Two Isomeric Magnesium Metal–Organic Frameworks with [24-MC-6

Communication pubs.acs.org/crystal

Two Isomeric Magnesium Metal−Organic Frameworks with [24-MC6] Metallacrown Cluster Lei Han,*,† Lan Qin,† Xiao-Zhi Yan,† Lan-Ping Xu,† Junliang Sun,‡ Lei Yu,† Hong-Bing Chen,† and Xiaodong Zou‡ †

State Key Laboratory Base of Novel Functional Materials and Preparation Science, Faculty of Materials Science & Chemical Engineering, Ningbo University, Ningbo, Zhejiang 315211, China ‡ Berzelii Centre EXSELENT on Porous Materials, Department of Materials and Environmental Chemistry, Stockholm University, SE-106 91 Stockholm, Sweden S Supporting Information *

ABSTRACT: Two novel Mg-based metal−organic framework isomers with the formula [Mg 2 (HCO 2 ) 2 (NH 2 -BDC)(DMF)2]n (NH2-BDC = 2-amino-1,4-benzenedicarboxylate) have been synthesized based on a 6-connected [24-MC-6] metallacrown secondary building unit (SBU), which display a two-dimensional (2D) 36 net (1) and three-dimensional primitive rhombohedral net (2) derived from a different extended orientation of SBU, respectively. The 2D framework of 1 exhibits relevant thermal stability, solvents stability, high CO2 adsorption, and strong luminescent properties.

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he design and synthesis of metal−organic frameworks (MOFs) have been a very important issue because of their intriguing structural versatility and their potential applications as functional crystalline materials in gas storage, separation, catalysis, sensor, and drug delivery.1 Framework isomerism is a remarkable phenomenon in the MOFs field due to not only the fabrication of multifunctional smart materials but also the investigation of their structure−property relationships.2 Although many MOFs have been reported as isomers, such as interpenetrated isomers,3 conformational isomers,4 and orientation isomers,5 controlling and predicting framework isomerism is still a challenging aspect. As defined, the framework isomers are generated by the same ligand and metal species that display different network structure;2c thus, besides the different conformations of organic linker that can result in framework isomers,4c,d the design of suitable secondary building units (SBUs) is another important synthetic strategy.4g,5 A number of defined metal carboxylate clusters, such as Cu2(CO2)4(H2O)2, Cr3O(CO2)6(H2O)3, and Zn4O(CO2)6 units, are potentially useful SBUs in the construction of MOFs with required properties.6 However, MOFs based on high nuclearity cyclic- and wheel-type SBUs are very scarce.7 Metallacrown clusters, compared to the organic crown ethers, are a class of novel complexes that could be considered as single-molecule magnets and molecular recognition agents.8 They can be also acted as excellent SBUs candidates to affect Figure 1. (a) [24-MC-6] metallacrown SBU (Mg, green; O, red; C, gray; N, blue). (b) View of the 6-connected hexagonal node in 1. (c) View of the 6-connected trigonal-antiprismatic node in 2. © XXXX American Chemical Society

Received: January 6, 2013 Revised: March 24, 2013

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Figure 2. (a) View of the 2D network of 1 along the c axis. (b) 2D 36 net of 1. (c) View of the 3D open framework of 2. (d) 3D primitive rhombohedral net of 2.

the structural topology and function of MOFs.9 For the [24MC-6] motif, there are few examples with a [M−O−C−O] repeat unit constructing the metallamacrocycle molecules.10 Only one type of [24-MC-6] metallacrown, [M6(CO2)6(HCO2)6(DMF)6] (M = Co, Mn, Fe, Ni, Mg, DMF = N,N-dimethylformamide), was used to construct isomorphous 2D (3,6)-connected coordination polymers with a 1,3,5-benzenetricarboxylate linker reported so far.11 Currently, we are interested in the construction of MOFs based on the high nuclearity metallacrown SBUs, which will open up the possibilities of the combination of porous properties of MOF with the function of the metallacrown cluster. Moreover, the ultimate goal of designed synthesis of a specific framework isomer would be implemented since the high nuclearity metallacrown SBUs could relatively induce the different conformations or extended orientations. MOFs have been described as derived from inorganic topology analogues by use of node, net, and vertex symbols in order to understand their structures in a simple way.12 In the uninodal framework structures, the 6-connected octahedral node can result in the three-dimensional (3D) primitive cubic net (pcu net),3a whereas the 6-connected hexagonal node can generate a two-dimensional (2D) 36 net.13 However, no exquisite examples based on 3D pcu and 2D 36 nets have been reported as framework isomers. As is known, a few metal ions would appear capable of forming an isolated 36 net, the challenge being to generate a 6-connected planar metalcentered node. Only a few 2D 36 nets have been reported with polynuclear metal clusters as hexagonal nodes.13 There-

