Aluminium hydride

Structure

Alane is a polymer. Hence, its formula is sometimes represented with the formula (AlH₃)_n. Alane forms numerous polymorphs, which are named α-alane, α’-alane, β-alane, γ-alane, δ-alane, ε-alane and ζ-alane. α-Alane has a cubic or rhombohedral morphology, whereas α’-alane forms needle-like crystals and γ-alane forms a bundle of fused needles. Alane is soluble in THF and ether. The rate of the precipitation of solid alane from ether varies with the preparation method.
The crystal structure of α-alane has been determined and features aluminium atoms surrounded by 6 hydrogen atoms that bridge to 6 other aluminium atoms. The Al-H distances are all equivalent (172pm) and the Al-H-Al angle is 141°.

α-Alane is the most thermally stable polymorph. β-alane and γ-alane are produced together, and convert to α-alane upon heating. δ, ε, and θ-alane are produced in still other crystallization conditions. Although they are less thermally stable, δ, ε, and θ polymorphs do not convert into α-alane upon heating.

Molecular forms of alane

Monomeric AlH₃ has been isolated at low temperature in a solid noble gas matrix and shown to be planar. The dimer Al₂H₆ has been isolated in solid hydrogen. It is isostructural with diborane (B₂H₆) and digallane (Ga₂H₆).

Preparation

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

3 LiAlH₄ + 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 LiAlH₄.

Several other methods exist for the preparation of aluminium hydride:

2 LiAlH₄ + BeCl₂ → 2 AlH₃ + Li₂BeH₂Cl₂2 LiAlH₄ + H₂SO₄ → 2 AlH₃ + Li₂SO₄ + 2 H₂2 LiAlH₄ + ZnCl₂ → 2 AlH₃ + 2 LiCl + ZnH₂

Electrochemical synthesis

Several groups have shown that alane can be produced electrochemically. Different electrochemical alane production methods have been patented. 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 an insoluble anode see reaction 1.

1. AlH₄⁻ - e⁻ → AlH₃ · nTHF + ½H₂ For soluble anodes, anodic dissolution is expected according to reaction 2,

2. 3AlH₄⁻ + Al - 3e⁻ → 4AlH₃ · 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 aluminum hydride from electrochemically generated alane has been demonstrated.

High pressure hydrogenation of aluminium metal

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

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, but in the solid phase it is dimeric with bridging hydrogen centres, (NMe₃Al(μ-H))₂. The 1:2 complex adopts a trigonal bipyramidal structure. Some adducts (e.g. dimethylethylamine alane, NMe₂Et · AlH₃) thermally decompose to give aluminium metal and may have use in MOCVD applications.

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

AlH₃ + (C₂H₅)₂O → H₃Al · O(C₂H₅)₂

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

AlH₃ + LiH → LiAlH₄

Reduction of functional groups

In organic chemistry, aluminium hydride is mainly used for the reduction of functional groups. 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.

Alane enables the hydroxymethylation of certain ketones, that is the replacement of C-H by C-CH₂OH). The ketone itself is not reduced as it is "protected" as its enolate.

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.

Nitro groups are not reduced by aluminium hydride. Likewise, aluminium hydride can accomplish the reduction of an ester in the presence of nitro groups.

Aluminium hydride can be used in the reduction of acetals to half protected diols.

Aluminium hydride can also be used in epoxide ring opening reaction as shown below.

The allylic rearrangement reaction carried out using aluminium hydride is a S_N2 reaction, and it is not sterically demanding.

Aluminium hydride even reduces carbon dioxide to methane under heating:

4 AlH₃ + 3 CO₂ → 3 CH₄ + 2 Al₂O₃

Hydroalumination

Aluminium hydride has been shown to add to propargylic alcohols. Used together with titanium tetrachloride, aluminium hydride can add across double bonds. Hydroboration is a similar reaction.

Fuel

Aluminium hydride have been discussed for storing hydrogen in hydrogen-fueled vehicles. AlH₃ contains up to 10% hydrogen by weight, corresponding to 148g/L, twice the Density of liquid H₂. Unfortunately, AlH₃ is not a reversible carrier of hydrogen. It is a potential additive to rocket fuel and in explosive and pyrotechnic compositions.

Precautions

Aluminium hydride is not spontaneously flammable, but it is highly reactive, similar to lithium aluminium hydride. Aluminium hydride decomposes in air and water. Violent reactions occur with both. With care AlH₃ can be handled safely in air, thought to be a result of a protective layer of aluminium oxide.