Synthesis and Structures of Two Novel Three-Dimensional Metal

Jun 30, 2010 - -bipy ligands, and they have been characterized structurally by single crystal X-ray diffraction studies and FT-IR and. TGA analysis. C...
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DOI: 10.1021/cg100463j

Synthesis and Structures of Two Novel Three-Dimensional Metal-Organic Frameworks: Comprising an Unprecedented Tetraflexure Helix

2010, Vol. 10 3311–3314

Zhe Dong, Yao-Yu Wang,* Rui-Ting Liu, Jian-Qiang Liu, Lin Cui, and Qi-Zhen Shi Key Laboratory of Synthetic and Natural Functional Molecule Chemistry of the Ministry of Education, Shaanxi Key Laboratory of Physico-Inorganic Chemistry, College of Chemistry & Materials Science, Northwest University, Xi’an 710069, P. R. China Received April 7, 2010; Revised Manuscript Received June 10, 2010

ABSTRACT: Two isomorphous three-dimensional metal-organic frameworks, [M(dtb)(4,40 -bipyridine)1.5(H2O)]n (1, M = Ni; 2, M = Co) [dtb = 5,50 -dithiobis(2-nitrobenzoic acid), 4,40 -bipy=4,40 -bipyridine], have been synthesized by ligating metal ions with V-shaped H2dtb and linear 4,40 -bipy ligands, and they have been characterized structurally by single crystal X-ray diffraction studies and FT-IR and TGA analysis. Complexes 1 and 2 exhibit fascinating interweaved meso-helices and represent the first examples of metal-organic frameworks with four spiral shafts in one helical chain, named a tetraflexural helix. The pitch of the single helical chain is up to 27.741 A˚, which is out of the normal scope of helical pitch. The flourishing scopes of crystal engineering of metal-organic frameworks (MOFs) have furnished a favorable junction stemming from the aesthetics of crystalline architectures and their potential applications, of which the ultimate termini is to successfully achieve target materials with tailored structures and physicochemical properties.1-3 The organization of the building blocks together can lead to MOFs with different dimensionalities. As we know, one of the major themes of one-dimensional (1D) supramolecular chemistry is helical systems, which are ubiquitous in our daily life and have attracted much attention from chemists because of their intrinsic aesthetic appeal and their potential applications, ranging from gas storage to heterogeneous catalysis.4,6 One key point of constructing desirable helical frameworks is the selection of multifunctional organic ligands containing appropriate coordination sites linked by a proper spacer with specific positional orientation. Up to now, great contributions have been made in this filed. For example, there is a mass of fascinating helical MOFs obtained by taking advantage of neutral N-donor ligands or anionic O-donor ligands in combination with a metal cation.5-7 On the basis of the pioneering work of Lehn and co-workers,8 a great number of amazing helical coordination polymers have been prepared9-11 and comprehensively discussed.12 In contrast, the meso-helical motifs are extremely infrequent, although meso-helices are common in nature, such as the tendrils of plants and telephone wires.13 Because it is still quite difficult to predict the product architecture resulting from the complicated mechanism of thermal reaction and the diversiform coordination mode of the V-shaped multicarboxylate acid,7 unforeseeable metal-organic extended structures will be obtained which may exhibit intriguing topology and have potential applications in functional materials. Just a few meso-helical MOFs have been constructed, which can be regarded as a three-dimensional presentation of a lemniscate.14-16 To the best of our knowledge, only three kinds of 1D meso-helical chain have been characterized up to now.14c,15,16 Inspired by the aforementioned considerations, we are interested in exploring various helical MOFs that can be formed in a mixed-ligand system containing aromatic carboxylates and neutral N-donor ligands. As a V-shaped aromatic polycarboxylate, 5,50 -dithiobis(2-nitrobenzoic acid) (H2dtb) features the bent orientation between two terminal coordinated groups and is considered as a nice candidate for the construction of novel helical coordination polymers. For

Figure 1. Coordination environment of the Ni atom in 1.

Figure 2. Schematic representation of the 2D layer structure stack in an ABAB fashion along the ab-plane with some displacement between A and B in 1.

