Reversible Phase Transformation in Three Dynamic Mixed-Ligand

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Reversible Phase Transformation in Three Dynamic Mixed Ligand Metal–Organic Frameworks: Synthesis, Structure and Sorption Study Arijit Halder, Biswajit Bhattacharya, Rajdip Dey, Dilip Kumar Maity, and Debajyoti Ghoshal Cryst. Growth Des., Just Accepted Manuscript • DOI: 10.1021/acs.cgd.6b00610 • Publication Date (Web): 18 Jul 2016 Downloaded from http://pubs.acs.org on July 21, 2016

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Crystal Growth & Design

Reversible Phase Transformation in Three Dynamic Mixed Ligand Metal– Organic Frameworks: Synthesis, Structure and Sorption Study Arijit Halder, Biswajit Bhattacharya, Rajdip Dey, Dilip Kumar Maity and Debajyoti Ghoshal* Department of Chemistry, Jadavpur University, Jadavpur, Kolkata, 700 032, India E-mail: [email protected]

______________________________________________________________ ABSTRACT Three new dynamic MOFs; {[Cd2(3,4-pyrdc)2(4,4′-bipy)(H2O)2].(H2O)4}n (1), {[Mn2(3,4pyrdc)2(bpee)(H2O)2].(H2O)}n (2) and {[Cu2(3,4-pyrdc)2(bpp)2(H2O)4].(H2O)5}n (3) based on 3,4-pyridinedicarboxylate (3,4-pyrdc) and three different N,N′-donor ligands [4,4′-bipy = 4,4′bipyridine, bpee = 1,2-bis(4-pyridyl)ethylene, bpp = bis-pyridylpropane] with various divalent transition metal ions, have been synthesized and characterized by single crystal and powder Xray diffractions and other physicochemical methods. In case of compounds 1 and 2, 3,4-pyrdc forms two-dimensional (2D) metal-carboxylates sheets that are connected by N,N′-donor ligands forming three-dimensional (3D) structures with water filled channels. For compound 3; 3,4pyrdc ligand affords a one-dimensional (1D) metal-carboxylates chains and these chains are connected by more flexible bpp ligand forming two-dimensional (2D) structures and extended to 3D supramolecular architecture by H-bonding. Compound 1 and 2 show reversible crystalline-tocrystalline phase transformation upon dehydration and rehydration whereas compound 3 exhibits interesting reversible crystalline-to-amorphous transformation. These transformations have been established and monitored by exhaustive X-ray powder diffraction study, elemental analysis, IR spectra, thermogravimetric analysis and morphology study. Dehydrated forms of 1-3 selectively adsorb CO2 over N2 and also exhibit stepwise water uptake. ______________________________________________________________________________ INTRODUCTION Materials with impressive flexibility can undergo chemical and physical changes in presence of several external stimuli like heat, light, pressure, etc; have recently taken the limelight of interest, due to their potential applications in the design of different molecular devices, such as sensors, data storage devices and switches.1-5 In this context, such flexibility in the structure of a 1 ACS Paragon Plus Environment


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molecule has been emerged as a new contrivance for the development of next generation functional materials.6-8 Thus in a natural way, the dynamic porous coordination polymers (PCPs) or metal–organic frameworks (MOFs) have also attracted significant attention in terms of functionality, as their structural flexibility allow them to alter their host structures by the aforesaid external stimuli.9-12 Because of this dynamic behavior, flexible MOFs are found to exhibit unique selective, stepwise adsorption which might be useful for the separation of gases and solvent vapors along with sensing of small molecules.13-17 It has been already established that the flexible or dynamic MOF based materials can be constructed by judicious choice of metal nodes and comparatively non-rigid organic linkers, or by utilizing the interplay of weak non-covalent forces (hydrogen bonds, π–π stacking and/or other supramolecular interactions).1819

It is most often that the structural transformations in such MOFs are associated with the

removal or exchange of guest molecules, changes in coordination number of metal atoms, bond rearrangement and conformational changes in flexible organic linkers.20-22 Recently, reversible single-crystal-to-single-crystal transformation or crystalline-to-microcrystalline transformation by various external stimuli has been reported in some MOFs.19 There are also some rare instances where it has been observed that although, the long-range crystallinity of MOFs may breaks down upon desolvation leading to the formation of amorphous phase and the original crystallinity can be restored upon desolvation, i.e., reversible crystalline-to-amorphous transformation.23-25 However, in implicational point of view, the crystalline-to-amorphous transformation is much more appealing because it might be associated with the radical changes in crystal structure dependent physical properties like colour; second-order nonlinear optical (NLO), magnetic and ferroelectric properties.26-29 However, the exact reason for such structural flexibility and specific structural information accounting the phase transition are still exist as a gray area of science and also the specific functional applications of these frameworks are yet to be explored. Recent studies have revealed that mixed ligand strategy30-34 for designing MOFs are found very effective for the incorporation of flexibility within the framework structure.35-38 Hence utilizing the mixed-ligand approach, here we have synthesized three new dynamic MOFs {[Cd2(3,4-pyrdc)2(4,4′-bipy)(H2O)2].(H2O)4}n (1), {[Mn2(3,4-pyrdc)2(bpee)(H2O)2]. (H2O)}n (2) and {[Cu2(3,4-pyrdc)2(bpp)2(H2O)4].(H2O)5}n (3) using 3,4-pyridinedicarboxylate (3,4-pyrdc) with different metal ions and three different N,N′-donor ligands; [4,4′-bipy = 4,4′-bipyridine, 2 ACS Paragon Plus Environment

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bpee = 1,2-bis(4-pyridyl)ethylene, bpp = bis-pyridylpropane] . The compounds are characterized by single crystal and powder X-ray diffractions as well as other physicochemical methods. Compound 1 and 2 exhibits reversible crystalline-to-crystalline phase transformation upon dehydration and rehydration, whereas compound 3 shows a rare reversible crystal-to-amorphous phase transformation accompanying a visible color change with respect to removal and addition of guest molecules. (Scheme 1) All three compounds after the phase transformation, exhibit nice selective adsorption of CO2 gas over N2 and step wise water adsorption indicating the flexible nature of frameworks.

