One-Pot Synthesis of Supramolecular Isomers with Two

(Bottom) The 2-fold interpenetrating supramolecular network in the structure of ...... Crystal data for 1: C26H24CdN4O2S2, M = 601.01, monoclinic, spa...
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One-Pot Synthesis of Supramolecular Isomers with Two-Dimensional 44 Grid and Three-Dimensional 64 · 82 NbO Frameworks: Solvothermal in Situ Ligand Formation and Conformational Isomers Separation

CRYSTAL GROWTH & DESIGN 2008 VOL. 8, NO. 10 3504–3507

Lei Han,*,† Wenna Zhao,‡ Yan Zhou,† Xing Li,† and Jianguo Pan† 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 24, 2008; ReVised Manuscript ReceiVed July 24, 2008

ABSTRACT: Two supramolecular isomers with two-dimensional 44 rhombic-grid and three-dimensional 64 · 82 compressed-NbO

frameworks, R-[Cd(SH)2(S-bztpy)] · 2H2O (1) and β-[Cd(SH)2(E-bztpy)] · 2H2O (2) (bztpy ) 1,2,4,5-tetra(4-pyridyl)benzene), have been synthesized in a one-pot reaction, involving solvothermal in situ ligand generation and conformational isomers separation of the bztpy ligand. The variation in structural topology results from the conformational freedom of the bztpy ligand through rotation of the CsC bonds between pyridine and benzene rings, which imparts them with intriguingly different luminescence properties. . The current interest in the crystal engineering of coordination polymers stems from their potential applications as materials for gas storage, separation, and catalysis, as well as their intriguing variety of topologies.1 The self-assembly of multidentate organic ligands and metal ions has resulted in many coordination polymeric networks, whose structures are influenced by the subtle interplay of many factors such as the geometric preference of metal ions, the size and shapes of the organic building blocks, templates, and solvent systems.2 Many recent examples have illustrated that solvothermal in situ metal/ligand reactions are usually performed under relatively high temperature and pressure in the presence of transition-metal ions, such as dehydrogenative carbon-carbon coupling, hydroxylation of aromatic rings, cycloaddition of organic nitriles with azide and ammonia, and transformation of inorganic and organic sulfur, which provide products that are inaccessible or not easily obtainable by conventional methods.1d,3 Evidently, the construction of functional coordination polymers involving in situ ligand synthesis is emergent and worth further exploration. Supramolecular isomerism is another interesting subject in the crystal engineering of coordination polymers.1c,4 Supramolecular isomerism often results from the existence of several different building units with little or no difference in formation energy, making it difficult to accurately predict final structures. The study of supramolecular isomerism is not only important in producing novel materials with structural diversity and interesting properties but may also be helpful in developing a fundamental understanding of the factors influencing crystal growth, such as conformational flexibility of ligands.1c However, the rational design and controlled synthesis of supramolecular isomers are still challenging topics.4,5 Although many coordination polymers have been reported as supramolecular isomers, a substantial number of them are based on the coexistence of different guest molecules. It would be more suitable to categorize these structures as pseudopolymorphs, rather than true isomers. Many higher dimensionality coordination polymer frameworks represent themselves as realistic targets of inorganic compounds or minerals in nature with topologies such as R-Po, PtS, Pt3O4, diamond, quartz, rutile, boracite, and so on.6 Frameworks based solely upon planar 4-connected nodes can have several topologies: the two-dimensional (2D) 44 square-grid, 44 rhombic-grid, 32 · 62 * To whom correspondence should be addressed. E-mail: [email protected]. † Ningbo University. ‡ Zhejiang University.

