Elucidation of the Formation Mechanisms of the Octahydrotriborate Anion (B₃H₈–) 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 (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.