Diamondoid Three-Dimensional Metal-Organic Framework Showing

Aug 7, 2009 - Structural Transformation with Guest Molecules ... coordination polymers show interesting 4-fold interpenetrating diamondoid 3D network...
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DOI: 10.1021/cg900504m

Diamondoid Three-Dimensional Metal-Organic Framework Showing Structural Transformation with Guest Molecules

2009, Vol. 9 4480–4486

Arshad Aijaz,† Elisa Barea,‡ and Parimal K. Bharadwaj*,† †

Department of Chemistry, Indian Institute of Technology Kanpur, 208016, India, and ‡Departamento de Quı´mica Inorg anica, Universidad de Granada, Avenida Fuentenueva S/N, 18071 Granada, Spain

Received May 8, 2009; Revised Manuscript Received July 20, 2009

ABSTRACT: The potential of carboxylate and imidazole-based mixed ligands for forming metal organic frameworks (MOFs) is demonstrated with 4-(1H-imidazole-1-yl)benzoic acid (HIBA) to generate a flexible material. Reaction of HIBA with Cd(NO3)2 3 6H2O under solvothermal condition in N,N-dimethylformamide (DMF), produces a three-dimensional MOF, {[Cd(IBA)2].2DMF}n (1⊃2DMF), which on heating at 60 and 200 °C turns opaque, forming {[Cd(IBA)2] 3 DMF}n (1⊃DMF) and {[Cd(IBA)2]}n (1), respectively, as characterized by PXRD, TGA, and elemental analysis. Either of these compounds on keeping in dry DMF for 15 days affords the original compound, i.e., showing reversibility deduced from PXRD. When a crystal of 1⊃2DMF is immersed in moist N,N-diethylformamide (DEF) in a vial open to atmosphere, a new compound, {[Cd(IBA)2] 3 DEF 3 H2O}n, (2) is formed in 1 month in a single-crystal to single-crystal (SC-SC) fashion. Also, a crystal of 1⊃2DMF on immersing in moist DMF produces another interesting 3D framework {[Cd(IBA)2.H2O] 3 DMF 3 3H2O}n, (3) in a SC-SC transformation. These coordination polymers show interesting 4-fold interpenetrating diamondoid 3D network structures.

Introduction Synthesis of coordination polymers has been an area of rapid growth in recent years. With the availability of a large number of organic multidentate ligands and various coordination tendencies of metal ions, modulation of size and shape of the pores as well as properties of the final porous materials are possible.1 Among the various architectures, interpenetrating and intervening phenomena are frequently observed especially in adamantoid and polyrotaxane networks.2 Topologically related coordination polymers where tetrahedral metal ions or octahedral metal ions are linked together with organic spacer ligands in tetrahedral fashion display the tendency to form diamondoid networks with various degrees of interpenetration including 2-12-fold degeneracy.3 The particular interest in diamondoid structures is attributable to the general robustness of three dimensionally interconnected nets. Compared to rigid bridging ligands, flexible bridging tectons can produce some unique frameworks with beautiful aesthetics and useful properties because of their flexibility and conformational freedom. However, porous materials that have both high framework stability and flexibility are not so common.4 In particular, MOFs with flexible and dynamic frameworks that reversibly change their structures and properties in response to external stimuli,5 are of great interest. Such materials may find different applications, for example, as sensors,6 highly selective guest accommodation,7 magnetic bistability,8carriers of specific chemicals, and so on.9 Generally, hydrogen bonds and weak interactions with flexible lengths and angles10 are responsible for most of the reported dynamic nature of coordination polymers. We have embarked on a systematic synthesis of new series of ligands incorporating both imidazole and aromatic carboxylate groups connected by a rotatable C-N bond to generate *Corresponding author. E-mail: [email protected]. pubs.acs.org/crystal

