Exploiting Dimensional Variability in Cu Paddle-Wheel Secondary

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Exploiting dimensional variability in Cu paddle-wheel SBU based mixed valence Cu(II)/Cu(I) frameworks from a bispyrazole ligand by solvent/pH variation Kapil Tomar, Ashish Verma, and Parimal K. Bharadwaj Cryst. Growth Des., Just Accepted Manuscript • DOI: 10.1021/acs.cgd.8b00002 • Publication Date (Web): 08 Mar 2018 Downloaded from http://pubs.acs.org on March 12, 2018

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Crystal Growth & Design

Exploiting dimensional variability in Cu paddle-wheel SBU based mixed valence Cu(II)/Cu(I) frameworks from a bispyrazole ligand by solvent/pH variation

Kapil Tomar, Ashish Verma, and Parimal K. Bharadwaj* Department of Chemistry, Indian Institute of Technology Kanpur, Kanpur 208016, India

ABSTRACT:

Four

coordination

polymers,

[CuII(L)H2O]·2DMF

(1),

[CuICuII2L2(H2O)(pyridine)2]·(NO3)·4DMF·3H2O (2), [CuICuII2L2(pyridine)3]·(NO3)·DMF·4H2O (3), and [CuII2L(µ-O)(DMF)]·4DMF·5H2O (4) have been synthesized under solvothermal conditions using a single methylene-bispyrazole based carboxylic acid ligand, L and Cu(NO3)2. All the compounds were comprised of copper paddle wheel Secondary Building Units (SBUs). The structural differences in 1-4 arise due to different composition of the solvent system as well as pH of the medium. Compound 1 exhibited a 1D double chain structure with a porous 3D supramolecular network while 2 and 3 represent mixed Cu(II)/Cu(I) based porous frameworks. MOF 2 and 3 were obtained by systematic binding of pyridine molecules on the axial positions of Cu paddle-wheel SBU, having a rare topology, sur; bbf-3,4-Cccm and a new topology, respectively. MOF 4 is a 3D porous network containing Cu2(µ-O)2 moiety and exhibits 3,4,4-c net. It contains open channels with 46.9% void volume. Gas sorption studies of 4 showed the -1

micro porous nature of the framework with 66 cc g-1 of CO2 uptake at 273 K and 31 cc g at 298 K with high Qst value of 30.7 kJ mol-1 at zero loading.

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INTRODUCTION As a class of newly developed crystalline porous materials, metal organic frameworks (MOFs), constructed from inorganic and organic building units through coordination bonds, exhibits intriguing topologies and have shown great potential for gas storage1, separation2, heterogeneous catalysis3, sensing4, and so on. The functions of these materials are directly related to their structures and topologies.5Although some strategies have been developed for obtaining desired structural features6-8 there are still mixed opinions as to weather a real design in their synthesis can be applied.9-11 The self-assembly process of MOF formation involves competing reversible and simultaneous interactions among the metal, ligand, counter anion and the solvent. This makes the predictability of the final network an open challenge to the researchers.12 The concept of secondary building unit (SBU) as a synthetic module for the construction of robust frameworks has made a paradigm shift in this area of research.13 In this regard, the copper paddle-wheel SBU, [Cu2(COO)4], based MOFs14 represent a distinctive subclass and have been thoroughly studied. The most common synthetic strategy to obtain such MOFs is solvothermal method in presence or absence of acidic medium. Although a successful strategy, but is mostly limited to obtaining only a single framework with no further exploration of obtaining multiple frameworks from a single ligand. Rare, studies have been performed to obtain more than one networks from Cu paddle-wheel SBU based systems by altering the solvent medium.15 One of them is reported by Zhao et al., where two Cu paddle-wheel SBU based MOFs were constructed from a single ligand by changing the solvent system.16 Much rarer are the systems where different dimensionalities are obtained from the same ligand and metal ion having the same SBU.17


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Scheme 1. Schematic diagram of the ligand, H2L, and synthetic scheme for 1-4. Herein, we explored the Cu paddle-wheel SBU based networks, ranging from, 1D to 3D with different porosities and topologies employing a single ligand, methylenebispyrazole based dicarboxylic acid, H2L (Scheme 1). We have shown how the SBU can be tuned to obtain different architectures by altering the solvent and pH conditions which also lead to mixed Cu(I)/Cu(II)

