Boranes

Generic formula of boranes

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

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.

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⁻ octahydridopentaborate(1−)

Optionally closo− nido− etc. can be added:

B₅H₉, nido−pentaborane(9)

B₄H₁₀, arachno−tetraborane(10)

B₆H₆²⁻, hexahydrido−closo−hexaborate(2−)

Understandably many of the compounds have abbreviated common names.

Cluster types

The geometries of boron clusters are related. Generally they are described as (i) deltahedra, polyhedra with triangular faces or (ii) those same deltahedra with one or more vertices missing. Common deltahedra that are found in boron-hydride clusters are shown below.

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.

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 deltahedron cluster, with 2n+2 framework electrons e.g., icosahedral B₁₂H₁₂²⁻

nido- (from the Latin for "nest") B occupies n vertices of an n+1 deltahedron with 2n+4 framework electrons, e.g., B₅H₉ an octahedron missing 1 vertex

arachno- (from the Greek for "spiders web") B occupies n vertices of an n+3 deltahedron with 2n+6 framework electrons e.g. B₄H₁₀ an octahedron missing 2 vertices

hypho- (from the Greek for "net") B occupies n vertices of an n+2 deltahedron, e.g., B₈H₁₆ which is trigonal prismatic

klado- (from the Greek for "branch")

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. In the 1950s and early 1960s, the US and USSR investigated boron hydrides as high energy density jet fuel ("zip fuel") (ethylboranes, for example) for high speed aircraft, 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.