Influence of Charge Delocalization on Manganese Catalyzed Transfer Hydrogenation

Hydrogen transfer reactions promoted by Mn(I)-based catalysts are a recently emerged field of research. Herein, we demonstrate the crucial effect of charge delocalization on the catalytic activity in transfer hydrogenation. Therefore, we synthesized pyridyl-pyrazole ligands and a pyridyl-pyridazinone ligand which exhibit resonance structures upon deprotonation with varying capability to transfer charge to the manganese center. The corresponding Mn(I) complexes were characterized and their activity as catalyst in transfer hydrogenation of acetophenone was determined using in-situ IR techniques. Kinetic data and DFT calculations reveal that the ligands are deprotonated throughout the catalytic cycle and that the catalysts in which resonance structures point towards more negative charge at the metal center are also the more active ones. Thus, we deliver ample evidence that for effective hydrogen transfer the manganese catalyst requires a ligand with a deprotonable moiety allowing efficient delocalization to the metal while geometrical access of the acidic proton to the substrate is not essential. Our findings highlight the importance of an anionic hydrido intermediate, where mesomerism allows an effective charge transfer to the metal. This may support the ongoing development of new manganese-based catalysts. Charge delocalization was found to be a crucial factor in transfer hydrogenation reactions catalyzed by manganese complexes. A series of catalysts was synthesized and characterized. The catalytic activity of those complexes was studied using in-situ IR spectroscopy and computational methods, revealing the crucial effects of mesomerism and its influence on catalytic activity. image