Kagome´ lattice, the three-dimensional (3D) 64 · 82 NbO, the 65 · 8 CdSO4, the “dense” 75 · 9 and the unusual 42 · 84 topologies.7 However, only a few exquisite examples based on square-planar nodes have been reported of supramolecular isomers that crystallized separately from different solvents.8 The organic ligand 1,3-bis(4-pyridyl)propane (bpp) is a long and flexible polyfunctional substance, which can adopt different conformations with respect to the relative orientations of the CH2 groups. And a series of unprecedented structural topologies with bpp have been isolated such as helical, interwoven, and other species.9 Recently, two intriguing in situ reactions of bpp, a dehydrogenative coupling into 1,2,4,5-tetra(4-pyridyl)benzene) (bztpy)10 and both dehydrogenative coupling and hydroxylation into a,a-1,4-dihydroxy-e,e,e,e-1,2,4,5-tetra(4-pyridyl)cyclohexane) (chtpy),11 have also been observed with new functional building blocks. In this contribution, following our recent reports on the in situ metal/ ligand redox reaction12 and the coordination polymers of bpp ligand,9c,d we report the first example of one-pot synthesis of supramolecular isomers with 2D 44 rhombic-grid and 3D 64 · 82 compressed-NbO frameworks, R-[Cd(SH)2(S-bztpy)] · 2H2O (1) and β-[Cd(SH)2(E-bztpy)] · 2H2O (2) (bztpy ) 1,2,4,5-tetra(4-pyridyl)benzene), involving solvothermal in situ ligand generation and conformational isomers separation of the bztpy ligand. The variation in structural topology results from the conformational freedom of the bztpy ligand through rotation of the CsC bonds between pyridine and benzene rings, which imparts them with intriguingly different luminescence properties. Two kinds of crystals with distinct differences in morphology, cubic for 1 and rhombohedron for 2, were synthesized from the in situ solvothermal reaction, one-pot of Cd(SCy)2, sulfur, thiourea, with KSCN and bpp in H2O/DMF/EtOH solution at 190 °C for five days.13 It is noteworthy that the high temperature in the reaction is crucial to synthesize the title complexes due to the effect of temperature and pressure on crystallization. Although the bulk products are cocrystallized in one-pot, the yield of compound 2 (30%) is higher than that of compound 1 (10%) estimated on the basis of the morphology of the crystals. Furthermore, 1 and 2 are air-stable and insoluble in water and most organic solvents. Singlecrystal X-ray diffraction analyses14 revealed that two kinds of crystals are ascribed to supramolecular isomers, namely, R-[Cd(SH)2(Sbztpy)] · 2H2O (1) and β-[Cd(SH)2(E-bztpy)] · 2H2O (2), respectively. For both complexes of 1 and 2, statically identical hexa-coordinate environments are found at the CdII centers, which are bonded to four pyridine groups of different bztpy ligands located at the

10.1021/cg800424t CCC: $40.75  2008 American Chemical Society Published on Web 08/29/2008

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Scheme 1. Two Conformational Isomers: S-bztpy and E-bztpy

equatorial positions and two SH- groups at the axial positions. Such unusual structure containing SH- ions probably results from the decomposition of CyS- moieties in the higher temperature solvothermal reaction.15 Each bztpy ligand links to four CdII centers as the basic building unit for constructing their metal-organic frameworks. The CdsN bond distances range from 2.392(3) to 2.436(3) Å, and the CdsS bond lengths are in the range of 2.557(10)-2.572(13) Å, which are all within the normal range found in other reported experiments.10,15a One of the most striking features of the synthesis is the in situ formation of tetradentate bztpy molecule through dehydrogenative coupling of two bpp units. Although this discovery has been reported previously,10 the isolation of two kinds of crystals with different morphologies is unprecedented. Moreover, supramolecular isomers cocrystallized in a one-pot reaction with in situ ligand formation have not been reported to date. The generation of the bztpy ligand is essential in establishing two supramolecular isomers of 1 and 2 with the same chemical formula and solvent system, because of conformational freedom of the bztpy ligand through rotation of the CsC bonds between pyridine and benzene rings. Interestingly, two kinds of conformation isomers of bztpy, staggered and eclipsed (Scheme 1), were separated in 1 and 2, respectively. The staggered isomer is found in compound 1, of which the dihedral angles between the centered benzene ring and pyridine rings are 45.3 and 53.2°, respectively, and the dihedral angle of two opposite, staggered pyridine rings is 98.4°. The eclipsed isomer is observed in compound 2, while in sharp contrast, the dihedral angles between the centered benzene ring and pyridine rings are identical being 131.8°, and the two opposite pyridine rings are completely eclipsed in 2. The discovery of the staggered and eclipsed conformations in 1 and 2 reveals that the bztpy ligand displays the geometrical

Figure 1. The valence-bonded 2D 44 metal-organic framework in the structure of 1 with guest water molecules.

