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MOFs with PCU Topology for the Inclusion of Onedimensional Water Cages: Selective Sorption of Water Va-pour, CO2 and Dyes and Luminescence Properties Avishek Dey, Dipayan Bairagi, and Kumar Biradha Cryst. Growth Des., Just Accepted Manuscript • Publication Date (Web): 19 May 2017 Downloaded from http://pubs.acs.org on May 23, 2017
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Crystal Growth & Design
MOFs with PCU Topology for the Inclusion of Onedimensional Water Cages: Selective Sorption of Water Vapour, CO2 and Dyes and Luminescence Properties Avishek Dey, Dipayan Bairagi, and Kumar Biradha* Department of Chemistry, Indian Institute of Technology, Kharagpur-721302, India. KEYWORDS. MOFs, PCU, SBUs, Gas sorption. ABSTRACT: A rigid pyridine-3,5-bis(5-azabenzimidazole) ligand (L) and linear dicarboxylate sodium terephthalate (TPA) with Cd(II), Zn(II) and Co(II) under solvothermal conditions afforded water stable and nitrogen rich MOFs with general formulae [Cd2L2(TPA)2]·12H2O}n, 1, {[Zn4L4(TPA)4]·(DMF)·16H2O}n, 2 and {[Co2L2(TPA)2]·12H2O}n, 3, respectively. These MOFs are found to contain doubly interpenetrated pcu networks with hydrophilic channels. The networks are propagated by M2(CO2)2 SBUs (intermetallic distances 4.0 Å) which leads to the formation of two-dimensional grid structure of metal-terephthalate. The shape of grids varies from rhomboidal to squaric depending on the presence of metal centre. The channels of these MOFs are occupied by the one-dimensional water cages. In the pcu network, each cuboid is found to have dimensions of ~12.5 Å as width and 18.5 Å as length. They have shown an excellent ability for water vapour sorption (180-225 cc/g at 298 K). Further, CO2 is found to be preferred sorbent (70 cc/g at 273 K) over N2. They were also shown to be capable of dye (methylene blue and crystal violet) removal from aqueous solutions through adsorption. Further, Cd(II) and Zn(II) MOFs found to exhibit solid state luminescence properties.
Introduction The burning of fossil fuel creates significant problems to the human civilization due to the evolution of CO2 gases.1-3 Carbon dioxide (CO2) acquisition in the atmosphere creates greenhouse effect as it raises pollution and global warming. The rising level of anthropogenic carbon dioxide (CO2) has a direct impact on our environment. The capturing of CO2 from the atmosphere is being considered as one of the effective methods for the reduction of greenhouse effects.4-6 In this regard, the design of the materials capable of adsorption of CO2 gases had gained great attention. MOFs have been regarded as one of those potential adsorbents given their designable structures, porosity, high surface area, large pore volume and facile pore functionalization.7-12 Therefore, several recent studies on MOFs have been focused on the fabricating frameworks with large pores and high surface areas. One of the main focuses of the area is selective sorption of CO2 over N2 or CH4 by the exploitation of series of MOFs that are synthesized by the change of metal ions or by changing the nature of pores through various functionalizations on organic part.13-19 Several MOFs reported so far for CO2 adsorption and selectivity utilizing organic ligands containing carboxylate, imidazole, tetrazole, triazole or pyrazole moieties.20-28 Yaghi group had explored zeolitic imidazolate frameworks (ZIFs) for the adsorption of the gases and concluded that the pore size has a little effect for the selective sorption of CO2 gas, while the interaction of gas with framework plays a significant role in the selective sorption of the gases.29-30 Family of CPM-33 MOFs were shown to exhibit superior CO2 adsorptiondesorption capacity.31-32 However, there are no previous reports on exploiting the MOFs containing azabenzimidazole for such purposes. The metal carboxylate binding is known to form several SBUs to propagate 3D-framework structures. Among those
the most utilized and attractive are six connected metal tetrahedron with M4(O)(carboxylate)6 and butterfly motif with M2(carboxylate)4.33-37 The coordination unsaturation of the metal carboxylate SBUs can be utilized further for the binding of neutral ligands containing bipyridyl/ imidazoles/ pyrazole in effect the dimensionality of the network can be enhanced.3841 Generally, the curved nature of the ligands expected to result in the formation of voids for the incorporation of guest molecules. Recently, we have reported the use of curved bis(azabenzimidazole) ligands and curved dicarboxylates in producing 2D-networks which have a capability for inclusion of water molecules and water vapour sorption. However, in these networks the angular dicarboxylates consistently formed metal-carboxylate 1D-chains consisting of SBUs of M2(COO)2. The ligand L has exhibited the propensity to link such 1D-chains to 2D-layers.42 From these results it is envisaged that the use of linear dicarboxylates may result in the formation of 2D layers of metal carboxylate that can be linked by L to 3D-networks, MOFs. These MOFs can be regarded as N-rich systems as they contain as many as seven N-atoms (two are imines) per ligand which create hydrophilic cavities. MOFs with such cavities are important to capture water molecules and to stabilize water clusters with various geometries within the channels/cavities. The study of geometry of water clusters is of importance as they have implications in understanding bulk water behaviour. A variety of water clusters have been reported to date include hexagons, pentagons, one dimensional networks, twodimensional layers, three-dimensional networks all these are encapsulated/templated by host network of CPs.43-49 The water stability of the MOFs is another important aspect that is detrimental for its industrial use as an adsorbent material.50-51 Herein we would like to report our studies on MOFs of L and terephthalate and their gas sorption properties. The complexa-
