The Use of Carboranes in Electrolyte Design and Ligand Development
The discovery of polyhedral boranes in the early 1900’s by Alfred Stock and coworkers opened the door to an entirely new realm of structural chemistry. Boron’s rather unique property of catenation, the ability to build molecules of unlimited size by covalently bonding to itself, has been exploited to develop the field of polyhedral borane chemistry. In 1960 the first icosahedral carborane was synthesized, B12H122- dianion, by Pitochelli and Hawthorne. Aside from the interesting structure and bonding of the cluster it was found to be incredibly thermally stable, withstanding heats above 800 °C, and chemically inert to almost all reagents. These properties have led to the B12H122- being called the most stable molecule known. The isoelectronic analogues of this structure where one or more BH has been replaced by a CH+ are known as carboranes. The first of this family was prepared in the industrial setting in the 1950’s but went unreported until 1963. The post-World War II government programs (ZIP and HEF) were purposed with developing borane-based rocket fuel that would utilize the higher energies generated by the combustion of boron hydrides in contrast to hydrocarbons. In 1957 ortho-carborane, 1,2-C2B10H12 was isolated by Reaction Motors Inc, however this ground-breaking work on the o-carborane wasn’t published until 1963 when the groups at Thiokol and Olin-Mathieson released a series of papers. Years later, the monoanionic cluster HCB11H111- was synthesized by Knoth in 1967. The related closed 10-vertex anion HCB9H91- was also prepared by Knoth in 1971 and displays many of the same characteristics as the 12-vertex mono-anion. These monoanionic closed cluster carboranes have found application most notably as weakly coordinating anions but are also used in medicine, material science, electrochemistry, and transition metal catalysis. It is these monoanionic carborane clusters that will be the primary focus of this thesis.
A major part of this work is directed at functionalizing these carborane clusters and incorporating them into imidazolium salts. The imidazoliums bearing functionalized carboranes can then converted into the corresponding N-heterocyclic carbenes (NHC) to be used as ligands for transition metal complexes. This thesis will also contain work in the area of electrolyte development exploiting the weakly coordinating nature of the carborane anion. Both the 10- and 12-vertex carborane clusters can be paired with various cations for their implementation in secondary batteries. Magnesium electrolytes were developed and tested in collaboration with the Guo lab at UC Riverside. Solid-state electrolytes were also developed for various cations and tested by our collaborators at Sandia National Laboratory, Argonne National Laboratory, and the Jet Propulsion Laboratory.