Topology diversity and reversible crystal-to-amorphous transformation properties of 3D cobalt coordination polymers from a series of 1D rodlike dipyridyl-containing building blocks and a flexible tripodal acid with additional amide groups

Abstract

Three novel cobalt(II) coordination polymers based on 1D rodlike dipyridyl-containing chains and a flexible tripodal acid with additional amide groups with unprecedented (3, 5) and (4, 4, 5) connected topology with a Schläfli symbol of (5, 6²) (5³, 6⁴, 7, 8²), and (5, 6⁵)₂ (5³, 6⁶, 7)₂ (6⁴, 8, 10) were reported along with interesting reversible crystal-to-amorphous transformation properties.

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Coordination assemblies based on highly symmetrical multi-topic ligands are of high interest due to their fascinating topologies and interesting properties.¹ Among them, rigid ligands are often employed in a designed strategy to construct coordination complexes with almost predictable topologies,² while flexible ligands with additional functional groups can adopt more versatile conformations and coordination modes according to the geometric requirements of different metal ions, which may lead to unpredictable intriguing topologies and properties.³ Recently, systematic investigations on a series of flexible tripodal acid ligands containing the –OCH₂– group, –NHCH₂– group, –SCH₂– group, and amide groups have been reported by us, which led to the complexes with interesting topologies and properties.⁴ Meanwhile, a building block methodology applying for a wide range of interesting network topologies has attracted great attention, such as the strategy of the secondary building units (SBUs),⁵ and the supermolecular building block (SBB) approach⁶ as well as the pillared-layer method.⁷ On the other hand, a comprehensive search in the literature and in the Cambridge Structural Database (CSD, version 5.30, November 2008) reveals that 4,4′-bipyridyl derivatives are widely used to form rodlike chains and exist as the building units for the construction of 3D coordination polymers (114 entries in the CSD).⁸ Thus, taking inspiration from a combination of the above considerations, N,N,′N′′-tris(carboxymethyl)-1,3,5-benzenetricarboxamide (TCMBT) are introduced to cobalt(II) coordination polymers of a series of rodlike building blocks based on three N,N′-donor ligands, 4,4′-bpy and its flexible derivatives, bpe and bpp {4,4′-bpy (4,4′-bipyridine), bpe [1,2-bi(4-pyridyl)ethane], and bpp (1,3-di(4-pyridyl)propane)} to afford three fascinating topological structures: {[Co_1.5(TCMBT)(4,4′-bpy)_1.5(H₂O)₃](H₂O)₅}_n (1), 3D novel ternary (3,4,4)-connected network; [Co_1.5(TCMBT)(bpe)_1.5(H₂O)₂]_n (2), 3D unprecedented ternary (4,4,5)-connected topology, and {[Co₂(TCMBT)(bpp)₂(NO₃)(µ₂-H₂O)₂]·(H₂O)₂} _n (3), 3D binary (3,5)-connected network (Fig. 1). Furthermore, an interesting combined effect of the two organic ligands and reversible crystal-to-amorphous transformation properties were also observed.

Fig. 1 View of the whole structures of complexes 1–3. In the view of 1D rodlike chains, the gray balls and the blue rods represent the metal ions and the pyridine-containing coligands, respectively. The red triangles with three arms represent the tripodal TCMBT ligands. 3D structures are obtained by the connection of rodlike chains and TCMBT.

Complex 1 was prepared by a hydrothermal reaction of Co(NO₃)₂·6H₂O, TCMBT and 4,4′-bpy in water. X-Ray structure analysis shows that 1 crystallizes in the monoclinic space group P2₁/c with the asymmetric unit consisting of one formula (Fig. S1).† Both Co1 and Co2 centers are coordinated in a distorted octahedral geometry with two carboxylate oxygen atoms from two distinct TCMBT and two aqua molecules in the equatorial plane, two nitrogen atoms from different 4,4′-bpy in apical positions. The 4,4′-bpy ligands act as exo-bidentate bridges to link the Co(II) centers to form typical one-dimensional chains along the a-axis with the Co⋯Co distance being 11.502 Å.^8a,b And each TCMBT ligand acts as a µ₃-trismonodentate ligand and adopts cis, cis, trans conformation (Scheme S1a).† It should be noted that all the chains running parallel are further linked to each other by TCMBT to form the 3D structure (Fig. 1). If each TCMBT is considered as a 3-connected node and both Co1 and Co2 centers act as two 4-connected nodes, the structure of 1 can be interpreted topologically as a special 3D (3,4,4)-connected ternary net with a Schläfli symbol (6³)₂ (6⁴, 8, 10)₃ (Fig. S2).†

