High-frequency / High-field Electron Paramagnetic Resonance Investigations on Metal-Organic Molecular Compounds

In this thesis detailed studies of the magnetic properties of different metal-organic molecular compounds are presented. The experimental investigations are mainly performed by high-frequency / high-field electron paramagnetic resonance (HF EPR) spectroscopy in an external magnetic field up to 16 T, accompanied by direct (dc) or alternating current (ac) susceptibility measurements. The results allowed to precisely determine the anisotropy parameters for a set of tetrahedrally coordinated Co(II) compounds with the general formula [Co(L1)4]X2, and thereby deliver a direct experimental proof for the influence of the second coordination sphere on the magnetic properties of the respective complexes. On the other hand, strong zero-field splitting induced by a highly axial anisotropy for an octahedrally coordinated mononuclear Co(II) complex was determined by pulsed field magnetisation measurements up to 55.5 T. The influence of the spatial coordination geometry on the g-anisotropy and slow relaxation of magnetisation behaviour was studied by the investigation of two isomeric Co(II) low spin (S = 1/2) complexes in a square-pyramidal and trigonal-bipyramidal coordination geometry, respectively. For a pentagonal-bipyramidal coordinated V(III) complex [V(III)(DAPBH)(CH3OH)2] Cl·CH3OH a planar crystal field anisotropy as well as a finite dimer-like intermolecular interaction was quantified by both, HF EPR investigations and low temperature magnetisation measurements. The exploration of small intermolecular coupling pathways is further a comprehensive thread running through the study of all the aforementioned complexes. In a second part 4f rare earth metal containing compounds are addressed. The evaluation of ac susceptibility data on the monomeric Dy(III) compound [DyIIIL1]OTf reveal an interesting two step magnetic relaxation. The magnetic relaxation behaviour also motivated a comparative HF EPR study on two novel Er(III) complexes figuring a pentagonal-bipyramidal coordination surrounding. In this context it was found that the differences in magnetic relaxation behaviour can directly be traced back to the changes in electronic structure induced by the crystal field. Beyond the investigation of monomeric 3d or 4f complexes, the coupling between those two species of magnetic ions is studied and quantified by HF EPR investigations on a Cu(II)-Ln-Cu(II) sample set with Ln = Gd, Tb, Dy and La