Boranes

In chemistry, boranes comprise a large group of compounds with the generic formula of B_xH_y. These compounds do not occur in nature. Many of the boranes readily oxidise on contact with air, some violently. The class is named after the parent chemical called "borane" itself, chemical formula BH₃. This compound is only known to exist as a transient intermediate since it dimerises to form diborane, B₂H₆. The larger boranes all consist of boron clusters that are polyhedral. In addition to the charge-neutral boranes, a large number of anionic boron hydrides are known. The most important boranes are diborane B₂H₆ and two of its pyrolysis products, pentaborane B₅H₉ and decaborane B₁₀H₁₄. The development of the chemistry of boron hydrides led to new experimental techniques and theoretical concepts. Boron hydrides have been studied as potential fuels, for rockets and for automotive uses, but the only commercial applications involve derivatives of borane.

Generic formula of boranes

The four series of single-cluster boranes have the following general formulae, where "n" is the number of boron atoms:

Type	formula	notes
closo−	BnHn2−	No neutral BnHn+2 boranes are known
nido−	BnHn+4
arachno−	BnHn+6
hypho−	BnHn+8	only adducts established

There also exists a series of substituted neutral hypercloso-boranes that have the theoretical formulae B_nH_n. Examples include B₁₂(OCH2Ph)₁₂, a derivative of the unstable hypercloso-B₁₂H₁₂.^[1]

Naming conventions

The naming of neutral boranes is illustrated by the following examples, where the Greek prefix shows the number of boron atoms and the number of hydrogen atoms is in brackets:

• B₅H₉ pentaborane(9)
• B₆H₁₂ hexaborane(12)

The naming of anions is illustrated by the following, where the hydrogen count is specified first followed by the boron count, and finally the overall charge in brackets:

• B₅H₈⁻ octahydropentaborate(1−)

Optionally closo− nido− etc. (see above) can be added:

• B₅H₉, nido−pentaborane(9)
• B₄H₁₀, arachno−tetraborane(10)
• B₆H₆²⁻, hexahydro−closo−hexaborate(2−)

Understandably many of the compounds have abbreviated common names.

Cluster types

It was realised in the early 1970s that the geometry of boron clusters are related and that they approximate to deltahedra or to deltahedra with one or more vertices missing. The deltahedra that are found in borane chemistry are (using the names favoured by most chemists)

Vertices Deltahedron Image 5 Trigonal bipyramid 6 Octahedron 7 Pentagonal bipyramid 8 Dodecahedron (snub disphenoid)

Vertices Deltahedron Image 9 Tricapped trigonal prism (triaugmented triangular prism) 10 Bicapped square antiprism (gyroelongated square bipyramid) 11 Octadecahedron 12 Icosahedron

One feature of these deltahedra is that boron atoms at the vertices may have different numbers of boron atoms as near neighbours. For example, in the pentagonal bipyramid, 2 borons have 3 neighbors, 3 have 4 neighbours, whereas in the octahedral cluster all vertices are the same, each boron having 4 neighbours. These differences between the boron atoms in different positions are important in determining structure, as they have different chemical shifts in the ¹¹B NMR spectra.

• 

  Diborane, B₂H₆
• 

  Tetraborane, B₄H₁₀
• 

  Pentaborane-[9], B₅H₉
• 

  Decaborane-[14], B₁₀H₁₄
• 

  B₁₂H₁₂²⁻

B₆H₁₀ is a typical example. Its geometry is, in essence, a 7-boron framework (pentagonal bipyramid), missing a vertex that had the highest number of near neighbours, e.g., a vertex with 5 neighbours. The extra hydrogen atoms bridge around the open face. A notable exception to this general scheme is that of B₈H₁₂, which would be expected to have a nido- geometry (based on B₉H₉²⁻ missing 1 vertex), but is similar in geometry to B₈H₁₄, which is based on B₁₀H₁₀²⁻.

The names for the series of boranes are derived from this general scheme for the cluster geometries:-

• hypercloso- (from the Greek for "over cage") a closed complete cluster, e.g., B₈Cl₈ is a slightly distorted dodecahedron
• closo- (from the Greek for "cage") a closed complete cluster, e.g., icosahedral B₁₂H₁₂²⁻
• nido- (from the Latin for "nest") B occupies n vertices of an n+1 deltahedron, e.g., B₅H₉ an octahedron missing 1 vertex
• arachno- (from the Greek for "spiders web") B occupies n vertices of an n+2 deltahedron e.g. B₄H₁₀ an octahedron missing 2 vertices
• hypho- (from the Greek for "net") B occupies n vertices of an n+3 deltahedron possibly B₈H₁₆ has this structure, an octahedron missing 3 vertices
• conjuncto- 2 or more of the above are fused together, e.g., the edge or two vertex fused B₁₉H₂₂¹⁻, face or three vertex fused B₂₁H₁₈¹⁻, and four vertex fused B₂₀H₁₆

