New C_2v- and chiral C₂-symmetric olefin polymerization catalysts based on nickel(II) and palladium(II) diimine complexes bearing 2,6-diphenyl aniline moieties: Synthesis, structural characterization, and first insight into polymerization properties

Four new 1,4-diaza-2,3-dimethylbutadiene ligands (Ar-N=C(CH₃)-(H₃C)C=NAr; Ar: 3a = 2,6=diphenylphenyl; 3b = 2,6-di(4-OCH₃-phenyl)phenyl; 3c = 2,6-di(4-tert-butyl-phenyl)phenyl); 3d = 2,6-di(3,5-dimethylphenyl)phenyl as well as the palladium dichloride complexes 4a-c and methyl monochloride derivatives 5a-c were prepared, and their polymerization behavior was investigated. The corresponding nickel species 6a-c were tested for the insertion polymerization of ethene by in situ reactions of 3a-c with (DME)NiBr₂. The ligands are accessible by a three-step procedure. Aryl boronic acids were prepared by Grignard reactions of substituted aryl bromides and were coupled with 2,6-dibromo aniline according to a Suzuki cross-coupling protocol to give the corresponding terphenyl anilines 2a-d. Condensation of 2,3-butanedione with the corresponding aniline afforded the formation of the diimines 3a-d. The corresponding palladium dichloride complexes 4a-c are accessible by reaction with (PhCN)₂PdCl₂. The structures of 4a-c could be determined by X-ray analysis. While the terphenyl complex 4a adopts a C_2v-symmetry, 4c exists in a chiral C₂-symmetric coordination geometry, due to the repulsive interactions of the sterically demanding tert-butylphenyl substituents of the aniline moieties. All palladium and nickel complexes are catalysts for the polymerization of ethene. However, the chiral Ni(II) complex 6c shows by far the highest polymerization activity up to 2 × 10⁴ kg(PE) [mol (Ni) h]^-1. The polyethenes obtained with the palladium methyl monochloro catalysts activated with Na[(3,5-(CF₃)₂C₆H₃)₄B] and the nickel dibromo complexes activated with MAO are linear and show in the case of the Ni(II) derivatives molecular weights up to 4.5 × 10⁶ g mol^-1 (M_w/M_n ≈ 2), which can be controlled by addition of hydrogen.