|3D model (Jmol)||Interactive image|
|Molar mass||30.865 g mol−1|
|Density||780 mg mL−1|
|Melting point||104 °C (219 °F; 377 K)|
|Tetragonal at B and N|
|Tetrahydral at B and N|
|S-phrases||S14, S15, S26, S36/37/39|
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
|what is ?)(|
Ammonia borane (also systematically named amminetrihydridoboron), also called borazane, is the chemical compound with the formula H3NBH3. The colourless or white solid is the simplest molecular boron-nitrogen-hydride compound. It has attracted attention as a medium for the chemical storage of hydrogen fuel. It has also been used in the laboratory as a precursor material for the chemical vapor deposition (CVD) of hexagonal boron nitride monolayers and thin films. The pyrolysis of ammonia borane between approximately 100°C - 150°C releases molecular hydrogen and gaseous boron-nitrogen compounds. These boron nitrogen compounds can be used for the CVD of hexagonal boron nitride but represent an impurity in the release of hydrogen intended for use as fuel in a fuel cell.
- BH3(THF) + NH3 → BH3NH3 + THF
Properties and structure
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.
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.
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 (NH2BH2)n, then to (NHBH)n, which ultimately decomposes to boron nitride (BN) at temperatures above 1000oC. It is more hydrogen-dense than liquid hydrogen and also able to exist at normal temperatures and pressures.
Many analogues have been prepared from primary, secondary, and even tertiary amines:
- Borane tert-butylamine (tBuNH2→BH3)
- Borane trimethylamine (Me3N→BH3)
- Borane isopropylamine (iPrNH2→BH3)
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.
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- Boese, R.; Niederprüm, N.; Bläser, D. (1992). Maksic, Z. B.; Eckert-Masic, M., eds. Molecules in Natural Science and Medicine. Chichester, England: Ellis Horwood. ISBN 978-0135615980.
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- 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.
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