Chinese Chemical Letters  2015, Vol.26 Issue (09): 1059-1064   PDF    
Preparations and characterizations of two MOFs constructed with hydroxylphenyl imidazole dicarboxylate
Ji-Feng Wang, Bing-Bing Shi, Gang Li     
College of Chemistry and Molecular Engineering, Zhengzhou University, Zhengzhou 450001, China
Abstract: Using the hydrothermal reactions of Mn(II) and Ba(II) salts with 2-(3-hydroxylphenyl)-1H-imidazole-4,5-dicarboxylic acid (m-OHPhH3IDC), two novel metal-organic frameworks, namely, {[Mn(m-OHPhHIDC)(H2O)]·2H2O}n (1) and {[Ba(m-OHPhH2IDC)2(H2O)3] ·2H2O}n (2) have been synthesized and structurally characterized by single-crystal X-ray crystallography, elemental analyses, and IR spectroscopy. Complex 1 features a novel non-interpenetrated three-dimensional (3,4)-connected network with one-dimensional open channels. Complex 2 exhibits a two-dimensional layered structure with rhombic grids. The role of the central metals in the formation of final architectures has been discussed. Furthermore, luminescent and thermal properties of the two complexes have been studied.
Key words: Imidazole dicarboxylate     Metal-organic frameworks     Manganese     Barium     Property    
1. Introduction

A variety of metal-organic frameworks (MOFs) have been rationally designed and constructed using conscientious strategies [1, 2, 3, 4, 5]. Rapid progress in this flourishing field has been made, but it still remains a great challenge to design predetermined architectures. There are many factors contributing to the complexity and uncertainty of the MOFs, such as temperature, pH values of the reaction solution, solvent system and the ratio of ligand and metal, etc. [6, 7, 8, 9]. Therefore great contributions have been focused on investigating the related effects. In practice, tactical synthesis or selection of the suitable organic ligand plays a crucial role. The organic ligands containing N and O donors, especially the N-heterocyclic carboxylates have been a long-standing fascination of chemists and much attention has been paid to the pyridine-, imidazole-, pyrazine- and triazole-based carboxylic acids. We have witnessed the reactivity of this kind of ligands with different metal ions, resulting in novel molecular structures with interesting properties [10, 11].

In this context, the imidazole-4, 5-dicarboxylic acid (H3IDC) ligand and its derivatives substituted at the 2-position with diverse groups such as alkyls, and aryls have attracted particular attention [12, 13, 14]. These ligands are particularly attractive building blocks and worthy to be investigated for two reasons: (1) These ligands containing two N and four O atoms and can be used as multidentate ligands; (2) The ligands can be partially or fully deprotonated at different pHvalues to exhibit various binding properties. Motivated by these possibilities, we have synthesized a series of attractive MOFs based on the 2-substituted-1H-imidazole-4, 5-dicarboxylic acid ligands and its derivatives, which have the potential to coordinate with metal ions as a new type of building block [15, 16, 17, 18, 19]. As a continuation of our systematic contribution, 2-(3-hydroxylphenyl)- 1H-imidazole-4, 5-dicarboxylic acid (m-OHPhH3IDC) has been selected and investigated by our research group. To the best of our knowledge, no relevant systematic studies concerning molecular self-assemblies of Mn(II) and Ba(II) ions based on m-OHPhH3IDC ligand have been presented in public domain.

On the other hand, the central metals also have a significant effect on the structural construction of complexes and their properties. As a result, two m-OHPhH3IDC-based MOFs with different dimensionalities, namely {[Mn(m-OHPhHIDC)(H2O)]· 2H2O}n (1) and {[Ba(m-OHPhH2IDC)2(H2O)3]·2H2O}n (2) were isolated via a hydrothermal method and structurally characterized by elemental analysis, IR spectroscopy, thermogravimetric analysis (TGA) and X-ray crystallography.

