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Auxiliary Ligand-Assisted Structural Variation of Cd(II) Metal-Organic Frameworks showing 2D#3D Polycatenation and Interpenetration: Synthesis, Structure, Luminescence Properties and Selective Sensing of Trinitrophenol DIVYENDU SINGH, and C. M. Nagaraja Cryst. Growth Des., Just Accepted Manuscript • Publication Date (Web): 01 Jun 2015 Downloaded from http://pubs.acs.org on June 4, 2015

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Crystal Growth & Design

Auxiliary Ligand-Assisted Structural Variation of Cd(II) Metal-Organic Frameworks

showing

2D→3D

Polycatenation

and

Interpenetration:

Synthesis, Structure, Luminescence Properties and Selective Sensing of Trinitrophenol Divyendu Singh and C. M. Nagaraja* Department of Chemistry, Indian Institute of Technology Ropar, Rupnagar 140001, Punjab, India.

Abstract Four

new

metal-organic

frameworks

[{Cd(fdc)(bpee)1.5}.3(H2O)](2),

(MOFs)

of

Cd(II)

[Cd(fdc)(3bpdb)(H2O)](3)

2(4bpdb)3}.1.5(4bpdp).2(H2O)](4)

ion,

[Cd(fdc)(bipy)1.5](1), and

[{Cd2(fdc)-

(where, fdc = 2, 5-furandicarboxylate dianion, bipy = 4,4'-

bipyridine, bpee = 1,2-bis(4-pyridyl)ethylene, 3bpdb = 1,4-bis(3-pyridyl)-2,3-diaza-1,3butadiene and 4bpdb = 1,4-bis(4-pyridyl)-2,3-diaza-1,3-butadiene) have been successfully synthesized using mixed ligand systems and characterized by single crystal X-ray analysis and other physicochemical studies. Structural determination revealed that compounds 1 and 4 possess a rare 2D→3D polycatenated pillared-bilayer structure with {48.62} SP 2-periodic net topology. Compound 2 has a novel 3-fold interpenetrating 3D hexagonal framework structure with {46.64} sqc-net topology. Whereas, compound 3 features a 2D Zig-Zag network with {44.62} sql-net topology. Interestingly, compounds 1 and 4 are the first examples of Cd(II) MOFs based on fdc ligand and bipy/4bpdb spacers exhibiting an unusual 2D→3D polycatenation of bilayers. Photoluminescence investigation revealed emissions from compounds 1-4 owing to ligand based charge transfer (n→π* and π→π*) transitions. The luminescent emission of 2 can be quenched by addition of trace amount of 2,4,6-trinitrophenol (TNP), demonstrating its potential application for highly selective and sensitive detection of TNP. The influence of N,N'-donor spacers on the structure, topology and the functionality of the resulting MOFs has been demonstrated.

1

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Introduction Porous coordination polymers (PCPs) or metal-organic frameworks (MOFs) have attracted an immense attention during the past few years not only due to their fascinating capability to form diverse structural architectures but also for their novel properties.1-7 The high surface areas, tailored pore size and the functionality makes MOFs useful materials for hydrogen storage,8-13 carbon dioxide capture,14-20 separation,21-26 catalysis,27-33

magnetism34-40

as well as drug

delivery41-43 applications. MOFs exhibiting interesting luminescent property are of high significance for the development of chemo sensors for various sensing applications including selective sensing of nitro explosives.44-47 Rapid detection of nitroaromatic explosives is important for the sake of environmental safety and national security.48-50 Particularly, selective detection of TNP has gained a special interest since it possesses high explosive power and has been identified as a toxic pollutant. It's ingestion can cause various health problems including liver or kidney damage, aplastic anemia, cyanosis as well as damage to respiratory organs and skin irritation.51-56 Therefore, it is highly desirable to develop materials for selective and sensitive detection of TNP. However, selective detection of TNP in the presence of other competing nitroaromatics is quite challenging due to their strong electron affinities.57-61 It has been well established that the auxiliary ligands play an important role in directing the structure, topology and the functionality of the resulting MOFs. In this regard, several multidentate ligands have been employed as building blocks for the construction of designer MOF materials. Among the various ligands, aliphatic and aromatic carboxylic acids having versatile coordination modes are the most preferred ones to construct MOFs with diverse structure and topology.62-67 Furthermore, it has been observed that the use of rigid, long chain organic ligands or secondary building units (SBUs) has often resulted in the formation of interpenetrated networks which are attracting much attention due to their intriguing artistic and the structure dependent properties.68-73 The origin of interpenetration in a framework has been attributed to the presence of large voids in it and generally the interpenetration of nets leads to high stability/rigidity of the framework. On the otherhand, polycatenation of low dimensional (1D/2D) structures has also known to generate high-dimensional (3D) MOFs with high stability.74-75 It is interesting to note that the interpenetration and the polycatenation are quite different in the sense, the component motifs in polycatenation have lower dimensionality compared to that of resulting framework, while the component motifs in interpenetration have 2


