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 B-(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.