Investigating the effects of para-methoxy substitution in sterically enhanced unsymmetrical bis(arylimino)pyridine-cobalt ethylene polymerization catalysts

A group of five bis(arylimino)pyridine-cobalt(II) chloride complexes, [2-{(2,6-(Ph2CH)2-4-MeOC6H2)N=CMe}-6-(ArN=CMe)C5H3N]CoCl2 (Ar = 2,6-Me2C6H3 Co1, 2,6-Et2C6H3 Co2, 2,6-iPr2C6H3 Co3, 2,4,6-Me3C6H2 Co4, 2,6-Et2-4-MeC6H2 Co5), each containing one N-4-methoxy-2,6-dibenzhydrylphenyl group and one smaller sterically/electronically variable N-aryl group, have been synthesized in good yield (>71%) from the corresponding neutral terdentate nitrogen-donor precursor, L1–L5. All complexes have been characterized by 1H NMR and FT–IR spectroscopy with the former highlighting the paramagnetic nature of these cobaltous species and the unsymmetrical nature of the chelating ligand. The molecular structures of Co3 and Co4 emphasize the steric differences of the two inequivalent N-aryl groups and the distorted square pyramidal geometry about the metal centers. In the presence of MAO or MMAO, Co1–Co5 collectively displayed high activities for ethylene polymerization producing high molecular weight polyethylenes that, in general, exhibited narrow dispersities (Mw/Mn values: 2.12 – 4.07). Notably, the least sterically hindered Co1 when activated with MAO was the most productive (6.92 × 106 g PE (mol Co)-1 h-1) at an operating temperature of 60 oC. Conversely, the most sterically hindered Co3/MMAO produced the highest molecular weight polyethylene (Mw = 6.29 × 105 g mol-1). All the polymers displayed high linearity as demonstrated by their melting temperatures (>130 oC) and their 1H and 13C NMR spectra. By comparison of Co1 with its para-methyl, -chloro and -nitro counterparts, the presence of the para-methoxy substituent had the most noticeable effect of enhancing the thermal stability of the catalyst.