Modelling an Fe-III High-Valent Pincer-type Transition Metal Complex for Dehydrogenation of Ammonia-Borane.

Abstract

Development of efficient and cost-effective catalysts for the dehydrogenation of Ammonia-Borane (AB) has been a challenge which affects the advancement of the hydrogen economy. Over the last decades, pincer-type transition metal complexes have been known to show promising results in catalyzing many chemical reactions ranging from CO₂ reduction to C-H bond activation. In this work we investigate the ability of a high-valent Ni-III-Cl complex (complex 1) for the dehydrogenating AB. Our results show that complex 1 can dehydrogenate two equiv. of AB under reaction conditions slightly higher than room temperature. Although the abstraction of H₂ from AB can occur at room temperature, higher temperature is required due to relatively higher free-energy barriers for the formation of molecular H₂. However, when the Ni-III center is substituted by a Fe-III center (complex 2), AB dehydrogenation can occur at room temperature for one equiv. of AB with a free-energetic span of 21.07 kcal/mol, but this does not remain the same for the second catalytic cycle for complex 2 and the free-energy energetic span increases to 36.1 kcal/mol. Therefore, for the initial cycle of AB dehydrogenation, the Fe-III complex has better functionality and this work exhibits the impact of metal mono-substitution, specifically Fe in activating AB dehydrogenation at room temperature and further paves the way for simple modelling of transition metal-based complexes as catalysts for such reactions.