group to be involved in strong Hbonding interaction and in turn the aryl carboxylates remain uncoordinated. Similar to compound 2, in 4, the aliphatic carboxylate (C−CH2−CH2−C) of the cpp ligands adopted nearly cis conformation having a dihedral angle of 69.0(7)° (C1−C2−C3−C4) in between them. It was found that compound 4 contains a repeated [Cd2(cpp)2(bpy)4(H2O)4] unit which can be considered as a SBU. The distance between two Cd(II) centers in the SBU is 3.98(1) Å (Figure 5a). Further, each SBU is connected to two SBUs by only bpy linkers and is extended in a 1D chain along the a axis (Figure 5b). The Cd(II)···Cd(II) distance between the two SBUs is 11.625(1) Å. Unlike in compounds 1−3, in 4 the aryl carboxylates do not participate in connecting the SBUs and that restricts the extension of SBUs either in the b and c axis to form a 2D or 3D network. In the noted 1D chain, the bpy linkers are associated with parallel displaced π···π interaction of 3.925 Å between their pyridine rings, and the cpp ligands are orientated above and below the π stacked bpy linker. That

means the bpy linkers are sandwiched between the cpp ligands along the 1D chain. Also it was noticed that the two pyridine rings of the neutral bpy linkers are not in the same plane and are tilted at an angle of 20.81°. Detail packing analysis reveals that each 1D chain is linked to nearby 1D chains through hydrogen bonding interaction between the coordinated water molecules and free carboxylate group of each SBU with the lattice water molecules down the a axis, which results in 3D supramolecular self-assembly (Figure 5c). In the noted 3D selfassembly, a rarely occurring distorted hydrogen bonded cube between coordinated water molecules, free carboxylate group of cpp ligands, and lattice guest water molecules was perceived (Figure 5d). Thermal Stabilities. To examine the thermal stability of compounds 1−4, thermogravimetric analysis (TGA) was carried out in the temperature range of 30−600 °C under N2 flow with a heating rate of 10 °C min−1 (Figure S23 in Supporting Information). A weight loss of 6.4% (calcd. 6.5%) observed in compound 1 in the range of 100−200 °C, which can be assigned to the loss of one coordinated water molecule, and the dehydrated framework was stable up to 373 °C followed by rapid weight loss due to the loss of cpp ligands in the framework. Compound 2 shows gradual weight loss of 10% (calcd. 9.2%) in the temperature range of 50−120 °C, which might be due to the loss of lattice water and MeOH molecules from pores. The dehydrated framework was stable up to 290 °C, followed by a sharp weight loss, which indicates decomposition due the loss of the cpp ligand and 4-bpmh linkers. Compound 3 shows a gradual weight loss of 10% H



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(calcd. 9%) in the temperature range of 30−150 °C, which corresponds to the loss of lattice water and methanol molecules. The desolvated framework was found to be stable up to 320 °C. Similarly, compound 4 demonstrates a weight loss of 10% (calcd. 9%) in the temperature range of 60−180 °C, which corresponds to the loss of coordinated water molecules, and lattice water molecules. The desolvated framework was found to be stable up to 240 °C. Adsorption Studies. To examine the porous nature of all compounds, we have carried out the gas sorption studies with N2, CO2, and CH4 at low temperature. The desolvated framework of compound 1 shows type-III isotherm for N2 at 77 K with the amount of 14 cm3 g−1, suggesting the nonporous nature of the framework (Figure 6), whereas compound 2

molecules to pass through, which results in only surface adsorption.26,10a The CH4 and CO2 adsorption isotherms of compounds 1 and 2 measured at 195 K shown in Figures 6 and 7 respectively. Both compounds do not show any uptake for CH4 . Interestingly, compounds 1 and 2 adsorb a significant amount of CO2 with the uptake amount of 71.4 cm3 g−1 and 55.81 cm3 g−1 at 1 bar pressure respectively. Compound 1 shows a TypeIII adsorption profile, whereas compound 2 shows a Type-I adsorption profile according to the IUPAC classification.27 This high uptake of CO2 over N2 in the case of 1 might be due to the interaction between electron-donating uncoordinated oxygen atoms of the cpp ligand and CO2 molecules, which results in the enhancement of CO2 uptake,19d,25 whereas in 2 all the carboxylate oxygen atoms are in coordination with metal centers, but the selective uptake of CO2 over N2 could be due to the presence of base functionalities (-NN- group) of 4bpmh on the pore walls, which can strongly interact with the quadruple moment (−1.4 × 10−35 C·m) of CO2 molecules.28,10a The surface area of compound 2 calculated from the CO2 profile using the Langmuir equation was found to be 197.67 m2 g1. The sorption profile of 3 and 4 shows a negligible amount of uptake for all the gases (Figures S25 and S26 in Supporting Information). The probable reason for much less uptake in the case of 3 might be due to the typical 2D sheetlike structure, though it has good solvent-accessible void volume, but less uptake for the gases could be due to the absence of continuous channels in 2D sheet to pass through whereas in 4 it is due to the 1D chain structure. To understand the interaction of host frameworks with solvent molecules, we have carried out a sorption study with different solvents (water (H2O), methanol (MeOH), ethanol (EtOH)) of varying sizes and polarity at 298 K. Interestingly, compound 1 shows a typical Type-1 isotherm for H2O with steep uptake at low pressure regions signifying the strong interaction of H2O molecules with the pore surface and attained a value of 118.2 mL g−1 at P/Po ≈ 1.00 bar pressure (Figure 8). Steep uptake adsorption in the case of H2O at low pressure region (P/Po ≈ 0.04) can be justified by considering the strong interactions between the adsorbate with adsorbent after filling the unsaturated Zn(II) site by one H2O molecule which experiences strong interaction.29 The strong interaction with unsaturated Zn(II) site of H2O is further supported by the

Figure 6. Gas sorption isotherms of compound 1 at low temperature, N2 at 77 K (blue), CH4 at 195 K (green), and CO2 at 195 K (red).

