Carboranes (or carbaboranes) are electron-delocalized (non-classically bonded) clusters composed of boron, carbon and hydrogen atoms.
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 carbon-alkyl and boron-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 Polyhedron 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 .[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".
]
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) 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
Monocarboranes 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, , and formaldehyde. For example, monocarbadodecaborate () is produced from decaborane and formaldehyde, followed by addition of borane dimethylsulfide.
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-.
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. Pentaborane9 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 cluster-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
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.
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 for transition metals, generating , 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.
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.
The compound is a superacid, forming an isolable salt with arenium ion cation, (benzenium cation).
See also
External links
