π–π Interactions-Driven Ethylene Polymerization Using “Sandwich” Bis(imino)pyridyl Iron Catalysts

Weak noncovalent interactions are an effective strategy for modulating catalytic olefin polymerization but have never been observed in bis(imino)pyridyl iron catalysts. In this paper, a series of "sandwich" bis(imino)pyridyl iron complexes with substituted 8-(p-R-phenyl)naphthylamine (R = OMe, Me, H, CF3) were designed and synthesized for ethylene polymerization. The π–π interactions between the capping aryl groups and the pyridyl ring are clearly observed in "sandwich" bis(imino)pyridyl iron complexes by single crystal X-ray diffraction analysis, UV–vis, and photoluminescence (PL) spectra. The intramolecular π–π interactions make the naphthyl rings tilt away from the iron center in the horizontal direction, thereby causing a more open horizontal space within iron complexes for ethylene coordination. Ethylene polymerization results show that π–π interactions are a crucial driving force rather than the electronic effects of ligands. Unprecedentedly, bulky "sandwich" bis(imino)pyridyl iron catalysts produce low-molecular-weight PE with a bimodal distribution, which originates from β-H transfer to monomer modulated by the π–π interactions. Density functional theory (DFT) calculations mechanistically demonstrate that the coordination of ethylene to the iron center is a crucial step in ethylene polymerization.