Are Boryl Radicals from Amine-Boranes and Phosphine-Boranes the Most Stable Radicals?

The relative stability of the radicals that can be produced from amine-boranes and phosphine-boranes is investigated at the G3-RAD level of theory. Aminyl ([RNH](center dot):BH3) and phosphinyl ([RPH](center dot):BH3) radicals are systematically more stable than the boryl analogues, [RNH2]:BH2 center dot and [RPH2]:BH2 center dot. Despite similar stability trends for [RNH](center dot):BH3 and [RPH](center dot):BH3 radicals with respect to boryl radicals, there are significant dissimilarities between amine-and phosphine-boranes. The homolytic bond dissociation energy of the N-H bond decreases upon association of the amines with BH3, whereas that of the P-H bond for phosphines increases. The stabilization of the free amine is much smaller than that of the corresponding aminyl radical, whereas for phosphines this is the other way around. The homolytic bond dissociation energy of the B-H bond of borane decreases upon complexation with both amines and phosphines.