2. Experimental 2.1. Materials

All chemicals were of reagent grade and were obtained from commercial sources and used without further purification. The organic ligands m-OHPhH3IDC were prepared according to the literature procedures [20]. The C, H and N microanalyses were carried out on a FLASH EA 1112 analyzer. IR spectra were recorded on a Nicolet NEXUS 470-FTIR spectrophotometer as KBr pellets in the 400-4000 cm-1 region. TG-DSC measurements were performed by heating a crystalline sample from 20 ℃ to 850 8C for 1, and from 20 ℃ to 890 ℃ for 2 at a rate of 10 8C min-1 in air on a Netzsch STA 409PC differential thermal analyzer. Fluorescence spectra were characterized at room temperature using an F-4500 fluorescence spectrophotometer. X-ray powder diffraction (PXRD) measurements were recorded on a PANalytical X’pert PRO X-ray diffractometer.

2.2. Preparations of {[Mn(m-OHPhHIDC)(H2O)]·2H2O}n (1) and {[Ba(m-OHPhH2IDC)2(H2O)3]·2H2O}n (2)

For 1, a mixture of MnCl2·4H2O (19.8 mg, 0.1 mmol), m- OHPhH3IDC (24.7 mg, 0.1 mmol), Et3N (0.056 mL, 0.4 mmol) and CH3CH2OH/H2O (3/4, 7 mL) was sealed in a 25 mL Teflon-lined bomb and heated at 150 ℃ for 96 h. The reaction mixture was then allowed to cool to room temperature at a rate of 10 ℃ h-1. Colorless flake-shaped crystals of 1 were collected, washed with distilled water, and dried in air (58% yield based on Mn). Anal. Calcd. for C11H12N2O8Mn: C, 37.18; H, 3.38; N, 7.89%. Found: C, 37.31; H, 3.25; N, 8.06%. IR (cm-1, KBr): 3472 (m), 1565 (m), 1469 (s), 1417 (m), 1392 (m), 1357 (s), 1307 (w), 1243 (s), 1164 (w), 815 (w), 796 (s), 734 (s), 684 (w), 589 (w), 549 (m).

For 2, a mixture of BaCl2·2H2O (24.4 mg, 0.1 mmol), m-OHPhH3IDC (24.7 mg, 0.1 mmol), Et3N (0.056 mL, 0.4 mmol) and CH3CH2OH/H2O (3/4, 7 mL) was sealed in a 25 mL Teflon-lined bomb and heated at 150 ℃ for 96 h. The reaction mixture was then allowed to cool to room temperature at a rate of 10 ℃ h-1. Brown crystals of 2 were collected, washed with distilled water, and dried in air (68% yield based on Ba. Anal. Calcd. for C22H24N4O15Ba: C, 36.57;H, 3.32;N, 7.76%. Found: C, 36.62;H, 3.31; N, 7.56%. IR (cm-1, KBr): 3405 (m), 1692 (w), 1564 (w), 1473 (m), 1378 (m), 1282 (w), 1279 (s), 1116 (s), 880 (m), 788 (s), 732 (s), 583 (w), 532 (m).

2.3. Crystal structure determinations

Suitable single crystals of 1 and 2 were carefully selected under an optical microscope and glued to thin glass fibers. Single-crystal data (in Table 1) for 1 and 2 were obtained on a Bruker smart APEXII CCD diffractometer with a graphite-monochromated Mo-Kα radiation (λ = 0.71073 Å). All data were collected at room temperature using the ω-2θ scan technique and corrected for Lorenz-polarization effects. Furthermore, a correction for secondary extinction was applied.

Table 1
Crystallographic data for complexes 1 and 2.

The two structures were solved using the direct methods and expanded using the Fourier technique. The non-hydrogen atoms were refined with anisotropic thermal parameters. The hydrogen atoms of the ligands were positioned geometrically and refined using a riding model. The hydrogen atoms of the water molecules were found at reasonable positions in the differential Fourier map. All the hydrogen atoms were included in the final refinement. The final cycle of full-matrix least squares refinement was based on 2196 observed reflections and 211 variable parameters for 1 and 6196 observed reflections and 418 variable parameters for 2. All calculations were performed using the SHELX-97 crystallographic software package [21]. Selected bond lengths and angles are listed in Tables 2 and S1 in Supporting information.

