Highly Stereoregular 1,3-Butadiene and Isoprene Polymers through Monoalkyl-N-Aryl-Substituted Iminopyridine Iron Complex-Based Catalysts: Synthesis and Characterization

Some novel monoalkyl-N-aryl-substituted iminopyridine iron chloride complexes, differing in the nature of the substituent at the iminic carbon and at the ortho position of the aryl ring, were synthesized and characterized. For one of them, single crystals were obtained, which allowed for the determination of its crystalline structure, in which the iron center is coordinated to the chlorides and to the two nitrogen atoms of the ligand. The coordination around iron is distorted tetrahedral, a coordination mode rarely identified for FeCl₂ adducts with bidentate nitrogen ligands. All the complexes were used, in combination with methylaluminoxane, for the polymerization of 1,3-butadiene and isoprene, providing syndiotactic 1,2 poly­(1,3-butadiene)­s and poly­(isoprene)­s with a predominantly cis-1,4/3,4 alternating structure, in which short cis-1,4 sequences of three or five units, whose length depends on the nature of the ligand on the iron atom, are present. A detailed NMR characterization (¹H-, ¹³C-, and 2D experiments) of the resultant poly­(isoprene)­s is reported, and a tentative scheme for the formation of the novel isoprene polymers is proposed.