Redox-Active Carboranyl-Based Lewis Acids for Targeted N-H Bond Weakening

University Of California-Santa Barbara, Santa Barbara CA

Investigators

Abstract

With the support of the Chemical Synthesis Program of the Chemistry Division, Gabriel Menard of the Department of Chemistry at University of California – Santa Barbara will investigate the ability of a class of compounds, called carboranes, to activate N-H bonds in ammonia. This project is motivated by the need to develop energy storage technologies to facilitate the sustained growth of renewable energy (RE) production and distribution. For the practical use of RE, efficient energy storage is vital. This is because excess energy is obtained, for example, during periods when the sun is shining and must be stored in order to provide energy when the sun is not shining. An emerging storage technique involves use of ammonia. Ammonia, the second most produced chemical in the world, is a low-cost, energy-dense molecule with a high hydrogen content and an established global distribution infrastructure. However, its use in RE storage relies on its efficient conversion to hydrogen – a clean fuel – and nitrogen. This project will develop a set of molecules, called carboranes, that are capable of utilizing electrical energy to transform ammonia into hydrogen and nitrogen. The work focuses on the maximizing the ability of the carboranes to weaken the chemical bonds in ammonia, which prompts the fundamental steps involved in the conversion to hydrogen and nitrogen. The project will lead to the training of highly skilled chemists, including women and underrepresented minority scientists. In addition, Dr. Menard hosts monthly “Science Pub Night” event, where scientists from campus present their research to the public in an open and accessible way. This research aims to develop carborane reactivity for initiating the activation of the strong N–H bonds in ammonia for hydrogen release (or its equivalents). The underlying challenge in converting NH3 (ammonia) lies in activating the strong N–H bonds sufficiently to prompt subsequent N–N bond formation. A general strategy is being developed to systematically control N–H bond dissociation free energy by synthesizing proximal, redox-active, carborane (Cb)-appended Lewis acidic centers that allow for the decoupling and control of the thermochemical parameters by tuning the Cb cage electronics and Lewis acidic properties of these compounds. This will allow the systematic control and weakening of the N–H bonds of coordinated NH3 and target its partial splitting to hydrazine and hydrogen equivalents. The Specific Aims include: 1) the synthesis of Cb-appended, proximal Lewis acidic systems; 2) targeting the spontaneous release of H2 through thermochemical and structural control, and; 3) closing the cycle using an electrochemical reactor. The project involves a significant effort to recruit and retain a diverse group of students and an outreach component target non-traditional audiences for science education. This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.