Catalytic systems based on nickel(ii) complexes with bis(3,5-dimethylpyrazol-1-yl)methane – impact of PPh3 on the formation of precatalysts and selective dimerization of ethylene

Abstract

This study was aimed at elucidating the role of Ph₃P in the formation of Ni(II) complexes which are active in ethylene oligomerization. Two ionic Ni(II) complexes bearing bis(3,5-dimethylpyrazol-1-yl)methane ligand [NiL₂(CH₃CN)₂]²⁺[NiBr₃(PPh₃)]₂⁻ and [NiL₂Br]⁺[NiBr₃(PPh₃)]⁻ have been synthesized. The structures of these compounds have been confirmed by X-ray diffraction. Individual and in situ complexes with PPh₃ are in equilibria between different forms in solution. These forms include the free complex and free PPh₃, and molecular and ionic complexes with coordinated triphenylphosphine. All individual compounds were active in ethylene dimerization upon activation with Et₂AlCl, producing a mixture of butenes with activities up to 960 kg mol[Ni]⁻¹ h⁻¹ atm⁻¹ and high selectivity (up to 100% of butenes and up to 90.5% of butene-1). The catalytic influence of triphenylphosphine has been evaluated – it increases the activity of the system up to 1800 kg mol[Ni]⁻¹ h⁻¹ atm⁻¹ when 2 mol. equiv. of additive has been applied and completely changes the selectivity of the process towards internal olefins.