Experimental Evidence Supporting Related Mechanisms for Ru(II)-Catalyzed Dehydrocoupling and Hydrolysis of Amine-Boranes

A family
of ruthenium­(II) half-sandwich complexes was tested for
the hydrolytic decomposition of amine-boranes. The analysis of the
catalytic results, together with a multilateral approach based on 1H, 11B NMR, and ESI-MS were used to propose a plausible
and conceptually unified mechanism for both the hydrolysis and competitive
dehydrogenation of amine-boranes. We propose the intermediacy of solvent-stabilized
borenium cations during the catalytic cycle, evolving toward dehydrogenation
products in distilled THF or releasing amine-hydroxyboranes in aqueous
media. Both reaction pathways would liberate up to 1 equivalent of
hydrogen through a metal-catalyzed process, but an out-of-cycle low-barrier
hydrolysis of amine-hydroxyboranes would produce the 2 additional
equivalents of hydrogen in aqueous solutions. Metal-catalyzed deuteration
of (non hydrogen-productive) trisubstituted amine-boranes by using
D2O as deuterium source was observed, and included as part
of the mechanism proposal.