A New 10-Connected Coordination Network with Pentanuclear Zinc

Communication pubs.acs.org/crystal

A New 10-Connected Coordination Network with Pentanuclear Zinc Clusters as Secondary Building Units Kun-Huan He, Wei-Chao Song, Yun-Wu Li, Yong-Qiang Chen, and Xian-He Bu* Department of Chemistry and Tianjin Key Lab on Metal and Molecule Based Materials Chemistry, Nankai University, Tianjin 300071, China S Supporting Information *

ABSTRACT: An unusual 3D 10-connected metal−organic framework [Zn5(μ3-OH)2(1,4-ndc)4(1,4-bix)2] (1, 1,4-ndc = 1,4-naphthalenedicarboxylic acid; 1,4-bix = 1,4-bis(imidazol-1ylmethyl)benzene) has been hydrothermally synthesized and structurally characterized. Topological analysis indicates that 1 is a 3D 10-connected self-penetrating framework based on [Zn5(μ3-OH)2] pentanuclear zinc clusters as secondary building units with the short Schläfli symbol of 364345362. This represents highly connected a uninodal network topology presently known for self-penetrating systems. Moreover, the TGA and luminescence properties of 1 were investigated.

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INTRODUCTION In recent years, rational design and synthesis of metal−organic frameworks (MOFs) as porous crystalline functional materials have attracted considerable attention not only for their diversity of architectures and fascinating topologies1 but also for their potential applications in gas storage/adsorption,2 catalysis,3 drug delivery,4 luminescence,5 and so on. One of the most important goals in the synthesis of new crystalline functional materials with desirable properties and expected structures derives from inorganic and organic building blocks.6−9 Considerable efforts have been devoted to the design and synthesis of highly connected MOFs because of their potential advantage in enhancing the stability of the frameworks and providing a blueprint for targeting particular packing arrangements. However, the steric hindrance of most organic ligands and limited coordination sites of single-metal ions centers still represent great challenges in construction of high-connected MOFs.10 To overcome these obstacles, Wuest11 and Yaghi and co-workers12 demonstrated some effective synthetic strategies to construct MOFs with highly connected nets by means of multinuclear clusters or metal−organic polyhedra (MOP) as edge transitive net.13 And some successful examples of 9-, 10-, 12-, and 14-connected nets have been obtained from highnuclear clusters, such as hexa-, hepta-, octanuclear, and larger ones.13 On the other hand, due to the intriguing topological structures and potential applications, much effort has been devoted to construct self-penetrating networks, and only a limited number of high-connected cases are known.13a A 2D 2-fold interpenetrating MOF with mononuclear Zn2+ as node based on Zn2+, 1,4-ndc, and 1,4-bix has been reported by Li et al.13d We selected 1,4-ndc and 1,4-bix as the ligand, in view of its rich coordination chemistry, attempting to construct cluster based-MOFs with unique structure types and associated © 2012 American Chemical Society

properties. Herein, we present a 3D uninodal network topology presently known for self-penetrating systems with a 10connected net constructed by pentazinc clusters based on the following considerations: (i) under hydrothermal conditions, hydrolysis of metal salts generates hydroxyl ions and metal ions by sharing corners (μ3-OH) to form metal−oxygen clusters; (ii) with the aid of rigid carboxylate ligands to chelate metal ions affording metal clusters; (iii) a long flexible 1,4-bix ligand with small steric hindrance, which can link polynuclear clusters into an extended 3D network. An effective synthetic strategy has been proposed to construct high-connected MOFs under proper pH value conditions by in situ reaction generating multinuclear zinc clusters as secondary building units (SBUs).13d The reaction of Zn(NO3)2·6H2O with mixed ligands H2ndc and 1,4-bix under hydrothermal conditions afforded colorless crystals of complex 1.14 The as-synthesized complex was characterized and formulated by elemental microanalysis and single-crystal Xray diffraction study as [Zn5(1,4-ndc)4(OH)2(1,4-bix)2].15 As shown in Figure 1a, X-ray analysis reveals that complex 1 crystallizes in monoclinic space group P21/c and contains three crystallographically independent Zn(II) centers (Zn1, Zn2, and Zn3), which are linked via one μ3-OH group to form a [Zn3(μ3OH)] core (Figure S1 of the Supporting Information). The Zn1 atom, sitting on an inversion center, is six-coordinated by four carboxylate oxygen atoms (Zn1− O1 2.098(3), Zn1−O4 2.123(2) Å) from four different 1,4-ndc ligands and two μ3-OH oxygen atoms (Zn −O9 2.085(3) Å) in an octahedral environment. Zn3 has a similar coordination environment to Received: November 1, 2011 Revised: February 14, 2012 Published: February 16, 2012 1064

