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

Six examples of 2-(1-arylimino)ethyl-9-arylimino-5,6,7,8-tetrahydrocycloheptapyridine-cobalt(II) chloride complexes, [2-(1-ArN)C(2)H(3)-9-ArN-5,6,7,8-C(5)H(8)C(5)H(3)N]CoCl(2), (Ar = 2-(C(5)H(9))-6-MeC(6)H(3) Co1, 2-(C(6)H(11))-6-MeC(6)H(3) Co2, 2-(C(8)H(15))-6-MeC(6)H(3) Co3, 2-(C(5)H(9))-4,6-Me(2)C(6)H(2) Co4, 2-(C(6)H(11))-4,6-Me(2)C(6)H(2) Co5, and 2-(C(8)H(15))-4,6-Me(2)C(6)H(2) Co6), were synthesized by the direct reaction of the corresponding ortho-cycloalkyl substituted carbocyclic-fused bis(arylimino)pyridines (L1–L6) and cobalt(II) chloride in ethanol with good yields. All the synthesized ligands (L1–L6) and their corresponding cobalt complexes (Co1–Co6) were fully characterized by FT-IR, (1)H/(13)C-NMR spectroscopy and elemental analysis. The crystal structure of Co2 and Co3 revealed that the ring puckering of both the ortho-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 × 10(6) 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 ortho-cycloalkyl substituent: Cyclopentyl (Co1 and Co4) > cyclohexyl (Co2 and Co5) > cyclooctyl (Co3 and Co6) for either R = H or Me and afforded strictly linear polyethylene (T(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 (M(w): 9.2~25.3 kg mol(−1)).