Abstract:

Although at first glance, the old reaction of B₂H₆ (diborane) with NH₃ seemed to be simple, it has been shown that the reaction was complex and had perplexed boron chemists for decades. The reaction was highly sensitive to the experimental conditions, and one or the mixture of the products, such as diammoniate of diborane (DADB, [NH₃BH₂NH₃][BH₄]), ammonia borane (AB, NH₃·BH₃), aminodiborane (ADB, NH₂B₂H₅), could be obtained. It was known that the asymmetrical cleavage of the double bridge bond of B₂H₆ produced the ionic product DADB, whereas symmetrical cleavage afforded the covalent product, AB. The different cleavage patterns of B₂H₆ under particular conditions attracted interest to study the key points of the selectivity including steric effect, inductive effect, solvent effect, and dielectric constant, but it is still not possible to predict the products of the reactions of B₂H₆ with Lewis base. B₂H₆ was present in THF as the THF·BH₃ adduct, and the products of the reactions of Lewis bases with THF·BH₃or B₂H₆ were of the same types. The formation mechanisms of AB and DADB were explored by an ingenious design using THF·BH₃ as a proxy for more reactive B₂H₆, and the reaction of THF·BH₃ with NH₃could be carried out under mild conditions. It was found that dihydrogen bonds played a critical role in the competing formation of AB and DADB. Both experimental and computational results demonstrated that dihydrogen bond was the dominant factor in the complex formation mechanism of ADB.

Key words: Boron-nitrogen-hydrogen compound, Diborane, Diammoniate of diborane, Ammonia borane, Aminodiborane, Dihydrogen bond, Lewis base