Scheme 1. Reversible phase transformation in 1−3 EXPERIMENTAL SECTION Materials. Highly pure Cd(NO3)2·4H2O, MnCl2·4H2O, Cu(NO3)2·3H2O, 4,4′-bipyridine, 1,2bis(4-pyridyl)ethylene (bpee), bis-pyridylpropane (bpp) and 3,4-pyridinedicarboxylic acid were purchased from the Sigma-Aldrich Chemical Co. All other reagents and solvents were purchased from commercial sources and were used without further purification. Physical Measurements. Elemental analyses (carbon, hydrogen, and nitrogen) were performed using a Heraeus CHNS analyzer. Infrared spectra (4000–400 cm−1) were taken on KBr pellet, 3 ACS Paragon Plus Environment


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using Perkin–Elmer Spectrum BX-II IR spectrometer. Thermal analysis (TGA) was carried out on a METTLER TOLEDO TGA 850 thermal analyzer under nitrogen atmosphere (flow rate: 30 cm3 min-1) at the temperature range 30-600º C with a heating rate of 2º C/min. X-ray powder diffraction (PXRD) patterns in different states of the sample were recorded on a Bruker D8 Discover instrument using Cu-Kα radiation. Morphology of the compounds in thin film was studied by a field emission scanning electron microscope (FESEM) FEI Inspect F50. Synthesis of {[Cd2(3,4-pyrdc)2(4,4′-bipy)(H2O)2].(H2O)4}n (1). 3,4-Pyridinedicarboxylic acid (H2-3,4-pyrdc) (1 mmol, 168 mg) was deprotonated in aqueous solution (20 ml) by NaOH (2 mmol, 80 mg) which was mixed slowly with methanolic solution (20 ml) of 4,4′-bpy (1 mmol, 156 mg) and stirred for 30 min to mix well. Cd(NO3)2·4H2O (1 mmol, 0.308 g) was dissolved in 20 mL water and 4 mL of this Cd(II) solution was slowly and carefully layered with the 8 ml of aforesaid mixed-ligand solution, using 4 mL buffer (1:1 of water and MeOH) mixture. Colorless block shaped single crystals suitable for X-ray diffraction analysis was obtained at the wall of the tube after one week. The crystals were separated and washed with MeOH and dried under air (yield: 82%). Anal. Calc. for C24H26Cd2N4O14 (1, %): C, 35.18; H, 3.20; N, 6.83. Found: C, 34.79; H, 3.31; N, 6.52. IR spectra (KBr pellet, 4000-450 cm1

): 3406(br), 1608(s), 1582 (s), 1490(m), 1404(s), 1380(s), 1223(m), 1067(m), 1008(m), 806(m),

684(m), 631(m), 451(m). Synthesis of {[Mn2(3,4-pyrdc)2(bpee)(H2O)2].(H2O)}n (2). The same diffusion technique as that of 1 was employed for the synthesis of compound 2 using the MnCl2·4H2O (1 mmol, 0.198g), and bpee (1 mmol, 0.182g) in place of Cd(NO3)2·4H2O and 4,4′-bpy respectively. Pale yellow crystals were found after two weeks. The crystals were separated and washed with MeOH and dried under air (yield: 69%). Anal.calc for C26H22Mn2N4O11 (2, %): C, 46.17; H, 3.28; N, 8.28. Found: C, 46.25; H, 3.11; N, 8.14. IR spectra (KBr pellet, 4000-450 cm-1): 3280(br), 1637(s), 1610 (s), 1573(s), 1412(s), 1061(m), 1012(m), 987(m), 834(s), 709(m), 677(s), 551(s), 435(m). Synthesis of {[Cu2(3,4-pyrdc)2(bpp)2(H2O)4].(H2O)5}n (3). Similar diffusion technique as that of 1 was adopted for the synthesis of compound 3 using the Cu(NO3)2·2.5H2O (1 mmol, 0.241g), and bpp (1 mmol, 0.198g) in place of Cd(NO3)2·4H2O and 4,4′-bpy respectively. Blue-colored crystals were appeared after twenty days. The crystals were separated and washed with MeOH and air dried (yield: 58%). Anal.calc for C40H52Cu2N6O17 (3, 4 ACS Paragon Plus Environment

