Aluminium hydride

Aluminium hydride (also known as alane and alumane) refers to a collection of inorganic compounds with the formula AlH₃. As a gas, alane is a planar molecule. When generated in ether solutions, it exists as an ether adduct. Solutions of alane polymerizes to a solid, which exists in several crystallographically distinguishable forms.^[1]

Aluminium hydride

Unit cell spacefill model of aluminium hydride
Names
Preferred IUPAC name Aluminium hydride
Systematic IUPAC name Alumane
Other names Alane Aluminic hydride Aluminium(III) hydride Aluminium trihydride Trihydridoaluminium
Identifiers
CAS Number	7784-21-6 Y
3D model (JSmol)	Interactive image
ChEBI	CHEBI:30136 Y
ChemSpider	13833 Y 17625618 (3H3) Y
ECHA InfoCard	100.029.139
Gmelin Reference	245
PubChem CID	14488 14399066 (2H3) 16721258 (3H3)
UNII	KZJ3T010RQ Y
CompTox Dashboard (EPA)	DTXSID80895037
InChI InChI=1S/Al.3H Y Key: AZDRQVAHHNSJOQ-UHFFFAOYSA-N Y InChI=1S/Al.3H Key: AZDRQVAHHNSJOQ-UHFFFAOYSA-N InChI=1/Al.3H/rAlH3/h1H3 Key: AZDRQVAHHNSJOQ-FSBNLZEDAV
SMILES [AlH3]
Properties
Chemical formula	AlH3
Molar mass	30.006 g·mol−1
Appearance	white crystalline solid, non-volatile, highly polymerized, needle-like crystals
Density	1.477 g/cm3, solid
Melting point	150 °C (302 °F; 423 K) starts decomposing at 105 °C (221 °F)
Solubility in water	reacts
Solubility	soluble in ether reacts in ethanol
Thermochemistry
Heat capacity (C)	40.2 J/(mol·K)
Std molar entropy (S⦵298)	30 J/(mol·K)
Std enthalpy of formation (ΔfH⦵298)	−11.4 kJ/mol
Gibbs free energy (ΔfG⦵)	46.4 kJ/mol
Related compounds
Related compounds	Lithium aluminium hydride, diborane
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa). Y verify (what is YN ?) Infobox references

Structure

Alane can adopt 3-, 4-, or 6-coordination, depending on conditions.

Gaseous alane

Monomeric AlH₃ has been isolated at low temperature in a solid noble gas matrix. It was shown to be planar.^[2] The dimeric form, Al₂H₆, has been isolated in solid hydrogen. It is isostructural with diborane (B₂H₆) and digallane (Ga₂H₆).^[3][4][5]

Solid alane

Solid alane, which is colorless and nonvolatile, precipitates from etherial solutions over the course of hours at room temperature. Numerous polymorphs can be obtained, which have been labeled α-, α’-, β-, γ-, ε-, and ζ-alanes.^[6] The best characterized solid alane is α-alane. According to X-ray crystallography, adopts a cubic or rhombohedral morphology. It features octahedral AlH6 centers interconnected by Al-H-Al bridges. The Al-H distances are all equivalent (172 pm) and the Al-H-Al angles are 141°.^[7] α’-Alane forms needle-like crystals, and γ-alane forms bundles of fused needles.^{[citation needed]}

Crystallographic Structure of α-AlH3[8]
The α-AlH3 unit cell	Aluminium coordination	Hydrogen coordination

Handling

Alane is not spontaneously flammable.^[9] Even so, it should be handled like Li[AlH₄] (the chemical reagent, lithium aluminium hydride) as its reactivity is comparable to this related reducing reagent.^[1] For these reagents, both preparations in solutions and isolated solids are "highly flammable and must be stored in the absence of moisture".^[10] Laboratory guides recommend alane use inside a fume hood.^[1][why?] Solids of this reagent type carry recommendations of handling "in a glove bag or dry box".^[10] After use, solution containers are typically sealed tightly with concomitant flushing with inert gas to exclude the oxygen and moisture of ambient air.^[10]

Passivation^{[clarification needed]} greatly diminishes the decomposition rate associated with alane preparations.^{[citation needed]} Passivated alane nevertheless retains a hazard classification of 4.3 (chemicals which in contact with water, emit flammable gases).^[11]

Reported accidents

Alane reductions are believed to proceed via an intermediate coordination complex, with aluminum attached to the partially reduced functional group, and liberated when the reaction undergoes protic quenching. If the substrate is also fluorinated, the intermediate may explode if exposed to a hot spot above 60°C.^[12]

