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Carboranes are electron-delocalized (non-classically bonded) clusters composed of
boron Boron is a chemical element with the 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 th ...
,
carbon Carbon () is a chemical element with the symbol C and atomic number 6. It is nonmetallic and tetravalent In chemistry, the valence (US spelling) or valency (British spelling) of an element is the measure of its combining capacity with o ...
and
hydrogen Hydrogen is the chemical element with the symbol H and atomic number 1. Hydrogen is the lightest element. At standard conditions hydrogen is a gas of diatomic molecules having the formula . It is colorless, odorless, tasteless, non-toxic, an ...
atoms.Grimes, R. N., ''Carboranes 3rd Ed.'', Elsevier, Amsterdam and New York (2016), . Like many of the related
boron hydrides Boranes is the name given to compounds with the formula BxHy and related anions. Many such boranes are known. Most common are those with 1 to 12 boron atoms. Although they have few practical applications, the boranes exhibit structures and bond ...
, these clusters are
polyhedra In geometry, a polyhedron (plural polyhedra or polyhedrons; ) is a three-dimensional shape with flat polygonal faces, straight edges and sharp corners or vertices. A convex polyhedron is the convex hull of finitely many points, not all on t ...
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 Boranes is the name given to compounds with the formula BxHy and related anions. Many such boranes are known. Most common are those with 1 to 12 boron atoms. Although they have few practical applications, the boranes exhibit structures and bond ...
, these clusters are
polyhedra In geometry, a polyhedron (plural polyhedra or polyhedrons; ) is a three-dimensional shape with flat polygonal faces, straight edges and sharp corners or vertices. A convex polyhedron is the convex hull of finitely many points, not all on t ...
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) B12H122– 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) Pentaborane(9) is an inorganic compound with the formula B5H9. It is one of the most common boron hydride clusters, although it is a highly reactive compound. Because of its high reactivity toward oxygen, it was once evaluated as rocket or jet fue ...
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 Cyanide is a naturally occurring, rapidly acting, toxic chemical that can exist in many different forms. In chemistry, a cyanide () is a chemical compound that contains a functional group. This group, known as the cyano group, consists of ...
,
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 a naturally occurring organic compound with the formula and structure . The pure compound is a pungent, colourless gas that polymerises spontaneously into paraformaldehyde (refer to section F ...
. For example, monocarbadodecaborate ( B11H12sup>−) is produced from
decaborane Decaborane, also called decaborane(14), is the borane with the chemical formula B10 H14. This white crystalline compound is one of the principal boron hydride clusters, both as a reference structure and as a precursor to other boron hydrides. It ...
and
formaldehyde Formaldehyde ( , ) (systematic name methanal) is a naturally occurring organic compound with the formula and structure . The pure compound is a pungent, colourless gas that polymerises spontaneously into paraformaldehyde (refer to section F ...
, followed by addition of
borane dimethylsulfide Borane dimethylsulfide (BMS) is a complexed borane reagent that is used for hydroborations and reductions. The advantages of BMS over other borane reagents, such as borane-tetrahydrofuran, are its increased stability and higher solubility. BMS ...
. 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 ...
s.


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 Diborane(6), generally known as diborane, is the chemical compound with the formula B2H6. It is a toxic, colorless, and pyrophoric gas with a repulsively sweet odor. Diborane is a key boron compound with a variety of applications. It has attracte ...
: C2B6H8, C2B8H10, and C2B7H9.


Icosahedral carboranes

The
icosahedral In geometry, an icosahedron ( or ) is a polyhedron with 20 faces. The name comes and . The plural can be either "icosahedra" () or "icosahedrons". There are infinitely many non- similar shapes of icosahedra, some of them being more symmetrica ...
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 Ortho-carborane is the organoboron compound with the formula C2B10H12. The prefix ''ortho'' is derived from ortho. It is the most prominent carborane. This derivative has been considered for a wide range of applications from heat-resistant polyme ...
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
ligand In coordination chemistry, a ligand is an ion or molecule (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 electr ...
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 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 In chemistry, a superacid (according to the classical 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 ...
, forming an isolable salt with protonated benzene, C6H. It protonates
fullerene A fullerene is an allotrope of carbon whose molecule consists 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 seven atoms. The molecule may be a hollow sphere, ...
, C60.


See also

*
Azaborane Azaborane usually refers a boranes, borane Cluster chemistry, cluster where BH vertices are replaced by N or NR (R = H, organic substituent). Like many of the related boranes, these clusters are polyhedra and can be classified as ''closo-'', ''nid ...
* Heteroborane * Metallacarboranes *
Organoboron chemistry Organoborane or organoboron compounds are chemical compounds of boron and carbon that are organic derivatives of BH3, for example trialkyl boranes. Organoboron chemistry or organoborane chemistry is the chemistry of these compounds. Organoboron ...
* Dicarbollide *
Carborane superacid Carborane acids (X, Y, Z = H, Alk, F, Cl, Br, CF3) are a class of superacids, some of which are estimated to be at least one million times stronger than 100% pure sulfuric acid in terms of their Hammett acidity function values (''H''0 ≤ –18) ...


References


External links


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