Table 2
Hydrogen bonds distances (Å) and angles (°) for 1 and 2.
3. Results and discussion

X-ray single-crystal diffraction reveals that compound 1 crystallizes in a trigonal system with the space group R3c and exhibits a charming 3D motif with 1D open channels. The coordination environment around each Mn(II) ion can be described as a slightly distorted [MnNO5] octahedron geometry. The Mn(II) atom is coordinated by four oxygen atoms and one nitrogen atom from three individual m-OHPhHIDC2- (O2c, O3d, O4, O5, N1), the remaining one site is occupied by a water molecule to complete the six-coordination configuration. Among the three m-OHPhHIDC2- ligands, one is bis-monodentate with O, N chelating donors to forma stable five-membered chelating ring, one is also bis-monodentate with two carboxyl oxygen atoms, while the other acts as a monodentate O-donor ligand. TheMn-O bonds span the range of 2.104(4)-2.217(4) A˚ and the Mn-N distance is 2.327(4) Å and the O/N-Mn-O/N bond angles range from 74.00(14)° to 177.28(17)°, which are in good agreementwith the bond lengths and bond angles observed in other Mn(II) compounds. Similar structural features can be found in the reported structural motifs by us [22]. All the m-OHPhHIDC2- ligands adopt the same coordination mode μ3-kN, O:kO, O′:kO″ to bridge three Mn(II) ions in N, O-chelating, O, O′-chelating, and monodentate fashions, as depicted in Scheme 1a. Each Mn metal center is bridged by the m-OHPhHIDC2- ligands in a bridging coordination mode forming a one-dimensional (1D) zigzag chain (Fig. 1b) through the fusing of five- and seven-membered rings in the c direction. Moreover, the -COO group further connected to another Mn(II) center in a bridging mode, which results in the formation of a three-dimensional (3D) metal-organic framework along the crystallographic c axis. Meanwhile, infinite 1D open channels are observed along the c-axis direction (Fig. 1c), and the disordered phenyl groups from the imidazole-4, 5-dicarboxylic acid ligands are all located around the 1D channels in this network. The guest water molecules occupy the void interspace region through O-H…N/O hydrogen-bonding interactions with the m-OHPhHIDC2- units.

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Scheme 1.Coordination modes of the imidazole dicarboxylate ligand.

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Fig. 1.(a) Coordination environment of Mn(II) atom in polymer 1 (H atoms omitted for clarity); (b) view the zigzag chain of 1; (c) view of the 3D framework of 1 with 1D open channels; and (d) (3,4)-connected topological network with the point symbol 8(2).8.8.

To understand the 3D structure better, the simplification of the building blocks for polymer 1 is helpful. From a topological point of view, the m-OHPhHIDC2- coordinates to three Mn ions, so it can be viewed as a 3-connected node. The Mn atom is surrounded by three m-OHPhHIDC2- ligands and one water ligand, which can be represented by a 4-connected node. Then the 3D framework of 1 can be described as a (3, 4)-connected net with 8(2).8.8 topology (Fig. 1d) by the software Olex and Diamond.

In contrast, the structure of compound 2 is a two-dimensional (2D) network. As illustrated in Fig. 2a, the asymmetric unit of 2 consists of one Ba(II) ion, three m-OHPhH2IDC- ligands, three coordinated water molecules. The Ba(II) ion is coordinated by five oxygen atoms from differentm-OHPhH2IDC- ligands, three oxygen atoms from the coordinated water molecules. The Ba-Owater bond lengths are 2.7916(18) Å, 2.8262(19) Å, 2.8715(18) Å, respectively, and the Ba-Ocarboxylate bond lengths are in the range of 2.6425(15)- 2.8352(17) Å . Unlike the polymer 1, there are two coordination modes (Scheme 1b and c) for the m-OHPhH2IDC- ligand in 2. One carboxylate groups adopts the μ2-kO:kO′ bridging mode, and the other is in the μ3-kO:kO:kO′ coordination mode, whereby both oxygen atoms coordinate to the two Ba(II) center in a chelating mode and one of the oxygen atom of the -COO group further connected to another Ba(II) center in a bridging mode. The μ3- kO:kO:kO′ coordination of carboxylate group of two different m- OHPhH2IDC- ligands results in the formation of a Ba2O2 dimer with the Ba…Ba distance of 4.58 Å . Each dimer connects to the other four surrounding units using eight m-OHPhH2IDC- linkers to form a 2D grid-like layer. The connectivity of the dimers with m- OHPhH2IDC- leads to the formation of single-strand mesohelical chains with alternating left and right turns of the strand, which results in the formation of a 2D metal acid layer. The guest water molecules occupy the void interspace region through O-H…O/O- H…N hydrogen-bonding interactions with the coordinated water molecules and m-OHPhH2IDC- units. From the topological point of view, if one m-OHPhH2IDC- ligand acts as a three-connected node, each Ba ion is an eight-connected node and the other m- OHPhH2IDC- ligand is a linker, compound 2 displays a (3, 8) topological (Fig. 2d) framework with 3(4).4(2).5.6(4).7(4) by the software Olex and Diamond. It is quite unusual to find a divalent ion connected to five different ligands and thus the resulting topology for 2 is unprecedented in the literature. To the best of our knowledge, there are six reported Ba(II) polymers derived from 2-substituted imidazole-4, 5-dicarboxylate ligands [23, 24, 25, 26, 27, 28], one of which shows 1D chains constructed with the N-containing bridging coligand phen, the other five present different charming 3D structures constructed from imidazole dicarboxylate ligands bearing aromatic substituents at the 2-position. The successful preparation of the 2D polymer 2 herein provided a unique opportunity to study the structures and properties of the related polymers.

