Ammonia borane

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Ammonia borane
Ammonia-borane-from-xtal-3D-balls.png
Ammonia borane.png
Names
IUPAC name
Ammoniotrihydroborate
Other names
Borazane [1] [2] [3]
Identifiers
3D model (JSmol)
ChemSpider
ECHA InfoCard 100.170.890 OOjs UI icon edit-ltr-progressive.svg
EC Number
  • 642-983-4
PubChem CID
UNII
  • InChI=1S/BH6N/c1-2/h1-2H3 X mark.svgN
    Key: PJYXJGDRFASJSB-UHFFFAOYSA-N X mark.svgN
  • [BH3-][NH3+]
Properties
H3NBH3
Molar mass 30.87 g·mol−1
AppearanceColorless crystals
Density 0.78 g/cm3
Melting point 104 °C (219 °F; 377 K)
Structure
I4mm, tetragonal
Tetragonal at B and N
Tetrahedral at B and N
5.2 D
Hazards [4]
GHS labelling:
GHS-pictogram-flamme.svg GHS-pictogram-exclam.svg
Danger
H228, H302, H315, H319, H332, H335
P210, P240, P241, P261, P264, P264+P265, P270, P271, P280, P301+P317, P302+P352, P304+P340, P305+P351+P338, P317, P319, P321, P330, P332+P317, P337+P317, P362+P364, P370+P378, P403+P233, P405, P501
Related compounds
Related compounds
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
X mark.svgN  verify  (what is  Yes check.svgYX mark.svgN ?)

Ammonia borane (also systematically named ammoniotrihydroborate[ citation needed ]), 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 source of hydrogen fuel, but is otherwise primarily of academic interest.

Contents

Synthesis

Reaction of diborane with ammonia mainly gives the diammoniate salt [H2B(NH3)2]+ [BH4] (diammoniodihydroboronium tetrahydroborate). Ammonia borane is the main product when an adduct of borane is employed in place of diborane: [5]

BH3(THF) + NH3 → BH3NH3 + THF

It can also be synthesized from sodium borohydride. [6] [7] [8]

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). [9]

Resonance structures of ammonia-borane Ammonia-borane-resonance-2D.png
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. [10] [11] 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 Ammonia-borane-xtal-3D-balls.png
Part of the crystal structure of ammonia borane
Comparison of bond lengths in simple boron-nitrogen hydrides
MoleculeAmmonia borane [12] Aminoborane [13] Iminoborane [14]
FormulaBNH6BNH4BNH2
Class amine-borane aminoborane iminoborane
Analogous hydrocarbon ethane ethylene acetylene
Analogous hydrocarbon class alkane alkene alkyne
Structure Ammonia-borane-dimensions-MW-1983-2D.png Aminoborane-dimensions-MW-1987-2D.png Iminoborane-dimensions-IR-1987-2D.png
Ball-and-stick model Ammonia-borane-from-xtal-3D-balls.png Aminoborane-from-MW-1987-double-3D-balls.png Iminoborane-from-IR-1987-triple-3D-balls.png
Hybridisation of boron and nitrogensp3sp2sp
B-N bond length1.658 Å1.391 Å1.238 Å
Proportion of B-N single bond100%84%75%
Structure determination method microwave spectroscopy microwave spectroscopy infrared spectroscopy

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 (NH2BH2)n, then to (NHBH)n, [15] which ultimately decomposes to boron nitride (BN) at temperatures above 1000 °C. [16] It is more hydrogen-dense than liquid hydrogen and also able to exist at normal temperatures and pressures. [17]

Ammonia borane finds some use in organic synthesis as an air-stable derivative of diborane. [18] It can be used as a reducing agent in transfer hydrogenation reactions, often in the presence of a transition metal catalyst. [19]

Analogous amine-boranes

Many analogues have been prepared from primary, secondary, and even tertiary amines:

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. [8]

See also

Related Research Articles

<span class="mw-page-title-main">Boron hydride clusters</span>

Boron hydride clusters are compounds with the formula BxHy or related anions, where x ≥ 3. Many such cluster compounds are known. Common examples are those with 5, 10, and 12 boron atoms. Although they have few practical applications, the borane hydride clusters exhibit structures and bonding that differs strongly from the patterns seen in hydrocarbons. Hybrids of boranes and hydrocarbons, the carboranes are also well developed.

