Investigating the origins of slow magnetic relaxation of S = ½ Ni(III) cyclams
Date
2023
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Abstract
This dissertation describes the syntheses and characterizations of several Ni(III) and Ni(II) complexes in an attempt to better understand the origin of slow magnetic relaxation, or spin reversal, in S = ½ systems by utilizing Ni(III) cyclam (1,4,8,11-tetraazacyclotetradecane) as a toy model system. The content is organized as follows: Chapter 1 provides the historical context and theory surrounding the class of materials called single molecule magnets (SMMs). Therein I describe the prototypical SMM and its primary figures of merit and characteristics, such as S and D, followed by the observation of how S = ½ systems, which have previously been shown to act as SMMs, do not fit within the context currently provided by the literature. The choice of using the Ni(III) cyclam system is then elaborated upon, along with its quirks and foibles. In Chapter 2 I describe the synthesis and magnetic characterization of three Ni(III) cyclams. The first two contain halides in the axial positions, which are 100% abundant in isotopes containing nuclear spin, and the third complex has perchlorate bound in the axial position, where oxygen is nearly nuclear spin free. Neither halide systems showed slow magnetic relaxation, but it was not clear whether it was due to the superhyperfine coupling between the nuclear and electronic spins or due to the antiferromagnetic interactions present at low temperatures. The perchlorate containing complex did show slow magnetic relaxation, consistent with the literature and our predictions. Chapter three describes the crystallographic tuning tools and corresponding magnetic properties of novel S = ½ Ni(III) cyclam complex salts: strong antiferromagnetic coupling in sulfate-bridged chain {[Ni(cyclam)(µ2-SO4)]ClO4·H2O}n and field-, temperature-, and size-dependent slow magnetic relaxation in molecular [Ni(cyclam)(HSO4)2]HSO4. I have reported two methods of manipulating the dynamic magnetic response of these coordination molecules: particle size selection and deuteration. I find that particle size dependency, which I attribute to the phonon bottleneck effect, for the magnetic dynamics in the parent protiated compound is removed in deuterated isotopologue, revealing only the faster molecular relaxation mode(s). Chapter 4 describes the synthesis and characterization of four novel Ni(III) cyclams utilizing neutral ligands in the axial positions as opposed to the anionic ones considered previously, namely [Ni(cyclam)(acetonitrile)2]X3 (X = OTf, ClO4, BF4) and [Ni(cyclam)(butyronitrile)2]OTf3. Through these complexes we probe the role of ligand charge, identity, and subtle differences in the hydrogen-bonding network on the slow magnetic relaxation of the Ni(III) ion. Chapter 5 describes the solution phase studies of [Ni(cyclam)(MeCN)2]OTf3 and [Ni(cyclam)(butyronitrile)2]OTf3 in glassy and non-glassy solvents, as well as their suitability for studying other novel species in situ that may not be able to be synthesized and measured traditionally. We find that there are significant differences in the magnetic relaxation of the Ni(III) cyclams between glassy and non-glassy solutions and discuss the possibilities these findings present. In Chapter 6 I summarize the key findings from Chapters 2-5 and propose new avenues of research for further investigating this phenomenon. Finally, in Chapter 7 I describe a different ligand involving intra-ligand π-π interactions and explore the feasibility of using such interactions for intelligently controlling and tuning the first coordination sphere geometry and electronic structure. By introducing new substituents, changes to the aromaticity, and oxidation of the ligand we are able to exhibit rational control over the crystallographic and electronic structure of the metal center.