By using the longer and more flexible bpe and bpp instead of the rigid 4,4′-bpy, complexes 2 and 3 were obtained, respectively, which crystallize in the orthorhombic Iba2 and Fdd2. The asymmetric unit of 2 consists of one formula and all the Co(II) centers are coordinated in a distorted octahedral geometry. The Co1 center is coordinated with three oxygen atoms from two monodentate carboxylate groups and one bridging carboxylate group of three different TCMBT and one aqua molecule in the basal plane, and two nitrogen atoms from different bpe in apical positions. Comparably, the Co2 center is coordinated with two carboxylate oxygen atoms from different bridging TCMBT and two aqua molecules in the equatorial plane, two nitrogen atoms from different bpe in apical positions (Fig. S3).† Each bpe ligand connects two Co(II) centers in anti-conformation to form another rodlike chain along the b-axis with the Co⋯Co distance being 13.632 Å. TCMBT adopts µ₂-η¹η¹ and monodentate coordination fashions in cis, cis, trans conformation (Scheme S1b†) to link the four neighboring chains to form the 3D structure (Fig. 1). From a topological view, if each TCMBT is considered as a 4-connected node, and the Co(II) centers are considered as 5-connected nodes (Co1) and 4-connected nodes (Co2), respectively, complex 2 shows a rare 3D (4,4,5)-connected topology with an unprecedented Schläfli symbol (5, 6⁵)₂ (5³, 6⁶, 7)₂ (6⁴, 8, 10) (Fig. S4†).⁹

The asymmetric unit of 3 consists of one formula and all the Co(II) centers are coordinated in a distorted octahedral geometry with two carboxylate oxygen atoms from different TCMBT ligands, two nitrogen atoms from different bpp and two µ₂-bridging water molecules (Fig. S5).† The two cobalt(II) centers are linked by two µ₂-bridging water molecules to form a metal dimer. The most fascinating structural feature of 3 is that the metal dimers are connected by pairs of bpp ligands in TG configuration,¹⁰ to form an interesting double-stranded chain of loops along the c-axis, in which the Co⋯Co distances in the single chain are 10.053 and 11.845 Å. These double chains are further cross-linked by the similar TCMBT compared with those in 2 to extend to a 3D structure (Fig. 1). With topological analyses, if the metal dimer is considered as a 5-connected node and TCMBT is defined as a 3-connected node, the structure of 3 can be symbolized as an unusual 3D binary (3,5)-connected topology with a Schläfli symbol (5, 6²) (5³, 6⁴, 7, 8²) (Fig. S6).† To the best of our knowledge, 3D networks based upon mixed three- and five- connected nodes are rare.¹¹

In our case, various rodlike building blocks based on 4,4′-bpy, bpe and bpp as well as cobalt(II) centers link with the flexible tripodal acid ligands, TCMBT, to construct three interesting 3D coordination polymers 1–3. As shown in Table 1, by changing the dipyridinium-based components with different size, the coordination modes of the TCMBT ligand increase from trismonodentate in 1 to tetradentate in 2 and 3. Meanwhile, interestingly, the amounts of metal, TCMBT, and dipyridinium-based components as well as the density appear to follow a clear decreasing progression. Two main factors may be responsible for the significant contrast. The first is the 4,4′-bpy and its flexible derivatives act as the effective spacer with different size to enhance the intermetal distance to form three different rodlike chains, which may facilitate the coordination between the metal ion and bulky organic molecules due to the minimization of crowding. The second, but most important factor is the combined effect of TCMBT and the 4,4′-bpy and its derivatives with different flexibility, which depends on the competition and adaptability of the components. For complex 1, the rigid 4, 4′-bpy ligands bridge the metal centers to form straight linear chains with the TCMBT ligand attached on them, in which the Co⋯Co distance is 11.502 Å. Since the TCMBT and bpe in 2 possess moderate flexibility, the bpe ligands in the anti-conformation lead to the formation of another kind of linear chains, which results in the longer Co⋯Co distance, 13.632 Å. Most interestingly, although the bpp ligand is longer than 4,4′-bpy and bpe, the bpp ligands in 3 adapt to the flexible acid in the TG configuration to satisfy the coordination requirement of the metal ions, which link the metal dimers to form unusual double chains with an unexpected shorter Co⋯Co distance (10.053 and 11.845 Å).

