A new 8-connected self-penetrating metal–organic framework based on dinuclear cadmium clusters as secondary building units

Chinese Chemical Letters 24 (2013) 691–694

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Original article

A new 8-connected self-penetrating metal–organic framework based on dinuclear cadmium clusters as secondary building units Kun-Huan He, Yun-Wu Li, Yong-Qiang Chen, Ze Chang * Department of Chemistry, and Tianjin Key Lab of Metal and Molecule-Based Material Chemistry, Nankai University, Tianjin 300071, China

A R T I C L E I N F O

A B S T R A C T

Article history: Received 12 March 2013 Received in revised form 10 April 2013 Accepted 16 April 2013 Available online 30 May 2013

A new metal–organic framework (MOF) based on metal clusters as secondary building units (SBU), has been synthesized and structurally characterized. The reported MOF presents an interesting 8-connected self-penetrating coordination network based on dinuclear cadmium cluster with a 424563 topology. Moreover, the thermal stability and luminescence property of this compound have been investigated. ß 2013 Ze Chang. Published by Elsevier B.V. on behalf of Chinese Chemical Society. All rights reserved.

Keywords: Metal–organic framework Secondary building units Coordination network Luminescence property

1. Introduction The rational design and synthesis of highly connected metal– organic frameworks (MOFs) remain an intensive research area, with much efforts geared toward the design of functional materials tailored to diverse potential applications [1]. The topological analysis of multitudinous networks serves as an important tool for simplifying complicated coordination networks and plays a key role in the rational design of porous functional materials with desirable properties [2]. Recently, O’Keeffe and Yaghi have devoted much effort to enumerating nets [3]. They focused on deconstructing the crystal structures of MOFs and related functional materials into their underlying nets. Such knowledge is also essential to the designed synthesis of MOFs and related functional materials potential for practical applications. In addition, network topology is an important aspect in the design and analysis of complicated coordination polymers and is also helpful in understanding the assembly of supramolecular structures [4,5]. However, examples of eight-, nine-, ten-, and twelve-connected coordination networks are extremely rare due to the limited coordination capacity of the metal centers and the steric hindrance of the organic ligands. It is still a great challenge to construct highly connected MOFs [6]. On the other hand, entangled systems as one of the major themes of supramolecular chemistry have attracted considerable attention due to their intriguing variety of architectures and

* Corresponding author. E-mail address: [email protected] (Z. Chang).

fascinating new topologies, such as catenanes, rotaxanes, and molecular knots [7]. Furthermore, self-penetration network is a type of entanglement in which the smallest topological circuits from the same networks are passed through by rods of the same net with each other [8]. A number of self-penetrating networks have been obtained in MOFs, but achieving highly connected selfpenetrating net in MOFs is still a great challenge in coordination chemistry [9]. For example, Su and co-workers have provided different strategies to construct highly connected self-penetrating coordination networks based on asymmetric ligands or long flexible or rigid ligands [10]. As for our own interest in MOFs with self-penetrated networks, we preferred using asymmetric and long rigid ligands to construct MOFs as their structural properties favor the formation of self-penetrating networks [11]. Herein we reported the construction of a MOF with unusual eight connected self-penetrating network. Using 1,4-naphthalenedicarboxylic acid (H2ndc) and 1,4-bis(imidazol-1-yl)benzene (bib) as co-ligands, a Cd MOF [Cd2(ndc)2(bib)]n (1) with eight connected network was constructed. In addition to the structural analysis, the thermal stability and luminescence property of this MOF have also been investigated. 2. Experimental 2.1. Materials and physical measurements All reagents were purchased commercially and used without further purification. The ligand 1,4-bis(imidazol-1-yl)benzene (bib) was synthesized according to the literature procedure [12].

