Design of lanthanide coordination compounds with unusual ligands and high symmetry for molecular magnetism

In the field of single molecule magnetism, big improvements on controlling relaxation phenomena have led to creating systems where high energy barriers are actually an obstacle to magnetisation reversal. Key here is to control the symmetry and lattice based processes such as spin-phonon coupling. In this thesis both these aspects are investigated. Dysprosium ions which are most commonly used for lanthanide single molecule magnets were chosen in order to develop a testbed system. Such a testbed system is important because it allows the impact of seemingly small structural changes to be gaged. The system is always five-fold coordinated to 2,6-diacetylpyridinebis(2'- pyridylhydrazone) (H${}_2$dapp) in the equatorial plane leaving axial sites available for coordination by secondary ligands such as H${}_2$O, Cl${}^{-}$, NO${}_{3^{-}}$ and OAc${}^{-}$. The first compound described is [Dy(H${}_2$dapp)(H${}_2$O)${}_4$]Cl${}_3$. For all compounds, the pentadentate ligand shows a helical distortion which can be quantified and is correlated to further factors. Expanding on this, the mononuclear units were coupled by fluoride or peroxide bridges which are rarely seen for lanthanide complexes. It is shown that the nature of the bridge can have positive or negative effects on the slow relaxation depending on the nature of the coupling. In a second section, molecules with higher symmetry are investigated. The optimised synthesis for an isotopically enriched sample of a literature known pentagonal bipyramidal compound is presented which was used for investigations on the vibrational mode of the central dysprosium ion. It could be shown that the gadolinium derivative has single molecule magnet properties which is rare for gadolinium which usually has an isotropic magnetic ground state. Finally, a case study on two new octahedral dysprosium compounds is presented to show the limits of low level calculations on lanthanide single ion complexes.