DOI: 10.1021/cg1005677
Generation of 2D and 3D (PtS, Adamantanoid) Nets with a Flexible Tetrahedral Building Block
2010, Vol. 10 3843–3846
Jian Tian, Radha Kishan Motkuri, and Praveen K. Thallapally* Energy and Environment Directorate, Pacific Northwest National Laboratory, Richland, Washington 99352 Received April 28, 2010; Revised Manuscript Received July 2, 2010
ABSTRACT: The self-assembly of the flexible tetrahedral linker tetrakis[4-(carboxyphenyl)oxamethyl]methane acid with various transition metals (Cu, Co, and Mg) results in a 2D layered structure and 3D frameworks with PtS and adamantanoid topology. The PtS net exhibits permanent porosity, as confirmed by BET and gas adsorption experiments. Metal-organic frameworks (MOFs) represent a new class of solid-state materials that have great potential in gas separation, catalysis, and controlled dug release applications.1-11 The conceptual approach for synthesizing these materials is based on the selfassembly of cationic systems acting as nodes with polytopic organic ligands acting as linkers, resulting in the construction of MOFs with zero- to three-dimensional network structures with pore sizes ranging from 5 to 20 A˚.12-14 Due to the versatile coordination chemistry of metal centers and polytopic organic ligands, MOFs can have an almost infinite variety of structures, and such diversity is not possible with any other solid-state materials known to date.15-20 One of the biggest advantages of these materials includes postsynthetic modification of functional linkers that appears to be a very valuable alternative.21-24 This approach consists of modifying the organic part of the material by a chemical reaction that takes place within the porous framework. Several research groups, including PNNL, have reported storage of various gases such as CO2, methane, and hydrogen using these materials.25-33 The adsorption of various gases in certain MOFs exceeds that of the standard zeolites due to their high surface area and pore volume. In this regard, we have recently reported a few papers on breathing phenomena as a function of gas loading and catalytic properties derived from tetrakis[4-(carboxyphenyl)oxamethyl]methane (1), a semirigid tetracarboxylic linker with a Zn2 cluster.28,34 For topological consideration, combination of a tetrahedral building block and Zn2 paddlewheel clusters lead to the formation of interpenetrated micro- and mesoporous supramolecular isomers with PtS, adamantanoid, and lonsdaleite networks having differences in solid-state packing, amount of gas uptake, selectivity toward other gases, and shape selective catalytic properties.9,10 Similarly, when auxiliary ligands such as bipyridine were used during the synthesis, a 2-fold interpenetrated PtS network and the two PtS nets connected by a bipyridine were observed with contraction and expansion of the framework upon solvent removal and CO2 uptake.9 We herein extend our research toward the synthesis and characterization of nonporous and microporous metal-organic frameworks built upon the copper, magnesium, and cobalt based SBUs and the flexible tetrahedral organic building block, 1. The synthesis of the ligand tetrakis[4-(carboxyphenyl)oxamethyl]methane (1) was accomplished by the reported procedure.35 Our initial efforts in getting single crystals for copper, cobalt, and magnesium failed in pure dimethylformamide (DMF) solvent. Thus, after modifying the synthetic procedure, single crystals of 2 (blue rectangular) were obtained by heating the reactants (ligand/ copper nitrate = 1:2 molar ratio) in a solvent mixture of DMF/ ethanol/water (v/v = 2/1/1) at 105 °C for 72 h. It is interesting to note that various attempts have been made to synthesize 2 under different reaction conditions, including pure DMF, pure DMA, and *Corresponding author. E-mail:
[email protected]. r 2010 American Chemical Society
Figure 1. Porous network structure of complex 2, with the corresponding PtS network on the bottom.
