Bis(imino)pyridines fused with 6- and 7-membered carbocylic rings as N,N,N-scaffolds for cobalt ethylene polymerization catalysts

Abstract

The unsymmetrical diketone, 1,2,3,7,8,9,10-heptahydrocyclohepta[b]quinoline-4,6-dione, based on a central pyridine unit fused by both 6- and 7-membered rings, has been synthesized via a sequence of reactions including ruthenium-catalyzed coupling cyclization. Templating this diketone with a mixture of cobalt(II) chloride hexahydrate and the corresponding aniline in acetic acid at reflux afforded five examples of carbocyclic-fused bis(arylimino)pyridine-cobalt(II) chlorides (aryl = 2,6-Me₂Ph Co1, 2,6-Et₂Ph Co2, 2,6-i-Pr₂Ph Co3, 2,4,6-Me₃Ph Co4, 4-Me-2,6-Et₂Ph Co5) in good yield. All cobalt complexes have been fully characterized including by ¹H NMR spectroscopy which reveals broad but assignable paramagnetically shifted peaks. The molecular structures of Co1, Co3 and Co4 highlight the inequivalency of the two fused rings with the cobalt center adopting a distorted trigonal bipyramidal geometry. Treatment of Co1–Co5 with MAO gave highly active catalysts (up to 5.03 × 10⁶ g PE mol⁻¹ (Co) h⁻¹ at 40 °C, with Co4 > Co5 > Co1 > Co2 > Co3) for ethylene polymerization generating strictly linear vinyl-terminated polymers with low molecular weights (M_w range: 1.53–22.77 kg mol⁻¹). By comparison, polymerizations conducted using Co1–Co5/MMAO were less active and displayed a lower selectivity for unsaturated polymers. Common to both MAO and MMAO, the most sterically hindered precatalyst Co3 gave the highest molecular weight polymer of the series (up to 22.77 kg mol⁻¹) but exhibited the lowest activity.