A borane is a compound with the formula BxHy or a related anion. 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 bonding that differs strongly from the patterns seen in hydrocarbons. Hybrids of boranes and hydrocarbons, the carboranes are also well developed. [1]
The development of the chemistry of boranes led to innovations in synthetic methods as well as structure and bonding. First, new synthetic techniques were required to handle diborane and many of its derivatives, which are both pyrophoric and volatile. Alfred Stock invented the glass vacuum line for this purpose. [2]
The structure of diborane was correctly predicted in 1943 many years after its discovery. [3] The structures of the boron hydride clusters were determined beginning in 1948 with the characterization of decaborane. William Lipscomb was awarded the Nobel prize in Chemistry in 1976 for this and many subsequent crystallographic investigations. These investigations revealed the prevalence of deltahedral structures, i.e., networks of triangular arrays of BH centers.
The bonding of the clusters ushered in Polyhedral skeletal electron pair theory and Wade's rules, which can be used to predict the structures of boranes. [4] These rules were found to describe structures of many cluster compounds.
Interest in boranes increased during World War II due to the potential of uranium borohydride for enrichment of the uranium isotopes and as a source of hydrogen for inflating weather balloons. In the US, a team led by Schlesinger developed the basic chemistry of the anionic boron hydrides and the related aluminium hydrides. Schlesinger's work laid the foundation for a host of boron hydride reagents for organic synthesis, most of which were developed by his student Herbert C. Brown. Borane-based reagents are now widely used in organic synthesis. Brown was awarded the Nobel prize in Chemistry in 1979 for this work. [5]
Borane clusters are classified as follows, where n is the number of boron atoms in a single cluster: [1] [6] [7]
| Cluster type | Chemical formula | Example | Notes |
|---|---|---|---|
| hypercloso- | BnHn | Unstable; derivatives are known [8] | |
| closo- | [BnHn]2− | Caesium dodecaborate | |
| nido- | BnHn+4 | pentaborane(9) | |
| arachno- | BnHn+6 | pentaborane(11) | |
| hypho- | BnHn+8 | Only found in adducts |
The International Union of Pure and Applied Chemistry rules for systematic naming is based on a prefix denoting a class of compound, followed by the number of boron atoms and finally the number of hydrogen atoms in parentheses. Various details can be omitted if there is no ambiguity about the meaning, for example, if only one structural type is possible. Some examples of the structures are shown below.
The naming of anions is illustrated by
The hydrogen count is specified first followed by the boron count. The -ate suffix is applied with anions. The ionic charge value is included in the chemical formula but not as part of the systematic name.
Boranes are nonclassically–bonded compounds, that is, there are not enough electrons to form 2-centre, 2-electron bonds between all pairs of adjacent atoms in the molecule. A description of the bonding in the larger boranes was formulated by William Lipscomb. It involved:
Lipscomb's methodology has largely been superseded by a molecular orbital approach. This allows the concept of multi-centre bonding to be extended. For example, in the icosahedral ion [B12H12]2−, the totally symmetric (Ag symmetry) molecular orbital is equally distributed among all 12 boron atoms. Wade's rules provide a powerful method that can be used to rationalize the structures in terms of the number of atoms and the connectivity between them.
![Structure of the conjuncto boron hydride cluster
[B19H22]. GAKFEF.png](http://upload.wikimedia.org/wikipedia/commons/thumb/d/d0/GAKFEF.png/220px-GAKFEF.png)
Although relatively rare, several multi-cluster boranes have been characterized. For example, reaction of a borane cluster with B2H6 (as a source of BH3) can lead to the formation of a conjuncto-borane species in which borane cluster sub-units are joined by the sharing of boron atoms. [10]
Other conjuncto-boranes, where the sub-units are joined by a B-B bond, can be made by ultra violet irradiation of nido-boranes. Some B-B coupled conjuncto-boranes can be produced using PtBr2 as catalyst. [11]
Analogous to Wade's Rules, electron counting scheme has been developed to predict or rationalize multicluster boranes.
| Prefix | Meaning | Example |
|---|---|---|
| klado- | branched clusters | |
| conjuncto- | conjoined clusters | |
| megalo- | multiple conjoined clusters |
The lowest borane, BH3 exists only transiently, dimerizing instantly to form diborane, B2H6. Its adducts BH3· THF and BH3· DMSO are sufficiently stable to be used in hydroboration reactions. Reminiscent of the behavior of diborane, some lower boranes react with air very exothermically, even explosively. By contrast, many closo-borane cluster, such as [B12H12]2−, do not react with air.
The boron hydride clusters are so diverse that generalizations on their reactions are not possible.
