Carborane Figure

Carboranes are electron-delocalized (non-classically bonded) clusters composed of boron, carbon and hydrogen atoms.Grimes, R. N., ''Carboranes 3rd Ed.'', Elsevier, Amsterdam and New York (2016), . Like many of the related boron hydrides, these clusters are polyhedra or fragments of polyhedra. Carboranes are one class of heteroboranes.

In terms of scope, carboranes can have as few as 5 and as many as 14 atoms in the cage framework. The majority have two cage carbon atoms. The corresponding C-alkyl and B-alkyl analogues are also known in a few cases.

Structure and bonding

Carboranes and boranes adopt 3-dimensional cage (cluster) geometries in sharp contrast to typical organic compounds. Cages are compatible with sigma—delocalized bonding, whereas hydrocarbons are typically chains or rings.

Like for other electron-delocalized polyhedral clusters, the electronic structure of these cluster compounds can be described by the Wade–Mingos rules. Like the related boron hydrides, these clusters are polyhedra or fragments of 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-''), three (''hypho-''), or more vertices. Carboranes are a notable example of heteroboranes.
The essence, these rules emphasize delocalized, multi-centered bonding for B-B, C-C, and B-C interactions.

Structurally, they can be considered to be related to the icosahedral (''I''_h) B₁₂H₁₂^2– via formal replacement of two of its BH^– fragments with CH.

Isomers

Geometrical isomers of carboranes can exist on the basis of the various locations of carbon within the cage. Isomers necessitate the use of the numerical prefixes in a compound's name. The closo dicarbadecaborane can exist in three isomers: 1,2-, 1,7-, and 1,12-C₂B₁₀H₁₂.

Preparation

Carboranes have been prepared by many routes, the most common being addition of alkynyl reagents to boron hydride clusters to form dicarbon carboranes. For example, the high-temperature reaction of pentaborane(9) with acetylene affords several closo-carboranes as well as other products:
: ''nido''-B₅H₉ + C₂H₂ $\overset$ ''closo''-1,5-C₂B₃H₅, ''closo''-1,6-C₂B₄H₆, 2,4-C₂B₅H₇

When the reaction is conducted at lower temperatures, an open-cage carborane is obtained:
: ''nido''-B₅H₉ + C₂H₂ $\overset$ ''nido''-2,4-C₂B₄H₈

Other procedures generate carboranes containing three or four cage carbon atoms ()

Monocarba derivatives

''Mono''carboranes are clusters with B_''n''C cages. The 12-vertex derivative is best studied, but several are known.

Typically they are prepared by the addition of one-carbon reagents to boron hydride clusters. One-carbon reagents include cyanide, isocyanides, and formaldehyde. For example, monocarbadodecaborate ( B₁₁H₁₂sup>−) is produced from decaborane and formaldehyde, followed by addition of borane dimethylsulfide.
Monocarboranes are precursors to weakly coordinating anions.

Dicarba clusters

Dicarbaboranes can be prepared from boron hydrides using alkynes as the source of the two carbon centers. In addition to the ''closo''-C₂B_''n''H_''n''+2 series mentioned above, several open-cage dicarbon species are known including ''nido''-C₂B₃H₇ (isostructural and isoelectronic with B₅H₉) and ''arachno''-C₂B₇H₁₃.

Syntheses of icosahedral ''closo''-dicarbadodecaborane derivatives (R₂C₂B₁₀H₁₀) employ alkynes as the R₂C₂ source and decaborane (B₁₀H₁₄) to supply the B₁₀ unit.

Classification by cage size

The following classification is adapted from Grimes's book on carboranes.

Small, open carboranes

This family of clusters includes the nido cages CB₅H₉, C₂B₄H₈, C₃B₃H₇, C₄B₂H₆, and C₂B₃H₇. Relatively little work has been devoted to these compounds. Pentaborane reacts with acetylene to give nido-1,2-C₂B₄H₈. Upon treatment with sodium hydride, latter forms the salt Na ,2-C₂B₄H₇

Small, closed carboranes

This family of clusters includes the closo cages C₂B₃H₅, C₂B₄H₆, C₂B₅H₇, and CB₅H₇. This family of clusters are also lightly studied owing to synthetic difficulties. Also reflecting synthetic challenges, many of these compounds are best known as their alkyl derivatives. 1,5-C₂B₃H₅ is the only known isomer of this five-vertex cage.

Intermediate-sized carboranes

This family of clusters includes the closo cages C₂B₆H₈, C₂B₇H₉, C₂B₈H₁₀, and C₂B₉H₁₁.

Isomerism is well established in this family: 1,2- and 1,6-C₂B₆H₈, and 2,3- and 2,4-C₂B₅H₇, as well as in open-cage carboranes such as 2,3- and 2,4-C₂B₄H₈ and 1,2 and 1,3-C₂B₉H₁₃. In general, isomers having non-adjacent cage carbon atoms are more thermally stable than those with adjacent carbons, so that heating tends to induce mutual separation of the carbon atoms in the framework.

Carboranes of intermediate nuclearity are most efficiently generated by degradations starting with carborane as summarized in these equations, starting from C₂B₉H₁₂⁻:
:C₂B₉H₁₂⁻ + H⁺ → C₂B₉H₁₃
:C₂B₉H₁₃ → C₂B₉H₁₁ + H₂
In contrast, smaller carboranes are usually prepared by building-up routes, e.g. from pentaborane + alkyne.

Chromate oxidation of this 11-vertex cluster results in deboronation, giving C₂B₇H₁₃. From that species, other clusters result by pyrolysis, sometimes in the presence of diborane: C₂B₆H₈, C₂B₈H₁₀, and C₂B₇H₉.

Icosahedral carboranes

The icosahedral charge-neutral ''closo''-carboranes, 1,2-, 1,7-, and 1,12- C₂B₁₀H₁₂ (informally ''ortho''-, ''meta''-, and ''para''-carborane) are particularly stable and are commercially available. The ortho-carborane forms first upon the reaction of decarborane and acetylene. It converts quantitatively to the meta-carborane upon heating in an inert atmosphere. Producing meta-carborane from ortho-carborane requires 700 °C, proceeding in ca. 25% yield.

B₁₁H₁₂sup>− is also well established.

Reactions

Base-induced degradation of carboranes give anionic nido derivatives, which can be employed as ligands for transition metals, generating metallacarboranes, which are carboranes containing one or more transition metal or main group metal atoms in the cage framework. Most famous are the dicarbollide, complexes with the formula M ₂B₉H₁₁sub>2-.

Research

Dicarbollide complexes have been evaluated for many applications for many years, but commercial applications are rare. The bis(dicarbollide) o(C₂B₉H₁₁)₂sup>− has been used as a precipitant for removal of ¹³⁷Cs⁺ from radiowastes.

The medical applications of carboranes have been explored. C-functionalized carboranes represent a source of boron for boron neutron capture therapy.

The compound H(CHB₁₁Cl₁₁) is a superacid, forming an isolable salt with protonated benzene, C₆H. It protonates fullerene, C₆₀.

See also

* Azaborane
* Heteroborane
* Metallacarboranes
* Organoboron chemistry
* Dicarbollide
* Carborane superacid

References

External links

Material Safety Data Sheet

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Organoboron compounds
Cluster chemistry
Superacids