Theoretical investigation of mechanism and ligand effects on half-sandwich iridium complexes for direct reductive amination

Abstract

The reaction mechanisms of direct reductive amination catalyzed by half-sandwich Cp*Ir complexes and the possible side reactions — transfer hydrogenation of ketones and N-formylation of amines have been explored by density functional theory (DFT). The reaction involves two steps: (1) the hydridoiridium formation as a rate-determining step, and (2) the hydride transfer to obtain amines. The relationships between the structure and activity of Cp*Ir complexes were analyzed by natural population analysis (NPA), Bader's atoms in molecules theory (AIM) and frontier molecular orbital (FMO). In general, the picolinamidato ligands are more favorable than the sulfonamidato ones due to the electronic effect. The catalyst with the phenyl ring bearing functional groups on the N-amide exerts the positive effect on the catalytic activity. Furthermore, the electron-donating group on the pyridine ring provides better performance owing to the higher NPA charge on the Ir atom, the lower electron density at the Ir-Cl bond critical point, and the destabilization of the highest occupied molecular orbital (HOMO).