Chemistry:Ammonia borane

Ammonia borane

File:Ammonia borane.png
Names
IUPAC name Ammoniotrihydroborate
Other names Borazane
Identifiers
CAS Number	13774-81-7 Y
3D model (JSmol)	Interactive image
ChemSpider	371215 Y
EC Number	642-983-4
PubChem CID	419330
UNII	XR7O5A5MH6 Y
InChI InChI=1S/BH6N/c1-2/h1-2H3 N Key: PJYXJGDRFASJSB-UHFFFAOYSA-N N
SMILES [BH3-] [NH3+]
Properties
Chemical formula	H 3NBH 3
Molar mass	30.87 g·mol−1
Appearance	Colorless crystals
Density	0.78 g/cm3
Melting point	104 °C (219 °F; 377 K)
Structure
Crystal structure	I4mm, tetragonal
Coordination geometry	Tetragonal at B and N
Molecular shape	Tetrahedral at B and N
Dipole moment	5.2 D
Hazards
GHS pictograms
GHS Signal word	Danger
GHS hazard statements	HH228Category:GHS errors, HH302Category:GHS errors, HH315Category:GHS errors, HH319Category:GHS errors, HH332Category:GHS errors, HH335Category:GHS errors
GHS precautionary statements	PP210Category:GHS errors, PP240Category:GHS errors, PP241Category:GHS errors, PP261Category:GHS errors, PP264Category:GHS errors, PP264+P265Category:GHS errors, PP270Category:GHS errors, PP271Category:GHS errors, PP280Category:GHS errors, PP301+P317Category:GHS errors, PP302+P352Category:GHS errors, PP304+P340Category:GHS errors, PP305+P351+P338Category:GHS errors, PP317Category:GHS errors, PP319Category:GHS errors, PP321Category:GHS errors, PP330Category:GHS errors, PP332+P317Category:GHS errors, PP337+P317Category:GHS errors, PP362+P364Category:GHS errors, PP370+P378Category:GHS errors, PP403+P233Category:GHS errors, PP405Category:GHS errors, PP501Category:GHS errors
Related compounds
Related compounds	Sodium borohydride Borazine Borane Diborane Ethane
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
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Infobox references

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Ammonia borane (also systematically named ammoniotrihydroborate), also called borazane, is the chemical compound with the formula H
3NBH
3. The colourless or white solid is the simplest molecular boron-nitrogen-hydride compound. It has attracted attention as a source of hydrogen fuel, but is otherwise primarily of academic interest.

Synthesis

Reaction of diborane with ammonia mainly gives the diammoniate salt [H
2B(NH
3)
2]+
[BH
4]−
(diammoniodihydroboronium tetrahydroborate). Ammonia borane is the main product when an adduct of borane is employed in place of diborane:

BH
3(THF) + NH
3 → BH
3NH
3 + THF

It can also be synthesized from sodium borohydride.

Properties and structure

The molecule adopts a structure similar to that of ethane, with which it is isoelectronic. The B−N distance is 1.58(2) Å. The B−H and N−H distances are 1.15 and 0.96 Å, respectively. Its similarity to ethane is tenuous since ammonia borane is a solid and ethane is a gas: their melting points differing by 284 °C. This difference is consistent with the highly polar nature of ammonia borane. The H atoms attached to boron are hydridic (negatively charged) and those attached to nitrogen are acidic (positively charged).

The structure of the solid indicates a close association of the NH and the BH centers. The closest H−H distance is 1.990 Å, which can be compared with the H−H bonding distance of 0.74 Å. This interaction is called a dihydrogen bond. The original crystallographic analysis of this compound reversed the assignments of B and N. The updated structure was arrived at with improved data using the technique of neutron diffraction that allowed the hydrogen atoms to be located with greater precision.

Comparison of bond lengths in simple boron-nitrogen hydrides

Molecule	Ammonia borane	Aminoborane	Iminoborane
Formula	BNH 6	BNH 4	BNH 2
Class	amine-borane	aminoborane	iminoborane
Analogous hydrocarbon	ethane	ethylene	acetylene
Analogous hydrocarbon class	alkane	alkene	alkyne
Structure
Ball-and-stick model
Hybridisation of boron and nitrogen	sp3	sp2	sp
B-N bond length	1.658 Å	1.391 Å	1.238 Å
Proportion of B-N single bond	100%	84%	75%
Structure determination method	microwave spectroscopy	microwave spectroscopy	infrared spectroscopy

Uses

Ammonia borane has been suggested as a storage medium for hydrogen, e.g. for when the gas is used to fuel motor vehicles. It can be made to release hydrogen on heating, being polymerized first to (NH
2BH
2)
n, then to (NHBH)
n, which ultimately decomposes to boron nitride (BN) at temperatures above 1000 °C. It is more hydrogen-dense than liquid hydrogen and also able to exist at normal temperatures and pressures.

Ammonia borane finds some use in organic synthesis as an air-stable derivative of diborane. It can be used as a reducing agent in transfer hydrogenation reactions, often in the presence of a transition metal catalyst.

Analogous amine-boranes

Many analogues have been prepared from primary, secondary, and even tertiary amines:

• Borane tert-butylamine ((CH
  3)
  3C–NH
  2→BH
  3)
• Borane trimethylamine ((CH
  3)
  3N→BH
  3)
• Borane isopropylamine ((CH
  3)
  2CH–NH
  2→BH
  3)

The first amine adduct of borane was derived from trimethylamine. Borane tert-butylamine complex is prepared by the reaction of sodium borohydride with t-butylammonium chloride. Generally adduct are more robust with more basic amines. Variations are also possible for the boron component, although primary and secondary boranes are less common.

See also

• Phosphine-borane (R
  3P→BH
  3)
• borane dimethylsulfide ((CH
  3)
  2S→BH
  3)
• borane–tetrahydrofuran (THF→BH
  3)