Carborane
Carboranes (or carbaboranes) are electron-delocalized (non-classically bonded) clusters composed of boron, carbon and hydrogen atoms.<ref name="grimes">Grimes, R. N., Carboranes 3rd Ed., Elsevier, Amsterdam and New York (2016), Template:ISBN.</ref> Like many of the related boron hydrides, these clusters are polyhedra or fragments of polyhedra. Carboranes are one class of heteroboranes.<ref name="Greenwood&Earnshaw2d">Template:Greenwood&Earnshaw2nd</ref>
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 bondingEdit
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-, 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 heteroboranes.<ref name="Greenwood&Earnshaw2d"/><ref>The Wade–Mingos rules were first stated by Kenneth Wade in 1971 and expanded by Michael Mingos in 1972:
They are sometimes known as simply "Wade's rules".
- Template:Cite journal</ref>
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 (Ih) [[Dodecaborate|Template:Chem2]] via formal replacement of two of its Template:Chem2 fragments with CH.
IsomersEdit
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-Template:Chem2.
PreparationEdit
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 derivativesEdit
Monocarboranes are clusters with Template:Chem2 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 (Template:Chem2) is produced from decaborane and formaldehyde, followed by addition of borane dimethylsulfide.<ref>Template:Cite journal</ref><ref>Template:Cite journal</ref> Monocarboranes are precursors to weakly coordinating anions.<ref>Template:Cite journal</ref>
Dicarba clustersEdit
Dicarbaboranes can be prepared from boron hydrides using alkynes as the source of the two carbon centers. In addition to the closo-Template:Chem2 series mentioned above, several open-cage dicarbon species are known including nido-Template:Chem2 (isostructural and isoelectronic with Template:Chem2) and arachno-Template:Chem2.
Syntheses of icosahedral closo-dicarbadodecaborane derivatives (Template:Chem2) employ alkynes as the Template:Chem2 source and decaborane (Template:Chem2) to supply the Template:Chem2 unit.
Classification by cage sizeEdit
The following classification is adapted from Grimes's book on carboranes.<ref name="grimes"/>
Small, open carboranesEdit
This family of clusters includes the nido cages Template:Chem2. Relatively little work has been devoted to these compounds. Pentaborane[9] reacts with acetylene to give nido-1,2-Template:Chem2. Upon treatment with sodium hydride, latter forms the salt [1,2-Template:Chem2.
Small, closed carboranesEdit
This family of clusters includes the closo cages Template:Chem2. 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-Template:Chem2 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:
- Template:Chem2 nido-2,3-Template:Chem2
- Template:Chem2 closo-2,3-Template:Chem2
Intermediate-sized carboranesEdit
StructuresEdit
This family of clusters includes the closo cages Template:Chem2 and their derivatives. Isomerism is well established in this family:
- 2,3- and 2,4-Template:Chem2
- 2,3- and 2,4-Template:Chem2
- 1,2- and 1,6-Template:Chem2
- 1,10-, 1,6-, and 1,2-Template:Chem2<ref>Template:Cite journal</ref>
- 1,2 and 1,3-Template:Chem2.
SynthesesEdit
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 Template:Chem2,<ref>Template:Cite book</ref> which can be manipulated with oxidants, protonation, and thermolysis.
Chromate oxidation of 11-vertex clusters results in deboronation, giving Template:Chem2. From that species, other clusters result by pyrolysis, sometimes in the presence of diborane: Template:Chem2.<ref name="grimes"/>
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 carboranesEdit
The icosahedral charge-neutral closo-carboranes, 1,2-, 1,7-, and 1,12- Template:Chem2 (informally ortho-, meta-, and para-carborane) are particularly stable and are commercially available.<ref>Template:Cite journal</ref><ref>Template:Cite journal</ref> 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.<ref name="grimes"/>
Template:Chem2 is also well established.
ReactionsEdit
The metalation of carboranes is illustrated by the reactions of closo-Template:Chem2 with iron carbonyl sources. Two closo Fe- and Template:Chem2-containing products are obtained, according to these idealized equations:
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 Template:Chem2, where M stands for metal.<ref name=Sivaev>Template:Cite journal</ref>
ResearchEdit
Dicarbollide complexes have been investigated for many years, but commercial applications are rare. The bis(dicarbollide) Template:Chem2 has been used as a precipitant for removal of Template:Chem2 from radiowastes.<ref>Template:Cite journal </ref>
The medical applications of carboranes have been explored.<ref>Template:Cite journal</ref><ref>Template:Cite journalTemplate:Open access</ref> C-functionalized carboranes represent a source of boron for boron neutron capture therapy.<ref>Template:Cite journal</ref>
The compound Template:Chem2 is a superacid, forming an isolable salt with protonated benzene cation, Template:Chem2 (benzenium cation).<ref>Template:Cite book</ref> The formula of that salt is Template:Chem2. The superacid protonates fullerene, Template:Chem2.<ref>Template:Cite journal</ref>