A Dynamic Microporous Metal–Organic Framework with BCT Zeolite Topology: Construction, Structure, and Adsorption Behavior Sheng Hu,† Jie-Peng Zhang,‡ Hao-Xiang Li,† Ming-Liang Tong,*,† Xiao-Ming Chen,*,† and Susumu Kitagawa‡
CRYSTAL GROWTH & DESIGN 2007 VOL. 7, NO. 11 2286–2289
MOE Laboratory of Bioinorganic and Synthetic Chemistry/State Key Laboratory of Optoelectronic Materials and Technologies, School of Chemistry and Chemical Engineering, Sun Yat-Sen UniVersity, Guangzhou 510275, People’s Republic of China, and Department of Synthetical Chemistry and Biological Chemistry, Graduate School of Engineering, Kyoto UniVersity, Katsura, Nishikyo-ku, Kyoto 615-8510, Japan ReceiVed June 30, 2007; ReVised Manuscript ReceiVed August 14, 2007
ABSTRACT: A new microporous metal–organic framework (MOF) material [Ni4(dpa)4(pyz)4(H2O)8] · 11H2O (1) with BCT zeolite topology has been hydrothermally synthesized. The framework components undergo dynamic structural transformation in response to removal and rebinding of the suitable guest molecules. Microporous metal–organic framework (MOF) materials have received increasing attention mainly because of their potential application in adsorption, ion exchange, and catalysis, as well as intriguing architectures and topologies.1,2 In particular, dynamic porous MOF materials retain crystallinity after some structural transformations, including stretching, rotational, “breathing”, and scissoring mechanisms, responding to external stimuli, which is essentially distinct from that of the rigid classical porous materials.3 Those reversibly dynamic structural changes, being induced by removal/readsorption of guest molecules and/or caused by the removal/addition of ligands from/to the host framework itself, may be used for the accommodation and separation of specific molecules. However, it is still a challenge to control the pore size and chemical characteristics of the internal surface as well as decorate the topology of dynamic porous MOF materials.4 A promising route to such materials is the rational choice of suitable inorganic compositions as secondary building units (SBUs) and flexible organic ligands as the spacers. 1,1′-Biphenyl-2,2′-dicarboxylic acid (H2dpa) is an effective twisted aromatic dicarboxylate ligand, in which the phenyl rings can rotate around the C–C bond.5,6 Here, we report a new microporous MOF material with mixed flexible dpa and rigid pyrazine (pyz) ligands, [Ni4(dpa)4(pyz)4(H2O)8] · 11H2O (1), which can be dehydrated to give new solidphase [Ni4(dpa)4(pyz)4] (1′). 1′ exhibits not only permanent microporosity but also dynamic structural transformation triggered by the removal/readsorption of guest molecules. Compound 1 was prepared by the hydrothermal reaction of Ni(NO3)2 · 6H2O, H2dpa, pyz, KOH, and H2O in the molar ratio of 3:2:1:6:1111 at 175 °C (see the Supporting Information). X-ray structural analysis reveals that there are two crystallographically unique NiII ions (Ni1 and Ni2) in the asymmetric unit with similar coordination environments (Figure 1) and that each nickel ion is in a N2O4 octahedral coordination environment surrounded by two trans-related oxygen atoms from different bismonodentate dpa ligands [Ni–O ) 2.046(3)-2.091(3) Å], two cisrelated pyz ligands [Ni–N ) 2.083(4)–2.132(4) Å], and two cisrelated water molecules [Ni–Oaqua ) 2.031(4)–2.095(4) Å]. Both carboxylate groups of a dpa ligand are approximately perpendicular to each other [91.619(4)°] to form a L-shaped coordination vector.5,6 Four nickel ions are bridged by four L-shaped dpa ligands to form a quasi-planar 36-membered Ni4(dpa)4 macrocycle (Scheme 1). The intracycle adjacent and opposite Ni · · · Ni distances are 6.942 and * To whom correspondence should be addressed. E-mail: tongml@ mail.sysu.edu.cn (M.-L.T.);
[email protected] (X.-M.C.). † Sun Yat-Sen University. ‡ Kyoto University.
