Trimetallic Nickel–Lanthanum and Nickel–Gadolinium Metallomesogens

Abstract

Adducts were formed between a mesomorphic Ni(salen) complex [salen=2,2′-N,N′-bis(salicylidene)ethylenediamine] with six terminal alkoxy chains and a lanthanide nitrate (Ln=La, Gd). Different alkoxy chain lengths were used: OC¹²H²⁵, OC¹⁴H²⁹, OC¹⁶H³³ and OC¹⁸H³⁷. Trinuclear nickel–lanthanum and nickel–gadolinium complexes [Ln(NO³)³{Ni(salen)}²] were obtained. The compounds exhibit a wide-temperature-range hexagonal columnar mesophase (Col^H) with rather low melting points. The mesophase stability ranges of both the parent nickel complexes and the nickel–lanthanide complexes decrease with increasing chain length. A decrease in the mesophase stability range over the lanthanide series was also observed. The results are compared with those of similar copper–lanthanide complexes. A marked difference is the higher thermal stability of the nickel–lanthanide complexes in comparison with the copper–lanthanide complexes.

Keywords:

• Liquid crystals
• Metallomesogens
• Columnar mesophases
• Rare earths
• Lanthanides

Acknowledgements

K.B. thanks the F.W.O.-Flanders (Belgium) for a postdoctoral fellowship and for financial support (research project G.0117.03). Financial support of the K.U. Leuven to K.L. (PhD grant) and to K.B. (research project GOA 03/03) is gratefully acknowledged. CHN microanalyses were performed by Mrs Petra Bloemen.

Notes

The Future of Supramolecular ChemistrySupramolecular chemistry is a rapidly growing branch of chemistry, each year attracting the interest of more and more scientists. Intense research in the field has led to many discoveries, and different principles have been demonstrated: self-assembly, recognition of cations, anions and neutral molecules, biomimetic systems, supramolecular crystal engineering, molecular machines and so on. However, very few supramolecular systems lead to (industrial) applications. Before supramolecular compounds can be transformed into competitive new materials, more efficient synthetic routes to self-assembling building blocks have to be developed. Compounds which are accessible only after multiple tedious synthesis steps will be too expensive for applications. I think that a major goal in supramolecular chemistry should be the development of easy and cheap methods for the synthesis of target compounds. The quest for new synthetic routes and of new materials is also of importance in the subfield of supramolecular chemistry I am working in: metal-containing liquid crystals (metallomesogens). This research field has been dominated by chemists, not by physicists. During the last two decades, many different classes of metallomesogens have been discovered. At present, only a limited number of metals have not yet been incorporated in liquid crystals: hafnium, tantalum, niobium, scandium, gallium and indium (not to mention the radioactive elements). It is to be expected that the methodology which leads to the development of metallomesogens with the lanthanides or zirconium as the central metal ion can also be successfully applied to the design of metallomesogens containing the above-mentioned metal ions. There is still a need to develop new types of ligands for metallomesogens, because many of the metallomesogens described in the literature have very high transition temperatures or are thermally unstable. In comparison with the classic liquid crystals, very little is known about the physical properties of metallomesogens. Workers in the field of metallomesogens should encourage physicists to look in more detail at the properties of metal-containing liquid crystals. Because of the presence of a (transition) metal ion, metallomesogens can exhibit very interesting electrochemical, electronic or magnetic behaviour. Other properties of interest could be thermochromism, information storage and switching ability (in electric and/or magnetic fields). Most of the metallomesogens described in the literature are thermotropic liquid crystals. Lyotropic metallomesogens have been largely neglected so far. Many transition complexes have the right chemical structure to exhibit lyomesomorphism, not only in polar solvents but also in apolar solvents.Prof. Dr Koen Binnemans was born in Geel, Belgium, in 1970. He obtained his M.Sc. (1992) and Ph.D. (1996) in Chemistry at the Catholic University of Leuven (Belgium), under the supervision of Prof. C. Görller-Walrand. In 1996, he became a Postdoctoral Fellow of the Fund for Scientific Research Flanders (Belgium). He did postdoctoral work with Prof. J. Lucas (Rennes, F) and Prof. D. W. Bruce (Exeter, UK). In 2000, he received the first ERES Junior Award (ERES: European Rare-Earth and Actinide Society), and since 2002, he has been Associate Professor of Chemistry at the Catholic University of Leuven. Koen Binnemans is an author of more than 100 papers in international scientific journals. His current research interests are metal-containing liquid crystals (metallomesogens), MRI contrast reagents, lanthanide-doped sol–gel glasses, lanthanide spectroscopy and applications of lanthanide compounds as reagents for organic synthesis. Other interests include reading, collecting minerals, Egyptology and history of artificial lighting.