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%): C, 47.29; H, 5.16; N, 8.27. Found: C, 47.14; H, 5.28; N, 8.15. IR spectra (KBr pellet, 4000450 cm-1): 3385(br), 1621(s), 1588 (s), 1461(m), 1397(s), 1370(s), 1123(m), 1070(s), 1032(m), 823(m), 718(m), 684(m), 615(s), 518(m), 465(m). The bulk compounds of 1-3 have been synthesized in powder form by the direct mixing of the corresponding ligands and metal salt in water-methanol mixture, at their equal molar ratio. Compounds purity was verified by PXRD, which give good correspondence between simulated and bulk-phase PXRD patterns indicating high purity of the bulk compounds. The purity of the bulk sample was further confirmed by the results of elemental analysis and IR spectra as well, which also found in accordance with the data obtained for the single crystals. To obtain the desolvated frameworks of 1′-3′, compounds 1-3 were heated to their corresponding transformation temperature, obtained from VTPXRD; under reduced pressure for 4h. We were unable to obtain the single crystals of the transformed products (1′-3′) even we started with the single crystals of 1-3 due to the loss of single crystallinity upon dehydration. Single-Crystal Structure Analysis. The single crystals of 1-3 were mounted on thin glass fibers with commercially available glue. X-ray single crystal data collection of all three crystals were collected at room temperature using Bruker APEX II diffractometer, equipped with a normal focus, sealed tube X-ray source with graphite monochromated Mo-Kα radiation (λ= 0.71073Å). The data were integrated using SAINT39 program and the absorption corrections were made with SADABS.40 All three structures were solved by SHELXS 9741 using Patterson method and followed by successive Fourier and difference Fourier synthesis. Full matrix least-squares refinements were performed on F2 using SHELXL-9741 with anisotropic displacement parameters for all non-hydrogen atoms. During refinement of 3, three lattice water molecules (O3W, O4W and O5W) were found disordered, and their occupancies were fixed at 0.5 before final refinement. In 1 (O5W and O6W) and 2 (O3W, O4W and O5W), the oxygen atoms of lattice water molecules were isotropically refined without fixing hydrogen atoms. Potential solvent accessible area or void space was calculated using PLATON42 multipurpose crystallographic software. All the hydrogen atoms were fixed geometrically by HFIX command and placed in ideal positions in case of all three structures. Calculations were carried out using SHELXL 9741, SHELXS 9741, PLATON v1.15,42 ORTEP-3v2,43 WinGX system Ver-1.8044 and TOPOS.45,46 Data collection and structure refinement parameters along with crystallographic 5 ACS Paragon Plus Environment


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data for 1-3 are given in Table 1. The selected bond lengths, bond angles are given in supporting information as Table S1-S3. Sorption Measurements Low pressure volumetric N2 gas adsorption study was carried out at 77 K with the desolvated samples of 1 (1′), 2 (2′) and 3 (3′), maintained by a liquid-nitrogen bath, with pressures ranging from 0 to 1 bar using a Quantachrome Autosorb-iQ adsorption instrument. CO2 adsorption measurements were also performed at 195 K (dry ice-acetone cold bath) with same pressure range and on same instrument. High purity gases were used for the adsorption measurements (nitrogen, 99.999%; carbon dioxide, 99.95%). The adsorbates were placed into the sample tubes, and then the change of the pressure was monitored and the degree of adsorption was determined with the change in pressure at the equilibrium state. All operations were computer-controlled and automatic. The water adsorptions studies of 1′, 2′ and 3′ at 298 K were performed in the vapor state by using same instrument. RESULTS AND DISCUSSION Synthesis Compounds 1−3 were synthesized by slow diffusion technique in methanol/water medium using 3,4-pyrdc dianion along with three different N,N′-donor linkers such as 4,4′-bpy (1), bpee (2), and bpp (3) respectively and the corresponding metal(II) solution [Cd(II), Mn(II) and Cu(II)] at room temperature (Scheme 2). Structure determination of compounds 1−3 reveals that they exhibit different structural topologies due to the different bridging modes of 3,4-pyrdc ligands, which might have affected the final structural arrangements as well as sorption properties. In each synthesis, the addition of NaOH was used as base to neutralize the 3,4-pyridinedicarboxylic acid. The 3,4-pyrdc ligands were completely deprotonated upon coordination to metal centers which is evidenced by IR spectra with the absence of the expected absorption band in the range of 1760-1680 cm-1 for the protonated carboxylate groups (Figures S1-S3). The results are well consistent with the single crystal structure as well. The IR spectrum of all compounds 1−3 show a broad peak around 3600-3350 cm-1 for O-H stretching frequency corresponding to the presence of free or coordinated water molecules (Figures S1-S3). The powder XRD patterns of 1−3 are in very good correspondence with the simulated patterns of the single crystal, indicating the phase

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purity of bulk samples. The purity of the samples was further confirmed by IR spectra and elemental analysis.

Scheme 2. Synthetic Scheme of Compounds 1−3 Structural Description of {[Cd2(3,4-pyrdc)2(4,4′-bipy)(H2O)2].(H2O)4}n (1). Single-crystal X-ray analysis reveals that compound 1 is a 3D porous framework of Cd(II) bridged by 3,4-pyrdc and 4,4′-bipy linkers. It crystallizes in the monoclinic space group P21/c. The asymmetric unit of 1 comprises two crystallographically independent Cd(II) metal centers (hepta-coordinated Cd1 and hexa-coordinated Cd2), two 3,4-pyrdc ligand, one 4,4′-bipy ligand and two coordinated water along with four guest water molecules. Each hepta-coordinated Cd1 with CdO5N2 environment is surrounded by two pairs of oxygen atoms (O1, O2, O7b and O8b) coming from two 3,4-pyrdc ligands, one water molecules (O1W), two nitrogen atoms (N2 and N3) from one 4,4′-bipy and one 3,4-pyrdc ligand, respectively, composing a distorted pentagonal bipyramidal geometry (Figure 1a). Cd1–O and Cd1–N distances around Cd1 are in the range of 2.297(3)−2.472(3) Å and 2.318(3)−2.400(3) Å, respectively (Table S1). However, each hexacoordinated Cd2 with CdO4N2 environment is coordinated to three carboxylate oxygen atoms (O4, O5a and O6a) from two different 3,4-pyrdc ligands, one pyridyl nitrogen atom (N1b) of another 3,4-pyrdc ligand and a coordinated water molecule (O2W). The sixth coordination site is 7 ACS Paragon Plus Environment


