Aluminum Trihydride-Diethyl Etherate

3LiAlH4 + A1C13 diethy eth 4[A1H3 0.3(C2Hs )20] + 3LiCl 

Methods for the preparation of aluminum trihydride-diethyl etherate, AlHa O.SffC Hs )20], t have been published,1,2 but the absence of complete experimental details makes duplication difficult. The following procedure is a modification of that reported by Finholt, Bond, and Schlesinger.1 Problems inherent in previous methods, such as premature precipitation, decomposition of the alane, and lithium chloride contamination, are avoided. 

Premature precipitation is controlled by maintaining a low temperature (—5°C.) in the reaction mixture. Purified reactants, as well as minimal exposure to light and higher temperatures, 

JEbulliometric molecular weight determinations obtained in diethyl ether (concentration range, 0.25-1.0 Af) indicates molecular weights only slightly higher than 30. This is consistent with studies reported by Wiberg and Uson.3 It must be assumed that in solution, A1H, exists essentially as the monomeric form. 

Chloride is removed from the ether solution of aluminum trihydride by the addition of sodium tetrahydroborate. The LiBH4 remains in solution, while the sodium chloride precipitates and is removed by filtration. Best results are obtained when NaBH4t is dried under vacuum at 60°C. for 8 hours and ground to 1 /urn. or smaller. This last step is particularly important since both the reaction rate and efficiency of chloride removal are a function of NaBH4 particle size. 

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The volume of clear aluminum trihydride solution is reduced to about 600 ml. by nitrogen-stream evaporation and then allowed to warm to room temperature. After 6 to 8 hours, the aluminum trihydride-diethyl etherate precipitates, which leaves the small amount of soluble LiBH4 and aluminum trihydride-diethyl etherate in the ether solution. The product is filtered, washed twice with 100 ml. of anhydrous ether, and dried under high vacuum for 10 to 12 hours. A yield of approximately 30 g. (58%) is obtained. A nal. Calcd. for A1H3 0.3[(C2HS )20] J Al, 50.78 C, 28.21 H, 11.61 Hydrolytic H2,5.69. Found Al, 50.60 C, 28.08 H, 11.62 Hydrolytic H2,5.68. 

The aluminum trihydride-diethyl etherate is a colorless solid which reacts violently with water or in a damp atmosphere. ( Caution. Sometimes this reaction is explosive ) The material is unstable to prolonged exposure to light at ambient temperatures, but can be stored at — 10°C. in a sealed container up to one year. The ratio of aluminum trihydride to ether (C2H2)20 in the solid varies between 0.29 and 0.33, depending upon the time under vacuum. The solubility of aluminum trihydride-diethyl etherate in diethyl ether is 0.2Af it is very soluble in tetrahydrofuran. Its infrared spectrum has broad bands in the... 

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