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Fig. 2.(a) Coordination environment of Ba(II) atom in polymer 2 (H atoms omitted for clarity); (b) 1D chain constructed by the connectivity of Ba2O2 dimer and m-OHPhH2IDC- ligands; (c) two-dimensional layer formed due to connectivity of Ba(II) with m-OHPhH2IDC- ligands; and (d) schematic description of the (3,8)-connected 2D network with 3(4).4(2).5.6(4).7(4) topology (green for 3-connected μ3-m OHPhH2IDC- node and purple for 2-connected μ2-m-OHPhH2IDC- node).

In order to assess the influence of metal ionic radii in tuning the dimensionality, we tried to correlate the formation of diverse architectures with different ionic radii of the metal ions used in this study. Compound {[Ba(m-OHPhH2IDC)2(H2O)3]·2H2O}n (2) can be obtained from compound {[Mn(m-OHPhHIDC)(H2O)]·2H2O}n (1) by replacing the metal center Mn(II) with Ba(II) under the same reaction conditions (temperature, molar ratio of reactants and pH value). Even though the nodes in both compounds contain m- OHPhH3IDC molecules and water molecules, the metal acid architectures are not identical in their respective crystal structures. A close inspection of coordination spheres of 1 and 2 reveals that, around the Mn(II) center (1), four carboxylate oxygen atoms from three m-OHPhH2IDC- linkers are present, whereas in the case of the Ba(II) metal center (2) five carboxylate oxygen atoms from five m-OHPhH2IDC- linkers form the coordination sphere. Compared to the ionic radius of Mn(II) ion (67 pm), the ionic radius of Ba(II) metal ion (135 pm) is relatively large. As a result, it has the tendency to adopt a higher coordination number (up to 8) and can accommodate more bulkiness in its coordination sphere, The structures clearly indicate that the crowdedness around the Ba(II) center in compound 2 is higher compared to that around the Mn(II) center in compound 1. Generally higher coordination numbers can be adopted along with the increase of the ionic radius of metal ion, forming a complicated or higher dimensional structures. However in compound 2, when the ionic radius increases, the metal atom includes more bulkiness by concentrating the linkers in its coordination sphere and the dimension reduces. This observation seems to be peculiar and we attempt to prepare the manganese analog with m-OHPhH3IDC, but amorphous powders were obtained. Overall, the structural features of the two compounds can be summarized as follows: it is the central metal that drives the formation of the final structures of complexes 1 and 2, although the influence the coordination modes of the m-OHPhH3IDC ligand can be important. The differences of the atomic radius of the central metal ions and the coordination modes of ligands are the key factors for producing diverse structures. These results prompted us to study these effects in similar systems, which will be published in due course.

In addition, the pH value of the reaction system is also a key influencing factor in the synthesis of coordination polymers. Through several parallel experiments in the synthesis of coordination coordination polymers 1 and 2, the optimal pH value was found to be around 8.0. A slight deviation would cause a failure.