Published on Web 08/07/2009

porous coordination polymers. Herein is reported a novel MOF constructed with Cd(II) and 4-(1H-imidazole-1yl)benzoic acid (HIBA) that shows guest dependent dynamic behavior. Framework {[Cd(IBA)2] 3 2DMF}n (1⊃2DMF), exhibits reversible single-crystal-to-single-crystal (SC-SC) structural transformations upon guest exchange as well as forms a new coordination polymer {[Cd(IBA)2. H2O] 3 DMF 3 3H2O}n, (3), with an expansion of coordination number in SC-SC fashion. Our interest in ligand HIBA is based on the fact that although the two rings are rigid, they are connected by rotatable C-N bond. Different conformers arising from this flexibility can be utilized to engineer different MOF architectures. Earlier, this ligand was used to construct coordination polymers with Co(II)1e and Cd(II).1f The MOF built with Cd(II) and HIBA has an interesting 4-fold interpenetrating diamondoid coordination network that exhibits flexibility based entirely on coordination bonds.12 Therefore, studies of such species not only add to structural diversity of coordination polymers but also provide new insights into the relationship between structure and properties of these materials. Experimental Section Materials and Physical Measurements. Ethyl-4-fluorobenzoate, N,N-diethylformamide, and metal salts were purchased from Aldrich and used as received. All solvents, imidazole, and K2CO3 were procured from S. D. Fine Chemicals, India. All solvents were purified prior to use. Spectroscopic data were collected as follows: IR (KBr disk, 4004000 cm-1) Perkin-Elmer model 1320; 1H NMR and 13C NMR spectra were recorded on a JEOL JNM-LA400 FT (400 and 100 MHzm respectively) instrument in CDCl3 or in DMSO-d 6 with Me4Si as the internal standard; ESI mass spectra were recorded on a WATERS Q-TOF Premier mass spectrometer; X-ray powder pattern (Cu KR radiation at a scan rate of 3°/min, 293 K) Phillips PW 100 X-ray generator; thermogravimetric analysis (heating rate of 5 °C/min) Perkin-Elmer Pyris 6. Microanalyses for the compounds r 2009 American Chemical Society

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Table 1. Crystal and Structure Refinement Data for 1⊃2DMF, 2, and 3 empirical formula fw T (K) radiation, wavelength (A˚) cryst syst space group a (A˚) b (A˚) c (A˚) U (A˚3) Z Fcalcd (Mg/m3) μ (mm-1) F(000) no. of reflns collected no. of independent reflns refinement method GOF final R indices [I > 2σ(I)] R indices (all data)

1⊃2DMF

2

3

C26H28N6O6Cd 632.94 100 Mo KR, 0.71073 orthorhombic Pbc21 8.254(3) 18.896(5) 18.006(7) 2808(2) 4 1.497 0.827 1288 33411 5193 full-matrix least- squares on F2 1.079 R1 = 0.0436 wR2 = 0.0921 R1 = 0.0623 wR2 = 0.0921

C25H25N5O6Cd 603.90 100 Mo KR, 0.71073 orthorhombic Pbca 16.720(6) 17.081(5) 18.432(7) 5264(3) 8 1.52 0.878 2447.6 33089 6545 full-matrix least- squares on F2 1.075 R1 = 0.0637 wR2 = 0.1777 R1 = 0.0951 wR2 = 0.2315

C22H21N4.5O8.5Cd 696.83 100 Mo KR, 0.71073 orthorhombic Pbcn 15.540(4) 17.665(3) 17.519(6) 4809(2) 8 1.664 0.967 2424 30571 5992 full-matrix least- squares on F2 1.078 R1 = 0.0452 wR2 = 0.1110 R1 = 0.0662 wR2 = 0.1294

