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Stoichiometry Controlled Structural Variation in 3D Zn(II)–Frameworks: Single-Crystal to SingleCrystal Transmetalation and Selective CO2 Adsorption Dinesh De, Subhadip Neogi, and Parimal K. Bharadwaj Cryst. Growth Des., Just Accepted Manuscript • DOI: 10.1021/acs.cgd.6b00795 • Publication Date (Web): 15 Aug 2016 Downloaded from http://pubs.acs.org on August 15, 2016

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Crystal Growth & Design

Stoichiometry Controlled Structural Variation in 3D Zn(II)–Frameworks: Single-Crystal to Single-Crystal Transmetalation and Selective CO2 Adsorption Dinesh De,a Subhadip Neogi,b,* and Parimal K. Bharadwaja,* a

Department of Chemistry, Indian Institute of Technology, Kanpur 208016, India

b

Inorganic Materials & Catalysis Division, Central Salt & Marine Chemicals Research Institute (CSIR), Bhavnagar-364002, Gujarat, India.

ABSTRACT: Solvothermal reaction of the tripodal linker, tris(4′-carboxybiphenyl)amine (H3L) and the co-linker 3,5-di(pyridin-4-yl)-4H-1,2,4-triazol-4-amine (dpta) in different molar ratio produce

two-fold

interpenetrated

but

structurally

different

3D

Zn(II)–frameworks:

{[Zn3(L)2(dpta)(DMF)]·18DMF·3H2O}n (1) and {[Zn3(L)2(dpta) (DMF)]·14DMF·3H2O}n (2) (L = L3-, DMF = N,N′-dimethylformamide). Both the structures are built with a common [Zn3(COO)6] secondary building unit (SBU). While 1 is a pillared-bilayer framework with (43·624·8)(43)2 topology, the construction of 2 is different due to altered disposition of SBUs and is describable by the Schlafli symbol (43.58.68.78.8)(43)2. The structural variation alters the electronic environment and pore sizes in these frameworks, which allow the activated framework 2' to uptake better N2 and H2 gases at 77 K, and CO2 at 273 K, than 1'. Compared to 1', framework 2' gives better selectivity of CO2 adsorption over N2, and H2 at 273 K; although, the selectivity of CO2 over CH4 is reversed. Both 1 and 2 undergo transmetalation reactions with Cu(II) at room temperature keeping crystalinity intact to generate 1Cu and 2Cu, respectively. The Cu-exchanged frameworks are characterized by single-crystal X-ray structures while transmetalation kinetics are confirmed by energy-dispersive X-ray spectroscopy. In contrast to the activated Zn(II) frameworks, the activated 1Cu and 2Cu are unstable and show no gas uptake.

ACS Paragon Plus Environment


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INTRODUCTION Over the past few years porous metal-organic frameworks (PMOFs) have drawn immense research focus due to their synthetic control that allows the understanding of high surface area and adjustable pore dimensions for optimizing selective adsorption of gases as well as other specific applications.1-10 A combination of the coordination inclinations of metal ions and various topologies of organic linkers have provided a tool for the construction of extended porous frameworks via crystal engineering.11-12 Since the process of self-assembly can frequently be modulated by various external factors, such as metal source, ligands, solvents, templates, pH value, temperature, and so on.13-20 The same reactants can result in completely different structures because of the extreme sensitivity of self-assembly to the reaction conditions. Although, several reports on the utilization of various structure directing agents (templates) are available for the synthesis of different MOFs,21-27 the concentration of the starting components is the most fundamental determinant in MOF synthesis. In case of the MOF-5 system, it has been observed that dilution of reactants leads to the formation of non-interpenetrated structures (IRMOF-10, -12, -14, and -16), possessing higher porosity.28,

29

Nevertheless, obtaining

concentration dependent, phase-pure, porous MOFs are still challenging because of the possibilities of amorphous product formation or channel clogging. In this article, we address how systematic variation of the pillar : linker ratio (Scheme 1), keeping all other conditions unaltered, affords two structurally dissimilar 3D frameworks {[Zn3(L)2(dpta)(DMF)]·18DMF·3H2O}n (1) and {[Zn3(L)2(dpta)(DMF)]·14DMF·3H2O}n (2), having common [Zn3(COO)6] secondary building units (SBUs). Critical inspection of the crystal structures unveils that the structural variation originates from different disposition of the SBUs, rendering the framework 1 as a


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pillared-bilayer

MOF

with

(43·624·8)(43)2 topology,

while

2

shows

the

topology

(43.58.68.78.8)(43)2.

