Diborane pyrolysis

It was shown that the rate of decomposition of diborane follows a rate law that corresponds to d(B2H6) = k B2H(, 2. This is consistent with the dissociation of diborane into BH3 as the rate determining step with formation of B3H9. This intermediate then decomposes with hydrogen evolution to give unstable B3H7 which on reaction with further BH3 produces B4H10 as the first isolable poly-borane  

B8H12 + B3H7 — BgH + B2Hg B9H13 + B3H7 — BioHi4 + B2H6 

Most of these reactions are reversible. Thus, heating tetraborane(lO) in the presence of a large excess of hydrogen reforms diborane. Pentaborane(ll) is thermally 

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Pentaborane(9), generated some time ago on a pilot plant scale via diborane pyrolysis as part of the fuel development program mentioned above, is currently stockpiled in large quantity by the U.S. Government. A convenient laboratory-scale preparation utilizes the bromotriborohydride anion ... 

Decaborane(14), the smallest solid borane and an important precursor to higher hydrides as well as carboranes (see below), was formerly prepared via diborane pyrolysis however, this approach has been rendered obsolete by the development of several high-yield routes based on the BH or B5Hg anions (the latter species is easily obtained via deprotonation of B5H9). 

G in the presence of a catalytic amount of a Lewis base such as dimethylether, (GH2)20. In addition to the gas-phase pyrolysis of diborane, can be prepared by a solution-phase process developed at Union Garbide Gorp. Decaborane is a key intermediate in the preparation of many carboranes and metaHa derivatives. As of this writing, this important compound is not manufactured on a large scale in the western world and is in short supply. Prices for decaborane in 1991 were up to 10,000/kg. 

Boron is also obtained by the pyrolysis of diborane. Only moderate temperature is required (400-900°C) with a pressure up to 1 atm [6][7]... 

At about the same time, the pyrolysis of diborane was studied by Bragg et al.88 in the temperature range 90-130 °C. These workers again used a static system (reaction vessel volume 212 cm3) and followed the conversion both by measurement of pressure increase and by determination of the amount of hydrogen formed. The system was also examined by mass spectrometric analysis. The empirical rate law was found to be... 

NaB2Hs + 4HC1 2B4H10 + 4NaCl + 8H2 Higher boranes can be produced from diborane by pyrolysis. 

Caution. Diborane(6) reacts explosively with atmospheric oxygen and is extremely toxic. This highly volatile substance must be handled in a vacuum line at all times. The pyrolysis bulb must be inspected as detailed in the synthesis o/[(CH3)2NBH2]2 (p. 32). The aminoboranes should be regarded as toxic. 

The compound bis-/i-(dimethylamino)-diborane(6), [(CH3)2NBH2]2, has been prepared by the pyrolysis of dimethylamine-borane, (CH3)2NH BH3,1-3 and by a novel iodination-deprotonation of (CH3)2NH BH3,4 The gas-phase pyrolysis described here is the simplest and most convenient method for the preparation of small samples, since no solvent is involved and separation problems are thus minimized. The basic vacuum-line techniques necessary for this synthesis are outlined in reference 7. 

Salts of this dianion can also be prepared via the pyrolysis of Et4N BH and the treatment of Et3NBH3 with diborane at 170°C. 

Diborane. Se. Vol 2, p B253-R, under Boron Hydrides and p B255-L, under Boron Hydride Fuels. See also the following addnl Refs Refs 1) F.R. Price, JACS 72, 5361-65 (1950) 8e CA 45, 2755(1951)(First 8e 2nd pressure limits of espln of Diborane-Oxygen tnixts) 2) R.P. Clarke R.N. Pease, JACS 73, 2132-34(1951) 8s CA 45, 7418(1951) (Kinetics of pyrolysis of Diborane) 3) J-K-Bragg et al, JACS 73, 2134-40(1951) 8s CA 45, 7418(l951)(Kinetics of pyrolysis of Diborane) 4) A.T. Whatley 8s R.N. Pease, JACS 76, 1997-99(1954) 8s CA 48. 8543(1954) (Thermal expln of Diborane-oxygen mixts)... 

An outstanding feature of borane chemistry is the large number of reactions which result in the conversion of one or more boranes into others. These reactions make it possible to develop the whole of borane chemistry from one simple starting material, diborane. This point is emphasized in Chart 24.2, which shows only a small part of the very complex chemistry of boranes. Pyrolysis of B2H6 yields a number of the lower boranes including, for example, the thermally unstable BsHi 1, but this compound is preferably prepared by utilizing the equilibrium... 

A number of compounds [R2PBH2] have now been prepared (4-1, HO), some by pyrolysis of borane adducts obtained from diborane and secondary phosphines, for example,... 

The unsubstituted cyclic boranamine polymers (H2NBH2) cannot be prepared by pyrolysis of NH3BH3. These compounds can be formed by preparation of the uns)nnmetrical cleavage product of diborane in liq NHj and subsequent treatment with... 

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