Two-Dimensional Coordination Polymers Comprising Mixed Tripodal

Article pubs.acs.org/crystal

Two-Dimensional Coordination Polymers Comprising Mixed Tripodal Ligands for Selective Colorimetric Detection of Water and Iodine Capture Yadagiri Rachuri,†,‡ Kamal Kumar Bisht,†,‡ and Eringathodi Suresh*,†,‡ †

Analytical Discipline and Centralized Instrument Facility and ‡Academy of Scientific and Innovative Research (AcSIR), CSIR−Central Salt and Marine Chemicals Research Institute (CSIR-CSMCRI), Council of Scientific and Industrial Research (CSIR), G. B. Marg, Bhavnagar 364 002, Gujarat, India S Supporting Information *

ABSTRACT: Two ternary coordination polymers (CPs) {[Co3(BTC)2(TIB)2(H2O)4]·(H2O)4}n (1) and {[Cu3(BTC)2(TIB)2(H2O)2]·(H2O)6}n (2) comprising tripodal linkers 1,3,5benzenetricarboxylate (BTC) and 1,3,5-tris(imidazol-1ylmethyl)benzene (TIB) are synthesized and characterized by single crystal X-ray diffraction and other physicochemical techniques. Adsorption experiments show that the materials can uptake molecular iodine in the vapor phase and can endure the adsorbed guest at ambient conditions. 27.92 and 28.62 wt % iodine uptake was observed for 1 and 2 respectively, which corresponds to the 1.88 and 1.96 molecules of iodine per formula units of 1 and 2 respectively. The adsorbed iodine can be slowly released in organic solvents. An interesting chromism phenomenon was also observed in the case of 1 which shows intense pink color in its pristine (hydrated) form and dark blue color in dehydrated forms which also may be attributed to the change in coordination environment around Co(II) centers. Interestingly, reversible blue to pink color transition can only be accomplished in the presence of water. Dehydrated framework retained its blue color in a range of organic solvents, advocating the ability of material to selectively sense water in liquid as well as vapor forms. Comprehensive structural description, thermal stability and luminescence behavior for both the CPs has also been presented.

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INTRODUCTION Research on the design and synthesis of coordination polymers (CPs)/metal organic frameworks (MOFs) has witnessed overwhelming growth in the past two decades for their numerous applications.1 Promising results have been achieved for gas storage, separation, catalysis, sensing, and magnetism.2−4 More recently, CPs/MOFs have been utilized for controlled capture and release of various analytes including drugs and radioactive wastes. Though there have been notable work on the effect of iodine encapsulation on conductivity, magnetism, and luminescence properties of MOFs/CPs, it is only recently when special emphasis has been paid on the capture of iodine as the radioactive isotope, 129I, a constituent of gas mixture liberated during the reprocessing of spent nuclear fuel.5,6 The long lifetime of radioactive iodine and its susceptibility to indulge in metabolic pathways are the issues of major concern. Recently Nenoff et al. have demonstrated the high iodine uptake capacities of ZIF-8 and HKUST-1 materials.5a−c They have further shown that permanent trapping of iodine within MOFs could be achieved through pressure-induced amorphization. 5d Cheetham and co-workers have also shown irreversible trapping of iodine in a series of zeolitic imidazolate frameworks (ZIFs).5i These and a few other studies may be considered as experimental inception of MOF materials for application toward the nuclear waste management.5 © XXXX American Chemical Society

At this juncture, it is relevant to note that a variety of iodine species such as neutral molecular iodine, tri-, and pentaiodide species can be encapsulated in the framework materials. Early work of Seff on the encapsulation of molecular as well as ionic iodine species in zeolite materials is noteworthy.6j−l Recently, Cooper et al. has shown the porous organic cages can efficiently encapsulate angular pentaiodide species.6m In another important report, neutral as well as ionic iodine species have been trapped in the crystal lattice of a Cd(II)-based MOF. It was shown that neutral molecular iodine can be trapped by vapor or solution phase experiments; however, ionic species were incorporated in crystal lattice when purely ionic or a mixture of ionic and neutral components were used as sorbate.6d On the contrary, dynamic transformation is one of the prominent structural features of layered CPs/MOFs relying on the ability of pillaring linkers to act as dynamic component. The choice of metal nodes, secondary building units, and ligand flexibility seems to play an important role to accomplish such structural merits. Zeng and co-workers have shown reversible structural transformations upon removing and rebinding the Received: February 5, 2014 Revised: May 28, 2014