DMA/CH3CN, but single crystal quality was only obtained in DMF and in the ethanol and water mixture. Similarly, purple block crystals of 3 were synthesized with cobalt nitrate (ligand/Co = 1:2.5) in a solvent mixture of DMA/CH3CN (v/v 2/1) at 110 °C for 72 h in a Teflon lined autoclave. On the contrary, complex 4 was obtained by heating the reactants (ligand/Mg = 1:2) in a mixture of DMF/H2O (5/1) at 110 °C for 24 h in a 20 mL glass vial, respectively. All of these complexes were successfully reproduced in bulk. The single crystal X-ray measurements on 2 show a tetrahedral ligand connected to two cupper atoms in a paddle-wheel fashion, as shown in Figure 1. In-depth analysis of 2 indicates that each Published on Web 07/20/2010
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Figure 2. 2-fold interpenetrated adamantanoid network structure of complex 3.
Figure 3. Two dimensional layered structure of complex 4. Notice the noncoordinated acid groups between the layers.
paddle-wheel binuclear Cu2 unit is connected by two water molecules to complete the coordination geometry around the paddle-wheel units. Overall, the network topology of 2 can be best described as a PtS net filled with the solvent DMF molecules (Figure 1 and Supporting Information). In this regard, each tetrahedral ligand connects four neighboring Cu2 paddle wheels to form a 3D porous framework with channels of 2.7 9.9 A˚ in diameter along the [001] axis and larger channels of pore diameters 5.1 A˚ 8.3 A˚ run along the [010] axis. Complex 2 has a total solvent-accessible volume of 51.7%, as calculated using the PLATON/SQUEEZE routine. Similarly, each tetrahedral ligand in 3 is connected to four trinuclear Co3 clusters that in turn connected to a DMA molecule to complete the coordination geometry around cobalt. Overall, the structure of 3 looks like a 2-fold interpenetrated hexagonal adamantanoid network filled with coordinated guest DMA molecules (Figure 2 and Supporting Information), whereas 4 adopts a trinuclear magnesium cluster connected by a semirigid tetracarboxylate linker, 1. Every SBU in 4 connects to six tetracarboxylate linkers, and every linker connects three SBUs by the coordination of six carboxylate O atoms from three carboxylate groups and leaves the fourth carboxylate group uncoordinated. The Mg3 SBUs are interconnected through the organic linkers to generate a 2D layer. Each layer is further interconnected by weak hydrogen bonds between the uncoordinated carboxylate groups of 1 and terminal aqua ligands of Mg3 SBUs from the neighboring layers (Figure 3 and Supporting Information). As a consequence, a hydrogen-bonded 3D framework is afforded. It is noted that the unsaturated coordination of carboxylate linkers in MOFs is rarely observed. Thermogravimetric analysis of 2 and 3 shows a gradual weight loss of about 25% and 30% between RT and 250 °C, corresponding to the loss of DMF and water molecules in 2 and of DMA and
Figure 4. BET surface area analysis on complex 2 at 77 K and 1 bar of nitrogen.
acetonitrile in 3, respectively. No further weight loss was observed upon heating to 400 °C in the case of complexes 3 and 4, whereas complex 2 started degrading after 350 °C. The powder XRD patterns of 2 before and after activation at 200 °C suggest the stability of the framework upon solvent removal. But, since complex 4 has a hydrogen bonded network, we performed the activation of the sample at three different temperatures;65 °C, 125 °C, and 200 °C;and tested PXRD of all three samples. From XRD studies, we observed that the structure of complex 4 is collapsing after heating the sample at 125 °C. As confirmed by the structural stability of complex 2 upon solvent removal using PXRD, BET surface area measurements were performed to find out the microporous nature of the material. The complex 2 was activated at high temperature under vacuum and the BET
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Figure 5. Gas adsorption (Δ) and desorption (r) isotherms of 2 at low (left) and high pressure (right). Notice the selectivity between CO2 over methane and nitrogen.
