Borane

Trihydridoboron, also known as borane or borine, is an unstable and highly reactive molecule with the chemical formula . 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.

Structure and properties

BH₃ is a trigonal planar molecule with D_3h symmetry. The experimentally determined B–H bond length is 119  pm.

In the absence of other chemical species, it reacts with itself to form diborane. Thus, it is an intermediate in the preparation of diborane according to the reaction:
:BX₃ +BH₄⁻ → HBX₃⁻ + (BH₃) (X=F, Cl, Br, I)
:2 BH₃ → B₂H₆
The standard enthalpy of dimerization of BH₃ is estimated to be −170 kJ mol⁻¹.
The boron atom in BH₃ has 6 valence electrons. Consequently, it is a strong Lewis acid and reacts with any Lewis base ('L' in equation below) to form an adduct:
:BH₃ + L → L—BH₃
in which the base donates its lone pair, forming a dative covalent bond. Such compounds are thermodynamically stable, but may be easily oxidised in air. Solutions containing borane dimethylsulfide and borane–tetrahydrofuran are commercially available; in tetrahydrofuran a stabilising agent is added to prevent the THF from oxidising the borane.Hydrocarbon Chemistry, George A. Olah, Arpad Molner, 2d edition, 2003, Wiley-Blackwell A stability sequence for several common adducts of borane, estimated from spectroscopic and thermochemical data, is as follows:
: PF₃ < CO< Et₂O< Me₂O< C₄H₈O < C₄H₈S < Et₂S< Me₂S< Py < Me₃N< H⁻

BH₃ has some soft acid characteristics as sulfur donors form more stable complexes than do oxygen donors. Aqueous solutions of BH₃ are extremely unstable.
: + 3 → +

Reactions

Molecular BH₃ is believed to be a reaction intermediate in the pyrolysis of diborane to produce higher boranes:
:B₂H₆ ⇌ 2BH₃
:BH₃ +B₂H₆ → B₃H₇ +H₂ (rate determining step)
:BH₃ + B₃H₇ ⇌ B₄H₁₀
:B₂H₆ + B₃H₇ → BH₃ + B₄H₁₀
:::::⇌ B₅H₁₁ + H₂
Further steps give rise to successively higher boranes, with B₁₀H₁₄ as the most stable end product contaminated with polymeric materials, and a little B₂₀H₂₆.

Borane ammoniate, which is produced by a displacement reaction of other borane adducts, eliminates elemental hydrogen on heating to give borazine (HBNH)₃.

Borane adducts are widely used in organic synthesis for hydroboration, where BH₃ adds across the C=C bond in alkenes to give trialkylboranes:
:(THF)BH₃ + 3 CH₂=CHR → B(CH₂CH₂R)₃ + THF
This reaction is regioselective, Other borane derivatives can be used to give even higher regioselectivity. The product trialkylboranes can be converted to useful organic derivatives. With bulky alkenes one can prepare species such as BR₂sub>2, which are also useful reagents in more specialised applications. Borane dimethylsulfide which is more stable than borane–tetrahydrofuran may also be used.

Hydroboration can be coupled with oxidation to give the hydroboration-oxidation reaction. In this reaction, the boryl group in the generated organoborane is substituted with a hydroxyl group.

As a Lewis acid

Phosphine-boranes, with the formula R_3−nH_nPBH₃, are adducts of organophosphines and borane.

Borane(5) is the dihydrogen complex of borane. Its molecular formula is BH₅ or possibly BH₃(η²-H₂). It is only stable at very low temperatures and its existence is confirmed in very low temperature. Borane(5) and methanium (CH₅⁺) are isoelectronic.A Life of Magic Chemistry: Autobiographical Reflections Including Post-Nobel Prize Years and the Methanol Economy, 159p Its conjugate base is the borohydride anion.

See also

*Butterfly cluster compound

References

{{Hydrides by group