Elucidation of the Formation Mechanisms of the Octahydrotriborate Anion (B_3H_8~-) through the Nucleophilicity of the B-H Bond

摘要

Boron compounds are well-known electrophiles. Much less known are their nucleophilic properties. By recognition of the nucleophilicity of the B–H bond, the formation mechanism of octahydrotriborate (B_(3)H_(8)~(–)) was elucidated on the bases of both experimental and computational investigations. Two possible routes from the reaction of BH_(4)~(–) and THF·BH_(3) to B_(3)H_(8)~(–) were proposed, both involving the B_(2)H_(6) and BH_(4)~(–) intermediates. The two pathways consist of a set of complicated intermediates, which can convert to each other reversibly at room temperature and can be represented by a reaction circle. Only under reflux can the B_(2)H_(6) and BH_(4)~(–) intermediates be converted to B_(2)H_(5)~(–) and BH_(3)(H_(2)) via a high energy barrier, from which H_(2) elimination occurs to yield the B_(3)H_(8)~(–) final product. The formation of B_(2)H_(6) from THF·BH_(3) by nucleophilic substitution of the B–H bond was captured and identified, and the reaction of B_(2)H_(6) with BH_(4)~(–) to produce B_(3)H_(8)~(–) was confirmed experimentally. On the bases of the formation mechanisms of B_(3)H_(8)~(–), we have developed a facile synthetic method for MB_(3)H_(8) (M = Li and Na) in high yields by directly reacting the corresponding MBH_(4) salts with THF·BH_(3). In the new synthetic method for MB_(3)H_(8), no electron carriers are needed, allowing convenient preparation of MB_(3)H_(8) in large scales and paving the way for their wide applications.