Convenient preparation of lithium borohydride
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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) (../rhodium/pdf /lithium.boro
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.
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3.2. Lithium Tetrahydridoborate
Properties . Lithium tetrahydridoborate (known industrially as lithium borohydride or lithium boranate), LiBH4 , is a white, microcrystalline powder that decomposes at the melting point (280 °C) with considerable evolution of hydrogen. It is stable in vacuum to about 200 °C. It is extremely hygroscopic, being rapidly decomposed by moist air, sometimes with spontaneous ignition. It is therefore stored and handled in an atmosphere of nitrogen or argon. It is hydrolyzed only slowly by water at 0 °C:
LiBH4 + 2 H2O __> LiBO2 + 4 H2
By acidification of the solution or by addition of certain metal salts as catalysts, the hydrolysis is rapid and large volumes of hydrogen are liberated (4.11 L/g at STP).
Lithium borohydride is a more powerful reducing agent than sodium borohydride although much less reactive than lithium aluminum hydride.
Lithium borohydride is used for the selective reduction of esters, carboxylic acids, amides, and epoxides in preference to other functional groups such as nitriles, nitro compounds, and halides (5). The effect of solvents has been investigated (40); diethyl ether and tetrahydrofuran are preferred. The addition of a stoichiometric amount of methanol leads to a very large increase in reducing power without signif icantly affecting the selectivity. Among the reactions of lithium borohydride with inorganic compounds (1) , (5) , (41) , special mention should be made of reactions yielding borohydrides or hydrides of other metals, e.g.,
4 LiBH4 + ZrCl4 __> Zr(BH4)4+ 4 LiCl
Production. Industrial production of lithium borohydride makes use of the heterogeneous metathetical reaction between LiCl or LiBr and sodium borohydride in ethers (diethyl ether, tetrahydrofuran) or amines:
NaBH4 + LiX __> LiBH4 + NaX
X = Cl, Br
The mixture is ball-milled while the reaction takes place in order to ensure a satisfactory rate of reaction and a good yield (up to 90 %) (42). Synthesis from the elements is not used industrially.
The commercial product (Chemetall, Ventron) has a bulk density of 0.6 – 0.7 g/cm3 and a purity of 94 %.
Uses. Lithium borohydride is used to a limited extent as a selective reducing agent in organic chemistry. It has been investigated as a means of storing hydrogen, but has not been used on an industrial scale for this purpose (43).
(5) A. Hajos: Complex Hydrides, Elsevier, Amsterdam 1979.
(40) K. Soai, A. Ookawa, J. Org. Chem. 51 (1986) 4000 – 4005.
(41) T. J. Marks, J. R. Kolb, Chem. Rev. 77 (1977) 265 – 293.
(42) H. C. Brown, Y. Moon Choi, S. Narasimhan, Inorg. Chem. 20 (1981) 4456 – 4457.
(3) Compagnie Francaise de Raffinage S.A., DE-OS 2 728 109, 1976 (A. Muller, F. Mathey, J. Bensoam).
Taken from Ullman’s encyclopedia of industrial chemistry__>