Proposing late transition metal complexes as frustrated Lewis pairs--a computational investigation

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 (AB) 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 AB dehydrogenation. In this study, we investigated two recently synthesized C-H activation catalysts, 1 (CoIV-dinitrate complex) and 2 (NiIV-nitrate complex), and demonstrated their efficacy for AB dehydrogenation. Using density functional theory calculations and a detailed analysis, we elucidated the AB dehydrogenation mechanism of these complexes. Our results revealed that both complexes 1 and 2 can efficiently dehydrogenate AB 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 2 can release two equivalents of H2 at a temperature slightly higher than room temperature. Furthermore, we investigated AB dehydrogenation kinetics and thermodynamics in iron (Fe)-substituted systems, complexes 3 and 4. Our results showed that the strategic alteration of the central metal atom, replacing Ni in complex 2 with Fe in complex 4, resulted in enhanced kinetics and thermodynamics for AB dehydrogenation in the initial cycle. These results underscore the potential of high-valent first-row transition metal complexes for facilitating AB dehydrogenation at room temperature. Additionally, our study highlights the beneficial impact of incorporating iron into such mononuclear systems, enhancing their catalytic activity.

Mechanistic Insight Into the Reactivity of Frustrated Lewis Pairs: Liquid-State NMR Studies

Frustrated Lewis pairs (FLPs) have been widely investigated as promising catalysts due to their metal-free feature and ability to activate small molecules. Over the last few years, the structure, dynamics and interactions between the Lewis centers and their effects on the reactivity with different substrates have been studied. Nuclear magnetic resonance (NMR) is a powerful tool in studying the reaction intermediates, kinetics and mechanism of frustrated Lewis pairs (FLPs). Various NMR experiments have been applied to precisely determine the association or cooperativity of FLPs and one or two-dimensional spectra were obtained. Herein, insights coming from NMR spectroscopy for FLPs are presented, the structure and reactivity of FLPs in solution are described, and their effects on the kinetics and mechanism of different substrates are also illustrated in this review.

Unraveling the Mechanistic Details of Ru-Bis(pyridyl)borate Complex Catalyst for the Dehydrogenation of Ammonia Borane

Ru-Bis(pyridyl)borate complex (CAT) is an efficient catalyst for ammonia borane (AB) dehydrogenation. Although the mechanistic pathway of this catalyst has been theoretically investigated previously, the gap between the experimental findings and the computational results could not be bridged thus far. In our study, using density functional theory calculations, we elucidate the mechanism of AB dehydrogenation of CAT at a variable degree of ligand hydrogenation. Our results confirm that the acetonitrile ligands get reduced in the presence of AB and remain hydrogenated. Moreover, in line with experiments, we find that AB dehydrogenation on CAT proceeds via a concerted mechanism (with the free energy energetic span between 25.4 and 32.5 kcal/mol). We find that the ligand reduction alters the electronic structure and activity of CAT and the highest activity of the catalyst is expected at the fifth degree of hydrogenation of ligands with an energetic span of 25.4 kcal/mol. Additionally, the mechanism for the removal of molecular H₂ from the catalysts also alters with the degree of ligand hydrogenation. Furthermore, our results show that optimal H₂ binding free energy calculations can be used as a descriptor to identify the most active sites. Finally, this work demonstrates that ligand reduction improves the activity of the catalyst. These results highlight the importance of ligand hydrogenation in probing the activity and operating mechanism of the Ru-bis(pyridyl)borate complexes for AB dehydrogenation. Further, we identify a plausible dimer structure and rationalized experimental observation that the deactivation chemistry of this catalyst is different from the Shvo's catalyst.

Parallels between Metal-Ligand Cooperativity and Frustrated Lewis Pairs

Metal ligand cooperativity (MLC) and frustrated Lewis pair (FLP) chemistry both feature the cooperative action of a Lewis acidic and a Lewis basic site on a substrate. A lot of work has been carried out in the field of FLPs to prevent Lewis adduct formation, which often reduces the FLP reactivity. Parallels are drawn between the two systems by looking at their reactivity with CO2, and we explore the role of steric bulk in preventing dimer formation in MLC systems.