Nickel(II) complexes of bidentate N-N' ligands containing mixed pyrazole, pyrimidine and pyridine aromatic rings as catalysts for ethylene polymerisation

Abstract

This work describes the synthesis and characterisation of Ni(II) complexes of the following neutralbidentate nitrogen ligands containing pyrazole (pz), pyrimidine (pm) and pyridine (py) aromatic rings: 2-pyrazol-1-yl-pyrimidine (pzpm), 2-(4-methyl-pyrazol-1-yl)-pyrimidine (4-Mepzpm), 2-(4-bromo-pyrazol-1-yl)-pyrimidine (4-Brpzpm), 2-(3,5-dimethyl-pyrazol-1-yl)-pyrimidine (pz*pm), 2-pyrazol-1-ylpyridine (pzpy) and bis(3,5-dimethylpyrazol-1-yl)phenylmethane (bpz*mph). The complexes[NiBr2(pzpm)] (1), [NiBr2(4-Mepzpm)] (2), [NiBr2(4-Brpzpm)] (3), [NiBr2(pz*pm)] (4), [NiBr2(pzpy)] (5)and [NiBr2(bpz*mph)] (6) were tested as catalysts for ethylene polymerisation, in the presence of thecocatalysts methylaluminoxane (MAO) or diethylaluminium chloride (AlEt2Cl), the catalyst systems 1-3/MAO showing moderate to high activities up to the temperature of 20 C only in the presence of MAO,whereas 4-6/MAO revealed to be inactive. Other related Pd(II) complexes, already reported in previousworks, such as [PdClMe(pzpm)], [PdClMe(pz*pm)], [PdClMe(pzpy)] and [PdClMe(bpz*mph)], alsoshowed to be inactive in the polymerisation of ethylene, when activated by MAO or AlEt2Cl. Selectedsamples of polyethylene products were characterised by GPC/SEC, 1H and 13C NMR and DSC, showing tobe low molecular weight polymers with Mn values ranging from ca. 550 to 1500 g mol1 and unusuallylow dispersities of 1.2e1.7, with total branching degrees generally varying between 2 and 12%, meltingtemperatures from 40 to 120 C and crystallinities from 40 to 70%.