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.

N‐Heterocyclic Phosphenium Complex of Manganese: Synthesis and Catalytic Activity in Ammonia Borane Dehydrogenation

“…[27][28][29] Based on these findings, it was suggestedt hat participation of NH 2 BH 2 in the dehydrogenation reactionm ay have ac rucial influence on the observation of class Iorclass II behaviour of acatalyst. [28,30] Interestingly,t he trapping reagent does not disturb the formationo fp olyaminoborane by class Ic atalysts because the dehydrogenation and polymerisation of NH 3 BH 3 take place directly at the metal and no free NH 2 BH 2 is involved. [28,30] Interestingly,t he trapping reagent does not disturb the formationo fp olyaminoborane by class Ic atalysts because the dehydrogenation and polymerisation of NH 3 BH 3 take place directly at the metal and no free NH 2 BH 2 is involved.…”

“…[21,27,28] In the latter case, NH 2 BH 2 hasb een identifieda sa ni mportant intermediate [29] and DFT calculations indicate that free NH 2 BH 2 can by itself dehydrogenate AB to polyborazylene. [21,27,28] In the latter case, NH 2 BH 2 hasb een identifieda sa ni mportant intermediate [29] and DFT calculations indicate that free NH 2 BH 2 can by itself dehydrogenate AB to polyborazylene.…”

“…[21,27,28] In the latter case, NH 2 BH 2 hasb een identifieda sa ni mportant intermediate [29] and DFT calculations indicate that free NH 2 BH 2 can by itself dehydrogenate AB to polyborazylene. [28] Experimentally,t he presence of NH 2 BH 2 can be detected by its scavenging with cyclohexanetoyield the formal hydroboration product Cy 2 BNH 2 (Cy = cyclohexyl). [28] Experimentally,t he presence of NH 2 BH 2 can be detected by its scavenging with cyclohexanetoyield the formal hydroboration product Cy 2 BNH 2 (Cy = cyclohexyl).…”

“…[28] Experimentally,t he presence of NH 2 BH 2 can be detected by its scavenging with cyclohexanetoyield the formal hydroboration product Cy 2 BNH 2 (Cy = cyclohexyl). [28,29] The observation of the formation of borazine and polyborazylene in the phosphenium-complex-catalysed AB dehydrogenation suggests that compound 2 behaves like ac lass II catalyst. [28,29] The observation of the formation of borazine and polyborazylene in the phosphenium-complex-catalysed AB dehydrogenation suggests that compound 2 behaves like ac lass II catalyst.…”

“…The formation of borazine and polyborazylenes [24] and as peeding up of the AB dehydrogenation [25] during the cyclohexene trapping of NH 2 BH 2 hadp reviously been observed, althougha ne xplanation for the latter is as yet unavailable. [27][28][29] With the aim to get further insighti nto the functioning of the phosphenium complex catalysis, we carriedo ut aD FT study on the reaction of the model complex 2 Me with AB. The outcomeo fo ur AB dehydrogenation studies furthers uggest that the presence of the phosphenium complexi si ndispensable to initiate the reaction, althoughi ti sp resumably not involved in every reaction step andt he formation of the various dehydrocoupling products mayinpart be autocatalytically promoted by NH 2 BH 2 (or,i fc yclohexene had been added, by Cy 2 BNH 2 ).…”