Synthesis, characterization and butadiene polymerization studies of cobalt(II) complexes bearing bisiminopyridine ligand

A series of 2,6-bis(imino)pyridyl Co(II) complexes of the general formulas [2,6-(ArNCMe)2C5H3N]CoCl2 (Ar = –C6H5, 3a; 2-MeC6H4, 3b; 2-EtC6H4, 3c; 2-iPrC6H4, 3d; 2,6-iPr2C6H3, 3e; 4-iPrC6H4, 3f; 4-FC6H4, 3g; 4-CF3C6H4, 3h; 2-FC6H4, 3i; 2,6-F2C6H3, 3j; 2-Me-4-FC6H3, 3k and 2,6-Me2-4-FC6H2, 3l) and [2,6-(ArNCH)2C5H3N]CoCl2 (Ar = –C6H5, 3m; 2-EtC6H4, 3n and 4-iPrC6H4, 3o) have been synthesized and characterized. The structures of new complexes 3a, 3f–3h and 3m–3o are further confirmed by X-ray crystallography. All complexes adopt distorted trigonal bipyramidal configuration with the equatorial plane formed by the pyridyl nitrogen atoms and the two chlorine atoms. In the complexes 3m and 3o, three aromatic rings are essentially coplanar, which is in sharp contrast to the other complexes, where three rings are almost orthogonal to each other. With methylaluminoxane (MAO) as cocatalyst in toluene at room temperature, the complexes show moderate to high conversion (42–99%) in butadiene polymerization, producing polybutadiene with tunable cis-1,4 structure (77.5–97%) and controllable molecular weight and molecular weight distribution. The catalytic activity, selectivity as well as the molecular weight and molecular weight distribution of the resultant polymer are found to be dependent on the size and nature of substituents on iminoaryl rings and their positions located. By deliberately tuning the ligand structure, more efficient catalyst in terms of high activity and high selectivity can be obtained.

Graphical abstractA series of new CoCl2 complexes ligated with bis(iminophenyl)pyridine have been prepared, well characterized and examined in 1,3-butadiene polymerization. All complexes, activated by MAO, polymerized 1,3-butadiene to give high and tunable cis-1,4 selectivity with high activity. More efficient catalyst can be accessible by modifying the ligand structure.Figure optionsDownload full-size imageDownload as PowerPoint slideHighlights► The cis-1,4 polybutadiene is the most important elastomer. ► New CoCl2 complexes with bisiminopyridine are prepared and examined in butadiene polymerization. ► All complexes, activated by MAO, show high activity and tunable cis-1,4 selectivity. ► More efficient catalyst can be accessible by modifying the ligand structure.