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UC Berkeley Electronic Theses and Dissertations UC Berkeley UC Berkeley Electronic Theses and Dissertations Title Synthesis and Magnetic Properties of Two-Coordinate Transition Metal Complexes Permalink https://escholarship.org/uc/item/05r034mm Author Bunting, Philip Colin Publication Date 2019 Peer reviewed|Thesis/dissertation eScholarship.org Powered by the California Digital Library University of California Synthesis and Magnetic Properties of Two-Coordinate Transition Metal Complexes by Philip C. Bunting A dissertation submitted in partial satisfaction of the requirements for the degree of Doctor of Philosophy in Chemistry in the Graduate Division of the University of California, Berkeley Committee in charge: Professor Jeffrey R. Long, Chair Professor Christopher J. Chang Professor Michael F. Crommie Spring 2019 Synthesis and Magnetic Properties of Two-Coordinate Transition Metal Complexes © 2019 Philip C. Bunting Abstract Synthesis and Magnetic Properties of Two-Coordinate Transition Metal Complexes by Philip C. Bunting Doctor of Philosophy in Chemistry University of California, Berkeley Professor Jeffrey R. Long, Chair 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 3 3 1 electronic structure, (dx2–y2, dxy) (dxz, dyz) (dz2) . 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 CoꞏꞏꞏO 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 1 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 0/− [Fe(SiR3)2] 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. 2 Table of Contents Acknowledgements ......................................................................................................................... ii Chapter 1: Single-Molecule Magnetism Through the Lens of Linearly Coordinated Transition Metal Ions Section 1.1 Introduction ...................................................................................................................2 Section 1.2 A Note on Terminology ................................................................................................2 Section 1.3 Magnetic Anisotropy ....................................................................................................3 Section 1.4 Magnetic Relaxation .....................................................................................................9 Section 1.5 Outlook .......................................................................................................................15 Section 1.6 References ...................................................................................................................16 Chapter 2: Synthesis and Single-Molecule Magnetism of Transition Metal Dialkyl Complexes Section 2.1 Introduction .................................................................................................................22 Section 2.2 Experimental Information ...........................................................................................22 Section 2.3 Results and Discussion ...............................................................................................24 Section 2.4 References ...................................................................................................................36 Chapter 2 Supporting Information .................................................................................................39 Chapter 3: A Linear Cobalt(II) Complex with Maximal Orbital Angular Momentum from a Non-Aufbau Ground State Section 3.1 Introduction .................................................................................................................68 Section 3.2 Results and Discussion ...............................................................................................69 Section 3.3 Outlook .......................................................................................................................77 Section 3.4 Methods.......................................................................................................................78 Section 3.5 Acknowledgements .....................................................................................................84 Section 3.6 References ...................................................................................................................85 Chapter 3 Supporting Information .................................................................................................91 Chapter 4: Pursuit of M(SiR3)2 Complexes for Enhanced Magnetic Anisotropy Through Heavy-Ligand Effect Section 4.1 Introduction ...............................................................................................................127 Section 4.2 Results and Discussion .............................................................................................128 Section 4.3 Outlook .....................................................................................................................134 Section 4.4 References .................................................................................................................135 Chapter 4 Supporting Information ...............................................................................................137 i Acknowledgements These are perhaps the hardest few pages I’ll have to write during my PhD. For one, they are the only pages that will be read by the majority of people who touch my dissertation.1 Secondly,
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