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Azo Functionalized 5-nitro-1,3-benzenedicarboxylate Based Coordination Polymers with Different Dimensionality and Functionality Dilip Kumar Maity, Arijit Halder, Saheli Ghosh, and Debajyoti Ghoshal Cryst. Growth Des., Just Accepted Manuscript • DOI: 10.1021/acs.cgd.6b00946 • Publication Date (Web): 01 Aug 2016 Downloaded from http://pubs.acs.org on August 5, 2016

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Crystal Growth & Design

Azo Functionalized 5-nitro-1,3-benzenedicarboxylate Based Coordination Polymers with Different Dimensionality and Functionality Dilip Kumar Maity, Arijit Halder, Saheli Ghosh, and Debajyoti Ghoshal* Department of Chemistry, Jadavpur University, Jadavpur, Kolkata, 700 032, India E-mail: [email protected] ___________________________________________________________________________ ABSTRACT Five azo-functionalized coordination polymers (CPs) namely [Zn(azbpy)(NO2-bdc).H2O]n (1),

[Zn(azbpy)(NO2-bdc)]n,3H2O

{[Mn(azbpy)2(NO2-bdc)]2}n

(4)

and

{[Cd(azbpy)(NO2-bdc).H2O].2H2O}n

(2),

(3),

{[Co(azbpy)(NO2-bdc)(H2O)2][Co(azbpy)0.5(NO2-

bdc)(H2O)3]}n (5) have been synthesized using different transition metal salts with 5-nitro1,3-benzenedicarboxylate (NO2-bdc2-) and 4,4´-azobispyridine (azbpy) ligand using slow diffusion technique at room temperature. The complexes 1-5 were characterized by single crystal X-ray diffraction analysis, elemental analysis, infrared spectroscopy (IR), powder Xray diffraction (PXRD) and thermo gravimetric analysis (TGA). In solid state, compound 1 shows a wavy 1D ladder; constructed through the N,N´-donor azbpy and NO2-bdc2- ligands with the metal centers; whereas compound 2 exhibits a bilayer 2D sheet containing a wavy 1D ladder of metal-carboxylate and compound 3 shows a stair like wavy 2D sheet. Compound 4 exhibits a novel twofold interdigitated 2D sheet of two similar layers containing pendent azbpy ligands whereas compound 5 displays a polythreaded 2D structure with intercalated 1D chain into the pore. The solid state luminescence properties of 1-3 along with free N,N´-donor azbpy ligand have been performed at room temperature; where all the complexes 1-3 show azbpy ligand based luminescence property. Gas and solvent vapor adsorption study have been performed for the compounds 2-4 and the dehydrated frameworks of compounds 2-4 exhibit selective CO2 adsorption at 195 K over N2 (at both 77 and 195 K) due to the strong interactions between polar pore walls of dehydrated frameworks with CO2 molecule having quadruple moment. ___________________________________________________________________________

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INTRODUCTION The cogent design of coordination polymers (CPs) with desired dimensionality and topology has become an amphitheatre of chemical research in last two decades.1-4 The field creates alluring interest among the contemporary chemists due to the challenging outcome involved in terms of their functionality which is essential factors of their structures. It has been well established in structure design methodology of coordination polymers (CPs) that the achievement of fascinating architectures with controllable network and topology depends on some key factors e.g. metal ions,5,6 nature of organic ligands,7-9 reaction temperatures,10,11 pH of the medium,12,13 solvents14,15 and counter anions16-18 which might bring the diversity in the structural topology of CPs through their smart tuning. It has been commonly observed in the syntheses of CPs, that having all prerequisites of reactions, the desired crystals are not at all afforded; or sometimes terminated with the crystals of completely undesired product. This evidently indicates not only the chemical binding but also the ease of crystallization is very crucial in the design of CPs to get a meaningful outcome. Sometimes the steric and electronic factors individually or additively facilitate the process of crystallization of CPs and that could be a very important precondition for the design of CPs. Moreover, as mentioned before the design and syntheses of CPs has drawn an enormous interest not only in terms of their diverse structural topology but also for their potential applications, in gas storage and seperation,19-23 photo luminescence,24-26 magnetism,27-30 ion exchange,31-35 conductivity,36-38 and many more.39-42 Out of several applications, in last few years our group has concentrated on the design of porous coordination polymers (PCPs)43-50 showing selective adsorptions of CO2 considering alarming rise of CO2 level in the atmosphere which may cause global warming. In this way we have precisely focused on the modification of pore wall that contains a polar core which can interact with CO2 to make that PCPs selectively towards CO2 over other gases. Here using same reaction conditions we have synthesized five mixed ligand based CPs with diverse dimensionality namely [Zn(azbpy)(NO2-bdc)·H2O]n (1), [Zn(azbpy)(NO2bdc)]n,3H2O (2), {[Cd(azbpy) (NO2-bdc)·H2O]·2H2O}n (3), {[Mn(azbpy)2(NO2-bdc)]2}n (4) and {[Co(azbpy)(NO2-bdc)(H2O)2][Co(azbpy)0.5(NO2-bdc)(H2O)3]}n (5); only by changing the transition metal ions. Though the bridging modes of 5-nitro-1,3-benzenedicarboxylate (NO2-bdc2-) is similar to that of non substituted or other 5-substituted 1,3benzenedicarboxylate51,52 but the presence of nitro group can play an active role in the synthesis and stabilization of CPs; as the nitro group (-NO2) can not only act as a hydrogen 2 ACS Paragon Plus Environment

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bond acceptor, but also it can exerts some spatial effects52 in the polymeric structure formation during self-assembly process.

