Title : Catalytic hydrogen release from Amine Boranes
Abstract:
The reaction of a bidentate pyrrole based rhenium tetracarbonyl complex (1a) with dimethylamine borane at room temperature results in rapid dehydrogenation. Hydrogen generation was detected at temperatures as low as 238K and NMR spectroscopy provided evidence for the initial formation of a Re-H-B ? bound adduct at these temperatures. The rate of the dehydrocoupling reaction was significantly influenced by the electron density on the metal center and the identity of the alkyl group (CH3 or CF3) on the ketone carbon of the pyrrole ligand. Thus, increasing the electron density on the metal center by substituting the ? acid CO ligand with ? donor groups resulted in significantly decreased reactivity while replacement of the electron donating CH3 group on the ketone carbon with CF3 yielded a ten-fold reduction in reaction rate. DFT calculations supported the experimental findings and suggested that the overall reaction occurs in three main steps to include, de-chelation of the organic carbonyl group from the metal center and coordination of the amine-borane to the resulting vacant site, activation of the B-H and N-H bonds, and finally metal mediated heterolytic coupling of H+ and H- to yield H2. The bifunctional nature of 1a incorporating both an acidic Re center and basic oxygen center, coupled with the hemi-lability of the organic carbonyl group, are key motifs of this catalyst that result in efficient thermal dehydrogenation of dimethylamine borane.