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the same dimensionality as that of the resulting MOF. Therefore, polycatenation can improve the dimensionality of the framework, while the interpenetration can increase the stability/rigidity of the framework. Literature study revealed that the reports of polycatenation of bilayers into 3D framework are rare.76-77 In this context, we were interested to see the results of the combination of Cd(II) ion with a rigid dicarboxylate ligand, 2,5-furandicarboxylic acid (H2fdc) and various bipyridine spacers.78-82 Herein, we report the syntheses, structures and luminescent property of four new MOFs of Cd(II) ion, [Cd(fdc)(bipy)1.5] (1), [{Cd(fdc)(bpee)1.5}.3(H2O)](2), [Cd(fdc)(3bpdb)(H2O)](3) and [{Cd2(fdc)2(4bpdb)3}.1.5(4bpdp).2(H2O)] (4) (fdc = 2, 5-furandicarboxylate dianion, bipy = 4,4'-bipyridine, bpee = 1,2-bis(4-pyridyl)ethylene, 3bpdb =1,4-bis(3-pyridyl)-2,3-diaza-1,3butadiene and 4bpdb = 1,4-bis(4-pyridyl)-2,3-diaza-1,3-butadiene) obtained using solvothermal condition (Scheme 1). Single crystal structural analysis revealed that compounds 1 and 4 possess a rare 2D→3D polycatenated pillared-bilayer structure with {48.62}, SP 2-periodic net topology. Compound 2 has a novel 3-fold interpenetrating 3D hexagonal framework structure with {46.64} sqc-net topology. Whereas, compound 3 shows a 2D Zig-Zag network structure with {44.62} sqlnet topology. To the best of our knowledge compounds 1 and 4 are the first examples of Cd(II) MOFs constructed from fdc ligand and bipy/4bpdb spacers exhibiting polycatenation of bilayers. Photoluminescence investigation revealed emissions from all the four compounds owing to intraligand charge transfer (n→π* and π→π*) transitions. Furthermore, the luminescence emission of 2 can be quenched by addition of trace amount of 2,4,6-trinitrophenol (TNP) demonstrating its potential application for highly selective and sensitive detection of TNP. The present study demonstrates that the nature of auxiliary ligand has profound effect on the structure, topology and the functionality of the resulting MOFs. Experimental Section Materials All the reagents employed were commercially available and used as provided without further purification. The Cd(NO3)2.4H2O, 2,5-furandicarboxylicacid (fdcH2), bipy = 4,4'-bipyridine (bipy) and 1,2-bis(4-pyridyl)ethylene (bpee) were obtained from Sigma Aldrich chemical Co. 1,4-bis(3-pyridyl)-2,3-diaza-1,3-butadiene (3bpdp) and 4bpdb = 1,4-bis(4-pyridyl)-2,3-diaza-1,3 -butadiene were synthesized following the previously reported procedure.83-84

3



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Physical Measurements Elemental analyses of C, H and N were carried out using a Thermo Fischer Flash 2000 Elemental Analyzer. IR spectra were recorded as KBr pellets on a Thermo ScientificNicolet iS10 FT-IR Spectrometer in the region 4000-400 cm-1. Thermogravimetric analyses (TGA) of the compounds were carried out using Metler Toledo Thermogravimetric analyzer in nitrogen atmosphere (flow rate of 50 mL min-1) in the temperature range of 30 – 600°C (heating rate of 5°C min-1) for 1/4 and with temperature range of 30 - 550 °C (heating rate of 3°C min-1) for 2/3. The phase purity of the as-prepared compounds was confirmed by powder XRD recorded on a PANalytical’s X’PERT PRO diffractometer using CuKα radiation (k = 1.542 Å; 40 kV, 20 Ma). Photoluminescence spectra were recorded at room temperature on a Perkin Elmer LS55 fluorescence spectrophotometer. Synthesis of [Cd(fdc)(bipy)1.5](1) Compound 1 was synthesized by employing solvothermal condition at 100°C. Cd(NO3)2.4H2O (0.060 g, 0.20 mmol) was dissolved in 4 ml of deionized water to which an aqueous solution (2ml) of H2fdc (0.031 g, 0.20 mmol) neutralized with NaOH (0.016 g, 0.4 mmol) was added drop wise with constant stirring. To this solution an ethanolic solution (2 ml) of bipy (0.031 g, 0.20 mmol) was added and the contents were stirred for 30 min and then taken in a 30 ml glass vial sealed with parafilm and heated at 100 oC for 2 days. After being cooled to room temperature, colorless needle like crystals of 1 were isolated. Yield ~55 % based on Cd(II) ion. Phase purity was confirmed by comparing the PXRD patterns of the as-synthesized sample and a simulated one from the single crystal X-ray data (Figure S1, Supporting Information). Anal. Calcd. for C21H14N3O5Cd: C, 50.32; H, 2.79; N, 8.38. Found: C, 50.54; H, 2.86; N, 8.32. IR (KBr disc, ν cm-1): 3060(w), 1615(s), 1602(s), 1556(s), 1440(s), 1335(s), 1245(s), 815(s), 760(s) cm-1. Synthesis of [{Cd(fdc)(bpee)1.5}.3(H2O)](2) Compound 2 was synthesized following the same procedure as that of 1 using bpee (0.036 g, 0.20 mmol) in the place of bipy. The colorless rectangular shaped crystals of 2 were isolated. Yield ~ 60 % based on Cd(II) ion. Phase purity was confirmed by comparing the PXRD patterns of the as-synthesized sample and a simulated one from the single crystal X-ray data (Figure S2, Supporting Information). Anal. Calcd. for C24H17N3O8Cd: C, 48.74; H, 3.38; N 7.10. Found: C, 48.69; H, 3.32; N, 7.18. IR (KBr disc, ν cm-1): 3401 (bw), 3100 (w), 1659 (s), 1602(s), 1583(s), 1402(s), 1368(s), 1252(s), 819(s), 787(s) cm-1. 4


