Do rhodium bis(σ-amine-borane) complexes play a role as intermediates in dehydrocoupling reactions of amine-boranes?

摘要

The recently synthesized rhodium complex [Rh{P(C_5H_9) _2(η~2-C_5H_7)}(Me_2HNBH_3)_2]BAr~F _4 (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.