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Figure 1. (a) Ball-and-stick and (b) polyhedral views of the pentanuclear zinc cluster of 1. (c) Each pentanuclear zinc cluster surrounded by 12 organic ligands.

link four Zn(II) centers, and 1,4-bix acts as a bidentate bridging ligand via N1 and N4 atoms to join two neighboring Zn(II) centers. In addition, because of the π−π stacking interaction as supramolecular force to govern the process of self-assembly and recognition of 1,17 the face-to-face and centroid−centroid distances are about 3.52 and 3.76 Å, respectively, indicating significant π−π interaction. A C−H···π interaction with an H(3)···centroid distance of 3.62 Å is also found to interlink the adjacent 1,4-bix and 1,4-ndc ligands (Figure S2). Each pentanuclear Zn(II) cluster of 1 is surrounded by twelve organic ligands, eight bridging 1,4-ndc and four bridging 1,4-bix that are essential parts of the connectivity of the network. Although each Zn(II) cluster is ligated by twelve bridging ligands, it is virtually linked to ten nearest neighbors, with distances of 11.395−15.737 Å (from center to center), because two pairs of 1,4-bix ligands form two “double-bridges” (Figure 1c). From the topological point of view, this cluster can be defined as a 10-connected node. Thus, the overall structure of 1 is a 3D 10-connected framework with the short Schläfli symbol of 364345362 (td10 = 3761).18 It should be noted that the two shortest four-member cycles (green and red fourmember cycles) in 1 are catenated, as shown in Figure 2; thus, the topological structure of 1 can be classified as a 10connected 3D self-penetrating MOF. Recently, Yang and Zhai reported two 10-connected self-penetrating frameworks based on a tetranuclear Co cluster and a sixteen-nuclear Cd cluster,18d,e respectively. To the best of our knowledge, complex 1 represents the highest-connected uninodal network topology polynuclear Zn clusters as SBUs in self-penetrating systems.

Zn1, which is coordinated by four carboxylate oxygen atoms (Zn3−O2 2.139(3), Zn3−O3 2.040(3), Zn3−O6 2.177(3), Zn3−O7 2.196(3) Å) from four 1,4-ndc ligands, one oxygen atom (Zn3−O9 2.046(3) Å) from one μ3-OH group, and one nitrogen atom (Zn3−N1 2.048(3) Å) from 1,4-bix, forming a slightly distorted octahedral coordination polyhedron. Zn2 has a different coordination environment from Zn1 and Zn3; it is coordinated by two carboxylate oxygen atoms (Zn2−O5 1.935(3), Zn2−O8 2.017(3) Å) from two 1,4-ndc dianions, one oxygen atom (Zn2−O9 1.936(3) Å) from a μ3-OH group, and one 1,4-bix (Zn2−N4 1.993(3) Å) nitrogen atom in a tetrahedral geometry. Although all the Zn−O bond distances are comparable to those documented values in the previous literature,16 the average Zn1−O bond length (2.085(3) Å) is slightly longer than those of Zn2−O (1.936(3) Å) and Zn3−O (2.046(3) Å). Zn1 links Zn2, Zn3, and two symmetry related Zn2a and Zn3a (symmetry code: −x, 2 − y, 1 − z) via two μ3OH groups, forming a unique pentanuclear [Zn5(μ3-OH)2] cluster. The pentanuclear zinc cluster can also be considered as two [Zn3(μ3-OH)] cores sharing a Zn1 vertex. Clearly, five Zn(II) centers form a [Zn5(μ3-O)2(OCO)8] cluster with 6.3 × 6.8 × 8.9 Å3, which can been seen as an unusual 10-connected SBU. It is notable that the five zinc(II) centers within the pentanuclear cluster are strictly coplanar. The nonbonding Zn1···Zn2, Zn1...Zn3, and Zn2...Zn3 distances are 3.625, 3.220, 3.407 Å, respectively, which are considerably shortened by the presence of a Zn−O−Zn linkage. It is interesting to note that there is one binding mode for both carboxylate groups of 1,4ndc ligands adopting a dimonodentate coordination mode to 1065

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Figure 3. Comparison of the simulated and experimental XRPD patterns of 1 in the angular range 2θ = 5−50°.

