Closocarboranes

Closocarboranes are a class of boron–carbon cluster compounds derived from the closo-icosahedral borane framework by replacing two adjacent boron vertices with carbon atoms. The parent neutral cluster has the formula C2B10H12 and is known in three geometric isomers depending on the positions of the two carbon atoms on the icosahedral skeleton: ortho-, meta-, and para-carborane.

Structurally, closocarboranes possess an icosahedral closo framework with two carbon vertices and ten boron vertices. The carbon atoms occupy two vertices of the B12 core, and each carbon is typically bound to hydrogen and to adjacent boron or carbon atoms. Electron counting for the neutral cluster follows Wade’s rules for closo species, with a total of 26 skeletal electrons, consistent with a stable closo-hedral cage.

Synthesis and characterization: Closocarboranes are prepared by substitution of boron vertices in closo-B12H12^2− with carbon-containing units, or by rearrangements of starting carboranes under conditions that promote C–B scrambling. The three isomers can interconvert under certain conditions but can be separated and purified. They are commonly characterized by multinuclear NMR (1H, 11B, and 13C) and X-ray crystallography.

Reactivity and applications: The carbon vertices confer unique reactivity and functionalizability, enabling derivatization at either B or C sites. Closocarboranes serve as rigid, three-dimensional scaffolds in organometallic chemistry and materials science, and have been explored as boron delivery agents for BNCT and as building blocks for boron-rich polymers and ligands.