[CuII(L)H2O]·2DMF

networks:

(1),

[CuICuII2L2(H2O)(pyridine)2]·(NO3)·4DMF·3H2O (2), [CulCuII2L2(pyridine)3]·(NO3)·DMF·4H2O (3), and [CuII2L(µ-O)(DMF)]·4DMF·5H2O (4). While 1 is a 1D double chain polymer with open channels, MOF 2, 3 and 4 have rare topologies, namely, sur; bbf-3,4-Cccm, 3,3,4,4-c net, and 3,4,4-c net having 2D/3D porous frameworks. All the MOFs have appreciable accessible void volumes, 28.4%, 43.8%, 19% and 46.9% in 1, 2, 3, and 4, respectively. Also, gas adsorption studies of 4 confirm the microporous nature of the framework with appreciable CO2 gas adsorption. This study presents an amazing example of obtaining multiple architectures via



alteration in pH/solvent system from the assembly of a single ligand and copper paddle-wheel SBU with control over the topology and porosity in the resulting networks. EXPERIMENTAL SECTION Materials and measurements. The metal salt and other reagent-grade chemicals were procured from Sigma-Aldrich and used as received. All solvents were procured from S. D. Fine Chemicals, India. These solvents were purified following standard methods prior to use. The details of spectroscopic techniques and X-ray structural studies are provided in the Supporting Information. Synthesis of ligand The ligand (H2L) was synthesized in two steps details of which are given in the SI (Scheme S1). [CuII(L)H2O]·2DMF (1). A mixture of H2L (20 mg, 0.05 mmol) and Cu(NO3)2·6H2O (30 mg, 0.1 mmol) in DMF (2 mL) was heated at 70 °C under autogenous pressure in a Teflon-lined stainless steel autoclave for 48 h, followed by cooling to room temperature at a rate of 10 °C per hour to afford 1 as blue block-shaped crystals in 61% yield. The crystals were repeatedly washed with water followed by acetone and air-dried. Anal. Calcd for C31H31CuN6O7: C, 56.15; H, 4.71; N, 12.67. Found: C, 55.42; H, 4.58%; N, 12.42. IR (cm–1): 3422 (broad), 2921 (s), 1608 (s), 1569 (s), 1407(s), 1354 (m), 1320 (s), 1274 (m), 1173 (w), 1102 (m), 1046 (s), 1016 (w), 861 (m). [CuICuII2L2(H2O)(pyridine)2]·(NO3)·4DMF·3H2O (2). A mixture of H2L (20 mg, 0.05 mmol) and Cu(NO3)2·6H2O (30 mg, 0.1 mmol) in DMF (2 mL) and pyridine (0.3 mL) was heated at 70 °C under autogenous pressure in a Teflon-lined stainless steel autoclave for 72 h, followed by cooling to room temperature at a rate of 10 °C per hour to afford 2 as blue crystals in 57% yield. The crystals were repeatedly washed with water followed by acetone and air-dried. Anal. Calcd for C72H90Cu3N15O18: C, 52.59; H, 5.52; N, 12.78. Found: C, 52.32; H, 5.28; N, 12.56. IR (cm–1):