Figure 2. (a) Simplified views of the ten connected node. (b) Schematic representation of the 10-connected net. (c) Schematic representation of the 10-connected self-penetrating 3D net framework. The red spheres are individual Zn5 units.

Only a few examples involved in high-connected 3D selfpenetrating MOFs are reported until now, which could help us deeply understand the nature of Zn clusters based metal− organic frameworks (Zn-MOFs) and better design porous crystalline functional materials. As discussed above, to date, there are reports of six different uninodal 10-connected nets featuring the 312·426·57, 312·424·59, 312·428·55, 36·423·513·63, and 36·431·56.62, 312·430·52·6 topologies. A further detailed discussion of the 36·434·53·62 topology is described below. Schröder and co-workers have proposed new approaches to the analysis of high connectivity frameworks based upon 44- and 36-sub net tectons.19b The 10-fold connectivity of complex 1 can be described as being formed from parallel 36 nets, where each center provides four links to four different centers in adjacent nets, two on each side. However, as far as we know, linkers used in the complexes with the 2D 36-hxl net are all rigid or semirigid ligands, and the triangles are all equilateral. In these cases, a mixed-linker synthetic strategy has been proven to be useful in yielding extremely rare topologies in 1. So far, the entries of higher 10connected self-penetrating nets are extremely rare. Our result provides rational design metal clusters and appropriate choices of (rigid/flexible) organic spatial linkers as building blocks for constructing a new highly connected self-penetrating network. To confirm the phase purity and to examine the crystallinity of bulk samples, the X-ray powder diffraction pattern was recorded. As shown in Figure 3, the peak positions of simulated and experimental patterns are in good agreement with each other, demonstrating the phase purity of the product. The dissimilarities in intensity may be due to the preferred orientation of the crystalline powder samples. A thermogravimetric (TGA) experiment was carried out to study the thermal stability of complex 1. The TGA curve (Figure 4) of 1 shows that the first weight loss takes place at 350 °C and corresponds to the loss of 1,4-naphthalenedicarboxylic acid and 1,4-bix ligands (obsd, 71.46%; calcd, 71.53%) in the temperature range 350−550 °C. From then on, almost no weight loss is observed until 550 °C. The decomposition

Figure 4. TGA curve for complex 1.

process ended at about 550 °C, and the final residue was probably ZnO (found, 28.54%; calc, 28.47%). The TAG analysis for 1 shows that complex 1 has excellent thermal stability in the class of MOFs.20 Much work has been carried out on hybrid frameworks as solid state lighting materials which are typically more stable than organic materials and have certain advantages over inorganics. Luminescent properties of coordination polymers are promising candidates for potential applications, such as chemical sensors, white light-emitting diodes (LEDs), and electroluminescent materials (OLEDs) for displays.21 Thus, the photoluminescence properties of 1 as well as free ligands were examined in the solid state at room temperature as shown in Figure 5. It can be observed that complex 1 exhibits an intense emission band with a maximum at 404 nm upon excitation at 310 nm. One robust emission peak at 508 nm with the excitation at 360 nm was observed in the range 450−550 nm of free 1,4-ndc ligand, which is attributable to the π*−π transitions. In comparison with the emission peaks of free 1,4-ndc, the maximum emission wavelength of complex 1 undergoes a blue-shift from 508 to 404 nm, we can presume that these emissions are neither ligand-to-ligand charge transfer (LLCT) nor metal-to-ligand charge transfer (MLCT) in nature. Therefore, the emission of 1 may be ascribed to a chargetransfer transition between ligands and metal centers, namely ligand-to-metal charge transfer (LMCT), similar to the reported results on coordination polymers with benzenedicarboxylate ligands.21b Because the luminescent polynuclear metal systems Zn(II) ion is difficult to oxidize or to reduce due 1066

