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] (Fe1: HL1 = pyridine-2-aldoxime; Fe2: HL2 = 6-methylpyridine-2-aldoxime; Fe3: HL3 = phenyl-2-pyridylketoxime; Fe4: HL4 = picolinaldehyde O-methyl oxime) were prepared and characterized by elemental analysis and IR spectroscopy. The crystal structure of Fe2, 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 Npyridine and Noxime. 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 Fe1, Fe3 and Fe4 were found to be highly active (up to 6.5 × 106 g/mol·h) in cis-1,4-alt-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 cis-1,4-alt-3,4-polyisoprene with a molecular weight of 140–420 kg/mol.

Catalytic Behavior of Cobalt Complexes Bearing Pyridine–Oxime Ligands in Isoprene Polymerization

“…Moreover, the N oxime atoms of each ligand have a stronger coordination than N pyridine at the cobalt center according to the bond lengths [Co-N1 = 2.188 (17), Co-N2 = 2.098 (16), Co-N3 = 2.219 (16), Co-N4 = 2.132 (16)]. Consistent with the previous work of pyridine-oxime iron complex [65], the oxime proton is unionized. (15).…”

“…2). However, it needs to be emphasized that the pyridine-oxime ligated iron complex bearing one 6-methyl substituent at pyridine ring showed lower activity because of steric hindrance in the previous work [65]. Zhang's group also reported that the substituent at the 6-position of the pyridine ring significantly influenced the catalytic performances of the iminopyridine ligated Co complexes.…”

“…Ligands L1 and L3 were purchased and used directly without treatment. The ligands L2 and L4 were synthesized and characterized according to the previous report [65]. A typical procedure for the synthesis of ligand was carried out as follows.…”

“…Due to our ongoing burning desire to develop polymerization catalysts with high performance, we are interested in transition metal catalysts coordinating with N,N-bidentate ligands for diene polymerization [58][59][60][61][62][63][64]. In a recent piece of work, we reported that the pyridine-oxime ligand exhibited high performance in iron catalyzed isoprene polymerization with very low amount of methylaluminoxane [65]. Confirming the fundamental role played by the nature of the pyridine-oxime ligand and the type of cobalt catalyst in determining the catalytic activity and selectivity [66,67], herein we describe a series of pyridine-oxime ligated cobalt catalysts and study their catalytic properties for isoprene polymerization.…”

Synthesis, Characterization, and Catalytic Behaviors in Isoprene Polymerization of Pyridine–Oxazoline-Ligated Cobalt Complexes

“…In 2018, Gong’s group demonstrated that cis -1,4- alt -3,4 polyisoprene can be achieved with aminophosphine(ory)-fused bipyridine-based cobalt complexes in combination with MMAO [ 3 ]. Subsequently, Wang and coworkers investigated the effect of fluorine substituents on isoprene polymerization catalyzed by iminopyridine-supported cobalt catalysts activated by AlEt 2 Cl or MAO/[Ph 3 C][B(C 6 F 5 ) 4 ] [ 32 , 33 , 34 ]. The binary system exhibited high catalytic activity and cis -1,4 selectivity, and also obtained high-molecular-weight polyisoprene.…”