Mechanistic Insight of High-Valent First-Row Transition Metal Complexes for Dehydrogenation of Ammonia Borane

Designing an efficient and cost-effective catalyst for ammonia borane (<b>AB</b>) dehydrogenation remains a persistent challenge in advancing a hydrogen-based economy. Transition metal complexes, known for their C-H bond activation capabilities, have emerged as promising candidates for <b>AB</b> dehydrogenation. In this study, we investigated two recently synthesized C-H activation catalysts, <b>1</b> (Co^IV-dinitrate complex) and <b>2</b> (Ni^IV-nitrate complex), and demonstrated their efficacy for <b>AB</b> dehydrogenation. Using density functional theory calculations and a detailed analysis, we elucidated the <b>AB</b> dehydrogenation mechanism of these complexes. Our results revealed that both complexes <b>1</b> and <b>2</b> can efficiently dehydrogenate <b>AB</b> at room temperature, although the abstraction of molecular H₂ from these complexes requires slightly elevated temperatures. We utilized H₂ binding free energy calculations to identify potentially active sites and observed that complex <b>2</b> can release two equivalents of H₂ at a temperature slightly higher than room temperature. Furthermore, we investigated <b>AB</b> dehydrogenation kinetics and thermodynamics in iron (Fe)-substituted systems, complexes <b>3</b> and <b>4</b>. Our results showed that the strategic alteration of the central metal atom, replacing Ni in complex <b>2</b> with Fe in complex <b>4</b>, resulted in enhanced kinetics and thermodynamics for <b>AB</b> dehydrogenation in the initial cycle. These results underscore the potential of high-valent first-row transition metal complexes for facilitating <b>AB</b> dehydrogenation at room temperature. Additionally, our study highlights the beneficial impact of incorporating iron into such mononuclear systems, enhancing their catalytic activity.