These two factors together may results the

formation of intriguing architecture through close packing in the crystallization of CPs. Here the dimension and topology of the complexes 1-5 are observed completely different from each other and gradually changes from their simple to more complicated form. The variation of dimension/topology in complexes 1-5 are achieved probably due to the different coordination modes of NO2-bdc2- ligand with different metal(II) ions as the other conditions remain same for the preparation of the complexes. In complexes 3 and 4, though the bridging modes of NO2-bdc2- is same but the structures are totally different suggesting the role of metal ions5,6 used in the formation of CPs. Notably, the NO2-bdc2- ligand bridges through bis-chelating and bis-monodentate fashion with Mn(II) and Co(II) in complexes 4 and 5 resulting the formation of two novel two fold interdigitated 2D sheet53,54 and polythreaded55-57 2D structure with intercalated 1D chain into the pore respectively. Moreover, the –NO2 groups are found very effective in the design of CO2 selective pore if the steric factors are critically overcome.50 Here also we have afforded three porous structures where the –NO2 groups are functionalized towards the pore. The gas (N2 and CO2) and solvent vapor (H2O and EtOH) adsorption of the said frameworks of 2-4 have also been measured. In case of 2 and 3 the steric factors have overcome to make them suitable material for selective CO2 adsorption whereas in case of 4 although the pore is functionalized by the – NO2 groups but the voids are sterically challenged to make it almost nonporous.

Scheme 1. Synthetic Outline of Complexes 1-5.



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All the complexes were characterized by single crystal X-ray crystallography, IR spectroscopy, powder X-ray diffraction (PXRD), elemental analysis, thermo gravimetric (TG) analysis. The photo luminescence properties of the solid complexes 1-3 with free N,N´donor azbpy ligand have also been investigated at room temperature. EXPERIMENTAL SECTION Materials The 4,4´-azobispyridine (azbpy) ligand was prepared according to earlier reported procedures.58,59 The starting materials for the aforesaid ligand synthesis i.e. 4-amino pyridine were purchased from Sigma-Aldrich Chemical Co. Inc. and used as received. Manganese(II) chloride

tetrahydrate,

Cobalt(II)nitrate

hexahydrate,

Zinc(II)nitrate

hexahydrate,

Cadmium(II)nitrate tetrahydrate and 5-nitro-1,3-benzenedicarboxylic acid (NO2-H2bdc), were also purchased from Sigma-Aldrich Chemical Co. Inc. and used as received. Sodium salt of 5-nitro-1,3-benzenedicarboxylic acid (NO2-Na2bdc) was synthesized by the gradual addition of AR grade Na2CO3 to the aqueous suspension of NO2-H2bdc in 1:1 ratio. The neutralization was checked by the pH of the solution and then it was allowed to stand for 40 mins. Finally, the solvent was evaporated in water bath to dryness. All other chemicals and solvents were AR grade and were used as received. Physical Measurements Elemental analyses (C, H and N) were performed using a Heraeus CHNS analyzer. Infrared spectra (4000–400 cm−1) were taken on KBr pellets, using a PerkinElmer Spectrum BX-II IR spectrometer. Thermal analysis (TGA) was carried out on a METTLER TOLEDO TGA 850 thermal analyzer under nitrogen atmosphere (flow rate: 10 cm3 min−1), at the temperature range 30–600 °C with a heating rate of 20 °C min−1. X-ray powder diffraction (PXRD) patterns of the bulk sample were recorded in Bruker D8 Discover instrument using Cu-Kα radiation. Solid state emission spectra were recorded on a HORIBA JobinYvon (Fluoromax3) fluorescence spectrophotometer. Sorption Measurements The adsorption isotherms of N2 (at 77 and 195 K) and CO2 (at 195 K) were measured for dehydrated 2-4 using Quantachrome Autosorb-iQ adsorption instrument. All operations were computer-controlled and automatic. High purity gases were used for the adsorption measurements (nitrogen, 99.999%; carbon dioxide, 99.95%). At the beginning the as 4 ACS Paragon Plus Environment

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synthesized compounds of 2, 3 and 4 (~90 mg each) were placed in the sample tube and dehydrated at 393, 423 and 373 K respectively, under a 1×10−1 Pa vacuum for about 4 hrs prior to measurement of the isotherms. Helium gas (99.999% purity) was introduced in the gas chamber and allowed to diffuse into the sample chamber to measure the dead volume. Taking dehydrated samples of 2-4 the N2 adsorptions were carried out at 77 K maintained by a liquid-nitrogen bath whereas N2 and CO2 adsorptions were also measured at 195 K (temperature maintained by dry ice-acetone cold bath) in the pressure range from 0 to 1 bar. The amount of gas adsorbed were calculated from the pressure difference (Pcal - Pe), where

Pcal is the calculated pressure with no gas adsorption and Pe is the observed pressure at equilibrium. The adsorption isotherms for different solvents (i.e. H2O and dry EtOH) were measured at 298 K in the pressure region from 0 to 24 torr and 0 to 46 torr for H2O and EtOH respectively, in their vapour state by taking the dehydrated compounds of 2-4 using the same instrument. The samples of 2, 3 and 4 (~90 mg each) were activated under similar conditions as mentioned earlier. The different solvent molecules used to generate the vapour were degassed fully by repeated evacuation. The dead volume was measured with helium gas. The adsorbate was placed into the sample tube, then the change of pressure was monitored and the degree of adsorption was determined by the decrease in pressure at the equilibrium state. Crystallographic Data Collection and Refinement The single crystals of compounds 1-5 were mounted on the tip of thin glass fibers with commercially available super glue. X-ray single crystal data collection of all five crystals were carried out at room temperature using Bruker APEX II diffractometer which is equipped with a graphite monochromated normal focus, sealed tube X-ray source with Mo-Kα radiation (λ= 0.71073Å). The data were integrated with SAINT60 program and the absorption corrections were made by SADABS.61 All the structures were solved by SHELXS 9762 using Patterson method and followed by successive Fourier and difference Fourier synthesis. Full matrix least-squares refinements were performed on F2 using SHELXL9762 with anisotropic displacement parameters for all non-hydrogen atoms. All the hydrogen atoms were fixed geometrically by HFIX command and placed in ideal positions in all cases before the final refinement. For complex 2, the solvent molecules are highly disordered which were subtracted from the reflection data by the SQUEEZE method as implanted in PLATON program.63 The exact determination of solvent molecules in 2 was obtained from the thermo gravimetric analysis (TGA) curves and elemental analysis (EA) results, which corroborates the presence of three lattice water molecules per formula unit of 2. Moreover, the coordinated 5 ACS Paragon Plus Environment


