Diborane, diammoniate

Critical temp 16.7 critical press. 39-5 atm Cp at 25° 13 60 cal/mOle/ C, Dec at red heat to B + H2. at lower temps to H and other boron hydrides. Spontaneous ignition temp in air about 40-50° presence of coni ami nants may lower the temp tjmii so that ignition or detonation of diborane(6)-air mixtures may occur at or bdow room temp. Hydrolyzes in water to H2 + HjBO-j, Sol in CS2. Reacts with NHj to form diborane diammoniate reacts slowly with Br2 and explosively with Cl2 io form boron halides reacts with hydrocarbons Or organoboron compds to give alkyl- or arylboron compds reacts with metal alkyls to form metal horohydrides reacts with strong electron pair donors to form borane addn compds. 

CHEMICAL PROPERTIES thermally unstable water reactive hydrolyzes in water to hydrogen and boric acid reacts with ammonia to form diborane diammoniate reacts slowly with bromine to form boron bromides reacts with hydrocarbons or organoboron compounds to give alkyl- or aryl-boron compounds reacts with metal alkyls to form metal borohydrides reacts with strong electron pair donors to form borane addition compounds FP (-90°C, -130°F) LFL/UFL (0.9%, 98%) AT (40-50°C, 104-122°F) HF (35.6 kJ/mol gas at 25°C). 

Amine—Boronium Cations. The most extensively studied boronium cation is the diammoniate of diborane [23777-63-1] (16—18). 

This works well for all nido- and arachno-boranes but not for the c/o o-dianions, which are much less reactive. Reactions of B2H6 with NH3 are complex and, depending on the conditions, yield aminodiborane, H2B(/r-H)(/r-NH2)BH2, or the diammoniate of diborane, [BH2(NH3)2]-[BH4] (p. 165) at higher temperatures the benzene analogue borazine, (HNBH)3, results (see p. 210). 

Shore, S. G., and R. W. Parry Chemical Evidence for the Structure of the Diammoniate of Diborane , III. The Reactions of Borohydride Salts with Eithium Halides and Aluminium Chloride. J. Amer. chem. Soc. 80, 12 (1958). 

The diammoniate of diborane, which was first prepared by Stock and Kuss in 1923, is formed as the sole reaction product on introduction of gaseous diborane, diluted with nitrogen, into liquid ammonia at —78°. While previous methods allowed the preparation of only small amounts of the compound, scaling of the present procedure to yield practically any desired quantity appears to be possible. Of the several laboratory methods available for generating diborane, a procedure adapted from the method of Shapiro and co-workers is described below. 

The reaction between ammonia and diborane depends upon the conditions. Excess of ammonia at low temperatures produces the salt-like diammoniate, [(NH3)2BH2]" BH4 , because with NH3 there is an unsymmetrical cleavage (BHa" + BH4 ) in contrast to the symmetrical one (BH3 -f BHg) resulting from reactions with amines. The monomer BH3.NH3 has indeed been made by the action of an ammonium halide on a borohydride ... 

Development from aminoboron chemistry led to phosphino- and arsino-boron compounds. Diborane and phosphine react at —110° to give B2Hg.2PHg, much less stable than the diammoniate. Trimethylamine displaces phosphine from it quantitatively at —40°, suggesting the ionic structure H3P+.BH3- ... 

More recently another mode of reaction of some of the boranes has been recognized. Certain reagents under rather carefully controlled conditions induce nonsymmetrical cleavage of the hydrogen bridge bonds. As will be discussed in more detail later (Section II,B), formation of the diammoni-ate of diborane probably involves this mechanism. 

Discovery of the diammoniate of diborane (B2He, 2NH3) and bor-azene (B3N3H6), better known by its older name of borazole, by Stock (131) indicated that the boron hydrides could be used to prepare interesting boron-nitrogen compounds. This approach to boron chemistry was rapidly extended when the principles discussed in the preceding section were clearly recognized. Thus the compound (CH3)3N BH3, the first borane adduct to be characterized (39), pointed the way to the synthesis of a number of similar substances directly from diborane and amines for exam-... 

The effect of ammonia on diborane is very complex. A variety of products are produced depending on conditions. At —120° using very specific techniques, diborane and excess of ammonia form the diammoniate, B2Hr, 2NH3 (89,110, 131). The latter was at first formulated as (NH4+)2(B2H4)-2 (131,145), but this structure was later shown to be incorrect (26,110), con-... 

Parry, R. W. Shore, S. G., Chemical evidence for the structure of the diammoniate of diborane. III. The reactions of borohydride salts with lithium halides and aluminum chloride, J. Am. Chem. Soc. 1958, 80, 15-20. 

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