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dx.doi.org/10.1021/cg500188n | Cryst. Growth Des. XXXX, XXX, XXX−XXX

Crystal Growth & Design

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radiation. Single crystal structures were determined using BRUKER SMART APEX (CCD) diffractometer. Solid state UV−Vis spectra were recorded using a Shimadzu UV-3101PC spectrometer and BaSO4 as a reference. Luminescence spectra were recorded at room temperature utilizing a Fluorolog Horiba Jobin Yvon spectrophotometer. Synthesis of {[Co3(BTC)2(TIB)2(H2O)4]·(H2O)4}n (1). H3BTC (75 mg, 0.36 mmol), TIB (75 mg, 0.24 mmol), and Co(NO3)2·6H2O (150 mg, 0.56 mmol) were dispersed in a mixture of 4 mL of 1,4-dioxane and 3 mL of water when sealed in a Teflon-lined autoclave (15 mL capacity), which was then heated at 408 K for 50 h. After slow cooling to room temperature pink single crystal blocks, suitable for single crystal X-ray analysis, were obtained with good yield and homogeneity (yield ≈ 69%). Elemental analysis (%), cal. For Co3C54H58N12O20: C, 47.28; H, 4.26; N, 12.25; found: C, 47.06; H, 4.106; N, 12.88. IR cm−1 (KBr): 3420 (br), 3186 (w), 2928 (w), 2358 (s), 1743 (s), 1612 (s), 1541 (s), 1429 (m), 1371 (s), 1235 (w), 1098 (s), 1026 (w), 947 (w), 832 (w), 724 (m), 659 (w), 551 (w). Synthesis of {[Cu3(BTC)2(TIB)2(H2O)2]·(H2O)6}n (2). H3BTC (75 mg, 0.36 mmol), TIB (75 mg, 0.24 mmol), and Cu(NO3)2·3H2O (150 mg, 0.66 mmol) were dispersed in mixed solvents (3 mL of each 1,4dioxane, methanol, and water) and then sealed in a Teflon-lined autoclave (23 mL capacity), which was heated at 408 K for 50 h. After slow cooling to room temperature block-shaped blue single crystals, suitable for single crystal X-ray analysis, were obtained with good yield and homogeneity (yield ∼71%). Elemental analysis (%), Cal. For Cu3C54H58N12O20: C, 46.80; H, 4.22; N, 12.13; found: C, 42.49; H, 4.13; N, 12.20. IR cm−1 (KBr): 3420(br), 2928 (m), 2861 (w), 2367(w), 1728 (w), 1617 (s), 1564 (m), 1556(m) 1438(w), 1364 (s), 1289 (w), 1113 (s),1029 (w), 946(w), 840(w), 759(w), 734(s), 658(w), 467(w). X-ray Crystallography. Summary of crystallographic data and details of data collection for 1 and 2 are given in Table 1. Single crystals with suitable dimensions were chosen under an optical microscope after immersing in paratone oil and mounted on a glass fiber for data collection. Intensity data for both crystals were collected using MoKα (λ = 0.71073 Å) radiation on a Bruker SMART APEX diffractometer equipped with CCD area detector at 150 K. The data integration and reduction were processed with SAINT software.10 An empirical absorption correction was applied to the collected reflections with SADABS.11 The structures were solved by direct methods using SHELXTL and were refined on F2 by the full-matrix least-squares technique using the program SHELXL-97.12,13 All non-hydrogen atoms were refined anisotropically until convergence was reached. Hydrogen atoms attached to the organic moieties present in both CPs are either located from the difference Fourier map or fixed stereochemically. After refining the coordination polymeric networks with one water molecule each for 1 and 2 located from the difference Fourier maps, diffused scattered peaks with residual electron density ranging from ∼2.5 Å−3 to 3 Å−3 were observed for 2, which can be attributed to lattice solvent molecule (five water molecules) present in the crystal lattice and isolated peak with residual electron density ≈ 3.0 Å−3 for 1 corresponding to one disordered water molecule in the difference Fourier map. Attempts made to model these disordered peaks were unsuccessful since residual electron density of the peaks obtained was diffused and there was no obvious major site occupations for the lattice solvent molecule. PLATON/SQUEEZE14 was used to correct the diffraction data for the contribution from disordered lattice solvent molecule. Final cycles of least-squares refinements improved both the R-values and goodness of fit significantly with the modified data set after subtracting the contribution from the disordered solvent molecules using SQUEEZ program. Topological analysis of compounds was performed by TOPOS15 software.

