Carboranes (or carbaboranes) are electron-delocalized (non-classically bonded) clusters composed of

boron Boron is a chemical element; it has symbol B and atomic number 5. In its crystalline form it is a brittle, dark, lustrous metalloid; in its amorphous form it is a brown powder. As the lightest element of the boron group it has three ...

carbon Carbon () is a chemical element; it has chemical symbol, symbol C and atomic number 6. It is nonmetallic and tetravalence, tetravalent—meaning that its atoms are able to form up to four covalent bonds due to its valence shell exhibiting 4 ...

and

hydrogen Hydrogen is a chemical element; it has chemical symbol, symbol H and atomic number 1. It is the lightest and abundance of the chemical elements, most abundant chemical element in the universe, constituting about 75% of all baryon, normal matter ...

atoms.Grimes, R. N., ''Carboranes 3rd Ed.'', Elsevier, Amsterdam and New York (2016), . Like many of the related

boron hydrides Boron hydride clusters are compounds with the formula or related anions, where x ≥ 3. Many such cluster compounds are known. Common examples are those with 5, 10, and 12 boron atoms. Although they have few practical applications, the borane hyd ...

, these clusters are

polyhedra In geometry, a polyhedron (: polyhedra or polyhedrons; ) is a three-dimensional figure with flat polygonal faces, straight edges and sharp corners or vertices. The term "polyhedron" may refer either to a solid figure or to its boundary su ...

or fragments of polyhedra. Carboranes are one class of

heteroborane Heteroboranes are classes of boranes in which at least one boron atom is replaced by another Chemical element, elements. Like many of the related boranes, these clusters are polyhedra and are similarly classified as Boranes#Chemical formula and nam ...

s. 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 In organic chemistry, an alkyl group is an alkane missing one hydrogen. The term ''alkyl'' is intentionally unspecific to include many possible substitutions. An acyclic alkyl has the general formula of . A cycloalkyl group is derived from a cy ...

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

, these clusters are

or fragments of polyhedra, and are similarly classified as ''closo-'', ''nido-'', ''arachno-'', ''hypho-'', ''hypercloso''-, ''iso''-, ''klado''-, ''conjuncto''- and ''megalo''-, 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

s. 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) via formal replacement of two of its 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-.

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

''Mono''carboranes are clusters with 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 In chemistry, cyanide () is an inorganic chemical compound that contains a functional group. This group, known as the cyano group, consists of a carbon atom triple-bonded to a nitrogen atom. Ionic cyanides contain the cyanide anion . This a ...

isocyanide An isocyanide (also called isonitrile or carbylamine) is an organic compound with the functional group –. It is the isomer of the related nitrile (–C≡N), hence the prefix is ''isocyano''.IUPAC Goldboo''isocyanides''/ref> The organic fragme ...

s, and

formaldehyde Formaldehyde ( , ) (systematic name methanal) is an organic compound with the chemical formula and structure , more precisely . The compound is a pungent, colourless gas that polymerises spontaneously into paraformaldehyde. It is stored as ...

. For example, monocarbadodecaborate () is produced from

decaborane Decaborane, also called decaborane(14), is the inorganic compound with the chemical formula B10 H14. It is classified as a borane and more specifically a boron hydride cluster. This white crystalline compound is one of the principal boron hydri ...

and

, followed by addition of

borane dimethylsulfide Borane dimethylsulfide (BMS) is a chemical compound with the chemical formula . It is an adduct between borane molecule () and dimethyl sulfide molecule (). It is a complexed borane reagent that is used for hydroborations and reductions. The adv ...

. Monocarboranes are precursors to

weakly coordinating anion Anions that interact weakly with cations are termed non-coordinating anions, although a more accurate term is weakly coordinating anion. Non-coordinating anions are useful in studying the reactivity of electrophilic cations. They are commonly found ...

Dicarba clusters

Dicarbaboranes can be prepared from boron hydrides using alkynes as the source of the two carbon centers. In addition to the ''closo''- series mentioned above, several open-cage dicarbon species are known including ''nido''- (isostructural and isoelectronic with ) and ''arachno''-. CAHBOR01

Syntheses of icosahedral ''closo''-dicarbadodecaborane derivatives () employ alkynes as the source and decaborane () to supply the 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 . Relatively little work has been devoted to these compounds. Pentaborane reacts with acetylene to give ''nido''-1,2-. Upon treatment with sodium hydride, latter forms the salt [1,2-.

Small, closed carboranes

This family of clusters includes the closo cages . 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- 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: : ''nido''-2,3- : ''closo''-2,3-

Intermediate-sized carboranes

Structures

This family of clusters includes the closo cages and their derivatives. Isomerism is well established in this family: *2,3- and 2,4- *2,3- and 2,4- *1,2- and 1,6- *1,10-, 1,6-, and 1,2- *1,2 and 1,3-.

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 , which can be manipulated with oxidants, protonation, and thermolysis. : : : Chromate oxidation of 11-vertex clusters results in deboronation, giving . From that species, other clusters result by pyrolysis, sometimes in the presence of diborane: . 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- (informally ''ortho''-, ''meta''-, and ''para''-carborane) are particularly stable and are commercially available. The ortho-carborane forms first upon the reaction of

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. is also well established.

Reactions

The metalation of carboranes is illustrated by the reactions of ''closo''- with iron carbonyl sources. Two ''closo'' Fe- and -containing products are obtained, according to these idealized equations: : : Base-induced degradation of carboranes give anionic ''nido'' derivatives, which can also be employed as

ligand In coordination chemistry, a ligand is an ion or molecule with a functional group that binds to a central metal atom to form a coordination complex. The bonding with the metal generally involves formal donation of one or more of the ligand's el ...

s 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 , where M stands for

metal A metal () is a material that, when polished or fractured, shows a lustrous appearance, and conducts electrical resistivity and conductivity, electricity and thermal conductivity, heat relatively well. These properties are all associated wit ...

Research

Dicarbollide complexes have been investigated for many years, but commercial applications are rare. The bis(dicarbollide) has been used as a precipitant for removal of 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 is a

superacid In chemistry, a superacid (according to the original definition) is an acid with an acidity greater than that of 100% pure sulfuric acid (), which has a Hammett acidity function (''H''0) of −12. According to the modern definition, a superacid i ...

, forming an isolable

salt In common usage, salt is a mineral composed primarily of sodium chloride (NaCl). When used in food, especially in granulated form, it is more formally called table salt. In the form of a natural crystalline mineral, salt is also known as r ...

with protonated benzene

cation An ion () is an atom or molecule with a net electrical charge. The charge of an electron is considered to be negative by convention and this charge is equal and opposite to the charge of a proton, which is considered to be positive by convent ...

, (benzenium cation). The formula of that salt is . The superacid protonates

fullerene A fullerene is an allotropes of carbon, allotrope of carbon whose molecules consist of carbon atoms connected by single and double bonds so as to form a closed or partially closed mesh, with fused rings of five to six atoms. The molecules may ...

, .

References

External links

Material Safety Data Sheet
{{Authority control Organoboron compounds Cluster chemistry Superacids