were obtained from the Central Drug Research Institute, Lucknow, India. Solid-gas adsorption isotherms were measured on a Micromeritics Tristar 3000 volumetric instrument. Prior to measurement, powder samples were heated at 433 K for 12 h and outgassed to 1 10-6 Torr using a Micromeritics Flowprep. Synthesis of Benzoic Acid, 4-(1H-imidazol-1-yl)-, Ethyl Ester. The ligand benzoic acid, 4-(1H-imidazol-1-yl)-, ethyl ester was prepared following13 a modified literature method. A mixture of imidazole (1.1 mmol), ethyl-4-fluorobenzoate (1 mmol), and K2CO3 (1.5 mmol) in 10 mL of DMF was heated at 100 °C for 48 h with constant stirring. Thereafter, all the solvent was removed under reduced pressure; the residue was washed with water and the compound extracted with ethyl acetate. Removal of ethyl acetate afforded a crude product that was recrystallized from methanol to obtain the ester as a pale yellow crystalline solid. Yield: 86.5%. IR (KBr): 3150.3(m), 3125(m), 2993.5(m), 2980.1(m), 2960.2(w), 2931.5(w), 2905.8(w), 1700.7(s), 1611.1(s), 1526.7(s), 1484.8(m), 1367.9(m), 1270.4(s), 1137.8(m), 1270.4(s), 1063.8(s), 960.6(m), 854.8(m), 766.5(m), 751.5(m) cm-1. 1H NMR (CDCl3): δ (ppm) 8.38 (d, 2H, ArH), 8.15 (s, 1H, ArH), 7.68 (d, 2H, ArH), 7.56 (s, 1H, ArH), 7.45 (s, 1H, ArH), 4.60 (q, 2H, OCH2), 1.61(t, 3H, CH3). 13C NMR (CDCl3): δ (ppm) 160.84, 135.92, 130.71, 126.81, 126.35, 124.66, 115.88, 113.09, 56.67, 9.78. ESI-MS (m/z): 217.0972(100%) [M þ 1]þ. 4-(1H-imidazole-1-yl)benzoic Acid (HIBA). This acid was obtained by hydrolyzing the ethyl ester mentioned above with 5% aqueous NaOH solution in ethanol followed by acidification with 1N HCl. The desired acid precipitates as a white solid. Yield: 82%. IR (KBr): 3158.7(m), 3121.8(m), 2924.2(w), 1692(s), 1606.7(s), 1526(s), 1491.2(m), 1431(m), 1371.7(m), 1310.4(s), 1251.7(m), 1059.8(s), 963.1(m), 854.5(m), 836(m), 772.7(s), 730.5(m) cm-1. 1 H NMR (CDCl3): δ (ppm) 8.39 (s, 1H, ArH), 8.04 (d, 2H, ArH), 7.85 (d, 2H, ArH), 7.31 (dd, 1H, ArH), 7.14 (s, 1H, ArH). ESI-MS (m/z): 189.0662(100%) [M þ 1]þ. Synthesis of {[Cd(IBA)2] 3 2DMF}n (1⊃2DMF). A mixture of Cd(NO3)2 3 6H2O (0.5 mmol) and HIBA (0.25 mmol) was taken in DMF (3 mL) and heated at 110 °C under autogenous pressure in a Teflon-lined steel bomb for 2 days, followed by slow cooling (5 °C h-1) to room temperature. The colorless block-shaped crystals formed were collected, washed with DMF, and dried in air. Yield: 74%. Anal. Calcd for C26H28N6O6Cd: C, 49.33; H, 4.45; N, 13.27. Found: C, 49.39; H, 4.49; N, 13.32%. IR (KBr): 3434.5(mb), 2925.3(w), 1659.2(w), 1605.2(w), 1385(s), 1181.6(w), 1062.4(w), 858.6(m), 784(m), 722.5(w) cm-1. Synthesis of {[Cd(IBA)2] 3 DMF}n (1⊃DMF) and {[Cd(IBA)2]}n (1). These were obtained by heating 1⊃2DMF at 60 and 200 °C, respectively, for 2 h under reduced pressure. Synthesis of {[Cd(IBA)2] 3 DEF 3 H2O}n (2). Compound 2 was obtained by dipping the crystals of 1⊃2DMF or 1 in DEF

