Ethylene Polymerization with Ni(II) Diimine Complexes Generated from 8-Halo-1-naphthylamines: The Role of Equilibrating Syn/Anti Diastereomers in Determining Polymer Properties

Abstract

A series of 8-halonapthalen-1-amines (6a–d, X = F, Cl, Br, I) were prepared and converted to the α-diimines of 2,3-butanedione (7a–d). The nickel dibromide and zinc dibromide complexes of these diimines (3a–d and 8a–d, respectively) were obtained in good yields from standard precursors. NMR spectroscopic analysis of the zinc diimine complexes show the existence of syn and anti diastereomers with syn/anti ratios of ca. 2:1 (F), 2:1 (Cl), 1:1.5 (Br), and ca. 1:22 (I). Variable temperature NMR spectroscopy was used to calculate barriers to interconversion of these diastereomers which fall in the range 17–18.5 kcal/mol. Activation of nickel dibromide complexes 3a–d with modified methyl alumoxane (MMAO) yields cationic diimine complexes in which the 8-halo substituents lie over the axial coordination sites. Ethylene polymerization using these activated complexes is reported. The anti diastereomer of the diiodo catalyst (10d) in which both axial sites are blocked yields high molecular weight PE (ca. 106 g/mol) as the major fraction with a high turnover frequency at 40 °C. The minor syn diastereomer of 10d in which only one axial site is blocked produces low molecular weight PE as a minor fraction. PE formed from the dichloro catalyst 10b also exhibits a bimodal polymer distribution, but the high molecular weight fraction from the anti diastereomer is minor, while the syn diastereomer produces low molecular weight PE. The dibromo complex (10c) is unique in that interchange of diastereomers is on a time scale such that the PE produced shows a very broad molecular weight distribution (MWD ca. 14), spanning the range from the low to high molecular weight regimes. Catalyst 10a, bearing the small fluoro substituents, yields very low molecular weight PE (ca. 600 g/mol).

Cationic α-Diimine Nickel and Palladium Complexes Incorporating Phenanthrene Substituents: Highly Active Ethylene Polymerization Catalysts and Mechanistic Studies of syn/anti Isomerization

“…The contribution of activation entropies to the free energies of activation is very small, as expected. For comparison, interconversion between syn and anti isomers in zinc 8-halonaphthyl-α-diimine complexes proceeds with much lower barriers of 16.9–19.1 kcal/mol …”

“…An increase of temperature from 25 to 40 °C leads to a significant increase in TOF (from 0.82 × 10 6 to 1.89 × 10 6 h –1 , entry 3) and again a bimodal distribution is observed with a MWD of 6.6 (Figure S8). The branching density increases and the melting temperature decreases with increasing polymerization temperature, as is commonly observed using other nickel catalysts. , Specifically, complex 16a produces nearly linear polyethylene (1 branch per 1000 C) at 25 °C, and lightly branched polyethylene (9 branches per 1000 C) at 40 °C. The increase in branching with an increase in temperature is commonly ascribed to an increase in the rate of chain walking relative to the rate of chain growth .…”

“…Recently, we reported a series of complexes derived from 8halonapthalen-1-amine imines that are related to sandwich catalysts by virtue of substituting the capping aryl substituents with halogen atoms. 24 These complexes exist in solution as mixtures of syn and anti isomers with syn/anti ratios of 2/1 (X = F), 2/1 (Cl), 1/1.5 (Br), and 1/22 (I) (Figure 1, 3). The halo-substituted nickel complexes exhibit performance in ethylene polymerization dependent on the syn/anti ratios and their rates of interconversion.…”

“…For comparison, interconversion between syn and anti isomers in zinc 8-halonaphthyl-α-diimine complexes proceeds with much lower barriers of 16.9−19.1 kcal/mol. 24 In contrast to the palladium complexes derived from phenanthrene, complex 14b containing 6,7-dimethyl substituents exists as a single conformer. The 1 H NMR spectrum of 14b in methylene chloride-d 2 contains only one set of resonances.…”

“…If a rapid interconversion were taking place, a monomodal GPC trace would be observed. The behavior of complex 16a is consistent with that seen for nickel α-diimine catalysts generated from 8-halo-1-naphthylamines 24 and from unsymmetrical ligands. 34,35 The use of other cocatalysts including iBu 3 Al, Et 2 AlCl, and EtAlCl 2 (Table 2, entries 5−7; Figures S10−12) decreases the molecular weight distributions to 4.0, 3.4, and 3.0, respectively.…”