Metal–organic supramolecular architecture containing cationic cavities: synthesis and single crystal investigation of {[Co(bpe)₂(H₂O)₂](ClO₄)₂·(H₂O)₂}_n

---

Abstract

The one-dimensional cavity-containing cationic complex [Co(bpe)₂(H₂O)₂](ClO₄)₂·(H₂O)₂ is synthesized and characterized. Single crystal analysis reveals that the metal centres are doubly bridged via bpe ligands (bpe⊕=⊕1,2-bis(4-pyridyl)ethane) creating cationic channels with dimensions 9.29⊕×⊕9.94 Å across Co–Co(1) and C(6)–C(6A) atoms respectively, which are occupied by perchlorate anions and lattice water molecules along the c axis. The linear 1D polymeric chains run along the b axis and the versatile bpe ligand bridges the metal centres in a syn conformation.

---

Introduction

Molecular architecture constructed from metal-based coordination is an expanding field which leads to the development of new classes of functional materials. These complexes have potential application in the field of magnetic materials,¹ molecular adsorption,² host–guest chemistry³ and catalysis.⁴ However, coordination polymers with open framework structures are of interest from a structural point of view and their mimicry of microporous solids. The various factors which can influence the structural topologies in coordination polymers are the coordination geometry of the metals, structure of the spacer ligand, counter ions and reaction conditions. A building block approach has been utilized for the design of coordination polymers, which are assembled from suitable metal centres and organic ligands of different size and nature. A family of metal bridging building blocks containing 4-pyridyl donors having two coordination sites of the rigid ligands are often used to construct coordination polymers.⁵ Because of their ability to form H-bonded networks, the incorporation of the flexible 4-pyridyl ligands as linkers opens up more research avenues for coordination polymers to create high-dimensional networks. We chose 1,2-bis(4-pyridyl)ethane as it exhibits different conformations: anti (extended), syn (folded) (Fig. 1) and also because of its versatile nature as a spacer in any extended system. The dimensions of the polymer motif can be increased by cross linking the low-dimensional polymer by weaker intermolecular interactions such as hydrogen bonding and stacking in the solid state. In an endeavour to understand the network topology of the flexible bridging ligand 1,2-bis(4-pyridyl)ethane) (bpe) with the Co(II) metal centre by taking advantage of its conformational flexibility and the versatile binding mode, the {[Co(bpe)₂(H₂O)₂](ClO₄)₂·2H₂O}_n complex was synthesized and structurally characterized.

Fig. 1 Schematic diagram showing the (a) syn and (b) anti conformations of the bpe ligand.

Experimental

All reagents were commercially available and used as received. Elemental analysis (C, H, N) was performed on a model 2400 Perkin-Elmer elemental analyzer. The FTIR spectrum was recorded from KBr pellets in the range 4000–400 cm⁻¹ on a Perkin-Elmer Spectrum GX FTIR spectrophotometer.

Synthesis of {[Co(bpe)₂(H₂O)₂](ClO₄)₂·(H₂O)₂}_n

To a solution of Co(NO₃)₂·6H₂O (0.291 g, 1 mmol) in an EtOH–water mixture (15 ml, 2∶1 (v/v)), a solution of bpe (0.368 g, 2 mmol) in EtOH (10 ml) was added slowly with stirring over 15 min at 65 °C. To this a solution of sodium perchlorate (0.245 g, 2 mmol) in water (10 ml) was added and the resulting solution was refluxed on a water bath for 1 additional hour. The yellow solution was filtered, kept at 25 °C and stable crystals suitable for diffraction studies appeared within one week. Calc: C⊕=⊕41.26, H⊕=⊕4.29, N⊕=⊕8.02. Found: C⊕=⊕41.34, H⊕=⊕4.31, N⊕=⊕8.20%. υ/cm⁻¹: 3224–2931 br, 1613 vs, 1561 m, 1503 m, 1423 vs, 1225 m, 1090 sh, 1061 m, 930 w, 830 sh, 811 m, 622 sh, 549 sh.