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water molecule (O4W) in 5 was also disorder; which was refined without fixing H atoms on it. All calculations were carried out using SHELXS 97,62 SHELXL 97,62 PLATON v1.15,63 ORTEP-3v2,64 WinGX system Ver-1.8065 and TOPOS.66,67 Data collection and structure refinement parameters and crystallographic data for all complexes are given in Table 1. SYNTHESES [Zn(azbpy)(NO2-bdc).H2O]n (1) and [Zn(azbpy)(NO2-bdc)]n,3H2O (2). The aqueous solution (10 mL) of Na2(NO2-bdc) (1 mmol, 0.255 g) was mixed with the methanolic solution (10 mL) of 4,4´-azobispyridine (azbpy) (1 mmol, 0.184 g) and stirred for 25 mins to mix it well. Zn(NO3)2.6H2O (1 mmol, 0.297 g) was dissolved in 10 mL of water in a separate beaker. Then in a crystal tube 2 ml of Zn(II) solution was slowly and carefully layered with the 4 ml of above mentioned mixed-ligand solution using a 3 ml of buffer solution in between the two solution; where the buffer solution was made with the mixing of H2O and MeOH in equal volume (v/v = 1:1) in a separate beaker. The tube was sealed and kept undisturbed at room temperature. After four weeks two types of crystals with different color and shape have been appeared that are suitable for single crystal X-ray diffraction analysis. Out of two, one is diamond shaped red colored crystals of 1 (Yield 45%) and the other is needle shaped orange colored crystals of 2 (Yield 32%). The two types of crystals were separated manually by using needle with the help of ultra high microscope and washed with a 1:1 methanol-water mixture and dried under air. For 1, Anal. Calc. of C18H13N5O7Zn (%): C, 45.35; H, 2.75; N, 14.69. Found: C, 45.42; H, 2.69; N, 14.75. IR spectra (in cm-1): ν(H2O), 3431; ν(CH-Ar), 3096; ν(N=N), 1610; ν(C=C), 1614-1423; ν(N=O), 1567(as) and 1345(s) and ν(C-O), 1225 (Figure S1). For 2, Anal. Calc. of C18H17N5O9Zn (%): C, 42.16; H, 3.34; N, 13.66. Found: C, 42.22; H, 3.28; N, 13.71. IR spectra (in cm-1): ν(H2O), 3408; ν(CH-Ar), 3088; ν(N=N), 1625; ν(C=C), 1566; ν(N=O), 1542(as) and 1344(s) and ν(C-O), 1226 (Figure S2). {[Cd(azbpy)(NO2-bdc).H2O].2H2O}n (3). This has been synthesized by the same procedure as that of 1 or 2 using Cd(NO3)2.6H2O (1 mmol, 0.308 g) instead of Zn(NO3)2.6H2O (1 mmol, 0.297 g). The red colored block shaped crystals suitable for X-ray diffraction analysis were obtained after two weeks. The crystals were separated and washed with a methanol-water (1:1) mixture and dried under air (Yield 55%). Anal. Calc. for C18H17N5O9Cd(%): C, 38.62; H, 3.06; N, 12.51. Found: C, 38.58; H, 3.11; N, 12.47. IR spectra (in cm-1): ν(H2O), 3413; ν(CH-Ar), 3086; ν(N=N), 1624; ν(C=C), 1599; ν(N=O), 1567(as) and 1367(s) and ν(C-O), 1232 (Figure S3).


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{[Mn(azbpy)2(NO2-bdc)]2}n (4). This has been also synthesized by the same procedure as that of 1 or 2 using MnCl2.4H2O (1 mmol, 0.198 g) instead of Zn(NO3)2.6H2O (1 mmol, 0.297 g). The red colored block shaped crystals suitable for X-ray diffraction analysis were obtained after three weeks. The crystals were separated and washed with a methanol-water (1:1) mixture and dried under air (Yield 62%). Anal. Calc. for C28H19N9O6Mn (%): C, 53.18; H, 3.03; N, 19.93. Found: C, 53.25; H, 3.09; N, 19.88. IR spectra (in cm-1): ν(CH-Ar), 3088; ν(N=N), 1613; ν(C=C), 1607-1455; ν(N=O), 1546(as) and 1383(s) and ν(C-O), 1219 (Figure S4). {[Co(azbpy)(NO2-bdc)(H2O)2][Co(azbpy)0.5(NO2-bdc)(H2O)3]}n

(5).

This

has

been

.

synthesized by the same procedure as that of 1 or 2 using Co(NO3)2 6H2O (1 mmol, 0.297 g) instead of Zn(NO3)2.6H2O (1 mmol, 0.297 g). The red colored block shaped crystals suitable for X-ray diffraction analysis were obtained after two weeks. The crystals were separated and washed with a methanol-water (1:1) mixture and dried under air (Yield 75%). Anal. Calc. for C31H28N8O17Co2 (%): C, 41.26; H, 3.13; N, 12.42. Found: C, 41.3; H, 3.17; N, 12.39. IR spectra (in cm-1): ν(H2O), 3412; ν(CH-Ar), 3090; ν(N=N), 1602; ν(C=C), 1600-1421; ν(N=O), 1553(as) and 1358(s) and ν(C-O), 1231(Figure S5). The bulk compounds of all complexes except 1 and 2 have been synthesized in powder form by the direct mixing of the ligands solution and corresponding M(II) salt solution in water at their equal-molar ratio; where complexes 1 and 2 have been synthesized by crystallization process mentioned above followed by their manual separation under ultra microscope. The purity of the complexes was verified by PXRD, which give good correspondence with the simulated PXRD patterns. The purity of the bulk samples were further confirmed by the results of elemental analysis and IR spectra as well, which also found in accordance with the data obtained for their corresponding single crystals. RESULTS AND DISSCUSSION Synthesis Complexes

1-5

have

been

synthesized

using

disodium

salt

of

5-nitro-1,3-

benzenedicarboxylate (NO2-bdc2-) along with the N,N′-donor 4,4´-azobispyridine (azbpy) ligand with the variation of transition metal(II) salts [e.g. Zn(II), Cd(II), Mn(II) and Co(II)] at room temperature by slow diffusion technique (Scheme 1). Sodium salt of 5-nitro-1,3benzenedicarboxylic acid (NO2-H2bdc) was synthesized by the continuing addition of AR grade Na2CO3 to the aqueous suspension of NO2-H2bdc in 1:1 ratio. The neutralization was



