This article is a must-have on borohydride chemistry!
Addition compounds of alkali metal hydrides. 22.
Convenient preparation of lithium borohydride from sodium borohydride (or BH3*Me2S) in ether solvents
Herbert C. Brown, Yong Moon Choi, S. Narasimhan
Inorg. Chem. 21(10), 3657-3661 (1982) (https://www.thevespiary.org/rhodium/Rhodium/pdf/lithium.borohydride.prep.review.pdf)
(https://www.thevespiary.org/rhodium/Rhodium/pdf/lithium.borohydride.prep.review.pdf)
Abstract
The preparation of LiBH4 in various ether solvents from the readily available reagents NaBH4 and lithium halides is described. The reactivity of lithium halides toward the metathesis reaction generally follows the order LiBr > LiI > LiCl. The heterogeneous reactions proceed satisfactorily with vigorous magnetic stirring. However, attempting to increase the scale of the preparations utilizing mechanical stirrers resulted in incomplete reactions and decreased yield. On the other hand, when the heterogeneous mixture was stirred with mechanical stirrers fitted with Teflon paddles and a mass of glass beads, the rate of the reaction increased considerably, producing quantitative yields of LiBH4 in greatly decreased reaction times. The ease of conversion of NaBH4 into LiBH4 in various solvents follows the order isopropylamine > 1,3-dioxolane > monoglyme > tetrahydrofuran = ether. The isolation of solvent-free LiBH4 from the various solvates was attempted under different conditions. In most cases, normal distillation at 100 or 150°C produced a strong 1:1 solvate, LiBH4.Solvent. Only in the case of ethyl ether is the solvent of solvation readily removed at 100°C at atmospheric pressure. In the other cases, both higher temperatures, up to 150°C, and lower pressures, down to 0.1 mm, are required to produce the unsolvated material. Thus the ease of isolating unsolvated LiBH4 is diethyl ether > IPA > THF > 1,3-Dioxolane = monoglyme. Consequently, ethyl ether is the medium of choice for the preparation of LiBH4 by the metathesis of NaBH4 and LiBr. LiBH4 can also be conveniently prepared by the reaction of LiH with BH3*Me2S in ethyl ether. Dimethyl sulfide is readily removed, along with ethyl ether of solvation, at 100°C (atmospheric pressure). These procedures make LiBH4 readily available.