Elsevier

Polyhedron

Volume 22, Issue 21, 15 September 2003, Pages 2933-2941
Polyhedron

Construction of coordination polymers of cadmium(II) with mixed hexamethylenetetramine and terephthalate or thiocyanate ligands

https://doi.org/10.1016/S0277-5387(03)00404-2Get rights and content

Abstract

Three new polymeric complexes [Cd(hmt)(SCN)2(H2O)2]n (1), [Cd32-hmt)2(SCN)6(H2O)2]n (2), and [Cd2(hmt)2(tp)2(H2O)6]n (3), [hmt=hexamethylenetetramine, tp=terephthalate] have been synthesized and characterized by single crystal X-ray diffraction. Both the compounds 1 and 2 are 1-D polymers where Cd units are linked by double end-to-end thiocyanate bridges but in 2 the chain is wider containing three cadmium atoms instead of one as found in 1. In both compounds the coordination environment around cadmium atom is distorted octahedral. Compound 3 is a three-dimensional polymer having water-filled microporous channels. Both tp and hmt are μ2-bridged. One of the acid groups of tp is coordinated in chelating bidentate and the other in monodentate fashion. Half of its Cd atoms are hexa-coordinated and the rest are hepta-coordinated. Thermogravimetric analysis and X-ray diffraction study of 3 show that its framework remains intact upon removal of water molecules. The flexibility of coordination number around cadmium atoms (six or seven) probably plays an important role in establishing the rigidity of the framework.

Hexamethylenetetramine forms two different one-dimensional polymeric chains with Cd(II) thiocyanate, [Cd(hmt)(SCN)2(H2O)2]n and [Cd32-hmt)2(SCN)6(H2O)2]n. With Cd(II) terephthalate it forms a three-dimensional polymer having water-filled microporous channels. X-ray single crystal structures of all three compounds are determined.

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Introduction

The design and synthesis of supramolecular architectures analogous to important minerals such as zeolites using the principle of crystal engineering are of much current interest [1], [1](a), [1](b). The crystal engineering of coordination polymers are usually achieved by using bi- or multidentate ligands to bind the metal centres. Two types of ligands, neutral organic e.g. 4,4′-bipyridine, pyrazine etc. [2], [2](a), [2](b), [2](c), [3], [3](a), [3](b), [3](c), [3](d), [3](e), [3](f), [3](g), [3](h), [4] and anionic e.g. thiocyanate, azide [5], [6], [7], [8], [9], polyaromatic acids [10], [11], [12], [13] etc. are used to synthesize one-, two- or three-dimensional polymers. Combination of these two types of ligands [14], though less frequently, has also been used to synthesize coordination polymers with variable cavities or channels. Among the anionic ligands, the terephthalate [15], [15](a), [15](b), [15](c), [15](d), [15](e) and thiocyanate ions exhibit a variety of bridging abilities and have strong tendencies to form large, tightly bound metal cluster aggregates. The polycyclic tertiary amine, hexamethylenetetramine [16], [17], [18], [19] is a potentially tetradentate neutral ligand but mostly coordinates to the meal centre in the bidentate fashion to produce several interesting polymers. The metal ion, Cd(II) is well suited for construction of such materials as its electronic configuration as well as size permits a wide variety of geometries and coordination numbers.

To explore the combination effects of neutral hmt and anionic thiocyanate or terephthalate ligands we synthesize coordination polymers of Cd(II) containing these ligands. Considering the variable coordination behaviour of hmt we changed the Cd(II): hmt ratio and found that two different 1-D polymers could be obtained in case of thiocyanate but only one type of 3-D polymer containing both hexa- and hepta- coordinated Cd(II) is obtained with tp, irrespective of the metal: hmt ratio. Reported herein are the details of the synthesis, crystal structure and other relevant physicochemical studies of these three polymers.

Section snippets

Materials and measurements

Elemental analyses (carbon, hydrogen and nitrogen) were performed using a Perkin–Elmer 240C elemental analyser. The infrared spectra in KBr (4500–500 cm−1) were recorded using a Perkin–Elmer RXI FT-IR spectrophotometer. Cd(OAc)2·2H2O, hexamethylenetetramine, and NH4SCN were purchased from EMerck Germany and used as received. All other chemicals used were AR grade. The thermal analysis (TG-DTA) were carried out on a Metler Toledo TGA/SDTA 851 thermal analyser in a dynamic atmosphere of

Results and discussion

Compounds 1 and 2 are prepared by varying the hmt concentration keeping other constituents constant. Interestingly, it has been observed that when the cadmium:hmt molar ratio is varied between 1:0.25–1:0.75 compound 2 is obtained as revealed by C, H, N, thermal and X-ray powder diffraction analysis whereas when the Cd:hmt molar ratio is maintained in the range 1:1–1:2 complex 1 is obtained. On the other hand, complex 3 is the only product in the concentration ranges of cadmium:hmt=1:0.2–1:2.

Conclusion

In this work, we reported two 1-D polymers of the formula [Cd(hmt)(SCN)2(H2O)2]n (1) and [Cd32-hmt)2(SCN)6(H2O)2]n (2) with double end-to-end thiocyanate bridges formed by varying the hexamine ratio and a noninterpenetrating rigid 3-D coordination framework with [Cd2(hmt)2(tp)2(H2O)6] as building blocks whose novelty lies in two different hexa and hepta coordinated Cd environment in the same structural motif bridged by hexamine and terephthalate to form hydrophilic channels where guest water

Supplementary material

Crystallographic data for the structural analysis have been deposited with the Cambridge Crystallographic Data Centre, CCDC Nos. 190884–190886 for complexes 13 respectively. Copies of this information may be obtained free of charge from The Director, 12 Union Road, Cambridge, CB2 1EZ, UK (fax: +44-1223-336033; e-mail: [email protected] or www: http://www.ccdc.cam.ac.uk).

Acknowledgements

We gratefully acknowledge financial support from EPSRC and the University of Reading, UK for funds for the Image Plate system. One of the authors (S.B.) is thankful to University Grants Commission, India for awarding a Junior Research Fellowship (Sanction no. UGC/548/jr. Fellow Sc.2001/2002). We also like to thank Dr. T. Maji and Mr. Jaydip Gangopadhyay, , IACS, Kolkata, and Dr. G. Mostafa, Berhampur, K.N.college, Murshidabad for their valuable suggestions regarding the structure of the

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