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Synthesis of [Cd(fdc)(3bpdb)(H2O)](3) Compound 3 was synthesized following the same procedure as that of 1 using 3bpdb (0.042 g, 0.20 mmol) in the place of bipy. After being cooled to room temperature, yellow colored crystals of 3 were isolated. Yield ~ 65% based on Cd(II) ion. Phase purity was confirmed by comparing the PXRD patterns of the as-synthesized sample and a simulated one from the single crystal Xray data (Figure S3, Supporting Information). Anal. calcd. for C18H14N4O6Cd : C, 43.65; H, 2.82; N, 11.30. Found: C, 43.73; H, 2.75; N, 10.93. IR (KBr disc, ν cm-1): 3335(bw), 3099(w),1678(s), 1626(s), 1586( s), 1429(s), 1370(s), 1309(s), 819(s), 795(s). Synthesis of [{Cd2(fdc)2(4bpdb)3}.1.5(4bpdp).2(H2O)](4) Compound 4 was synthesized following the same procedure as that of 1 using 4bpdb (0.042 g, 0.20 mmol) in the place of bipy. After being cooled to room temperature, yellow colored crystals of 4 were isolated. Yield ~ 45 % based on Cd(II) ion. Phase purity was confirmed by comparing the PXRD patterns of the as-synthesized sample and a simulated one from the single crystal Xray data (Figure S4, Supporting Information). Anal. Calcd. for C66H49N18O12Cd2: C, 52.20; H, 3.62; N, 16.61. Found: C, 52.36; H, 3.88; N, 16.79. IR (KBr disc, ν cm-1): 3050(w),1667(s), 1620(s), 1570( s), 1450(s), 1345(s), 1290(s), 805(s), 790(s). X-ray Crystallography X-ray single crystal structural data of compounds 1-4 were collected on a Bruker D8 Venture PHOTON 100 CMOS diffractometer equipped with a INCOATEC micro-focus source and graphite monochromated Mo Kα radiation (λ = 0.71073 Å) operating at 50 kV and 30 mA. The program SAINT85 was used for integration of diffraction profiles and absorption correction was made with SADABS86 program. All the structures were solved by SIR 9287 and refined by full matrix least square method using SHELXL-201388 and WinGX system, Ver 2013.3.89 All the non hydrogen atoms were located from the difference Fourier map and refined anisotropically. All the hydrogen atoms were fixed by HFIX and placed in ideal positions and included in the refinement process using riding model with isotropic thermal parameters. Compounds 3 and 4 show residual peaks of 2.91, 2.83 e/A3and 2.47, 4.09 e/A3 respectively appearing near the Cd(II) ion which can be ascribed due to absorption. Potential solvent accessible area or void space was calculated using the PLATON multipurpose crystallographic software.90 All the crystallographic and structure refinement data of compounds 1-4 are summarized in Table 1. Selected bond lengths and angles are given in Table 2. 5



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Results and discussion Compounds 1-4 were synthesized by solvothermal reaction of Cd(II) ion with deprotonated H2fdc ligand and N,N'-donors (bipy, bpee, 3bpdp and 4bpdp, respectively) at 100°C (Scheme 1). Crystal structure of [Cd(fdc)(bipy)1.5](1) Compound 1 crystallizes in the monoclinic crystal system with the P21/n space group. X-ray structure determination reveals a rare 2D→3D polycatenated pillared-bilayer structure. The asymmetric unit consists of a Cd(II) ion, a fdc dianion and one and half molecules of bipy spacers (Figure 1). The Cd(II) ion is in a distorted pentagonal bipyramidal geometry with CdO4N3 chromophore satisfied by four oxygen (O1, O2, O3, O4) atoms of a bridging fdc and three 4-pyridyl nitrogen (N1, N2 and N3) atoms of one and half molecules of bipy spacer (Figure 1). The Cd-O and Cd-N bond lengths are in the range 2.395(3) - 2.449(3) Å and 2.347(3)2.409(3) Å, respectively (Table 2). Here, the fdc dianion coordinates to Cd(II) ion through chelating bidentate fashion forming a [Cd-fdc-Cd]n 1D chain that are connected by bipy spacers to generate 2D, [Cd(fdc)(bipy)] network (Figure 2a). Two such 2D layers are further pillared by bipy spacers via Cd-N bonds to generate pillared-bilayer network (Figure 2b). As it can be seen from Figure 2c the 2D pillared-bilayer network houses large rectangular 1D channels of dimension ~ 6.16 × 6.63 Å2 along the crystallographic b–axis. Interestingly, the 2D pillaredbilayers undergo polycatenation into 3D framework (Figure 2c) stabilized by C-H…π interactions between the benzene rings of adjacent bilayers with a distance of ~3.5Å (Figure 2d). From the topological view, if one assumes the carboxylate coordination to Cd(II) ion as monodentate then each Cd(II) center acts as 5-connected node and the overall structure has the SP 2-periodic net topology with Schläfli point symbol {48.62} as revealed from TOPOS91 analysis (Figure 2e). The distance between the two adjacent Cd….Cd centers along Cd…fdc…Cd and Cd…bipy…Cd are 9.885 Å and 11.933 Å, respectively. Due to the polycatenated nature, the structure is nonporous and shows a negligible solvent accessible void volume of 2.8% (2020.5 Å3) determined by PLATON90 analysis. Structural description of [{Cd(fdc)(bpee)1.5}.3(H2O)](2) Compound 2 crystallizes in the monoclinic crystal system with the P21/c space group. Single crystal X-ray structure determination reveals a 3-fold interpenetrating 3D framework structure constituted by fdc dianion and bpee spacer. The asymmetric unit consists of a Cd(II) ion, a fdc dianion, one and half molecules of bpee spacers including three guest water molecules (Figure 6