Figure 1. Coordination environment of the Ni atoms in 1. Symmetry code: (a) –x + 1, –y + 1, –z + 2 and (b) x, –y + 3/2, z – 1/2.
7.079 Å and 9.464 and 10.346 Å, respectively. It is remarkable that all aqua ligands point toward the center of the Ni4(dpa)4 circle, forming a hydrophilic cavity (Figure S1 in the Supporting Information). Each Ni4(dpa)4 cyclic unit connects eight neighboring ones by eight µ-pyz spacers with the intercycle shortest Ni · · · Ni distances of 6.939 and 7.051 Å, forming a new three-dimensional (3D) MOF with 1D channels along the c axis (Figure 2a). The diagonal lengths and effective window size of the channels are ca. 4.33 × 4.10 and 2.99 × 3.78 Å, respectively, taking into account the van der Waals radii for constituent atoms. The Vvoid of 1′ (without guest and aqua ligands) is 22.1% as calculated by PLATON.7 The 11 water guest molecules per molecular formula determined by the X-ray singlecrystal diffraction analysis and following thermalgravimetric analysis (TGA) are located in the hydrophilic channels. One-dimensional hydrogen-bonded water columns with fused 5-, 6-, 7-, 8-, and 10membered cyclic water cluster units are formed among the lattice and coordinated water molecules via plentiful hydrogen-bonding interactions (Figure S2 in the Supporting Information). New water clusters have received scientific interest for their unusual properties in many physical, chemical, and biological processes.8
10.1021/cg070602v CCC: $37.00 2007 American Chemical Society Published on Web 09/26/2007
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Crystal Growth & Design, Vol. 7, No. 11, 2007 2287
Scheme 1. Connectivity of the 36-Membered Ni4(dpa)4 Macrocycle in 1
The coordination network of 1 can be simplified as a uninodal 4.65 topology (the long symbol is 4 · 62 · 6 · 6 · 6 · 6) by considering the NiII ions as tetrahedral nodes and the dpa and pyz ligands as linkers. It is interesting to note that this quasi-regular topological net is homeomorphic to that of BCT zeolite and CrB4 (Figure 2b), which is rarely found in MOF materials9 because the common networks with tetrahedral nodes are diamond or quartz nets. The thermal stability of 1 was examined by TGA and variabletemperature powder X-ray diffraction (VTPXRD). The TG curve exhibits two significant mass changes over the temperature range of 25–220 °C (Figure 3). A first weight loss of 10.5% (calcd 10.7%) occurs below 75 °C and corresponds to the loss of lattice water in the channels. The departure of the coordinated water molecules occurs between 75 and 120 °C (found, 7.4%; calculated, 7.7%). The host MOF of 1′ starts to decompose up to 220 °C. VTPXRD studies (inset of Figure 3) show that 1 is stable up to 220 °C and crystallinity is retained upon the loss of all water molecules. The XRD patterns recorded between 25 and 70 °C show no evident changes, which could be associated with the removal of guest water molecules. The XRD patterns recorded above 120 °C show that a few new diffraction peaks at 2θ ) 6.9, 14.8, 15.2, and 17.3° appeared and the diffraction peak 2θ ) 8.6 (120) disappeared, although most other diffraction peaks could be observed up to 220 °C with the removal of eight coordinated water
Figure 3. TGA (green), guest-free (magenta), and reload water (blue) curves of 1. (Inset) X-ray thermodiffractogram of 1 under air from 30 to 330 °C. The patterns of red (30–110 °C), blue (130–230 °C), and green (250–330 °C) are corresponding to the structural changes (see the text).
molecules. These observations suggest that the departure of aqua ligands is followed by a significant structural deformation, which should be ascribed to the local distortion of the coordination environments around the Ni ions. The possibility of rearrangement of the coordination geometry of NiII from octahedron to square can be ruled out because both carboxylate groups are coordinated in trans positions and are rather difficult to adjust to be in cis positions in the 3D MOF structure. Therefore, adjusting the coordination mode of the carboxylate groups from former monodentate to chelate to complete a distorted octahedral coordination sphere is most likely. Above 220 °C, the departure of the organic moieties leads to the collapse of the framework, and the resulting amorphous phase crystallizes at higher temperatures to give NiO.10 The phenomenon of significant positional and rotational rearrangements of the MOF components upon external stimuli has received much attention.11 Enlightened by the results of TG and VTPXRD studies on the reversible hydration–dehydration process between 1 and 1′,12 we measured the adsorption properties of porous phase 1′ with different guests to investigate the details of the structural changes induced by the removal/readsorption of coordinated water molecules.
