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Abstract

The aim of this thesis is to combine experimental and computational methods in order to gain a deeper understanding of the ligand field effects of lanthanide(III) ions. Because the magnetic properties of mononuclear lanthanide complexes are determined primarily by the electronic structure of the isolated ion, a thorough understanding of the ligand field effects is essential in order to rationally design new complexes to function as single molecule magnets (SMMs). In this work a variety of experimental techniques are utilised in combination with a detailed theoretical analysis by way of a highly powerful ab initio approach. The approach involves the determination of CASSCF wave functions, from which spin-orbit coupled states are calculated by construction of a state interaction matrix. The magnetic properties are determined by the effective Hamiltonian theory, which yields parameters such as the electron-Zeeman interaction g and also ligand field parameters. Importantly, the validation of the ab initio approach is illustrated not only by comparison with magnetometric results but also by using the highly sensitive optical spectroscopic technique, magnetic circular dichroism. Thorough ligand field analysis of the experimental data was possible utilising the ab initio calculated ligand field parameters, illustrating the accuracy of the computational method.

The first part of this thesis investigates the sensitivity of the electronic structure of terbium(III) and dysprosium(III) to very slight changes of the ligand field. Two different ligands, L1 and L2, were employed that provide coordination spheres comprised of eight homoleptic oxygen donors, meaning the differences in the ligand field of these complexes is purely of geometrical origin. Differences in the magnetic susceptibility of all four complexes revealed different splitting of the ground state J multiplet induced by the two ligand fields. Loose powder magnetisation measurements indicated differences in the ground state g values, which were in qualitative agreement with the calculated values. High frequency electron paramagnetic resonance (HF-EPR) studies of the terbium(III) complexes provided insight into the composition of the ground state MJ levels. Ab initio calculations are utilised to rationalise the experimental results and further illustrate the effect of the structural features on the electronic and magnetic properties of the different complexes. Magnetic circular dichroism (MCD) spectra of the dysprosium(III) complexes illustrate fine details highlighting the differences in the splitting of the J multiplets and allowed for a thorough ligand field analysis. The analysis utilised the ab initio calculated ligand field parameters in order to produce a reasonable fit of the experimental data, illustrating the accuracy of the computational methods. The calculated properties indicated no significant SMM behaviour of the different complexes, leading to the design of a new set of ligands based on L1, which are explored in the second part of this thesis.

The second part of this thesis focuses on the difference in f-electron density of dysprosium(III) and erbium(III), and how more pronounced differences in the ligand field affect the electronic structures of the oblate and prolate ions, respectively. Two ligands were designed that, upon coordination to a lanthanide(III) ion, provide strong electron density in the axial L3 and equatorial regions L4, respectively. The calculated properties indicate that the more strongly axial ligand field promotes strong anisotropy and a large magnetic blocking of dysprosium(III), whereas the strongly equatorial ligand field induces extensive mixing of the MJ states and a large transversal moment. Conversely, the properties of the complexes of erbium(III), having a prolate f electron density, produce the opposite effect. MCD spectra of dysprosium(III) and erbium(III) complexes of ligands L3 and L1 are compared. As in the first part of the thesis, a thorough ligand field analysis was possible utilising the ab initio methods.