Microwave-Assisted Solvothermal Synthesis of a Dynamic Porous Metal-Carboxylate Framework Xiao-Feng Wang,†,‡ Yue-Biao Zhang,† Hong Huang,⊥ Jie-Peng Zhang,*,† and Xiao-Ming Chen*,†
CRYSTAL GROWTH & DESIGN 2008 VOL. 8, NO. 12 4559–4563
MOE Key Laboratory of Bioinorganic and Synthetic Chemistry, School of Chemistry and Chemical Engineering, Sun Yat-Sen UniVersity, Guangzhou 510275, P. R. China, School of Chemistry and Chemical Engineering, UniVersity of South China, Hengyang 421001, P. R. China, and Instrumental Analysis and Research Center, Sun Yat-Sen UniVersity, Guangzhou 510275, P. R. China ReceiVed June 13, 2008; ReVised Manuscript ReceiVed August 25, 2008
ABSTRACT: By treating Cu(NO3)2 · 3H2O with a V-shaped ligand 4,4’-oxydibenzoic acid (H2oba), a dynamic metal-carboxylate framework [Cu2(oba)2(DMF)2] · 5.25DMF (MCF-23) was synthesized, which features a wavelike layer with rhombic grids based on the paddle-wheel secondary building units. These layers stack via strong offset π-π stacking of the phenyl groups of oba ligands to give three-dimensional porosity. MCF-23 synthesized by conventional solvothermal methods always contains considerable and intractable impurities. In contrast, a microwave-assisted solvothermal method was proven to be a faster and greener approach to synthesize phase-pure MCF-23 in high yield. More interestingly, larger crystals suitable for single-crystal diffraction could be obtained by the multistep microwave heating mode. A powder X-ray diffraction study of MCF-23 reveals the dynamic structural transformation accompanying the release and reabsorption of the guest molecules. The N2 (77 K) and CO2 (195 K) gas sorption isotherms of the guest-free phase MCF-23a were measured. Introduction Porous coordination polymers (PCPs) have been enjoying greater and greater prominence worldwide owing to the creation of novel supramolecular architectures and their unique functionalities and potential applications in storage, separation, and heterogeneous catalysis.1-16 As a new category of porous materials, PCPs have the advantages of complete order, high porosity, and designable frameworks, in contrast to conventional porous materials. The past two decades have witnessed remarkable progress in the field of PCPs in the rational design, controllable synthesis, and pores modification. Nevertheless, PCPs is still in its infancy of development, exploration of rational design and synthetic approaches are highly demanded. In this context, new approaches such as high-throughput,5 ionothermal,17,18 microwave-assisted,19 and microwave ionothermal20 syntheses have been introduced to the synthesis of PCPs. In contrast to the conventional heating solvothermal method, which requires a long time (typically half to several days) and high electric power (over a thousand Watts), microwave-assisted heating is a greener approach to synthesize materials in a shorter time (several minutes to hours) and with lower power consumption (hundreds of Watts) as a consequence of directly and uniformly heating the contents. Consequently, it emerges as a feasible approach in organic synthesis,21 inorganic hybrid materials22 and for nanoscale particle preparations.23,24 One can reasonably expect that microwave-assisted solvothermal synthesis (MASS) is a promising preparation technique that has advantages in effectively saving reaction time, hastening the crystallization process, and producing phase-pure products of PCP materials in high yield and large scale,25-29 although its application in the crystal growth of large single crystals suitable for single-crystal diffraction has been scarcely reported.30 * To whom correspondence should be addressed. E-mail: zhangjp7@ mail.sysu.edu.cn (J.-P.Z.) or
[email protected] (X.-M.C.). † School of Chemistry and Chemical Engineering, Sun Yat-Sen University. ‡ University of South China. ⊥ Instrumental Analysis and Research Center, Sun Yat-Sen University.
Table 1. Crystallographic Data for MCF-23 empirical formula formula weight T/K crystal size/mm cryst syst space group a/Å b/Å c/Å V/Å3 Z Dc/g · cm-3 µ/mm-1 R1a (I > 2σ) wR2 (all data) goodness-of fit on F2 ∆Fmin/max (e/Å3) a
C44.25H54.25Cu2N5.25O15.25 1030.77 293(2) 0.20 × 0.16 × 0.12 orthorhombic Pnna 23.567(5) 15.899(3) 17.476(4) 6548(2) 4 0.797 0.684 0.0716 0.1784 0.998 1.493/-0.768
R1 ) ∑|Fo| - |Fc|/∑|Fo|. wR2 ) [∑w(Fo2 - Fc2)2/∑w(Fo2)2]1/2.
