Magnetism of metal-nitroxide compounds involving bis-chelating imidazole and benzimidazole substituted nitronyl nitroxide free radicals

doi:10.1016/j.ccr.2005.06.008

Coordination Chemistry Reviews

Volume 249, Issue 23, 1 December 2005, Pages 2591-2611

https://doi.org/10.1016/j.ccr.2005.06.008 Get rights and content

Abstract

Coordination compounds based on imidazole and benzimidazole substituted nitronyl nitroxide radicals with transition metal ions and trivalent lanthanide ions are described from the perspective of their magnetic properties.

For the transition metal compounds the crystal structures show various metal-nitroxide dimensionalities including mononuclear (0D), one-dimensional (1D) and two-dimensional (2D) complexes. The mononuclear complexes were isolated with most metal ions of the first transition series. One copper(II) complex shows a copper(II)–radical ferromagnetic coupling (J = +75 cm⁻¹) while for the other mononuclear compounds, mainly with manganese(II), the metal–radical interactions are antiferromagnetic. The one-dimensional and two-dimensional complexes are manganese(II) compounds which show canting effects leading to weak ferromagnetism.

For the trivalent lanthanide ions [La(III), Gd(III) and Eu(III)], three series of mononuclear complexes were obtained in which the metal center is bound to four, two or one nitroxide radicals depending on the counter ions and ancillary ligands. Unexpectedly, in most gadolinium(III) complexes, the Gd(III)–radical interactions were found to be antiferromagnetic in contradiction with other foundings and previous theoretical models. In support to the magnetic studies, the optical properties of the lantanide complexes have also been investigated and are briefly described.

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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ReviewMagnetism of metal-nitroxide compounds involving bis-chelating imidazole and benzimidazole substituted nitronyl nitroxide free radicals

Abstract

Introduction

Section snippets

Background

Scope of the review

Conclusions and perspectives

Acknowledgements

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Review
Magnetism of metal-nitroxide compounds involving bis-chelating imidazole and benzimidazole substituted nitronyl nitroxide free radicals