Assembly of a Cluster-Based Coordination Polymer from a Linear Trimetallic Building Block Surrounded by Flexible Hingelike Ligands Lei Han,*,† Yan Zhou,† and Wen-Na Zhao‡
CRYSTAL GROWTH & DESIGN 2008 VOL. 8, NO. 7 2052–2054
State Key Laboratory Base of NoVel Functional Materials & Preparation Science, Faculty of Materials Science & Chemical Engineering, Ningbo UniVersity, Ningbo, Zhejiang, 315211, P. R. China, and Key Laboratory for Molecular Design and Nutrition Engineering of Ningbo, Ningbo Institute of Technology, Zhejiang UniVersity, Ningbo, Zhejiang 315100, P. R. China ReceiVed April 20, 2007; ReVised Manuscript ReceiVed May 27, 2008
ABSTRACT: A linear trimetallic cluster {[Cd3(bhnq)3(H2O)2](DMF)(H2O)3}n (1) has been synthesized from Cd(II) salts and a flexible hingelike ligand, 2,2′-bis(3-hydroxy-1,4-naphthoquinone). Sequential assembly of the linear trimetallic cluster as a new controlled second building unit with rigid covalent linker resulted in a one-dimensional hybrid cluster-based coordination polymer, {[Cd3(bhnq)3(4,4′bipy)](DMF)(H2O)3}n 2, which is further assembled into a three-dimensional supramolecular framework with cavities via self-complementary π-π interactions.
Introduction. Metal organic frameworks (MOFs) are hybrid materials where metal ions or small clusters are linked into one-, two-, or three-dimensional (1D, 2D, or 3D) structures by multifunctional organic linkers. The rational design and synthesis of MOFs are of great interest because of their intriguing structural versatility and their potential applications as functional crystalline materials.1 The most important goal in the field is always to reach a real “design” of hybrid crystalline solids, that is, the way to obtain tailor-made solids with the required structures and properties starting from well-characterized inorganic and organic species.2–5 Hence, the number of synthetic routes that use a controlled approach based on secondary building units (SBUs) is helpful in directing the construction of a given structure. Two efficient SBUs used in the construction of MOFs were the tetrameric Zn4O clusters6 and the paddle-wheel dimeric Cu2 clusters7 formed by carboxylate ligands. A few solids were also obtained recently with the oxo-centered metal(III) trimeric clusters.8 However, in the cases mentioned above their synthesis conditions are not a complete designed SBUs approach, and they are in situ cooperative synthesis in reaction with the carboxylates moieties. Our synthetic strategy focuses on designing new “controlled SBUs” as a target, which has better solubility and keeps the structural integrity of the inorganic precursor during the formation of the polymeric solids. One-dimensional nanostructures, such as nanowires, nanorods, and nanotubes, represent the smallest dimension for efficient transport of electrons and excitons and thus are ideal building blocks for hierarchical assembly of functional nanoscale electronic and photonic structures. One-dimensional cluster-based hybrid crystalline networks have also received much impetus due to their interesting optical and magnetic properties.9 On the other hand, hierarchical chemical structures that incorporate some of the complexity typical of biological assemblies now exist and are being developed for use as highly selective catalysts, sensors, and drugtransport materials.10 Therefore, we design herein a new and controlled SBU of a luminescent linear trimetallic cluster with “free” donor groups at axial sites. We also report the successful use of the SBU in the rational synthesis of a rigid 1D hybrid cluster-based coordination polymer that is hierarchical assembled into a 3D MOF containing solvent-filled cavities via self-complementary π-π interactions. To design novel functional SBUs, we chose a hingelike ligand with conformational and geometrical flexibility as supported * To whom correspondence should be addressed. E-mail:
[email protected]. † Ningbo University. ‡ Zhejiang University.
Figure 1. Side view of the linear trimeric SBU in 1 (left) and paddlewheel type of arrangement of SBU viewed through the intermetal axis (right).
