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 4f(1) to 4f(5) electronic configurations the (in-organic radical) interaction is antiferromagnetic. Conversely, this interaction was found to be ferromagnetic for the configurations 4f(7) to 4f.(10) These interactions have been clearly indicated for each Ln(III).