Table 1 Comparison of structural data in the three compounds

	Co–Co^a/Å	Coordination mode of TCMBT	Density/g cm^−3	Co/1000 Å^−3	TCMBT/1000 Å^−3	Dipyridyl ligand/1000 Å^−3
a The Co–Co refers to the distance between Co(II) centers bridged by a dipyridyl ligand.
1	11.502	Trismonodentate	1.583	1.692	1.128	1.692
2	13.632	Tetradentate	1.439	1.675	1.117	1.675
3	10.053	Tetradentate	1.283	1.508	0.754	1.508
	11.845

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All three complexes show an interesting reversible crystal-to-amorphous transformation, which is similar to the examples reported by Kitagawa's groups.^7b The dark red residues were obtained by heating the crystals of 1, 2 and 3 to 120 °C for 12 h. Powder XRD analysis shows that the crystallinity of each product was lost and the arrangement of the framework was disordered by dehydration (Fig. S7–9).† Interestingly, when these solids were suspended in water for 12 h, the same PXRD patterns as those of the original crystals were regenerated. Thus, the dehydrated solids of these complexes may be potential reversible adsorbent materials for water molecules and the absorptive capacities are 38.3, 23.3 and 33.3%, respectively, which are much higher than those in our previous work.^4c,12

In summary, three novel coordination polymers were obtained based on a series of different rodlike chains and TCMBT. Noticeably, complexes 1–3 show three intriguing topologies, including the ternary (3,4,4)-connected topology in 1, the ternary (4,4,5)-connected topology in 2, and the binary (3,5)-connected topology in 3. Due to the different size and flexibility of 4,4′-bpy, bpe and bpp, the interesting combined effect is observed between the 4,4′-dipyridyl ligands and TCMBT, which results in diversity of the topological structures and a decreasing progression of the contents in these complexes. An interesting reversible crystal-to-amorphous transformation and high absorptive capacities of water are obtained in these three complexes.

Acknowledgements

The authors gratefully acknowledge support from the Major State Basic Research Development Programs (Nos. 2006CB806104 and 2007CB936302), the NSFC (No. 20771058), the Science Foundation of Innovative Research Team of NSFC (No. 20721002), and the Specialized Research Fund for the Doctoral Program of the Ministry of Education of China (No. 200802840011).

