Aluminium hydride (also known as alane and alumane) is an inorganic compound with the formula AlH3. Alane and its derivatives are part of a family of common reducing reagents in organic synthesis based around group 13 hydrides. In solution—typically in etherial 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, some forms of alane are, as of 2016, active candidates for storing hydrogen and so for power generation in fuel cell applications, including electric vehicles. 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.
Solid aluminium hydride, or alane, is colorless and nonvolatile, and in its most common reagent form it is a highly polymerized species (i.e., has multiple AlH3 units that are self-associated); it melts with decomposition at 110 °C. While not spontaneously flammable, alane solids and solutions require precautions in use akin to other highly flammable metal hydrides, and must be handled and stored with the active exclusion of moisture. Alane decomposes on exposure to air (principally because of adventitious moisture), though passivation — here, allowing for development of an inert surface coating — greatly diminishes the rate of decomposition of alane preparations.
Form and structure
Alane is a colorless and nonvolatile solid that melts with decomposition at 110 °C. The solid form, however, often presents as a white solid that may be tinted grey (with decreasing reagent particle size or increasing impurity levels). This coloration arises from a thin surface passivation layer of aluminium oxide or hydroxide.
Under common laboratory conditions, alane is "highly polymeric", structurally. This is sometimes indicated with the formula (AlH3), where is left unspecified. Preparations of alane dissolve in tetrahydrofuran (THF) or diethyl ether (ether), from which pure allotropes precipitate.
Structurally, alane can adopt numerous polymorphic forms. By 2006, "at least 7 non-solvated AlH3 phases" were known: α-, α’-, β-, γ-, ε-, and ζ-alanes; the δ- and θ-alanes have subsequently been discovered. Each has a different structure, with α-alane being the most thermally stable polymorph. For instance, crystallographically, α-alane adopts a cubic or rhombohedral morphology, while α’-alane forms needle-like crystals and γ-alane forms bundles of fused needles. The crystal structure of α-alane has been determined, and features aluminium atoms surrounded by six octahedrally oriented hydrogen atoms that bridge to six other aluminium atoms (see table), where the Al-H distances are all equivalent (172 pm) and the Al-H-Al angle is 141°.
When β- and γ-alanes are produced together, they convert to α-alane upon heating, while δ-, ε-, and θ-alanes are produced in still other crystallization conditions; although they are less thermally stable, the δ-, ε-, and θ-alane polymorphs do not convert to α-alane upon heating.
Under special conditions, non-polymeric alanes (i.e., molecular forms of it) can be prepared and studied. Monomeric AlH3 has been isolated at low temperature in a solid noble gas matrix where it was shown to be planar. The dimeric form, Al2H6, has been isolated in solid hydrogen, and it is isostructural with diborane (B2H6) and digallane (Ga2H6).
Handling
Alane is not spontaneously flammable. Even so, "similar handling and precautions as... exercised for LiAlH4" (the chemical reagent, lithium aluminium hydride) are recommended, as its "reactivity [is] comparable" to this related reducing reagent. For these reagents, both preparations in solutions and isolated solids are "highly flammable and must be stored in the absence of moisture". Laboratory guides recommend alane use inside a fume hood. Solids of this reagent type carry recommendations of handling "in a glove bag or dry box". After use, solution containers are typically sealed tightly with concomitant flushing with inert gas to exclude the oxygen and moisture of ambient air.
Passivation greatly diminishes the decomposition rate associated with alane preparations. Passivated alane nevertheless retains a hazard classification of 4.3 (chemicals which in contact with water, emit flammable gases).