Scheme 1. Schematic diagrams of the ligands used in this work.

This structural variation leads to different pore sizes with altered electronic environment of the channels in the two PMOFs, which is further reflected in the higher N2, CO2 and H2 gas uptake for the activated framework 2ʹ compared to 1ʹ. Although, the selectivity of CO2 over N2 and H2 at 273 K follows the order 1ʹ < 2ʹ, the selectivity of CO2 over CH4 is reversed. We also report the room temperature, single-crystal to single-crystal (SC-SC) metal ion exchange with Cu(II), producing 1Cu and 2Cu that are not accessible through the de novo synthesis. This drawback has led us to consider post-synthetic SC-SC approach as a possible option.30, 31 SC-SC transmetalation has been a freshly developed area32,33 and holds the promise to achieve multifunctional MOFs that are hardly accessible by direct solvothermal synthesis.34, 35 Moreover, transmetalation helps to fabricate the interior as well as exterior of MOFs with improved properties.36-38 Although several linker substitution in MOFs has been realized in SC-SC fashion,39-41 metal exchange is limited to only a handful of examples42-45 since this process involves drastic cleavage/formation of coordinate bonds between the ligands and the metal ion, where structural integrity and particularly, single crystallinity is lost. Therefore, to better



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elucidate the exchange mechanism, investigations of the metal replacement behavior in MOFs through SC-SC mode are indispensable.

EXPERIMENTAL SECTION Materials and Method. The metal salts and other reagent grade chemicals were procured from Sigma-Aldrich and used as received. All the solvents were from S. D. Fine Chemicals, India. These solvents were purified following standard methods prior to use. Physical Measurements. All physical measurements to characterize the linkers and MOFs were carried out as reported earlier.46 Energy-dispersive X-ray spectroscopic data (EDS) were recorded on a JSM-6010A; JEOL Tungsten-Electron Microscope (W-SEM). XRF experiment was carried out on a Rigaku WD-XRF system at 60 kV and 150 mA. Low pressure gas adsorption measurements were performed using automatic volumetric BELSORP-MINI-II adsorption equipment as well as using a static volumetric system (Micromeritics ASAP 2020). Prior to BET adsorption measurements, as-synthesized compounds were immersed in acetone for 5 d at room temperature to replace lattice guest molecules. The solvent-exchanged frameworks were then heated to 120 °C for 12 h under vacuum to produce guest free compounds 1′ and 2′. X-Ray Structural Studies. Single crystal X-ray data were collected at 100 K on a Bruker SMART APEX CCD diffractometer using graphite-monochromated Mo Kα radiation (λ= 0.71073 Å) as described earlier.11 To give an account of disordered electron densities associated with solvent molecules, the “squeeze” protocol in the PLATON package47 was applied. The crystal and refinement data are listed in Table S1 of the Supporting Information. Synthesis of the Ligands. The ligand tris(4ʹ-carboxybiphenyl)amine (H3L) and the coligand 3,5-di(pyridin-4-yl)-4H-1,2,4-triazol-4-amine (dpta) (Scheme 1) have been synthesized


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following previously reported procedures.48-49 Their characterizations were done by elemental, 1

H and 13C NMR and ESI-MS analyses (Supporting Information). Synthesis of {[Zn3(L)2(dpta)(DMF)]·18DMF·3H2O}n (1). H3L (20 mg, 0.033 mmol),

