Diborane

Diborane(6), generally known as diborane, is the chemical compound with the formula B₂H₆. It is a toxic, colorless, and pyrophoric gas with a repulsively sweet odor. Diborane is a key boron compound with a variety of applications. It has attracted wide attention for its electronic structure. Several of its derivatives are useful reagents.

Structure and bonding

The structure of diborane has D_2h symmetry. Four hydrides are terminal, while two bridge between the boron centers. The lengths of the B–H_bridge bonds and the B–H_terminal bonds are 1.33 and 1.19 Å respectively. This difference in bond lengths reflects the difference in their strengths, the B–H_bridge bonds being relatively weaker. The weakness of the B–H_bridge compared to B–H_terminal bonds is indicated by their vibrational signatures in the infrared spectrum, being ≈2100 and 2500 cm⁻¹ respectively.

The model determined by molecular orbital theory describes the bonds between boron and the terminal hydrogen atoms as conventional 2-center 2-electron covalent bonds. The bonding between the boron atoms and the bridging hydrogen atoms is, however, different from that in molecules such as hydrocarbons. Each boron uses two electrons in bonding to the terminal hydrogen atoms and has one valence electron remaining for additional bonding. The bridging hydrogen atoms provide one electron each. The B₂H₂ ring is held together by four electrons forming two 3-center 2-electron bonds. This type of bond is sometimes called a "banana bond".

B₂H₆ is isoelectronic with C₂H₆²⁺, which would arise from the diprotonation of the planar molecule ethylene. Diborane is one of many compounds with such unusual bonding.

Of the other elements in group IIIA, gallium is known to form a similar compound digallane, Ga₂H₆. Aluminium forms a polymeric hydride, (AlH₃)_''n''; although unstable, Al₂H₆ has been isolated in solid hydrogen and is isostructural with diborane.

Production and synthesis

Extensive studies of diborane have led to the development of multiple syntheses ways. Most preparations entail reactions of hydride donors with boron halides or alkoxides. The industrial synthesis of diborane involves the reduction of BF₃ by sodium hydride (NaH), lithium hydride (LiH) or lithium aluminium hydride (AlLiH₄):
: 8 BF₃ + 6 LiH → B₂H₆ + 6 LiBF₄

Two laboratory methods start from boron trichloride with lithium aluminium hydride or from boron trifluoride ether solution with sodium borohydride. Both methods result in as much as 30% yield:
: 4 BCl₃ + 3 LiAlH₄ → 2 B₂H₆ + 3 LiAlCl₄
: 4 BF₃ + 3 NaBH₄ → 2 B₂H₆ + 3 NaBF₄

Older methods entail the direct reaction of borohydride salts with a non-oxidizing acid, such as phosphoric acid or dilute sulfuric acid.
: 2 BH₄⁻ + 2 H⁺ → 2 H₂ + B₂H₆
Similarly, oxidation of borohydride salts has been demonstrated and remains convenient for small-scale preparations. For example, using iodine as an oxidizer:
: 2 + → 2 NaI + +
Another small-scale synthesis uses potassium hydroborate and phosphoric acid as starting materials.

Reactions

Diborane is a highly reactive and versatile reagent.

Air, water, oxygen

As a pyrophoric substance, diborane reacts exothermically with oxygen to form boron trioxide and water:
: 2 B₂H₆ + 6 O₂ → 2 B₂O₃ + 6 H₂O ( Δ''H''_r = −2035 k J/ mol = −73.47 kJ/ g)

Diborane reacts violently with water to form hydrogen and boric acid:
: B₂H₆ + 6 H₂O → 2 B(OH)₃ + 6 H₂ (Δ''H''_r = −466 kJ/mol = −16.82 kJ/ g)

Diborane also reacts with alcohols similarly. Methanol for example give hydrogen and trimethylborate:
: B₂H₆ + 6 MeOH → 2 B(OMe)₃ + 6 H₂

Lewis acidity

One dominating reaction pattern involves formation of adducts with Lewis bases. Often such initial adducts proceed rapidly to give other products. For example, borane-tetrahydrofuran, which often behaves as an equivalently to diborane, degrades to borate esters. Its adduct with dimethyl sulfide is an important reagent in organic synthesis.

With ammonia diborane forms the diammoniate of diborane, DADB with small quantities of ammonia borane as byproduct. The ratio depends on the conditions.

Hydroboration

In the hydroboration reaction, diborane also reacts readily with alkenes to form trialkylboranes. This reaction pattern is rather general and the resulting alkyl borates can be readily derivatized, e.g. to alcohols. Although early work on hydroboration relied on diborane, it has been replaced by borane dimethylsulfide, which is more safely handled.

Other

Pyrolysis of diborane gives hydrogen and diverse boron hydride clusters. For example, pentaborane