1001-8417/$ – see front matter ß 2013 Ze Chang. Published by Elsevier B.V. on behalf of Chinese Chemical Society. All rights reserved. http://dx.doi.org/10.1016/j.cclet.2013.04.042

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K.-H. He et al. / Chinese Chemical Letters 24 (2013) 691–694

Elemental analyses of C, H, and N were performed on a PerkinElmer 240C analyzer. IR spectra were measured on a TENSOR 27 (Bruker) FT-IR spectrometer with KBr pellets. The X-ray powder diffraction (XRPD) was recorded on a Rigaku D/Max-2500 diffractometer at 40 kV, 100 mA for a Cu-target tube and a graphite monochrometer. Thermogravimetric analyses (TGA) were carried out on a Rigaku standard TG–DTA analyzer with a heating rate of 10 8C min1 from ambient temperature to 700 8C using an empty Al2O3 crucible as reference. Fluorescence spectra were recorded on a Cary Eclipse fluorescence spectrophotometer at room temperature. 2.2. Synthesis of [Cd2(ndc)2(bib)]n (1) A mixture of Cd(NO3)26H2O (148.7 mg, 0.5 mmol), NaOH (1.0 mmol, 40 mg), H2ndc (0.5 mmol, 106 mg), and bib (0.5 mmol, 112 mg) in 15 mL solvent (H2O/CH3OH = 2/1, v/v) was sealed in a Teflon-lined autoclave and heated to 165 8C for 4 days. After the autoclave was cooled to room temperature at 10 8C h1, colorless block crystals suitable for single crystal X-ray crystallographic analysis were obtained. The crystals were rinsed three times with ethanol (5 mL  3) and dried in air. Yield: 31% based on Cd. Anal. Calcd. for C18H11N2O4Cd: C 50.08, H 2.57, N 6.49. Found: C 50.22, H 2.62, N 6.38. IR (KBr, cm1): 3355w, 2930m, 2363s, 1591w, 1525m, 1400s, 1356s, 1306s, 1260s, 1055m, 952w, 834m, 776m, 636w, 540w. 2.3. Cystallographic studies X-ray diffraction data were collected on a SCX-mini diffractometer with graphite monochromated Mo-Ka radiation (l = 0.71073 A˚) at 293 K. Absorption corrections were applied by using multi-scan program SADABS. The structures were solved by direct method and refined with full-matrix least-squares technique using SHELXTL. All non-hydrogen atoms were refined with anisotropic displacement parameters. 3. Results and discussion The reaction of Cd(NO3)26H2O, H2ndc, bib, and NaOH in a mixture of water/methanol yielded block-like, colorless crystals of complex 1. The phase purity of 1 was confirmed by XRPD (Fig. S1 in Supporting information), and several extra diffraction peaks observed might be attributed to the slight change of framework structure upon the grinding of sample. Single-crystal X-ray diffraction study revealed that 1 crystallizes in the monoclinic space group C2/c [13]. The asymmetric unit of 1 contains one CdII cation (Cd1), one ndc2 anion, and half bib ligand (Fig. 1a). The CdII ion is six-coordinated and exhibits a distorted octahedral geometry. Each Cd1 is surrounded by five carboxylate oxygen atoms (O1, O1A, O2, O3, O4, Cd1–O1 2.337(4), Cd1–O1A 2.397(4), Cd1–O2 2.588(4), Cd1–O3 2.189(4), Cd1–O4 2.299(4) A˚) from five different ndc2 ligands, and one nitrogen atom (N1, Cd1–N 12.287(4) A˚) from one bib ligand (see Table S1 in Supporting information for detail). The octahedral coordinated Cd1 is connected to one adjacent Cd1A center in a corner-sharing mode to form a [Cd2(m3-O)2]4+ core via m2-carboxylate oxygen atoms (O1) with a nonbonding CdCd distance of 3.8182(11) A˚ (Fig. 1b). The Cd–O average length is in the range of 2.189(4)–2.588(2) A˚, which is comparable to those of CdII cluster-based MOFs [14]. The dinuclear cadmium units are bridged by carboxylate groups to form a linear arrangement, which is further connected by ndc2 and bib linkers to form a three dimensional (3D) framework (Fig. 2). To fully understand the network structure of 1, a topological approach is adopted to simplify the 3D framework. Apparently, the

Fig. 1. (a) Coordination environment of the ligands and CdII cations in 1 (b) views of the eight-connecting dinuclear cadmium cluster. CdII cations are presented as polyhedron.

