Advances with Ammonia‐Borane: Improved Recycling and Use as a Precursor to Atomically Thin BN Films

Abstract

ammonia-borane · boranes · boron nitride · hydrogen storage · hydrogenation Amine-boranes, exemplified by ammonia-borane (AB), H 3 N·BH 3 , represent some of the simplest and best-known inorganic molecules. Over the past decade these compounds have attracted considerable attention as portable hydrogen storage materials, due in the main part to their high gravimetric hydrogen contents (ca. 20 % by weight for AB).[1] This has motivated the development of new synthetic methods for convenient hydrogen release, and a variety of highly active catalysts that function under mild conditions are now known. These procedures have recently been adapted to allow the preparation of new polymeric materials. For example, polyaminoboranes, (RNH-BH 2 ) n , have been prepared by catalytic dehydrogenation of primary amine-boranes, such as MeNH 2 ·BH 3 .[2] Amine-boranes have also been widely used as reducing agents, including for the preparation of metal nanoparticles, and in hydrogen-transfer reactions involving organic substrates.[3] Moreover, as a consequence of the expanded interest in hydrogen release, the field of amineborane coordination chemistry is now under active development.[4] Discoveries in this new area promise improved fundamental understanding of the activation and transformation of these fascinating molecules at transition-metal centers, processes that appear central to both their dehydrogenation and polymerization chemistries.The dehydrogenation of AB [1] may be viewed in terms of three distinct exergonic steps, each associated with the loss of one equivalent of hydrogen. The challenge with developing AB as a practical hydrogen-storage material therefore lies not only in the liberation of hydrogen but also in the regeneration of the spent fuel. Furthermore, this problem becomes particularly acute if the reaction proceeds with complete dehydrogenation to form boron nitride, which is thermodynamically very stable.In practice, the near-ambient-temperature routes to dehydrogenate AB do not result in the formation of boron nitride but yield a spent fuel whose precise nature depends on the method of hydrogen release. Polyborazylene (BNH x (0 < x < 2), Scheme 1) functions as a model for such species, as it contains the fused borazine rings and BÀH and NÀH bonds that should be common to any dehydrogenated material. A viable synthetic route to convert polyborazylene back to AB would therefore have important implications for the feasibility of using the latter for hydrogen storage.An impressive recent advance by Gordon and co-workers describes the successful conversion of polyborazylene to AB upon treatment with hydrazine in liquid ammonia.[5] Polyborazylene was found to react with hydrazine at room temperature to afford predominantly hydrazine-borane. Exchange of the hydrazine in this compound for ammonia, however, could not be achieved at this temperature owing to the greater thermodynamic stability of the hydrazine adduct. Heating to 60 8C was therefore employed to cleave the BÀN bond and afford AB by combination of the l...

Structure, Properties and Applications of Two‐Dimensional Hexagonal Boron Nitride

“…Borazane (BH 3 NH 3 ), or ammonia borazine, one of the most popular solid precursors for h-BN synthesis, undergoes multiple steps of thermal decomposition at different temperatures. [204][205][206] Borazine, polyaminoborane, polyiminoborane, aminoborane, and hydrogen are a few among the several decomposition products. This makes the reaction temperature extremely crucial since it can control the formation of desirable products.…”

2D III‐Nitride Materials: Properties, Growth, and Applications

“…[ 181–185 ] The common precursors and substrates used to develop h‐BN system by CVD are summarized in Table 6 . [ 180,186–213 ]…”

Lewis base assisted B–H bond redistribution in borazine and polyborazylene

“…Not surprisingly there has been a multitude of publications on AB and amine-boranes, with several comprehensive reviews on the topic. [1][2][3][4] Since the formation and regeneration of B-H bonds is energetically expensive, 5,6 the ability to utilize and recover the maximum number of B-H bonds is important for the efficient use of AB. Herein we describe the recovery of B-H bonds from two common products that result from AB dehydrocoupling, borazine and polyborazylene (PB).…”