Exploration of new routes to boranes and carboranes

Abstract

The crystal chemistry of binary metal borides in the composition range M(_4) to MB(_12) and the hydrolytic chemistry of reactive metal borides to give boranes are reviewed. The thermal interconversion reactions of the smaller volatile boranes from B(_2)H(_6) to B(_10)H(_14) is critically examined and a rationalisation of their reactions with the transient Lewis acid species {BH(_3)), (B(_3)H(_7)) and (B(_4)H(_8)) is developed. This approach reveals a pattern of step-wise cage-growth for nido boranes which first seem to ligate a BH(_3) unit and then eliminate H(_2) giving the next higher homologue. Also, a structure-reactivity relationship for B(_2)H(_6) and the arachno boranes is suggested where the terminal hydrogens of the BH(_2) groups are thought to possess hydridic character which dominates their reactions with the Lewis acid intermediates. This work is partially extended to the cage-expansion and pyrolysis reactions of the anionic boron hydrides. A detailed review is presented of the pyrolysis of tetraalkylammonium borohydrides to give [B(_10)H(_10)](^2-) and [B(_12)H(_12)](^2-) in high yield. A study of the acidic hydrolysis of MgB(_2) under a wide range of conditions is reported. The reactions of MgB(_2) with 100% H(_3)PO(_4) and with 7M H(_3)PO(_4) under the influence of ultrasonics are found to be promising methods for increasing the borane yield. The anions [B(_12)H(_120](^2-), [B(_10)H(_10)](_2) and possibly [B(_9)H(_14)](^-) are shown to be important products from MgB(_2) hydrolysis. A mechanism is proposed for this reaction incorporating initial production of CB(_6)H(_9)](^-) which is protonated to give B(_6)H(_10); degradation of this borane in the acidic solution is thought to account for generation of B(_5)H(_9) and B(_4)H(_10) which are the only other major volatile boron hydride products. Finally, some attempted "one-pot” carborane syntheses using MgB(_2) as the boron source are discussed. [brace not closed]