Magnetism of Inorganic-Organic Molecular Materials Featuring First-Row Transition Metals

This thesis covers a broad range of molecular materials with a variety of magnetic properties. The linking theme in these studies is the inclusion of Fe(II) or Co(II) in their high-spin state coordinated by bridging organic ligands. The diversity of these transition metal centers in their coordination geometry, ligands, dimensionality, and coupling yield an assortment of materials from magnetic metal organic frameworks, to single chain magnets, to spin crossover molecules with cooperative organic diradical modulation. Chapter 1 is an introduction covering the background and basics of the field of molecular magnetism and briefly touches upon the current state of several fields within this broad area of research. Possible applications, current drawbacks, and the potential advantages over traditional magnetic materials are discussed. Chapter 2 investigates novel metal-organic frameworks of the type M(BDC)(pyz) and M(BDC)(bipy) with M = Fe(II) and Co(II). These materials exhibit unique structures with high crystallinity and permanent porosity over 1,300 m^2/g. The diamagnetic ligands (i.e. aromatic heterocycles and carboxylates) which bridge the spin centers engender weak antiferromagnetic superexchange as investigated by dc magnetic measurements, and the importance of extended structure on magnetic behavior is highlighted. Chapter 3 discusses a new set of 2- and 3-dimensional coordination polymers of the MIL-53 family type. These coordination polymers feature chains of Co(II) and a bridging N-oxide. Slow magnetic relaxation observed by ac magnetic measurements suggests that these chains may feature spin canted antiferromagnetism. The material Co(BPDC)(IQNO) also has significant interchain ordering, leading to bulk (3D) magnetic ordering. Chapter 4 describes the magnetic properties of a set of Fe(II) compounds with NNN-pincer ligands. While the two discrete complexes do not show remarkable magnetic behavior, the coordination polymer which utilizes EE-bound azide to bridge Fe(II) centers exhibits slow magnetic relaxation characteristic of a single chain magnet when investigated by ac magnetometry. In Chapter 5, sulfur-based ligands are introduced in Fe(II) and Co(II) containing coordination polymers. Sulfur-based ligands were targeted in place of more common oxygen and nitrogen containing ligands due to superior energy matching of sulfur to first-row transition metals which should allow for improved electron delocalization and coupling. These materials are primarily investigated by SXRD and magnetometry. Finally, in Chapter 6, the complex (FeTPA)2TTFtt is investigated. This complex contains the sulfur-based ligand TTFtt which is redox active at mild potentials. The doubly oxidized complex [(FeTPA)2TTFtt][BArF4]2 features ligand-centered oxidation yielding a largely closed shell, diamagnetic TTFtt2- core at room temperature. Upon cooling, however, the Fe(II) centers undergo spin transition from high-spin to low-spin. Simultaneously, there is a change in the electronic structure of the TTFtt2- unit to yield significant open-shell, diradical character. These phenomena are investigated by a variety of experimental techniques, particularly SXRD, EPR, Mössbauer, dc magnetometry, NMR, and UV-Vis-NIR spectroscopy