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

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).