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checked by measuring pH of the solution and then it was allowed to stand for 40 mins. Finally, the solvent was evaporated in water bath to dryness. Infrared Spectra The IR spectra of complexes 1-5 discussed in the synthesis part of experimental section, exhibit the sharp and strong peaks ranging from 1210 to 1320 due to the presence of C-O moiety of the deprotonated carboxylate groups. The absence of any absorption bands at 1720−1670 cm−1 confirms that the NO2-H2bdc ligand adopts completely deprotonated NO2bdc2- form. For complexes 1-5, the absorption bands ranging from 1575 to 1630 cm−1 corroborate the presence of -N=N- group of the 4,4´-azobispyridine ligand. Moreover, the absorption bands from 1345 to 1385 cm−1 and from 1515 to 1560 cm−1 signify the symmetric and asymmetric stretching frequency of –NO2 group of the NO2-bdc2- ligand respectively in complexes 1-5. Structural Descriptions of the CPs As mentioned before the spectacular structural diversity are created only by varying the metal centers. The structural diversity of five azo functionalized 5-nitro-1,3-benzenedicaroxylate (NO2-bdc2-) based CPs are depicted in Scheme 1; which are originated principally due to various coordination modes of NO2-bdc2-dianion (Scheme 2) in complexes 1-5. The structural details for all the complexes are discussed here.

Scheme 2. Different bridging modes of 5-nitro-1,3-benzenedicarboxylate (NO2-bdc2-): (a) bis-monodentate with oxo-bridging mode in 1. (b) bidentate-chelating mode in 2. (c) bischelating mode in 3 and 4. (d) bis-mondentate mode in 5. [Zn(azbpy)(NO2-bdc).H2O]n (1). Single-crystal X-ray analysis reveals that compound 1 crystallizes in the triclinic Pī space group with Z value of 2. The structure analysis indicates the formation of a wavy 1D ladder constructed by NO2-bdc2- and azbpy linker with Zn(II) ion. The asymmetric unit of 1 contains one Zn(II) ion, one azbpy, one NO2-bdc2- and one coordinated water molecule. The penta coordinated Zn(II) centers forms a distorted trigonal


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bipyramidal geometry with ZnO4N coordination environment (Figures 1a and S6). The four oxygen atoms (O1, O4a, O4b and O1W) and one N atom (i.e N1) from three different NO2bdc2- ligand, one coordinated water molecule and one pendent azbpy ligand respectively are ligated to Zn1. The Zn-O bond lengths vary from 1.941(3) to 2.658(3) Å; and Zn-N bond length is 2.035(3) Å (Table S1). Here, each NO2-bdc2- ligand bridges with the three different Zn(II) centers through bis-monodentate oxo-bridging fashion to form the 1D ladder (Figure 1b) where the azbpy ligands and coordinated water molecules are pendent. The two H-atoms of each coordinated water molecule in the 1D ladder are strongly H-bonded with the pendent carboxylate O-atom of NO2-bdc2- ligand and also the pendent N-atom of azbpy ligand by means of intermolecular H-bonding (Figure 1c and Table S2) to form a supramolecular 2D sheet. Finally, the H-bonded 2D sheet exhibits a supramolecular 3D structure with the help of intermolecular π-π interactions (Figure 1c and Table S3). The TOPOS66,67 analysis reveals that the structure of 1 can be represented as a 3-connected uninodal ladder (Figure 1d) with the corresponding Schläfli symbol {42.6}. [Zn(azbpy)(NO2-bdc)]n,3H2O (2). Single-crystal X-ray analysis reveals that compound 2 crystallizes in the monoclinic P2/c space group with Z value of 4. The structure analysis designate the formation of a 2D sheet structure constructed by Zn(II) ion, NO2-bdc2- and azbpy linker. The asymmetric unit of 2 contains one Zn(II) ion, one azbpy, one NO2-bdc2and three lattice water molecules which are removed through squeeze method implanted by PLATON63 program. The hexa coordinated Zn(II) centers forms a distorted octahedral geometry with ZnO4N2 coordination environment (Figures 2a and S7). The four oxygen atoms (O1, O2, O3a and O4b) of three different NO2-bdc2- ligands and two nitrogen atoms (N1 and N4c) of two different azbpy linkers are ligated to Zn1 center. The Zn-O bond lengths vary from 2.0586(11) to 2.2957(13) Å; and Zn-N bond lengths vary from 2.1304(13) to 2.1610(13) Å (Table S4). Here, NO2-bdc2- bridges with three Zn(II) centers by chelatingbidentate fashion to form a Zn2(CO2)4 SBU unit which finally results the formation of a 1D metal carboxylate ladder (Figure 2b) along crystallographic a-axis. The one dimensional ladders are further linked by bridging azbpy ligand to form a 2D bilayer structure in bc plane (Figure 2c). These 2D bilayer structure contains a 1D guest water filled channels of dimension 6×3 Å2 along c-axis (Figure S21c). Upon removal of all guest solvent molecules the total solvent accessible void estimated by PLATON is 758.9 Å3 which is 31.44% of the total crystal volume of 2414 Å3. Finally, the 2D bilayers are stacked by intermolecular π-π interaction to form a supramolecular 3D structure (Figure S11, Table S5). The TOPOS66,67 9 ACS Paragon Plus Environment


