This thesis describes the insight sought and gained for a range of molecule-based magnetic
systems with special emphasis on a class of trinuclear manganese(III) complexes.
In chapter 2 a series of Mn3 SMMs with different equatorial R-sao2- oximate ligands are studied. The unique structural property of these Mn trinuclear complexes is that due to the restrictive ligand surrounding along the JT-axes of the Mn(III) ions, the molecule becomes very rigid and the addition of a bulkier group on the sao2- ligand (from H, over Me and Et to Ph) leads a breaking of the trigonal symmetry. The molecules with the different sized ligands therefore show deviation from C3 symmetry to different degrees. Using INS the influence of the symmetry on the ground spin state multiplet splitting and the relaxation barrier was studied.
Chapter 3 represents a continuation of the authors master thesis[1]. Here, the possibilities of
enhancing the effective barrier height Ueff by alignment of the single center anisotropy axes were studied. The alignment was achieved by exchanging a bridging tripodal ligand ClO4
- with the larger analogue ReO4 . This has already been attempted once [2], but the effect could not be observed due to a lack of isostructural systems. As the changes in the magnetic behavior are expected to be small for such a small structural change, it is crucial to minimize such complications. Therefore the experiment was improved by employing a different system where the trigonal symmetry was maintained and intermolecular interactions were inhibited by larger distances between the molecules.
In chapter 4 several systems showing inter-molecular exchange interactions, such as the ones
mentioned above, were studied using INS. The studied systems show unusually large relaxation barriers, which cannot solely be explained by a combination of the single center anisotropies.
Chapter 5 shows the results of other trinuclear compounds. Instead of the triangular Mn3 SMMs, the molecules are linear, heterometallic Mn(III)-M(III)-Mn(III) (M = Ir, Ru, Os). The SMMs of the form (NEt4)[Mn2(5-Brsalen)2(MeOH)2M(CN)6] has a Schiff-base ligand on each Mn center, that both are linked to a central hexacyanidometallates(III) (4d/5d) ion by cyanide bridges. First the molecule containing the diamagnetic Ir(III) was measured using EPR and INS as well as magnetic measurements, to determine the anisotropy on the Mn sites DMn and EMn and to obtain a maximum value for the exchange coupling between the two Mn through Ir. The DMn is then used as a guiding value for the fitting of the experimental data of the analogous compounds with Os and Ru. These compounds show anisotropic exchange between Mn and the central metal ion. Instead of using M(CN)6 3- as a building block for larger SMMs a new module was found with [ReF6]2-. (PPh4)2[ReF6]2 H2O was synthesized and its magnetic properties characterized by magnetic measurements as well as INS and High-Field and X-band EPR spectroscopy. Interestingly the mononuclear compound has a
positive ZFS value, but shows in spite of that, peaks in the out-of-phase components of the Ac susceptibility χ can be observed if a small magnetic field is applied. Furthermore, chains with reoccurring motif M(viz)4[ReF6] (M = Zn, Ni) have been synthesized and the intra-chain exchange coupling parameter for the Ni(viz)4[ReF6] chain has been determined and found to be large by fitting of dc susceptibility data using two different approaches. An additional exchange interaction between neighboring chains can be observed leading to stabilization of an antiferromagnetic ground state at low temperatures. The exchange interactions through fluoride bridges were further investigated by comparing four different fluoride bridged Gd(III)-Cr(III) compounds, from a simple dinuclear complex over squares with alternating Gd and Cr to a triangle of Gd3 that is “capped” by one Cr(III) above and one below the Gd3-plane each. Magnetic measurements and DFT calculations showed a
correlation between the exchange coupling interactions and the bending of the F- bridges. Lastly a series of trigonal Er(trensal) compounds with slightly varying ligand field surroundings has been synthesized and characterized by INS spectroscopy and magnetic measurements.