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(C 5H 9) 2(η 2-C 5H 7)}(Me 2HNBH 3) 2]BAr F4 (2), which incorporates two amine-boranes coordinated to the rhodium center with two different binding modes, namely η 1 and η 2, 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 [BH 2NMe 2] 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(C 5H 9) 2(η 2-C 5H 7)}B] + to be important for the whole dehydrocoupling process. © 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.