A family of six unsymmetrical N,N′-diiminoacenaphthene-nickel(II) bromide complexes, [1-{2,6-(Ph₂CH)₂-4-MeC₆H₂N}-2-(ArN)C₂C₁₀H₆]NiBr₂ (Ar = 2-(C₆H₁₁)-6-MeC₆H₂Ni1, 2-(C₅H₉)-6-MeC₆H₂Ni2, 2-(C₈H₁₅)-6-MeC₆H₂Ni3, 2-(C₆H₁₁)-4,6-Me₂C₆H₂Ni4, 2-(C₅H₉)-4,6-Me₂C₆H₂Ni5, 2-(C₈H₁₅)-4,6-Me₂C₆H₂Ni6), each bearing one ring-size variable 4-R-2-methyl-6-cycloalkyl-substituted N-aryl group and one N′-4-methyl-2,6-dibenzhydrylphenyl group, have been prepared and fully characterized. The molecular structures of Ni1, Ni2, Ni3 and Ni5 reveal distorted tetrahedral geometries with different degrees of steric protection imparted by the two inequivalent N-aryl groups. On activation with either EASC or MMAO, all the precatalysts are highly active (up to 17.45 × 10⁶ g PE mol⁻¹ (Ni) h⁻¹) for ethylene polymerization at 20–50 °C with their activities correlating with the type of cycloalkyl ortho-substituent: cyclooctyl (Ni6, Ni3) > the cyclopentyl (Ni5, Ni2) > cyclohexyl (Ni4, Ni1) for either R = H or Me. Moderately branched to hyperbranched polyethylenes (T_m's as low as 44.2 °C) can be obtained with molecular weights in the range 2.14–6.68 × 10⁵ g mol⁻¹ with the branching content enhanced by the temperature of the polymerization. Dynamic mechanical analysis (DMA) and monotonic tensile stress–strain tests have been employed on the polyethylene samples and reveal the more branched materials to show good elastic recovery properties (up to 75.5%).

The syntheses of six bis(imino)-5,6,7,8-tetrahydrocycloheptapyridine-iron(II) chloride complexes, [2-{(Ar)NCMe}-9-{N(Ar)}C₁₀H₁₀N]FeCl₂ (Ar = 2-(C₅H₉)-6-MeC₆H₃Fe1, 2-(C₆H₁₁)-6-MeC₆H₃Fe2, 2-(C₈H₁₅)-6-MeC₆H₃Fe3, 2-(C₅H₉)-4,6-Me₂C₆H₂Fe4, 2-(C₆H₁₁)-4,6-Me₂C₆H₂Fe5, 2-(C₈H₁₅)-4,6-Me₂C₆H₃Fe6), are reported in which the ring size of the ortho-cycloalkyl group has been varied as has the type of para-substituent. The molecular structures of Fe3 and Fe6 reveal square pyramidal geometries at iron while the ortho-cyclooctyl rings adopt boat-chair conformations. On treatment with either methylaluminoxane (MAO) or modified methylaluminoxane (MMAO), all six complexes showed optimal activities at 80 °C [up to 1.9 × 10⁷ g of PE per mol Fe per h for Fe5/MMAO] for ethylene polymerization forming linear polyethylene (T_m's > 126 °C). Notably, the catalytic activities showed a marked correlation with the ring size of the ortho-cycloalkyl substituent: cyclohexyl (Fe2 and Fe5) > cyclooctyl (Fe3 and Fe6) > cyclopentyl (Fe1 and Fe4) for either para-substituent, H or Me. Furthermore, this family of iron catalysts exhibited remarkable thermostability by remaining highly active even at temperatures as high as 100 °C (1.1 × 10⁷ g of PE per mol Fe per h); the wide variation in polymer molecular weights (M_w: 2.4–166 kg mol⁻¹), influenced through choice of precatalyst and co-catalyst as well as by temperature and pressure, further highlights the versatility of these catalysts.

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 Co1, 2-(C₆H₁₁)-6-MeC₆H₃ Co2, 2-(C₈H₁₅)-6-MeC₆H₃ Co3, 2-(C₅H₉)-4,6-Me₂C₆H₂ Co4, 2-(C₆H₁₁)-4,6-Me₂C₆H₂ Co5, and 2-(C₈H₁₅)-4,6-Me₂C₆H₂ 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, ¹H/¹³C-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 × 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 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).

A one-pot template strategy has been utilized to synthesize sterically enhanced bis(imino)cyclohepta[b]pyridine-cobalt(II) chlorides, [2-{(Ar)NCMe}-9-{N(Ar)}C₁₀H₁₀N]CoCl₂ (Ar = 2-(C₅H₉)-4,6-(CHPh₂)₂C₆H₂Co1, 2-(C₆H₁₁)-4,6-(CHPh₂)₂C₆H₂Co2, 2-(C₈H₁₅)-4,6-(CHPh₂)₂C₆H₂Co3, 2-(C₁₂H₂₃)-4,6-(CHPh₂)₂C₆H₂Co4, 2,6-(C₅H₉)₂-4-(CHPh₂)C₆H₂Co5). All five complexes have been characterized by a combination of FT-IR spectroscopy, elemental analysis and single crystal X-ray diffraction. The molecular structures of Co1, Co3 and Co5 highlight the substantial steric hindrance imparted by the 2-cycloalkyl-6-benzhydryl or 2,6-dicyclopentyl ortho-substitution pattern; distorted square pyramidal geometries are exhibited in each case. On activation with methylaluminoxane (MAO) or modified methylaluminoxane (MMAO), all the complexes (apart from Co4/MAO) were active ethylene polymerization catalysts (up to 3.70 × 10⁶ g PE per mol (Co) per h for Co5/MMAO), operating effectively at temperatures between 50 °C and 60 °C, producing polyethylenes with high molecular weights (up to 589.5 kg mol⁻¹ for Co3/MAO). Furthermore, all polymers were highly linear (T_m > 130 °C) with narrow dispersities (M_w/M_n range: 2.0–3.0). The coexistence of two chain termination pathways, β-H elimination and transfer to aluminum, has been demonstrated using ¹³C/¹H NMR spectroscopy.