fore, this work inspired us to construct framework isomers by using 6-connected [24-MC-6] metallacrown SBU of [M6(CO2)6(HCO2)6(DMF)6].11 Successfully, we have isolated two Mg-based framework isomers by selecting the 2-amino-1,4benzenedicarboxylate (NH2-BDC) organic linker and controlling synthetic condition. Two isomers have the identical components, [Mg2(HCO2)2(NH2−BDC)(DMF)2]n, and display a 2D 36 net (1) and 3D primitive rhombohedral net (elongated or squashed pcu net along C3 axis) (2), respectively, which is derived from different extended orientation of metallacrown SBU. Solvothermal reaction of Mg(NO3)·6H2O and NH2-BDC in DMF at 100 °C for 3 days afforded hexagonal prismatic crystals of compound 1, while compound 2 was obtained as rhombic dodecahedral crystals with a relatively lower yield by using of Mg(CH3COO)2·4H2O under similar conditions [see the Supporting Information (SI)]. It is noteworthy that the formate group came from the in situ hydrolysis of DMF molecules. The compounds were characterized by elemental analysis (CHN) and IR spectra. Single-crystal X-ray analyses reveal that 1 crystallizes in P3̅ space group and gives a 2D network, while 2 crystallizes in R3̅ space group and gives a 3D network.14 The most important structural feature of 1 and 2 is the presence of a [Mg6(CO2)6(HCO2)6(DMF)6] cluster as SBU, which can be described as a [24-MC-6] metallacrown.10b As shown in Figure 1a, six Mg atoms are arranged in a chair conformation, with six carboxylate groups from NH2-BDC capping the edges of the chair. Three formates reside above the chair. Each of these formates bridges three Mg atoms, two B

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Figure 3. (a) PXRD patterns of simulated and synthesized 1 and 2. (b) TG curves of 1 and 2. (c) Variable temperature in situ PXRD patterns of 1. (d) PXRD patterns of 1 after immersion in water or organic solvents for one week.

adjacent atoms through one O atom, and the third through the other O atom. This motif is repeated on the other side of the chair with three more formates. The bond lengths and bond angles around Mg centers are normal, and similar to the lightweight Mg MOFs.15 The distances of adjacent two Mg atoms in 1 and 2 are 3.541 and 3.561 Å, and the angles of the chair Mg6 structure in 1 and 2 are 101.38 and 102.32°, respectively. Comparing the metallacrown SBUs in 1 and 2, the spatial locations of coordinated DMF molecules are different because of the free rotation of CO bond in DMF. Furthermore, the different extended modes of six NH2-BDC ligands can explain the framework isomerism of 1 and 2 with different topologies. As shown in Figure 1b,c, six NH2−BDC ligands around the SBU in 1 are almost coplanar, resulting in a 6-connected hexagonal node. In 2, however, six NH2-BDC ligands are alternately arranged with SBU up and down, which generates a 6-connected trigonal-antiprismatic node. It is noteworthy that the steric hindrance and spatial locations around SBU play an important role in the formation of extended MOF structures. Therefore, the 2D 36 net of 1 and 3D primitive rhombohedral net of 2 are formed (Figure 2). This 3D primitive rhombohedral net of 2 could also be considered as an elongated or squashed pcu net along the C3 symmetry axis. To the best of our knowledge, this is the first example of framework isomers based on 6-connected 2D 36 net and 3D pcu net constructed from [24-MC-6] metallacrown SBU. As known, Mg-based MOFs represent a new class of lightweight materials that have great potential in applications like gas storage. Recent research on Mg-based MOFs showed that some Mg-MOFs have the capability to store a significant