In this paper, we report the synthesis and characterization of a porous metal-carboxylate framework [Cu2(oba)2(DMF)2] · 5.25DMF (MCF-23, H2oba ) 4,4′-oxydibenzoic acid). Since conventional solvothermal reactions did not produce a satisfactory product, we have developed a multistep MASS approach to efficiently synthesize phase-pure MCF-23 in high yield. The dynamic structural transformation of MCF-23 upon release and reabsorption of guest molecules has also been studied by thermogravimetric analysis (TGA), powder X-ray diffraction (PXRD), and gas adsorption studies. Experimental Section General Considerations. All chemicals were commercially available and used as received without further purification. The C, H and N elemental analysis was carried out on a Vario EL elemental analyzer. The infrared (IR) spectrum was recorded on a Bruker Tensor 27 spectrometer using KBr pellets. The scanning electron micrographs (SEM) were obtained using a Quanta 400 thermal FE environment scanning electron microscope. TGA was performed on a NETZSCH TG 209 F3 thermogravimetric analyzer in flowing N2 with a heating rate of 10 °C · min-1. The PXRD data were collected on a Bruker D8
10.1021/cg800623v CCC: $40.75 2008 American Chemical Society Published on Web 11/08/2008
4560 Crystal Growth & Design, Vol. 8, No. 12, 2008
Wang et al.
Figure 1. Coordination environment of the dinuclear Cu(II) unit (a), (4 × 4) sheet (b), ABA layer stacking featuring offset face-to-face π-π stacking interactions of oba phenyl groups (c) in MCF-23. diffractometer with Cu KR radiation (λ ) 1.5418 Å). The adsorption isotherms of N2 and CO2 were measured respectively at 77 and 195 K using a BEL-max adsorption instrument (BEL Japan). Conventional Solvothermal Preparation of [Cu2(oba)2(DMF)2] · 5.25DMF (MCF-23). Cu(NO3)2 · 3H2O (0.121 g, 0.5 mmol) and H2oba (0.129 g, 0.5 mmol) were dissolved in 30 mL of DMF, and the mixture was then sealed in a 50-mL Teflon-lined reactor, which was heated to 160 °C and kept for 72 h. The autoclave was cooled over a period of 12 h at a rate of 10 °C h-1 to room temperature. Green block crystals with considerable brown impurities were obtained (ca. 50% ‘yield′ based on Cu and impurities not separated). Single-Step MASS Preparation of MCF-23. Cu(NO3)2 · 3H2O (0.121 g, 0.5 mmol) and H2oba (0.129 g, 0.5 mmol) were dissolved in 30 mL of DMF, and then sealed in a microwave- specified 60-mL Teflon reactor and kept for a specific time (1, 5, 15, 30 or 150 min). The reactor was cooled to room temperature in 2 h. Green microcrystals were collected by filtration, washed with DMF, and dried in air (60% yield based on Cu). Multistep MASS Preparation of MCF-23. Cu(NO3)2 · 3H2O (0.121 g, 0.5 mmol) and H2oba (0.129 g, 0.5 mmol) were dissolved in 30 mL of DMF, and then sealed in a microwave- specified 60-mL Teflon reactor. The reactor was heated to 80 °C and kept for 30 min, and then heated to 120 °C and kept for 30 min, and finally heated to 160 °C and kept for 1 h. The reactor was cooled to room temperature in 2 h. Green block crystals were collected by filtration, washed with DMF, and dried in air (80% yield based on Cu). These crystals are stable in the mother liquid but easily lose guest molecules and effloresce into microcrystals in air. Anal. Calc. for the guest-free product (C28H16Cu2O10, M ) 639.51): C, 52.59, H, 2.52; Found: C, 52.11; H, 2.43%. IR (KBr/cm-1):
Figure 2. Viewing the channels along the b-axis (a), (111) direction (b), and (01j1) direction (c). 539(w), 663(m), 777(m), 872(m), 1012(w), 1101(m), 1237(s), 1400(s), 1499(w), 1608(s), 1665(m), 2375(w) and 3431(m). X-ray Crystallographic Analysis. A green block crystal (0.20 × 0.16 × 0.12 mm3) was sealed in a capillary with mother liquid for diffraction measurement on a Bruker Smart Apex CCD diffractometer with graphite monochromated Mo KR radiation (λ ) 0.71073 Å) at 293(2) K. All intensity data were corrected for Lorentz and polarization effects (SAINT), and empirical absorption corrections based on equivalent reflections were applied (SADABS). The structure was solved by direct methods and refined by the full-matrix least-squares method on F2 with SHELXTL program package.31 Except those of DMF ligands, all non-hydrogen atoms of the framework were refined with anisotropic displacement parameters. The organic hydrogen atoms were placed in calculated positions with isotropic displacement parameters set to 1.2 × Ueq of the attached atom. The solvent molecules were highly disordered and were impossible to refine using conventional discrete-atom models; thus the contribution of solvent electron density was removed by the SQUEEZE routine in PLATON.32 The final chemical formula was estimated from the SQUEEZE results combined with the TGA results. The R1 value is 0.1007 before SQUEEZE was applied and the electron density is 819 electrons per cell which match
Dynamic Porous Metal-Carboxylate Framework
Crystal Growth & Design, Vol. 8, No. 12, 2008 4561
Figure 3. SEM images of MCF-23 synthesized via single-step MASS: (a) 1 min, (b) 5 min, (c) 10 min, (d) 30 min, (e) 150 min at 160 °C, and (f) photo of single crystals synthesized via multistep MASS. that of the solvent molecules. Selected crystallographic data and structure determination parameters are given in Table 1, and selected bond lengths and angles in Table S1, Supporting Information.
