Carborane Anions: As Weakly Coordinating Counterions and Coordinating Ligands for Catalyst Design
- Author(s): Wright, James Henry
- Advisor(s): Lavallo, Vincent
- et al.
1-carba-closo-dodecaborates, more commonly referred to as carborane anions, are icosahedral CB11 cages with substituents at each vertex. These substituents are highly variable allowing for a large range of properties. Such properties include being weakly basic, chemically inert, non-nucleophilic, and resistant to reduction or oxidation. Due to these properties, carborane anions have been given the term "weakly coordinating" to describe their interactions with cations. The isolation of numerous reactive species, superacids, and coordinatively unsaturated cations are testaments to the weak coordinating ability of carboranes.
Recently carborane anions have begun being transition from being used for fundamental chemistry to applied chemistry with their utility in Lewis acid catalysis being explored by a number of research groups. This dissertation follows this transition as the exploration and discovery of new cations is explored in Chapter 2. Next by careful substitution at the C-vertex the synthesis of carborane anions that can coordinate to transition metals was attempted. By limiting the coordination of the transition metal to the substituent off the C-vertex
the carborane ligand can stabilize traditional transition metal catalysts. The two specific substituents targeted in this research are phosphines and azides.
Phosphines are an obvious choice as their use in transition metal catalysis is ubiquitous. Chapter 3 reports the synthesis and coordination to a gold (I) center of the diisopropyl undecachlorocarborane phosphine anion, iPr2P(CB11Cl11)-.Upon coordination to the gold, zwitterionic and anionic complexes were isolated. These compounds success in hydroamination of alkynes with amines serve as a proof of principle in the use of carboranes in ligand design.
Azides arethe next target for the synthesis of new carborane anion ligands, since the azide can be a versitile synthon. Chapter 4 discusses the attempted synthesis of the undecachlroinated carboranyl azide, N3CB11Cl11-, and the competing side reaction discovery. This cycloaddition results it the cleavage of a typically inert B-Cl bond.
The discovery of a new reaction involving both a carbon and a boron vertex being substituted resulted in the expansion of possible carborane fused triazole anions. Before these can be used as ligands their fundamental chemistry is studied in Chapters 5 and 6. The study of the carborane fused heterocycle resulted in the synthesis of a triazolium carborane zwitterion and a triazolium carborane radical anion.