coordinated water molecule in a two-dimensional (2D) bilayer cobalt MOF.3a They have also shown that the removal of coordinated water from the framework increases storage capabilities for various analytes. The wide spectrum of applications and structural versatility of MOFs can apparently be attributed to the modular and tunable synthetic methodologies as well as judicious choice of precursors.2−4 Incidentally, tripodal linkers have been among the premium choices for the construction of frameworks with diverse topological, structural as well as functional features. Perhaps benzenetricarboxylate is the most celebrated tripodal ligand used for MOF design with successful MOFs like HKUST-1 to its credit.7 Recently, Bu and co-workers have utilized the BTC linker as one of the constituents to fabricate a highly intricate framework material showing cage-in-cage and framework-in-framework features.7o Similarly, CPs/MOFs derived from imidazole-based flexible or rigid linkers are also acclaimed for their thermal stability and structural diversity. An early report by Kang and co-workers features the first threedimensional (3D) MOF comprising the tripodal ligand TIB.8i Sufficient reports on the CPs/MOFs comprising TIB derivatives as one of the ligands are available in the literature.8 We and others have extensively studied the ternary CPs derived from dicarboxylates and imidazole-based linkers.1a,9 However, ternary CPs/MOFs comprising dual tripodal linkers are seldom explored.7b In this contribution we present the synthesis and crystal structure of two ternary CPs derived from tripodal linkers BTC and TIB (Scheme 1). Both the compounds exhibit 2D motifs Scheme 1. Molecular Structures of Tripodal Linkers Utilized in Present Study

with intriguing structural and topological features. Enduring intake of iodine vapor by these materials at 80 °C and its controlled release in organic solvents have been systematically studied. Additionally, an interesting chromism phenomenon was also observed for the Co(II) CP, which reversibly displays an intense color change from pink to blue in its pristine (hydrated) and dehydrated forms, respectively.

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EXPERIMENTAL SECTION

Materials and General Methods. 1,3,5-Benzenetricarboxylic acid (H3BTC), precursors for 1,3,5-tris(imidazol-1-ylmethyl)benzene (TIB), metal salts, and solvents were purchased from commercial sources. Distilled water was used for synthetic manipulations, and the reagents and solvents were used as received without further purification. TIB was synthesized by the procedure described in the literature.8a CHNS analyses were done using elementar vario MICRO CUBE analyzer. IR spectra were recorded using the KBr pellet method on a Perkin−Elmer GX FTIR spectrometer. For each IR spectra, 10 scans were recorded at 4 cm−1 resolution. 1H NMR spectra for the ligand TIB was recorded on Bruker AX 500 spectrometer (500 MHz) at temperature 25 °C and was calibrated with respect to internal reference TMS. TGA analysis was carried out using Mettler Toledo Star SW 8.10. X-ray powder diffraction data were collected using a PANalytical Empyrean (PIXcel 3D detector) system with CuKα

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RESULTS AND DISCUSSION Structural Description of 1. 1 represents a 2D framework and crystallizes in the triclinic space group P1̅. The asymmetric unit of 1 consists of a half of Co1 at special position, Co2 occupying at general position, and one BTC anion, one TIB B



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Table 1. Crystal Data and Refinement Parameters for Compounds 1 and 2

a

identification code

1

2

chemical formula formula weight crystal color crystal size (mm) temperature (K) crystal system space group a (Å) b (Å) c (Å) α (deg) β (deg) γ (deg) Z V (Å3) density (Mg/m3) absorption coefficient (mm−1) F(000) reflections collected independent reflections R(int) 2θ range for data collection (deg) number of parameters S (goodness of fit) on F2 final R1a/wR2b (I > 2σ(I) weighted R1a/wR2b (all data) largest diff peak/hole/e·Å−3 CCDC number

C54H60Co3N12O21 1389.93 pink 0.10 × 0.06 × 0.05 150 triclinic P1̅ 8.990(2) 12.728(3) 13.252(3) 91.192(4) 92.755(4) 109.691(4) 1 1425.0(6) 1.620 0.955 717 10 968 5526 0.0626 1.70−26.00 412 0.964 0.0644/0.1480 0.0945/0.1609 1.317/ −0.740 977 332

C54H58Cu3N12O21 1401.74 blue 0.14 × 0.07 × 0.03 150 triclinic P1̅ 8.7506(15) 10.7300(18) 18.933(3) 92.997(3) 103.096(3) 102.456(3) 1 1681.1(5) 1.385 1.018 721 14 325 7419 0.0340 1.95−27.50 393 1.029 0.0545/0.1254 0.0713/0.1329 0.612/ −0.406 977 333

