Nitrogen-Based Ligands Accelerate Ammonia Borane Dehydrogenation with the Shvo Catalyst

We previously reported that quantitative
poisoning, a test for
homogeneous catalysis, behaves oddly in the dehydrogenation of ammonia
borane (AB) by Shvo’s catalyst, whereas the “poison”
1,10-phenanthroline (phen) accelerates catalysis and apparently prevents
catalyst deactivation. Thus, we proposed a protective role for phen
in the catalysis. Herein we account for the mechanistic origin of
this accelerated AB dehydrogenation in the presence of phen and define
the relevance boundaries of our prior proposal. In so doing, we present
syntheses for novel amine- and pyridine-ligated homologues of the
Shvo catalyst and show their catalytic efficacy in AB dehydrogenation.
These catalysts are synthetically easy to access, air stable, and
rapidly release over 2 equiv of H2. The mechanisms of these
reactions are also discussed.