X-Ray crystallographic analysis

Data was collected on an Enraf-Nonius CAD-4 diffractometer with graphite monochromatised MoKα radiation (λ⊕=⊕0.7107 nm). Cell constants and the orientation matrix for data collection were obtained from least squares refinement using 25 high-angle reflections in the θ range 8–12°. The ω-2θ scan mode with a maximum θ value of 22.5° was used to collect the intensity data for the complex. Preliminary data, cell refinement and intensity data collection were carried out using the program CAD4-PC⁶ and the data reduction using NRCVAX.⁷ The data were corrected for Lorentz Polarization effects but not for absorption. Structure solution was carried out using the Patterson method with full-matrix least squares refinement on F² using SHELX-97.⁸ Scattering factors and anomalous dispersions were taken from international tables for X-ray crystallography.⁹ Hydrogen atoms were located either from difference Fourier maps or kept fixed using the riding model, except for the lattice water hydrogens which were not available from the difference Fourier map. Crystallographic data and refinement for the compound are presented in Table 1.

Table 1 Crystal data for complex 1^a

a Click b009706n.txt for full crystallographic data (CCDC 154062)
Formula	C12H16ClCo0.5N2O6
M	349.18
Crystal system	Monoclinic
Space group	C2/c
Crystal dimensions/mm	0.16⊕×⊕0.08⊕×⊕0.06
a/Å	18.522(7)
b/Å	9.293(5)
c/Å	18.782(5)
β/°	104.20(3)
V/Å^3	3143(2)
D c/g cm^−1	1.480
F(000)	1444
Total number of reflections	2061
Number of observed reflections [I⊕>⊕2σ(I)]	1196
Number of refined parameters	223
R1	0.0608
wR2	0.1491

---

Results and discussion

An ORTEP¹⁰ view of the cationic linear coordination network is shown in Fig. 2. The structure of [Co(bpe)₂(H₂O)₂] consists of linear cationic chains extending along the crystallographic b axis. The coordination sphere around Co(II) can be defined by four nitrogen atoms from the bpe ligands and two oxygen atoms from the water molecules forming slightly distorted octahedral geometry. The equatorial positions are occupied by four nitrogen atoms from doubly bridged bpe ligands, whereas the axial coordination is provided by the two oxygen atoms of the symmetry-related water molecules. The average Co–N_bpe (2.15 Å) and Co–O_H2O (2.143 Å) distances are well within the range of reported values¹¹ and all the angles around the metal centre are closer to the ideal value giving rise to a slightly distorted octahedral geometry around the Co²⁺. The molecule possesses a centre of symmetry passing through the metal centre which occupies the (0.5, 0.275, 0.25) postion. The Co(II) ion makes a good equatorial plane with the coordinated bpe nitrogens; both the trans nitrogens are slightly displaced in opposite directions by only ±0.026 Å. Each Co(II) centre is doubly bridged by the terminal nitrogen atoms of the bidentate bpe ligands to form a 22-membered cyclic ring extending the linear polymeric chain along the b axis. The adjacent metal centres bridged by the bpe ligands are separated by 9.293 Å.

Fig. 2 ORTEP view of the molecular structure of complex 1 at a 50% probability level. Perchlorate anions and lattice water molecules are omitted for clarity.Click image or 2.htm to access a 3D representation.

A PLATON¹² diagram showing the packing viewed down the c axis is shown in Fig. 3. The linear polymeric chain runs along the b axis with adjacent chains staggered by half the b axis with respect to each other in the ab plane to form effective close packing. The neighbouring chains which are almost aligned along the c axis are separated by 9.39 Å. The bpe units doubly bridged with the metal centre form through channels down the c axis with dimensions 9.293⊕×⊕9.413 Å,¹³ across the Co–Co(1) and C(6)–C(6A) atoms, respectively. These channels along the c axis are occupied by the perchlorate anions and the lattice water molecules which form alternate layers with the cationic framework. The CPK diagram is shown in Fig. 4.

Fig. 3 Packing diagram viewed down the c axis showing the staggering of adjacent polymeric chains along the ab plane and through the cavity along the c axis filled with perchlorate anions and lattice water molecules.

Fig. 4 CPK diagram showing the cationic cavity filled with perchlorate anions and lattice water molecules.

The intrachain metal–metal distance is shorter (9.293 Å) than the one found in the Co(II)·μ-4,4′-bipyridine complexes,^{11a, 14} forming linear polymers, as a result of the folded syn conformation adopted by the bpe ligand. The torsion angle involving the four carbon atoms C3, C6, C7 and C8 (central ethyl carbon atoms and the connecting pyridyl carbon atom) is 63.7(4)°, clearly show the folding of the coordinated bpe ligand. The interchain metal–metal distance down the c axis is 9.417 Å, whereas the nearest metal–metal distance of the polymeric chains which are staggered in the ab plane is 10.361 Å. The cavity-enclosed bpe pyridyl rings are rotated by 44.7(3)° with respect to each other.

