Ammonia borane

Ammonia borane 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.

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Synthesis

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

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 NH and the BH centers.^[2] 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.^[3]

Borane-ammonia finds some use in organic synthesis as an air-stable derivative of diborane.^[4]

References

1. ^ Shore, S. G.; Boeddeker, K. W. “Large Scale Synthesis of H₂B(NH₃)₂⁺BH₄⁻ and H₃NBH₃. Inorganic Chemistry 1964, volume 3, pages 914-15. DOI: 10.1021/ic50016a038
2. ^ Klooster, W. T.; Koetzle, T. F.; Siegbahn, P. E. M.; Richardson, T. B.; Crabtree, R. H., "Study of the N-H...H-B Dihydrogen Bond Including the Crystal Structure of BH₃NH₃ by Neutron Diffraction", Journal of the American Chemical Society, 1999, volume 121, pages 6337-6343. DOI: 10.1021/ja9825332. Boese, R.; Niederprüm, N.; Bläser, D. In "Molecules in Natural Science and Medicine; Maksic, Z. B., Eckert-Masic, M., Eds.; E. Horwood: Chichester, England, 1992.
3. ^ "Hydrogen gets on board", Maciej Gutowski and Tom Autrey, Royal Society of Chemistry
4. ^ Andrews, G. C. "Borane–Ammonia" in Encyclopedia of Reagents for Organic Synthesis (Ed: L. Paquette) 2004, J. Wiley & Sons, New York. DOI: 10.1002/047084289.