In 1964, a lab explosion was reported during the reduction of perfluorosuccinimide using lithium aluminum hydride, which produces similar aluminum hydride complexes. Following the explosion, the lab tested various LAH complexes containing perfluorinated substrates, each of which were found to be explosive.^[13]

In 2020, a lab reported an explosion when cold methanol was used to quench a solution containing alane complexed with a perfluorinated nitrile. ^[12]

Preparation

Many different Synthesis processes of Aluminium hydride uses Li[AlH₄] which can be synthesized by the following process under high pressure and temperature in step 1 and a salt metathesis reaction in step 2:

[Step 1] Na + Al + 2H₂ → Na[AlH₄]

[Step 2] Na[AlH₄] + LiCl → Li[AlH₄] + NaCl

Aluminium hydrides and various complexes thereof have long been known.^[14] Its first synthesis was published in 1947, and a patent for the synthesis was assigned in 1999.^[15][16] Aluminium hydride is prepared by treating lithium aluminium hydride with aluminium trichloride.^[17] The procedure is intricate: attention must be given to the removal of lithium chloride.

3 Li[AlH₄] + AlCl₃ → 4 AlH₃ + 3 LiCl

The ether solution of alane requires immediate use, because polymeric material rapidly precipitates as a solid. Aluminium hydride solutions are known to degrade after 3 days. Aluminium hydride is more reactive than Li[AlH₄].^[18]

Several other methods exist for the preparation of aluminium hydride:

2 Li[AlH₄] + BeCl₂ → 2 AlH₃ + Li₂[BeH₂Cl₂]

2 Li[AlH₄] + H₂SO₄ → 2 AlH₃ + Li₂SO₄ + 2 H₂

2 Li[AlH₄] + ZnCl₂ → 2 AlH₃ + 2 LiCl + ZnH₂

2 Li[AlH₄] + I₂ → 2 AlH₃ + 2 LiI + H₂

2 Li[AlH₄] + CH₃COOH → 2 AlH₃ + Li[CH₃CH₂O] + LiOH

Electrochemical synthesis

Several groups have shown that alane can be produced electrochemically.^{[19][20][21][22][23]} Different electrochemical alane production methods have been patented.^[24][25] Electrochemically generating alane avoids chloride impurities. Two possible mechanisms are discussed for the formation of alane in Clasen's electrochemical cell containing THF as the solvent, sodium aluminium hydride as the electrolyte, an aluminium anode, and an iron (Fe) wire submerged in mercury (Hg) as the cathode. The sodium forms an amalgam with the Hg cathode preventing side reactions and the hydrogen produced in the first reaction could be captured and reacted back with the sodium mercury amalgam to produce sodium hydride. Clasen's system results in no loss of starting material. For insoluble anodes, reaction 1 occurs, while for soluble anodes, anodic dissolution is expected according to reaction 2:

1. [AlH₄]⁻ − e⁻ + n THF → AlH₃·nTHF + 1/₂ H₂
2. 3 [AlH₄]⁻ + Al − 3 e⁻ + 4n THF → 4 AlH₃·nTHF

In reaction 2, the aluminium anode is consumed, limiting the production of aluminium hydride for a given electrochemical cell.

The crystallization and recovery of aluminium hydride from electrochemically generated alane has been demonstrated.^[22][23]

High pressure hydrogenation of aluminium

α-AlH₃ can be produced by hydrogenation of aluminium at 10 GPa and 600 °C (1,112 °F). The reaction between the liquified hydrogen produces α-AlH₃ which could be recovered under ambient conditions.^[26]

Reactions

Formation of adducts with Lewis bases

AlH₃ readily forms adducts with strong Lewis bases. For example, both 1:1 and 1:2 complexes form with trimethylamine. The 1:1 complex is tetrahedral in the gas phase,^[27] but in the solid phase it is dimeric with bridging hydrogen centres, [(CH₃)₃NAlH₂(μ-H)]₂.^[28] The 1:2 complex adopts a trigonal bipyramidal structure.^[27] Some adducts (e.g. dimethylethylamine alane, (CH₃CH₂)(CH₃)₂N·AlH₃) thermally decompose to give aluminium and may have use in MOCVD applications.^[29]

Its complex with diethyl ether forms according to the following stoichiometry:

AlH₃ + (CH₃CH₂)₂O → (CH₃CH₂)₂O·AlH₃

Similar adducts are assumed to form when alane is generated in THF from lithium aluminium hydride.