Even though structural reports of bpe ligands with transition metals are limited in number, conformational flexibility (both syn and anti) and the versatile coordination ability of the bpe ligand play vital roles in the generation of the various three-dimensional coordination networks. It is interesting to note that the isostructural Cd¹⁵ and Co¹⁶ structures [M₂(μ-syn-bpe)₂(μ-anti-bpe)(NO₃)₄]_n (M⊕=⊕Cd²⁺, Co²⁺) are reported, in which two syn bpe spacers link the metal centres in a similar fashion to that reported here to create a cyclic closed structure. These macrocyles are further linked viaanti-bpe ligand spacers at the metal centres to create linear polymeric chains. In the same report on Co(II)bpe complexes, Hennigar et al.¹⁶ structurally characterized two more isomeric forms [Co(II)(μ-syn-bpe)(μ-anti-bpe)₂(NO₃)₄]_n, [Co(II)(μ-anti-bpe)₂(μ-anti-bpe)(NO₃)₄]_n, which are possible only due to the conformational flexibility of the bpe ligands. Similar syn coordination of the bpe ligand bridging the metal centres blocked at the cis positions by ethylenediamine to create a dimer with a square cavity has been reported by Fujita et al. where no counter ions or solvent molecules are encapsulated.¹⁷ Ferbinteanu et al. reported the interesting structural motif {[Co(II)(μ-anti-bpe)(anti-bpe)(syn-bpe)(H₂O)₂](ClO₄)₂·0.5(anti-bpe)·(H₂O)}_n,¹⁸ where the bpe moiety adopts both conformations creating a novel supramolecular architecture utilizing both coordinate and hydrogen bonding interactions. In the structural report of the neutral [Fe(bpe)₂(NCS)₂]_n polymeric compound,¹⁹ the bpe ligands exhibit a similar type of coordination to that reported in the present investigation. Both the bpe ligands are in the syn disposition bridging the adjacent metal centres creating the linear polymeric chains, showing weak antiferromagnetic coupling through a M–(bpe)₂–M bridge, which is extended via a C–H⋯S H-bonding interactions between the adjacent chains resulting in a two-dimensional sheet-like architecture. The coordinated water molecule in complex 1 makes strong H-bonding interactions with the lattice water molecule O2W and the perchlorate oxygen atom O2 where O1W acts as a donor [O1W⋯O2W⊕=⊕2.728(1) Å, <O1W–H11O⋯O2W⊕=⊕152.64(2)°, symmetry code⊕=⊕1/2⊕+⊕x, 1/2⊕+⊕y, z; O1W⋯O2⊕=⊕3.160(2) Å,<O1W–H21O⋯O2⊕=⊕162.234(2)°, symmetry code⊕=⊕1/2⊕+⊕x, −1/2⊕+⊕y, z]. Although we could not locate the H-atoms of the lattice water molecule it is making good short contacts with the perchlorate oxygens O1 and O2 (O2W⋯O1⊕=⊕2.963 Å, O2W⋯O2⊕=⊕2.939 Å). These hydrogen bonding interactions play a crucial role in the stabilization of the molecule in the crystal lattice and the alignment of the polymeric chains along the c axis creating the cationic cavity.

Conclusions

The cavity-containing cationic linear coordination polymeric network {[Co(bpe)₂(H₂O)₂](ClO₄)₂·(H₂O)₂}_n has been synthesized and characterized by various physicochemical techniques including single crystal X-ray diffraction. The metal centres are bridged via exobidendate bpe ligands from either end through both terminal nitrogen atoms to create cationic channels which are occupied by the lattice water molecules and the perchlorate anions. The structure reported here illustrates the ability of the flexible bpe ligand to create a cavity-containing coordination network. Selective guest inclusion or anion exchange properties of this class of coordination polymer are yet to be investigated.

Acknowledgements

We acknowledge Dr P. K. Ghosh, Director and Dr R. V. Jasra, Deputy Director, CSMCRI, Bhavnagar for their interest and encouragement in this work.

