Síntesis y caracterizacion de nuevos complejos moleculares magnéticos

Résumé

Magnets and magnetism play undoubtedly a key role in the modern technology. Particularly the coercive field from the magnetic hysteresis phenomena associated with magnetic materials are exciting: hysteric materials retain part of their magnetization and thus can store information bellow their critical temperature. During the last decades, magnetism has experimented a significant revolution yielding a new class of magnetic materials with physical and chemical properties far away from those observed in the classical magnets. This new class of materials, called molecule-based magnets, display low density, transparency, electrical insulation, and low-temperature fabrication, as well as combine magnetic ordering with other properties. Coordination polymers (CPs) belong to this class of materials. The CPs are formed from multitopic organic ligands combined with a huge number of metal ions and polynuclear species called secondary building units (SBUs). The combination of these two building blocks provides an almost endless number of possibilities for creating new CPs with new structures, topologies and magnetic properties. The subject of this thesis is the synthesis and characterization of magnetic CPs under three different approximations.. The first approach deals with the study of the structural and magnetic properties for two series of 3d-4f CPs involving Cu(II) and Co(II) as metal transitions and some rare earth elements as 4f ions. The unusual crystal growth methods as well as the characterization of a flexible structure for the Cu(II) based compounds together with an accurate magnetic analysis for the Co(II) based samples are probably the most representative goals of this section. The second approach is devoted to study the controversial magnetic behavior of the [Co2(bta)]n compound, which shows a rich magnetic behavior showing magnetic hysteresis, metamagnetism, antiferromagnetic order among other. This compound has two analogs, the [Mn2(bta)]n and [Fe2(bta)]n compounds, all they are antiferromagnets. This study is performed through the combination of neutron diffraction and magnetic susceptibility measurements. The main goal of this section lies on the synthesis and magnetic study of two series of metal-organic solid solutions [Co1-xMx(bta)]n M = Mn(II) and Fe(II) (x= 0.2, 0.4, 0.6, 0.8). Here, we show how magnetic properties like the metamagnetism, coerive field or the order temperatures are extremely dependent of the metal contents and its magnetic nature. The third section displays the exploratory study performed over the slightly investigated 1,2,3,4-cyclobutantetetracarboxylic acid. The section is divided in two parts, one devoted to analyze the magnetism of a novel Co(II) based compound, which shows metamagnetism, antiferromagnetic order and magnetic hysteresis. While the second part reveals how this versatile ligand behaves under different synthetic conditions giving rise to six new Cu(II) compounds where three of them have been synthesized under mild conditions by using the thermally transformed ligand. This fact, together with the theoretical studies presented on this section open new possibilities within the crystal-engineering field for the study of new coordination polymer involving this ligand.