Design and Synthesis of High-Performance Lanthanide-Based Single-Molecule Magnets

Field: Chemistry (Physical Sciences)

This dissertation describes the synthesis and characterization of various lanthanide-based single-molecule magnets. Analysis of these compounds reveals design strategies to increase the operating temperature of molecular magnets. In particular, methods to minimize deleterious through-barrier magnetic relaxation pathways in metallocenes and radical-bridged complexes are outlined. This dissertation also investigates rare electronic structures that arise in lanthanide-based single-molecule magnets through the lens of fundamental chemistry principles, such as aromaticity and Hund’s rules.Chapter 1 provides a general introduction to single-molecule magnet research and the electronic structure of lanthanide complexes. Key figures of merit are defined, and factors that control the magnetic properties of a single-molecule magnet are outlined. In particular, the importance of mitigating deleterious through-barrier relaxation pathways to enable higher operating temperatures is discussed, and progress towards this goal in metallocenes and radical-bridged complexes is highlighted.Chapter 2 describes the synthesis of the first trinuclear lanthanide single-molecule magnet. Magnetic characterization of this species and comparison with analogous dinuclear compounds demonstrates that increased nuclearity does not result in enhanced single-molecule magnet performance. Instead, the strength of magnetic exchange coupling is shown to control the operating temperature.Chapter 3 describes the synthesis of a series of radical-bridged dilanthanide complexes. Variation of the substituent on the organic radical bridging ligand tunes the strength of magnetic exchange coupling interactions in these compounds, which in turn impacts the rate of magnetic relaxation. An empirical correlation is derived from the magnetic properties of this series which can be used to guide the synthesis of radical-bridged dysprosium complexes with higher operating temperatures.Chapter 4 describes the synthesis of a novel binucleating ligand which binds one lanthanide ion on each face of a central benzene dianion diradical. Magnetic susceptibility measurements reveal the strongest magnetic exchange coupling yet observed between a lanthanide ion and a bridging ligand. In addition, structural and computational analysis reveal Baird aromaticity, a reversal of Hückel’s rule that has been predicted for molecules with 4n π-electrons in the triplet state.Chapter 5 describes the synthesis of a series of dysprosium metallocenium cation salts. Variation of the substituents on the cyclopentadienyl ligands results in substantial changes to molecular structure, which in turn impact magnetic relaxation. Magneto-structural correlations are derived to guide the synthesis of dysprosium metallocenium cation salts with higher operating temperatures.Chapter 6 describes the synthesis of divalent lanthanide metallocenes. Magnetic characterization of these compounds demonstrates that lanthanide reduction enhances the operating temperature for terbium and diminishes it for dysprosium. Electronic structure analysis reveals j–j coupling, a deviation from Hund’s rules that has only yet been reported for lanthanide ions in the gas phase.