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3572 (broad), 3486 (s), 1630 (s), 1607 (s), 1514 (s), 1401(s), 1384 (m), 1150 (s), 1109 (m), 987 (w), 851 (m), 786 (s), 617 (w). [CulCuII2L2(pyridine)3]·(NO3)·DMF·4H2O (3). A mixture of H2L (20 mg, 0.05 mmol) and Cu(NO3)2·6H2O (30 mg, 0.1 mmol) in DMF (2 mL) and pyridine (0.6 mL) was heated at 70 °C under autogenous pressure in a Teflon-lined stainless steel autoclave for 72 h, followed by cooling to room temperature at a rate of 10 °C per hour to afford 3 as blue crystals in 64% yield. The crystals were repeatedly washed with water followed by acetone and air-dried. Anal. Calcd for C68H74N13O16Cu3: C, 52.59; H, 5.52; N, 12.78. Found: C, 52.34; H, 5.39; N, 12.53. IR (cm–1): 3571 (broad), 3486 (s), 1607 (s), 1568 (s), 1515 (s), 1403(s), 1384 (m), 1107 (s), 1015 (m), 886 (w), 786 (m), 707 (s), 644 (w). [CuII2L(µ-O)(DMF)]·4DMF·5H2O (4). A mixture of H2L (20 mg, 0.05 mmol) and Cu(NO3)2·6H2O (30 mg, 0.1 mmol) in DMF (2 mL) and HNO3 (0.5mL) was heated at 70 °C under autogenous pressure in a Teflon-lined stainless steel autoclave for 72 h, followed by cooling to room temperature at a rate of 10 °C per hour to afford 4 as blue crystals in 66% yield. The crystals were repeatedly washed with water followed by acetone and air-dried. Anal. Calcd for C40H67N9O15Cu2: C, 46.15; H, 6.49; N, 12.11. Found: C, 46.25; H, 6.24; N, 12.37. IR (cm–1): 3443 (broad), 2922 (s), 1607 (s), 1563 (s), 1514 (s), 1406(s), 1367 (m), 1305 (s), 1172 (m), 1014 (w), 866 (m), 786 (s), 705 (w).

RESULTS AND DISCUSSIONS Structural description of [CuII(L)H2O]·2DMF (1) A solvothermal reaction of H2L and Cu(NO3)2·6H2O in DMF at 70 °C aﬀords blue crystals of compound 1. The X-ray single crystal analysis reveals that 1 crystallize in P-1 space group with an asymmetric unit that contains one each of Cu(II) ion, ligand, L, coordinated water along 5 ACS Paragon Plus Environment


Figure 1.1D double chain in 1 with open windows having Cu paddlewheel SBU.

(a)

(b)

Figure 2. (a) 3D porous supramolecular network in 1 with open channels along the b-axis, and (b) Connolly surface of the pores in 1. with two DMF solvent molecules in the lattice. Hydrogen atoms on lattice DMF solvents could not be located because of the disorder and large thermal ellipsoids of the atoms. Detailed structural analysis reveal that each Cu(II) ion is square-pyramidally coordinated by four carboxylate oxygen atoms at the basal positions with one water molecule at the apical position. Each pair of Cu(II) ions is bridged by four carboxylate groups of L to generate the well-known paddle-wheel SBU with Cu···Cu distance of 2.631 Å (Fig. 1). Due to the bent shape of the ligand, L, two {Cu2(COO)4} SBUs and two ligand moieties connect together to form a [2+2] rhombic metallacycle which further extends into a 1D chain structure along the bc-plane. This type of architecture was also reported by Yaghi et al. where they have used a bent dicarboxylate ligand.18 Interestingly, the nitrogen atoms of the pyrazole moiety of L does not coordinate to the metal ion and remains free. The 1D chains with open cavities are packed into a 3D porous supramolecular 6 ACS Paragon Plus Environment

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structure via weak non-covalent interactions (Fig. 2 (a)). The resulted 3D porous network contains 1D open channel along the b-axis with dimensions of ~6 x 12 Å (Fig. 2 (b)). These channels are filled up with DMF solvent molecules. The space taken by solvent molecules is 28.4% (419.1/1478.1 Å3) of the total crystal volume as calculated by PLATON.19

Figure 3. Coordination environments of Cu(II) and Cu(I) in 2. Structural description of [CuICuII2L2(H2O)(pyridine)2]·(NO3)·4DMF·3H2O (2) When ligand L and Cu(II) salt were reacted in the presence of stoichiometric amounts of pyridine, a 3D mixed Cu(II)/Cu(I) ion based MOF resulted, [CuICuII2L2(H2O)(pyridine)2] ·(NO3)·4DMF·3H2O (2). Formation of Cu(I) ions from Cu(II) salts under basic/solvothermal conditions is a well-known phenomenon and various mixed valence MOFs are reported in literature.20-23 Here, the addition of pyridine acts as a reducing agent since without any addition of pyridine, only CuII ion based MOF resulted (MOF 1). MOF 2 crystallizes in I 2/a space group with asymmetric unit consists of one Cu(II) ion, half of Cu(I) ion, one ligand, L, and one each of coordinated pyridine, and water molecule, respectively. One nitrate anion, four DMF and three water molecules per formula unit have also been confirmed by elemental analysis (EA), thermogravimetric analysis (TGA), and infrared spectroscopy (IR).24 The peak at 1384 cm-1 in the IR spectrum of 2 (Fig. S4) confirms the presence of nitrate anion as reported in literature.25