dpta (8 mg, 0.033 mmol) and Zn(NO3)2·6H2O (40 mg, 0.134 mmol), (1: 1: 4 ratio) were taken in anhydrous DMF (3 mL). The mixture was placed in a Teflon-lined stainless steel autoclave and heated under autogenous pressure to 90 °C for 72 h and then allowed to cool to room temperature at the rate of 1 °C per min. Yellowish green block shaped crystals of 1 were collected by filtration, washed first with DMF followed by acetone, and finally dried in air. Yield ∼68 %. Anal. Calcd. for C147H197N27O34Zn3: C, 57.28; H, 6.44; N, 12.27%. Found: C, 57.55; H, 6.61; N, 12.19 %. FT-IR (KBr pellet, cm-1): 3434 (broad), 2930 (m), 1667 (s), 1598 (s), 1389 (s), 1099 (s), 785(s). Synthesis of {[Zn3(L)2(dpta)(DMF)]·14DMF·3H2O}n (2). H3L (20 mg, 0.033 mmol) dpta (16 mg, 0.067 mmol,) and Zn(NO3)2·6H2O (40 mg, 0.134 mmol), (1: 2: 4 ratio) were taken in anhydrous DMF (3 mL). The mixture placed in a Teflon-lined stainless steel autoclave and heated under autogenous pressure to 90 °C for 72 h and then allowed to cool to room temperature at a rate of 1 °C per min. Yellowish green block shaped crystals of 2 were collected by filtration, washed first with DMF followed by acetone, and finally dried in the air. Yield∼76%. Anal. Calcd. For C135H169N23O30Zn3: C, 58.11; H, 6.11; N, 11.55 %. Found: C, 58.32; H, 6.24; N, 11.43%. FT-IR (KBr pellet, cm-1): 3426 (broad), 2929 (m), 1667 (s), 1598 (s), 1387 (s), 834 (s), 785(s). Synthesis

of

{[Cu3(L)2(dpta)(DMF)]·20DMF·2H2O}n

(1Cu)

and

{[Cu3(L)2(dpta)(DMF)]·16DMF·4H2O)}n (2Cu) via transmetalation. The as-synthesized single crystals of 1 and 2 were soaked in DMF for 2 d to remove any remaining reactants and unwanted



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products that may be present. During the soaking process, the solvent was refreshed twice. After that, the crystals of 1 and 2 were immersed in DMF solution of Cu(NO3)2·3H2O (0.1 M) at room temperature for about 15 d. During this period, the solution was replaced with a fresh solution of Cu(NO3)2·3H2O in DMF after every 48 h. The Cu-exchanged crystals were washed thoroughly with DMF and kept in DMF for 4 d to remove any excess metal salt from the pores of the frameworks. Anal. Calcd. For C153H209N29O35Cu3 (1Cu): C, 57.33; H, 6.57; N, 12.67 %. Found: C, 57.61; H, 6.73; N, 12.84%. Anal. Calcd. For C141H185N25O33Cu3 (2Cu): C, 57.43; H, 6.32; N, 11.88 %. Found: C, 57.69; H, 6.54; N, 12.07%.

RESULTS AND DISCUSSION Given that synthesis of mixed ligand MOFs using polycarboxylates and N-donor spacers has been established as a promising strategy to manipulate the overall structures with required functionality, we used C3-symmetric carboxylate ligand (H3L) and bent bipyridyl linker (dpta) (Scheme 1) for the construction of channel functionalized porous frameworks, using “node and bridging ligand” approach. Single-crystal X-ray structure determination revealed that 1 crystallizes in the triclinic space group P-1. The asymmetric unit contains two L ligands, one dpta ligand, three crystallographically independent Zn(II) ions and one metal bound DMF molecule. Three Zn(II) ions are bridged together by six carboxylate groups from six different L ligands to form a metallic trimer [Zn3(COO)6], which is regarded as the secondary building unit (SBU). The average distance between the neighboring Zn(II) ions in the SBU approximates to 3.54 Å. The coordination environment around each Zn(II) center in the SBU is depicted in Figure 1, which reveals that terminal Zn1 and Zn3 ions exhibit six and four coordination, with NO5 and NO3 donor sets, adopting distorted octahedral and tetrahedral geometries, respectively.


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The middle Zn2 ion is five-coordinated, with O5 donor set, and adopts distorted trigonal bipyramidal geometry. Each trinuclear SBU further links six neighboring SBUs through long aromatic arms of six L ligands forming a 2D bi-layer framework with large hexagonal rings (Figures 2a & 2b) of dimension, 22.80 × 22.48 Å2 (excluding van der Waals radii) in the ac plane.

Figure 1. The coordination environment of Zn(II) ions in 1 (H-atoms, except for the coordinated DMF molecule and the –NH2 group, are omitted for clarity). Noticeably, the aromatic rings of ligand L in the bilayer are not parallel but make certain angles with the neighboring aromatic rings to maximize C–H…π interactions (Figure S7, Supporting Information). Bent dpta ligands occupy the axial sites of the terminal Zn(II) ions in the SBUs and connect adjacent bi-layers (Figure 3a) to generate the overall 3D open framework with large 1D rhombus channels of size, 15.71 × 10.84 Å2 (excluding van der Waals radii) along the a-axis (Figure 2c).



