UCLA

Icosahedral boron clusters (e. g. C2B10H12, [CB11H12]1-, [B12H12]2-) are aromatic molecules that can serve as 3-dimensional scaffolds in molecular materials. Vertex-selective functionalization of boron cluster B−H bonds allows tuning of the electronic properties and spatial arrangement of boron cluster-bound moieties. Vertex-selective functionalization of icosahedral boron clusters is desirable for making complex molecules based on boron cluster scaffolds. Thus new functionalization methods for boron clusters are needed which can append a variety of functional groups with atomic precision. Vertex-specific functionalization of polyhedral boranes is possible by leveraging the anisotropic electron density and inductive effects of the icosahedral boron clusters to control the site of functionalization. By adapting existing palladium-catalyzed cross-coupling chemistry to polyhedral boranes we are able to form B−O, B−N, and B−C bonds with amines, heterocycles, alkoxides, boronic acids, and small nucleophiles (e.g. −F, −OH, −NH2, and −CN). Our discovery of Pd-catalyzed isomerization (“cage-walking”) of bromo-meta-carborane (9-Br-m-C2B10H11) into four Br-m-C2B10H11 regioisomers enables synthesis of several regioisomer products from a single starting material. Incorporation of the cage-walking process into Pd-catalyzed cross-coupling allows synthesis of all four boron-vertex (B(9), B(5), B(4), and B(2)) functionalized meta-carborane regioisomers from 9-Br-m-C2B10H11. Additionally, vertex-selective functionalization enables tuning of physical properties, such as melting point and electrochemical behavior, that expand the utility of polyhedral boranes to new application. We are interested in tuning the chemical stability and weak coordination properties of polyhedral borane anions for ionic liquid electrolytes and flow-battery applications.