Design of FeIII–LnIII Binuclear Complexes Using Compartmental Ligands: Synthesis, Crystal Structures, Magnetic Properties, and Ab Initio Analysis

Three new binuclear FeIII–LnIII complexes, with similar structures, have been synthesized using the end-off compartmental proligand, H2valpn, resulting from the Schiff condensation between o-vanillin and 1,3-propylenediamine: [FeIIILnIII(valpn)(hfac)2(OAc)Cl] (Ln = Gd, Tb, Dy; hfac− = hexafluoroacetylacetonate; AcO− = acetate). The metal ions are triply bridged by two phenoxido oxygen atoms and by the acetato ligand (syn–syn bridging mode). Among all these compounds the FeIII–LnIII exchange interaction was found to be ferromagnetic (JFeGd = +1.004(4) cm−1; H = −JSFeSGd). The [FeIIIDyIII] derivative shows field-induced slow relaxation of the magnetization. The interpretation of the magnetic properties for the [FeIIIGdIII], [FeIIITbIII] and [FeIIIDyIII] is done through ab initio calculations. It was concluded that the temperature behavior of the relaxation time in the [FeIIIDyIII] derivative is due to dominant Orbach relaxation processes between states at the opposite sides and opposite heights of the blocking barrier. From this understanding, the relaxation mechanism for similar systems where a transition metal is in exchange coupling with a strong anisotropic lanthanide ion is elucidated, which could provide a strategy to synthesize other binuclear complexes with better magnetic properties.