Ammonia borane

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Ammonia borane
Ball and stick model of ammonia borane
Identifiers
CAS number 13774-81-7 N
PubChem 419330
ChemSpider 371215 YesY
Jmol-3D images Image 1
Properties
Molecular formula BNH
6
Molar mass 30.865 g mol−1
Appearance Colorless crystals
Density 780 mg mL−1
Melting point 104 °C (219 °F; 377 K)
Structure
Crystal structure I4mm, tetragonal
Coordination
geometry
Tetragonal at B and N
Molecular shape Tetrahydral at B and N
Dipole moment 5.2 D
Hazards
R-phrases R5
S-phrases S14, S15, S26, S36/37/39
Related compounds
Related compounds
Except where noted otherwise, data are given for materials in their standard state (at 25 °C (77 °F), 100 kPa)
 N (verify) (what is: YesY/N?)
Infobox references

Ammonia borane (or borazane) is the chemical compound with the formula H3NBH3. The colourless solid is the simplest molecular boron-nitrogen-hydride compound and has attracted attention as a source of hydrogen fuel.

Synthesis[edit]

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:[1]

BH3(THF) + NH3 → BH3NH3 + THF

Properties and structure[edit]

The molecule adopts a structure like 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 borane-ammonia 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 and those attached to nitrogen are somewhat acidic.

Resonance structures of ammonia-borane

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.[2][3] 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.

Part of the crystal structure of ammonia borane[2]

Uses[edit]

Ammonia borane has been suggested as a storage medium for hydrogen when the gas is used to fuel motor vehicles. It can be made to release hydrogen on heating, being polymerized first to (NH2BH2)n, then to (NHBH)n, [4] which ultimately decomposes to boron nitride (BN) at temperatures above 1000oC.[5] It is more hydrogen-dense than liquid hydrogen and also able to exist at normal temperatures and pressures.[6]

Ammonia borane finds some use in organic synthesis as an air-stable derivative of diborane.[7]

References[edit]

  1. ^ Shore, S. G.; Boddeker, K. W. (1964). "Large Scale Synthesis of H2B(NH3)2+BH4 and H3NBH3". Inorganic Chemistry 3 (6): 914–915. doi:10.1021/ic50016a038. 
  2. ^ a b Klooster, W. T.; Koetzle, T. F.; Siegbahn, P. E. M.; Richardson, T. B.; Crabtree, R. H. (1999). "Study of the N-H···H-B Dihydrogen Bond Including the Crystal Structure of BH3NH3 by Neutron Diffraction". Journal of the American Chemical Society 121 (27): 6337–6343. doi:10.1021/ja9825332. 
  3. ^ Boese, R.; Niederprüm, N.; Bläser, D. (1992). Maksic, Z. B.; Eckert-Masic, M., ed. Molecules in Natural Science and Medicine. Chichester, England: Ellis Horwood. ISBN 978-0135615980. 
  4. ^ Gutowski, M.; Autrey, T. (2006). "Features: Hydrogen gets onboard". Chemistry World 3 (3). 
  5. ^ Frueh, S.; Kellett, R.; Mallery, C.; Molter; T.; Willis, W. S.; King'ondu, C.; Suib, S. L. (2011). "Pyrolytic Decompostion of Ammonia Borane to Boron Nitride". Inorganic Chemistry 50 (3): 783–792. doi:10.1021/ic101020k. 
  6. ^ Stephens, F. H.; Pons, V.; Baker, R. T. (2007). "Ammonia–Borane: The Hydrogen Source par excellence?". Dalton Transactions 2007 (25): 2613–2626. doi:10.1039/b703053c. 
  7. ^ Andrews, G. C. (2004). "Borane–Ammonia". In Paquette, L. Encyclopedia of Reagents for Organic Synthesis. New York: John Wiley & Sons. doi:10.1002/047084289.