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3). The Cd(II) ion is in a distorted pentagonal bipyramidal geometry with CdO4N3 chromophore fulfilled by four oxygen (O1, O2, O3 and O4) atoms from two chelated carboxylate groups of the fdc ligand and three 4-pyridyl nitrogen (N1, N2 and N3) atoms from one and half molecules of bpee spacers (Figure 3). The Cd-O/N bond lengths vary from 2.305(8) to 2.559(8)Å (Table 2). Similar to compound 1 the fdc dianion bridges two Cd(II) ions through chelating bidentate fashion and if one assumes the carboxylate coordination as monodentate, then each Cd(II) centre acts as 5-connecting node which are connected with each other by fdc and bpee spacers in three dimensions to

generate a 3D hexagonal framework (Figure 4a). Topological analysis by

TOPOS91 confirms the presence of 5-connected Cd(II) nodes and the overall structure has hexagonal sqc-net topology with Schläfli point symbol {46.64}. As shown in the Figure 3b the 3D hexagonal framework houses large 1D channels with dimension of ~8.33 x 20.04Å2 in the crystallographic a-direction. The void space is sufficiently large enough to facilitate interpenetration of two other hexagonal nets into the empty space to generate a 3-fold interpenetrating 3D framework (Figure 4b and c). Interpenetration analysis by TOPOS91 confirms the presence of 3-fold interpenetration with the Schläfli vertex symbol [4.4.4.4.4.4.6(3).6(3).6(3).6(3)] (Figure 4d). Due to interpenetration the pore size reduces significantly resulting an effective solvent accessible void volume of barely ~6.3% (2566 Å3) calculated using PLATON90 after the removal of guest molecules. Structural description of [Cd(fdc)(3bpdb)(H2O)](3) Compound 3 crystallizes in the triclinic crystal system with the Pī space group. X-ray structure determination reveals a 2D network constituted by a bridging fdc ligand and a 3bpdb spacer. The asymmetric unit comprising of a Cd(II) ion, a fdc dianion and a 3bpdb spacer including one coordinated water molecule (Figure 5a). Similar to compounds 1 and 2, the Cd(II) ion is in a distorted pentagonal bipyramidal geometry with CdO5N2 chromophore completed by four oxygen (O1, O2, O3 and O4) atoms from two chelated carboxylate groups of the fdc ligand and an oxygen (O1w) atom of a water molecule forming the pentagon. The axial positions are occupied by two 3-pyridyl nitrogen (N1 and N2) atoms of a 3bpdb spacer (Figure 5a). The Cd-O bond lengths vary from 2.230(4) to 2.488(4) Å and the Cd-N bond lengths vary from 2.342(5) to 2.366(5) Å (Table 2). Here again the fdc dianion coordinates to Cd(II) ion through chelating bidentate fashion forming a [Cd-fdc-Cd]n 1D chain, which are further connected by 3bpdb spacers to form a 2D Zig-Zag network (Figure 5b). These 2D sheets are further stacked with 7



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each other in the crystallographic a-direction via π-π interactions to generate a 3D supramolecular framework (Figure 5c). From the topological view, each Cd(II) center acts as 4connecting node and the overall structure has the 2D sql-net topology with Schläfli point symbol {44.62} revealed by TOPOS91 analysis (Figure 5d). The distance between the two adjacent Cd….Cd centers along Cd…fdc…Cd and along Cd…3bpdb…Cd are 9.96 Å and 12.63 Å, respectively. Crystal structure of [{Cd2(fdc)2(4bpdb)3}.1.5(4bpdp).2(H2O)](4) Compound 4 crystallizes in the monoclinic crystal system with the P21/c space group. X-ray structure determination reveals the presence of 2D→3D polycatenated bilayer structure similar to that of compound 1. The asymmetric unit consists of two Cd (II) ions, two fdc dianions and three 4bpdb spacers including guest molecules of one and half 4bpdp spacers and two water molecules (Figure 6). The Cd1/Cd2 ions are in a distorted pentagonal bipyramidal geometry with CdO4N3 chromophore satisfied by four oxygen (O1, O2, O3, O4)/(O5, O6, O7, O8) atoms from two bridging fdc and three 4-pyridyl nitrogen (N1, N2 and N3)/(N4, N5, N6) atoms of three 4bpdb spacers, respectively (Figure 6). The Cd-O and Cd-N bond lengths are in the range 2.323(7) - 2.561(6) Å and 2.331(7)-2.389(7) Å, respectively (Table 2). Similar to compounds 13, the fdc dianion brides Cd(II) ions through chelating bidentate fashion forming a [Cd-fdc-Cd]n 1D chains, that are linked by 4bpdb spacers to form 2D sheet (Figure. 7a). Two such 2D sheets are further pillared by 4bpdb spacers to generate pillared-bilayer network (Figure.7b). As observed in compound 1, polycatenation of the 2D bilayers results in a 3D framework stabilized by C-H…π interactions involving the benzene rings of different bilayers (Figure S5). From the topological view each Cd(II) ion acts as a 5-connecting node and the overall structure has SP 2periodic net topology with Schläfli point symbol {48.62} analyzed by TOPOS91 (Figure 7c). The distance between the two adjacent Cd….Cd centers along Cd…fdc…Cd and along Cd…4bpdb…Cd are 9.936Å and 16.099 Å, respectively. Effects of N, N'-donor spacers on the structures of MOFs 1-4 It is noteworthy that the auxiliary ligands play a significant role in directing the structure and topology of the resulting MOFs. Here, four different N,N'-donor (bipy, bpee, 3bpdp and 4bpdp) spacers were selected to investigate their effects on the structure, dimensionality and the topology of the resulting frameworks 1-4 (Scheme 1). In compounds 1, 2 and 4 the Cd(II) ion acts as 5-connecting node, while in 3 it is 4-connected. The use of bipy/4bpdb spacers resulted 8