Figure 2. Three-dimensional MOF structure with the Ni4(dpa)4 macrocyclic unit (a) and topological network (b) viewed along the c axis of 1. The guest water molecules in the channels are highlighted in blue spheres. The dpa and pyz ligands are represented by red and blue lines, respectively.
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Scheme 2. Reversible/Irreversible Structural Transformations Responding to Various Guest Molecules
Figure 4 shows the adsorption isotherms for the different adsorbates. The adsorption isotherm of H2O shows the anticipated behavior. Below a specific pressure (P/P0 ) 0.18), the amount adsorbed in the micropore increases slightly from 0 to 42.3 mL/g (2.9 H2O molecules per formula unit for 1′). Above the specific pressure, the amount adsorbed increases suddenly to 190.5 mL/g at P/P0 ) 0.25 (12.9 H2O molecules per formula unit). Finally, the isotherm reaches the saturation point of approximately 243.6 mL/g (16.5 H2O molecules per formula unit). A similar sorption phenomenon was also observed for the use of MeOH as the adsorbate at 298 K. The isotherm shows an abrupt rise at P/P0 ) 0.5 and reaches 124.5 mL/g (8.4 MeOH molecules per formula unit) at P/P0 ) 0.52. Finally, the isotherm reaches the saturation point of approximately 163.5 mL/g (11.1 MeOH molecules per formula unit). Such adsorption behaviors of H2O and MeOH are typical of an adsorption process in which a guest-induced phase transition takes place.13 This characteristic adsorption profile supports the conversion of a phase 1′ to a phase 1, with some of the water molecules chemically binding to Ni centers and the dpa ligands going back into their former positions (Scheme 2). The absorption amount of acetone and CO2 is small, and there is no evident structural change that can be evaluated from the isotherms. The steep rise in the adsorption isotherm of CO2 (20.4 mL/g) was observed at extremely low pressures relative to the saturation vapor pressure of CO2 (P/P0 ) 0.02), and this initial
uptake indicates the presence of intrinsic micropores in [Ni4(dpa)4(pyz)4] (1′). The maximum adsorbed amount of CO2 is about 37.1 mL/g at P/P0 ) 0.95 and 2.5 molecules per formula unit. The adsorption isotherm for Me2CO is a typical physical adsorption. Me2CO cannot be adsorbed apparently because its kinetic diameter (6.60 × 4.13 × 5.23 Å) is greater than the effective pore window size. Such selectivity for different adsorbates (H2O, MeOH, CO2, and Me2CO) is excellent for porous metal–organic frameworks, which may arise from the differentiations on size or chemical characteristics on host–guest interactions. In summary, we have constructed a new dynamic microporous MOF material with 1D hydrophilic channels and rare BCT zeolite net topology by using flexible dpa ligand and rigid pyrazine spacers. The activated phase 1′ obtained by removal of the guest and coordinated water molecules functions as a dynamic microporous MOF material responding to H2O and MeOH, as evidenced by TGA, high-temperature X-ray diffraction (HTXRD), and gas/solvent adsorption studies.
Acknowledgment. This work was supported by the NSFC (20525102 and 20471069), the FANEDD of China (200122), and the Scientific and Technological Project of the Guangdong Province (04205405). Supporting Information Available: Synthesis and two structural plots of 1 and an X-ray crystallographic file, in CIF format, for the structure determination of 1. This material is available free of charge via the Internet at http://pubs.acs.org.
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Figure 4. Adsorption isotherms for different adsorbates in 1′: (a) H2O (298 K), (b) MeOH (298 K), (c) CO2 (195 K), and (d) Me2CO (298 K) and desorption isotherms for different adsorbates in 1′: (a′) H2O (298 K), (b′) MeOH (298 K), (c′) CO2 (195 K), and (d′) Me2CO (298 K) (where P/P0 is the relative vapor pressure).
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