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Mechanistic Details of Ru–Bis(pyridyl)borate Complex Catalyzed Dehydrogenation of Ammonia–Borane: Role of the Pendant Boron Ligand in Catalysis

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journal contribution
posted on 2016-05-13, 00:00 authored by Sourav 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.

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