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 α,α′-bis(imino)-2,3:5,6-bis(pentamethylene)pyridyl-cobalt(II) chlorides, [2,3:5,6-{C₄H₈C(N(2-R¹-4-R³-6-R²C₆H₂))}₂C₅HN] CoCl₂ (R¹ = Me, R² = R³ = CH(p-FPh)₂Co1; R¹ = Et, R² = R³ = CH(p-FPh)₂Co2; R¹ = i-Pr, R² = R³ = CH(p-FPh)₂Co3; R¹ = Cl, R² = R³ = CH(p-FPh)₂Co4; R¹ = F, R² = R³ = CH(p-FPh)₂Co5; R¹ = F, R² = R³ = CHPh₂Co5′′, R¹ = R² = Me, R³ = CH(p-FPh)₂Co6; R¹ = R³ = Me, R² = CH(p-FPh)₂Co7), have been synthesized by a one-pot template reaction of α,α′-dioxo-2,3:5,6-bis(pentamethylene)pyridine, cobalt(II) chloride and the respective aniline in n-butanol. By contrast, the mixed cobalt(II) chloride/acetate complex, [2,3:5,6-{C₄H₈C(N(2-F-4,6-(CH(p-FPh)₂)₂C₆H₂))}₂C₅HN]CoCl(OAc) (Co5′), was isolated when the corresponding template reaction was carried out in acetic acid. Structural characterization of Co4, Co5 and Co5′′ revealed distorted square pyramidal geometries while six-coordinate Co5′, incorporating a chelating acetate ligand, exhibited a distorted octahedral geometry. On activation with either MAO or MMAO, 2-fluoride-4,6-bis{di(p-fluorophenyl)methyl}-substituted Co5 showed maximum catalytic activity for ethylene polymerization at a high operating temperature of 60 °C (up to 2.1 × 10⁷ g (PE) mol⁻¹ (Co) h⁻¹), producing highly linear (T_ms > 121 °C), low molecular weight polyethylene waxes (M_w range: 1.5–5.0 kg mol⁻¹) with narrow dispersity (M_w/M_n range: 1.7–2.9). End-group analysis of the waxes reveals β-H elimination as the dominant chain transfer process.

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.

Based on iminopyridine ligands (L1–L3), Fe(II) (Fe1–Fe3) and Co(II) (Co1–Co3) complexes were synthesized and characterized well. According to single-crystal XRD studies, Co3 exhibits a dimeric structure with a four-coordinated metal center. Whereas, the iron complexes (Fe1 and Fe3) coordinate with two ligands to form a distorted-octahedral and distorted-trigonal bipyramidal geometry, respectively. All these catalysts were tested for the polymerization of isoprene. Using MAO as the co-catalyst, the Fe3 catalyst containing a bulky substituent can provide trans-1,4-unit (64%) polyisoprenes, while the Fe1 catalyst afforded trans-1,4-unit polyisoprenes (32%). Conversely, the combination of EASC as the co-catalyst with Fe1–Fe3 catalysts can give 1,4-unit polyisoprenes with high trans-1,4-unit selectivity (up to 88%). The central metal atom of the catalysts plays a crucial role in isoprene polymerization and significantly affects their activity. Fe(II) catalysts showed the highest activity (up to 99% yield), and afforded polyisoprenes with high molecular weights (191 kg mol⁻¹). However, Co(II) systems exhibited lower activity (60% yield) and the obtained polyisoprenes were of lower molecular weights (2.15 kg mol⁻¹) with broad molecular weight distribution.

Six examples of dinuclear bis(imino)pyridine-cobalt(II) complexes, [1,5-{2-(CMeN)-6-(CMeN(2,6-R¹ ₂-4-R²-C₆H₂))C₅H₃N}₂(C₁₀H₆)]Co₂Cl₄ (R¹ = Me, R² = H Co1; R¹ = Et, R² = H Co2; R¹ = ⁱPr, R² = H Co3; R¹ = Me, R² = Me Co4; R¹ = Et, R² = Me Co5; R¹ = CHPh₂, R² = Me Co6), have been prepared from the corresponding bis(tridentate) compartmental ligands (L1–L6) in reasonable yields. The molecular structures of Co3 and Co5 revealed two N,N,N-cobalt dichloride units to adopt anti-positions about the 1,5-naphthyl linking unit, with each cobalt center exhibiting a distorted trigonal bipyramidal geometry. On activation with either MAO or MMAO, Co1–Co6 were shown to promote both polymerization and oligomerization of ethylene with high overall activities (up to 1.03 × 10⁷ gPE per·mol(Co) per·h for Co1/MAO at 70 °C). Curiously, on increasing the reaction temperature a larger proportion of polymer was noted, while at lower temperature an enhanced selectivity for oligomer was seen. In general, the oligomeric products displayed Schulz–Flory distributions with high selectivities for α-olefins (>99%). On the other hand, the highly linear polymers displayed narrow dispersities and comprised both fully saturated and unsaturated chain ends with the vinyl content (–CHCH₂) found to rise with the reaction temperature. By modulating the steric hindrance exerted by the ortho-R¹ substituents in the precatalyst, polyethylenes displaying a remarkably broad range of molecular weights could be obtained [from 4.52 kg mol⁻¹ (R¹ = Me) to 246.7 kg mol⁻¹ (R¹ = CHPh₂)].