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)(2)(eta(2)-C5H7)}(Me2HNBH3)(2)]BArF4 (2), which incorporates two amine-boranes coordinated to the rhodium center with two different binding modes, namely h 1 and h 2, has been used to probe whether bis(samine- borane) motifs are important in determining the general course of amine-boranes dehydrocoupling reactions. DFT calculations have been car-ried 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 offmetal, intramolecular dehydrogena-tions provide two coordinated amineborane 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)(2)(eta(2)-C5H7)}B](+) to be important for the whole dehydrocoupling process.