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analysis reveals that the structure of 2 can be represented as a 3,5-connected binodal net (Figure 2d) with stoichiometry (3-c)(5-c) and the corresponding Schläfli symbol for the net is {42.67.8}{42.6}. {[Cd(azbpy)(NO2-bdc).H2O].2H2O}n (3). Single-crystal X-ray analysis shows that the compound 3 crystallizes in the triclinic Pī space group with Z value of 2. The structural analysis reveals that the formation of a stair like wavy 2D sheet structure constructed by Cd(II) ion, NO2-bdc2- and azbpy linkers. The asymmetric unit of 3 contains one Cd(II) ion, one azbpy, one NO2-bdc2- one coordinated water molecule and two lattice water molecules. Here the hepta coordinated Cd(II) center forms a distorted pentagonal bipyramidal geometry with CdO5N2 environment (Figures 3a and S8). The five oxygen atoms (O1, O2, O3a, O4a and O1W) from two different NO2-bdc2- ligand and one coordinated water molecule respectively; along with two nitrogen atoms (N1 and N3) of two different azbpy linkers are ligated to Cd1. The Cd-O bond lengths vary from 2.268(3) to 2.646(4) Å; and Cd-N bond lengths vary from 2.345(4) to 2.349(4) Å (Table S6). Here, NO2-bdc2- bridges with two adjacent Cd(II) centers through bis-chelating fashion to form a 1D metal carboxylate chain (Figure 3b) along a-axis and these 1D chains are further linked by azbpy ligand to form a wavy 2D sheet in bc plane (Figure 3c). This 2D sheet contains a 1D channel of dimension 10.8×3.8 Å2 (Figure S12a) along b-axis occupied by guest water molecules. Upon removal of the guest water molecules the total solvent accessible void value estimated by PLATON is 218.2 Å3 which is 19.5% of the total crystal volume of 1118 Å3. Moreover, the 2D sheets are stacked by intermolecular π-π interaction to form a supramolecular 3D structure (Figure S12b, Table S7). The TOPOS66,67analysis reveals that the structure of 3 can be represented as a 4-connected uninodal wavy 2D sheet (Figure 3d) with the corresponding Schläfli symbol {44.62}. {[Mn(azbpy)2(NO2-bdc)]2}n (4). Single-crystal X-ray analysis reveals that compound 4 crystallizes in the monoclinic P21 space group with Z value of 4. Here each asymmetric unit contains two independent polymeric fragments of [Mn(azbpy)2(NO2-bdc2-)] and the structural analysis reveals that the formation of a twofold interdigitated 2D layer structure. The asymmetric unit of 4 contains two crystallographically independent [Mn(azbpy)2(NO2-bdc2-)] core, where both hepta coordinated Mn(II) centers Mn1 and Mn2 forms a distorted pentagonal bipyramidal geometry with MnO4N3 coordination environment (Figures 4a and S9) in each case. The four oxygen atoms (O1, O2, O3a and O4a) of two different NO2-bdc2ligand and three nitrogen atoms (N1, N4b and N5) of three different azbpy linkers are ligated 10 ACS Paragon Plus Environment

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to Mn1 whereas, Mn2 is surrounded by four oxygen atoms (O7, O8, O11c and O12c) from two different NO2-bdc2- ligand and three nitrogen atoms (N10, N13d and N14e) of three different azbpy linkers. The Mn-O bond lengths for Mn1 and Mn2 vary from 2.129(3) to 2.716(3) Å; and Mn-N bond lengths vary from 2.286(3) to 2.317(2) Å (Table S8). Here, 5nitro-1,3-benzenedicarboxylate (NO2-bdc2-) bridges with two metal centers by bis-chelating fashion resulting the formation of a 1D metal carboxylate chain (Figure 4b) along a-axis and these 1D chains are further linked by azbpy ligand to form a 2D layer in ac plane (Figure 4c). Interestingly, the formation of 2D sheets are originated around both Mn1 and Mn2 centers and they are interwoven to each other to form a unique not so common interdigitated 2D layer53,54 which is shown in Figure 4d. These 2D sheets are stacked by intermolecular π-π interaction and C-H…π interaction to form a supramolecular 3D structure (Figure S13a, Table S9). The TOPOS66,67 analysis reveals that the structure of 4 can be represented as a 4connected uninodal net with point symbol {44.62} (Figure S13b). {[Co(azbpy)(NO2-bdc)(H2O)2][Co(azbpy)0.5(NO2-bdc)(H2O)3]}n (5). Single-crystal X-ray structure analysis reveals that compound 5 crystallizes in centro-symmetric triclinic Pī space group with Z value of 2 and the structural analysis reveals that the formation of a polythreaded55-57 2D layers with an intercalated 1D chain within the pore; where both the moities made up with Co(II) ion, NO2-bdc2-and azbpy linkers. In the asymmetric unit of 5, there are three different Co(II) atoms has been observed. Similar type of Co(II) center Co1 and Co2 with 0.5 occupancy, one coordinated azbpy ligand, one NO2-bdc2- and two coordinated water molecules created the 2D sheets whereas Co3 center, half coordinated azbpy ligand, one NO2-bdc2- and three coordinated water molecules created the 1D chain. In both 2D and 1D systems, Co(II) centers show a distorted octahedral geometry with two different coordination environment having CoO4N2 and CoO5N environment respectively (Figures 5a and S10). In 2D sheets, each hexa coordinated Co(II) center possessed of four carboxylate oxygen atoms (O1, O1a, O1W and O1Wa for Co1 and O5, O5b, O2W and O2Wb for Co2) from a pair of NO2-bdc2- ligands, two coordinated water molecules and two nitrogen atoms (N1, N1a for Co1 and N4, N4b for Co2) of azbpy linkers respectively. For Co1 and Co2 center the Co-O bond length varies from 2.104(2) to 2.1298(16) Å and 2.0671(19) to 2.129(2) Å respectively; whereas Co-N bond lengths are 2.132(2) and 2.150(2) respectively (Table. S10). In case of 1D system, which exists in the pores of the 2D structures; each Co3 centers are connected with five oxygen atoms (O7, O12c, O3W, O4W and O5W) of two different NO2-bdc2- ligands and three coordinated water molecules respectively along with one 11 ACS Paragon Plus Environment