Figure 2. A perspective view of the framework 1 before interpenetration, showing (a) planar hexagonal ring formed by three SBUs, (b) 2D bi-layer with hexagonal pores along the ac plane, and (c) 1D rhombus channels along the a-axis.

The distance between two adjacent bi-layers is 14.43 Å. The axial linker is not vertical to the 2D bi-layer plane but makes an angle of 97.46° (Figure 3a). The large channel dimension favors mutual interpenetration with another equivalent independent framework, generating a 2-fold interpenetrating 3D architecture (Figure 3b). Despite the 2-fold interpenetration, the overall structure possesses a void volume of 56.87% per unit cell, as calculated by PLATON.47 Channel A

14.43 Å

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Channel B

97.46°

(a)

(b)

(c)

Figure 3. (a) Side view of the adjacent bi-layers, connected by bent dpta ligands, before and (b) after interpenetration. (c) Top view of two hexagonal channels after interpenetration.


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The double interpenetration divides each hexagonal channel into two different types of channels: channel A and channel B (Figure 3c). The DMF molecule, coordinated to Zn2 centre, is directed towards the centre of channel A (aperture dimension: 12.58 × 5.48 Å2); channel B (aperture dimension: 16.10 × 10.46 Å2) contains solvent molecules in the voids. Location of these solvent molecules was not possible due to their highly disordered nature and the solvent composition (18DMF+3H2O) was calculated from the thermogravimetric weight-loss and elemental analysis that were consistent with the PLATON calculated results. A broad band at 3434 cm-1 in the IR spectrum (Figure S20, Supporting Information), indicates presence of the lattice water molecules while sharp peaks in the range 1667−1655 cm-1 point to the C=O stretching vibrations of the DMF molecules. Close matching between the experimentally observed and the simulated (single crystal data) powder X-ray diffraction patterns (Figure S11, Supporting Information) indicates bulk phase purity. Crystal Structure of 2. Yellow crystals of 2 were grown by hydrothermal reaction of H3L, dpta and Zn(NO3)2·6H2O in 1:2:4 molar ratio. In spite of the weakly diffracting crystal, we were able to collect single crystal data, which revealed the formation of a different framework {[Zn3(L)2(dpta) (DMF)]·14DMF·3H2O}n (2), where space group changed to Fdd2, with considerable changes in the lattice parameters (Table S1, Supporting Information). Bulk phase purity of 2 was confirmed by perfect agreement of the experimental PXRD pattern with the simulated one (Figure S12, Supporting Information). Structural analysis indicates that although, the asymmetric unit of 2 resembles to that of 1, orientations of the ligand L, dpta co-ligand, and the coordination environment around the Zn(II) centres are different. A comparison of the structure between 1 and 2 divulges that the [Zn3(COO)6] SBUs, metal bound DMF as well as the coordination modes of dpta co-ligand are common. However, unlike in 1, the terminal Zn1 and



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Zn3 in 2 adopt four and five coordination with NO3 and NO4 donor sets, respectively. Also, the central Zn2 ion is hexa-coordinated, with five carboxylate oxygen and one DMF oxygen atom in a distorted octahedral geometry (Figure 4). Moreover, the average Zn···Zn distance in the trinuclear SBU (3.67 Å) is bit larger than that in 1. It should be noted here that the pyridyl ring of dpta co-ligand and coordinated DMF molecule are highly distorted in 2.

Figure 4. The coordination environment of Zn(II)ions in 2 (H-atoms, except for the coordinated DMF molecule and the –NH2 group, are omitted for clarity). Although, each trinuclear SBU in both the frameworks links six neighboring SBUs through the linker L, the key structural differences arise from their positioning and orientation. As observed in 1 (vide supra), all six L linker arms run in a parallel manner and lie in the same plane, with connecting SBUs being positioned perpendicular to the plane of the bilayer (Figure 5a). In 2, however, four L units lie in the same common plane to construct one half of the hexagonal ring. The tripodal ligands are slanted and orient away to make an angle of 116.78° between the SBU and aromatic arms of L (Figure 5b). A close inspection of the relative orientations of [Zn3(COO)6] SBUs in 2 reveals that unlike 1, two SBUs are located parallel to the planar halfhexagonal ring, allowing the outward positioning of the L ligand (Figure 5b). Due to this distinct orientation of L with respect to SBUs, framework 2 precludes any formation of a perfect 2D


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bilayer, and embraces a different structure to that of 1. The hitherto unknown network topology in 2 may be simplified by considering the ligands and metal ions as three- and eight-connecting nodes, respectively. This generates a 3D binodal (3,8)-connected network (Figure S8, Supporting Information) with the Schlafli symbol (43.58.68.78.8)(43)2. Framework 1 also shows (3,8)connected binodal 3D network, but owns a different, yet unique Schlafli symbol, (43·624·8)(43)2 (Figure S8, Supporting Information).

