New tetradentate N,N,N,N-chelating α-diimine ligands and their corresponding zinc and nickel complexes: synthesis, characterisation and testing as olefin polymerisation catalysts

Abstract

A series of zinc complexes of the general formula {[ZnCl(ArNC(An)–C(An)NAr)]⁺}₂[Zn₂Cl₆]²⁻ (where Ar = 2-(1-benzyl-1H-1,2,3-triazol-4-yl)phenyl 2a, 2-(1-(1-phenylethyl)-1H-1,2,3-triazol-4-yl)phenyl 2b, 2-(1-phenyl-1H-1,2,3-triazol-4-yl)phenyl 2c; An = acenaphthene backbone) were prepared by the condensation of acenaphthenequinone with the corresponding o-triazolyl-substituted anilines (2-(1-benzyl-1H-1,2,3-triazol-4-yl)aniline 1a, 2-(1-(1-phenylethyl)-1H-1,2,3-triazol-4-yl)aniline 1b, 2-(1-phenyl-1H-1,2,3-triazol-4-yl)aniline 1c) which were formed by the copper(I)-catalyzed Huisgen[3+2] dipolar cycloaddition between 2-ethynylaniline and the corresponding azides in high yields, using anhydrous ZnCl₂ as the metal template, in boiling glacial acetic acid. Zinc complexes of the type [ZnCl(ArNC(An)–C(An)NAr)]⁺[ZnCl₃(NCCH₃)]⁻ (4a–c) were synthesized by crystallisation of the corresponding complexes 2a–c in acetonitrile, at −20 °C. After removal of zinc dichloride from complexes 2a–c by the addition of potassium oxalate, in dichloromethane, the tetradentate N,N,N,N-chelating α-diimine ligands of the type ArNC(An)–C(An)NAr (5a–c) were obtained. The new ligand precursors and zinc complexes were characterised by elemental analysis, ¹H and ¹³C{¹H} NMR spectroscopy, two-dimensional NMR spectroscopy, and X-ray diffraction. Reaction of the ligand precursors 5a–c with [NiBr₂(DME)], in dichloromethane, gave nickel complexes of the type [NiBr₂(ArNC(An)–C(An)NAr)] (6a–c). The results of single crystal X-ray diffraction characterisation and magnetic susceptibility measurements demonstrated that nickel complexes 6a–c possess octahedral geometries around the nickel atoms with variable configurations, the Br atoms of which can be ionized when dissolved in methanol. In preliminary catalytic tests, complexes 6a–c revealed to be active as catalysts for the polymerisation of norbornene and styrene, when activated by cocatalyst MAO. The characterisation of the polymers by ¹H and ¹³C{¹H} NMR spectroscopy, gel permeation chromatography/size-exclusion chromatography (GPC/SEC) revealed that these polymers were formed by a coordination addition mechanism.