Cobalt dichloride complexes with pyridyl-phosphine bidentate ligands: synthesis, characterization, and behavior in the polymerization of 1,3-butadiene

Cobalt catalysts Pyridyl-phosphine ligands Stereospecific polymerization Polybutadiene 1. Introduction Cobalt-based catalytic systems have been extensively investigated since the beginning of the stereospecific polymerization of conjugated dienes thanks to their ability to provide all possible polymers from 1,3-butadiene (cis-1,4; trans-1,4; 1,2) according to the chosen catalytic formulation [1]. Catalysts based on cobalt complexes with mono- dentate, aliphatic and aromatic, phosphines, have been reported to be able to provide poly(1,3-butadiene)s with a highly cis-1,4 structure, highly syndiotactic 1,2 structure or mixed cis-1,4/1,2 structure depending on the nature of the phosphine, thus highlighting a noticeable effect of the ligand on the polymerization selectivity [2-10]; catalysts based on cobalt complexes with bidentate ligands containing N as donor atoms (e.g., pyridyl-imino ligands, bis-imine ligands) were reported to give poly(1,3-butadiene)s with an essentially cis-1,4 structure (>=90 %) regardless of the type of ligand coordinated to the cobalt atom, thereby meaning a negligible effect of the ligand on the polymerization selec- tivity [11-41]. To try to understand the reasons for this diametrically opposite behavior of the above cobalt complexes (i.e., cobalt complexes with * Corresponding author. 1 E-mail address: giovanni.ricci@scitec.cnr.it (G. Ricci). 2 Orcid::http://orcid.org/0000-0001-6977-2920 3 Orcid::http://orcid.org/0000-0003-0161-4963 Orcid::http://orcid.org/0000-0002-5020-5544 https://doi.org/10.1016/j.ica.2023.121424 ABSTRACT New cobalt dichloride complexes with pyridyl-phosphine bidentate ligands were synthesized and characterized. Single crystals were obtained, and their molecular structure was determined by single crystal X-ray diffraction method. The complexes were then used in combination with methylaluminoxane (MAO) for the polymerization of 1,3-butadiene, obtaining predominantly cis-1,4 polymers or with a mixed cis-1,4/1,2 structure depending on the type of ligand and on the MAO/Co molar ratio. A plausible interpretation for this behavior is provided.