Figure 1. Coordination geometry of the Cd(II) center in 1⊃2DMF. (diethylformamide) at room temperature for 1 month. Anal. Calcd for C25H27N5O6Cd: C, 49.55; H, 4.49; N, 11.55. Found: C, 49.49; H, 4.54; N, 11.52%. IR (KBr): 3414.9(wb), 3135.1(w), 2970.9(w), 1689.2(s), 1606.8(s), 1547.9(s), 1522.6(s), 1406.8(s), 1304.9(m), 1249.1(m), 1120.9(m), 1061(s), 855.2(m), 785.2(s) cm-1. Synthesis of {[Cd(IBA)2(H2O)] 3 DMF 3 3H2O}n (3). Compound 3 was prepared at room temperature by keeping crystals of 1⊃2DMF in moist DMF for 2 weeks. The crystals do not change color or shape but become shinier. Anal. Calcd for C23H29N5O9Cd: C, 43.71; H, 4.62; N, 11.08. Found: C, 43.81; H, 4.69; N, 11.05%. IR (KBr): 3387.2(sb), 3142.9(w), 1665(m), 1606.5(m), 1552.1(w), 1522.1(w), 1494.8(w), 1401.8(s), 1302.6(m), 1248.1(m), 1116.5(m), 1061.4(s), 964.2(m), 930(m), 858.4(s), 785(s), 724.8(m) cm-1. Single-Crystal X-ray Crystallography. Single-crystal X-ray data were collected at 100 K on a Bruker SMART APEX CCD diffractometer using graphite-monochromated Mo KR radiation (λ = 0.71073 A˚). Linear absorption coefficients, scattering factors for the atoms and the anomalous dispersion corrections were taken from International Tables for X-ray Crystallography. Data integration and reduction were processed with SAINT14 software. An empirical absorption correction was applied to the collected reflections with SADABS15 using XPREP.16 The structure was solved by the direct method using SHELXTL17 and refined on F2 by fullmatrix least-squares technique using the SHELXL-97 program package. Non-hydrogen atoms were refined anisotropically. All

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Figure 2. Perspective view of the structure of 1⊃2DMF, showing a single adamantane cage with HIBA connections between cadmium ions.

Figure 3. View of the structure of 1⊃2DMF, showing the 4-fold interpenetration of diamondoid networks. hydrogen atoms were either located in successive difference Fourier maps or added at calculated positions, and they were treated as riding atoms using SHELXL default parameters. However, hydrogen atoms on OW1 in 2 and one hydrogen atom of OW2 in 3 could not be located. OW3, OW4 in 3 are refined isotropically. The crystal and refinement data are collected in Table 1.

Results and Discussion 1⊃2DMF was synthesized by the solvothermal reaction of HIBA and Cd(NO3)2.6H2O in DMF at 110 °C for two days as colorless block-shaped crystals. It was formulated as {[Cd(IBA)2] 3 2DMF}n by elemental microanalysis and single-crystal X-ray diffraction studies. Once isolated, it is stable in air and insoluble in common organic solvents. Solid state structure of 1⊃2DMF reveals that the asymmetric unit consists of one Cd(II), two coordinated ligands and two free DMF molecules. Each metal ion shows distorted octahedral geometry with equatorial coordination from two bidentate carboxylates while imidazole N atoms occupy the axial sites. This way, each metal is bonded to four different IBA- ligand units (Figure 1). The Cd-O as well as Cd-N bond distances (see Table 2 in the Supporting Information) are comparable18 to those observed in other octahedral complexes of the metal ion. The imidazole and the benzene moieties are not coplanar but slightly twisted with respect to each other (torsion angles of 29.7 and 29.1°). This binding mode leads to the formation of large adamantane-like cages (Figure 2) with a common Cd 3 3 3 Cd distance of 12.4 A˚. Each cage is delineated by four cyclohexane-like windows in chair conformation showing the maximum dimension as 21.72  23.44  26.17 A˚3 (the channel size is measured by