Figure 5. Perspective views of (a) half hexagon formed by four parallel L and three perpendicular SBU to the average plane in 1, (b) half hexagon formed in 2 by four parallel L, one perpendicular SBU and two parallel SBUs to the average plane, (c) elongated pentagonal channels in 2, (d) 2-fold interpenetration in 2 and (e) framework 2 along the c axis showing rhombus shaped cavities.

The dpta linkers in 2 perfectly gets accommodated between two parallel SBUs and can be regarded here as a linking ligand rather than a pillar. Because of this linking, framework 2 shows



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two nearly pentagonal channels (Figure 5c and 5d) of dimensions: 23.89 × 12.88 Å2 (channel A) and 15.96 × 13.84 Å2 (channel B; excluding van der Waals radii). These large channels are filled by two-fold interpenetration. In spite of the interpenetration, (Figure 5d) framework 2 possesses 1D rhombus channels (dimension 7.17 × 6.73 Å2; excluding van der Waals radii) along c direction (Figure 5e), which are occupied by guest DMF and water molecules. Like 1, the highly disordered guest solvent molecules cannot be mapped by single crystal XRD. The PLATON calculated solvent-accessible void volume of the channels is about 61.03%, which accounts for the total number of guest molecules (14 DMF+3 H2O).

Single-Crystal to Single-Crystal Metal-ion Exchange Reactions in 1 and 2. Direct synthesis of the Cu(II) analogues of 1 and 2 by replacing Zn(NO3)2·6H2O with Cu(NO3)2·3H2O remained unsuccessful. However, presence of the trinuclear [Zn3(COO)6] SBUs and their accessibility from large channels in the frameworks, prompted us to explore the possibility of metal ion exchange reactions. Crystals of 1 and 2 were separately dipped in DMF solutions of Cu(NO3)2·3H2O (0.1 M) at room temperature. Optical microscopic examination showed that the original size and shape of the crystal were maintained, while color of both the crystals changed to greenish blue in 2 d, and then to more intense blue after 15 d (Figure S28– S30, Supporting Information). The metal replacements of the intense blue products were initially verified by the EPR spectrum (Figure S26 and S27, Supporting Information) and X-ray fluorescence (XRF) spectroscopic studies. Although, the blue crystals, obtained through transmetalation of 1, were poorly diffracting, we were able to obtain the unit cell parameters, which showed good agreement with that of its mother Zn(II) framework (Table S2, Supporting Information). Based on this result, the formula of the metal exchanged framework was expressed


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as, {[Cu3(L)2(dpta)(DMF)]·20DMF·2H2O}n (1Cu), which bears good concurrence with the elemental analysis results. Moreover, the PXRD pattern of 1Cu is comparable to that of the mother Zn(II)-framework (Figure S11, Supporting Information) and does not show any formation of a new phase, demonstrating that the framework is preserved throughout the metal exchange process. On the other hand, single crystal X-ray data (Table S1, Supporting Information) of 2 after Cu(II) exchange could be collected, which unveils the formula: {[Cu3(L)2(dpta)(DMF)]·16DMF·4H2O)}n (2Cu). Here, the solvent compositions were established from a combination of elemental analysis, TGA and PLATON squeeze results. The PXRD pattern of 2Cu matches well with the simulated pattern of 2Cu, as well as with the simulated and experimental patterns of 2, confirming the bulk phase transformation as well (Figure S12 in the Supporting Information).

