Designing an Effective Metal-Free Lewis Acid Catalyst for Ammonia-Borane Dehydrogenation: A DFT Investigation on Triarylboranes

5 years ago

Designing an Effective Metal-Free Lewis Acid Catalyst for Ammonia-Borane Dehydrogenation: A DFT Investigation on Triarylboranes

Ankan Paul, Sourav Bhunya

The catalytic dehydrogenation of ammonia-borane (NH3BH3) is dominated largely by transition-metal catalysts. Metal-free catalysis for NH3BH3 dehydrogenation is a rarity. It is well known that mono-boron-based Lewis acids are largely ineffective to facilitate the catalytic dehydrogenation of NH3BH3. Herein, through theoretical investigations, we have identified the routes with catalytic potential for B(C6F5)3 and its congeners and also the factors that are likely to prevent effective catalysis for these systems. Our findings reveal for triarylboranes that potential catalytic dehydrogenation routes comprise of two main events: ion pair formation from NH3BH3 in the presence of a catalyst assisted by a nucleophile and subsequent H2 release from the ion pair. Donor solvents and the B−H hydride of NH3BH3 act as a nucleophile to facilitate ion-pair formation from NH3BH3 and the Lewis acid catalyst in donor and nondonor solvents, respectively. A good nucleophilic solvent decreases the activation barrier of ion-pair formation but it increases the activation barrier associated with the subsequent H2 release process. The reverse is true for nondonor solvents, in which case NH3BH3 acts as a nucleophile. Our studies reveal that by the careful tuning of the hydride affinity of the Lewis acid catalyst in combination with nondonor solvents, rate-limiting barriers for dehydrogenation can be reduced to approximately 19–20 kcal mol−1, which would enable catalytic turnovers at room temperature. Striking a fine balance: The effectiveness of triarylborane catalysts in ammonia-borane dehydrogenation is determined by the reactivity of the ion pair formed in situ, which is controlled by the hydride affinity of triarylboranes and the nature of solvent. By fine tuning these two factors, triarylborane catalysts effective at room temperature have been designed theoretically.

Publisher URL: http://onlinelibrary.wiley.com/resolve/doi

DOI: 10.1002/cctc.201700416