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nitrogen atom (N6) of azbpy linker. Co3-O and Co3-N bond lengths are in the range of 2.046(2)-2.137(2) Å and 2.145(2) Å, respectively (Table S10). In the 2D fragment consist of Co1 and Co2, NO2-bdc2- bridges with two metal centers by bis-monodentate fashion and extend along a-axis; these metal carboxylate chains are further linked by bridging azbpy ligand to form a 2D layer (Figures 5d and 5e). On the other hand, two adjacent Co3 centers are joined by two NO2-bdc2- ligands in bis-monodentate fashion to form a metal-carboxylate dimeric unit of [Co(NO2-bdc2-)(H2O)3] which is further linked by azbpy linker, resulting the formation of a 1D coordination polymer (Figures 5b and 5c). These 1D chains are intercalated perpendicularly into the voids of the 2D layers. The simplified diagram of these intercalated arrangements has been shown in Figure 5f. The TOPOS66,67 analysis reveals that the structure of 5 can be represented as a 4-connected uninodal net with point symbol {44.62}. Powder Diffraction (PXRD) Analysis To confirm the phase purity of the bulk materials with the simulated pattern, powder X-ray diffraction (PXRD) experiments were carried out at room temperature using powder samples of all compounds individually and shown in Figures S14-S19. In all the compounds, good correspondence of the entire peak positions in the simulated and as-synthesized patterns which suggests the phase purity of the as-synthesized bulk compounds. Moreover, to check the framework rigidity of the dehydrated frameworks of 2 (activated at 120 oC), 3 (activated at 150 oC) and 4 (activated at 100 oC) PXRD measurement (Figures S15, S17 and S18) have also been carried out for the above mentioned dehydrated frameworks which show the almost identical peak position to their corresponding as synthesized pattern. This matching of peak of the dehydrated frameworks with their corresponding as synthesized patterns, clearly indicates the framework rigidity of the aforesaid compounds after complete removal of the water molecules. Thermo Gravimetric Analysis (TGA) Thermo gravimetric analysis (TGA) of compounds 1-5 were carried out at the temperature range 30-600 °C with a flow rate of 10 cm3 min−1 and depicted in Figure S20. Compound 1 shows a weight loss 3.72% (calcd, 3.78%) at 125 °C for one coordinated water molecule which starts to release at 105 °C (Figure S20). Then the dehydrated framework of 1 is stable up to 250 °C without any further weight loss; after that it collapses into unidentified product upon further heating. For 2, the weight lost has been observed 10.75% (calcd. 10.53%) at 113 °C which corroborates the loss of three lattice water molecules that are started to loss at 87 °C


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(Figure S20). After complete removal of all guest water molecules the dehydrated framework is stable up to 270 °C and finally collapses into unidentified product upon further heating. The TGA plot of compound 3 (Figure S20) shows two step weight loss where the first step indicates the loss of two lattice water molecules at 101 °C (found, 6.77%; calcd, 6.43%), and the second step for strongly bounded one coordinated water molecule (found, 3.11%; calcd, 3.43%) at 140 °C. The dehydrated framework is stable up to 315 °C without any further weight loss; after that it has been collapsed into unidentified product on further heating. In complex 4, the TGA curve shows no significant weight loss up to 350 °C due to the absence of any solvent molecules, finally leading to the formation of unidentified products on further heating (Figure S20) after 350 °C. Finally, compound 5 shows the weight loss of 9.9% (calcd, 9.97%), within the temperature range of 150-200 °C; which is attributed to the release of five coordinated water molecules and the dehydrated framework is stable up to 370 °C without any weight loss; which is finally collapsed into unidentified product during further heating above 370 °C (Figure S20). Gas and Solvent Vapor Sorption Studies Considering the large solvent accessible void in the dehydrated frameworks of 2 and 3 along with their framework stability supported by TGA and PXRD measurements, we have performed the sorption study of the dehydrated frameworks using different gases (e.g. N2 at both 77 and 195 K along with CO2 at 195 K) and vapors (e.g. H2O and EtOH at 298 K). The similar gas and solvent sorption study have also been performed for nonporous interdigitated framework of 4. From the space filling model of compound 2 (Figure S21), it is observed that it contains the guest water filled 1D channels only along c-axis; not in other direction. As a result the dehydrated framework of 2 shows a type-II (Figure 6) surface adsorption for N2 with a very low uptake. This is possibly due to the blocking of the windows of unidirectional pore channels through the adsorption of larger sized N2 molecules (kinetic diameter = 3.6 Å). Interestingly, it shows selective CO2 uptake over N2 (Figures 6 and S23) up to a value of 31 cc/gm at 195 K and 1 bar pressure with a moderately large hysteresis in between the adsorption-desorption curve. This hysteresis can be attributed to the strong adsorbateadsorbent interaction and slow kinetics of adsorption.68-72 The CO2 uptake of dehydrated 2 can be commensurate to the interaction of quadrupolar CO2 with the electron rich azo (– N=N–) moiety and uncoordinated polar –NO2 group additively or individually.50 Dehydrated framework of 2 also shows 11cc/g and 5 cc/g for CO2 adsorption at 273 K and room 13 ACS Paragon Plus Environment


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temperature (i.e. at 298 K) respectively (Figure S24) with a moderate hysteresis in between the adsorption and desorption curve corroborating the strong adsorbate-adsorbent interaction.68-72 Using Clausius Clapeyron equation on the CO2 adsorption data of 2 measured at 273 and 298 K, the isosteric heat of CO2 adsorption (Qst) at zero loading has been calculated which is ~36 KJ/mol (Figure S25); the value reduces with the uploading of adsorbate suggesting the filling of their maximum affinity sites.73 The value of isosteric heat of CO2 adsorption (Qst) of 2 is comparable to the other MOFs reported 74,75 in literature at the same condition. The dehydrated framework of 3 exhibits type-V selective CO2 adsorption over N2 (Figures 7 and S26). In the dehydrated framework of 3, unidirectional guest water filled channels (Figure S22) that are exist only along b-axis. As discussed before, it is quite obvious to show nearly non porosity on N2 adsorption due to its larger size (kinetic diameter = 3.6 Å) compared to CO2 gas molecules (kinetic diameter = 3.3 Å) possibly due to the blocking of unidirectional channels by incoming N2 molecules. But the dehydrated 3, shows very appreciable uptake of CO2 at 195 K with type-V adsorption profile. It has been observed that there is no considerable uptake occurs up to 0.36 bar; after that a sharp uptake up to 119 cc/g at 1 bar pressure. This gate-opening phenomena76,77 in CO2 adsorption profile of 3 at P = 0.36 bar which may be corroborated to the expansion of flexible wavy 2D sheets present in 3. The adsorption profile generates a reversible incomplete desorption branch with a large hysteresis which can be ascribed to the similar adsorbate-adsorbent interaction 68-72 mentioned earlier. This highly selective CO2 adsorption for 3 can be explained in terms of strong interaction in between Lewis acidic C atom of quadrupolar CO2 molecule and Lewis basic polar azo (–N=N–) moiety of azbpy ligand and uncoordinated polar –NO2 groups. Though the –NO2 groups and –N=N– moities are pore oriented in both the dehydrated frameworks of 2 and 3, the larger pore size of 3 (10.8×3.8 Å2) in comparison to 2 (6×3 Å2) can be attributed simply for the higher uptake capacity of CO2 for 3. At 273 K and room temperature (i.e. at 298 K), the dehydrated framework of 3 exhibits 13 cc/g and 4 cc/g for CO2 adsorption respectively (Figure S27). Like 2 using Clausius Clapeyron equation the isosteric heat of CO2 adsorption for 3 has been calculated which (Qst) is 36 KJ/mol (Figure S28) and behaves similarly like 2 discussed above. For 3 the Qst value of CO2 adsorption is also comparable to the other reported MOFs at the same condition.74,75 Finally, the interdigitated framework of 4 exhibits negligible amount of surface adsorption for N2 and ~14 cc/g for CO2 at 195 K up to 1 bar pressure due to its nonporous nature (Figures 8 and S29). A small hysteresis was observed in between the adsorption and desorption profile of CO2 due to the interaction of


