Unraveling the Crucial Role of Metal-Free Catalysis in Borazine and Polyborazylene Formation in Transition-Metal-Catalyzed Ammonia–Borane Dehydrogenation

Abstract

Though the recent scientific literature is rife with experimental and theoretical studies on transition-metal (TM)-catalyzed dehydrogenation of ammonia–borane (NH₃·BH₃) due to its relevance in chemical hydrogen storage, the mechanistic knowledge is mostly restricted to the formation of aminoborane (NH₂BH₂) after 1 equiv of H₂ removal from NH₃·BH₃. Unfortunately, the chemistry behind the formation of borazine and polyborazylene, which happens only after more than 1 equiv of H₂ is released from ammonia–borane in these TM-catalyzed homogeneous reactions, largely remains unknown. In this work we use density functional theory to unravel the curious function of “free NH₂BH₂”. Initially, free NH₂BH₂ molecules form oligomers such as cyclotriborazane and <i>B</i>-(cyclodiborazanyl)­aminoborohydride. We show that, through a web of concerted proton and hydride transfer based dehydrogenations of oligomeric intermediates, cycloaddition reactions, and hydroboration steps facilitated by NH₂BH₂, the development of the polyborazylene framework occurs. The rate-determining free energy barrier for the formation of a polyborazylene template is predicted to be 25.7 kcal/mol at the M05-2X­(SMD)/6-31++G­(d,p)//M05-2X/6-31++G­(d,p) level of theory. The dehydrogenation of BN oligomeric intermediates by NH₂BH₂ yields NH₃·BH₃, suggesting for certain catalytic systems that the role of the TM catalyst is limited to the dehydrogenation of NH₃·BH₃ to maintain optimal amounts of free NH₂BH₂ in the reaction medium to enable polyborazylene formation. TM catalysts that fail to produce borazine and polyborazylene falter because they rapidly consume NH₂BH₂ in TM-catalyzed polyaminoborane formation, thus preventing the chain of events triggered by NH₂BH₂