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

This study was aimed at elucidating the role of Ph 3 P 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 [NiL 2 (CH 3 CN) 2 ] 2+ [NiBr 3 (PPh 3 )] 2 − and [NiL 2 Br] + [NiBr 3 (PPh 3 )] − have been synthesized. The structures of these compounds have been confirmed by X-ray diffraction. Individual and in situ complexes with PPh 3 are in equilibria between different forms in solution. These forms include the free complex and free PPh 3 , and molecular and ionic complexes with coordinated triphenylphosphine. All individual compounds were active in ethylene dimerization upon activation with Et 2 AlCl, 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.