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Here, two different type of Cu coordination geometries are evident, one is the usual Cu(II) paddlewheel SBU and another is the three coordinated trigonal planar Cu(I) ion which is coordinated from two nitrogens atoms of the pyrazole moiety and one oxygen atom of the water molecule (Fig. 3). Although the CuII···O bond distance (1.939 Å) in the paddlewheel SBU is similar to others reported in literature,26 the CuI···N distance in three coordinated Cu(I) ion is very small, 1.888 Å, confirming the +1 oxidation state of Cu1 ion.27 Here, the role of pyridine addition brings two changes, first, the reduction of Cu(II) to Cu(I) and, second, coordination of

Figure. 4 3D packing view of 2, showing the 1D open channels along with the helical chain of L and three connected Cu(I) ion.

(a)

(b) 8 ACS Paragon Plus Environment

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Figure 5. (a) Comparison of the coordination modes of L in 1 and 2, (b) Topological representation of 2 showing the 3,4-c binodal net. the resulted Cu(I) ion to the nitrogen of pyrazole moiety (Fig. 5(a)). Therefore, by the addition of pyridine, we have seen a small change in the Cu paddlewheel SBU which in turn brings a change in the dimensionality of the resulted network from 1D in 1 to 3D in 2. Oxidation states of Cu ions in 2 were further analyzed by X-ray photoelectron spectroscopy (XPS) (Fig. S12(a)). The peaks of 2p3/2 and 2p1/2 at 932.07 eV and 952.06 eV confirms the presence of Cu(I) ions while small peaks at 942.02 eV and 961.08 eV indicate the presence of Cu(II) ions.28 Despite the presence of similar Cu paddlewheel SBUs in 1 and 2, a [2+2] rhombic metallacycle motif is not observed in 2. Due to the bulky pyridine molecules on the axial positions, the motif deviates from planarity and acquires the shape of a helical chain which increases the dimensionality in 2 compared to 1 (Fig. 4). Also, changes in the ligand geometry were noticed, the angle in C-CH2-C moiety of L changes from, 118° in 1 to 110° in 2 (Fig. 5(a)). The plane of the two pyrazole rings in L were also tilted by different angles, 75° and 87° in 1 and 2, respectively. The resulted porous 3D network in 2 has 1D open channels along the b-axis with dimensions of ~16 x 15 Å which are filled up with disordered solvent molecules. Calculations using PLATON showed that 2 has about 43.8% (4351.3/9934 Å3) of the total volume available for guest inclusion. Topological analysis using TOPOS29 reveals that, the Cu paddlewheel SBU can be considered as a four connected node while the ligand itself can be viewed as a three connected node. Therefore, the network has a 3,4-c binodal net having rare topological type: sur; bbf-3,4Cccm (binodal.ttd). The point symbol for the net is {62.82.102}{62.8}2 (Fig. 5(b)). Structural description of [CulCuII2L2(pyridine)3]·(NO3)·DMF·4H2O (3)



When

excess

pyridine

is

used

with

Cu(II)

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ion

and

ligand

L,

MOF

3,

[CulCuII2L2(pyridine)3]·(NO3)·DMF·4H2O, resulted. It contains a unique type of SBU which resulted after the disruption of Cu-paddlewheel SBU by pyridine molecules. As shown in Fig. 6, we can imagine the successive SBU disruption in 2 and 3, where a sequential attack of pyridine molecules on the axial positions of the paddlewheel occured in 2 and 3. MOF 2 has two pyridine molecules bound on axial positions of the SBU, but, in MOF 3, one more pyridine attacked and replaces one of the O atom of the carboxylate group and resulted in an unprecedented SBU. The asymmetric unit of 3 contains, two Cu(II) ion, two ligand L, one Cu(I) ion, three coordinated pyridine molecules, one disordered nitrate anion and disordered solvent molecules in the lattice. One of the pyridine molecule was found to be in severely disordered state with large thermal ellipsoids due to which hydrogen atoms cannot be located. Unlike in 2, the charge balancing nitrate anion has been located in the structure and is present in a pocket where it is surrounded by several methyl groups of pyrazole moiety and showed weak Vander wall interactions.

