Synthesis, Characterization and Catalytic Property Studies for Isoprene Polymerization of Iron Complexes Bearing Unionized Pyridine-Oxime Ligands

Abstract

Iron complexes of the types [Fe(HL)₂Cl₂] (<b>Fe1</b>: HL¹ = pyridine-2-aldoxime; <b>Fe2</b>: HL² = 6-methylpyridine-2-aldoxime; <b>Fe3</b>: HL³ = phenyl-2-pyridylketoxime; <b>Fe4</b>: HL⁴ = picolinaldehyde O-methyl oxime) were prepared and characterized by elemental analysis and IR spectroscopy. The crystal structure of <b>Fe2</b>, determined by single-crystal X-ray diffraction, featured a distorted octahedral coordination of the iron center binding with two ligands of HL². The X-ray structure and infrared spectral data indicated that pyridine-oxime ligands act as unionized bidentate ligand by coordinating with N_pyridine and N_oxime. The catalytic performance for isoprene polymerization, catalyzed by these pyridine-oxime-ligated iron complexes, was examined. For a binary catalytic system combined with MAO, complexes <b>Fe1</b>, <b>Fe3</b> and <b>Fe4</b> were found to be highly active (up to 6.5 × 10⁶ g/mol·h) in <i>cis</i>-1,4-<i>alt</i>-3,4 enchained polymerization, with average molecular weights in the range of 60-653 kg/mol and narrow PDI values of 1.7-3.5, even with very low amounts of MAO (Al/Fe = 5). Upon activation with [Ph₃C][B(C₆F₅)₄]/AlR₃ for the ternary catalytic system, theses complexes showed extremely high activities, as well about 98% yield after 2 min, to afford <i>cis</i>-1,4-<i>alt</i>-3,4-polyisoprene with a molecular weight of 140-420 kg/mol.