(a) (b) Figure 6. Kinetic profiles of the framework transmetalation from Zn(II) to Cu(II) for (a) 1 and (b) 2. For both the frameworks, the kinetics of the Zn2+ to Cu2+ ion exchange were monitored by energy dispersive X-ray spectroscopy (EDS), which indicated 60% of Zn2+ ions in 1 could be replaced by Cu2+ within 5 d, while 96% exchange took place in 10 d. For 2, 60% replacement of Zn2+ ions by Cu2+ within 5 d, while 96% exchange took place in 12 d. Complete replacement of



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Zn(II) ions by Cu(II) ions required about 12 d for 1 and 15 d for 2 (Figure 6). This difference in metal exchange kinetics can be attributed to the different structures of 1 and 2. Reverse transmetalation of the Cu(II) exchanged products, or replacement of Zn(II) ions in 1 and 2 by other transition metal ions do not take place supporting the order of Irving–Williams series.

Thermal Stability. Thermogravimetric analyses (TGA) (Figures S15 and S16, Supporting Information) give weight-loss of 46.80 % (calculated 46.78%) and 41.10 % (calculated 41.20%), for 1 and 2 respectively, between 30–300 °C, that correspond to the loss of lattice guest molecules and coordinated DMF molecules. Both the frameworks are stable up to 400 °C, beyond which decomposition starts. The variable-temperature powder X-ray diffraction (VTPXRD) studies for both the framework reveal that crystallinity and overall framework integrity is maintained at least up to 150 °C (Figures S9 and S10, Supporting Information). In contrast, the TGA curves of 1Cu and 2Cu reveal a continuous loss of solvent molecules, starting from room temperature, without a plateau (Figure S17, Supporting Information) showing their limited thermal stability. Also, the VTPXRD data of the transmetalated frameworks show complete loss of crystallinity above 50 °C (Figures S13 and S14, Supporting Information).

Gas Adsorption Studies. The porous and robust nature of 1 and 2, together with the presence of –NH2 functionalized channels and possibility of creating unsaturated metal center (UMC) in the SBU upon activation, satisfy the essential prerequisites for gas sorption studies. Activation of the samples could be achieved by dipping crystals of 1 or 2 in acetone for 5 d, followed by heating at 120 °C under vacuum for 12 h to produce the guest free frameworks 1ʹ and 2ʹ, respectively. The activated frameworks were first subjected to N2 gas sorption


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measurements at 77 K up to a relative pressure of 1.0, which showed type-I behavior in both cases, suggesting their microporous nature (Figure 7). The Brunauer-Emmett-Teller (BET) surface area for 1ʹ and 2ʹ were estimated as 300 and 340 m2/g, respectively. Based on N2 adsorption isotherms, the pore volumes of 1ʹ and 2ʹ were calculated to be 0.15 and 0.18 cm3/g, respectively, while the pore sizes are found to be 0.73 nm for 1′ and 0.76 nm for 2′ (Figures S18, Supporting Information). The superior N2 adsorption capacity in 2ʹ is in full agreement with its larger opening of channels (vide supra). However, it is worth mentioning that compound 2′ shows adsorption–desorption hysteresis in the relative pressure range 0.40 to 0.95. This behavior suggests that the adsorbed N2 is not immediately released on reducing the external pressure and may be trapped within the framework.

Figure 7. Nitrogen physisorption isotherms of 1′ (adsorption/desorption ▲/∆) and 2′ (adsorption/desorption ●/○) at 77 K.

We next studied the H2 storage performances of the activated frameworks at 77 K up to the relative pressure of 1.0. As depicted in Figure 8, the H2 sorption for 1′ and 2′ show typical type-I behavior, where a steep uptake is realized at low-pressure region, with gradual increase in uptake amount upon rising pressure. Noticeably, the desorption curve does not follow the



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adsorption one and show hysteresis for both the frameworks. The final adsorption amounts to be 0.27 wt% (29.85 cm3/g) for 1′ and 0.54 wt% (60.22 cm3/g) for 2′ (Figure 8).

Figure 8. Hydrogen physisorption isotherms of 1′ (adsorption/desorption (adsorption/desorption ●/○) at 77 K.

/

) and 2′

The –NH2 group decorated channels in these frameworks prompted us to explore their CO2 capture properties. The sorption isotherms of CO2 at 273 and 298 K for 1ʹ and 2ʹ are probed up to the relative pressure (p/p0) of 1.0. As depicted in Figure 9, the maximum CO2 uptake capacities at 273 and 298 K for 1′ are 28.00 and 15.95 cm3/g, respectively, with small hysteresis, while those values for 2′ are 54.03 and 38.54 cm3/g, respectively. It should be mentioned that although, the adsorption–desorption experiments in 2′ were conducted several times with fair consistency; the reproducibility for 1′ was poor after two cycles.


