Heptahydrodiborate

Standard high-vacuum techniques6 8 are used throughout this preparation. 

Methyltriphenylphosphonium heptahydrodiborate(l —) is a white, crystalline, air-sensitive compound. It is moderately stable at room temperature, but decomposes readily at 100°. It is soluble in methylene chloride, chloroform, and 1,1,2,2-tetrachloroethane, but insoluble in benzene, pentane, tetrahydrofuran (THF), diethyl ether, dimethyl ether, and diglyme [bis(2-methoxyethyl) ether]. 

The H nmr spectrum of [P(C6H5)3CH3][B2H7] in methylene chloride at room temperature consists of a phenyl resonance (t2.2), a methyl doublet (t7.02, Jph = 13.2 Hz), a broad quartet (t8.6, Jbh = 106 Hz) due to terminal B—H, and a very broad unresolved resonance (rl4.2) due to bridge B—H—B. The UB nmr spectrum is a broad quartet (524.5 relative to BF3-0(C2H5)2, Jbh = 83Hz).4 

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In ether or diglyme, triphenyl borate (but not trimethyl borate) produces diborane with either LiAlHi or NaAlHi, although in restricted yield (7, 8). In this case it is essential to add a solution of the aluminum compound to the ester, for no diborane results if the order of addition is reversed. A disadvantage of this reaction is that the metal tetrahydroborate and possibly the heptahydrodiborate occur as byproducts. ... 

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