Frustrated Lewis Acid–Base-Pair-Catalyzed Amine-Borane Dehydrogenation

• Published in: Inorganic chemistry 2020-01, Vol.59 (2), p.1046-1056
• Main Authors: Bhattacharjee, Ishita, Bhunya, Sourav, Paul, Ankan
• Format: Article
• Language: English
• ISSN: 0020-1669; 1520-510X
• DOI: 10.1021/acs.inorgchem.9b02561

Metal-free catalysis by sterically encumbered Lewis Acid–Base pairs, popularly known as frustrated Lewis pairs (FLPs), is gaining importance by the day due to its promise of providing a greener alternative to transition-metal-based catalysis. One of the stumbling blocks in achieving catalytic dehydrogenation of amine-boranes is catalyst deactivation by the reaction product. Herein, we have theoretically investigated the routes of a dimethylxanthene-derived B,P-FLP-catalyzed dehydrogenation of dimethylamine-borane (DMAB), a rare instance which avoids catalyst inhibition by the reaction product. Our computational findings reveal that the dehydrogenation proceeds via formation of the ion pair [FLP-H] – and [HMe 2 N-BH 2 -H-BH 2 -NMe 2 H] +. This step is followed by indirect B–H activation assisted by a second DMAB molecule and further H2 release via deprotonation by the PPh2 center. It is revealed that the binding of NMe 2 BH 2 to the FLP is unfavorable which ensures smooth propagation of the catalytic cycle. Catalytic dehydrogenation by the same mechanistic pathway is somewhat inhibited in the case of ammonia-borane by the same FLP due to the latent stabilization provided by strong hydrogen bonding interaction to FLP-NH 2 BH 2 adduct which renders partial deactivation of the catalyst.