Aluminium hydride is an inorganic compound with the formula AlH3. It presents as a white solid and may be tinted grey with decreasing particle size and impurity levels. Depending upon synthesis conditions, the surface of the alane may be passivated with a thin layer of aluminum oxide and/or hydroxide. Alane and its derivatives are used as reducing agents in organic synthesis.
Structure
Alane is a polymer. Hence, its formula is sometimes represented with the formula 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 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 and digallane.
Preparation
Aluminium hydrides and various complexes thereof have long been known. Its first synthesis was 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. 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:
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 wire submerged in mercury 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− → AlH3 · nTHF + ½H2 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.
α-AlH3 can be produced by hydrogenation of aluminium metal at 10GPa and. The reaction between the liquified hydrogen produces α-AlH3 which could be recovered under ambient conditions.
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, 2. The 1:2 complex adopts a trigonal bipyramidal structure. Some adducts thermally decompose to give aluminium metal and may have use in MOCVD applications. Its complex with diethyl ether forms according to the following stoichiometry: The reaction with lithium hydride in ether produces lithium aluminium hydride:
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. 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. Aluminium hydride even reduces carbon dioxide to methane under heating:
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
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. AlH3 contains up to 10% hydrogen by weight, corresponding to 148g H2/L, or twice the hydrogen density of liquid H2. In its unpassivated form, alane is also a promising rocket fuel additive, capable of delivering impulse efficiency gains of up to 10%.
Precautions
Alane is not spontaneously flammable. It should be handled similarly to that of other complex metal hydride reducing agents like lithium aluminium hydride. Alane will decompose in air and water, although passivation greatly diminishes decomposition rate. Passivated alane is generally assigned a hazard classification of 4.3.