Figure 6. Step by step formation of distorted Cu paddlewheel SBU along with the formation of [2+2] metallacycle motif in 3. 10 ACS Paragon Plus Environment

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Figure 7. A portion of the 2D sheet structure in 3, depicting the CuII/CuI coordination modes.

Here, two types of Cu ion geometries are observed, one is the disrupted Cu paddlewheel SBU another is the three connected Cu(I) ion which is coordinated from three nitrogen atoms of the pyrazole moieties (Fig. 7). The presence of Cu(I) ions was also detected by XPS spectrum of 3 (Fig. S12(b)). The peaks at 932.08 and 952.07 eV confirms the presence of Cu(I) ions in 3. The Cu···Cu distance has increased to 3.388 Å in the disrupted Cu paddlewheel SBU. Detailed structural analysis reveals that, here also, the [2+2] rhombic metallacycle motif is present with a disrupted Cu paddlewheel SBU which is shown in Fig. 6. A prime distinct of 3 from 2 is that the Cu(I) ion is coordinated to three nitrogen atoms of the pyrazole moiety in a

(a)

(b) 11 ACS Paragon Plus Environment


Figure 8. (a) Coordination modes of L in 3, and (b) topological representation showing the 3,3,4,4-c tetra nodal net.

trigonal planar geometry and resulted in a 2D layered structure (Fig. 8(a)). Furthermore, the 2D sheets arrange and pack themselves in a supramolecular 3D arrangement with a dense packing. The void volume is calculated to be 19% (1325.8/6991.5 Å3) which is occupied by disordered nitrate anions and solvent molecules. A sharp distinct of addition of extra pyridine is visible in 3, where the paddlewheel SBU has changed which ultimately resulted in a 2D network as compared to a 3D network in 2. Applying the topological analysis, the dinuclear distorted Cu paddlewheel and the ligand L can be considered as four connected nodes while the three connected Cu(I) ion is a three connected node. Therefore, the network can be envisioned as a 3,3,4,4-c tetra nodal net with a new topology. Point symbol for net is {4.63.82}{4.82}{42.6.83}{6.82}.

Figure 9. 1D chain in 4, showing the [2+2] metallcyclic motif.

Structural description of [CuII2L(µ-O)(DMF)]·4DMF·5H2O (4) When acidic medium (HNO3) was used with Cu(II) ion and the ligand L in DMF, [CuII2L(µ-O)(DMF)].4DMF.5H2O (4), resulted with an unprecedented Cu2µ-(O)2 bridging 12 ACS Paragon Plus Environment

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moiety. Although copper dioxygen, Cu2µ-(O)2 cores have been studied in small crystal structures for studying the biological systems,30-31 no such moiety was observed in the metal organic frameworks till date. The asymmetric unit of 4 contains, two Cu(II) ions, one ligand, L, one oxo anion, one coordinated DMF and disordered solvent molecules in the lattice. Structural analysis reveals that 4 consists of similar paddlewheel SBU Cu2(COO)4 (Cu···Cu distance, 2.607 Å) with DMF molecules bound on axial positions. Also, the [2+2] rhombic metallacycle motif is retained here similar to 1. This resulted in a 1D chain formation with open windows similar to MOF 1 (Fig. 9).

Figure 10. 3D packing diagram in 4 showing Cu2(µ-O)2 linkages and axially bound DMF molecules (in yellow spheres). Unlike in 1, these 1D chains are connected to each other by Cu2µ-(O)2 linkers through the nitrogen atoms of the pyrazole ring (Fig. 10), with Cu···O bond distance of 2.292 and 2.438 Å. The effect of acidic medium (HNO3) compared to basic medium (pyridine) in 2, 3 and 4 is evident here. While the presence of basic system in 2 and 3 promoted the reduction of Cu(II) to Cu(I) ion in 2 and 3, acidic medium in 4 promoted the formation of Cu2µ-(O)2 inorganic pillars which connects the 2D sheets into a 3D porous network. It should be noted that absence of HNO3



