Highly Linear Polyethylenes Achieved Using Thermo-Stable and Efficient Cobalt Precatalysts Bearing Carbocyclic-Fused NNN-Pincer Ligand

Abstract

Six examples of 2-(1-arylimino)ethyl-9-arylimino-5,6,7,8-tetrahydrocycloheptapyridine-cobalt(II) chloride complexes, [2-(1-ArN)C₂H₃-9-ArN-5,6,7,8-C₅H₈C₅H₃N]CoCl₂, (Ar = 2-(C₅H₉)-6-MeC₆H₃ <b>Co1</b>, 2-(C₆H₁₁)-6-MeC₆H₃ <b>Co2</b>, 2-(C₈H₁₅)-6-MeC₆H₃ <b>Co3</b>, 2-(C₅H₉)-4,6-Me₂C₆H₂ <b>Co4</b>, 2-(C₆H₁₁)-4,6-Me₂C₆H₂ <b>Co5</b>, and 2-(C₈H₁₅)-4,6-Me₂C₆H₂ <b>Co6</b>), were synthesized by the direct reaction of the corresponding <i>ortho</i>-cycloalkyl substituted carbocyclic-fused bis(arylimino)pyridines (<b>L1</b>⁻<b>L6</b>) and cobalt(II) chloride in ethanol with good yields. All the synthesized ligands (<b>L1</b>⁻<b>L6</b>) and their corresponding cobalt complexes (<b>Co1</b>⁻<b>Co6</b>) were fully characterized by FT-IR, ¹H/¹³C-NMR spectroscopy and elemental analysis. The crystal structure of <b>Co2</b> and <b>Co3</b> revealed that the ring puckering of both the <i>ortho</i>-cyclohexyl/cyclooctyl substituents and the one pyridine-fused seven-membered ring; a square-based pyramidal geometry is conferred around the metal center. On treatment with either methylaluminoxane (MAO) or modified methylaluminoxane (MMAO), all the six complexes showed high activities (up to 4.09 × 10⁶ g of PE mol^-1 (Co) h^-1) toward ethylene polymerization at temperatures between 20 °C and 70 °C with the catalytic activities correlating with the type of <i>ortho</i>-cycloalkyl substituent: Cyclopentyl (<b>Co1</b> and <b>Co4</b>) > cyclohexyl (<b>Co2</b> and <b>Co5</b>) > cyclooctyl (<b>Co3</b> and <b>Co6</b>) for either R = H or Me and afforded strictly linear polyethylene (<i>T</i>_m > 130 °C). The narrow unimodal distributions of the resulting polymers are consistent with single-site active species for the precatalyst. Furthermore, compared to the previously reported cobalt analogues, the titled precatalysts exhibited good thermo-stability (up to 70 °C) and possessed longer lifetime along with a higher molecular weight of PE (<i>M</i>_w: 9.2~25.3 kg mol^-1).