Metal Mediated Reduction of Borazines for the Regeneration of Boron-Nitrogen Hydrogen Storage Materials.

Abstract

Boron-nitrogen materials such as ammonia borane and cyclotriborazane are promising candidates for hydrogen storage due to their high gravimetric and volumetric hydrogen capacities. Though many examples of hydrogen release from these materials have been reported in the scientific literature, low-energy regeneration of the spent fuel products (such as borazine and polyborazylene) remains a significant challenge. Therefore, developing reactions that directly convert these spent fuel products back to useful hydrogen storage materials would be a key advance towards realizing practical application of B-N hydrogen storage materials. This thesis explores transition-metal coordination as a promising strategy to mediate indirect hydrogenation of B=N bonds in these materials and establishes proof of concept for metal-mediated B=N bond reduction.

The reactivity of Cr(CO)3 and Mn(CO)3+ coordination complexes of hexamethylborazine (a model for B-N spent fuel) with respect to hydride and proton addition is examined. Reduction of these complexes by hydride addition is shown to produce stable dearomatized borazine complexes; which is the most energy intensive step in recycling borazine to useful hydrogen storage materials such as cyclotriborazane. Hydride delivery is accomplished through direct addition of chemical hydrides, as well as one and two electron transfer pathways. In each case, the mechanism of these transformations is studied in detail, to provide a basis for adaptation of this methodology to a catalytic scheme for the regeneration of B-N materials. Protonation of a dearomatized borazine complex is also demonstrated resulting in complete B=N bond reduction. By establishing proof-of-concept for metal mediated B=N bond reduction, this work builds a foundation for later studies which will seek to develop a comprehensive, low-energy, B-N spent fuel regeneration pathway.