Ammonia-borane, H3NBH3, is an intriguing molecule for chemical hydrogen storage applications. With both protic N-H and hydridic B-H bonds, three H atoms per main group element, and a low molecular weight, H3NBH3 has the potential to meet the stringent gravimetric and volumetric hydrogen storage capacity targets needed for transportation applications. Furthermore, devising an energy-efficient chemical process to regenerate H3NBH3 from dehydrogenated BNHx material is an important step towards realization of a sustainable transportation fuel. In this perspective we discuss current progress in catalysis research to control the rate and extent of hydrogen release and preliminary efforts at regeneration of H3NBH3.

Liberating hydrogen: Strong Lewis or Brønsted acids react with ammonia–borane (H3NBH3) to form an in situ boronium cation, resulting in the formation of a mixture of cyclic and acyclic BNHx oligomers and the liberation of H2 (see scheme). A proposed mechanism is supported by an examination of the reaction thermodynamics using density functional theory.

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Dabei kommt Wasserstoff heraus: Starke Lewis‐ oder Brønsted‐Säuren erzeugen in situ aus dem Ammoniak‐Boran‐Komplex (H3NBH3) ein Boroniumkation, das unter Freisetzung von H2 zu einer Mischung aus cyclischen und acyclischen BNHx‐Oligomeren reagiert (siehe Schema). Der vorgeschlagene Mechanismus wird durch Dichtefunktionalrechnungen zur Thermodynamik der Reaktion gestützt.

Reactions of nitriles RCN with the sterically encumbered Mo(N[t-Bu]Ar)3 (1, Ar = 3,5-C6H3Me2) or the somewhat less hindered Mo(H)(η2-Me2CNAr)(N[i-Pr]Ar)2 (2) have been investigated. Where R = Me or Ph, reaction with 1 results in reductive nitrile coupling and the formation of a diiminato product [μ-NC(R)C(R)N][1]2. In contrast, reaction of 1 with Me2NCN surprisingly results in a stable, albeit highly congested, η2 adduct of the nitrile. When the less sterically hindered 2 is used, reaction with PhCN gives the diiminato product analogous to the one mentioned for the tert-butyl system, [μ-NC(Ph)C(Ph)N][Mo(N[i-Pr]Ar)3]2, where molybdaziridine-hydride 2 has provided access to the three-coordinate Mo(N[i-Pr]Ar)3 (3) moiety. Use of a more bulky nitrile such as MesCN (Mes = 2,4,6-C6H2Me3) results in formation of a bis-η1 compound, (η1-MesCN)2[3]. Use of 9-anthracenylcarbonitrile results in head-to-tail C−C coupling of two monomers via the anthracenyl moiety. Detailed variable-temperature EPR and 2H NMR data are included for both molybdenum-containing starting materials and selected reaction intermediates and products.