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the formation of porous 2D pillared-bilayer network which undergo polycatenation to generate nonporous 3D polycatenated framework. Whereas the use of bpee spacer resulted a 3D hexagonal framework which houses large 1D hexagonal channels and the void space available is sufficiently large enough to assist nucleation of two other 3D nets to generate a novel 3-fold interpenetrating 3D framework, 2. On the other hand, use of an angular spacer, 3bpdp resulted a non-interpenetrating 2D network structure. Thus the nature of ancillary ligand has profound effect on the structure, dimension and the topology of the resulting frameworks. It is to note that interpenetration and polycatenation are two types of entanglements commonly observed in MOFs. Mainly, two classes of 2D→3D polycatenated arrays have been reported, in the first, 2D layers are interlaced in a parallel fashion by stacking with their offsets and mostly observed in the 2D bilayers or 2D periodic nets, while in the second class the layers are inclined by a certain nonzero angle.92-93 Further, polycatenation in a single layer differ from that of bilayers in the sense: for a single planar layer, the dihedral angle (α) between the layers should be in the range of 0 < α ≤ 90 and it is the inclined or staggered polycatenation while, for a bilayer the dihedral angles between the layers should fall in the range of 0 ≤ α ≤ 90 and if α = 0 represents the condition of parallel polycatenation.76 More importantly, the reports of 2D→3D polycatenated bilayers are rare and most of these arrays possess mainly hcb or sql net topology.92 In this regard, compounds 1 and 4 described here represents the rare examples of 2D→3D parallel polycatenated arrays with SP 2-periodic net topology. Luminescent Properties Luminescent properties of MOFs based on d10 metal ions are of special interest due to their prospective applications in electroluminescent displays, nonlinear optical (NLO) devices and so on.94-96 In this context, luminescent emission of 1-4 dispersed in different organic solvents such as ethanol, methanol, acetonitrile and dimethyl formamide (DMF) were investigated. Among these solvents, ethanolic dispersion of 1-4 shows an intense emission (Figure. 8) and since ethanol is considered as a green solvent compared to those of other solvents used, we chose ethanol solvent to investigate the luminescent properties of MOFs. As shown in Figure 8, compounds 1 and 2 exhibit luminescent emissions around 420-430 nm, while 3 and 4 shows emissions around 460 nm (λ

ex

= 300 nm) which can be assigned due to ligand based charge

(n→π* and π→π*) transfer transitions. Further, 2 exhibits an intense emission which might be 9



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due to the high rigidity of the framework arising due to 3-fold interpenetration. To investigate the potential application of 2 for selective sensing of nitro explosives, the luminescence spectra were recorded with addition of various nitro analytes. Therefore, the emission spectra were recorded with addition of different nitro analytes (0.02 M in ethanol) such as, nitromethane (NM), nitrobenzene (NB), 1,3-dinitrobenzene (DNB), 2,6-dinitrotoulene (DNT), 2,4,6-trinitrotoluene (TNT), 1,3,5-trinitroperhydro-1,3,5-triazine (RDX) and 2,4,6-trinitrophenol (TNP). Interestingly, the emission spectra shows that different nitro analytes exhibit different fluorescence quenching efficiency towards the emission intensity of 2 that can be attributed to their different electron deficient nature.45 Amongst all the nitro analytes, TNP exhibits the highest fluorescence quenching of the emission intensity of 2 (Figure S6). Further, to test the sensitivity of 2 for TNP the emission spectra were recorded with addition of increasing concentration of TNP. As shown in the Figure.9 the fluorescence intensity of 2 decreases gradually with increasing concentration of TNP indicating its high sensitivity of fluorescence quenching. Significantly, even small (60 µM) amount of TNP was good enough to quench ~ 83% of the fluorescence intensity of 2. The Stern-Volmer (SV) quenching constant (Ksv) estimated from the plot of relative fluorescence intensity (I₀/I) against the concentration of nitro analytes and by applying linear curve fitting was found to be 6.64 X 104 M-1 (Figure S7). The estimated value of Ksv is comparable to the value reported for MOF based sensors for selective detection of TNP.97 The limit of detection (LOD) for TNP by MOF(2) dispersed in ethanol was found to be 1.145 ppm (5µM) (Figure S8). The mechanism of fluorescence sensing of TNP by 2 can be attributed due to photoinduced electron transfer from an excited MOF to the electron deficient TNP adsorbed on the surface of the MOF through interspecies contacts.98-101This observation is in line with the mechanism of fluorescence quenching through donor-acceptor electron transfer. Furthermore, the observed non-linear trend of the SV plot for TNP indicates that the mechanism of fluorescence quenching occurs through resonance energy transfer. Higher the spectral overlap between the absorbance spectrum of the analyte and the emission spectrum of the MOF, greater the probability of energy transfer and hence fluorescence quenching. In accordance with this the highest spectral overlap is observed between absorption spectrum of TNP with emission spectrum of 2 (Figure 10) supporting the high selectivity for TNP. The above observations clearly support that 2 can act as highly selective and sensitive sensor for the detection of TNP even in the presence of other competing nitro analytes. Furthermore, the detection ability of 2 can be restored and the MOF 10


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can be recycled for significant number of cycles by centrifuging the dispersed solution after use and washing several times with ethanol. Remarkably, the initial fluorescence intensity of 2 was almost regained even after four cycles indicating a high photostability and recyclability for detection applications (Figure S9). Moreover, powder XRD analysis of the isolated samples of 2 after immersing in different nitro analytes revealed that the original framework structure is retained (Figure S10).