CdII dimers are regarded as the SBU in the construction of this network. As illustrated in Fig. 3a, there are eight organic ligands (six ndc2 and two bib) surrounding each Cd2 unit. Therefore, each Cd2 unit defines an eight-connected node, which is further linked to eight nearest neighbors at distances of 6.77–15.823 A˚ through bridging ligands. Thus, the overall structure of 1 is a 3D eightconnected network with an ilc topology with a short Schla¨fli symbol of 424563 (TD10 = 3321) [15]. It should be noted that the two shortest four-member cycles (red and blue four-member cycles) in 1 are catenated (Fig. 3b). Thus, the topological structure

[(Fig._2)TD$IG]

Fig. 2. Projection view of 3D framework of 1 toward the ab plane.

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Fig. 3. Topological representation of the network of 1 showing the 424563 topology (a) and schematic representation of the self-penetrating in the network (b). The dinuclear cadmium clusters are represented by nodes.

of 1 can be classified as an 8-connected 3D self-penetrating framework. To the best of our knowledge, only eleven different eightconnected nets have been reported, which featured the Schla¨fli symbols of 42464, 42068, 416612, 4165864, 41258677, 4166118, 4451767, 424563, 36414576, 334155862, and 354115864, respectively [16]. It is obvious that 1, with the characteristic connections of 424563, is different from the previously reported nets, and it contains the features of the intersection of 36 and 44 nets. Hill and co-workers have proposed a new approach to analyze highly connected frameworks [17], namely the visualization of structures as combinations of interconnected 2D sheets or subnet sections instead of considering 3D structures solely in terms of topology via ‘‘Schla¨fli’’ symbols. To further understand the complicated structure of 1, this approach is used here. Using this method, the eight-connected ilc net can be thus described as a combination of interconnected 2D (4,4) sheets (green), which are cross-linked by zigzag chains (rose), and the zigzag chain in the inter-layer region bridges across the diagonal of a single window in the (4,4) network (Fig. S2). Previous studies have shown that coordination polymers containing cadmium oxide or hydroxide motifs may exhibit interesting photoluminescent properties [18]. The solid state luminescent properties of 1 and free ligand H2ndc have been investigated at room temperature. Complex 1 exhibits luminescence and show intense broad emission bands at lem = 506 nm upon excitation at 390 nm (Fig. 4). These emissions are neither metal-to-ligand charge transfer (MLCT) nor ligand-to-metal charge transfer (LMCT) in nature for the reason that CdII ions are difficult

[(Fig._4)TD$IG]

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to oxidize or reduce with their d10 configuration. Comparing the emission peak of H2ndc ligand at 418 nm, the emission peaks of complex 1 at 506 nm can be tentatively assigned to a charge transfer (CT) process between ndc2 and bib ligand combined with intra-ligand p*–p transitions emission states of ndc2 [19]. The enhancement of luminescence of 1 may be attributed to the ligand chelation to the metal clusters, which effectively increases the rigidity of the coordination framework and reduces the loss of energy through radiationless decay. In order to investigate the thermal stability of complex 1, thermogravimetric analyses (TGA) were carried out from 25 8C to 700 8C. As shown in the TGA curve of complex 1 (Fig. S3), there are two obvious weight loss steps. Complex 1 is thermally stable until around 350 8C. In the temperature range of 350550 8C, a whole weight loss of about 62.44% is observed, which corresponds to the removal of ndc2 and bib ligands (Calcd. 68.33%). The component of final residue (Found: 37.56%) after the removal of organic ligands is in good agreement with the expectation of CdO residuum (Calcd. 37.37%). 4. Conclusion In conclusion, we have synthesized a cluster-based MOF with a novel highly connected self-penetrated network. In this MOF, the [Cd2(OCO)6] clusters act as eight-connected node to give an unusual ilc net with a (424563) topology. Our results not only provide an intriguing example of highly connected and selfpenetrated topology, but also confirm the significant potential of constructing new highly connected coordination networks using polynuclear metal clusters modulated by N-donor ligands as spatial linkers. Acknowledgments This work was supported by the National Science Foundation of China (No. 51073079), the Natural Science Fund of Tianjin, China (No. 10JCZDJC22100), and the Fundamental Research Funds for the Central Universities. 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.2013.04.042. References

Fig. 4. Emission spectra of complex 1 and ligand H2ndc (L) in solid state at room temperature.

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