Combining Protons and Hydrides by Homogeneous Catalysis for Controlling the Release of Hydrogen from Ammonia–Borane: Present Status and Challenges

Ammonia–borane (AB) has been in the spotlight for its much touted potential as an onboard vehicular hydrogen delivery material. Over the past decade, catalyzed dehydrogenation/dehydrocoupling reactions for releasing H2 from the maximum available 3 equiv in AB have gained significant momentum. In this Perspective, we focus on the homogeneous AB dehydrogenation catalysis, by both transition metal (TM)-based and metal-free systems. Several questions pertaining to underlying mechanisms, nature of intermediates, and catalyst efficacy have surfaced as the multitude of discoveries in the field has built up at a fast pace. The varied fate of the dehydrogenation reactions of AB with different catalysts yielding different end products ranging from polyaminoborane (PAB) to polyborazylene (PBZ) and the ability/inability of catalysts to release more than 1 equiv of H2 from AB have fuelled the genesis of several mechanistic hypotheses. However, the copious investigations on the experimental and theoretical fronts have led to some convergent views. We try to highlight the general consensus on mechanistic underpinnings and the crucial role of important intermediates in determining the fate of catalysis for this family of dehydrocoupling reactions. We also point out the unresolved issues along with a short note on the regeneration of AB.