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occupied by one N atom (N4) from the 4,4′-bipy linkers (Figure 1a). Here the Cd-O and Cd-N bond lengths around Cd2 are in the range of 2.225(4)−2.436(3) Å and 2.305(4)−2.356(3) Å, respectively (Table S1). The bond angles around Cd1 and Cd2 are in the range of 53.69(10)−168.07(12)° and 53.38(12)−167.68(12)°, respectively (Table S1). Here, one 3,4-pyrdc ligand bridged three different Cd(II) centers through two chelating carboxylate groups and pyridyl nitrogen atom whereas the other 3,4-pyrdc ligand also connects three metal centers via chelated-monodentate carboxylate group and pyridyl nitrogen (Figure S4). Thus, two geometrically different Cd(II) centers connect with each other via two different 3,4-pyrdc ligands to form an wave-like 2D arrangement in the crystallographic ac plane (Figure 1b). The 4,4′-bipy linkers connect the 2D metal-carboxylate sheets in a criss-cross and canted fashion to create a 3D arrangement (Figure 1c) with two types (hexagonal and rectangular) of 1D channels along the crystallographic a axis which are occupied by guest water molecules. The dimensions of the hexagonal and rectangular channels are about 5.3×3.5 Å2 and 4.7×2.0 Å2, respectively, which provides a void space of ∼29.3% to the total crystal volume.42 The 3D structure is also stabilized by intermolecular π-π interaction (centroid-cetroid distances are in the range of 4.112−4.434 Å) The network analysis by TOPOS45,46 showed that the overall structure consists of (3,4)connected tri-nodal nets with the point symbol (Schlӓfli symbol) {63}{65.8} (Figure 1d). Structural Description of {[Mn2(3,4-pyrdc)2(bpee)(H2O)2].(H2O)}n (2). Single-crystal X-ray analysis reveals that compound 2 is a 3D framework of Mn(II) bridged by 3,4-pyrdc and bpee linkers. Compound 2 crystallizes in monoclinic C2/c space group. The asymmetric unit of 3 contains one Mn(II) ion, one 3,4-pyrdc, half bpee ligand along with one coordinated water and half guest water molecule. In 2, each Mn(II) center is hexa-coordinated with MnO4N2 environment coming from three carboxylate oxygen atoms (O2a, O3b and O4c) of three different 3,4-pyrdc ligands, one coordinated water (O1W), one nitrogen atom (N1) of 3,4pyrdc ligands and one nitrogen atom (N2) from bpee ligand forming a distorted octahedral geometry (Figure 2a). The Mn−O bond lengths are in the range of 2.1057(15)− 2.2147(17) Å, whereas the Mn−N bond lengths are in the range of 2.2913(17)−2.2928(15) Å. The bond angles around Mn1 center are in the range of 80.54(6)−178.87(6)° (Table S2). Here, the coordination mode of 3,4-pyrdc are different from the previous structure (Figure S4). Each 3,4-pyrdc connects four different Mn(II) centers via its pyridyl nitrogen atom as well as monodentate and bidentate


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carboxylate groups; forming a 2D metal-carboxylate layer (Figure 2b). Here, bpee ligand connects the adjacent metal carboxylate layers, extending the structural motif into three dimension which has been further supported by intermolecular π-π interaction (centroid-cetroid distances are in the range of 3.5072-4.2657 Å, Table S3). The formation of 3D structure is associated with the construction of small cavities along the crystallographic c axis that are occupied by lattice water molecules (Figure 2c). The effective solvent accessible void space, calculated by PLATON42, found 10.6% void space relative to the total crystal volume. The network analysis by TOPOS45,46 showed (Figure 2d) that the overall structure consists of (4,5)connected bi-nodal nets with the point symbol (Schlӓfli symbol) {44.62.104}{44.62}.45,46 The coordinated (O1W) and lattice water molecules (O2W) are in short contact with the free carboxylate oxygen atom (O1) of the 3,4-pyrdc ligand through hydrogen bonding and also both the water molecules are hydrogen bonded with each other (Table S4). Structural Description of {[Cu2(3,4-pyrdc)2(bpp)2(H2O)4].(H2O)5}n (3). Compound 3 crystallizes in the orthorhombic crystal system with Pna21 space group. X-ray structural determination reveals a 2D coordination framework made up of Cu(II), 3,4-pyrdc, and bpp linkers. The asymmetric unit of 3 contains one Cu(II) ion, one 3,4-pyrdc ligand, one bpp linker, two coordinated water molecules along with two and half lattice water molecules. Each Cu(II) center is hexa-coordinated with CuO3N3 environment with two nitrogen atoms (N1 and N2a) from two different bpp ligands, one nitrogen atom (N3b) from one 3,4-pyrdc ligand, one oxygen atom (O1) from another 3,4-pyrdc ligand and two coordinated water molecules (O1W and O2W) to exhibit a distorted octahedral geometry (Figure 3a). The Cu−O bond lengths are in the range of 1.973(5)−2.746(7) Å, whereas the Cu−N bond lengths are in the range of 2.017(5)−2.018(3) Å. The bond angles around Cu1 center are in the range of 84.68(14) −175.9(14)° (Table S5). The equatorial Cu–O and Cu–N bond distances are comparable to similar reported Cu(II) structures,47,48 while the axial Cu–N or Cu–O bond distances are significantly longer which is commensurate with the Jahn–Teller sensitive Cu(II) ion. Here, the 3,4-pyrdc ligand acts as linear ligand and binds Cu(II) centers only via carbxylate group of 4position and pyridyl nitrogen atom, the carboxylate group of 3-position is free (Figure S4). Each 3,4-pyrdc bridges two Cu(II) centers in monodentate fashion to form a 1D metal-carboxylate chain. These 1D chains are further connected by bpp linkers, leading to the formation of a 2D



