Balancing the Seesaw in Mn-Catalyzed N-Heteroarene Hydrogenation: Mechanism-Inspired Catalyst Design for Simultaneous Taming of Activation and Transfer of H2

Recently, Mn-catalyzed hydrogenation reactions have gained significant interest due to their importance in various challenging transformations. Among several classes of organic substrates, including amides, ketones, carbamates, urea-derivatives, (cyclic) carbonates/polycarbonates, CO2, and CO2 derivatives, N-heteroarene hydrogenation using a Mn-based system is severely underdeveloped. Only a handful of reports are known in the literature, which mainly use expensive phosphine-based ligand systems for designing effective Mn catalysts. Mechanistically, for the known pincer Mn catalysts, the H2-activation step was facilitated by a typical metal- ligand bifunctional mechanism through the so-called "Mn-amino"/"Mnamido" platform, while the H2-transfer step (in the form of hydride and proton transfer) was triggered by judiciously designed stereoelectronically tuned P-and/or N-donor ligands attached at the Mn center. Interestingly, these two steps, i.e., H2-activation and H2-transfer, often counteract owing to opposite stereoelectronic demands at the metal center, and the existing PNP-Mn and NNP-Mn catalysts indeed suffer from disbalanced energetics in the case of these two steps. With a rationally analyzed approach, herein, we show how these two crucial steps can be simultaneously tamed and their energetics can be balanced to optimize the kinetic and thermodynamic demand. Thus, the exchange of both the P and N arms in the PNP/NNP ligand framework with a better sigma-donor, moderate pi-acceptor, and sterically nonhindering planar ligands such as N-heterocyclic carbenes (NHCs), keeping the central amino/amido-based bifunctional H2-activating motif intact, facilitated both the critical steps in the catalytic cycle by reducing the corresponding activation barriers and balancing the thermodynamic stability of the Mn-hydride species (Delta G# H2-cleavage: 19.2 kcal/mol; Delta G# H2 (hydride/proton)-transfer: 22.2 kcal/mol; Delta G H2-cleavage step: -2.8 kcal/mol). Eventually, the bis-NHC-armed CNC-Mn pincer catalyst, designed through the mechanism-inspired approach, was proved apt in efficient catalytic hydrogenation of a variety of N-heteroarenes, under 10-60 bar of H2 pressure and 60-120 degrees C temperature within 6-12 h of reaction time.