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]

Borane
Structural formula of 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
3D model (JSmol)
ChEBI
ChemSpider
44
Properties
BH3
Molar mass 13.83 g·mol−1
Appearance colourless gas
Conjugate acid Boronium
Thermochemistry
187.88 kJ mol−1 K−1
106.69 kJ mol−1
Structure
D3h
trigonal planar
0 D
Related compounds
Related compounds
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
Infobox references

Contents

Structure and propertiesEdit

BH3 is trigonal planar molecule with D3h 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
+ 3H2OB(OH)
3
+ 3 H
2

ReactionsEdit

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)Edit

Borane(5) is the dihydrogen complex of borane. Its molecular formula is BH5 or possibly BH32-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.

ReferencesEdit

  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. doi:10.1063/1.461942. ISSN 0021-9606.
  4. ^ a b c Greenwood, Norman N.; Earnshaw, Alan (1997). Chemistry of the Elements (2nd ed.). Butterworth-Heinemann. ISBN 0-08-037941-9.
  5. ^ M. Page, G.F. Adams, J.S. Binkley, C.F. Melius "Dimerization energy of borane" J. Phys. Chem. 1987, vol. 91, pp 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. ^ a b 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: 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 – via pubs.rsc.org.
  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 – via ACS Publications.
  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. doi:10.1063/1.468496 – via AIP Publishing.
  15. ^ A Life of Magic Chemistry: Autobiographical Reflections Including Post-Nobel Prize Years and the Methanol Economy, 159p