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network along the crystallographic b axis (Figure 3b). The 2D framework of 3 can be viewed in a simplified way using TOPOS45,46 software, as a 4-connected uninodal net with the point symbol (Schlӓfli symbol) {44.62} (Figure 3c). In the crystal packing, the coordinated water molecules (O1W and O2W) are held together with the uncoordinated carboxylate oxygen atoms (O2 and O4) of the 3,4-pyrdc ligand and lattice water molecule (O3W) by means of H-bonding forming a supramolecular three-dimensional (3D) structure in the crystallographic c axis (Figure 3d, Table S4). These types of H-bonding probably made carboxylate oxygen atoms unavailable for coordination to Cu1 prompting the monodentate bridging mode of the 3,4-pyrdc. The dimension of the channel is about 6.8×2.6 Å2 and upon removal of the lattice water molecules the framework contains about 25.35% void space to the total crystal volume as suggested by the PLATON42 crystallographic software. Structural Transformation: Thermogravimetric (TG), Powder X-ray Diffraction (PXRD) and Morphology Analysis. To check the thermal stability of the compounds reported in this paper, TGA analyses of 1-3 were carried out. TGA profile of 1 (Figure S6) indicates a gradual weight loss of 13.5% below 120 °C, which corresponds to the releases of four guest and two coordinated water molecules (cal. 13.18%) in a single step. There was no further weight loss observed for this compound up to 240 °C and after that the framework gradually decomposed. For compound 2, a weight loss of ∼8.3% was observed up to 120 °C, which can be assigned to the loss of one coordinated and half guest water molecules (cal. 7.98%) per formula. After this, it remains without any weight loss up to 270 °C followed by gradual decomposition (Figure S7). The TGA curve for compound 3 showed a gradual weight-loss step of 16.3% (30–65 °C), corresponding to escape of all coordinated and guest water molecules (cal. 15.95%) and after that, it gradually decomposes into unidentified products (Figure S8).

The bulk-phase purity of three compounds (1-3) was confirmed by comparison of the experimentally observed powder X-ray diffraction (PXRD) patterns with the patterns simulated from the single-crystal structures. The PXRD peaks of the as-synthesized form, matched well with the simulated pattern (Figures 4-6) in all three cases. Considering the thermal stability of aforesaid compounds, an in situ variable temperature powder X-ray diffraction (VTPXRD) studies were performed. Samples were taken in the high temperature cell of the instrument and 10 ACS Paragon Plus Environment

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heated gradually with a rate of 1 °C per second. At the required temperature data was collected after a delay time of 2 minute. Variable-temperature PXRD of all three compounds show considerable change in the powder pattern at high temperature (Figures S9-S11) and this could be caused by phase transformation upon solvent removal. In case of 1, the diffraction patterns below 70 °C remained almost unchanged, indicating the retention of the crystal lattice. After 70 °C, the decrease in intensity and slight shifts of some peaks of the PXRD patterns involve that the framework was partially amorphous after the loss of lattice as well as coordinated solvent molecules (Figure 4 and S9). The VTPXRD patterns (Figure S10) of 2 reveal that the framework also exhibited crystalline phase transformation, started from 40 °C where two phases simultaneously present and at 50 °C it is exclusively transformed to desolvated phase. It is also evident from FESEM images (Figures S11, S12) that 1 and 2 shows a crystalline-to-crystalline transformation with similar kind of morphology with different particle size for mother compound and dehydrated compound. VTPXRD patterns of 3 shows crystal-to-amorphous transformation2325

upon removal guest molecules also associated with a distinct color change (Figure S13). When

heated, 3 started to transform to an amorphous phase after 50°C, understandable by the decrease in intensity and slight shifts of some peaks in the PXRD pattern and finally at 110 °C it completely transformed to the desolvated phase. The TGA curves of 1′-3′ show single step weight loss at high temperature, indicating the complete absence of coordinated solvent molecules (Figures S6-S8). When the dehydrated phases of all three compounds were exposed to water vapor environment for 7 days, they returned to their original crystalline phases, indicating the reversible nature of the structural transformations even in case of 3′ which is basically an unusual amorphous-to-crystal transformation. Nevertheless, we also exposed the dehydrated phases (1′-3′) of above mention three compounds to different polar coordinating solvents (MeOH, EtOH and THF) but surprisingly, could not observed any structural change in 1′-3′ upon exposure to MeOH, EtOH and THF (Figure S14-S16). The above results reveal that the reversible structural change observed in these compounds is highly selective for water. IR and UV Spectra In order to get the possible insight of the structural changes that happened after the removal of solvent molecules from framework, we also performed IR spectra of as-synthesized, dehydrated, 11 ACS Paragon Plus Environment