Figure 7. Gas sorption isotherms of compound 2 at low temperature, N2 at 77 K (green), CH4 at 195 K (blue), and CO2 at 195 K (red).

shows very less uptake for N2 (Figure 7), although the framework contains the comparable pore window dimensions to permit the passage of N2 (kinetic diameter, 3.6 Å) (as calculated from pore size distribution as shown in Figure S24 in Supporting Information). This is probably due to the presence of a 1D channel system in 2, and no additional channels are available along the crystallographic b and c axes25 (Figure 3c). Therefore, at low temperature (77 K) N2 molecules might be strongly interacting with pore aperture and block other

Figure 8. Vapor adsorption isotherms of 1. I



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Luminescent Properties. Luminescent MOFs have received significant attention in view of various prospective applications, such as in chemical sensors,31 photochemistry,32 electroluminescent display, 33 and so on. Generally, d 10 complexes are promising materials to exhibit photoluminescence properties,34 and we have done solid state luminescent property study of H2cpp, 3-bpmh, bpy, and compounds 1, 3, and 4 at room temperature. It was found that H2cpp, 3-bpmh, and bpy ligands show stronger emission at 337 nm (λex = 272 nm), 316 nm (λex = 243 nm), and 350 nm (λex = 231 nm), which can be attributed to the π* → π and/or π*→ n transitions free ligands.35 Upon photo excitation, compounds 1, 3, and 4 exhibit an emission band with maxima at 357 nm (λex = 271 nm), 355 nm (λex = 270 nm), and 358 nm (λex = 243 nm) respectively (Figure 11). The emission bands of 1, 3, and 4 indicate significant red shifts, which can be attributed to the charge transfer of metal centers to ligands (MLCT).36

incomplete desorption profile of H2O. It is worth mentioning that the adsorption isotherms of MeOH and EtOH show TypeIII, suggesting only surface adsorption with the amount of 46 mL g−1 and 47 mL g−1 respectively. In other hand, both compounds 2 and 4 show almost a linear uptake of water vapor at low pressure, and it start rising sharply at a relative vapor pressure of P/Po ≈ 0.63 and reaches a final uptake value of ∼158 mL g−1 and ∼123 mL g−1 at P/P0 ≈ 1 bar (Figures 9 and 10). Low uptake of H2O at the low pressure

Figure 9. Vapor adsorption isotherms of 2.

Figure 11. Solid-state emission spectra for ligands used and complexes 1, 3, and 4.

In addition, the larger red shifts are observed for the emissions of compounds 1, 3, and 4. Comparing with the emissions of H2cpp ligand, this might be directly related the local linker surroundings determined by their orientation and separation of conjugated linkers in the respective crystal structure, which has an influence on the luminescence properties.13 The above photoluminescent properties of compounds 1, 3, and 4 reveal that they can be used as smart materials for possible applications.

Figure 10. Vapor adsorption isotherms of 4.

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region in the case of 2 signifies the hydrophobic nature of the pore surface and sharp uptake at P/P0 ≈ 0.6, indicating diffusion of H2O molecules in the pore surfaces, whereas in the case of compound 4, low pressure adsorption can be attributed to the central part of the pore where water molecules formed hydrogen bonding with pendent carboxylate groups, and sharp uptake indicates reaccumulation of H2O molecules inside the pore and coordination sites.30 The desorption isotherm of both compounds 2 and 4 does not retrace the adsorption profile and showing hysteresis indicates strong interaction with framework. The adsorption isotherms of MeOH for both compounds 2 and 4 show a Type-1 profile with the uptake amount of 72 mL g−1 and 84 mL g−1 respectively. An EtOH vapor shows uptake capacities of 43 mL g−1 and 56 mL g−1 for compounds 2 and 4 respectively. The less adsorption amount of uptake in the case of MeOH and EtOH can be justified by correlating their large molecular diameter (MeOH - 3.8 Å, EtOH - 4.3 Å) and less polarity in comparison with H2O (2.6 Å).18c,19d

CONCLUSION We have successfully synthesized four new compounds (two CPs and two MOFs) by using both rigid and flexible H2cpp ligand along with different N,N′-donor neutral linkers with varying lengths. The effects of both rigidity (aryl carboxylate) and flexibility (aliphatic carboxylates) of cpp ligands as well their bridging modes and the orientation of nitrogen atoms of the neutral linkers have been studied. Further, the role of dihedral angle of the aliphatic carboxylates of cpp ligand on the final structural arrangement has been deliberated. Gas adsorption studies reveal that compounds 1 and 2 show selective adsorption of CO2 over other gases (N2, CH4 ) at low temperature, whereas 3 and 4 shows no uptake. Vapor sorption studies reveal that compounds 1, 2, and 4 show high uptake capacities for H2O over MeOH and EtOH vapors. Solid state luminescence studies of compounds 1, 3, and 4 display significant red shifts compared to free ligands. We are currently J



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working with this versatile cpp ligand along with different flexible neutral linkers to explore their effect to generate novel dynamic “soft” materials.

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ASSOCIATED CONTENT

* Supporting Information S

PXRD patterns of compounds 1−4, IR data, TGA, and some additional figures and tables. This information is available free of charge via the Internet at http://pubs.acs.org/.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS S.P. and S.S. thank IISER Bhopal for the Ph.D. fellowships, and H.S.J. and K.T. are thankful for a postdoctoral fellowship. S.K. thanks CSIR, Government of India (Project No. 01/(2473)/ 11/EMRII), and IISER Bhopal for generous financial and infrastructural support.

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