considering the diagonals of a cyclohexane window in a single adamantane unit). Because of the spacious nature of a single network, it allows another three identical diamondoid network to penetrate it leading to 4-fold interpenetrated structure (Figure 3). If we considered only a single diamondoid network when viewed down the crystallographic a or c axes, big channels can be found (Figure 4) that are filled with DMF molecules. Notably, even with this interpenetration, the framework is still highly open, containing three-dimensional channels of 6.5  6.3 A˚2 dimension (the channel size is measured by considering the van der Walls radii of the constituting atoms) extending along the crystallographic a axis that are enclosed by four intertwined helices coming from four interpenetrating lattices (Figure 5). As mentioned already, these channels are occupied by DMF molecules that stabilize the overall 3D structure with weak interactions between imidazole C-H and DMF oxygen atoms (H 3 3 3 O, 2.55 and 2.60 A˚). Calculation with PLATON19 shows that the effective volume for the inclusion is 39.6% of the crystal volume. In the IR spectrum, strong peaks appear at 1385 and 1605 cm-1 diagnostic of coordinated carboxylate, whereas the peak due to DMF appears at 1650 cm-1 (see Figure S8 in the Supporting Information). Thermal analysis shows that compound 1⊃2DMF loses lattice DMF molecules in two stages (see Figure S2 in the Supporting Information). At 40 °C, it loses one DMF (calcd, 11.5%; found, 11.5%) molecule cleanly to form 1⊃DMF and the second DMF molecule is lost at 170 oC (calcd, 11.5%; found, 11.4%) to afford {[Cd(IBA)2]}n (1). The first DMF coming at low temperature

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Figure 4. (a) Single 3D-diamondoid network of 1⊃2DMF. (b) Space filling model of a single 3D-diamondoid framework of 1⊃2DMF along the b-axis.

Figure 5. (a) Schematic presentation of the overall 3D network of 1⊃2DMF, having DMF guest molecules in the cavity. (b) View of the channel after interpenetration along the a-axis. (c) View of the nanotube composed by 4 helix of interpenetrating framework.

may be associated with the stability of 1⊃DMF and also with the reduction of stress of the dynamic framework. Compound 1 can be obtained by heating the original compound (1⊃2DMF) at 200 °C under reduced pressure for 2 h. Removal of solvent guest molecule causes significant changes in the framework that are readily detected by PXRD and IR. The PXRD patterns of 1⊃DMF and 1 are completely different from that of 1⊃2DMF, indicating that 1⊃DMF and 1 are two new forms. We presume that such a significant change in PXRD pattern might be attributed to the framework squeeze upon guest expulsion given the interpenetrating nature of the framework. Further, this structural transformation triggered by guest molecules is confirmed by immersing 1⊃2DMF in pure chloroform to obtain a material showing a completely different PXRD pattern. Thermal analysis of this product indicates the presence of one chloroform molecule that is lost

cleanly at 110 °C and possibly no DMF is present, as no loss due to DMF is seen (see Figure S3 in the Supporting Information). Thus, the structure of the framework here changes with the size, shape, and quantity of the guest molecules. However, all attempts to make single crystals of these compounds remained unsuccessful. On the other hand, when any of these materials is kept in dry DMF for 15 days, the original compound (1⊃2DMF) is recovered, confirmed by the coincidence of the peak positions and relative intensities in the PXRD pattern (see Figure S1e in the Supporting Information). This signifies that these changes are completely reversible and dynamic nature of framework may be associated with the rotational motion of the aromatic rings as well as sliding of the interpenetrating frameworks. Replacement of DMF with DEF, obtained by dipping crystals of 1⊃2DMF in DEF for 1 month, leads to a different

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Figure 6. (a) Simplified view of the structure of 2 showing square channels along b-axis (hydrogen atoms are omitted for clarity). (b)View showing the inclusion of DEF molecules in the cavity of 2.

