Carborane

A carborane is a cluster composed of boron and carbon 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.

Interesting examples of carboranes are the extremely stable icosahedral closo-carboranes.

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. This compound is called super aromatic because it obeys Huckel's rule and exhibits high thermal stability. At 420 °C o-carborane converts to the meta isomer. In comparison, benzene requires a >1000 °C to induce skeletal rearrangement. Like arenes, carboranes also undergo electrophilic aromatic substitution.
 * [[Image:O-carborane.png|150px|o-carborane, hydrogen atoms connected to boron omitted for clarity]]

Another important carborane is the negatively charged CHB11H12—, which has been used to make solid superacids.

The carborane superacid H(CHB11Cl11) is one million times stronger than sulfuric acid. The reason for this high acidity is that the acid anion CHB11Cl11- is very stable and substituted with electronegative substituents. H(CHB11Cl11) is the only acid known to protonate C60 fullerene without decomposing it. .

Dicarbadodecaborane
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.

History
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. Simultaneously a group from the USSR published similar work. 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, and described the transformation of substituents without destroying the carborane cluster. and demonstrated the ortho to meta isomerization.

Dicarbollide
Numerous studies have been made on derivatives of the so-called dicarbollide anion, [B9C2H11]2-. This anion forms sandwich compounds with many metal ions and some exist in otherwise unusual oxidation states. The dianion is a nido cluster prepared by degradation of the parent dicarborane:
 * B10C2H12 +  3 CH3OH  +  KOH  → KB9C2H12  +  B(OCH3)3  +  H2O  +  H2

Carborynes
Carboryne, or 1,2-dehydro-o-carborane, is an unstable derivative of ortho-carborane with the formula B10C2H10. The hydrogen atoms on the C2 unit in the parent o-carborane are missing. The compound resembles and is isolobal with benzyne. A carboryne compound was first generated in 1990 starting from o-carborane. The hydrogen atoms connected to carbon are removed by n-butyllithium in tetrahydrofuran and the resulting lithium dianion is reacted with bromine at 0°C to form the bromo monoanion.


 * [[Image:Carboryne synthesis.png|600px|Carboryne synthesis, main chemical bonds involving carbon in red]]

Heating the reaction mixture to 35 °C releases carboryne, which can subsequently be trapped with suitable dienes:
 * [[Image:Carboryne reactions.png|600px|carboryne reactions]]

such as anthracene (to afford a trypticene-like molecule) and furan in 10 to 25% chemical yield.

Carborynes react with alkynes to benzocarboranes in an adaptation of the above described procedure. O-carborane is deprotonated with n-butyllithium as before and then reacted with dichloro-di(triphenylphosphino) nickel to a nickel coordinated carboryne. This compound reacts with 3-hexyne in an alkyne trimerization to the benzocarborane.


 * [[Image:BenzocarboraneSynthesis.png|600px|benzocarborane synthesis]]

Single crystal X-ray diffraction analysis of this compound shows considerable bond length alternation in the benzene ring (164.8 pm (!) to 133.8 pm) ruling out aromaticity.