Scheme 1. The Ligand H2bhnq
moieties. The ligand 2,2′-bis(3-hydroxy-1,4-naphthoquinone) (H2bhnq, Scheme 1) is a dimer of the natural product 3-hydroxy1,4-naphthoquinone (lawsone). The degree of π conjugation in H2bhnq is influenced by the dihedral angle between two lawsone groups. Metal-directed assembly of bhnq2- has provided structural diversity such as zigzag, helix, square, and dimer architectures.11 Because of the virtue of allowing rotation about the C-C bond linking the two lawsone units, it is readily envisaged that the new structural feature could be formed upon the ligation of the appropriate metal ions. Fortunately, reaction of Cd(II) salts with H2bhnq in mixed DMF/MeOH solution afforded red block crystals of {[Cd3(bhnq)3(H2O)2](DMF)(H2O)3}n 1.12 The compound 1 crystallizes in the triclinic space group P1j, and the structure consists of a linear trimeric metallic cluster [Cd3(bhnq)3(H2O)2] and solvent DMF and water molecules.13 As shown in Figure 1, the trimetallic cluster is surrounded by three bhnq2- anion ligands, and results in a paddle-wheel feature with C3-symmetry. The linearity of the trimetallic array can be gauged by the Cd-Cd-Cd angle, which shows a near linear arrangement (177.37(2)°). The Cd-Cd distances are 3.1898(10) and 3.1914(10) Å, respectively. Of the three metal ions present in the molecular assembly, the two terminal metal ions have an O7 coordination environment, while the central metal ion has an O6 coordination sphere. The geometry around the metal
10.1021/cg070381t CCC: $40.75 2008 American Chemical Society Published on Web 06/17/2008
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Crystal Growth & Design, Vol. 8, No. 7, 2008 2053
Figure 2. View of the one-dimentional rigid coordination polymer in 2.
centers can be described as single-cap octahedron and triangularprism arrangement, respectively. A paddle-wheel type of architecture is readily noticed by viewing the molecule along the intermetal axis. This type of arrangement has been observed in other linear trimetallic derivatives.14,15 Thus, the paddle-wheel arrangement of the ligands may be viewed as a signature structural feature for linear trimetallic complexes. Furthermore, a comparison of the structural feature of the trimetallic cluster with those known in the literature is instructive. Most of homo- and heterotrinuclear assemblies prepared by Cotton and co-workers14 using dipyridylamine ligand are neutral and are of the type M3(dpa)4X2 (X ) Cl, Br; M ) Cu(II), Co(II), Cr(II), Ni(II)). In these complexes, the terminal and central metal ions have N4X and N4 coordination environments, respectively. It could be concluded that the present linear trimetallic cluster in 1 is analogous to previously reported complexes based on structural characteristic, and to our best knowledge, it is the first linear trinuclear Cd(II) cluster. It is worth noting that the coordinated water molecules at axial sites of 1 are “free” donor groups, which can be easily substituted by strong donor atoms for further binding. Additionally, the hingelike ligands could enhance intermolecular interactions using weak noncovalent interactions such as π-π stacking and hydrogen-bonding. Therefore, 1 may serve as a new and controlled SBU to assembly of novel solid-state functional metal organic frameworks with covalent linkers as well as supramolecular interactions. As expected, the assembly of 1 with rigid covalent linker 4,4′bipyridine generated a 1D hybrid cluster-based coordination polymer {[Cd3(bhnq)3(4,4′-bipy)](DMF)(H2O)3}n 2.16 X-ray analyses indicated that 2 consists of the coordination chain [Cd3(bhnq)3(4,4′bipy)]n and interstitial DMF and water molecules.17 As shown in Figure 2, each trimetallic SBU contributes two axial Cd(II) sites to interact with two 4,4′-bipyridine through Cd-N bonds (2.296(4) and 2.298(4) Å), affording the 1D cluster-based network with alternating skeleton. The Cd-Cd distances are 3.2185(5) and 3.2395(5) Å, respectively, which are longer than those in 1. Interestingly, the 1D network can be considered the presence of a sticky surface based on the π conjugated lawsone groups, and thereby further generating a 3D MOF containing solvent-filled cavity directed by self-complementary π-π interactions in a hexagonal close packing manner. The centroid-to-centroid distances of the meaning rings is about 3.51 Å.18 The basic solvent-filled cavity structure in the 3D MOF is shown in Figure 3, the interstitial DMF molecules are included. This indicates that the assembly of sticky rodlike architecture through specific π-π interaction results in the formation of a potentially microporous host framework. Futhermore, the guest DMF molecules play an important role on the formation and stability of the supramolecular network. PLATON calculations19 show that the guest accessible volume (2223.9 Å3 per unit cell) comprises 65.8% of the crystal volume. The preliminary photoluminescence of complexes 1 and 2 in the solid state have been investigated, and interesting results were obtained. At ambient temperature, the free H2bhnq shows red emission with a peak maximum at 620 nm upon excitation at 350 nm, which can be assigned to intraligand π-π* transition. Excitations of solid samples of 1 at 320 nm and 2 at 315 nm produce
Figure 3. View of the cage structures in 3D packing framework of 2.
strong blue emissions with peak maxima at 500 and 450 nm, respectively. The larger blue-shift emission could be tentatively assigned to originating from the ligand-to-metal charge transfer (LMCT). The fact may be ascribed to the presence of the linear Cd3-oxolation core, as the ligand may tighten the whole skeleton, thus resulting in much weaker vibrations. More detailed theoretical and spectroscopic studies are necessary for a better understanding of the luminescent mechanism. In conclusion, we have succeeded in constructing a novel 3D framework based on a new and controlled linear trimetallic SBU. Hierarchical assembling the SBU into a 1D cluster-based crystalline superstructure with covalent linker, and further into a cage-based MOF via self-complementary π-π interactions demonstrates a new strategy of organizing solid-state functional materials. We are actively moving this synthesis strategy toward new high dimensional molecule-based functional materials with other multifunctional organic covalent linkers in our laboratory.