References

1. (a) S. R. Seidel and P. J. Stang, Acc. Chem. Res., 2002, 35, 972; (b) M. Hong, Y. Zhao, W. Su, R. Cao, M. Fujita, Z. Zhou and A. S. C. Chan, J. Am. Chem. Soc., 2000, 122, 4819; (c) S. Ma and H. C. Zhou, J. Am. Chem. Soc., 2006, 128, 11734; (d) H. K. Chae, J. Kim, O. D. Friedrichs, M. O'Keeffe and O. M. Yaghi, Angew. Chem., Int. Ed., 2003, 42, 3907; (e) J. P. Costes, F. Dahan and F. Nicodeme, Inorg. Chem., 2003, 42, 6556.
2. (a) M. Eddaoudi, J. Kim, M. O'Keeffe and O. M. Yaghi, J. Am. Chem. Soc., 2002, 124, 376; (b) B. F. Abrahams, M. G. Haywood, R. Robson and D. A. Slizys, Angew. Chem., Int. Ed., 2003, 42, 1112; (c) H. Chun, D. Kim, D. N. Dybtsev and K. Kim, Angew. Chem., Int. Ed., 2004, 43, 971.
3. (a) E. Lee, J. Kim, J. Heo, D. Whang and K. Kim, Angew. Chem., Int. Ed., 2001, 40, 399; (b) C. Qin, X. L. Wang, E. B. Wang and Z. M. Su, Inorg. Chem., 2005, 44, 7122; (c) R. J. Hill, D. L. Long, N. R. Champness, P. Hubberstey and M. Schroder, Acc. Chem. Res., 2005, 38, 335.
4. (a) S. N. Wang, H. Xing, Y. Z. Li, J. Bai, Y. Pan, M. Scheer and X. Z. You, Eur. J. Inorg. Chem., 2006, 3041; (b) S. N. Wang, J. Bai, Y. Z. Li, Y. Pan, M. Scheer and X. Z. You, CrystEngComm, 2007, 9, 228; (c) R. Sun, Y. Z. Li, J. Bai and Y. Pan, Cryst. Growth Des., 2007, 7, 890; (d) R. Sun, S. N. Wang, H. Xing, J. Bai, Y. Z. Li, Y. Pan and X. Z. You, Inorg. Chem., 2007, 46, 8451; (e) S. N. Wang, J. Bai, H. Xing, Y. Z. Li, Y. Song, Y. Pan, M. Scheer and X. Z. You, Cryst. Growth Des., 2007, 7, 747; (f) S. N. Wang, H. Xing, J. Bai, Y. Z. Li, Y. Pan, M. Scheer and X. Z. You, Chem. Commun., 2007, 2293; (g) S. N. Wang, J. Bai, Y. Z. Li, Y. Pan, M. Scheer and X. Z. You, CrystEngComm, 2007, 9, 1084; (h) S. N. Wang, R. Sun, X. S. Wang, Y. Z. Li, Y. Pan, J. Bai, M. Scheer and X. Z. You, CrystEngComm, 2007, 9, 1051; (i) S. N. Wang, Y. Yang, J. Bai, Y. Z. Li, M. Scheer, Y. Pan and X. Z. You, Chem. Commun., 2007, 4416.
5. (a) N. W. Ockwig, O. D. Froedrichs, M. O'Keeffe and O. M. Yaghi, Acc. Chem. Res., 2005, 38, 176; (b) H. Li, M. Eddaoudi, M. O'Keeffe and O. M. Yaghi, Nature, 1999, 402, 276.
6. (a) A. J. Cairns, J. A. Perman, L. Wojtas, V. Ch. Kravtsov, M. H. Alkordi, M. Eddaoudi and M. J. Zaworotko, J. Am. Chem. Soc., 2008, 130, 1560; (b) F. Nouar, J. F. Eubank, T. Bousquet, L. Wojtas, M. J. Zaworotko and M. Eddaoudi, J. Am. Chem. Soc., 2008, 130, 1833.
7. (a) R. Kitaura, K. Fujimoto, S. Noro, M. Kondo and S. Kitagawa, Angew. Chem., Int. Ed., 2002, 41, 133; (b) S. Kitagawa, R. Kitaura and S. I. Noro, Angew. Chem., Int. Ed., 2004, 43, 2334.
8. Some examples of 3D coordination polymers composed of the rodlike building blocks based on 4,4′-bpy, bpe and bpp are described in: (a) B. Chen, C. Liang, J. Yang, D. S. Coutreras, Y. L. Clancy, E. B. Lobkovsky, O. M. Yaghi and S. Dai, Angew. Chem., Int. Ed., 2006, 45, 1390; (b) J. Zhang and X. Bu, Chem. Commun., 2008, 1756; (c) B. Rather and M. J. Zaworotko, Chem. Commun., 2003, 830; (d) R. Baldomá, M. Monfort, J. Ribas, X. Solans and M. A. Maestro, Inorg. Chem., 2006, 45, 8144; (e) J. Zhang and X. Bu, Angew. Chem., Int. Ed., 2007, 46, 6115; (f) L. Hu, J. Fan, C. Slebodnick and B. E. Hanson, Inorg. Chem., 2006, 45, 7681.
9. K. V. Domasevitch, P. V. Solntsev, I. A. Gural'skiy, H. Krautscheid, E. B. Rusanov, A. N. Chernega and J. A. K. Howard, Dalton Trans., 2007, 3893.
10. (a) L. Carlucci, G. Ciani, D. W. V. Gudenberg and D. M. Proserpio, Inorg. Chem., 1997, 36, 3812; (b) F. M. Tabellion, S. R. Seidel, A. M. Arif and P. J. Stang, J. Am. Chem. Soc., 2001, 123, 11982; (c) E. Q. Gao, Z. M. Wang, C. S. Liao and C. H. Yan, New J. Chem., 2002, 26, 1096.
11. (a) B. F. Abrahams, S. R. Batten, B. F. Hoskins and R. Robson, Inorg. Chem., 2003, 42, 2654; (b) K. A. Brown, D. P. Martin and R. L. LaDuca, CrystEngComm, 2008, 10, 1305; (c) R. Heck, J. Bacsa, J. E. Warren, M. J. Rosseinsky and D. Bradshaw, CrystEngComm, 2008, 10, 1687.
12. X. L. Hong, Y. Z. Li, H. Hu, Y. Pan, J. Bai and X. Z. You, Cryst. Growth Des., 2006, 6, 1221.

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Footnote

† Electronic supplementary information (ESI) available: Detailed experimental procedures, crystal data, coordination modes, asymmetric units, topologies and X-ray powder diffraction. CCDC reference numbers 735371–735373. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/b911380k

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