Ammonia borane

Synthesis

Reaction of diborane with ammonia mainly gives the diammoniate salt (diammoniodihydroboronium tetrahydroborate). Ammonia borane is the main product when an adduct of borane is employed in place of diborane: : 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.

Uses

Main article: Dehydrogenation of amine-boranes

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 , then to , 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 ()
• Borane trimethylamine ()
• Borane isopropylamine ()

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.

References

References

1. "Method for producing borazane".
2. (2015). "Decomposition processes of diborane and borazane (ammonia-borane complex) on hot wire surfaces". Thin Solid Films.
3. "Results for Borazane".
4. "Ammonia borane".
5. (1964). "Large Scale Synthesis of H₂B(NH₃)₂⁺BH₄⁻ and H₃NBH₃". Inorganic Chemistry.
6. (January 1977). "New synthetic approaches to ammonia-borane and its deuterated derivatives". Journal of Inorganic and Nuclear Chemistry.
7. (2014). "Ammonia-mediated, large-scale synthesis of ammonia borane". Dalton Trans..
8. (2007). "Room-Temperature Structure of Ammonia Borane". Australian Journal of Chemistry.
9. Klooster, W. T.. (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]].
10. Boese, R.. (1992). "Molecules in Natural Science and Medicine". Ellis Horwood.
11. (1983). "Microwave spectrum, torsional barrier, and structure of BH₃NH₃". [[The Journal of Chemical Physics.
12. (1987). "Microwave spectrum and molecular structure of aminoborane, BH₂NH₂". [[Journal of Molecular Spectroscopy.
13. (1987). "Detection of HBNH by infrared diode laser spectroscopy". [[The Journal of Chemical Physics.
14. (2006). "Features: Hydrogen gets onboard". Chemistry World.
15. (2011). "Pyrolytic Decomposition of Ammonia Borane to Boron Nitride". Inorganic Chemistry.
16. (2007). "Ammonia–Borane: The Hydrogen Source ''par excellence''?". Dalton Transactions.
17. (2008). "Borane–Ammonia". [[John Wiley & Sons]].
18. (August 2023). "Ammonia borane-enabled hydrogen transfer processes: Insights into catalytic strategies and mechanisms". Green Energy & Environment.