Four examples of para-nitro substituted 2-(arylimino)pyridine-nickel(II) bromide complexes of general formula, [2-{(2,6-R-4-NO₂C₆H₂)N=CMe}C₅H₄N]NiBr₂, but differentiable by the steric/electronic properties displayed by the ortho-groups [R = i-Pr (Ni1), Et (Ni2), CHPh₂ (Ni3), CH(4-FPh)₂ (Ni4)], have been prepared in good yield. For comparative purposes, the meta-nitro complex, [2-{(2,6-i-Pr₂-3-NO₂-4-(4-FPh)₂C₆H)N=CMe}C₅H₄N]NiBr₂ (Ni5), has also been synthesized. The molecular structures of mononuclear Ni3·xH₂O (x = 2, 3) and bromide-bridged dinuclear Ni4 and Ni5 are disclosed. Upon activation with either ethylaluminum dichloride (EtAlCl₂) or modified methylaluminoxane (MMAO), all precatalysts displayed good catalytic performance at operating temperatures between 30 °C and 60 °C with higher activities generally seen using EtAlCl₂ [up to 4.7 × 10⁶ g PE (mol of Ni)⁻¹ h⁻¹]: Ni2 ~ Ni5 > Ni1 ~ Ni4 > Ni3. In terms of the resultant polyethylene (PE), Ni4/EtAlCl₂ formed the highest molecular weight of the series (M_w up to 1.4 × 10⁵ g mol⁻¹) with dispersities (M_w/M_n) ranging from narrow to broad (M_w/M_n range: 2.2–24.4). Moreover, the melting temperatures (T_m) of the polymers generated via EtAlCl₂ activation fell in a narrow range, 117.8–126.0 °C, which resembles that seen for industrial-grade linear-low density polyethylene (LLDPE). Indeed, their ¹³C NMR spectra revealed significant amounts of uniformly distributed long-chain branches (LCBs), while internal vinylene groups constituted the major type of chain unsaturation [vinylene:vinyl = 5.3:1 (EtAlCl₂) and 9.9:1 (MMAO)].

The cobalt(II) chloride N,N,N-pincer complexes, [2-{(2,4-(C₁₅H₁₃)₂-6-FC₆H₂)N=CMe}-6-(ArN=CMe)C₅H₃N]CoCl₂ (Ar = 2,6-Me₂C₆H₃) (Co1), 2,6-Et₂C₆H₃ (Co2), 2,6-i-Pr₂C₆H₃ (Co3), 2,4,6-Me₃C₆H₂ (Co4), 2,6-Et₂-4-MeC₆H₂ (Co5), and [2,6-{(2,4-(C₁₅H₁₃)₂-6-FC₆H₂)N=CMe}₂C₅H₃N]CoCl₂ (Co6), each containing at least one N-2,4-bis(dibenzosuberyl)-6-fluorophenyl group, were synthesized in good yield from their corresponding unsymmetrical (L1–L5) and symmetrical bis(imino)pyridines (L6). The molecular structures of Co1 and Co2 spotlighted their distorted square pyramidal geometries (τ₅ value range: 0.23–0.29) and variations in steric hindrance offered by the dissimilar N-aryl groups. On activation with either MAO or MMAO, Co1–Co6 all displayed high activities for ethylene polymerization, with levels falling in the order: Co1 > Co4 > Co5 > Co2 > Co3 > Co6. Indeed, the least sterically hindered 2,6-dimethyl Co1 in combination with MAO exhibited a very high activity of 1.15 × 10⁷ g PE mol⁻¹ (Co) h⁻¹ at the operating temperature of 70 °C, which dropped by only 15% at 80 °C and 43% at 90 °C. Vinyl-terminated polyethylenes of high linearity and narrow dispersity were generated by all catalysts, with the most sterically hindered, Co3 and Co6, producing the highest molecular weight polymers [M_w range: 30.26–33.90 kg mol⁻¹ (Co3) and 42.90–43.92 kg mol⁻¹ (Co6)]. In comparison with structurally related cobalt catalysts, it was evident that the presence of the N-2,4-bis(dibenzosuberyl)-6-fluorophenyl groups had a limited effect on catalytic activity but a marked effect on thermal stability.

Given the great importance of cobalt catalysts supported by benchmark bis(imino)pyridine in the (oligo)polymerization, a series of dibenzopyran-incorporated symmetrical 2,6-bis(imino) pyridyl cobalt complexes (Co1–Co5) are designed and prepared using a one-pot template approach. The structures of the resulting complexes are well characterized by a number of techniques. After activation with either methylaluminoxane (MAO) or modified MAO (MMAO), the complexes Co1–Co4 are highly active for ethylene polymerization with a maximum activity of up to 7.36 × 10⁶ g (PE) mol⁻¹ (Co) h⁻¹ and produced highly linear polyethylene with narrow molecular weight distributions, while Co5 is completely inactive under the standard conditions. Particularly, complex Co3 affords polyethylene with high molecular weights of 85.02 and 79.85 kg mol⁻¹ in the presence of MAO and MMAO, respectively. The ¹H and ¹³C NMR spectroscopy revealed the existence of vinyl end groups in the resulting polyethylene, highlighting the predominant involvement of the β-H elimination reaction in the chain-termination process. To investigate the mechanism underlying the variation of catalytic activities as a function of substituents, multiple linear regression (MLR) analysis was performed, showing the key role of open cone angle (θ) and effective net charge (Q) on catalytic activity.