*To whom correspondence should be addressed. E-mail: wyaoyu@nwu. edu.cn.

this purpose, 5,50 -dithiobis(2-nitrobenzoic acid) with 4,40 -bipy and nickel/cobalt have been here chosen as precursors to construct novel helical structures and investigate the coordination abilities of dtb ligands. Noteworthily, it remains blank related to the MOFs based on H2dtb in the present scientific literature. Herein we successfully constructed two isomorphous (3D) metal-organic frameworks, which are fascinating interweaved meso-helices

r 2010 American Chemical Society

Published on Web 06/30/2010

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based on two distinct ligands and Ni(II)/Co(II) ions. Interestingly, these two complexes represent the first examples of metal-organic frameworks with four spiral shafts in one helical chain and can be named a tetraflexural helix. Single-crystal X-ray analyses reveal that complexes 1 and 2 exhibit isostructural 3D coordination frameworks. So the following description and discussion will be focused only on complex 1.18 Complex 1 crystallizes in the monoclinic space group C2/c. As illustrated in Figure 1, the repeat unit consists of two crystallographically independent Ni(II), two dtb ligands, six 4,40 -bipy ligands, and two coordinated water molecules, in which both of them are

Figure 3. View of the single-stranded meso-helices of 1 with a tetraflexural nod.

Dong et al. six-coordinated, exhibiting distorted octahedral coordination geometries. Ni1 is coordinated by two carboxylate O atoms from two different dtb ligands, and four pyridyl N atoms of four distinct 4,40 -bipy ligands. However, the coordination environments of the Ni1 and Ni2 centers are slightly different. Compared with Ni1, the Ni2 is coordinated by two pyridyl N atoms of distinct 4,40 -bipy ligands and four O atoms, of which two come from water molecules and the other two from different dtb ligands. The Ni-O and Ni-N distances are within the range 2.177(2)-2.543(2) A˚ and 2.317(2)-2.389(2) A˚, respectively, which drop into the normal scope of Ni-O and Ni-N bond lengths.14c,17 The coordination angles around Ni1 vary within the range 83.86(13)°-96.11(13)°, and those of Ni2 vary from 88.17(11)° to 91.83(11)°. Based on these connection modes, each dtb ligand bridges two Ni(II) ions via the carboxylate O atoms, of which both come from the two carboxylate groups, taking bis(monodentate) coordination fashion into an infrequent single-strand meso-helix along the c-axis. The pitch of the single helix is 27.741 A˚, which is out of the normal scope of helical pitches.4-6,8,9,14c,17 On the other hand, a 2D rectangular grid, plumb to the b-axis, is developed by 4,40 -bipy ligands and the Ni atoms with the approximate dimensions of 11.1  22.0 A˚2. The 2D layers arrange in an offset way along the ab-plane with the grid of each layer occupied by groups from the adjacent ones (as shown in Figure 2). The meso helixes act as pillars to support the 2D rectangle grids into a 3D framework. Surprisingly, a further investigation reveals that the configuration of this type of helix in complex 1/2 shows a unique feature, and the term “tetraflexural helix” is suggested for it, meaning one helix consists of four flexures in one single strand. The most interesting feature of complex 1/2 is each helical chain is a mesohelical chain which entangles four spiral shafts. The four shafts are entangled as the sequence b-c-a-d (Figure 3). All the points of flexure on the edge of the spiral line are the twists at the -C22-S1S2-C24- groups of the dtb ligand. We can easily find that when viewing from the c direction, the projection of the helical chain on the ab plane looks like a four-leaf clover, which pertains to a kind of variation of trifolium (Figure 4a and b). Moreover, the vicinal “four-leaf clover” shares a and c shafts with the right helical chain and co-uses the b and d shafts with the left helical chain

Figure 4. (a) Four-leaf clover in nature; (b) projection of the helical chain on the ab plane; (c) co-use shafts between the adjacent helices (a axis, yellow; b axis, purple; c axis, green; d axis, orange.).

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crystallographic data in CIF format for this paper are available free of charge via the Internet at http://pubs.acs.org.

References

Figure 5. Single topology representation for 1, Ni1 as 6-connected (green) and Ni2 as 4-connected (red).