Acknowledgment. This work is supported by the National Science Foundation of China (20701022) Ningbo Municipal Natural Science Foundation (2007A610024), Open Foundation of Municipal Key Laboratory of Ningbo (2007A22003), and the K. C. Wong Magna Fund in Ningbo University. Supporting Information Available: X-ray crystallographic files in CIF format and PXRD patterns for complexes 1 and 2. This material is available free of charge via the Internet at http://pubs.acs.org.
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(13)
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(10) (a) Han, L.; Wang, R.-H.; Yuan, D.-Q.; Wu, B.-L.; Lou, B.-Y.; Hong, M.-C. J. Mol. Struct. 2005, 737, 55. (b) Sudik, A. C.; Cote, A. P.; Wong-foy, A. G.; O’Keeffe, M.; Yaghi, O. M. Angew. Chem., Int. Ed. 2006, 45, 2528. (11) (a) Yamada, K.; Yagishita, S.; Tanaka, H.; Tohyama, K.; Adachi, K.; Kaizaki, S.; Kumagai, H.; Inoue, K.; Kitaura, R.; Chang, H. C.; Kitagawa, S.; Kawata, S. Chem. Eur. J. 2004, 10, 2648. (b) Yamada, K.; Tanaka, H.; Yagishita, S.; Adachi, K.; Uemura, T.; Kitagawa, S.; Kawata, S. Inorg. Chem. 2006, 45, 4322. (12) Preparation of 1: A DMF (3 mL) solution of H2bhnq (70 mg) was transferred to a glass tube. A 4 mL MeOH solution of CdCl2 · 2.5H2O (46 mg) was poured into the tube without mixing the two solutions. The solutions were allowed to stand at room temperature for a month
(17)
(18) (19)
to give red block crystals for 1 with 50% yields. Anal. Calcd. for [C63H41Cd3NO24]: C 49.35, H 2.70, N 0.91; found: C 48.43, H 3.51, N 1.32. Crystal data for 1: C63H41Cd3NO24, M ) 1533.17, triclinic, P1j (No. 2), a ) 14.767(3), b ) 14.926(3), c ) 15.933(3) Å, R ) 79.58(3), β ) 73.47(3), γ ) 81.16(3)°, V ) 3291.5(11) Å3, Z ) 2, Dc ) 1.547 g/cm3, F(000) ) 1524, µ ) 1.038 mm-1, S ) 0.966, R1 ) 0.0612, wR2 ) 0.1840. (a) Clerac, R.; Cotton, F. A.; Daniels, L. M.; Dunbar, K. M.; Kirschbaum, K.; Murillo, C. A.; Pinkerton, A. A.; Schultz, A. J.; Wang, X. J. Am. Chem. Soc. 2000, 122, 6226. (b) Berry, J. F.; Cotton, F. A.; Daniels, L. M.; Murillo, C. A.; Wang, X. Inorg. Chem. 2003, 42, 2418. (a) Heinicke, J.; Peulecke, N.; Karaghiosoff, K.; Mayer, P. Inorg. Chem. 2005, 44, 2137. (b) Chandrasekhar, V.; Azhakar, R.; Andavan, G. T. S.; Krishnan, V.; Zacchini, S.; Bickley, J. F.; Steiner, A.; Butcher, R. J.; Kogerler, P. Inorg. Chem. 2003, 42, 5989. Preparation of 2: A DMF (5 mL) solution of 1 (60 mg) was transferred to a glass tube. A 3 mL MeOH solution of 4,4′-bipy (105 mg) was poured into the tube without mixing the two solutions. Deep-red prism crystals of 2 began to form within one month with 40% yields. Anal. Calcd. for [C73H45Cd3N3O22]: C 53.03, H 2.74, N 2.54; found: C 52.63, H 3.07, N 2.82. Crystal data for 2: C73H45Cd3N3O22, M ) 1653.32, triclinic, P1j (No. 2), a ) 12.6395(4), b ) 14.7442(7), c ) 18.4913(7) Å, R ) 86.597(6), β ) 82.283(4), γ ) 82.239(7)°, V ) 3380.6(2) Å3, Z ) 2, Dc ) 1.624 g/cm3, F(000) ) 1648, µ ) 1.016 mm-1, S ) 0.939, R1 ) 0.0580, wR2 ) 0.1726. Janiak, C. J. Chem. Soc., Dalton Trans. 2000, 3885. Spek, L. PLATON, version 1.62; University of Utrecht: Utrecht, The Netherlands, 1999.
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