Bonding in boranes

Boranes are electron-deficient and pose a problem for conventional descriptions of covalent bonding that involves shared electron pairs. BH₃ is a trigonal planar molecule (D_3h molecular symmetry). Diborane has a hydrogen-bridged structure, see the diborane article. The description of the bonding in the larger boranes formulated by William Lipscomb involved:

• 3-center 2-electron B-H-B hydrogen bridges
• 3-center 2-electron B-B-B bonds
• 2-center 2-electron bonds (in B-B, B-H and BH₂)

The styx number was introduced to aid in electron counting where s = count of 3-center B-H-B bonds; t = count of 3-center B-B-B bonds; y = count of 2-center B-B bonds and x = count of BH₂ groups.

Lipscomb's methodology has largely been superseded by a molecular orbital approach, although it still affords insights. The results of this have been summarised in a simple but powerful rule, PSEPT, often known as Wade's rules, that can be used to predict the cluster type, closo-, nido-, etc. The power of this rule is its ease of use and general applicability to many different cluster types other than boranes. There are continuing efforts by theoretical chemists to improve the treatment of the bonding in boranes — an example is Stone's tensor surface harmonic treatment of cluster bonding. A recent development is four-center two-electron bond.

Synthesis

Diborane is made industrially by the reduction of BF₃, and is the starting point for preparing the higher boranes. It has been studied extensively. The higher (larger) boranes are all derived from diborane. Heating this compound generates the reactive monomer BH₃, which combines with diborane to produce higher boranes.

Reactivity of boranes

Boranes are all colourless and diamagnetic. They are reactive compounds and some are pyrophoric. The majority are highly poisonous and require special handling precautions.

closo−

There is no known neutral closo borane. Salts of the closo anions, B_nH_n²⁻ are stable in neutral aqueous solution, and their stabilities increase with size. The salt K₂B₁₂H₁₂ is stable up to 700 °C.

nido−

Pentaborane(9) and decaborane(14) are the most stable nido−boranes, in contrast to nido−B₈H₁₂ that decomposes above -35^o.

arachno−

Generally these are more reactive than nido−boranes and again larger compounds tend to be more stable.

Typical reactions of boranes are

• electrophilic substitution
• nucleophilic substitution by Lewis bases
• deprotonation by strong bases
• cluster building reactions by pyrolysis and by condensation with borohydrides, which serves as a Lewis base.
• reaction of a nido-borane with an alkyne to give a carborane cluster

Boranes can function as ligands in coordination compounds. 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.

Boranes can react to form hetero-boranes, e.g., carboranes or metalloboranes (clusters that contain boron and metal atoms).

History

The development of the chemistry of boranes posed two challenges to chemists. First, new laboratory techniques had to be developed to handle these very reactive compounds; second, the structures of the compounds challenged the accepted theories of chemical bonding.

The German chemist Alfred Stock first characterized the series of boron-hydrogen compounds. His group developed the glass vacuum line and techniques for handling the compounds. However, exposure to mercury (used in mercury diffusion pumps and float valves) caused Stock to develop mercury poisoning, which he documented in the first scientific papers on the subject. The chemical bonding of the borane clusters was investigated by Lipscomb and his co-workers. Lipscomb was awarded the Nobel prize in Chemistry in 1976 for this work. PSEPT (Wades rules) can be used to predict the structures of boranes.

Interest in boranes increased during World War II due to the potential of uranium borohydride for enrichment of the uranium isotopes. In the US, a team led by Schlesinger developed the basic chemistry of the boron hydrides and the related aluminium hydrides. Although uranium borohydride was not utilized for isotopic separations, 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. For example, sodium borohydride is the standard reagent for converting aldehydes and ketones to alcohols. Brown was awarded the Nobel prize in Chemistry in 1979 for this work.^[2] In the 1950s and early 1960s, the US and USSR investigated boron hydrides as high-energy fuels (ethylboranes, for example) for high speed aircraft,^[3] such as the XB-70 Valkyrie. The development of advanced surface-to-air missiles made the fast aircraft redundant, and the fuel programs were terminated, although triethylborane (TEB) was later used to ignite the engines of the SR-71 Blackbird.^[4]

General references

1. Fox, Mark A.; Wade, Ken (2003). "Evolving patterns in boron cluster chemistry". Pure Appl. Chem. 75 (9): 1315–1323. doi:10.1351/pac200375091315. 
2. Greenwood, Norman N.; Earnshaw, Alan (1997). Chemistry of the Elements (2nd ed.). Butterworth-Heinemann. ISBN 0080379419. 
3. Cotton, F. Albert; Wilkinson, Geoffrey; Murillo, Carlos A.; Bochmann, Manfred (1999), Advanced Inorganic Chemistry (6th ed.), New York: Wiley-Interscience, ISBN 0-471-19957-5 

Footnotes