Notes and references

1. (a) H. O. Stumpt, L. Ouahab, Y. Pei, D. Gajindran and O. Khan, Science, 1993, 261, 447; (b) J. A. Real, E. Anders, M. C. Munoz, M. Julve, T. Granier, A. Bousseksou and F. Varet, Science, 1995, 268, 265.
2. (a) M. Kondo, T. Yoshitomi, K. Seki, H. Matsuzaka and S. Kitagawa, Angew. Chem., Int. Ed. Engl., 1997, 36, 2076; (b) W. Mori, F. Inoue, K. Yosida, H. Nakayama, S. Takamizawa and M. Kishita, Chem. Lett., 1997, 1219.
3. (a) G. B. Gardner, D. Vekataraman, J. S. Moore and S. Lee, Nature, 1995, 374, 792; (b) D. Vekataraman, G. B. Garner, S. Lee and J. S. Moore, J. Am. Chem. Soc., 1995, 117, 11600; (c) O. M. Yaghi, G. Li and H. Li, Nature, 1995, 378, 703; (d) S. Subramanin and M. J. Zawarotko, Angew. Chem., Int. Ed. Engl., 1995, 34, 2127; (e) O. M. Yaghi, H. Li and T. L. Groy, Inorg. Chem., 1997, 36, 4292; (f) R. Robinson and M. J. Zaworotko, J. Chem. Soc., Chem. Commun., 1995, 2413; (g) O. M. Yaghi and H. Li, J. Am. Chem. Soc., 1996, 118, 295; (h) O. M. Yaghi and H. Li, J. Am. Chem. Soc., 1995, 117, 10401.
4. (a) J. S. Seo, D. Whang, H. Lee, S. I. Jun, J. Oh, Y. J. Jeon and K. Kim, Nature, 2000, 404, 982; (b) M. Fujita, Y. J. Kwon, S. Washizu and K. Ogura, J. Am. Chem. Soc., 1995, 116, 1151.
5. (a) S. Kitagawa and M. Kondo, Bull. Chem. Soc. Jpn., 1998, 71, 1739 and references therein.; (b) C. S. Hong and Y. Do, Inorg. Chem., 1998, 37, 4440; (c) G. D. Munno, D. Armeatano, T. Poerio, M. Julve and J. A. Real, J. Chem. Soc, Dalton Trans., 1999, 1813.
6. CAD4-PC Software, Version 5, Enraf-Nonius, Delft, Netherlands, 1989..
7. E. I. Gabe, Y. Le Page, I. P. Charland, F. L. Lee and P. S. White, J. Appl. Crystallogr., 1989, 22, 384.
8. G. M. Sheldrick, SHELX-97. Program for Crystal Structure Determination, University of Göttingen, Germany, 1997..
9. International Tables for X-Ray Crystallography, Kynoch Press, Birmingham, 1974, vol. IV..
10. C. K. Johnson, ORTEP, Report ORNL-3794, Oak Ridge National Laboratory, Oak Ridge, TN, USA, 1976..
11. (a) Y. B. Dong, M. D. Smith, R. C. Layland and H. C. Loye, J. Chem. Soc., Dalton Trans., 2000, 775; (b) J. Lu, C. Yu, T. Niu, T. Paliwala, G. Crisci, F. Somosa and A. Jacobson, Inorg. Chem, 1998, 37, 4637.
12. A. L. Spek, PLATON 92, University of Utrecht, Utrecht, The Netherlands, 1992..
13. The cavity dimensions described here are crystallographic scalar quantitities only and do not take into account the van der Wals radii of the atoms defining the cavity..
14. (a) P. Losier and M. J. Zaworotko, Angew. Chem., Int. Ed. Engl., 1996, 35, 2779; (b) L. M. Zheng, X. Fang, K. H. Li, H. H. Song, X. Q. Xin, H. K. Fun, K. Chinnakali and A. Razak, J. Chem. Soc., Dalton Trans., 1999, 2311.
15. M. Fujita, Y. J. Kwon, M. Miyazawa and K. Ogura, J. Chem. Soc., Chem Commun., 1994, 1977.
16. T. L. Hennigar, D. C. MacQuarrie, P. Losier, R. D. Rogers and M. J. Zaworotko, Angew. Chem., Int. Ed. Engl., 1997, 36, 972.
17. M. Fujita, S. Nagao, M. Iida, K. Ogata and K. Ogura, J. Am. Chem. Soc., 1993, 115, 1574.
18. M. Ferbinteanu, G. Marinescu, H. W. Roesky, M. Noltemeyer, H. G. Schmidt and M. Anrudh, Polyhedron, 1998, 18, 243.
19. M. L. Hernàndez, M. G. Barandika, M. K. Urtiaga, R. Cortés, L. Lezama, M. I. Arriortua and T. Rojo, J. Chem. Soc., Dalton Trans., 1999, 1401.

---