NiII-LnIII 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-II-Gd-III 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-II-Gd-III interactions are present in the heterodinuclear Ni-II-Gd-III entities, whereas ferromagnetic Ni-II-Ni-II and Ni-II-Gd-III interactions dominate above 2 K in the tetranuclear Ni-2(II)-Gd-2(III) compounds, devoid of any Gd-III-Gd-III 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.