R1 = Σ∥Fo| − |Fc∥/Σ|Fo|. bwR2 = Σ[w(Fo2 − Fc2)2]/Σ[w(Fo2)2]1/2.

ligand, two coordinated water molecules along with one water as solvent of crystallization (S1). Hexacoordinated Co1 and Co2 metal centers adopt distorted octahedral geometry. Octahedral coordination around Co1 occupying at special position is provided by a pair of each symmetrically disposed TIB (N4 atom) and BTC (chelating carboxylate atoms, O3 and O4) linkers as presented in Figure 1b. For Co2 the square base is furnished by carboxylate oxygen atoms (O1 and O5) from a different BTC moiety in monodenate coordination mode and water molecules (O7, O8). Axial coordination is provided by

imidazole nitrogens (N1 and N5) from different TIB ligands. Co−O distances range from 2.100(3) Å to 2.169(3) Å, and the Co−N distance is Co2−N1 = 2.103(4) Å, Co2−N6 = 2.106(6) Å with respect to Co2 and for the center of symmetric Co1, Co1−O3 = 2.148(3) Å; Co1−O4 = 2.164(3) Å and Co1−N4 = 2.0666(3) Å respectively. BTC anions bridge the pairs of Co2 centers generating onedimensional loops which are aligned down the a-axis diagonal to the bc-plane. The same Co2···Co2 pair is further bridged by two TIB units resulting in a Co2(BTC)2(TIB)2 cage, which acts as a four connected secondary building unit (SBU-I) to link another four connected secondary building unit (SBU-II) comprising Co1 nodes (Figure 1a,b). The Co2···Co2 separation in the Co2(BTC)2(TIB)2 cage is 7.94 Å, and the Co1···Co1 distance bridged by SBU-I is 18.56 Å. SBU-I and SBU-II are superimposed on another such unit down the a-axis and connected together to generate a 2D (4,4) network in 011 plan (Figure 1c,d). The coordination modes of BTC and TIB ligands are presented in (S2). In an attempt to understand the supramolecular interactions, packing and hydrogen bonding interactions were analyzed in detail. Lattice water molecules oriented between the layers of (4,4) nets are involved in O−H···O contacts linking the adjacent 2D framework. As shown in Figure 2, the 2D framework is staggered for the closer approach of Co1 to Co2 from the adjacent layers to make an effective hydrogen bonding interaction in the supramolecular assembly. In addition to the above O−H···O interactions, coordinated water molecules act as donors and are involved in O−H···O interaction with carboxylate oxygen and O8 as an acceptor in making C−H···O

Figure 1. Depiction of (a) cage-type dinuclear four connected SBU (SBU-I) with coordination around Co2; (b) simple four connected SBU (SBU-II) with coordination around Co1; (c) (4,4) network comprising alternate arrangement of SBU-I and SBU-II in 1 and (d) topologically simplified (4,4) net. C



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Figure 2. Depiction of inter and intramolecular hydrogen bonding interactions present in 1.

contacts with imidazole hydrogen H23. Besides the aforementioned inter-/intramolecular interactions, methylene hydrogen, H24 is also involved in C−H···O contact with the carboxylate oxygen O6 in stabilizing the supramolecular network. Details of all these pertinent H-bonding interactions with symmetry codes are given in Table 2. Table 2. Details of Hydrogen Bonding Interactions Observed in CPs 1 and 2 D−H···A O(7)−H(7C)···O(2)1 O(7)−H(7D)···O(3)2 O(8)−H(8C)···O(4)3 O(8)−H(8D)···O(6)4 O(9)−H(9C)···O(2)2 C(23)−H(23)···O(8)5 C(24)−H(24B)···O(6)6 O(7)−H(7C)···O(3)1 O(7)−H(7D)···O(2)2 C(10)−H(10)···O(7)3 C(12)−H(12)···O(8)4 C(20)−H(20A)···O(2)3 C(21)−H(23)···O(3)3

d(H···A) (Å) Compound 1a 1.68(2) 1.92(4) 1.89(4) 1.73(2) 1.95(4) 2.53(2) 2.41(3) Compound 2a 1.98(4) 1.94(4) 2.33(2) 2.54(4) 2.30(3) 2.50(5)

d(D···A) (Å)

∠D−H···A (deg)

2.622(5) 2.851(4) 2.793(4) 2.620(4) 2.873(7) 3.276(6) 3.365(6)

170(5) 170(4) 162(3) 160(5) 167(4) 137(5) 168(3)

