Bis(imino)pyridines Incorporating Doubly Fused Eight-Membered Rings as Conformationally Flexible Supports for Cobalt Ethylene Polymerization Catalysts

Five distinct α,α′-bis(arylimino)-2,3:5,6-bis(hexamethylene)pyridine-cobalt(II) chloride complexes, [2,3:5,6-{C 5 H 10 C(NAr)} 2 C 5 HN]CoCl 2 (Ar = 2,6-Me 2 C 6 H 3 , Co1; 2,6-Et 2 C 6 H 3 , Co2; 2,6-i-Pr 2 C 6 H 3 , Co3; 2,4,6-Me 3 C 6 H 2 , Co4; 2,6-Et 2 -4-MeC 6 H 2 , Co5), have been synthesized by one-pot template reaction of α,α′-dioxo-2,3:5,6-bis(hexamethylene)pyridine with the corresponding aniline and cobalt(II) chloride in acetic acid. The molecular structures of Co2 and Co3 reveal distorted square pyramidal geometries with two conformationally flexible eight-membered rings, a feature of the ligand manifold. On activation with either methylaluminoxane (MAO) or modified MAO (MMAO), all five complexes displayed high catalytic activities for ethylene polymerization (up to 3.62 × 10 6 g PE mol -1 (Co) h -1 ) generating strictly linear polyethylenes with the activity decreasing in the orders: Co4 > Co5 > Co1 > Co2 > Co3 (MAO) and Co1 > Co4 > Co2 > Co5 > Co3 (MMAO). Moreover, the nature of the aluminoxane cocatalyst employed had a marked effect on the molecular weight and distribution (M w /M n ) of the polymeric material obtained. For example using MAO, high molecular weight polyethylene (M w ≈ 10 5 g mol -1 ) with a narrow monomodal molecular weight distribution (2.1-3.6) was a characteristic, while using MMAO, the polyethylene was of lower molecular weight (M w ≈ 10 4 g mol -1 ) and exhibited a mono- to bimodal distribution (10.1-23.8) depending on the ratio of MMAO employed, temperature, and run time.