Synthesis and characterization of aminopyridine iron(ii) chloride catalysts for isoprene polymerization: sterically controlled monomer enchainment

In this study, a series of 2-R-6-(1-(alkylamino)methyl)pyridine-iron complexes [alkyl: (CPh₃) Fe1_H; (CHPh₂) Fe2_H; (CHPh₂) Fe3_Me; (CHMePh) Fe4_H; (CH₂Ph) Fe5_H; (CHMe₂) Fe6_H; (C₆H₁₁) Fe7_H; (CH₂(4-OMe)Ph) Fe8_H; (CH₂(4-CF₃)Ph) Fe9_H; (CH₂(2,4,6-Me₃)Ph) Fe10_H; (CH₂Ph) Fe11_Me] were synthesized and well characterized by ATR-IR spectroscopy, HRMS spectroscopy and elemental analysis. In addition, Fe3_Me, Fe4_H, Fe7_H and Fe11_Me were characterized by X-ray diffraction analysis: Fe3_Me and Fe11_Me adopted distorted tetrahedral geometries in the solid state while Fe4_H and Fe7_H were found in dimeric or polymeric forms respectively in which chlorides acted as bridging ligands. The catalytic capacities of these iron complexes were investigated for isoprene polymerization. Upon activation with a MAO cocatalyst, the catalytic activities of complexes varied as a function of the steric and electronic influences of substituents. In general, the catalysts bearing the least steric groups and electron-withdrawing groups exhibited relatively high activities. An outstanding activity of 190.6 × 10⁴ g·mol^-1·h^-1 was obtained by Fe5_H [CH₂Ph]. Moreover, changes in the steric hindrance around the metal center showed a notable effect on the selectivity of monomer enchainment. In particular, most of the polymers obtained by these complexes bearing flexible frameworks were in favor of 3,4-enchainment.