2.725(4) 2.655(3) 3.258(4) 3.283(5) 3.270(4) 3.404(4)

164(4) 158(4) 172(3) 137(4) 179(5) 164(3)

Figure 3. Depiction of coordination environments around Cu1 and Cu2 nodes in 2 (a); 1D channel down the a-axis (b); 2D bilayer spreading along the ab plane (c); and simplified topological view of bilayer metal organic sheet (d).

the trigonal base and the axial coordination is endowed by the imidazole nitrogen atoms N1 and N4 from different TIB ligands. The Cu···O distance ranges from 1.992(2) Å to 2.179(2) Å while the Cu1−N1 = 1.975(3) Å and Cu1−N4 is 1.977(3) Å. Cu2 possesses a distorted octahedral coordination in which the square base is offered by the chelated coordination of the carboxylate oxygen atoms (O5 and O6) from the symmetrically disposed BTC ligands (Cu2−O5 = 2.653(2); Cu2−O6 = 1.968(2)). Axial coordination comes from the imidazole nitrogen N6 (Cu2−N6 = 1.972(3)) of the symmetrically disposed TIB moiety. Versatile coordination of BTC and TIB independently engender ladder-type motifs as depicted in S4. Offset stacked [Cu(BTC)]n ladders are bridged by the imidazole nitrogen from the TIB ligand generating a bilayer 2D sheet as depicted in Figure 3c. Dihedral angles between the central phenyl ring (C14 to C19) to that of the imidazole rings (C10−12N1N2, C25−27N5N6 and C21−23N3N4) in the case of 1 (77.91, 77.81, and 87.58°) in which the trans imidazole ring is almost orthogonal to the central phenyl ring, while for 2 all the imidazole rings are almost orthogonal (87.91, 81.24, and 84.17°) to make effective coordination with the metal centers. Packing and hydrogen bonding interaction viewed down the b-

Symmetry code: 1. x, y, z; 2. −x, 2−y, 1−z; 3. 1+x, y, z; 4. −x, 2−y, −z; 5. x, −1+y, z; 6. −x, 1−y, −z. aSymmetry code: 1. 1−x, −y, −z; 2. x, y, z; 3. 1−x, 1−y, −z; 4. 1+x, y, z. a

Structural Description of 2. X-ray single-crystal diffraction analysis reveals that 2 crystallizes in triclinic space group P1̅, generating a 2D bilayer network. The asymmetric unit of 2 consists of Cu1 occupying at general position, half a molecule of Cu2 at special position, one molecule each of TIB, BTC and water coordinated to the metal center along with water molecules as solvent of crystallization. Metal nodes Cu1 and Cu2 adopt different coordination geometry in 2 as depicted in Figure 3a. Thus, trigonoal bipyramidal coordination around Cu1 is provided by monodentate carboxylate oxygen from different BTC moiety (O1 and O4) along with one water molecule (O7) generating D



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removal of coordinated water or the encapsulation of iodine does not affect the carboxylate coordination to the metal nodes. This in turn reflects the structural integrity of pristine, activated, and iodine loaded samples. OH stretching broad bands around 3420 cm−1 and OH bending mode at ca. 1743 and at ca. 1728 are attributable to the coordinated and lattice water molecules for pristine 1 and 2. For activated samples 1150, 1180, 2150, and 2180 the OH banding signal disappears confirming the eviction of lattice as well as coordinated water molecules. Experimental PXRD patterns of bulk pristine samples 1, 1120, 2, and 2120 corroborate well with those simulated from respective single crystal data, confirming the stability of the bulk samples at elevated temperature and their phase purity (S6). The removal of lattice/coordinated water and rebinding as well as quantitative amount of encapsulated iodine were ascertained from the thermal and structural analyses and described in the following sections where appropriate. Thermal Analysis and Colorimetric Dehydration− Rehydration Process. Thermal stability of synthesized compounds was ascertained by TGA and PXRD. 1 with robust 2D frameworks showed stability up to 310 °C. 1 expels all of its lattice water molecules (obs. 5.9%; calc. 5.50%) in the temperature range 50−140 °C without loss in its crystalline nature. Further heating leads to 5% weight loss in the temperature range 140−200 °C, which may be ascribed to the exit of coordinated water molecules (calc. 5.03%). The expulsion of lattice water molecules alone or lattice as well as coordinated water molecules could selectively be achieved by heating the pristine sample in a hot air oven for 120 and 180 °C respectively. The loss of coordinated water molecules presumably leads to the change in the octahedron coordination environment around Co2 to the tetracoordinated one with significant loss in the crystalline nature and a distinct color change from pink to blue. Thus, the observed color change may be attributed to the change in the coordination geometry of Co(II) metal center. Pink color is typical of Co(II) complexes in Oh symmetry, whereas the blue color can be ascribed to the Td coordination geometry of the Co(II) center attained upon heating the pink (octahedral) sample resulting in the removal of coordinated water molecules. The mentioned structural transitions were characterized by TGA and PXRD analyses. The solid state absorbance showed distinct d−d transition bands at ca. 521 and 577 nm for pink and blue forms respectively (S7). The completely dehydrated blue sample of cobalt CP 1 can uptake the water in liquid or vapor forms to regain the original pink color with revival of starting crystalline framework (Figures 5 and S9). Interestingly, moistening of dehydrated blue sample with water droplets results in quick hydration (10− 20 min) and regeneration of the crystalline pink framework. The gradual regeneration of pink hydrated form could also be achieved by means of exposing the dehydrated sample to water vapors for a longer time, normally 4−12 h. The dehydrated blue framework remains intact in a variety of common organic solvents such as methanol, ethanol, chloroform, acetonitrile, acetone, dimethylformamide, dimethylacetamide, nitromethane, and nitrobenzene, which is noteworthy. The blue dehydrated network was soaked in the mentioned solvents for 24 h and was found intact by visual inspections as well as PXRD patterns. Furthermore, the blue color of framework gradually turns pink when it is soaked in the moistened solvents. Thus, partial regeneration of the pink hydrated framework was observed when the dehydrated blue