<span class="mw-page-title-main">Lewis acids and bases</span> Chemical bond theory

A Lewis acid (named for the American physical chemist Gilbert N. Lewis) is a chemical species that contains an empty orbital which is capable of accepting an electron pair from a Lewis base to form a Lewis adduct. A Lewis base, then, is any species that has a filled orbital containing an electron pair which is not involved in bonding but may form a dative bond with a Lewis acid to form a Lewis adduct. For example, NH3 is a Lewis base, because it can donate its lone pair of electrons. Trimethylborane () is a Lewis acid as it is capable of accepting a lone pair. In a Lewis adduct, the Lewis acid and base share an electron pair furnished by the Lewis base, forming a dative bond. In the context of a specific chemical reaction between NH3 and Me3B, a lone pair from NH3 will form a dative bond with the empty orbital of Me3B to form an adduct NH3•BMe3. The terminology refers to the contributions of Gilbert N. Lewis.

<span class="mw-page-title-main">Diborane</span> Chemical compound

Diborane(6), commonly known as diborane, is the chemical compound with the formula B2H6. It is a highly toxic, colorless, and pyrophoric gas with a repulsively sweet odor. Given its simple formula, borane is a fundamental boron compound. It has attracted wide attention for its electronic structure. Several of its derivatives are useful reagents.

<span class="mw-page-title-main">Hemiaminal</span> Organic compound or group with a hydroxyl and amine attached to the same carbon

In organic chemistry, a hemiaminal is a functional group or type of chemical compound that has a hydroxyl group and an amine attached to the same carbon atom: −C(OH)(NR2)−. R can be hydrogen or an alkyl group. Hemiaminals are intermediates in imine formation from an amine and a carbonyl by alkylimino-de-oxo-bisubstitution. Hemiaminals can be viewed as a blend of aminals and geminal diol. They are a special case of amino alcohols.

<span class="mw-page-title-main">Dihydrogen bond</span>

In chemistry, a dihydrogen bond is a kind of hydrogen bond, an interaction between a metal hydride bond and an OH or NH group or other proton donor. With a van der Waals radius of 1.2 Å, hydrogen atoms do not usually approach other hydrogen atoms closer than 2.4 Å. Close approaches near 1.8 Å, are, however, characteristic of dihydrogen bonding.

In organic chemistry, hydroboration refers to the addition of a hydrogen-boron bond to certain double and triple bonds involving carbon. This chemical reaction is useful in the organic synthesis of organic compounds.

<span class="mw-page-title-main">Sodium cyanoborohydride</span> Chemical compound

Sodium cyanoborohydride is a chemical compound with the formula Na[BH3(CN)]. It is a colourless salt used in organic synthesis for chemical reduction including that of imines and carbonyls. Sodium cyanoborohydride is a milder reductant than other conventional reducing agents.

<i>tert</i>-Butylamine Chemical compound

tert-Butylamine (also erbumine and other names) is an organic chemical compound with the formula (CH3)3CNH2. It is a colorless liquid with a typical amine-like odor. tert-Butylamine is one of the four isomeric amines of butane, the others being n-butylamine, sec-butylamine and isobutylamine.

<span class="mw-page-title-main">Catecholborane</span> Chemical compound

Catecholborane (abbreviated HBcat) is an organoboron compound that is useful in organic synthesis. This colourless liquid is a derivative of catechol and a borane, having the formula C6H4O2BH.

<span class="mw-page-title-main">Borane dimethylsulfide</span> Chemical compound

Borane dimethylsulfide (BMS) is a chemical compound with the chemical formula BH3·S(CH3)2. It is an adduct between borane molecule and dimethyl sulfide molecule. It is a complexed borane reagent that is used for hydroborations and reductions. The advantages of BMS over other borane reagents, such as borane-tetrahydrofuran, are its increased stability and higher solubility. BMS is commercially available at much higher concentrations than its tetrahydrofuran counterpart and does not require sodium borohydride as a stabilizer, which could result in undesired side reactions. In contrast, BH3·THF requires sodium borohydride to inhibit reduction of THF to tributyl borate. BMS is soluble in most aprotic solvents.

<span class="mw-page-title-main">Trimethylborane</span> Chemical compound

Trimethylborane (TMB) is a toxic, pyrophoric gas with the formula B(CH3)3 (which can also be written as Me3B, with Me representing methyl).

