carboranes

Carboranes are a class of boron–hydrogen–carbon cluster compounds in which carbon atoms are incorporated into a boron hydride cage. The most studied species are closo-carboranes with the formula C2B10H12, which possess an almost ideal icosahedral geometry. The two carbon atoms can occupy different positions on the cage, leading to three isomeric families: ortho-carborane (carbons adjacent), meta-carborane (carbons separated by one boron), and para-carborane (carbons opposite each other).

Carboranes are renowned for exceptional thermal and chemical stability and for their versatile chemistry. They can be prepared by methods that introduce carbon into a boron framework, such as cycloaddition or substitution reactions on boron–hydrogen vertices, often starting from boron-hydride clusters. Functionalization at the carbon vertices allows the synthesis of a wide range of organocarborane derivatives, which serve as ligands, reagents, or building blocks.

Carborane chemistry is governed by the concept of closo clusters and Wade’s rules; many derivatives can exist as neutral molecules or as ionic species. Their cages delocalize electron density, imparting aromatic-like stabilization. C–H and C–C bonds at the carbon vertices can be selectively modified, enabling attachment of bulky, hydrophobic substituents that alter solubility and reactivity.

Applications span organometallic chemistry, materials science, and medicine. Carboranes are explored as ligands for transition metals, as boron sources in boron neutron capture therapy (BNCT), and as components of boron-rich polymers and ceramics. The field was established by work of Bartlett and others in the 1950s and has continued to grow, with thousands of derivatives described.