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 inte raction with a second spin carrier such as a transition metal ion or an org anic radical have been described. However, except for the isotropic Gd(III) ion, very little is known concerning the nature (ferro- or antiferromagnet ic) and mechanism of the interaction involving a Ln(III) ion. This study ad dresses the problem of the interaction between the Ln(III) ions displaying spin-orbit coupling and an organic radical. The magnetic properties of a se ries of isostructural compounds, {Ln(organic radical)(2)(NO3)(3)} comprisin g a Ln(III) ion (Ln = Ce to Dy) surrounded by two N,O-chelating aminoxyl ra dicals (3-nitronyl nitroxide-4,5-dimethyltriazole) have been investigated. The experimental approach used to get insight into the {Ln-Radical} couplin g occurring within these compounds is based on the knowledge of the intrins ic paramagnetic contribution of the metal ion. This contribution has been d educed 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 o f the magnetic susceptibilities of {Ln(organic radical)(2)(NO3)(3)} and the corresponding {Ln(Nitrone)(2)(NO3)(3)} derivatives then permitted the natu re 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 investig ation of the isostructural series of compounds allowed the evolution of the se interactions to be compared as a function of the electronic configuratio n of the 4f orbitals. For the Ln(III) with 4f(1) to 4f(5) electronic config urations the (in-organic radical) interaction is antiferromagnetic. Convers ely, 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).