Thermal stability of compounds 1-4 TGA of 1 shows that the framework is stable up to 300 ºC and a major weight loss of ~78% observed in the temperature regime 300 - 400 ºC corresponding to the loss of one and half bipy molecules and a fdc ligand (calc. wt % 77.8) (Figure S11). TGA of 2 shows a weight loss of ~8.8 % around 75-120 ºC corresponding to the loss of three guest water molecules (calc. wt % 9.1) and the dehydrated sample, [Cd(fdc)(bpee)1.5] is stable up to 280 ºC. The second weight loss of ~26 % was observed in the temperature regime 280-320 ºC which corresponds to the loss of one fdc ligand (calc. wt % 26.5). The third weight loss of ~ 46% was observed in the temperature regime 320-430 ºC corresponding to loss of bpee linker (calc. wt% 46.2) (Figure S12). TGA of 3 shows a weight loss of ~3.7% around 150 ºC which can be assigned due to the loss of one coordinated water molecule (calc. wt% 3.8) and the dehydrated sample is stable up to 250 ºC. The second major weight loss of ~76.7% was observed in the temperature regime 280-380 ºC, corresponding to loss of 3bpdb spacer and the fdc ligand (calc. wt% 74.0) (Figure S12). TGA of 4 shows a weight loss of ~2.5% around 105 ºC corresponding to the loss of two guest water molecules (calc. wt % 2.4). The second weight loss of ~62% in the temperature regime 125 – 350 ºC was observed corresponding to the loss of one and half molecules of guest and three coordinated 4bpdb spacers (calc. wt % 62.6). The third weight loss ~20% was observed around the temperature range 360 – 460 ºC due to the loss of two fdc molecules (calc. wt % 20.6) (Figure S11). Thus compound 1 exhibits relatively higher thermal stability (300 ºC) compared to 2-4, while the desolvated samples of 2 and 3 show moderate thermal stability (290 ºC) and 4 shows the least thermal stability (105 ºC). The above analysis suggests that the loss of coordinated solvent or guest molecules (in case of 2-4) results in lowering of the thermal stability of the compounds. Generally the coordinated solvent molecules and the guest molecules of crystallization are known to stabilize framework structure and their release leads to loss of the framework structure and resulting in lowering of thermal stability. 11



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Conclusions In summary, we have constructed four new MOFs of Cd(II) ion using mixed ligand systems and structurally characterized them. Structural analyses revealed that compounds 1 and 4 possess a rare 2D→3D polycatenated pillared-bilayer framework structure. Compound 2 has a novel 3-fold interpenetrating 3D hexagonal framework structure, while 3 exhibits a 2D Zig-Zag network structure. Remarkably, compounds 1 and 4 are the first examples of Cd(II) MOFs constituted by fdc ligand and bipy/4bpdp spacers exhibiting polycatenation of 2D bilayers. Herein, we have shown that depending upon the nature of N,N'-donor spacer used one can construct MOFs which can exhibit interesting structural architectures such as, polycatenation or interpenetration of networks. All the four compounds exhibit luminescence emission owing to ligand based charge transfer (n→π* and π→π*) transitions. Remarkably, the emission of 2 can be selectively quenched by the addition of small quantity of TNP demonstrating the potential application of 2 for highly selective and sensitive detection of nitroexplosive. The present study demonstrates the influence of auxiliary ligands on the structure, topology and the functionality of the resulting MOFs. Supporting Information Experimental and simulated PXRD patterns for the compounds, fluorescence quenching spectra, and TGA plots of the compounds. This material is available free of charge via the internet at http://pubs.acs.org/. The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.cgd.5b00272. ACKNOWLEDGMENTS C.M.N gratefully acknowledges the financial support from the Department of Science and Technology (DST), Government of India (Fast Track Proposal). D.S is thankful to the Council of Scientific and Industrial Research (CSIR), Government of India for the JRF. References (1) Yaghi,O. M.; O'Keeffe, M.; Ockwig, N. W.; Chae, H. K.; Eddaoudi, M.; Kim, J. Nature 2003, 423, 705. (2) Moulton, B.; Zaworotko, M. J. Chem. Rev. 2001, 101, 1629. 12


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Scheme 1. Synthesis scheme of compounds 1-4.

18


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Table 1.Crystal data and structure refinement parameters for compounds 1-4

a

Parameters

1

Chemical formula Formula mass 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) T(K) λ(Mo Kα)(Å) θmin(°) θmax(°) Total data Unique data Rint Data[I>2σ(I)] a R1 b wR2 S

C21H14N3O5Cd 500.76 Monoclinic P21/n 11.7783(9) 9.8847(7) 18.2783(15) 90.00 108.294(3) 90.00 2020.5(3) 4 1.646 1.119 996 293 0.71073 2.3 28.3 57886 5030 0.082 3774 0.0441 0.0840 1.09

2

3

C24H17N3O8Cd 587.82 Monoclinic P21/c 10.010(5) 14.104(5) 18.255(5) 90.00 95.281(5) 90.00 2566.3(17) 4 1.521 0.902 1176 293 0.71073 2.2 28.4 35677 6148 0.167 3781 0.0852 0.2404 1.14