axis is depicted in Figure 4. Even though hydrogen atoms for the lattice water molecules could not be located from the

Figure 4. Depiction of (a) inter and intramolecular hydrogen bonding interactions present in 2; the stitching of 2D sheets by O−H···O interactions is noteworthy; (b) topological view of 2D sheet depicting the 1D open channels disposed parallel to each other along the a-axis.

difference Fourier map, O8 is held by C−H···O interactions between the imidazole hydrogen H12 and making short contact with the carboxylate oxygen O5 (O3···O8 = 2.832 Å). Coordinated water molecule O7 acts as a donor in intra-/ intermolecular O−H···O interaction with the carboxylate oxygen (O2 and O3) and donor in C−H···O interaction with imidazole hydrogen H23 and methylene hydrogen H10 in bridging the adjacent 2D network. In addition to the above Hbonding methylene hydrogen H20A and imidazole hydrogen H23 is also involved in intermolecular C−H···O interaction in bridging the 2D sheets and stabilizing the 2D supramolecular architecture. Details of all these hydrogen bonding interaction and their pertinent symmetry codes are given in Table 2. Topological analysis of the bilayer sheets created by the coordination of the [Cu(BTC)]n ladder motif connected by the imidazole nitrogen atoms of the tripodal ligand revealed a (4c-, 3c-) binodal network identified with vertex symbol {42.204.324.364.44} (Figures 3d and 4b). Interestingly in both the CPs the TIB ligands adopt cis, cis, trans conformation of the imidazole rings in bridging the M[BTC]n layers in the 2D network. FTIR and PXRD Analyses. The lattice guests and composition of 1 and 2 were ascertained by TGA, elemental analysis, and single crystal X-ray crystallography. FTIR spectra (S5) revealed valuable information about the coordination modes of the carboxylate moiety of BTC ligand. The pristine cobalt and copper CPs revealed carbonyl stretching bands at 1612, 1541, 1429, 1371 and at 1617, 1564, 1438, 1364 cm−1 respectively. These peak positions remain persistent for the activated and iodine loaded samples as well, indicating that the E



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molecules residing on the crystal surfaces and finally dried in air for 1 h. The iodine loaded samples, I2@1120, I2@1150, I2@1180, I2@2120, I2@2150 and I2@2180, thus obtained were characterized by TGA and PXRD analysis (Figures 6, 7, and S9).

