3d and/or 4f coordination compounds derived from chiral Schiff bases: magnetic and spectroscopic study

Abstract

This PhD thesis has been developed in the Molecular Magnetism Group of the University of Barcelona. As its name indicates, the Molecular Magnetism Group is mostly interested in the magnetic study of coordination complexes, mainly with d metals but also with lanthanide cations, which permit magnetic studies but also luminescence studies, which could allow the syntheses of multifunctional products. The group has also interest in supramolecular chemistry and chirality. The main objective of the thesis was the synthesis, magnetic and spectroscopic characterization of coordination complexes derived from chiral Schiff bases, used as organic ligands, and, due to the chirality of the organic ligands, the study of the chirality transfer from ligand to metal. This series of organic ligands, have been combined with different metals: With 3d metals, like manganese, iron or nickel, but also with 4f metals or with combinations of both types. The metals used as paramagnetic cations have been used in the search of Single Molecule Magnet (SMM) behavior and also to develop a series of magnetostructural correlations. In other cases, the interesting part of the final product was related with their supramolecular chemistry. All the products in this thesis have been magnetically characterized to try to elucidate their kind of magnetic interaction (ferromagnetic, antiferromagnetic or ferromagnetic), and, in the case that they present SMM behavior, to study their slow relaxation of the magnetization. Moreover, for complexes derived from lanthanide cations which are highly anisotropic due to the ion itself, other characterization techniques have been performed: Cantilever Torque Magnetometry (CTM) for the complete resolution of the magnetic anisotropy, and spectroscopic techniques like Electronic Paramagnetic Resonance (EPR), in X and W bands, in powder and monocrystal samples. Due to the chiral nature of the used organic ligands, all the products in this thesis have been characterized by Electronic Circular Dichroism (ECD). This technique has been used to demonstrate the chirality of the products and the differences in the solution vs. solid structures. The induced chirality in the final products due to the use of enantiopure reactants could allow the future study of the derived properties, like for example the emission of polarized light due to the presence of the lanthanide cation in a chiral environment, or the ferroelectricity, which should achieve the multiferrocity in the final materials. The thesis contains a final chapter with a discussion about how the different ligands behaved with the different metals, about how the chirality of the reactants affected the final supramolecular structure, demonstrating that the chiral and racemic forms of a reactant not always yield in the same final structure, and about how the metals behave magnetically in each situation.

L’objectiu principal de la tesi ha estat la síntesi i caracterització, magnètica i espectroscòpica, de complexes de coordinació derivats de bases de Schiff quirals. Les bases de Schiff han estat sintetitzades, o bé a partir d’una diamina condensada amb dos grups aldehid, o bé a partir d’un aminoalcohol condensat amb un grup aldehid. Aquesta sèrie de lligands orgànics, s’ha combinat amb diferents metalls, 3d¸4f o barreges dels dos. En d’altres casos, la part que ens ha interessat del complexes sintetitzats ha estat la seva química supramolecular. Tots els productes has estat caracteritzats magnèticament, per intentar esbrinar el seu tipus d’interacció magnètica (acoblament ferromagnètic, ferrimagnètic o antiferromagnètic) i, en cas de que presentessin propietats d’imant unimolecular, per esbrinar com era la seva relaxació magnètica. A més a més, per als compostos derivats de ions lantànid, que presenten una elevada anisotropia associada al catió, s’han dut a terme altres tècniques de caracterització magnètica, com l’anomenada Cantilever Torque Magnetomentry (CTM), i de caracterització espectroscòpica amb Ressonància Paramagnètica Electrònica (EPR) en banda X i en banda W, tan de compostos en pols com de monocristall. Degut a la naturalesa quiral dels lligands utilitzats, s’han pogut caracteritzar tots els productes mitjançant Dicroisme Circular Electrònic (ECD), que ens ha servit per demostrar tant la quiralitat dels productes com la seva estructura comparada en sòlid i en dissolució. La quiralitat induïda als productes mitjançant l’ús de reactius enentiomèricament purs, ha de permetre en el futur estudiar-ne les propietats que se’n deriven, com per exemple l’emissió de llum polaritzada deguda també a la presencia del catió lantànid, o la ferroelectricitat, que permetria aconseguir la síntesis de compostos multiferroics.