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tion reactions of L and sodium terephthalate with Cd(II) or Zn(II) or Co(II) metal salts have been conducted in solvothermal process to obtain the single crystal of the corresponding MOFs (Scheme 1). All the three MOFs are found to contain plethora of water molecules in their channels and found to exhibit water sorption. Further, these materials have shown selective sorption of CO2 over N2 gas. The Cd(II) and Zn(II) MOFs have exhibited excellent luminescence properties at room temperature. Scheme 1: Structural drawings for L, and TPA anion N
N
N
HN
N H
N
N L
O
O
O
O TPA
Results and Discussion The ligand L was synthesized by simple condensation reaction between 3,4-diamino pyridine and pyridine-3,5-dicarboxylic acid. The single crystals of MOFs (1-3) were obtained by the solvothermal reaction of L, sodium terephthalate and Cd(NO3)2 or Zn(NO3)2 or Co(NO3)2 in DMF/H2O or DMA/H2O at 100°C. The single crystal diffraction analyses reveal that the complexes 1-3 have formulae [Cd2L2(TPA)2]·12H2O}n, {[Zn4L4(TPA)4]·(DMF)·16H2O}n and {[Co2L2(TPA)2]·12H2O}n respectively. The crystal structure analysis suggests that all three are isostructural and contains doubly interpenetrated 3D-networks containing pcutopology. The pertinent crystallographic information is given in Table 1. The complexes 1-3 crystallize in monoclinic crystal system and exhibits isostructurality. The complexes 1 and 3 exhibit P2(1)/c while 2 exhibits P2(1) space group. The asymmetric units of 1 and 3 are constituted by two metal atoms, two ligands, two TPA units and 12 uncoordinated water molecules. While that of 2 is constituted by four each of Zn(II) centers, ligands and TPA units, one DMF molecule and 16 uncoordinated water molecules. The unit cell volumes of 2 & 3 are less than that of 1 by 4%. In all cases, the metal centers have distorted octahedral coordination geometry with MO4N2 configuration. Further, in all three complexes one of the carboxylate of TPA is involved in the formation bimetallic SBU (M2(COO)2) with metal-to-metal separation about 4 Å while the other carboxylate links the SBUs via chelation of metal centers. As a result, metal carboxylate layer containing rhomboidal cavities is formed in bc-plane in 1 and 3 and in abplane in 2. The grids are more rhomboidal in 1 (18·4x12·6 Å2) than in 2 and 3 (16·8x13·6 Å2). These layers are interlinked to 3D-network with the double pillaring of the layers by L units such that the interlayer separation is of 18 Å in 1 and 16.4 Å in 2 and 3. The huge channels of the 3D-netwrok are partially filled by double interpenetration. The interpenetration occurs through insertion of two of the L units (double
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pillar) into each and every rhomboidal cavity of the metalterephthalate layers. The double pillar units of L are formed via π- π interactions with a plane to plane separation of 3·63·7 Å. The interpenetration creates channels along c-axis in case of 1 and 3 and along a-axis in case of 2. The representation of the network as nodes and spacers by reducing the SBUs as six connected nodes and double pillars as spacers illustrates the doubly interpenetrated 3D-pcu network (Figure 1). All three MOFs are found to have 26% void space (PLATON) that are occupied solely by water molecules in 1 & 3 and by water and DMF in 2. These guest molecules located and refined in 1 and 2 but could not be located in 3. The central pyridine of L in all three cases remain uncoordinated, in 1 the pyridine units of both the ligands involved in O-H···N (O···N: 2.870 & 3.055 Å) hydrogen bonding with the channelled water molecules. Whereas in 2, three of the four pyridine moieties have such O-H···N (O···N: 2.849, 2.903, 2.891 Å) hydrogen bonding and the fourth one has close proximity to electrophilic C-atom of DMF (C···N: 3.337 Å). Interestingly, the channels that are formed between interpenetrated networks are filled by column of hydrogen bonded water molecules in 1, whereas in 2 they are occupied by the column of hydrogen bonded water and DMF molecules. In case of 1, it forms (H2O)12 cluster as repeating unit of 1Dcage. The water cluster consists of two five membered rings (A and B) and one each of six (C) and four membered rings (D). These clusters propagate as 1D cage by the formation of another six membered ring (E) (Figure 2). Among twelve water molecules only one of it connects to four of its neighbours with distorted tetrahedral geometry (dO-O= 2.6, 2.8, 2.8 and 2.7 Å,
