1,3-Butadiene Polymerizations Catalyzed by Cobalt and Iron Dichloride Complexes Bearing Pyrazolylimine Ligands

A series of pyrazolylimine ligated Co(II) and Fe(II) complexes with general formula of(PhC=N(C 6 H 3 (R 1 )2-2,6)(C 3 HN 2 (R 2 )2-3,5)MtCl 2 (R 1 = Me, R 2 = H, Mt = Co(1 a), Fe(2 a); R 1 = Me, R 2 = Me, Mt = Co(1b), Fe(2b); R 1 = iPr, R 2 = H, Mt = Co(1 c), Fe(2 c); R 1 = i Pr, R 2 = Me, Mt = Co(1 d), Fe(2 d); R 1 = i Pr, R 2 = Ph, Mt = Co(1 e), Fe(2 e)) were synthesized and thoroughly characterized.Determined by single crystal X-ray diffraction, complexes 1 b and 2 b revealed dimeric structures, in which distorted trigonal bipyramid geometries were adopted for each metal centers. In the presence of ethylaluminum sesquichloride(EASC), all the cobalt complexes displayed high activities in 1,3-butadiene polymerization, affording polybutadienes with predominant cis-1,4 contents(up to 97.0%).Influences of ligand structure and polymerization parameters on catalytic performance were investigated systematically. For pyrazolylimine iron(II) dichloride complexes, the catalytic activities and microstructures of the resultant polybutadienes were highly dependent on ligand structures and polymerization conditions. For complex 2 a, changing cocatalyst from trialkyl aluminums to methyl aluminoxane(MAO) led to an shift of selectivity from high cis-1,4-to trans-1,4-/1,2-manner. Being activated by MAO, complexes 2 a and 2 b gave trans-1,4-/1,2-binary polybutadienes, while complexes 2 c, 2 d, and 2 e afforded cis-1,4-enriched polymers.