amount of carbon dioxide (8.08 mmol/g at 298 K) and hydrogen (1.96 wt % at 77 K).15 However, their structure topology and thermal stability (especially extended 3D structure) are difficult to control because Mg2+ has a high affinity for oxygen donor atoms of water and other polar solvents. Furthermore, some Mg-MOF is sensitive to water and unstable in water due to the competitive coordination by water or other solvents. Herein we systematically investigated the thermal stability and the stability in solvents of compounds 1 and 2. Powder X-ray diffraction (PXRD) experiments on the bulk material of 1 and 2 show that all major peaks match well with simulated PXRD, indicating their crystalline phase purity (Figure 3a). TGA in a nitrogen flow revealed that 1 exhibits higher thermal stability than 2. The framework of 1 can be stable up to 200 °C, while 2 is thermal instability (Figure 3b). The reason can be assigned to the unstable geometry of SBU in 2. The thermal stability of 1 is further confirmed by variable temperature in situ PXRD experiments (Figure 3c). Compound 1 is unstable in H2O but can be stable in other common solvents, such as CH3OH, C2H5OH, CH3CN, THF, DMF, DMSO, and acetone, which was confirmed by PXRD results (Figure 3d). Compound 2 can be soluble in water and unstable in the above-mentioned solvents, which further confirmed the unstable geometry of SBU in 2. Another important structural feature of 1 and 2 is the presence of Lewis basic amino groups on the surface of their pores. Recent research on amine-functionalized MOFs revealed that they have high CO2 adsorption capacity at lower pressure due to the potential interaction between amino groups and CO2.16 This feature prompted us to examine the material’s CO2 adsorption propeties. Considering that compound 1 is stable C

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Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China (Nos. 21071087, 91122012, 21171102), the Special Project of 973 Program (No. 2011CB612306), the Swedish Research Council (VR) and the Göran-Gustafsson Foundation for Nature Science and Medical Research, the Scientific Research Foundation for the Returned Overseas Chinese Scholars, the Outstanding Dissertation Growth Fundation of Ningbo University (No. PY2012017), the Discipline Fund in Ningbo University (No. XKL11051), and the K. C. Wong MagnaFund at Ningbo University.

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Figure 4. (a) CO2 gas adsorption isotherms of 1 at 273 K. (b) Excitation and emission spectra of NH2-BDC ligand (L), 1 and 2.

and has one-dimensional channels in its AA fashion packing structure (Figure 2a), we collected the preliminary CO2 isotherm at 273 K. As shown in Figure 4a, it is completely reversible, exhibits a steep rise at low pressures, and reaches a maximum of 3.99 mmol·g−1 at 1 bar. The fluorescent properties of the NH2-BDC ligand, compounds 1 and 2 have also been investigated in the solid state at room temperature. As shown in Figure 4b, 1 and 2 exhibit similar emission peaks at 415 nm (λex = 370 nm) and 420 nm (λex = 392 nm), respectively, which show the large blue shift compared with the NH2-BDC ligand emission (λem = 555 nm, λex = 392 nm). These emissions could be assigned to the ligand-to-metal charge transfer (LMCT).4f In conclusion, two novel framework isomers of 2D 36 and 3D primitive rhombohedral net have been designed and synthesized from [24-MC-6] metallacrown SBU. Compound 1 displays relevant thermal stability, solvent stability, high CO2 adsorption, and strong luminescent properties. The exploration of other framework isomers with metallacrown SBU is underway in our laboratory.

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ASSOCIATED CONTENT

S Supporting Information *

Experimental preparation, materials and physical measurements, and crystal structure data. This material is available free of charge via the Internet at http://pubs.acs.org.

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Corresponding Author

*E-mail: [email protected]. D

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