Results and Discussion Crystal Structure. The crystal structure of MCF-23 consists of two-dimensional (2-D) frameworks constructed by connections of the well-known paddle-wheel dinuclear Cu2(COO)4 secondary building units (SBUs)33 with the oxydiphenyl spacers in a (4,4) topology (Figure 1).34 As a ditopic linker, the V-shape oba with an ether-oxygen site (CsOsC ) 121.6(6)°) and two noncoplanar phenyl rings (torsion angle 65.1°) is very flexible in helping the packing of layers in the solid state. In fact, such layers are undulated and are distinct from the planar layers of [Cu2(DBC)2]35 and [Zn2(DBC)2(H2O)2] · 2DMF,36 leading to very strong off-set π-π stacking (interplanar distance 3.2 Å) of the oba phenyl groups between adjacent layers. Consequently, the adjacent layers are stacked in an ABAB fashion into a robust supramolecular 3-D structure, featuring potential porosity. When the terminal DMF ligands of dinuclear SBUs are ignored, the supramolecular structure of MCF-23 contains 3-D channels. As shown in Figure 2, MCF-23 exhibits nanoscale channels running along the b-axis (12 × 9 Å2) (van der Waals radii included), which is slightly smaller than the cavity size of an individual layer because of the slight displacement between adjacent layers. Viewed along (111) and (01j1) directions, there are elliptic and hourglass-shaped channels, which are 6.8 × 5.5 Å2 and 6.8 × 4.4 Å2, respectively. Synthesis. MCF-23 was initially synthesized by a conventional solvothermal reaction of Cu(NO3)2 and H2oba with DMF as solvent, which is dissimilar to our previous [Cu(oba)(dmso)] (MCF-21) prepared by similar reactants in a different solvent DMSO.37 However, concomitant impurity was always observed in the products, and it is very difficult to isolate the phase-pure product because the impurity is wrapped within the crystals. Thus, an alternative approach for the phase-pure product is necessary. When the single-step MASS method was applied to prepare MCF-23 by directly heating to 160 °C and incubating for 1 min, a crystalline product with an average size ca. 2 µm was obtained. However, when the incubation time was increased to
30 min, the average crystal size was not significantly increased. When the time was extended up to 150 min, the crystal size slightly increased (Figure 3), while the yields in all trials were almost the same at ca. 60%. These results demonstrate that single-step MASS is difficult to generate single crystals large enough for X-ray single-crystal diffraction, even although the PXRD reveals a longer reaction time could lead to a higher degree of crystallinity (see Figure S1, Supporting Information). Although the crystals of micron size are appropriate for practical applications as porous materials, a larger size crystal is indispensable to explore single-crystal diffraction, which is important for discovering new PCPs. Therefore, we employed a multistep MASS approach with a stepwise warming technique, and succeeded in generating crystals with an average size of ca. 50 µm along with a few single crystals of size ca. 0.2 mm (see Figure 3f). The result implies that multistep MASS may be a new efficient approach for the growth of large single crystals suitable for single-crystal X-ray analysis on a common diffractometer. Moreover, the as-synthesized product was phasepure in a high yield of 80%, and no further purification is necessary. Actually, the PXRD patterns of the as-synthesized MCF-23 via single- or multistep MASS matched well with the simulated one, while that synthesized via conventional solvothermal method (see Figure 4d) matched very poorly owing to considerable impurities (the product contain MCF-23, as its crystals can be selected for single-crystal X-ray analysis). Thermal Stability and Structural Dynamics. The cavities of MCF-23 are occupied by 5.25 solvent DMF molecules per dinuclear unit, as estimated by SQUEEZE and TGA. The TGA curve (Figure 5) of the as-synthesized MCF-23 shows a twostep loss below 250 °C. The first and gradual weight loss occurred between 30 and 100 °C, which corresponds to partial removal of the lattice solvents, while the second weight loss between 100 and 200 °C is attributed to the removal of the remaining lattice and ligated DMF molecules. The total weight loss 45.1% corresponds to 5.25 solvent and 2 ligated DMF molecules (calc. 45.3%). A reversible structural transformation is observed by PXRD from the as-synthesized MCF-23 to the guest-free phase (denoted as MCF-23a), accompanied with DMF desorption/ readsorption. By heating the as-synthesized sample up to 180
4562 Crystal Growth & Design, Vol. 8, No. 12, 2008
Wang et al.
Figure 6. The N2 sorption isotherms of MCF-23a (inset shows the lower P/Po region). Figure 4. The PXRD patterns of MCF-23 by different synthesis methods: (a) simulated; (b) multistep MASS; (c) single-step MASS; (d) conventional solvothermal.
Figure 5. The TGA curves of the as-synthesized (straight) and guestfree (dash) samples.
°C under vacuum (