Figure 5. Digital photographs depicting the chromic response (pink to blue) observed for cobalt CP, 1 with controlled thermal treatment (150 °C, 8 h) and revival of original color (blue to pink) on rehydration (a); PXRD patterns recorded for pristine 1, 1 subjected to controlled heating at 120 and 150 °C and the sample prepared by rehydrating the 1 heated to 150 °C for 8 h (b) and TGA traces for respective samples (c). Figure 6. TGA traces for activated and iodine loaded cobalt and copper CPs clubbed with respect to activation temperature. TG curve of respective pristine sample (1 or 2) is also plotted in each set for the sake of comparison (see text for description).

sample was soaked in the methanol or ethanol sample comprising 1% water for 24 h. These unique chromic responses of 1 in dehydrated and hydrated forms advocate its suitability as detector for vapor and liquid phases of water. Relevant reports by Kurmoo and Real groups on rapid detection of water by MOFs species are worth mentioning at this juncture.3b,6a TGA analysis of 2 showed the framework stability up to 260 °C. CP 2 rapidly starts losing its loosely bound lattice water molecules as soon as it is taken out of mother liquor (observed 20 wt %, calculated 7.79 wt %) and forms its anhydrous state before 100 °C. Further heating indicates a gradual weight loss (observed 2.2 wt %, calculated 2.59 wt %) in a wide temperature range 90−200 °C, which could be ascribed to two coordinated water molecules. The expulsion of coordinated water in the case of 2 was immediately followed by the decomposition of framework at a temperature beyond 260 °C. Unlike 1, no chromic response was observed toward water and other solvents in the case of dehydrated 2. Iodine Uptake and Controlled Release. Considering the ability of 1 and 2 to selectively expel the lattice solvent molecules at elevated temperatures without disrupting the framework, their capacity toward the molecular I2 capture was studied. I2 loading experiments are presented in S8. The results demonstrate that the void space in the activated frameworks of 1 and 2 could accept the molecular I2. Crystals of 1 and 2 were heated at 120, 150, and 180 °C to achieve activated samples 1120, 1150, 1180 and 2120, 2150, 2180. These activated samples were placed in closed containers and exposed to the iodine vapor at 80 °C for 4 h. After being cooled to room temperature, all the iodine loaded samples were thoroughly washed with acetonitrile and hexane to remove the iodine

Attempts to collect single crystal diffraction data for the iodine loaded samples were not successful due to the significant

Figure 7. Photographs depicting the color change observed for 2 after activation at 180 °C followed by iodine loading (top); PXRD patterns for copper CP samples activated at different temperatures and the same samples after iodine encapsulation (bottom). F



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which sets off the release of encapsulated iodine by solvent extraction or heating. The release of iodine from loaded samples in hexane was monitored by time-dependent visible absorption spectra. Iodine release commences quickly and progresses linearly for an initial 3 h period, whereas the release rate slows down with time (Figures 8 and S11−S13). The release of iodine with time can

loss in crystallinity or probable long-range disorder in encapsulated iodine molecules. The amount of iodine uptake in all the cases was established by corroborating crystallographic data and the thermal analysis which showed 8.60, 9.70, and 27.92 wt % iodine was encapsulated in the cobalt CP samples I2@1120, I2@1150, and I2@1180, which correspond to 0.50, 0.51, and 1.88 molecules of iodine per formula unit of respective dehydrated frameworks. The values turned out to be 18.15, 16.88, and 28.62 wt % in the case of iodine loaded copper CP samples I2@2120, I2@2150, and I2@2180 respectively, which indicates the encapsulation of 1.14, 1.15, and 1.96 molecules of iodine per formula unit of respective samples. The accessible void volumes per unit cell estimated by PLATON/ SOLV for 1 and 2 after the complete removal of the lattice solvent molecule are 7.3% (104.2 Å3) and 27.9% (468.6 Å3) respectively, which corroborate well with the higher uptake of I2 in the larger voids of Cu coordination polymer 2 at moderate activation temperatures. The amount of adsorbed iodine was further estimated by the iodometric titration, which complements the described results (S15). Enhanced iodine uptake capacity for both 1 and 2 after activation at higher temperatures may be ascribed to the removal of coordinated water creating more room for guest molecules as well as the dynamic structural transformations. However, in the case of compound 2, the collective effect of minor conformational changes and removal of lattice/coordinated water molecules result in the modification of useful voids in such a way that a slightly lesser amount of iodine could be incorporated in the case of I2@2150 (16.88 wt %) than that in the case of I2@2120 (18.15 wt %). Samples 1150 and 1180 exhibit loss of crystallinity, and it is proposed that amorphization of sample at higher temperature results the higher uptake of iodine. Therefore, the higher I2 uptake observed for 1180 could be ascribed to the complete amorphization of the framework. Significant uptake of iodine in amorphous MOF materials has been recently explored, and the studies suggest that stability of such iodine loaded assemblies could be ascribed to the plausible electron donor−acceptor interactions between iodine molecules and aromatic sextets of ligand moieties.5d,i Similar dynamic transformation in the context of MeOH, EtOH, and benzene encapsulation in a cobalt MOF has been studied by Zeng et al.3a Thermal analysis carried out after keeping the samples exposed to the ambient conditions for 10 days showed a good match with the data obtained for the freshly prepared samples, which suggests the permanent encapsulation of I2 guest molecules within the frameworks. However, retention of I2 molecules within framework was permanent at ambient conditions; the encapsulated I2 can be extracted in organic solvents such as acetonitrile, hexane, or cyclohexane by soaking the loaded samples in the solvent of choice. As soon as the iodine loaded sample is soaked in solvent, encapsulated iodine molecules come in contact with solvent due to the open pore structures of 1 and 2. Eventually, movement of iodine species from MOF pore to bulk of the solvent commences to attain a dynamic equilibrium of iodine species in- and outside of the framework. Thus, repeated replacement of solvent with extracted iodine with fresh solvent would result in the complete extraction of iodine from the MOFs. In fact, iodine encapsulation inside the voids can be attributed to the weak supramolecular interactions between the iodine molecules and framework walls.5d Interactions with solvent molecules or gentle heating provides with sufficient energy to overcome these weak supramolecular interactions