The reaction with lithium hydride in ether produces lithium aluminium hydride:

AlH₃ + LiH → Li[AlH₄]

Various alanates have been characterized beyond lithium aluminium hydride. They tend to feature five- and six-coordinate Al centers: Na
₃AlH
₆, Ca(AlH
₄₎)
₂, SrAlH
₅).^[30]

Reduction of functional groups

Alane and its derivatives are reducing reagents in organic synthesis based around group 13 hydrides.^[31] In solution—typically in ethereal solvents such tetrahydrofuran or diethyl ether—aluminium hydride forms complexes with Lewis bases, and reacts selectively with particular organic functional groups (e.g., with carboxylic acids and esters over organic halides and nitro groups), and although it is not a reagent of choice, it can react with carbon-carbon multiple bonds (i.e., through hydroalumination). Given its density, and with hydrogen content on the order of 10% by weight,^[6] some forms of alane are, as of 2016,^[32] active candidates for storing hydrogen and so for power generation in fuel cell applications, including electric vehicles.^{[not verified in body]} As of 2006 it was noted that further research was required to identify an efficient, economical way to reverse the process, regenerating alane from spent aluminium product.

In organic chemistry, aluminium hydride is mainly used for the reduction of functional groups.^[33] In many ways, the reactivity of aluminium hydride is similar to that of lithium aluminium hydride. Aluminium hydride will reduce aldehydes, ketones, carboxylic acids, anhydrides, acid chlorides, esters, and lactones to their corresponding alcohols. Amides, nitriles, and oximes are reduced to their corresponding amines.

In terms of functional group selectivity, alane differs from other hydride reagents. For example, in the following cyclohexanone reduction, lithium aluminium hydride gives a trans:cis ratio of 1.9 : 1, whereas aluminium hydride gives a trans:cis ratio of 7.3 : 1.^[34]

Stereoselective reduction of a substituted cyclohexanone using aluminium hydride

Alane enables the hydroxymethylation of certain ketones (that is the replacement of C−H by C−CH₂OH at the alpha position).^[35] The ketone itself is not reduced as it is "protected" as its enolate.

Functional Group Reduction using aluminium hydride

Organohalides are reduced slowly or not at all by aluminium hydride. Therefore, reactive functional groups such as carboxylic acids can be reduced in the presence of halides.^[1]

Functional Group Reduction using aluminium hydride

Aluminium hydride reduces ester in the presence of nitro groups.^[1]

Ester reduction using aluminium hydride

Aluminium hydride reduces acetals to half protected diols.^[1]

Acetal reduction using aluminium hydride

Aluminium hydride reduces epoxide to the corresponding alcohol:^[1]

Epoxide reduction using aluminium hydride

The allylic rearrangement reaction carried out using aluminium hydride is a S_N2 reaction, and it is not sterically demanding:^[1]

Phosphine reduction using aluminium hydride

Aluminium hydride will reduce carbon dioxide to methane with heating:^{[citation needed]}

4 AlH₃ + 3 CO₂ → 3 CH₄ + 2 Al₂O₃

Hydroalumination

See also: Carbometalation § Carboalumination

Akin to hydroboration, aluminium hydride can, in the presence of titanium tetrachloride, add across multiple bonds.^[36][37] When the multiple bond in question is a propargylic alcohols, the results are Alkenylaluminium compounds.^[38]

Hydroalumination of 1-hexene

Fuel

In its passivated form, alane is an active candidate for storing hydrogen, and can be used for efficient power generation via fuel cell applications, including fuel cell and electric vehicles and other lightweight power applications.^[39] AlH₃ contains up 10.1% hydrogen by weight (at a density of 1.48 grams per milliliter),^[6] or twice the hydrogen density of liquid H₂.^{[citation needed]} As of 2006, AlH₃ was described as a candidate for which "further research w[ould] be required to develop an efficient and economical process to regenerate [it] from the spent Al powder".^{[6][needs update]}

Alane is also a potential additive to solid rocket fuel and to explosive and pyrotechnic compositions ^{[citation needed]} due to its high hydrogen content and low dehydrogenation temperature.^[39] In its unpassivated form, alane is also a promising rocket fuel additive, capable of delivering impulse efficiency gains of up to 10%.^[40] However, AlH₃ can degrade when stored at room temperature, and some of its crystal forms have "poor compatibility" with some fuel components.^[39]

Deposition

Heated alane releases hydrogen gas and produces a very fine thin film of aluminum metal.^[41]