Nickel complexes based on BIAN ligands: transformation and catalysis on ethylene polymerization

Abstract

Treatment of bis(arylimino)acenaphthene (^ArBIAN) with Ni(COD)₂ in toluene afforded ^dmpBIANNi(COD) (2a, dmp = 2,6-Me₂C₆H₃) and ^dippBIANNi(COD) (2b, dipp = 2,6-ⁱPrC₆H₃), respectively, in moderate yields. Complexes 2a and 2b can be oxidized by a small amount of oxygen at low temperature leading to oxygen-bridged dinuclear Ni(II) complexes (^dmpBIANNi)₂(μ-O)₂ (4a) and (^dippBIANNi)₂(μ-O)₂ (4b), respectively, as a purple powder. The reaction of ^ArBIAN with 0.5 equiv of Ni(COD)₂ or Ni(Ph₃P)₄ gave bisligated complexes (^dmpBIAN)₂Ni (3a) and (^dippBIAN)₂Ni (3b), which can be considered as Ni(0) complexes supported by two neutral BIAN ligands. Oxidation of the bisligated nickel complexes 3a and 3b with [Cp₂Fe][B(C₆F₅)₄] afforded cationic Ni(I) complexes [(^dmpBIAN)₂Ni][B(C₆F₅)₄] (5a) and [(^dippBIAN)₂Ni][B(C₆F₅)₄] (5b), respectively, in which the Ni(I) centre is chelated by two neutral Ar-BIAN ligands. These complexes were characterized by NMR and IR spectroscopy and DFT calculation, and the molecular structures of 3b, 4b, and 5b were well established by X-ray diffraction analysis. These complexes were evaluated as catalysts for ethylene polymerization in which 2b showed high activity in the presence of AlMe₃. ¹³C NMR analysis of polymers showed that the 2b/AlMe₃ catalytic system gave less-branched polymers when compared to that obtained with ^dippBIANNiBr₂ under the same conditions.