Urothermal in situ ligand synthesis to fabricate a metal-organic framework with(3,4)-connected tfi topology
Hui Yang a,b, Tie hu Li b, Yao Kang a, Fei Wang a,⁎a State Key Laboratory of Structural Chemistry, Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences, Fuzhou, Fujian, 35002, Chinab School of Materials Science and Engineering, Northwestern Polytechnical University, Xi'an 710072, China
A new 3D metal-organic framework [Cu(en)(ntd)](e-urea)(H2O) (1; en=1,2-ethylenediamine, ntd=1,4-Naphthalenedicarboxylic acid, e-urea=2-imidazolidinone hemihydrate), has been synthesized by urother-mal method. Structural analysis shows that compound 1 possesses unprecedented (3,4)-connected tfitopology. 2-imidazolidinone, one of the urea derivatives, plays multiple roles in the self-assembly ofcompound 1.
Metal-organic frameworks (MOFs) have recently attracted consider-able interest not only for their potential applications but also forfascinating architectures and topologies [1,2]. Chemical synthesis ofMOFs is largely dependent on the use of solvents, and the well-knownsynthetic methods include hydrothermal, solvothermal and ionothermalsynthesis [3–5]. Recently, urothermal synthesis that uses urea derivativesas solvents opened a new approach toward the creation of porousframework materials with promising applications [6]. One particularfeature of urothermal method is that urea derivatives exhibit a range ofbinding affinity for variousmetal cationic sites, which is strong enough toallowthemtobondcompetitively tometal sitesduring crystallization, andyet weak enough to be readily removed after synthesis to generateporosity and open metal sites. At the current stage, the urothermalsyntheses of MOFs remain largely unexplored. Herein, we report theurothermal synthesis and structural characterization of a new compound[Cu(en)(ntd)](e-urea)(H2O) (1), based on 1,4-Naphthalenedicarboxylicacid (ntd) and in situ obtained 1,2-ethylenediamine (en) by using ureaderivative 2-imidazolidinone (e-urea) as solvent, which shows anunprecedented (3,4)-connected tfi net. 2-imidazolidinone plays multipleroles in the self-assembly of compound 1.
Compound 1 was synthesized by reacting Cu(NO3)2·3H2O with1,4-Naphthalenedicarboxylic acid and e-urea at 120 °C [7]. Single-crystal X-ray diffraction analysis reveals that compound 1 crystallizesin the monoclinic space group C2/c [8], including two crystallograph-ically independent copper atoms, having a half occupancy factor(Fig. 1). Cu1 has distorted octahedral coordination; it is ligated by two
O2 and two O4 atoms in an equatorial plane and by two axial O1atoms. Because of the John–Teller effect, the Cu1-O1 (2.703 Å) isobviously longer than that of Cu1-O2 (1.965 Å) and Cu1-O4 (1.956 Å).
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Cu2 also has elongated octahedral coordination; it is ligated by two N3and two N4 atoms in an equatorial plane and two axial O1 atoms.Similarly, The Cu2-O1 distance of 2.477 Å is longer than that of Cu2-
Fig. 3. View of the 3D framework of 1, the en and H atoms were omitted for clarity; (b) the dideal (3,4)-connected tfi net.
N3 (2.015 Å) and Cu2-N4 (2.014 Å). The N atoms from two 1,2-ethylenediamine, generated by the decomposition of e-urea. Each ntdligand acts as a μ3-bridge to link two Cu1 and one Cu2 atoms. So the
efined 3-connected and 4-connected nodes; (c) the (3,4)-connected tfi net of 1; (d) the
1697H. Yang et al. / Inorganic Chemistry Communications 14 (2011) 1695–1697
Cu(1) atoms are linked by ntd ligands to form a 2D layer (Fig. 2). TheCu(2)(en)2 units are between the adjacent layers and linked by the O1atoms of the carboxyl groups to form 3D framework (Fig. 3a).
An interesting structural feature of 1 is the presence of theunprecedented (3,4)-connected tfi net. Topological analysis was per-formed by Topos [9] and Systre [10] programs. The ntd ligand can bereduced as a planar 3-connected node (L) and Cu1 atom as a planar 4-connected node (Cu)(Fig. 3b). So the framework topology can berepresented as a (3,4)-connected tfi net (Fig. 3c) with point symbol of(62,84)(62,8)2. Further analysis by the Systre program reveals the ideal tfinet is centrosymmetry with space group P42/mmc (Fig. 3d), in which theligand acts as a regular triangular 3-connected node and the Cu acts as thetetrahedral 4-connectednode, thedistancesof thenodes are almost equal.From the above structure analysis, it is obvious that the ntd ligand acts asthe planar 3-connected node, but the angles are away from 60°; the Cuatomacts as the planar 4-connected node; furthermore, the Cu(en)2 unitsact as the μ2-bridges to link two ntd ligands, whichmade the length of L-Lnodesmuch longer than that of L-Cunodes. Allmentioned abovedecreasethe symmetry of the net. To the best of our knowledge, compound 1represented the first example of the MOFs with the tfi net.
It is interesting to note that the e-urea exhibits distinct roles in theconstruction of compound 1. The first role of e-urea is to act as thesolvent, and it has good solubility to the metal salts and organicligands. The second role of e-urea is to act as the guest molecules. Thelast one is to act as chelating ligand through decomposition, which isfirstly demonstrated in compound 1 by the urothermal method.Morris and co-workers [11] have recently reported that the e-urea indeep-eutectic solvents (DESs) can decompose to deliver ethylenedia-mine, which in the protonated form can direct the formation ofzeolite-type metal phosphates. We just used the e-urea as solvent toconstruct the compound 1, also called urothermal method, which isdifferent from the DES solvents. The 1,2-ethylenediamine, which alsogenerated by the decomposition of e-urea molecules in compound 1,is not protonated but neutral considering the charge balance, and actsas an in situ generated ligand to chelate one metal center.
Thermal gravimetric analysis (TGA) of 1 indicates that the frameworkis stable up to ca. 180 °C (Figure S1). After that temperature, theframework begins to decompose. The low thermal stability of 1 may beattributed to the long Cu-O bond length (Cu2-O1=2.477 Å), whichsuggests the weak interactions between the [Cu1(ntd)] layers and Cu2(en) units.
In summary, we have successfully constructed a new compound bythe urothermal synthetic method. The compound 1 represented the firstexample of the MOFs with the tfi net, which enrich the topology types ofthe MOFs. The e-urea solvent exhibits distinct roles in 1. The resultsdemonstrated urothermal synthesis may be a useful method for theconstruction of MOFs with new topology.
Acknowledgments
This work was supported by the Natural Science Foundation ofFujian Province (2009J01045, 2010J05039).
Appendix A. Supplementary material
Supplementary data associated with this article can be found, inthe online version, at doi:10.1016/j.inoche.2011.07.009.
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