sorption experiments were performed using liquid nitrogen at 77 K. The nitrogen adsorption isotherm of 2 shows type I behavior, characteristic of microporous materials, confirming the permanent porosity with reversible adsorption and desorption of nitrogen from the host cavity. Calculated from the nitrogen adsorption data, the Langmuir surface area of complex 2 is 685 m2 g-1 (434 m2 g-1 BET, Figure 4). The estimated BJH pore diameter and micropore volume are 13 A˚ and 0.10 cc/g. BET surface area measurements on 3 and 4 show these two complexes were found to be nonporous and consistent with PXRD measurements. Low and high pressure gas sorption measurements were performed on 2 using a volumetric gas analyzer (HPVA-100). Complex 2 is activated at 200 °C under dynamic vacuum for 12 h, gas adsorption sorption experiments were performed by introducing a known amount of CO2 gas into the sample chamber at regular intervals, and the volume adsorbed per gram of material was plotted against the pressure. Due to the availability of the large pore and surface area, complex 2 adsorbed 9.7 wt % (2.21 mmol/g) of CO2 at 1 bar pressure. The sorption measurements were repeated by evacuating the sample and pressurizing with CO2 again, and almost the same weight percentage was obtained. From the CO2 sorption plot for complex 2, it suggests that there is no saturation at 1 bar; therefore, high pressure CO2 experiments show complex 2 sorbs 24 wt % (5.47 mmol/g) at 35 bar (Figure 5). Similar sorption experiments with methane and nitrogen gas on complex 2 indicate an uptake of only 4.0 (2.5 mmol/g) and 1.5 wt % (0.5 mmol/g) at 30 bar. From these experiments, it is clear that complex 2 can preferentially sorb more CO2 over methane at a given pressure but did not observe a significant amount of nitrogen at the same conditions. The pore size of complex 2 shows 5.1 A˚ 8.3 A˚, which is large enough to accommodate all the gases, but the gas sorption isotherms indicate no uptake of nitrogen. This could be explained by considering the quadrupole moment and polarizability of the gases. The higher uptake of CO2 is explained due to the higher quadrupole moment and polarizability compared to CH4 and N2 whereas the higher uptake of methane over nitrogen can be explained due to the higher polarizability of methane over nitrogen. Similarly, complex 4 was activated at three different temperatures (65, 125, and 200 °C) and performed gas sorption experiments, respectively. Only the complex activated at 65 °C was found to be sorptive and have a relatively low soprtion of CO2 (4 wt % at 1 bar pressure). This relatively low sorption value can be attributed to the structural deformation of the hydrogen-bonded network during the thermal activation of 4. Similarly, we did not observe any noticeable uptake in 3. During the preparation of this manuscript, we noticed a paper by Hong-Bin Du and co-workers36 reporting the synthesis and gas sorption properties of [Cu2(1)(H2O)2] 3 (DMA)4(H2O)2 (20 ), synthesized from dimethyl acetamide, which has identical network topology and crystallographic cell parameters as we observed.
According to Du and co-workers, complex 20 collapses by heating at 250 °C. The Langmuir surface area (507 vs 685 m2/g) and uptake of CO2 (50 vs 120 cm3/g) and methane (24 vs 60 cm3/g) were very low compared to what we reported in the present paper. This could be due to the fact that the authors activated complex 20 at 100 °C for 10 h, which may not be enough to remove the trapped solvent molecules completely from the host lattice. In conclusion, we have successfully synthesized three new metalorganic frameworks from a flexible organic linker and Cu, Co, and Mg based SBUs. Complex 2 results from a tetrahedral carboxylate linker and Cu2 paddle-wheel units to form a PtS network topology with 24 wt % of CO2 uptake at 35 bar with selectivity over nitrogen and methane. We also observed a rare noncoordinated carboxylate linker in complex 4, which could potentially be used for generating a mixed valence metal-organic framework. Acknowledgment. P.K.T. thanks the U.S. Department of Energy, Office of Basic Energy Sciences, Division of Materials Sciences and Engineering, under Award KC020105-FWP12152, for the synthesis and characterization part and the Office of Fossil Energy for CO2 uptake measurments. The Pacific Northwest National Laboratory is operated by Battelle for the U.S. Department of Energy under Contract DE-AC05-76RL01830. Supporting Information Available: Crystal data for 2-4, structural images, PXRD patterns, TGA patterns, BET plot, and BJH desorption. This material is available free of charge via the Internet at http://pubs.acs.org.
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