The Catalyst Review September 2024 - 13

Experimental Abstracts
π−π Interactions‑Driven Ethylene Polymerization Using " Sandwich "
Bis(imino)pyridyl Iron Catalysts
Bis(imino)pyridyl iron catalysts play a prominent role in ethylene polymerization/oligomerization reactions due to their nontoxic
properties, high metal abundance, and insensitivity to air and moisture. In contrast to α-diimine nickel/palladium catalysts, bis(imino)
pyridyl iron catalysts produce entirely linear polyethylene of high molecular weight and possessing a bimodal distribution. Herein, the
authors explore the modulating effect of noncovalent, π−π, interactions on catalytic olefin polymerization using a series of " sandwich "
bis(imino)pyridyl iron complexes with substituted 8-(p-R-phenyl)naphthylamine (R = OMe, Me, H, CF3
). They found that bulky " sandwich "
complexes in this series produce low-molecular-weight PE with a bimodal distribution, originating from β-H transfer to monomer
modulated by π−π interactions.
Preparation of the " sandwich " bis(imino)pyridyl iron complexes (Figure 1) was followed by extensive analysis, including single crystal
X-ray diffraction analysis, UV−vis, and photoluminescence (PL) spectra. The results of these studies confirm the existence of the π−π
interactions between the capping aryl groups and the pyridyl ring in both solid and solution. 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 the ethylene coordination despite large, buried volumes.
Figure 1. Synthetic Routine of the Bis(imino)pyridyl Ligands and Their Iron Complexes
The four bulkier iron complexes were then used as catalyst precursors for ethylene polymerization after activation with 1000 equiv.
of MAO at different reaction temperatures. The detailed ethylene polymerization results are summarized in Table 1. The " sandwich "
Fe-Me and Fe-H precursors were found to be inactive for ethylene polymerization. At the same time, Fe-OMe and Fe-CF3
exhibited
good activity, even though they are bulkier than Fe-Me and Fe-H based on the calculated buried volumes. This significant difference in
ethylene polymerization activity among four iron catalysts is clearly not due to the calculated buried volumes.
Unexpectedly, bulky " sandwich " bis(imino)pyridyl iron catalyst Fe-CF3
produces low-molecular-weight PE with a bimodal distribution.
-catalyzed ethylene
Two chain transfer pathways, including β -H transfer to ethylene monomer and chain transfer to aluminum, were identified by separating
the two fractions. The β -H transfer to the ethylene monomer is the dominant chain transfer pathway in Fe-CF3
polymerization, resulting in a substantial proportion of low-molecular-weight unsaturated polyethylene. The authors ascribe these
unusual ethylene polymerization behaviors to weak noncovalent π−π interactions rather than the electronic effects of ligands. In
addition, density functional theory (DFT) calculations show that the coordination of ethylene to the iron center is a crucial step in ethylene
polymerization. These findings represent the first reported weak noncovalent interactions observed in bis(imino)pyridyl iron catalysts,
thus opening a new pathway for modulating these complexes' structures and catalytic polymerization properties. Cheng Z, Gao H, Qiu Z,
et al. (2024). ACS Catal., 14, 7956−7966
Table 1. Ethylene Polymerization Results Using Bis(imino)pyridyl Iron Catalystsa. aPolymerization conditions: 2.4 μmol iron catalyst, Al(MAO)/Fe=1000, 10 atm ethylene
pressure, 30 min, 68mL of toluene and 2mL of CH2
Cl2. bHammett constants of substituent R. cActivity in 106
at 150°C. eDetermined by DSC, second heating. fNo activity and not determined.
The Catalyst Review
September 2024
13
g PE/ (mol Fe·h). dDetermined by GPC in 1,2,4-trichlorobenzene

The Catalyst Review September 2024

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