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quadrupolar CO2 with azo-functionalized framework. Like 2 and 3, it contains the void spaces in 2D sheet but may be compensated due to the presence of additional mono coordinated azbpy ligand into the pore and mutual interwovening of two 2D sheets. The dehydrated framework of 4 exhibits the CO2 adsorption capacity up to 8 cc/g and 1.4 cc/g at 273 K and 298 K (Figure S30) respectively with a moderate hysteresis corroborating the adsorbate-adsorbent interaction.68-72 Like 2 and 3, the calculation of isosteric heat of CO2 adsorption (Qst) for the dehydrated framework of 4 at zero coverage shows 47 KJ/mol (Figure S31) signifying moderately high CO2 interaction with the polar framework of 4;78 which gradually decreases with uploading of adsorbents due to the similar reason discussed above.73 On account to the solvent vapor (e.g H2O and EtOH) sorption, due to very common reason of smaller size and more polar nature of H2O than EtOH, all the dehydrated frameworks of compounds 2-4 exhibit selective H2O adsorption capacity over EtOH. Dehydrated 2 shows type-II adsorption profile (Figure S32) with a maximum uptake of 27 cc/g at P/P0=0.9 for water and 5 cc/g at P/P0=0.9 for EtOH respectively, with a large hysteresis and incomplete desorption branch; which possibly due to the aforesaid strong adsorbate-adsorbent interaction.68-72 Similarly, dehydrated 3 shows much selective water adsorption (i.e. ~212 cc/g) over EtOH (i.e. ~73 cc/g). It shows a typical type-V H2O (Figure S33) adsorption where no such uptake occurs up to P/P0=0.2; after that it sharply reaches up to 165 cc/g which may be due to the presence of open metal sites31 after activation at 150 oC. For both the vapor sorption profile, there occurs a hysteresis which may be aroused for adsorbate-adsorbent interaction.68-72 Finally, the dehydrated framework of 4 shows similar type of adsorption capacity for both polar solvents (Figure S34) such as H2O (~21 cc/g at 298 K and P/P0=0.9) and EtOH (~11 cc/g at 298 K and P/P0=0.9). In both the sorption isotherms of H2O and EtOH, 4 exhibits a large hysteresis in between the adsorption and desorption profile signifying the strong adsorbate-adsorbent interaction68-72 like previous two cases. Luminescence Study Recently, the luminescent properties of CPs with d10 metal centres has drawn a large interest owing to their potential application in chemical sensors,79,80 non linear optical (NLO) properties81,82

and

electroluminescence

display24-26

etc.

At

the

solid

state,

the

photoluminescence study has been performed for d10 complexes 1-3 and free azbpy ligand at room temperature; which are depicted in Figure 9. The azbpy ligand and complexes 1-3 exhibit the emission maxima at 760 nm (λex = 467 nm), 766 nm (λex = 480 nm), 765 nm (λex = 482 nm) and 764 nm (λex = 471 nm) respectively where they exhibit almost similar 15 ACS Paragon Plus Environment


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excitation spectra shown in Figure S35. It is cleared from Figure 9 that the complexes 1-3 show similar type of ligand based emission spectra which are slightly red shifted compared to the emission spectra of free N,N´-donor azbpy ligand. The emission spectrum of free azbpy ligand with aromatic skeleton may be due to the π*–n or π*–π transition; similarly complexes 1-3 exhibit their emission spectra through intra ligand charge transfer (ILCT) process from π*–n or π*–π probably due to having no possibility to occur the metal-centered MLCT/LMCT charge transfer, since the d10 metal ions are difficult to be reduced or oxidized.83-85 Basically, the enhancement of luminescent intensity in the CPs can be tuned through the different coordinating capability of metal ions with the N,N´-donor azbpy ligand by reducing the energy loss through the non-radiative decay.86 CONCLUSION Five azo-functionalized 5-nitro-1,3-benzenedicarboxylate based CPs of different transition metals have been successfully synthesized and characterised by IR, elemental analysis, PXRD and TGA measurement. It has been noticed that all the complexes exhibit completely different intriguing architecture of diverse dimensionality and topology along with their different functionality which are originated by changing the transition metal ions. The steric effect of –NO2 group exerts implication on the versatile coordination modes of NO2-bdc2coligand which might have facilitated the formation of diverse structures with different topology. Regarding the functionality, porous compounds 2 and 3 show their significant CO2 adsorption over N2 selectively along with H2O over EtOH which has been explored in the light of their framework structure. The appreciable amount of CO2 uptake by both the 2 and 3 have been occurred probably due to the strong interaction in between the electron rich Lewis basic azo (–N=N–) moiety of azbpy ligand and uncoordinated polar –NO2 groups with Lewis acidic CO2 molecule which has a quadruple moment. In compound 3, the water uptake capacity after P/P0=0.12 is sharply increases indicating the presence of open metal sites in the dehydrated framework of 3. Compound 4 shows lower although selective uptake capacity for CO2 over N2; due to its lower solvent accessible void. In summary, way of designing can provide a new window for constructing intriguing structural topology of CPs with polar pore wall, to adsorb CO2 selectively for their utilization in selective CO2 capture from industrial flue gas by carbon capture and sequestration (CCS) method. ASSOCIATED CONTENT Supporting Information


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The IR, PXRD patterns and TGA of compounds 1-5 along with different structural and application based Figures (i.e. Figures S1-S35) and tables related to the crystal structures (i.e. Tables S1-S10) reported in this paper are available as SI. Accession Codes CCDC

1468132−1468134,

1406215

and

1406216

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 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 DebajyotiGhoshal, 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/IC06/2014, grant to DG] and DRDO, Govt. of India (Grant No. ERIP/ER/1103938/M/01/1501 grant to DG). DKM acknowledges UGC for the research fellowship. Authors are grateful to Prof. K. K. Rajak for the photoluminescence study.

