Ni^II-Ln^III complexes with: O -vanillin as the main ligand: Syntheses, structures, magnetic and magnetocaloric properties

ortho-Vanillin in the presence of nickel and lanthanide ions yields three types of heteronuclear Ni-Gd complexes, ranging from dinuclear to tetranuclear defect dicubane complexes, as demonstrated by their structural determinations. These complexes are dependent on the solvents used during the reaction processes and on the retained lanthanide ions, as characterized by an increase in their Lewis acid character on moving from lighter to heavier lanthanide ions. Intramolecular ferromagnetic Ni-Gd interactions are present in the heterodinuclear Ni-Gd entities, whereas ferromagnetic Ni-Ni and Ni-Gd interactions dominate above 2 K in the tetranuclear NiII2-GdIII2 compounds, devoid of any Gd-Gd interaction. The effect of a magnetic field on the magnetic entropy and adiabatic temperature changes is maximum near the liquid-helium boiling temperature, mainly determined by the relative weakness of the magnetic interactions.