Borane

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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
hydrolyses
Solubility in Ammonia 3.2 mol L−1
Thermochemistry
187.88 kJ mol−1 K−1
106.69 kJ mol−1
Structure
D3h
trigonal planar
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

Borane (systematically named trihydridoboron), also called borine, is an inorganic compound with the chemical formula BH
3
. It is a colourless gas that cannot be concentrated in pure form. Borane is both the simplest member of the boranes, and the prototype of the monoboranes.

Structure and properties[edit]

Structure[edit]

The structure of BH3 is trigonal planar (D3h molecular symmetry) with an experimentally determined B–H bond length of 119 pm.[1] This the same as the terminal B–H bond length in diborane(6).[citation needed] When occurring in an adduct, the geometry of borane is drastically altered to a degree determined by the nature of the lewis base. Alterations include the deformation into a triangular pyramidal shape due to the boron centre being tetrahedrally coordinated, as well as a change in bond length. For this reason, borane is highly polarisable.

Solute properties[edit]

Borane is an amphiphilic (nonpolar) solute. While displaying a preference for solvents with low relative static permittivity (dielectric constants), it dissolves not only in nonpolar compounds, but also in polar compounds such as dimethyl sulfide and tetrahydrofuran. Borane has a solubility in liquid ammonia of 3.2 mol L−1, precipitating out as the monoammoniate (BH
3
•NH
3
). Its solubility in dimethyl sulfide and pyridine is 12.9 and 11.7 mol L−1, respectively. In aprotic coordinating solvents, the following equilibrium exists:

2 [BH
3
L]
[B
2
H
6
]
+ 2 L

Therefore, when selecting an appropriate solvent for a particular reaction, it is important to consider a solvent that will shift this equilibrium as far as possible towards the borane-solvent complex on the left, without significantly interfering with the reaction.

When desolvating borane, it is necessary to maintain a low concentration, due to its propensity to dimerise. One such known method involves thermolysing the solvate in a carrier gas at reduced pressure. Many solvates cannot be thermally desolvated, but instead undergo molecular rearrangement, eliminating elemental hydrogen.

THF and DMS solutions are commercially available, although the former requires a stabilising agent to prevent the THF from oxidising the borane.[2]

Amphotericity[edit]

As borane is an electron-deficient compound, its dominant behaviour is to dimerise to form diborane, with a reaction enthalpy which calculated to be near -170 kJ mol−1.[3]

2 [BH
3
]
[B
2
H
6
]

The boranyl group (−BH
2
) in borane can incorporate an electron pair from a Lewis base into the molecule by adduction:

[BH
3
]
+ :L → [BH
3
L]

Because of this capture of the Lewis base (L), borane has Lewis acidic character. Borane is a strong Lewis acid, able to capture one Lewis base. A stability sequence for several common adducts of borane, which was estimated spectroscopically and thermochemically is as follows:-[4]

PF3 < CO < Et2O < Me2O < C4H8O < C4H8S < Et2S < Me2S < Py < Me3N < H

BH3 has some soft acid characteristics (sulfur donors are more stable than oxygen donors).[4]

The boranyl group can also incorporate a hydrogen cation into the molecule by dissociative protonation:

BH
3
+ H+
BH+
2
+ H
2

Because of this capture of the proton (H+
), borane and its adducts also has basic character. They are weakly basic, able to absorb one proton. Borane's conjugate acid is boranylium (BH+
2
). An aqueous solution contains only a portion of borane moeities that are protonated.

BH
3
+ H
3
O+
BH+
2
+ H
2
O
+ H
2

However, aqueous solutions are extremely unstable due to rapid hydrolysis of both the borane moeity and boranylium cation. The latter reaction readily occurs even at a temperature of −78 °C (−108 °F):[5]

BH
3
+ 3 H
2
O
B(OH)
3
+ 3 H
2
[6]
BH+
2
+ 4 H
2
O
B(OH)
3
+ H
3
O+
+ 2 H
2
BH+
2
+ 2 H
2
O
+ HO
B(OH)
3
+ 2 H
2

From the second reaction, it can be seen that borane hydrolysis is catalysed by acidic conditions. Protic solvents containing an oxygen-hydrogen bond tend to oxidise the boranylium ion and borane moeity in the same way, yielding borate esters instead of boric acid. In other protic solvents, the ion may be stable towards oxidisation. For example, the primary product of the reaction of diborane with ammonia is boranylium boranuide diammoniate, which is isolatable.

Production[edit]

Unadducted borane can be produced as a minor product alongside diborane from the reaction of laser ablated atomic boron with hydrogen.[7] Studies of gas phase diborane have detected monomeric BH3.

Diborane symmetric cleavage[edit]

Borane can be made for use in chemical research is made by the symmetric cleavage of diborane. In symmetric cleavage, diborane and a soft Lewis base react to produce a monoborane adduct according to the equation:

[B
2
H
6
]
+ 2 L → 2 [BH
3
L]
, (L = Lewis base)

The process involves no other compounds as intermediates, and occurs in a single step. No catalyst is needed for the symmetric cleaving. The most common Lewis base used is tetrahydrofuran.

