Amine- and Phosphine-Borane Adducts: New Interest in Old Molecules

A review of various studies conducted in the field of borane adducts of nitrogen and phosphorus-donors is presented. After the discovery of ammonia-trifluoroborane in 1809, the first amine-borane containing only hydride substituents on boron was reported, a species formed by direct reaction of trimethylamine and diborane. Studies have shown that substituted boranes can be used for the preparation of amine-borane adducts. The formation of the adduct bond leads to pyramidalization of the borane moiety to produce an approximately tetrahedral geometry, with a change in hybridization at boron from sp 2 to sp3. In ammonia-borane, the central dative bond has a measured dissociation enthalpy of 27.2 kcal/mol at 25°C, and the central covalent bond in ethane is more than three times stronger under the same conditions, 90.1 kcal/mol.