Systematic Investigation of the Nature of The Coupling between a Ln (III) Ion (Ln = Ce (III) to Dy (III) ) and Its Aminoxyl Radical Ligands. Structural and Magnetic Characteristics of a Series of Ln (organic radical) 2 Compounds and the Related Ln (Nitrone) 2 Derivatives

Numerous molecular compounds in which a paramagnetic Ln (III) ion is in interaction with a second spin carrier such as a transition metal ion or an organic radical have been described. However, except for the isotropic Gd (III) ion, very little is known concerning the nature (ferro- or antiferromagnetic) and mechanism of the interaction involving a Ln (III) ion. This study addresses the problem of the interaction between the Ln (III) ions displaying spin−orbit coupling and an organic radical. The magnetic properties of a series of isostructural compounds, Ln (organic radical) 2 (NO3) 3, comprising a Ln (III) ion (Ln = Ce to Dy) surrounded by two N, O-chelating aminoxyl radicals (3-nitronyl nitroxide-4, 5-dimethyltriazole) have been investigated. The experimental approach used to get insight into the Ln-Radical coupling occurring within these compounds is based on the knowledge of the intrinsic paramagnetic contribution of the metal ion. This contribution has been deduced from the corresponding Ln (Nitrone) 2 (NO3) 3 derivatives (Nitrone stands for 3-N-tert-butylnitrone-4, 5-dimethyltriazole), where the Ln (III) ion is now in a diamagnetic surrounding. A simple mathematical difference of the magnetic susceptibilities of Ln (organic radical) 2 (NO3) 3 and the corresponding Ln (Nitrone) 2 (NO3) 3 derivatives then permitted the nature of the correlations within the Ln (organic radical) 2 (NO3) 3 compounds to be established. Moreover, these results have been confirmed by the field dependence of the magnetization for each derivative. A systematic investigation of the isostructural series of compounds allowed the evolution of these interactions to be compared as a function of the electronic configuration of the 4f orbitals. For the Ln (III) with 4f1 to 4f5 electronic configurations the Ln-organic radical interaction is antiferromagnetic. Conversely, this interaction was found to be ferromagnetic for the configurations 4f7 to 4f. 10 These interactions have been clearly indicated for each Ln (III).