Figure 2. (Top) The schematic representation of the hydrogen-bonded 3D (4,6)-connected supramolecular network in the structure of 1. The blue sticks represent the valence-bonded 2D 44 metal-organic framework, and the green sticks represent hydrogen-bonded linkers. (Bottom) The 2-fold interpenetrating supramolecular network in the structure of 1.

flexibility in spite of the identical coordinated directions of its pyridine groups. Consequently, it is readily envisaged that the variety structural motifs could be constructed upon the ligation of the appropriate metal ions and the suitably coordination environments. Such conformational flexibility of the bztpy ligand may account for the supramolecular isomerism of 1 and 2 with different topologies. To the best of our knowledge, no similar type of isomerism has been previously reported. The complex 1 crystallizes in monoclinic, space group P2/n (No. 13). As shown in Figure 1, the complex 1 displays a valence-bonded 2D layer metal-organic framework. By considering both the Cd atoms and the tetradentate ligands as square-planar nodes, 1 can be defined as a simple 44 rhombic-grid topology. There are two grid sizes in the layer, of which the diagonal separations are 9.91 and 12.75 Å based on CdsCd distances, respectively. The guest water molecules are clathrated in the larger grids. Adjacent independent layers are then arranged in an orderly manner as ABAB-alternating stacking patterns, of which the nearest interlayer Cd · · · Cd distance is 7.10 Å. Interestingly, the alternating layers are linked into 3D hydrogen-bonded supramolecular network through S-H · · · O hydrogen-bonds between coordinated -SH groups and guest water molecules. The S · · · O distances range from 3.313 to 3.309 Å. Consequently, a 2-fold interpenetrating supramolecular network is formed in the structure of compound 1. From a topological perspective, the hydrogen-bonded 3D supramolecular network of 1 can be simplified to a unique 2-fold interpenetrating (4,6)-connected net by considering the Cd atoms as 6-connecting nodes and the tetradentate ligands as square-planar nodes (Figure 2). Compared with the (4,6)-connected nets in the literature,16

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Figure 3. (Left) The compressed NbO framework in 2 with the encapsulation of (H2O)6 circle clusters in the cavity. (Right) The left- and righthanded helical substructures in 2 along a 3-fold screw axis.

Figure 5. Emission spectra of complexes 1 (dash line) and 2 (solid line) in the solid state at room temperature.

Figure 4. The 3D open-framework with 1D channels in the structure of 2. The guest water molecules are omitted for clarity.

R-Al2O3 and others, the topology of 1 represents an uncommon example of (4,6)-connected nets and has not been observed among the 3D coordination polymers and hydrogen bonded nets so far. The complex 2 crystallizes in trigonal, space group R3jm (No. 166) and exhibits a 3D noninterpenetrated 64 · 82 NbO framework by considering both the Cd atoms and the tetradentate ligands as square-planar nodes. The compressed NbO unit in 2 is shown in Figure 3. Remarkably, the disordered circle (H2O)6 groups are encapsulated in the cavity. The diameter of the ring is 8.6 Å based on O · · · O separation. Along the c-axis, the 3D open-framework displays cross-sectional hexagonal channels of about 1.2 nm diameter where water clusters are clathrated inside (Figure 4). The relative orientation of the eclipsed bztpy ligands imparts handedness to the trigonal channels so that left- and right-handed helical channels alternate around the hexagonal voids. As shown in Figure 3, the left- and right-handed helical substructures in 2 are running along 3-fold screw axis. The pitch of the 31 helices is about 6.6 Å. Interestingly, the whole honeycomb structure of 2 is noninterpenetrating and has a total solvent-accessible volume of 3798.8