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and rehydrated forms of compounds 1-3 (Figures S1-S3). In case of assynthesized forms of compounds 1-3 show broad band at 3406 cm-1 (for 1), 3280 cm-1 (for 2) and 3385 cm-1 (for 3); respectively, corresponding to water molecules, and these band almost completely disappeared in dehydrated forms of all these compounds (Figures S1-S3). In case of the dehydrated forms of all three compounds, the corresponding carboxylate C=O (1640-1573 cm-1) and C−O (1412-1573 cm-1) band become broad, representing change in the coordination environment around metal center (Figure 7 and Figures S1-S3). Upon exposure of water vapor for seven days these broad bands in the dehydrated phase of compounds 1-3 regenerate the original bands present in the parent compound (Figure 7 and Figures S1-S3). The UV-vis absorption spectra of assynthesized, dehydrated, and rehydrated forms of compounds 1-3 were measured in solid state at room temperature (Figures S15-S16 and Figure 8). The absorption spectra of compounds 1 and 2 were very similar in their different forms (Figures S17, S18), but compound 3 shows a reversible visible color change corresponding to hydrated and dehydrated phases (Figure 8). The assynthesized blue compound (3) has an absorbance maximum at 623 nm, which changes its color to pale blue upon dehydration (3′) shifting the absorbance maxima at 700 nm. The color of the de-solvated sample changes again from pale blue to original blue with an absorbance maximum at 623 nm when the de-solvated crystals were exposed to water vapor (Figure 8). Adsorption Properties Encouraged by the single crystal X-ray structure and structural flexibility of 1-3, we have determined the gas uptake capacities of the desolvated (activated) frameworks. Before the measurement, powder compounds of 1-3 evacuated at 120 °C for 4 h to obtain their activated forms 1′-3′. The N2 adsorption isotherm of the activated compounds 1′-3′ at 77 K, show type-II adsorption profile with very low uptake, suggesting only surface adsorption (Figure 9). Surprisingly, even though these compounds are non porous to N2, we found that these are porous to CO2 at 195 K, for which these compounds reveal reversible type-I isotherms. Adsorption isotherms of compounds 1′-3′ with CO2 at 195 K show 39 cm3 g-1 (7.7 wt%), 19 cm3 g-1 (3.7 wt%) and 47 cm3 g-1 (9.2 wt%) of uptake respectively (Figure 9). It is interesting to note that the desorption curve does not coincide with adsorption curve, instead showing a large hysteretic sorption profile. The presence of the huge hysteresis in adsorption-desorption isotherms indicate that the quadruple moment of CO2 molecules interact strongly with the framework. Stronger


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interaction can be attributed to the presence of unsaturated M(II) sites within the host network and free oxygen atoms of carboxylate groups in desolvated states on the pore walls which can effectively interact with the CO2 molecules as the carbon atom in CO2 is electron deficient (Lewis acid) in nature. This interaction provides extra energy of adsorption and attributed to the higher uptake of CO2 in the said compounds (Figure 9). In comparison to compounds 1′ and 3′, less CO2 uptake amount in the case of 2′ is observed which is possibly due to the low solvent accessible voids. To investigate the selectivity of CO2 of the three compounds 1′, 2′ and 3′ over N2, sorption measurements of CO2 and N2 have also been performed at ambient temperature, 298 K up to 1 partial pressure (Figures S19). For selectivity measurements, the initial slopes of the single component adsorption isotherms were calculated individually, and the ratios of the initial slopes of respective isotherms were used to estimate the adsorption selectivity of CO2 over N2 for the three compounds.49 Based on that, the CO2/N2 selectivity for 1′ is 8.8 while for 2′, this is 3.7 and for 3′, it is 9.3 (Figures S20-S22). To understand the interaction of water molecules with flexible frameworks we also performed water vapor adsorption measurements of activated samples of 1′-3′ at ambient conditions (Figure 10). All three compounds exhibit interesting water sorption profiles. Compound 1′ shows very high H2O uptake (~37 cm3 g-1) at lower P/P0 values (0.0−0.05) and the final uptake amount reaches 177.5 mL g−1 which corresponds to 5.6 molecules of H2O per formula (Figure 10). For 2′, the H2O adsorption profile exhibits a nice gate opening type profile;50,51 almost no uptake occurs at low pressure (P/P0 ∼ 0.1) but after this, a steep rise leads to final uptake of 121.5 cm3 g−1 (~3.4 molecules of H2O per formula) (Figure 10). In case of 3′, up to a relative pressure of P/P0 ∼0.2, there is almost no uptake and after that there is a gate opening which reaches to an amount of 243 cm3 g−1 (~7.2 molecules of H2O per formula) (Figure 10). The water isotherm of 3′ displays highest water uptake among the all three compounds at P/P0 ∼0.9 whereas at low pressure region (P/P0 ∼0-0.5), the water uptakes of 3′ is lower than 1′ and 2′. Low water uptake of 3′at low pressure region illustrates the amorphous nature of the framework along with orientation of hydrophobic -CH2 moieties of the bpp ligand towards the pore surface. The sharp uptake after that in higher partial pressure indicates the probable diffusion of H2O molecules in the pore surfaces. Moreover, the strong hysteresis and retention of water molecules even at the very low pressure at the end of desorption, signifies a strong interaction of water molecules with 13 ACS Paragon Plus Environment