Figure 7. (a) Coordination geometry of Cd(II) center with seven co-ordinations in 3. (b) Space-filling model of a single 3D-diamondoid framework of 3 along the b-axis.

framework {[Cd(IBA)2] 3 DEF.H2O}n (2) in a single-crystal to single-crystal transformation. We had taken the single-crystal data at various intervals during this transformation. Initially, up to 26 days it diffracted well and gave the same cell as 1⊃2DMF. After 30 days, the cell parameters changed significantly and we collected data on this crystal. Compound 2 crystallizes in the orthorhombic space group Pbca, with two IBA- units, one Cd(II) ion, one free DEF and one water molecule in the asymmetric unit. The structure shows the same connectivity and 4-fold interpenetrating diamondoid network. The cadmium centers adopt a distorted octahedral geometry (Cd-O, 2.23-2.51 A˚; Cd-N, 2.25 A˚). Although overall connectivity in 2 remains the same, the important change taken place here, is the rotation of the imidazole ring with respect to the benzene ring. The dihedral angles between the benzene and imidazole rings are now 31.2 and 41.9° from the original value of 29.7 and 29.1° in 1⊃2DMF. Similar examples are reported in the literature.10c,11 In 2, the interpenetrated structure forms square channels when viewed down b-axis (Figure 6). Stability of 2 is further investigated by thermogravimetric analysis (see Figure S4 in the Supporting Information). The weight loss of 20.1% in the range 60200 °C corresponds to loss of the DEF and water molecules

(calculated 19.6%). Decomposition of the framework begins at 330 °C as in the case of 1⊃2DMF. The solvent-accessible free volume of 2 calculated by PLATON is estimated to be 35.9% of the total crystal volume and is somewhat lower than 1⊃2DMF (39.6%). Compound 2 is stable in air and insoluble in water and common organic solvents. We had dipped a single crystal of 2 in dry DMF to obtain 1⊃2DMF in one weak showing the process to be reversible. This process was repeated three times and found reproducible. Such type of flexible coordination polymers that exhibits reversible SC-SC transformations by specific guest molecules could be the advanced materials for science. In an attempt to replace guest DMF molecules in 1⊃2DMF with water molecules, a crystal of 1⊃2DMF is kept in DMF exposed to air for 2 weeks at room temperature. The crystal becomes shinier and affords another interesting 3D-framework {[Cd(IBA)2(H2O)] 3 DMF 3 3H2O}n (3) without losing crystallinity. Since 1⊃2DMF is insoluble in DMF, DEF or water, we rule out the possibility of dissolution followed by crystallization and believe the transformations of 1⊃2DMF to 2 and 3 as mentioned above, take place in single-crystal to single-crystal fashion. Compounds 2 and 3 cannot be obtained via solvothermal reactions of HIBA and Cd(NO3)2.6H2O with DEF and wet DMF respectively. Only solids of indefinite

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Scheme 1. Schematic Illustration of the Framework Transformations

Figure 8. Overall 3D-framework of 3 along the b-axis (all the solvent molecules are omitted for clarity).

