Release from Tetrahydroborates

Diborane (or, more correctly, borane) may be regarded as existing already preformed in tetrahydroborates. Metal tetrahydroborates have been the subject of a recent article in this series (163). A variety of methods for releasing diborane from them are now known, although aqueous and hydroxylic reagents that react with it are useless. Until recently, however, all reported methods incorporated one or more avoidable practical disadvantages. 

The reaction of boron trifluoride etherate with a solution of lithium tetrahydroborate in a simple ether or in dioxan (278, 280) is of importance, not only as the first reaction of this kind to be discovered, but also as the first reaction of any kind found to produce 

In trials with a pilot plant, a yield of 90.9% of diborane (purity 95%) was obtained (41), although under ordinary laboratory conditions this is hardly ever achieved. The reaction can also be carried out in tetrahydrofuran, but the yield is much lower (87). Instead of BF3 Et20, LiBF in ether may be employed with good results (87), although the reaction appears to be reversible (322)  

There is, however, a major disadvantage in using boron trifluoride or tetrafluoroborates in the preparation of diborane, for they result in a product that is contaminated with BF3, which is not readily removed (29). Particularly when the diborane is required for selective reductions, this is a most undesirable impurity traces of BF3 enhance the reducing powers of B2Hg and catalyze hydrogenolysis at centers that would not otherwise be attacked (24). 

The iron is left as a black pyrophoric residue. An interesting featiire of these reactions is that the borohydrides Fe(BH )2 and CuBHi, which can be isolated 

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A schematic illustration of a typical inlet apparatus for separating volatile hydrides from the analyte solution, in which they are generated upon reduction with sodium tetrahydroborate. When the mixed analyte solution containing volatile hydrides enters the main part of the gas/liquid separator, the volatiles are released and mix with argon sweep and makeup gas, with which they are transported to the center of the plasma. The unwanted analyte solution drains from the end of the gas/liquid separator. The actual construction details of these gas/liquid separators can vary considerably, but all serve the same purpose. In some of them, there can be an intermediate stage for removal of air and hydrogen from the hydrides before the latter are sent to the plasma. 

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