Woods, Toby John (2016-07). Geometric and Electronic Control of the Magnetic Properties of First-Row Transition Metal Complexes. Doctoral Dissertation.
Thesis
The realization that [Mn12(CH3COO)16(H2O)4O12]o2CH3COOHo2H2O (Mn12OAc) displayed magnetic hysteresis, a phenomenon usually associated with permanent bulk magnets, was a truly remarkable discovery since Mn12OAc is a zero-dimensional molecular system in which there is no magnetic interplay between individual Mn12OAc units. In essence, every Mn12OAc molecule behaves as a tiny bar magnet of 1 nm in size, capable of maintaining its magnetization even after removal of the magnetizing field. The term Single Molecule Magnet (SMM) was introduced to describe molecules that displayed hysteresis behavior similar to Mn12OAc. In the intervening years, many new examples of molecules that display SMM properties have been discovered but success at increasing the operating temperature has been limited. Despite these challenges, interest in this field remains high due to the potential applications of these materials as elements in magnetic storage devices as well as in spintronics and quantum computing. In an effort to better understand the relationships between molecular structure, spin, anisotropy, and SMM behavior and, ideally, to discover new SMMs with enhanced properties, this work focuses on exploring the use of two ligand types that have been underexplored in the field of molecular magnetism - radical bridging ligands and fluoride as a bridging ligand. A series of dinuclear compounds in which two transition metal centers are bridged by the radical anion form of the tetrazine-based ligand bmtz (bmtz = 3,6-bis(2'-pyrimidyl)-1,2,4,5-tetrazine) have been synthesized; the complexes exhibit the desired strong metal-radical magnetic exchange coupling and SMM behavior. In an effort to expand the library of molecular magnets that contain unsupported fluoride bridges, a series of mixed-valence metal-fluoride cages were prepared and studied. Single crystal X-ray diffraction experiments revealed that these cages resemble a classic Keggin ion in structure but with the oxide bridges of the Keggin ion being replaced by fluoride bridges. Further studies using single crystal neutron diffraction methods have revealed that a small percentage of the bridging fluoride ligands are actually [OH]^- ligands. Finally, an investigation into how careful control of the molecular geometry can affect the magnetic behavior of a series of mononuclear cobalt compounds was undertaken.