Figure 8. Photographs showing a color change with time due to the release of encapsulated iodine in 1.5 mL of hexane after soaking 10 mg of I2@2180 (a); Time-dependent UV−vis spectra depicting enhancement in absorbance (λmax 524 nm) suggesting an increase in iodine concentration with time (b); (c) absorbance−time profile for same set indicates the fast and linear iodine release for initial 3 h; however, the release rate slows down with a further increase in time.

very easily assessed with the naked eye showing an increase in intensity of the purple color of the hexane solution. Calibration experiments were performed to assess the amount of released iodine after a given time using the absorbance of standard iodine solutions in hexane (S10). In a typical iodine release experiment, 10 mg I2@2180 soaked in 1.5 mL of hexane released 0.28 mg of iodine in starting 3 h, whereas only 0.09 mg of additional iodine was released in the next 2 h. Thus, the materials 1 and 2 owing to their quick and stable iodine uptake capacity add to the subset of functional CPs having the potential for nuclear waste management.

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CONCLUSION Two novel CPs {[Co3(BTC)2(TIB)2(H2O)4]·(H2O)4}n (1) and {[Cu3(BTC)2(TIB)2(H2O)2]·(H2O)6}n (2) comprising tripodal linkers BTC and TIB are synthesized by solvothermal techniques and unambiguously characterized by various physicochemical techniques. Single crystal X-ray diffraction studies reveal 2D monolayer and bilayer structures for 1 and 2 respectively with voids filled with lattice guests. The materials showed good thermal stability, and the lattice guests as well as the coordinated water molecules could be thermally removed by activation protocols without damaging the MOF. Cobalt CP showed a visible color change for its pristine (pink) and activated (blue) forms which may be attributed to structural transformation of one of the two Co(II) centers from hexacoordinated to tetracoordinated geometry. The pink to blue color transition is reversible and can only be accomplished G



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by water. Dehydrated framework retained its blue color in a range of organic solvents, advocating the ability of material to selectively sense water in liquid as well as vapor forms. Moreover, the activated frameworks 1180 and 2180 showed an uptake of approximately two molecules of iodine per formula unit of these activated frameworks. Interestingly, the iodine uptake process was highly enduring as the iodine loaded sample did not show any loss in guest content even after been exposed to the ambient atmosphere for 10 days. The encapsulated iodine can be desirably released by soaking the loaded materials in polar or nonpolar organic solvents such as acetonitrile or hexane. The permanent iodine uptake capacity of these novel materials advocates their potential application in the field of nuclear waste management.

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ASSOCIATED CONTENT

S Supporting Information *

ORTEP diagrams, crystallographic figures, bond length and bond angle table, FTIR, PXRD, UV−vis spectra, fluorescence spectra, activation procedures, iodine uptake-release protocols, crystallographic information files. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]; sureshe123@rediffmail.com. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Authors acknowledge the CSIR, India (HYDEN Project, Grant no. CSC 0122), for financial support, Mrs. Monika Gupta for TGA data, Ms. Riddhi Laiya for XRPD data, Mr. Satyaveer Gothwal for elemental data, Mr. Viral Vakani for IR data, and Dr. P. Paul for all around analytical support. Y.R. and K.K.B. acknowledge UGC and CSIR (India) for JRF and SRF, respectively. Publication Registration Number: CSIR-CSMCRI No.: 020.

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