Influence of imine‐carbon substituent in 6‐bromo‐2‐iminopyridine‐based cobalt(II) complexes on 1,3‐butadiene polymerization

Abstract

6-Bromo-2-iminopyridine cobalt(II) complexes bearing different imine-carbon substituents (Co1-Co7) were synthesized and subsequently employed for 1,3-butadiene polymerization. All the complexes were identified using Fourier transform infrared spectra and elemental analysis, and complexes Co1 and Co3 were further characterized using single-crystal X-ray diffraction analysis, demonstrating they adopted distorted trigonal bipyramidal and tetrahedral geometries, respectively. Activated by methylaluminoxane, these complexes exhibited high cis-1,4 selectivity, and the activity was highly dependent on the substituent at the imine-carbon position of the ligand. Addition of PPh 3 to the polymerization systems could enhance the catalytic activity and simultaneously switched the selectivity from cis-1,4 to cis-1,2 manner. On the basis of the obtained results, a plausible mechanism involving the regulation of selectivity and activity is proposed.

Dienes Polymerization: Where We Are and What Lies Ahead

“…Highly cis -1,4-polybutadienes (∼95%) were obtained with catalysts based on bidentate phosphine Co complexes (CoCl 2 [R 2 P­(CH 2 ) n PR 2 ]–MAO; R = Me, Et, Ph; n = 1, 2), independently of the type of bidentate phosphine ligand bonded to the cobalt atom. ,, Predominantly cis -1,4-polybutadienes were also recently obtained with several new catalytic systems based on cobalt complexes with bi- and tridentate ligands containing N and/or O as donor atoms (e.g., bis-imines, bis-imino-pyridines, keto-imines, and keto-pyridinimines), regardless of the nature of the ligand coordinated to the cobalt atom. ,,,,− …”

“…190 High trans-1,4 polybutadiene (>90%) was obtained by Lee et al by using a quaternary cobalt system, which was composed of Co(2-ethylhexanoate) 2 /HOPh/Al( i Bu) 2 H/phosphite (phosphate). 191 The fact that almost all of the above-mentioned catalytic systems based on cobalt complexes with bi-or tridentate ligands, containing N and/or O as donor atoms, provide essentially cis-1,4 polymers from butadiene, regardless of the nature of the ligand, while we have observed how the systems based on cobalt complexes with monodentate phosphine ligands provide polybutadiene with different structure depending on the phosphine coordinated to the cobalt atom, may give rise to some doubts/perplexities about the actual structure of the active catalytic center in these systems and suggest the following interpretation. One of the hypotheses sketched during the many fruitful discussions on this subject with Prof. Porri was the following.…”

“…High trans -1,4 polybutadiene (>90%) was obtained by Lee et al by using a quaternary cobalt system, which was composed of Co­(2-ethylhexanoate) 2 /HOPh/Al­( i Bu) 2 H/phosphite (phosphate) …”