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.