Carborane

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 this reason, the great majority of carborane have two carbon vertices.

Monocarba derivatives

Monocarboranes are clusters with B_nC 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 ([CB₁₁H₁₂]⁻) 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_nH_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[9] reacts with acetylene to give nido-1,2-C₂B₄H₈. Upon treatment with sodium hydride, latter forms the salt [1,2-C₂B₄H₇]⁻Na⁺.

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 the five-vertex cage. It is prepared from the reaction of pentaborane(9) with acetylene in two operations beginning with condensation with acetylene followed by pyrolysis (cracking) of the product:

B₅H₉ + C₂H₂ → nido-2,3-C₂B₄H₈ + BH₃

C₂B₄H₈ → closo-2,3-C₂B₃H₅ + BH₃

Intermediate-sized carboranes

Structures

This family of clusters includes the closo cages C₂B₆H₈, C₂B₇H₉, C₂B₈H₁₀ and C₂B₉H₁₁ and their derivatives. Isomerism is well established in this family:

• 2,3- and 2,4-C₂B₄H₈
• 2,3- and 2,4-C₂B₅H₇
• 1,2- and 1,6-C₂B₆H₈
• 1,10-, 1,6-, and 1,2-C₂B₈H₁₀
• 1,2 and 1,3-C₂B₉H₁₃.

Syntheses

Carboranes of intermediate nuclearity are most efficiently generated by degradations from larger clusters. In contrast, smaller carboranes are usually prepared by building-up routes, e.g. from pentaborane + alkyne, etc. For example ortho-carborane can be degraded to give [C₂B₉H₁₂]⁻, which can be manipulated with oxidants, protonation, and thermolysis.

[C₂B₉H₁₂]⁻ + Fe³⁺ → C₂B₈H₁₂ + "B⁺" + Fe²⁺

[C₂B₉H₁₂]⁻ + H⁺ → C₂B₉H₁₃

C₂B₉H₁₃ → C₂B₉H₁₁ + H₂

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

In general, isomers having non-adjacent cage carbon atoms are more thermally stable than those with adjacent carbons. Thus, heating tends to induce mutual separation of the carbon atoms in the framework.

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 decaborane 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.

[CB₁₁H₁₂]⁻ is also well established.

Reactions

The metalation of carboranes is illustrated by the reactions of closo-C₂B₃H₅ with iron carbonyl sources. Two closo Fe- and Fe₂-containing products are obtained, according to these idealized equations:

C₂B₃H₅ + Fe₂(CO)₉ → C₂B₃H₅Fe(CO)₃ + Fe(CO)₅ + CO

C₂B₃H₅Fe(CO)₃ + Fe₂(CO)₉ → C₂B₃H₅(Fe(CO)₃)₂ + Fe(CO)₅ + CO

Base-induced degradation of carboranes give anionic nido derivatives, which can also 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 M²⁺[C₂B₉H₁₁]²⁻, where M stands for metal.

Carborane substituents on an arene are electron-withdrawing.

Research

Dicarbollide complexes have been investigated for many years, but commercial applications are rare. The bis(dicarbollide) [Co(C₂B₉H₁₁)₂]⁻ 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 cation, [C₆H₇]⁺ (benzenium cation). The formula of that salt is [C₆H₇]⁺[CHB₁₁Cl₁₁]⁻. The superacid protonates fullerene, C₆₀.