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Designing an Effective Metal‐Free Lewis Acid Catalyst for Ammonia‐Borane Dehydrogenation: A DFT Investigation on Triarylboranes
Author(s) -
Bhunya Sourav,
Paul Ankan
Publication year - 2017
Publication title -
chemcatchem
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.497
H-Index - 106
eISSN - 1867-3899
pISSN - 1867-3880
DOI - 10.1002/cctc.201700416
Subject(s) - dehydrogenation , ammonia borane , chemistry , catalysis , nucleophile , lewis acids and bases , hydride , photochemistry , medicinal chemistry , inorganic chemistry , organic chemistry , metal
The catalytic dehydrogenation of ammonia‐borane (NH 3 BH 3 ) is dominated largely by transition‐metal catalysts. Metal‐free catalysis for NH 3 BH 3 dehydrogenation is a rarity. It is well known that mono‐boron‐based Lewis acids are largely ineffective to facilitate the catalytic dehydrogenation of NH 3 BH 3 . Herein, through theoretical investigations, we have identified the routes with catalytic potential for B(C 6 F 5 ) 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 NH 3 BH 3 in the presence of a catalyst assisted by a nucleophile and subsequent H 2 release from the ion pair. Donor solvents and the B−H hydride of NH 3 BH 3 act as a nucleophile to facilitate ion‐pair formation from NH 3 BH 3 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 H 2 release process. The reverse is true for nondonor solvents, in which case NH 3 BH 3 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.