Preparation, crystal structures, and magnetic features for a series of dinuclear [Ni(II)Ln(III)] Schiff-base complexes: evidence for slow relaxation of the magnetization for the Dy(III) derivative.

A series of dinuclear [Ni(II)Ln(III)] Schiff-base complexes (using a Schiff-base dicompartmental ligand derived from o-vanillin [H(2)valpn = 1,3-propanediylbis(2-iminomethylene-6-methoxy-phenol)]) with Ln = La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, and a hydroxo-bridged tetranuclear [Ni(II)Yb(III)] are reported. The crystal structures have been solved for 10 dinuclear complexes revealing four arrangements for the dinuclear units, which are modulated by the coordinated solvent molecules and the nitrato-anion interactions. The magnetic behaviors have been investigated, and the nature of the Ni(II)-Ln(III) exchange interaction has been emphasized by comparison with the behavior of the related [Zn(II)Ln(III)] derivatives. This allowed for establishing that the interaction within these compounds is antiferromagnetic with the 4f ions of the beginning of the Ln series and turns ferromagnetic from Gd(III) toward the end of the series. AC susceptibility investigations clearly show the occurrence of slow relaxation processes of the magnetization close to 2 K for the dinuclear [Ni(II)Dy(III)] complex.

Di- and triheteronuclear Cu-Gd and Cu-Gd-Cu complexes with dissymmetric double bridge.

The study of the Cu-Gd interaction is simplified by the use of heteronuclear Cu-Gd complexes of low nuclearity, such as dinuclear Cu-Gd or trinuclear Cu-Gd-Cu complexes. In the large majority of cases published until now, this interaction presents the advantage of being ferromagnetic. Among the known examples, the complexes with the largest J values have been prepared with use of symmetric Schiff base compartmental ligands, so that the active cores of the molecules involve two phenoxo bridges between the copper and gadolinium ions. Keeping this remark in mind, we decided to synthesize simple complexes in which two different bridges link the copper and gadolinium ions. The structural determinations of a heterodinuclear Cu-Gd and a trinuclear Cu-Gd-Cu complex confirm the asymmetry of the central cores, involving phenoxo and hydroxo bridges. The magnetic studies evidence again the presence of ferromagnetic interactions. These results corroborate preponderance of the planarity of the Cu-O2-Gd core over the dissymmetry of the bridges.