Chapter 1 of this dissertation provides an introduction to the field of single-molecule magnetism through the lens of two-coordinate transition metal complexes. Single-molecule magnets are a class of materials which display magnetic properties, such as magnetic hysteresis, typically only observed for bulk materials. Given their small size, with many single-molecule magnets utilizing only one magnetic ion, these materials are also subject to quantum effects. The marriage of classical and quantum properties provides intriguing possibilities for the applications of these materials. From the perspective of basic research, they provide the challenge of controlling the electronic structure of these molecules down to the magnetic microstate level. Two-coordinate transition metal complexes provide excellent insight to the field as a whole, as it is possible to understand the relationships between molecular structure, electronic structure, and magnetic microstate structure and mixing. By understanding their electronic structure and magnetic anisotropy, it is also possible to gain insight into the mechanisms by which magnetic relaxation occurs.
Chapter 2 and Chapter 3 are closely related. Calculations on a hypothetical complex which featured a linear C–Co–C moiety, Co(C(SiMe3)3)2, predicted a magnetic anisotropy near the physical limit (determined by the Co(II) spin-orbit coupling constant) arising from a non-Aufbau electronic structure, (dx2–y2, dxy)3(dxz, dyz)3(dz2)1. These calculations ignited a synthetic endeavor to isolate the first dialkyl Co(II) complex. Chapter 2 details the first success in this endeavor with the isolation of Co(C(SiMe2OPh)3)2. However, long-range Co···O interactions led to a significantly bent C–Co–C axis, and thus Co(C(SiMe2OPh)3)2 behaved as an unremarkable single-molecule magnet. Other complexes of the type M(C(SiMe2OPh)3)2 (M = Cr, Mn, Fe, Zn) were synthesized in an effort to understand the deviation from linearity. Ultimately the bend in the in the C–Co–C axis arises from a compromise of several stabilizing forces. The ligand field stabilization energy is relatively weak and interligand non-covalent interactions are essential for the stability of the complexes; when those interligand interactions are not sufficiently strong, the metal moves closer to the nearby oxygen atom for additional stabilization. Though this metal-oxygen distance is longer than an actual metal-oxygen bond, the interaction is sufficient to both stabilize the molecule and destroy the magnetic anisotropy expected from the linear moiety.
Chapter 3 details the end result of a search for a ligand which would support a linear C–Co–C moiety. By moving from phenyl to naphthyl substituents, the interligand non-covalent interactions were greatly enhanced. The complex, Co(C(SiMe2ONaph)3)2, exhibits a non-Aufbau ground state—an unprecedented electronic structure for a transition metal molecule—which arises from an extremely weak and high symmetry ligand field. The electronic structure is confirmed by dc magnetic susceptibility, ab initio calculations, and experimental charge density maps. Additionally, the electronic structure has the maximal orbital angular momentum for a transition metal complex, a property that was only recently observed in cobalt adatoms and is novel for a molecule. Due to the unquenched orbital angular momentum this molecule displays magnetic anisotropy that is near a physical limit for transition metals. The spin-reversal barrier, determined by a combination of variable-field far-IR spectroscopy and ac magnetic susceptibility, is the largest spin-reversal barrier (450 cm−1) for any transition metal containing molecule.
Chapter 4 provides the beginning of a new synthetic endeavor for linear transition metal complexes. Magnetic anisotropy is typically limited by the spin-orbit coupling constant of the constituent magnetic ions. For mononuclear transition metal complexes, metal-ligand covalency nearly always diminishes magnetic anisotropy compared to the free-ion values. One possible exception to this trend is through the use of heavy ligands (i.e. ligands of the 4p, 5p, etc. rows), which have been shown to enhance magnetic anisotropy. Thus, complexes of the type [Fe(SiR3)2]0/− were targeted. While no such complex was successfully synthesized, there are several promising leads in this direction. Specifically, the novel ligands [Si(carbazole)3]− and [Si(2,7-dimethylcarbazole)3]− provide several interesting new structures, including a new two-coordinate zinc complex, Zn(Si(carbazole)3)2.