Ammonia borane

 ( Inorganic Compounds ) 

Ammonia borane (also systematically named ammoniotrihydroborate), also called borazane, is the chemical compound with the formula . The colourless or white solid is the simplest molecular boron-nitrogen-hydride compound. It has attracted attention as a source for hydrogen fuel, but is otherwise primarily of academic interest.

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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.

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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 hydride (negatively charged) and those attached to nitrogen are acidic (positively charged).

The structure of the solid indicates a close association of the N H and the B H 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.

R. Boese (1992). 9780135615980, Ellis Horwood. ISBN 9780135615980

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.

+Comparison of bond lengths in simple boron-nitrogen hydrides

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Uses 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.

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Analogous amine-boranes Many analogues have been prepared from primary, secondary, and even :

• 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.

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See also

• Tert-butylamine borane ( tBuNH₂→BH₃)
• Phosphine-borane ()
• Borane dimethylsulfide ()
• Borane–tetrahydrofuran ()

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