Do Rhodium Bis(σ-amine-borane) Complexes Play a Role as Intermediates in Dehydrocoupling Reactions of Amine-boranes?



The recently synthesized rhodium complex [Rh{P(C5H9)22-C5H7)}(Me2HNBH3)2]BArF4 (2), which incorporates two amine-boranes coordinated to the rhodium center with two different binding modes, namely η1 and η2, has been used to probe whether bis(σ-amine-borane) motifs are important in determining the general course of amine-boranes dehydrocoupling reactions. DFT calculations have been carried out to explore mechanistic alternatives that ultimately lead to the formation of the amine-borane cyclic dimer [BH2NMe2]2 (A) by hydrogen elimination. Sequential concerted, on- or off-metal, intramolecular dehydrogenations provide two coordinated amine-borane molecules. Subsequent dimerization is likely to occur off the metal in solution. In spite of the computationally confirmed presence of a BH⋅⋅⋅NH hydrogen bond between amine-borane ligands, neither a simple intermolecular route for dehydrocoupling of complex 2 is operating, nor seems [Rh{P(C5H9)22-C5H7)}B]+ to be important for the whole dehydrocoupling process.