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(2012CB821700), and the Natural Science Fund of Tianjin, China (10JCZDJC22100)

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(1) (a) Lehn, J. M. Supramolecular Chemistry: Concepts and Perspectives; VCH: Weinheim, 1995. (b) Cotton, F. A.; Lin, C.; Murillo, C. Acc. Chem. Res. 2001, 34, 759. (2) (a) Zhao, X.; Xiao, B.; Fletcher, A.; Thomas, K. M.; Bradshaw, D.; Rosseinsky, M. J. Science 2004, 306, 1012. (b) Rowsell, J. L. C.; Yaghi, O. M. Angew. Chem., Int. Ed. 2005, 44, 4670. (c) Li, J. R.; Kuppler, R. J.; Zhou, H. C. Chem. Soc. Rev. 2009, 38, 1477. (3) (a) Forster, P. M.; Cheetham, A. K. Top. Catal. 2003, 24, 79. (b) Ma, L.; Abney, C.; Lin, W. Chem. Soc. Rev. 2009, 38, 1248. (c) Lee, J.; Farha, O. K.; Roberts, J.; Scheidt, K. A.; Nguyen, S. T.; Hupp, J. T. Chem. Soc. Rev. 2009, 38, 1450. (d) Zhao, J. A.; Mi, L. W.; Hu, J. Y.; Hou, H. W.; Fan, Y. T. J. Am. Chem. Soc. 2008, 130, 15222. (4) Horcajada, P.; Serre, C.; Vallet Regi, M.; Sebban, M.; Taulelle, F.; Férey, G. Angew. Chem., Int. Ed. 2006, 45, 5974. (5) (a) Allendorf, M. D.; Bauer, A. A.; Bhakta, R. K.; Houk, R. J. T. Chem. Soc. Rev. 2009, 38, 1330. (b) Cui, Y.; Yue, Y.; Qian, G. D.; Chen, B. L. Chem. Rev. 2012, 112, DOI:10.1021/cr200101d. (6) (a) Rebecca, H. L.; McInnes, E. J. L. Eur. J. Inorg. Chem. 2004, 2811. (b) Schubert, U. Chem. Soc. Rev. 2011, 40, 575. (7) (a) Lan, Y. Q.; Wang, X. L.; Li, S. L.; Su, Z. M.; Shao, K. Z.; Wang, E. B. Chem. Commun. 2007, 4863. (b) Morris, J. J.; Noll, B. C. Chem. Commun. 2007, 5191. (c) Xiang, S.; Wu, X.; Zhang, J.; Fu, R.; Hu, S.; Zhang, X. J. Am. Chem. Soc. 2005, 127, 16352. (8) (a) Sun, D.; Ma, S.; Ke, Y.; Collins, D. J.; Zhou, H. C. J. Am. Chem. Soc. 2006, 128, 3896. (b) Zou, R. Q.; Zhong, R. Q.; Du, M.; Kiyobayashi, T.; Xu, Q. Chem. Commun. 2007, 2467. (9) (a) Cairns, A. J.; Perman, J. A.; Wojtas, L.; Kravtsov, V. C.; Alkordi, M. H.; Eddaoudi, M.; Zaworotko, M. J. J. Am. Chem. Soc. 2008, 130, 1560. (b) Nouar, F.; Eubank, J. F.; Bousquet, T.; Wojtas, L.; Zaworotko, M. J.; Eddaoudi, M. J. Am. Chem. Soc. 2008, 130, 1833. (10) (a) Ke, X. J.; Li, D. S.; Du, M. Inorg. Chem. Commun. 2011, 14, 788. (b) Hubberstey, P.; Lin, X.; Champness, N. R.; Schröder, M. Highly Connected Metal-Organic Frameworks 2010, 131−163. (c) Batten, S. R. Topol. Interpenetration 2010, 91. (11) (a) Hosseini, M. W. Acc. Chem. Res. 2005, 38, 313. (b) Wuest, J. D. Chem. Commun. 2005, 5830. (c) Hosseini, M. W. CrystEngComm 2004, 6, 318. (12) Ockwig, N.; Delgado-Friedrichs, O.; O’Keeffe, M.; Yaghi, O. M. Acc. Chem. Res. 2005, 38, 176. (13) (a) Wang, X. L.; Qin, C.; Wang, E. B.; Su, Z. M.; Xu, L.; Batten, S. R. Chem. Commun. 2005, 4789. (b) Li, D.; Wu, T.; Zhou, X. P.; Zhou, R.; Huang, X. C. Angew. Chem., Int. Ed. 2005, 44, 4175. (c) Hao, Z. M.; Fang, R. Q.; Wu, H. S.; Zhang, X. M. Inorg. Chem. 2008, 47, 8197. (d) Li, Y. P.; Sun, D. J.; Zang, H.; Su, G. F.; Li, Y. L. Acta Crystallogr. 2009, C65, m340. (e) Song, W. C.; Pan, Q.; Song, P. C.; Zhao, Q.; Zeng, Y. F.; Hu, T. L.; Bu, X. H. Chem. Commun. 2010, 4890. (14) Synthesis and analytical data of 1: A mixture of Zn(NO3)2·6H2O (0.5 mmol, 148.7 mg), NaOH (1 mmol, 40 mg), H2(1,4-ndc) (0.5 mmol, 108 mg), and 1,4-bix (0.5 mmol, 120 mg) in 15 mL of H2O was sealed in a Teflon-lined autoclave and heated to 170 °C for 4 days. After the autoclave was cooled to room temperature at 10 °C h−1, colorless block crystals suitable for single crystal X-ray crystallographic analysis were obtained. The crystals were rinsed three times with ethanol (8 mL × 3) and dried in air. Yield: 20% based on Zn. Anal. Calcd for C76H54N8O18Zn5: C, 53.83; H, 3.18; N, 6.61%. Found: C, 53.73; H, 3.18; N, 6.49%. IR (KBr, cm−1): 3450m, 2365m, 1595s, 1519s, 1359s, 1246m, 1098m, 961w, 831m, 728m, 598w, 523 m. (15) Crystal data for 1: C76H54N8O18Zn5, Mr = 1694.22, monoclinic, P21/c, a = 15.309(3) Å, b = 15.799(3) Å, c= 16.424(3) Å, α = γ = 90°, β = 117.56(3)°, V = 3521.6(12) Å3, λ = 0.71073 Å, T = 293.0(2) K. Z = 2, Dcalcd = 1.598 g cm−3, R(int) = 0.0682, F(000) = 1720, μ (mm−1) = 1.758, final R1 = 0.0494, wR2 = 0.0937 (all data), GOF = 1.077.

