Any of a class of organometallic compounds containing carbon, boron, and hydrogen. The general carborane chemical formula is C₂B_math.nH_{math.n + 2}, in which math.n is an integer; carboranes with math.n ranging from 3 to 10 have been described. Carboranes can form three-centre bonds (one pair of electrons is shared between three atoms) and classical two-centre bonds (one pair of electrons is shared between two atoms). As a result, carboranes can form complex polyhedral structures that are based on networks of boron and carbon atoms. Carboranes are generally prepared by reaction of acetylene or acetylene derivatives with boron hydrides.

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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.

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.

Heating the reaction mixture to 35 °C releases carboryne, which can subsequently be trapped with suitable dienes:

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.

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.

References

External links

• The Reed Group