posted on 2016-05-13, 00:00authored bySourav Bhunya, Lisa Roy, Ankan Paul
The role of pendant
boron ligands in ammonia–borane (AB)
dehydrogenation has been investigated using hybrid density functional
theory for two very efficient ruthenium-based catalysts developed
by Williams and co-workers. Our findings reveal that the catalytic
action initiates through opening of the labile metal–ligand
bridging group associated with the boron-based pendant ligand arm
for both catalysts. In case of the hydroxyl-bridged catalyst, the
ligand (B–OH moiety) backbone plays an active role along with
the metal center to perform concerted dehydrogenation of ammonia–borane
by overcoming a free energy activation barrier of 24.3 kcal/mol, and
this dehydrogenation step is the rate-determining step of the catalytic
cycle. However, for the trifluoroacetate-bridged complex, H2 is released in a stepwise fashion with active participation of the
solvent. It involves formation of a boronium cation with a rate-determining
free energy activation barrier of 23.7 kcal/mol for the solvent-assisted
B–H bond-breaking step, while the pendant boron ligand acts
as a spectator. Overall, our detailed theoretical study illustrates
that the chemical nature of the pendant boron ligand is decisive in
the AB dehydrogenation pathway. Further computational investigations
indicate that greater amounts of hydrogen are released from AB by
the dual participation of free NH2BH2 and the
Ru catalysts.