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the unsaturated metal sites on the pore surface which may also additionally responsible for the large water uptake by 3′ at higher pressure. CONCLUSION In summary, by using 3,4-pyrdc and three different N,N′-donor ligands with various divalent transition metal ions, we have constructed three dynamic metal organic frameworks. Compounds 1-2 exhibit reversible crystalline-to-crystalline phase transformations upon dehydration and rehydration whereas compound 3 shows a guest water responsive reversible crystal-to-amorphous phase transformations accompanying with a visible color change. The reversible structural transformation of the compounds was monitored and confirmed by IR spectroscopy, FESEM, TGA and PXRD data. It is also worth to mention that the dehydrated forms of all these frameworks selectively adsorb CO2 over N2 and thus the adsorption study of 3′ reveals the formation of a rare amorphous MOF. Moreover, water adsorption profiles show stepwise adsorptions in all three cases which are attributed to their structural transformation based on very strong coordination affinity of water molecules with the unsaturated metal ions and H-bonding affinity of water molecules with the free O atom of the carboxylate group in the framework. Therefore, it may be concluded that the close monitoring of the reversible structural transformation in all three cases will enlighten us for the understanding of crystal-to-crystal or crystal-to-amorphous transformation which is essentially useful for the design of such frameworks having the ability of inter-conversion in presence of external stimuli. There is no doubt that such smart materials will serve the purpose of sensing and detection in future along with several interesting application. ASSOCIATED CONTENT Supporting Information The figures related to IR, UV-Vis spectral study; PXRD patterns, FESEM images and TGA of compounds along with different structural figures and tables related to the crystal structures reported in this paper are available as SI. Accession Codes CCDC 1474475−1474477 contains the supplementary crystallographic data for this paper in CIF format. These data can be obtained free of charge via www.ccdc.cam.ac.uk/data_request/cif, or


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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 Corresponding Author Debajyoti Ghoshal, e–mail: [email protected], FAX: +9133 2414 6223 Note: The authors declare no competing financial interest. ACKNOWLEDGMENTS Authors gratefully acknowledge the financial assistance given by SERB (No. SB/S1/IC-06/2014) and DRDO, Govt. of India (Grant No. ERIP/ER/1103938/M/01/1501). DKM acknowledges UGC for the research fellowship. Dr. P. P. Ray of Department of Physics, Jadavpur University is gratefully acknowledged for the FESEM study. REFERENCES (1) Irie, M.; Fukaminato, T.; Matsuda, K.; Kobatake, S. Chem. Rev. 2014, 114, 12174−12277. (2) Kundu, P. K.; Olsen, G. L.; Kiss, V.; Klajn, R. Nat. Commun. 2014, 5, 3588. (3) Mura, S.; Nicolas, J.; Couvreur, P. Nat. Mater. 2013, 12, 991–1003. (4) Narasimha, K.; Jayakannan, M. ACS Appl. Mater. Interfaces 2014, 6, 19385−19396. (5) Huang, N.; Ding, X.; Kim, J.; Ihee, H.; Jiang, D. Angew. Chem. Int. Ed. 2015, 54, 8704-8708. (6) Kobatake, S.; Takami, S.; Muto, H.; Ishikawa, T.; Irie, M. Nature 2007, 446, 778−781. (7) Yan, X.; Wang, F.; Zheng, B.; Huang, F. Chem. Soc. Rev. 2012, 41, 6042–6065. (8) Yagai, S.; Okamura, S.; Nakano, Y.; Yamauchi, M.; Kishikawa, K.; Karatsu, T.; Kitamura, A.; Ueno, A.; Kuzuhara, D.; Yamada, H.; Seki, T.; Ito, H. Nat. Commun. 2014, 5, 4013. (9) Coudert, F.-X. Chem. Mater. 2015, 27, 1905−1916; (10) Chang, Z.; Yang, D.-H.; Xu, J.; Hu, T.-L.; Bu, X.-H. Adv. Mater. 2015, 27, 5432–5441. (11) Sakata, Y.; Furukawa, S.; Kondo, M.; Hirai, K.; Horike, N.; Takashima, Y.; Uehara, H.; Louvain, N.; Meilikhov, M.; Tsuruoka, T.; Isoda, S.; Kosaka, W.; Sakata, O.; Kitagawa, S. Science 2013, 339, 193−196. (12) Bhattacharya, B.; Halder, A.; Maity, D. K.; Ghoshal, D. CrystEngComm 2016, 10.1039/c5ce01952d. (13) Zhang, J.; Wu, H.; Emge, T. J.; Li, J. Chem. Commun. 2010, 46, 9152–9154. 15 ACS Paragon Plus Environment


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(32) Banerjee, K.; Roy, S.; Biradha, K. Cryst. Growth Des. 2014, 14, 5164−5170. (33) Mukherjee, S.; Mukherjee, P. S. Cryst. Growth Des. 2014, 14, 4177−4186. (34) Manna, P.; Tripuramallu, B. K.; Das, S. K. Cryst. Growth Des. 2014, 14, 278−289. (35) Sanda, S.; Biswas, S.; Konar, S. Inorg. Chem. 2015, 54, 1218−1222. (36) Lyndon, R.; Konstas, K.; Ladewig, B. P.; Southon, P. D.; Kepert, C. J.; Hill, M. R. Angew. Chem. Int. Ed. 2013, 52, 3695 –3698. (37) Li, L.; Wang, Y.; Yang, J.; Wang, X.; Li, J. J. Mater. Chem. A, 2015, 3, 22574–22583. (38) Sikdar, N.; Bonakala, S.; Haldar, R.; Balasubramanian, S.; Maji, T. K. Chem. Eur. J. 2016, 22, 6059 – 6070. (39) SMART (V 5.628), SAINT (V 6.45a), XPREP, SHELXTL, Bruker AXS Inc., Madison, WI, 2004. (40) Sheldrick, G. M. SADABS (Version 2.03), University of Göttingen, Germany, 2002. (41) Sheldrick, G. M. SHELXS-97, Acta.Crystallogr. 2008, A64, 112−122. (42) Spek, A. L. Acta.Crystallogr. 2009, D65, 148−155. (43) Farrugia, L. J. J. Appl. Crystallogr. 1997, 30, 565−566. (44) Farrugia, L. J. WinGX, J. Appl. Crystallogr. 1999, 32, 837−838. (45) Blatov, V. A.; Shevchenko, A. P.; Serezhkin, V. N. J. Appl. Crystallogr. 2000, 33, 1193. (46) Blatov, V. A.; Carlucci, L.; Ciani, G.; Proserpio, D.M. CrystEngComm 2004, 6, 377−395. (47) Jia, Y.-Y.; Liu, B.; Liu, X.-M.; Yang, J.-H. CrystEngComm 2013, 15, 7936−7942. (48) Wei, M.-L.; Sun, J.-J.; Duan, X.-Y. Eur. J. Inorg. Chem. 2014, 2, 345–351. (49) Maity, D. K.; Halder, A.; Bhattacharya, B.; Das, A.; Ghoshal D. Cryst. Growth Des. 2016, 16 , 1162–1167. (50) Fukushima, T.; Horike, S.; Inubushi, Y.; Nakagawa, K.; Kubota, Y.; Takata, M.; Kitagawa, S. Angew. Chem. Int. Ed. 2010, 49, 4820−4824. (51) Yamada, T.; Shirai, Y.; Kitagawa, H. Chem. Asian J. 2014, 9, 1316–1320.