composition are obtained in these cases. We had also dipped the single crystals of 1⊃2DMF in pure water for complete exchange of DMF with water molecules, but the crystals turn opaque within 2 h and no cell parameter can be obtained. Compound 3 also forms a 4-fold interpenetrated network similar to 1⊃2DMF with large adamantane-like cages of dimension, 18.03  21.40  28.4 A˚3 (the channel size is measured similar to 1⊃2DMF by considering the diagonals of a cyclohexane window in a single adamantane unit). Here, each Cd(II) center exhibits hepta-coordination. The seventh coordination comes from a water molecule at a Cd-O bond distance of 2.358 A˚ keeping other six connectivities same as in case of 1⊃2DMF (Figure 7). This expansion of coordination number on the metal ion with a drastic change of the coordination geometry is an example of truly structural transformation in SC-SC fashion. Vittal has given a brief review on the transformation of structures by the expansion of coordination numbers.1g Also, three water molecules replace one of the two DMF molecules in the cavity. As a result, the pore shape changes with concomitant shrinking from 6.5 6.3 A˚2 in 1⊃2DMF to 5.74.6 A˚2 in 3 (the channel dimension is measured by considering the van der Walls radii of the constituting atoms) (Figure 8). The dihedral angles between the benzene and imidazole rings also change and become 28.6 and 29.9° due to the rotation about the C-N bond. While the Cd-N bond distances (2.28-2.31 A˚) are normal and compares well with those of 1⊃2DMF, the four Cd-O distances (2.30-2.62 A˚) are longer compared to those in 1⊃2DMF. Thermal analysis of 3 (see Figure S5 in the Supporting Information) shows the first weight-loss of 11.4% at 70 °C corresponding to all the four water molecules while the second weight-loss of 11.5% at 210 °C corresponds to the loss of the DMF molecule. The framework is stable at least up to 300 °C. The solvent accessible volume is calculated to be 24.8% of the total volume. Dipping single crystals of 3 in DMF for several days does not lead to any changes in cell parameters and PXRD patterns. However, heating 3 at 200 °C for 4 h under a vacuum leads to complete desolvation to afford 1 as monitored by its PXRD pattern. Similar to case 2, we had taken the single-crystal data at several intervals during this transformation but did not get any different cell parameter or intermediate state. Only after 15 days we observed a different

look of crystal (giving more shining) and different cell parameters. So this is an example of SC-SC transformation as well as complete structural transformation. The dynamic nature and structural transformations of the framework are illustrated in Scheme 1. To examine permanent porosity and storage capability, the adsorption properties of desolvated 1 have been performed with N2 and CO2 gases at 77 and 273 K, respectively (see Figures S6 and S7 in the Supporting Information). The adsorption measurements lead to the conclusion that neither nitrogen molecules (kinetic diameter ∼3.64 A˚) nor carbon dioxide (∼3.3 A˚) diffuse into the micropores at all. If the original cross-section of 1 were retained, these molecules would be able to diffuse through the channels. Hence, it is presumed that the pores in 1⊃2DMF contracts significantly upon removal of solvent molecules. Exchange of the DMF molecules with more volatile CHCl3 does not improve this situation. This is also corroborated by the PXRD studies that indicate different structures for the two frameworks. Probably, this behavior comes from the plastic nature of this network, which collapses upon guest removal or to the formation of a densely packed interpenetrating framework in the guest-free state that refuses entry of the gas molecules at low pressures.5d However, the type of dynamic framework transformation are supported by the following: (a) Cd(II) ion is flexible in adopting different coordination geometries, (b) flexibility include conformational changes of the ligand having rotatable C-N bond, and (c) interpenetrated diamondoid network having significant flexibility. Conclusions In conclusion, we have successfully constructed a threedimensional 4-fold interpenetrating diamondoid metal organic framework, showing reversible guest-dependent dynamic behavior and interesting single-crystal to single-crystal transformations. Such types of flexible coordination polymers could be useful models for exploring the sorption, inclusion, and catalytic properties. Further studies of dynamic MOFs with similar organic ligand-based porous MOFs are in progress in our laboratory, as these materials are expected to afford potential applications. Acknowledgment. We gratefully acknowledge the financial support received from the Department of Science and

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Technology, New Delhi, India (to P.K.B.), and a SRF from the CSIR to A.A. E.B. thanks the Spanish Ministry of Education and Science (CTQ-2008-0037/PPQ), University of Granada (Contrato de Incorporaci on de Doctores), and Junta de Andalucia for financial support. The authors acknowledge the suggestions of Prof. Jorge A. R. Navarro from the University of Granada. Supporting Information Available: Crystallographic data for 1⊃2DMF, 2, and 3 in CIF format; IR, TGA, X-ray powder diffraction patterns, NMR, and ESI-MS (PDF). This material is available free of charge via the Internet at http://pubs.acs.org.

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