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(a) (b) Figure 9. Carbon dioxide physisorption isotherms of 1′ (adsorption/desorption ●/○) and 2′ (adsorption/desorption ■ /□) at (a) 273 K and (b) 298K.

To gain more insights on the interaction of the adsorbate with the frameworks, we calculated the isosteric heat of CO2 adsorption (Qst) from the isotherms obtained at 273 and 298 K utilizing the Clausius-Clayperon equation (Figure 10). At low loading, Qst values are 25.98 kJ/mol for 1′ and 28.83 kJ/mol for 2′, indicating moderate interaction between individual activated framework and CO2. The Qst values are superior to that of MAF-26 (23 kJ/mol),51 CuBTTri (21 kJ/mol),52 IRMOF-3 (19 kJ/mol)53 and UMCM-1 (12 kJ/mol),54 although, they are inferior than reported amine functionalized famous MOFs like NH2-MIL-53(Al) (50 kJ/mol)55, CAU-1 (48 kJ/mol)56 and bio-MOF-11 (45 kJ/mol).57



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Figure 10. Isosteric heats of CO2 adsorption (Qst) for 1′ (■) and 2′ (●). To further establish the potential of the activated Zn(II)-frameworks for gas separation under ambient conditions, the adsorption isotherms for N2, H2, and CH4 were probed at 273 K and up to 1 bar relative pressure. As shown in Figure 11, both N2 and H2 gases do not diffuse into the pores of either 1ʹ or 2ʹ, while small amount of CH4 gas enters. The N2, H2 and CH4 uptake values for 1ʹ are 1.65 cm3 g−1, 1.11 cm3 g−1, and 3.27 cm3 g−1, respectively, while those values for framework 2ʹ are 0.61 cm3 g−1, 2.03 cm3 g−1, and 7.48 cm3 g−1, respectively. Clearly, the uptake values of these three gases are much less compared to that of CO2 for 1ʹ or 2ʹ (vide supra). The superior inclusion of CO2 in both the activated frameworks can be explained on the basis of its larger quadrupole moment (13.4 × 10−40 Cm2) in comparison to N2 (4.7 × 10−40 Cm2), H2 (4.7 × 10−40 Cm2), and CH4 (0 Cm2)58 that induce better interaction with the channels and interior wall of the MOFs, composed of basic –NH2 functionalities. The significant challenge regarding gas separation under ambient conditions, plus the favorable distinguishing adsorption behavior of both the activated Zn(II)-frameworks encouraged us to examine their selective CO2 capture ability over N2, H2, and CH4 gases at 273


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K. The selectivity (S) of CO2 adsorption over N2 is calculated from the single-component isotherm data. For CO2 capture, this value typically reports the ratio of the adsorbed amount of CO2 at 0.15 bar to the adsorbed amount of N2 at 0.75 bar. The value is normalized for the pressures chosen according to the equation: S = (qCO2/qN2)/(pCO2/pN2),59 where q is the amount adsorbed and p is the relative pressure. For 1ʹ, the CO2 selectivity over N2 is found to be 40, while the value for 2ʹ is 185. To the best of our knowledge, such high CO2 selectivity over N2, as observed in 2ʹ, falls in the domain of well-known values in MOFs60 reported to date. It is worth mentioning that the CO2/N2 adsorption amount ratios at 0.16 atm61 (typical partial pressure of CO2 in industrial flue gas) for 1ʹ and 2ʹ are 17 and 255, respectively. To our delight, the value for 2ʹ is comparable or higher to the values reported for MOFs62-64 under similar measurement conditions.

(a) (b) Figure 11. CO2, N2, H2 and CH4 physisorption isotherms of (a) 1′, and (b) 2′ at 273 K. We presume that diversity of the pore sizes and electronic environments, due to the dissimilar structure of these two Zn(II) frameworks, play crucial roles towards superior CO2 selectivity of 2ʹ over 1ʹ. Moreover, 2ʹ possess larger surface area than 1ʹ (vide supra). Given the