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Tables Table 1. Crystallographic and Structural Refinement Parameters for 1−3 formula formula weight crystal system space group a/ Å b/Å c/ Å α/° β/° γ/° V/ Å3 Z Dc/ g cm–3 µ /mm–1 F(000) θ range/° reflections collected unique reflections reflections I > 2σ(I) Rint goodness-of-fit (F2) R1 (I > 2σ(I)) [a] wR2(I > 2σ(I)) [a] a

1 C24H26Cd2N4O14 819.30 monoclinic P21/c 7.6297(2) 26.8079(5) 15.0922(3) 90 90.838(1) 90 3086.57(12) 4 1.746 1.449 1592 1.5-27.5 51580 7087 6276 0.032 1.08 0.0352 0.1110

2 C26H22Mn2N4O11 676.36 monoclinic C2/c 22.7591(13) 11.1312(5) 11.6810(5) 90 111.652(4) 90 2750.4(2) 4 1.633 0.987 1376 1.9-27.6 22503 3176 2931 0.025 1.02 0.0309 0.0834

3 C40H52Cu2N6O17 1015.98 orthorhombic Pna21 23.9719(6) 11.9127(3) 8.9852(2) 90 90 90 2565.90(11) 2 1.307 0.898 1044 1.7-27.6 40562 5904 5013 0.044 1.11 0.0588 0.1956

R1 = ΣFo–Fc/ΣFo, wR2 = [Σ (w (Fo 2 – Fc2 ) 2 )/ Σw (Fo 2 )2] ½.


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Figures

Figure 1. (a) Coordination environment around Cd(II) ions in 1; Cd (hepta-coordinated Cd1 in green and hexa-coordinated Cd2 in cyan), N (blue), O (red), C (gray). (b) View of wave-like infinite 2D arrangement constructed from hepta-coordinated Cd1 and hexa-coordinated Cd2 centers linked with each other through 3,4-pyrdc ligands in the crystallographic ac plane. (c) View of 3D porous structure of 1 with hexagonal- and rectangular-shaped water filled channels along crystallographic a axis. (d) Schematic representation of (3,4)-connected 3D net of compound 1.



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Figure 2. (a) Coordination environment around Mn(II) ions in 2; Mn (green), N (blue), O (red), C (black). (b) 2D sheet formed by Mn(II) and 3,4-pyrdc. (c) View of 3D pillared-layer frameworks of 2 (lattice guest water molecules are present in the small cavities). (d) Schematic representation of (4,5)-connected 3D net of compound 2.


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Figure 3. (a) Coordination environment around Cu(II) ions in 3; Cu (green), N (blue), O (red), C (black). (b) The 2D undulated net of 3. (c) Schematic representation of 4-connected 2D net of compound 3. (d) Supramolecular 3D arrangements in 3 by locking the 2D sheets by H-bonding (H-bonding: cyan dotted lines).



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Figure 4. Powder X-ray diffraction patterns of compound 1 in different state.

Figure 5. Powder X-ray diffraction patterns of compound 2 in different state. 22 ACS Paragon Plus Environment

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Figure 6. Powder X-ray diffraction patterns of compound 3 in different state.



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Figure 7. Comparison between the FT-IR spectra in different phases for 1(a), 2(b), and 3 (c).


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Figure 8. UV-Vis spectrum of compound 3 in different state

Figure 9. CO2 (Filled and open circles represent adsorption and desorption respectively) and N2 (Filled and open triangles represent adsorption and desorption respectively) adsorption isotherms of 1′ (red), 2′ (blue) and 3′ (green) measured at 195 K for CO2 and 77 K for N2.



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Figure 10. Water sorption isotherms of 1′ (red), 2′ (blue) and 3′ (green) measured at 298 K. Filled and open triangles represent adsorption and desorption respectively.


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For Table of Content Use Reversible Phase Transformation in Three Dynamic Mixed Ligand Metal– Organic Frameworks: Synthesis, Structure and Sorption Study Arijit Halder, Biswajit Bhattacharya, Rajdip Dey, Dillip Kumar Maity and Debajyoti Ghoshal

Three dynamic MOFs, showing reversible crystalline-to-crystalline and crystalline-to-amorphous phase transformation upon dehydration/rehydration. All dehydrated species selectively uptake CO2 over N2 along with stepwise water adsorption.


Reversible Phase Transformation in Three Dynamic Mixed-Ligand

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