C18H14N4O6Cd 494.74 Triclinic Pī 5.9502(3) 9.9575(5) 16.4531(9) 74.197(2) 84.359(2) 89.099(2) 933.38(8) 2 1.760 1.215 492 293 0.71073 2.6 28.3 29958 4608 0.054 4309 0.0461 0.1314 1.26

R1 = ∑║F0│-│Fc║/∑│Fo│, bwR2 = [ ∑w(Fo2-Fc2)2/∑w(Fo2)2]1/2

19


4 C66H48N18O12Cd2 1511.05 Monoclinic P21/c 22.317(5) 9.927 (5) 31.901(5) 90 109.521(5) 90 6661(4) 4 1.507 0.714 3052 150 0.71073 2.3 28.4 114508 16613 0.099 12358 0.0813 0.1820 1.16


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Table 2. Selected band lenths (Å) and angles (°) for compounds 1-4. Cd1-O1 Cd1-O2 Cd1-O3 Cd1-O4 Cd1-N1 Cd1-N2 Cd1-N3

2.437(3) 2.433(3) 2.449(3) 2.395(3) 2.361(4) 2.409(3) 2.347(3)

Cd1-O1 Cd1-O2 Cd1-O3 Cd1-O4 Cd1-N1 Cd1-N2 Cd1-N3

2.559(8) 2.305(8) 2.555(8) 2.385(8) 2.361(9) 2.337(9) 2.363(10)

O1-Cd1-O2 O1-Cd1-O3 O1-Cd1-O4 O1-Cd1-N1 O1-Cd1-N2 O1-Cd1-N3 O2-Cd1-O3 O1-Cd1-O2 O1-Cd1-O3 O1-Cd1-O4 O1-Cd1-N1 O1-Cd1-N2 O1-Cd1-N3 O2-Cd1-O3

53.56(9) 132.47(10) 78.81(10) 90.48(11) 140.19(10) 90.83(9) 70.96(11) 52.7(3) 168.2(2) 138.3(3) 98.8(3) 84.0(3) 89.1(3) 138.9(2)

Cd1-O1 2.488(4) Cd1-O2 2.332(4) Cd1-O3 2.485(4) Cd1-O4 2.390(4) Cd1-O1w2.230(4) Cd1-N1 2.342(5) Cd1-N2 2.366(5)

O1-Cd1-O1w83.12(16) O1-Cd1-O2 54.78(13) O1-Cd1-O3 165.57(12) O1-Cd1-O4 138.42(13) O1-Cd1-N1 81.49(13) O1-Cd1-N2 104.65(14) N1-Cd1-N2 72.12(16)

Cd1-O1 Cd1-O2 Cd1-O3 Cd1-O4 Cd1-N1 Cd1-N2 Cd1-N3 Cd2-O5 Cd2-O6 Cd2-O7 Cd2-O8 Cd2-N4 Cd2-N5 Cd2-N6

O1-Cd1-O2 O1-Cd1-O3 O1-Cd1-O4 O1-Cd1-N1 O1-Cd1-N2 O1-Cd1-N3 O2-Cd1-O3 O2-Cd1-O4 O2-Cd1-N1 O2-Cd1-N2 O2-Cd1-N3 O3-Cd1-O4 O3-Cd1-N1 O3-Cd1-N2

2.369(5) 2.524(5) 2.578(5) 2.309(5) 2.360(6) 2.338(5) 2.327(6) 2.375(5) 2.456(5) 2.479(4) 2.383(5) 2.384(5) 2.358(5) 2.349(5)

54.67(16) 136.35(16) 83.51(17) 89.2(2) 91.0(2) 137.29(19) 168.77(15) 137.87(17) 95.98(19) 88.10(19) 82.73(18) 52.89(16) 87.15(18) 89.37(18)

Compound 1 N1-Cd1-N2 N1-Cd1-N3 O2-Cd1-O4 O2-Cd1-N1 O2-Cd1-N2 O2-Cd1-N3 O3-Cd1-O4

88.48(10) 177.10(11) 132.31(9) 87.16(13) 86.64(10) 91.53(11) 53.96(10)

O3-Cd1-N1 O3-Cd1-N2 O3-Cd1-N3 O4-Cd1-N1 O4-Cd1-N2 O4-Cd1-N3 N2-Cd1-N3

86.10(13) 87.16(10) 94.93(11) 91.36(12) 141.00(10) 91.44(11) 88.86(9)

Compound 2 O2-Cd1-O4 O2-Cd1-N1 O2-Cd1-N2 O2-Cd1-N3 O3-Cd1-O4 O3-Cd1-N1 O3-Cd1-N2

85.7(3) 91.8(3) 136.7(3) 92.0(3) 53.2(2) 83.7(3) 84.4(3)

O3-Cd1-N3 O4-Cd1-N1 O4-Cd1-N2 O4-Cd1-N3 N1-Cd1-N2 N1-Cd1-N3 N2-Cd1-N3

88.8(3) 84.8(3) 137.6(3) 88.6(3) 93.1(3) 172.0(3) 88.9(3)

Compound 3 O1w-Cd1-O2 O1w-Cd1-O3 O1w-Cd1-O4 O1w-Cd1-N1 O1w-Cd1-N2 O2-Cd1-O3 O2-Cd1-O4

133.78(17) 84.13(16) 37.98(16) 91.97(16) 83.99(16) 39.48(13) 87.22(14)

