Ammonia- and Amine-Borane and Borohydride Adducts: Synthesis, Characterization, and Dehydrogenation Behaviors

Beaumont, Paul
Journal Title
Journal ISSN
Volume Title
[Honolulu] : [University of Hawaii at Manoa], [May 2015]
High theoretical densities, favorable dehydrogenation temperatures and kinetics continue to make ammonia-borane (AB) a tantalizing option for a hydrogen storage material. However, in order to become a viable storage material, several challenges must be overcome. Several strategies have been explored addressing these challenges associated with AB, namely the polymerization of the dehydrogenated material and subsequent regeneration. To investigate limiting polymerization through solvation, AB was dissolved in several imidazoles, in a 1:1 molar ratio. The solutions were subjected to a range of increasing temperatures for periods of 20 hours. It was discovered that the through transamination, the borane would form an adduct with the solvent, followed by the subsequent dimerization forming imidazabole isomers. Reactions performed in the presence of an Ir-based pincer catalyst, IrH2(1,3-C6H3-(OPtBu2)2), demonstrated that AB would undergo dehydrogenation at room temperature in small yields before the occurrence of transamination. Higher reaction temperatures were found to result in the formation of only the imidazabole isomers with little released H2. Substituted amine adducts of borane were explored as a method to limit the degree of polymerization of the dehydrogenation products. Furthermore, N bound cycloalkyl groups were chosen as a means to provide additional H2 during dehydrogenation through their aromatization while dehydrogenation of the borane adducts across the B-N bonds were found to occur. However, no C-H bond activation occurred, even when treated with a known alkane dehydrogenation catalyst, IrH2(1,3-C6H3-(OPtBu2)2). In another attempt to limit polymerization, two additional novel ammonium borohydride derivatives were synthesized, meta- and para-bis(ammonium borohydride)benzene. This proved more successful as dehydrogenation experiments demonstrated the release of high purity H2 with trace amounts of diborane. Additionally it was found that treatment of the dehydrogenated material with hydrazine sulfate at 40 °C for 7 days resulted in partial regeneration to the ammonia-borane derivative. Investigations were made into understanding the relationship of the ammonia present in LiSc(BH4)4∙4NH3 and its dehydrogenation behavior. IR analysis of the complex revealed new peaks in the B-Ht stretching region that can be assigned to the interaction of the terminal H atom of the borohydride with a neighboring NH3 group. No significant differences observed in the B-Ht stretching frequency for complexes synthesized with NH3 and ND3 suggest that the B-Ht interaction my not be subject to isotopic perturbation or that the interaction does not involve N—Hδ+∙∙∙-δH—B bonding, but rather a direct interaction of the N bound lone pair on the terminal H atom of borohydride. Through this research, many variations to AB were proposed and explored to improve the viability of using AB and AB derivatives as a hydrogen storage material. Therefore, allowing for the expansion and solidification of what is currently known about this material and related derivatives.
Ph.D. University of Hawaii at Manoa 2015.
Includes bibliographical references.
Ammonia-borane, hydrogen, storage material, ammoniate, borohydride
Access Rights
Email if you need this content in ADA-compliant format.