Aluminium hydride

Aluminium hydride, chemical formula AlH3, is a chemical reagent used as a reducing agent. It is used in hydroalumination of alkynes, allylic rearrangements, and storing hydrogen in hydrogen-fueled vehicles. . It is a colourless polymeric solid, (AlH3)n. The molecular AlH3 species are not stable. 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 and is isostructural with diborane.

History
Aluminium hydride was reported as impurities, amines and ether complexes throughout the history, until its first synthesis published in 1947 by Finholt, Bond, and Schlesinger from the George Herbert Jones Laboratory at University of Chicago. A U.S. patent for the synthesis was assigned to Petrie et al. in 1999 with the U.S. Pat. No. 6228338.

Structure and physical properties
Aluminium hydride is formed as numerous polymorphs: α-alane, α’-alane, β-alane, δ-alane, ε-alane, θ-alane, and γ-alane. α-alane has a cubic or rhombohedral morphology, while α’-alane forms needle like crystals and γ-alane forms a bundle of fused needles. Alane is soluble in THF and ether, and its precipitation rate from ether depends on the preparation method.

The structure of α-alane has been determined and contains 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°.

Chemical properties
α-Alane is the most thermally stable polymorph. β-alane and γ-alane are produced together, and will turn into α-alane upon heating. δ, ε, and θ-alane are produced in different crystallization condition. Though they are less thermally stable, they do not convert into α-alane upon heating.

AlH3 readily forms adducts with strong Lewis bases e.g. 1:1 and 1:2 complexes with trimethylamine. The 1:1 complex with trimethylamine is tetrahedral in the gas phase, but in the solid phase it is dimeric with bridging hydrogens, (NMe3Al(μ-H))2. The 1:2 complex has 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.

Preparation
Aluminium hydride is generally prepared by treating an ether solution of lithium aluminium hydride (LAH) with aluminium trichloride. An ether solution aluminium hydride is prepared after precipitation of lithium chloride. The dissolving process of aluminium trichloride requires the addition of 0.5-4 mol equivalents of borohydride salt, which is very expensive and not recovered. This makes the synthesis of aluminium hydride expensive.

3 LiAlH4 + AlCl3 → 4 AlH3 + 3 LiCl

The ether solution of aluminium hydride requires immediate use, because polymeric material with ether will precipitate with AlH[3] otherwise. Aluminium hydride solutions are known to degrade after 3 days. Aluminium hydride is more reactive than LAH, but the procedure to handle aluminium hydride should be similar to that of LAH.

There are also several other methods to prepare aluminium hydride:

2 LiAlH4 + BeCl2 → 2 AlH3 + LiBeH2Cl2

2 LiAlH4 + H2SO4 → 2 AlH3 + Li2SO4 + 2 H2

2 LiAlH4 + ZnCl2 → 2 AlH3 + 2 LiCl + ZnH2

Precautions
Aluminium hydride is not spontaneously flammable, but it is highly reactive. It is recommended to handle the chemical similar to the handling and precaution procedures for lithium aluminium hydride. It is known to degrade in a relatively short time, 3 days. It is required to be used in a fume hood.

Reduction of functional groups
In organic chemistry, aluminium hydride is mostly 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.

It has selectivity different from other hydride reagents. For example, in the following cyclohexanone reduction, lithium aluminium hydride gives a trans:cis ratio of 1.9 : 1, while aluminium hydride gives a trans:cis ratio of 7.3 : 1.



Corey et al. have developed a procedure to hydroxymethylate certain ketones. (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 SN2 reaction, and it is not sterically demanding.



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.



Hydrogen storage
Aluminium hydride may be a useful material for storing hydrogen in hydrogen-fueled vehicles. It contains up to 10% hydrogen by weight and can store up to 148g/L, twice the density of liquid H2. However, currently there are no ways to turn the aluminium byproduct back into AlH3. It also shows promise as an additive to rocket fuel. Aluminium hydride is used in explosive and pyrotechnic compositions.