(Figure 4c). The helix is built from alternating linkages of Ni1 and Ni2 atoms by the dtb ligand and is decorated by 4,40 -bipy at two sides, having similar handedness to the lemniscate type.16 A representation of the exceptional tetraflexural helix is here available, when compared with the only three types of spiral lines presently known (including the ordinary single-handed helix and the rarely observed meso-helix).4b,15,16 Due to the intense flexibility of the dtb ligand, the dihedral angle of the two phenyl rings is 66.1° and the helical pitch is much longer than earlier reported.4-6,8,9,17 To the best of our knowledge, this new type of helical complex has never been observed prior to this work and may bring a fresh feature into the isomers related to the helices. A better insight into the nature of these intriguing frameworks of 1 and 2 can be achieved by the application of a topological approach. While making the Ni atoms as nodes and connections between metal centers as rods, Ni1 and Ni2 motifs can be viewed as 6-connected (green) and 4-connected nodes (red), respectively (Figure 5). Thus, the whole framework of 1/2 can be rationalized as a (4,6)-connected topology with a rare Schl€afli symbol of (42 3 52 3 62)(42 3 56 3 64 3 73). The phase purity samples of complexes 1 and 2 have been confirmed by X-ray power diffraction measurements, which result in an experimental pattern closely resembling the simulated one from the single-crystal diffraction analysis. To examine the thermal stability of complexes 1 and 2, thermogravimetric analyses (TGA) were conducted. The TGA curves of 1 and 2 are almost the same. From the first plateau we can conclude that complexes 1 and 2 can be stable up to 238 °C/218 °C. Upon further heating, the structures begin to collapse, following the ligand molecules being released. In conclusion, based on the flexible V-shaped ligand 5,50 -dithiobis(2-nitrobenzoic acid) and rigid assistant ligand 4,40 -bipy, we have designed and synthesized two isomorphous 3D metal-organic frameworks. The two complexes show intrinsic aesthetic appeal, comprising tetraflexural helixes, which look like four-leaf clovers and 2D rectangular grids in different directions. Finally, the tetraflexural helix, a unique type of supramolecular isomer related to the meso-helical system, has not been observed in experiments. Acknowledgment. We gratefully acknowledge financial support of this work by the National Natural Science Foundation of China (Grant No. 20771090), State Key Program of National Natural Science of China (Grant No. 20931005), Natural Science Foundation of Shaanxi Province (Grant No. 2009JZ001), and Specialized Research Found for the Doctoral Program of Higher Education (Grant No. 20096101110005). Supporting Information Available: Crystallographic data for complexes 1 and 2 have been deposited at the Cambridge Crystallographic Data Center as supplementary publications (CCDC771771 and 771772). These data can be obtained free of charge at www.ccdc.cam.ac.uk/data_request/cif or from the Cambridge Crystallographic Data Center, 12 Union Road, Cambridge CB2 1EZ, U.K.; fax, (internat.) þ44-1223/336-033; e-mail, deposit@ ccdc.cam.ac.uk. Additional figures, TG, and the supplementary

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(15) Plasseraud, L.; Maid, H.; Hample, F.; Saalfrank, R. W. Chem.; Eur. J. 2001, 7, 4007. (16) Qin, C.; Wang, X. L.; Yuan, L.; Wang, E. B. Cryst. Growth Des. 2008, 8, 2093. (17) Zang, S. Q.; Su, Y.; Li, Y. Z.; Zhu, H. Z.; Meng, Q. J. Inorg. Chem. 2006, 45, 2972. (18) Complex 1 was prepared by hydrothermal reaction of Ni(NO3)2 3 6H2O (0.1 mmol), dtb (0.1 mmol), and 4,40 -bipy (0.15 mmol). The pH was adjusted to 6 with NaOH (1 M), 15 mL of deionized water was added by dropper into the vessel at 160 °C for 72 h, and the reaction

Dong et al. mixture was slowly cooled down to room temperature, which afforded green block crystals. Elem anal. Calcd for C29H20N5NiO9S2 (%): C, 53.87; H, 3.12; N, 10.83. Found: C, 53.69; H, 3.23; N, 10.74. IR (KBr, cm-1): 3421 s, 1614 s, 1559 m, 1512 s, 1390 m, 1362 m, 1336 m, 1220 w, 1145 w, 809 m, 625 w. Complex 2 was synthesized in a similar procedure as that for 1, by using CoCl2 3 6H2O in place of Ni(NO3)2 3 6H2O. Red block crystals of 2 were obtained. Elem anal. Calcd for C29H20N5CoO9S2 (%): C, 49.38; H, 2.86; N, 9.93. Found: C, 49.07; H, 2.62; N, 10.03. IR (KBr, cm-1): 3420 s, 1615 s, 1560 m, 1512 s, 1390 m, 1362 m, 1336 m, 1220 w, 1144 w, 808 m, 624 w.