PPh2-ImPy Ligands with an Unsymmetric N-Aryl Ring for Efficient Acrylate Synthesis Reaction from Ethylene and CO2

Abstract

Building on the previously reported PPh₂-ImPy ligand system which showed an excellent reactivity in the acrylate synthesis reaction, a new ligand design strategy was applied to further improve reaction efficiency. While maintaining the structural stability of the original ligand system, two substituents with different steric bulkiness were introduced into the N-aryl ring. The resulting unsymmetric steric environment around the Ni center has the possibility that could induce certain catalytic steps to proceed on more or less hindered side depending on their steric demands. This strategy is likely to facilitate ligand dissociation step on the more hindered side, while promoting oxidative coupling with CO2 step on the less hindered side, thereby enabling an efficient progression of the catalytic cycle. As a result, this may lead to enhanced catalytic performance in the acrylate synthesis reaction. Of the four different ligands with an unsymmetric N-aryl ring, the PPh₂–ImPy ligand (TON 146) with i-propyl and n-propyl substituents at the ortho positions of the N-aryl ring showed the highest reactivity in the acrylate synthesis reaction. Also, it exhibited more improved reactivity than the previously reported PPh₂–ImPy ligand (TON 122) with symmetric N-aryl ring. The result showed the possibility that the unsymmetric steric environment tuned by the difference in the steric bulkiness and positional arrangement of each substituent on N-aryl ring could lead to the improvement of the catalytic efficiency in the acrylate synthesis reaction.