Some function as electron donors owing to the relative basic character of the B−Hterminal groups. Boranes can function as ligands in coordination compounds. [13] Hapticities of η1 to η6 have been found, with electron donation involving bridging H atoms or donation from B-B bonds. For example, nido-B6H10 can replace ethene in Zeise's salt to produce Fe(η2-B6H10)(CO)4.
They can also act as Lewis acids, with concomitant opening of the cluster. An example involving trimethylphosphine:
Some higher boranes, especially those with bridging hydrogen atoms, can be deprotonated with a strong base. An example:
Acidity increases with the size of the borane, with B10H14 having a pKa value of 2.7. [14] [15]
![Structure of
[(CH3)4N ]2[Fe(C2B9H11)2], showing only one
Me4N. KIWJOP.png](http://upload.wikimedia.org/wikipedia/commons/thumb/7/71/KIWJOP.png/180px-KIWJOP.png)
For the boron hydride chemist, one of the most important reactions is the building up process by which smaller boron hydride clusters add borane to give larger clusters. This approach also applies to the synthesis of metallaboranes,
Reminiscent of the behavior of diborane and its adducts, higher boranes participate in hydroboration. When boron hydrides add an alkyne, the carbon becomes incorporated into the cluster, producing carboranes, e.g. C2B10H12. [17]
Diborane and its monomeric adducts borane–tetrahydrofuran or borane–dimethylsulfide are useful reagents. They are often used for hydroboration in organic synthesis. Some cobalt derivatives of carboranes have been commercialized for sequestering 137Cs from radioactive waste. [18]
Boranes have a high specific energy of combustion compared to hydrocarbons, making them potentially attractive as fuels or igniters. Intense research was carried out in the 1950s into their use as jet fuel additives, but the effort did not lead to practical results.
Because 10B has a very high neutron-capture cross section, boron-hydride derivatives have often been investigated for applications in Neutron capture therapy of cancer. [19]
William Nunn Lipscomb Jr. was a Nobel Prize-winning American inorganic and organic chemist working in nuclear magnetic resonance, theoretical chemistry, boron chemistry, and biochemistry.
Hydroboration–oxidation reaction is a two-step hydration reaction that converts an alkene into an alcohol. The process results in the syn addition of a hydrogen and a hydroxyl group where the double bond had been. Hydroboration–oxidation is an anti-Markovnikov reaction, with the hydroxyl group attaching to the less-substituted carbon. The reaction thus provides a more stereospecific and complementary regiochemical alternative to other hydration reactions such as acid-catalyzed addition and the oxymercuration–reduction process. The reaction was first reported by Herbert C. Brown in the late 1950s and it was recognized in his receiving the Nobel Prize in Chemistry in 1979.
Diborane(6), commonly known as diborane, is the chemical compound with the formula B2H6. It is a toxic, colorless, and pyrophoric gas with a repulsively sweet odor. Given its simple formula, borane is a fundamental boron compound. It has attracted wide attention for its electronic structure. Several of its derivatives are useful reagents.
Decaborane, also called decaborane(14), is the borane with the chemical formula B10H14. 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 is toxic and volatile, giving off a foul odor, like that of burnt rubber or chocolate.
Carboranes are electron-delocalized clusters composed of boron, carbon and hydrogen atoms. Like many of the related boron hydrides, these clusters are polyhedra or fragments of polyhedra. Carboranes are one class of heteroboranes.
Hexaborane, also called hexaborane(10) to distinguish it from hexaborane(12) (B6H12), is an inorganic compound with the formula B6H10. It is a colorless liquid that is unstable in air.
A three-center two-electron (3c–2e) bond is an electron-deficient chemical bond where three atoms share two electrons. The combination of three atomic orbitals form three molecular orbitals: one bonding, one non-bonding, and one anti-bonding. The two electrons go into the bonding orbital, resulting in a net bonding effect and constituting a chemical bond among all three atoms. In many common bonds of this type, the bonding orbital is shifted towards two of the three atoms instead of being spread equally among all three. Example molecules with 3c–2e bonds are the trihydrogen cation and diborane. In these two structures, the three atoms in each 3c-2e bond form an angular geometry, leading to a bent bond.
Organoboron chemistry or organoborane chemistry studies organoboron compounds, also called organoboranes. These chemical compounds combine boron and carbon; typically, they are organic derivatives of borane (BH3), as in the trialkyl boranes.
In organic chemistry, hydroboration refers to the addition of a hydrogen-boron bond to certain double and triple bonds involving carbon. This chemical reaction is useful in the organic synthesis of organic compounds.