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Figure

Figure 1. (a) Coordination environment around the Zn(II) ions in 1; Zn (green), N (blue), O (red), C (black). (b) Diagram of 1D ladder in 1 (azbpy ligands are omitted for clarity). (c) Supramolecular 3D structure in 1 with the help of intermolecular π-π and H-bonding interactions. (d) Simplified topological representation of 3-connected uninodal wavy 1D ladder in 1.


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Figure 2. (a) Coordination environment around the Zn(II) ions in 2; Zn (green), N (blue), O (red), C (black). (b) Diagram of 1D metal carboxylate ladder in 2 along a-axis (azbpy ligands are omitted for clarity). (c) Diagram of bilayer 2D sheet in 2 with the help of both NO2-bdc2and azbpy ligands. (d) Simplified topological representation of (3,5)-connected binodal 2D bilayer sheet in 2.



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Figure 3. (a) Coordination environment around the Cd(II) ions in 3; Cd (green), N (blue), O (red), C (black). (b) Diagram of 1D metal carboxylate chain in 3 along a-axis (azbpy ligands are omitted for clarity). (c) Diagram of wavy 2D sheet in 3 with the help of both NO2-bdc2and azbpy ligands. (d) Simplified topological representation of 4-connected uninodal net in 3.


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Figure 4. (a) Coordination environment around the Mn(II) atoms in 4; Mn (green), N (blue), O (red), C (black). (b) Diagram of 1D metal carboxylate chain along a-axis (azbpy linkers are omitted for clarity). (c) Single 2D layer having pendent azbpy linkers. (d) Diagram of two interdigitated 2D layer in 4.



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Figure 5. (a) Coordination environment around the Co(II) ions in 5; Co (green), N (blue), O (red), C (black). (b,c) 1D chain of [Co(azbpy)0.5(NO2-bdc2-)(H2O)3] in ball-stick model (coordinated water molecules are omitted for clarity) and topological representation respectively in 5. (d,e) The 2D sheet of [Co(azbpy)(NO2-bdc2-)(H2O)2]in ball-stick model and topological representation of 5, respectively. (f) Overall diagram showing an intercalated 1D chain into the pore of 2D sheet in 5.

Figure 6. Selective sorption isotherms of CO2 (at 195 K) over N2 (at 77 K) by dehydrated 2. Filled symbols signify adsorption and the open symbols indicate desorption profile.


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Figure 9. Emission spectra of the complexes 1-3 with free azbpy ligand. Tables Table 1. Crystallographic and Structural Refinement Parameters for Complexes 1-5. 1 2 3 4 Formula C18H13N5O7Zn C18H17N5O9Zn C18H17N5O9Cd C28H19N9O6Mn Formula Weight 476.72 512.76 559.78 632.46 Crystal System Triclinic monoclinic Triclinic Monoclinic Space group Pī P2/c Pī P21 a/ Å 7.808(5) 10.1322(3) 10.2155(4) 9.9843(3) b/Å 9.127(5) 13.2675(3) 11.2411(5) 20.6625(6) c/ Å 14.502(5) 18.5298(5) 11.2815(5) 13.5839(4) α/° 98.101(5) 90 103.303(2) 90 β/° 92.463(5) 104.292(1) 108.634(2) 96.012(2) γ/° 110.479(5) 90 104.594(2) 90 V/ Å3 953.7(9) 2413.85(11) 1117.75(9) 2786.95(14) Z 2 4 2 4 –3 Dc/ g cm 1.660 1.262 1.657 1.507 µ /mm–1 1.342 1.055 1.035 0.535 F(000) 484 928 556 1292 θ range/° 1.4-27.6 1.5-27.5 2.0-27.8 1.8-27.5 Reflections collected 15778 40416 18481 48275 Unique reflections 4365 5554 5109 12767 Reflections I > 2σ(I) 3673 4515 4379 9896 Rint 0.068 0.041 0.031 0.052 goodness-of-fit (F2) 1.02 1.02 1.06 0.99 [a] R1 (I > 2σ(I)) 0.0666 0.0291 0.0442 0.0475 wR2(I > 2σ(I)) [a] 0.1943 0.0749 0.1207 0.1165 -1.62, 2.19 -0.30, 0.25 -0.62, 1.38 -0.37, 0.57 ∆ρ min / max /e Å3 [a] R1 = ΣFo–Fc/ΣFo, wR2 = [Σ (w (Fo 2 – Fc2 ) 2 )/ Σw (Fo 2 )2] ½. 28 ACS Paragon Plus Environment

5 C31H28 N8O17Co2 902.47 Triclinic Pī 11.6520(4) 13.2244(4) 13.3737(4) 103.482(1) 110.658(1) 94.442(1) 1846.02(10) 2 1.620 0.986 916 1.6- 27.5 29772 8444 6999 0.028 1.04 0.0448 0.1495 -0.54, 1.49

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Table of Contents Azo Functionalized 5-nitro-1,3-benzenedicarboxylate Based Coordination Polymers with Different Dimensionality and Functionality Dilip Kumar Maity, Arijit Halder, Saheli Ghosh, and Debajyoti Ghoshal*

Tuned syntheses of 5-nitro-1,3-benzenedicarboxylate based azo functionalized CPs with diverse topology viz. 1D ladder, 2D bilayer, wavy 2D sheet, interdigitated 2D sheet and polythreaded 2D sheet with intercalated 1D chain; have been synthesized and most of the compounds show interesting functionality like selective gas adsorption and represented ligand based emission.


Azo Functionalized 5-Nitro-1,3 ... - ACS Publications

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