Ammonia (NH₃) is one of the most important chemicals in modern industry. It is a key component of nitrogen-based fertilizers, which enhance crop yields and support global food production. Beyond agriculture, ammonia plays a vital role in the production of plastics, dyes, and pharmaceuticals. Additionally, the catalytic oxidation of ammonia has spurred interest in its potential as a carbon-free fuel for power generation and transportation.
However, direct utilization of ammonia for chemical synthesis presents significant challenges. One major obstacle is the high energy required to break its first N–H bond. Furthermore, ammonia tends to form stable adducts with transition metal complexes rather than undergoing bond activation. Due to these difficulties, only a handful of studies have reported the successful oxidative addition of ammonia N–H bonds to a metal center. Developing new methods for efficient and sustainable ammonia bond activation and oxidation remains an important research goal.
A promising approach to overcome metal catalyst poisoning caused by the strong binding of ammonia to metal centers is to explore metal-free systems for N–H bond activation. In a recent study published in the Journal of the American Chemical Society, the USC team of graduate students Amanda Humphries and Gabrielle Tellier, and Prof. Dmitry Peryshkov, in collaboration with Prof. Anthony Chianese from Colgate University, tackled this challenge.
The researchers developed novel redox-active phosphine reagents based on boron clusters. These icosahedral cluster molecules exhibit unique electronic structure that enables redox activity which can drive chemical transformations. By incorporating external donor groups, such as phosphines, these metal-free molecules composed of Earth-abundant elements can mimic the behavior of precious metals in bond activation and catalysis.
In this study, the N–H bonds of ammonia were activated at room temperature across two phosphine centers, forming P–NH₂ and P–H groups. Unlike many ammonia activation examples in literature, this system operates under ambient conditions and is tolerant of both air and water. Experimental and computational analyses revealed that the boron cluster electron-accepting properties enhance the electrophilicity of the phosphine groups, facilitating the initial coordination and bond cleavage of ammonia.
Furthermore, the activated product can undergo complete oxidation, breaking all remaining N–H bonds to form a cyclic phosphazenium cation. This work represents the first example of ammonia activation and subsequent hydrogen atom abstraction, which are the key chemical steps necessary for its potential use as an alternative energy storage medium, by a metal-free, main-group element system.
Humphries, A. L.; Tellier, G. A.; Smith, M. D.; Chianese, A. R.; Peryshkov, D. V. N–H Bond Activation of Ammonia by a Redox-Active Carboranyl Diphosphine Journal of the American Chemical Society, 2024, 146, 33159–33168.