In chemistry the polyhedral skeletal electron pair theory (PSEPT) provides electron counting rules useful for predicting the structures of clusters such as borane and carborane clusters. The electron counting rules were originally formulated by Kenneth Wade, and were further developed by others including Michael Mingos; they are sometimes known as Wade's rules or the Wade–Mingos rules. The rules are based on a molecular orbital treatment of the bonding. These rules have been extended and unified in the form of the Jemmis mno rules.
Boron compounds are compounds containing the element boron. In the most familiar compounds, boron has the formal oxidation state +3. These include oxides, sulfides, nitrides, and halides.
Borohydride refers to the anion [BH4]−, which is also called tetrahydridoborate, and its salts. Borohydride or hydroborate is also the term used for compounds containing [BH4−nXn]−, where n is an integer from 0 to 3, for example cyanoborohydride or cyanotrihydroborate [BH3(CN)]− and triethylborohydride or triethylhydroborate [BH(CH2CH3)3]−. Borohydrides find wide use as reducing agents in organic synthesis. The most important borohydrides are lithium borohydride and sodium borohydride, but other salts are well known. Tetrahydroborates are also of academic and industrial interest in inorganic chemistry.
Caesium dodecaborate is an inorganic compound with the formula Cs2B12H12. It is a salt composed of caesium and dodecaborate(12) ions. The [B12H12]2− anion has been of great theoretical interest to the chemistry community.
Borane, also known as borine, is an unstable and highly reactive molecule with the chemical formula BH
3. The preparation of borane carbonyl, BH3(CO), played an important role in exploring the chemistry of boranes, as it indicated the likely existence of the borane molecule. However, the molecular species BH3 is a very strong Lewis acid. Consequently, it is highly reactive and can only be observed directly as a continuously produced, transitory, product in a flow system or from the reaction of laser ablated atomic boron with hydrogen. It normally dimerizes to diborane in the absence of other chemicals.
Pentaborane(11) is inorganic compound with the chemical formula B5H11. It is an obscure boron hydride cluster, especially relative to the heavily studied pentaborane(9) (B5H9). With two more hydrogen atoms than nido-pentaborane(9), pentaborane(11) is classified as an arachno- cluster.
1,2-Dimethyldiborane is an organoboron compound with the formula [(CH3)BH2]2. Structurally, it is related to diborane, but with methyl groups replacing terminal hydrides on each boron. It is the dimer of methylborane, CH3BH2, the simplest alkylborane. 1,2-Dimethyldiborane can exist in a cis- and a trans arrangement. 1,2-Dimethyldiborane is an easily condensed, colorless gas that ignites spontaneously in air.
Dimethylborane, (CH3)2BH is the simplest dialkylborane, consisting of a methyl group substituted for a hydrogen in borane. As for other boranes it normally exists in the form of a dimer called tetramethyldiborane or tetramethylbisborane or TMDB ((CH3)2BH)2. Other combinations of methylation occur on diborane, including monomethyldiborane, trimethyldiborane, 1,2-dimethylborane, 1,1-dimethylborane and trimethylborane. At room temperature the substance is at equilibrium between these forms. The methylboranes were first prepared by H. I. Schlesinger and A. O. Walker in the 1930s.
Trimethyldiborane, (CH3)3B2H3 is a molecule containing boron carbon and hydrogen. It is an alkylborane, consisting of three methyl group substituted for a hydrogen in diborane. It can be considered a mixed dimer: (CH3)2BH2BH(CH3) or dimethylborane and methylborane. called 1,2-dimethyldiborane. Other combinations of methylation occur on diborane, including monomethyldiborane, 1,2-dimethyldiborane, tetramethyldiborane, 1,1-dimethylborane and trimethylborane. At room temperature the substance is at equilibrium between these forms, so it is difficult to keep it pure. The methylboranes were first prepared by H. I. Schlesinger and A. O. Walker in the 1930s.
Methyldiborane, CH3B2H5, or monomethyldiborane is the simplest of alkyldiboranes, consisting of a methyl group substituted for a hydrogen in diborane. As with other boranes it exists in the form of a dimer with a twin hydrogen bridge that uses three-center two-electron bonding between the two boron atoms, and can be imagined as methyl borane (CH3BH2) bound to borane (BH3). Other combinations of methylation occur on diborane, including 1,1-dimethylborane, 1,2-dimethyldiborane, trimethyldiborane, tetramethyldiborane, and trimethylborane (which is not a dimer). At room temperature the substance is at equilibrium between these molecules.
Borane carbonyl is the inorganic compound with the formula H3BCO. This colorless gas is the adduct of borane and carbon monoxide. It is usually prepared by combining borane-ether complexes and CO. The compound is mainly of theoretical and pedagogical interest.
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