Tetrahydridoborate oxidation[edit]

Borane may be prepared by the partial oxidation of a tetrahydridoborate salt. In this process, a tetrahydridoborate salt and a mild oxidising agent such as trifluoroborane react to produce borane according to the equation:

4 BF
3
+ 3 BH
4
3 BF
4
+ 4 BH
3

This process involves diborane as an intermediate, and occurs in two steps. A strongly-coordinating aprotic Lewis base is required for the symmetric cleaving (step 2).

  1. 4 BF
    3
    + 3 BH
    4
    3 BF
    4
    + 2 [B
    2
    H
    6
    ]
  2. [B
    2
    H
    6
    ]
    + 2 L → 2 [BH
    3
    L]
    , (L = Lewis base)

This is the preferred process to produce the borane-dimethyl sulfide adduct. By using iodine as the oxidant, it is possible to eliminate the production of diborane as an intermediate, as done in the production of the THF adduct. By using a protonated amines, it is possible to produce an adduct where the deprotonated amine plays the role of the Lewis base.

BH
4
+ NR
3
H+
[BH
3
(NR
3
)] + H
2
, (R = organic group or hydrogen atom)

This process is commonly used in the production of the ammonia and pyridine adducts.

Trifluoroborane reduction[edit]

Borane may also be prepared by the reduction of trifluoroborane. In this process, trifluoroborane and a hydride salt react to produce borane according to the equation:

4 BF
3
+ 3 H
3 BF
4
+ BH
3

Like tetrahydridoborate oxidation, a strongly-coordinating aprotic Lewis base is required for the symmetric cleaving (step 2).

  1. 8 BF
    3
    + 6 H
    6 BF
    4
    + [B
    2
    H
    6
    ]
  2. [B
    2
    H
    6
    ]
    + 2 L → 2 [BH
    3
    L]
    , (L = Lewis base)

Reactions[edit]

Inorganic chemistry[edit]

Upon treatment with a lewis base, borane converts to an adduct. Upon treatment with a standard acid, borane and its adducts convert to a boranyl derivative and elemental hydrogen.[5] Oxidation of borane gives boric acid. Since borane dimerises, reactions requiring borane as opposed to diborane(6) must be carried out using a borane adduct.

Borane ammoniate, which is produced by a displacement reaction of other borane adducts, eliminates elemental hydrogen on heating to give borazine (HBNH)3.[8]

Molecular BH3 is believed to be a reaction intermediate is 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.

Reactions with organic compounds[edit]

Borane is commonly featured as a reagent in hydroboration, in which an organoborane is generated by the syn-addition of boron and one of the hydrogen atoms across a carbon multiple bond of another molecule.[9] This reaction is rapid, quantitative and reversible with alkenes. As borane is electrophilic in nature, hydroboration typically yields anti-Markovnikov products. Borane derivatives can be used to give even higher regioselectivity.[9]

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'.

Uses[edit]

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, and 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 the THF adduct of borane adduct.[10]

History[edit]

In 1937, the discovery of carbonyltrihydridoboron, the adduct of borane with carbon monoxide, among other borane adducts, played an important role in exploring the chemistry of "normal" boranes at a time when three-centre two-electron bonding was not yet known.[11] This discovery also implied the existence of borane, however, it was not until some years later that direct evidence was observed.

References[edit]

  1. ^ 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. 
  2. ^ Hydrocarbon Chemistry, George A. Olah, Arpad Molner, 2d edition, 2003, Wiley-Blackwell ISBN 978-0471417828
  3. ^ 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
  4. ^ a b c Greenwood, Norman N.; Earnshaw, Alan (1997). Chemistry of the Elements (2nd ed.). Butterworth-Heinemann. ISBN 0-08-037941-9. 
  5. ^ a b Finn, Patricia.; Jolly, William L. (August 1972). "Asymmetric cleavage of diborane by water. The structure of diborane dihydrate". Inorganic Chemistry (PDF). ACS Publications. 11 (8): 1941–1944. doi:10.1021/ic50114a043. 
  6. ^ D'Ulivo, Alessandro (May 2010). "Mechanism of generation of volatile species by aqueous boranes". Spectrochimica Acta Part B: Atomic Spectroscopy. Elsevier B.V. 65 (5): 360–375. doi:10.1016/j.sab.2010.04.010. 
  7. ^ 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. 
  8. ^ 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. 
  9. ^ a b Burkhardt, Elizabeth R.; Matos, Karl (July 2006). "Boron reagents in process chemistry: Excellent tools for selective reductions". Chemical Reviews. ACS Publications. 106 (7): 2617–2650. doi:10.1021/cr0406918. 
  10. ^ Kollonitisch, J., "Reductive Ring Cleavage of Tetrahydrofurans by Diborane", J. Am. Chem. Soc. 1961, volume 83, 1515. doi: 10.1021/ja01467a056
  11. ^ Burg, Anton B.; Schlesinger, H. I. (May 1937). "Hydrides of boron. VII. Evidence of the transitory existence of borine ({{Chem|BH|3}}): Borine carbonyl and borine trimethylammine". Journal of the American Chemical Society. 59 (5): 780–787. doi:10.1021/ja01284a002.