O2-Cd1-N1 O2-Cd1-N2 O3-Cd1-O4 O3-Cd1-N1 O3-Cd1-N2 O4-Cd1-N1 O4-Cd1-N2

98.85(16) 8.80(16) 53.85(13) 92.22(13) 80.67(14) 89.45(15) 89.06(16)

Compound 4 O3-Cd1-N3 O4-Cd1-N1 O4-Cd1-N3 N1-Cd1-N2 N1-Cd1-N3 N2-Cd1-N3 O5-Cd2-O6 O5-Cd2-O7 O5-Cd2-O8 O5-Cd2-N4 O5-Cd2-N5 O5-Cd2-N6 O6-Cd2-O7

86.35(18) 87.4(2) 139.20(19) 175.1(2) 92.7(2) 90.5(2) 53.91(15) 136.02(15) 81.44(15) 139.23(16) 97.47(18) 92.54(17) 169.67(16)

O6-Cd2-O8 O6-Cd2-N4 O6-Cd2-N5 O6-Cd2-N6 O7-Cd2-O8 O7-Cd2-N4 O7-Cd2-N5 O7-Cd2-N6 O8-Cd2-N4 O8-Cd2-N5 O8-Cd2-N6 N4-Cd2-N5 N4-Cd2-N6

135.34(15) 85.34(16) 97.00(17) 93.24(17) 54.65(15) 84.74(16) 79.75(16) 89.31(16) 139.32(16) 87.19(19) 88.99(16) 87.8(2) 88.17(16)

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Figures with captions

Figure 1.Thermal ellipsoid probability (50%) plot of the asymmetric unit of 1 showing the coordination environment around Cd(II) ion, the hydrogen atoms are omitted for clarity.

21



(a)

(c)

(e)

a

c

b

a

(b)

c

(d)

11.77Å

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Page 22 of 31

a

Figure 2. (a) View of the 2D, [Cd(fdc)(bipy)] sheet in the crystallographic ab-plane. (b) Perspective view of the 2D pillared-bilayer structure. (c) The 2D→3D polycatenation among different bilayers. (d) C-H...π interactions between the benzene rings of adjacent bilayers. (e)Topological representation of the 2D→3D polycatenated network in 1 (three different 2D nets are shown in three different colors).

22


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Figure 3. Thermal ellipsoid probability (50%) plot of the asymmetric unit of 2 showing the coordination environment around Cd(II) ion; the hydrogen atoms and the guest water molecules are omitted for clarity.

23



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Figure 4. (a) Perspective view of a single 3D hexagonal framework of 2 formed by pillaring of 2D, [Cd(fdc)(bpee)] sheets showing large 1D channels along the a-axis. (b) and (c) shows the 3fold interpenetrating 3D framework of 2 and its topological representation is shown in (d) (three different 3D hexagonal nets are shown in three different colors).

24


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Figure 5. (a) Thermal ellipsoid probability (50%) plot of the asymmetric unit of 3 showing coordination environment around Cd(II) ion; the hydrogen atoms are omitted for clarity. Perspective view of the 2D sheet along the bc-plane. (c) Stacking of the 2D sheets along crystallographic a-axis via π-π interactions forming 3D supramolecular framework. Topological representation of the 2D network of 3.

25


the (b) the (d)


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Page 26 of 31

Figure 6. Thermal ellipsoid probability (50%) plot of the asymmetric unit of 4 showing the coordination environment around the Cd(II) ions, the hydrogen atoms, the guest molecules of 4bpdb and H2O are omitted for clarity.

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Figure 7. (a) View of the 2D [Cd(fdc)(4bpdp)] sheet in the crystallographic bc-plane. (b) The 2D pillared-bilayer network formed by pillaring of 2D sheets by 4bpdp spacers. (c) The 2D→3D polycatenation of different bilayers. (d) Topological representation of the 3D polycatenated network of 4 (three different 2D nets are shown in three different colors).

27



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Figure 8. Room temperature emission spectra of compounds 1-4 (inset shows visual emission observed under UV-light.

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Figure 9. Change in the fluorescence emission intensity of 2 in ethanol upon incremental addition of 0.1mM solution of TNP in ethanol.

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Figure 10. Spectral overlap between the normalized absorption spectra of nitro compounds and the normalized emission spectra of 2 in ethanol.

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For Table of Contents Use Only Auxiliary Ligand-Assisted Structural Variation of Cd(II) Metal-Organic Frameworks

showing

2D→3D

Polycatenation

and

Interpenetration:

Synthesis, Structure, Luminescence Properties and Selective Sensing of Trinitrophenol

Divyendu Singh and C. M. Nagaraja*

Four

new

MOFs

of

[Cd(fdc)(3bpdb)(H2O)](3)

Cd(II) and

ion,

[Cd(fdc)(bipy)1.5)](1),

[{Cd(fdc)(bpee)1.5}.3H2O](2),

[{Cd2(fdc)2(4bpdb)3}.1.5(4bpdp).2(H2O)](4)

have

been

successfully synthesized using mixed ligand systems and characterized by single crystal X-ray diffraction and other physicochemical studies. By the variation of N,N'-donor spacers, MOFs with diverse structural architectures such as, 2D→3D polycatenated bilayers (1/4), 3-fold interpenetrating 3D hexagonal framework (2) or a non-interpenetrating 2D network (3) have been synthesized. All the four compounds exhibit ligand based luminescence emission owing to n→π* and π→π* transitions. Compound 2 exhibits a highly selective and sensitive detection of TNP via fluorescence quenching mechanism. 31


Auxiliary Ligand-Assisted Structural Variation of Cd ... - ACS Publications

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