Schiff-based bipyridine ligands. Unusual coordination features and mesomorphic behaviour

Recent investigations from our laboratory have described compelling experimental evidence that Schiff-based pyridine, bipyridine (bipy), terpyridine (terpy), phenanthroline (phen), naphthyridine (naphthy) and pyridine-pyridazine (pyridaz) ligands exhibit a rich coordination chemistry towards transition metal complexes providing unusual coordination modes. This is well illustrated with bipy and phen frameworks which exhibit a bridging type of coordination compared to the classical chelating behaviour. In such a way metallo-helicates are produced in a straightforward manner. A similar helicoidal complex is prepared with the naphthy ligand while a tetranuclear tissue-like complex is obtained with the pyridaz frame. The stability constants for these copper(I) complexes were determined by spectrophometric titrations and global analytical techniques. Each step of the assembly process could be deciphered and the association constants of the final complexes are high. An important caveat of these studies is that positive cooperativity is not mandatory to obtain a helicate template as the final product. Judicious grafting of flexible and paraffinic tails to these multichelating ligands insure the formation of mesomorphic material after a selective metal-induced wrapping of the non-mesomorphic strands around the copper(l) cations. By careful design of the system either columnar or columnaro-smectic mesophases are observed at room-temperature. This simple approach of using Schiff-based scaffoldings which are easy to obtain in a pure form, also facilitates the extension of the size of the supramolecular assembly and the alteration of the molecular shape. Other metals such Ni(II), Fe(II), Cd(II) could be complexed easily which testify to the remarkable properties of this family of compounds. Finally, some consideration is given to the preparation of larger systems and their possible use in molecular based-devices. (C) 2001 Elsevier Science B.V. All rights reserved.