Figure 11. (a) 3D packing view of 4 along c-axis, (b) along b-axis, (c) conolly surface view, and (d) topological representation of 4 (purple and red = 4 connected nodes, green = 3 connected node). does not produce Cu2µ-(O)2 moieties as discussed in formation of 1. This resulted in a porous 3D framework with open 1D channels along c-axis with dimensions ~16 x 11 Å (Fig. 11(a) and (b)) and void volume of 46.9% (4065.4/8660.8 Å3) which increases to 54.3% (4699.8/8660.8 Å3) when axially bound DMF molecules were removed. Topological analysis reveal that both Cu paddlewheel SBU and Cu2O2 moiety acts as 4 connected nodes while the ligand L is viewed as a 3 connected node which resulted in a 3,4,4-c net having a new topology with point symbol, {6.72}4{62.72.82}{72.82.102}. (Fig. 11(d)). Comparison of the SBUs, coordination modes, and dimensionality in 1-4


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The basic skeleton of Cu paddlewheel SBUs were found to be similar in 1, 2 and 4, whereas, a deformation was observed in 3. The effect of addition of acidic and basic components was also observed on the axial bound molecules of the SBU. When only DMF was used as solvent, water is found on the axial positions (in 1), while pyridine molecules were found in 2 and 3. The coordinated DMF molecules were found when acidic medium was used (in 4). Similarly, coordination modes of the pyrazole moiety were also found to depend on acidic/basic medium in 1-4. When pyridine was used, pyrazolic N atom coordinates to CuI ion (in 2 and 3) while in acidic medium (HNO3) it is coordinated to CuII2(µ-O)2 moiety (in 4). When no additive was used, pyrazolic N atom was found to be non-coordinated (in 1). A comparison of all the geometrical parameters of 1-4 has been tabulated in table 1. It is interesting to note that the CuIIO bond distance in the paddlewheel increases as the dimensionality of MOFs increases (2, 4 > 3 > 1). Also, the angle between the pyrazole planes was found to be smallest for the 1D compound 1, whereas it drastically increases in 2-4. The angle of the methylene moiety joining the two pyrazole units is also found to be smallest in 2 while it is almost similar in other compounds. All these differences in geometrical parameters occurs due to the different coordination environments around the metal ion due to which the ligand, L also adopts different angles in 1-4. Table 1 MOF

Cu-O/N distance (Å)

1

(CuII-O) = 1.972

2

(CuII-O) = 1.939 (CuI-N) = 1.888 (CuII-O) = 1.964, 1.952 (CuI-N) = 1.925, 2.058

3 4

(CuII-O) = 1.919 (CuII-N) = 1.955, 1.980

C-CH2-C angle (°) 116.80

Angle b/w Pz planes (°) 70.27

Cu···Cu distance (Å) 2.631

Dimensionality 1D

110.57

87.70

2.609

3D

115.40, 116.98

88.69, 89.84

3.388

2D

115.69

85.54

2.607

3D



PXRD and TGA studies To examine the thermal stability and phase purity of the frameworks, we have carried out the TG analysis and PXRD measurements. TGA of all four networks showed that after loss of lattice solvent molecules, the frameworks are stable up to a moderately high temperature. For compound 1 (Fig. S7 (a)), the TG curve reveal a weight loss of ~24% (calculated 24.4%) between 30 and 260°C which corresponds to loss of two DMF solvent molecule and one coordinated water molecule per formula unit. Then, the framework collapses with an abrupt weight loss with decomposition of the framework. MOF 2 (Fig. S7 (b)) showed a weight loss of ~32% (calculated 32.6%) between 30 to 325°C which corresponds to loss of four lattice DMF, four water (three lattice and one coordinated), and two coordinated pyridine molecules after which the framework starts collapsing. A similar decomposition process is observed for MOF 3 (Fig. S7 (c)) where a weight loss of ~25 % (calculated 25.2%) is observed in the temperature range, 30 to 260 °C, which corresponds to loss of one DMF, four water, and three pyridine molecules after which the framework decomposes. MOF 4 (Fig. S7 (d)) showed a weight loss of ~43% (calculated, 43.7%) in the temperature range 30 to 320°C which corresponds to loss of four DMF and five water molecules after which the framework is stable up to ~390°C without any framework decomposition. The similarity of the PXRD data with the simulated patterns confirms the phase purity of the as synthesized materials 1-4 (Fig. S8-S11). With high thermal stability of 4, we have also checked the PXRD of the desolvated phase (4ʹ) after evacuation at 130 °C which retains the crystallinity (Fig. S11). The stable, robust nature and presence of open metal sites in 4 prompted us to study its gas sorption properties.