The IR spectra display characteristic absorption bands for water molecules, the carboxylate, imidazole and phenyl units. Compounds 1 and 2 show strong and broad absorption bands in the range of 3400-3500 cm-1, which indicates the presence of the nN-H and the υO-H stretching frequencies from the imidazole ring and coordinated water molecules, respectively. The carboxyl can be observed from the absorption bands in the frequency range 1357- 1696 cm-1 as a result of υas(COO-) and υs(COO-) vibrations, respectively. The characteristic IR band of the phenyl ring at 840- 860 cm-1 is due to δ(=C-H) vibrations, which can be found at 815 cm-1 for 1, and 879 cm-1 for 2. In conclusion, the infrared spectral data of the complexes 1 and 2 are consistent with crystal structure analysis.

To confirm the phase purity of the two polymers, the XRD patterns were recorded. Most of the peak positions of the simulated and experimental patterns are in good agreement (Fig. 3). Wealso performed a thermal gravimetric analysis for these polymers, because thermal stability is an important quality for the application of metal-organic crystalline materials. The TGA curves for complexes 1 and 2 are also provided in Fig. 4.

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Fig. 3.PXRD of polymers 1 and 2.

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Fig. 4.TG analysis profiles of 1 and 2.

The TGA curve of 1 shows that after the first weight loss step, corresponding to the removal of the two free water molecules (observed 10.25%, calculated 10.23%), a plateau region is observed from 140.0 ℃ to 155.0 ℃. After that, the second weight loss step begins (observed 5.19%, calculated 5.11%), corresponding to the loss of the coordinated water molecule (155-217.5 ℃). When the temperature is higher than 217.5 ℃, the decomposition of the remaining organic units of m-OHPhHIDC2- can be observed (observed 64.40%, calculated 64.49%). The residue is MnO (calculated 20.17%, observed 20.16%).

The polymer 2 experiences three steps of weight lost. It firstly loses the two free water molecules (observed 4.82%, calculated 4.99%). The second gross weight loss step is 7.61%, which occurs in the range of 175.0-245.0 ℃ corresponding to the removal of the three coordinated water molecules (calculated 7.48%). When the temperature is higher than 270.0 ℃, the decomposition of the framework starts (observed 66.34%, calculated 66.34%). Finally, a plateau region is observed from 735.0 ℃ to 884.9 ℃ showing the final residue being BaO (observed 21.23%, calculated 21.19%).

Many metal-organic frameworks exhibit high thermal stability and the ability to tune emission intensity of the free-organic ligands after coordination with metals. The luminescent properties of complex 2 and the free ligand were investigated in the solid state at room temperature. Their emission spectra are illustrated in Fig. 5. The free m-OHPhH3IDC ligand shows luminescence with an emission maximum at 468 nm by selective excitation at 400 nm, which may be attributed to ligand-centered π*n or ππ* electronic transitions. Compared with the free ligand, the intensity of the luminescence spectrum of compound 2 was enhanced slightly, showing an emission maximum at 460 nm by selective excitation at 400 nm. Due to the electronic configuration of the organic ligands, the emission band of compound 2 is attributed to the ligand-to-ligand charge transfer [29], so we can observe a slight blue shift of 2 comparing to the free ligand. Unfortunately, the polymer 1 shows very weak emissions and it is visible that Mn2+ shows fluorescence quenching for the m-OHPhH3IDC.

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Fig. 5.Emission spectra of the free ligand and polymer 2 in the solid state at room temperature.
4. Conclusion

In this contribution, based on the newly prepared multifunctional organic ligand m-OHPhH3IDC, two different-dimensional (2D/3D) MOFs with architectural diversity have been successfully synthesized under hydrothermal conditions. The results show that the metal centers and the different coordination modes of the ligand play dominating roles in directing the formation of structures. Furthermore, photoluminescence properties of the complexes were studied in the solid state at room temperature. Further investigations for the influence of metal ionic radii in the self-assembly of coordination networks are underway in our laboratory.

Supplementary data

Crystallographic data for the structure reported in this paper in the form of CIF file has been deposited with the Cambridge Crystallographic Data Centre as supplementary publication Nos. 948693 and 948606 for 1 and 2, respectively. Copy of the data can be obtained free of charge on application to CCDC, 12 Union Road, Cambridge CB2 IEZ, UK (fax: +44 1223 336 033; e-mail: deposit@ccdc.cam.ac.uk).

Acknowledgments

We gratefully acknowledge the financial support by the National Natural Science Foundation of China (No. 21341002), and the Natural Science Foundation of Henan Education Department (No. 13A150655).

Appendix A. Supplementary data

Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.cclet.2015.04.022.

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