<span class="mw-page-title-main">Borane–tetrahydrofuran</span> Chemical compound

Borane–tetrahydrofuran is an adduct derived from borane and tetrahydrofuran (THF). These solutions, which are colorless, are used for reductions and hydroboration, reactions that are useful in synthesis of organic compounds. The use of borane–tetrahydrofuran has been displaced by borane–dimethylsulfide, which has a longer shelf life and effects similar transformations.

Dehydrogenation of amine-boranes or dehydrocoupling of amine-boranes is a chemical process in main group and organometallic chemistry wherein dihydrogen is released by the coupling of two or more amine-borane adducts. This process is of due to the potential of using amine-boranes for hydrogen storage.

Borane, also known as borine, is an unstable and highly reactive molecule with the chemical formula BH
3
. The preparation of borane carbonyl, BH3(CO), played an important role in exploring the chemistry of boranes, as it indicated the likely existence of the borane molecule. However, the molecular species BH3 is a very strong Lewis acid. Consequently, it is highly reactive and can only be observed directly as a continuously produced, transitory, product in a flow system or from the reaction of laser ablated atomic boron with hydrogen. It normally dimerizes to diborane in the absence of other chemicals.

<span class="mw-page-title-main">Borinic acid</span> Chemical compound

Borinic acid, also known as boronous acid, is an oxyacid of boron with formula H
2
BOH
. Borinate is the associated anion of borinic acid with formula H
2
BO
; however, being a Lewis acid, the form in basic solution is H
2
B(OH)
2
.

<span class="mw-page-title-main">1,2-Dimethyldiborane</span> Chemical compound

1,2-Dimethyldiborane is an organoboron compound with the formula [(CH3)BH2]2. Structurally, it is related to diborane, but with methyl groups replacing terminal hydrides on each boron. It is the dimer of methylborane, CH3BH2, the simplest alkylborane. 1,2-Dimethyldiborane can exist in a cis- and a trans arrangement. 1,2-Dimethyldiborane is an easily condensed, colorless gas that ignites spontaneously in air.

<span class="mw-page-title-main">Tetramethyldiborane</span> Chemical compound

Dimethylborane, (CH3)2BH is the simplest dialkylborane, consisting of a methyl group substituted for a hydrogen in borane. As for other boranes it normally exists in the form of a dimer called tetramethyldiborane or tetramethylbisborane or TMDB ((CH3)2BH)2. Other combinations of methylation occur on diborane, including monomethyldiborane, trimethyldiborane, 1,2-dimethylborane, 1,1-dimethylborane and trimethylborane. At room temperature the substance is at equilibrium between these forms. The methylboranes were first prepared by H. I. Schlesinger and A. O. Walker in the 1930s.

<span class="mw-page-title-main">Trimethyldiborane</span> Chemical compound

Trimethyldiborane, (CH3)3B2H3 is a molecule containing boron carbon and hydrogen. It is an alkylborane, consisting of three methyl group substituted for a hydrogen in diborane. It can be considered a mixed dimer: (CH3)2BH2BH(CH3) or dimethylborane and methylborane. called 1,2-dimethyldiborane. Other combinations of methylation occur on diborane, including monomethyldiborane, 1,2-dimethyldiborane, tetramethyldiborane, 1,1-dimethylborane and trimethylborane. At room temperature the substance is at equilibrium between these forms, so it is difficult to keep it pure. The methylboranes were first prepared by H. I. Schlesinger and A. O. Walker in the 1930s.

<span class="mw-page-title-main">Methyldiborane</span> Chemical compound

Methyldiborane, CH3B2H5, or monomethyldiborane is the simplest of alkyldiboranes, consisting of a methyl group substituted for a hydrogen in diborane. As with other boranes it exists in the form of a dimer with a twin hydrogen bridge that uses three-center two-electron bonding between the two boron atoms, and can be imagined as methyl borane (CH3BH2) bound to borane (BH3). Other combinations of methylation occur on diborane, including 1,1-dimethylborane, 1,2-dimethyldiborane, trimethyldiborane, tetramethyldiborane, and trimethylborane (which is not a dimer). At room temperature the substance is at equilibrium between these molecules.

<span class="mw-page-title-main">Boranes</span> Class of chemical compounds

A borane is a compound with the formula BRxHy although examples include multi-boron derivatives. A large family of boron hydride clusters is also known. In addition to some applications in organic chemistry, the boranes have attracted much attention as they exhibit structures and bonding that differs strongly from the patterns seen in hydrocarbons. Hybrids of boranes and hydrocarbons, the carboranes, are also a well developed class of compounds.

References

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