Ammonia borane

Chembox new
Name = Ammonia borane
ImageFile = Ammonia-borane-3D-balls.png ImageName =
IUPACName = Ammonia borane
OtherNames = Borazane
Section1 = Chembox Identifiers
SMILES = [NH3+] [BH3-]
CASNo = 13774-81-7
RTECS = none
Section2 = Chembox Properties
Formula = H₆NB
MolarMass = 30.86534 g/mol
Appearance = colorless, waxy solid
Density = 0.780 g/cm³
Solubility = good
Solvent = other solvents
SolubleOther = MeOH, Et₂O
MeltingPt = 104°C
BoilingPt = decomposes
pKb =
Section3 = Chembox Structure
Coordination = ethane-like
CrystalStruct = I4mm (orthorhombic, < 200K)(tetragonal, >200K)
Dipole = 5.2 D
Section7 = Chembox Hazards
ExternalMSDS =
MainHazards = flammable
FlashPt =
RPhrases = 5
SPhrases = 14-15-26-36/37/39
Section8 = Chembox Related
OtherCpds = NaBH₄,
B₃N₃H₆,
BH₃, C₂H₆

Ammonia borane (or borazane) is the chemical compound with the formula H₃NBH₃. The species is the most fundamental molecular boron-nitrogen compound and has attracted recent attention as a stable storage medium for hydrogen.

ynthesis

Reaction of diborane with ammonia mainly gives the diammoniate salt [H₂B(NH₃)₂] ⁺, (BH₄)⁻. Ammonia borane is the main product when an adduct of borane is employed in place of diborane:cite journal | last = Shore | first = S.G. |coauthors = Boeddeker, K.W. | title = Large Scale Synthesis of H₂B(NH₃)₂⁺BH₄⁻ and H₃NBH₃ | journal = Inorganic Chemistry | year = 1964 | volume = 3 | pages = pp. 914&ndash;15 | doi = 10.1021/ic50016a038] :BH₃(THF) + NH₃ → BH₃NH₃ + THF

Properties and structure

It can be described as an hydride of nitrogen and boron, but its behavior indicates that the H atoms attached to boron are hydridic and those attached to nitrogen are somewhat acidic. It is a solid, formally isoelectronic with ethane. Obviously its similarity to ethane is tenuous since borane-ammonia is a solid and ethane is a gas (m.p. −181 °C), their melting points differing by 284 °C. Understanding this difference has been the subject of an ongoing discussion for many years.

The B-N distance is 1.58(2) Å. The B-H and N-H distances are 1.15 and 0.96 Å, respectively.

The structure of the solid indicates a close association of the N"H" and the B"H" centers.citation | last = Klooster | first = W.T. | coauthors = Koetzle, T. F.; Siegbahn, P. E. M.; Richardson, T. B.; Crabtree, R. H. | title = Study of the N-H...H-B Dihydrogen Bond Including the Crystal Structure of BH₃NH₃ by Neutron Diffraction | journal = Journal of the American Chemical Society | year = 1999 | volume = 121 | pages = pp. 6337&ndash;6343 | doi =10.1021/ja9825332] citation | last = Boese | first = R | coauthors = Niederprüm, N.; Bläser, D. | title = Molecules in Natural Science and Medicine | editor = Maksic, Z.B., Eckert-Masic, M. | Publisher = E. Horwood | location = Chichester, England | year = 1992] 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.

Uses

Ammonia borane has been suggested as a source of hydrogen for use in motor vehicles as it can be made to release hydrogen on heating, being polymerized first to (NH₂BH₂)_n, and then to (NHBH)_n. [" [http://www.rsc.org/chemistryworld/Issues/2006/March/HydrogenOnBoard.asp Hydrogen gets on board] ", Maciej Gutowski and Tom Autrey, Royal Society of Chemistry]

Borane-ammonia finds some use in organic synthesis as an air-stable derivative of diborane.citation | last = Andrews | first = G.C. | contribution = Borane–Ammonia | title = Encyclopedia of Reagents for Organic Synthesis | editor-first = L. | editor-last = Paquette | year = 2004 | publisher = John Wiley & Sons | location = New York | doi = 10.1002/047084289]

References