Å3, which approximately corresponds to 45.1% of the crystal volume (8423.0 Å3), as calculated by PLATON.17 The preliminary photoluminescent properties of complexes 1 and 2 in the solid state at room temperature have been investigated and are shown in Figure 5. Clearly, 1 and 2 exhibit green-yellow emission, with peak wavelengths at 500 and 520 nm, respectively. These emissions could be tentatively assigned as originating from the ligand-to-metal charge transfer (LMCT). Accordingly, the spectral differences between 1 and 2 are intrinsic, and could plausibly be due to the aforementioned structural variation. The red shift of emission in 2 can tentatively be rationalized by its robust 3D valence-bonded framework than in 1. On the other hand, thermogravimetric analysis (TGA) experiments were conducted to determine the thermal stability of the crystalline samples of 1 and 2 in the range 25-800 °C. The results indicate that 1 and 2 lost their guest H2O molecules below 110 °C. Then, the remaining samples suffer incessant weight loss at 310-630 °C and 320-680 °C corresponding to the removal of the organic species of 1 and 2, respectively. In conclusion, this work describes the first example of one-pot synthesis of supramolecular isomers involving solvothermal in situ ligand generation and conformational isomers separation of ligand. The variation in structural topology results from the conformational freedom of the organic ligand, which imparts to them intriguingly

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different luminescence properties. This synthesis strategy may open a broad interest in the construction of novel valence-bonded metalorganic frameworks or inorganic-organic hybrid materials.

Acknowledgment. This work is supported by the National Science Foundation of China (20701022), Ningbo Municipal Natural Science Foundation (2007A610024, 2008A610045, 2008A610048), 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 for complexes 1 and 2. This material is available free of charge via the Internet at http://pubs.acs.org.

