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

Abstract

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 ...