Elucidation of the Formation Mechanisms of the Octahydrotriborate Anion (BH) through the Nucleophilicity of the B-H Bond.

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 (BH) was elucidated on the bases of both experimental and computational investigations. Two possible routes from the reaction of BH and THF·BH to BH were proposed, both involving the BH and BH 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 BH and BH intermediates be converted to BH and BH(H) via a high energy barrier, from which H elimination occurs to yield the BH final product. The formation of BH from THF·BH by nucleophilic substitution of the B-H bond was captured and identified, and the reaction of BH with BH to produce BH was confirmed experimentally. On the bases of the formation mechanisms of BH, we have developed a facile synthetic method for MBH (M = Li and Na) in high yields by directly reacting the corresponding MBH salts with THF·BH. In the new synthetic method for MBH, no electron carriers are needed, allowing convenient preparation of MBH in large scales and paving the way for their wide applications.