Purifying LiAlH₄

Whilst commercial grade (typically 95 % purity) LiAlH₄ is routinely used super-stoichiometrically as a reducing agent in both organic and inorganic research laboratories, pure LiAlH₄ (that is actually colourless and completely soluble in Et₂O) is essential for stoichiometric reactions where is it more commonly used as the source of aluminium (i.e. to make AlH₃ or Al(ORᶠ)₄ weakly-coordinating anions).

Step 1: Transfer the commercial grade, grey LiAlH₄ into a filter frit equipped with a large Schlenk flask and magnetic stir bar. This is easiest to do in a glovebox as it ensures that the glassware is sufficiently dry and free of air which is essential for later steps. Adding a stir bar to the crude LiAlH₄ in the frit allows for better mixing and extraction. Cycle the glassware onto the Schlenk line.

Crude LiAlH₄ loaded into a filter frit.

Step 2: Replace the Schlenk cap with a rubber septum under a flow or inert gas and then extract the LiAlH₄ with portions of Et₂O via cannula transfer. Five 100 mL portions of Et₂O should give at least 90% recovery of pure LiAlH₄ (from 20-25 grams of crude material).

Transferring the Et₂O into the filter frit.

Step 3:  Filter the LiAlH₄ slurry through the frit and then repeat the extraction/filtration process (see above).

Filtering the LiAlH₄/Et₂O slurry.

Step 4: Replace the filter frit with a greased glass stopper to seal the LiAlH₄/Et₂O solution. Quench the residual grey material in the filter frit with toluene/IPA under a flow of inert gas. Remove the Et₂O under vacuum. Ensure that the solution has adequate stirring during evaporation to prevent bumping. Manual stirring/agitation with a strong external magnet helps to create a fine and dry solid.

Removing the Et₂O under vacuum.

Once all of the Et₂O has been removed, the pure LiAlH₄ should be a colourless or very pale grey solid that can then be isolated and stored in the glovebox.

Purified LiAlH₄ isolated in a glovebox.
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