ReviewMagnetism of metal-nitroxide compounds involving bis-chelating imidazole and benzimidazole substituted nitronyl nitroxide free radicals
Introduction
From the general point of view, the strategy to get magnetic materials by coordination chemistry, relies on building extended polymetallic networks in which magnetic metal centres are connected through bridging ligands. The latters should both assemble the metal centres and mediate strong magnetic interactions, and in such a way to have a bulk non-zero magnetic moment. In this context, the use of free radicals as the bridging ligands is particularly pertinent. First, the direct bonding of the spin carriers, that is the metal centers and the free radical, should favour strong magnetic interactions. Second, the combination of inorganic and organic spin carriers allows a great versatility in the design of magnetic networks with various topologies and dimensionalities.
Whereas the metal–radical approach should not be restricted to the only nitroxide free radicals, the latter ones have gained predominance and dragged much works in the last two decades. This should be ascribed to the fact that nitroxide radicals are among the most persistent free radicals even in the presence of metal ions and may be obtained in almost limitless forms thus allowing the fine design of a large panel of magnetic bridging ligand.
The scope of the present review with respect to magnetic materials is not to cover all the results involving nitroxide–metal complexes for which some reviews have already appeared [1], [2], [3], [4], [5]. It concerns mostly the complexes of nitronyl or imino nitroxide grafted on imidazolyl or benzimidazolyl groups (Scheme 1) and is mainly based on our published work supported by some unpublished results.
Section snippets
Background
Nitroxide free radicals are one of the best characterized groups of free radicals. Their prior interest was mainly as probes in biological systems and most of the early studies on metal-nitroxide interactions were related to this field [6]. It is in the last two decades that they became increasingly appealing as spin carriers in magnetic materials. Thus, the first organic magnets were nitroxide free radicals [7], [8] and their discovery in the 1990s gave rise to extending research works
Scope of the review
The present article is mainly a review of our cumulative work with nitronyl and imino-nitroxide grafted on to imidazole or benzimidazole groups (Scheme 1). These ligands have also been studied by others but mainly as organic building blocks [64], [65], [66], [67]. In our case, these radicals were specially designed in order to have, after deprotonation of the imidazolyl amino group, two symmetrical chelating sites incorporating the NO groups (Scheme 5).
We had in mind that the resulting
Conclusions and perspectives
This review illustrates the versatility of bis-chelating nitroxide free radicals in magnetic engineering. They are particularly promising spin carriers for designing extended exchange-coupled networks as illustrated with the one-dimensional and two-dimensional manganese(II) compounds exhibiting magnet like behaviour with relatively high Curie temperature in the case of the 2D systems. Moreover, these 2D-complexes are appealing for the development of intercalation compounds. Work is also in
Acknowledgements
We would thank all our co-workers on synthesis, magnetic theory, low temperature magnetic measurements and optical studies: Christophe Lescop, Karine Fegy, Elie Belorizky, Carley Paulsen, Christian Reber, Guillaume Bussière, Rémi Beaulac. This work was supported by the Commissariat à l’Energie Atomique (CEA), the Centre de la Recherche Scientifique (CNRS), the European Community through TMR programmes, the Centre Jacques Cartier and the Commission Permanente de Coopération Franco-Québécoise.
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