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(8) (a) Chen, B.; Fronczek, F. R.; Maverick, A. W. Chem. Commun. 2003, 2166. (b) Lee, I. S.; Shin, D. M.; Chung, Y. K. Chem.-Eur. J. 2004, 10, 3158. (c) Rather, B.; Moulton, B.; Walsh, R. D. B.; Zaworotko, M. J. Chem. Commun. 2002, 694. (d) Wang, C.-C.; Lin, W.-Z.; Huang, W.-T.; Ko, M.-J.; Lee, G.-H.; Ho, M.-L.; Lin, C.-W.; Shih, C.-W.; Chou, P.-T. Chem. Commun. 2008, 1299. (9) (a) Carlucci, L.; Ciani, G.; Proserpio, D. M.; Rizzato, S. CrystEngComm 2002, 4, 121. (b) Luan, X.-J.; Wang, Y.-Y.; Li, D.-S.; Liu, P.; Hu, H.-M.; Shi, Q.-Z.; Peng, S.-M. Angew. Chem., Int. Ed. 2005, 44, 3864. (c) Han, L.; Zhou, Y. Inorg. Chem. Commun. 2008, 11, 385. (d) Han, L.; Valle, H.; Bu, X. H. Inorg. Chem. 2007, 46, 1511. (10) Zheng, N. F.; Bu, X. H.; Feng, P. Y. J. Am. Chem. Soc. 2002, 124, 9688. (11) Hu, S.; Chen, J.-C.; Tong, M.-L.; Wang, B.; Yan, Y.-X.; Batten, S. R. Angew. Chem., Int. Ed. 2005, 44, 5471. (12) (a) Han, L.; Hong, M. C.; Wang, R. H.; Wu, B. L.; Xu, Y.; Lou, B. Y.; Lin, Z. Z. Chem. Commun. 2004, 2578. (b) Han, L.; Bu, X. H.; Zhang, Q. C.; Feng, P. Y. Inorg. Chem. 2006, 45, 5736. (13) One-pot solvothermal preparation of 1 and 2: A mixture of Cd(SCy)2 (122 mg), sulfur (45 mg), thiourea (34 mg), KSCN (62 mg), 1,3bis(4-pyridyl)propane (102 mg), H2O (5 mL), DMF (3 mL), EtOH (3 mL) was homogenized at room temperature for 30 min, then the final solution was sealed in a 20 mL stainless-steel autoclave at 190 °C for 5 days. Two kinds of crystals with distinct differences in morphology, cubic for 1 and rhombohedron for 2, were obtained after the solution was cooled to room temperature. Approximate yields: ca. 10% for 1 and 30% for 2 based on Cd. Anal. Calcd. for 1 C26H24CdN4O2S2 (%): C, 51.96; H, 4.02; N 9.32; S, 10.67. Found (%): C, 51.73; H, 4.24; N, 9.19; S, 10.56. Anal. Calcd. for 2 C26H24CdN4O2S2 (%):C, 51.96; H, 4.02; N 9.32; S, 10.67. Found (%): C, 51.77; H, 4.22; N, 9.23; S, 10.51. IR (KBr) for 1: ν(SsH) 2560(w) cm-1; IR (KBr) for 2: ν(SsH) 2556(w) cm-1. (14) Crystal data for 1: C26H24CdN4O2S2, M ) 601.01, monoclinic, space group P2/n (No. 13), a ) 10.322(3), b ) 9.907(2), c ) 12.752(3) Å, β ) 105.846(4)°, V ) 1254.5(5) Å3, Z ) 2, Dc ) 1.591 g/cm3, F000 ) 608, Mo-KR radiation, λ ) 0.71073 Å, µ ) 1.068 mm-1, T ) 298(2) K, 2θmax ) 56.6°, 7482 reflections collected, 2965 unique (Rint ) 0.0899). Final GOF ) 0.990, R1 ) 0.0457, wR2 ) 0.0932, R indices based on 2132 reflections with I > 2σ(I). Crystal data for 2: C26H24CdN4O2S2, M ) 601.01, trigonal, space group R3jm (No. 166), a ) b ) 27.012(2), c ) 13.3298(17) Å, V ) 8422.8(15) Å3, Z ) 9, Dc ) 1.066 g/cm3, F000 ) 2736, Mo-KR radiation, λ ) 0.71073 Å, µ ) 0.716 mm-1, 2θmax ) 56.5°, 29435 reflections collected, 2470 unique (Rint ) 0.0569). Final GOF ) 1.107, R1 ) 0.0372, wR2 ) 0.1475, R indices based on 2225 reflections with I > 2σ(I). The high Uiso for O atom in 2 is related to the very disordered electron density of guest water molecules. CCDC-685690 and 685691 contains the supplementary crystallographic data for this paper. These data can be obtained free of charge from the Cambridge Crystallographic Data Centre via http://www.ccdc.cam.ac.uk/data_request/cif. (15) (a) Li, S.-L.; Wu, J.-Y.; Tian, Y.-P.; Tao, X.-T.; Jiang, M.-H.; Fun, H.-K. Chem. Lett. 2005, 34, 1186. (b) Han, L.; Shi, L.-X.; Zhang, L.-Y.; Chen, Z.-N.; Hong, M.-C. Inorg. Chem. Commun. 2003, 6, 281. (16) (a) Xiao, D.-R.; Wang, E.-B.; An, H.-Y.; Li, Y.-G.; Su, Z.-M.; Sun, C.-Y. Eur. J. 2006, 12, 6528. (b) Kutasi, A. M.; Harris, A. R.; Batten, S. R.; Moubaraki, B.; Murray, K. S. Cryst. Growth Des. 2003, 3, 605. (c) Xue, L.; Lou, F.; Che, Y.-X.; Zheng, J.-M. J. Mol. Struct. 2007, 832, 132. (d) Chen, P.-K.; Che, Y.-X.; Zheng, J.-M. Inorg. Chem. Commun. 2007, 10, 187. (17) Spek, A. L. PLATON, A Multipurpose Crystallographic Tool; Utrecht University: Utrecht, The Netherlands, 2003.

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