Borane

Trihydridoboron, also known as borane or 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.[1] 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.[2]

Not to be confused with bornane.

This article is about a specific compound. For the class of compounds, see boranes.

Borane

Ball-and-stick model of borane Spacefill model of borane
Names
Systematic IUPAC name borane (substitutive) trihydridoboron (additive)
Other names borine boron trihydride
Identifiers
CAS Number	13283-31-3
3D model (JSmol)	Interactive image
ChEBI	CHEBI:30149
ChemSpider	6091
Gmelin Reference	44
PubChem CID	6331
InChI InChI=1S/BH3/h1H3 Key: UORVGPXVDQYIDP-UHFFFAOYSA-N
SMILES B
Properties
Chemical formula	BH3
Molar mass	13.83 g·mol^−1
Appearance	colourless gas
Conjugate acid	Boronium
Thermochemistry
Std molar entropy (S^o298)	187.88 kJ mol^−1 K^−1
Std enthalpy of formation (ΔfH^⦵298)	106.69 kJ mol^−1
Structure
Point group	D3h
Molecular shape	trigonal planar
Dipole moment	0 D
Related compounds
Related compounds	diborane
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
Infobox references

Structure and properties

BH₃ is trigonal planar molecule with D_3h symmetry The experimentally determined B–H bond length is 119 pm.[3]

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:[4]

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.[5] The boron atom in BH₃ has 6 valence electrons. Consequently it is a strong Lewis acid and reacts with any Lewis base, L 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.[6] 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.[4] Aqueous solutions of BH₃ are extremely unstable.[7][8]

BH
₃ + 3H
2O → B(OH)
₃ + 3 H
₂

Reactions

Molecular BH₃ is believed to be a reaction intermediate in the pyrolysis of diborane to produce higher boranes:[4]

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.[9]

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.[10] The product trialkylboranes can be converted to useful organic derivatives. With bulky alkenes one can prepare species such as [HBR₂]₂, which are also useful reagents in more specialised applications. Borane dimethylsulfide which is more stable than borane–tetrahydrofuran may also be used.[11][10]

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.

Reductive amination is an extension of the hydroboration-oxidation reaction, wherein a carbon–nitrogen double bond is undergoing hydroboration. The carbon–nitrogen double bond is created by the reductive elimination of water from a hemiaminal, formed by the addition of an amine to a carbonyl molecule, hence the adjective 'reductive'.

Borane(5)

Borane(5) is the dihydrogen complex of borane. Its molecular formula is BH₅ or possibly BH₃(η²-H₂).[12] It is only stable at very low temperatures and its existence is confirmed in very low temperature.[13][14] Borane(5) and methanium (CH₅⁺) are isoelectronic.[15] Its conjugate base is the borohydride anion.

References

1. Burg, Anton B.; Schlesinger, H. I. (May 1937). "Hydrides of boron. VII. Evidence of the transitory existence of borine (BH
   ₃): Borine carbonyl and borine trimethylammine". Journal of the American Chemical Society. 59 (5): 780–787. doi:10.1021/ja01284a002.
2. Tague, Thomas J.; Andrews, Lester (1994). "Reactions of Pulsed-Laser Evaporated Boron Atoms with Hydrogen. Infrared Spectra of Boron Hydride Intermediate Species in Solid Argon". Journal of the American Chemical Society. 116 (11): 4970–4976. doi:10.1021/ja00090a048. ISSN 0002-7863.
3. Kawaguchi, Kentarou (1992). "Fourier transform infrared spectroscopy of the BH₃ ν₃ band". The Journal of Chemical Physics. 96 (5): 3411. Bibcode:1992JChPh..96.3411K. doi:10.1063/1.461942. ISSN 0021-9606.
4. Greenwood, Norman N.; Earnshaw, Alan (1997). Chemistry of the Elements (2nd ed.). Butterworth-Heinemann. ISBN 978-0-08-037941-8.
5. Page, M.; Adams, G.F.; Binkley, J.S.; Melius, C.F. (1987). "Dimerization energy of borane". J. Phys. Chem. 91 (11): 2675–2678. doi:10.1021/j100295a001.
6. Hydrocarbon Chemistry, George A. Olah, Arpad Molner, 2d edition, 2003, Wiley-Blackwell ISBN 978-0471417828
7. Finn, Patricia; Jolly, William L. (August 1972). "Asymmetric cleavage of diborane by water. The structure of diborane dihydrate". Inorganic Chemistry. 11 (8): 1941–1944. doi:10.1021/ic50114a043.
8. D'Ulivo, Alessandro (May 2010). "Mechanism of generation of volatile species by aqueous boranes". Spectrochimica Acta Part B: Atomic Spectroscopy. 65 (5): 360–375. doi:10.1016/j.sab.2010.04.010.
9. Housecroft, C. E.; Sharpe, A. G. (2008). "Chapter 13: The Group 13 Elements". Inorganic Chemistry (3rd ed.). Pearson. p. 336. ISBN 978-0-13-175553-6.
10. Burkhardt, Elizabeth R.; Matos, Karl (July 2006). "Boron reagents in process chemistry: Excellent tools for selective reductions". Chemical Reviews. 106 (7): 2617–2650. doi:10.1021/cr0406918.
11. Kollonitisch, J. (1961). "Reductive Ring Cleavage of Tetrahydrofurans by Diborane". J. Am. Chem. Soc. 83 (6): 1515. doi:10.1021/ja01467a056.
12. Szieberth, Dénes; Szpisjak, Tamás; Turczel, Gábor; Könczöl, László (19 August 2014). "The stability of η2-H2 borane complexes – a theoretical investigation". Dalton Transactions. 43 (36): 13571–13577. doi:10.1039/C4DT00019F. PMID 25092548.
13. Tague, Thomas J.; Andrews, Lester (1 June 1994). "Reactions of Pulsed-Laser Evaporated Boron Atoms with Hydrogen. Infrared Spectra of Boron Hydride Intermediate Species in Solid Argon". Journal of the American Chemical Society. 116 (11): 4970–4976. doi:10.1021/ja00090a048.
14. Schreiner, Peter R.; Schaefer III, Henry F.; Schleyer, Paul von Ragué (1 June 1994). "The structure and stability of BH₅. Does correlation make it a stable molecule? Qualitative changes at high levels of theory". The Journal of Chemical Physics. 101 (9): 7625. Bibcode:1994JChPh.101.7625S. doi:10.1063/1.468496.
15. A Life of Magic Chemistry: Autobiographical Reflections Including Post-Nobel Prize Years and the Methanol Economy, 159p