Supramolecular architecture of cadmium(II)–terephthalate complexes having a tridentate or tetradentate Schiff base as blocking coligand
Graphical abstract
Two new cadmium(II)–terephthalate complexes, one with the tridentate ligandN1,N1-diethyl-N2-(1-(pyridin-2-yl)ethylidene)ethane-1,2-diamine and other with the tetradentate ligandN,N′-bis-(1-pyridin-2-yl-ethylidene)-ethane-1,2-diamine, have been synthesized. The compound having the tridentate ligand is a 1D polymeric chain whereas the compound with the tetradentate ligand is a very rare type of dinuclear complex.
Introduction
Metal-organic coordination frameworks (coordination polymers) with various intriguing topologies have been extensively studied for their versatile chemical and physical properties and potential applications in functional materials [1], [2], [3], [4], [5], [6], [7], [8], [9]. The combination of metal–ligand coordination and non-covalent interactions, such as hydrogen bonding, provides a powerful method for creating complex structures from simple building blocks [10], [11], [12], [13]. In addition, the π-interactions and host–guest solvent interactions offer the propensity for packing control in the solid crystalline state [14], [15], [16], [17]. Usually bi- or multidentate ligands containing N or O donors, e.g., 4,4′-bipyridine, azide, pyrazine, 4,4′-trimethylenedipyridine and polyaromatic acids, etc., are used to bridge metal centres [18], [19], [20], [21], [22]. Multi-carboxylic ligands with suitable spacers are good choices as building blocks, especially benzoic acid based ligands [23], [24]. As an example, terephthalates (ta) have been used in many synthetic systems because they can form short bridges via one carboxylato end or long bridges via the benzene ring, leading to a great variety of structures [25], [26]. Among the metal ions, Cd(II) coordination polymers with terephthalate have varying topologies as the metal coordination number can be expanded up to seven and the directional property of the coordination bond is somewhat relaxed by the absence of any crystal field stabilization energy in the d10 electronic configuration [27], [28], [29]. The terminal or blocking co-ligands, which are usually used along with the bridging ligand to complete the metal coordination sphere, can alter the supramolecular assembly and consequently the type of structure formed. The literature data show that for cadmium–terephthalate systems either monodentate or chelating bidentate terminal ligands have been used [26], [27]. Only one compound has been reported with a tetradentate terminal ligand and the resulting compound is mononuclear [30]. No compound with a tridentate blocking ligand is reported to date. Schiff base type ligands are easily prepared, highly variable and offer a good control on the metal coordination sphere by occupying a square plane of an octahedron or trigonal plane of a trigonal biypramid and leaving the axial bonding sites free for the desired extended coordination [31], [32].
In this paper, we report the synthesis of two complexes of cadmium terephthalate using a tridentate (L1) and a tetradentate (L2) terminal Schiff base type ligand by the conventional solution method and their crystal structure analyses.
Section snippets
Materials and physical measurements
Commercially available solvents, 2-acetyl-pyridine, N,N′-diethylethylenediamine, ethylenediamine, cadmium acetate dihydrate and sodium terephthalate were used without further purification. Elemental analyses were carried out using a Perkin–Elmer 240C elemental analyzer. IR spectra were recorded in KBr (4500–500 cm−1) using a Perkin–Elmer R XI FT-IR spectrophotometer. The thermal analyses (TG-DTA) were carried out on a Mettler Toledo TGA/SDTA 851 thermal analyser in a dynamic atmosphere of
IR spectra
For complexes 1 and 2 strong absorption bands appear at 1572 and 1570 cm−1 for the asymmetric stretching and 1398 and 1381 cm−1 for the symmetric stretching of the carboxylate groups, respectively. The binding modes of the carboxylate ligands can be accessed on the basis of the difference (Δ) between the asymmetric stretching νa and symmetric stretching νs [40]. The Δ value for 1 is 174 cm−1 and for 2 is 189 cm−1. The separation is consistent with binding modes as found in the
Conclusion
Herein, we reported two cadmium(II) complexes of terephthalate having nitrogen containing tridentate and tetradentate Schiff-base blocking ligands. Compound 1, with a tridentate ligand, is a 1D polymeric chain whereas compound 2 with tetradentate ligand is a very rare type of dinuclear complex. In both complexes the water of crystallization is involved in a strong H-bonding network. In complex 2 the intemolecular packing is controlled by typical π-stacking interactions between pyridyl planes.
Acknowledgements
This work was supported by the University Grants Commission (UGC) (Fellowship for S. Banerjee, Sanction No. UGC/548/jr. Fellow Sc 2001/2002), the Deutsche Forschungsgemeinschaft (Grant Ja466/10-1,-2) and the Fonds der Chemischen Industrie.
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