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(a)

(b)

(c)

Figure 12. (a) N2 adsorption isotherm at 77 K and CO2 adsorption isotherm of 4ʹ at 195 K, (b) CO2 adsorption isotherm of 4ʹ at 273 and 298 K, and, (c) isosteric heat of adsorption (Qst) for CO2 gas. Gas adsorption studies Activation of 4 was done by keeping the powdered sample in anhydrous methanol for four days followed by heating at 130 °C under high vacuum for 12 hrs to produce guest free MOF 4ʹ. The result of PXRD measurement demonstrates that desolvated 4ʹ retains its structural integrity (Figure S11). The porosity of 4ʹ was examined by the gas sorption measurements, which indicates that it can adsorb N2, and CO2 at low and at ambient temperatures. As shown in Figure 12(a), N2 adsorption measurement at 77 K and 1 atm revealed type-I isotherm with saturated N2 uptake of 158 cc g–1 (STP), which is characteristic of a microporous material, corresponding to a BET surface area of 432.5 m2 g–1. CO2 adsorption isotherm demonstrated that 4ʹ can store up to 130 cc g–1 (STP) CO2 at 195 K/1 atm. Further, the CO2 sorption was performed at 0 °C and near room temperature to study the interactions between CO2 and the host framework of 4ʹ. As shown in Figure 12(b), the CO2 uptakes at 1 bar reaches upto 66 cc g–1 at 273 K and 31 cc g–1 at 298 K. In order to understand the interactions between CO2 and the framework of 4ʹ, we calculated the isosteric heat (Qst) of CO2 by fitting the 273 and 298 K isotherms to the virial equation (see the



SI).32 We note that the value of the initial isosteric heats of adsorption for 4 is 30.7 kJ/mol (Figure 12(c)), which implies that the framework of 4ʹ has relatively high binding affinity toward CO2. The obtained Qst value of 4 is comparable to the well-known HKUST-1 (31.2 kJ/mol) and higher than NOTT-101 (26.5 kJ/mol).33

CONCLUSIONS In this study, four new Cu paddlewheel based frameworks were constructed with a novel bispyrazole based carboxylic acid ligand. While MOF 1 and 4 are Cu(II) paddlewheel SBU based porous MOFs, 2 and 3 are mixed Cu(I)/Cu(II) based porous MOFs with rare topologies, sur; bbf-3,4-Cccm, and 3,3,4,4-c net. These frameworks present the first example where a single ligand and Cu paddlewheel SBU based system has yielded multiple topological networks just with slight change in the pH (from acidic to basic) of the solvent system. Gas adsorption studies of 4 also confirmed the microporous nature of the framework with high CO2 adsorption at 273 and 298 K.

ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website at DOI: X-ray crystallographic data of 1-4 (CCDC 1814232-1814235) (CIFs)


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Materials and methods, X-ray structural studies, ESI-MS spectra, IR spectra, TGA, PXRD, NMR spectra, XPS spectra, and additional Figures, (PDF) AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] Notes The authors declare no competing financial interest. ACKNOWLEDGMENTS We gratefully acknowledge the financial support received from the DST and MNRE, New Delhi, India (to P.K.B.), and SRF from the University Grants Commission (UGC), New Delhi, India to A.V. K.T. acknowledges SERB ((YSS/2015/001088/CS)), India for Young Scientist research fellowship.

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For Table of Contents Use Only Exploiting dimensional variability in Cu paddlewheel SBU based mixed valence Cu(II)/Cu(I) frameworks from a bispyrazole ligand by solvent/pH variation Kapil Tomar, Ashish Verma, and Parimal K. Bharadwaj*

Synthesis and properties of four metal organic frameworks have been discussed which were prepared with a bispyrazole based ligand by varying the pH of the system from basic to acidic. Rare topologies were obtained with micro porous channels having high CO2 absorption uptake at 273 K.


Exploiting Dimensional Variability in Cu Paddle-Wheel Secondary

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