Figure 5. Solid state emission spectrum of 1,4-ndc and complex 1 at room temperature.

to its d10 configuration, the emission of bulk ZnO material was observed at λem = 380 nm.21a At the same time, the free 1,4-bix exhibits weak fluorescent emission bands at ca. 423 nm upon excitation at 250 nm, which further supports this interpretation. On one hand, this may be caused by a change in the HOMO and LUMO energy levels of deprotonated 1,4-ndc anions and neutral ligands coordinating to metal centers, a charge-transfer transition between ligands and metal centers.21a On the other hand, a weak luminescence emission may be attributed to flexibility of the 1,4-bix ligand to the metal cluster, which effectively increases the flexibility of the metal−organic framework and the loss of energy by radiationless decay. This observation indicates that 1 may be an excellent candidate for potential photoactive materials. In conclusion, we have prepared and characterized a new 10connected self-penetrating coordination network based on pentanuclear zinc clusters. This work shows that polynuclear metal cluster entities as secondary building units have been proven as an effective and powerful synthetic strategy in constructing new high-connected MOFs. This work enriches the construction of highly connected nodes based on polynuclear metal cluster SBUs and provides a rational design of metal clusters and appropriate choices of (rigid/flexible) organic spatial linkers as building blocks for constructing new highly connected topological structures and types in the near future.

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ASSOCIATED CONTENT

* Supporting Information S

Experimental and crystallographic details, selected bond lengths and angles, IR spectrum, and X-ray crystallographic files in cif format for 1. This material is available free of charge via the Internet at http://pubs.acs.org.

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REFERENCES

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Fax: +86-22-23502458. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the National Science Foundation of China (21031002 and 51073079), the 973 Program of China 1067

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