Structure of ligands would play a crucial role in carbon-carbon coupling reactions catalyzed by N-heterocyclic carbene palladium (NHC-Pd) complexes. Not only the modification of NHC ligands but also the synergistic action of the second ligands were required to achieve better catalytic performance. However, the synergistic effect of the second ligands was often overlooked because of their considerably weaker coordination ability. Three NHC-Pd complexes composed of the same structural NHC ligand and different structural imine ligands were utilized to catalyze the Suzuki-Miyaura cross-coupling reaction. The results showed that the three imine-ligand-assisted NHC-Pd complexes exhibited significantly different catalytic effects under the same reaction conditions. Further theoretical calculations were applied to investigate the catalytic reaction mechanism of these imine-ligand-assisted NHC-Pd complexes in depth. By calculations of the complete catalytic cycle process, it was found that although imine ligands were not involved in the catalytic cycle process, competitive coordination between imine ligands and chlorobenzene should be metioned. The difference in their coordination ability with NHC-Pd(0) directly lead to the difference in the concentration of effective active centers actually involved in the catalytic cycle. The large site-blocking and electron-deficient imine-ligand-assisted NHC-Pd complexes were more favorable for the reaction. The study elucidated the mechanism of imine ligands in catalytic carbon-carbon coupling reactions and provided new strategies for the structural modulation of NHC-Pd complexes.

Key words: N-heterocyclic carbene ligand, imine ligand, Suzuki-Miyaura cross coupling reaction, reaction mechanism, density function theory (DFT) calculation