Mechanistic insight into borrowing-hydrogen N-alkylation catalyzed by an MLC catalyst with dual proton-responsive sites

Abstract

Metal–ligand cooperation (MLC) catalysis is one of the most important concepts in the field of organometallic catalysis. However, diverse functional ligands result in ambiguous mechanisms and constrain the understanding of MLC catalysis. Herein, a theoretical study based on DFT calculations is performed to shed light on the mechanistic preference of borrowing-hydrogen N-alkylation catalysed by a ruthenium complex with dual proton-responsive sites. The results suggest that the reaction pathway mediated by the α-NH site requires overcoming a higher activation energy barrier (31.4 kcal mol⁻¹) compared with the γ-NH site due to the ligand distortion after protonation. Nevertheless, the instability caused by the ligand distortion does not transform into catalytic activity for the subsequent hydrogenation reaction. In contrast, the γ-NH site facilitates the rate-determining hydride transfer step (21.1 kcal mol⁻¹) via non-covalent interaction instead of participating in the bond formation and breaking process, which is found to be a more plausible mechanism. These findings demonstrate the versatile role of ligand N–H functionality, which may provide useful guidance for the design of new MLC catalysts in the future.