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importance to develop protocols for reducing anthropogenic emission of CO2, the biggest contributor of greenhouse gas, the present results suggests that 2ʹ may find potential applications in the separation of CO2 from industrial flue gas and selective CO2 capture from a binary CO2– N2 mixture. Gas sorption studies on individual activated Cu-exchanged frameworks at 77 K show negligible N2 uptake (maximum 22.73 cm3/g for 1Cuʹ and 38.83 cm3/g for 2Cuʹ) (Figures S31, Supporting Information), compared to their corresponding mother Zn(II) frameworks. Also, both the transmetalated frameworks do not exhibit any H2 sorption at 77 K or CO2 sorption at 273 K. The Cu-exchanged frameworks are unstable even at 75 °C, as evidenced from the VTPXRD patterns (Figure S13 and S14, Supporting Information). The TGA curves also reveal a continuous loss of solvent molecules, starting from room temperature, without a plateau (Figure S17, Supporting Information) showing their limited thermal stability. Exchange with other low boiling solvents like MeOH, Me2CO, CHCl3 or CH2Cl2, followed by activation under mild conditions always led to amorphous materials as confirmed from PXRD measurements (Figure S32 and S33, Supporting Information).

CONCLUSION In conclusion, by stoichiometric variation of the bridging dipyridyl linkers we have synthesized two

structurally

different,

3D

porous

metal-organic

frameworks

(PMOFs)

{[Zn3(L)2(dpta)(DMF)]·18DMF·3H2O}n (1) and {[Zn3(L)2(dpta) (DMF)]·14DMF·3H2O}n (2), incorporating a tripodal ligand (H3L) and Zn(II) ions, under solvothermal condition. Both the structures are 2-fold interpenetrated and posses common [Zn3(COO)6] secondary building units (SBUs). The origin of the structural variation emerges from different disposition of SBUs in the


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framework, which renders 1 as a pillared-bilayer MOF with (43·624·8)(43)2 topology, while 2 holds the Schlafli symbol (43.58.68.78.8)(43)2. The structural variation changes the electronic environment and pore sizes in these frameworks, allowing the activated framework 2ʹ to uptake better N2, H2 gases at 77 K and CO2 gas at 273 K, than 1ʹ. At 273 K, framework 2ʹ shows better selective CO2 adsorption over N2, and H2 than 1ʹ, while the selectivity of CO2 over CH4 is reversed. Another interesting finding is that all the Zn(II) ions of SBU in both the frameworks could be completely exchanged by Cu(II) ions under ambient condition, via a single-crystal to single-crystal transmetalation reaction, producing 1Cu and 2Cu. However, gas sorption behavior of activated Cu-exchanged frameworks show negligible uptake compared to the mother frameworks, signifying their unstable character.

ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website at DOI: Several spectroscopic, thermogravimetric analysis, powder X-ray diffraction patterns and photographs of crystal, X-ray crystallographic data and figures. (PDF) Accession Codes CCDC 1481914−1481916 contains the supplementary crystallographic data for this paper. These data can be obtained free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by emailing [email protected], or by contacting The Cambridge Crystallographic Data Centre, 12, Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033.

AUTHOR INFORMATION



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Corresponding Author *E-mail: [email protected] *E-mail: [email protected] Notes The authors declare no competing financial interest.

ACKNOWLEDGEMENTS We gratefully acknowledge the financial support from the Department of Science and Technology, New Delhi, India (to PKB) and SRF from the Council of Scientific and Industrial Research, New Delhi, India to DD. SN acknowledges CSC-0102. Support from analytical division, CSMCRI is gratefully acknowledged. CSIR-CSMCRI Communication No. 084/2016.

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For Table of Contents Use Only Stoichiometry Controlled Structural Variation in 3D Zn(II)–Frameworks: Single-Crystal to Single-Crystal Transmetalation and Selective CO2 Adsorption Dinesh De,a Subhadip Neogi,b,* and Parimal K. Bharadwaja,*

Stoichiometric variation of the bridging dipyridyl linkers allows to construct two structurally different PMOFs, with altered topology and pore size, yet having common [Zn3(COO)6] SBU. All the Zn(II) ions in both the frameworks could be exchanged by Cu(II) ions. Activated Zn(II) frameworks show selective CO2 adsorption.


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Stoichiometric variation of the bridging dipyridyl linkers allows to construct two structurally different PMOFs, with altered topology and pore size, yet having common [Zn3(COO)6] SBU. All the Zn(II) ions in both the frameworks could be exchanged by Cu(II) ions. Activated Zn(II) frameworks show selective CO2 adsorption. 246x133mm (96 x 96 DPI)


Stoichiometry Controlled Structural Variation in ... - ACS Publications

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