Carborane

A carborane is a cluster composed of boron, carbon and hydrogen atoms. Like many of the related boranes, these clusters are polyhedra and are similarly classified as closo-, nido-, arachno-, hypho-, etc. based on whether they represent a complete (closo-) polyhedron, or a polyhedron that is missing one (nido-), two (arachno-), or more vertices. Carboranes are a notable example of heteroboranes.

Interesting examples of carboranes are the extremely stable icosahedral closo-carboranes. These boron-rich clusters exhibit unique organomimetic properties with chemical reactivity matching classical organic molecules, yet structurally similar to metal-based inorganic and organometallic species

A prominent example is the charge-neutral C₂B₁₀H₁₂ or o-carborane with the prefix o derived from ortho, which has been explored for use in a wide range of applications from heat-resistant polymers to medical applications. The electronic structure of these compounds is best described by Wade-Mingos rules for cluster molecules. Another important carborane is carborane acid, a chlorinated superacid H(CHB₁₁Cl₁₁), which has an anion structure shown at right.

The most heavily studied carborane is C₂B₁₀H₁₂, m. p. 320 °C. It is often prepared from the reaction of acetylene with decaborane. A variation on this method entails the use of dimethyl acetylenedicarboxylate to give C₂B₁₀H₁₀(CO₂C H₃)₂, which can be degraded to the C₂B₁₀H₁₂.

The 1,2-closo-dicarbadodecaboranes (usually simply called carboranes), were reported simultaneously by groups at Olin Corporation and the Reaction Motors Division of Thiokol Chemical Corporation working under the U.S. Air Force and published in 1963. Heretofore, decaborane derivatives were thought to be thermally unstable and reactive with air and water. These groups demonstrated the unprecedented stability of the 1,2-closo-dodecaborane group, presented a general synthesis, described the transformation of substituents without destroying the carborane cluster, and demonstrated the ortho to meta isomerization.

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