Investigation of ligand field modifications on lanthanide${}^{III}$ complexes. Structure, magnetism and optical properties

This work deals with lanthanide complexes, their structure, magnetism and optical properties. Scandium, yttrium and the lanthanides reaching from lanthanum to lutetium are the rare-earth elements. They generally show similar chemical and physical properties. Starting with cerium and ending with lutetium, the 4f orbitals are progressively filled. These elements generally adopt a stable oxidation state of +3. The first focus of this work is on the molecular structures of lanthanide complexes. A ligand system was designed which can be easily modified, in order to understand how changes in the structure of the ligand affect the geometry of the resulting complexes. The changes to the ligand were intentionally kept small to understand better the effects of these changes. In total, seven related ligands were used. In addition, different lanthanide ions were tested for some of these systems. Due to the lanthanide contraction, their ionic radius decreases sharply across the series. This can change the coordination number, the metal-ligand bond length and the molecular structure. In the context of this work, the molecular structure of sixteen complexes, ten of which are dinuclear compounds and six are mononuclear compounds were determined by X-ray crystallography. The investigations are supported by SHAPE analyses of the coordination polyhedra. The dinuclear compounds are composed of seven phenolate and three acetate bridged structures. The second focus is on the magnetic properties of these complexes. Since Dy${}^{III}$ ions are most commonly used for lanthanide-based single-molecule magnets (SMMs) due to their combination of a strong uniaxial magnetic moment and a high m${}_J$ value of ±15/2, the focus of this work is on the synthesis of Dy${}^{III}$ complexes. Magnetic investigations of five dinuclear Dy${}^{III}$ compounds were carried out on a SQUID magnetometer and AC measurements were carried out to better understand the SMM behaviour. Three of the presented compounds show fast relaxation processes and one shows no SMM behaviour. On the other hand, compound [Dy${}_2$(L2)${}_2$(CH${}_3$COO)${}_4$(CH${}_3$OH)${}_2$] (6) turns out to be an excellent SMM. It has an energy barrier U${}_{eff}$ of 50(10) K and a relaxation time ${\tau }_0$ of 7(5)∙10${}^{-6}$ s and shows SMM behaviour up to a temperature of 15 K. Furthermore, ab-initio calculations were performed for these five compounds to estimate the energy levels of the excited Kramer doublets and the preferred orientation of the anisotropic axes in the ground doublets. The anisotropy axes were also calculated using MAGELLAN and compared with the ab-initio results. The calculated Kramer doublets agree well with the measured magnetic behaviour. The alignment of the anisotropy axes illustrates why the acetate-bridged dimers are better suited for preventing spin reversal than the phenolate-bridged dimers. The third focus is on the optical properties of such lanthanide complexes. Extensive series of measurements were carried out on the optical absorption behaviour of the ligand 2,6-(1-(2-(1H-benzimidazol-2-yl)hydrazineylidene)ethyl)pyridine (HL7) and its complexes with all lanthanide ions. An equilibrium in solution was discovered by adding acid and base, between the isolated protonated ligand, the protonated complex and the deprotonated complex. Concentration-dependent measurements show that this equilibrium also changes at different concentrations, indicating an intermolecular interaction between the molecules in solution.